2,445 m. The gob of each model was divided into 3 zones of different permeability: ...... inorganic chlorides such as NaCl and CaCl. Using the same ... bentonite spread in a foam-liquid solution reduces the surface exposed to air, thus reducing the ...... 0.0950. 0.0900. 0.0850. 0.0800. 0.0750. 0.0700. 0.0651. 0.0601. 0.0551.
COMPUTATIONAL STUDY COMPUTATIONAL FLUID FLUID DYNAMICS DYNAMICS SIMULATION SIMULATION STUDY ON ON HOT HOT SPOT SPOT LOCATION LOCATION IN IN A A LONGWALL LONGWALL MINE MINE GOB
by by Samuel Samuel Atta Atta Lolon
submitted to the faculty of A thesis submitted of The University of of Utah fulfillment of of the requirements for the degree of of in partial fulfillment
of Science Science Master of
Engineering Department of Mining Engineering Utah The University of Utah 2008 December 2008
Copyright Copyright © Samuel Atta Lolon 2008 All Rights Reserved
THE
UNIVERSITY
OF UTAH
GRADUATE
SCHOOL
SUPERVISORY COMMITTEE APPROVAL
of a t hesis submitted by
Samuel Atta Lolon
This thesis has been read by each member of the following supervisory com m i ttee and by majori£)' vote has been found to be satisfactory.
Chair:
Felipe CaIJzaya
Michael K. McCarter
D, Kip Solomon
THE UNIVERSITY
OF UTAH
GRADUATE SCHOOL
FINAL READING APPROVAL
To the Graduate Councj I of the Unive rs i ty of Utah:
Samuel Atta Lolon
I have read the and have
thesis of fo u nd that (1)
in its final fonn
its fonnat, citations, and bibliographic style are consistent and
acceptabJe; (2) its illustrative materials includ i ng figures, tables, and charts are in place; and
(3)
the finaJ
m a n uscript
is satisfactory to
submission to The Graduate School
Date
the
supervisory committee and is ready for
.
..
Felipe Calizaya Chair: Supervisory Commltlee
Approved for the Major Department
Michael K. McCarter ChairlDean
Approved for the Graduate Council
C'Q�
. David S. Chapman
Dean of The Graduate School
____
�
ABSTRACT
Spontaneous combustion combustion is one of of the main sources for mine fires in underground coal mines. Most of of these fires are initiated in the longwall gob (caved area) by coal oxidation. Because coal oxidation generates heat, this phenomenon phenomenon is called the selfheating process. This process will eventually create hot spots under conditions, i.e.,
of at least 5% (by volume) and gob temperatures of of 100°C. Coal oxygen concentrations of ventilation system are properties, gob permeability, self-heating self-heating characteristics, and the ventilation of these hot spots. the key variables for the formation formation of of hot spots. The study is based on A study was carried out to identify identify the location of ventilation surveys, laboratory experiments, and gob simulations using mine ventilation Computational Fluid Dynamics (CFD). Ventilation Ventilation surveys were conducted conducted in an existing Computational laboratory experiments were longwall mine located in the western United States; the laboratory performed on a physical gob model to investigate permeability permeability (k) and airflow performed airflow behavior in the distribution; and the CFD models were simulated to investigate the flow behavior gob, the oxidation of of coal, and heat transfer transfer phenomena. Four CFD models were formulated formulated and solved, three utilized a bleeder bleeder ventilation ventilation system, and the fourth a bleederless ventilation ventilation system. For these models, the gob length varied from 912 m to of each model was divided into 3 zones of of different different permeability: 2,445 m. The gob of unconsolidated (k = 4.68 xlO" semi-consolidated (k= (k= 3.15 x 1010"8 m 2), unconsolidated (k= xl0- 7 m 2), ), semi-consolidated ), and 7
2
consolidated 10"9 m 2). consolidated (k == 7.98 x 10). 9
2
8
2
The simulation results showed that in the models ventilated by a bleeder system,
the hot spot was located in the consolidated consolidated zone near the return side of of the gob. Once initiated, it propagated propagated along the tailgate side as the gob progressed. The leakage flow through the gob played an important important role in determining the size and location of of the hot
spot. In models ventilated by a bleederless system, the hot spot was located in the gob by the face line. This is mainly caused by the air leakage from the headgate T junction (face) and between further into the gob depending on the gob between the shields. It may extend further permeability and the fan pressure. permeability In addition, these gob simulation simulation exercises have shown that the hot spot areas in
all cases can be located located accurately. This information information can be used to develop suitable control methods. The parametric studies have indicated indicated that the ventilation system and permeability are the major of hot spots. gob permeability major contributing factors for the formation formation of Although the gob models were developed for specific specific dimensions and ventilation ventilation system, the results can be applied to other schemes with minor adjustments. minor adjustments.
vv
To my parents: Jan and Elisabeth Lolon, for their love and prayers
TABLE TABLE OF OF CONTENTS
ABSTRACT .. . . . . . .. .. . . . . .. . . .. . .. . ... .. . .. . .. . .. ... ... . .. ... ... . . . .. . .. .... . . .. .. .. .. .. . ..... ... ABSTRACT
IV iv
LIST OF OF TABLES TABLES ..... .. ..... ...... .. .. .. .... .... .... .. .. ... .. ... .. .. .... ..... . ... . .... .... .. ... LIST
xx
LIST OF OF FIGURES FIGURES... .. ... .. ... ........... .... .. .... .. ........ ... ..... ....... ........ ......... . xii XlI LIST ACKNOWLEDGMENTS.... ... ... .. .... ... ...... ..... ........... ... ........................ ACKNOWLEDGMENTS
xv
CHAPTER CHAPTER INTRODUCTION.................... .................... ....... .. .... .. .... .. ..... .......... 1. INTRODUCTION
11
1.1 1.2
Statement of of Problems Problems.... ........ ..... .. .. ........ .. .... . ....... .. .... ..... ........... Statement
11
Thesis Overview. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ............
4
1.2
Thesis Overview
4
2. BACKGROUND AND LITERATURE REVIEW ....... ....... ... .. ............ .....
6
2. BACKGROUND AND LITERATURE REVIEW
6
2.1 2.1 2.2
Longwall Mines in the United States.................. ...... .......... ...... ......
6
3.1 3.1 3.2
Longwall Mine Gob....... ...... ............. .. .. .. ...... ........................... . 36 Gob 36 and Its Characteristics............................ Characteristics 40 Gob Material and ........ ........... 40
Longwall in the States VentilationMines Systems forUnited Longwall Mines .. .. .. .... .. .. .. .. .. .. .. .. .... .. ...... .. . 106 2.2.1 U-Tube System... ... .. . ... ... ... ... ... . .... . ... .. . . . . ... . .. ......... . .. . ..... 10 2.2 Ventilation Systems for Longwall Mines 10 2.2.2 Y System. ... ......... ...... .... ... .. ..... .... ............... .................. 12 2.2.1 U-Tube System 10 2.2.3 Wrap-Around System.............. .............. .. ......................... 14 2.2.2 Y System 12 2.3 2.2.3 Spontaneous Combustion in the Gob .... ... .... , .. . . .. ... . . . . .. . . . ...... . . .. .. . . .. 15 Wrap-Around System 14 Mechanism of Self-Heating 2.3 2.3.1 Spontaneous Combustion in the GobProcess .... .... .... ... .. .. ............... ... 16 15 2.3.2 Spot Occurrence. 2.3.1 Requisites Mechanismfor of Hot Self-Heating Process .. . .. .. . . .. ... . .. .. .... . . .. . .... .. .. . 18 16 2.3.3 Prediction of Spontaneous Combustion Potential. .. ... . .. .. . ..... ... .. 19 2.3.2 Requisites for Hot Spot Occurrence 18 2.3.4 to Self-Heating Process.............. ........ .. .... 23 2.3.3 Contributing Prediction of Factors Spontaneous Combustion Potential 19 2.3.5 Control Methods........ .. ... ... .......... ..... .... . .... ............. ........ 26 2.3.4 Contributing Factors to Self-Heating Process 23 2.4 Spontaneous Combustion 2.3.5 Control Methods Studies Using CFD .. ...... ........ ...... .. .. ......... 28 26 2.5 Porous Medium. ....... ..... ...... ... .... ..... .............. ... ... ..... ..... ..... ... .. 30 2.4 Spontaneous Combustion Studies Using CFD 28 2.5 2.5.1 Porous Particle MediumSize Distribution. ..... ........... .... ...... ... ................. .... 30 30 Porosity........................ ... ... ... .. ..... .. .. .... .... ... .. ... ... ......... 31 2.5.2 2.5.1 Particle Size Distribution 30 2.5.3 2.5.2 Specific Porosity Permeability..................... . ....... .. ...... .. .. ... ... ........ 32 31 2.5.3 Specific Permeability 32 3. CHARACTERISTICS OF GOB MATERIAL ............................ ........ ...... 36 36
3.2.1 Particle Particle Size Size Selection Selection.................................. ........ ............ 3.2.1 3.2.2 Packing Packing and and Particle Particle Shape Shape.................................. ............... 3.2.2 3.3 Permeability Permeability Tests Tests............................................................ ........ 3.3 Preparation........................................ .................. 3.3.1 Sample Sample Preparation 3.3.1 3.3.2 Water-Based Method............................................ ........... 3.3.2 Water-Based Method 3.3.3 Air-Based Air-Based Method Method........................................................... 3.3.3 3.4 Specific Specific Permeability Permeability of ofGob Gob Material Material...... ........ .......... .................... 3.4
40 40 41 41 42 42 43 43 44 44 50 50 55 55
4. RESEARCH RESEARCH METHODOLOGIES METHODOLOGIES .................................... '" .. . .......... .. 4.
58 58
4.l 4.1
4.2 4.2
4.3
Physical Model Model ... '" ........................................................... " .... Physical 4.1.1 Simulated Simulated Airway Airway. .. ... .. .... ....... .. ... .. .. . .... . .. . .. ......... . .... .. .. .... 4.1.1 4.l.2 Fan Fan and and Regulator Regulator ............................................................ 4.1.2 Computational Fluid Fluid Dynamics Dynamics Model Model ............... '" .. . .. .... . .... .. ...... . ... Computational Introduction................................................................... 4.2.1 Introduction 4.2.2 Airflow Airflow Simulation Simulation (Without (Without Oxidation) Oxidation) ................................ . 4.2.2 Model Similitude Similitude ................................................................... .. Model 4.3.1 Similitude Similitude Concept Concept .......................................................... . 4.3.2 Similitude Similitude Validation Validation ....................................................... . 4.3.2 Model Calibration Calibration ........................................................... . 4.3.3 Model
5. HOT HOT SPOT SPOT LOCATION LOCATION - CFD SIMULATION SIMULATION EXERCISES EXERCISES .....................
5.1
5.2
5.3
Assumptions................................................................... Basic Assumptions Geometry.................................................. 5.l.1 Longwall Longwall Mine Geometry 5.1.1 5.1.2 Input Parameters Parameters............................................................... Distribution - A Base Case Case.................. ........................ 5.1.3 Flow Distribution Simulation Simulation Exercises ................................................................ , 5.2.1 Bleeder Ventilation System: Models A, B, and C ........................ 5.2.2 Bleederless Ventilation System: Model D ................................. Preliminary Conclusions .............................................................
58 60 65 66 66 68 74 74 76 77 79
80 80 83 89 91 91 91 100 103
6. DISCUSSION OF GOB SIMULATION STUDIES .................................. 106 6.1
6.2
Physical Model. Model . . . . . . . . ... . ... ...... .. ... .. .. . .... . . . .. . ... . .. . .. . . . . .. .. . ... . .. ..... 6.l.1 6.1.1 Limitations.................................................................... Limitations .... .. .... ........ ...... .... .. ...... .... 6.1.2 Fluid Effects Effects on Permeability........ Permeability 6.1.3 Permeability - Particle Size Relationship........... . .... .. . .. ........ .. .. Relationship Computational Fluid Dynamics Model ............................................ 6.2.1 Limitations ................................................................... , 6.2.2 6.2.2 Hot Spot Spot Locations ........................................................... 6.2.3 6.2.3 Effect Effect of Permeability on Hot Spot Spot Formation........................... Formation 6.2.4 6.2.4 Effect Effect of of Gob Width Width on on Hot Spot Spot Formation............................. Formation 6.2.5 Hot Hot Spot Spot Control Control through through Gas Gas Injections Injections .................................. 6.2.5
viii V111
106 106 106 108 108 110 110
112 112 112 112 114 114 118 118 122 122 124 124
CONCLUSIONS AND AND RECOMMENDATIONS RECOMMENDATIONS .. ... ... ... .. .. ...................... 129 7. CONCLUSIONS 7.1
Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 129 Conclusions
7.2
Recommendations for Future Research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 131
7.2
Recommendations for Future Research
131
APPENDICES APPENDICES
A
PERMEABILITY TEST DATA ...... .. . .. .. .. ..... .. . . .. .. . . .. .. . . . ... .. .. . .. . ... .. 133
A
PERMEABILITY TEST DATA
133
B
SAMPLE OF PERMEABILITY CALCULATIONS ............. ..... ........ ... 138
B
SAMPLE OF PERMEABILITY CALCULATIONS
138
C
CALIBRATION OF CFD MODEL ...... ... ............... ............ ............. . 141
C
CALIBRATION OF CFD MODEL
141
D
CALCULATION OF COAL INJECTION RATE ..... .. ................... .. .. .. 147
D
CALCULATION OF COAL INJECTION RATE
147
E
PHASES INVOLVED IN SELF-HEATING PROCESS ... .. .............. ...... 150
E
PHASES INVOLVED IN SELF-HEATING PROCESS
150
REFERENCES ........ ... .. ... ... .... ..... ........ ... .. .. ......... .... ... .. ..... .... .. .......... 153 REFERENCES
153
ix IX
LIST OF TABLES
Table
Page
2.1.
Parameters for SPONCOM Program........... ..... ......... ..... ... . ... ... ......... SPONCOM Program
22
2.2.
Experimental Experimental specific specific permeability permeability of of Utah coals .... ............. .. ... ........ ..
34
2.3 2.3..
Experimental permeability of Experimental specific specific permeability of broken rocks... rocks ....... ... ... .. ......... ..
35
3.1.
Specific permeability for rock and coal samples using water-based tests tests..... ... Specific permeability
50
3.2.
Specific Specific permeability for rock samples using air-based tests .. ....... ... ....... .
55
3.3.
Specific permeability for simulated gob materials ..... ...... . ...... ............ . Specific permeability
57
4.1.
Leakage percentage through crosscuts. crosscuts .. . .. . .. . .. . .. .. . . . . . .. .. . . .. .. ... . ...... . . ..
63
4.2.
Type of of regulators used for ventilation controls. controls .. .. . . .. .. . . .. .. . . . . .. .. .... .. ... .
65
4.3.
Input parameters used in Fluent for airflow airflow simulations. simulations .. ....... .. .......... ......
69
4.4.
Ventilation survey data for Mine A and Physical model model.. . . . . ..... .. ..... .. ..... .
77
5.1.
Input parameters used for a single-phase model .. .. .... .. .. ............ ...... .. ...
84
5.2.
Input parameters used for a two-phase model ........ .. .... .......... .. .. .... ......
86
5.3.
Input parameters for the self-heating ........ .................. ... self-heating process.......... process
89
6.1
Summary Summary of of hot spot locations - Models A through D .......... ........... .. ...
115
6.2
Specific Specific permeabilities used for parametric studies. studies .. .. . . . . . .. . . . . .. . .. . . . . . ... . 118
6.3
Input parameters for injection injection simulations .... ........ ........... ... .. ... .........
125
AI. A l.
Water-based .. ............... Water-based test data for 0.28-mm 0.28-mm diameter samples.......... samples
134
A2.
Water-based test data for 3.22-mm 3.22-mm diameter samples.... samples Water-based .... .. .... ........ ......
134
A3.
Water-based Water-based test test data data for for 5.74-mm 5.74-mm diameter diameter samples samples.......................... .
135
A4. A4.
Air-based Air-based test test data data for for 5.74-mm 5.74-mm diameter diameter rock rock samples samples........ .. ..............
135
A5.
Air-based Air-based test test data data for for 7.73-mm 7.73-mm diameter diameter rock rock samples samples.. . ............ ....... ..
136
A6.
Air-based test test data data for for 8.72-mm 8.72-mm diameter diameter rock rock samples samples.. . ....... .. .. ........ .. Air-based
136
A7.
samples.......... .......... .... Air-based test test data data for for 9.71-mm 9.71-mm diameter diameter rock samples Air-based
137
B 1. B1.
for permeability permeability calculation calculation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Sample data for
139
Cl. CI.
for the physical model model.................................. ...... ..... Measured data for
142
C2.
Calculated air velocity velocity....................................... . ....... ................ Calculated
144
C3 . C3.
Number (NR) (NR) of of airflow airflow.......................................... .... .... Reynolds Number
144
C4.
Fluent.. ........ ............................ .... Parameters used for validation in Fluent
145
C5.
CFD modeling results ...................................... .. .... .. ...... ............ . 145
C6.
Comparison of of results - Physical model versus CFD model model. ... ... . .. ... .. ..... Comparison
146
Dl. D l.
injection parameters parameters................................ .. .... ..... .... ............. Coal injection
149
E l1..
Primary and mixture phase properties ........... ....... ..... . .................... .
151
E2.
Secondary phase and gob material properties properties................................ ..... 152
xi Xl
LIST LIST OF OF FIGURES
Figure Figure
Page
2.1. 2.1 .
Typical longwall mine Typicallongwall mine layout layout used used in in the the United United States States ....... ........ .........
77
2.2. 2.2.
Longwall Longwall equipment equipment and and coal coal transportation transportation system system ... . . . . .. .. . . .. . . . .. . . .. . . ..
9
2.3. 2.3.
system............................. .............. ....... Typical U-tube U-tube ventilation ventilation system Typical
11
2.4. 2.4.
Typical Y Y ventilation ventilation system system...... ............ .. .. .. .. .. ...... .... .. .... .... .. .... .. Typical
13
2.5.
Typical Wrap-Around Wrap-Around ventilation ventilation system system........................................ . Typical
14
2.6.
Schematic of of fire triangle triangle.................... .. .................. .. ........ .. ........ . Schematic
16
2.7. 2.7.
SPONCOM SPONCOM result for the sample mine mine...................... .......... ...... ......
23
3.1.
section ...... ............ .......... ...... Gob and strata zones in a longwall mine section
38
3.2.
Permeability network for water-based method........................... .... Permeability test network water-based method
45
3.3.
Water head-flow ... ... ..... head-flow rate relationships for coal and rock samples...... samples
48
3.4.
Longwall mine ventilation model at the University of of Utah. Utah . . . .... .. . .. .. . .. . .
51 51
3.5.
The permeameter permeameter for air-based test.. test .. .......................................... .. .
52
3.6.
Particle size effect effect on broken rock sample permeability using air-based tests
56
3.7.
Specific permeability distribution in gob. gob ....... .. .... ... ........ .... .. ......... ..
57
4.1.
Mine ventilation model schematic............................................ ....... schematic
59
4.2.
Pressure gradients for the physical model.... .. .. .... ...... ...... .. .... .... ....... model
62
4.3.
Leakage percentage through four crosscuts...................................... . crosscuts
64 64
4.4. 4.4.
Type of regulator for physical model used in this study. study .. ... . .. ... . .... . .. . .. ..
66 66
4.5.
Gambit The CFD model created in Gambit...... .. .. ...... ...................... ...........
69 69
4.6.
Velocity contours contours for for the sample sample model model....................... . .. .. .......... ... .. Velocity
71
4.7.
Velocity contours contours for for the U-section V-section. ... .. ....... .. .......... .... ... ...... .......... Velocity
71
4.8.
Velocity profiles profiles for for two simulated simulated openings openings..................... ........... .... . Velocity
72
4.9.
Static pressure pressure contours contours for for the sample sample model model ... ... ......................... .... , Static
72
4.10. 4.10.
pressure contours contours for for the U-section V-section.............. ... ... ... ..... ... ...... .... Static pressure
73
4.11.
Pressure drop through through porous medium medium.. ... . .. . ... .. . ... .. . . .. . . . . .. .. . ... ... ... . . Pressure
74
5.1.
Model schematic schematic for a typical typicallongwall mine .... ...... .... ... ................ .. .. Model longwall mine
81
5.2.
Location Location of of injection injection ports in the simulated simulated mine gob gob........... .. ........... ...
82
5.3.
of airflow airflow distribution distribution.. .. .. .... .. .... ................. . ...... .... ........ Base case of
90
5.4.
Velocity vectors in gob for a bleeder bleeder system system ......... ..... ....... ........... ... .. ' Velocity
92
5.5.
Oxygen Oxygen concentration concentration contours for model A ........... . ........ .. .... ........ .. .
93
5.6.
Temperature contours for model A ................................................. Temperature
94
5.7.
location for model A .... ......... ......... .................. ... ' Potential hot spot location
94
5.8.
Oxygen Oxygen concentration concentration contours for model B ...... .. ........... ........ ... .... ...
96
5.9.
Temperature contours for model B ...... ... ............ ..... ... .... .. .... ..... .. ...
96
5.10.
Potential hot spot location for model B ..................... ..... . ...... ...........
97
5.11.
Oxygen concentration concentration contours for model C ............................... ... ... '
98
5.12.
Temperature contours for model C .......... .. ........ .. .......... .... ... .... ... ...
99
5.13.
Potential hot spot locations for model C .. .. ... .. ................. .. .... .. ........ , 100
5.14.
Velocity vectors in gob for a bleederless system .................................
101
5.15.. 5.15
Oxygen concentration contours for model D .... .... ....... .. ... ... ..............
102
5.16.
Temperature contours for model D ...... ... ..... ...... ... ..... ... ... ............... 102
5.17.
Potential hot spot location for model D ........ .. .. .... ... ... ..... ................ , 103
xiii Xlll
6.1.
Fluid Fluid effects effects on on rock rock sample sample permeability permeability. .. ... . .. . . . ..... .. . . . . .. .. . ... . ... .... . .
6.2.
Velocity Velocity contours contours through through the extended extended permeameter permeameter .... . ... . .. .... .... . .. . . . .. 111 Ill
6.3.
Pressure Ill Pressure contours contours through through the extended extended permeameter permeameter ...... . ............. ... . . . . 111
6.4.
Particle Particle size size effect effect on on broken broken rock rock sample sample permeability permeability. .. ..... . .. .. ...... .....
112
6.5.
Oxygen Oxygen concentration concentration contours contours for for case 1 ...... . .. ... . ....... . .... .. .. ... .... .....
119
6.6.
Temperature contours contours for for case case 1 .. ......... ............ . . . . ... .... ... . .... . .. ...... . 120 Temperature
6.7. 6.7.
Oxygen Oxygen concentration concentration contours for for case 2 ... .. ... . .. ... . . .. ... ... .. ......... ......
121
6.8.
Temperature Temperature contours contours for case 2 .... . .. .... ....... . .. . ..... .... . . ... ... . .. . .... .. ...
121
6.9.
Oxygen concentration concentration contours contours for model model E . . .. . . . . ......... ....... .... ...... .. .. Oxygen
123
6.10. 6.10.
Temperature contours for model E ..... .. .............. . . . ...... . . . ... ... . .. . ..... . Temperature
123
6.11.
Temperature Temperature contours contours for model A with with a vertical injection injection...... ..... ... . .. ..
126
6.12.
Nitrogen Nitrogen concentration concentration contours for model D with horizontal horizontal injection injection holes hole1 126
6.13.
Temperature Temperature contours for model D with horizontal injection injection holes . . .... .. . ..
127
C1. CI.
ventilation model schematic. Mine ventilation schematic .. . .. . .. . ... .. .... . .... ...... .... .. ...... . ... ...
143
D l1..
Assumed gob shape and dimensions ................................... ......... . . . 148
xiv XIV
108
ACKNOWLEDGMENTS ACKNOWLEDGMENTS
This thesis would not have been possible without the financial support of the William C. Browning Graduate Scholarship. I would like to express my sincere sincere encouragement appreciation to my advisor, Dr. Felipe Calizaya, for his constant encouragement
throughout this study and his invaluable advices on the research work. I gratefully gratefully acknowledge the helpful helpful guidance, advice and comments of of my thesis committee committee members: Dr Michael K. McCarter and Dr. D. Kip Solomon. Recognition is also due to Pamela Hoffman Hoffman of of the Mining Engineering Department Department for helping me with paperwork and administration, and Robbie for his assistance in performing performing the paperwork experiments. I also sincerely appreciate the assistance and friendship friendship given by all graduate fellows of of the Mining Engineering Department, Sonny Suryanto and his family, and Darrel Cameron Cameron for reviewing some sections of of this thesis. Finally, my parents, Jan and Elisabeth; my my brothers and sisters, special thanks are given to my Elyezer, Daniel, Olivia, Yunita; and the last but not the least, Zilva.
CHAPTERl CHAPTER 1
INTRODUCTION INTRODUCTION
Spontaneous combustion combustion in underground underground coal mines has become a serious particularly in the caved area (gob). Recent statistics have shown that problem, particularly approximately 17% of underground coal mine fires in the United States are approximately of a total of of 87 underground attributed to spontaneous combustion (De Rosa, 2004). Spontaneous combustion attributed combustion results accumulated heat, if from a self-heating self-heating process in exothermic conditions. The accumulated if not removed, is conducive to the rapid increase of of temperature and may result in mine fires or of such fires is expected to increase in the future as wider explosions. The incidence of
increased consumption consumption of of low rank coals panels and deeper coal seams are mined, and increased becomes more prevalent. The effects effects of of spontaneous combustion are often often associated
of life and damage to property. The crucial step in reducing these effects with loss of effects is locating the ignition point of of spontaneous combustion combustion (hot spot). This study is an effort effort to obtain potential hot spot locations in mine gobs from the best gathered information. information.
1.1 Statement Statement of of Problems In the past decades, much has been written on the subject subject of of spontaneous spontaneous
of coal, including self-heating combustion. The characteristics of self-heating temperature and rank of of coal, have been the subjects of many experiments. In the late 1980s, the Bureau of of Mines subjects of
2 perfonned extensive studies on this matter, developed an empirical expression of performed of coal's self-heating temperature, and identified identified several contributing factors (Smith and Lazarra, self-heating spontaneous 1987). It is widely accepted that lower rank coals are more susceptible to spontaneous
combustion than higher rank coals mainly due to their innate properties. However, such of coal properties in spontaneous combustion. studies merely appear to explain the role of often originates in the gob area, then the problem is more complex Since this combustion often than just permeability of of the gob material is the major major just a rank-related phenomenon. The penneability contributing factor for the self-heating self-heating process. The resistances of of the porous media change over time. This is the result of of stress changes during the mining process. A better understanding of of gob penneability permeability must be developed to simulate the mine gob and determine the possible location of of self-heating self-heating areas. However, a thorough knowledge of of detennine permeability of studies have penneability is impossible because the gob is inaccessible. A number number of been devoted to detennining determining the characteristics of of gob material. Brunner Brunner (1985) constructed a model that was correlated to measured field data. Later research by Pappas constructed and Mark (1993) included a photoanalysis approach and laboratory tests on gob material. A more recent study developed by Balusu (2002) used a tracer gas (SF6)) to predict gob 6
of these studies are crude estimates of of gob caving characteristics. However, the results of profiles further investigation. profiles and the problem calls for further
In foreign countries, limiting the oxygen supply to the gob using a bleederless ventilation spontaneous ventilation system has been chosen as the best alternative to control spontaneous combustion (Koenning, 1989). The regulations in the Unites States require longwall
generated in the gob (30 mines to utilize a bleeder system to dilute and remove gases generated if not maintained correctly, may cause a CFR section 75.334). The bleeder system, ifnot
3 substantial volume of of coal left left in the gob to be exposed to critical conditions under which sufficient quantity of sufficient of air is supplied to promote oxidation, but inadequate to remove heat. may induce the self-heating self-heating of of coal due to an improper improper utilization of The system may of ventilation air, thus creating favorable conditions to sustain spontaneous combustion. ventilation practice of of coursing air into the gob becomes more complex when The ventilation the dynamic aspects of of longwall mining are considered. The overburden overburden depth and the mining rate determine the gob compaction compaction behind the shields and the entry resistances to the airflow. In the gob, the caved caved material expands to fill the void and the roof roof pressure is transferred transferred to the gob, thus reducing the gob porosity and increasing the airway resistances. This dynamic aspect has been barely considered considered in the past.
bleeder ventilation The self-heating self-heating mechanism, the gob permeability, the required bleeder system, and the dynamic aspects of of the longwall mining method have magnified magnified the problem of of spontaneous combustion, making it difficult difficult to solve empirically. However, of supercomputers, the problem problem can be investigated investigated easily in more detail. with the advent of phenomena has produced produced better The application of of numerical methods to simulate these phenomena and more accurate results. Computational Computational fluid dynamics has been used successfully successfully to model caved areas (Balusu et aI., a l , 2002), gob wells (Ren and Edwards, 2000), and air leakage through stoppings and seals (Calizaya, Duckworth, and Wallace, 2004). Such
simulation studies provided provided a better approximation of of certain components of of longwall simulation mechanism of mining but not the self-heating self-heating mechanism of coal. The final goal of of this study is locating potential self-heating self-heating sources within a longwall gob. A series of of experiments consisting of simulation exercises have been of physical models, field investigations, and computer computer simulation conducted evaluation performed performed in this study highlights conducted to determine these locations. An evaluation
4
the importance of of using the Computational Fluid Dynamics (CFD) program to simulate all the involved phenomena phenomena in the self-heating self-heating process.
1.2 Thesis Overview This thesis develops a method used to simulate the location of of a hot spot in a
longwall mine gob ventilated by either a bleeder system or a bleederless system using self-heating coefficients, coefficients, and CFD. Parameters such as gob permeability, panel geometry, self-heating ventilation system are the major major factors that affect affect the development of of a hot spot. These of existing mines and laboratory experiments. parameters are obtained from field surveys of effectiveness of of both bleeder and bleederless ventilation systems to control hot spot The effectiveness
are analyzed. As a reminder, the simulation results presented presented herein are valid to the different gob geometries or ventilation conditions stated in this study. To simulate different modified accordingly. systems, the base model should be modified
After After collecting background information information and defining defining the parameters, computer of airflow airflow inside the gob and to simulations are carried out to show the distribution of predict the potential locations of of hot spot. In this step, four CFD models are built to represent a longwall mine with different different gob lengths and ventilation systems. The results of stress redistribution on the gob are simulated by zones of of different different permeability. The of adjacent to the face, filled with less consolidated material, is characterized characterized by a gob zone adjacent of high permeability. This permeability of the porous medium medium of permeability decreases with the distance of zone from the face; the further further the distance, the lower the permeability. The permeability of each zone is determined based on laboratory tests, field surveys, and computer of
5
simulations. Based on these data and information, infonnation, the hot spot location is primarily defined by two parameters: temperature and oxygen concentration. defined A detailed analysis of of the collected data, the geometry of of the panel, and the parameters used in the simulations are presented; the locations of of potential hot spots in the gob are identified identified and the effect effect of of ventilation systems and gob characteristics on
these locations are discussed. Finally, some conclusions and recommendations recommendations for future future work in this area are presented.
CHAPTER 2 CHAPTER
BACKGROUND AND LITERATURE BACKGROUND LITERATURE REVIEW REVIEW
2.1 Longwall Mines in the United States Longwall mining is the most efficient of mining coal. The most recent efficient method of report issued by the U.S. Energy Information Administration Administration shows that longwall mines Energy Information accounted for 49% of approximately of the 2006 nation's underground underground coal output. Today, approximately 51 underground underground coal mines in the United States utilize the longwall method. If If the trend
for more energy sources prevails, there will be a higher demand for coal, thus calling for a longer and wider panel to increase recovery and decrease the cost. method is utilized for horizontal or nearly flat seams that have The longwall method uniform thickness and are fairly free from discontinuities. According to the relatively uniform of Federal Regulations in the United States, a three- or four-entry Code of four-entry panel in development development is used in longwall mines, although a two-entry panel is allowed under special circumstances. A typical longwalllayout longwall layout of of a three-entry system is shown in of a panel is generally 330 m (1,000 ft) wide and 3100 m Figure 2.1. The geometry geometry of
(10,000 ft) long. Development Development work usually requires 9 months to 1 year, depending on of the panel. In contrast contrast to the advancing-type method method used in Europe, a the size of retreating method method is widely used in the United States. With this method, coal extraction
starts from the farthest end of of the panel and proceeds toward the main entries.
••••••! ••••••
••••••••
;t----,
Mining direction ~
D• •••••• Main entries
entries
Intake air R Regulator . . . - - Return air """'* Overcast Seal ~ Bleeder air Belt conveyor C Curtain .......- Pennanent stopping
D
Typical longwall mine layout used in the United States Figure 2.1 Typicallongwall
8 The development entries are connected at the back of of the panel by another set of of entries called "bleeder "bleeder entries." Each entry is about 3 m (10 ft) high and 6 m (20 ft) wide. The entries are connected in regular intervals by crosscuts and separated by a number of pillars with an average dimension of24 of 24 m (80 ft) wide and 50 m (165 ft) long, depending
on seam and cover conditions. The set of of entries used for transportation of of coal, workers, and equipment is called a "headgate." These entries are also used to deliver intake air. On the opposite side of of the panel, a "tailgate" is used for return air. The main equipment used
to extract coal from the face is illustrated in Figure 2.2. It includes a shearer going back and forth across the face, a set of of shields, and a chain conveyor. For an average panel of of coal cutting from headgate to tailgate takes about 45 330 m wide, a continuous trip of minutes. As the shearer moves along the face, the cutting drums detach coal from the
face. The broken fragments fragments are gathered and pushed onto the chain conveyor by a ramp plate as the shields advance forward. A chain conveyor conveyor transports the broken coal to a surface. loading point on a stage loader and to a belt conveyor, which delivers coal to surface.
The support system, a side-by-side arrangement of of hydraulic shields, is used not only to hold the roof roof during the extraction and push the face conveyor, but also to provide a safe workspace. Today, longwall mines utilize more than 100 shields per panel. After a panel has been mined out completely, the relocation of of equipment takes from 3 to 4 weeks. This is the major major regular delay to production in longwall mining. With this method, the overlying strata are allowed to cave behind the shield as soon as the coal is
extracted. The caved area is later referred referred to as the "gob." The void due to coal seam extraction produces abutment pressure heaped up around the gob. Under this condition,
Figure 2.2 2.2 Longwall Longwall equipment equipment and and coal coal transportation transportation system system (after (after Oitto, 1979; 1979; Ramani, 1981; 1981 ; and and Peng, Peng, 1984) 1984) Figure
10 the caved area expands laterally to the nearest entry of of outward gob (Peng 1985). The
area is mainly filled with coal, caved-in roof, and heaved-up floor materials representing media with different different porosities. Its consolidation behavior changes over time as a further from the face face response to changes in stress pattern. It is accepted that the gob further becomes more consolidated consolidated over time and has less porosity than that behind the shields.
2.2 Ventilation Systems for Longwall Mines
The ventilation system is described as the lifeblood lifeblood of of underground mines. The airflow in sufficient sufficient system ensures safe working conditions in the mine by providing airflow
quantity and quality. In addition, ventilation air also dilutes contaminants and hazardous gases to safe levels. Importantly, the mining and geologic conditions need to be examined examined to determine the proper ventilation system. The primary function function of of a ventilation system
in an underground underground coal mine is to dilute methane gas to less than 11% % by volume and keep the respirable dust levels below 2 mglm mg/m 3 in all work areas. 3
For longwall mines, various ventilation systems have been developed. In the
United States, three systems are commonly commonly applied: U-tube, Y system, and Wrap-around (McPherson, 1993). It is a common practice to use the same system for the entire panel of each system and their layout are presented life. The main features of presented in this section. The legend shown in Figure 2.1 applies to alliongwall all longwall mines.
2.2.1 U-Tube System In the U-tube ventilation system, air is brought to the face from the headgate and airflow schematic is shown in Figure 2.3. 2.3. This is exhausted through the tailgate. The airflow
11
•••••••••••
0 0
• •••• •
ventilation system Figure 2.3 Typical U-tube ventilation
number of preferred to limit the leakage flow to the gob and reduce the number system is preferred of seals. It is used in Australian and European mines.
modified U-tube ventilation system is used in the United States. This system is A modified sometimes referred referred to as "bleederless" system. In this modified modified system, seals are constructed in entries and crosscuts to isolate the gob. Air is directed up the headgate constructed entries, across the face, and back along the return. The entry in the headgate adjacent adjacent to the longwall panel can be used either as intake or return. Most mines use this entry as secondary intake since it is also the belt entry. Only two entries in the tailgate are
12 available for ventilation since the outer entry is caved during mining of of the previous panel. These two entries are used to exhaust the return air. This system is considered
simpler and more cost-effective cost-effective compared to the "Y system" related to the number of of preferred in mines with the potential problem of utilized seals. It is preferred of spontaneous combustion. The main disadvantage of of this system is that in gassy mines, methane could corner of of the gob in the tailgate side. Therefore, this system is accumulate at the back comer
more suitable for non-gassy non-gassy mines.
2.2.2 Y System The Y system, sometimes called a "bleeder" system, utilizes both panel entries outside the face as intakes and a tailgate bleeder as return. Figure 2.4 shows a typical
setup of of this system. The fresh air flushes the face from the headgate to tailgate, and the contaminated contaminated air exits through the outside return entry of of the tailgate. This system also of fresh air to flow across the gob to dilute gasses generated inside allows some portions of the caved area. It also provides an additional quantity of of fresh air to the face near the tailgate. It is used for gassy mines to control the gas concentrations in the tailgate comer. corner. A bleeder fan installed on the surface as exhauster creates the pressure difference difference to ventilate the panel. Despite the fact that most of of the longwall mines employ this system, the Y
method is less suitable than other methods to control spontaneous combustion in mine If the air flushing the gob does not have enough velocity to carry away the heat of of gobs. If the self-heating self-heating process, it could be trapped inside the gob, creating "dead-lock" pockets of of coal. of air that would initiate a continuous oxidation of
13 13
9
•
Bleeder fan on the surface
•
'
~====;DDDDDDDD
•
•
•
•
•
•
•
•
•?r_p n n n r j
• Ci
•[ f
• Ci •
[
GOB
Gc ] G G ]
• GOB
-e
• C: • •
[ [
•
[
Li I
]
] ]
• D
a
_=DT[!JC: F^GD D D D D D D Q D Figure 2.4 Typical Y ventilation ventilation system
Besides the spontaneous of the Y spontaneous combustion combustion problem, another another disadvantage disadvantage of system is that the effectiveness effectiveness of of this method method relies on the conditions conditions of of bleeder bleeder entries. In practice, practice, these entries will become become high-resistance high-resistance airways as the panel retreats. The overburden overburden weight weight could could causes roof roof and pillar pillar failures. The difficulties difficulties to maintaining the initial entry entry conditions conditions may may extend extend to other other entries and and increase increase the airway resistances, thus demanding demanding greater greater pressure pressure of of the bleeder bleeder fan. In some cases, additional pillars are set set up to keep the return paths paths open, open, thus reducing reducing the quantity quantity of of air circulated through through the face. However, However, the presence presence of of pillars may may increase increase the overall overall resistance resistance of of the mine, thus decreasing decreasing the total flow flow rate. rate.
14 2.2.3 Wrap-Around Wrap-Around System
In the wrap-around wrap-around system, the bleeder bleeder entries are located at the back of of the mined-out panels. These entries are used to ventilate the gob. Similar to the Y -type mined-out Y-type system, a bleeder bleeder or exhaust fan is used to create the pressure difference. difference. Figure 2.5 shows a typical layout of of this system. Permanent Permanent ventilation controls such as stoppings and seals are required to isolate the gob. The entries of of the headgate are used as intake and escape paths. The air is then split. Part of of it is used to ventilate the face, and the remainder remainder directed through through the gob and the bleeder bleeder entries. The major major advantage of of this system is that the distance between between the fan and the panel decreases as the panel retreats,
Wrap-Around ventilation ventilation system Figure 2.5 Typical Wrap-Around
15 15 thus increasing the quantity at the face. However, the flow rates through the gob and and bleeder entries may suffer suffer due to the difficulties difficulties of of maintaining narrow entries, especially especially of deep cover. cover. under coal seams of
The success of of all ventilation systems depends on the geologic conditions and mining practices. Well-maintained entries and regularly inspected control devices are of air. Ventilation Ventilation essential to provide all workplaces with the required quantities of
simulators such as VnetPC can be used to optimize the design parameters. VnetPC, a commercial program developed between the 1960s and 1990s by McPherson, allows efficient one (McPherson, 1993). An users to evaluate alternatives and select the most efficient of alternatives is crucial to ventilation planning since longwall mining is a evaluation of of such a simulator is restricted to fixed fixed dynamic process. However, the application of resistance networks. Longwall mine gobs are difficult difficult to simulate, though some efforts efforts of of doing so have been reported (Brunner, 1985; Prosser and Oswald, 2006). Finite volume programs such as Fluent Fluent are now used to study the flow distribution in the gob. This will be discussed in more detail in the following following sections.
Spontaneous Combustion Combustion in the Gob 2.3 Spontaneous Spontaneous combustion combustion is a major major safety safety concern in underground underground coal mines. It Spontaneous approximately 17% of of the total number number of of fires recorded recorded in the United United States accounts for approximately of coal is most most likely initiated initiated by by a self-heating Spontaneous combustion combustion of since 1990. Spontaneous self-heating process is well described described as the temperature temperature rise due to oxidation oxidation of of coal. process. This process of physical physical and chemical process is a complex complex phenomenon phenomenon involving involving a wide wide range of This process processes. In longwall longwall mines, the problem problem becomes becomes complex complex mainly mainly because because the
16 processes take place inside the gob, thus restricting field investigations. The processes,
contributing factors, and spontaneous combustion ccmbustion control methods are described below.
2.3.1 Mechanism of Self-Heating Process of Self-Heating The The spontaneous combustion combustion follows the principle of of the fire triangle, as shown in Figure 2.6. The legs of of the triangle represent three elements of of fire. These are oxygen,
fuel, and ignition source. In the self-heating self-heating process, carbon, pyrite, and other combustible matters left left in the gob represent represent the fuel. The oxygen element is delivered to the gob by the ventilation system, influenced influenced by mining and geologic conditions. The
contact of of oxygen and combustible of coal. The contact combustible matters initiates the exothermic oxidation of rapid increase of of heat, at last, can ignite the fuel and eventually eventually develop a fire.
Today, it is well accepted interaction between oxygen and coal substances accepted that the interaction of opinion is the main cause for spontaneous combustion. There has been much diversity of of various components components of about the tendency of of coal to react with oxygen. However, it is agreed that some factors such as pyrite, moisture, and bacteria bacteria play playaa secondary role to the self-heating self-heating of of coal. Therefore, they are not included in this study.
Ignition
Fuel
of fire triangle Figure 2.6 Schematic of
17 Coal oxidation occurs as coal comes into contact with air. The process is suitably explained in terms of of heat transfer, chemical surface surface absorption, and energy balance related to inherent properties of of coal. According to Wang et al. (2003), the oxidation process of of coal involves oxygen transport to the surface of of coal particles, chemical
interaction between between coal and oxygen, and release of of heat and gaseous products. Chamberlain and Hall (1973), and also Cliff Cliff and Bofinger Bofmger (1998) have confirmed confirmed the Chamberlain complexity of such phenomena; however, the overall reaction can be simplified simplified using the complexity of suggested by Mitchell (1996): following reactions as suggested
0
C+O -> CO C 0 2 + heat 0 2 -7
(65 (65°- 94°C)
CO C Q 2 + C -7 -> 2CO + heat
(100 (100°- 150°C)
2
(2.1)
_
2
0
_
2
(2.2)
of coal of pyritic sulfur, FeS2, FeS , can initiate the spontaneous heating of The presence of 2
(Banerjee, 2000). Such a process is represented by:
2FeS2 2FeS + 70 7 0 2 + 16H 1 6 H200 -7 ^ 2H 2 H2S0 S 0 4 + 22FeS04 F e S 0 .• 7H 7 H20 + 316 kcal (heat) 2
2
2
2
4
4
2
(2.3)
This reaction, however, is not that frequent frequent because the amount of of pyritic sulfur sulfur in coal is
usually less than 11%. %. As indicated in Equations 2.1 and 2.2, the carbon (C), constituent of of coal, reacts with oxygen (0 ( 0 2)) within the temperature range of65 of 65 to 94°C 94 C producing carbon dioxide 2
(C0 ( C 0 2)) and heat. Subsequent Subsequent reaction of of CO C 0 2 and C at higher temperature generates CO 2
2
and heat. Both processes occur in exothermic states. The process temperature, once above o
100°C, 100 C, begins to accelerate, though the heating can still be interrupted. The reaction
18 process accelerates as temperature climbs beyond 150°C 150 C and then a spontaneous ignition
ensues. The temperature at which the coal reaches thermal runaway is called the selfheating temperature (SHT) (Smith and Lazarra, 1987; Koenning, 1989). Equations 2.1 and 2.2 clearly imply the dependency of of the reaction on temperature. The relationship between reaction rate and temperature obeys Arrhenius' law, which is given by:
Rate == A [exp] (-E/RT)
(2.4)
where A = pre-exponential pre-exponential factor, K S-1 s"
1
E =
activation energy of of coal, kJ mor mol"l
1
l1
R = = molar gas constant, 8.314472 JKI JK" mor mol" 1
T = temperature, K Wiemann (1985), Smith and Lazarra (1987), and Mitchell (1996) noted that the rate Wiemann of coal oxidation does not produce a significant significant rise in temperature, as long as the oxygen of concentration concentration in the air mixture is below 5% by volume. This finding, together with SHT values, is used in the following sections to explain the hot spot occurrence.
2.3.2. Requisites for Hot Spot Occurrence
The term "hot spot" used in this study refers to a potential location for an ignition source due to spontaneous combustion. Hot spot is a result of ofthe self-heating process. the self-heating produced by continuous oxidation. This condition is characterized characterized by a high temperature produced of heat and reaction Energy released from this exothermic reaction is in the forms of
19 products. Exothermic reaction implies that the higher the temperature, the more rapid the reaction. Once the reaction temperature climbs above 100°C, 100 C, it progresses so intensely
that it produces spontaneous combustion combustion (Mitchell, 1996). This finding suggests a o
minimum temperature of 100°C for hot spot occurrence in the simulation. Several other minimum temperature of 100 C for hot spot occurrence in the simulation. Several other
experiments of thermal runaway beyond coal's SHT in oxidation present a solid experiments of thermal runaway beyond coal's SHT in oxidation present a solid
foundation for this study. foundation for this study.
Oxygen, one of the two reactants in Equation 2.2, also represents an important Oxygen, one of the two reactants in Equation 2.2, also represents an important
factor to the oxidation process. A minimum supply of oxygen should be available to factor to the oxidation process. A minimum supply of oxygen should be available to
ensure continuation of oxidation. The oxygen concentration must be at least 5% by ensure continuation of oxidation. The oxygen concentration must be at least 5% by
volume in the air mixture. Methane, gases from oxidation, and the volume of fresh air volume in the air mixture. Methane, gases from oxidation, and the volume of fresh air
supply determine the oxygen concentration in the gob. supply determine the oxygen concentration in the gob.
These two parameters, temperature and oxygen concentration, are used to These two parameters, temperature and oxygen concentration, are used to
determine a hot spot. For simulation purposes, the area where temperature and oxygen determine a hot spot. For simulation purposes, the area where temperature and oxygen o concentration are above 100°C and 5%, respectively, obviously becomes a hot spot. Other
concentration are above 100 C and 5%, respectively, obviously becomes a hot spot. Other
factors mentioned in section 2.3.4 such as moisture content are included in simulating the
factors mentioned in section 2.3.4 such as moisture content are included in simulating the
hot spot under input variables of CFD. Chapter 5 describes the details of these variables.
hot spot under input variables of CFD. Chapter 5 describes the details of these variables.
2.3.3. Prediction Prediction of of Spontaneous Combustion Potential The self-heating self-heating temperature was proved to have a direct relationship with the of coal (Chamberlain, 1973; Smith and Lazarra, 1987; Koenning, 1989). It is widely rank of combustion of of coal is a rank-related rank-related phenomenon, meaning accepted that spontaneous combustion that young coals such as sub-bituminous or lignite are more susceptible susceptible to spontaneous spontaneous
combustion than higher rank coals such as anthracite. combustion
20 A number of of laboratory tests and computer simulation exercises have been developed to predict the propensity of of coal to spontaneous combustion. However, none is
Bofinger, 1998). There are many generally agreed upon and used universally (Cliff (Cliff and Bofmger, unanswered questions about the reliability of of such tests to represent real conditions because the spontaneous combustion problem is not merely a coal rank problem. It also
depends on other factors such as geologic condition, mining method, and mine ventilation. A most recent report on spontaneous combustion justified these facts (Cliff combustion justified (Cliff and Bofinger, Bofinger, 1998). However, a global fact shown in these experiments indicates that significant cause for mine fires and explosions. spontaneous combustion may be a significant
2.3.3.1. Development Development ofSPONCOM of SPONCOM Program In the 1990s, the U.S Bureau of of Mines developed a ranking method to predict the combustion propensity of combustion propensity of coal based on the temperature. The experiment was carried out to detennine determine the minimum temperature at which coal starts generating heat that is retained until flaming. Coals with minimum self-heating self-heating temperature of of below 70°C 70 C are
considered to have a high propensity to spontaneous combustion; those with temperatures considered temperatures o
t
o
between 70 and 100°C, a medium propensity; and those with temperatures above 100°C, between 70 and 100 C, a medium propensity; and those with temperatures above 100 C,
low propensity. In general, the rank of coal is agreed to have a high correlation with this low propensity. In general, the rank of coal is agreed to have a high correlation with this
propensity. According to Smith (1992), if the coal is lignite or sub-bituminous, the coal is propensity. According to Smith (1992), if the coal is lignite or sub-bituminous, the coal is
automatically assigned a high spontaneous combustion potential. If the rank is anthracite, automatically assigned a high spontaneous combustion potential. If the rank is anthracite,
the coal is a low spontaneous combustion potential. For bituminous coal, Smith suggested the coal is a low spontaneous combustion potential. For bituminous coal, Smith suggested
that the self-heating temperature can be approximated by: that the self-heating temperature can be approximated by:
21
SHT, °c °C = 139.7 - [6.6 x 0O2,, %(DAF)] 2
(2.5)
where O 0 2 is the oxygen percentage in the coal on a dry-ash free basis (DAF). As 2
between the combustion combustion risk through selfindicated, the equation shows a correlation between heating temperature and the oxygen oxygen content.
Based on field and laboratory studies, the former former U. S. Bureau Based Bureau of of Mines a1., 1996). This program, developed an expert system called SPONCOM (Smith et al., written in ANSI C language, is designed to assess the spontaneous combustion combustion potential of coal based on coal properties, geologic conditions, and mining practices. Since the of mining methods used in the United States are longwall and room-and-pillar, this program
program output includes the spontaneous combustion is limited to those methods. The program potential of of the coal, its rank, and its self-heating self-heating temperature. The results of of this program are crucial for mine operators at the planning and production production stages.
2.3.3.2. SPONCOM SPONCOM Program - A Case Study This section demonstrates the application application of of SPONCOM SPONCOM to assess the from a longwall mine located in the spontaneous combustion combustion of of coal samples taken from of 3 m and a cover western U.S. In this mine, the coal seam has an average thickness of production since 1941, initially as a between 335 m and 700 m. This mine is in production ranging between room-and-pillar coal mine and more recently as a longwall operation. Currently, the room-and-pillar of clean coal. coa1. For a comprehensive comprehensive analysis, data are also production is 7.9 Mt of annual production gathered from from references references and reports conducted conducted by the third-parties such as USGS core gathered drilling analysis, geologic properties, etc. (www.energy.er.usgs.gov). Several
22 (Calizaya and assumptions are also made based on experience and field survey data (Calizaya Miles, 2006). Table 2.1 lists all parameters used in the program.
SPONCOM Program Table 2.1 Parameters for SPONCOM 1. Coal Properties (as received):
3. Geologic Properties:
Proximate Analysis: Proximate
Concentration rating of: of: Concentration
Moisture (%) (%)
6.32
Joints
50
Matter (%) Volatile Matter (%)
35.43
Channel Deposits
50
Carbon (%) Fixed Carbon (%)
45.92
Dikes
0
Ash (%) (%)
12.33
Clay Veins
0
Ultimate Analysis:
Coal seam thickness (ft):
Hydrogen (%) Hydrogen (%)
5.12
Max.
19.5
Carbon (%) Carbon (%)
64.28
Min.
6.7
Nitrogen (%) Nitrogen (%)
1.16 1.16
Sulfur (%) Sulfur (%)
0.5
Max.
3.0
16.21
Min.
1.0 1.0
Oxygen (%) Oxygen (%) Ash (%) (%)
BTUllb BTU/lb
12.33 11,302
Seam gradient (%):
Average
Overburden range (ft): Overburden
13.1
2.0
1001-1500
Pyritic sulfur sulfur
0.13
Presence of of rider seam seam in roof
No
Coal contains impurities such as resins
Yes
Presence of of rider seam in floor floor
Yes
Coal bed show signs of of previous oxidation
Yes
bed (ft) Distance from from coal bed
Friability rating (0 - 100) Friability
25
2. Mining Conditions
4
Presence of of pyrite in roof
No
of pyrite in floor floor Presence of
No
Face cleats (Number/ft) (Number/ft)
10
of floor Rating of floor heave in the mine entries
0
Butt cleats (Number/ft) Butt (Number/ft)
10
of rib sloughage in mine entries Rating of
25
of geothermal sources Presence of
No
of mine air ( F) Ambient temperature temperature of Ambient
78
Presence of of burn bum zones
Yes
of significant Presence of significant faults
Yes
encountered self-heating Have you encountered self-heating events in: Gobs/worked out areas Gobs/worked
No
Entries or gateroads
No
Mining Practices 4. Mining
Pillars
No
Mining technique used:
Other in-mine areas
No
Average seam thickness (ft) (ft)
Transport Transport
No
Stockpiles Stockpiles or silos
No
Longwall production rate (ton/day)
Quantity of of ventilating air Quantity on face near headgate (cfm) (cfm) in tailgate return (cfm) (cfm) of gob (ft) height of Caving height
50,000 5,000 15.0
Longwall rate of Longwall of advance/retreat (ft/day) advance/retreat (ft/day)
Longwall Longwall 8.3 22700
75.0
Longwall panel dimension: width (ft) (ft)
906
length (ft) (ft)
18,240
23 The output of of the SPONCOM SPONCOM program is shown in Figure 2.7. An evaluation of of
these data reveals that coal in this mine is highly susceptibility susceptibility to spontaneous combustion. The coal rank is classified classified as high volatile B with a self-heating self-heating temperature of 54 DC (less than the critical temperature of of 70DC). of 54°C 70°C). This result is crucial to determine the appropriate ventilation ventilation system in planning and also evaluating the effectiveness effectiveness of of the current system.
Contributing Factors to Self-Heating Self-Heating Process 2.3.4. Contributing combustion results from the exothermic oxidation oxidation of The problem problem of of spontaneous combustion of coal. The oxidation process depends on intrinsic and extrinsic factors. The intrinsic factors are represented of coal, including self-heating self-heating represented by the inherent properties of temperature, pyrites and moisture contents, volatile matter, friability, and particle size.
m
Jsl«l
Company Name : Company X U s e r Name : SL D a t e : N/ft
< 70 deg C
Mine : Mine X C o a l b e d : N/ft C o a l Rank : H i g h v o l a t i l e B EitrntrtTrot*©..;;/^ S e l f - H e a t i n g T e m p e r a t u r e :/1>4 d e g C "X S p o n t a n e o u s C o m b u s t i o n P o t e n t i a l : HIGH ) The f o l l o w i n g p a r a m e t e r s were i d e n t i f i e d c a n i n c r e a s e t h e r i s k of s e l f - h e a t i n g : RATING
as f a c t o r s
that
RISK
P r e s s any key
SPONCOM result for the sample mine Figure 2.7 SPONCOM
24 These combustible combustible properties properties influence influence the oxidation oxidation and heat heat generation These generation process leading ventilation system, geologic to a fire. The extrinsic factors are represented represented bby y the ventilation of coal coal are unalterable, the problem conditions, and mining practices. Since the properties properties of and mining problem of combustion m may be solved solved bby providing a good good ventilation ventilation system of spontaneous spontaneous combustion a y be y providing system compatible with with the mining mining practice. The The fire triangle, as illustrated illustrated in Figure Figure 2.6, compatible requires both both intrinsic and and extrinsic extrinsic factors to initiate a fire. The absence absence of of either requires either one m a y stop the process, at least temporarily. may least temporarily.
The self-heating self-heating is found found through through experiments experiments to vary vary with with the rank rank of The of coal. Coals most susceptible susceptible to self-heating self-heating are found found in the low-rank low-rank classification, classification, nnamely most a m e l y the subbituminous and oxygen contents bituminous and lignite, containing containing high high pyrite, moisture, and and oxygen contents which which have self-heating temperatures a s initially suspected to be self-heating temperatures below below 70°C. Pyrite Pyrite content content w was initially suspected be a major constituent to initiate found that enhances major constituent initiate the oxidation. Later, it was was found that pyrite pyrite only only enhances y generating a y assist the the reaction reaction of of the coal coal bby generating heat heat during during the oxidation oxidation process. It m may oxidation carbonaceous matrix y breaking o w n coal into smaller fragments and oxidation of of carbonaceous matrix bby breaking ddown smaller fragments exposing larger (Banerjee, 2000). exposing larger surface surface area area to the air (Banerjee,
of moisture moisture contained contained in coal is also a contributing The amount The amount of contributing factor factor to Groundwater and and extraneous moisture kknown oxidation. Groundwater extraneous moisture n o w n as adventitious moisture are readily evaporated. evaporated. Moisture Moisture held held within within the coal coal itself, kknown inherent moisture, is readily n o w n as inherent analyzed and sshown h o w n in Table oxidation process, the heat-ofheat-ofanalyzed Table 2.1 (Ward, (Ward, 1984). In the oxidation wetting adsorption of self-heating, the wetting stage begins begins with with adsorption of moisture. For For coals capable capable of of self-heating, evolved from moisture can bbe e as m u c h as 2.5 times evolved heat heat from moisture can much times greater greater than in dry air, and the heat of wetting oxidation (Kuchta (Kuchta et aaI., l , 1980). If If moisture moisture drains, wetting can can be be greater greater than of of oxidation heat of adsorbed If oxygen oxygen is adsorbed, adsorbed, m o r e heat be more heat must must be adsorbed gases will replace replace the void void space. If
25
dissipated (Cliff (Cliff et aI., al., 1996) and the system grows to an exothermic exothermic condition condition and induces potential potential for combustion. combustion.
The surface surface area of of coal plays an important important role in the oxidation oxidation process. Winmill Winmill with the fineness of A study (1915-16) observed observed that the rate of of oxidation oxidation increased increased with of coal. A study conducted heating oven also confirms conducted by b y Smith Smith and Lazarra Lazarra (1987) using an adiabatic heating confirms self-heating process potential this statement. The self-heating potential increases with with the increase of of surface surface of particle area or decrease of particle size.
Mining practices m may Mining practices a y contribute contribute to self-heating self-heating mainly mainly bby y extending extending the production period period and increasing production increasing the combustible combustible matter matter left left in the gob. The T h e mining mining rate determines the incubation particular m mine which incubation period for a particular i n e gob in w h i c h self-heating self-heating of of coal may A reduction reduction in mining rate, often m a y develop. A often caused caused by b y frequent frequent delays, gives where the selfsufficient time for heat buildup buildup in the gob. The location sufficient location of of critical velocity velocity where heating tends to occur predicted to be near near the face (Koenning, 1989). A rapid retreat heating occur is predicted A rapid mining development of of spontaneous spontaneous combustion. The combustible combustible materials mining avoids the development left in the gob may m a y also increase increase the oxidation oxidation rate. In addition, timber timber sets and steel left wrecks m a y cause voids inside the gob, creating creating paths for airflow. Air Air leakage is often often may spontaneous combustion combustion process cited as the cause for initiating the spontaneous process (Koenning, 1989).
Another extrinsic factor factor that contributes to spontaneous Another spontaneous combustion combustion is the ventilation regulations require the use of bleeder ventilation ventilation system for ventilation air. Current Current regulations of a bleeder for longwall ventilation air to travel through the gob and longwall mines. The T h e system system allows the ventilation remove of oxidation. This injection injection of of air may m a y cause a remove harmful harmful gases and the heat of substantial vvolume o l u m e of of broken broken coal to be oxidized. In foreign foreign countries, a conventional conventional substantial bleeder system has been been recognized m i n e with spontaneous bleeder system recognized as being hazardous hazardous in a mine with high spontaneous
26 combustion potential potential (Oitto, 1979). The The bleeder bleeder system system m may prevent the self-heating combustion a y prevent self-heating process if if the quantity quantity of of air air course passing passing through through the gob is large enough enough to remove process heat of However, the difficulty of supplying supplying sufficient airflow quantity the heat of oxidation. However, difficulty of sufficient airflow quantity to may result in preferable preferable local conditions conditions for a continuous continuous self-heating self-heating of all gob areas m a y result of and heat heat buildup. Air Air leakage leakage through through the stoppings factor that that contributes coal and stoppings is another another factor contributes to the self-heating self-heating process. The y using The leakage leakage flow flow can can bbee minimized minimized bby using heavy heavy duty doors and and regulators. In In panels panels with with severe geologic geologic structures, faults and joints joints provide
courses for airflow. airflow. These These also contribute contribute to spontaneous spontaneous combustion. courses combustion.
Control Methods 2.3.5. Control Methods Early detection detection of of spontaneous combustion is a preventive preventive method. Monitoring Early spontaneous combustion Monitoring combustion products products of of CO, CO 2 , and the oxygen oxygen deficiency deficiency in the gob is carried carried out to combustion C O , CO2, detect a m mine However, this practice practice m may be very very effective prevention of i n e fire. However, a y not be effective in early early prevention of fire. Time Time is not not a friend friend in in a mine fire (Mitchell, 1996). Currently, there are four four control methods to reduce reduce the risk risk of spontaneous combustion: utilization utilization of methods of spontaneous of an inhibitor, inhibitor, inertization, mining mining practices, and and a ventilation ventilation system. An inhibitor is a chemical chemical substance that can can bbee used used to prevent prevent the physical A n inhibitor substance that
contact between between oxygen oxygen and and combustible combustible materials. The The inhibitors used used in mines mines include contact inorganic chlorides chlorides such such as N NaCI CaC!. Using Using the same same principle principle of protecting steel inorganic a C l and CaCl. of protecting products liquid form through a products from from corrosion, corrosion, an inhibitor inhibitor is injected injected in liquid form into coal seams through borehole substance propagates of this borehole before before mining. This This substance propagates to the coal seam. The success of application of application depends depends on on the borehole borehole depth, injection injection pressure, and and the presence presence of fissures in the seam. It is then substance will protect from fissures then expected expected that that this substance protect the coal from
27 being oxidized oxidized during and after cover of being after the panel extraction. In addition, a cover of limestone or bentonite spread bentonite spread in a foam-liquid foam-liquid solution solution reduces the surface surface exposed exposed to air, thus reducing the oxidation reducing oxidation of of coal (Chamberlain, (Chamberlain, 1973; Banerjee, Banerjee, 1985). Inertization is the process of Inertization of injecting injecting an inert gas into the gob to replace the oxygen oxygen content content in the affected affected area. Nitrogen Nitrogen and carbon carbon dioxide are the common c o m m o n inert gasses used for this purpose. In longwall m mine preferred to carbon i n e gobs, nitrogen nitrogen is preferred carbon safety reasons (Banerjee, (Banerjee, 2000). San Juan Juan Coal mine m i n e is the only only longwall longwall dioxide for for safety inertization in the United United States. The m i n e requires a mine that continues continues to apply gob inertization mine of nitrogen. Pipes of of 100 - 150 m m in diameter diameter are continuous supply supply of of 0.007 m 3//ss of mm continuous 3
normally used to deliver deliver nitrogen nitrogen gas into the gob from from crosscuts (Bessinger (Bessinger et aI., al., 2005). normally used
The development wider and longer panels panels increases the mining development of of wider mining period, providing m more providing o r e time for coal oxidation. This extension extension allows for a longer longer incubation incubation period (Koenning, 1989). Hazardous Hazardous situations period situations may m a y result due to the exponential exponential relationship between between time and temperature temperature rise (Wang relationship (Wang and Dlugogorski, 2003). A little increase result in a fire. A larger increase in time could could cause a thermal runaway runaway and result fire. A larger panel also gives a chance for the development development of of a hot hot spot. An ventilation system A n adequate ventilation system can be used to reduce spontaneous spontaneous combustion combustion in bleeder or wrap-around wrap-around entries are used, oxygen the gob. If If bleeder oxygen is allowed allowed to percolate percolate through the gob, thus supporting of coal. If through supporting the oxidation oxidation of If this condition condition is allowed allowed to occur long period period of buildup and spontaneous occur for a long of time, oxidation oxidation may m a y result in heat buildup spontaneous combustion. Therefore, used only when Therefore, this system system should should be used w h e n the gob condition permits permits ventilation without any chance for heat The ventilation air to pass through the gob without heat buildup. The bleederless system system should should be considered considered if if the resistance to airflow airflow is so high that it can bleederless
28 create critical conditions bleederless systems, conditions for self-heating self-heating of of coal in the gob. In the bleederless isolated bby y m e a n s of of seals and stoppings, thus reducing the risk of of self-heating. self-heating. the gob is isolated means self-heating of of coal can still occur occur near the face. A n adequate ventilation ventilation An However, the self-heating n u m b e r of of potential locations of of heat buildup without neglecting its without neglecting design can reduce the number function in providing providing fresh air to working (Hartman et aI., al., 1997; McPherson, McPherson, main function working areas (Hartman Banik et aI., al., 1994; Cliff, Rowlands, and Sleeman, 1996). 1993; Banik
Spontaneous Combustion Combustion Studies Using CFD CFD 2.4 Spontaneous
Many been done on spontaneous M a n y studies have h a v e been spontaneous combustion, but but only a few utilize Computational Computational Fluid Dynamics D y n a m i c s (CFD) software software in their investigations. CFD C F D has initially initially been used in a wide mechanics-related engineering been w i d e variety of of fluid mechanics-related engineering applications. It provides n u m e r o u s options for modeling laminar and turbulent studying provides numerous modeling laminar turbulent flows, studying multiphase fluids, representing representing complex complex chemical reactions, etc. Often, Often, results are multiphase
achieved FORTRAN For combustion achieved bby y applying applying user-defined user-defined F O R T R A N subroutines. For combustion studies, CFD CFD is a powerful powerful tool to simulate simulate conductive, convective, and radiative processes. In Australia, a CFD C F D modeling modeling investigation investigation was carried carried out by b y the
Commonwealth C o m m o n w e a l t h Scientific Scientific and Industrial Research Organization Organization ((CSIRO) C S I R O ) to develop develop airflow airflow patterns for spontaneous spontaneous combustion combustion control (Balusu (Balusu et aI., al., 2002). The studies studies involved CFD C F D modeling, validation, and calibration calibration of of initial models obtained involved models using data obtained from field O n e of of the models showed showed that the oxygen oxygen distribution distribution in the gob from field studies. One from 2% 2 % to 21 2 1 %. %. A concentration of of up to 22% % was w a s detected detected in the A low low concentration ranging from consolidated zone. This zone was characterized characterized bby y low insitu permeability. Although Although no consolidated explanation w a s presented presented on this permeability, used w a s about about 1 x 101 0 "17 m m2 explanation was permeability, the value used was 17
2
29 similar permeability determined western coals bby Hucka similar to insitu coal permeability determined for western y H u c k a (1992). This of coal to spontaneous information is substantial determine the susceptibility information substantial to determine susceptibility of spontaneous study did not not specify specify areas with with potential potential heat heat buildup. combustion. Yet, the study Lowndes used CFD improve the design In the u.K., U.K., L o w n d e s et al. (2002) also used C F D modeling modeling to improve design of surface for degasification minimizing the leakage leakage of of air, which may of surface gob wells for degasification while minimizing may lead to the danger danger of spontaneous combustion combustion of of coal. Importantly, the permeability permeability of of spontaneous gob material material w was An method was was developed developed for a s discussed discussed in this study. A n experimental experimental method measuring of scaled-down measuring the permeability permeability of scaled-down rock rock fragments fragments under under increasing increasing stress. F L A C , a two-dimensional a s used simulate strata FLAC, two-dimensional finite difference difference modeling modeling package, w was used to simulate behavior The permeability behavior in in association association with with permeability permeability changes. The permeability used used in this 14 8 10" m spaced 150 m m simulation ranged from 1 x 1010" to 1 xX 10simulation ranged from m2.• Three Three 0.18-m 0.18-m boreholes boreholes spaced 8
14
2
suction pressure of -4,000 Pa found to yield p t i m u m results for the apart with with a suction pressure of Pa were were found yield the ooptimum degasification spontaneous degasification study. Even Even though though they they have have no direct direct correlation correlation with with spontaneous combustion, h e n simulating simulating inert combustion, these results can can be be taken taken into consideration consideration w when inert gas injection o x y g e n level injection to reduce reduce oxygen level in the gob. Pressurized Pressurized air air or or inert gas can can be used used to reduce reduce or or eliminate eliminate the the heat heat buildup buildup in the gob due to oxidation.
U.S., recent study National Institute Institute for Occupational In the U . S . , a recent study conducted conducted at the National Occupational Safety and ( N I O S H ) utilized F D to investigate Safety and Health Health (NIOSH) utilized C CFD investigate the self-heating self-heating process process of of coal. Yuan Yuan et al. (2006 (2006 - 2007) 2007) studied studied the ventilation ventilation flow flow paths paths in the gob and and the likelihood
of heating in longwall longwall gob. Gob permeability, permeability, as the important important input input variable of spontaneous spontaneous heating for simulation, w a s obtained L A C . Using L A C model, the of the FFLAC simulation, was obtained from from FFLAC. Using the results of 10"12 m m 2.• permeabilities 10"9 and 5 x 10permeabilities for for the five zones zones were were determined determined to be be between between 1 xx 109
12
2
analyzed in terms In this study, the preferable preferable condition condition for spontaneous spontaneous combustion combustion was analyzed
30 of defined as insufficient insufficient airflow airflow to rremove of critical velocity. Critical airflow airflow is defined e m o v e the heat but sufficient maintain the oxidation study confirmed due to oxidation, but sufficient to maintain oxidation process. This study confirmed existence of of a critical critical velocity behind the shields in the gob for for a bleederless the existence velocity zone behind system. In addition, for a three-entry three-entry bleeder bleeder system, the critical velocity velocity zone m may a y also occur back end of the gob. occur at the back end of Although these studies outlined with spontaneous spontaneous combustion combustion potential, Although outlined the areas with they hot spots in the gob. Besides they did not specify specify the location location of of the hot Besides critical critical velocity, other parameters parameters such concentration and temperature temperature should should be be considered considered in other such as oxygen oxygen concentration the simulation simulation study.
Porous Medium 2.5 Porous Medium Porous m medium can be defined defined as the solid solid or or loose bbody that contains Porous e d i u m simply simply can o d y that o d y refers o u n d material of bbound material while while a loose loose body open cavities. A A solid bbody refers to a packed packed form form of consists of granular particles. The system are often often consists of granular The interconnected interconnected pores pores in a porous porous system
called effective pores pores play play an important important role in fluid fluid flow called effective effective pores. In practice, the effective flow h e detailed detailed description description of of porous e d i u m is sometimes sometimes intuitive, through porous porous media. T The porous m medium exact properties difficult to describe so that that the exact properties are difficult describe (Scheidegger, (Scheidegger, 1957). A A statistical review e d i u m , including including particle-size review of of porous porous m medium, particle-size distribution, distribution, porosity, and and permeability, described bby y Bear i n e gob characteristics. Bear (1972), is necessary necessary to understand understand m mine as described
2.5.1. Particle Particle Size Distribution Distribution Granular materials are best best described described by their particle-size particle-size distribution. It is Granular materials b y their generally irregular material material particle particle size cannot cannot be be easily easily defined defined as a generally accepted accepted that irregular
31
sphere or or cube. Each particle shape The measurement measurement results depend depend on the sphere Each particle shape is unique. The particle dimensions dimensions and and the method method of of measurement. measurement. For For particles particles larger larger than 0.06 mm, particle mm, sieve determine the size distribution distribution (Bear, 1972). used to determine sieve analysis can can bbee used In sieve analysis, a pile of forced to pass of material material is forced pass through through a sieve of of a certain opening u m b e r of of sieves are used define the particle distribution graphs. opening size. A A nnumber used to define particle distribution However, larger than a y slip However, using using this method, method, particles particles with with lengths lengths larger than the sieve opening opening m may through alter the particle Side assessments assessments are necessary necessary to eliminate eliminate through and and alter particle distribution. Side y screening screening the material same sieve. such a possibility, possibility, for for example, bby material twice with with the same
2.5.2. Porosity Porosity
The major major properties required to simulate mine gob are porosity porosity and properties required simulate a mine Porosity is defined of void void vvolume volume of of a permeability. Porosity defined as the ratio of o l u m e to the total volume packed body. Mathematically, Mathematically, it is given given by: packed
Vv xx 100 nn = = —z100 % % V T V
(2.6) (2.6)
where where = the porosity porosity
n V Vv
= the pore volume volume
V VTT
= the total volume volume
V
For porosity depends on For consolidated consolidated materials, the porosity on the degree of of cementation, cementation, porosity of unconsolidated or loose material material depends packing of while the porosity of unconsolidated depends on the packing of the arrangement, grains, their their shape and and size distribution distribution (Bear, 1972). Depending Depending on on their their arrangement,
32 of the total volume. non-uniform-sized particles m may change the porosity porosity of non-uniform-sized particles a y change volume. Small Small particles may occupy the space space between between the large particles, particles, and reduce reduce the porosity. Compaction m a y occupy Compaction and consolidation are other factors that that affect affect porosity. In In the case of and consolidation other factors of gob material, compaction is caused caused bby pressure of of overlying overlying strata strata varying varying with with the depth of compaction y the pressure of overburden and and age of overburden of the gob.
2.5.3. Specific Specific Permeability Permeability
Another parameter parameter used used to characterize characterize the porous porous m media Another e d i a is the specific specific permeability, sometimes sometimes just just called called permeability. This parameter parameter indicates permeability, indicates the ability of of consolidated material to transmit transmit fluids. Specific permeability is of consolidated or unconsolidated unconsolidated material Specific permeability of great o m m o n unit for great importance importance in determining determining the airflow airflow behavior behavior in the gob. A A ccommon
permeability is the darcy darcy (D), or or more more ccommonly millidarcy (mD), in which which 1 darcy permeability o m m o n l y the millidarcy
toequals 9.87 equals 9.87 x 10
13
m 2.• D Darcy' m a r c y ' ss law is expressed expressed by: 2
Q=CA Q = C A
!1h Ah
(2.7)
L
where where
Q Q
3 airflow rate, m airflow rate, m Is /s
C C
hydraulic conductivity, mls hydraulic conductivity, m/s
A
sectional area of of porous porous sample, m cross sectional m
Ah Llh
pressure difference difference between between two points, points, m pressure m
L
3
=
length of porous porous sample, m length of m
2
33
Equation 2.7 is restricted restricted for a laminar The Equation laminar flow flow condition. T h e proportionality proportionality k n o w n as D a r c y ' s velocity. Based specific constant, C, is also known Darcy's Based on on this coefficient, coefficient, the specific permeability permeability is given given by:
k = =
f.1 C
(2.8)
r
7
where where k
=
2 specific permeability, m specific permeability, m
y
=
specific weight of fluid, N/m specific weight of N/m
Il
=
dynamic viscosity of dynamic viscosity of fluid, Ns/m2 Ns/m
2
22
of porous media, Since porosity and and specific permeability measure measure the structure structure of Since porosity specific permeability they ought ought to be be related. M Many investigators have studied relationship between they a n y investigators studied the relationship between these
An empirical correlation proposed bby A modified two parameters. A n empirical correlation was proposed y Carman C a r m a n in 1937. 1937. A modified version of of this w work Carman-Kozeny equation equation (Scheidegger, o r k is kknown n o w n as the Carman-Kozeny (Scheidegger, 1957):
2
3
d k* = d; nn3 k* = 180 (1 - - n) 2 180 (1 -
n)
(2.9) (2.9)
2
where
k* k*
theoretical specific specific permeability, permeability, m = theoretical m
dm
=
m
22
the mean m e a n particle particle size, m m
Equation clearly indicates that that specific permeability is dependent dependent on the mean Equation 2.9 clearly specific permeability assuming uniform-size particle diameter diameter and theoretically obtained obtained bby particle and porosity, and is theoretically y assuming uniform-size particles particles are arranged arranged in cubic packing.
34 Table 2.2 Experimental Experimental specific specific permeability permeability of of Utah Utah coals (Hucka, 1992) Permeability ( xx 10Permeability 10"17 m 2)) Perpendicular to bedding Parallel to bedding Perpendicular 4.1 3.9 1.4 0.5 1.4 6.3 9.6 1.1 1.6 1.6 17
Mine
Coal Seam
Castle Gate
Sub 3 Rock Canyon Sunnyside L. Sunnyside
Soldier Creek Soldier
Sunnyside
2
Using D Darcy's Hucka (1992) found permeability values for Using a r c y ' s law, H u c k a (1992) found the specific specific permeability for Utah's The coal samples samples used bby Hucka were U t a h ' s coals (Table 2.2). The y H ucka w e r e taken taken from from coal mines and laboratory with found and tested tested in the laboratory with nitrogen nitrogen as a fluid. The permeability permeability values values were were found
to be influenced influenced bby cleat direction direction and whether whether the coal sample y the cleat sample is parallel or perpendicular to the bedding, fracture, and other other geological geological structures. perpendicular To characterize characterize the gob material, it is necessary necessary to consider consider the specific specific permeability of of the broken broken coal-rock coal-rock mixture mixture behind behind the face. Therefore, Therefore, the gob specific permeability specific permeability u c h higher specific permeability shown in permeability should should bbee m much higher than the specific permeability of of fresh fresh coal shown Table 2.2. Investigations e r e conducted conducted at the Table Investigations of of experimental experimental specific specific permeability permeability w were University h e results are shown shown in Table These values adjusted University of of Utah. T The Table 2.3. These values have have been been adjusted for air air as fluid fluid rather rather than than nitrogen. The The materials materials used used were were the broken broken rock rock with with various sizes and are tested tested bby y X-ray X-ray microtomography microtomography and and constant-head constant-head techniques techniques (Gold, 2004; Lin, 2005; 2 0 0 5 ; Videla, 2008). These comparable with These values values should should be be comparable with those used used in this simulation. simulation.
35
Table 2.3 Experimental broken rocks Table Experimental specific specific permeability permeability of of broken Particle Range Size Mesh Standard Standard (mm)
2 Permeability Permeability (m ))
Investigators
Method
Lin et al. (2005)
X-Ray Tomography Tomography
Gold (2004)
Constant-head Constant-head
Videla (2008)
Constant-head
2
No. 200 - 1 in. No.
00.074 . 0 7 4 - 225.40 5.40
1.400 1.400 x x 10,07 10"
No, 170 No. 1 7 0-- 3/4 / iin. n.
00,088 . 0 8 8-- 19.00
10,11 6.340 xx 10" 6.340
No. 40 - 1 in. No.
0.420 0 . 4 2 0 - 225.40 5.40
3.450 x 10" 10' "
No. 100 1 0 0-- NNo. o . 40
00.149 . 1 4 9 - 00.420 .420
4.034 x 10' 10""
No. 4400 - NNo. o . 10
0.420 - 2.000
3.475 3.475 X x 10,1 10" 0
No. 10 No. 1 0 - 1118 / 8 in.
22.000 . 0 0 0 - 33.175 .175
2.023 x 10'09 10'
3
4
07
11
11
n
10
09
CHAPTER CHAPTER 3
CHARACTERISTICS CHARACTERISTICS OF GOB MATERIAL MATERIAL
Knowledge conditions is a critical element K n o w l e d g e oflongwall of longwall gob conditions element in the study study of of spontaneous combustion. combustion. Currently, the interpretation interpretation of of events events taking taking place inside the spontaneous gob is unclear o m e cases, merely o o f caving major causes unclear and, in ssome merely guesses. R Roof caving is one of of the major impeding investigators search for further further details on impeding investigators to search on air-gas air-gas flow, although, recently, m u c h work S o m e of works are used much work has been been done done to understand understand this behavior. behavior. Some of these works used as the foundation foundation of of this study. Ventilation Ventilation air distribution, distribution, panel panel dimensions, and particle distribution of of gob material factors in the design size distribution material are important important factors design of of physical physical and computer-based For this study, a number tests hhave a v e been been conducted conducted to computer-based gob models. For number of oftests better of these studies, assumptions assumptions better understand understand the gob material material characteristics. Results Results of made significance of made and and the significance of laboratory laboratory experiments experiments are described described in this section.
Longwall Mine Gob 3.1. Longwall Mine Gob i n e is influenced y several factors, factors, The development development of of the gob in a longwall longwall m mine influenced bby including overlying and including the geologic geologic conditions conditions of of the overlying and underlying underlying strata, panel
and the depth depth of presence of dimensions, and of the coal seam. The presence of joints, fractures, and any other geologic features will change other geologic change the characteristics characteristics of of the gob, the caving time, and the size of The most parameter of considered in this of the broken broken material. The most important important parameter of the gob considered
37 study permeability. This parameter parameter is strongly by porosity and study is specific specific permeability. strongly affected affected b y gob porosity particle size. particle Peng (1984) states that that coal extraction extraction using using the longwall mine method induces induces a Peng longwall m i n e method of events: abutment pressure and roof-to-floor roof-to-floor convergence convergence in the entries and face series of abutment pressure face rock strata, and surface surface subsidence. Figure illustrates the typical area, m movement o v e m e n t of of rock Figure 3.1 illustrates extraction in retreat retreat longwall longwall mining. The response to mining mining is result of of coal extraction T h e initial strata response failure failure of of the immediate immediate roof, thus creating creating a void void over over the caved caved material. A Ass the minedspan increases, the strata o l u m e of of the broken out span strata failure continues continues and the vvolume broken material overlying strata gradually gradually fills the void void space. Eventually Eventually the overlying strata rest rest oonn the caved caved material which some degree longwall face retreats further, further, the full weight which offers offers some degree of of support. A Ass the longwall of overlying strata strata will rest upon the gob material, reducing reducing the void of the overlying rest upon void spaces in the n investigation y the U . S . Mine Safety and Health 2002 gob. A An investigation bby U.S. Mine Safety Health Administration Administration in 2002 of the caved caved zone zone m a y range from from 1 to 10 times the mining mining reported reported that the height height of may height, depending depending on the geologic condition condition of of the roof. Other Other investigators investigators state that that the caved zone m a y extend height of of the coal M u c h o et al., caved may extend from from 4 to 6 times the height coal bed ((Mucho aI., 2000). A b o v e the caved strata do not detach by Above caved zone, the strata detach from from each other other but are linked linked by connecting extends from from 9 to 60 times connecting cracks. This This is called the dilated dilated zone. This zone extends the mining a y cause e a m deformation. b o v e this is the fractured mining height height and m may cause bbeam deformation. A Above fractured zone. Surface fracture about 50 ft deep m a y occur occur due to tension subsidence zone. of about may tension in the subsidence Surface fracture of
The gob materials, such as those from roof and heaved heaved floor, can cause from caved caved roof variable resistance airflow if if a bleeder h e amount amount of resistance to airflow bleeder system system is utilized. T The of void void spaces and h o w they affect the resistance. Research significant how they are connected connected affect Research indicates indicates that that a significant portion of of voids is located he m a x i m u m particle of portion located in the area behind behind the shields. T The maximum particle size of
surface surface
Shearer/Shields Shearer/Shields
Zone 1
Zone 2
Zone 3
Bleeder Bleeder Entry
Figure 3.1 3.1 Gob Gob and and strata strata zones zones in in aa longwall longwall mine mine section section (after (after MSHA, MSHA, 2002) Figure w 00
39
gob material in this area is about 550 mm (Pappas and Mark, 1993). Conversely, smaller particles are found near the bleeder entries. The reduction of of void space is due to of the overlying strata; the longer the compaction process, the lower the void compaction of
space. The shape of of the particles depends on the way these are arranged inside the gob. Densely consolidated, blocky material tends to break into large slabs, and creates large porous spaces. Laminated Laminated fragments tend to be more compacted compacted than the blocky of the large fragments fragments materials, thus decreasing the void space. The initial shape of compaction. changes over time due to compaction.
permeability depends on the void space distribution in the caved area. With Gob permeability the knowledge of of the material size, shape of of broken fragments, and packing mode, the gob can be divided into three permeability permeability zones: unconsolidated, semiconsolidated, and characterized by their porosity as high, consolidated (Figure 3.1). These zones are characterized medium, and low, respectively. The size, shape, and packing of of gob material may change of studies and become more compact over time due to overburden weight. A number of
have found that the permeability permeability of lxlO -13 to 1x10" lxl0- 5 of the gob material ranges from 1x10" (Brunner, 1985; Ren et aI., al., 1985; Ezterhuizen and Karacan, 2007). The experimental values used in this study are presented in Section 3.4. Although the step of of dividing the gob into 3 zones is a fair assumption and
supported by several studies, the boundaries of of each zone are difficult difficult to define. define. Longwall mining is a dynamic process. The gob permeability permeability decreases gradually from from the face to the bleeder area over time. Therefore, more permeability preferable permeability zones are preferable
for simulation to reflect reflect gradual permeability permeability changes. However, iteration time and complexity of the model are the limitations for having unlimited zones. Investigations complexity of
40 conducted by Balusu Balusu et al. in 2002 and 2005 with tracer gas (SF6) and a gas monitoring 6
system presented information information to characterize and determine the boundaries of of each zone. unconsolidated zone, characterized characterized by tracer gas, is hypothesized hypothesized to extend up to 150 The unconsolidated m behind the face. A lower concentration zone is assumed to extend from 150 m up to
simulation purposes, zone 600 m, and the third zone, almost degassed beyond 600 m. For simulation 1 extends extends up to 150 150 m from from the the face face line, line, zone 22 from 150 150 m to 600 m, and and zone zone 33 from from these zones is presented ofthese presented in Section 5.1. 600 m up to the bleeder bleeder area. The schematic of
3.2. Gob Material and Its Characteristics
of physical and computational computational models in simulating hot spots The reliability of depends on how closely the simulated gob material emulates the real conditions. Even though there is no simple way to quantify quantify real gob conditions, some studies have been conducted distribution through the gob. The gob is often often conducted to approximate the air distribution represented of fixed volume filled with particles of of given size distribution. The represented by a zone of particle size and packing mode affect airflow distribution distribution and the self-heating affect the airflow self-heating process
of broken coal. of crushed rock and coal. Particle represented by crushed For this study, the gob material is represented size and packing modes are discussed in this section. These properties affect affect the porosity permeability of of the porous media, and eventually eventually the fluid transport process. and permeability
3.2.1 Particle Size Selection
coal-rock particles are more likely to be located in the area In the field, the largest coal-rock of the behind the shields. This material is freshly freshly broken and unconsolidated. The size of
41
broken particles in this area is based on a study carried out by analyzing images taken from the area behind the shields in three coal mines in the United States (Pappas and maximum particle size in the gob area behind the Mark, 1993). The results show that the maximum shields is about 550 mm mm with a mean of of 122 mm. This average size is used in the present study to determine the permeability permeability values for the unconsolidated unconsolidated gob material. For other zones, such as those located near the bleeder bleeder entries, the size should be assessed through simulations. This is due to the lack of of experimental information information in these gob zones. Through simulations, the mean particle sizes for the semi-consolidated semi-consolidated and
of the consolidated zones were 0.02 and 0.006 m, respectively, smaller than those of unconsolidated unconsolidated zone. These were determined based on laboratory experiments, permeability permeability tests, and numerical simulations (Section 3.4).
3.2.2 3.2.2 Packing and Particle Shape understand the relationship between between particle structure and porosity, To understand investigators have established established the concept of of stable packing (Scheidegger, 1957; Bear, 1972; Freeze and Cherry, 1979). Stable packing is approximated approximated by a motionless arrangement of uniform spheres. By studying the various modes of arrangement of uniform of stable packing, a correlation between grain size, structure, and porosity can be determined mathematically.
The uniform uniform packing concept has been used by other investigators to generate computer simulated porous media media (Scheidegger, 1957; Bear, 1972). However, this concept only of porous media. The natural condition includes approximates the natural condition of differs from from that of of spheres. They are seldom uniform uniform in particles whose shape and size differs
42
nonunifonnity permits penn its the smaller size and shape. This nonuniformity smaller particles to fill the spaces between the larger ones, thus reducing the void space of of the porous media.
represent the gob material. In this study, both crushed coal and rock are used to represent Penneability Permeability tests have shown that though crushed coal and rock samples have identical different values of of porosity and permeability penneability (Section particle sizes, they may still have different 3.3). The way each particle is arranged in the porous media plays an important important role in defining the permeability of the porous media. The shape of of coal particle is usually more defining penneability of
angular than that of of rock. These factors cause coal particles to have a denser packing than noncoal particles. However, the experiments carried out in this study indicate that, on the average, the difference difference in permeability penneability between between coal and rock samples is within 20%.
computational simulators While longwall gob does not exhibit spherical packing, computational such as Fluent use the spherical packing concept. Therefore, physical measurement measurement and computer computer modeling results are expected expected to differ differ to some degree. A calibration factor factor can be used to convert physical rock or coal permeability computer model penneability. permeability. penneability to computer
Then, this factor factor can be used to modify modify the Kozeny-Carman Kozeny-Cannan relationship used with Fluent (Section 4.3.2).
3.3. Permeability Penneability Tests A series ofpenneability of permeability tests were conducted conducted at the University of of Utah using water and air as fluids. The objective objective of of these tests was to determine specific detennine the specific permeability concept of penneability of of simulated gob materials. These tests followed followed Darcy's concept of fluid flow through porous media. During each test, laminar flow conditions were maintained maintained in the permeameter penneameter (container). Fluid flow rates, pressure differences, differences, and room
43 temperatures were recorded systematically. These data were used to calculate specific specific permeability of the material. This section describes the process of of determining permeability of permeability of broken coal and rock, data interpretation, and conclusions. permeability of
3.3.1 Sample Preparation
The granular materials such as crushed rock and coal are best described by their particle-size distribution (Bear, 1972). Six sieve sizes were used to classify classify the rock and
coal samples: 150-um, I50-l1m, 425-um, 425-l1m, 1.70-mm, 4.75-mm, 6.73-mm, and 12.5-mm I2.5-mm sizes. The diameter of of permeameter permeameter used to hold material defined defined the largest size. After After sieving, the
crushed rock and coal samples were divided into 6 size ranges based on the sieves. The mean sizes for each group were 0.28,3.22,5.74, 0.28, 3.22, 5.74, 7.73, 8.72, and 9.71 mm, respectively.
Tests were conducted conducted circulating either water or air through the permeameter. permeameter. water-based standard used to measure the permeability permeability of ASTM Method D2434-68, a water-based of followed for these tests using the "constant granular soils, was followed "constant head method." For this method, 30 tests were performed of 0.28, 3.22, performed using 3 sample groups with mean sizes of and 5.74 mm. The permeameter permeameter size restricted the tests for larger particles.
The air-based tests were carried out using the same constant-head constant-head method. Permeameter different than those used in water-based Permeameter dimensions used in this test were different test. Therefore, the sample groups were different. different. Thirty six tests using 4 sample groups with mean sizes of of 5.74, 7.73, 8.72, and 9.71 mm were performed. performed. The first sample group was tested using both fluids (air and water) to explore the effect of fluid to permeability. effect of of both experiments is presented presented in the following sections. A detailed description of
44 3.3.2 Water-Based Water-Based Method water-based method was used to measure the specific permeability of The water-based specific permeability of the simulated gob material by maintaining maintaining constant water head (pressure). The pressure drop measured by the difference difference in height of through the porous medium is measured of two water columns. To determine permeability permeability using Darcy's law, constant flow must be established maintaining the water column in the container established first. This is achieved by maintaining constant. The ASTM standard standard states stringent prerequisites for permeability permeability tests: (a) continuity of of flow with no material volume change, (b) flow with the material voids saturated with water water and no air bubbles, and (c) steady state flow with no changes in saturated explained in the following sections. hydraulic gradient. These prerequisites are explained
3.3.2.1 Testing Apparatus Figure 3.2 shows the apparatus used for the test. It includes a carbon dioxide gas tank, a water container, a material column (permeameter), a flask, and tubings. At the
of each test, carbon dioxide was flushed through the permeameter permeameter to eliminate beginning of air bubbles trapped in the material voids. This gas was selected due to its inertness and safety. A valve attached to the tank outlet controlled controlled gas flow rate. The maximum maximum gas pressure in the tank was 689 kPa (100 psi). However, only 3.5 kPa (0.5 psi) of of gage pressure was used to flush the permeameter. It took from 10 to 15 minutes to flush out the air bubbles from from the column. This was monitored monitored visually.
container of of fixed head. The The energy source was represented by a water container container container was joined joined to the permeameter permeameter through control valves and tubings. tUbings. The permeameter was filled with granular permeameter granular samples and saturated with distilled water. The
r--~
Gas Pressure Gauge
Relief pressure taps
Top screen
Water Container
Permeameter
Flask
Bottom screen
Figure 3.2 3.2 Permeability Penneability test network network for water-based water-based method Figure
46 permeameter permeameter cylinder was 60 mm in diameter. This was 8 to 12 times larger than the
prescribed by the ASTM standard. Two porous screens with maximum particle size as prescribed openings smaller than the particle size were attached to two ends of of the specimen. The screen openings were larger than the material voids but smaller than the particle diameter to prevent prevent the movement movement of of particles. The permeameter permeameter had two taps to allow water to flow. The specimen specimen height in the permeameter permeameter was at least twice the diameter of of the cylinder. A metal spring was attached to the top screen to avoid changes in specimen height during the test. Two pressure relief relief valves in the permeameter permeameter lid are used to eliminate pressure buildup between the water level and the permeameter permeameter lid. The overflow overflow water water collected collected in the flask was used to determine the flow rate through the specimen. speCImen.
3.3.2.2 Testing Procedure 3.3.2.2
The permeability permeability of of crushed samples (coal and rock) was determined experimentally using the following procedure: experimentally 1. Place crushed permeameter and setup the network (Figure 3.2). crushed material in the permeameter 2. Flush the specimen with carbon dioxide at a gage pressure of3.5 of 3.5 kPa (0.5 Psi).
3. Once the air bubbles are removed, close the gas control valve and open the water valve.
water level in the container constant by feeding it continuously. 4. Maintain the water 5. Collect the fluid overflow overflow in the flask and record the water volume (V). Also, record the collection time (t).
difference in water head (Ah) (~h) and sample length in permeameter permeameter (L). 6. Measure the difference
47
7. Record the room temperature and atmospheric pressure. 8. Repeat steps 1 through 7 for different different flow rates and particle sizes. water-based tests were performed performed using the above procedure. The gathered Thirty water-based presented in Appendix A. data from these tests are presented
3.3.2.3 Testing Results For the water-based water-based method, 30 experiments were considered considered large enough to produce reliable results. Besides the sample size, another concern was laminar condition requirement for the experiments. Regression analysis was performed performed to check this of condition. Using a standard permeameter, and measuring the water heads, quantity of overflow, and the Darcy's specific permeability (A:) for material samples can be Darcy's law, the specific permeability (k) calculation can be found in Appendix A. of such a calculation calculated. An example of
permeability tests conducted Figure 3.3 shows the results of of 30 permeability conducted for the same flow conditions. These were carried out using crushed crushed rock and coal samples of of three different different particle sizes. This figure also shows the relationship between the water head and flow rates for rock and coal samples. The linear regression analysis on each graph shows an upward trend of of head with flow rates showing that the experimental conditions followed followed the Darcy's concept. Data points that lay outside these regression trends may indicate the presence of of turbulent flow. From the observations, such data generally occur at either very low or
R-squared value (R 2), called the correlation high flow rates. The R-squared correlation coefficient, coefficient, represents how well the regression line matches the original data points and ranges from from 0 (no 2
(perfect match). The high R R2 values shown on the regression regression lines implied match) to 1 (perfect
48
A. Sample size: 0.3.5 0.15 - 0.42 0.42 mm mm 00.15 .15 ., - - - - - - -— - - -— - - - -— -------------------------------, • Rock .. Coa l - - Linear (Rock) -- -.-.-.... Linear (Coal)
I
;§
•
~
0.10
~
y = 14282x
CII
:t:
::;;
~
R' = 0 .965 0.0 5 1-·-·-....·--·..·........·......................-·-..·......·..·-·..·......:..·..............:;;;;~~,,:.:.............-....................-............ -.... - ................ ---· ..·..·..·........ -·......··....·. ·..·_..-..·....· ..1
:r:
+-----.----.,---.........,-.
00.00 .00 4
-t—
O.E+OO O.E+00
2.E-07
l.E-0 7 1..E-07
3.E-07 3.E'()7
44.E-07 .E-0 7
5.E-07 5.E.. 07
6 .E..07 6.E-07
7.E-07
88.E-07 .E-07
m3/s) Flow rate, QQ ((m3/s)
B. Sample size: 1.68 1.68 - 4.75 4.75 mm mm 0.15 0 .15 ..,.....----------------- --
----------- - - - - - - - - - - - - - - - - -----,
r
• Rock .. Coal - - - Linear (Rock)
;§
~
y = 29 705 •
•
R' = 0.906
00.10 .10 ..~........-.... ..-.. .-.-.-. L.....in....e..._a.r••(-c...o..a-.I-) .--.-....--..------~:---....-.-.-~~....- .•-~~
o.o2
:r:
0 .00 0.00 O.E+OO 0.E+O0
^
•
1330.x y == 1330 .)( R '"* 00.902 R' .9 0 2 2
^ ^ ^ ^
•• •• 5 .E..06 5.E-06
l.E·05 l.E-05
III
2 .E-0 5 2 .E-05 3.E·05 2.E-05 2.E-05 3.E-05 Flow Flow rate, Q Q (m3/s) (m3/s)
33.E-05 .E·0 5
4 .E.. 0 5 4.E-05
4.E .. 05 4.E-05
Figure 3.3 Water head-flow rate relationships for coal and rock samples
49 that the lines can be used to predict values that were not observed within the size ranges. The graphs also illustrate the effect effect of of particle size by showing the slope change
of of the regression lines. In graph A, the rock and coal particle regression lines almost overlap each other. The gap between the lines is larger with an increase in particle size. This effect effect is shown in graphs Band B and C, implying that the smaller the grain size is, the significant angular shape is on packing mode. In other words, the angularity of less significant of particles becomes smaller as the size decreases. This affects affects the properties of of porous media significantly. significantly. In the gob, the material directly behind the shields, made up by large-size broken particles, will always have high porosity. In contrast, the gob area adjacent to the bleeder compaction will have small porosities. bleeder entries due to compaction adjacent
laminar flow conditions were During the experiment, the prerequisites for the laminar examined frequently. The test apparatus and its arrangement examined arrangement were checked for any of air bubbles in the voids, and transient possibility of possibility of material volume changes, presence of state. Several external factors may still affect affect the results. The critical ones include the
following: (1) the permeameter permeameter specifications: length, specimen diameter, tap hole of invisible air bubbles and tubing; and (3) diameters, and particle size; (2) presence of material placement placement in the permeameter. permearneter. For coal, care must be taken to avoid abrading the particles and washing out the fines during the test.
summary of of specific permeabilities for crushed crushed rock and coal Table 3.1 shows a summary specific permeabilities of these results shows the specific permeability of samples. A comparison comparison of specific permeability of coal specimen is consistently consistently higher than that of of rock specimen.
50 Table 3.1 Specific Specific permeability penneability for rock and coal samples using water-based water-based tests Particle Size Range (mm) 0.15-0.42 0.15-0.42 1.68 - 4.75 1.68-4.75 4.75 4 . 7 5 --66.73 .73
Mean size
(mm) 0.28 3.22 5.74
22
Specific Permeability, k (m (m )) Rock uli 3.34 x lO10" uli 6.51 x lO10" 09 7.49 x 1010'
09
09
09
Coal Uli 3.49 x 1010" 09 6.62 xX 10~ 108.54 xx 10" 10-u~ 8.54
09
09
09
3.3.3 Air-Based Air-Based Method This method is used to determine detennine the specific specific permeability penneability of of rock particles by passing air through a porous medium in a physical model. These tests were conducted by of applying the same principles used with the constant-head constant-head method. The prerequisites of laminar flow conditions were also required in these tests. For this purpose, part of of an existing longwall model was modified permeameter. modified to serve as a penneameter.
conducting these tests: first, since the model There are several advantages for conducting resembles a longwall panel, the expected results should better approximate real
circulated through the porous medium medium instead of conditions; second, air is circulated of water; third, the permeameter larger rock particles. The penneameter can be used to measure the permeability penneability of oflarger
combined results can predict predict the gob permeability penneability more accurately. Finally, these results can be compared compared with those of of water-based water-based tests for the same particle size. This comparison specific penneability. permeability. comparison can be used to determine detennine the fluid effect effect on the specific
3.3.3.1 Testing Apparatus Figure 3.4 shows the ventilation model used for this test. This physical model was pressurized by a 1. 75-kW blower blower fan. The maximum maximum fan constructed 1.75-kW constructed of of PVC pipes and pressurized speed was 60 rpm. The pipes were arranged in a U-shape system (Figure 3.5). It included
Figure 3.4 Longwall mine ventilation model at the University of of Utah
Return
Steel Screen
Stat 10 B
A
Simulated Gob Material
D
C
Fan
Stat 1
Stat 2
Stat 4
55.75 cm
Stat 5
Intake Steel Screen
Pressure tap Crosscut
permeameter for air-based test Figure 3.5 The penneameter
to
53 one intake, one return and four crosscuts (A, B, C and D). There were 10 pressure taps (stat 1 to 10) to measure velocity and static pressures. Each crosscut had one slot where a regulator of of fixed resistance (porous medium) could be inserted. For the permeability completely blocked while the last crosscut was tests, the first three crosscuts were completely represented a longwall panel. A detail description of regulated. This arrangement arrangement represented of the presented in Chapter Chapter 4. model is presented modified permeameter. It consists of Figure 3.5 also shows the modified of a cylindrical container container 14 cm in diameter diameter and 55.75 cm in length. It was filled with rock particles. Two of 4.7 mm spacing were attached to the top and bottom ends of of the steel screens of permeameter. The screen size was selected to minimize the resistance to airflow. This of the limited the particle size that could be tested in the permeameter. The height of sample-column sample-column in the permeameter permeameter was at least twice its diameter (31.25 cm). The pressure drop through the porous medium was measured manometer at measured by reading a manometer
Stations 5 and 6. The resistances caused by two elbows were also measured and considered considered in the calculation. The air quantity was determined based based on velocity heads monitored at Stations 4, 5 and 7.
3.3.3.2 Testing Procedure
following procedure: An air-based test was carried out using the following 1. Inspect the model and the monitoring instruments (i.e. manometers, pitot tubes, regulators, and tubings).
permeameter from from the mine model. Fill it up with particles of 2. Disassemble the permeameter of predetermined height. predetermined
54 3. Install the top steel screen and reassemble the permeameter.
4. Energize the blower blower fan and set the initial frequency frequency to 30 Hz. Run the fan for about 2 minutes to reach a steady state condition. 5. Record static and velocity heads at Stations 1,5,6, 1, 5, 6, and 7. 6. Measure the room temperature and barometric pressure. 7. Repeat the procedure for different different specimen specimen heights (312.5, 468.8, and 557.5 mm) and fan frequencies frequencies (45 and 60 Hz).
Thirty seven experiments were carried out to determine a relationship between particle size and permeability. Four different 8.72 different particles sizes were tested: 5.74, 7.73, 8.72 and 9.71 mm, respectively. The first experiment experiment was carried out with an empty
permeameter to determine the model's resistance to airflow frictions and shock permeameter airflow due to frictions losses. The model was inspected inspected carefully carefully for leakage. The remaining tests were carried
out with the permeameter permeameter filled with dried rock particles. There were nine tests for each particle size (3 sample heights x 3 fan frequencies). frequencies).
3.3.3.3 Testing Results In these tests, rock particles were used as the porous medium. The particle size ranging from 4.7 to 12.7 mm were divided into four groups. Their mean sizes were 5.74, 7.73,8.72, conducted by changing one of 7.73, 8.72, and 9.71 mm. A parametric study was conducted of the three variables at a time: particle size, specimen height, and fan speed. For example, the first test was conducted conducted with a fan frequency frequency of 30 Hz, mean particle size of of30 of 5.74 mm, and of 3,125 mm. For the next test, the fan fequency fequency was increased increased to 45 Hz sample height of
while maintaining maintaining the particle size and the sample height constant. Thirty six experiments
55 were carried out to complete this study. Out of of these, only 12 yielded reasonable results. These were achieved by setting the fan frequency frequency at 45 Hz. At speeds higher than this, the air leakage became a problem and at lower speeds, the instrument instrument accuracy became questionable. Table 3.2 shows the results of of this experiment. An evaluation of of the figures figures in this table shows that the permeability permeability increases with the particle size and remains unchanged with the sample height. This follows the Karman-Cozeny Karman-Cozeny concept of
permeability permeability and porosity relationship (Bear, 1972). For the sample sizes used in the 8 experiment (5.7 - 9.7 mm), the specific permeability permeability varied between 1.117 1.117 x 1010" and 8
2 1.405 m. 1.405 xX 1010"8 m 8
2
3.4. Specific Permeability of Specific Permeability of Gob Material The specific specific permeability permeability of of gob material is one of of the key parameters in the combustion study. A number of of gob investigations have been carried out to spontaneous combustion of tracer gas (Koenning, 1989; determine the gob permeability, including the use of al., 2002) and photoanalyses (Pappas and Mark, 1993). Such investigations investigations Lowndes et aI.,
found that the gob permeability permeability varies from 1x10 lxlO"5 m 2 .. This variation in 1 x 1 0-13 to 1x10_13
5
permeability permeability zones: unconsolidated, permeability is addressed in this study by using three permeability semiconsolidated and consolidated, as explained in Section 3.1. semiconsolidated
permeability for rock samples using air-based tests Table 3.2 Specific Specific permeability (fan frequency frequency 45 Hz) 8 2 Specific m )) Specific Permeabilit Permeability,, k (x 1010" m 8
2
Mean size (mm) 5.74
(half-packed) (half-packed) 1.109 1.109
(3/4-packed) 1.011
(fully-packed) (fully-packed) 1.231
Average 1.117 1.117
7.73 8.72 9.71
1.017 1.017 1.229 1.229 1.118 1.118
1.185 1.185 1.137 1.137 1.261
1.281 1.748 1.748 1.830 1.830
1.161 1.371 1.403 1.403
56
Mark (1993) reported that an average particle size of Pappas and Mark of gob material permeability of of the unconsolidated unconsolidated zone was behind the shields is 122 mm. The permeability determined based on this information. For other zones, semiconsolidated semiconsolidated and consolidated, their permeabilities permeabilities were determined by CFD simulations (Section 5.1.3). The permeability permeability experiments carried out in this study were used to determine the particle size - permeability permeability relationship. This relationship, once adjusted adjusted for packing effect, effect, was then used to generate input input parameters for CFD modeling. Figure 3.6 shows this modified modified relationship for air-based permeability permeability tests. tests . Based on this relationship, for the unconsolidated specific permeability unconsolidated zone (particles size: 0.122 m), the specific permeability was estimated at 7
2
4.203 x 10- m .
information on material characteristics in zones 2 and 3, the Due to limited information permeabilities for these zones were estimated estimated using CFD simulations. Section 5.1.3 5.1.3
1.4E-07
1.2E-07
y= 2E-OSJ 363 K) K)
304
298 298 293
of Total Temperature Temperature
