Combustion Science and Technology Effect of Pile ...

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Effect of Pile Height on Spontaneous Heating of Coal Stockpiles a

AHMET ARISOY & FEHMI AKGÜN

a

a

Istanbul Tecnical University, Faculty of Mech. Eng , Gum¨şuyu, Istanbul, 80191, Turkey Published online: 17 Sep 2008.

To cite this article: AHMET ARISOY & FEHMI AKGÜN (2000) Effect of Pile Height on Spontaneous Heating of Coal Stockpiles, Combustion Science and Technology, 153:1, 157-168, DOI: 10.1080/00102200008947257 To link to this article: http://dx.doi.org/10.1080/00102200008947257

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AHMET ARISOY· and FEHMI AKGON Istanbul Tecnical University, Faculty of Mech. Eng., 80191, Gumiissuyu, Istanbul, Turkey (Received September 21. 1999) The purpose of this study is to predict the safe storage height of coal stockpiles. To accomplish this, a one dimensional non - steady - state model with the effect of stockpile height approximated by a heat sink in the one dimensional energy equation has been develepod. The model consists of conservation equations for oxygen. water vapour, inherent moisture of the coal. and energy for both gaseous and solid phases. The heal sink term taken from literature is bused on an assumed sinusoidal variation in temperature in the vertical direction. Numerical solution of the model equations gives the time-dependent maximum temperature in the coal stockpile, The effect of stockpile height on the maximum temperature has been analysed parametrically for four different low-rank Turkish coals. When the safe storage timc is specified, the critical height can be determined. By defining the storage time as three months. it has been found that these critical values varied between 0.85 and 1.8 m. depending on the liability of the coals to spontaneous heating. Keywords: Self combustion; Spontaneous combustion

INTRODUCTION The spontaneous ignition of coal stockpiles is an economic and safety problem that is the result of several complex pyhsical and chemical processes and presents difficulties of practical control to restrict the temperature increase in the coal pile. The spontaneous heating rate depends on many parameters which can be divided into three general groups such as controllable, uncontrollable and conditionally controllable parameters respectively. It is therefore necessary to determine the critical values of controllable parameters, notably pile height, governing significantly the heating rate of the coal stockpile under a certain con-

* Correspondence Author: Tel. (90) (212) 25295 57). Fax: (90) (212) 245 07 95. e-mail: arisoy@burgaz,lI1kn,itu.edu.Lr 157

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158

AHMET ARISOY and FEHMI AKOUN

dition. These critical values enable the designer of coal stockpiles to design them to meet safe conditions. Although considerable effort involved both experimental and theoretical studies has already been reported in literature investigating the process of spontaneous combustion, contribution of these results to the practical applications is very limited. However, the requisite conditions are, of course, known in broad qualitative terms in consequence of distillations of decades of field observations and major coal-using countries have standart on governing parameters. The stand arts are biased towards safety but, even so, they are also known to be neither fully exclusive not fully inclusive. Several mathematical models have been developed to predict the conditions under which coal in a pile could undergo spontaneous combustion and to determine the influence of factors contributing to the spontaneous heating. Numerical solution of these models enable predicting of the dynamic behaviour of the coal stockpile, investigating the affecting limits on the process and determining the safe storage conditions and period. One of the more complete models belongs to Nordan (1970). This model consists of the equations for oxygen, water vapour and energy conservation and includes particular transport terms for convection, diffusion or conduction. This kind of modelling efforts has been continued and more developed models have been obtained (Brooks et al. (1988), Schmal (1989), Edwards (1990), Arisoy and Akgun (1994)). However, it is still necessar to develop a more sophisticated model considering all of the affecting parameters sufficiently and to give more realistic results for practical applications. Models solely in one - dimensional can consider only the vertical or the horizontal variations of parameters in the pile. However, actual processes are at least two - dimensional. But the numerical solution of two - dimensional model equations requires much more computer time due to large scale of the pile and time dependency of the process. It may therefore be necessary to make an acceptable assumption to avoid time consumption. Buurn (1981) assumes that heat conduction in the coal pile is effective only in the vertical direction and he solves the one - dimensional energy equation analytically in this direction. He investigated critical ignition conditions depending on coal reactivity. A sinusoidal temperature distrubution is assumed in the vertical direction in Baums model. Present model, extended from the prior study of Arisoy and Akgun (1994), is one dimensional non - steady - state with the effect of stock pi Ie height approximated by a heat sink term adapted from Baum's assumption. This model has been used to evaluate the effect of the stockpile height on the prosess. Critical stockpile height for a safe storage period of three months have been determined for four different Turkish coals by numerical solution of the mathematical model.

SPONTANEOUS HEAT OF COAL

159

The considered Turkish coals in this study are from Can, Tuncbilek, Seyitomer and Gediz regions. Calorific value losses of coals during the preignition periods are also calculated and given in this study.


MATHEMATICAL FORMULATION Modelling of the spontaneous heating of a coal stockpile is based on the mathematical formulation of heat and mass transfer in a chemically reacting porous medium. These processes involved complex interaction between physical and chemical properties of coal and envirolmental factors. For this reason, numerical solutions are only possible under the circumstances of simplifying assumptions and approaches. The one-dimensional model and its solution has been described in detail elsewhere (Akgun (1994)). Only the conservation equations and the modifications in the equations will be given here. Conservation of oxygen in the gas phase: (1 _ a) apl'!. " ut

2 _ OCe) a Plo _

+ yaplo

,,ux

10 ux "2

k

aC:Plo

Oxygen mas balance in the coal particle:

!r2 ~ ar

(oce) ap20 r 2) 20

ar

= P20k

Conservation of moisture in the gas phase: ( 1 - a )ap!W at

+ yaPlw ax

= 0(e)a 2P1w _ l w ax2 ar w

Moisture mass balance in the coal particle: W~aW at = r.,

»«:

Conservation of energy in the gas phase: a Tg (1 - a )( Pg C pg ) ...,,ut

g

+ \ T( Pg Cpg )aT ---n- -_ ux

2T

3 ( Ce)a g Ag ~ - -Rah T g ux

-

Ts

)

Conservation of energy in the solid phase: aTs

_

(e)

apsCpSat - As

a

2T

s

3

.

ax 2 - ilah(Tg - T s) - o~Hwrw

+ ac:~HoPlok + Q,

The evaporation (or condensation) rate for coal is indicated r w in these equations and given below:

AHMET ARISOY and FEHMI AKGON

160

W~. w r w = p,-h WO

rw \\

-

W)

Q2 term in energy equation represents the heat conduction in the vertical direction. Assuming that the temperature at the upper and lower surface of the pile are the same and this temperature corresponds to the ambient temperature. The vertical distribution of temperature can be expressed as

T7, = (T, - Ta)Sin

(Zrrz) + T a

This yields the heat conduction term in the vertical direction as:


2

-T) Qz-- _~\(e)(T Z'2/\o s a The calorifc value loss of coal depends on heat generation due to oxidation or slow combustion of coal. Calorific value loss can be expressed by the ratio of the heat produced to the lower calorific value of coal:

{)I

LlHo

-{)t = oCPlokp, H a Initial and boundary conditions for the equations and soluttion procedure can be found in Akgun (1994).

RESULTS AND DISCUSSION Kinetic data from Akgun and Arisoy (1994) have been used in this study. Other important parameters for standard conditions which are assumed in this study are give in Table I. TABLE I Some parameters used for standard condition Wind-induced gas now velocity in the stockpile Mean coal particle size

Compaction degree of the stockpile

1 "'1O=4

m 51

7.5 nun

0.7

Amhient temperature

20'C.

Initial coal stockpile temperature

20'C.

Generally the rate of tempereture rise in the coal bed is the simplest and most common criterion to evaluate the liability of coal to the spontaneous heating. For this reason the maximum temperature variation with time within the coal stokpile for four different Turkish coals have been predicted considering various stock-

161


--.140r------,-----r---~r_--------_,


oo

.

20

40

60

eo

100

120

TIME (DAY) FIGURE I Variation of maximum temperature with time. Inllucnccof stock heighI for Can coal bed

pile heights. These maximum temperature variations are given in Figure I, Figure 2, Figure 3 and Figure 4 for Can. Tuncbilek, Seyitomer and Gediz coals receptively. Stock heights are parameters in these figures. The effect of stock height on temperature rise can be seen clearly in these figures. Heat loss increases with decreasing stock height. Increased heat losses cause to slow down the heating rate and also to stay the temperature for longer time at a stable level. Thus spontaneous heating or self - combustion of coal is delayed or even eliminated by reducing the coal stockpile height. As can be seen quantitatively from these figures. stock height is a very useful tool to avoid the spontaneous heating problem. The critical stock height for a specific coal can be determined with the help of the related figures for the considered safe storage time. The safe storage period will be different depending on the purpose of the application. Critical height and the critical time can be determined from the slope of the time dependent maximum temperature curves. Thermal runaway can be assumed to start when the slope of the curve reaches a critical value. Heat liberation depends strongly upon the coal reactivity. On the other hand heat dissipation depends upon the convective and conductive heat transfer. The

162

AHMET ARISOY and FEHMI AKOON

140 . . . - - - - - - - - - - - - - - - - , - - - - - - - - - - ,

oo .

: Z=c>:

,"

-

....

. Z=1.8 m: Z=1.2 in


Z=.1 m

20

40

60

50

100

120

TIME (DAY) FIGURE 2 Variation of maximum temperature with lime. Influence of stock height for Tuncbtlek coal hed

larger portion of the heat transfer corresponds to the moisture from wet coal particle. For this reason, time - dependent temperature increase shows two different stages due to the initial moisture content of the coal. Moisture content of the Can, Tuncbilek, Seyitorner and Gediz coals are 16.8 %, 18.3 %, 35.4 % and 1.8% respectively. Generally the maximum temperature increases steadily with time for low-moisture content coals. However in the case of high-moisture content coals, temperature increases rapidly at the beginning and then evaporation becomes effective and the temperature approaches to a steady state value. In this stage, heat liberation due to chemical reaction and heat dissipation due to evaporation and other transfer mechanisms are approximately equal. This steady temperature value is maintained until the evaporation process decays and the coal becomes dry. After that point, temperature increases rapidly again, With the comparison of Figure I, Figure 2, Figure 3 and Figure 4, the effect of evaporation on the temperature increase can be seen clearly. 11 can be see from Figure I that, safe storage time for Can coal of I m stock

height is 47 days, According to the same figure, these safe storage times are


163

~ 120 , . . - - - - - - - - - - - - - - - - - - - - - - - ,

oo

.......

W 100

. Z=oo

a::::

~

.

0..

:i: 60

·Z=1.2m·

W

I-


.

·Z=1.8 m·

W

:i:

::::>

:i:

~

:i:

......................'--'-~ ............l...-...........- - ' - - ' -.........- ' - -...........

O'"-'~~-'--'--'--"-"-~

o

20

40

60

80

100

120

TIME (DAY) FIGURE 3 Variation of maximum temperature with time. Influence of stock height for Seyitomcr

coal bed

92 days for 0.85 m height and 28 days for 1.2 m height. This coal is very reactive and spontaneous heating of this coal is very sensitive to the stock height. However Seyitomer coal in Figure 3 is safe in terms of the thermal runaway for 120 days, independent of the stock height. This safety is due to the very high moisture content of this coal. Figure 4 indicates that less reactive but very dry gediz coal can be stored safely only 57 days for 2.4 m stock height. Safe storage heights for a there-month period are defined by the help of the proposed model as 0.85 m for (an coal. 1.2 m for Tuncbilek coal and 1.8 m for Gediz coal. Seyitorner coal is safe for every stock height. At the end of there-month period maximum temperature in the coal stockpile reaches 65°C for (an and Tuncbilek coals and 55°C for Gediz coal. By taken Qz term as zero in the equations which represents an infinite dimension in the vertical direction, one dimensional model solved in Akgun (1994). calorific value losses of the coal along the pile are given in Figure 5, Figure 6 and Figure 7 for (an, Tuncbilek and Seyitomer coals respectively. Storage time is the parameter in these figures. Effect of volatilisation on the calorific value loss has been neglected.

164

AHMET ARISOY and FEHMI AKOON

140 r - - - - - - - - - - - - - , - - - - - - - , - - - - - - - - ,

oo '-'120 UJ

a::: ~

100

~

UJ Q..

:iE

UJ


f-

:iE

:::l

:iE

~ '0 L-...~~_'___~~_'__~~............~~~.!........o~~_'___~___..__' 80 100 120 40 60 o 20

TIME (DAY) FIGURE 4 Variation of maximum temperature with time. Influence of stock height for Gcdiz coal

hed

Calorific value loss reaches a maximum at the high oxygen depletion region in the coal pile. Total calorific value loss can be calculated approximately by integrating thc local losses along the horizontal length of the coal pile. The maximum calorific value loss for very thick Can coal stockpile occurs at the cross section which is at a 0.8 m horizontal distance from the edge of the pile for a storage period of 20 days. The loss value at this cross section 20%. In the case of Tuncbilek coal stockpile, the maximum calorific value loss reaches II % for a storage period of 40 days.

CONCLUSION Generated heat should be transferred to the ambient air to avoid self heating of coal stockpiles. There are factors intluencing heat liberation and heat dissipation. Most of these factors can not be controlled easily. However the height of the storage influences the process very strongly and this factor can be controlled easily in practice.

165


___ 0.3 r - - - - - - - - - - - - - - - - - r - - - - - - ,

o o'"

Time (day) 10

1=

~

15

en en 0.2

20

o ~

25

w ::> o-l


-c ;>

u &::

0.1

§ ~

U

2

4

6

8

10

DISTANCE FROM INLET (m) FIGURE 5 Calorific value losses of Can coal along the pile. Time is parameter

--'" 0

0.3

Time (day)

0

10

......

I-

20

~

30

0

50

en en

40

~

60

U.l

::>

o-l

« ;>

u ...... ~

0.1

i:2 0

~ U

0.0 0

2

4

6

8

DISTANCE FROM INLET (m) FIGURE 6 Calorific value losses of Tuncbilck coal along the pile. Time is parameter

10

166

AHMET ARISOY and FEHMI AKGON


The safe storage height of the coal pile can be defined regarding the storage time. Stock height should be reduced for a longer storage time. Critical height of the coal stockpiles can be calculated by the numerical solution of the developed mathematical model. The results of the model prediction for four different Turkish coals are given in this study. Defining storage time as 3 months, critical heights of the stockpiles have been calculated as 0.85 m. for Can coal, 1.2 m. for Tuncbilek coal and 1.8 m. for Gediz coal. Seyitomer coal does not reach thermal runaway point during the three months period.

G ;::I o 1=

0.10

r----------------.,....---------, Time (day)

40

eo

;2

120

CIl CIl

"

o

....J

~

~ 0.05

::> U ...... ~

i:2 o...l

....

""

"-, "

""

-,

tS 0.00 L.....~"'--"'__.L..-~~~_'_ _ _~...!_~~~...L.~~~....J 4 6 8 10 o 2

DISTANCE FROM INLET (m) FIGURE 7 Calorific value losses of Seyid6mer coal along the pile. Time is parameter

Calorific heating value loss of coal due to slow oxidation reaction during storage time is essentially dependent on coal reactvity. Can coal is the most reactive among the investigated four coals. Heating value lost for this coal during the safe storage time reaches 20% at the point where maximum temperature exists.


NOMENCLATURE


Cp : D:

Specific heat capacity (Jkg- 1K- I) Diffusion coefficient (m 2s- l)

h:

Heat transfer coefficient (Wm-2K- 1)

Ha: 6H o:

Lower calorific value of coal (Jkg- I)

6H w :

Heat of reaction of oxygen with coal (Jkg- 10 2) Heat of evaporation of water (Jkg- IH

I:

Calorific value loss ratio

k:

Reaction rate constant (s-I)

Kw : Qz:

Evaporation (or condensation) rate constant (s-I) Heat conduction in vertical direction (Wm- 3)

r:

Radius (m)

rw : R:

Evaporation (condansation) rate of water vapour (kgH 20m- 3s- l) Radious of particle (m)

t:

Time (s)

T:

Temperature (K)

Tz:

Vertical temperature distribution (I