ORIGINAL RESEARCH PAPER
Optimisation of COREX process P. S. Assis*1, L. Guo2, J. Fang2, T. R. Mankhand3, G. F. Salierno4 and G. de Arau´jo Filho1 Coal and energy consumption in the COREX process can be minimised from the balance between the required and generated reduction gas. Metallisation ratio is calculated at the balance, which corresponds to the effective calorific value of smelting coal. The proper metallisation ratio will be defined in this paper, after which it is demonstrated that an effective calorific value of smelting coal is needed to minimise the consumption of energy in the process. In addition, heat stability is also an important parameter when using coal instead of coke to insure a useful fixed bed in the melter gasifier. By considering the above points the energy consumption in the COREX process can be optimised. Keywords: Effective calorific value, Heat stability, Smelting heat, Energy consumption
; List of symbols Ad ash content in coal, dry basis,% C coal consumption per one tonne hot metal in melter gasifier, kg t21 hot metal E total energy consumption, GJ t21 hot metal Hdaf hydrogen content in smelting coal in a dry and ash free condition, % M moisture content in smelting coal, % Mco CO gas quantity produced by one kilogram coal, kmol/kg coal Mco2 CO2 gas quantity produced by one kilogram coal, kmol kg21 coal Mg gas quantity produced by one kilogram coal, kmol kg21 coal MH2 H2 gas quantity produced by one kilogram coal, kmol kg21 coal MH2 O H2O gas quantity produced by one kilogram coal, kmol kg21 coal Mp the gas quantity produced by smelting coal in melter gasifier for one tonne hot metal, kmol t21 hot metal Mr the quantity of gas needed in reduction shaft in order to deoxidise iron ore to certain metallisation ratio sponge iron for one tonne hot metal, kmol t21 hot metal Ndaf nitrogen content in dry and ash free coal, % Oc degree of secondary oxidisation of CO. It is the per cent of CO in total CO produced which is oxidised to CO2 in an oxygen excess atmosphere, % OH degree of secondary oxidisation of H2. It is the percent of H2 in total H2 produced which
;
1
Escola de Minas; REDEMAT-UFOP/CETEC/UEMG, Brazil Hebei University of Science and Technology, Tangshan, China Banaras Hindu University, Varanasi, India 4 Control and Automation Engineering – Escola de Minas – Federal University of Ouro Preto, Brazil 2 3
*Corresponding author, email
[email protected]
is oxidised to H2O in an oxygen excess atmosphere, % Qdc heat of decomposition of one kilogram coal. It can be measured by experiment or calculated by experiential formula,kJ kg21 coal Qe effective calorific value of smelting coal in function Rm, calculated kJ kg21 coal Qo the sum of energy consumptions for reducing FeO to Fe and smelting Fe to hot metal in one tonne hot metal, kJ t21 hot metal Qs smelting heat of iron ore for one tonne hot metal, kJ t21 hot metal Rm metallisation ratio of sponge iron as reduction product SRm calculated metallisation ratio by using the balance between produced and required gas xH mol fraction of H2 and H2O in H2, H2O, CO and CO2 g reduction gas utilisation ratioNone go theoretical reduction gas utilisation ratio
Introduction Coke and coking coal shortages worldwide led to the development of the COREX process, which is currently the only commercialised smelting and reduction process. Its product is liquid iron.1 The advantages of this process, in comparison with the blast furnace, are the use of coal instead of coke and the possible utilisation of fine ores. However, the technology must be improved, especially because of its high energy consumption and the necessity still to use some coke.2 Two factors primarily influence the energy consumption in COREX. One is the smelting and reduction energy consumption, and the other is the physical heat found in the excess coal gas. In normal production the former aspect is set, so the energy consumption is controlled by the amount of excess coal gas.3 In this paper a method has been developed to calculate how energy consumption can be reduced in
ß 2008 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 14 September 2005; accepted 25 October 2006 DOI 10.1179/174328108X269478
Ironmaking and Steelmaking
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:00 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
2008
VOL
20
NO
000
1
Assis et al.
Optimisation of COREX process
@
1 View of COREX process4
the COREX process by using the balance of required and produced amounts of reduction gas, and the proper effective calorific values of smelting coal. The proper metallisation ratios and the gas balance for choosing coal to optimise COREX process was then defined.
Coal consumption and gas producing of smelting shaft Figure 1 shows a schematic of the COREX process from an Indian company, and consists of two parts: the reduction shaft and the smelter gasifier. In the smelter gasifier, the specific coal consumption c in kg t21 hot metal depends on the smelting heat of iron ore and the effective calorific value of coal. The specific formula, equation (1), is written as c~55z
(1)
The amount of produced gas can be calculated according to equation (2) Qs Qe
(2)
where Qs is the smelting specific heat of iron ore and can be determined by equation (3)5 Qs ~Q0 {4634396Rm
(3)
Qe is the effective calorific value of smelting coal. This value for one kilogram dried coal is determined by equation (4)6 Qe ~ð5634Cdaf {17600Hdaf {1336Ndaf Þð1{Ad Þ{ ð1841z9916SiO2 {10088CaOÞAd { Qdc {15389
M 1{M
(4)
Hdaf is the hydrogen content in smelting coal under a dry and ash free condition. Qdc is the decomposition heat, which can be measured by experiment or calculated by formula. M is the water content in smelting coal. Ndaf is the nitrogen content in coal under a dry and ash free
2
Mco ~
Cdaf ð1{OC Þð1{Ad Þ 12
Cdaf OC ð1{Ad Þ 12 Hdaf M ð1{Ad Þz ð1{OH Þ MH2 ~ 18ð1{M Þ 2 Mco2 ~
MH2 O ~
Qs Qe
Mp ~Mg
condition. Ad is the ash content of coal in a dry basis. All symbols are fully defined at the end of the article. Mg is the gas quantity produced by 1 kg of coal and can also be calculated by equations (5) to (8). These show each gas (CO, CO2, H2 and H2O) produced by unit of coal.
Ironmaking and Steelmaking
2008
VOL
20
NO
000
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:01 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
Hdaf M OH ð1{Ad Þz 18ð1{M Þ 2
(5) (6)
(7)
(8)
The total gas amount produced by one unit of coal is the sum of four kinds of gas quantities as shown in equation (9) Mg ~Mco zMco2 zMH2 zMH2 O
(9)
By knowing the chemical compositions of smelting coal, Mp can be calculated based on the formulae above. Table 1 shows the composition of 11 types of smelting coal and Table 2 shows the composition of Sishen iron ore. go is theoretical reduction gas utilisation, what can be calculated using equation (10)5,6 x{1zRm g~(0:49{0:28xH ) 1 3 zRm
(10)
where Rm is the metallisation ratio of sponge iron. Supposing that it is 0?92, and x is 1?5. This formula can be simplified as g~0:555{0:317xH
(11)
From this research, it is shown that the real gas utilisation ratios approaches the theoretical ones, so equation (11) can be considered directly in some calculations.
Assis et al.
Optimisation of COREX process
2 Relation between required gas quantity and Rm
Gas requirement in reduction shaft
3 Relation between supplied and required gas and metallisation ratio using effective calorific value coal
In the reduction shaft, the required gas per hot metal unit is not directly influenced by the iron ore composition. It depends upon the metallisation ratio and reduction gas utilisation ratio. One kg of hot metal has 16?96 mol of Fe. The amount of reduction gas to deoxidise this iron ore to sponge iron with a certain metallisation ratio is defined by equation (12)7 8:48z16:96Rm (12) Mr ~ g
produced gas will be more than that required, thus there will be excess reduction gas, which means loss of energy. If the metallisation ratio is .0?92, the gas produced will be less than the required gas, thus there will not be enough gas for reduction. The solution is to add smelting coal in the melter gasifier to produce more gas until this relation is achieved. This then leads to an excess smelting heat in the melter gasifier, resulting in an increase in coal gas temperature. As a result, there will also be wasted energy consumption. Therefore, at the point where SRm is 0?92, there is an optimisation of smelting and reduction and energy consumption reaches its minimum. Not all kinds of smelting coal can achieve this balance due to their different effective calorific values. A metallisation ratio is calculated supposing that all producing gas is used for the reduction shaft. Table 3 shows the calculated metallisation ratios and the effective calorific value for eleven kinds of coal. Since the balance is an assumption, the calculated Rm value can be .1. For example, Ng coal’s effective calorific value is low; hence, using this kind of coal for smelting, the producing gas quantity is high. If all the producing gas is used for
The average value of gas utilisation ratios (for the eleven kinds of coal) is 46?7%, so the formula can be simplified as shown in equation (13) Mr ~17:78z35:56Rm
(13)
Figure 2 shows the relationship between metallisation ratio and reduction gas needed for producing 1 t of hot metal.
Balance between required and produced gas Since there is no excess coal gas from the balance between the required and produced reduction gas, the energy consumption minimises at this state. The metallisation ratio from the balance can be related to Mp and Mr, as shown in Fig. 3. This figure shows the above relationships for ‘Sys’ smelting coal and ‘Sishen’ ore. It is known from Fig. 3 that the amount of produced gas is the same as that required when the metallisation ratio is equal to 0?92. If the metallisation ratio is ,0?92, the
Table 2 Iron ore analysis Qo, kJ t21
Composition, % Name
Type
TFe
FeO CaO SiO2 MgO
Sishen Natural 66.30 0.21 0.09 3.26
0.06
5 835 800
Table 1 Parameters of coal and gas Coal
Gas Composition, %
Composition, % 21
Coal name
Coal type
Ad
Vdaf
Cdaf
Hdaf
Qe, kJ kg
CO
CO2
H2
H2O
xH
Mg, kmol kg21
go, %
Xlz Qls Yq Sf Dt Nm Sys Dwk Tx Ng Tg
Soft Soft Anthracite Soft Soft Soft Soft Anthracite Anthracite Soft Soft
6.35 11.04 13.21 6.90 15.89 19.23 8.51 12.58 11.50 10.64 8.69
37.26 22.55 10.97 33.83 30.25 16.56 40.12 11.52 10.02 44.42 38.82
83.08 92.36 91.55 80.76 82.42 89.10 83.48 91.37 91.88 67.98 61.30
5.45 4.40 4.20 4.87 4.93 4.58 5.68 4.08 4.09 3.99 3.56
2432 3670 3132 1978 895 1854 2765 3135 3167 794 1214
68.89 74.66 76.28 70.49 72.26 73.37 68.17 75.72 75.76 71.00 71.19
2.87 3.11 2.14 2.94 3.12 3.06 2.84 3.15 3.18 2.96 2.97
26.83 21.12 19.50 25.24 24.39 22.39 27.54 20.07 20.03 24.74 24.55
1.41 1.11 2.08 1.33 0.23 1.18 1.45 1.06 1.05 1.30 1.29
0.28 0.22 0.22 0.27 0.25 0.24 0.29 0.21 0.21 0.26 0.26
0.09035 0.08804 0.08333 0.08533 0.07675 0.07847 0.08963 0.08439 0.08586 0.06845 0.06290
46.62 48.53 48.62 46.94 47.52 47.89 46.31 48.84 48.84 47.26 47.26
Ironmaking and Steelmaking
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:06 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
2008
VOL
20
NO
000
3
Assis et al.
Optimisation of COREX process
4 Relation between effective calorific value and metallisation ratio
5 Relation between gas supplying, needed amounts and metallisation ratio when using high effective calorific value coal
reducing, based on equation (13), the sponge iron’ Rm is .1. In reality, of course, Rm cannot be .1, but is used to indicate that too low or too high effective calorific values are not appropriate. If the effective calorific value is too low, as with Ng coal, the gas amount produced is much higher than that needed in the reduction shaft. If it is too high, the producing gas quantity is insufficient compared to reduction shaft requirements. Figure 4 shows the relation between the metallisation ratios and the coal effective calorific values. SRm is the calculated metallisation ratio by using the balance. From Fig. 4, it is clear that SRm decreases as effective calorific value increases. In the COREX process, the feasible metallisation ratio, simplified as Rm, varies between 0?85 and 0?95. Using an effective calorific value between 2500 and 3200 kJ kg21, the SRm from 0?85 to 0?95 can be obtained using the balance. This way using coals with that effective calorific value the energy can be minimised. Values of SRm from 0?85 to 0?95 cannot be obtained for all kinds of coal, as can be seen in Table 3. When the effective calorific value is too high, its SRm using the balance is ,0?85 from Fig. 5, so its SRm and the balance cannot be obtained by using this kind of coal. A feasible Rm is from 0?85 to 0?95, then the produced gas is less than the needed gas. When Rm is 0?85 the lack of gas is the smallest, as can be seen in Fig. 5. Consequently, energy consumption also reaches its minimum when Rm is 0?85 in the range that goes from 0?85 to 0?95. Between these values, coal consumption and produced gas are determined by the reducing process. From Fig. 6, it can be seen that when the effective calorific value is too low, its SRm is .0?95, so its SRm and the balance can not be obtained by using this kind of coal. From Fig. 6, it can be said that feasible Rm is from 0?85 to 0?95, then gas produced is more than the needed gas. The excessive gas amount is least when Rm is 0?95. Then, energy consumption also reaches its minimum when Rm is 0?95 (in the range of 0?85 to 0?95). In this case, coal consumption and produced gas are determined by the smelting process. Figure 7 shows the relationship between coal consumption and effective calorific value of smelting coal. It
6 Relation between supplied and needed gas and metallisation ratio by using low effective calorific value coal
7 Relation between effective calorific value and coal consumption
is clear that coal consumption increases as effective calorific value reduces; obviously when the effective calorific value is low, the coal consumption is high. The influence of effective calorific value on coal consumption weakens when effective calorific value increases. When the effective calorific value rises to proper bound, the coal consumption reaches the minimum. Figure 8 shows the relationship between energy consumption and effective calorific value. As it can be
Table 3 Influence of effective calorific value on calculated Rm
4
Name
Ng
Dt
Tg
Nm
Sf
Xlz
Sys
Yq
Dwk
Tx
Qls
Qe, kJ kg21 SRm
794 1.11
895 1.12
1214 1.03
1854 0.99
1978 0.99
2432 0.95
2765 0.92
3132 0.87
3135 0.87
3167 0.87
3670 0.84
Ironmaking and Steelmaking
2008
VOL
20
NO
000
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:09 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
Assis et al.
Optimisation of COREX process
Table 4 Heat stabilities of smelting coal Name
Xlz
Qls
Yq
Sf
Nm
Dwk
Tx
RWz6, % 92.78 83.68 98.68 82.85 95.17 89.00 88.78
8 Relation between total energy consumption and effective calorific value
seen the influence of effective calorific value on energy consumption is similar to the coal consumption.
1. Balancing the required and supplied reduction gas in the COREX process is the effective way to minimise energy consumption. 2. The optimisation of smelting and reduction occurs when the range of metallisation ratio lies between 0?85 and 0?95. 3. Maintaining an effective coal calorific value is a precondition for COREX process optimisation. The optimum value is 2700 kJ kg21. 4. Heat stability is also an important index of coal for COREX process optimisation. Usually the higher the value, the better the process as high stability coal can decrease coke consumption in the melter gasifier.
Acknowledgments
Heat stability of smelting coal The COREX process uses a certain proportion of coke to add additional calorific value and to ensure an effective fixed bed.8 The fixed bed plays a very important role in the melter gasifier. Reactions on the fixed bed are complex, such as direct reduction, carbonisation and separation impurity from liquid iron. High heat stability of coal can avoid the use of coke, which means that heat stability is another important index of smelting coal for COREX process optimisation. The index of coal heat stability is defined as RWz6 and is determined by the following process. One kilogram of coal is put into drying oven. This coal should have a grain size between 5 and 15 mm. This oven is heated to 1100uC where the coal will be maintained in the furnace for 0?5 h without air. After that, the coal is taken out and screened using 6 mm screen. RWz6 is the percentage of coal bigger than 6 mm obtained after this process.9 For high heat stability of smelting coal, its RWz6 reaches 98%. Table 4 shows seven kinds of smelting coal’s heat stability.
Conclusions Based on this research, the following conclusions can be reached.
This work was made under the financial support of FAPEMIG (Research Foundation in the Minas Gerais State, Brazil), Brazilian Academy of Sciences and Indian National Sciences Academy-INSA. We are heartily thankful for that. We would like to acknowledge Lilian Bambirra de Assis for helping us with the ultimate correction in English.
References 1. H. M. W. Delport and P. J. Holaschke: Proc. ‘Corex Symposium 1990’ Conf., The South African Institute of Mining and Metallurgy – Book commissioned by Iscor and Voest Alpine, Johannesburg, 1990. Apud: Assis, P. S. & Sampaio, R. S. Novos processos de produc¸a˜o de ferro prima´rio, Ed. ABM, Out. 1995, 250. 2. Gudenau et al.: Steel Res., 1989, 3, 138. 3. Puehringer et al: Stahl und Eisen, 1991, 111, 37–44. 4. S. K. Gupta: Ironmaking Technology, Proc. VAI Int. Ouro Preto Conf., March 2002, 12. 5. J. Fang, X. J. Wang and L. Guo: Steel Res., 2005, (9), 705–708. 6. J. Fang: ‘Smelting reduction and direct reduction’, 67–107; 1996, Shenyang, NEU Press. 7. J. Fang and M. S. Chu: J. Northeastern Univ., Jan. 2002, 23, (1), 32–34. 8. P. Subrata and A. K. Lahiri: Process Metall. Mater. Process. Sci., Feb. 2003, 34, (1), 115–119. 9. Z. Y. Pi: ‘Study on heat stability measuring method for industrial briquette’, Coal Technology/National Coal Quality Supervision Test Center, Beijing100013, China, December, 2003, (31), 37–39. .
Ironmaking and Steelmaking
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:13 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
2008
VOL
20
NO
000
< = >
?
5
Authors Queries Journal: Ironmaking and Steelmaking Paper: 2012 Title: Optimisation of COREX process Dear Author During the preparation of your manuscript for publication, the questions listed below have arisen. Please attend to these matters and return this form with your proof. Many thanks for your assistance
Query Reference
Query
Remarks
1
Author: Please confirm the corresponding author.
2
Author: Please provide other authors.
3
Author: Please provide other authors.
4
Author: Please provide place of event and organisation.
5
Author: The reference is from a journal? Please clearify.
6
Author: Please confirm the quality of figure 1.
Ironmaking and Steelmaking IRS2012.3d 10/3/08 18:42:13 The Charlesworth Group, Wakefield +44(0)1924 369598 -
Rev 7.51n/W (Jan 20 2003)
