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Future potential for biomass use in blast furnace ironmaking
19th European Symposium on Computer Aided Process Engineering – ESCAPE19 J. JeĪowski and J. Thullie (Editors) © 2009 Elsevier B.V. All rights reserved.
567
Future potential for biomass use in blast furnace ironmaking Jarmo Söderman, Henrik Saxén, Frank Pettersson Heat Engineering Laboratory, Åbo Akademi University, Biskopsg. 8, FI-20500 Åbo, Finland,{jsoderma,hsaxen,fpetters}@abo.fi
Abstract Iron- and steelmaking is an energy intensive industrial sector using mainly coal as the heat source and reduction agent. The industry gives rise to about 7 % of the anthropogenic CO2 emissions in the world. In the absence of economically feasible and efficient methods of capturing and storing such enormous quantities of CO2, means for suppressing the emissions must be explored. The work reported in this paper studies the potential of injecting biomass to partially replace fossil reductants in the blast furnace process. The ironmaking blast furnace process is described mathematically by a thermodynamic simulation model, including realistic operational constraints. The model has been applied extensively to evaluate the use of biomass (e.g., wood chips) as auxiliary reductant, creating a simplified linear model on the basis of the results. The model is used to throw light on the feasibility of biomass injection under future price scenarios. Even though the coke replacement ratio of biomass is low, in the order of 25 %, it is demonstrated that the use of biomass as reductant can be a feasible alternative under future price scenarios of coke and emissions. Keywords: Blast furnace, biomass, optimization
1. Introduction Global concern of climate change has brought up into the discussion the role of iron and steel industry as a CO2 source. About 7 % of the anthropogenic CO2 emissions in the world are emitted from iron- and steelmaking industries. The industry uses vast amount of coal in form of coke in the blast furnaces. The purpose is not only to gain the needed heat into the ironmaking process but also to use carbon as reductant for the oxygen containing iron ore. By injection of biomass the fossil CO2 emissions could be lowered. The cost of biomass and the CO2 emission cost are main factors when optimal biomass use in blast furnace process is discussed. Biomass injection rate is, on the other hand, limited to a maximum specific rate in proportion to the hot metal production. The work reported in this paper studies the potential of injecting biomass into the blast furnace in order to partially replace fossil reductants and hereby reduce the fossil CO2 emissions.
2. Model In the present work a thermodynamic blast furnace model was applied [2,3], which is based on the fundamental concepts introduced by Rist et al. [1]. The model utilizes a division of the process into two main control volumes, with thermal and chemical equilibrium approached on the boundary, the reserve zone, between the two volumes. A
J. Söderman et al.
568
linear model was developed with a set of binary variables to control a nonlinear relation of two process variables. The blast furnace operational costs are predicted effectively by the model. Additionally, the revenues of electric power and district heat that can be produced at the plant were obtained. The input variables in the linear process model were volumetric flow rate of air, volumetric flow rate of oxygen, specific oil rate, blast temperature, specific rate of pellets, specific rate of limestone and specific injection rate of biomass, shown in Eq. 1.
+ K i ,5
Vair
VO 2
moil T + K i ,3 + K i , 4 bl + °C kg/t hm km n/h km 3 n/h mpel m mbio + K i ,6 lime + K i ,7 kg/t hm kg/t hm kg/t hm
Yi = K i ,0 + K i ,1
3
+ K i,2
(1)
The objective function F is given by
m pel c pel m coke,own ccoke,own m coke,b c coke,b c F § m = ¨ sin ⋅ sin + ⋅ + ⋅ + ⋅ + €/t hm © t/h €/t t/h €/t t/h €/t t/h €/t VO2 cO2 m c m c m c + oil ⋅ oil + lime ⋅ lime + bio ⋅ bio + ⋅ + 3 t/h €/t t/h €/t t/h €/t km n/h € km 3 n m CO2 c CO2 c el Q c heat · m hm P ¸ + ⋅ − ⋅ + dh ⋅ t/h € t MW € MWh MW € MWh ¸¹ t hm /h
(2)
For the specific cost of hot metal production the mass and volume flow rates are multiplied by specific mass or volumetric costs, ci. Coke rate is included as own and bought coke, denoted by subscripts coke,own and coke,b. The price of electricity is denoted by cel and heat by cheat. The mass flow rate of fossil CO2 emissions is calculated as the ratio of the molecular weights of CO2 and carbon and taking also into account the share of fossil carbon in the total carbon input to the blast furnace.
3. Illustrative case The thermodynamic blast furnace model was run under a large number of input combinations, with input variables uniformly distributed within their admissible ranges. The injection rate of dry biomass was limited to max. 120 kg/t hm. The process was optimized with the price data shown at Table 1. The biomass and CO2 emission costs were varied: biomass price between 50 and 110 €/t and CO2 emission cost between 0 and 80 €/t.
Future Potential for Biomass Use in Blast Furnace Ironmaking
569
Table 1. Input price data for the test cases.
oil limestone pellets sinter coke, own coke, bought oxygen gas
150.0 €/t 30.0 €/t 110.0 €/t 90.0 €/t 200.0 €/t 300.0 €/t 50.0 €/t
El power, sold Heat, sold
50.0 €/MWh 10.0 €/MWh
biomass price CO2 emission
50.0 - 110 €/t 0.0 - 80 €/t
4. Solutions
Production cost, €/t hm
The model was first tested by a series of runs with fixed biomass price of 50 €/t and CO2 emission cost of 0 €/t at different hot metal production rates as a base case, Case 1. Secondly, a series of test runs was made by the biomass price of 100 €/t and CO2 emission cost of 30 €/t, Case 2. The optimal production costs, predicted by the MILPmodel, are shown in Figure 1 for different hot metal production rates in a blast furnace. The minimum production costs were obtained at the production rate of about 145 t hm/h.
250 case 1 case 2
200 120
130
140
150
160
Hot metal prod, t hm/h
Figure 1. Optimal production costs at different hot metal production rates. The variations of two process variables, specific injection rate of biomass and oil, with different hot metal production rates are shown in Figure 2 for Case 1 and Case 2.
J. Söderman et al.
570
Case 1 biomass Case 1 oil
Injection rate, kg/t hm
150.0
Case 2 biomass Case 2 oil
100.0
50.0
0.0 120
130
140
150
160
Hot metal prod, t hm/h
Figure 2. Optimal specific injection rates of biomass and oil to blast furnace at different hot metal production rates.
Biomass price €/t
Biomass injection (kg/t hm)
120 100
60
80
70 80
60
90
40
110
20 0 0
10
20
30
40
50
60
70
80
CO2 cost (€/t)
Figure 3. Optimal specific injection rates of biomass at a production rate of 150 t hm/h with different prices for biomass and CO2 emissions. The optimal specific injection rates of biomass at different CO2 costs are shown in Figure 3, with fixed hot metal production rate of 150 t hm/h. The injection rates vary with the cost of CO2 emissions for a selected set of prices of biomass. The curves in Figures 2 and 3 demonstrate some aspects of the complex nature of the blast furnace process.
Future Potential for Biomass Use in Blast Furnace Ironmaking
571
Partial pyrolysis of biomass Injection of dry wood chips has been shown to be possible up to the level of about 25% of the coke. Limitation is caused by the high oxygen content of wood chips. Biomass pyrolysis has been suggested as a possible way to further increase of the biomass injection rate. A partial pyrolysis of the biomass lowers its oxygen content, but decreases the yield. Additionally there are losses of carbon and hydrogen. The blast furnace MILP-model was rewritten so that it could also take into account the pyrolysis process step. The solutions of the test series with the pyrolysis model are presented by authors in [7].
5. Conclusion An MILP-model for the blast furnace process was tested with the focus on replacement of fossil reductants oil and coke by injection of biomass under consideration of CO2 emission costs. The feasibility of biomass use was revealed at different hot metal production rates and with different price levels for biomass and CO2. A partial pyrolysis of biomass was studied with a modified model and optimal pyrolysis temperatures were found for different biomass and emission costs.
Acknowledgements The financial support from the Academy of Finland to the GreenSteel project is gratefully acknowledged.
References [1] A. Rist, N. Meysson: “A dual graphic representation of blast-furnace mass and heat balances”, Journal of Metals, 19 (1967), 50. [2] H. Saxén, M. Brämming, J.-O. Wikström and P. Wiklund: “Theoretical limits on operation under high oxygen enrichment in the blast furnace”, Proc. 60th Ironmaking Conf., ISS, Warrendale, PA, (2001), 721. [3] F. Pettersson, H. Saxén: “Model for economic optimization of iron production in the blast furnace”, ISIJ International, 46 (2006), 1297.
[4] Y. Kim and E. Worrel: “Intenational comparison of CO2 emission trends in the iron and steel industry”, Energy Policy, 30 (2002), 827. [5] T. Ariyama and M. Sato, “Optimization of ironmaking process for reducing CO2 emissions in the integrated steel works”, ISIJ International, 46 (2006), 1736. [6] M. Takekawa, K. Wakimoto, M. Matsu-ura, M. Hasegawa, M. Iwase and A. McLean: “Investigation of waste wood as a blast furnace injectant”, Steel Research, 74 (2003), 347. [7] H. Saxén, F. Pettersson, J. Söderman, M. Helle. and H. Helle, "Optimization of biomass use as auxiliary reductant in the blast furnace", Recent Progress in Mathematical modeling in Ironmaking 2008 (Ed. T. Ariyama), Tokyo, October 2008, ISIJ, pp. 95-100.
567
Future potential for biomass use in blast furnace ironmaking Jarmo Söderman, Henrik Saxén, Frank Pettersson Heat Engineering Laboratory, Åbo Akademi University, Biskopsg. 8, FI-20500 Åbo, Finland,{jsoderma,hsaxen,fpetters}@abo.fi
Abstract Iron- and steelmaking is an energy intensive industrial sector using mainly coal as the heat source and reduction agent. The industry gives rise to about 7 % of the anthropogenic CO2 emissions in the world. In the absence of economically feasible and efficient methods of capturing and storing such enormous quantities of CO2, means for suppressing the emissions must be explored. The work reported in this paper studies the potential of injecting biomass to partially replace fossil reductants in the blast furnace process. The ironmaking blast furnace process is described mathematically by a thermodynamic simulation model, including realistic operational constraints. The model has been applied extensively to evaluate the use of biomass (e.g., wood chips) as auxiliary reductant, creating a simplified linear model on the basis of the results. The model is used to throw light on the feasibility of biomass injection under future price scenarios. Even though the coke replacement ratio of biomass is low, in the order of 25 %, it is demonstrated that the use of biomass as reductant can be a feasible alternative under future price scenarios of coke and emissions. Keywords: Blast furnace, biomass, optimization
1. Introduction Global concern of climate change has brought up into the discussion the role of iron and steel industry as a CO2 source. About 7 % of the anthropogenic CO2 emissions in the world are emitted from iron- and steelmaking industries. The industry uses vast amount of coal in form of coke in the blast furnaces. The purpose is not only to gain the needed heat into the ironmaking process but also to use carbon as reductant for the oxygen containing iron ore. By injection of biomass the fossil CO2 emissions could be lowered. The cost of biomass and the CO2 emission cost are main factors when optimal biomass use in blast furnace process is discussed. Biomass injection rate is, on the other hand, limited to a maximum specific rate in proportion to the hot metal production. The work reported in this paper studies the potential of injecting biomass into the blast furnace in order to partially replace fossil reductants and hereby reduce the fossil CO2 emissions.
2. Model In the present work a thermodynamic blast furnace model was applied [2,3], which is based on the fundamental concepts introduced by Rist et al. [1]. The model utilizes a division of the process into two main control volumes, with thermal and chemical equilibrium approached on the boundary, the reserve zone, between the two volumes. A
J. Söderman et al.
568
linear model was developed with a set of binary variables to control a nonlinear relation of two process variables. The blast furnace operational costs are predicted effectively by the model. Additionally, the revenues of electric power and district heat that can be produced at the plant were obtained. The input variables in the linear process model were volumetric flow rate of air, volumetric flow rate of oxygen, specific oil rate, blast temperature, specific rate of pellets, specific rate of limestone and specific injection rate of biomass, shown in Eq. 1.
+ K i ,5
Vair
VO 2
moil T + K i ,3 + K i , 4 bl + °C kg/t hm km n/h km 3 n/h mpel m mbio + K i ,6 lime + K i ,7 kg/t hm kg/t hm kg/t hm
Yi = K i ,0 + K i ,1
3
+ K i,2
(1)
The objective function F is given by
m pel c pel m coke,own ccoke,own m coke,b c coke,b c F § m = ¨ sin ⋅ sin + ⋅ + ⋅ + ⋅ + €/t hm © t/h €/t t/h €/t t/h €/t t/h €/t VO2 cO2 m c m c m c + oil ⋅ oil + lime ⋅ lime + bio ⋅ bio + ⋅ + 3 t/h €/t t/h €/t t/h €/t km n/h € km 3 n m CO2 c CO2 c el Q c heat · m hm P ¸ + ⋅ − ⋅ + dh ⋅ t/h € t MW € MWh MW € MWh ¸¹ t hm /h
(2)
For the specific cost of hot metal production the mass and volume flow rates are multiplied by specific mass or volumetric costs, ci. Coke rate is included as own and bought coke, denoted by subscripts coke,own and coke,b. The price of electricity is denoted by cel and heat by cheat. The mass flow rate of fossil CO2 emissions is calculated as the ratio of the molecular weights of CO2 and carbon and taking also into account the share of fossil carbon in the total carbon input to the blast furnace.
3. Illustrative case The thermodynamic blast furnace model was run under a large number of input combinations, with input variables uniformly distributed within their admissible ranges. The injection rate of dry biomass was limited to max. 120 kg/t hm. The process was optimized with the price data shown at Table 1. The biomass and CO2 emission costs were varied: biomass price between 50 and 110 €/t and CO2 emission cost between 0 and 80 €/t.
Future Potential for Biomass Use in Blast Furnace Ironmaking
569
Table 1. Input price data for the test cases.
oil limestone pellets sinter coke, own coke, bought oxygen gas
150.0 €/t 30.0 €/t 110.0 €/t 90.0 €/t 200.0 €/t 300.0 €/t 50.0 €/t
El power, sold Heat, sold
50.0 €/MWh 10.0 €/MWh
biomass price CO2 emission
50.0 - 110 €/t 0.0 - 80 €/t
4. Solutions
Production cost, €/t hm
The model was first tested by a series of runs with fixed biomass price of 50 €/t and CO2 emission cost of 0 €/t at different hot metal production rates as a base case, Case 1. Secondly, a series of test runs was made by the biomass price of 100 €/t and CO2 emission cost of 30 €/t, Case 2. The optimal production costs, predicted by the MILPmodel, are shown in Figure 1 for different hot metal production rates in a blast furnace. The minimum production costs were obtained at the production rate of about 145 t hm/h.
250 case 1 case 2
200 120
130
140
150
160
Hot metal prod, t hm/h
Figure 1. Optimal production costs at different hot metal production rates. The variations of two process variables, specific injection rate of biomass and oil, with different hot metal production rates are shown in Figure 2 for Case 1 and Case 2.
J. Söderman et al.
570
Case 1 biomass Case 1 oil
Injection rate, kg/t hm
150.0
Case 2 biomass Case 2 oil
100.0
50.0
0.0 120
130
140
150
160
Hot metal prod, t hm/h
Figure 2. Optimal specific injection rates of biomass and oil to blast furnace at different hot metal production rates.
Biomass price €/t
Biomass injection (kg/t hm)
120 100
60
80
70 80
60
90
40
110
20 0 0
10
20
30
40
50
60
70
80
CO2 cost (€/t)
Figure 3. Optimal specific injection rates of biomass at a production rate of 150 t hm/h with different prices for biomass and CO2 emissions. The optimal specific injection rates of biomass at different CO2 costs are shown in Figure 3, with fixed hot metal production rate of 150 t hm/h. The injection rates vary with the cost of CO2 emissions for a selected set of prices of biomass. The curves in Figures 2 and 3 demonstrate some aspects of the complex nature of the blast furnace process.
Future Potential for Biomass Use in Blast Furnace Ironmaking
571
Partial pyrolysis of biomass Injection of dry wood chips has been shown to be possible up to the level of about 25% of the coke. Limitation is caused by the high oxygen content of wood chips. Biomass pyrolysis has been suggested as a possible way to further increase of the biomass injection rate. A partial pyrolysis of the biomass lowers its oxygen content, but decreases the yield. Additionally there are losses of carbon and hydrogen. The blast furnace MILP-model was rewritten so that it could also take into account the pyrolysis process step. The solutions of the test series with the pyrolysis model are presented by authors in [7].
5. Conclusion An MILP-model for the blast furnace process was tested with the focus on replacement of fossil reductants oil and coke by injection of biomass under consideration of CO2 emission costs. The feasibility of biomass use was revealed at different hot metal production rates and with different price levels for biomass and CO2. A partial pyrolysis of biomass was studied with a modified model and optimal pyrolysis temperatures were found for different biomass and emission costs.
Acknowledgements The financial support from the Academy of Finland to the GreenSteel project is gratefully acknowledged.
References [1] A. Rist, N. Meysson: “A dual graphic representation of blast-furnace mass and heat balances”, Journal of Metals, 19 (1967), 50. [2] H. Saxén, M. Brämming, J.-O. Wikström and P. Wiklund: “Theoretical limits on operation under high oxygen enrichment in the blast furnace”, Proc. 60th Ironmaking Conf., ISS, Warrendale, PA, (2001), 721. [3] F. Pettersson, H. Saxén: “Model for economic optimization of iron production in the blast furnace”, ISIJ International, 46 (2006), 1297.
[4] Y. Kim and E. Worrel: “Intenational comparison of CO2 emission trends in the iron and steel industry”, Energy Policy, 30 (2002), 827. [5] T. Ariyama and M. Sato, “Optimization of ironmaking process for reducing CO2 emissions in the integrated steel works”, ISIJ International, 46 (2006), 1736. [6] M. Takekawa, K. Wakimoto, M. Matsu-ura, M. Hasegawa, M. Iwase and A. McLean: “Investigation of waste wood as a blast furnace injectant”, Steel Research, 74 (2003), 347. [7] H. Saxén, F. Pettersson, J. Söderman, M. Helle. and H. Helle, "Optimization of biomass use as auxiliary reductant in the blast furnace", Recent Progress in Mathematical modeling in Ironmaking 2008 (Ed. T. Ariyama), Tokyo, October 2008, ISIJ, pp. 95-100.