Optimization Study of Steelmaking under Novel Blast Furnace

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Optimization Study of Steelmaking under Novel Blast Furnace Operation Combined with Methanol Production Hamid Ghanbari,* Mikko Helle, Frank Pettersson, and Henrik Sax
en Thermal and Flow Engineering Laboratory, Department of Chemical Engineering, Åbo Akademi University, Åbo, Finland ABSTRACT: The opportunities to improve the performance of an existing production concept by plant retrofit are largely dependent on the available knowledge of the best operational state of the plant and its parameters and conditions. In this paper, nonlinear programming was used to analyze the economic potential of the use of large volumes of gases in a steel plant to produce methanol as a valuable byproduct in steelmaking. Conventional blast furnace operation was compared with the option of operating the blast furnace with top gas recycling after carbon dioxide stripping. The optimal integration of the processes was investigated by minimizing the cost of liquid steel production, considering the cost of raw materials and fuels, CO2 emission, and stripping, as well as credits for power, district heat, and methanol production. It was found that the novel way of operating the blast furnace with cold oxygen blowing and top gas recycling was well suited for combination with a polygeneration system using the residual gases of the steel plant.

1. INTRODUCTION Carbon dioxide emissions from the combustion of fossil fuels have been considered a primary factor contributing to climate change and global warming, which has led to actions to limit CO2 emissions in the world. In order to fulfill the Kyoto protocol requirements in a cost proficient way, the European Union Emission Trading Scheme (EU ETS) was introduced.1 It expects steel industry to operate existing processes more efficiently and to develop new environmentally benign processes. During the last 40 years, the steelmaking companies in Japan, Korea, and Europe have reduced the specific emissions and energy use considerably more than any other industrial sector. However, the production rates have simultaneously grown, so further reductions of the carbon dioxide emissions are needed to adapt to post-Kyoto emission levels. Rising energy prices will also act as a driver for a strong reduction in the energy demand. In Europe, the ULCOS (Ultra Low CO2 Steelmaking) consortium formed by European steel industry2 studies means by which the carbon dioxide emissions in steelmaking can be reduced dramatically. One option investigated is top gas recycling in the blast furnace.3 The process includes stripping of the top gas carbon dioxide through chemical absorption, preheating the remaining gas, and blowing it back into the furnace. Top gas recycling in the blast furnace was tested over a long period of time in Toulachermet in Russia during 1985
90 at a specifically constructed plant, where the operation results verified that such a process would reduce the coke rate and increase the iron production rate.4 Today, the most attractive benefit of the process would be to reduce the emissions of steelmaking, if the stripped CO2 could be stored. Blast furnace operation under top gas recycling has been studied mathematically for both shaft and tuyere injections. A two-dimensional mathematical model5 for four phases (gas, solid, liquid, and fines), including heat and mass transfer and chemical reactions, was used to evaluate simple replacement of normal blast gases with recycled top gas and was predicted to decrease the production rate and increase the fuel rate. Oxygen enriched blast replacement showed similar effects, while r 2011 American Chemical Society

stripping of CO2 from the recycled gas led to an increased production rate and a simultaneous decrease in the fuel rate at fixed metal temperature.5 Also, operation with natural gas injection was investigated, and the results showed a decrease in the CO2 emission from the system while the decrease in the energy input was small compared to conventional blast furnace operation with pulverized coal.6 Blast furnace operation with top gas recycling and injection of solid fuel has also been studied as an option for minimizing CO2 emissions.7 Some investigators have also analyzed the optimal operating conditions of a blast furnace with top gas recycling. The optimal recycling states of the process were studied using a simple thermodynamic first principles model,8 applying linearization to make the optimization robust and efficient. When the operation costs of an integrated steelmaking plant with one blast furnace are minimized and top gas recycling and high oxygen enrichment of the blast are applied, the costs of CO2 emission and carbon capture and storage (CCS) were found to considerably affect the optimal state. As expected, a high emissions cost led to high top gas recycling rates while high CCS costs reduced the optimal degree of recycling, but the transitions between the states were found rather complex. A multiobjective analysis, where both emissions and costs were minimized, furthermore demonstrated that the final emission reductions were extremely expensive and sometimes only shifted the emissions outside the balance boundary of the system.9 More recently, a multiobjective optimization formulation based on genetic algorithms was used for the system, and the results demonstrated the feasibility of the different blast furnace operation concepts with respect to production costs and emissions.10 However, it is not straightforward to develop efficient strategies for solving optimization problems under inequality constraints using stochastic algorithms,

Received: June 4, 2011 Accepted: September 22, 2011 Revised: September 9, 2011 Published: September 23, 2011 12103

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Figure 1. Integrated Steel Plant. CP: Coke Plant; SP: Sinter Plant; ST: Hot Stoves; CCP: CO2 Capturing Plant; BF: Blast Furnace; BOF: Basic Oxygen Furnace; CHP: Combined Heat and Power Plant; GR: Gas Reforming unit; and MP: Methanol Plant.

and the numerical burden associated with the final convergence may become prohibitive for systems with large number of variables. In order to further study means to suppress emissions and costs, the possibilities to integrate a steel plant with a chemical plant was considered.11 Recent studies show that polygeneration systems are potentially attractive technologies for energy utilization and that they could be a feasible solution for decreasing emissions. A polygeneration system has better energy efficiency due to the tight integration of the power generation and the chemical synthesis section, and it can also be a producer of alternative fuels and energy carriers.12 Conventional steelmaking gives rise to large volumes of residual gases which are traditionally used for preheating of the blast in hot stoves and for generation of electricity in a power plant or electricity and district heat in a combined heat and power (CHP) plant. Integration of steelmaking with chemical processes has been investigated,13,14 and some commercial plants have been built. For instance, the Shanxi Tianho Chemical Company Ltd. in the Shanzi province, China, commissioned the first phase of a methanol process project based on coke oven gas with an annual capacity of 300 000 tons of methanol.15 The objective of the research reported in the present paper is to mathematically analyze the potential of integration of a steel plant with a polygeneration system, including gas reforming and methanol units and CHP, using residual gases (coke oven gas, blast furnace gas, and basic oxygen furnace gas) which arise in steelmaking. The blast furnace in the system can apply top gas recycling with tuyere injection of the CO2-stripped gases and is described by a blast furnace model.16 The optimal states of steelmaking integrated with the polygeneration plant are estimated by nonlinear programming. In interpreting the results, it should be noted that the optimal states found are strongly influenced by the selected cost structure of raw materials, energy and emissions, and naturally, by the constraints of the process.

2. PROCESS MODELS AND EMISSIONS The main processes in the steel plant and polygeneration system are presented in Figure 1. 2.1. Steel Plant Models. The core of the system is the blast furnace, for which a more detailed description is used, while the

Table 1. Blast Furnace Input (First Six) and Some of the Output (Next 12) Variables and Their Constraints, as well as Sinter and Coke Mass Production Rate Constraints of the Planta variable

range

BF production rate recycled top gas

131
157 thm/h 0
220 km3n/h

blast oxygen content

21
99 vol %

specific oil rate

0
120 kg/thm

blast temperature

250
1200 °C

specific pellet rate

0
600 kg/thm

specific coke rate

g0 kg/thm

flame temperature

1800
2300 °C

top gas temperature bosh gas volume

115
250 °C 150
220 km3n/h

solid residence time

6.0
9.5 h

slag rate

g0 kg/thm

top gas volume

g0 km3n/h

top gas CO content

g0 vol %

top gas CO2 content

g0 vol %

top gas H2 content

g0 vol %

top gas N2 content top gas heating value

g0 vol % g0 MJ/m3n

sinter feed flow

0
160 t/h

own coke feed flow

0
55 t/h

a

The BF production rate limits (expressed in tons of hot metal, thm) were set to yield a steel production rate within [150,180] tls/h.

other unit processes in the steel plant are described in a simplified way.8 The nonlinear blast furnace model is expressed as a function of six input variables: the production rate, the volume flow rates of recycled top gas, the oxygen enrichment, the specific oil injection rate, and the temperature of the injected tuyere gases (here called “blast temperature”), as well as the specific pellet rate. In the present study, three alternative ways of operating the blast furnace were considered: (1) Conventional blast furnace (CBF) operation: The blast is heated and no top gas is recycled. 12104

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Industrial & Engineering Chemistry Research (2) Recycled top gas is injected cold together with hot blast (TBF). (3) Recycled top gas is heated and injected together with cold oxygen (OBF). For the cases with top gas recycling (TBF, OBF), it was assumed that 95% of the CO2 of the recycled gas was removed17 and that this quantity was stored. The excess nitrogen was assumed to be purged from the system. Table 1 lists the inputs and some outputs of the blast furnace model, as well as the corresponding lower and upper limits. The production rate of the blast furnace was constrained to yield an hourly steel production in the range of 150
180 t liquid steel (tls), imposing internal (e.g., flame and top gas temperature, as well as slag basicity) and external (e.g., maximum sinter and coke production) constraints in accordance with the conditions at a reference plant.8 For more details about the models, the reader is referred to Helle et al.16 2.2. Polygeneration Plant. The polygeneration plant uses the residual gases from the coke oven, blast furnace, and basic oxygen furnace, as well as external oil. The gases available for use are 50% of the basic oxygen furnace gas, all coke oven gas, and the nonrecycled blast furnace top gas. In the CHP, an empirical factor distributes the energy between power and heat according to local demand and the overall efficiency of the power plant is assumed to be 79%.8 In the gas reforming (GR) unit, the chemical reaction (CH4 + H2O = CO + 3H2) is very endothermic and favored by high temperature and low pressure. Depending on the total CO/H2 available in the system, the water-gas shift reaction (CO2 + H2 = CO + H2O) may be needed to balance the feed ratio to the methanol unit. In other words, for CH4 from coke oven gas, the CO2/CH4 molar feed ratio should be 1:3 to get a CO/H2 ratio of 1:2 for MeOH synthesis (3CH4 + 2H2O + CO2 = 4CO + 8H2), though any incomplete conversion of CO2 would call for a slightly higher feed ratio. The excess of CO or H2 is used as a fuel, and unconverted CO2 is purged from the synthesis loop. The net reaction of methanol synthesis in a Lurgi Catalytic Converter is CO + 2H2 = CH3OH, and the converter is modeled as a cooled multitubular reactor running at 5
8 MPa and 250
260 °C. Liquid entrained micrometer-sized copper-based catalysts can convert as much as 25% per pass, and the heat of reaction is directly used to generate high pressure steam. Methanol is condensed by both heat exchange and pressure reduction, and the condensed methanol is collected and purified. The steam demand within the unit usually closely matches the steam produced in the plant. The material and energy balances of methanol plant for the whole process (GR and MP) are stated and solved using overall hydrogen and carbon dioxide utilization factors of 0.999 and 0.99, respectively,18 and a total heat output to the cooling water in heat exchanger, cooler condenser, and methanol separation of 10.12 MJ/kgMeOH. The temperature dependent enthalpies were estimated on the basis of values reported in the literature.18,19 The balance equations yield the material flows and the heat input to the methanol plant in the form of steam in the heat exchanger and methanol separation units. 2.3. Emissions. The CO2 emissions from the system are calculated on the basis of a carbon balance equation, including all carbon-bearing inputs (coal, oil, external coke, limestone) and excluding the outflows of carbon with liquid steel, methanol, and stripped CO2. The feed rate of limestone is the sum of the requirement from the sinter plant, blast furnace, and basic oxygen furnace; the feed flow of oxygen is the sum of the requirements by the blast furnace and the basic oxygen furnace, while the mass flow rate of external (bought) coke is the difference between the

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coke requirement in the blast furnace and the supply from the coke plant.8 The emissions associated with the production of external raw materials (e.g., pellets, oxygen) and electricity were not considered, as the units were outside the balance boundaries of the system.

3. OPTIMIZATION PROBLEM The optimal state of the system is considered to be that corresponding to minimal operation costs of liquid steel production, expressed in specific terms (i.e., per ton liquid steel, tls) F ¼ h=tls



m_ pel cpel m_ ore core m_ coal ccoal m_ coke ccoke þ þ þ t=h 3 h=t t=h 3 h=t t=h 3 h=t t=h 3 h=t

þ

m_ quartz cquartz m_ oil coil m_ lime clime þ þ 3 t=h h=t t=h 3 h=t t=h 3 h=t

þ

m_ scrap cscrap m_ CO2 cCO2 V_ O2 cO2 þ þ 3 3 t=h h=t t=h h=t t=h 3 h=t

m_ CCS cCCS m_ MeOH cMeOH P cel

MW 3 h=MWh t=h 3 h=t t=h 3 h=t    m_ ls cdh Q_ dh
= MW 3 h=MWh t =h þ

ð1Þ

ls

where m_ i, V_ i, and ci are mass flow rates, volume flow rates, and unit costs. Observe that the (net) outflows of methanol, power (el), and district heat (dh) from the system appear in the last three credit terms. The nonlinear programming problem, min (F), was solved numerically by sequential quadratic programming subject to constraints of the variables and to upper limits on some of the raw material flows shown in Table 1. The fixed cost factors used in this study are core = 80 h/t, cpel = 120 h/t, ccoal = 145 h/t, ccoke = 300 h/t, coil = 150 h/t, cquartz = 30 h/t, clime = 30 h/t, cscrap = 100 h/t, cO2 = 50 h/km3n, cMeOH = 250 h/t, cel = 50 h/MWh, and cdh = 10 h/MWh. It should be stressed that the values of the costs are indicative only, as actual costs could not be used for reasons of confidentiality. However, the internal relation between the costs of, e.g., the fuels, is considered appropriate. Still, one should keep in mind that the selected cost setup, as well as the internal and external constraints imposed on the system and its units, will affect the optimal solutions found. Therefore, in interpreting the results, the main emphasis should be put on the observed overall trends rather than on the absolute values reported.

4. RESULTS The system was studied for a steel production rate of [150,180] tls/h with CCS and carbon dioxide emission costs both in the range of [10,50] h/tCO2 for a steel plant integrated with a polygeneration plant. 4.1. Optimal Conditions at Constant Steel Production Rate. The first case to be illustrated is the optimal operational

conditions at a steel production rate of 180 tls/h for the three different blast furnace concepts (CBF, TBF, OBF) using the moderate CO2 capturing cost of cCCS = 10 h/t in a steel plant or a steel plant integrated with methanol production (INT). Figure 2a shows the specific emissions from the system expressed per tons of liquid steel, while the specific cost of liquid steel is depicted in Figure 2b. In all scenarios, integration with methanol production (lines with markers) is seen to considerably decrease the production costs. Furthermore, the results show that there is potential to decrease the emission from the system by process 12105

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Figure 2. (a) Specific emissions and (b) specific costs of steel production for a steel plant with (lines with markers) or without (lines without markers) methanol production for a steel production rate of 180 tls/h and a CCS cost of 10 h/tCO2. Cases optimized: Conventional (CBF, solid lines), top gas recycling with hot blast (TBF, dashed lines) or cold oxygen (OBF, dotted lines).

Figure 3. (a) Recycled top gas rate and (b) feed flow of external coke for a steel plant with (lines with markers) or without (lines without markers) methanol production for a steel production rate of 180 tls/h and a CCS cost of 10 h/tCO2. Cases optimized: Conventional (CBF, solid lines), top gas recycling with hot blast (TBF, dashed lines) or cold oxygen (OBF, dotted lines).

integration and by increased top gas recycling. Another feature noted for these (and later) results is that sudden changes in the states may occur as a result of a transition between two optimal states, where, e.g., a material flow experiences a major change. A transition of this kind is observed at cCO2 = 35
40 h/t for OBFINT, where the recycled top gas (cf. Figure 3a) suddenly increases. Such changes may be triggered by constraints becoming active or inactive. However, the objective, the specific steelmaking cost, changes smoothly at these points (cf. Figure 2b).

For the plant without methanol production (lines without markers), the OBF is clearly more economical than the other two alternatives. The fact that CBF and TBF yield almost identical steelmaking costs is due to the very modest optimal top gas recycling in the latter, as indicated in Figure 3a. The strong recycling applied in the case of OBF seen in the same figure is the reason for the small slope of the OBF steelmaking costs in Figure 2b and also for lower emissions since the recycled top gas flow is subjected to CCS. It should be noted that at the highest emission costs the OBF recycles 12106

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Figure 4. (a) Specific coke rate and (b) oil rate in the blast furnace of a steel plant with (lines with markers) or without (lines without markers) methanol production for a steel production rate of 180 tls/h and a CCS cost of 10 h/tCO2. Cases optimized: Conventional (CBF, solid lines), top gas recycling with hot blast (TBF, dashed lines) or cold oxygen (OBF, dotted lines).

Figure 5. (a) Methanol production rate and (b) extra oil needed in utility in a steel plant integrated with methanol production for a steel production rate of 180 tls/h and a CCS cost of 10 h/tCO2. Cases optimized: Conventional (CBF, solid lines), top gas recycling with hot blast (TBF, dashed lines) or cold oxygen (OBF, dotted lines).

a maximum amount of BF top gas (leaving only a small flow for purging the nitrogen). Figure 3b further indicates that the CBF and TBF concepts both require external coke while in the OBF there is no need for this external raw material. Figure 4 shows the specific coke and oil rates in the blast furnace at the optimum states. It is clear that cold oxygen injection and top gas recycling (OBF) reduces both quantities. At high emission costs, the oil rate is very low, which indicates that oil-less operation could be beneficial, considering the fact

that heavy oil injection gives a higher sulfur load in the furnace. However, there is a clear mirror relation between the coke and oil injection, so decreasing one will increase the other. Figure 5a shows the potential for producing methanol from residual gases of the integrated plant, considering the empirical factors for availability from each unit, while Figure 5b illustrates the amount of oil that would be needed instead of the use of residual gases in burners. The ratio of methanol produced to extra (heavy residual) oil needed is estimated to 2.5
3.3. 12107

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Figure 6. Specific costs of steelmaking (solid lines) and specific CO2 emissions (dashed lines) of steel plant with conventional blast furnace operation without (upper panels) and with (lower panels) methanol production. Emission costs of 25 h/tCO2 (left panels) and 50 h/tCO2 (right panels), but no costs of CCS were considered.

Figure 7. Influence of CCS cost and production rate on specific CO2 emissions (in tCO2/tls, upper panels) and costs of steelmaking (in h/tls, lower panels) for TBF without methanol production. Emission costs of 25 h/tCO2 (left panels) and 50 h/tCO2 (right panels).

The corresponding ratio for a 2000 t/d methanol plant with heavy residual oil gasification has been reported to be about 1.2,20 which shows the potential of integration of the methanol plant with steelmaking. Interestingly, the OBF system (cf. Figures 2a and 5a) shows two different levels of carbon dioxide emission rates and methanol production at low and high emission costs, but the overall costs (cf. Figure 2b) are relatively stable.

4.2. Optimal Conditions at Variable Steel Production Rate and CCS Cost. Figure 6 shows how the production costs (solid lines)

and the specific emissions (dashed lines) for a plant with conventional blast furnace without (upper panels) and with (lower panels) methanol production vary with the steel production rate, using two different costs of emissions (left: 25 h/tCO2, right: 50 h/tCO2). The results illustrate that the specific CO2 emissions decrease with the 12108

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Figure 8. Influence of CCS cost and production rate on specific CO2 emissions (in tCO2/tls, upper panels) and costs of steelmaking (in h/tls, lower panels) for TBF with methanol production. Emission costs of 25 h/tCO2 (left panels) and 50 h/tCO2 (right panels).

Figure 9. Influence of CCS cost and production rate on specific CO2 emissions (in tCO2/tls, upper panels) and costs of steelmaking (in h/tls, lower panels) for OBF without methanol production. Emission costs of 25 h/tCO2 (left panels) and 50 h/tCO2 (right panels).

production rate and that the emissions are clearly lower for the integrated concept. As for the steelmaking costs, there is a small increase with the production rate, but the plant with integrated methanol production shows clearly lower costs. An increase in the emission cost of 25 h/tCO2 is, furthermore, seen to bring about an increase in the cost of liquid steel of about 40 h/tls, which reflects the fact that a conventional steel plant (with the present system boundaries) has a specific emission rate of about 1.6
1.7 tCO2/tls.

The effect of CCS costs and production rate on specific costs of liquid steel and emission for specific emission costs of 25 h/tls and 50 h/tls are presented in Figures 7
10 as contour plots for a steel plant with a blast furnace with recycling of top gas and hot blast blowing (TBF) and recycling of top gas with cold oxygen injection (OBF). For a steel plant without integrated methanol production and TBF, the lower left panel of Figure 7 shows that at the lower emission price the CCS cost does not substantially 12109

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Figure 10. Influence of CCS cost and production rate on specific CO2 emissions (in tCO2/tls, upper panels) and costs of steelmaking (in h/tls, lower panels) for OBF with methanol production. Emission costs of 25 h/tCO2 (left panels) and 50 h/tCO2 (right panels).

Table 2. Optimal Process Variables for a Steel Production Rate of 180 t/h, Costs of Emissions cCO2 = 50 h/t and Capturing cCCS = 10 h/ta variable blast volume (km3n/h) oxygen enrichment (vol %) blast furnace top gas volume (km3n/h)

TBF

TBF-INT

OBF-INT

51.6

52.6

27.0

26.3

86.6 192

84.4 193

99.0 203

99.0 215

blast furnace recycling gas volume (km3n/h)

109

108

201

212

sinter feed rate (t/h)

160

160

160

160

coal feed rate (t/h)

79.1

79.1

79.1 280

79.1

specific coke rate (kg/thm)

312

311

specific oil rate (kg/thm)

120

120

specific pellet rate (kg/thm)

513

513

513

513

1800 1200

1800 1200

1800 1200

1800 1200

flame temperature (°C) blast temperature (°C)

26.0

297 10.5

bosh gas volume (km3n/h)

197

197

197

204

top gas temperature (°C)

115

115

178

193

burden residence time (h) slag rate (kg/thm) coke oven gas volume (km3n/h) basic oxygen furnace gas volume (km3n/h) oil needed for other units than BF (t/h) bought coke (t/h)

7.0

7.0

215

215

sold district heat (MW) specific emission (tCO2/tls) specific steel cost (h/tls)

198

7.3 199

17.6

17.6

17.6

6.5

6.5

6.5

6.5

1.49

9.0 1.23

7.2 3.66

sold methanol (t/h) sold electricity (MW)

7.6

17.6

sold coke (t/h)

a

OBF

26.2

0.91 18.0

35.0 178

36.4

1.17

1.13

288.1

274.7

0.54 251.6

0.48 232.7

Variable values at their bounds are written in bold face.

affect the cost of liquid steel, since the degree of top gas recycling (and associated CCS) is small. The specific cost of steel (lower panel)

is seen to increase with the production rate, while the emissions (upper panel) decrease. At the higher emission price (right panels), 12110

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coke which is spared due to the efficient top gas recycling. The main energy flows into and out of the system (excluding the heat losses and off-gases) are depicted in Figure 11. In summary, the results show that integration of steelmaking with methanol production has a potential of lowering both steelmaking costs and arising CO2 emissions, if recycling of the blast furnace top gas is applied. Among the alternatives studied, injection of hot recycled gas (after CO2 stripping) and cold oxygen (OBF) seems the most promising recycling concept. However, the analysis did not consider the emissions associated with CCS and additional oxygen production, so the global sustainability of the process should be evaluated further.

Figure 11. Energy inflows (upper panel) and outflows (lower panel) for TBF and OBF without and with methanol production (cf. Table 2).

the two variables are seen to be rather independent of the production rate while the CCS cost will affect both. Furthermore, the higher emission cost is seen to lower the specific emissions, in particular at lower CCS cost due to the increased degree of top gas recycling. A steel plant integrated with methanol production, Figure 8, shows similar trends as those of Figure 7 for the steelmaking costs, but the dependence of the specific emissions on the production rate and the CCS costs are different. Still, the most notable difference is the lower costs and emissions shown by the integrated TBF concept (of Figure 8). Figure 9 illustrates the results for a steel plant with the OBF concept but without integrated methanol production. At the lower emission cost (left panels), the specific emissions depend nonlinearly on the production rate, but the production costs are rather insensitive to production rate changes. The same holds true for the higher emission costs (right panels) but the changes in the emissions are more linear, and the emission rate is lower and the production costs are higher. The specific emissions of OBF with methanol production, Figure 10, also exhibits a rather complex pattern at lower emission cost (top left panel), with specific emissions increasing with the production rate at high CCS cost but decreasing at low CCS costs. This serves to illustrate the complexity of the system under study. The steelmaking costs, in turn, show very similar behavior as in the case without methanol production (cf. Figure 9) but at a lower level. As for the optimal states of operation, Table 2 shows some key process variables for the TBF and OBF concepts at a production rate of 180 tls/h, as well as cCO2 = 50 h/tCO2 and cCCS = 10 h/tCO2. The optimal TBF states are seen to apply high, but not maximum, oxygen levels in the blast, higher coke rates, and clearly higher oil rates. Furthermore, the BF top gas temperature is at its lower limit. The OBF concepts, in turn, apply practically full top gas recycling and “pure” oxygen injection. For all optimal states, the blast temperature is at its upper and the flame temperature at its lower bound. The former can be explained by fact that it is practically always beneficial to apply maximum gas preheating in the blast furnace to save coke, while the latter follows from a need to inject large volumes of gases, part of which is cold. The plants with integrated methanol production are seen to export neither electricity nor district heat. The OBF plant without methanol production exports only little heat but some

5. CONCLUSIONS AND FUTURE WORK The work reported in this paper has investigated the possibility to suppress CO2 emissions in primary steelmaking by integrating the process with methanol production. Three alternative steelmaking concepts were studied, i.e., conventional blast furnace operation (CBF), blast furnace operation with recycling and carbon capture and storage (CCS) of cold top gas combined with hot blast injection (TBF), or top gas recycling with CCS with heated recycled gas combined with injection of cold oxygen (OBF). The steel works studied was modeled mathematically using simple models for the other unit processes but a more detailed simulation model of the blast furnace. The task of finding the optimal state of the system was written as a nonlinear optimization problem, where the operating costs of the plant, expressed in terms of specific costs of liquid steel, were minimized under a process and raw material constraints. The effect of steel production rate, costs of CO2 emission, and CCS was analyzed. The results demonstrated that steelmaking with top gas recycling in the blast furnace combined with oxygen blowing could be a promising concept to be combined with methanol production: this alternative was characterized by low emission levels and reasonable costs. This can be ascribed to the fact that the top gas in a blast furnace operated with oxygen blast and recycled top gas has a suitable composition for methanol synthesis. In the future work, the analysis should be extended to a conceptual design of the CCS and polygeneration systems, including investment cost for the suggested units. In conjunction with this, the analysis should also pay attention to the emissions arising in the CCS unit and oxygen plant in order to make a fair evaluation of effect on the total emissions. An MINLP optimization model will be used to evaluate how other raw materials, fuels, and products affect the optimal state of the system, and the analysis should also consider the time dependence of district heat and electricity demand for different periods of the year. This would resolve problems caused by local minima, indications of which can be discerned in Figures 2
4 at cCO2 ≈ 40 h/tCO2. Forthcoming research could also evaluate whether biomass would be useful as a partial feedstock in the system for further suppression of CO2 emissions.14,21 ’ AUTHOR INFORMATION Corresponding Author

*E-mail: hamid.ghanbari@abo.fi.

’ ACKNOWLEDGMENT Financial support from the Academy of Finland and the Fortum foundation is gratefully acknowledged. We also thank 12111

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the anonymous reviewers for valuable comments helping us improve the quality of the paper.

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