Reduction of Blast Furnace Ironmaking Carbon Footprint through Process Integration

Transcription

1 Reduction of Blast Furnace Ironmaking Carbon Footprint through Process Integration Ka Wing Ng CanmetENERGY 1 Haanel Drive, Ottawa, ON, Canada, K1A 1M1 Phone: (613) Fax: (613) kng@nrcan.gc.ca Louis Giroux CanmetENERGY 1 Haanel Drive, Ottawa, ON, Canada K1A 1M1 Phone: (613) Fax: (613) lgiroux@nrcan.gc.ca Tony MacPhee CanmetENERGY 1 Haanel Drive, Ottawa, ON, Canada, K1A 1M1 Phone: (613) Fax: (613) tmacphee@nrcan.gc.ca Ted Todoschuk ArcelorMittal Dofasco Inc Burlington Street East, Hamilton, ON, Canada, L8N 3J5 Phone: (905) Fax: (905) ted.todoschuk@arcelormittal.com Key words : Process Integration, Oxygen Blast Furnace, CO 2 Capture and Storage, Biomass INTRODUCTION The feasibility of reducing the carbon footprint of the steelmaking process by integrating blast furnace ironmaking with combined cycle power generation was examined. Using highly-enriched oxygen blast, the CO 2 content in the furnace top gas is significantly increased thus facilitating the capturing of CO 2 for longterm storage to reduce the environmental impact of ironmaking. Moreover, energy content of the blast furnace gas is enhanced and can be used for combined cycle power generation for meeting the energy demand of the process. Heat and mass balance simulations of the integrated blast furnace-combine cycle process was performed. It was observed that the net CO 2 emission of the process was related to the amount of blast furnace top gas recycled for ironmaking. By optimizing the top gas recycle ratio, it is possible to reduce the net CO 2 emission of the ironmaking process to 230 Nm 3 /thm, which corresponds to only 30% of the emission level of the conventional process. The incorporation of a gasifier in the proposed BF-CC process increased the flexibility of the choice of fuel for direct injection. Applying direct gasification of sustainable biomass in the BF-CC process can further reduce the net emission of the ironmaking process to 176 Nm 3 /thm or a 77% reduction as compared to the conventional blast furnace. BACKGROUND Steelmaking is an energy-intensive process. In 2007, the energy consumption of Canadian iron and steel manufacturing was 224 petajoules 1. The major energy sources are coke, coke oven gas and natural gas with these three sources accounting for over 80% of the energy consumed. This heavy dependence on nonrenewable energy sources makes the iron and steel industry a major greenhouse gas (GHG) emission point source. In Canada, emissions from the Iron and Steel industry were 15.5 Mt CO 2eq in

2 The conversion of iron ore into steel in the integrated steelmaking process is carried out by the blast furnace-basic oxygen furnace route. The blast furnace ironmaking process uses carbon as reductant for reducing iron ore to produce molten metallic iron. The major carbon source introduced to the furnace is in the form of coke, produced by carbonization of coal blends. In the blast furnace, nearly all carbon input is gasified and eventually released to the atmosphere as CO 2. The amount of CO 2 produced by the ironmaking process is directly linked to the productivity of the blast furnace. In other words, the amount of CO 2 released by the production of one tonne of molten iron cannot be reduced unless a reductant other than carbon is used. Biofuel application utilizes GHG neutral carbon sources to mitigate the environmental impact of the process. However, it does not actually reduce the amount of CO 2 released to the atmosphere. In order to decrease the actual amount of CO 2 released to the atmosphere without lowering the productivity of the blast furnace, the only option is via Carbon Dioxide Capture and Storage (CCS). CCS consists of separation of CO 2 from industrial and energy-related sources, transportation to a storage location and long-term isolation from the atmosphere 3. Applying CCS to the blast furnace ironmaking process is challenging due to the dilute CO 2 content in the blast furnace offgas. In a typical industrial blast furnace operation, the sum of CO and CO 2 content in the furnace offgas is only about 50% and balanced mainly by nitrogen. The presence of a significant amount of nitrogen makes the capturing of CO 2 difficult and costly. OXYGEN BLAST FURNACE IRONMAKING The high nitrogen content in blast furnace gas results from the use of air or oxygen enriched air as the injected blast. Nitrogen is not involved in any chemical reaction occurring in the furnace. In terms of the process chemistry, nitrogen can be eliminated without affecting the conversion of iron ore into molten iron. Nitrogen-free blast furnace ironmaking has been extensively studied in the 1980s and 1990s The challenge in operating the nitrogen-free blast furnace arises from the use of high oxygen content blast which intensifies the heat generation in the hearth of the furnace. Moreover, the elimination of nitrogen significantly reduces the volumetric gas flow rate in the furnace and leads to improper distribution of heat in the furnace. In the literature, the methodology commonly adopted to address this challenge is by recycling of the furnace top gas. Based on the same idea, Ultra Low Carbon dioxide (CO2) Steelmaking (ULCOS) has recently carried out a comprehensive research program to examine the feasibility of using a highly enriched oxygen blast in the ironmaking process and developed the so-called Top Gas Recycled Blast Furnace (TGRBF) 15, Figure 1. The primary goal of the TGRBF is to reduce the carbon consumption for producing a tonne of hot metal. The process involves the use of Vacuum Pressure Swing Adsorption (VSPA) for capturing the CO 2 in the furnace top gas. The captured CO 2 can then be directed for long-term storage. After CO 2 capture, the processed gas is reheated and recycled back into the furnace either through the tuyere or injected directly into the shaft of the furnace. The reduction in carbon consumption by the TGRBF ironmaking process is achieved by separating CO and CO 2 in the furnace top gas and recycling only CO back into the furnace. Using this approach, the amount of input carbon leaving the system in the form of CO 2 is increased. Therefore, the carbon efficiency of the process is also increased. Hence, it is desirable to maximize the amount of furnace top gas delivered to the VSPA to minimize the carbon input to the furnace. Pilot-scale trials of the TGRBF concept were conducted recently. It was concluded that recycling 90% of the furnace top gas was possible, which leads to a reduction of 123 kg/thm 16 in fuel input. It was also reported that the CO 2 emission of the TGRBF was reduced by 76% with the assumption that the CO 2 extracted by the VSPA was sequestrated for long-term storage.

3 Figure 1. Top Gas Recycling Blast Furnace- ULCOS The reported reduction in CO 2 emission refers only to the CO 2 directly emitted from the blast furnace. However, in order to implement the proposed TGRBF, it is necessary to carry out CO 2 capturing, recycle gas preheating and also production of oxygen. All these steps require input of energy which is not necessary in the conventional blast furnace process. Unless the energy required by these operations originates from renewable energy sources, the CO 2 emission associated to meet the demand of these operations should be taken into consideration. Depending on the sources of energy, the reduction in CO 2 emission by the process may thus be significantly less than anticipated. INTEGRATION OF IRONMAKING AND POWER GENERATION Integration of the oxygen blast furnace with combined cycle power generation to reduce the carbon footprint of the manufacturing process is proposed. In the oxygen blast furnace operation, the heating value of the top gas is enhanced as compared to the conventional blast furnace process due to the elimination of nitrogen. When CO 2 capture is applied, the heating value of the process gas is further increased. Moreover, the furnace top gas is clean and free of sulphur, which makes it suitable for gas turbine power generation. Using a portion of the top gas after removal of CO 2 for combined cycle power generation can produce power required to support the necessary operating units for implementation of the oxygen blast furnace process. Hence, the oxygen blast furnace can be implemented to facilitate CCS without importing energy from external sources. The flowsheet of the proposed integrated blast furnace combined cycle process (BF- CC) is shown in Figure 2. The detailed description of the integrated BF-CC flowsheet is summarized as follows. Oxygen produced by air separation is used as the blast. The purity of the oxygen blast is assumed to be 95%. Gasification of coal is carried out in an oxygen medium using an entrained flow type gasifier. Instead of using direct injection of pulverized coal into the hearth of the blast furnace, injection of gasified coal is employed to increase the process flexibility and prepare for biofuel application in the BF-CC process, which will be discussed in more detail. All of the blast furnace top gas is directed to the VSPA for capturing of CO 2. After CO 2 capture, a portion of the processed gas is mixed with the gaseous products from the coal gasifier and returned to the blast furnace. Both injection of recycled gas via tuyere only and tuyere plus shaft are considered. The remaining portion of the processed gas after CO 2 capture is used as fuel for combined cycle power generation.

4 Figure 2. Integrated Blast Furnace-Combined Cycle Process Injection of gasified coal instead of solid pulverized coal eliminates the limitation on injection rate imposed by the combustibility of coal. Moreover, the raw gas produced by coal gasification is sufficiently hot (~1500 o C) to provide the necessary heat required for reheating the recycled gas after CO 2 capture. The introduction of a gasifier in the process also allows the usage of different types of solid fuels in the ironmaking process. As discussed above, the total fuel rate (coke + coal) required for producing a tonne of hot metal is related to the amount of CO recycled back into the furnace. The proportion of gas recycled to the blast furnace after CO 2 capture is termed the recycle ratio. In terms of fuel consumption, it is desirable to maximize the recycle ratio. However, in the proposed flowsheet, increasing the recycle ratio implies that the amount of fuel available for power generation decreases. Minimizing the fuel rate will require importation of energy from external sources to support CO 2 capture and oxygen production, which may increase the CO 2 emission associated with operating the oxygen blast furnace ironmaking process. Therefore, there is an optimal recycle ratio that minimizes the overall CO 2 emission of the ironmaking process. HEAT AND MASS MODELING OF THE BF-CC PROCESS A numerical model was developed to simulate the material and energy flow of the proposed BF-CC process as shown in Figure 2. The operating conditions of the blast furnace were determined by a heat and mass balance model developed at CanmetENERGY. The modeling results were validated against the published ULCOS-TGRBF pilot tests results 17 as well as operating data of industrial conventional blast furnaces 18 and information published in the literature 19. The CO 2 capturing efficiency of VSPA suggested in the literature was used 20. The low heating value of the processed gas was estimated and was used for calculating the power output of the combined cycle based on the efficiency reported in the literature 21. The energy consumed in CO 2 capturing 22 and oxygen production 3 was calculated using the information published in the literature. While numerous coal gasifiers are available, the Shell gasifier was selected for the current process. This is justified since the Shell gasifier produces gases containing high CO and H 2 contents, thus suitable for blast furnace injection. The yield and composition of the gas produced by the Shell gasifier was obtained from the literature 23. Three coal rates to the gasifier, 100 kg/thm, 140 kg/thm and 180 kg/thm, were examined to illustrate the effect of change in gasifier coal input on the CO 2 emission of the process. A wide range of recycle ratios were examined. Only those meeting both of the following criteria are reported:

5 1. Flame temperature (RAFT) between 2000 and 2200 o C, and 2. Bosh gas temperature greater than 1500 o C In the calculation, it was assumed that all oxygen injected into the hearth was completely converted into CO. The adiabatic temperature of the gas produced was reported as the RAFT. In most industrial blast furnaces, the RAFT is typically between 2000 o C and 2200 o C to ensure the proper operation of the furnace. The bosh gas temperature refers to the temperature of the gas ascending to the shaft after transferring heat for melting of the hot metal produced and for energy consumed by the solution loss reaction. The bosh gas temperature was set above 1500 o C to ensure there is sufficient heat in ascending gas to meet the heat demand in the shaft of the furnace. With the above mentioned two criteria satisfied, the heat demand in the hearth and shaft of the furnace can both be met for ensuring the ironmaking ability of the furnace. Calculation results not meeting both of the above criteria were rejected from discussion. DISCUSSION OF SIMULATION RESULTS Figure 3 shows the relationship between coke rate and recycle ratio and coal input of the BF-CC process. As expected, coke rate decreases with increasing recycle ratio and coal rate input to the gasifier. As discussed above, this is because of the proportion of carbon leaving the system in the form of CO 2 is increased by increasing the recycle ratio. Hence, the carbon efficiency of the furnace is also increased and less coke is required for producing a tonne of hot metal. Figure 3. Coke Rate of the BF-CC Process Figure 4 shows the direct CO 2 emission of the BF-CC process. Direct emission refers to the CO 2 directly released to the atmosphere after combustion in the gas turbine power generation. The CO 2 captured by the VSPA was assumed to be sequestrated for long-term storage and thus was not included. As can be seen, direct emission decreases with increasing recycle ratio and slightly decreases as coal input to the gasifier is lowered. The direct CO 2 emission of the process is related to the total fuel rate (coal + coke) required to support the operation. As the recycle ratio increases, the carbon efficiency of the process is improved and less fuel is required to support the ironmaking process. Hence, direct emission of CO 2 is also reduced. Based on the direct CO 2 emission, it appears beneficial to maximize the recycle ratio of the process. However, as discussed above, the energy consumed by CO 2 capture and oxygen production needs to be included since it may represent a significant contribution to CO 2 emission.

6 Figure 4. Direct CO 2 Emission of the BF-CC Process Figure 5 shows the power output of the BF-CC process. The power output refers to the net energy output with the energy demand by CO 2 capture and oxygen production subtracted from the energy produced by the combine cycle. Therefore, positive power outputs in Figure 5 signify that electricity produced by combine cycle is sufficient to cover the energy demands of CO 2 capture and oxygen production with a surplus. When the energy demand exceeds the electricity produced, the power output becomes negative as shown in the figure. As can be seen in Figure 5, the power output of the process decreases with increasing recycle ratio. This is because the fuel available for power is reduced as the recycle ratio increases. With up to 40% recycling of the furnace gas after CO 2 capture, the power produced by the combine cycle is still sufficient to meet the demand of the process. Once the recycle ratio is increased above about 45%, the energy demand of the process cannot be met and importation of energy from an external source is needed. Figure 5. Power Output of the BF-CC Process For the purpose of illustrating the relationship between power output and CO 2 emission of the proposed BF-CC process, the CO 2 emission of a coal-fired power generation is used. Life Cycle Assessment of coalfired power generation carried out by the National Renewable Energy Laboratory reported that the CO 2 emission is 0.11 Nm 3 /MJ 24. Using this factor, the CO 2 emission associated with the power output of the BF- CC process was calculated. A positive power output of the BF-CC process implies that there is a CO 2 credit since the surplus electricity can be used somewhere in the mill. The calculated emission is subtracted from the direct emission of the process. In the case of a negative power output, energy input from an external source is required. The associated CO 2 emission is added to the direct emission of the process.

7 CO 2 emission of the BF-CC process including consideration of power output is reported as net emission, Figure 6. Net emission associated with producing a tonne of hot metal decreased slightly as the recycle ratio increased up to 30%. Once the recycle ratio increased above 40%, net emission increased rapidly. At recycle ratio levels over 40%, power produced by the combined cycle was insufficient to meet the demand of the process and thus required importation of energy from external sources. Energy importation significantly contributed to CO 2 emission of the process. As a consequence, net emission of the process was increased despite reduction in direct emission as the recycle ratio increased. Figure 6. Net Emission of the BF-CC Process The case considering a top gas recycle ratio of 30% and gasifier coal input rate of 140 kg/thm was selected to further elaborate the BF-CC process. The furnace conditions and energy demand of the BF-CC process are summarized in Table I. In the same table, two conventional blast furnace operations were also shown for comparison. The all coke operation refers to no auxiliary fuel injection. The base case is the one with pulverized coal injection at 140 kg/thm. In the all coke operation, air blast is used and no external energy input is required. Therefore, only direct emission is associated with the process. For the base case, the oxygen content in the blast being slightly enriched, a small amount of input energy is needed for the production of oxygen. Hence, on top of the direct emission, there is a small amount of CO 2 released indirectly. The net CO 2 emissions of both cases are 733 Nm 3 /thm and 772 Nm 3 /thm, respectively. For the BF-CC process with the selected conditions, the coke rate is reduced as compared to the base case due to recycling 30% of the top gas. Moreover, 408 MJ of energy surplus is produced. This corresponds to a CO 2 credit of 45 Nm 3 further contributing to the reduction in CO 2 emission. The net emission of the process is 230 Nm 3 /thm, which represents only 30% of the emission in the base case. As shown in Table I, 395 Nm 3 /thm of CO 2 was captured for long term storage in the BF-CC process. In case that long-term storage of CO 2 is not available and the captured CO 2 is released to the atmosphere, the emission of the process becomes 625 Nm 3 /thm or only 19% reduction as compared to the base case. Therefore, the development of CO 2 long-term storage is crucial to achieve significant reduction in CO 2 emission.

8 Table I. Comparison between blast furnace ironmaking processes (/thm) All Coke Base Case BF-CC Blast (%O 2 ) Coal Rate(kg) Coke Rate (kg) RAFT ( o C) Top Gas Volume (Nm 3 ) Direct CO 2 Emission (Nm 3 ) Energy CO 2 Capture (MJ) O 2 Production (MJ) CC Power Generation (MJ) Power Output (MJ) Indirect CO 2 Emission (Nm 3 ) Net CO 2 Emission (Nm 3 ) CO 2 Sequestration (Nm 3 ) FUTURE WORK For the specific case of the BF-CC process considered, the amount of direct CO 2 emission was 275 Nm 3 /thm as shown in Table I. The two sources of carbon contributing to direct emission are coke input to the blast furnace and coal input to the gasifier. The contribution to direct CO 2 emission by coal input to the gasifier is about 36%. Introduction of a gasifier in the process flowsheet offers a great flexibility in the choice of solid fuel that can be used and thus potentially further reducing direct CO 2 emission. One of the possibilities is employing gasification of biomass instead of coal. Since CO 2 originating from biomass is GHG neutral, direct emission of the process can be further reduced to 176 Nm 3 /thm if sustainable biomass is used. A lot of effort had been spent on investigating the gasification of biomass for electricity generation resulting in a good understanding of the composition of gas produced. However, the effect of injecting gasified biomass on the blast furnace conditions requires further study. Another important observation is that the furnace top gas volume of the BF-CC is only about 50% that of the conventional blast furnace. Even though the material and energy flows in the oxygen blast furnace considered are sufficient to meet the demands of ironmaking, the reduction in volumetric gas flowrate inside the furnace may impose significant challenges to directly switching from a conventional blast furnace to oxygen blast operation. It is generally anticipated that oxygen blast operation could increase the productivity of the furnace due to the CO content increase in the gas phase. It is likely that the internal volume of the oxygen blast furnace can be reduced while maintaining the same productivity as the conventional blast furnace. To confirm these suppositions, a more detailed study on the geometry, gas distribution and hydrodynamics of the oxygen blast furnace is needed. In all of the above discussions, it was assumed that the CO 2 captured was sequestrated for long-term storage. Since it contributes significantly to the reduction in GHG achievable, further development of longterm CO 2 storage is crucial. CONCLUSIONS Integration of the oxygen blast furnace ironmaking with combined cycle power generation was studied. 1. The proposed BF-CC process makes use of a portion of the furnace top gas as fuel for combined cycle power generation to meet the energy demands of the operating units for implementing oxygen blast furnace ironmaking and CO 2 capturing. 2. Heat and mass balance simulation of the proposed BF-CC process revealed that recycling more than 40% of the furnace gas resulted in insufficient fuel for generating power to meet the energy demand. Despite the total fuel rate of the process decreasing with increasing recycle ratio, net emission of the process was significantly increased upon taking into account the CO 2 emission associated with producing power required from external sources.

9 3. Simulation results suggested that the optimal recycle ratio was attained when the power produced by the combined cycle was just sufficient to cover the energy demand of the process, which occurs at recycling 30% of top gas back into the blast furnace. Under this circumstance, the net emission of the ironmaking process is reduced to 230 Nm 3 /thm, which corresponds to only 30% of the net emission of the conventional process. 4. The incorporation of a gasifier in the proposed BF-CC process increased the flexibility on the choice of fuel for direct injection. Applying direct gasification of sustainable biomass in the BF-CC process can further reduce the net emission of the ironmaking process to 176 Nm 3 /thm or 77% reduction as compared to the conventional blast furnace. 5. Development of long-term CO 2 storage is crucial. ACKNOWLEDGEMENTS We would like to express thanks to the Canadian Carbonization Research Association and to the Canadian Federal Government ecoeti program for supporting this work. REFERENCES 1. Report on Energy Supply and Demand in Canada- 2007, Statistics Canada, Cat. No X. 2. Canada s GHG Emission by Sector, Office of Energy Efficiency, Natural Resources Canada, retrieved on January 18, E. Rubin, L. Meyer and H. de Coninck, Technical Summary, IPCC Special Report on Carbon Dioxide Capture and Storage, Prepared by Working Group III of the International Panel on Climate Change, B. Metz, O. Davidson, GH.C. de Coninck, M. Loos, and L.A. Meyer (eds), Cambridge University Press, Cambridge, UK, 2005, pp W-K. Lu and R. Vasant Kumar, The Feasibility of Nitrogen-Free Blast Furnace Operations, ISS Transactions, Vol. 5, 1984, pp D. S. Gathergood, Nitrogen-Free Blast Furnace Operation, Energy-Conscious Iron- and Steelmaking, 1981, pp D. Ma, L. Kong and W-K. Lu, Heat and Mass Balance of Oxygen-Enriched and Nitrogen-Free Blast Furnace Operations with Coal Injection, 47 th Ironmaking Conference, 1988, pp M. Qin and N. Yang, A Blast Furnace Process with Pulverized Coal Oxygen and Gas Circulation for Reduction, Scand. J. Metall., Vol. 15, No. 3, 1986, pp M. Qin, Z. Gao, G. Wang and Y. Zhang, Blast Furnace Operation with Full Oxygen Blast, Ironmaking & Steelmaking, Vol. 15, no. 6, 1988, pp D.M. Kundrat, Redesign of the Blast Furnace Process to Decrease Coke Rate and Increase Productivity- The Near N 2 -Free BF, Metallurgical Processes for the Early Twenty-First Century Vol. II. Technology and Practice, 1994, pp Y. Ohno, M. Matsuura, H. Mitsufuji and T. Fukukawa, Process Characteristics of a Commercial-scale oxygen Blast Furnace Process with Shaft Gas injection, ISIJ Int., Vol.32, No. 7, 1992, pp A. Poos and N. Ponghis, Potentials and Problems of High Coal Injection Rates, Ironmaking Conference Proceedings, Vol. 49, 1990, pp H. Yamaoka and Y. Kamei, Theoretical Study on an Oxygen Blast Furnace Using Mathematical Simulation Model, ISIJ Int., Vol. 32, No. 6, 1992, pp

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