Wood Pellets for Ironmaking from a Life Cycle Analysis Perspective

Transcription

1 Wood Pellets for Ironmaking from a Life Cycle Analysis Perspective 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 : Biomass, Charcoal, LCA, GHG mitigation, Ironmaking, Blast Furnace INTRODUCTION Replacement of fossil carbon by bio-carbon is one of the effective approaches to reduce the CO 2 emission intensity of the ironmaking process. The Canadian Carbonization Research Association (CCRA) has partnered with CanmetENERGY in conducting research on bio-carbon application in iron and steelmaking. In the long-term, it is anticipated that a new ironmaking process supported solely by biomass materials will be developed to achieve drastic GHG reduction in the Canadian steel industry. To facilitate the development of a technological pathway accelerating the implementation of a bio-ironmaking process, the feasibility of using wood pellets to support ironmaking was examined from a life cycle analysis perspective. Results of the analysis showed that GHG emission in the carbon life cycle of bio-ironmaking via this pathway was kg CO 2eq /thm compared to 1552 kgco 2eq /thm in the conventional blast furnace process. The drastic reduction, 83%, arises from the GHG neutral characteristic of renewable biomass materials. This would result in a reduction of GHG emission associated with Canadian hot metal production from 12 Mt/yr to 2 Mt/yr. In comparison to choosing an alternative bio-ironmaking pathway examined previously that utilises charcoal, fewer technological barriers are encountered in the wood-pellet pathway, which would accelerate its industrial-scale implementation. It is also identified that the low charcoal yield in wood pyrolysis imposes the most significant technical challenge on bio-ironmaking. Further research to improve the charcoal yield at the industrial-scale level is essential.

2 BACKGROUND Blast furnace ironmaking is the major greenhouse gas (GHG) emitting process in integrated steel mills. In the blast furnace, the reducing gases required to convert iron ore into metallic iron and the heat needed to produce molten pig iron is generated mainly by combustion of coke, formed via the carbonization of coal. All gases formed in blast furnace operation are eventually released to the atmosphere after retrieval of their residual energy. It has been estimated that the blast furnace ironmaking process releases about 1.5 tonne of CO 2 to the atmosphere per tonne of hot metal produced. The Canadian iron and steel industry has devoted continuous and substantial efforts in reducing energy consumptions and CO 2 emission associated with its manufacturing processes. In comparison with 1990, the energy intensity of Canadian iron and steelmaking and absolute GHG emission in 2008 was reduced by 26% and 17%, respectively. On a per tonne basis of shipped steel, GHG emission has been reduced by almost 30% 1. The Canadian Carbonization Research Association (CCRA) 2, in partnership with CanmetENERGY, is conducting research on bio-carbon application in iron and steelmaking to further reduce the GHG emissions of the Canadian steel industry. Bio-carbon refers to carbon sources originating from recent biological materials. As for other fossil carbon sources, combustion of bio-carbon also releases CO 2. However, CO 2 released by combustion of biomass from renewable sources is balanced by CO 2 absorbed during its growth period. Since the duration of this natural carbon cycle is relatively short compared to that of fossil fuel, CO 2 originating from combustion of renewable bio-carbon sources is considered not to contribute to the increase in atmospheric GHG concentration. Hence, bio-carbon is considered as GHG neutral. The advantage of using bio-carbon in blast furnace ironmaking is the fact that the carbon requirements for reduction and melting in the production process remain unchanged. Therefore, upon utilising bio-carbon, CO 2 emission can be reduced without affecting blast furnace productivity. Research efforts have focused on two approaches for applying bio-carbon in conventional cokemakingironmaking system in preparation for industrial implementation in the near future. First, bio-carbon can be used to partly substitute coal in blends for cokemaking 3-6. Second, it can also be used as an auxiliary fuel injected directly into the hearth of the furnace 7-8. It is estimated that implementation of both methods can reduce the GHG emission of the conventional blast furnace ironmaking process by 25%. A real concern on substituting fossil carbon by bio-carbon in industrial process is dealing with the transportation of bio-materials. Due to the low bulk density and low carbon content in raw biomass materials, long-range transportation of raw biomass materials is not desirable. Therefore, densification of the raw biomass for this means is necessary. In previous work, the present authors investigated increasing the carbon density by pyrolysis in converting raw biomass into charcoal prior to long-range transportation. Carbon life cycle analysis of a hypothetical bio-ironmaking process supported only by biomass revealed that emissions associated with such a process could drastically be reduced to 63 kgco 2eq /thm with respect to 1552 kgco 2eq /thm for the conventional blast furnace ironmaking process 9. The current charcoal-making capacity in Canada (~0.006 Mt/yr) is far too small to meet the estimated demand of bio-ironmaking (~6 Mt/yr) representing only 0.1% of the required level. Moreover, the raw biomass materials are widely dispersed in forest regions. The charcoal-making facility must have high mobility and flexibility to minimize transportation distance of the low bulk density and low carbon content raw materials. Needless to say, significant efforts and investments are required to advance charcoal-making technology and business development prior to its possible implementation at industrial-scale level. Alternatively, raw biomass materials can also be densified by pelletization. In comparison to the bulk density of wood chips (~350 kg/m 3 ) 10, that of wood pellets is significantly higher (~550 kg/m 3 ) 11, which thus substantially reduces the cost of transportation. Wood pellet production is a well developed industry in Canada. In 2010, Canada had 33 pellet plants with 2 Mt annual production capacity operating at about 65% capacity 12. Only about 15% of wood pellets produced are consumed domestically. About 1 Mt of pellets produced in 2009 was exported to either the US or Europe 11.

3 In this paper, the pathway using wood pellets to support industrial-scale ironmaking is studied from a carbon life cycle perspective. The system examined commences at the stage of harvesting raw biomass materials for pellet production and ends at consumption of bio-carbon in a hypothetical bio-ironmaking process. The goal of this study is to examine the feasibility of this technological pathway based on information available and to identify obstacles and challenges that may be encountered in implementing bio-ironmaking at industrial-scale. CARBON LIFE CYCLE IN CONVENTIONAL BLAST FURNACE IRONMAKING Carbon life cycle in conventional blast furnace ironmaking was examined for the purpose of establishing a base case for comparison. Figure 1 illustrates the system boundary. The functional unit of this analysis is defined as one tonne of hot metal produced. The pulverized coal injection (PCI) rate of the process was assumed to be 140 kg/thm (tonne of hot metal). The system considered consists of 4 sub-systems, namely coal mining, coal transportation, cokemaking and ironmaking. Emission contribution of each sub-system was calculated and details of the calculation can be accessed elsewhere 9. As shown in Figure 1, the total emission in the carbon life cycle associated with producing one tonne of hot metal by the conventional blast furnace process is 1552 kg CO 2eq /thm. This will be compared with the bio-ironmaking route discussed in the following sections. Figure 1. System Boundary of Carbon Life Cycle in Conventional Ironmaking CARBON LIFE CYCLE IN WOOD PELLET MAKING FOR BIO-IRONMAKING Figure 2 shows the carbon life cycle in using wood pellets to support ironmaking. The system boundary considered was based on the life cycle analysis performed by Zhang et al 13 on their examination of wood pellets for electricity production in the Nanticoke power plant located in southern Ontario, Canada. Due to the fact that the majority of integrated steel mills in Canada are also situated in southern Ontario and the similarity of pathways between the two systems, life cycle inventory information collected in Zhang et al s work was used in the current analysis with some modifications.

4 Figure 2. System Boundary of Carbon Life Cycle in Wood Pellets for Bio-Ironmaking The carbon life cycle commences at harvesting of raw biomass materials in the Great Lakes St. Lawrence (GLSL) Forest Region. The GLSL region refers to the forest area found along the St. Lawrence River across central Ontario. This forest region contains a large amount of lower quality hardwoods, which are not ideal for saw log and pulp production but suitable for pellet production 13. GHG Emissions of each sub-system on a per tonne basis of hot metal production are summarized in Table 1. Using the Global Warming Potential for 100 years horizon suggested by IPCC 14, the total emission of each sub-system is expressed in g CO 2eq /thm. Table 1: Sub-Systems GHG Emissions Emissions Harvesting Raw Biomass Pellet Pellet Pellet Bio- Transportation Production Transportation Pyrolysis Ironmaking CO 2 (g/thm) CH 4 (g/thm) N 2 O (g/thm) GHG Emission (g CO 2eq /thm) In this analysis, raw biomass materials are purposely grown for pellet production. The emissions associated with their growth must also be taken into consideration. Therefore, beside emissions associated with the harvesting operation (harvesting, skidding and slashing), the harvesting sub-system also includes emissions related to the use of fuel and herbicide for forest renewal and road construction and maintenance. The calculated total GHG emission associated with the harvesting of raw biomass materials is 82.7 kg CO 2eq /thm. The harvested raw biomass materials was transported to the pellet plant by self-loading pulp truck with a transportation distance assumed as 115 km. The emission related to transportation of raw materials for pelletization is 61.5 kg CO 2eq /thm. In the pellet plant, the size of raw biomass material is reduced through initial grinding in a hammer mill. The energy required to reduce the moisture content of the grinded raw biomass is provided by combustion of a portion of raw biomass materials. Finally, the raw biomass is compressed into wood pellets. Fuel and electricity consumption and emissions associated with grinding, drying and compression are all taken into consideration as contributions to the GHG emission related to pellet production. The calculated emission for pelletizing raw biomass is 76.9 kg CO 2eq /thm.

5 Location of the pellet plant was assumed to be close to rail lines. The wood pellet produced is transported by rail (180 km) to the port and carried by vessel (890 km) directly to the steel mill located in southern Ontario. The energy consumed for loading and unloading of transported pellets as well as direct emission from the transportation fuel are all taken into account. The emission associated with long-range transportation of wood pellet was found to be 39.1 kg CO 2eq /thm. It is anticipated that the carbon density in wood pellet needs to be enhanced prior to use in the ironmaking process. As for carbonization of coal into coke in the conventional process, wood pellet was assumed to be carbonized by slow pyrolysis in the steel mill. It is further assumed that the Brazilian rectangular kiln with an improved off gas collection system is used for pyrolysis of wood pellet. Since CO 2 emissions during pyrolysis originate from raw biomass, which is GHG neutral, the total emission of this sub-system is only 1.6 kg/thm. Raw biomass materials are assumed to originate from sustainable sources. CO 2 emitted from the bioironmaking process is balanced by the growth of raw biomass. Hence, the GHG emission of the bioironmaking sub-system is assumed to be zero. In the scenario of utilising wood pellets to support the ironmaking process, total GHG emissions in the carbon life cycle is kg CO 2eq /thm. The contribution of each sub-system to the total emission is shown in Figure 3. Emission associated with the pyrolysis of biomass contributes only a very small portion of the total emission in view of the GHG neutral characteristic of raw materials. Transportation of materials (biomass and wood pellets) in the process account for about 40% of the total emission. The two major emission sources are harvesting of raw materials and pellet production. Figure 3. Contribution of Sub-Systems of Wood Pellet Making to Total Emissions CHOICE OF TECHNOLOGICAL PATHWAY Compared to the conventional BF ironmaking process, GHG emission of bio-ironmaking process is significantly reduced, Table 2. The degree of reduction achievable depends on the pathway chosen to support the process. In Table 2, the emission of the technological pathway examined in the previous work, charcoal bio-ironmaking, and that of the current work, wood pellet bio-ironmaking, are included for comparing the effect of pathway chosen on the process emission.

6 Table 2. Comparison of Emissions in Different Ironmaking Pathways Conventional Ironmaking Emission (kg CO 2eq /thm) Charcoal Bio- Ironmaking Emission (kg CO 2eq /thm) Wood Pellet Bio-Ironmaking Emission (kg CO 2eq /thm) Coal Mining 14 Residues Collection and Transportation (80 km) Coal Transportation (46 km rail, 434 km barge) 26 Charcoal Transportation (Truck: 1200 km) 20.3 Harvesting and Transportation (115 km) 41 Pellet Transportation (Rail: 180 km, Vessel: 890 km) Cokemaking 133 Charcoal Making 1.5 Pellet Production 78.5 and Pyrolysis Ironmaking 1379 Ironmaking 0 Ironmaking 0 Total 1552 Total 62.8 Total The charcoal bio-ironmaking pathway 9 involved the collection of forestry residues as raw materials and their conversion into charcoal prior to long-range transportation. The use of side products or residues from forestry operation as raw materials leads to emissions associated with their growth and harvesting. However, as this is part of regular forestry operations, these emissions are excluded from the calculation. Moreover, the transportation of high-carbon density charcoal also lowers emission related to long-range transportation of materials. As a result, the total emission of the charcoal bio-ironmaking process is only 62.8 kg CO 2eq /thm. In the pathway considered in the current work, wood pellet bio-ironmaking, raw materials are purposely grown for pellet production. Therefore, emissions associated with the growth of raw biomass materials and harvesting must be included, which significantly increases emissions associated with the collection and transportation of raw materials compared to the charcoal bio-ironmaking process. The carbon density of wood pellet is relatively low compared to charcoal. Then, despite the fact that transportation efficiency is improved for the former, its actual emission level is not significantly improved. Furthermore, this pathway also involves a pellet production step, which is not necessary in the charcoal bio-ironmaking process. Consequently, the total emission of this pathway increased to kg CO 2eq /thm, which is more than four times that of the charcoal bio-ironmaking process. The charcoal bio-ironmaking pathway considered previously represents the best case scenario in terms of GHG emission for implementing bio-ironmaking. However, in that scenario, the supply of raw material highly depends on the level of activity in forestry operations. A very close collaboration between the steel and forestry industries is therefore needed to maintain a stable supply of raw biomass materials. Moreover, the charcoal-making capacity in Canada must also be significantly increased to meet the demand of ironmaking. Given the barriers to industrial-scale implementation, it is anticipated that the charcoal bioironmaking pathway will not be able to be implemented in the near future. The barriers in wood pellet bio-ironmaking pathway appear to be slightly easier to overcome. As raw biomass is purposely grown for pellet production, it is possible to maintain a sustainable and stable supply of raw material via proper management of forestry operations. Pellet production is a mature technology in Canada and the production capacity can be increased to meet the demand. The technological barrier needed to be overcome to meet the demand of the steel industry is less challenging than the need to expand the charcoal-making capacity. Moreover, the pyrolysis of wood pellet is expected to be carried out in the steel mill. Experience acquired by steelmakers in coal carbonization is expected to be useful in applying largescale pyrolysis of wood pellet. Therefore, even though the GHG emission level of the wood pellet bioironmaking pathway is higher than that of the charcoal pathway, choosing it could accelerate the industrial-

7 scale implementation of bio-ironmaking and still be capable of achieving a drastic reduction in GHG emission in the steel industry. The emission level of wood pellet bio-ironmaking is significantly lower compared with that of the conventional BF process. This is so since the most significant GHG emitting ironmaking step in the conventional process is replaced by a bio-ironmaking step supported by GHG neutral biomass material resulting in the elimination of emission associated with this step. Even though the emission in raw material collection and transportation of materials is increased, converting the conventional ironmaking process into pellet bio-ironmaking process could reduce the GHG emission in the carbon life cycle by more than 83%, from 1552 kg CO 2eq /thm down to only kg CO 2eq /thm. The five year ( ) iron productivity average from Canadian blast furnaces is 7.8 Mt/yr 15. The GHG emission associated with the ironmaking process can thus be reduced from 12 Mt/yr to 2 Mt/yr by replacing conventional blast furnace process by the wood pellet bio-ironmaking process. RAW BIOMASS DEMAND Figure 4 shows the demand of raw biomass materials for supporting the Canadian ironmaking capacity via the wood pellet bio-ironmaking pathway. As specified above, the annual hot metal production in Canada is about 7.8 Mt/yr. Heat and mass balance modeling of typical Canadian blast furnaces reveals that the demand of carbon for producing one tonne of hot metal is 426 kg/thm (carbon in coal + coke) 8. As the new bio-ironmaking process is still under development, it is assumed, in order to facilitate the estimation, that the carbon demand of that process is the same as that of the conventional blast furnace process. Figure 4. Raw Biomass Demand for Wood Pellet Bio-Ironmaking in Canada The yield of charcoal strongly depends on the heating rate during wood pyrolysis 16. Charcoal yield in typical industrial-scale production is roughly 25% 17, which is the level used in the current estimate. The yield of wood pellet in pelletization is about 84.7% based on industrial experience 11, with the major loss being the consumption of biomass via combustion for drying the raw materials. Using the information above, it was calculated that the raw biomass demand to support the hot metal production capacity in Canada is about 17.9 Mt/yr. This high demand of raw materials is not favourable for industrial-scale implementation of the bio-ironmaking process. The raw biomass available in the Great Lakes St. Lawrence Forest Region of Ontario, Canada is only 1.5 Mt/yr 13, which is far less than the demand of the steel industry. An insufficient supply means that it is necessary to explore forest regions located further away from the steel mills. This results in the increase in transportation costs and also GHG emissions associated with this activity. More importantly, it also means that a larger forest area needs to be managed to ensure a sustainable supply of raw materials but at the same time, this would also lead to a substantial increase in emissions associated with the harvesting of raw materials. Therefore, it is desirable to lower the raw materials demand as much as possible. From Figure 4, the high demand of raw materials mainly arises from the low yield achieved in wood pyrolysis. The theoretical yield limit of charcoal making ranges from 55% to 71% depending on the raw materials used in pyrolysis 18. The low charcoal yield in current industrial practice is a consequence of loss

8 of carbon materials in gaseous and vapour form during pyrolysis. Therefore, it is possible to increase the charcoal yield by employing new technologies and oven design. Antal et al 19 have developed a new wood pyrolysis technology capable of increasing the charcoal yield to as high as 47%. By increasing the charcoal yield to 47%, it was calculated that the raw biomass demand to support the Canadian hot metal production capacity was significantly reduced from the 17.9 Mt/yr to 9.5 Mt/yr. This herefore indicates that it is important to further develop the technology of charcoal making in order to accelerate the industrial-scale implementation of bio-ironmaking process. CONCLUSIONS CanmetENERGY has partnered with the CCRA to conduct research on the technical feasibility of applying bio-carbon in conventional coke and iron making to achieve GHG mitigation. To achieve a drastic reduction in GHG emission in the steel industry, it is anticipated that a bio-ironmaking process based solely on bio-carbon will need to be developed. To facilitate the development of a technological pathway that accelerates the implementation of such a bio-ironmaking process, the feasibility of using wood pellets to support ironmaking was examined from a life cycle analysis perspective. The sub-systems included in the analysis consist of the growth and collection of raw biomass material for pellet production, pelletization of raw biomass, pyrolysis/carbonization of wood pellets and the associated transportation of materials. Results of the analysis showed that GHG emission associated with the carbon life cycle in bio-ironmaking via this pathway was kg CO 2eq /thm in comparison to 1552 kgco 2eq /thm for the conventional blast furnace process. The drastic reduction, 83%, arises from the GHG neutral characteristic of renewable raw biomass, which could reduce the GHG emission associated with hot metal production in Canada from 12 Mt/yr to 2 Mt/yr. In comparison to the charcoal bio-ironmaking pathway examined in a previous paper, the technological barriers in the wood pellet bio-ironmaking pathway are lessened and fewer, which would preferentially identify it as the bio-ironmaking route of choice and thus lead to its accelerated industrial-scale implementation. It is also identified that the low charcoal yield in wood pyrolysis imposes the most significant technical challenge on bio-ironmaking. To facilitate the development of charcoal making and utilization, further research work to improve its yield at industrial-scale operation level is essential. ACKNOWLEDGEMENTS We would like to express thanks to the Canadian Carbonization Research Association (CCRA) and to the Canadian Federal Government ecoeti program for supporting this work. REFERENCES 1. Environmental Performance Report, Canadian Steel Producers Association, Retrieved on December 13, Canadian Carbonization Research Association (CCRA), 3. J.A. MacPhee, J.F. Gransden, L. Giroux and J.T. Price, Possible CO 2 mitigation via addition of charcoal to coking coal blends, Fuel Processing Technology, Volume 90, Issue 1, January 2009, Pages J.A. MacPhee, J.F. Gransden, L. Giroux and J.T. Price, CO 2 Mitigation via Addition of Charcoal to Coking Coal Blends, International Conference On Coal Science and Technology, Aug , 2007, Nottingham, UK. 5. K.W. Ng, L. Giroux, J.A. MacPhee and T. Todoschuk, CO 2 Mitigation by Incorporation of Charcoal into Coking Coal Blends, International Conference on Coal Science and Technology, Oct 26-29, 2009, Cape Town, South Africa. 6. K.W. Ng, L. Giroux, J.A. MacPhee and T. Todoschuk, Reactivity of Bio-Coke with CO 2, Fuel Processing Technology, Volume 92, Issue 4, April 2011, Pages

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