Diversification of primary energy sources is becoming increasingly important as concerns arise from the use of fossil fuels, specifically coal, in the generation of electrical power. Emission of greenhouse gases and particulate matter associated with the combustion of coal threaten the state of the environment from a climate disruption and human health perspective.
A complete overhaul of the Canadian energy utility, while technologically possible, is prohibitively expensive and would require political will and substantial modification of existing infrastructure, making it unlikely in the short term. An alternative which would make use of existing generation facilities and would not require major modifications to the existing grid is to co-fire biomass with coal. Co-firing involves combusting coal mixed with biomass in varying ratios to achieve the same generation capacity with a reduced emission footprint. It is a first step in the decarbonization of our energy generation system that can be achieved with comparative ease.
Co-firing can be done in a variety of ways, including:
- Direct co-firing: biomass and coal enter the boiler and are combusted together
- Parallel co-firing: biomass and coal supply separate boilers which feed to a common header
- Indirect co-firing: biomass is gasified prior to combustion with coal (Maciejewska, et al. 2006).
Direct co-firing is the cheapest and most straightforward option, requiring the least initial investment. However, drawbacks are encountered relating to the differences in the properties of the mixed feedstocks, the mixing of coal and biomass ashes, and limitations associated with the type of biomass used in the process (Livingston 2005).
Parallel co-firing keeps the fuel streams separate, and the fuel preparation and feeding are physically independent. This configuration is limited more often in retrofitted facilities by the downstream infrastructure, such as steam turbine capacity. While more cost-intensive than direct co-firing, it has the advantage of being able to use a wider variety of biomass sources, has the possibility of optimizing the combustion process, and keeps the biomass and coal ashes separate (Zuwala and Sciazko 2005).
Indirect co-firing is the most costly option, but provides potential for the highest co-firing ratios (i.e. the highest proportion of biomass for combustion). It is the least developed of the three, but demonstration plants have been constructed in Europe (European Commission 2000).
Many hurdles must be overcome for biomass co-firing implementation, associated primarily with the material properties. Biomass is significantly less dense than coal, leading to increased fuel transportation costs. This is exacerbated by the low heating value of biomass, so more fuel mass is required to produce the same amount of power. However, there are benefits to using biomass in a co-firing operation, including reduced emissions of mono-nitrogen oxides (NOX), Sulfur dioxide (SO2) and carbon dioxide (CO2), increased boiler efficiency, and decreased fuel cost (Demirbas 2003).
According to the National Energy Board, energy generation in Canada has an installed capacity of approximately 137 GW, 9% of which is supplied by coal-fired power plants. The emission factor for CO2 from the combustion of coal has been determined as 98*10-6 kg/kJ, corresponding to a CO2 emission from coal-fired plants in Canada of between 38 and 45 million tons (Mt) per year (Bulucea, et al. 2011). Currently, displacement of coal using natural gas is gaining ground due to the low cost and availability of fuel. While the emission factor for CO2 from natural gas combustion is approximately half of that from coal, it is still a fossil fuel, and contributes to the increase in atmospheric carbon. Since biomass is an active participant in the global carbon cycle, it can be seen as a fuel source that is very nearly carbon-neutral (Ragauskas, et al. 2006).
The most appropriate biomass-derived fuel for co-firing currently in use is compacted wood pellets. The pelletizing process produces a dry, homogeneous, high energy density fuel that addresses the high transportation costs often associated with biomass (Maciejewska, et al. 2006). Canadian wood pellet producers have an installed annual production capacity of approximately 3 million tons, but only operate at approximately 65% total capacity. Canadian exports grew from 1.3 to 1.6 million tons annually from 2012 to 2013 as European, North American, and Asian demands increased (Statistics Canada 2014). Integration of wood pellets into the domestic energy supply would ideally be done without significant disruption to the export market.
If the current trends continue, Canada could be exporting close to 2 million tons annually, which is still not in excess of the total production capacity. Increasing the operation of the pellet plants to 100% capacity would produce an additional 1 million tons of wood pellets annually. Integration of this surplus into domestic energy generation could displace approximately 4.4% of the energy obtained from coal combustion, representing a co-firing ratio of approximately 7-8% (Francescato, Antonini and Bergomi 2008), well within the operational limits of direct co-firing (Livingston 2005). Co-firing at this rate would prevent the emission of between 1.67 and 1.98 Mt of CO2, making use of existing facilities without disrupting the export market.
In short, Canada has the production capacity to displace a significant portion of coal used for energy using direct co-firing of wood pellets. This can be achieved without significant modifications to existing generation facilities or wood pellet production capacity, and without disrupting the current export market for wood pellets. Co-firing at a rate of 7-8% would reduce carbon emissions by 1.67 to 1.98 Mt and contribute to the transition into a low-carbon energy sector.
Bulucea, C., A. Jeles, N. Mastorakis, C.A. Bulucea, and C. Brindusa. “Assessing the environmental pollutant vector of combustion gases emission from coal-fired power plants.” Recent researches in geography, geology, energy, environment, and biomedicine, 2011: 35-42.
Demirbas, A. “Sustainable cofiring of biomass with coal.” Energy conversion and management (Pergamon) 44 (2003): 1465-1479.
European Commission. “Addressing the constraints for successful replication of demonstration technologies for co-combustion of biomass/waste.” Booklet, 2000.
Francescato, V., E. Antonini, and L. Bergomi. Wood Fuels Handbook. Legnaro: Italian Agriforestry Energy Association, 2008.
Livingston, W.R. A review of the recent experience in Britain with the co-firing of biomass with coal in large pulverized coal-fired boilers. Workshop presentation, Mitsui Babcock, Renfrew, Scotland: IEA Exco, 2005.
Maciejewska, A., H. Veringa, J. Sanders, and S.D. Peteves. Co-firing of biomass with coal: constraints and role of biomass pre-treatment. Institute for Energy, European Communities, 2006.
National Energy Board. Canada’s Energy Future 2013. Ottawa: Government of Canada, 2013.
National Energy Board. Canadian Energy Dynamics 2013. Energy Market Assessment, Ottawa: National Energy Board, 2014.
Ragauskas, A., et al. “The path forward for biofuels and biomaterials.” Science 311, no. 5760 (2006): 484-489.
Statistics Canada. “Domestic exports – Wood and articles of wood.” Canadian International Merchandise Trade Database. Government of Canada, 2014 27-11.
Zuwala, J., and M. Sciazko. Co-firing based energy systems – modelling and case studies. Paper, Paris, France: 14th European Biomass Conference, 2005.
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