A life cycle assessment of two bioenergy conversion pathways: Cogeneration versus stand-alone power generation

Background/Question/Methods

Future demand for bioenergy will be met in large part by electricity generated in large coal plants, which can be retrofitted to co-fire biomass, including wood that is normally left behind after harvest. Since coal emits more greenhouse gases (GHGs) than other fossil fuels, replacing it with harvest residue is thought to be an effective means of GHG mitigation. However, centralized electricity production suffers two disadvantages that offset the GHG reductions achieved by displacing coal: 1) while large plants are efficient at converting biomass to electricity, they do not utilize excess heat, unlike smaller cogeneration plants; 2) harvest residues must be processed and transported, often as highly-processed wood pellets. Thus, greater GHG reductions may be achieved using local harvest residues to cogenerate heat and electricity in small industrial plants. Here, we use life cycle assessment (LCA) to compare the energy and carbon balance of two alternative conversion pathways in Ontario: 1) chipping and gasifying harvest residues at a local sawmill that would use a portion of the heat to dry lumber in a kiln; 2) pelletizing and firing the residues in a large power plant that was recently retrofitted to phase out the use of coal in the province.  

Results/Conclusions

Cogeneration of heat and electricity avoids GHG emissions by displacing coal-fired electricity as well as propane that would have been used to dry wood in the kiln.  The use of recovered heat for drying wood chips and lumber more than compensates for the lower conversion efficiency of gassification: non-biogenic CO2 emissions are 21 g/kWh for cogeneration, compared to 59 g/kWh for stand-alone electricity generation, most of which is emitted during the transportation and processing of wood pellets. However, removing harvest residues reduces the amount of carbon stored in the forest, so no net reduction in GHG emission is achieved until the so-called “carbon debt” is paid off. The payback time depends on the conversion pathway: 5 years for cogeneration, and 11 years for stand-alone electricity generation. Cogeneration achieves carbon neutrality earlier due to higher net energy efficiency and lower life cycle emissions, as well as the additional displacement of propane.