Assessment of unutilized woody biomass energy and the cost and greenhouse gas emissions of woody biomass power plants in Hokkaido, Japan

With the adoption of the Paris Agreement at the 21st Conference of the Parties organized under the United Nations Framework Convention on Climate Change in 2015, many countries agreed to increase their efforts to reduce greenhouse gas (GHG) emissions. The introduction of renewable energy is expected to reduce emissions in the electricity and heat use sectors, where emissions are particularly high. Among renewable energies, woody biomass energy is attracting attention as a stable, largescale source of power; however, this energy source must be used sustainably because it originates from forests, which also absorb CO₂. In Japan, the quantity of power generated using woody biomass energy is increasing each year, but only a small percentage of the woody feedstock is derived from domestic sources. In this study, we estimated the unutilized woody biomass energy potential in Hokkaido, Japan, then modeled the GHG emissions and costs of using woody biomass in a large-scale woody biomass power plant. Both costs and GHG emissions increased as increasing proportions of unutilized wood were used as feedstock. Compared with the cost per unit of electricity generated for a woody biomass power plant in Japan (29.8 JPY·kWh^-1), the cost of using only unutilized wood was estimated to be 38.6–127 JPY·kWh^-1. Assuming that woody biomass energy is not carbon-neutral, the GHG emissions generated by wood combustion were 1.30–1.41 kg-CO₂ eq·kWh^-1, which were larger than the GHG emissions from coal-fired power plants. For unutilized thinned lumber and forest residues, the costs per unit of heat value and the GHG emissions associated with the use of tractors for lumber collection were higher than the costs and GHG emissions associated with other machinery; these costs and emissions increased along with increases in the amount of wood used. The costs per unit of heat and GHG emissions from the trucks used for transporting prunings were higher than the costs and GHG emissions of other machines. These costs increased as the collection area expanded because of the increase in the amount of woody biomass energy used.