木質バイオマス発電および熱電併給のライフサイクル環境影響と外部コストの評価

抄録

Objective. A feed-in tariff (FIT) scheme came into effect in Japan in 2012. Because fossil fuels are consumed in the harvest, transportation, and manufacture of fuel chips, it is necessary to perform a life cycle assessment (LCA) of woody biomass-based energy production systems (woody biomass systems). In recent years, there has been increasing demand not only for environmental impact assessment related to global warming, such as the generation of carbon dioxide and other greenhouse gases, but also for comprehensive assessment of other environmental impact categories. However, the environmental impacts of woody biomass systems have, as yet, not been assessed. Therefore, the present study aims to determine all the characteristics of such systems with respect to their environmental performances.
Results and Discussion. Two systems were examined using LCA under the assumption that 100,000 m³ (log equivalent) / year of woody biomass is available: system B₁, which is a power plant that consumes the woody biomass to produce electricity; and system B₂, in which the woody biomass is consumed by a cogeneration (combined heat and power: CHP) plant, which produces both heat and electricity. We compared these systems to the corresponding alternative fossil fuel systems, systems F₁ and F₂, respectively. The external costs associated with woody biomass systems are at least 36.8% (system B₁) and at most 72.4% (system B₂) lower than those associated with fossil fuel systems F₁ and F₂, respectively. In woody biomass systems, 57.8% of the total external costs are associated with the disposal of combustion ash. If electricity only was produced from woody biomass by using a steam turbine system with a sufficiently low power generation efficiency and all of the combustion ash and residue were landfilled, the expected external costs would be higher than those of the alternative fossil fuel system. For this reason, when the woody power generation efficiency is less than the threshold of 15.3%, the external costs are higher than those of a commercial power system.
Conclusions. The external costs of woody biomass systems are smaller compared to those of the alternative fossil fuel systems. In addition, this reduction in the external costs is greater for CHP systems, which produce both heat and electricity, than systems that produce electricity only.