Comparative analyses of different biogenic CO₂ emission accounting systems in life cycle assessment

The biomass-derived CO₂ emission is usually treated as neutral to climate change. However, due to the stay of biomass-derived CO₂ in the atmosphere, many researchers believe that biomass-derived CO₂ also has climate change benefit. Therefore, many methods to account the global warming potential of biomass-derived CO₂ (GWP_bio) were proposed. Based on those new methods, we developed an accounting system for climate change impact of biomass utilization in this study, and compared it with the conventional accounting system which follows the carbon neutral assumption. A case study of caragana-to-pellet bioenergy production system was simulated to test the performance of the GWP_bio accounting system. The CENTURY model was used to simulate carbon dynamics of caragana plantation in the Loess Plateau in China, and life cycle assessment (LCA) model was developed to estimate the life cycle emissions of the caragana-to-pellet system. Attributed to short rotation of caragana plantation and fast biomass accumulation after harvest, the GWP_bio values around 0.044 were obtained. When the GWP_bio was applied to LCA, significant high life cycle CO₂ emission was found in comparison to the conventional method. However, the GWP_bio accounting system has lower positive climate change impact than the conventional accounting system in assessing the overall impact of biomass utilization. This indicated that the application of GWP_bio accounting system would encourage the utilization of biomass and allow a fair comparison with fossil fuels. In the sensitivity analysis, we found the accounting system was sensitive to biomass accumulation and all the corresponding factor affecting biomass accumulation.