Energy, exergy and environmental analyses of conventional, steam and CO2-enhanced rice straw gasification

Abstract

In this study, air, steam and CO2-enhanced gasification of rice straw was simulated using Aspen PlusTM simulator and compared in terms of their energy, exergy and environmental impacts. It was found that the addition of CO2 had less impact on syngas yield compared with gasification temperature. At lower CO2/Biomass ratios (below 0.25), gasification system efficiency (GSE) for both conventional and CO2-enhanced gasification was below 22.1%, and CO2-enhanced gasification showed a lower GSE than conventional gasification. However at higher CO2/Biomass ratios, CO2-enhanced gasification demonstrated higher GSE than conventional gasification. For CO2-enhanced gasification, GSE continued to increase to 58.8% when CO2/Biomass was raised to 0.87. In addition, it was found that syngas exergy increases with CO2 addition, which was mainly due to the increase in physical exergy. Chemical exergy was 2.05 to 4.85 times higher than physical exergy. The maximum exergy efficiency occurred within the temperature range of 800 oC to 900 oC because syngas exergy peaked in this range. For CO2-enhanced gasification, exergy efficiency was found to be more sensitive to temperature than CO2/Biomass ratios. In addition, the preliminary environmental analysis showed that CO2-enhanced gasification resulted in significant environmental benefits compared with stream gasification. However improved assessment methodologies are still needed to better evaluate the advantages of CO2 utilization.