Abstract
A new method for improving the efficiency of oxyfuel gasification in biomass energy with carbon capture and storage (BECCS) cycles using carbon dioxide recycled from exhaust gases is described and modelled. Thermodynamic simulations in ASPEN Plus show this process can increase the indicated efficiency of a representative cycle by up to 10.3 % in part by reducing the oxygen requirements for the gasification reaction. Details of syngas production, process cold gas efficiency (CGE), and resulting system temperatures reveal the thermodynamic mechanisms contributing to the observed trends in overall cycle energy efficiency. Exhaust recycling is also shown to have a practical limit beyond which the syngas fuel becomes highly diluted, resulting in low combustion and exhaust temperatures which negatively influence the gasification process. For the system presented here, CO2-enhanced oxy–gasification is thermodynamically limited to oxygen equivalence ratios above λ = 0.13 and equilibrium temperatures above 576 °C. This thermodynamically limited case produced an indicated system efficiency of 26.9 % based on supplied biomass lower heating value (LHV). Further simulations using both ideal cycles and detailed numerical models highlight the influence of several operational settings on the thermodynamic conditions of the gasification process. Principally, the coupling between exhaust temperatures, allothermal heat, and syngas quality are shown to govern the performance of the gasification reactions.
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