Thermodynamic and exergy assessment of a biomass oxy-fuel combustion with supercritical carbon dioxide cycle under various recycle flue gas conditions

The ongoing development of second-generation oxy-fuel combustion system with higher oxygen concentration have presented a potential pathway for efficient biomass utilization. It also signified the meticulous role of recycled flue gas in altering the combustion behaviour in the combustor. Accordingly, investigation into different recirculation modes focusing on recycle ratio, wet/dry conditions, and excess oxygen ratio are conducted with an indirect supercritical power cycle integrated biomass oxy-fuel combustion system. Energy and exergy analyses, as well as composition of flue gas produced are presented. Energy and exergy efficiencies increased with reducing recycle ratio, with respective highest efficiency of 31.33% and 44.37% obtained at excess oxygen ratio of 1.01 and recycle ratio of 0.5 under wet condition. Investigation of CO₂ and H₂O concentration in produced flue gas depicted a separated stage profile under dry recycle condition, different from the slight diverging profile observed under wet condition when the recycle ratio varied. Concentration of NOx and SOx increased with decreasing recycle ratio and are generally higher under dry condition. Selective exergy analyses of the proposed power cycle presented highest exergy loss in the combustor, followed by the air separation unit, heat exchanger, CO₂ compression and purification unit, and turbine generator. Based on the obtained result, wet recycle flue gas enhanced the combustion performance in the combustor but slightly impaired the convective heat transfer in the heat exchanger. The performance of proposed biomass power cycle is optimized at low recycle ratio of 0.5 under wet recycle condition.