Technical, economic, and environmental analysis of coal co-firing with biomass: A comparative study of gasification co-firing, torrefaction co-firing, and direct co-firing

Co-firing biomass with coal is recognized as an effective pathway to reduce carbon emissions from conventional power plants. However, existing research has predominantly focused on direct co-firing, leaving significant gaps in studies on gasification co-firing and torrefaction co-firing. To address these limitations, this study systematically models key co-firing processes—including dedicated coal combustion, direct co-firing, gasification co-firing, and torrefaction co-firing—with particular emphasis on integrating the torrefaction process with existing power plant systems to minimize energy consumption and carbon emissions.Based on the models, a systematic comparative study is conducted to evaluate the energy efficiency, exergy efficiency, unit electricity production cost, and CO2 emission under different co-firing processes, biomass types, and co-firing ratios. Through sensitivity analysis, the main factors influencing production costs and CO2 emissions are investigated, and recommendations for controlling costs and CO2 emissions are proposed. The main conclusions of this study are as follows: The boiler efficiency of torrefaction co-firing is higher than that of gasification co-firing but lower than that of direct co-firing. Among the three co-firing methods, gasification co-firing exhibits the lowest exergy efficiency, followed by torrefaction co-firing, while direct co-firing demonstrates the highest exergy efficiency. For sawdust biomass, when the co-firing ratio increases from 0 to approximately 25%, the CO2 emissions of direct co-firing, gasification co-firing, and torrefaction co-firing decrease from 1.041 kg/kWh to 0.72, 0.63, and 0.71 kg/kWh, respectively, while the unit production costs increase by 8.5%, 7.3%, and 4.8%, respectively.This study offers important practical references for reducing carbon emissions through biomass co-firing in coal-fired power plants.