[Objective] Biomass gasification-coupled co-firing is a feasible low-carbon retrofit route for existing coal-fired units. This study evaluates the carbon reduction effect and economic performance of a biomass gasification-coupled co-firing power system based on a 640 MW coal-fired unit. The effects of different co-firing ratios are examined at 70%, 50%, and 30% load, so as to provide a basis for low-carbon upgrading of large coalfired power plants, flexible power system operation, and future engineering practice. 

[Methods] A framework integrating life cycle assessment (LCA) and techno-economic analysis (TEA) was built. Pure-coal cases and biomass gasification-coupled co-firing cases were compared under different loads and co-firing ratios. The system boundary included fuel supply, biomass gasification, coupled combustion, power generation, auxiliary material use, and emission control. Fossil-based CO2 emission intensity was used to assess the direct carbon reduction caused by biomass substitution, while global warming potential (GWP) was used to measure life-cycle greenhouse gas emissions. Economic performance was assessed by levelized cost of electricity (LCOE) and electricity profit. Biochar revenue and carbon trading revenue were also included to test their effects on system profitability. 

[Results] The results show that biomass gasification-coupled co-firing can greatly reduce the overall fossil-based CO2 emission intensity of the system. Compared with the corresponding pure-coal cases, the 15% co-firing cases reduce fossil-based CO2 emissions by 16.9%, 11.5%, and 10.9% at 70%, 50%, and 30% load, respectively. Contribution analysis shows that direct combustion is still the main emission source, accounting for about 60%~70% of total emissions. Thus, the overall emission reduction is mainly driven by the substitution of fossil coal with biomassderived syngas. In terms of life-cycle greenhouse gas emissions, the GWP of CO70-15, CO50-15, and CO30-15 decreases by 17.3%, 15.1%, and 17.8%, respectively, compared with the corresponding pure-coal benchmarks. 

[Conclusion] Economically, the coupled system generally has a higher LCOE than the pure-coal benchmark because of added biomass supply, gasification, and system integration costs. However, when biochar revenue and carbon trading revenue are included, electricity profit rises with the co-firing ratio. Overall, biomass gasification-coupled co-firing can reduce fossil-based CO2 emissions and life-cycle greenhouse gas emissions, while showing a certain level of economic feasibility for large coal-fired units in practice. These findings further indicate that the proposed system can support low-carbon retrofitting of coal-fired power plants when biomass use, by-product recovery, and carbon market mechanisms are jointly considered.