Analysis of exergy and carbon emission of torrefied biomass gasification coupled with coal-fired power generation system based on heat recovery

Biomass gasification coupled coal-fired power generation is an effective way to achieve low-carbon transformation of power generation systems. Due to the frequent changes of biomass types in the gasification unit and the large fluctuation range of moisture content, the quality of syngas is unstable and the energy consumption of the system is high. This paper proposes a new process combining biomass torrefaction and waste heat recovery. The system uses the tail flue gas is used for biomass torrefaction, and the gasification gas is used to cool the heat to heat the water as the gasification agent of the gasifier. A 600 MW coal-fired plant model was built using Aspen Plus, with exergy, energy, economic, and environmental analyses conducted. The results show that torrefaction pretreatment is beneficial to improve the quality of gasification syngas, and higher torrefaction temperatures increase combustible gas percentage, gas production, and heat production. Controlling syngas temperature between 360 ℃ and 440 ℃, with an S/B ratio of 1.04–1.24, reduces syngas temperature and LHV but increases G_v and H_v. The new process boosts syngas yield by 1.27 % and 1.25 %, reduces boiler exergy loss by 7.98 % and 2.2 %. The system's energy efficiency is 51.66 %, with a 6.63-year payback period, and it can reduce annual carbon emissions by 94,000 tons, offering better thermal economy and lower carbon emissions.