Co-firing of coal and biomass under pressurized oxy-fuel combustion mode in a 10 kW_th fluidized bed: nitrogen and sulfur pollutants

Co-firing coal and biomass under pressurized oxy-fuel combustion (POFC) mode in fluidized beds is a novel and promising technology to capture CO₂ at a low cost and with eco-friendly waste disposal. However, large knowledge gaps exist in combustion characteristics and pollutant generation owing to the tremendous lack of practices. Following our successful testing of co-firing coal and biomass under POFC mode in a10 kW_th pressurized fluidized bed (PFB) [Chem. Eng. J. 2022, 431,133457], this study conducted a special and in-depth investigation of nitrogen and sulfur pollutants. Based on a comprehensive analysis of flue gas and solid residues, the effects of key operating parameters, such as combustion pressure (P), biomass blending ratio (M_b), and combustion temperature (T), on the emissions of gaseous and solid pollutants, transformation and distribution of nitrogen and sulfur, sulfur self-retention (SSR) process, and ash properties and potentials of deposition and agglomeration, were investigated. The results show that increase in P and M_b significantly reduces NOx, N₂O, SO₂, and CO emissions and improves the SSR efficiency. Notably, the pressurized and co-firing conditions have a synergistic effect on SO₂ emission reduction. The T dependence of these products under POFC mode is clearly less than that under atmospheric oxy-fuel combustion mode. Within the typical operating temperature range of an oxy-fuel PFB, SSR occurs through the direct sulfation route. The ash and bed material have an overall low potential for slagging, fouling, and agglomeration, with no indication of ash-related problems during testing. This work will be helpful for the development of oxy-fuel co-firing in PFBs for CO₂ capture.