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

Pressurized oxy-fuel combustion (POFC) of solid fuels in fluidized beds possess the potential for CO₂ capture at low cost. However, the practical experience of oxy-coal combustion in pressurized fluidized beds (PFB) is still very limited, and there is a lack of attempts on the co-firing of coal and other fuels. In this study, the co-firing of coal and biomass in a POFC mode at a 10 kW_th PFB was tested. The dynamic behaviors of the start-up process and combustion mode switching were investigated. The effects of key operating parameters, including combustion pressure (P), biomass blending ratio (M_b), and excess oxygen coefficient (α), on the temperature distributions, CO₂ enrichment and conversion, pollutant emissions (CO, NO_X, SO₂), and solid residues were methodically studied. The results show that the stable, pressurized, and oxy-fuel combustion mode with coal and biomass mixtures as fuels can be successfully realized in a fluidized bed. Increasing P and M_b not only conduces to better temperature distribution, more CO₂ enrichment in flue gas, and higher combustion efficiency but also has the advantage of reducing NO_x and SO₂ emissions by over 30%. In the oxy-fuel PFB, the positive effect of α on the combustion performance is more significant than that under atmospheric conditions. As P increases, the fly ash surface is more cracked, and the particle size distribution of fly ash decreases, while the bottom slag surface is smoother. Besides, the increase in P results in a decreased specific surface area and cumulative pore volume but an increased average pore diameter in fly ash.