Study of the thermal behaviour of coal/biomass blends during oxy-fuel combustion by thermogravimetric analysis

During oxy-fuel combustion, the gas composition inside the boiler differs greatly from that of conventional combustion with air, involving consequences for different aspects in fuel combustion. Research on oxy-fuel combustion is needed to understand which factors influence the process, especially for coal and biomass co-firing. In this study, the combustion behaviour of coal/biomass blends was determined by thermogravimetric studies (TG) with different CO2/O-2 mixtures and compared with similar results for conventional combustion. This approach determines the appropriate conditions for the oxy-fuel combustion for future studies that will be carried out in lab- and bench-scale combustors. One sub-bituminous coal (Puertollano coal) and two Spanish biomasses (olive grove and thistle) were the fuels selected for the study. The combustion behaviour of each pure fuel and several coal/biomass blends, under air and oxy-fuel conditions (70 %CO2-30 %O-2, 60 %CO2-40 %O-2), was studied. Results obtained for the pure fuels have shown that the temperatures of maximum reaction rate, T (max), determined under oxy-fuel combustion were lower than those found during conventional combustion. Similar pattern was encountered for the different coal/biomass blends studied (varying from 80 % coal/20 % biomass to 20 % coal/80 % biomass), with a more reactive behaviour in oxy-fuel conditions than in conventional air combustion. The values of temperatures at maximum mass loss, T (m), obtained for these blends in an oxy-fuel atmosphere were 100-200 A degrees C lower than the values found for the air atmosphere. T (m) values determined for the blends were also dependent on the oxy-fuel conditions, with larger differences observed with the 60 %CO2-40 %O-2 mixture than with the 70 %CO2-30 %O-2 atmosphere with respect to air combustion. However, the greatest decreasing effect compared to air of biomass addition on T (m) values was found for the blend with the lowest biomass content (20 % biomass w/w).