Abstract
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/O2 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 %O2, 60 %CO2–40 %O2), was studied. Results obtained for the pure fuels have shown that the temperatures of maximum reaction rate, Tmax, 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, Tm, obtained for these blends in an oxy-fuel atmosphere were 100–200 °C lower than the values found for the air atmosphere. Tm values determined for the blends were also dependent on the oxy-fuel conditions, with larger differences observed with the 60 %CO2–40 %O2 mixture than with the 70 %CO2–30 %O2 atmosphere with respect to air combustion. However, the greatest decreasing effect compared to air of biomass addition on Tm values was found for the blend with the lowest biomass content (20 % biomass w/w).
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