Effect of Recycled Flue Gas Ratio on Combustion Characteristics of Lignite Oxy-Combustion in a Circulating Fluidized Bed

Oxy-combustion and oxygen-enriched combustion appear as promising options for CO2 capture. In this study, the effect of the recycled flue gas (RFG) ratio on lignite oxy-combustion has been experimentally studied in comparison with oxygen-enriched (O2/N2) combustion in a 30 kWth circulating fluidized bed reactor. Oxygen concentrations range from 21 to 28% for both cases, while RFG ratios vary between 0.72 and 0.63 in the oxy-combustion case. The influence of oxygen concentration in the combustion atmosphere on temperature, gaseous emissions, and carbon-based combustion efficiency were investigated by keeping the excess oxygen ratio constant at 1.45 for all cases. Temperatures in the combustor increase with increasing O2 concentration. CO2 emissions are much higher in O2/RFG combustion. CO and N2O emissions decrease with O2 enrichment, while NO and SO2 emissions are promoted for both modes. Under O2/RFG mode, CO, NO, N2O, and SO2 mass concentrations (mg/MJ) are lower compared to O2/N2 mode. Higher oxygen concentrations increase combustion efficiencies for both cases. Similar O2 concentrations result in similar combustion efficiencies. Oxy-combustion results are considered as the most favorable option for CO2 capture along with the competitive combustion efficiencies and less emissions per unit energy.