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 (O-2/N-2) combustion in a 30 kW h 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 O-2 concentration. CO2 emissions are much higher in O-2/RFG combustion. CO and N2O emissions decrease with O-2 enrichment, while NO and SO2 emissions are promoted for both modes. Under O-2/RFG mode, CO, NO, N2O, and SO2 mass concentrations (mg/MJ) are lower compared to O-2/N-2 mode. Higher oxygen concentrations increase combustion efficiencies for both cases. Similar O-2 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.