A comparative study on the air, the oxygen-enriched air and the oxy-fuel combustion of lignites in CFB

This paper deals with the comparative combustion processes of two lignites in the air, the oxygen-enriched air and the oxy-fuel-environments. Combustion experiments were conducted in a 30 kWth circulating fluidized bed (CFB) combustor. The air combustion and the oxygen-enriched air combustion were conducted with different excess air ratios and excess oxygen percentages, respectively. Emissions of major pollutants such as CO, CO2, NOx and SO2 were measured during experiments via on-line gas analysis systems. Similarly, temperatures along the combustor axis were measured at various positions. The oxy-fuel combustion experiments were done with O2/RFG mixture. In all environments the lignites sustained stable and steady combustion processes in the temperature range of 700–950 °C under the conditions applied. Emissions of both NOx and SO2 appeared to be considerably affected by combustion atmospheres. In the oxygen-enriched combustion both the NOx and SO2 emissions were higher than that in the air combustion and increased with increase in the O2 concentration in the feed gas. In comparison to the air and the oxygen-enriched air combustion processes, however, the NOx emissions were considerably depressed in the oxy-fuel combustion. In the oxyfuel combustion processes, CO2 concentrations of up to 96% in the flue gases were reached on dry basis. •Air, oxygen-enriched air and oxyfuel combustion of lignite are studied.•Pollutants’ emissions were affected by the combustion atmosphere.•NOx emissions decreased as: oxygen-enriched air > air > oxyfuel combustion.•In the oxyfuel combustion of lignites 96% CO2 in the dry flue gases was reached.•The maximum bed temperature gradient in oxyfuel combustion varied in 90–105 °C.