Effect of operating conditions on SO2 and NOx emissions in oxy-fuel mini-CFB combustion tests

Abstract

Anthropogenic CO2 production is caused primarily by fossil fuel combustion. In consequence, it is increasingly necessary to find ways to reduce these emissions when fossil fuel is used. CO2 capture and storage (CCS) appears to be among the most promising. All of the CCS technologies involve producing a pure stream of CO2 either by concentrating it from the flue gases, or by using pure oxygen as the combustion gas. The latter option, oxy-fuel combustion, has now been well studied for pulverized coal combustion, but has received relatively little attention to date in the case of oxy-fuel circulating fluidized bed combustion. Recently, oxy-fuel CFBC has been examined in a 100 kW pilot plant operating with flue gas recycle at CanmetEnergy. The results strongly support the view that this technology offers all of the advantages of air-fired FBC, with one possible exception. Emissions such as CO or NOx are lower or comparable to air firing. It is possible to switch from air-firing to oxy-firing mode easily, with oxygen concentrations as high as 60-70%, and flue gas recycle levels of 50-60%. Only sulphur capture is poorer. However, this result is not in good agreement with other studies, and the reasons for this discrepancy need further exploration. Here, longer tests have confirmed previous findings from CanmetEnergy with two coals and a petroleum coke. It also appears that changing from direct to indirect sulphation with the petroleum coke improves sulphur capture efficiency, although a similar effect could not be confirmed with coal from these results.