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Anthropogenic CO 2 production is primarily driven by fossil fuel combustion, and the current energy demand situation gives no indication that this will change in the near future. In consequence, it is increasingly necessary to find ways to reduce these emissions when fossil fuel is used. CO 2 capture and storage (CCS) appears to be among the most promising approaches. All of the CCS technologies involve producing a nearly pure stream of CO 2 , either by concentrating it in some manner 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 to date has received relatively little attention in the case of oxy-fuel circulating fluidized bed combustion (CFBC). Recently, oxy-fuel FBC 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 NO x are lower or comparable to those of air firing. It is possible to switch from air firing to oxy firing easily, with oxygen concentrations as high as 60-70%, and flue gas recycle levels of 50-60%. Only sulfation is poorer, which is not in good agreement with other studies, and the reasons for this discrepancy need further exploration. However, longer tests have confirmed these findings with two coals and a petroleum coke. It also appears that changing from direct to indirect sulfation with the petroleum coke improves the sulfation, although a similar effect could not be confirmed with coal from these results.

Oxy-fuel technology uses effectively pure oxygen for fossil fuel combustion in order to obtain a highly concentrated CO2 stream, suitable for direct compression and sequestration. It is an effective technology to reduce greenhouse gas emissions to the atmosphere from large point sources such as power generation plants. Oxy-fuel FBC technology has the combined advantage of producing high CO2 concentration flue gas and allowing excellent fuel flexibility. In addition, with external cooling of the recirculated solids, the flue gas recirculation ratio can be reduced. CETC-Ottawa has carried out oxy-fuel fluidized bed combustion with flue gas recirculation on its modified mini-CFBC. The mini-CFBC has an internal diameter of 100 mm and internal height of 5000 mm. Both bituminous and sub-bituminous coals were fired. Limestone was premixed with coal and fed to the mini-CFBC. Recirculated solids were cooled in the return leg of the mini-CFBC. The bed temperature was controlled at about 850 °C, while the oxygen concentration in the primary gas was ∼25% and in the secondary gas was ∼50%. With flue gas recycle, the CO2 concentration in the flue gas reached 82–90%. Sulfur capture efficiency and CO and NOx concentrations were also measured and were all at acceptable levels. The transition from air firing to oxy-fuel firing was a fast and relatively smooth process, and operation of the mini-CFBC under oxy-fuel firing conditions was similar to that of air firing.

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