Conventional pf coal-fired boilers use air for combustion in which the nitrogen from the air dilutes the CO2 concentration in the flue gas. The capture of CO2 from such dilute mixtures using amine stripping is relatively expensive. As shown on the Figure, during oxy-fuel combustion, combustion with oxygen typically of greater than 95% purity and recycled flue gas is used for combustion of the fuel. By recycling the flue gas, a gas consisting mainly of CO2 and water is generated, ready for compression and sequestration.
Figure. Illustrative flowsheet for the Oxy-fuel process, with additional unit operations for a conventional pf power plant for carbon capture showed bold.
The process can be applied as a retrofit to pf boilers and also to fluidised bed boilers, again with recycled flue gas, but possibly with adjusted heat extraction in the bed compared to air operation.
Currently there are no full-scale plants using oxy-fuel combustion in operation. However, laboratory and pilot-scale studies have provided an understanding of the relevant design parameters and operational issues. Current technology demonstrations involved on the O-fWG will lead to commercial deployment.
The technology will be further developed. The above description is for the first-generation oxy-fuel technology. Ongoing research is developing the next generations of systems which would reduce or avoid the recycled gas flow, and also use other means of providing oxygen than the ASU.