Pre-Combustion

How is the CO2 Captured?

Pre-combustion is a process where the fuel source is gasified to create syngas, a mixture of hydrogen and carbon monoxide. The carbon monoxide then undergoes a shift reaction to generate hydrogen and CO2 which can then be captured prior to combusting the gas mixture.

Pre-combustion capture systems basically involve de-carbonizing the fuel source prior to combustion, a process that is widely used in the manufacture of hydrogen and fertilizer (IPCC, 2005). The fuel source can be converted to a syngas, which consists mostly of a mixture of carbon monoxide (CO) and hydrogen. This conversion can be done using gasification, partial oxidation or steam reforming technology. Gasification is most often used for solid fuels, partial oxidation for liquids, and steam reforming for gases. Then the CO is converted into CO2 through a shift conversion process which also produces a stream of hydrogen. The most valuable by-product of pre-combustion is the hydrogen, and as the world shifts towards a hydrogen-based economy it will become even more valuable as a fuel source for transportation or distributed generation.

Compared to other combustion processes, the incremental energy penalty of pre-combustion capture is low at 6% (IEA, 2003); because of the relatively favourable CO2 concentrations in the process (which range from 15 to 80%) and the high pressure involved (IPCC, 2005). Both factors make the separation and compression of CO2 in pre-combustion systems relatively efficient.

Pre-combustion systems are costly and questions exist regarding the reliability of using gasification technology on low-rank Canadian coals such as the sub-bituminous and lignite coals in western Canada. Shift converters weren't made for fuels like coal, and process-related ash particles will result in system damage. Other problems include hot gas clean-up and the issues related to pure hydrogen-fired turbines. While integrated gasification combined cycle (IGCC) technology (gasification technology) has been commercially demonstrated in other settings around the world, it has yet to be proven technically feasible using Canada's variety of low rank coals.

Therefore, second generation IGCC concepts are needed; ones that incorporate improved membrane processes for the water gas shift reaction and for hydrogen/CO2 separation. These concepts will need to include more energy efficient CO2 and multi-pollutant capture processes that apply to low rank coals. Ultimately, a pilot-scale gasification facility is needed so that industrial operators conducting research on new configurations (specifically designed for heat, power and hydrogen production) can test their new designs in an economic setting. IGCC technology, optimized for generating power in Canada, would enable the roll-out of a whole new fleet of power generation facilities across the country.

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