Pre Combustion Capture

The pre-combustion capture process involves converting fossil fuel into hydrogen and CO2, usually by gasification, and is appropriate for IGCC plants. During pre-combustion, coal is first transformed into syngas, a mixture of carbon monoxide and hydrogen. In a second step the carbon monoxide is shifted further with water to carbon dioxide and an extra amount of hydrogen. In a CO2 separation unit, the carbon dioxide is separated from the hydrogen using a solvent. The hydrogen is subsequently combusted in the gas turbine of the power plant. Electricity is generated by combusting hydrogen in a gas turbine with minimal CO2 emissions.

Further efficiencies are gained by using waste heat to power a steam turbine. Compared to oxyfuel combustion, this process requires much less oxygen per unit of fuel feedstock or net power output as CO2 can be recovered in a dry condition, at moderate pressure with the use of little or no steam. The result is a significant reduction in both the CO2 compressor capital and power requirements, which reduce net capacity and efficiency losses. Additionally, the hydrogen produced in this process can be used to generate electricity in a fuel cell, a promising attribute for future generation technologies. In principal hydrogen can be produced out of any fuel, either of fossil origin or from biomass. The carbon dioxide can also be removed from the hydrogen by a physical recovery process. The carbon dioxide is recovered in almost pure form.

The most logical power plant technology to apply pre-combustion capture technology will be the integrated coal-fired combined cycle (IGCC). Two additional steps need to be added to this process: shift from the coal gas to a carbon dioxide rich gas stream and separation of the carbon dioxide from the hydrogen. Capture is also applicable to natural gas-fired combined cycle plants (NGCCs). It should be realized that for the latter type of plants the natural gas should be converted in steam reformer reactor to a synthesis gas (i.e. an additional step compared to capture from IGCC), and the carbon content is considerably lower in natural gas than in coal. These aspects make it generally more expensive for natural gas than for coal per Kg of carbon dioxide captured.

Pre-combustion has a competitive advantage in the energy market: IGCC (coal gasification) has the highest potential for economically feasible CO2 capture applications (for both existing and new plants). The costs for CO2 capture for an IGGC plant range from 20 to 25 /tonne CO2 for new build, while the costs for conventional coal plants range from 30 to 50 /tonne CO2 .

Near-term applications of CO2 capture from pre-combustion systems will likely involve physical or chemical absorption processes, with the current state of the art being a glycol-based solvent called Selexol.

Mid-term to long-term opportunities to reduce capture costs through improved performance could come from membranes and sorbents currently at the laboratory stage of development. Membrane separation units that can selectively permeate H2 and retain CO and CO2 are also promising for IGCC power plants. Under US-Department of Energy funded research, ionic liquid membranes and absorbents are being developed for capture of CO2 from power plants. Ionic liquid membranes have been developed at NETL that surpass polymers in terms of CO2 selectivity and permeability at elevated temperatures for pre-combustion applications. A novel CO2 capture membrane is being developed by Los Alamos National Laboratory and SRI International. This polymeric membrane system is being developed to create a pre-combustion capture system that can operate at higher temperatures and pressures than the state-of-the-art Selexol-based system and therefore reduce parasitic power loss and capture cost.

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