Pre-combustion capture is applicable to IGCC power plants and refers to removal of the CO2 from the syngas prior to its combustion for power production. A simplified process schematic for pre-combustion CO2 capture is shown below.
|Process Schematic of Pre-combustion Capture (click to enlarge)
Unlike a combustor, a gasifier carefully controls the amount of air or oxygen available inside it so only a small portion of the fuel burns completely. This "partial oxidation" process provides heat to drive gasification reactions. Rather than burning, most of the coal is chemically broken apart by the heat and pressure in the gasifier, setting into motion chemical reactions that produce syngas. Syngas is primarily H2 and carbon monoxie (CO) but can include other gaseous constituents whose compositions vary depending on coal characteristics and the conditions in the gasifier. After the syngas is produced, it is further processed in a WGS reactor to prepare it for pre-combustion capture. A WGS reactor is typically a fixed-bed reactor containing shift catalysts to convert CO and water into additional H2 and CO2. Following WGS, sulfur compounds and CO2 are separated from the H2 in an AGR system. Acid gases in a gasification process typically consist of hydrogen sulfide (H2S), carbonyl sulfide (COS), and CO2. Sulfur compounds and CO2 can be removed either simultaneously or selectively, depending on the shifted syngas composition and conditions, as well as the end fuel gas specifications. After CO2 removal, the H2 is used as a fuel in a combustion turbine combined cycle to generate electricity. Another application, currently being developed under DOE's Fuel Cell Program, utilizes the H2 to power solid oxide fuel cells (SOFCs) to significantly increase the overall plant efficiency.
The current state-of-the-art pre-combustion CO2 capture technologies that could be applied to IGCC systems (the glycol-based Selexol™ process and the methanol-based Rectisol® process) employ physical solvents that preferentially absorb CO2 from the syngas mixture. Several Selexol and Rectisol systems are in use at commercial scale, though not at IGCC power plants. For example, the Rectisol system is used for CO2 capture at the Dakota Gasification Company's substitute natural gas (SNG) plant in North Dakota, which is designed to remove approximately 1.5 million tons of CO2 per year from the syngas. The CO2 is purified, transported via a 320-kilometer pipeline, and injected into the Weyburn oilfield in Saskatchewan, Canada.