Advantages of Gasification - Designs for CO2 Capture
How Plant Design Changes
Gasification has a distinct advantage with addressing greenhouse emission reduction compared to conventional coal power plants, as its overall process scheme can be easily modified (or retrofitted) to allow for economic pre-combustion carbon dioxide (CO2) capture. All gasification-based conversion processes require removal of hydrogen sulfide (H2S; an acid gas) from the synthesis gas (syngas) as part of the overall plant configuration. Typical acid gas removal (AGR) processes employed for gasification design are either a chemical solvent system (e.g., methyl diethanolamine [MDEA]) or a physical solvent system (e.g., Rectisol or Selexol). Process selection is mostly dependent on the syngas cleanup requirement and costs. Conventional chemical/physical AGR processes using MDEA, Rectisol or Selexol are commercially proven technologies and can be designed for selective removal of CO2 (also an acid gas), in addition to H2S, from a syngas stream. For significant capture of CO2 from a gasification plant (e.g., > 80%) the CO in the syngas must first be converted to CO2 and hydrogen (H2) via a water-gas-shift (WGS) process upstream of the AGR plant.
NETL has recently completed a comprehensive study comparing the performance and cost of various fossil fuel-based power generation technologies, with and without CO2 capture1. The study has a companion set of integrated gasification combined cycle (IGCC) designs, using GE’s gasification technology, which can be used to illustrate the design changes needed for CO2 capture.
Current Technology – IGCC Plant Design
Figure 1 shows a simplified block flow diagram (BFD) of a market-ready IGCC design without CO2 capture. As shown, the IGCC plant consists of the following processing islands, of which a more detailed description of each can be found in the cited NETL referenced report: 1
- Gasification – GE Energy (GEE) based technology, operating in radiant cooling mode, with high temperature syngas heat recovery.
- Air Separation Unit (ASU) – Integrated into the power train extracting 16% of its air from the gas turbine compressor and producing a high-pressure nitrogen (N2) stream for turbine combustor NOX control.
- Quench and Syngas Scrubbing – For additional syngas cooling and the removal of entrained fine particulates as well as water soluble contaminates (e.g., ammonia and chlorides) from the syngas.
- COS (carbonyl sulfide) Hydrolysis – Converts the carbonyl sulfide (COS) in the syngas to H2S before sulfur is removed downstream by the acid gas removal (AGR) unit.
- Gas Cooling, Boiler Feed Water (BFW) Heating, and Knockout – Cools the syngas leaving COS hydrolysis, before it is fed to Hg removal and the AGR unit, by heat exchanging with the boiler feed water. The produced water condensate is sent to the sour water stripping plant.
- Sour Water Stripper – Takes the produced water/condensate from the low-temperature cooling processes, including that from the scrubber, and strips off the gas contaminants such as H2S, ammonia (NH3) and CO2 etc., before the water can be sent onto the plant waste water treatment facility.
- Mercury Removal – via activated carbon bed.
- Selexol – A conventional Selexol unit is used for H2S removal.
- Claus Plant & Hydrogenation Reactor and Gas Cooler – A conventional Claus process to recover sulfur from H2S as a salable solid sulfur product, followed by hydrogenating the Claus tail gas and recycling it back to the Selexol unit.
- Syngas High Pressure Reheat/Syngas Expander – Heat removal and expansion of the high pressure, clean syngas to recover additional energy before sending it onto the power train.
- Combined-Cycle Power Train - GE’s advanced F Class gas turbine, which includes a heat recovery steam generator (HRSG) and a steam turbine.
|Figure 1 – IGCC without CO2 Capture Block Flow Diagram (click to enlarge)
IGCC Design Change to Enable CO2 Capture
Figure 2 shows a companion simplified block flow diagram (BFD) of a market-ready IGCC design with CO2 capture. The design is for 90% CO2 capture from the IGCC plant. As shown, in comparison with the no-capture design, there are two key process plant modifications needed:
- Shift Reactors - A water-gas-shift (WGS) reactor is added to replace the COS Hydrolysis plant. This was done to convert all carbon monoxide (CO) in the syngas to CO2 and H2, so that the CO2 can be removed downstream in the AGR plant. Since the WGS catalyst also serves to hydrolyze COS, a separate COS plant is no longer needed.
- Modify the conventional Selexol plant to enable selective removal of both H2S and CO2. In principle, this involves adding an additional absorber column and flash drums onto the plant, of which the technology vendor, UOP, can help with the detailed design, once the syngas flow and composition, and the CO2 product specifications, are clearly defined. Figure 3 shows a simplified BFD for a selective Selexol design for both H2S (acid gas to the Claus plant) and CO2 removal.2
|Figure 2 – BFD of an IGCC Plant Configuration with CO2 Capture (click to enlarge)
An additional processing step of CO2 dehydration and compression (shown in Figure 2 as CO2 Compression) is added to bring the produced CO2 to pipeline transport pressure and purity specifications.
Figure 3 – BFD of a Selective Selexol Design for H2S and CO2 Removal