Sorbent Enhanced Water Gas Shift-rethinking Carbon Capture in IGCC

Abstract

A dry sorbent pre-combustion CO2 capture process to reduce carbon emissions and enhance the water gas shift (WGS) reaction in integrated gasification combined cycle (IGCC) power plants was investigated. The approach aims to eliminate the need for WGS reactors by maximizing the amount of H2 through the removal of CO2 and resulting changes in syngas chemical equilibrium. This concept can limit the energy penalty typically required of a WGS reactor by maintaining the higher temperature and pressure conditions present at a coal gasifier outlet and desired at the gas turbine. The sorbent needed for this process must have high reactive surface area, high CO2 capacity, be able to tolerate required operating conditions (as high as 1000 °C and 40 bar), and must have a long lifetime. A first principles approach was taken (thermodynamic and molecular modelling) to develop a list of candidate sorbents. Different approaches were taken to synthesize sorbents, including ultrasonic spray pyrolysis (USP), which led to materials with novel properties. These sorbents were characterized (e.g., SEM, TEC, XRD), screened (e.g., TGA) and ultimately tested in laboratory scale reactor systems at high temperatures and pressures. A technoeconomic assessment based on laboratory results and a sorbent enhanced WGS (SEWGS) process, which takes advantage of the high heat of adsorption (ΔHads) of calcium oxide to generate turbine quality steam, is presented. The additional gross energy output resulting from the ΔHads helps offset the parasitic losses typically encountered for CO2 capture. A slipstream of produced H2 was used to regenerate the calcium sorbent in a ‘regenerating boiler’; waste heat from this operation was recovered. Process designs were evaluated which increased the overall gross energy output of an IGCC by 40%, or from 737 MWe without CO2 capture to 1,028 MWe with CO2 capture for a fixed amount of coal. The energy produced from the ΔHads alone was estimated to contribute 429 MWe. In fact, the overall dynamics of produced energy are shifted from the majority of energy being produced from the gas turbine to a large fraction being produced by the CO2 capture process itself. In order to realize this SEWGS approach, scientific and engineering challenges must be met. Included are well designed adsorption and regeneration reactors which limit thermal shock, can efficiently remove the ΔHads, and can withstand H2 combustion in the presence of a solid sorbent. Advancement in sorbent materials must also continue; sorbent replacement accounts for a large portion of O&M costs. Capital costs were projected to be high, but optimization should reduce this disadvantage. Even in light of these challenges, SEWGS with the ‘regenerating boiler’ concept can approach targeted increases in COE over IGCC without CO2 capture of $81.30 per MWh through a rethinking of energy production in concert with CO2 capture, rather than incremental cost reductions through evolutionary refinement of power generation augmented with CO2 capture. The current optimal SEWGS case has a COE of $97.50 per MWh compared to $119.40 per MWh for IGCC with CO2 capture.