Front-End Engineering Design (FEED) Study for a Carbon Capture Plant Retrofit to a Natural Gas-Fired Gas Turbine Combined Cycle Power Plant (2x2x1 Duct-Fired 758-MWe Facility with F Class Turbines)

Abstract

A comprehensive front-end engineering design (FEED) study has been undertaken for a post-combustion capture (PCC) unit located at Panda’s Sherman natural gas–combined cycle (NGCC) power plant in Sherman, Texas. This is described in a full and unredacted FEED study report with all supporting documents, numbering over 150. The PCC unit uses 35 wt% MEA as the solvent to capture 85% of the CO2 in a slipstream, corresponding approximately to the flow at minimum stable generation to maximize use of the new investment. MEA was selected because extensive information is available for it in the public domain and a previous NGCC retrofit study has shown that it would be comparable in performance and overall costs to a range of proprietary solvents. It is also widely available at low prices. At the design flue gas throughput of 704 kg/s, the PCC unit captures 129 t/h of CO2. Energy consumption for solvent regeneration is estimated as 3.65 GJ/tCO2 (of which 0.14 GJ/tCO2 is provided by waste heat recovery from the CO2 compressors), with design lean and rich loadings of 0.254 and 0.475 mole CO2/mole MEA respectively, and an absorber liquid/gas ratio of 1.07. To maintain a low level of degradation products in the circulating solvent, a semi-continuous thermal reclaimer is used to process the equivalent of a complete plant solvent inventory every 28 days. Solvent recovery in the reclaimer is estimated at ≥90%, with net solvent consumption 2 kg MEA/tCO2 captured. Reclaimer bottoms disposal costs are estimated at $500/t. This cost has been estimated at the high end of liquid hazardous waste haulage and disposal due to lack of actual waste characterization. In addition to heat recovery from the reclaimer, energy consumption for solvent regeneration is reduced by flashing some of the rich solvent, using heat from the CO2 compressor intercoolers, and returning the resulting semi-lean solvent partway down the absorber. The overall capital cost for the PCC retrofit is estimated at $477M, including indirect costs, owner’s and contractor’s costs, and interest during construction; completion is 30 months from notice to proceed. A centrifugal compressor, send-out pump, and dehydration are used, delivering 151 bara CO2. Estimated baseline CO2 capture costs (both annualized capital and annual O&M together) are $114.50/tCO2, dominated by capital recovery charges. For a 70/30 debt-to-equity ratio with 6% interest rate on debt over 15 years and 12% return on equity, plus PCC operation for an average of 5,000 hours per year, these total $83.10 $/tCO2 for just the annualized capital portion. Power plant net output is reduced by 67.3 MW when supplying PCC steam and electricity requirements. The PCC plant is not operated during those limited periods when the Texas ERCOT power grid has elevated prices (up to $9,000/MWh) (nor when the power plant is not operating), and average foregone electricity revenues are assessed at $25/MWh, contributing $13.00/tCO2 captured. At the time of this FEED study, no full-scale NGCC power plants with PCC have been built anywhere in the world. This leads to a lack of data for process simulation model validation under conditions of interest for commercial NGCC+PCC plants, that is, low CO2 concentrations, relatively low absorber packing heights, and low liquid-to-gas ratios. MEA concentrations above 30 wt% have also received little previous attention in public domain testing. Slight differences in performance modeling predictions were observed between ProMax® and the MEA Steady-State Model from the Carbon Capture Simulation Initiative (CCSI) running under Aspen Plus® 10, leading to some uncertainty in precise values for the FEED design parameters. In addition, model predictions do not consider the actual variation in performance that can be expected under nominally constant process conditions. A pilot testing program is therefore proposed to resolve most of these design uncertainties, generally duplicating all process elements of the full-scale PCC unit apart from CO2 product compression. The pilot plant will operate solely on flue gas from the Sherman power plant, accommodating the same frequent shutdowns of up to 48 hours. The primary pilot plant test objectives are to confirm the base case mass and energy balances for the PCC unit under design steady-state conditions; assess long-term solvent management requirements and reclaimer performance; confirm emissions levels of MEA, ammonia, and other degradation products in the absorber vent gas during long-term operations. Initial confirmation of CO2 recovery and heat and mass balances should be possible within a 30- to 60-day test program, but credible confirmation of amine degradation rates, reclaiming effectiveness, emissions rates, and system corrosion should be based on at least a 12-month run, but with additional runs—for example, at reduced or increased packing heights—the overall test program would last 18–24 months.