Selective Exhaust Gas Recycle with Membranes for CO₂ Capture from Natural Gas Combined Cycle Power Plants

Abstract

Low natural gas prices are contributing to rapid growth in natural gas combined cycle (NGCC) power production in the United States. CO2 capture from the exhaust gas of these plants is complicated by the relatively low CO2 concentration in this flue gas (3%–4%). A membrane process using incoming combustion air as a sweep stream in a selective exhaust gas recycle configuration can be used to preconcentrate CO2 from 4% to 15%–20% with almost no energy input. Depending on the process configuration, the selective recycle membrane design reduces the minimum energy of a CO2 capture step by up to 40%. An all-membrane design using a capture step in series with a selective recycle membrane can capture 90% of CO2 from an NGCC power plant using less energy and at a lower cost than the base-case amine process analyzed by the U.S. Department of Energy. The current state-of-the-art membranes for use in this process have a CO2 permeance of 2200 gpu and a CO2/N2 selectivity of 50. Higher CO2 permeance will improve the economics and reduce the footprint of a membrane CO2 capture system, while higher CO2/N2 selectivity is of less benefit, because the process is limited by the affordable pressure ratio.

Off-design point modelling of a 420 MW CCGT power plant integrated with an amine-based post-combustion CO2 capture and compression process

“…However, the EGR ratio is limited to 35-40% in practice (Li et al, 2011 andMerkel et al, 2013) as increased an increasing EGR ratio reduces the level of oxygen that is available in the air to the combustion chamber. The O 2 concentration in the inlet air to the combustion chamber should be greater than 16% to prevent the production of significant quantities of unburned hydrocarbons and carbon monoxide (Li et al, 2011).…”

Process Analysis of Selective Exhaust Gas Recirculation for CO2 Capture in Natural Gas Combined Cycle Power Plants Using Amines

“…New process alternatives are currently being investigated to overcome these limitations and optimize CO2 capture processes in gas-fired power plants. This is the case of exhaust gas recirculation (EGR) [5][6][7][8][9] and selective exhaust gas recirculation (S-EGR) [10] processes. The EGR process is shown in Figure 1.…”

“…To overcome this limitation and achieve a further increase in the CO2 content of the flue gas, Merkel et al. [10] proposed the application of selective exhaust gas recirculation. The idea behind the S-EGR concept is to selectively recirculate CO2 upstream of the post-combustion CO2 capture process, from the gas turbine exhaust back through the inlet of the engine, thus largely increasing the CO2 concentration of the flue gas sent to the capture plant.…”

“…For this purpose, S-EGR makes use of a membrane that selectively separates the CO2 contained within the flue gas. The two S-EGR schemes proposed by Merkel et al [10] are shown in Figures 2a and 2b, where the membrane and the CO2 capture plant are arranged in parallel or series, respectively. In both cases, a fraction of the flue gas is passed through a membrane, where mainly CO2 permeates and mixes with an air sweep stream that flows countercurrently.…”

“…In both cases, a fraction of the flue gas is passed through a membrane, where mainly CO2 permeates and mixes with an air sweep stream that flows countercurrently. According to Merkel et al [10], the CO2 separation in the membrane is driven by the different CO2 concentration in the permeate and retentate streams, and no compression or vacuum is required, apart from that needed to overcome the pressure drop in the membrane. The CO2-enriched air is then introduced to the compressor of the gas turbine, whereas the remaining fraction of the flue gas that has not been recirculated is taken to a CO2 capture plant.…”