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

Abstract

Post-combustion CO2 capture from natural gas combined cycle (NGCC) power plants is challenging due to the large flow of flue gas with low CO2 content (~3-4%vol.) that needs to be processed in the capture stage. A number of alternatives have been proposed to solve this issue and reduce the costs of the associated CO2 capture plant. This work focuses on the selective exhaust gas recirculation (S-EGR) configuration, which uses a membrane to selectively recirculate CO2 back to the inlet of the compressor of the turbine, thereby greatly increasing the CO2 content of the flue gas sent to the capture system. For this purpose, a parallel S-EGR NGCC system (53% S-EGR ratio) coupled to an amine capture plant using MEA 30%wt.was simulated using gCCS (gPROMS). It was benchmarked against an unabated NGCC system, a conventional NGCC coupled with an amine capture plant (NGCC+CCS), and an EGR NGCC power plant (39% EGR ratio) using amine scrubbing as the downstream capture technology. The results obtained indicate that the net power efficiency of the parallel S-EGR system can be up to 49.3% depending on the specific consumption of the auxiliary S-EGR systems, compared to the 49.0% and 49.8% values obtained for the NGCC+CCS and EGR systems, respectively. A preliminary economic study was also carried out to quantify the potential of the parallel S-EGR configuration. This high-level analysis shows that the cost of some scenarios. However, further benefits with respect to the EGR configuration will depend on future advancements and cost reductions achieved on membrane-based systems.

Techno-economic analysis of a hybrid CO2 capture system for natural gas combined cycles with selective exhaust gas recirculation

“…This is the case of natural gas combined cycle configurations that make use of exhaust gas recirculation (EGR) or selective exhaust gas recirculation (S-EGR) schemes, where a fraction of the inlet air is replaced by a chilled recycled gas flow. As a result, a smaller flue gas flowrate with a higher CO2 concentration is generated [6,7,[9][10][11][12][13][14][15][16][17][18][19]. In the EGR process, this is achieved by taking a fraction of the flue gas exiting the heat recovery steam generator (HRSG) back to the inlet of the compressor, after passing through a cooling and a water knockout stage.…”

“…Therefore, only modest recirculation ratios can be achieved in these systems to avoid issues related to flame stability and/or combustion efficiency [21][22][23][24] that may require major combustor redesign [25]. It is for this reason that many previous works have considered maximum EGR ratios of around 40% [6,11,14,15,18,22,26,27]. This leads to oxygen levels at the combustor inlet no less than 16%vol., thus preventing undesired combustion effects from happening when using combustors currently available [21,22,24,25].…”

“…Both configurations have specific benefits and limitations. The size of the amine capture plant and the area of the selective membrane are substantially reduced in the parallel S-EGR system with respect to the series configuration, because of the decreased flue gas flowrate with high CO2 concentration treated in both units [13][14][15]19]. Nevertheless, the CO2 capture efficiency in the amine system, as well as that in the selective membrane, should be pushed to very high levels (values equal or above 95% are usually considered) to retain a large fraction of the CO2 generated during combustion as well as all the recirculated CO2 and attain the required high overall CO2 capture efficiencies (typically close to 90%) [13][14][15].…”

“…The size of the amine capture plant and the area of the selective membrane are substantially reduced in the parallel S-EGR system with respect to the series configuration, because of the decreased flue gas flowrate with high CO2 concentration treated in both units [13][14][15]19]. Nevertheless, the CO2 capture efficiency in the amine system, as well as that in the selective membrane, should be pushed to very high levels (values equal or above 95% are usually considered) to retain a large fraction of the CO2 generated during combustion as well as all the recirculated CO2 and attain the required high overall CO2 capture efficiencies (typically close to 90%) [13][14][15]. In contrast, the capture plant in the series S-EGR configuration can operate at a much lower capture efficiency (values in between 30 and 58% have been investigated [13,14]), since the subsequent selective membrane unit strips the remaining CO2 needed to achieve the overall capture target (also ~90% in these works) [13,14].…”

“…Only a limited number of works have investigated the potential of coupling NGCC power plants with post-combustion capture systems and the S-EGR processes described above [13][14][15]19]. Merkel et al initially proposed the concept and carried out an analysis of the configurations in Figures 1a and 1b, using Polaris membranes for the selective recycle and a generic CO2 capture unit [13].…”