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Carbon capture & storage (CCS) with high capture rates (90-99%) will be necessary to keep natural gas around in a low-carbon future. Furthermore, as intermittent renewable energy expands, natural gas power plants will have to operate at low-load conditions more frequently. To meet these demands, we propose a natural gas combined cycle (NGCC) power plant with a hybrid CCS system that can attain high capture rates and easily cycle between high-load and low-load conditions. More specifically, we propose an integrated system where natural gas exhaust is first processed by a membrane CCS system and then by a solid sorbent CCS system. We integrated models of an NGCC plant, a membrane carbon capture system and a solid sorbent carbon capture system into a single optimization platform to maximize the net present value (NPV) and carbon capture rate of the integrated system. The optimization results indicate a 99.3% carbon capture rate from the inlet natural gas stream at high-load conditions, and a 99.6% carbon capture rate from the inlet natural gas stream at low-load conditions. Compared to a baseline NGCC system without carbon capture, the integrated system has a higher NPV, which indicates that performing carbon capture is more profitable than operating an NGCC system without carbon capture in a future with CO2 taxes. Compared to a baseline NGCC system with an amine-based CCS system that performs 90.7% carbon capture, the proposed integrated system captures more carbon and emits less CO2, while still maintaining a competitive CO2 capture cost.

Novel carbon capture systems are necessary to help natural gas power plants approach net zero CO2 emissions. We propose a hybrid carbon capture system attached to a natural gas combined cycle (NGCC) power plant that consists of a membrane system and a solid sorbent system, with this work focusing on the design of the solid sorbent system. We modeled fixed bed adsorbers that are packed with metal-organic framework (MOF) solid sorbents that adsorb CO2 and undergo temperature swing desorption using steam from the power plant. Parametric simulation of adsorber conditions showed that ten 5-m diameter beds adsorbing in parallel with 4.9 bars inlet gas pressure and 1.5 bars of steam pressure at a flow rate of 0.15 kmol/s led to optimized performance. This study enabled us to determine that the MOF bed adsorber can attain 86.6 % and 85.4 % carbon capture during peak and off-peak operation, respectively. When combined with the membrane capture system, this results in overall capture rates of 98.4 % and 98.9 % during peak and off-peak operation, respectively. Although we were unable to attain net-zero or net-negative emissions in this study, we are confident that net-negative operation could be obtained in future work by selecting a solid sorbent better suited to direct air capture conditions so that more air could be processed by the solid sorbent system.