Novel Natural Gas Plant Design Attains Over 99% Carbon Capture

Natural gas-fired combined cycle (NGCC) power plants are popular for their efficiency, but natural gas generation emits high levels of carbon dioxide (CO₂). To address this problem, University of Pittsburgh researchers developed a hybrid carbon capture system for NGCC plants that can capture more than 98% of carbon dioxide during peak- and off-peak operations. 

 

Natural gas combined cycle plant. Image used courtesy of Wikimedia Commons

 

The model system, which can be retrofitted for existing NGCC plants, integrated a solid sorbent material downstream of a membrane system. The membrane is the primary point-source capture system, while a sorbent counterpart captures CO₂ from the air and membrane exhaust. The combined solution is adaptable to changing grid conditions.

 

NGCC Plants: Design, Efficiency, Emissions

In conventional simple-cycle combustion turbine plants, hot gas generates electricity through a turbine. Newer combined-cycle systems use simple-cycle turbines, heat recovery, and steam turbine generators for improved efficiency and dispatch times. NGCC plants typically have a higher capacity factor than simple-cycle systems, supplying base and intermediate loads to the grid. NGCC systems combining steam and combustion turbines represent more than half of U.S. natural gas-fired generating capacity, according to the U.S. Energy Information Administration.

Since gas power plants can turn on and off quickly, they’re ideal for responding to seasonal and short-term demand spikes. In the transition to renewables, natural gas plants serve a critical role as dispatchable resources to counter intermittent solar and wind generation. NGCC plants operate efficiently at a partial load, ensuring electricity generation stays stable as more variable resources join the grid. This pivotal role as a transition fuel keeps natural gas relevant in the electricity mix, while coal’s share has steadily declined. Natural gas represents about 40% of electricity generation in the U.S. and more than 20% globally. 

NGCC plants still have a significant carbon footprint, even though combustion in the gas turbine produces less emissions than coal-fired power plants. Plants have installed various carbon capture and storage (CCS) technologies with high capture rates to mitigate emissions. For example, some use post-combustion capture systems to pull CO₂ from the flue gas via a solvent. 

The proposed design by the University of Pittsburgh researchers boasts high capture rates and easy cycling between load conditions—meeting two vital requirements for successful CO₂ capture technologies at NGCC plants. 

 

Researchers developed a novel hybrid membrane + solid sorbent system design for NGCC plants. Image used courtesy of the University of Pittsburgh

 

Introducing a Hybrid Carbon Capture System 

First, the natural gas exhaust is processed by a membrane CCS system and then by a solid sorbent material. The researchers integrated the models into one optimization platform to achieve the highest net present value and capture rate. 

 

Optimized results for a natural gas power plant using the hybrid membrane and solid sorbent system design. Image used courtesy of the study’s authors (Creative Commons) -- graphical abstract

 

In selecting adsorbers, the researchers opted for solid sorbents rather than aqueous solvents, as the former can accommodate low CO₂ concentration capture with less energy and operating costs. Their solid sorbent would need to capture CO₂ under conditions ranging from unpressurized air with 400 parts per million (ppm) of CO₂ to NGCC exhaust with about 40,000 ppm of CO₂. 

For the solid sorbent system, the researchers selected a tetraamine-appended metal-organic framework (MOF) material developed by University of California–Berkeley researchers who demonstrated the MOF for CO₂ partial pressures from 400 ppm to 50,000 ppm. The material offers stable and high adsorption capacity under humid conditions and a 90% capture rate when used on post-combustion natural gas exhaust. It’s also capable of adsorption at high temperatures, with a smaller temperature swing between adsorption and desorption, thus reducing operating costs. It can be regenerated using low-temperature steam, making it ideal for an NGCC plant.

When combined with a membrane, the integrated system provides high carbon capture rates and additional flexibility to grid dynamics. Membranes are well-suited for variable electric grids because they respond quickly. Carbon capture membranes can also recover CO₂ at high purity with little energy. However, since these systems may struggle to attain high CO₂ capture rates independently, the researchers selected a fast-responding solid sorbent-based technology to offset the CO₂ from the membrane process. 

 

How Does the Integrated System Work Together? 

In developing the solid sorbent system, the researchers modeled fixed bed adsorbers containing MOF solid sorbents that adsorb CO₂. Using steam from the plant, the sorbents then perform temperature swing desorption. The MOF bed adsorber design alone can offer 86.6% carbon capture during peak operation and 85.4% off-peak. Combining it with the membrane system boosts the capture rate to over 98%. 

 

The top two charts compare the system’s power output and efficiency performance in Modes 1 and 2. The bottom chart shows the capture rates for each component. Image used courtesy of the study’s authors (Creative Commons) -- Figures 9 (top) and 10 (bottom) 

 

The membrane captures CO₂ from the NGCC exhaust gas, while the solid sorbent removes CO₂ from the membrane system under Mode 1 operations with high load conditions. In Mode 2 (low load conditions), the solid sorbent draws CO₂ from the membrane system exhaust and the compressed air (via the surroundings and NGCC compressor). The system offers a 99.3% carbon capture rate from the inlet natural gas stream during high electricity demand and 99.6% during low demand conditions.

Compared to a baseline NGCC system without carbon capture, the novel system offers a higher net present value. This means its performance is more profitable than operating without it in a future scenario that assumes CO₂ taxes of $100 per tonne are imposed. Also, the proposed system captures more carbon with less emissions than an alternative CCS system using an amine solvent with a 90.7% capture rate. 

The researchers mentioned that net-negative emissions could be achieved in future projects by selecting a solid sorbent better suited to DAC conditions, enabling more air to be processed.