Carbon Capture by Allam Power Cycle: A Comparative Analysis

Energy generation from natural gas power plants is on the rise. The Allam cycle offers an environmentally friendlier alternative to traditional natural gas combined cycle plants.

Natural gas power plants account for 40% of the electricity generation in the U.S. (1) and 617 million tons of CO₂ emissions per year (2). While energy generation from wind and solar continues to grow, 19.3 GW of natural gas electricity capacity was added to the grid in 2018, accounting for more than 60% of the total capacity added that year from all sources of electricity (3).

Of that 19.3 GW of additional capacity, almost 90% utilized natural gas-fired combined cycle (NGCC) technology (4), which relies on combined Brayton and Rankine cycles to increase the efficiency of a traditional simple-cycle turbine. Due to the massive scale of the natural gas market and the threat of climate change, efforts are increasing to develop technologies to capture and sequester CO₂ from natural gas power plants.

Carbon capture in natural gas power plants. Post-combustion capture, where CO₂ is separated directly from the fluegas using amine-based absorption, is the most popular carbon capture technology. The Petra Nova project in Texas captured 1.6 million tons of CO₂ per year from post-coal-combustion fluegas, before shutting down due to the decline in oil prices that occurred during the COVID-19 pandemic. While Petra Nova demonstrated the advantage of retrofitting existing plants with carbon dioxide removal (CDR) systems, there are significant energetic and economic costs associated with the large amount of fluegas that must be processed, the heat of regeneration in the solvent, and the compression of CO₂ from ambient conditions to pipeline specifications.

The Allam cycle, patented in 2013 by Rodney Allam, utilizes oxyfuel combustion and a supercritical CO₂ stream as the working fluid to produce high-purity liquid CO₂ and energy. NET Power demonstrated the Allam cycle at a 50-MW plant in 2018, but the economics of a larger, 300-MW plant are not well documented. In this article, the economics of both post-combustion capture and the Allam cycle are compared with the more profitable, but less environmentally friendly, traditional NGCC.

This article details a comparative analysis for three natural gas-fired power plants: Allam cycle, NGCC, and NGCC with post-combustion CDR. Each was simulated in Aspen Plus to calculate power, utility, and raw material production/consumption. The economic and environmental impacts of the three power plants were compared under similar thermodynamic performance and costing assumptions. Although rigorous modeling of post-combustion capture in the NGCC is beyond the scope of this project, capital and operating costs of amine scrubbing units are well documented from sources such as the National Energy Technology Laboratory (NETL) (3).

Capturing CO₂, whether via post-combustion or Allam cycle, was determined to be less economical than venting to the atmosphere in a traditional NGCC. However, if tax incentives for capturing CO₂ were to increase, the Allam cycle process could become more economically favorable.

Tax credit 45Q. Section 45Q of the U.S. tax code provides a tax credit on a per-ton basis for CO₂ that is sequestered; this section was updated in the passage of the 2018 Bipartisan Budget Act. The credit is available for 12 years, which begins once the plant is in service (4). For taxpayers who dispose of qualified CO₂ in secure geological storage spaces, an incentive of $22.66 per m.t. was available in 2017 with linear increases to $50 per m.t. by 2026. To qualify for the tax credit, the facility or plant must either properly dispose of the CO₂ in secure geological storage spaces or use it for certain approved purposes, such as a tertiary injectant for a natural gas extraction process.

The lack of carbon capture projects in natural gas-fired power plants is indicative of the shortcomings of the current tax policy and technology. This article calculates a breakeven credit that renders the Allam cycle economically equivalent to the industry-standard NGCC. This breakeven credit could be funded by a CO₂ tax credit or by selling the CO₂ as a feedstock for enhanced oil recovery (EOR), or a combination of both...