The agency recently took another step in that direction, publishing a white paper detailing ways gas-plants can reduce their CO2 emissions.
The paper presents opportunities for emission cuts, like the co-firing of natural gas with alternative fuels such as hydrogen; using carbon capture, utilization, and storage (CCUS) technologies; and the co-location with energy storage. Although it doesn’t set policy or establish standards, the EPA “anticipates that the white paper may be useful to inform future rulemaking efforts.”
The Clean Air Act requires new engines and equipment sold or distributed in the United States to be certified to meet EPA-established emissions requirements to protect public health and the environment from air pollution.
Under the latest standards from 2015, new natural gas power plants can emit no more than 1,000 pounds of carbon dioxide per MW of electricity produced. New coal-fired power plants can emit no more than 1,400 lbs CO2/MWh. The standards apply to sources on or after the date of publication of the proposed standards, June 18, 2014.
It’s not clear to what extent the new EPA rules will rely on CCUS technology. The agency’s white paper notes while there is increased interest in carbon capture for natural gas-fired plants, most CCUS efforts have focused on coal plants.
Examples of carbon capture installed on coal plants include the slip stream capture facilities AES Warrior Run in Maryland and AES Shady Point in Oklahoma. In both cases, the captured CO2 is used in the food processing industry. The EPA also cited Southern Company’s Plant Barry in Alabama and AEP’s Mountaineer in West Virginia as having CCUS technology.
Use of CCUS on combined cycle plants include the Bellingham, Massachusetts plant, which used Fluor’s Econamine FG PlusSM capture system. The 40 MW slipstream capture facility operated from 1991 to 2005 and captured 85 to 95 percent of the CO2 for use in the food industry.
The paper notes some emission control technologies that are currently available and some still in the research and development phase.
EPA officials noted the U.S. Department of Energy (DOE), utilities and other organizations are developing processes that use solvents, polymeric membranes, a combination of the two, or solid sorbents for separating and capturing CO2.
Fuel cells configured for emissions capture have also emerged as a CCUS technology. In this process, the flue gas from a plant is “routed through a molten carbonate fuel cell that concentrates the CO2 as a side reaction during the electric generation process in the fuel cell.”
The white paper also mentions oxygen combustion, the use of a mixture of oxygen and recycled flue gas in place of ambient air for combustion. An oxy-combustion power plant consists of an air separation unit (ASU), which generally requires a significant amount of energy. However, alternative oxygen separation methods are being researched for possible commercial-scale development. These include ion transport membranes (ITM), ceramic autothermal recovery, oxygen transport membranes, and chemical looping.
The EPA noted because oxy-combustion produces a flue gas that contains primarily CO2 and water vapor, minimal post-combustion cleanup is required prior to compression, transportation, and injection for use in geological storage, enhanced oil or gas recovery, or some other use. However, a potential constraint of oxy-combustion is the ability of the air separation unit to respond to variable loads.
The Allam-Fetvedt Cycle, which combusts natural gas with oxygen instead of air, uses supercritical carbon dioxide as a working fluid to drive a turbine instead of steam. This theoretically eliminates all air emissions and inherently produces pipeline-quality CO2 that can be sequestered.
There are several announced commercial projects proposing to use the Allam-Fetvedt cycle. These include the 280-MW Broadwing Clean Energy Complex in Illinois and the 280-MW Coyote Clean Power Project on the Southern Ute Indian Reservation in Colorado. Final investment decisions on the U.S. projects are expected in 2022 and commercial operations could commence by 2025.
EPA is asking for public comment on the white paper through June 6, 2022.
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