Researchers at the University of North Dakota Energy & Environmental Research Center (EERC) announced in February major strides in coal power plant efficiency and emission reductions by burning a combination of pure oxygen and coal to generate electricity in an advanced, super-heated power system. The process could be the pathway to development of zero emission coal plants.
The material used in the system — oxide dispersion strengthened (ODS) super alloy — is able to withstand the extremely high temperatures demanded by the process. It is the same alloy used to make F-16 fighter jet engines, and this is the first time the material has been used in a coal-fired power system.
In a demonstration conducted at the EERC, a combination natural gas/coal-fired system was used to test a very high temperature heat exchanger – the heart of an advanced high-efficiency power plant. The technology is known as “indirectly fired combined cycle” (IFCC). In theory, the process could produce zero emissions. The process also represents a step beyond existing subcritical, supercritical and ultrasupercritical technology, which operate at steam temperatures of 1000-1100 F and steam pressures of 2600-4000 psi.
A plant using IFCC technology burns coal to heat air to a much higher temperature in the heat exchanger and uses the hot air to turn the turbine, resulting in a significantly higher efficiency. The EERC has used the system’s heat exchanger to produce air at 2000 F, considered to be a world record for pressurized coal-fired heat exchangers. In an IFCC system, the hot air would be sent to a gas turbine, and the waste heat used to produce steam for a steam turbine. The system is much like a natural gas-fired combined-cycle plant, except that coal is used to heat the gas entering the turbine, rather than firing natural gas. Because an IFCC plant produces only half as much steam as a typical steam-cycle plant, water requirements are also cut in half.
“Results of previous demonstrations while firing with air prove that the efficiency of a power plant could improve by about a third using this technology and produce cheaper, cleaner energy,” said EERC Senior Research Manager John Hurley.
“This technology offers a potentially cost-effective method to enhance efficiency, reduce emissions, and contribute
to a cleaner, healthier environment worldwide,” said EERC Director Gerald Groenewold. Because of its high efficiency, an IFCC plant is a natural choice for operation with oxygen-blown firing to significantly reduce emissions.
The benefit of using pure oxygen is that excluding nitrogen from the flame would result in a 50% reduction of NOx. Furthermore, the volume of gas is reduced, so the plant could be much smaller than an air-blown plant, and less expensive to construct. Although firing with pure oxygen rather than air reduces system efficiency because of the energy required to make the oxygen, the use of pure oxygen produces flue gas consisting almost totally of water vapor and carbon dioxide. The former could be collected and stored for reuse within the plant while the later could be sequestered, leaving no gas to be emitted to the atmosphere.
“If a time comes when taxes are imposed on carbon dioxide emissions, it will be very important to remove the CO2 from the gas stream and sequester it, and that will be much easier if the system is fired with oxygen rather than air,” said Hurley.
While the EERC system has been tested with several types of coal, it can also be configured to use a variety of fuels such as biomass (fast-growing trees and grasses, and municipal waste) or liquid fuels. Through the Xcel Energy Renewable Development Fund, the EERC has recently completed tests using 20% biomass and 80% sub-bituminous coal. In addition to the Xcel funding, much of the work is funded through the U.S. Department of Energy and managed through the DOE National Energy Technology Laboratory.