The efficient capture of a greenhouse gas

In the past year 35 years, Lehigh’s Energy Research Center (ERC) has developed a variety of technologies and solutions that improve the operating efficiency of power plants while reducing emissions of toxic substances and greenhouse gases.

One ERC technology has achieved a 70-percent reduction in mercury emissions from coal-fired power plants by modifying the physical conditions of boilers. A second promotes the capture of toxic acids, and the capture and reuse of water, by condensing water and acid vapors in separate heat exchangers. A third limits costly slagging on boiler tubes by integrating optical technologies and artificial intelligence.

And Boiler OP, a combustion optimization technology developed at the ERC in the mid-1990s, has been implemented at more than two dozen U.S. power plants, a coal-powered plant in China, and an oil-fired plant in Mexico.

Recently, the ERC was awarded a grant from the U.S. Department of Energy (DOE) to develop methods of recovering and reusing the heat that would be generated by the carbon-dioxide (CO2) compression process in a carbon capture system. Carbon capture, which involves the removal of CO2 from power-plant flue gas, would make it possible to generate electric power from coal while avoiding significant emissions of CO2 to the atmosphere.

The 30-month grant is being awarded through DOE’s National Energy Technology Laboratory (NETL). The funding was provided by the American Recovery and Reinvestment Act of 2009, the economic stimulus package passed by Congress in February.

The goal of the research project, says ERC director Edward Levy, is to facilitate carbon capture and sequestration, or storage (CCS), and thus limit the amount of CO2, a greenhouse gas, emitted into the atmosphere by coal-fired power plants.

Towards that end, the ERC will use the DOE grant to train graduate students to develop computational models of the methods that are used to capture and compress CO2 and to estimate the increases in efficiency that will result from each method.

The benefits of reusing captured heat

Coal-fired power plants produce half the electricity in the U.S. and account for about 75 percent of total power generation in China, which leads the world in coal consumption. At a coal-fired plant, finely ground coal is mixed with air and burned in a boiler, or furnace, where it heats water in the boiler pipes into steam that spins turbines to generate electric current. Meanwhile, carbon from the coal reacts with oxygen in the air to form CO2, which exits the power plant with the flue gas and enters the atmosphere.

CCS technologies separate CO2 from the flue gas and then compress the CO2 to high pressure. Compressed CO2 can be transported by pipeline and is currently used to help extract oil from underground reservoirs in a process known as enhanced oil recovery. Scientists are also evaluating the feasibility of injecting compressed CO2 one or two miles below the earth’s surface into saline aquifers whose geological features would sequester the CO2 underground.

The goal of the current ERC project, says Levy, is to recover heat that is generated when CO2 is compressed and to use that heat to improve the efficiency of the power plant’s operation.

“It requires a tremendous amount of pressure, about 2,200 pounds per square inch or close to 150 atmospheres, to compress CO2 to a supercritical state,” says Levy. “In the compression process, CO2 heats up, creating the potential for heat to be recovered and used beneficially within the power plant.

“All carbon capture schemes reduce power plant efficiency and increase the cost of generating electricity. We’re trying to mitigate this. We’re looking at different types of compressors to see how much heat can be recovered and what we can do with this heat to improve power plant efficiency.”

The ERC has conducted other research projects that promoted the reduction of carbon emissions. One project involved the recovery of water from flue gas and another removed water from high-moisture coals. Both resulted in improvements in power plant efficiency and reduction in the rates of CO2 formation.

In the new project, students will use a software tool called ASPEN, which is used widely to help engineers model chemical processes.