Kwangkook Jeong, a graduate student in mechanical engineering, works on the heat-exchanger project at a coal-fired power plant.
The technology, which utilizes six “staged” heat exchangers, was invented by engineers at Lehigh’s Energy Research Center
(ERC) and tested successfully last year at a coal-fired power plant in the eastern U.S. and at one of Lehigh’s two campus boilers facilities.
ERC senior research scientist Harun Bilirgen and ERC director Edward Levy said the unique deployment of the heat exchangers could prove beneficial to power plants in the western U.S.
Unlike power plants in eastern states, those in the more arid West often lack access to rivers, lakes and other sources of water needed for cooling. In addition, the subbituminous and other “low rank” coals from western states contain more moisture than the bituminous and anthracite coals that are more common in eastern states.
The heat exchangers in the new design, Bilirgen and Levy say, will recover water vapor from the flue gas exiting the power plant via the stacks, or chimney and condense the vapor into water.
The temperatures in the exchangers are set so that the water vapor and the vapors from toxic acids, especially sulfuric acid, condense in separate exchangers.
The recovered water is thus relatively untainted and can be used, with some treatment, in the power-plant process to replace water that evaporates from the power-plant cooling tower.
“As water grows more scarce,” said Bilirgen, “water recovery is becoming important for many countries as well as for states in the western U.S.
“We were able to recover 50 to 80 percent of the water from the flue gas in tests conducted to date. We believe this will ultimately make it possible to provide as much as 20 percent of the cooling water needed for a 600-MW power plant boiler burning Powder River Basin (Wyoming) coal and 30 percent of the cooling water needed for the same unit if it’s burning lignite.”
The new design will also make it possible to improve power-plant efficiency, reduce mercury emissions (a major concern with coal-fired plants) and capture sulfuric, hydrochloric and other acids.
And the availability of low-temperature flue gas with reduced acid and water vapor content would also reduce the costs of capturing carbon-dioxide in back-end amine and ammonia CO2 scrubbers, the researchers said.
“The heat exchanger series will be located along the duct that carries flue gas to the stacks,” said Bilirgen. “The existing temperature of approximately 300 degrees F in this duct will drop to less than 110 degrees F after the exchangers.
“This will be very helpful for post-combustion carbon-capture techniques, which require a flue gas temperature of 110 degrees F or lower for efficient operation.”
The ERC tests were conducted at a power plant that fires high-moisture coal and at one of the university’s boiler on Packer Avenue, which, together with boilers on the Mountaintop Campus, burns oil and natural gas to provide steam heat to the campus.
The experiments were part of a 30-month project funded by the U.S. Department of Energy (DOE).
The key advantage of the new ERC technology, said Levy and Bilirgen, is its ability to condense water vapors and acid vapors in separate heat exchangers.
The separation is made possible by the different temperatures at which water and sulfuric acid condense from a vapor to a liquid. Water in flue gas condenses at 95 to 130 degrees F while sulfuric acid condenses at 220 to 310 degrees F.
By exploiting this difference, the ERC has been able to control where the acids condense and to keep these liquids separate from the condensed water.
“We used six heat exchangers to reduce the flue gas temperature in stages,” Levy said at the ERC’s Annual Fossil Power Plant Technology Meeting held in May at Lehigh. The temperature dropped from 300 degrees F in the first exchanger that the flue gas enters to between 80 and 100 degrees F in the last exchanger, he said.
“The sulfuric acid condensed first in the hottest exchanger and continued to condense over the entire range of temperatures. The hydrochloric acid and nitric acid vapors condensed out with the water vapor at the lower temperatures.”
The segregated condensation, said Bilirgen, helped the ERC researchers overcome a problem that had complicated previous efforts to recover usable water from the flue gas.
“Other researchers have tried various heat-exchanger concepts,” said Bilirgen. “But all these concepts have been susceptible to corrosion damage to the heat exchanger tubes due to combined acid and water condensation.
“Because our technology condenses the water and the acids in different places, we end up with relatively clean water in the last three exchangers, where the water vapor is condensed. With some treatment, this water can be reused in the power-plant process.”
The reduction of flue gas temperature will also improve the power plant’s thermal efficiency, said Bilirgen.
“A considerable amount of waste energy was recovered by all of our heat exchangers. Flue gas typically leaves the system at 300 degrees F. We were able to reduce that to 110 degrees F and gain almost 200 degrees F in recovered heat that could be recycled back into the system to preheat the feed water. This would lead to a 1- to 2-percent improvement in turbine cycle heat rate and in overall power plant thermal efficiency.”
Curbing mercury and acid emissions
The new heat-exchanger system also resulted in a 60- to 65-percent reduction in total mercury emissions at the coal-fired power plant where tests were conducted. Bilirgen believes that the mercury reduction in the flue gas resulted from condensation of mercury through the heat exchangers. However, he said that further research and experiments need to be done to ensure the accuracy and repeatability of the field test results for such reductions in mercury emissions.
Mercury, a toxic element, is difficult to control because it is harmful in quantities as small as parts per billion. It enters the atmosphere as a gas, precipitates with rain and falls into bodies of water, where it is ingested by fish. People who consume contaminated fish can develop brain and nervous ailments. Coal-fired power plants are the largest known source of mercury emissions in the U.S. The U.S. Environmental Protection Agency has issued regulations mandating mercury reductions from power plants of 23 percent by 2010 and 69 percent by 2018.
The ERC researchers are continuing their studies of the condensing heat exchangers and their application to power plants.
“Over the next several years,” said Levy, “much of our effort will be focused on determining which materials to use for heat exchanger tubes in the high- and low-temperature regions of the heat exchanger. There would be obvious benefits if we can minimize the use of expensive corrosion-resistant tube materials. We are also beginning to scale up our heat-exchanger design for use in full-size power plants.”
Several mechanical engineering graduate students have worked on the heat-exchanger project. They include Christopher Samuelson, Kwangkook Jeong, Michael Kessen and Christopher Whitcombe.
Posted on Wednesday, July 16, 2008