Public Release: 

Lowering The Temperature On The SOx NOx Rox Box

Penn State

Las Vegas, Nev. -- Penn State researchers have bench and pilot tested a low temperature nitrogen oxide reduction catalyst with an eye to eventual use in small production boiler systems.

"Babcock & Wilcox have a high temperature process that provides combined sulfur dioxide, nitrogen oxide and particulate emissions control for coal fired boilers that they call the SOx NOx Rox Box," says Dr. Andre L. Boehman, assistant professor of fuel science. "We are trying to modify this process to permit combined emissions control at the much lower temperatures common in industrial scale boilers."

The researchers, who include Boehman; T.K. Anand and Manohar Vittal, graduate students in materials science, are testing a commercially available catalyst, supplied by Engelhard Corporation, at low temperatures to try to eliminate the need for flue gas reheating and high-temperature baghouse bags, which are expensive.

Emission control systems for boilers generally consist of a bag house that removes flyash and other particulate materials, a method for removing sulfur from the flue gas and a method of converting nitric oxide produced during combustion to environmentally harmless forms of nitrogen. Typically, a sorbent is injected into the flue gas to remove sulfur before the gas enters the baghouse.

Ammonia is also injected to facilitate the selective catalytic conversion of NOx. Frequently, NOx conversion takes place after the gas leaves the baghouse, but the researchers would like to place the conversion catalyst monoliths inside the baghouse.

The bench scale tests were carried out using synthetic flue gas containing sulfur. The first round of pilot-scale tests was run using a combustor fired by natural gas. The second round of pilot-scale tests will use a fuel containing sulfur. The eventual goal of this U.S. Department of Energy funded project is to test the system in a demonstration boiler.

"We need to ensure that sulfur removal occurs before the flue gas reaches the NOx catalyst, so that the sulfur does not form ammonium sulfates and foul the catalyst," Boehman told attendees at the fall meeting of the American Chemical Society today (Sept. 9) in Las Vegas.

"We are also investigating the variables that control the total proportions of nitric oxide converted and the proportions converted to nitrogen as opposed to nitrous oxide."

The conversion of nitric oxide to nitrogen is governed by temperature and the speed at which gas passes over the catalyst. The presence of sulfur and water also effects the outcome. The researchers can achieve NOx conversion at 95 percent, but with 50 percent of the conversion product nitrogen and 45 percent nitrous oxides at the temperatures under study.

"We have designed this system to work between 350 to 400 degrees Fahrenheit, which is in the range of temperature within the baghouse of our demonstration boiler," says Boehman. "The nitric oxide conversion is comparable to that seen in high temperature systems.

"But this low-temperature catalyst has a tendency to form nitrous oxide instead of nitrogen. We are looking for operating conditions that minimize the formation of nitrous oxide."

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Editor: Dr. Boehman may be reached at (814) 865-7839 or boehman@ems.psu.edu by email.

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