Microwave Process for NOx Abatement

The recent NOx State Implementation Plan calling for substantial reductions in NOx emissions continues to be debated. In a recent market analysis completed by McIlvaine Company, U.S. power plants are likely to spend an estimated $48 billion on advanced emission controls over the next 20 years. An estimated $22.5 billion will be spent on "scrubbers" to reduce fine particulate, hazardous air pollutants, and sulfur dioxide; $13 billion for "selective catalytic reduction" to control NOx; and $12.5 billion for additional technology to control other pollutants, including mercury.
The interest in discovering a better way to control emissions from combustion sources has engineers and scientists drawing on information from a variety of studies and disciplines to create high-tech solutions. Most recently, at the Air & Waste Management Association conference in Salt Lake City, Chang Yul Cha from the University of Wyoming, Charlie T. Carlisle from CHA Corporation, and Joseph D. Wander of Tyndall Air Force Base presented their findings from a Small Business Innovation Research project.
Their study investigated the feasibility of using a novel filter device (in which microwave energy is applied to destroy pollutants in a catalyst bed) to control unwanted combustion by-products in diesel engine exhaust gases. The microwave-based cleanup process for diesel engine exhaust gases has the potential to significantly reduce the cost of removing airborne pollutants from the exhaust of combustion processes. These pollutants include nitrogen oxides (NOx), carbon monoxide (CO), particulate matter less than 10 microns (PM10), products of incomplete combustion (PICs), volatile organic compounds (VOCs), and other hazardous air pollutants (HAPs).

Currently, no single device is available for simultaneous removal of NOx, CO, VOCs, and PM10 from oxygen-rich exhaust gases. Two methods for NOx reduction currently available are selective non-catalytic reduction (SNCR) and selective catalytic reduction (SCR). In both SNCR and SCR (processes originally developed for large, coal-fired power plants), a chemical reducing agent, commonly anhydrous ammonia or urea, is injected into the exhaust gas stream as it exits the combustion chamber. The costs of implementing and operating these processes are quite high, and emissions of small amounts of unreacted ammonia are likely. Furthermore, the service conditions of non-steady-state operations are ill-suited for SNCR and SCR processes.

The Technology
CHA Corporation has developed a process employing a carbon-based adsorbent and microwave energy to treat exhaust gases. An integrated prototype system capable of treating exhaust gases for a 58-hp diesel engine was designed, fabricated and demonstrated. The integrated system is a continuous adsorption/regeneration arrangement capable of 100% NOx removals, as well as destroying the VOCs and soot present in diesel exhaust. The pollutants are first removed from the flue gases by passing the gases through a bed of relatively inexpensive carbon adsorbent. NOx, SOx, and VOC gases and vapors are adsorbed and stored on the carbon during this step. The pollutants are then destroyed during regeneration of the carbon by microwaves. The microwave energy decomposes NOx to nitrogen and carbon dioxide, VOCs to carbon dioxide and water, and SOx to elemental sulfur and carbon dioxide.
A prototype unit was constructed and perfected through experimentation. The unit was then transported to McClellan Air Force Base in Sacramento, CA, in March of 1998 for demonstration. The field demonstration of the prototype was very successful. No mechanical or technical problems were encountered during frequent startup and shutdown operations. During the week of field demonstration, the outlet gas contained less than 1 ppm NOx. Additional research and modification has been completed to overcome inlet temperature and size limitations identified during experimentation.
The principles of operation are as follows:
1. Capture the carbonaceous soot in a ceramic filter and, using microwave energy, oxidize the soot into CO2 or CO.
2. Convert the NO into NO2 in an oxidative-catalytic section of the device. It was learned that this reaction proceeds very quickly in the presence of a platinum catalyst, and the reduction of NO2 by a reducing agent in the presence of oxygen is more favorable than that of NO.
3. Reduce the NO2 to nitrogen and CO2 using a commercially available NOx-reducing (three-way) catalyst and a reducing agent such as JP-8 jet fuel during irradiation by low-level microwave energy. This step destroys about 65 percent of the NOx.
4. Oxidize any CO and VOC materials to CO2 and water using an oxidizing catalyst in the presence of low-level microwave energy.

For more information on this technology contact Charlie T. Carlisle, CHA Corporation, by phone: 307.742.2829 or e-mail: ccarlisl@wyoming.com.