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Show greenhouse atmospheres) or many products (barley malt, powdered milk, and baked meats, etc.). Therefore, in many applications, direct-fired air-heating systems have been replaced with more costly electric heating systems or less efficient and more complex indirect-fired systems to avoid contact of the POCs with the products. One of the major applications for direct gas-fired hot-air systems is unvented space heating. Recent investigation by the U.S. EPA Clean Air Science AdviSOry Board has shown that concentrations of nitrogen dioxide (NOv in the 0.1 to 0.3 vppm (at 0% 0v range in indoor air may cause adverse health effects. Therefore, indoor air with less than 0.1 vppm N02 is considered IIgood qualityll air based on the Pollutant Standards Index (PSI) Range and Description category specified in the Code of Federal Regulation (CFR, 1989).1 According to the American National Standard (ANS, 1983) for gas-fired room heaters,2 this requires N02 emissions from a direct-fired burner to be no more than 1.75 vppm (at 0% 02, or 0.5 vppm at 15% 02)' In a recent survey conducted by the Gas Research Institute (GRI), however, 98% of unvented gas-fired space heaters cannot provide good air quality.3 Another important application for direct gas-fired hot-air systems is drying and process heating. In the food-drying and heating industries, for example, the primary concern is the potential formation of nitrosamines and the pink color in foods from the relatively high levels of NOx contained in the direct-heated air. Similar emissions concerned with "yellowing" problems in fibers and carpet-heating processes are also addressed. On the basis of European regulations, American Gas Association (A.G.A.) recommendations, and feedback from the U.S. Industry, GRI has determined that the POGs from natural gas-fired combustors for direct air heating must not contain more than 0.5 vppm NOx and 3 vppm CO. To date, no commercial gas-fired burners can achieve this target of emission levels. The gas industry in the United States is interested in developing advanced, economically feasible burners that produce ultra-clean products of combustion for direct use in space heating, process heating, and drying applications. The Institute of Gas Technology (IGT) has teamed with Maxon Corp. to develop a gasfired direct air heater based on the cyclonic combustion concept. This paper introduces this technology and presents the major experimental results obtained from a pilot-scale test combustor. CONCEPT AND TECHNICAL APPROACH NOx Formation in Combustion Processes Most nitrogen oxides (NOx) are produced in the combustion process in two forms: nitric oxide (NO) and nitrogen dioxide (NO~. NO is the initial oxidation stage in which the NOx are formed. Because the process of oxidation of the atmospheric nitrogen is endothermic, the formation of NO takes place in high-temperature regions of the flame. The NO is then oxidized to N02 in the zone of considerable excess air. It is commonly recognized that NO formation In combustion processes can be the result of three different mechanisms: 1. Thermal NO is produced by oxidation of molecular nitrogen. 2. Prompt NO is produced by high-speed reactions at the flame front. 3. Fuel NO is produced by oxidation of chemically-bound nitrogen contained in the fuel. 2 |