OCR Text |
Show temperature. It is the presence of such high temperatures which has led industrial furnaces to be substantial emitters ofNOx. Of the three NOx formation mechanisms (thermal, prompt, and fuel), thermal NOx formation is the most vexing in controlling emissions from industrial furnaces, due to two reasons. First, industries are moving away from 'dirty' fuels such as coke, producer gas, heavy fuel oils, and coal and are turning to natural gas. Recent surveys by the Gas Research Institute indicate that industrial use made up 44.5% of gas usage in 1991 (Gemmer, 1994). Second, at the temperatures of most high-temperature processes, the radical chain reaction process of the extended Zeldovich mechanism overwhelms the contribution of prompt NOx formation. The reduction of thermal NOx formation is a particular challenge for high-temperature industrial processes for the following reasons: • Performance and productivity of these furnaces are increased by high heat transfer from the reacting gasses to the charge, which is conveniently and effectively achieved by high reaction temperatures. • Common methods of in-flame NO" control, including flue-gas recirculation, steam or water injection, and staged combustion, succeed, when effective, by lowering the peak reaction temperature. This may not be possible when the process demands high temperatures and high heat transfer rates to the charge. Work in the area of in-flame emissions control of industrial furnaces has increased over the past six years. The primary method currently employed by most high-temperature burner manufacturers involves delayed or degraded fuel air mix~ng; this is accomplished either by adjusting the location or direction of fueVair injection into the primary combustion zone or through fuel staging. This method has the advantage of producing long flames in which heat release is spread over the length of the furnace. Lean-burn staged and pre-mixed combustion are also gaining popularity, with various strategies used to address the problems of flame stability and flashback. Recent research conducted at the UC Irvine Combustion Lab (UCICL) has focused on the role of fuel and air mixing as having the major potential role in limiting the formation of NO". By using a small model industrial burner in an open test enclosure, the UCICL has demonstrated that a fueVair mixing strategy exists that simultaneously yields low formation rates of NO" while maintaining high combustion efficiency (e.g., St. John, 1994). |