| OCR Text |
Show mechanisms involved are then identified. In high temperature processes NOx is formed almost entirely by the thermal mechanism. In moderate temperature processes both the thermal and prompt mechanisms are active, while the prompt mechanism is the sole source of NO x in low temperature processes. This informtion guides the approach taken to minimize NOx formation in a given process. Next, the gas use for a given process is estimated. The relative threat to gas use, or opportunity for gas to replace solid or liquid fuels, combined with the amount of gas being used by a particular process, is used to identify the appropriate research targets. Thus, the high temperature processes represent a combination of high gas use and high NOx emissions (over 1000 ppm for 1500° F preheated air, Figure 2). Research aimed at developing burners for forge furnaces and steel reheat furnaces was initiated in 1989. At moderate temperatures, the NOx levels are not as high, but the gas load is the greatest of any industrial sector, exceeding four quadrillion Btu in 1993. Here, research was initiated that first focused on reducing thermal NO", but since has entered the domain of prompt NOx' Finally, low temperature processes already produce extremely low NOx emissions, and are not likely to be threatened by air quality regulations. The only efforts in this segment have been driven by the possible deleterious effects of NO x on food quality. High Temperature Processes High temperature processes present a particular challenge to designers of low NOx combustion equipment. Because the process temperature itself is high, with flames frequently exceeding the 2600° F threshold for thermal NOx formation, very careful control of the combustion process is necessary if expensive post-combustion cleanup is to be avoided. In practical terms the focus of NOx control for high temperature processes has been on reducing the flame temperature by fuel or air staging combined with either external flue gas recirculation (FGR) or internal dilution of the flame with furnace gases. Over five years ago, GRI initiated two programs aimed at developing low NOx burners for high temperature processes. One of these is the subject of a companion paper (see "Development & Demonstration of a Low-NOx Burner for High-Temperature Processes"). The burner is designed for use in forge furnaces and similar high temperature applications. This development has been very successful and is nearing market introduction. The second program aimed at larger applications, such as steel reheat furnaces. This program, initially involving Eclipse Combustion teamed with Altex Technologies, developed a burner that was could provide excellent heat transfer characteristics and less than 50 ppm NOx with air preheat up to 1500° F (Figure 5). An interesting feature is the relatively low sensitivity of NO x emissions to temperature even at the highest preheat values. The data in the Figure are a compilation of tests run in the Altex laboratory and at Eclipse's R&D center. The burner is characterized by not requiring external ducts, fans, etc. to achieve the very low NOx values. It is simple in design, with no moving parts, and operates at excess air levels similar to conventional burners. Scaling the burner is straightforward, and the design is amenable to adaptation to 7 |
