OCR Text |
Show * CO must be controlled. Generally, conditions that limit NOx formation favor CO formation. In industrial applications, CO is readily formed by quenching the reacting gases by contact with heat sink surfaces or, in boilers, by gas leakage from the furnace into the convection section. * The design should be scaleable over a wide range of heat release rates: ideally from 4 million Btu/hour to 250 million Btu/hour, but in any event over as wide a range as possible. * The burner must operate down to a minimum rate of 10% of design capacity for boilers. * The design must be robust. * The burner must be stable under all of these conditions. In addition, ultra-low NOx should be achieved without the use of flue gas recirculation, although the burner should be capable of using FGR to lower NOx even further. A proposed solution was to use a very turbulent, rapid mix lean flame zone, one which has the flame characteristics of a premix flame, . but which avoids the size, stabi 1 ity and turndown limitations which Coen observed in premix burners. The burner consists of three parts. 1) The simulated premix flame zone (SPF zone) 2) A zone or zones of diffusion flame 3) A core-pilot flame These parts are designed to trade off the competing requirements of stability, prompt NO control, excess air, thermal NOx control, and CO control. Early in the project it became obvious that ultra-low NOx could be achieved. As the engineers reported, "The challenge of this project is not so much achieving low NOx as achieving and extending stability at the desired low NOx conditions. The most stab 1 e cond it ions a 1 so favor NOx formation." The SPF zone has a high turndown ratio and extremely short distance requirement for fuel/air mixing. The fuel lean conditions and absence of fuel rich interfaces greatly suppresses the prompt NO formation mechanisms. The core pilot flame provides stable flame propagation over the entire burner reacting gas mixture. The diffusion zone fuel jets ingest large Quantities of very low O2 combustion products before they interact with the oxygen-rich SPF zone. This fuel dilution reduces prompt NO. Thermal NOx is also controlled by conventional methods. Page 4 |