OCR Text |
Show 8 The FGR burner also uses a technique of NOx control called staged air combustion. This means that the burner has been designed to produce a stable flame at almost its L.E.L. limit thus producing a highly intense, very high speed flame with a significant amount of CO and H2. The rest of the combustion air is then supplied to the flame via a series of holes and a completely separate actuated staged air control system. The carbon monoxide and hydrogen formed in the high velocity flame mixed with the rest of the air inside the furnace chamber where the combustion reaction is complete. With such a burner fitted to the furnace one can expect to see CO levels within their usual limits and NOx levels as low as 5-7 parts per million. When the burner is observed it is almost impossible to see a flame. There is very little flame luminosity and for all intents and purposes the flame does not appear to be anchored to the burner tile. There is an obvious limitation to this technology in that if the burner were to be operated in a furnace where the temperature was less than 650 degrees C. the combustion of the carbon monoxide and hydrogen together with any unburnt methane may not proceed resulting in a potentially hazardous condition. Therefore, with this type of technology the control system has to be fitted with a safety system which would prevent the operator from selecting staged air combustion at temperatures too low for safe operation. The final safety code issue relates to the fact that the staged air burner is very difficult to prove firing. There is little flame luminosity therefore little UV radiation to be detected by the normal kinds of sensors we see in industry and the amount of ionised gas is also very small leading to problems of detecting the flame with a rectification probe. The only real practical method is to employ an FM style safety system which for all intents and purposes the majority of the burner suppliers have tried to avoid for the last ten years. In summary there is much work to do on the reaction kinetics, mathematical modelling of turbulent high temperature systems etc. to keep the academic community happy for a few years, however, I happen to believe that the majority of right minded people will be more concerned with what we can do now. In this talk I have tried to illustrate many simple things which can be done without incurring great cost however, major NOx reduction will only become an important consideration when industry purchases major equipment. If there is a code or regulation in place to level the playing field, aftd finally NOx control technology raises a number of questions which will affect the gas safety code and therefore the Ministry of Environment should be encouraged to consider any changes very carefully so that potential code conflict can be avoided. |