OCR Text |
Show The volatile N compounds are liberated in the primary flame zone and are subjected to competitive reactions forming molecular N2 and NOx as end products. Volatile N release from the coal is favoured by high flame temperatures [ 1], and is heavily dependent on coal composition [2]. The conversion rate of volatile N to NOx can be high, but is strongly affected by burner aerodynamic changes [ l]. Char combustion may also generate NOx, but conversion rates are observed to be low and account for 20 - 30% of the total NOx [ 1 ]. NOx formation from char N is much less affected by burner aerodynamic changes [ 3]. The following schematic summarises the contribution to NOx emissions of the different formation routes: Combustion Air Nitrogen 10 - 30 % total NOx Volatile N 50 - 70 % total NOx Fuel Nitrogen Char N 20 - 30 % total NOx Such experimental findings allow the formulation of a low NOx burner concept. Clearly NOx formation from volatile N compounds is substantial; however, fortunately this fuel N reaction route is most susceptible to combustion modification through burner aerodynamic changes. This last fact provides the basis for NOx control by staged combustion. This latter NOx control concept, embodied in an experimental staged combustion burner, is detailed in the burner schematic above. Here a primary fuel rich zone is established in the flame by splitting the combustion air into separate streams. The devola-tilized N species are consequently subjected to an oxygen deficient environment which promotes their decay to molecular N2. The remaining staged combustion air is delivered in the char burnout zone where combustion is completed. Although the staging air ports - 5 - |