OCR Text |
Show oxidation of volatile nitrogen species [1, 2]. The fixation of nitrogen in the - combustion air occurs predominantly by the Zeldov;ch mechanism with smaller amounts produced by the "prompt" route [3]. In practical pulverized-coal combustion systems, particularly utility boilers, flame temperatures and residence times are such that it is unlikely that NOx formation from the combustion air accounts for more than 20% of the total NOx budget [4]. Consequently, the aim of employing primary NOx reduction techniques is to reduce NOx formation from volatile coal nitrogen species. A common design strategy used to reduce volatile nitrogen generated NOx is combustion air staging. Generically, these systems rely on promoting devolatilisation in an oxygen deficient environment, by physically or aerodynamically separating (staging) a portion of the combustion air from the devolatilisation region. Air staging can be achieved by either external or internal air staging. For retrofit applications, internal air staging is most suitable as all the combustion air passes through the burner quarl. This obviates the modification of the furnace firing walls to incorporate staging air ports as is required for external air staging. 2 BACKGROUND 2.1 Nitrogen in Coal and Reactions During Combustion The nitrogen content of coal is usually between 0.5 and 2.0% [5]. The distribution and forms of nitrogen within the coal structure is the subject of much conjecture at present. Pyridine, pyrrole and amine type heterocyclic ring structures are believed to be present with the concentration of each structure being highly coal type dependent [6-8] . Some of the basic structures believed to exist in coal are shown in Figure 1. In a pulverized coal flame, coal is heated to a temperature in excess of 14000C at rates of the order of 10~oC/s. Under these conditions, primary coal pyrolysis occurs and results in solid and gaseous products. Of the gaseous products, nitrogen can be contained in small molecules such as HCN and NH3 or as part of larger tar molecules. In the tar molecules, some of the nitrogen containing heterocyclic ring structures noted above will exist. The distribution of nitrogen in the gaseous pyrolysis products and the residual char is critically dependant on: heating rate, peak temperature, residence time at high temperature, and nitrogen distribution within the coal structure [9, 10]. Subsequent to primary pyrolysis, tar molecules undergo secondary the nitrogen-containing decomposition reactions 2 |