OCR Text |
Show -5- Values of Kp NO are available in the JANAF tables [2]. have a general equation: d[NO]/dt = 1 + (k1b[NO]/k2f[02]) Thus we KIf is strongly dependent on the local temperature. [N2] and [02] are traditionally set to the equilibrium conditions at the prevailing temperature but in coal flames the temperatures are probably too low for this equilibrium assumption to be valid for [N2] and it is virtually impossible to measure [0] using currently available techniques. The above is a limited and greatly simplified approach to the theory of thermal NOx formation and is included to allow an appreciation of the complexity of the theory and the difficulty of making theoretical predictions of thermal NOx emissions. 2.2 Fue1 MOx Formation The mechanisms by which NOx is formed from the chemically bound nitrogen in the coal is extremely complex, even the structure of the nitrogen in the coal is subject to considerable conjecture. The nitrogen is believed to be in the form of pyridine, pyrrol and amine type structures, figure 1. The actual structure in any coal is believed to be strongly dependent on coal type (3,4). One study (5) showed that the predominant forms of nitrogen in most coals were the pyrrolic and pyridine forms and that the former tended to decrease with increasing coal rank. However, at present, the importance of the structure of the nitrogen in the coal on the final NOx emissions is not well established. When coal is burnt in suspension it is heated very rapidly to high temperatures and pyrolysis occurs, producing solid and gaseous products. The nitrogen present will divide between these with typically 20% of the nitrogen in the char and 80% in the gaseous phase, the latter both as the light fractions and tars. For any coal, the distribution of nitrogen between the gaseous phase and char is heavily dependent on the conditions in the flame such as heating rate, peak temperature, and residence time at high temperature. |