OCR Text |
Show When discussing therma l ~ IOx, it was shown that control of thermal NOx occurs thr ough reduction of the peak flame temperature. The method for controlling fuel NOx is quite different. The effect of temperature on fuel NOx is not clear. Opinion in the literature ranges from no effect of temperature to possible increases and decreases in NOx [3, 7-9, 14, 15]. It appears that the effect of temperature is dependant on the gaseous environment to which the volatile N2 species are exposed. The main factor affecting fuel NOx is the gaseous environment to which the volatile N2 species are exposed. The reasons for the effect of gaseous environment on fuel NOx can be understood by considering the mechanisms for formation and destruction of NOx by fuel N2 . A global mechanism for the formation of NOx from volatile Nz species is represented in Figure 1 ~ 2, 16, 17]. As can be seen, the volatile Nz species first decompose to HCN which leads to NHI via NCO. NHI can react to NOx in an oxidizing environment or Nz in a reducing environment. NOx can also be reduced to Nz by this path. Two very important aspects of NOx formation from coal volatile mat ~ er come from this mechanism. Firstly, the starting point of the mechanism is HCN. It is generally found that the lecomposition of coal volatile matter to HCN is fast under flame conditions [3, 11, 12, 15]. In models of NOx fo rmation and destruction, the chemical composition of pri ~ ary volatile species is ignored and assumed to be completely decomposed to HCN. The second important factor for ~ ~ e NOx via volatile N2 route is that the gaseous environm _nt strongly affects the formation of NOx. NOx is fo r~e d in an oxygen-rich environment; N2 is the predominant ~~ oduct in a oxygen-lean environment. This fact is che main information used in the design of low-NOx burners . Production of NOx by the heterogeneous reaction of )2 with char-bound N2 has different characteristics th a~ the volatile N2 path. Most of the char NOx forms downstr eam of the volatile release zone. In this zone, it is diff ~ cult for 02 to diffuse to the particle surface. The NO f o_~e d by the char path can be destroyed by reaction with C i n the char to form Nz and CO or C02. It is generally believed that this heterogeneous path of NOx destructi cn is the reason for the low efficiency of converting char N to NOx. Furthermore, it appears that most of the heterogeneous reduction occurs either in the coal pores or before the NOx leaves the char boundary layer, because contact of char with NOx under pulverized-coal combustion conditions seems to be unimportant in reducing NOx [18]. As with the volatile N2 path, the form of the Nz in the char is assumed to be one entity, nitrogen in a highly-condensed aromatic-ring structures. Furthermore, differences in reactivity due to different aromatic-ring 3 |