OCR Text |
Show N O + C H ; -* Products 0 < i <2 (VII) The CH; concentration is calculated from: CH; + OH = CHM + H20 / = 1,2,3,4 (VIII) and the resulting NO destruction rate can be expressed as d(NO)/dt = - (CH4) (NO)y[(OH),T] (IX) where (NO) and (CH4) are concentrations of NO and CH4 (g-moles/cm3), respectively. The function y[(OH),T], is given by f= 5.8301 x IO14 T-048377 exp(\2436/T) (OH)2/(H20)2 + 7.7328 x IO16 T-067788 _xz>(20730/T) (OH)3/(H20)3 + 1.1990 x 1014 T-029159 _X7?(40708/T) (OH)4/(H20)4 {cm3/g-mole-s) (X) The OH concentration, (OH)_q is obtained by assuming global equilibrium [9]: OH + H + M -> H20 + M (XI) H + H+M->H2+M (XH) from which, utilising the kinetic data of Levy et al [23] yields: (OH)^ 0.48 x 10VT05 exp( - 34480/T) [I^O] /[H2]05 (XIH) The OH equilibrium overshoot is considered using the following empirical correlation [9]: (OH) = (OH)Eq x 5.7837X10-4exp(\30\9/T) (XIV) The rate of additional HCN formation, from reaction (VII), is given by: d(HCN)/dt = (CH4) (NO)/ [(OH),T] (XV) 4. Results and Discussion 4.1 Experimental Results Unless specifically indicated, all NO reduction data in this section are reported relative to baseline N O emission of 600ppm (measured at the furnace exit before the start of coal feeding but with both the carrier gas and burnout air supply turned on) and are for a two-nozzle coal injector configuration. N 0 2 , which was typically only 5 to lOppm. of the total N O x , has not been included in the data presented. Figure 3 shows the typical axial variations in temperature, NO, CO and 02 measured from the start of the rebum zone for Thoresby coal using two rebum coal injectors. The inlet gas temperature (from the primary zone) was around 1300°C. The temperature increased slightly near the reburn-coal injection point to a peak of about 1325°C, due mainly to volatiles combustion. Further downstream the temperature dropped, due to heat losses through the walls, to a minimum of 1050°C near the burnout air injection point before increasing again due to combustion of remaining volatiles and char. The furnace exit temperature was 1120°C. The C O level was relatively high (0.75 - 1.0 % ) in the rebum zone but in the burnout zone C O quickly consumed by added oxygen and was less than 200ppm in the exit flue. |