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Show resulting in further generation of NOx by ammonia oxidation. This effect does not occur at the lower velocity (1.2 m/s) or with a shallower bed (22cm). In what could be expected to be a realistic level of ammonia injection (mole ratio in the range of 2 to 4), the NOx reduction was about 25 and 33% respectively at 2 mls and 22cm bed and about 30 and 40% respectively for 1.2 m/s and 3Ocm. In order to comply with the anticipate~ legislation which would require NOx emissions to be reduced by up to 50%, mole ratios of order 8 would be required. Clearly ammonia injection alone in conventional operation cannot achieve the required reduction in NOx• (4) The Effect of Gaseous Ammonia Injection During Staged Combustion Figure 5 shows a comparison of NO and NOx emissions for staged operation with ammonia injection. It can immediately be seen that NO and NOx reductions by ammonia are considerably enhanced if it is combined with air-staging (cf Fig. 4). For staged combustion at 2m/s NO and NOx reductions of about 50% are attained at a mole ratio of only 0.6, compared with about 12% for conventional unstaged operation. At a mole ratio of 3:1 the reduction in NO and NOx is about 60% for air staging, almost double that without air staging. In practice however a mole ratio not exceeding 1.1 would achieve a substantial reduction in NOx (-52%) compared with higher ratios which cannot be justified on the basis of further increases in NO reduction. At 1.7 mis, the reduction in NO and NOx emissions is sli2btly lower when compared to the reduction at 2m/s but higher than for 1.2 m/s. It is therefore apparent that effectiveness of ammonia injection varies with fluidising velocity all other operating condition being similar. This can be explained by the fact that at higher velocity the freeboard turbulence will be higher resulting in better mixing than at lower velocities. Due to the effect of bed expansion at 2m/s and 30cm bed height the NOx reduction achieved were very low, about 30% at NH3/NO molar ratio of 0.6:1. When the ammonia injection height was increased to 65cm above the distributor there was some improvement in NOx reduction, but this reduction was still quite low. The improvement in ammonia utilization during staged combustion can be explained by three reasons. Firstly, under staged combustion, there is more time for reaction 1 to occur than for unstaged combustion, due to the extended reducing zone below the secondary air injector. Secondly, there will be more unbumt volatile combustibles in a::.srrison to unstaged combustion, which can enhance NOx reduction with ammonia. p. y, due to staging, the superficial bed velocity will be lower (U/Umf = 7.6) and the bed surface (expanded bed beight 4Ocm) will be further from the ammonia injector resulting in a smaller recirculation of ammonia into the lower oxidation regions of the bed with subsequent improved ammonia utilisation. The next logical comparison is to estimate the extent of additional NO reduction achieved by using ammonia in the staged operating mode. Without ammonia injection the reductions in NO and NOx were 38 and 36% respectively. Fig. 6 shows the contribution of ammonia to the total NO and NOx reduction achieved during staged and unstaged combustion. For staged combustion at 2m/s and at the lowest NH3/NO' molar ratio of 0.6, the total NO and NOx reductions have been increased to about 50% and 46% respectively. This represents an enhancement of about 38% of the reduction achieved with air staging. At the lower velocity of 12m/s the total reduction of NO and NOx were 43 and 38% reS{>ectively an enhancement of 31% over unstaged combustion. As the NH3/NO ratio is Increased further the contribution by ammonia to the total reduction goes up considerably, at 2m/s with a mole ratio of 3, the enhancement is -58% and at 6 it IS about 79%, at 1.2m/s with a mole ratio of3 the enhancement is 56% and at 6 it is 64%. 5. |