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Show length of the freeboard. This is attributed to chemical destruction of NO. There is a reduction of 13.2% from a position 52cm above the distributor to the final NO level in the flue. Fig. 3 shows a profile of NO and NOy levels at various points along the freeboard for a range of fluidising velocities. Total conversion of fuel nitrogen was 14.6%. At 40% excess air the effect of an increase in fluidising velocity is to increase the levels of NO and NOx emission. However, this increase is slight (12%). At 10% excess air there was little effect of fluidising velocity on NO and NOy • Figures 2 and 3 show NO and NOy profiles in the freeboard for unstaged and staged combustion conditions at 1.7 and 2 m/s. NO and NOy for staged combustion levels are considerably lower (36%) than those obtained during unstaged combustion (cf Fig. 3). The effect of air staging is to increase the reducing zone above the freeboard where NOx reduction can occur with either CO or with char particles. The sharpest decrease occurs near the secondary air injection region probably due to dilution by secondary air. (2) NOx Reduction Using Ammonia Injection Section (1) has shown that considerable reductions in NOy emissions can be achieved by usin~ staged combustion, at the expense however of a small loss in combustion effiCIency . The reduction of NOx emission by ammonia injection was investigated under unstaged and staged conditions. Ammonia is known to react selectively with NO in the gas phase to produce nitrogen and water vapour. According to Lyon (3), the gas phase reactions are: 4NH3 + 6NO-+5N2 + 6 H20 4NH3 + 6NO + 4N2 + 6 H20 (1) (2) The theoretical (stoichiometric) mole ratio of ammonia to NO required to achieve 100% NO removal, based on the above equations, should be 1-1.5:1. Hence in order to estimate the efficiency of No removal by ammonia, it is appropriate to define the quantity of ammonia injected in all cases in terms of the theoretical mole ration of ammonia injected to the molar flow of NO at the injection point as NH3/NO. (3) The Effect of Gaseous Ammonia Injection on NOy Emissions for Conventional Unstaged Combustion: Ammonia gas injection was first carried out for conventional operating conditions so that NO reductions could be compared with similar tests carried out for air sta~n~. The effect of ammonia addition on NO and NOx emissions as the NH3/NO muo IS increased are presented graphically in Fig. 4. The addition of ammonia can be seen to decrease the NO and NO. emissions. There did not appear to be a particular ammonia flowrate (or NH3fNO ration) that was most effective, rather there was a steady progressive reductIon in the NO and NOy emissions as the ammonia flow was increased. These findings were consistent with our previous study (7). The ability of ammonia to reduce NOx at the lower fluid bed temperatures is explained by the presence of unbumt volatile hydrocarbon fragments (1). It can be seen that for the deeper bed (3Ocm) and the ammonia injector at 52 cm above the distnbutor the ability of ammonia to reduce NOx emission was considerably reduced. A probable explanation could be that, at 2 m/s and 30cm static bed height, the ammonia injector would be below the surface of expanded bed height and this would lead to some ammonia being circulated into the bed due to the back mixing, 4. |
