OCR Text |
Show (3) Ammonia Injection Techniques The additives used were gaseous ammonia, ammonial liquid and urea "solution". Gaseous ammonia was injected horizontally to the combustor axis at locations of 52 em and 65 em above the distributor via a 0.95 diameter stainless steel tube whose height could be adjusted. A metered flow of ammonia and nitrogen was injected uniformly across the main gas stream through eighteen holes (O.8mm Ld.) equi-spaced on either side of the injector tube (Fig. 1). typically, the exit velocity of the ammonia jets issuing from the injector tube holes at the lowest flowrate was around 2 mIse Ammoniacal liquor, and urea "solution" were injected midpoint into the freeboard of the combustor, 52cm above the distributor, through a water cooled injector. A stre~ of nitrogen assisted their injection and cold tests have shown, that this results in a spray emerging from the end of the injector. Powdered urea was supplied from a hopper via a calibrated rotary valve, and conveyed pneumatically to the combuster using au as the transport medium. It was then injected, through a stainless steel tube (1.2cm id), into the freebound, some 52cm above the distributor. 4. Operating Conditions The NO reduction tests were carried out using bituminous coal. The typical proximate and ultimate analyses are given in Table 1. The range of operating conditions covered by the experimental programme was: Combustion Mode: Unstaged and Staged Secondary Air Location l00cm above the distributor Bed temperature - 850°C. Excess Air Level - 10-50% FIuidising Velocity - O.76-2m/s Bed Depth - 22 & 30 com (static) Coal Particle Size - 3 - 16mm Ammonia/NO molar ratio - 0.5 to 10 Measurements of NO and NOx reported in this paper have been corrected to 6.4% flue gas oxygen. RESULTS AND DISCUSSIONS (1) Datum Conditions - With and Without Air Sta~n~ A number of runs of were carried out without air staging in order to determine datum levels of CO, NO, NOx and 02 in the flue and also to obtain profiles of CO, NO and N Ox in the freeboard. This would act as a guide for comparing the effectiveness of the various NOx reduction techniques which were under mvestigation, hence enabling selection of the most effective technique. Combustion efficiencies decreased for unstaged combustion from 96% at 2m/s to a minimum of 91 % at 1.2m/s and 10% excess air. For the fluidising velocity range of 0.76 to 2m/s, combustion efficiency was in excess of 98% for unstaged operation at 40% excess air. Combustion efficiency was slightly lower during staged operation. Combustion efficiency was 91 % at 2m/s and 96% at 1.7m/s. For 2m/s and 40% excess air average levels of NO and NOx in the flue were 460 and 480 ppm. There was a progressive decay in the NO and NOx concentration along the 3. |