OCR Text |
Show surf ace, and passes through a fuel rich layer, in which a part of the nitrogen oxides are reduced again. High temperature increases the oxidation rate, but increases also the reduction rate in the fuel rich layer. The resulting rate of formation of nitrogen oxides therefore, is comparatively independent of temperature, except at very high temperatures Thus the amount of NO. formed, is mainly dependent on the time necessary f or complete burnout of the char particles. 3 The conclusion of the discussion above is, that it is very important that the char is completely burned out before the reburning zone, even if this must be achieved at the cost of higher NOx concentration at the inlet of the reburning zone. H the char particles are not completely burned out, the burning rate will decrease in the reduction zone, and increase again in the final burnout zone, AA. The result will be new formation of NOx. These nitrogen oxides will not pass any reduction zone, and will therefore not be reduced to nitrogen. In addition the delayed combustion will cause CO in the waste gas from the boiler furnace, and increased carbon concentration in the flyash. A further consequence of incomplete burnout of the char in the primary zone will be that the unused primary oxygen will be carried over to the reburning zone and decrease or eliminate the reduction of NOx. PROJECT PLAN Three series of experiments in the boiler were conducted: 1 Baseline experiments before the installation of the reburning system. 2 Same experiments repeated, at 100 and 68% of full load after the installation. 3 Input - output experiments to study the influence of the various parameters. The baseline experiments were parallelled in the model, to establish confidence in the modelling results. Thereafter input/ output experiments were run in the model, in order to optimise the dimensions and positions of the reburning fuel nozzles, UFI, and the additional air nozzles, AA. Finally experiments, parallell with the reburning experiments at full load were run, with the objective to analyse and explain the results. BOILER AND EXPERIMENTAL METHODS Profiles of gas temperature, COl' CO, Ol NO and NOl were obtained by probes, inserted though openings in all the four walls at the 6 levels shown in fig 6, through the burners and overfire air ports: In addition sootblowers and lime injection nozzles were removed so that diagonal profiles from corner to corner could be obtained. The data were logged, together with relevant data from the boiler instrumentation: Boiler thermal load, air input, air temperatures, waste gas analysis, including NO.. Coal consumption was calculated from the boiler thermal load. Isotermal Model Gas flow and mixing was studied in a perspex model in scale 1:70 of the boiler, using the acid/alkaline method. In this method an alkalic water solution simulates the fuel, and an acid solution simulates the air. Neutralisation corresponds to combustion at stoichiometric air/fuel ratio. By adding an indicator to the "fuel", ie fenolphthalein or thymolphthalein , the area of uncomplete mixing can be visualised. Different stoichiometric air /fuel ratios can be represented by adjusting the concentratins of the two fluids. Geometric similarity was maintained, exept for burners and air ports. Their dimensions were adjusted according to the equivalent diameter concept of Thring.- Newby (1956) |