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Show Trials with reburning 2 Modelling 7 Reburning 5 Simultaneous NOx and Sox reduction 6 Analysis of flyash 8 NOx reduction by urea injection 9 Primary combustion improvements 10 Training of operators The main objectives of the total project were: to reduce sulphur emissions 40 - 50 per cent with lime injection, nitrogen oxides 40 - 60 per cent by urea injection and additives and 20 - 40 per cent by reburning. . The objectives of sUbproject 1 and 2 was to analyse and explain the results of the reburning installation, and to recommend optimal design and position of the secondary fuel and additional air nozzles. FORMATION AND REDUCTION OF NITROGEN OXIDES As a base for discussion of the project results, the kinetics of NOx formation and destruction will first be briefly discussed. Reburning is shown schematically in figure 1. Additional air .. Rebunrinl fuel, ~Puel Injector. UFI) Of A Fig 1 OFA- and reburning systems, schematic. In the primary zone the nitrogen in fuel and combustion air is oxidised to nitrogen oxide at high temperature. Between 80 and 90 % of the fuel is injected in this zone. The NO. formation can be reduced by limiting the temperature and available oxygen. This can be achieved by injecting the combustion air in stages, either through the burners - low NO. burners - or downstream of the burners - as overfire air, OFA. In the reburning zone a secondary fuel is injected. This creates a fuel rich area, in which the NOx concentration is reduced. Finally additional air, AA, is supplied in the burnout zone. In this zone a certain, but limited, reoxidation of nitrogen takes place.The process can be described in the following, schematic way:When a coal particle is ignited, a thermally controlled pyrolysis is first started. The pyrolysis causes release of combustible, gaseous components, volatiles. Left in the particle is char and ash. The main part of the fuel nitrogen is released together with the volatiles. Nl is released with a certain time lag, but at the same rate as the volatiles. This part of the fuel nitrogen is oxidised very rapidly, and the rate determining factors are the oxygen availability and temperature. Also the nitrogen in the air is oxidised very rapidly.The char bums much slower and NO. is formed by heterogeneous reactions at the particle surface. The oxidation products diffuse outwards from the 2 |