NOx- REDUCTION WITH STAGED COMBUSTION - COMPARISON OF EXPERIMENTAL AND MODELLING RESULTS 1Joint Meeting of the Portuguese, British, Spanish and Swedish Section of the Combustion Institute, Madeira, 1996NO x - REDUCTION WITH STAGED COMBUSTION - COMPARISON OF EXPERIMENTAL ANDMODELLING RESULTSH. C. Magel, U. Greul, U. Schnell, H. Spliethoff, K.R.G. HeinInstitute for Process Engineering and Power Plant Technology (IVD), University of Stuttgart Pfaffenwaldring 23, D-70550 Stuttgart (Germany)1. INTRODUCTIONThe IVD of the University of Stuttgart carries out investigations of in-furnace DeNOx technologies using an electrically heated bench-scale test facility where the different process parameters are evaluated independently . The DeNOx technologies of air and fuel staging have been demonstrated to be effective control techniques to reduce NO x from coal-fired utility boilers. In order to assure a good reduction efficiency the process parameters temperature, sto-ichiometry and residence time must be carefully controlled. The importance of and interest in applying mathematical models is growing as they are a tool giving information about the combustion behaviour inside full-scale power sta-tion combustors. Until today discretization has been fairly coarse, and simple ‘global’ reaction models are used for the pyrolysis of coal, the burnout of the char and the formation or reduction of NO.The NO formation and reduction in substoichiometric regions is strongly influenced by the kinetics of the nitro-gen chemistry and also by intermediate species of the hydrocarbon combustion, e.g. hydrocarbon and oxygen-con-taining radicals. Therefore the prediction of NO is very difficult and simple ‘global’ models must be proven on their validity to describe the nitrogen chemistry with sufficient accuracy for different combustion situations.2. STAGED COMBUSTION TEST FACILITYA gravimetric screw conveyor supplies coal at a constant feeding rate. The ceramic tube has a length of 2.5 m and an inner diameter of 200 mm. Fuel and air staging in the furnace with different residence times in each zone can be investigated. The flue gas composition in different combustion zones can be measured with a gas probe. A detailed description of the test facility can be found in Greul (1996).air staging fuel stagingFigure 1. Scheme of DeNOx technologies Figure 1 presents a schematic diagram of staged com-bustion technologies which are applied to the test facility.Air staging divides the combustion process into a primaryzone with a deficiency of air and a second reburn zone runwith excess air. Fuel staging or reburning is an effective method to reduce NO x emissions in the combustion cham-ber. One part of the fuel is used to establish a fuel-rich zone after the main combustion zone. In this substoichiometric zone NO x can be reduced e.g. by hydrocarbon or NH radi-cals. The delay in supplying the burnout air provides the res-idence time for NO x reduction. Both NO x reductiontechniques were studied in the electrically heated entrained flow combustion reactor with 5 regulated heating zones allowing a maximum temperature of 1400 ˚C. The combus-tion air can be heated up to 400 ˚C. Air staging at the burner,and swirled secondary air are possible. Fuel and air are fedthrough three coaxial pipes.
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