OCR Text |
Show 10 CONCLUSIONS: The results of an installation of a reburning system, with natural gas as the reburn fuel, in a 125 MW, coal fired hot water boiler for district heating has been verified by extensive measurements before and after the retrofit. Experiments in an isothermal flow model has been conducted in order to optimise the design of the natural gas and additional air supply systems, and to analyse and explain the results. Reburning at full load reduced th~ nitrogen oxide concentration in the waste gas from the boiler from 240 ppm at 3% 02 (133 mg/MJ), to 113 ppm (66 mg/MJ)- or more than 50 per cent. This is better than expected,but was achieved at the costs of an increase of carbon concentration in the flyash, from 8 % to 10 - 11%. In addition, the CO concentration in the outlet to the convection section was rather high at reburning with full load, but normal at reduced load. The stoichiometry in both the primary zone and the reburn zone was lower than values reported in literature. Also the temperature level in the reburning zone was lower. These facts indicate that the carbon particles were not fully burned out in the primary zone. This in turn would call for longer residence time in the primary zone. The available volumes in all the different zones however, allowed for much longer recidence times, than necessary according to the literature. All this indicates that the mixing between the fuel and air jets, and the furnace gases should be improved. This confirms an opinion, also expressed by other authors, that the aerodynamics of the injection is the single most important factor that influences the formation and destruction of nitrogen oxides in boilers./LisaukasI9826/ Structural members of the boiler limited the possibilities to install the reburn and additional air nozzles to the chosen levels. The experiments demonstrated however, that the size of the primary zone and the reburning zone can be influenced by the direction of the additional air nozzles. The experiments also showed that the distribution of air between the different burners and air ports in the primary zone did not have any too significant influence on NOx formation. As soon as the OFA ports were open, the other valves could be adjusted at will, without significantly influencing the NOx emission. This points to good possibilities to achieve satisfactory combustion, i.e. complete burnout and high efficiency, without increasing the NOx emission.The model experiments agreed very satisfactorily with the results from measurements in the boiler. ACKNOWLEDGEMENTS The project was carried out under a contract with ME Malmo Energi AB, and partly financed also by the National Energy Administration, Swedish Energy Development Corporation, and Thermal Engineering Research Association. Many members of the department of Heat and Furnace Technology at the Royal Institute of Technology in Stockholm were engaged in the project: Alf Malmgren and Marc Landtblom were responsible for each one part of the boiler trials. Dr Jiri Vaclavinek conducted the isothermal model experiments. Peter Carlsson was responsible for the mathematical modelling. Last but not least, we are deeply indepted to the staff of the Limhamn District heating plant for their enthusiastic support during the trials in the Limhamn boiler. |