Study On Characteristics Of Flow Fields In Furnace With Coal Reburning Technology And Its Engineering Application

Reburning with pulverized coal is one of the low NO_x combustion technologies. It is low cost, relatively high efficient and suitable for our country, in which the coal is considered as the main energy source. The project and demonstration of'Research on Low NO_x Combustion Technology with Micronized Coal Reburning'is supported by the National High Technology Research and Development of China (863 Programme). Combining with the first commercial application of reburning technology with pulverized coal on the No.3 boiler in Yanbaoshan Power Plant, the research work on the gas-particle two phase aerodynamics fields characteristics in furnace and experiments before and afer the retrofit are carried out. The experimental results show that the pulverized coal reburning technology can efficiently decrease the NO_x emission to meet the anticipated target.With PDA measurement system, the experiments have been conducted to investigate the effect of stoichiometric air coefficiency(α) in main combustion zone(MCZ), the bias of secondary air nozzles, the different air distribution in MCZ, the up-and-down bias of OFA air nozzles and the OFA reverse-tangent on the characteristics of aerodynamics fields in reburning zone and OFA zone. The experimental results indicate that: 1) with theαdecrease from 0.95 to 0.8, the vortex diameter in rebuning zone is increased due to the decrease of the swirling momentum; 2) with the air distribution changes in MCZ, e.g., block up different burners nearly does not effect the flow fields; 3) with the increase of the secondary air nozzles bias, the swirling intensity in furnace increases and the rigidity of the jets in reburning zone and OFA zone is weaken apparently, which causes the flow containing particles flush to the water wall, trending to form clinkering, and also the bad flow fullness in furnace reduces heat and mass transfer, which is greatly effect the coal particle combustion and NO_x deduction process; 4) the OFA reverse-tangient does not effect the jets rigidity; 5) the OFA air nozzles up or down bias for 5°, the rigidity of the jets attenuates greatly, up-bias 10°, the jets characteristics present as the free jets, down-bias 10°, the jets rigidity attenuates slightly, but the velocity of jets attenuated rapidly; 6) a relative high particle concentration area exits in the jets and near burner areas, and also exits in the near wall areas due to the particle centrifugation; 7) the particle concentration distribution is similar to the velocity distribution; 8) the turbulence intensity reaches peak in the two sides of the main jet flow where the jet flow is strongly conflict with the main vortex and upper-flow jet.The comparision with numerical simulation for the cold test and the PDA measurement results shows that with proper numerical models, the CFD software FLUENT can obtain the gas-particle two phase flow fields accurately sufficient for the industrial application.Base on the results of the cold experiments and the prediction of numerical simulation, the reburning technology retrofit schemes were brought out for No.3 boiler of Yuanbaoshan Power Plant. Before the reburning technology retrofit, two reference experiments were carried out with normal operation condition and low oxygen combustion condition. The results indicate that the low oxygen condition can greatly decrease the NO_x emission, but the boiler combustion efficiency also decreases slightly. After the retrofit, three experiments under typical operation conditions, such as air staging, reburning with H burners and reburning with G & H burners, were carried out. The experimental results indicate that under all the three conditions, the NO_x emission can all be decreased, especially, when reburning with G & H burners, NO_x emission can reach 243.3mg/m³(O₂=6%). The boiler combustion efficiency nearly did not decrease, and the clinkering is decreased, so the unit can operate stablely.Base on the camparision numerical simulation with the experimental operation conditions, the proper numerical models and parameters were applied to the further numerical prediction on the undown micronized coal reburning schemes. The results indicate that to compare with reburning with normal pulverized coal, rebuning with micronized coal can decrease the NO_x emission more effeciently, and carbon in fly ash decrease apparently, the boiler combustion efficiency increase slightly.