Micronized Coal Reburning In Tangentially Fired Boiler

The pollutant NOx emission which come from coal combustion in power plant and the technology of clean coal-fired electric power have been paid more and more attention on in the world. Fuel reburning technology is one of the most efficient methods in NOx reduction. In the early research, natural gas was always being considered as the reburn fuel. With the improvement in reaearch, the researchers found micronized coal, under given conditions, can make an effect the same as, even more efficient than the NOx reduction efficiency of natural gas. Moreover, micronized coal reburning can effectively reduce NOx while minimizing carbon loss and keeping high heat efficiency. There are many factors that influence the NOx emission and unburned carbon in fly ash (UBC) during micronized coal reburning. This paper considered the factors as two hands. One is the characteristic of reburn fuel. The other is the combustion condition. The optimization of combustion parameters by revealing NOx formation and reduction mechanisms in micronized coal reburning has become an urgent problem to be solved. Up to now, simulations and experiments of coal reburning were conducted mostly for bench-scale furnace or pilot-scale furnace, which focused on the NOx reduction efficiency and were with so few reports about the unburned carbon in fly ash.The typically tangentially fired boiler was aimed to build reasonable mathematical model and geometrical model in order to conduct three dimensional numerical simulation of micronized coal reburning. The models were established through the generalization and analysis of the pollutant NOx emission mechanisms and control technology in coal combustion. Moreover, this paper conducted a systematical research on the effects of the combustion parameters and reburn fuel characteristics on the NOx emissions, unburned carbon in fly ash and the boiler heat efficiency. The major work and conclusions of present paper are as followings.1. The mathematical model was established to predict turbulence, combustion, and heat transfer in the full-scale tangentially fired utility boiler furnace. The turbulence was described by Realizable k -εturbulence model. Mixture fraction / probability density function model was employed to describe the chemical reaction and heat transfer process accrued in furnace. The stoichastic tracking model was applied to analyze the gas-solid flow field. The energy equation was solved directly, and radiation was depicted by P-1 model. The devolatilization process was modeled by two competing reaction model. Char combustion was modeled by diffusion-kinetics model. Furthermore, the optimum scheme of furnace meshing was obtained. The computational results are in good agreement with the experimental results, indicating that the mathematical and geometrical models described above are feasible in simulation of general coal combustion process.2. Three dimensional numerical simulation of air-staged combustion process in a full-scale tangentially fired boiler was conducted with those numerical models described above and a NOx post-processing approach to study the effects of the Over-fire air (OFA) ratios and the OFA heights on the NOx emissions, the unburned carbon in fly ash and the boiler heat efficiency. The results indicated that the OFA height has optimum value where NOx reduction efficiency is the highest. With the increase of OFA ratio, NOx reduction efficiency increases noticeably. The comparison between the measured and predicted NOx emissions shows a good coincidence, indicating that the post-processing approach is valid in prediction of NOx emission.3. Coal reburning technology was proposed in the foundation of the air-staged combustion technology. NOx formation and UBC in micronized coal reburning were simulated by the mathematical model described above and three NOx models. It proved that diffusion-kinetics char combustion model can be applied to predict UBC and evaluated the validity and the applicability of the three NOx models by comparing computational results and experimental ones. The results indicated that the improved model can greatly improve the NOx simulation accuracy. Moreover, it indicated that micronized coal reburning technology can largely reduce the NOx emissions through the analysis of NOx formation characteristic in micronized coal reburning.4. Three dimensional numerical simulation of micronized coal reburning for different reburn coal particle sizes in full-scale tangentially fired boiler was conducted by a suitable NOx model to study the effects of the reburn zone length, the height of reburn nozzles, the stoichiometric ratio in reburn zone, the reburn fuel fraction and the reburn coal fineness on the NOx reduction efficiency, the unburned carbon in fly ash and the boiler heat efficiency. The results indicated that the NOx reduction efficiency reaches the largest value when the relative height of reburn nozzles is about 0.210 and the stoichiometric ratio is between 0.8 and 0.9 in reburn zone; NOx reduction efficiency increases with reburn zone length, reburn fuel fraction and the decrease of reburn coal particle size; the optimum reburn fuel fraction is between 10% and 20%; the optimum particle size of reburn fuel is between 20μm and 30μm; the smaller the coal particle size is, the better the burnout performance of coal is, and the higher the boiler heat efficiency is.5. Three dimensional numerical simulation of micronized coal reburning for different reburn coal types in full-scale tangentially fired boiler was conducted by a suitable NOx model to study the effects of the combustion parameters and fuel characteristics on the NOx reduction efficiency, the unburned carbon in fly ash and the boiler heat efficiency. The results indicated that the more volatiles the coal is, the more effective the NOx reduction is, and the lower the UBC is; the type of reburn coal has no influence on the optimum injection location, but the type of primary coal has influence on the optimum injection location; the type of reburn coal has significant influence on the optimum stoichiometric ratio in reburn zone.6. By means of the analysis of results described above, the relationship between primary coal volatile matter V_daf and optimum relative height of reburn nozzles h₀op is h₀op= 0.361V_daf^-0.155and the relationship between reburn coal volatile matter V_daf and optimum stoichiometric ratio in reburn zone SR₂op isSR₂op = 0.336 + 3.66×10^-2V_daf -5.87×10^-4V_daf²which can provide convenience for combustion parameter optimization in micronized coal reburning.7. A 400t/h tangentially fired boiler was reconstructed according to the optimum parameters obtained above. Four reconstruction projects had been promoted in this work. They are air staging, natural gas reburning, micronized bituminous coal reburning and micronized brown coal reburning. Numerical simulations had been processed on the four projects above. Temperature field, species concentration field, NOx concentration field, exit gas temperature, exit species concentration, exit NOx emission, exit soot emission and UBC for different projects were given respectively. The results showed that micronized brown coal reburning is the best reconstruction project of all with high NOx reduction efficiency, lower temperature rising at exit, and smaller heat loss due to exhaust gas. This project also can reduce the unburned carbon in fly ash, enhance the combustion efficiency and boiler heat efficiency, emit less soot than natural gas reburning, largely reduce the slag and control the environment pollution. Micronized brown coal is the best of the three reburn fuel. Brown coal is of superior burnout performance, and it was cheaper and with higher NOx reduction ratio than natural gas.8. A 400t/h boiler was reconstructed successfully according to the optimum parameters which obtained through numerical simulation of a 35t/h full-scale boiler. It proved that the computational results can be applied to optimize combustion parameters of larger boiler.The new ideas presented in this paper are as follows:Firstly, a improved NOx model considering the deoxidation property of H₂ gas and the effect of the fuel reburning on the content of HCN is established through the deep analysis of NOx formation and reduction mechanisms in micronized coal reburning and pyrogenation behavior of micronized coal under reburning condition, with which the NOx concentration is calculated. Compared with the experimental results, the deviations of present calculated results are within 9%, but the deviations of results calculated by the unimproved NOx model are within 26%.Secondly, based on the three dimensional numerical simulation of micronized coal reburning in full-scale tangentially fired boiler, systematic analyses have been carried out on the effects of the combustion parameters and reburn fuel characteristics on the NOx emissions, unburned carbon in fly ash and the boiler heat efficiency. Moreover, the optimum parameters are obtained for a better comprehensive effect. The quantitative relationship between primary coal volatile matter V_daf and optimum relative height of reburn nozzles h₀op, and the quantitative relationship between reburn coal volatile matter V_daf and optimum stoichiometric ratio in reburn zone SR₂op are established for the first time.Thirdly, comprehensive quantitative analysis and comparison of air staging, natural gas reburning, micronized bituminous coal reburning and micronized brown coal reburning have been conducted. The results show the temperature, the species and the pollution of NOx distributions in the furnace, and exit soot emission. Furthermore, the UBC is given through the analysis of the particle statistical data. Finally, according to the NOx reduction efficiency, the combustion efficiency, the boiler heat efficiency and the slag, the best reconstruction project and reburn fuel can be determined.