【作者】 贾艳艳；

【导师】 毕明树；

【作者基本信息】 大连理工大学 ， 化工过程机械， 2008， 博士

【摘要】 解决燃煤发电造成氮氧化物的污染问题，发展洁净煤发电技术是当前国际社会最为关心的问题之一。燃料再燃技术是一种有效的低NOx排放燃烧技术。早期的研究工作都把天然气作为再燃燃料，随着研究的深入，研究者发现以超细煤粉作为再燃燃料，在一定条件下可以实现类似、甚至高于天然气的NOx脱除效果，同时可减小未完全燃烧损失、提高NOx还原率，确保较高的锅炉燃烧效率。超细煤粉再燃技术中影响NOx生成量和飞灰含碳量的因素很多，可归结为再燃燃料本身特性和燃烧条件。通过揭示超细煤粉再燃过程NOx的生成和控制机理，研究工艺参数的最优配置是当前亟待解决的问题。虽然国内外对此进行了不少的试验与计算研究，但都局限在一维实验炉和小型中试炉，并多将重点集中在NOx脱除率上，对于飞灰含碳量的研究少有报道。本文在对煤粉燃烧产生NOx污染物的原理、控制技术进行归纳总结与分析的基础上，以我国典型的四角切圆燃烧煤粉锅炉为研究对象，建立合理的数学模型和几何模型对超细煤粉再燃过程进行全尺寸三维数值模拟，系统地研究了再燃燃料特性和燃烧参数对NOx排放、飞灰含碳量和锅炉热效率的影响规律。论文的主要工作和结论如下。（1）建立了模拟煤粉燃烧前期流场的数学模型，气相的湍流流动选择Realizablek-ε模型，气相湍流燃烧使用混合分数概率密度函数模型，煤粉颗粒相流动采用随机轨道方法，挥发分析出模型为双竞争反应热解模型，焦炭燃烧采用动力／扩散控制燃烧模型，用P-1辐射模型计算辐射传热。对四角切圆燃煤锅炉炉内热态流场进行了数值模拟，确定了其网格划分的最优方案，并且计算结果与实测结果基本相符，证明了上述数学模型和几何模型的有效性。（2）采用上述数学模型与NOx后处理模拟方法对四角切圆锅炉空气分级燃烧过程进行了数值模拟，探讨了OFA（火上风）风率和OFA喷口高度对NOx排放、飞灰含碳量和锅炉热效率的影响，结果表明，增加OFA风率可以降低NOx排放值，OFA喷口高度h对于NOx减排存在一个最佳值。NOx浓度的数值计算结果与实测结果吻合，表明了采用NOx后处理模拟方法研究氮氧化物排放情况的可靠性。（3）煤粉再燃技术是在空气分级的基础上实现燃料分级，利用上述数学模型与三个煤粉再燃还原NOx模型对超细煤粉再燃过程NOx生成特性与飞灰含碳量进行了数值模拟，将模拟后所得计算结果与试验结果进行了比较，证明了动力／扩散控制焦炭燃烧模型对UBC预测的有效性，评估了三个煤粉再燃还原NOx模型的有效性及适用范围。结果表明，改进模型大大提高了模拟NOx的精确度。另外，通过分析超细煤粉再燃过程NOx的生成特性，表明了超细煤粉再燃具有一种减少NOx排放量较大的综合潜力。（4）利用合适的煤粉再燃还原NOx模型对全尺寸锅炉的不同粒度的超细煤粉再燃过程进行了三维数值模拟，考察了再燃区长度、再燃燃料投射位置、再燃煤粉粒径、再燃量及再燃区过量空气系数对NOx排放、飞灰含碳量和锅炉热效率的影响。提出了燃烧工艺参数优化配置方案：对于不同粒度的安徽烟煤再燃煤粉，再燃燃料喷口相对高度h₀存在同一最佳值，为0．210左右，再燃区过量空气系数存在同一最佳值，在0．8～0．9之间，此时NOx的脱除率达到最高；超细煤粉再燃量宜在10％～20％之间选择；在实际的工程应用中，可以在保证燃尽要求的基础上增大再燃区长度；再燃煤粉的平均粒径宜在20μm～30μm之间选择。结果同时表明，以超细煤粉作为再燃燃料，对NOx还原效果明显改善，并能大幅度降低飞灰含碳量，提高锅炉燃烧效率与热效率。（5）利用合适的煤粉再燃还原NOx模型对全尺寸锅炉的不同煤种的超细煤粉再燃过程进行了三维数值模拟，综合研究了燃料特性与燃烧工艺参数对NOx排放、飞灰含碳量和锅炉热效率的影响。结果表明：在相同的条件下，褐煤再燃还原NOx的效果最好，其次是安徽淮南烟煤、神府烟煤、贫煤，无烟煤效果最差；在相同的条件下，褐煤再燃带来的飞灰含碳量最低，其次是安徽淮南烟煤、神府烟煤、贫煤，无烟煤再燃炉膛出口飞灰含碳量最高；对于不同煤种的超细再燃煤粉，再燃燃料喷口相对高度h₀存在同一最佳值，再燃燃料喷口最佳位置主要与主燃料的种类有关；当以不同煤种的超细煤粉为再燃燃料时，其对应不同的最佳再燃区过量空气系数。（6）通过分析上述结果，得到了主燃料煤粉干燥无狄基挥发分含量V_daf与再燃燃料喷口相对高度最佳值h₀op之间的关系和再燃煤粉干燥无灰基挥发分含量V_daf与再燃区过量空气系数最佳值SR₂op之间的关系当对锅炉实施煤粉再燃技术时，可根据主燃料煤粉和再燃煤粉干燥无灰基挥发分含量由上述关系式估算出其对应的再燃燃料喷口相对高度最佳值和再燃区过量空气系数最佳值，为燃烧参数的优化提供了便利的途径。（7）基于前文得到的优化参数对400t／h四角切圆煤粉炉设计了4个改造方案，空气分级、天然气再燃、超细烟煤粉再燃与超细褐煤粉再燃。模拟得出了各方案的温度场及出口温度、组分浓度场及组分出口平均浓度、NOx浓度场及其排放浓度、碳黑（soot）的排放浓度与飞灰含碳量。形成了从NOx脱除率、锅炉燃烧效率、锅炉热效率和锅炉结渣四个角度出发，评价不同改造方案优劣的方法。结果表明，超细褐煤粉再燃是4个改造方案中最理想的改造方案，NOx脱除率高，炉膛出口温升较小，排烟热量损失较小，并降低了炉膛出口不完全燃烧产物排放量和飞灰含碳量，增加了锅炉的燃烧效率与热效率，得到了比天然气再燃更低的碳黑排放量，更大程度的抑制了锅炉结渣，控制了环境污染。超细褐煤粉是三种再燃燃料当中最为理想的再燃燃料，它有优越的燃尽性，与天然气相比成本低并且对NOx有更强的还原性。（8）利用基于35t／h全尺寸锅炉数值试验得到的优化参数对容量更大（400t／h）、燃烧器喷口布置更复杂的锅炉设计了改造方案，分析改造方案的模拟结果可以看出，改造方案可以获得高效率、低污染的燃烧效果，从而在一定程度上说明了本文得到的优化燃烧参数放大的准确性。本文计算结果可用于更大型号的锅炉燃烧参数的优化。本文的创新点是：（1）通过深刻分析超细煤粉再燃过程NOx的生成／还原机理与超细煤粉再燃条件下的热解特性，基于焦炭N转化为NO模型，提出了考虑还原性组分H₂对NO的还原和燃料再燃对HCN含量的影响的煤粉再燃还原NOx改进模型，使NOx浓度的计算结果最大偏差由26％降低到9％。（2）基于全尺寸锅炉超细煤粉再燃过程的三维数值模拟，全面分析了再燃煤粉特性和燃烧工艺参数对NOx排放及脱除率、飞灰含碳量和锅炉热效率的影响规律，得到了锅炉燃烧综合效果较好时各参数的优化配置。并提出了最佳再燃燃料投射位置与主燃料煤粉干燥无灰基挥发分含量的定量关系式，最佳再燃区过量空气系数与再燃煤粉干燥无灰基挥发分含量的定量关系式。（3）针对大型四角切圆燃烧煤粉锅炉设计了改造方案，全方位定量分析比较了空气分级、天然气再燃、超细烟煤粉再燃与超细褐煤粉再燃4种改造方案，给出了各方案炉内详细的温度场，组分浓度场，NOx、碳黑等污染物的排放状况，并通过分析颗粒统计数据，确定了炉膛出口飞灰含碳量。提出了从NOx脱除率、锅炉燃烧效率、锅炉热效率和锅炉结渣四个角度出发，评价不同改造方案优劣的方法，从而确定了最优的改造方案和最佳的再燃燃料。更多还原

【Abstract】 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.更多还原