【作者】 王永征；

【导师】 路春美；

【作者基本信息】 山东大学 ， 热能工程， 2007， 博士

【摘要】 能源与环境是当今社会发展的两大问题。我国是煤炭大国，电力生产以燃煤火力发电为主。燃煤引起的煤烟型烟雾污染是我国大气污染的主要来源。严格控制燃煤污染物排放，实现燃煤的高效低污染燃烧，具有重大的社会经济效益和环保效益。本文以山东电力用煤为研究对象，采用试验研究、理论分析和模拟计算相结合的研究方法，结合大型电站锅炉实际运行测试数据，对电力用煤燃烧过程中硫、氮污染物的协同析出与排放特性进行了专门研究，以指导工程实践。由于煤的燃烧特性直接影响到燃烧过程中污染物的析出与排放特性，因此文中首先在热重分析仪上研究了电力用煤的燃烧特性，探讨了煤质特性对煤的着火特性、稳燃特性、燃尽特性的影响规律，并对混煤的燃烧特性进行了重点探讨。结果表明：对单一组分煤种，随着煤阶的增加，煤的活性下降，挥发分初始析出温度升高，着火温度增大，最大燃烧失重速率对应温度升高，固定碳燃尽率下降，稳燃指数减小，煤的着火及燃尽性能变差。对于各组分煤种燃烧性能相近的混煤，燃烧过程参数介于各组分煤种之间，且随着组分煤种掺混比例的增加，混煤越多地表现出该组分煤种的燃烧特性，但不同组分煤种的影响程度不同；对于各组分煤种燃烧性能差别较大的混煤，其燃烧特性变化与组分煤种差别较大，单一煤种的稳燃指数均高于混煤，总体上使混煤的着火燃烧特性变差。混煤的燃烧特性与各组分煤种的燃烧特性之间并不是简单的线性叠加关系。在煤粉燃烧试验平台、热重分析仪、高温反应器上较为系统地研究了电力用煤燃烧过程中硫、氮污染物的生成与排放特性。详细研究了不同煤种的煤粉在燃烧过程中硫污染物（SO₂、H₂S）和氮污染物（NO、NO₂）的生成机理和析出特性，并进一步探讨了不同煤种的煤质特性和运行工况参数对硫、氮污染物析出排放的影响规律。研究发现，SO₂的析出浓度曲线呈现出较为明显的“双峰”结构，这是由于煤中的低温硫和高温硫在不同阶段析出形成的；对于不同的煤种，煤中低温硫和高温硫的析出速率并不相同。H₂S的析出浓度曲线也呈现出“双峰”结构，但峰值出现时间提前。一般煤中含硫量越高，SO₂的析出浓度越大，但高挥发分煤中硫的析出较低挥发分煤中硫的析出提前。煤粉粒径增加，火焰中心H₂S浓度明显增加，SO₂生成量减少，硫析出时间变长；增加过量空气系数，可加快硫的析出，SO₂浓度较高，而H₂S浓度降低较大；随氧浓度增加，硫总体析出时间缩短。一、二次风比例增加，H₂S浓度降低，SO₂浓度增大。贴壁风可消除局部区域的还原性气氛，使H₂S浓度大幅下降；CO的存在使SO₂的析出速率、析出量明显降低，而H₂S的析出速率、析出量增加，硫的总转化率低于空气气氛下的数值；随温度升高，硫析出时间缩短，最终析出量增加。煤粉燃烧过程中氮的析出可分为前期快速生成和后期缓慢释放两个阶段，并具有“双峰”结构。NO_X主要在煤粉着火过程中产生，煤中含氮量越高，NO浓度越高，但其转化率随含氮量的增加而降低；在含氮量相同时，挥发分含量越大，NO排放浓度越高；随着煤化程度加深，NO浓度降低；煤中氧氮比越大，氮析出速率越高，氮析出量也越多。煤粉越细，NO转化率越小，细煤粉可达到较低的NO排放浓度；煤的湿度增加，可降低NO的排放量；过量空气系数增加，NO生成量增加；一、二次风比例变化对NO的生成影响非常明显，加入贴壁风可使NO浓度降低；由于CO对NO具有还原作用，使NO的生成减少。燃料氮的析出具有中温生成特性，在700℃～800℃时氮的析出量最大。NO₂的生成量与燃烧温度及其前驱物NO的浓度密切相关，温度升高时，NO₂大量分解；当NO浓度增加时，NO₂的浓度相应增大。综合考虑燃烧过程中硫、氮成分的相互作用与相互影响，对煤粉燃烧过程中硫、氮污染物的协同作用规律进行了探索性研究，重点探讨了煤中不同形态硫对氮析出的影响规律以及不同气氛下SO₂对NO生成的影响规律，同时对煤粉燃烧过程中氮对煤中硫析出的影响规律进行了初步探讨。结果表明：煤中硫的不同形态对氮的析出特性有较大影响，单质硫主要影响NO的析出峰值和峰宽，并且这种影响与煤种有关；黄铁矿硫对氮析出有显著影响，贫氧工况下煤中黄铁矿可以抑制煤半焦还原NO，有利于NO的生成。反应气氛不同，SO₂对氮析出特性的影响差异明显，随着SO₂浓度的提高，在氧化性气氛下NO生成量明显减少，而在还原性气氛下NO生成量却明显增加。NO影响煤中硫的析出特性，使得煤中硫的析出释放提前，SO₂初始生成浓度增加。并且随煤中挥发分含量的不同，SO₂析出特性表现出不同的规律。同时还发现温度对硫、氮污染物析出过程中的协同作用也有影响。对混煤燃烧过程中硫、氮污染物的协同析出与排放特性进行了试验研究，探讨了硫、氮污染物析出与排放过程中组分煤种的协同作用，以及掺入不同煤种对混煤燃烧硫、氮污染物析出与排放特性的影响规律。发现多组分煤种掺混燃烧可促进低温硫的析出，在高硫煤中掺烧低硫煤能够降低SO₂的最终排放量。混煤SO₂的析出浓度和最终析出率基本符合各组分煤种的加权计算值，且单组分煤种在混煤中所占比例越大，其析出特性就越接近该组分煤种；而H₂S的析出浓度则不同，可高于或低于组分煤种的加权计算值。混煤燃烧时NO_X的生成量与煤中的氮含量基本呈线性关系；混煤氮的析出曲线一般具有“双峰”结构，峰值区域较宽；混煤氮的析出时间、析出浓度基本位于各组分煤种之间，但与其加权平均值有一定差异。温度对混煤燃烧氮的析出特性有较大影响，在温度较低时氮析出曲线只形成单峰结构，当温度较高时才形成双峰结构，并且温度越高曲线的双峰结构越突出，氮析出量也越大。利用数学模型来模拟和预测大型电站锅炉燃煤污染物的析出与排放特性。根据煤粉燃烧特点和污染物的生成特性，运用多相流动、传热传质、湍流燃烧和污染物生成等方面的有关理论和多种数学模型以及相应的数值计算方法，对不同煤种在不同工况下炉内燃烧过程及污染物的生成排放规律进行数值模拟，得到了炉内速度场、温度场以及气相浓度场等的分布规律。结果表明：在模拟的燃烧工况条件下，整个炉膛内部湍流强度都比较强烈，煤粉气流的混合非常好，火焰充满度高，有利于煤粉的着火燃烧与燃尽；火焰在燃烧器区域附近炉膛中心形成最高温度区域，在该区域内生成的燃料型NO_X和热力型NO_X浓度都最高；减少二次风量，提高燃尽风量可使炉膛内燃烧器区域的最高温度降低，使高温区域向炉膛上部移动，NO_X的生成浓度降低。根据模拟计算的结果，结合锅炉实际运行数据，提出了优化燃烧、减少污染物排放的技术措施。建立软仪表测量模型，实现大型电站锅炉燃煤硫、氮污染物生成浓度的精确预估和在线测量。采用基于超闭球CMAC神经网络的软测量技术，在对超闭球CMAC神经网络的原理、结构、算法进行深入探讨的基础上，提出了大型电站锅炉燃煤污染物生成浓度的软测量模型。在模型中，采用通用着火特性指标F_z和煤中的含硫量S_ar、含氮量N_ar来描述煤质特性，以反映煤质特性对硫、氮污染物析出特性的影响；采用炉膛温度T和炉膛出口氧浓度C_o2（或过量空气系数α″₁）来反映炉内燃烧条件对硫、氮污染物析出特性的影响；通过对样本数据的训练与学习，得到燃煤硫、氮污染物生成浓度的软测量模型。对于混煤，模型中除各组分煤种的煤质参数和炉内燃烧条件参数外，同时包含了混煤的掺混比。在模型中也体现了硫、氮污染物在生成过程中的协同作用。计算仿真结果表明，采用CMAC（小脑模型）神经网络技术建立软仪表测量模型可实现大型电站锅炉燃煤硫、氮污染物生成浓度的精确预估和在线测量，有效地指导运行人员进行燃烧调整，以控制污染物的超标排放。综上所述，本文从煤的燃烧特性研究入手，较为全面地研究了煤燃烧过程中硫、氮污染物的析出与排放特性以及硫、氮的协同作用规律，并对混煤燃烧硫、氮污染物析出与排放过程中组分煤种的协同作用进行研究；同时利用数学模型对电站锅炉内煤粉燃烧过程及污染物的生成排放规律进行数值模拟，并采用CMAC神经网络技术建立软仪表测量模型，实现大型电站锅炉燃煤硫、氮污染物生成浓度的精确预估和在线测量，以指导运行人员进行燃烧调整。为火电机组燃用多变燃料及混煤，优化燃烧过程，降低污染物排放，提供理论依据。更多还原

【Abstract】 Energy and environment are two important problems in modern society. China is a country rich in coals, and power generation in China mainly relies on coal-fired thermal power generation. Coal smoke pollution caused by coal combustion is the main source of air pollution in China, so strictly controlling pollutant emission during coal combustion and simultaneously realizing high-efficiency, low-pollution combustion of coal are of great benefits in society, economy and environment. In this dissertation, coals used in power plant in Shandong province were selected as studied objects, by means of experimental investigation, theoretical analysis and calculating simulation, together with measuring data from large-scale coal-fired boiler in operation in power plant, characteristics of sulfur and nitrogen pollutants coordinated emission during coals combustion were specially studied to direct engineering practice.Because combustion characteristics of coals have direct effect on characteristics of various pollutants emission during coals combustion, combustion characteristics of coals used in power plant were studies with thermo-gravimetric analyzer in the beginning of the dissertation. Regularity of effect of coal property on ignition characteristics, combustion stability, burn-off characteristics were discussed, and combustion characteristics of blended coals were emphasized to study. The results showed that for single component coal, coal activity decreased with coal rank increasing , and initial temperature corresponding to volatile emission increased, ignition temperature increased, temperature corresponding to maximum weight loss rate of combustion increased, fixed carbon burn-off rate declined, combustion stability index decreased, so ignition and burn-off characteristics would become bad. For blended coals with approximately identical component coal rank in combustion characteristics, characteristic parameters of combustion were between that of component coals, and with the blended proportion of the component coal in the blended coal increasing, combustion characteristics of blended coals would be more near to that of the component coal, but extent of this effect would vary with different component coal rank. For blended coals with distinctly different component coal rank in combustion characteristics, the combustion characteristics would be strongly different from that of component coals. Combustion stability index of single component coal were larger than that of blended coals, so ignition and combustion characteristics of blended coals would become bad in general. Combustion characteristics of blended coals could not be simply figured out by the linear correlation with that of component coal ranks.Characteristics of sulfur and nitrogen pollutants emission during coal combustion in power plant were systematically studied by means of experimental system for pulverized coal combustion, thermo-gravimetric analyzer and high temperature reactor. Formation mechanism and emission characteristics of sulfur pollutants （SO₂, H₂S） and nitrogen pollutants （NO, NO2） were particularly studied during pulverized coal combustion for different coal rank. And regularities of effects of coal property and operating conditions on characteristics of sulfur and nitrogen pollutants emission were further investigated. It is found that concentration curve of SO₂ emission was of obvious two peaks in shape, and it was due to emission of sulfur in coal at different stages. For different coal rank, emission rate of sulfur in coal was different. Also concentration curve of H₂S emission was of two peaks in shape, but time corresponding peak value in concentration curve of H₂S was earlier than that of SO₂. Generally, the more sulfur content in coal was, the higher concentration of SO₂ emission was, but emission time of sulfur in coal of higher volatile content was earlier than that of lower volatile content. When particle size of pulverized coal increased, the concentration of H₂S in the center of flame increased, emission amount of SO₂ decreased, and emission time of sulfur would prolonged. As excess air coefficient increased, emission of sulfur in coal would quicken, and concentration of SO₂ increased, concentration of H₂S decreased substantially. With oxygen concentration increasing, the whole emission time of sulfur shorten. Increasing ratio of primary air to secondary air, concentration of H₂S decreased, and concentration of SO₂ increased. Adding near-wall air could clear reducing atmosphere in local area, and make concentration of SO₂ deeply decrease. At atmosphere contain CO, emission rate and emission amount of SO₂ decreased obviously, but that of H₂S increased, total conversion rate of sulfur was less than that at air atmosphere. With temperature increasing, emission time of sulfur shorten, and ultimate emission amount of sulfur increasedDuring pulverized coal combustion, emission process of nitrogen pollutants could be divided into two stage: early-stage prompt formation and later-stage gradual release, and characteristic curve of nitrogen pollutants emission was of two peaks in shape. Formation of NOx was mainly at the stage of pulverized coal ignition, and with nitrogen content in coal increasing, concentration of NO emission increased while the conversion rate of nitrogen in coal to NO decreased. In the case of the same nitrogen content in coal, the more volatile content was, the more concentration of NO emission would be. When coalification increased, concentration NO emission would decreased. With ratio of oxygen to nitrogen in coal increasing, emission rate of nitrogen increased, and emission amount of nitrogen increased. Conversion rate of nitrogen in coal to NO decreased with smaller pulverized coal fineness, so lower concentration of NO emission could be realized by smaller pulverized coal fineness. When humidity in coal increasing, emission amount of NO would decrease in general. The amount of NO formation increased with excess air coefficient increasing during coal combustion. Concentration of NO emission varied obviously with different ratio of primary air to secondary air, and adding near-wall air could reduce concentration of NO emission. Because of reducing action of CO to NO, concentration of NO emission decreased. Fuel-NO_x emission was of characteristics of formation in middle temperature, and amount of NO emission was maximal in temperature ranges from 700℃to 800℃. Amount of NO₂ formation were found to be strongly dependent on combustion temperature and precursor NO concentration. Large quantities of NO₂ decomposed when temperature increased. Formation concentration of NO₂ increased accordingly with concentration of NO increasing.Overall considering interaction of sulfur and nitrogen in combustion, regularity of coordinated action of sulfur and nitrogen pollutants during pulverized coals combustion were studied with exploring. Regularity of effect of different form of sulfur in coal on characteristics of nitrogen pollutants emission and regularity of effect of SO₂ concentration on NO formation at different atmosphere were emphasized to study, and regularity of effect of nitrogen on emission of sulfur in coal during pulverized coals combustion were investigated in the mean time. The results showed that there was great effect of different form of sulfur in coal on characteristics of nitrogen pollutants emission. Sulfur in the form of simple substance chiefly affected peak value and peak width of concentration curve of nitrogen pollutants emission, and it is relevant to coal rank. There was outstanding effect of sulfur in the form of pyrite on characteristics of nitrogen pollutants emission, pyrite in coal would restrain reaction reducing NO with semi-coke in the lean oxygen condition, and it is advantageous to NO formation. There was distinctively different effect of SO₂ concentration on NO formation at different atmosphere, when SO₂ concentration increased, amount of NO formation obviously decreased at oxidizing atmosphere, while it was just opposite at reducing atmosphere. NO also affected emission characteristics of sulfur in coal, and brought emission of sulfur in coal forward, initial concentration of SO₂ formation increased. Characteristics of SO₂ emission were of different regularity with different volatile content in coal. It was also found that there was effect of combustion temperature on coordinated action in emission of sulfur and nitrogen pollutantsCharacteristics of sulfur and nitrogen pollutants coordinated emission during blended coals combustion were experimental studied. Coordinated actions of component coal ranks in blended coals during sulfur and nitrogen pollutants emission were emphasized to investigate along with regularity of effect of blended coals with different coal ranks on sulfur and nitrogen pollutants emission. It was found that blended coal combustion can promote release of sulfur at first stage, and final emission amount of SO₂ may be reduced by means of blending high-sulfur coals with low-sulfur coals. On the whole, concentration and final emission amount of SO₂ during blended coals combustion accorded with calculating value based on the weighted means of the component coals, and with the blended proportion of the component coal in the blended coal increasing, characteristics of SO₂ emission would be more near to that of component coal. But concentration of H₂S emission was different from calculating value based on the weighted means of the component coals, and it would be larger or less than calculating value. For blended coals combustion, there was a linear correlation between formation amount of NOx and nitrogen contents in blended coals. Concentration curve of NO emission during blended coals combustion is generally of the two peaks in shape, and the region on the peak value of concentration of NO emission is wider. Concentration and time of NO emission is generally between that of component coals, but they were different from the calculating value based on the weighted means of the component coals to a certain extent. There was great effect of temperature on nitrogen pollutants coordinated emission during blended coals combustion, concentration curve of NO emission only showed one peak in shape at lower temperature, but two peaks appeared at higher temperature, and the higher temperature, the more prominent two peaks in shape, the more amount of nitrogen emission.Characteristics of pollutants emission during pulverized coal combustion in large-scale coal-fired boiler in power plant were simulated and forecasted by means of mathematical model. According to pulverized coals combustion features and pollutant formation characteristics, relevant theory and mathematical model such as multiphase flow, heat transfer, mass transfer, turbulent combustion and pollutant formation and so on, and corresponding numerical calculating method are used to numerically simulate combustion process in furnace and the regularity of pollutants formation in different operating conditions for different coal ranks, and regularity of velocity field distribution, temperature field distribution, gas-phase concentration field distribution in furnace and so on were obtained. Calculated results showed that in simulated combustion conditions, turbulent intensity in the whole furnace was very high, and mixture of pulverized coal and gas flow were quite uniform, flame permeating degree in furnace were high, so it is advantageous to ignition, combustion and burn-off of pulverized coal. Flame formed high temperature area adjacent to burner at the center of furnace, and formation concentration of both fuel-NO_x and thermal-NO_x in that area were highest. With decreasing secondary air flow rate and increasing burn-off air flow rate, highest temperature in the area adjacent to burner in furnace decreased, and high temperature area would move towards upside of furnace, formation concentration of NOx decreased. According to simulated calculation results, and using the measuring data from coal-fired boiler in operation, technical measures were put forward to optimize combustion and control pollutants emission.The soft measuring model was established to achieve precise prediction and on-line measurement of concentration of sulfur and nitrogen pollutants release from coals combusting in large-scale coal-fired boiler in the power plant. By means of soft measurement technology based on hyper CMAC neural network, soft measuring model for concentration of pollutants release from coals combusting in large-scale coal-fired boiler in the power plant were raised on the basis of an in-depth study on principle, structure and algorithm of hyper CMAC neural network. In this model, coal property is described by a general ignition characteristic index—Fz number, sulfur content S_ar and nitrogen content N_ar in coal, to reflect the effect of coal property on characteristics of sulfur and nitrogen pollutant emission; Furnace temperature T and oxygen concentration C_O2 or excess air coefficientα_l at furnace outletare used to reflect the effect of combustion condition on characteristics of sulfur and nitrogen pollutant emission. The soft measuring model for concentration of sulfur and nitrogen pollutants emission during coals combustion would be obtained through training and learning for sample data. For blended coal, blended proportion of the component coal in the blended coal needed to be included in the model besides parameters concerned with property of component coal and combustion condition in furnace. Coordinated action of sulfur and nitrogen pollutants during pulverized coals combustion was also taken into account in the model. Calculating and emulating results indicate that the soft measuring model based on CMAC neural network technology could implement accurate forecast and on-line measurement to concentration of sulfur and nitrogen pollutant emission during coals combustion in large-scale coal-fired boiler in the power plant. It would be effective to guide operator in the power plant to optimize combustion and control pollutants emission.To sum up the above, starting with studies on combustion characteristics of coals, characteristics of sulfur and nitrogen pollutants emission during coal combustion were systematically studied in the dissertation, along with regularity of coordinated action of sulfur and nitrogen pollutants were further investigated, and coordinated actions of component coal ranks in blended coals during sulfur and nitrogen pollutants emission were particularly studied. In the meantime, by means of mathematical model, combustion process in furnace and the regularity of pollutants formation were numerically simulated. Based on CMAC neural network technology, soft measuring model was established to achieve accurate forecast and on-line measurement of concentration of sulfur and nitrogen pollutants release from coals combusting in large-scale coal-fired boiler in the power plant, and further guide operator in the power plant to control combustion process. The theoretical foundation was supplied to utilize diverse coal rank and blended coal in the power plant, optimize combustion and control pollutants emission.更多还原