【作者】 况敏；

【导师】 李争起； 朱群益；

【作者基本信息】 哈尔滨工业大学 ， 热能工程， 2012， 博士

【摘要】 无烟煤和贫煤占我国动力用煤的40%以上。针对燃用无烟煤和贫煤而引进的W火焰锅炉近年来在我国得到广泛应用，目前已投运和在建的W火焰锅炉约达130台。但实际运行表明采用引进技术制造的W火焰锅炉存在煤粉气流着火晚、结渣严重、燃烧不对称、燃尽差和NO_x排放超高等诸多问题。根据新的《火电厂大气污染物排放标准》（GB13223–2011），W火焰锅炉NO_x最高允许排放浓度为200mg/m³（折6%O₂），靠尾部脱硝很难达到这一排放标准，必需同时借助于炉内低NO_x燃烧改造。此外，对于超临界W火焰锅炉，因水冷壁温度偏差过大而导致的水冷壁鳍片拉裂甚至是爆管事故，已严重影响到锅炉的安全运行。为全面解决锅炉存在的上述问题，本文提出了W火焰锅炉多次引射分级燃烧技术，即在拱上由炉膛中心向前后墙依次布置一次风、内二次风、乏气、外二次风，在拱下布置下倾三次风，在拱部靠近喉口处布置燃尽风。其技术原理在于：在静压差作用下，低速的一次风粉由内、外二次风和三次风依次引射携带下行，从而使煤粉颗粒获得较大的下射深度而维持较高燃尽；三次风下倾和特殊的燃烧器喷口布置方式建立对称燃烧；内、外二次风和三次风以及燃尽风分级给入构建了炉内深度分级燃烧，从而抑制NO_x生成；高速的内、外二次风近前后墙集中布置，三次风高速下倾喷入而贴向冷灰斗近壁区域，加之炉内形成对称燃烧，为控制水冷壁壁温和减小壁温偏差创造了有利条件。本文除对W火焰锅炉多次引射分级燃烧技术开展研究并将其投入工业应用外，还借助于冷态单相/气固两相模化试验、工业实验和数值计算，对斗巴W火焰锅炉炉内流动特性和运行状况进行了系统研究；与斗巴技术相比，采用多次引射分级燃烧技术的W火焰锅炉在炉内燃烧状况、NO_x排放、结渣趋势和锅炉安全运行上明显优越。分别对3台斗巴W火焰锅炉（即350MW和300MW亚临界锅炉和600MW超临界锅炉）进行工业实验，结果表明：3台锅炉炉内燃烧均严重不对称，前、后墙侧温差达300°C，采用改变配风等初级手段很难消除这一现象；满负荷下飞灰可燃物含量可高达15%，锅炉效率最低仅为83%，NO_x排放量为1100¹700mg/m³（折6%O₂）；下炉膛结渣严重，尤其是在翼墙及侧墙敷设卫燃带区域；此外，600MW超临界锅炉还存在水冷壁壁温偏差过大且壁温严重超出允许值的问题。对3台锅炉分别建立冷态模化试验台，借助于冷态单相/气固两相流模化试验，发现3台锅炉炉内均形成偏斜流场，它是导致炉内燃烧不对称的根本原因；偏斜流场的形成与锅炉配风、燃烧器结构和三次风喷射角度设计不合理有关。借助于300MW亚临界锅炉的冷态模化试验，发现采用低三次风率、三次风下倾、在靠炉膛中心侧封堵二次风喷口部分面积、构建二次风在前后拱（或三次风在前后墙）不对称分配等措施，均能有效缓解甚至消除流场偏斜。发现二次风离开喷口后迅速与一次风混合，这是引起煤粉气流着火晚和NO_x排放高的主要原因；另外，炉内分级燃烧有限（三次风率一般不超过15%）也是导致锅炉NO_x排放高的原因之一。发现二次风携带颗粒冲刷前后水冷壁、高温烟气携带颗粒向翼墙和侧墙膨胀而使颗粒物冲刷炉壁，这是造成下炉膛结渣严重的主要原因。以350MW亚临界锅炉为对象对多次引射分级燃烧技术开展了一系列研究：在冷态试验台上研究了三次风率、三次风倾角、燃尽风率和燃尽风倾角对炉内单相流动特性的影响，利用数值计算研究了燃尽风布置位置和主燃区化学当量比对炉内燃烧和NO_x生成的影响。基于这些研究结果，确定该技术的参数设置为：三次风率25%，三次倾角45°，燃尽风率20%，燃尽风倾角40°，燃尽风布置在拱部靠近喉口处，保持拱下燃烧器区域化学当量比为0.66时主燃区化学当量比宜取0.96。在此锅炉上用这一技术替代斗巴W火焰燃烧技术，计算结果表明先前不对称的流场、温度场和烟气组分浓度场均转变为对称分布；在前后墙和冷灰斗的近壁区形成有利于减轻结渣的低温高氧浓度区域；NO_x排放降幅达50%且不影响燃尽。最后，将这一技术在2台新建的600MW超临界W火焰锅炉上进行工业示范。未设置燃尽风时，冷态单相/气固两相模化试验结果表明炉内流场对称，在拱下燃烧器区域内靠向炉膛中心侧形成高颗粒浓度且低气流速度区（这有利于及时着火和抑制NO_x生成）。热态测试结果表明：设置燃尽风和针对三次风的改造均不影响满负荷下炉内对称燃烧的形成；未设置燃尽风时飞灰可燃物含量在4%以内，但NO_x排放量仍在1000mg/m³（折6%O₂）以上，且冷灰斗水冷壁还存在因火焰下冲过深而导致的热疲劳问题。增设燃尽风且将三次风倾角调小至20°后，炉内烟温下降但冷灰斗内烟温仍处于较高水平，在燃尽风挡板开度40⁷0%内开大燃尽风，NO_x排放量大幅下降而飞灰可燃物含量显著上升；在高燃尽风开度下，随三次风开大，飞灰可燃物含量下降而NO_x排放量有所升高；这一阶段NO_x排放量可控制在878mg/m³（折6%O₂）的同时飞灰可燃物含量约10%。将三次风调至水平喷入且加大三次风面积后，在高的二、三次风开度下（均为75%）燃尽效果得以改善且冷灰斗区域烟温降至较安全水平；飞灰可燃物含量受燃尽风影响较小，NO_x排放随燃尽风开大而显著下降；经此次改进后，NO_x排放可降至867mg/m³（折6%O₂）的同时控制飞灰可燃物含量为5.4%；此外，在300、450和600MW负荷下均无水冷壁壁温偏差过大和壁温严重超出允许值的情况。因而，相对于引进的W火焰锅炉燃烧技术，多次引射分级燃烧技术在保证锅炉安全运行、构建对称燃烧、防止结渣、实现高效燃尽和低NO_x排放上优势明显。更多还原

【Abstract】 Anthracite and lean coal account for more than40%of power coal in China.Down-fired boilers, designed specifically for the industrial firing of anthracite and leancoal, have become popular in China in recent years. Currently, approximately130down-fired boilers are either in service or are under construction in China. Operatingresults reveal that down-fired boilers suffer from various problems such as late coalignition, heavy slagging, asymmetric combustion, poor burnout, and high NO_xemissions. According to newly promulgated emission standards for air pollutants fromthermal power plants （GB13223–2011）, the allowed NO_xemission concentration fordown-fired boilers in power plants in China is200mg/m³at6%O₂. These newguidelines require that most down-fired boilers must be retrofitted with low-NO_xcombustion and flue gas denitration processes. Again, problems of fin cracking andtube bursting in water-cooled walls, caused by large differences in water-cooled walltemperatures and these temperatures exceeding allowable values excessively, affect thesafe operation of down-fired supercritical utility boilers. In this dissertation, a newdown-fired combustion technology, i.e., multi-injection multi-stage combustiontechnology, has been developed to resolve these problems. This new technologycombustion system is characterized by three features:（i）: in the burner configuration,the fuel-rich coal/air flow nozzles, inner secondary-air ports, fuel-lean coal/air flownozzles, and outer secondary-air ports are located in order on the furnace arches fromthe side near the furnace center to the side near the front and rear walls;（ii） tertiary airis fed into the lower furnace through the lower part of the front and rear walls; and （iii）over-fire air （OFA） is positioned on the furnace arches in the zone near the furnacethroat. Three technical principles govern the new down-fired combustion technology.（i）Air-driving mechanism—Because of static-pressure differences between the low-speedfuel-rich coal/air flow and three high-speed jets （i.e., inner secondary, outer secondary,and tertiary air）, the fuel-rich coal/air flow is guided downstream by these high-speedjets into the hopper region to prolong the residence time of the pulverized coal in thefurnace and enable efficient fuel combustion.（ii） Symmetrical combustion—Thedeclivitous tertiary air and unique burner configuration establish a symmetricalcombustion pattern in the furnace.（iii） Deep-air-staging combustion—Multiple airinjections consisting of inner and outer secondary air, tertiary air, and OFA alongthe flame travel form deep-air-staging conditions in the furnace, which inhibitsignificantly the NO_xformation. Moreover, a centralized layout of the high-speedinner and outer secondary air in the zone near the front and rear walls, the high-speedtertiary air jet in the near-wall zone in the hopper, and the symmetrical combustionformation, prevent large differences in water-cooled wall temperatures and these temperatures exceeding allowable values significantly. Besides the investigations intovarious parameter optimizations and industrial applications for the new down-firedcombustion technology, this dissertation also provides a study on the flowcharacteristics and operation status of Doosan Babcock （DB） down-fired boilers,determined by cold single-phase and gas/solid two-phase flow experiments, industrialmeasurements, and numerical simulation. In comparison with DB down-firedcombustion technology, down-fired boilers with this new technology exhibit goodburnout, low-NO_xproduction, low slagging tendency and safe operation.Industrial measurements in three DB down-fired boilers （i.e., two subcriticalboilers with capacities of350and300MWeand a600MWesupercritical boiler） revealthat severely asymmetric combustion characterized by gas temperature differences ashigh as300°C arising between the regions near the front and rear walls, developed inall three furnaces. Primary measures such as parametric tuning of operating conditionsfailed to eliminate this asymmetric combustion phenomenon. At normal full load, thehighest carbon content in fly ash, the lowest boiler efficiency, and NO_xemissions were15%,83%and1100–1700mg/m³（at6%O₂）, respectively. Heavy slagging occurredin the lower furnaces of the350and300MWeboilers, especially the wing-andside-wall regions with refractory coverage. Additionally, large differences inwater-cooled wall temperatures occurred in the600MWesupercritical boiler and thesetemperatures exceeded the allowable values significantly.Using cold single-phase and gas/solid two-phase flow experiments in small-scalemodel test facilities, it was found that a deflected flow field appeared in all three boilers.This explains the severely asymmetric combustion in real furnaces. Unreasonabledesigns in air distribution, burner arrangement, and staged-air supply direction favor theflow-field deflection formation. Cold small-scale airflow experiment results for the300MWeboiler showed that various methods such as reducing the tertiary-air ratio,inclining downward tertiary air, shortening the secondary-air port area in the side nearthe furnace center, and constructing an asymmetric secondary-air distributionbetween the front and rear arches （or asymmetric tertiary-air distribution between thefront and rear walls）, could mitigate or eliminate the flow-field deflection. Afterleaving the port outlet, secondary air with higher velocity than that of the fuel-richcoal/air flow mixes rapidly with the slower flow. This process dilutes the pulverizedcoal concentration, increases the fuel-rich flow velocity in the burner zone, andfacilitates the early ignition of pulverized coal in an oxygen-rich atmosphere, therebydelaying coal ignition and producing a large amount of fuel-NO_x. In conjunction withlow tertiary-air ratios （below15%） forming shallow staging conditions in these furnace,high levels of NO_xemissions are unavoidable. The high-velocity secondary air carryingthe fuel-rich coal/air flow to wash over the front and rear walls and thehigh-temperature flue gas entrained particles expanding towards the wing and side walls, are the main causes of the heavy slagging in the furnaces.Using cold small-scale airflow experiments and numerical simulations, theimpact of various parameters settings in the new technology such as tertiary-airratio, tertiary-air declination angle, OFA ratio, and OFA declination angle on theflow field was determined for the350MWeboiler. This was in addition to the effect ofOFA location and air stoichiometric ratio in the primary combustion zone （SRp） on thecombustion characteristics and NO_xformation in the furnace. Using a series ofoptimization investigations, the optimal setup for the new technology was finallydetermined, i.e.,25%and20%for the tertiary-air and OFA ratios,45°and40°forthe tertiary-air and OFA declination angles, the arch zone close to the furnace throatfor the OFA positioned location, and0.96for the SRpvalue when the airstoichiometric ratio in the burner zone is set at0.66, respectively. Application of thenew technology as a replacement for the prior DB art in the350MWeboiler led tothree furnace performance improvements as identified by numerical simulations.（i）The original asymmetric flow field, gas temperature, and gas componentdistributions in the furnace all develop a symmetrical pattern.（ii） Relatively low gastemperatures and high O₂concentrations exist in near-wall zones, therebymitigating slagging in the lower furnace.（iii） NO_xemissions decrease by as much as50%, without increasing levels of unburnt carbon in fly ash.Finally, the new technology was applied to two newly designed down-fired600MWesupercritical utility boilers. Before applying the OFA system, results from thecold single-phase and gas/solid two-phase flow experiments for one of the twoboilers revealed that a well-formed symmetrical flow field developed in the furnace.A high-particle-concentration and low-velocity zone existed in the burner zone inthe cold gas/particle flow field. This could facilitate timely coal ignition andfuel-NO_xreduction in the real furnace. Industrial measurements showed that thefurnace achieved well-formed symmetrical combustion and the symmetrical patternwas not affected by the latter OFA application and two retrofits in tertiary air.Before the OFA application, the furnace had below4%carbon in fly ash, relativelyhigh NO_xemissions （above1000mg/m³at6%O₂） and thermal fatigue ofwater-cooled walls in the hopper slope because the too deep flame penetration depthproduced gas temperatures of up to1350°C in the hopper region. After applying OFAand reducing the staged-air declination angle from45to20°, the hopper region stillmaintained relatively high gas temperature levels, despite gas temperaturesdecreasing in the lower furnace. With increasing the OFA damper opening in therange of40–70%, NO_xemissions decreased significantly and carbon in fly ashincreased sharply. Fixing the OFA damper at an opening of50%and increasing thestaged-air flux reduced carbon in fly ash and raised NO_xemissions slightly. Byadjusting the air distribution, NO_xemissions could be reduced to approximately878 mg/m³at6%O₂, with relatively high carbon in fly ash of approximately10%.Fortunately, the latter retrofit where tertiary air is supplied horizontally and thetertiary-air slot area is increased resulted in decreased gas temperatures in the hopperregion to safe levels of approximately1110°C and relatively low carbon in fly ash （i.e.,approximately5%） with high secondary-and tertiary-air damper openings of75%.This improved the previously deteriorated burnout and resolved the thermal fatigueproblem in the hopper region. Under these circumstances, opening OFA affected theburnout rate slightly but reduced s NO_xemissions significantly. The furnace couldfinally attain a well furnace performance with low NO_xemissions （867mg/m³at6%O₂） and relatively high burnout rate （carbon in fly ash of5.4%）. In addition, largedifferences in water-cooled wall temperatures and phenomenon of the water-cooledwall temperatures exceeding allowable values were absent at boiler loads of300,450,and600MWe. In comparison with the imported down-fired combustion technologiessuffering from the problems mentioned above, the proposed multi-injectionmulti-stage combustion technology has been confirmed to be excellent in attainingsafe boiler operation, symmetrical combustion, weak slagging tendency, goodburnout, and low NO_xemissions.更多还原