【作者】 斯东波；

【导师】 岑可法； 周昊；

【作者基本信息】 浙江大学 ， 工程热物理， 2008， 博士

【摘要】 氮氧化物(NO_x)是电站燃煤锅炉排放的主要污染物之一，为了满足国家日益严格的排放标准，如何有效降低我国大批燃煤锅炉的氮氧化物排放是当前能源环保领域关注的焦点之一。超细煤粉再燃和空气分级是两种典型的炉内燃烧脱硝技术，具有脱硝率较高，经济性较好的特点。其中超细煤粉再燃技术可以取得50％以上的脱硝效率，而且可以避免飞灰含碳量的升高；而燃用烟煤的超临界和超超临界锅炉在使用深度空气分级技术时可以将NO_x排放降低到350mg/Nm~3以下。本文的研究工作主要就是围绕这两种脱硝技术来展开。本文结合国家863子课题“利用超细煤粉再燃降低煤粉炉NO_x排放”，在一台200MW四角切圆燃烧煤粉锅炉上进行了超细煤粉再燃的工程示范。针对该机组采用中储式制粉系统的特点，采用三次风中的超细煤粉来实施再燃，并引入再循环烟气来降低三次风的含氧量。通过现场变工况调试试验，研究了各种主要因素对NO_x排放和锅炉运行的影响，主要包括燃尽风风速、制粉系统投运方式、是否投用烟气再循环、炉内整体氧量、入炉煤粉整体细度和三次风带粉量等。现场的长期运行试验结果表明，本文中的超细煤粉再燃系统在最佳运行条件下可以获得约40％的稳定脱硝效果，真正实现连续运行过程中的低NO_x排放，同时保证锅炉的安全经济运行。现场未能达到预期50％以上的脱硝率水平，主要是由于受现场实际条件的限制，三次风带粉率偏低且再燃区的停留时间不足。现场试验获得的数据十分有限，对炉内的燃烧和污染物生成情况缺乏全面的了解。本文借助数值模拟技术并结合现场试验数据对三次风再燃技术进行了较为详细的研究。通过对不同三次风带粉率工况下炉内流动、燃烧和污染物生成过程的综合数值模拟，分析了三次风带粉率对炉内燃烧的影响，主要包括炉内气相温度场和煤粉颗粒燃尽情况的变化，并预测了三次风带粉率增大后可以获得的脱硝效果。此外还通过数值模拟预测了OFA喷口高度提高以后的脱硝率和炉内燃烧状况，数值模拟结果表明，通过提高OFA喷口高度将再燃区停留时间增加到0．5s以后，在20％的三次风带粉率下，本文的三次风再燃系统可以达到56％的脱硝率。采用LNCFS燃烧系统的百万千瓦单炉膛双切圆燃烧锅炉是目前比较先进的一种锅炉技术。该型锅炉在炉内组织双切圆燃烧方式，并通过紧凑燃尽风(CCOFA)和分离燃尽风(SOFA)以及偏置二次风(CFS)来实现炉内纵向和水平方向的深度空气分级。本文通过冷态模化试验研究了其内部独特的空气动力场结构。试验过程中发现，在各角配风均匀的情况下，炉内形成了明显的双切圆流场，且左右侧炉膛内的气流切圆不会相互干扰，并各自体现出LNCFS燃烧系统的空气动力场特点；而如果某一侧炉膛的四角燃烧器出现配风不均匀的现象，两侧的切圆都将发生偏移，这不仅不利于炉内的稳定燃烧，还将影响到低NO_x燃烧的效果；通过SOFA风反切可以有效降低炉膛上部的扭转残余，但也容易引起炉内气流的反向旋转，在实际运行过程中，需要选择合理的反切层数和反切角度以获得更合适的反向旋转动量。本文提出了将燃尽风分级送入炉内的设想，其目的就是避免大量空气一次性进入炉内引起氧浓度过高，防止焦炭氮的集中氧化。本文以采用低NO_x同轴燃烧系统(LNCFS)的百万千瓦单炉膛双切圆燃烧锅炉为研究对象，设计了几种多级SOFA风方案，并通过数值模拟技术分析了其可行性。根据模拟结果得出，将SOFA风分成多级送入炉内可以避免SOFA风集中送入时焦炭氮的大量氧化，从而较为明显的降低第一级SOFA风加入后NO_x浓度的反弹幅度，而炉膛出口处最终的NO_x浓度还决定于后续几级燃尽风加入后NO_x浓度的上升情况；为了在炉膛出口获得更低的NO_x排放，要让尽可能多的焦炭氮在氧量相对较低的第一、二级燃尽区内析出并完成转化过程。多级SOFA风方案只是一种设想，而本文的这一部分工作可以认为是对这种方案的初步探索，若要将其投入实际运用，必须在有效性和可行性方面进行更为严格的论证。更多还原

【Abstract】 Nitrogen oxides (NO_x) are one of the major gaseous pollutants emitted from the coal-fired boilers. With the increasingly strict emission standards, the NO_x reduction for utility coal-fired boilers has been an important research topic.Micronized coal reburning and air staging are two kinds of typical in-furnace NO_x reduction technologies with advantages of high NO_x reduction efficiency and cost effective feature. Micronized coal reburning is capable of providing more than 50% NO_x reduction without increase in the fly ash carbon content. And the NO_x emissions can be reduced to 350mg/Nm~3 with the use of deep air staging in the supercritical and ultra-supercritical boiler firing bituminous coal. This work was involved in the study on these two NO_x reduction technologies for the utility coal-fired boilers.Based on 863 Programme, micronized coal reburning was demonstrated at a 200MW tangentially coal-fired boiler equipped with the ball type pulverizer system. Micronized coal contained in the tertiary air was used as the reburning fuel and the oxygen level of the tertiary air was reduced with the use of flue gas recirculation. During the performance tests, the effects of the operating variables on the NO_x emissions and boiler performance were investigated, including OFA velocity, pulverizing system operating mode, FGR, overall excess air, overall coal fineness and micronized coal content in the tertiary air. The results of the long-term tests show that NO_x emissions can be reduced by approximately 40% under the optimal operating condition, with no significant impact on boiler performance. The object of 50% NO_x reduction was not achieved mainly because of the insufficient reburning coal fraction and the short reburn zone residence time.In this work, the detailed numerical simulation on tertiary air reburning was carried out. During the simulation, the gas temperature profile in the furnace, flyash carbon content and NO_x emissions were predicted in different conditions of micronized coal content in the tertiary air and OFA nozzles elevation. The results indicate that NO_x reduction rate of 56% can be obtained without adverse impact on boiler performance with satisfying the following conditions: the OFA nozzles are shifted upward in a proper range, each zone in the furnace is operated at reasonable stoichiometry, and the micronized coal content in the tertiary air is increased to 20%. The 1000MW dual circle tangential firing boiler equipped with the low NO_x concentric firing system (LNCFS) is an advanced boiler-manufactured technology. Deep air staging was carried out with the use of CCOFA, SOFA and CFS jets. In this work, a physical isothermal flow model study was conducted to investigate the fluid mechanic performance. During the tests the following phenomena was observed. When the air flow was evenly distributed among all of the nozzles, dual tangential circles were established in the single furnace without mutual disturbance. Both circles will offset if the air distribution was nonuniform in half part of the furnace. The residual rotation at upper furnace can be eliminated by using the counter-tangential SOFA air, which is possible to cause the reverse air rotation in the upper furnace. So the levels and angles of counter-tangential SOFA nozzles should be chosen appropriately as to achieve the reasonable counter-tangential moment.In order to reduce the conversion of char-N to NO_x, a new idea was put forward in which the Over Fire Air was divided into several stages and entered the furnace at different elevations. Several multistage SOFA schemes were designed for the 1000MW dual circle tangential firing boiler equipped with the LNCFS system, and the numerical simulation technology was applied to evaluate the feasibility. The simulation results show that multistage SOFA schemes can reduce the rebound extent of NO_x concentration obviously when the first-stage OFA was injected into the furnace. The NO_x emissions at the furnace exit are determined by the NO_x formation quantity in the latter stages of burnout zone. In order to achieve lower NO_x emissions at the furnace exit, char-N should be promoted to release as much as possible in the first and/or second stage of burnout zone where the oxygen content was lower relatively. The effectiveness and feasibility of multistage SOFA schemes should be demonstrated more strictly before practical application.更多还原