【作者】 方庆艳；

【导师】 周怀春；

【作者基本信息】 华中科技大学 ， 热能工程， 2007， 博士

【摘要】 我国燃烧低挥发份煤发电占有相当大的比重。但是,低挥发份煤燃烧普遍存在燃烧效率偏低、污染物排放超标、炉膛结渣等问题。因此,低挥发份煤高效低污染燃烧仍然是我国亟需解决或改进的重要问题。电站锅炉炉内的煤粉燃烧是一个复杂的物理化学过程,要实现低挥发份煤的高效低污染燃烧,必须在充分掌握煤的燃烧特性基础上,采用适当的燃烧方式及燃烧器布置方式、合理的炉膛结构尺寸、优化的运行工况和新的燃烧技术。本文将按照这一思路,采用理论分析、数值模拟和试验相结合的研究方法,从低挥发份煤的燃烧动力学特性、混煤燃烧模拟和稳燃新技术、低挥发份煤锅炉的燃尽、NOx排放和结渣特性等方面开展研究,以期获得具有工业应用价值的研究成果。首先,本文提出将煤的燃烧特性看成是随燃尽率α变化的函数,采用两条不同升温速率下的燃烧热重分析曲线,建立化学动力学参数活化能E和指前因子A随燃尽率α的变化关系。使用热天平和沉降炉研究了一种低挥发份煤的燃烧动力学参数随燃尽率的变化特性,重点是随燃尽率变化的燃烧动力学参数对煤粉燃尽后期反应性降低的描述能力。结果表明,随燃尽率变化的燃烧动力学参数能够合理地预测煤粉燃尽后期反应活性降低的现象,比较准确地计算煤粉在沉降炉内燃尽率大于0.9以后的燃尽过程,燃尽率计算最大相对误差为4.98%,且计算精度随燃尽率增加而增加。随燃尽率变化的燃烧动力学参数将为本文中煤粉燃烧过程的数值模拟提供基础的输入数据。其次,本文以一台300 MW四角切圆锅炉为对象,开展了低挥发份煤混煤燃烧数值模拟研究。文中先采用双混合分数/PDF (Probability Density Function)方法和单混合分数/PDF方法两种方法,模拟了无烟煤和劣质烟煤在该炉炉内分层混合燃烧的过程。模拟及试验结果表明,炉内温度及氧浓度分布呈现出非均匀和对角对称分布特征。采用双混合分数/PDF方法的模拟结果更加符合两种煤在炉内分层混烧的实际燃烧过程。然后,在燃烧器区域敷设卫燃带,并采用新的混煤燃烧方式以提高锅炉运行安全性和经济性,开展了数值模拟和燃烧试验研究。结果表明,该方案能够有效提高锅炉安全性和经济性。试验确定了合理运行参数,为锅炉燃烧混煤的安全性和经济性提供了可靠的依据。接着,本文以W型火焰锅炉为对象,开展了低挥发份煤锅炉燃尽、NOx排放和结渣特性的研究。设计了W型火焰锅炉燃尽风空气深度分级技术研究方案,创新性地提出利用一次风风机风量裕度大和风压高的优势,将富余的部分一次风通过燃尽风喷口送入炉内,提高燃尽风风量和风速,开展了改善燃尽和NOx排放特性的试验研究。结果表明,该方法能够降低飞灰可燃物含量和NOx排放量,但是由于燃尽风风量不到5%,降低幅度较小。采用结渣模型并耦合气固两相燃烧模型,首次对三种W型火焰锅炉炉膛结渣特性开展了数值模拟,结合现场运行状况,对结渣位置、程度以及原因进行了深入地分析,并提出了防止结渣的方法。结果表明,W型火焰锅炉结渣特性是固有的,主要由炉内空气动力场特性决定的。切停侧边燃烧器、降低锅炉负荷以及燃用低结渣倾向的煤都能够有效防止炉膛结渣;增加燃尽风不会对锅炉结渣特性产生不利的影响。最后,在上述研究的基础上,本文对低挥发份煤稳燃新技术进行了探索性思考和初步研究。对煤粉射流吸热升温着火及稳燃机理进行了探索性的思考和分析,提出了关于煤粉射流着火稳燃机理的新解释。在此基础上,提出了一种可调节、自适应一次风内直流外旋流煤粉主燃燃器的设计思路,介绍了主燃烧器的结构和工作原理。将其应用于旋流燃烧器,在一台125 KW燃烧试验炉上对燃烧器燃烧特性进行了初步地数值模拟。结果表明,二次风旋流强度为0.67时,增加外一次风旋流强度能够有效改善低挥发份煤的着火特性和燃尽特性。研究结果可为可调节、自适应煤粉主燃烧器的进一步研究提供有益的指导。更多还原

【Abstract】 Thermal power generating capacity fueled with low-volatile coal (LVC) accounts for a sizeable proportion in China. Currently, practical applications suffer commonly from the problems of low combustion efficiency, the excess emission of pollutants, slagging et al.. Therefore, the study of the combustion of high efficiency and low pollution emission for LVC is still strongly needed and is of great significance in China. The combustion of the pulverized coal in utility boiler is a complex physical and chemical process. In order to achieve its safe, efficient and clean combustion, based on having fully grasped the coal combustion characteristics, the appropriate methods of combustion, reasonable size and structure of the furnace and burner layout, the optimization of operating conditions and new technology must be used. Following this line of thought, the researches of the efficient and clean combustion of LVC and their blends were conducted on several aspects of the combustion kinetics, the combustion simulation of coal blends, and novel stable combustion technology for LVC, and the characteristics of burnout, NOx emission and slagging for LVC boilers, with a view to obtaining the valuable research results for industrial applications.First, the kinetic parameters of chemical reaction, apparent pre-exponential factor A and activation energy E, are taken as variables changed with burnout degree, and they can be obtained from TG and DTG curves of thermo-gravimetric analysis (TGA) of a coal at two heating rates. A thermo-gravimetric analyzer and a drop tube furnace (DTF) were used to investigate the characteristics of combustion kinetic parameters changed with burnout degree of a LVC, with focus on the rationality of the kinetic parameters E(α) and A(α) describing the deactivation during the late burnout stage of coal char. The results show that the combustion kinetic parameters E(α) and A(α) can reasonably predict the low reactivity in the late burnout stage of the pulverized coal. The burnout process of the burnout degree bigger than 0.9 of this coal can be appropriate calculated. The calculated relative error of the burnout degree compared with experimental data is 4.98% in maximum, and decreases with the increase of the burnout degree. E(α) and A(α) will be used as the basic input data for the numerical simulation of the combustion process of pulverized coal. Secondly, the numerical simulation of the combustion processes of a LVC blend were carried out on a 300 MW tangentially coal-fired boiler. At first, the combustion process employing inferior bituminous coal and anthracite as fuel, injected at different floor levels, was simulated in two ways. One way was calculation by the two-mixture fraction/probability density function (PDF) approach, assuming the simultaneous presence of two kinds of coal of different property. Another way was using the single-mixture fraction/probability density function method, assuming a single coal blend of weighted average property. Simulation and actual test measured results show that the temperature and the oxygen concentration distributions in the furnace are non-uniform, but symmetric across corners and that the two-mixture fraction / PDF calculation approach is more in conformity with the actual combustion process of coal blends. Then, in order to improve the boiler performances in security and economics, refractory coverage was added on the water-cool wall region near the burns and a new mode of blended coal combustion was adopted. Numerical simulation shows that this program can effectively enhance the boiler combustion efficiency. On this basis, combustion experiments were performed to determine the reasonable operational parameters, such as the velocities of the primary and tertiary air, the distribution of the secondary air, excess air ratio, the operational mode of pulverizing system et al.. These provide the reliable directions for the optimum operation of the studied boiler.Then, the characteristics of burnout, NOx emission and slagging for LVC boilers were studied with W-flame boilers as examples. The plans for the reforming and experiments of the over fire air for W-flame boiler (OFAW)were then designed. The innovative use of the advantages of the adequate margin in air quantity and high pressure of the primary was proposed. So, some excess primary air treated as OFAW was sent into the furnace to ensure the air quantity and velocity of OFAW. The experimental results show that this method can reduce the carbon content in the fly ash and NOx emission, but the quantity is small because of the percentage of OFAW of less than 5%. The numerical simulations of slagging characteristics in three W-flame boilers were performed for the first time using the slagging models coupled with the gas-solid two phase flows combustion models. Combined with the operating conditions, the slagging position, extent and reasons were deep analyzed, the inhibition methods of slagging were presented. The results show that the slagging characteristics of W-flame boilers are inherent and determined by the aerodynamic characteristics inside the furnace. stopping side burners, reducing boiler load and using coals with low slagging tendency can effectively suppress the furnace slagging. OFAW has little affection on the the characteristics of slagging of W-flame boilers. Finally, based on the researches above, the exploratory discussion and primary investigations on the novel stable combustion technology of LVC were performaed. The mechanism of the heat absorbing, temperature increasing and ignition of the coal/air mixture flow and the mechanism for stabilizing combustion flame were deep analyzed. Some new interpretations were presented. On this basis, a kind of novel adjustable self-adaptive pulverized coal main burner and a combustion method of separating coarse and fine pulverized coal are being developed. The structure and working principle of the main burner were introduced in details. The combustion characteristics of the main burner which was applied to a swirl burner were simulated on a 125 KW down-fired furnace. The results show that an increase in the swirl strength of the outer primary air can effectively improve the ignition and burnout characteristics of LVC with the secondary air swirl intensity of 0.67. The results above may provide useful guidance for the research and development of the adjustable, self-adaptive main burner and combustion technology of separating coarse and fine pulverized coal.更多还原