【作者】 马培勇；

【导师】 林其钊；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2010， 博士

【摘要】 由于工业生产工程中产生的废气所含可燃成分(主要成分为甲烷)稀薄,难以被常规的燃烧技术所利用,通常被直接排空。虽然这些废气所含可燃气体成分低,但排放量巨大。将如此大量的甲烷直接排放到大气中,一方面会造成有限的不可再生资源的巨大浪费；另一方面甲烷又是很强的温室气体(温室效应约为CO2的21倍),还会造成对大气臭氧层的破坏,加剧大气污染。因此,发展低浓度可燃废气燃烧技术不仅能够“节能”,最重要的意义在于可以“减排”；探索贫燃料(超低热值可燃气体)自维持燃烧的方法对控制环境污染、回收能量都是十分有意义的。本文基于超焓燃烧原理,结合瑞士卷燃烧器和多孔介质燃烧器的技术特点和优势,设计一种新型的外置瑞士卷多孔介质燃烧器,应用于低浓度燃气燃烧处理；为设计该燃烧器,对低浓度甲烷的高温氧化特性进行了研究；探索了燃烧器的制造加工方法,并通过冷态、热态试验研究、数值计算分析研究了低浓度预混气体在外置瑞士卷多孔介质燃烧器的燃烧特性；并进一步提出将该燃烧器用于天然气富燃重整制氢,进行了化学动力学模拟与初步的实验研究。(1)为了探索贫燃料自维持燃烧方法,对目前降低燃气的贫可燃极限的方法进行了总结与分析,采取提高燃烧温度和添加催化剂是目前比较有效的方法：提高燃烧温度方法主要是采用超焓燃烧的思想,通过换热器或多孔介质蓄热体实现热再循环,极限回收燃烧产生的热量,使贫燃烧的温度提高,实现自维持燃烧；添加催化剂主要是通过催化燃烧的方式降低燃料着火的活化能,改善着火条件,拓展贫燃极限。或者采用两者的结合的复合燃烧方式,进一步降低燃料的贫可燃极限,用于可燃废气的处理与热能利用。基于以上分析,本文设计了一种集瑞士卷燃烧器和多孔介质燃烧器优点于一体的新型外置瑞士卷多孔介质燃烧器(简写为SRPC)物理模型,用于低浓度废气的燃烧处理。(2)为设计低浓度甲烷燃烧器,基于甲烷燃烧详细机理GRI 3.0,采用充分搅拌反应器(PSR)与柱塞流反应器(PFR)模型,对低浓度甲烷定压预混燃烧(氧化)过程进行了计算,研究了甲烷浓度、停留时间、预热温度、热损失等参数对超低浓度甲烷燃烧的氧化特性的影响。结果表明,这些参数对低浓度甲烷的氧化有较大影响。首先,由于低浓度甲烷预混气体单位体积内氧化放热量小,温度提升较低,化学反应时间增加,难以氧化完全,因此为增加低浓度甲烷的氧化速率,必须对其进行预热至一定温度,提高化学反应速率。另一方面,由于单位体积预混气体完全燃烧(或氧化)的热损速率与氧化反应温度成正比关系,而放热速率随甲烷浓度的减小而减小,若热损速率大于放热速率,反应温度下降,造成氧化反应速率也随之下降,反应不能继续进行,导致难以稳定自维持氧化。控制热损速率的关键因素在于增强保温措施,降低对外换热系数与外壁换热面积,采用热再循环降低排烟热损,预热低浓度甲烷预混气体；并且气流速度决定了低浓度甲烷氧化装置的氧化处理能力。研究结果可为设计低浓度甲烷氧化装置提供指导。(3)对气体在无瑞士卷结构的单向直流式多孔介质的流动特性进行了实验研究,研究了气流速度、孔密度(PPI)对阻力的影响；结果表明,在相同空隙率条件下,孔密度越大,气流速越大,流动阻力越大；分析了气流在颗粒填充床与泡沫陶瓷阻力产生的机理,得出若该两种多孔介质的比表面积及空隙率相等,且排列方式相同,两者的流动阻力近似相等；基于此,建立了泡沫陶瓷结构模型,求出两者相等时PPI与颗粒填充床的颗粒直径函数关系,可直接利用泡沫陶瓷的特征参数(PPI,空隙率),由颗粒填充床经典模型-Ergun方程来预测流动阻力；实验结果验证了此方法在预测高空隙率的泡沫陶瓷流动阻力是有效的；并确定了渗透阻力系数与惯性阻力系数的计算式,为燃烧器中心多孔介质的设计及其进行泡沫陶瓷多孔介质燃烧的数值模拟研究提供了参数计算依据。(4)采用矩形钢板拼接制作了矩形瑞士卷换热通道,并对对其进行了冷态实验,试验研究了不同风速、通道圈数对燃烧器的阻力损失分布和最大阻力的影响规律。结果表明,燃烧器通道内的压力随与送风点的距离增加而降低,弯头处的局部阻力对直管段的沿程阻力影响较大；压降随着双向逆流通道数目与风速的增加而增加；风速与总压呈介于线性和抛物线之间的非线性关系。基于沿程阻力和局部阻力公式,采用综合阻力系数与平均局部阻力系数方法对瑞士卷结构的流动阻力进行了数学描述,回归试验数据得到阻力计算公式,计算结果与实验结果吻合较好,结合多孔介质流动阻力研究结果,可为本燃烧器的设计与运行提供参考依据。该试验方法可推广应用于电力、水利、化工行业中具有多弯头的结构流动阻力的研究。其实验结果可为燃烧器通道结构优化设计、风机的初步选型、首次点火启动、热态安全运行提供必要的控制参数,同时为掌握燃烧器密封性能检测方法提供依据。(5)根据瑞士卷燃烧器初样、矩形瑞士卷燃烧器及内置刚玉管螺旋板换热器方案在加工与实验过程中存在的问题,不断优化燃烧器的设计方案与加工方法,最终确定了外置瑞士卷多孔介质燃烧器的最优设计加工方法,制作了燃烧器,并进行了燃烧特性实验,对其点火预热启动过程,及其不同预混气体浓度、流量等工况参数条件下的温度分布、排放进行了测量；分析了燃烧器的启动方法以及流量、预混气体浓度等有关工况参数对其燃烧特性的影响规律。实验发现,优化后的设计方法确保了燃烧器加工方便及其密封等特性要求；外置瑞士卷多孔介质燃烧器可有效拓展燃料的贫燃极限,具有较大的功率调节范围与较好的火焰稳定性；并对装置在实际工程化应用方面,提出了采用单元燃烧器优化设计方案。(6)提出将外置瑞士卷多孔介质燃烧器应用于天然气富燃制氢,可实现装置的自保温、产物自冷却,能量利用率高、结构紧凑,能够满足小规模现场制氢的要求；对富燃制氢过程进行了化学动力学模拟,计算结果表明,增大停留时间,提高反应温度,选择合适的化学当量比是提高制氢效率的关键所在；并且进行了天然气/空气预混气在当量比1.25-2.50,总流量60-120L/min条件下的富燃试验,研究了其自热重整制氢的燃烧特性。结果表明,富燃情况下可实现自维持燃烧,自稳定响应时间较快；瑞士卷结构的有效预热与中心多孔介质的蓄热,提高反应温度至1600K以上,可实现超绝热富燃制氢。(7)对气体在外置瑞士卷多孔介质燃烧过程进行了合理的简化,建立了外置瑞士卷多孔介质燃烧器的物理和数学模型,对甲烷空气预混气体在外置瑞士卷多孔介质的燃烧过程进行了二维数值模拟计算,将计算结果同试验结果进行了对比,分析了误差存在的原因；并分析了不同入口流速和当量比对燃烧器的性能影响,模拟结果与实验结果定性相同。更多还原

【Abstract】 The exhaust gas in the industrial processes contains little grade fuels (bases is methane). At such low concentration, it is difficult to convert this methane gas mixture to usable forms of energy with conventional technologies and is generally emitted into air. Although its concentration is very low, the discharge amount is huge. So large amounts of methane are discharged into the atmosphere with the exhaust gas, and it not only produces wasted energy resource but also adds undesirable pollutants to the atmosphere. Methane is also a greenhouse gas and its global warming potential is 21 times higher than that of CO2. The development of low concentration exhaust gas direct combustion technology can not only save energy, but it is important that it can reduce discharge. Therefore, it is very significant to explore sustain combustion of lean fuel for controlling environment pollution and using energy. Base the theory of excess enthalpy combustion, a new type porous media combustor with external Swiss-roll (abbreviated as SRPC) taking advantage of Swiss-roll combustor and porous combustor for combusting the low concentration fuel was proposed in the paper. For designing the very low concentration methane reactors, the high temperature oxidation characteristic of ultra-lean methane was theoretically investigated. The design method of the SRPC was studied and the combustion characteristics of lean gas mixture in the SRPC were carried out by not only experiments under cold state and thermal state, but also numerical simulations in this paper. Further, the SRPC used for conversion of natural gas to hydrogen under super adiabatic rich combustion is put forward in the paper and the kinetics simulation and the primary experiment was carried out.(1) For exploring the method of sustain combustion of lean fuel, the present methods of reducing the lean flammability limits of fuel were summed up and classified. It was concluded that raising the combustion temperature and putting catalyst are relatively effective methods. The method of rising the combustion temperature was based the idea of excess enthalpy combustion which can realize regenerative reheat cycle and recycle the combustion heat by heat interchanger and porous media heat accumulator. If the level of combustion temperature is raised to a certain extents, it can realize sustain combustion. The method of putting catalyst is to reduce the activation energy of the fuel, improve the ignition condition and extend lean flammability limits. Or the two combined combustion method is employed to reduce the lean flammability limits fatherly for the dispose and heat utilization of the exhaust gas. Based on the above analysis, a new type porous media combustor with external Swiss-roll bearing both advantages of Swiss-roll combustor and porous combustor was proposed in the paper.(2) For designing the very low concentration methane reactors, the high temperature oxidation characteristic of ultra-lean methane is theoretically investigated by the numerical simulations of methane oxidation in a perfectly stirred reactor PSR and plug flow reactor PFR with detailed reaction kinetics GRI 3.0. The effect of the process parameters such as methane concentration, residence time, preheating temperature, and heat loss of the oxidizer is explored. Results show that these parameters have important effect the lean methane oxidation. First, due to the low heat release of lean methane per unit volume, the temperature increase of the mixture is very low and the chemical reaction increases which cause the complete oxidation to be difficult. Therefore, in order to increase the oxidation speed of low concentration methane, it is necessary to warm up the lean methane mixtures to a certain temperature for increasing the oxidation speed. On the other hand, because the heat loss speeds of the complete oxidation of the mixture per unit volume is directly proportional to the oxidation temperature. Moreover, the heat release speed decreases with the decrease of the methane concentration. If the heat loss speed is faster than the heat release speed, the reaction temperature would decrease and it causes the reaction to stop. The crucial methods to control the heat loss speed are to strengthen heat preservation, reduce the contact area and heat transfer coefficient between the reactor and the environment and adopt heat recycle to reduce the heat loss of the exhaust and warm the lean methane mixture. The ability of low concentration methane oxidization is decided by gas velocity. The results provide some guidance for the design of ultra low concentration methane oxidation device.(3) The experimental study on the flow and combustion characteristics of uni-directional direct current porous media combustor without Swiss-roll structure was carried out. The effect of flow rate and pore density (PPI) on pressure drop were tested.Results show the resistance to flow increases with decrease of pore density and the increase of flow velocity. The mechanism of resistance to flow in foams ceramic and beds made of spherical particles was analyzed. It draws a conclusion that the resistance to flow in the two types of porous media is equal in the condition of the same surface area and same porosity. Based on the conclusion, the structure model of foam ceramic was established and the functional relations between PPI and particle diameter was created. From the functional relations and classical Ergun model for beds made of spherical particles, the resistance to flow in foams ceramic is calculated with two parameters (PPI, Porosity) provided by the manufacturer. The proposed model was successfully validated by experimental results, which fully confirm the validity of the model. The effect of flow rate and the number of channels on pressure drop were tested. The formulas for calculating permeability resistance and inertia resistance coefficient was obtained and it can provide parameters for simulation of foam ceramic porous media combustion. The experiments of gas mixture combustion in the porous with different materials and pore size were carried out.(4) The rectangle Swiss-roll heat exchange channels was made by split joint of many armor plate and cold state experiment was performed on it. The effects of flow rate and the number of channels on pressure drop were tested. Results showed that the pressure in the channels of the combustor increased with the increase of the distance from the inlet of gas. The local resistances caused by bend angle affected on-way resistance very much. The total pressure drop increased with the increase of the number of bidirectional countercurrent channels and flow rate. In the range of the experiment, the correlation of total pressure drop and the flow rate showed nonlinear relationship between line and parabola. Based on the formulas of line resistance and local resistance, the flow resistance of Swiss-roll combustor was described mathematically by synthetical resistance coefficient. A pressure drop formula was acquired by regression of experimental data. The calculation results form this formula agreed well with the experimental data. It can provide reference to the design and operation of the burner. Meanwhile, the method can also be widely applied to researches on the flow resistance of multi-elbow structures used in the electrical, water conservancy and chemical industries. The experiment results can provide necessary control parameters for the optimization design of the Swiss-roll structure, the preliminary selection of blower, first ignition and safe operation in the thermal state. Meanwhile, it can provide a test method for determining the sealing property of the combustor.(5) According to the problem of prototype sample, rectangle Swiss roll combustor, spiral-plate heat exchanger with internal alundum tube in the process of manufacture and experiment, the design proposal and manufacture method were continuously optimized. Finally an optimal method of design and manufacture was obtained. Based on the method, a SRPC was manufactured and some combustion experiments research were carried out. The temperature profiles and pollutant emission were measured in the startup process of preheating by ignition, and at various working parameters including fuel concentration and gas flow rate. The influences of the major parameters on the combustion characteristics of SRPC are analyzed. The experiments results shows the optimal design method met the challenge of convenient processing, leak tightness and so on. In the situation of stable combustion, the SRPC has higher efficiency wide range of adjustment and better flame stabilization than the combustor without Swiss-roll structure in the condition of the same methane concentration of premixed gas. For the actual project account, the optimal design method of unit combustor was proposed.(6) The SRPC used for conversion of natural gas to hydrogen under super adiabatic rich combustion is put forward, which can solve the problems of can solve the problems of flame drift, heat preservation, product cooling, and low transform efficiency. Due to its simple and compact structure, it is attractive for distributing hydrogen production system and solving the transportation and storage problems of hydrogen. The kinetics investigation on the hydrogen production from rich combustion of methane was carried out. The simulating results show that the key to increase hydrogen production is to increase residual time, raise the reaction temperation, and choose appropriate equivalence ratios. The primary rich combustion experiment was investigated over the methane-air mixture with equivalence ratio range of 1.5 to 5 and total flow range of 60 to 120 L/min. The combustion characteristics of auto-thermal reforming were studied. The experimental results show that it can realize the self-maintenance combustion in a certain extent and come to self-stabilization fast in the fuel-rich conditions. The preheating effects of Swiss-roll structure and the heat storage of porous media can raise the react temperature to more than 1600K, which realizes superadiabatic rich combustion.(7) The progress of gas mixture combustion in the SRPC was simplified reasonably, and physical models and mathematical model of the SRPC were built. Then two-dimensional numerical simulation of the SRPC for methane/air mixture combustion was conducted. And the veracity and feasibility of numeral combustion model could be verified by comparing the simulation and experimental results. The influence rules of different inlet velocity of flow and methane concentration on combustion characteristics. The simulation results were in qualitative agreement with experiment.更多还原