【作者】 王宝源；

【导师】 林其钊； 邢献军；

【作者基本信息】 中国科学技术大学 ， 工程热物理， 2010， 博士

【摘要】 常温空气无焰燃烧不仅与传统的有焰燃烧不同,而且其实现方式与以往的无焰燃烧也有所不同。它利用常温高速燃料射流、空气旋流和燃烧搅拌反应器实现无明显火焰前沿的燃烧,能够明显改善工业炉内的热工状态并大幅度节能,能够降低温室气体排放(单位能耗的CO2排放)和氮氧化物(NOx)等污染物排放,并且减小燃烧设备尺寸。对常温空气无焰燃烧技术的研究将有助于无焰燃烧理论的发展和该技术在冶金炉、锅炉和窑炉等行业的应用。本文结合数值模拟和实验测量研究了常温空气无焰燃烧的反应特征、常温空气无焰燃气锅炉炉膛的强化换热和污染物(氮氧化物)生成控制。搭建了一台立式燃气试验锅炉,以天然气为燃料,通过非预混的方式进行了常温空气无焰燃烧实验,进行了温度场采样、热效率测试和烟气排放测试。分别采用基于混合物分数的概率密度函数燃烧模型和耦合骨架化学反应机理的涡耗散概念(EDC)燃烧模型,对常温空气无焰锅炉炉膛的燃烧进行了三维准稳态数值模拟。通过对流场、氧浓度分布和温度场的分析,研究了常温空气无焰燃烧特征。结果表明,基于混合物分数的概率密度函数燃烧模型由于采用了即混即燃的假设,忽略了化学反应速率的影响,因而模拟的温度场与实测结果有较大的失真,特别是在锥形射流混合区域。这也说明,常温空气无焰燃烧在这个区域并非即混即燃,必须考虑湍流与化学反应过程之间的相互作用。而实测结果与EDC模型模拟结果基本一致,验证了该模型进行常温空气无焰燃烧模拟的正确性。对模拟结果的分析表明,常温空气无焰燃烧首先通过高速射流和旋流的方式提高入射反应物动量,配合入射反应流的常温条件,使得反应物在燃烧之前能卷吸更多的烟气,形成了一个相对低温的射流混合区；第二,借助燃烧搅拌器结构,提高了烟气循环量和热再循环量,保证了反应物浓度的降低和温度的提升；最终在射流下游的一个宽厚的空间里发生了化学反应,形成了无焰燃烧。通过简化分析的方式,对比了普通燃气锅炉和常温空气无焰燃气锅炉的换热情况,对常温空气无焰燃气锅炉辐射壁和水冷壁的热流密度分布进行了数值模拟分析,进行了锅炉热效率测试。结果表明,相比于普通燃气锅炉,常温空气无焰燃烧炉通过在其内部安置燃烧搅拌反应器,延长了烟气在炉膛中的流动回程,增加了燃烧搅拌反应器辐射壁与水冷壁之间的辐射传热,使锅炉水冷壁受到的辐射传热占其全部传热的80%以上,而且水冷壁受热均匀,锅炉的热效率高于现有同吨位的燃气锅炉,表明常温空气无焰燃烧方式确实强化了炉内换热过程。应用甲烷氧化过程的骨架化学反应机理,对常温空气无焰燃气锅炉的污染物浓度分布和生成速率进行了数值模拟分析,进行了污染物排放测试,研究了NOx的生成区域和浓度分布,分析了NOx的各种生成途径以及减排机理。结果表明,采用常温空气无焰燃烧可以实现超低NOx排放,NOx排放的质量浓度低于20×10-6 kg·m-3,NOx主要在射流下游周围一个较宽广的空间生成,亦即高温低氧的无焰反应区,由于消除了传统有焰燃烧的火焰锋面,热力型NO锐减,快速型NO极低,N2O转化型NO成为主要的NOx生成途径,NOx排放的实测结果与数值计算结果符合良好。更多还原

【Abstract】 Normal Temperature Air Flameless Combustion (NTAFC) is not only different from flame combustion, but also different from flameless combustion in its realization approach。NTAFC employs high speed jet flow of normal temperature reactants and stirred reactor to achieve combustion without a noticeable flame front. NTAFC is able to improve the heat transfer condition in industry boilers, save energy significantly, reduce green house gas and NOx emission, and reduce size of combustion facilities. Using numerical and experimental methods, the characteristics of NTAFC and heat transfer enhancement in NTAFC furnace and control of pollutant production were studied.A gas-burned experimental boiler was built for NTAFC using nonpremixed natural gas and air in normal temperature. Temperature was sampled and thermal efficiency as well as pollutant emission was tested. Using Probability Density Function (PDF) model based on mixture fraction and Eddy Concept (EDC) model simulated the quasi-steady 3D combustion in the furnace. The results indicated that mixture fraction model made a distortion in the simulation of conical jet flow area due to the assumption of "Mixed is burnt" and neglect of chemical reaction rate. This also indicated that the interaction of the turbulent and chemical processes must be considered in NTAFC. While the experimental results agreed well with EDC model results basically. The analysis showed that NTAFC boiler firstly employed high speed jet flow of reactants to increase the momentum, and on the condition that the reactants were in normal temperature, the jet engulfed and entrained more flue gas, and made a relatively low temperature mixing area; secondly, it took advantage of the stirred reactor to increase flue gas circulation and thermal circulation, ensured the concentration reduction and temperature increase of the reactants. Finally, a wild and thick reaction area around the downstream of the jet flow was formed, which is the flameless combustion region.The heat transfer in traditional gas-burned boiler furnace and NTAFC furnace was compared in a simplified analysis. The heat flux of the radiant plate of the stirred reactor as well as the water wall of the boiler was simulated and investigated. Thermal efficiency was tested experimentally. The result illustrated that, compared to traditional gas-burned boiler, the stirred reactor in the NTAFC boiler prolonged the journey of flue gas in the boiler furnace, increased the radiation by the radiant plates of the stirred reactor, and the proportion of radiation to the total heat transfer of the water wall was up to 80%, and the water wall was heated uniformly. The thermal efficiency of NTAFC boiler was higher than traditional gas-burned boiler for the same evaporation capacity, which corroborated the enhancement of heat transfer in NTAFC furnace.Coupling with reduced chemical kinetic reaction mechanisms of methane oxidation, the simulation of pollutants formation in NTAFC furnace was conducted and the exhaust gas at the outlet was tested and analyzed. The simulation results showed that NOx formed in a wide and broad region around the jet downstream, namely the flameless combustion region, and NOx emission at the outlet was below 20x10-6 kg-m-3. Due to the broadened reaction area, the maximum temperature was lower than 1700K and thermal NO formation was significantly suppressed compared with flame combustion. Prompt NO formation was minute, and N20->NO mechanism became the major route of NOx. And the results were agreed well with experiment.更多还原