摘要
循环流化床锅炉以其燃料适应性广、燃烧效率高、污染物排放低等独特的优点,而得到广泛的应用。在当前及今后仍以燃煤发电为主的情况下,循环流化床燃煤发电技术作为一种新的洁净煤燃烧发电技术势必在国内外得到更加迅速的发展。
目前,循环流化床锅炉正朝着大型化的方向发展。大型循环流化床锅炉的设计技术和对大型循环流化床锅炉的运行特性的认识,都需要了解和掌握炉内的流动、燃烧、温度等状况,而通过传统试验测量的方法只能够得到有限的数据,这成为循环流化床锅炉大型化发展的一个障碍,而今随着计算机和煤粉燃烧理论模型的发展,数值试验成为了炉内过程研究的另一个重要手段,而且通过数值试验的方法还可以弥补传统试验的不足。数值模拟是研究工程气固多相流中辅助试验方法的一种强有力的工具,本文用数值模拟的方法,对东锅DG440/13.7-Ⅱ2型循环流化床锅炉的燃烧进行了多个工况的计算分析,以期对锅炉的优化设计和运行提供有益的指导。
锅炉内的燃烧过程是非常复杂的湍流流动和反应过程,它涉及到三维湍流、多相、多组分流动;而热量的传递又包括对流换热、辐射换热、热传导;涉及到相关的化学反应又包括气相燃烧、颗粒相燃烧等。本文采用非预混燃烧模型,挥发分的气相湍流燃烧用混合分数法来模拟,用标准k ?ε紊流模型模拟气固两相间气相湍流输运,对于炉内燃烧时的辐射和对流采用P-1辐射模型,对气相流场采用非交错网格的SIMPLE方法求解N-S方程,对固体颗粒湍流扩散的求解则采用随机颗粒轨道模型,颗粒的挥发份析出及焦炭的氧化过程是颗粒反应的模拟重点,颗粒挥发份析出过程的模拟采用Kobayashi模型,焦炭燃烧过程模拟则采用动力/扩散控制反应速率模型,对于氮氧化物的生成主要考虑燃料氮和热力氮的生成,采用后处理的方法。数值模拟采用FLUENT6.2进行计算。
最后,本文对计算结果给出了直观的视图描述和定性分析。视图描述了炉内的流动、温度分布;氧气、二氧化碳和一氧化碳的浓度分布;燃料颗粒的轨迹;给煤粒径大小对燃烧的影响和分布;氮氧化物(NO)的排放,通过控制风速和添加还原性材料以在炉内营造还原性气氛从而控制NO的排放。数值模拟的结果趋势正确,符合机理分析,本文的研究内容对循环流化床的燃烧过程机理分析及其设计和运行有一定的指导意义和参考价值。
Circulating Fluidized Bed (CFB) boiler is widely used in the world, because of the characteristics of fuel flexibility, high combustion efficiency and low pollution emissions. CFB combustion is a new technology of coal cleaning combustion and will inevitably have a great development in all over the world due to its many merits.
At present, CFB is heading for the large-scale. There are many aspects such as design technology, the operation characteristics known for large CFB, which need to know and master the flow, combustion, temperature and so on in the boiler. But only finite data can be got through the measurement of traditional experiment, which becomes a barrier to develop CFB capability, these days, with the fast development of computer and the theory of pulverized coal’s combustion, numerical experiment, which can make up for the shortcoming of the traditional means, becomes another crucial method to study the above cases. Numerical simulation is a very powerful method assisting the test method in research of gas-solid multiphase flow, This paper focuses on the burning process of the DG440/13.7-Ⅱ2 CFB boiler, and several conditions are simulated, which hope to provide instructive guidance for the optimum design and operation of the boiler.
The combustion process in the boiler including turbulent flow and chemical reaction is very complex, which is concerned with three-dimensional turbulent flow, multiphase and multi-component flows; Heat transfer includes convection, radiation and conduction; while chemical reaction includes gas combustion, char combustion and so on. The paper adopted non-premixed combustion model, The equations for the mixture fraction are used to simulate volatile’s turbulent combustion, and the simulation of gas turbulent transport employs theκ-εturbulent model, then for the radiation P-1 model is used to simulate radiation heat transfer, Besides, it selects the SIMPLE difference methods of non-interleaving gridding for the gas flow field (solving the N-S equations), and accordingly the resolution of the turbulent diffusion about solid particle chooses the stochastic tracking model. The simulation emphases of the particle reaction are the separateness of the volatile and the oxidation of the char, the first one uses Kobayashi model, and the other one takes the kinetic/diffusion surface reaction rate model, NOx, including thermal NOx and fuel NOx, are post-processed from combustion.FLUENT6.2 is employed in total simulation.
Last, the paper shows the results of numerical simulation and analyzes it qualitatively. The results showed pay special attention to the following points: the boiler furnace flow; temperature distributions; oxygen species concentrations, carbon monoxide concentrations, carbon dioxide species concentrations; particles track; how the particles diameter of feed carbon affect the combustion and distribution; NOX (NO) emission and controlling the NOX (NO) emission by controlling the speed of air and adding reductive materials. The trend of the numerical simulation’results are correct and pretty reasonable to the mechanism analysis, The conclusions have a certain significance of guide and value to the mechanism analysis of combustion process, design and operation of the circulating fluidized bed.
引文
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