褐煤过热蒸汽气流干燥过程动力学模型研究
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摘要
我国褐煤资源相对比较丰富,己探明的保有储量就达1303亿吨,占全国煤炭储量的13%。随着经济的高速发展,我国的烟煤和无烟煤等优质煤炭资源已被充分利用。因此,褐煤已经成为我国能源生产和供应的重要组成部分。但褐煤是煤化程度较低的煤种,具有高水分、高灰分、高挥发分、低发热量和低灰熔点的特征,从而导致褐煤直接燃烧效率低、温室气体排放量大等问题。同时也造成其单位能量的运输成本高,不利于长途运输和储存的缺点。我国褐煤的开发利用遇到了严重的瓶颈。褐煤的利用,如液化、干馏和气化都需要把煤中水分降至10%以下。而迄今为止,褐煤的开发利用在我国还没有实际工业化展开。其主要原因在于缺乏一套有效的工艺和技术以满足褐煤干燥的安全、节能、经济和环保的要求。而有效的干燥工艺及其相关装备的形成在很大程度上依赖于对其机理的深入了解和设计、技术方法的可靠。
本文以极具能源战略意义的褐煤作为对象,分析了褐煤的特性及国内外褐煤干燥技术的应用现状。分析表明虽然国内外学者正积极研究开发各种褐煤干燥技术和工艺,但其研究仍未取得较大的进展,相关成果也并不充裕。寻找安全、节能、环保的先进褐煤干燥技术及建立实用、完整、可靠的相关理论仍是现阶段研究的重点。过热蒸汽干燥由于具有安全、节能、环保等诸多优点,非常适用于褐煤的干燥。虽然一些学者针对褐煤的过热蒸汽干燥开展了实验研究和模型研究,但这些研究仅处在起步阶段。描述物料在干燥过程中的水分、温度的变化规律,预测和优化物料的干燥效果一般通过建立干燥热、质传递模型并进行数值模拟来实现。目前,过热蒸汽干燥动力学特性通常由干燥曲线的测试结果建立的半经验模型来反映。但这种干燥模型与干燥条件、物料特性等密切相关。而绝大部分实际干燥过程中的干燥条件却随时处于变化当中。现有形式的干燥模型在干燥过程计算的应用中受到了严重的制约,其理论亟待补充、完善并使之实用化。
与烟煤和无烟煤相比,褐煤在物理、化学及热特性等方面均有所不同,而褐煤干燥过程中的热、质传递与这些特性却密切相关。因此,本文对褐煤的物理、化学和热特性进行了分析,并对以储量较大的内蒙古赤峰褐煤为煤样,对其其孔径分布、工业分析(水分、灰分、挥发分和固定碳)、元素分析(C、H、O、N和S)和发热量等进行了测试,为褐煤过热蒸汽干燥的工艺的选择及其干燥理论的进一步研究提供了基础数据。结果表明,褐煤的内部结构与常见的大孔径多孔介质相同,其水分传递过程遵循多孔介质的规律。
基于褐煤的理化特性及工业生产特点,褐煤干燥提质首先需要综合考虑安全、能耗、水耗、环保、产能、运行费用等指标。干燥方式的选择对这些指标的影响是至关重要的。本文对常用的包括转筒干燥、回转管式干燥、流化床干燥和气流干燥等干燥工艺进行了分析,讨论了各自的优缺点。鉴于气流干燥方式结构简单、各种参数易于测量、易于大型化和工业化等优点,且其热、质传递过程与其它流态化干燥方式(如流化床干燥、旋转闪蒸干燥、喷雾干燥等)具有相同的特点和相同的数学描述。因此,本文选定气流干燥方式作为代表性的研究对象。在同时吸收了过热蒸汽干燥的安全、节能和环保的优点的基础上,开发了一套适用于褐煤干燥的安全、.节能的名为“循环分级粉碎气流”的独特的过热蒸汽流态化干燥系统,并对其工艺技术进行了详细分析,为中试试验研究和模拟研究提供了方向。
为实现褐煤过热蒸汽气流干燥工艺,验证其工业生产的可行性,本文按上述设想建立了一套独特的测试功能齐全的中试试验系统。该系统的处理能力为20-300kg/h,气流管风速为10-30m/s,进气温度为400-650℃。试验物料选用初始含水量为30-50%、原煤平均粒径为0-3mm的赤峰平庄褐煤。在试验系统的各个关键位置设置了一系列的测试孔,用于测量该处的温度、静压、颗粒速度、气体流速等参数,并可通过特制的取样装置采集该测试孔处的物料样品完成其水分、成分、温度等指标的测试。通过对颗粒的停留时间、含水量分布、气流和颗粒的温度分布等进行测量,获得了大量可用于工业规模生产和理论模型验证的具有较高参考价值的运行和过程参数；通过对干燥管内颗粒及气流的运动特性和传热特性进行了分析,掌握了大量的干燥过程参数,为下一步过热蒸汽干燥过程的模型建立及数值模拟提供了参考和验证依据,也为褐煤过热蒸汽气流干燥工程设计提供了依据。结果表明该系统能够安全、稳定地运行,且干燥后产品的品质能够满足工业生产的要求,可以应用于实际工程的工艺选择之中。
实现准确的褐煤过热蒸汽气流干燥过程预测的前提是建立精确的数学模型,以描述褐煤颗粒的含水率、温度以及气体的流速、温度等参数变化规律。从本质而言,过热蒸汽流态化干燥过程是干燥机内气流与被干燥物料的动量、热量、质量传递的耦合。在组分传递方面,目前普遍采用的是不考虑物料颗粒内部含水量差异的集总参数模型。虽然该模型具有形式简单、便于工程计算等优点,但仅适用于表面水分蒸发的干燥过程。而在用于以内部水分迁移为主要因素的干燥过程的描述时,必然会导致严重的失真。本文在分析了干燥管中褐煤颗粒和过热蒸汽两相流状况的基础上,利用N-S方程,采用离散相模型(Discrete Phase Model)描述颗粒行为,并先忽略褐煤颗粒的内部传质阻力,建立了面向过热蒸汽流态化干燥的基础计算流体动力学模型。通过建立物理模型,进行适当的网格划分,并确定进口条件、出口条件和边界条件和求解及控制条件等工作,对褐煤过热蒸汽气流干燥过程进行了模拟。模拟结果显示,对于褐煤过热蒸汽干燥,忽略褐煤颗粒的内部传质阻力会导致其干燥过程模拟的严重失真,对干燥动力学理论的补充和完善是非常必要的。
虽然国内外学者对于如褐煤等的多孔介质的组分传递进行了大量的研究,建立了大量的干燥动力学的理论模型。但由于这些模型的表达方式过于复杂,且其中的孔隙率分布、平均空隙尺寸等仍需实验确定,将其应用于实际工程计算尚需极大的研究进展和巨大的研究努力。目前,对干燥动力学的研究仍以实验的方法为主,而干燥动力学特性主要由干燥特性曲线来体现。为此,本研究建立了一套独特的可用于热风干燥和过热蒸汽干燥机理研究的实验系统,并通过进行了大量的过热蒸汽干燥机理及其干燥曲线的实验研究与测试。实验发现,与热风干燥不同,过热蒸汽干燥存在三个阶段：冷凝段,含水率增加,物料温度同时升高；恒速段为物料颗粒表面水的蒸发过程,含水率直线下降；降速段为物料内部水分迁移至颗粒表面扩散的过程,含水率呈指数规律下降。在此基础上,本文进一步研究了过热蒸汽作为干燥气流介质的干燥动力学特性,建立了一套能够描述过热蒸汽干燥全过程包括蒸汽冷凝和物料内部传质阻力在内的干燥动力学模型,使得干燥动力学理论更加完整和准确。
在上述过热蒸汽干燥的三个明显的干燥阶段中,冷凝段和恒速段仅与物料颗粒表面水分相关,采用集总参数模型可以获得较高的计算精度。而内部传质阻力的影响主要呈现于降速干燥段。本文对干燥动力学的理论和实验的分析发现,对于特定的物料和一定的粒径范围,在不同的干燥条件下,其干燥曲线具有相同的变化规律,进行适当的无量纲处理后,这些降速段的干燥曲线可为同一数学表达式。该表达式独立于干燥条件之外。基于该特点,为将恒定干燥条件下获得的干燥曲线引入干燥条件处于变化中的干燥过程的计算和模拟中,本文首次提出了通用干燥动力学模型。而运用通用干燥动力学模型的要点是：确定一定干燥条件下恒速段的干燥强度、临界含水量及临界含水量随恒速段的干燥强度的变化关系；以临界含水率为节点,含水率大于此节点的干燥过程可用成熟且简便的集总参数模型进行描述,而进入降速干燥段后,干燥过程则运用通用干燥模型进行预测。
为褐煤过热蒸汽气流干燥过程进行预测,并对褐煤干燥工艺进行优化,本文在建立了上述基础流体力学计算模型和过热蒸汽干燥动力学模型的基础上,建立了针对以气流干燥为代表的褐煤过热蒸汽流态化干燥过程的CFD模型,并进行了模拟。与中试试验结果比较显示该模型满足工程设计和计算的精度要求,可对褐煤的干燥过程进行可靠的预测,为本文研究的过热蒸汽干燥理论在过程实际中的应用奠定了良好的基础。
In China, lignite is the abundant energy resource.130.3billion tons of lignite reserves, about13%in national coal reserves have been proven. The high-quality kinds of coal, such as bituminous coal and anthracite, have been fully utilized to match the rapid national economic development. Consequently, lignite has become an important part of energy production and supply. However, because of its low rank characteristics, such as high moisture, high ash and high volatile contents, low calorific value and low ash melting point and so on, there is a lot of problems to be resolved in the lignite utilization, including combustion efficiency and greenhouse gas emissions, as well as transportation costs and storage. The development and utilization of lignite encounters a serious bottleneck. The lignite utilization, such as liquefaction, pyrolysis and gasification, requires low moisture content, say lower than10%. So far, large scale industrial lignite utilization in China has not been actually carried into execution. One of the main reasons is the lack of an effective process and technology to match the requirements of safety, energy conservation, economic and environmental protection in lignite drying. Obviously, the development of an effective drying process and its associated equipment depend on the in-depth understanding on the mechanism and the reliabilities on the approaches of design and calculation..
This paper aims at the development on the theory and the process for drying of lignite, which is of high energy strategic significance. The characteristics of lignite and application status on lignite drying were global widely analyzed. It was found that despite the massive effort, for lignite the progress on drying technologies and processes is still insufficient. At present, the emphasis of research is still on the development of reliable theories and the development of proper drying technologies. Because of many advantages, such as safety, energy conservation and environmental protection, the superheated steam drying has been widely taken for a suitable drying approach for lignite. Although some scholars have being performed the researches on the experiments and models, the researches research of lignite superheated steam drying is only in infancy. The prediction of drying process and the calculation of parameters on drying process are generally realized by a heat and mass transfer model.
Drying kinetics can be described through a series of drying curves. The drying curves are usually based on experimental determination and theory analysis and, the drying curves related with its drying condition and physical characteristics. However, in practical drying process, the drying conditions are most likely variable. As a result, the drying kinetic model is difficult to apply to the practical drying process prediction. In order to obtain the basic information for the further research of heat and mass transfer, this paper performed the analyses and determinations properties, including the pore-size distribution, proximate analysis (moisture content, ash content, volatile and fixed carbon), elemental analysis (C、H、O、N and S) and calorific value. The testing sample was taken from Inner Mongolia, where reserves a great deal of lignite. It was found that lignite is of the same structure as the common porous media.
Because of the physical and chemical properties, the first-line factors to be considered in industrial lignite drying are safety, energy consumption, water consumption, environmental protection, capacity, operating costs and so on. The consequence depends on the selections of drying approach and drying equipment. Based on the analysis of existing drying equipment, including rotary dryer, rotary steam tube dryer, fluid bed dryer and pneumatic dryer, this paper selected pneumatic dryer as the objective dryer for the reasons of simple structure, feasible parameter measurement, and scaling-up and the carried heat and mass transfer mathematical description is also suitable to other fluidized drying method (such as fluidized bed drying, spin pneumatic drying, spray drying, etc.). Combining the advantages of the superheated steam drying approach, a unique drying system, named'Upgrading of Low Degree Coal by Superheated Steam Grading-pulverization Circulated Pneumatic Dryer (SPU)'was proposed in this paper.
In order to realize the superheated steam pneumatic drying process of lignite and verify the feasibility a pilot-scale testing system was carried out. This drying system can deal with an input from20to300kg/h. the gas (superheated steam) velocity in the pipe can be varied from10to30m/s with a variable inlet temperature in a range of400～650℃. The testing lignite material is also taken from Chifeng, Inner Mongolia, with an initial moisture content range of30～50%and an average particle size of0～3mm. a number of measuring holes were maken to perform the measurement of superheated steam flow's velocity, pressure and temperature. By taking sample, moisture content, temperature and composition distributions of lignite were also determined. A large number of tests provided a massive data, such as residence time and temperature distribution. These data are valuable and indispensable in the analysis of feasibility for engineering application, particularly in our further steps for the verification of developed drying kinetics and CFD models. The tests results also showed that the drying system provides high quality product and, works with a safety and stable performance.
One of the most important preconditions for realization of accurate prediction for lignite superheated steam pneumatic drying process is the development of a mathematical model to describe the parameters variations, such as moisture content, velocity and temperature in the drying process. Radically, drying process is the coupling of momentum, heat and mass transfer. At present, because of simple formation, the widely applied models for mass transfer in drying process usually ignore the inner mass transfer resistance. These models are only suitable for the cases that moisture transfers from the particle surface. It most likely results in large error, when the model is used to calculate the inner moisture transfer process. On the bases of N-S model and discrete phase model, this paper developed a basic CFD model. In the first step, the inner mass transfer resistance was ignored. However, the simulation result showed that covering the influence of inner mass transfer resistance in the drying kinetics model is necessary for lignite drying.
Despite the plentiful of achievement on for moisture transfer in porous media, there still a far way to be applied in practical calculations, due to the extreme complicated formulation and difficulties on the determination of key variables, such as porous distribution and average porous dimension. Until now, the simple and reliable method for drying kinetic research is still experiment. The feature of drying kinetics is usually denoted by drying curves. A unique experimental set was carried out to research the drying mechanism and to determine drying curves for both hot gas drying and superheated steam drying. The experiments showed that, different from hot gas drying the superheated steam drying process can be divided by three stages:vapor condensation, constant drying rate and falling drying rate. Consequently, to make drying kinetics complete and accurate, further research was carried out that a drying kinetics model, including steam condensation and inner mass transfer resistance, was proposed.
Among the above stages, the first two only connect with surface moisture that can be accurately described by the completed pure water drop model. The influence of inner mass transfer only appears in the falling drying rate stage. It was found that for a certain material with a certain particle dimension, despite different drying conditions, the drying curves show a same variation tendency. By a proper non-dimension treatment, the drying curves can be described by a same mathematical formula. The formula is independent from the drying conditions. Based on this performance, to apply the determined drying curves, which measured under a certain drying condition, this paper proposed a now drying model, named generalized drying model. The key steps are determine the drying rate in constant drying stage, critical moisture content and the relationship of drying rate and critical moisture constant. When the process is in the first two stages, the drying process can be modeled by pure water drop. While, when it is in the falling drying rate stage, the drying process should be described by generalized drying model.
Based on the above work, a complete CFD model fro the prediction of lignite superheated team dring and calculation of engineering processes was carried out. Comparason with the pilot experimental results was also performed in this paper. The comparason shwen that this CFD model satisfies engineering requirement, can be applied into the design and system optimization.
本文以极具能源战略意义的褐煤作为对象,分析了褐煤的特性及国内外褐煤干燥技术的应用现状。分析表明虽然国内外学者正积极研究开发各种褐煤干燥技术和工艺,但其研究仍未取得较大的进展,相关成果也并不充裕。寻找安全、节能、环保的先进褐煤干燥技术及建立实用、完整、可靠的相关理论仍是现阶段研究的重点。过热蒸汽干燥由于具有安全、节能、环保等诸多优点,非常适用于褐煤的干燥。虽然一些学者针对褐煤的过热蒸汽干燥开展了实验研究和模型研究,但这些研究仅处在起步阶段。描述物料在干燥过程中的水分、温度的变化规律,预测和优化物料的干燥效果一般通过建立干燥热、质传递模型并进行数值模拟来实现。目前,过热蒸汽干燥动力学特性通常由干燥曲线的测试结果建立的半经验模型来反映。但这种干燥模型与干燥条件、物料特性等密切相关。而绝大部分实际干燥过程中的干燥条件却随时处于变化当中。现有形式的干燥模型在干燥过程计算的应用中受到了严重的制约,其理论亟待补充、完善并使之实用化。
与烟煤和无烟煤相比,褐煤在物理、化学及热特性等方面均有所不同,而褐煤干燥过程中的热、质传递与这些特性却密切相关。因此,本文对褐煤的物理、化学和热特性进行了分析,并对以储量较大的内蒙古赤峰褐煤为煤样,对其其孔径分布、工业分析(水分、灰分、挥发分和固定碳)、元素分析(C、H、O、N和S)和发热量等进行了测试,为褐煤过热蒸汽干燥的工艺的选择及其干燥理论的进一步研究提供了基础数据。结果表明,褐煤的内部结构与常见的大孔径多孔介质相同,其水分传递过程遵循多孔介质的规律。
基于褐煤的理化特性及工业生产特点,褐煤干燥提质首先需要综合考虑安全、能耗、水耗、环保、产能、运行费用等指标。干燥方式的选择对这些指标的影响是至关重要的。本文对常用的包括转筒干燥、回转管式干燥、流化床干燥和气流干燥等干燥工艺进行了分析,讨论了各自的优缺点。鉴于气流干燥方式结构简单、各种参数易于测量、易于大型化和工业化等优点,且其热、质传递过程与其它流态化干燥方式(如流化床干燥、旋转闪蒸干燥、喷雾干燥等)具有相同的特点和相同的数学描述。因此,本文选定气流干燥方式作为代表性的研究对象。在同时吸收了过热蒸汽干燥的安全、节能和环保的优点的基础上,开发了一套适用于褐煤干燥的安全、.节能的名为“循环分级粉碎气流”的独特的过热蒸汽流态化干燥系统,并对其工艺技术进行了详细分析,为中试试验研究和模拟研究提供了方向。
为实现褐煤过热蒸汽气流干燥工艺,验证其工业生产的可行性,本文按上述设想建立了一套独特的测试功能齐全的中试试验系统。该系统的处理能力为20-300kg/h,气流管风速为10-30m/s,进气温度为400-650℃。试验物料选用初始含水量为30-50%、原煤平均粒径为0-3mm的赤峰平庄褐煤。在试验系统的各个关键位置设置了一系列的测试孔,用于测量该处的温度、静压、颗粒速度、气体流速等参数,并可通过特制的取样装置采集该测试孔处的物料样品完成其水分、成分、温度等指标的测试。通过对颗粒的停留时间、含水量分布、气流和颗粒的温度分布等进行测量,获得了大量可用于工业规模生产和理论模型验证的具有较高参考价值的运行和过程参数；通过对干燥管内颗粒及气流的运动特性和传热特性进行了分析,掌握了大量的干燥过程参数,为下一步过热蒸汽干燥过程的模型建立及数值模拟提供了参考和验证依据,也为褐煤过热蒸汽气流干燥工程设计提供了依据。结果表明该系统能够安全、稳定地运行,且干燥后产品的品质能够满足工业生产的要求,可以应用于实际工程的工艺选择之中。
实现准确的褐煤过热蒸汽气流干燥过程预测的前提是建立精确的数学模型,以描述褐煤颗粒的含水率、温度以及气体的流速、温度等参数变化规律。从本质而言,过热蒸汽流态化干燥过程是干燥机内气流与被干燥物料的动量、热量、质量传递的耦合。在组分传递方面,目前普遍采用的是不考虑物料颗粒内部含水量差异的集总参数模型。虽然该模型具有形式简单、便于工程计算等优点,但仅适用于表面水分蒸发的干燥过程。而在用于以内部水分迁移为主要因素的干燥过程的描述时,必然会导致严重的失真。本文在分析了干燥管中褐煤颗粒和过热蒸汽两相流状况的基础上,利用N-S方程,采用离散相模型(Discrete Phase Model)描述颗粒行为,并先忽略褐煤颗粒的内部传质阻力,建立了面向过热蒸汽流态化干燥的基础计算流体动力学模型。通过建立物理模型,进行适当的网格划分,并确定进口条件、出口条件和边界条件和求解及控制条件等工作,对褐煤过热蒸汽气流干燥过程进行了模拟。模拟结果显示,对于褐煤过热蒸汽干燥,忽略褐煤颗粒的内部传质阻力会导致其干燥过程模拟的严重失真,对干燥动力学理论的补充和完善是非常必要的。
虽然国内外学者对于如褐煤等的多孔介质的组分传递进行了大量的研究,建立了大量的干燥动力学的理论模型。但由于这些模型的表达方式过于复杂,且其中的孔隙率分布、平均空隙尺寸等仍需实验确定,将其应用于实际工程计算尚需极大的研究进展和巨大的研究努力。目前,对干燥动力学的研究仍以实验的方法为主,而干燥动力学特性主要由干燥特性曲线来体现。为此,本研究建立了一套独特的可用于热风干燥和过热蒸汽干燥机理研究的实验系统,并通过进行了大量的过热蒸汽干燥机理及其干燥曲线的实验研究与测试。实验发现,与热风干燥不同,过热蒸汽干燥存在三个阶段：冷凝段,含水率增加,物料温度同时升高；恒速段为物料颗粒表面水的蒸发过程,含水率直线下降；降速段为物料内部水分迁移至颗粒表面扩散的过程,含水率呈指数规律下降。在此基础上,本文进一步研究了过热蒸汽作为干燥气流介质的干燥动力学特性,建立了一套能够描述过热蒸汽干燥全过程包括蒸汽冷凝和物料内部传质阻力在内的干燥动力学模型,使得干燥动力学理论更加完整和准确。
在上述过热蒸汽干燥的三个明显的干燥阶段中,冷凝段和恒速段仅与物料颗粒表面水分相关,采用集总参数模型可以获得较高的计算精度。而内部传质阻力的影响主要呈现于降速干燥段。本文对干燥动力学的理论和实验的分析发现,对于特定的物料和一定的粒径范围,在不同的干燥条件下,其干燥曲线具有相同的变化规律,进行适当的无量纲处理后,这些降速段的干燥曲线可为同一数学表达式。该表达式独立于干燥条件之外。基于该特点,为将恒定干燥条件下获得的干燥曲线引入干燥条件处于变化中的干燥过程的计算和模拟中,本文首次提出了通用干燥动力学模型。而运用通用干燥动力学模型的要点是：确定一定干燥条件下恒速段的干燥强度、临界含水量及临界含水量随恒速段的干燥强度的变化关系；以临界含水率为节点,含水率大于此节点的干燥过程可用成熟且简便的集总参数模型进行描述,而进入降速干燥段后,干燥过程则运用通用干燥模型进行预测。
为褐煤过热蒸汽气流干燥过程进行预测,并对褐煤干燥工艺进行优化,本文在建立了上述基础流体力学计算模型和过热蒸汽干燥动力学模型的基础上,建立了针对以气流干燥为代表的褐煤过热蒸汽流态化干燥过程的CFD模型,并进行了模拟。与中试试验结果比较显示该模型满足工程设计和计算的精度要求,可对褐煤的干燥过程进行可靠的预测,为本文研究的过热蒸汽干燥理论在过程实际中的应用奠定了良好的基础。
In China, lignite is the abundant energy resource.130.3billion tons of lignite reserves, about13%in national coal reserves have been proven. The high-quality kinds of coal, such as bituminous coal and anthracite, have been fully utilized to match the rapid national economic development. Consequently, lignite has become an important part of energy production and supply. However, because of its low rank characteristics, such as high moisture, high ash and high volatile contents, low calorific value and low ash melting point and so on, there is a lot of problems to be resolved in the lignite utilization, including combustion efficiency and greenhouse gas emissions, as well as transportation costs and storage. The development and utilization of lignite encounters a serious bottleneck. The lignite utilization, such as liquefaction, pyrolysis and gasification, requires low moisture content, say lower than10%. So far, large scale industrial lignite utilization in China has not been actually carried into execution. One of the main reasons is the lack of an effective process and technology to match the requirements of safety, energy conservation, economic and environmental protection in lignite drying. Obviously, the development of an effective drying process and its associated equipment depend on the in-depth understanding on the mechanism and the reliabilities on the approaches of design and calculation..
This paper aims at the development on the theory and the process for drying of lignite, which is of high energy strategic significance. The characteristics of lignite and application status on lignite drying were global widely analyzed. It was found that despite the massive effort, for lignite the progress on drying technologies and processes is still insufficient. At present, the emphasis of research is still on the development of reliable theories and the development of proper drying technologies. Because of many advantages, such as safety, energy conservation and environmental protection, the superheated steam drying has been widely taken for a suitable drying approach for lignite. Although some scholars have being performed the researches on the experiments and models, the researches research of lignite superheated steam drying is only in infancy. The prediction of drying process and the calculation of parameters on drying process are generally realized by a heat and mass transfer model.
Drying kinetics can be described through a series of drying curves. The drying curves are usually based on experimental determination and theory analysis and, the drying curves related with its drying condition and physical characteristics. However, in practical drying process, the drying conditions are most likely variable. As a result, the drying kinetic model is difficult to apply to the practical drying process prediction. In order to obtain the basic information for the further research of heat and mass transfer, this paper performed the analyses and determinations properties, including the pore-size distribution, proximate analysis (moisture content, ash content, volatile and fixed carbon), elemental analysis (C、H、O、N and S) and calorific value. The testing sample was taken from Inner Mongolia, where reserves a great deal of lignite. It was found that lignite is of the same structure as the common porous media.
Because of the physical and chemical properties, the first-line factors to be considered in industrial lignite drying are safety, energy consumption, water consumption, environmental protection, capacity, operating costs and so on. The consequence depends on the selections of drying approach and drying equipment. Based on the analysis of existing drying equipment, including rotary dryer, rotary steam tube dryer, fluid bed dryer and pneumatic dryer, this paper selected pneumatic dryer as the objective dryer for the reasons of simple structure, feasible parameter measurement, and scaling-up and the carried heat and mass transfer mathematical description is also suitable to other fluidized drying method (such as fluidized bed drying, spin pneumatic drying, spray drying, etc.). Combining the advantages of the superheated steam drying approach, a unique drying system, named'Upgrading of Low Degree Coal by Superheated Steam Grading-pulverization Circulated Pneumatic Dryer (SPU)'was proposed in this paper.
In order to realize the superheated steam pneumatic drying process of lignite and verify the feasibility a pilot-scale testing system was carried out. This drying system can deal with an input from20to300kg/h. the gas (superheated steam) velocity in the pipe can be varied from10to30m/s with a variable inlet temperature in a range of400～650℃. The testing lignite material is also taken from Chifeng, Inner Mongolia, with an initial moisture content range of30～50%and an average particle size of0～3mm. a number of measuring holes were maken to perform the measurement of superheated steam flow's velocity, pressure and temperature. By taking sample, moisture content, temperature and composition distributions of lignite were also determined. A large number of tests provided a massive data, such as residence time and temperature distribution. These data are valuable and indispensable in the analysis of feasibility for engineering application, particularly in our further steps for the verification of developed drying kinetics and CFD models. The tests results also showed that the drying system provides high quality product and, works with a safety and stable performance.
One of the most important preconditions for realization of accurate prediction for lignite superheated steam pneumatic drying process is the development of a mathematical model to describe the parameters variations, such as moisture content, velocity and temperature in the drying process. Radically, drying process is the coupling of momentum, heat and mass transfer. At present, because of simple formation, the widely applied models for mass transfer in drying process usually ignore the inner mass transfer resistance. These models are only suitable for the cases that moisture transfers from the particle surface. It most likely results in large error, when the model is used to calculate the inner moisture transfer process. On the bases of N-S model and discrete phase model, this paper developed a basic CFD model. In the first step, the inner mass transfer resistance was ignored. However, the simulation result showed that covering the influence of inner mass transfer resistance in the drying kinetics model is necessary for lignite drying.
Despite the plentiful of achievement on for moisture transfer in porous media, there still a far way to be applied in practical calculations, due to the extreme complicated formulation and difficulties on the determination of key variables, such as porous distribution and average porous dimension. Until now, the simple and reliable method for drying kinetic research is still experiment. The feature of drying kinetics is usually denoted by drying curves. A unique experimental set was carried out to research the drying mechanism and to determine drying curves for both hot gas drying and superheated steam drying. The experiments showed that, different from hot gas drying the superheated steam drying process can be divided by three stages:vapor condensation, constant drying rate and falling drying rate. Consequently, to make drying kinetics complete and accurate, further research was carried out that a drying kinetics model, including steam condensation and inner mass transfer resistance, was proposed.
Among the above stages, the first two only connect with surface moisture that can be accurately described by the completed pure water drop model. The influence of inner mass transfer only appears in the falling drying rate stage. It was found that for a certain material with a certain particle dimension, despite different drying conditions, the drying curves show a same variation tendency. By a proper non-dimension treatment, the drying curves can be described by a same mathematical formula. The formula is independent from the drying conditions. Based on this performance, to apply the determined drying curves, which measured under a certain drying condition, this paper proposed a now drying model, named generalized drying model. The key steps are determine the drying rate in constant drying stage, critical moisture content and the relationship of drying rate and critical moisture constant. When the process is in the first two stages, the drying process can be modeled by pure water drop. While, when it is in the falling drying rate stage, the drying process should be described by generalized drying model.
Based on the above work, a complete CFD model fro the prediction of lignite superheated team dring and calculation of engineering processes was carried out. Comparason with the pilot experimental results was also performed in this paper. The comparason shwen that this CFD model satisfies engineering requirement, can be applied into the design and system optimization.
引文
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