A fundamental study on steam and air fluidized bed drying of Victorian brown coal

Almost 50% of the world’s coal resources are low rank coals. Efficiency for low rank coal utilization is reduced by high moisture contents and can decrease the conventional power plant efficiency by up to 9%. Drying coal is an effective way of increasing the efficiency of low rank coal combustion for power generation. A drying technology for use with Victorian brown coal is steam fluidized bed drying. However fundamental drying data is required for scaling up steam fluidized bed drying to a commercial scale. This study generates the data and also provides an understanding of the kinetics on fluidized bed drying in general, and steam fluidized bed drying in particular. Water in Victorian brown coal consists mainly of non-freezable and bulk water whereas similar coals such as Chinese lignite only contains bound and non-freezable water. The structure of bound water in Chinese Shenhua lignite also differs from that in Victorian brown coals –Loy Yang, Yallourn and Morwell–, displaying two separate freezing temperatures (-36°C and -48°C) compared to Victorian brown coal’s one (-47°C). When re-introducing water to dried coal, non-freezable water returns to previous levels, while the bound water is reduced. The difference in re-wetted coal-water interactions affects the energy requirements for re-wetted coals, and can impact agglomeration and slurry processes when water is added to coal. Observing different size fractions, smaller fractions exhibit lower bulk water mass, however the non-freezable and bound water mass remains relatively unchanged. The kinetics of both air and steam fluidized bed drying of Victorian brown coal show that increasing the fluidization velocity and bed temperature or decreasing the particle size results in a lower drying time. This trend occurs regardless of the fluidization medium, with drying ratio’s between drying temperatures and fluidization velocities remaining consistent between air and steam fluidization mediums. Through experiments in a 1 kg bed a drying time of 30 minutes was established to completely dry the coal and represents a practical drying time closer to commercial driers. To reconcile laboratory and larger drying kinetics a scaling co-efficient of 1.33 has been created in conjunction with a modified Page model. This model accurately describes the scaling of drying in a small steam fluidized bed to a large steam fluidized bed. Analysis of the chemical characteristics of dried coal indicates that the oxygen functional groups change with temperature. Synchrotron infrared experiments of single particle drying in Nitrogen indicates that functional group breakdown of Victorian brown coals occur between 140 and 250°C, while Chinese lignite’s begin to break down at 160°C. The loss of functional groups was also dependent on the residence time. At 130°C longer residence times show Chinese Shenhua lignite showing faster and different functional group loss than Yallourn brown coal. At 170°C a drop in COOH dimers is seen after 15 minutes, with additional functional group loss occurring at 25 minutes. These results show Shenhua lignite is more temperature sensitive and should be dried at temperatures below 200°C to avoid major functional group loss. During steam fluidized bed drying particle breakage occurs, with a 100 μm drop in average particle diameter observed. However a 100 μm average particle size drop occurs regardless of drying medium or method and is attributed to the transition from bulk/bound water to non-freezable water and not fluidization. At 30 and 60 minutes residence time, air fluidization decreases the average particle diameter by an additional 75 μm, while steam fluidization has no additional effect. Moisture re-adsorption of Victorian brown coal shows steam fluidized bed dried coal re-adsorbs up to 2% less moisture than air fluidized bed dried coals (9.6% vs. 7.6% for Loy Yang brown coal), and has been attributed to the change in oxygen functional groups. Unlike previously developed engineering models, a single particle model for steam drying has been developed for implementation into computational fluid dynamics (CFD) simulations. Using a lumped, master curve approach, a model has been developed to describe the surface temperature and moisture content of a coal particle during drying. This provided comparable results to other drying models in the literature, with a significantly simpler calculation method. The form of the equations allows for modelling of local dryer conditions and has practical applications in drier design and optimization. The characteristics of the dried coal were investigated through the combustion and gasification reactivity along with the ignition point analysis. The change in drying temperature and fluidization medium make no observable impact on either the reactivity or ignition point. This proves steam fluidized bed drying maintains similar combustion and gasification properties of dried coal when compared to other forms of drying. This thesis characterised the steam fluidized bed drying process, allowing for an accurate prediction of the output coal quality, from the moisture content to the resultant coal properties to the coals usability in combustion and gasification processes.全球近50%的煤炭资源为低阶煤（low rank coals）。高水分含量会降低低阶煤的利用效率，甚至可使常规电厂的发电效率最高降低9%。对煤炭进行干燥是提升低阶煤发电燃烧效率的有效手段。针对维多利亚褐煤的干燥技术为蒸汽流化床干燥（steam fluidized bed drying），但要将该技术放大至商业规模，仍需获取基础干燥数据。本研究不仅生成了所需的干燥数据，还对通用流化床干燥，尤其是蒸汽流化床干燥的动力学机理进行了阐释。维多利亚褐煤中的水分主要由不可冻结水与本体水组成，而类似煤种如中国褐煤（Chinese lignite）仅含有结合水与不可冻结水。中国神华褐煤（Shenhua lignite）的结合水结构也与维多利亚褐煤（包括Loy Yang、Yallourn及Morwell褐煤）存在差异：前者具有两个独立的冻结温度（-36℃与-48℃），而后者仅存在一个冻结温度（-47℃）。将水分重新加入干燥后的煤炭时，不可冻结水会恢复至初始水平，而结合水含量则会降低。复湿后煤-水相互作用的差异会影响复湿煤炭的能量需求，同时在向煤炭中加水时，还会对团聚过程及浆体工艺产生影响。对不同粒度级分的研究显示，粒度越小的样品其本体水含量越低，但不可冻结水与结合水的含量基本保持不变。维多利亚褐煤的空气流化床与蒸汽流化床干燥动力学研究表明：提升流化风速与床层温度，或减小颗粒粒径，均可缩短干燥时间。该规律不受流化介质类型影响，空气与蒸汽流化介质下，干燥温度与流化风速对应的干燥比值均保持一致。通过1kg级床层实验，本研究确定煤炭完全干燥所需时长为30分钟，该时长贴近商业干燥设备的实际干燥周期。为协调实验室与大型干燥装置的动力学数据，本研究结合修正Page模型（modified Page model）得到了1.33的缩放系数。该模型可精准描述小型蒸汽流化床至大型蒸汽流化床的干燥放大过程。对干燥后煤炭的化学特性分析显示，含氧官能团会随温度发生变化。氮气氛围下单颗粒干燥的同步加速器红外（synchrotron infrared）实验表明：维多利亚褐煤的官能团分解温度区间为140~250℃，而中国褐煤的官能团分解则始于160℃。官能团的脱除速率还与停留时间相关。在130℃下，延长停留时间时，中国神华褐煤的官能团脱除速率更快，且脱除路径与Yallourn褐煤存在差异。在170℃下，15分钟后即可观测到羧基二聚体（COOH dimers）含量下降，25分钟时则会发生额外的官能团脱除。上述结果表明，神华褐煤对温度更为敏感，为避免大量官能团脱除，其干燥温度应控制在200℃以下。蒸汽流化床干燥过程中会发生颗粒破碎，平均粒径可降低100μm。但无论采用何种干燥介质或干燥方式，平均粒径均会降低100μm，该现象源于本体水/结合水向不可冻结水的转变，而非流化作用。当停留时间为30分钟和60分钟时，空气流化会使平均粒径额外降低75μm，而蒸汽流化则无此额外效果。维多利亚褐煤的水分再吸附实验显示：经蒸汽流化床干燥的煤炭其水分再吸附量比空气流化床干燥的煤炭最高低2%（以Loy Yang褐煤为例，前者为7.6%，后者为9.6%），该现象源于含氧官能团的变化。与此前已开发的工程模型不同，本研究开发了可应用于计算流体动力学（computational fluid dynamics, CFD）模拟的蒸汽干燥单颗粒模型。本研究采用集总主曲线法，开发了可描述干燥过程中煤炭颗粒表面温度与水分含量的模型。该模型的计算方法更为简洁，所得结果与现有文献中的其他干燥模型结果具有可比性。该模型的方程形式可对干燥机的局部工况进行建模，在干燥机的设计与优化中具有实际应用价值。本研究通过燃烧与气化反应性分析，以及着火点测试，对干燥后煤炭的特性进行了研究。干燥温度与流化介质的变化对反应性与着火点均无显著影响。这表明，相较于其他干燥方式，蒸汽流化床干燥可保留干燥后煤炭相近的燃烧与气化性能。本论文对蒸汽流化床干燥工艺进行了系统表征，可精准预测干燥后煤炭的产品质量，包括水分含量、最终煤炭特性，以及煤炭在燃烧与气化工艺中的可用性。