A fundamental study on steam and air fluidized bed drying of Victorian brown coal
thesisposted on 03.03.2017 by Stokie, David Kee William
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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.