Characteristics And Mechanism Of Coal Gasification Fly-ash Scrubbing In Fixed Valve Tray Column

This study is motivated against Opposed Multi-Burner (OMB) entrained-flow gasification background. The study object is fixed valve tray column that is the key equipment used in syngas preliminary purification process. The multiphase flow fields, scrubbing characteristics and mechanism of fixed valve tray column have been investigated by experimental and numerical simulation. The detailed contents are as follows:1. Starting with the single-orifice bubbling with the double-sensor conductivity probe, the study lays stress on the gas-liquid two-phase flow on fixed valve trays with single and multiple orifices. The statistical laws of the phase interfacial parameters-gas holdup, interfacial area concentration, mean bubble size and mean bubble velocity along the axial height on the fixed valve tray have been obtained.2. The effect of gas and liquid flow rate, particle size, particle concentration and tray number on collection efficiency of fixed valve tray column have been studied through experiment. For particles larger than15μm, the collection efficiency can achieve about100%. For particles3-15μm, the collection efficiency will increase with increasing gas flow rate while for particles smaller than1μm, the collection efficiency decreases with increasing gas flow rate. The collection efficiencies of all particles increase with liquid flow rate. With increasing inlet particle concentrations, for particles larger than5μm the collection efficiencies increases slightly while for particles smaller than3μm, the collection efficiencies decrease apparently and for particle size between them the collection efficiency have no obvious improving. Comprehensive analysis the effects of bubble hydrodynamics, system property, and operation conditions on the collection efficiency, and a mathematical model based on mass balance was proposed to predict fly-ash particle collection efficiency in fixed valve tray column:3. The moment method was introduced to analysis the scrubbing process on fixed valve tray.The effect of gas-liquid flow characteristics and particle wettability on particle collection efficiency were also studied.The effect of gas-liquid flow characteristics on particle collection efficiency and particle size distribution properties in scrubbing process were analyzed. The results show that as particles pass through the bubble column, the geometric standard deviations of the size distribution of fly-ash particles decrease. The geometric mean diameter of fly-ash particles in the diffusion-dominant region increases, whereas it decreases in the impaction-dominant region. Bubble velocity, bubble size and gas hold-up play an important role in determining the particle collection efficiency of bubble columns. The collection efTiciency is enhanced in the entire particle size range either as bubble size decreases or as gas hold-up increases. With increase of bubble velocity, the particle collection efficiency in the diffusion-dominant region decreases, whereas it increases in the impaction-dominant region.4. The suspend particle concentration distribution in froth and particle entrainment rate above the froth of fixed valve tray column were investigated experimentally and theoretically. The experiment results indicate that only a fraction of the particle loading gets suspended in the froth. This fraction turns out to be largely dependent on the froth height, particle size, gas and liquid flow rates. However it is insensitive to particle loading variations. The results also reveal the particle entrainment rates above the froth depend more sensitively on the froth height, the bubbling rate and the vertical distance from the froth surface. With increasing gas flow rate or surface particle concentration, particle entrainment rate will increase while with increasing froth height, particle entrainment rate decreases. The surface particle concentrations will increase with increase superficial gas flow rate or decrease weir height. Equations of describing the particle concentration distribution in the froth are derived by force analysis and combine with liquid entrainment rate, a mathematical model was presented to predict fly-ash particle entrainment rates in fixed valve tray column.