Lignite Gasification In Supercritical Water Catalytic Methane

Supercritical water gasification is a new gasification technology which can realize the efficient gasification of high moisture content biomass. At present the aim of supercritical water gasification is making H2. While H2is dangerous than CH4and CH4liquefaction is easier to H2and is convenient to transport and use, also it contens more energy. For the production of CH4, to get the best methane-producing conditions, supercritical water gasification of lignite was investigated in a high temperature and pressure batch autoclave. The material used was lignite of Zhaotong in Yunnan Province. The main research contents are:(1) By crushing, screening pretreatment, industrial analysis and elemental analysis, we got material characteristics of Zhaotong lignite.(2) By changing water content in the batch autoclave, temperature and pressure changing profile was researched in order to understand the equipment characteristics.(3) The influences of residence time, mass ratio of H2O to lignite, temperature and mass ratio of KOH to lignite on gas composition, gas yield, gasification efficiency and carbon efficiency were examined.(4) The influences of basic catalysts KOH、K2CO3、Na2CO3、Ca(OH)2and chloride of metal catalysts ZnCl2、FeCl3、CuCl2、AlCl3on gas composition, gas yield, gasification efficiency and carbon efficiency were examined.(5) Pretreatment, characterization and qualitative analysis were carried out to liquid product and reaction pathway was proposed.(6) Qualitative characterization of scanning electron microscope were carried out to fresh lignite and solid residue after reaction. Main conclusions are:(1) At our reaction condition, for reaction has reached equilibrium content and yield of CH4tend to be the same. The suitable residence time is20min in the experiment.Higher mass ratio of H2O to lignite can lead lower gas CH4yield. The main reason might be that higher water densities leads to accelerate steam reforming reaction rates, bringing higher H2and CO2but lower CH4yields. Yield of CH4increases firstly and then decreases as the mass ratio of H2O to lignite increases. This is because very lower mass ratio of H2O to lignite leads lower GE and lower CH4yield.The suitable mass ratio of H2O to lignite is1:1in the experiment.As mass ratio of H2O to lignite increases, the color of liquid product gradually becomes dark. This is because high concentration coal-water slurry is good for liquefaction but not gasification and many liquid organic matter is produced. Solid residue is loose at low concentration coal-water slurry while accumulates together at high concentration coal-water slurry. Which also explains the drop of gasification efficiency. Yield of CH4increases6times as much as temperature increases from400℃to600℃. For the total gasification reaction is endothermic, external power is needed to continue the reaction. Also high temperature is good for radical reaction and suppress the ionic reaction.(2) Yield of CH4increases firstly and then decreases as mass ratio of KOH to lignite increases. The suitable mass ratio of KOH to lignite is10%in the experiment. Because basic catalysts can improve the water-gas shift reaction and OH exists in the form of radical·OH in supercritical water, which hasstrong oxidbillity and can promote the gasification of aromatics. Increased KOH can improve gasification reaction but not so evident when KOH increased to some extent.The highest yield of CH4was110.2ml/(g daf coal) which is almost double to that of no catalyst addition when KOH was added. The main principle is asic catalysts can improve the water-gas shift reaction. The effect of Ca(OH)2was well, which may reacts to CaCO3with CO2and to CaS with S contained in lignite. ZnCl2can improve yield of CH4, the reason may be ZnCl2can improve degradation of the intermediates to acid which is good for acid catalytic reaction.(3) Liquid reaction pathway can be concludes as follow:lignite hydrolyzes to glucose, then generates to phenol and ketone by reaction of aldol condensation, inverse aldol condensation, dehydration and isomerism.As temperature increases, kinds of liquid production decreases. The reason may be higher temperature is good for gasification not for liquefaction.Because chloride of metal catalysts can improve the degradation of lignite while basic catalysts can improve the water-gas shift reaction. At the same condition kinds of liquid production is more with ZnCl2as catalyst compared with KOH.(4) Fresh lignite is loose, granular and fine. Residue after reaction is smooth block with no catalyst and is rough when ZnCl2was added. Compared with adding no catalyst and ZnCl2, when KOH was added, residue after reaction is smoother and many tiny granules evenly cover on surface.