Transformation Of Heavy Metals And Sulfur In Coal During Supercritical Water Gasification

Coal,taking up to 65%of primary energy consumption in China,has great potential as a substitute of petroleum.However,the utilization of coal is facing with a number of strict environmental challenges.Coal supercritical water?SCW?gasification,as a promising clean and efficient coal convention technology,can directly converte the low rank coal without drying to hydrogen,hydrocarbons and oils at relatively low temperatures.However,the transformation mechanism of heavy metals?HMs?and sulfur during coal SCW gasification process is still unclear.In fact,the transfer of those elements is greatly affected by supercritical water.In this work,the migration of selected HMs under coal SCW gasification at various conditions,such as temperatures,coal-water ratios,reaction time,catalyst,atmosphere,de-ashing was thoroughly investigated in a bench autoclave.The quantitative compositions of HMs in char before and after SCW process were determined by sequential extraction,besides SEM and BET were also applied.Results showed that a sequence of their volatilities?from high to low:As>Cu>U?Cr>Pb?generally increased with raising temperatures,descending coal-water ratios,prolonging reaction time or adding catalyst,while the distributions of HMs in char had different changes accompany with series treatments.Water could erode coal matrix,augment pore volume and play an important role in HMs removal compared with pyrolysis.The efforts of SCW on sulfur transformation during coal gasification process were studied in the same autoclave.Sulfur morphological distribution of two different rank coals on solid,liquid and gas phase at different temperatures were thoroughly investigated during coal SCW gasification and also made comparation with coal pyrolysis.The sulfur forms in solid was tested by modified weight-centrifugal method.Compared with coal pyrolysis,the removal of sulfate-sulfur and pyritic-sulfur in solid state were enhanced during coal SCW gasification,especially with temperature rising.Both in coal pyrolysis and coal SCW gasification processes,the XRD analysis showed that the pyrite which blended with coal transformed into pyrrhotite,whereas pyrrhotite further converted into magnetite during SCW gasification.Supercritical water had the ability not only restrict thiophene forming,but also can oxidize organic-sulfur to produce sulfone,causing that the forming of organic-sulfur was restrained.The sulfate and sulfite were the main liquid-sulfur composition after coal pyrolysis,whereas sulfide also existed in spite of sulfate and sulfite after coal SCW gasification.The sulfide may be produced via H₂S absorbing in water.Because the sulfide can be oxidized into sulfate and sulfite by·OH radicals or coal matrix in SCW condition,the amount of sulfate and sulfite was much larger than that after coal pyrolysis.The gas sulfur after coal pyrolysis included H₂S and SO₂,but the abundant·H radicals provided by SCW system could promote the production of H₂S and inhibit the formation of SO₂.The sulfur transformation could be explained by free radical mechanism.Further researches on the basic properties of coal ashes,as well as the speciations and environmental implications of HMs and sulfur were implemented under pilot-scale SCW gasification,and then compared with bench test.The total concentration of HMs in cinder increased except As.Most enrichment factors?EF?of HMs were higher than that in bench test,while Ni had the highest EF.The risk assessment code?RAC?of As,Cd and Ni in cinder ash markedly increased from no risk or low risk into medium risk after SCW treatment,while it also greatly increased the bioactivity fractions of Ni,Pb,As and Cu.The potential ecological risk indexs?Er?values of HMs were 2-8 times higher after SCW treatment?except As?.The sulfur content from high to low was fine ash>cinder ash>slag ash and the main form in three ashes was organic sulfur,while quite a quantity of sulfate sulfur was in slag ash.This study of transformation properties was important for further environmental evaluation and the better using of coal SCW gasification technology with low sulfur pollution.