The Purification And Repairation Of Polluted Groundwater By Coal?rock And Gasification Semi-coke

Underground coal gasification?UCG?is a new technology for coal mining and exploitation by converting solid coal to gaseous fuel or chemical materials in situ,and it is of great economic and environmental benefits.The possible groundwater pollution caused by UCG has been proved in previous studies.Meanwhile,the residual semi-coke after UCG,which may be of similar physicochemical properties as activated carbons,may possess potential adsorption performance.By applying self-made UCG model test systems and some other equipment,the removal effects of NH₃-N and TOC in model UCG wastewater as well as the purification mechanism were systematically investigated through purification by bituminous coal,UCG semi-coke and surrounding rocks,i.e.sandstone,limestone and shale,before and after high temperature treatment under certain temperature and pressure conditions.Meanwhile,the possible groundwater contamination caused by coal and UCG residue semi-coke were also researched and estimated by measurement of NH₃-N and TOC in this paper:?1?Porous structures are observed in raw coal,gasification semi-coke and the heated surrounding rocks,and the relative larger specific surface area and total pore volume can be measured as 21.6 m²?g^-1 and 0.025 cm³?g^-1 in semi-coke by N₂adsorption.Oxygen containing groups,such as hydroxyl and carbonyl,can be also observed in raw coal,semi-coke and heated surrounding rocks which are collected from coal measure strata.?2?In order to investigate the purification of confined polluted groundwater,leaching experiments were carried out.The results show that the removal rates of NH₃-N and TOC by coal,semi-coke and heated rocks decrease with the increasing of leaching time and then remain almost constant under designed pressures.At experimental conditions,the maximum removal rates of TOC increase with the increasing of pressure,whereas the maximum removal rate of NH₃-N can be obtained at pressures different from that of TOC due to the competition adsorption.?3?On consideration of the relative higher temperatures after gasification and slow leaching in cavity,static adsorption experiments under designed temperatures and pressures were also carried out.With the increasing of time,the removal rates of NH₃-N and TOC by coal and semi-coke increase and then decrease;the removal of NH₃-N by heated limestone increases with the increasing of time,whereas the removal of TOC keeps decreasing.With the increasing of pressure,the removal rates of NH₃-N and TOC by coal increase,and the removal by semi-coke and heated limestone initially increase and then decrease,which are in accordance with the results of leaching experiments.The removal rates of NH₃-N and TOC by the above three samples increase with temperature rising,but when above 50?,the increasing trend becomes slow.?4?The adsorption of NH₃-N onto coal,semi-coke and heated limestone can be well fitted by Freundlich model and pseudo second-order equation,indicating a multilayer physisorption process and that the adsorption process is controlled by both liquid film diffusion and inner diffusion.?5?The NH₃-N and organic components in bituminous coal and semi-coke can be leachable and result in a slight pollution of groundwater under experimental conditions,of which the possible contamination caused by bituminous coal is more serious.Although the concentrations of NH₃-N and TOC are higher than that of relevant regulations,the relative preferable adsorption performances of coal and semi-coke are verified in this paper.Thus,it is feasible for the possible use of raw coal and residual semi-coke for in situ remediation of polluted groundwater.