Removal properties of hexavalent chromium and hazardous trace elements in UCG condensate water by semi-cokes

Abstract

Underground coal gasification (UCG) is an attractive clean coal technology by converting coal into combustible gases in situ, but it also poses a potential environmental risk to groundwater pollution. In this paper, the Hebi pyrolysis/gasification semi-cokes were prepared by applying tubular resistance furnace, model test of UCG were also carried out. The porous characteristics of obtained semi-cokes were characterized by measuring the amount of N2 adsorbed at 77 K, the removal of hexavalent chromium in simulated wastewater and hazardous trace elements in condensate water produced from UCG model test by pyrolysis semi-cokes were further investigated. The results indicate that the porous structure of semi-coke is closely related with applied preparation temperature and atmosphere (nitrogen, water vapor or CO2). The specific surface area (S BET) of semi-coke gasified by CO2 at 900°C reaches the largest, semi-coke pyrolyzed at 900°C (YM-900) then follows; Compared with CO2 gasification semi-coke, a lower S BET and total pore volume (V t) and a larger pore with average pore width of 4.9 nm than 3.06 nm are observed in the water vapor gasification semi-coke; An increased S BET and V t of pyrolysis semi-cokes are initially observed as the temperature increases, the largest can be obtained at 900°C and then decrease at higher temperatures. The removal results show that the highest removal rate of Cr+6 in simulated wastewater can be obtained by using YM-900, subsequently by CO2 gasification semi-coke; It is also found that hazardous trace elements in gas condensate water can be removed to certain extents by different pyrolysis semi-cokes, wherein the removal rates of Cd, Pb, Gr, Ni and Co can exceed 60%. However, parts of trace elements can be leached into groundwater from semi-coke samples, which should be paid more attention to.