The trace elements emission could result in harmness to the environment and human health during coal combustion.The removal of trace elements before coal combustion is an effective method.The transformation and removal behavior of arsenic in Wujiaping(WJP) coal during sub-critical water treatment was studied in a semi-continuous apparatus.Effects of temperature,pressure,residence time and water flow rate on the arsenic removal were discussed.The occurrence modes of arsenic in WJP raw coal and residuals treated by sub-critical water were also studied by float-sink experiments and sequential chemical extractions.The results show that with the increasing of temperature,pressure,residence time and water flow rate,the arsenic removal in WJP coal also increases;and at the condition of 380 ℃,15 MPa,60 min and water flow rate of 0.58 L/h,the arsenic removal is about 62%.Arsenic in WJP coal mainly is associated with sulfates & mono sulfides,disulfide & some organic matters and "insoluble forms".Sub-critical water treatment can decrease all these arsenic forms in coal and transfer them into the volatile form.
Pre-combustion desulfurization of coal is an important method to control SO2 emission in a coal fired power plant. Desulfurization of Huayinshan coal with sub-critical water in a semi-continuous reactor was studied. The effects of temperature （320℃450℃）, pressure （5MPa15MPa） and extraction time （10min100min） on the removal of sulfur and weight loss of coal were examined. The results show that with the increase of temperature, pressure and reaction time the weight loss of coal and the removal of total sulfur in coal increase, and with the increase of temperature and reaction time the removal of organic sulfur increases. At higher temperature, sub-critical water has the characteristic of selective desulfurization. The inorganic sulfur removal is mainly a physical process. Below 370℃ the organic sulfur removal is higher than inorganic sulfur removal, but above 370℃ it is opposite. It is suggested that considering the practicability, the suitable high temperature and low pressure should be taken in the sub-critical water operation condition.
