| Coal is a valuable asset given to human by nature, which serves as main source of energy the human's activities. During the coal's formation, the environment is different. And so the coal contains large amounts of coexisting elements, many of which are toxic, and will discharge in the course of human use of coal, doing harm to people's health and posing a great threat to the natural environment. To protect the environment and ensure the safety of production, it is urgent to lay out the limits of various toxic and hazardous substances in the raw coal. This paper takes the coal arsenic and mercury as the research object, though the combustion test of the coal with high arsenic and mercury, studies the migration law and distribution features of arsenic and mercury in the process of high-temperature combustion, in order to provide a scientific basis for further determining the arsenic and mercury limits in coal.This paper takes the drilling coal from a large-scale mine to be exploited in in northeastern China as the samples. By using high-temperature tube furnace for the combustion tests of coal samples, it is confirmed that the volatility of arsenic and mercury at high temperatures is the main source of arsenic and mercury pollution in raw coal, but the volatilization temperature of arsenic and mercury varies. Experiments show that the mercury in coal begins to volatilize at a low temperature, and in 300-600℃it nearly all volatilized; the volatilization temperature of arsenic is relatively high, and the arsenic only appears to volatilize in a large amount arsenic under 1100℃or higher temperature. By analyzing the residue, the flyingdolomite ash, and the absorption solution resulting from the test, it is noticed that, between 600℃and 1000℃, the arsenic in the raw coal, after coal combustion, mainly remains in the residues (about 80%), the rest present in the combustion flyingdolomite ash (about 20%), the arsenic escaping from combustion is really little (<1%); however, when the burning temperature is higher than 1000℃, the distributions mentioned above change dramatically. The amount of arsenic in the residues of arsenic decreases rapidly with the increase of combustion temperature, while the amount of arsenic in the flyingdolomite ash increases rapidly with the increase of combustion temperature and reaches its maximum at 1100℃, close to 100%, but the amount of the arsenic in the absorption solution changes little. This result proves that arsenic in coal combustion presents in the flyingdolomite ash mainly in the form of arsenic or arsenate, so these enterprises which use the raw coal as fuel must strengthen the treatment of dust to prevent the volatilization of the arsenic with the flyingdolomite ash from polluting the environment. Mercury is an easily volatile element which can volatilize in a low combustion temperature in the form of gaseous mercury. When the combustion temperature is between 300℃and 600℃, 40% of the mercury vapors, about 60% of the mercury finds the way into the flyingdolomite ash in the form of mercury compounds, and the residue almost has no presence of mercury and its compounds, but with increase of the combustion temperature, this distribution changes, and some mercury compounds go though high temperature chemical reaction, of which a part finds its way into the air in the form of volatile mercury, and another part into the residue in the form of refractory compounds. These give rise to the phenomenon that the distribution rate of mercury in flyingdolomite ash decreases, while that in the residue and the absorption increases. The bastard coal is the coal's gangue, and it also has a number of related properties of the raw coal. The author uses the self-designed spontaneous combustion test device of coal gangue, and obtains the migration law of arsenic and mercury in the spontaneous combustion of coal gangue. In the spontaneous condition, the arsenic volatilization rate is not high, and, for different types of gangue, about 60% to 85% of arsenic remains in the combustion bottom ash, and while about 90% mercury volatilizes, polluting the environment. Deducted from the experimental data of the combustion migration law, the limit of high arsenic coal should be determined below 90μg / g; while the limit of high mercury coal should be controlled under 0.20μg / g.To deal with the harm posed by the high arsenic coal to the environment, the author, taking advantage of the principle of high temperature chemical reaction, studies and designs a series of high temperature arsenic capture sorbent. Sodium bicarbonate between 1050℃and 1100℃has some function of arsenic-capturing, but the overall efficiency is very low, with the highest rate at 26.98%. The commercial sulfur capture sorbent TL-4 can play the dual role of sulfur-capturing and arsenic-capturing and it does well in the arsenic-capturing in the raw coal, with the highest rate of 81.7%. Single calcium carbonate or single calcium oxide reacts with arsenic compounds under high temperature and generates calcium arsenic which is hard to discompose. By adding a small amount of these substances, their arsenic-capturing rates are up to 73.0% and 47.4% respectively. It can also capture arsenic by add 3% commercial cement in the raw coal, with the arsenic-capturing rate of approximately 80% at 1050℃. The main components of natural Dolomite are calcium oxide and magnesium oxide, which can also play a good role in arsenic-capturing, with the highest rate close to 80%. The experiments above prove that the natural dolomite is the most promising arsenic capture sorbent thanks to that this arsenic capture sorbent is not only abundant, cheap, and easy to use, but also without secondary pollution, so it should be popularized. |
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