Experimental Study On Mercury Release Behavior During Coal Pyrolysis With Calcium Chloride Addition

Coal-fired power plants are the largest single source of anthropogenic mercury emissions worldwide. Mercury emission control becomes the focus of global environment governance. Activated carbon injection or adsorption studies have shown the most effective mercury removal technology, but its high cost and unstable nature limit its application. The control of mercury is closely related to species of mercury emissions form the coal-fired, and it’s shown that chlorine in coal can promote the transformation and removal of mercury. Wet FGD wastewater of limestone-gypsum, with complicate and high cost conventional treatment process, is rich in chloride ion and its main ingredient is CaCl2. In this study, chlorine from the wet FGD wastewater was used to promote mercury species transformation, thus contributing to the mercury control of the coal combustion in mercury pollution, and also achieving the purpose of waste to treat waste.In this study, firstly, three kinds of experimental coal were selected after measuring mercury content and chlorine contentment for eight kinds of coal. Then mercury release behavior and speciation during coal pyrolysis before and after different chlorine addition from CaCl2in mass were studied in a temperature-programmed tube furnace.The mercury content was measured by microwave digestion-cold vapor atomic fluorescence measurements. The results showed that the mercury content differences between the different coals, some high up to0.484mg/kg, and some low with content of0.047mg/kg, and we know the average mercury content for coal in China is0.22mg/kg. Methods of measuring chlorine content in coal were compared by testing results of two known chlorine content of standard coal. It was found that the test error is too large for national standard method GBT3558-1996aldrin sample fusion-potassium thiocyanate titration (Fu Er Hade method), but test error can be greatly reduced for method of oxygen bomb combustion-ion Chromatography law. So the latter methods was chosen to measuring the chlorine content for coal. Finally three types of coal,2#、4#and7#, were selected as the experimental coal.Continuous temperature-programmed coal pyrolysis experiments were studied, mainly on temperature, gas, heating rate in mercury release behavior. Research showed that the temperature is an important factor affecting the releasing of mercury. Mercury starts to release from about200℃, with slightly differences during different kinds of coal; The almost all mercury specie releasing was Hg0in pure N2atmosphere. However with the oxygen proportion in atmosphere increased, the Hg2+proportion was significantly increased, but the total mercury release rate slightly decreased; Heating rate also has an important impact on mercury precipitation. Faster heating rate can increase Hg2+precipitation proportion and meanwhile improve the total mercury release.Then continuous temperature-programmed coal pyrolysis experiments with CaCl2addition were studied. Studies show that, with the participation of O2in atmosphere and higher heating rate. CaCl2addition can significantly improve the Hg0oxidation rate, the degree of increase is also due to the type of coal,Hg2proportion of some kind of coal high up to about50%, but some low to about20%; Fourier online flue tests showed that CaCl2addition can increase HCl concentration greatly. In the participation of the O2, HCl then changed to Cl2, Cl, etc. which can oxidize Hg0strongly, thereby enhancing the rate of oxidation of Hg0; Compared with raw coal, CaCl2addition decreased the total mercury release rate, this is because with the processing of pyrolysis, coal gradually becomes semi-coke which could adsorb mercury. CaCl2added plays a modified role as to enhance the adsorption properties of the semi-coke.Continuous fixed-temperature-temperature pyrolysis experiments were conducted on4#coal during low temperatures. First at the heating rate of3℃/min, three releasing peak of mercuty can be observed, due to different precipitation temperature for different forms of mercury in coal, which leads to three distinct releasing peak. In this regard, four typical temperature point were chosen for continuous fixed temperature pyrolysis experiments. CaCl2Added can conjunct with some forms of mercury in coal, and these forms of mercury can be more likely to remain in the residue, results in reducing the release of total mercury.