褐煤地下气化过程汞析出规律的研究

Study on mercury emission during underground coal gasification of lignite

在褐煤地下气化模型试验的基础上 ,研究了褐煤中汞的赋存形态 ,褐煤地下气化煤气中汞含量的变化及存在形态 ,以及气化工艺的影响 ;分析了地下气化过程中汞的析出及反应机理 .实验结果表明 ,汞在试验褐煤中主要以硫化物结合态和残渣态形式存在 ;褐煤地下气化煤气中汞含量随气化时间的变化而变化 ,且主要以Hg0 (g)的形态存在 ;水蒸汽的存在会降低Hg0 (g)被其它气体氧化为Hg2 + (g)的趋势 ;与地面气化及燃煤过程不同 ,煤地下气化产物中 ,气态汞占总汞比例低于 2 0 % 。

underground coal gasification (UCG) is a process in which underground coal, without mining and transportation, is directly converted into combustible gas. The regularity of mercury emission during UCG of lignite was studied with aims of prevention and control of mercury emission released from UCG. Mercury form in lignite is analyzed by sequential chemical extraction. It was found that the average mercury content of the test lignite is 0.3986?μg·g^(-1) and mercury in lignite mainly occurs in bound-sulfide and residue, taking up 77.8% and 19.1%, respectively. Based on UCG simulation test of lignite, the change of mercury content in UCG gas varying with time and temperature and the influence of gasification technology were investigated. Elemental mercury, Hg^0(g), and divalent mercury, Hg^(2+)(g), in UCG gas was captured by Ontario Hydro Method (recommended by EPA in USA) and detected by atomic fluorescence spectrometer. In addition, the mechanism of mercury transformation and emission during UCG process was analyzed. Mercury exists mainly in the form of Hg^0(g) in UCG gas. Small amount of Hg^(2+)(g) is produced through chemical reaction of Hg^0(g) and the product gas in the gasification tunnel, and it easily decomposes into Hg^0(g) when the temperature is above 700K. The content of mercury in UCG gas changes with gasification time. At the initial stage of UCG, the content of Hg^0(g) is lower and Hg^(2+)(g) can not be detected. The content of both Hg^0(g) and Hg^(2+)(g) increases in the stable gasification process and decreases at the end of UCG due to different amounts of gasified coal in the different periods of UCG. The average content of Hg^0(g) is 50～60μg·m^(-3) and that of Hg^(2+)(g) is 0.30～0.35?μg·m^(-3). The highest content of Hg^0(g) is found to be 160 times as much as that of Hg^(2+)(g). Changes of Hg^0(g) and Hg^(2+)(g) content in UCG gas with temperature of gasification zone was obtained based on the results of temperature field. The content of Hg^0(g) and Hg^(2+)(g) increases with elevated temperature of gasification zone. The content of Hg^0(g) reaches saturation when the temperature is higher than 600K, similar to the results of surface gasification. Gaseous mercury contents in UCG gas under different technologies were compared. It indicates that UCG gas contains more Hg^(2+)(g) under oxygen gasification and less Hg^(2+)(g) under oxygen-steam gasification since steam slows down the oxidation tendency of Hg^0(g) to Hg^(2+)(g). Analysis of mercury balance in UCG process shows that gaseous mercury takes up less than 20% of the total mercury in UCG products, lower than that from surface gasification and coal burning, and solid mercury takes up more than 60%. Since mercury content in UCG ash is lower, the majority of solid mercury is particulate mercury absorbed by char and ash when UCG gas flows through the long air stream tunnel. UCG gas from simulation test contains higher percentage of Hg^0(g) than that from field test, since the tunnel length is shorter in simulation gasification.