The Products Distribution Of Each State From Supercritical Water Gasification Of Sewage Sludge Under Different Conditions

Sludge treatment has become the important research field for the researchers. People took all kinds of different methods for sludge reduction and harmless. However, supercritical water gasification sludge method not only has a good effect on sludge reduction. In the direction of the reaction faster rate, reuse of sludge and the treatment of heavy metal sludge has a good performance. So the supercritical water gasification of sludge became the significant direction of sludge harmless. This article select the sludge in a sewage sludge plant in Shen Zhen as raw material and treat them with drying process. By adjusting the reaction conditions of supercritical unit, to explore the influence of various reaction conditions on the sludge gasification and the regularity of heavy metals migration, which is for accumulating the original data for the industrialized produce in the future.By controlling the single variable, we examined the influence of reaction temperature, reaction time, reaction pressure and ratio of water and material on the sludge gasification. Obtained the highest relative content of hydrogen and methane,when the reaction happened under 23 Mpa pressure at 390 ℃with 30 minutes residence time and the feed ratio of 10:1. The catalyst with 5% mass ratio was used as catalyst for the sludge gasification. According to the content of hydrogen and methane produced by the reaction, the order of catalyst priorities is KOH > K2CO3 > Na OH > Na2CO3 > Na HCO3. Na HCO3 as the reaction catalyst which produced the minimum hydrogen production. The catalyst with KOH had the highest yield of Hydrogen. Hydrogen has the highest yield when using KOH as catalyst. The FT-IR analysis showed that there were some organic functional groups the original sludge,such as-CH3,-CH2,-OH,-NH2,-C=O, C-O, from which we can speculate that the original sludge contains proteins of hydroxy, ether, amine, alcohols, phenols and other organic matter. After supercritical water gasification, the most active groups, such as-OH,-NH2,-CH3,-CH2, are relatively easier to turn into other new substances. The relative quantity content of C, H, N, S in solid product decreased. They accounted for 35%、23%、21%、87% of the original sludge respectively. With the increase of reaction temperature, pressure and time, The content of C, H, N in solid product decreased, but the sulfur content increased. In the liquid products, with the increase of temperature, time, the content of the C, H elements decreased, and the content of N, S increased.With the change of reaction temperature, pressure, time, and the water material ratio, the heavy metal ions in exchange state dissolved in water and generated heavy metal in the reaction of supercritical water gasification in the sludge; Most carbonate in sludge is unstable, and they can produce oxide or hydroxide under the condition of supercritical water gasification; Oxidation state or organic state of iron and manganese partly dissolved in water to form ions under supercritical conditions, The rest did not change basically; Under the condition of the supercritical water gasification, part of the heavy metal ions dissolved in aqueous solution combined with complex organic combination and formed a relatively stable organic state. Some oxidated by oxidizing substances in the sludge and generated the iron and manganese oxidation which does not dissolve in water; Part of the heavy metal ions and some mineral crystal lattice, formed a special stable residue. We analyzed leaching law of Cr, Cu, Ni, Zn, Pb and Cd in solid sludge production with TCLP method and national standard, and the result of the different methods had the similar trend, which showed that the risk index of heavy metals reduced in different levels after supercritical gas water treatment. Among them huge changes of the risk index of Cd and Pb had taken place. The risk index of Cd ranged from 0.3475 to 0.0224 after the reaction, which reduce the nearly 15 times; risk index of Pb changed from 0.1225 to 0.0201 after the reaction, which decreased about 6 times. So its biological availability improved efficiently. Therefore the technique of supercritical water gasification sludge reduced environmental damage risk produced by heavy metal.