| As the basic energy in china, coal takes major position in our economical life. Many issues exsit in coal utilization, such as high transportation cost and severe pollution. To improve efficiency of coal utilization and reduce environmental pollution, advanced technology should be developed. Coal reaction in supercritical water has been concerned in recent years. The reaction takes place in water so that coal should be used without dryness. Water participates in the reaction in which H could improve heating value of gas and O could reduce consumption of oxygen. Gas products are clean due to the exsitence of water. Different products could be obtained through varying reaction conditions and meanwhile polluting elements could be removed.Lignite was selected for research in the paper. Characteristics of conventional products and pollutants migration were studied to improve efficiency of coal utilization and reduce pollution to environment.Distribution characteristics of conventional products in solid,liquid and gaseous phases were studied and influencing mechanism of reaction conditions was discussed to investigate utilizing potentiality of products for coal reaction in supercritical water. Conclusions were obtained. CO2, CH4and H2were main gaseous products and gas yield increased evidently with temperature and pressure increasing. As the important intermediate product in gas phase, CO content was low and it was transformed to CO2and H2through water-gas shift. With ER rising, yield of CO2increased while that of CH4and C2H6decreased, but H2presented inconsistent trend. Phenols took up about70%of organics in liquid phase. And various kinds of polycyclic aromatic hydrocarbon were detected in liquid. Comparing with raw coal, contents of C and N increased while that of H and S decreased in solid residue. Besides, ash content and calorific value were enhanced. Volatile matter was almost dissolved out completely at550℃。 Infrared absorption characteristic peaks of hydroxyl and aliphatics disappeared and that of oxygen-containing functional groups was weakened in solid residue. Laminar structure and pore structure were found on the surface of solid residue.Existing forms and transforming mechanism of sulfur in products were studied and migration paths of sulfur were deduced. In the experimental conditions of the paper, decomposition rate of S was at the range of58%-78%. Organic sulfur occupied the highest amount in raw coal in the paper. And80%of organic sulfur was dissolved after supercritical water reaction. Amounts of sulfate in solid residues exceeded that in raw coal when ER was higher than0.3. It illustrated that other forms of sulfur were transformed to sulfate deposited in solid during supercritical water reaction. SO42-and S2O32-were the main products in liquid, and contents of S2-,HS-SO32-,HSO3-were low. In the process of reaction in supercritical water for coal, no sulfur-containing compounds except H2S were detected in gas phase. Migration paths of sulfur in supercritical water reaction were as follows. Different forms of sulfur in coal were descomposed partly to produce free radical with sulfur which reacted with H2to generate H2S. Meanwhile free radical with sulfur possibly combined with large molecular organic matter to generate thiophenic sulfur and reacted with functional groups rich in oxygen or H2O2to produce SO2. Thiol and thioether could descomposed or hydrolyzed to generate H2S which could react with unsaturated hydrocarbon to form thiol and thioether. H2S was dissolved partly to water to gengerate S2-and HS-and part of H2S possibly combined with large molecular organic matter to generate thiophenic sulfur. The rest was released as gas. SO2was dissolved to form SO32-and HSO3-. Anions of sulfur with different valence could transmute into each other.Existing forms and intermediate products of nitrogen were studied and migration paths of sulfur were deduced. Comparing with S, decomposition rate of N was lower in supercritical water reaction, which was between26%-46%. Content of N in solid residues varied slightly. NH4+and NO3-were main forms in liquid for inorganic nitrogen. Nitrogen-containing compounds except N2were not detected in gas. Migration paths of nitrogen in supercritical water reaction were as follows. Organic nitrogen in coal reacted with supercritical water and H2O2, which led to cracking of nitrogen chain to produce NH3, nitro compounds and other organic compounds with nitrogen. Nitro compounds could combine with free radical H to generate NH3which was dissolved in water to produce NH4+. Hydrolysis of nitro compounds led to formation of NO3" which could react with NH4+to produce N2.Reaction characteristics of heavy metals were illustrated and speciations in products were comprared with raw coal. Also migration paths of heavy metals were deduced. Pb and Cd were semi-volatile elements, and decomposition rates were between16-27%and11-26%respectively. Mn,Cu and Zn were nonvolatile elements, and decomposition rates were lower than10%. Decomposition rates of Cr and Ni were negative, which demonstrated corrosion of the reactor. Enrichment abilities of Pb and Cd were relatively poor and that of Cu and Zn were similar to Mn while relative enrichment coefficients to Mn of Cr and Ni were far higher than1due to corrosion. Extraction ability of supercritical water at different conditions and solubilities of metal compounds were important to concentrations of heavy metals in liquid. Meanwhile, addition of water inhibited migration of heavy metals to gaseous product. Concentration of heavy metals in gaseous product was lower than detected limit. After supercritical water reaction, heave metals studied were transformed from F1and F2to relatively stable forms. Supercritical water reaction reduced environmental risk of heavy metals. Forms of F1and F2were unstable in supercritical water and could transform to relatively stable forms. Migration paths of heavy metals in supercritical water reaction were as follows. For heavy metals, Fl could dissolve into water through dissolution or ion exchange. F2was unstable in supercritical water, which could decomposed to oxides and CO2. Part of F3could dissolve into liquid. Part of F4together with organic meatter could dissolve in water. Heavy metals dissolved above reacted with H2O2and H2O to produce oxides or hydroxides precipitated in solid which belonged to F3. Heavy metals dissolved combined with organic matter to produce F4. Heavy metals dissolved combined with indissolvable organic matter or entered into lattice to transform to F5which precipitated in solid.Reaction characteristics of alkali metals were studied and speciations in products were comprared with raw coal. Also migration paths of alkali metals were deduced. Decomposition rates of alkali metals(K,Na) were studied that K was between1.1%-11.4%and Na was between0.3%-19.9%, which was closely related to water addition amount and solubility of compounds containing K and Na. Enrichment ability of Na was similar to Mn and that of K was a little more than Mn. Extraction ability of supercritical water at different conditions and solubilities of alkali metals were crucial. Meanwhile, addition of water inhibited migration of alkali metals to gaseous product. Concentration of alkali metals in gaseous product was lower than detected limit. Forms of water soluble(A1) and bound to carboxylate(A2) were unstable in supercritical water reaction, which could transformed to forms of bound to function group with oxygen or nitrogren(A3) and insoluble(A4). Concentration of alkali metals in liquid varied greatly. Migration paths of alkali metals were as follows. For alkali metals, A1was dissolved in water to generate alkali metal ions. A2and A3could be decomposed to release alkali metal ions while A4was relatively stable in experimental conditions in the paper. Alkali metal ions dissolved above combined with large molecular organic matter to generate A3and reacted with SiO2and minerals to generate A4.In conclusion, no pollutants were detected except H2S in gaseous products after coal reaction in supercritical water. A certain amount of heavy metals and alkali metals besides inorganic anions containing sulfur and nitrogen exsited in liquid products. Sulfur, nitrogen, heavy metals and alkali metals in solid products were transformed from unstable forms to more stable forms. Migration paths of sulfur, nitrogen, heavy metals and alkali metals in coal during supercritical water reaction were deduced.Looking forward to the future, classified utilization would be realized after coal reaction in supercritical water. Lower water content and higher heat value make solid products could use as fuel. Further study should be done to determine utilization forms of liquid products. Gaseous products were relatively clean for coal reaction in supercritical water and purification cost could be reduced. It is a promising gasification technology for IGCC and coal-based natural gas. |
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