【作者】 陈桂芳；

【导师】 马春元；

【作者基本信息】 山东大学 ， 热能工程， 2014， 博士

【摘要】 煤炭是中国重要的基础能源和化工原料,在国民经济中占据战略地位。煤炭的利用存在许多问题,如运输成本高、利用效率低、污染问题难以解决等。提高我国煤炭利用效率、减少环境污染的根本途径是研制和推广煤炭的分质利用技术。由于超临界水的特殊性质,煤在超临界水中的转化近年来倍受关注。其反应介质为水,因此即使是水分含量较高、难以利用的褐煤,也无需进行干燥；反应介质及褐煤中的水均可参与反应,水中的H原子可提高燃气热值,O原子可减少氧量消耗；反应后气体产物较为洁净；根据反应条件的不同,可获得不同的目标产物,从而实现煤炭的分质利用,同时可对煤中的污染性元素进行定向脱除。本文以褐煤作为研究对象,对煤在超临界水中反应的产物特性及污染物迁移特性进行研究,旨在提高煤利用效率的同时,降低煤中污染物对环境的影响。研究了除污染物外的常规产物在气、液、固的分布特性。CO2、CH4、H2是主要的气体产物,随着温度及压力的升高,气体产率均有明显上升。CO是重要的中间产物,含量较低,在超临界水中通过水煤气转换反应,生成C02和H2。随氧当量比(ER)的升高,C02产率上升,CH4及C2H6的产率下降,H2产率则呈现先上升后降低的趋势。液体产物中酚类所占比例最多,多环芳烃种类较多。与原煤相比,固体产物中的C、N含量升高,H、S含量降低,灰分比例及热值升高,挥发分比例降低。固体产物中羟基及脂肪类有机物的特征峰消失,含氧官能团的特征峰明显减小,表面出现片状的碎片及孔隙结构。探讨了煤中S在超临界水反应后三相产物中的赋存形态及转化机理,并推断出S的反应路径。文中试验条件下,S元素的反应率在58-78%之间。原煤中有机硫含量最高,经过超临界水反应后,80%以上的有机硫均已分解；硫化铁硫的固存率随温度、压力、ER的升高均呈降低趋势；硫酸盐硫的总量在ER≧0.3时,超过原煤中硫酸盐硫的总量。S042-及S2032-是液体产物中主要的无机含硫产物,S2-和HS-、SO32-和HS03-相对较少。文中试验条件下,煤在超临界水中反应后,气体中的含硫化合物仅检出H2S。反应路径如下：煤中不同形态的S部分分解产生含硫自由基,与H2反应生成H2S,与煤中大分子有机物结合生成噻吩硫,与煤中富氧官能团或加入的H202反应生成802；硫醇、硫醚等有机硫经过分解或水解也可产生H2S；生成的H2S部分溶于水生成S2-+HS-,部分与煤中有机物结合生成有机硫,部分以气态形式排放；生成的S02溶于水生成SO32-+HSO3-,S的不同价态的阴离子之间会发生相互转化。在文中的试验条件下,认为CaSO4未发生分解。分析了煤中N在超临界水反应后的存在形式及中间产物,得出了N的反应路径。褐煤中的N在超临界水中的反应率并不高,在26%-46%之间,固体产物中N含量的变化幅度较小。液体产物中NH4+、NO3为主要的无机氮存在形式。气体中除N2外未检出其他含氮化合物。反应路径为：煤中的大分子有机氮与超临界水、H202反应,导致氮链断裂,生成NH3、有机类硝基化合物及其他大分子含氮化合物,其中硝基化合物及大分子含氮化合物可能与H自由基结合,生成NH3,NH3溶于水生成NH4+,而硝基化合物在超临界水中发生水解,生成NO3-,NO3和NH4+发生氧化还原反应生成N2。探究了煤中重金属在超临界水反应过程中的挥发及富集特性,分析了反应前后赋存形态的变化,并进行了反应路径的推断。Pb、Cd为半挥发性元素,其反应率分别在16%-27%及11%-26%之间；Mn、Cu、Zn属于难挥发元素,反应率均低于10%；Cr和Ni的反应率出现负值,反应釜发生了腐蚀。固体产物中的重金属相比原褐煤均有了不同程度的富集。液体产物中重金属的含量与超临界水的萃取能力及重金属化合物的溶解度有密切关系。同时,水的加入一定程度上也阻止了重金属向气体产物的迁移,气体产物中重金属的浓度低于仪器检出限。利用Tessier萃取法将煤中的重金属分为可交换态、碳酸盐态、铁锰氧化物态、有机态及残渣态。经过超临界水反应后,不稳定的可溶态及碳酸盐态向稳定的形态发生了不同程度的转变,超临界水反应降低了煤中重金属的环境风险。反应路径如下：可交换态重金属通过溶于水或发生离子交换溶出到液体产物中,生成重金属离子；绝大部分碳酸盐态重金属分解产生重金属的氧化物及C02；超临界水可将部分铁锰氧化物态的重金属脱附到液体中；部分有机态的重金属连同有机物一起溶于超临界水中。上述溶解出的重金属与02、H20反应生成相应的氧化物及氢氧化物,归属于铁锰氧化态；与一般有机物结合会生成有机态；进入矿物晶格中即转化为残渣态。阐明了煤中碱金属在超临界水中的反应特性,分析了反应前后碱金属赋存形态的变化,得出了不同形态碱金属的反应路径。煤在超临界水反应过程中,K、Na的反应率分别在1.1%-11.4%及0.3%-19.9%之间,反应率的高低与K、Na化合物在水中的溶解度有密切的关系。固体产物中碱金属相比原褐煤有所富集。碱金属在液体产物中的浓度变化范围较大,与超临界水的萃取能力及碱金属的溶解度有关。同时,水的加入一定程度上阻止了碱金属向气体产物的迁移,气体产物中碱金属的浓度均低于仪器检出限。利用化学萃取法将煤中的碱金属分为水溶态、羧酸盐态、配位官能团态及硅铝酸盐态。水溶态及羧酸盐态的碱金属在超临界水中不稳定,可转化为配位官能团态及硅铝酸盐态,降低了碱金属的环境风险。反应路径如下：水溶态的碱金属首先溶于水中生成碱金属离子,羧酸盐态及配位官能团态的碱金属分解释放碱金属离子,硅铝酸盐态在试验条件下相对稳定。上述释放的碱金属离子与煤分解得到的大分子有机物结合会产生配位官能团态,与煤中的Si02、矿物质等反应会生成稳定的硅铝酸盐态沉积在固体产物中。综上所述,煤在超临界水中反应后,气体产物中除H2S外,未检出其他污染性化合物；液体产物中存在一定浓度的硫、氮无机阴离子及重金属、碱金属；固体产物中硫化铁硫及有机硫的反应率较高,二者可向硫酸盐硫转化,氮的反应率不高,重金属及碱金属均由不稳定的形态向稳定的形态转化；推断得出煤中硫、氮、重金属、碱金属在超临界水反应过程中的迁移路径。展望未来,煤经过超临界水反应后,可实现分质利用。固体产物水分较低,热值较高,可作为燃料使用；液体产物的利用有待进一步的探讨；气体产物较为洁净,可大大降低气体产物的净化成本,有望替代IGCC及煤制天然气的气化技术。更多还原

【Abstract】 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.更多还原