高碱低阶煤热解及热解半焦研究进展

我国碱金属、碱土金属(AAEM)含量高的低阶煤储量丰富。高碱含量造成锅炉受热面结渣沾污及气化炉结块腐蚀等难题,低阶煤内水高、氧含量高、挥发分高、发热量低以及易氧化自燃等特性为其储、运、用带来极大的难题。热解可生产优质燃料和高附加值化工原料,也是燃烧、气化、直接液化等过程的起始阶段和/或伴随反应,煤在热解阶段发生的反应、经历的变化,对煤转化利用的效率和清洁程度起重要、甚至决定性作用。笔者对煤热解与热解半焦研究及进展进行综述性评价,着重探讨煤中AAEM对热解过程及半焦的影响。结果表明,热解研究装置模拟的工况与现代煤化工过程中煤热解所处环境相差甚远,半焦样的代表性不强使热解研究成果的指导意义不大;对煤中不同赋存形态AAEM的分离方法有待完善,还需筛选、尝试新的萃取试剂;基本掌握了煤热解过程中AAEM的变迁行为,但尚缺乏控制煤中AAEM危害的有效方法。高碱低阶煤的安全高效洁净转化利用技术仍待突破。     

煤中碱(土)金属形态在煤转化过程中的作用及不同形态间转化的研究进展

煤中的碱（土）金属（alkali/alkaline earth metals,AAEM）既可导致设备结渣、腐蚀,对燃烧、气化等反应也有显著的催化作用。识别煤中AAEM的形态,揭示其在煤转化过程中的作用与产生机制,明晰不同形态间的迁移转化规律,是减缓或消除设备玷污、腐蚀,强化催化作用的前提和基础。论文对煤中AAEM赋存形态和分离方法、造成设备积灰腐蚀的机制以及对煤炭燃烧、气化等催化机理的研究做了较为系统的综述性评介,着重探讨AAEM不同形态间相互转化的可能性。结果表明,借助逐级萃取法可将煤中AAEM划分成水溶型、离子交换型、酸溶型和不溶型4种形态,但现有方法存在缺陷,可通过筛选萃取溶剂和规范操作流程进行完善,达到准确分离的目的;AAEM是造成设备玷污、腐蚀,还是催化燃烧、气化,取决于AAEM的物种,对AAEM所起作用与其形态间的关系尚不清晰,需通过研究进一步验证;采用水洗、酸洗、添加剂、混配煤等方法来缓解或消除AAEM有害影响具有局限性,基于不同形态的AAEM可以相互转化,促使有害形态的AAEM向有益形态转化,可为高碱低阶煤洁净高效利用提供新的思路。     

Transformation of alkali and alkaline earth metals in low rank coal during gasification

Transformation of alkali and alkaline earth metals (AAEM) in low rank coals during gasification was examined by combining computer-controlled scanning electron microscopy (CCSEM) and inductively coupled plasma-atomic emission spectroscopy (ICP-AES). Two sub-bituminous coals were pyrolyzed at 1500 °C using a drop tube furnace, and the resultant chars were then gasified in CO₂ atmosphere at the same temperature. Total amounts of AAEM species in the raw coals and the chars were determined by ICP-AES. Minerals in the raw coals and ash particles in the chars were analyzed by CCSEM.AAEM species were mainly present in the raw coals as dispersed species, organically associated cations or fine mineral particles (<1 μm), which cannot be quantified by CCSEM. It was found that the dispersed Ca species were first converted into fine ash particles upon the devolatilization and then most of the particles interacted with inherent clay minerals to form complex aluminosilicates. In the case of Na and K, the dispersed species mostly vaporized and the interaction with inherent minerals was not observed.     

Alkali and earth alkaline metal contents in the aluminium cathode in aluminium electrolysis

The contents of sodium, lithium, calcium and magnesium in aluminium in contact with NaF–AlF₃-based melts in laboratory and in industrial aluminium cells were investigated in the temperature range 950–1030 C. The experimental data were compared with a thermodynamic model. It was found that the addition of alumina or CaF₂ to the NaF–AlF₃ melts has only a minor effect on the equilibrium content of sodium in aluminium. Cathodic polarization enhances the content of sodium in aluminium. However, polarization has a smaller effect on the concentrations of lithium, calcium and magnesium in aluminium in industrial cells.     

Mallee wood fast pyrolysis: Effects of alkali and alkaline earth metallic species on the yield and composition of bio-oil

The purpose of this study was to investigate the effects of inorganic species in biomass, especially the alkali and alkaline earth metallic (AAEM) species (K, Na, Mg and Ca), on the yield and properties of bio-oil from the pyrolysis of biomass. A mallee wood sample from Western Australia was washed with water and a dilute acid solution to remove its AAEM species. The water-washed and acid-washed mallee wood samples were then pyrolysed in a fluidised-bed reactor at 500 °C under fast heating rate conditions. The removal of AAEM species did not result in significant changes in the yields of bio-oil and bio-char. However, the bio-oil properties, e.g. viscosity, were drastically affected by the removal of AAEM species. Our results indicate that the water-soluble AAEM species were not as important as the water-insoluble but acid-soluble AAEM species in influencing the bio-oil composition and properties. It is believed that the acid-soluble AAEM species (especially Ca) were more closely linked with the organic matter in biomass and thus were closely involved in the reactions during pyrolysis. The removal of AAEM species, especially the acid-soluble AAEM species, led to very significant increases in the yields of sugars and lignin-derived oligomers, accompanied by decreases in the yields of water and light organic compounds in the bio-oil.     

碱处理HZSM-5分子筛对神东煤热解产物分布的影响

使用NaOH溶液对HZSM-5分子筛进行处理,利用粉-粒流化床快速热解实验装置,考察了经过不同时间碱处理的HZSM-5分子筛对神东煤热解产物分布的影响。结果表明：适当的碱处理能够在不破坏HZSM-5分子筛晶相结构的情况下在晶体内部引入介孔,且随着碱处理时间的延长,分子筛的介孔孔容、介孔比表面积增加,孔径分布变宽,总酸量先增大后减小。碱处理0.5h的HZSM-5分子筛催化时焦油中苯、甲苯含量最大,较原煤热解分别增加了268%、296%;碱处理2h的HZSM-5分子筛催化下热解气体总含量达到最大,较原煤热解增加了24.8%;碱处理4h的HZSM-5催化时焦油中萘及萘的同系物、多环芳烃（除萘类）含量最大,分别较原煤增加了92%、192%。     

Na及矿物类型对高碱煤快速热解焦油及BTEXN产物分配的影响

轻质芳烃（BTEXN）如苯、甲苯、乙苯、二甲苯和萘作为高价值化学原料,在工业中具有广泛应用。煤干馏焦油的精制是BTEXN重要生产方法之一。本文分别选用红沙泉和将军庙两种典型新疆高碱煤,通过逐级萃取及外源添加Na两种预处理方式,利用Py-GC/MS半定量研究了高碱煤中钠形态和含量,以及矿物组分类型对煤快速热解焦油及BTEXN产物分布的影响。结果表明,两种高碱煤中固有矿物质均可提高焦油及BTEXN析出量,水溶性矿物质（Na）对此起关键作用。内源性水溶性矿物及醋酸铵溶性矿物使红沙泉煤焦油析出量提高22%和55%;而外源添加的水溶性NaCl和醋酸铵溶性CH₃COONa降低了将军庙煤焦油析出量。水溶性矿物及1.0%（质量）的NaCl和CH₃COONa分别使煤焦油中的BTEXN析出量提高45%、42%和62%,醋酸铵溶性物质及低含量[0.4%(质量)]Na则会抑制BTEXN的析出。     

反应气氛对煤热解过程中硫变迁与释放的影响

通过固定床、流化床热解脱硫过程,重点对比分析反应气氛对煤中硫变迁与释放行为的影响.研究结果表明:还原性气氛较惰性气氛和氧化性气氛更有利于煤中硫的释放.较高的热解温度有利于有机硫和硫酸盐硫的脱除,较长的热解时间可以使更多种类的硫发生分解反应.选择合适的热解条件参数是实现煤热解预脱硫、有效提高脱硫效率的关键.     

矿物质对煤中硫氮在热解气化过程中迁移变化的催化作用

从煤自身所含的矿物质和外加添加剂的观点出发,对煤热解、气化和燃烧过程中存在的催化作用进行了阐述,尤其是煤中N、S有害元素的催化转化作用。煤自身所含的矿物质及部分碱金属、碱土金属和过渡金属添加剂对煤热解、气化和燃烧过程具有一定的催化能力,同时对煤中N、S有害元素的转化也有明显作用。对具有单功能催化作用的金属催化剂进行合理组合、适当改性和优化添加方式,在NOx和SOx污染性气体形成前对其前驱体进行抑制和定向转化,是今后煤炭洁净转化的一个研究方向。     

煤热解活性及气相产物释放的关联性分析

煤热解产物分布及热解活性与煤的组成和结构密切相关,热解过程中的工艺条件也会对其产生影响;本文通过对煤样内在因素(组成特性)和热解过程的外在因素(工艺条件)考察,探讨各因素对煤热解活性及气相产物释放规律的影响.     

Release of alkali and alkaline earth metallic species during rapid pyrolysis of a Victorian brown coal at elevated pressures

Possible reaction mechanisms responsible for the release of Na and Mg during pyrolysis at elevated pressures are described in this paper. In order to evaluate these mechanisms a Victorian brown coal, Loy Yang coal, was pyrolysed in a wire-mesh reactor at pressures up to 6.1 MPa at a heating rate of 1000 °C s⁻¹. Release of Na and Mg were quantified as functions of temperature and pressure. The results demonstrated that increasing pressure suppresses or promotes release of Na and Mg depending on the combination of pressure and temperature. The results obtained have been explained qualitatively by the proposed reaction mechanisms. At temperatures of 600 °C and lower, the release of Na and Mg from the pyrolysing coal/char particles, as light carboxylates, other organic salts and/or metals, was controlled by their diffusion through the pore system of the particles and, therefore, was suppressed by increasing pressure. At higher temperatures, the release of Na and Mg seems to be affected by the changes in intra-particle mass transfer mechanism due to increasing pressure as well as by chemical reactions responsible for the formation of volatile Na and Mg species.     

Fertiliser influence on alkali release during straw pyrolysis

Comparative studies of alkali release from wheat and oat straws cultivated in the laboratory with chloride-rich and sulphate-rich fertilisers are reported. The release of alkali during pyrolysis in the temperature range 25-1060°C is measured using the sensitive surface ionisation technique. Straw cultivated with a chloride-rich fertiliser releases two to four times more alkali than straw fertilised with a sulphate-rich fertiliser, and the release takes place at lower temperatures. The temperatures for maximum alkali release are ∼800°C for straw cultivated with a chloride-rich fertiliser, and ∼870°C for straw supplied with a sulphate-rich fertiliser. The samples were also subject to simple water leaching, and alkali was observed to be easily removed by leaching irrespective of fertiliser. Application of chloride-free fertilisers is concluded to be a simple and efficient way to reduce the alkali release from the fuels, and the method can easily be combined with other methods in order to improve fuel quality.     

Dynamic behaviour of sulfur forms in rapid pyrolysis of coals with alkali treatment

Three kinds of subbituminous and bituminous coals with added potassium hydroxide were heated at 523 K in a nitrogen stream to transform thermally stable organic sulfur to reactive species. Extents of total sulfur removal were 27–52% during the course of alkali treatment, while weight loss was 8–13%. The extent of total sulfur removal was linearly proportional to the internal surface area of the parent coal. The parent coals and alkali-treated samples were pyrolysed rapidly in a free-fall reactor in a nitrogen stream at 1233 K. Under these conditions the alkali-treated samples lost more organic sulfur than did the parent coals. The observed changes in the content of sulfur forms were successfully simulated kinetically. The combined process of rapid pyrolysis with alkali leaching was effective for reduction of organic sulfur, except for a high-rank coal of small internal surface area.     

不同热解温度下玉米秸秆中碱金属K和Na的释放及半焦中赋存特性

为了解高含K玉米秸秆在不同热解温度下碱金属的析出规律,建立了一维水平管式炉,利用FTIR、SEM-EDX、XRD等测试手段及化学分馏法对碱金属的释放规律及热解半焦中赋存形式进行深入实验研究。结果表明：K是玉米秸秆的主要碱金属元素,且91%的K以无机K的形式存在;300~600℃主要为有机K的分解、释放阶段,大于600℃主要为KCl、K₂SO₄等无机K分解、释放。K元素由颗粒内部向表面迁移,并在700℃出现表面K盐富集,继续升温富集程度降低;Na和K析出规律相似,Ca和Mg热稳定性强,热解过程多以稳定的化合物形式存在于热解半焦中,不易析出到气相。     

Effects of volatile-char interactions on the volatilisation of alkali and alkaline earth metallic species during the pyrolysis of biomass

An important feature of a fluidised-bed gasifier is the continuous contact between volatiles and char. The aim of this study is to experimentally investigate the effects of volatile-char interactions on the volatilisation of AAEM species during pyrolysis of two sugarcane industry wastes, bagasse and cane trash. A two-stage quartz fluidised-bed/fixed-bed reactor was used for this fundamental study. Our results indicate that the volatile-char interactions could lead to the additional volatilisation of alkali and alkaline earth metallic (AAEM) species, particularly if the volatile-char interactions have resulted in additional char weight losses. The monovalent Na and K behaved differently from the divalent Mg and Ca in biomass. Our results provide circumstantial but clear evidence that the AAEM species in biomass could behave distinctly differently from those in brown coal, largely due to the differences in the structure and composition between biomass and coal. The development of biomass gasification technologies must consider the special thermochemical characteristics of biomass. Furthermore, even the bagasse and cane trash grown in the same area behave drastically differently, at least partly due to the different microstructures of bagasse and cane trash.     

Influence of alkali on pyrolysis of coals

Pyrolysis of several Indian coals of different rank in the presence of NaOH showed that the best yield of hydrogen was obtained from medium rank (C, 83.5%) Poniati-Girimint coal. With this coal, other alkalis (Ca(OH)₂, CaCO₃, Na₂CO₃) gave much less hydrogen, but significantly more methane in the product gas.     

铁镍复合助剂对煤热解过程中氮迁移规律的影响

为了减少煤炭燃烧过程中NO_x的排放,在管式炉中进行了煤与金属助剂（FeCl₃、NiCl₂）的热解实验,研究了助剂负载量、热解温度、助剂添加方式对氮迁移及N₂产率的影响并且对复合助剂作用机理进行了探讨。结果表明：随着助剂负载量的增加,氮脱除率及N₂产率呈现先增加后趋于稳定的趋势,且负载量以0.8%Fe复合1.0%Ni为最佳。在700~1000℃的热解温度范围内氮脱除率及N₂产率随热解温度的增加而增加。对煤进行溶胀处理添加复合助剂后,氮脱除率及N₂产率要优于未经处理的煤样。铁基助剂与镍基助剂在催化煤热解氮迁移过程中形成互补,铁基助剂的添加增加了镍基助剂的活性,弥补了单助剂的劣势,且复合助剂相比于单助剂有更强的氮脱除效果并且N₂产率达到最高39%。铁镍复合助剂对煤中N-5转化为N₂的催化效果更加明显,因为复合助剂对吡咯的内氢转移和开环有更强的催化作用。本研究能够为煤炭洁净化利用提供理论和实验依据。     

Observation of transient free radicals during coal pyrolysis

Electron spin resonance studies of the early stages of carbonization of a high-volatile A bituminous coal have been used to detect and quantify transient free radicals present in the coal pyrolysate. Free radical (spin) populations are converted from susceptibilities measured at elevated temperatures assuming that the population present after ≈1 h of pyrolysis does not change when the coal pyrolysate is cooled to 20 °C. Co-carbonization of 9–10 dihydroanthracene and anthracene with the coal suggest that disproportionation pathways may be preferred over combination pathways as temperatures of pyrolysis and of liquefaction processes are increased.     

添加木质素对污泥热解过程氮转化的影响

利用热重-质谱联用仪（TG-MS（在线检测添加不同比例木质素的污泥热解和主要含氮气体释放特性,并利用傅里叶红外光谱仪（FTIR（在线检测热解过程固体残焦表面官能团的变化规律,分析木质素对污泥热解过程氮转化的影响,从而揭示氮的迁移机理。结果表明：污泥热解过程NH₃的生成主要来自蛋白质的脱氨作用,加入木质素后热解过程中产生了大量羰基,抑制了氨基酸的分子内环化和分子间缩聚反应,而促进了直链酰胺的生成,最终在OH自由基攻击下转化成HNCO和HCN;NH₃的增加并非来自HCN和OH的反应,而是由未缩合的氨基脱除生成;另外,添加40%左右的木质素对于减少污泥热解过程中NO_x前驱物的排放有积极意义。     

典型碱金属、碱土金属对不同变质程度煤焦溶损反应的催化作用

试验研究发现,添加碱金属、碱土金属对低变质程度煤焦反应性的催化作用小于焦煤等中等变质程度煤焦反应性。它们对焦炭溶损反应的催化作用顺序为：K〉Na〉Ca〉Mg,且随着添加量的增加,催化作用越强。随反应时间和典型金属添加量的增加,以及温度的升高,焦炭转化率越大。