热提质对褐煤水分复吸特性的影响规律研究

The moisture content of lignite is greater than 30% by weight, which make it troublesome to storage or transport the coal as it is, so upgrade process include drying and pyrolysis is essential when they used for both energy source and chemical feedstock. With the reserve of high rank coal decreased and cost of coal winning increased, effective utilization of lignite resources must be treated seriously, thus the upgrade methods of lignite has been the focus in the field of coal processing and utilization. However, the upgraded products reabsorb water in humid air atmosphere just like sponge in water since the porous structure and hydrophilic nature of raw lignite, which will decrease the efficiency of upgrade process and engender bad influence on the subsequent processing and utilization. So it is of profound theoretical and practical significance to investigate the water re-adsorption of processed products..In this research, Baorixile lignite obtained from SHENHUA GROUP CORPORATION LIMITED will be selected as the main experimental material. Drying treatment or pyrolysis was first conducted on the raw lignite, and then the products processed under various experimental conditions (i.e. temperature, pressure and residence time) were acquired. Specific experiment was designed to investigate the water re-adsorption properties of these products, afterwards, correlate the experimental results with the operating conditions accordingly and confirm the relation between them. Chemical methods (FT-IR spectroscopy, Nuclear magnetic resonance, X-ray diffraction, carbon dioxide adsorption, Mercury intrusion porosimetry, etc) and physical methods (Ion exchange, alkylation, etc) were applied to analysis the products, which will offer information regarding to the change of coal structure, such as the content of hydrophilic functional group, pore structure, ultimate composition, aromaticity, the type of crystal, etc. In addition, some physicochemical properties of the products, like contact angle, wetting heat, Zeta potential, also need to be measured. With the application of gas chromatography and gas chromatography-mass spectroscopy, the component, molecular structure and elements distribution of the gaseous products and liquid products were investigated. Finally, by analyzing the above results comprehensively, the removal of hydrophilic functional group, destruction of pore structure, changes of surface wettability and electrical property during processing can be interpreted. The essence and control mechanism of the water re-adsorption reaction can be revealed based on the above analysis, which could serve as references for developing drying or pyrolysis process with optimized operating conditions.

褐煤水分通常大于30%，储、运困难，作为燃料或化工原料一般需热提质（干燥/热解）。随着我国优质煤资源的减少和开采难度增加，对褐煤提质的研究已经成为煤炭加工利用领域的热点。但是，褐煤提质后常发生不同程度水分复吸现象而影响提质加工的有效性和后续工利用。本研究首先对代表性褐煤进行不同温度、压力、时间、升温速率等条件下的干燥/热解提质；并在不同温度、湿度、时间、压力、粒级条件下，研究提质煤的吸水可逆性变化规律，明确水分复吸特性与提质条件的内在关系。采用化学分析及FTIR、XRD、BET等手段，分析新鲜褐煤/提质煤的亲水官能团含量、孔结构、元素组成、缩合芳香度、晶型变化等；经气相色谱、色-质联用等手段，分析热提质过程产生的气、液产物组成、结构、元素分布等，解析褐煤亲水官能团分解反应机理、孔隙结构破坏效应、表面润湿性等变化规律，揭示褐煤吸水可逆性的化学本质及可控机制，从而指导褐煤提质工艺条件选择与优化

褐煤/长焰煤提质后，会发生不同程度的水分复吸，且仍易发生自燃。采用固定床反应器对其进行干燥/热解提质，研究不同温、湿度条件下提质煤的水分复吸规律，确定水分复吸与提质条件的内在关系，分析提质煤理化结构变化对复吸特性的影响。结果表明，提质煤水分复吸能力受样品粒度和环境温度(10~40℃)影响较小；受提质温度和环境湿度影响较大。随着提质温度(105~900℃)增加，当RH＜60%，复吸量减小；当RH≥60%，复吸量先降低后又升高，在600℃/400℃提质的褐煤/长焰煤复吸量分别达到最低，主因是含氧官能团减少/中孔体积的降低。等温吸附试验表明，105/400℃提质煤和原煤的吸附等温线均属第II 类，符合Dent模型。提质温度增加，或煤阶增加(褐煤→长焰煤)，含氧官能团减少，第一位吸附量均减少；第二位吸附比重随环境湿度增大或孔结构发育而增加，随煤阶增加而降低。第一位吸附热高于第二位吸附热（接近吸附温度下的液化潜热），提质褐煤的第一位吸附热高于相应提质温度下的长焰煤。600/900℃提质煤的吸附等温线属第V 类，符合DS 模型。第一位吸附量极低，复吸水分主要来自中孔内的水分填充。吸附动力学分析表明，复吸历经快/慢速两个阶段，4h即可超过平衡含水量的85%；符合拟二级动力学模型。含氧官能团丰富(原煤/105℃提质煤)或孔结构发达(600/900℃提质煤)的样品，初始吸附速率均较高。与固定床相比，回转窑提质褐煤的快速吸附/吸附平衡时间分别增加30/10倍，延长干燥时间仅降低复吸量约3%；且回转窑提质后碎裂生成的颗粒越小，复吸能力越低。经自燃倾向性测定和FCC指标分析，提质温度增加，自燃倾向降低，但105~500℃提质煤仍易自燃或可自燃。提质褐煤/长焰煤的粒度分别大于1mm/0.18mm，颗粒内传热和传质扩散阻力效应显著，使其自燃倾向降低。提质褐煤经存放(发生空气氧化和水分复吸)后，自燃倾向明显增加，如105~400℃提质褐煤的自燃倾向达到与原煤基本一致。自燃倾向增加的原因：内部官能团和孔结构此消彼长的演变可增加吸氧量、促进热量聚集；复吸产生较高润湿热，加深预氧化程度；复吸水分在自燃氧化初期具有催化作用。但提质长焰煤经存放自燃倾向变化不明显。本研究为低阶煤提质和分质利用的产业化发展提供了理论支持。