Study On The Essence Of Quick Coal Liquefaction At High Temperature And A Kind Of Two-stage Liquefaction

It was started form common direct coal liquefaction in this paper. At first, all the inner and external factors that could influence Quick Coal Liquefaction at High Temperature (QCLHT) were analyzed in theory. And then, the reaction mechanism of QCLHT was studied through a series of experiments that employed these factors as research objects. It was proved that the essence of QCLHT was a coal liquefaction method that approaches"Ideal Liquefaction"most. At last, a two-stage liquefaction scheme in which the first stage was QCLHT and the second one was common catalytic liquefaction was studied. There are two types of bonds with very different bond energy that exist in coal structure at the same time, which are bridge bonds and aromatic bonds separately, and this scheme just fitted the characteristic. The above two types of covalent bonds were cracked separately in the most appropriate manner for them each in the two stages. The merits of thermal and catalytic liquefaction were integrated, and the highest conversion rate and the optimum product distribution were acquired at the same time.QCLHT was studied by using a 17ml tubular resonance agitation microautoclave reactor. Under the condition, low rank bituminous coal containing little mineral matter exhibited good liquefaction performance. Conversion was mainly influenced by hydrogen-donating ability of solvent. Contribution of H₂ was negligible, and conversion under N₂ atmosphere was basically equal to that under H₂ atmosphere. Effect of catalyst was unobvious. Coal particle size had little influence on conversion, whereas vibration of reactor had important influence. Based upon the above conclusions, the reaction mechanism of QCLHT was considered to be: At the high temperature zone of the primary pyrolysis temperature range of coal, which is usually near 500℃or so, for low rank bituminous coal, bridge bonds in its structure could be sufficiently broken, and large amounts of free radical fragments were thus formed. If enough excellent hydrogen donor solvent was used as hydrogen source, the free radical fragments could be stabilized and formed into liquid product, so rather high conversion could be achieved within dozens of seconds or a few minutes.The concept of Ideal Liquefaction was proposed, and through the study of direct coal liquefaction under the condition of higher temperature and enough hydrogen-donor solvent, it was found that the total conversion of Yanzhou coal had approached 80% when the actual liquefaction time was only 1 min. It was confirmed that the essence of QCLHT is a coal liquefaction method that approaches Ideal Liquefaction most. The further study under the conditions approaching Ideal Liquefaction showed that besides the contribution to hydrogen-donation the pressure itself of H2 pressure was also very important. Even if it was under under N2 atmosphere, to increase system pressure still could greatly increase the conversion. To increase the amount of hydrogen donor solvent could not completely compensate the conversion loss brought by the decrease in system pressure.The coal chemical structure is composed of bridge bonds and aromatic bonds, and the two types of bonds differ greatly in bond energy. According to the characteristic, a two-stage liquefaction scheme composed of thermal coal liquefaction and catalytic coal liquefaction was designed. The total conversion of Yanzhou coal -- which contained 11.8% fusinoid group -- reached 90.9%, and the product distribution was: gas 8.9%, oil 53.2%, asphaltene 24.5%, preasphaltene 4.3%, and residue 9.1%. The condition of thermal liquefaction was 490℃for 3 minutes, and that of the subsequent catalytic one was 400℃for 120 minutes. While the experiment was kept carrying on continuously, the drop of temperature from 490℃to 400℃needed about 4 minutes. The final total conversion had already been essentially determined during the first stage, and the second stage was mainly responsible for the obvious improvement of product distribution. The first stage reaction needed only several dozens of seconds or a few minutes, during which the active hydrogen was mainly donated by hydrogen-donating solvent. At the second stage, H2 obviously participated in reaction.