Study On The Cleavage Of Covalent Bonds In Coal Pyrolysis In Consecutive Temperature Ranges

Coal is a macromolecular polymer with complex structure and its composition and properties varies with coalification time greatly. After more than several century-long large scales industrial use of coal and extensive researches on many coal conversion technologies, it is still found that there is no way to interpret the organic portion of coal with a exact chemical formula. As a result, empirical terms or methods are used to characterize coal, such as the petrographic analysis, ultimate analysis and proximate analysis. There are many coal structure models have been proposed, but because of the limitation on the phenomenal explanation, they can not meet the requirements of the coal chemical reaction.The nature of all chemical reaction is the change of the chemical bonds. Although it is commonly recognized that coal structure is very complex, coal consists of only a dozen covalent bonds with different bond dissociation energies. How to use the formations and changes of chemical bonds to describe coal and its chemical reactions become an essential step to recognize the complex structures of coal and a direction to the development coal science. This information, however, is scarce in the literature.It is difficult to analyze the chemical structures in a static state owing to the multiple categories and complex structures of coal. Hence studying the dynamic processes of coal reactions become a feasible way to deduce the bonds structures. Therefore, the information of coal convention reactions and coal bonds structures can be found both, which is meaningful to the study of coal.Thermal decomposition of coal (pyrolysis) is the most direct chemical reaction of coal. It is also the common part of many chemical processes such as coking, gasification, liquefaction, combustion. Hence many researches, which are carried out in different conditions, provide huge samples to recognize the pyrolysis reactions and processes. However, most of theses studies were limited to duplication of kinetics reaction data for a few coal samples and fail to analyze the nature of the changes of coal structures.The pyrolysis of coal could be regarded as a two step reaction including rupture of bonds to generate free radicals (first step) and the coupling of free radical fragments (second step). The first step is a key to probe the bond structures of coal. Unfortunately, it is generally believed that the first step can not be observed directly on macro level due to the interference of second step. How to judge the extent of second step occurs in pyrolysis and design experiment to reduce or even eliminate the impact of second step and furthermore parse data to speculate bonds structure is an important subject in coal chemistry. Besides that, one of the requirements to probe the evolvement of coal structure is to conduct a systematic study in same condition by using enough variety of coals in different rank.The paper studies the coal pyrolysis in TGA and fixed-bed reactor to recognize three major issues:(1) Whether or not a method to measure the volatile radicals by pyrolysis which could not be affected by coupling of radicals can be established.(2) How to probe bond structure of coal by analysis in macro scale?(3) The variation law of yield and group composition of gaseous and liquid compounds from pyrolysis of the coals in various temperature ranges. In order to obtain the reliable conclusions, we conducted many relevant researches and the major conclusions obtained are as follows:1The formation of solid by coupling of primary radicals is not significant in high carrier gas rate, small sample amount and small H/D of crucible. It indicates that the DTG data of coal pyrolysis can be attributed mainly to the cleavage of covalent bonds in coals and the coupling of the volatile radical fragments affects little the TGA data. This conclusion provides an evidence of the change of bond structures of coal during pyrolysis.2The DTG curves of coals are fitted by5sub-curves and each of the5sub-curves is assigned to cleavage of a group of covalent bonds in coal. The lumped model of coal is attained by the analysis of peak parameters of34coals with different rank. The model is the first to explain the pyrolysis process of coal from a view point of chemical bond. Therefore it is beneficial to the efficient and clean utilization of coal.3The structural origin of CH4and H2in products of the coal pyrolysis process was analyzed from the view point of covalent bond. CH4can be attributed to the thermal cleavage of CH3-O, CH3-Caland CH3-Car bonds, while the H2mainly came from the rupture of H-Cal and H-Car bonds. Hence MS curves of CH4and H2generated by coal pyrolysis were fitted into2sub-curves which represent the bonds with different bond dissociation energies. The evolvement of the hydrogen-rich structure of coal can be obtained by analysis of peak parameters in coal pyrolysis.4The paper systematically studies the yields of tars and oils (hexane-soluble fraction) and the group composition of the oils obtained from pyrolysis of23coals in tubular fixed-bed reactor at temperature ranges of10-300,300-400,400-500,500-600and600-800℃. It is found that the yields of tars are generally greater than10wt.%of the coals (daf) with30-60wt.%being in the oils. The tars and oils produced in the temperature range of400-500℃generally account for50%of the total tars and the total oils, respectively. Aliphatic hydrocarbons dominate the oils obtained from the coals with C%less than80wt.%, while aromatics, especially the1-ring and2-ring aromatics, dominate the oil obtained from coals with C%greater than80wt.%.