Building Of Coal Pyrolysis Model Based On Grading Lignite Pyrolysis

Grading lignite pyrolysis process is one of the key technologies to achieve clean and efficient use of lignite resources. Pyrolysis is the core unit of lower-rank coal upgrading and refining, and the distribution of pyrolysis products and their properties determine the design of the downstream in grading technology. Coal pyrolysis model can be used to cost-effectively predict the pyrolysis products distribution, which can provide the primary data of pyrolysis unit for the grading lignite pyrolysis technology.In this paper, the coal pyrolysis model was divided into distribution model of coal pyrolysis products and component analysis model of coal tar. Firstly, coal structural units were constructed based on ultimate analysis and 13C-NMR spectra. And based on the structural model, the changes of the coal structure during pyrolysis and the formation mechanism of the volatiles were analysed. The coal was regarded as a two-dimensional structure which was composed of four types of functional groups. The distribution model of coal pyrolysis product was built based on this two-dimensional structure. The component analysis model of coal tar was built based on the mass/element conservation and the theory of non-linear programming. Summarizing all the research work, the main conclusions are listed as follow:1) Constructing the coal structural units of organic matter. By the TG and FT-IR experiments, the organic matter in coal was determined to be the main research object of coal pyrolysis model. Through the three different sizes of coal structural units, it was known that the aromatic clusters of the structural units were given priority to benzene and naphthalene and anthracene were complementary; The bridge bond mainly contained aliphatic bond and ether bond; The alkyl side chain mainly contained methyl and ethyl and oxygen containing functional groups contained hydroxyl and carboxyl. The heat value of the structural units whose molecular weight were 1229g/mol, 2028g/mol and 2846g/mol were 34829.93 k J/kg, 34607.12 k J/kg and 34716.55 k J/kg, respectively, which almost equaled to the heat value of dry ash-free basis raw coal. The structural unit with a molecular weight of 1229g/mol can correctly describe the structural feature of the organic matter and was chosen as the study object.2) Building the distribution model of coal pyrolysis products. According to the changes of the coal structure during pyrolysis and the functional group theory, with the increase of the pyrolysis temperature, lots of changes occurred, which included that the alkyl side chain and oxygen containing functional groups decomposed to generate light gases, the small molecular aromatic clusters coming from the rupture of bridge bond escaped to form the tar, and the nucleus of the char become larger. All these changes illustrated that the coal pyrolysis equaled to the decomposed of functional groups. So the coal was considered as a two-dimensional structure which was composed of four types of functional groups with different chemical bonds. The coal pyrolysis products distributions were predicted by the kinetic equations, and the two parameters of the initial amounts of functional groups and the potential yield of tar were determined by the modified Van Krevelen diagram and the semi-empirical correlation. The comparison between the simulated value and the experimental value verified that the model can correctly predict the coal pyrolysis products distributions. Except for H2 and tar, all other products had two or three different sources. The different product curves was not obvious since the interaction effect between the initial amounts of functional groups and the rate constants.3) Building the component analysis model of coal tar. The proximate analyses of different temperature char were determined by the mass conservation of the coal pyrolysis unit. With the increase of the pyrolysis temperature, the volatile yield of the char decreased, while the contents of fixed carbon and ash increased. With the objective function of the correlation coefficient, the functional equations between the ultimate analysis and proximate analysis of three different metamorphic grade coals were fitted by curve regression analysis software. The ultimate analyses of the different temperature char were calculated by these functional equations. The correlation coefficient of carbon in the functional equation of lignite, high-volatile bituminous coal, and bituminous coal were 0.84, 0.91 and 0.89, respectively. And it of hydrogen in the functional equation of these three types of coal were 0.78, 0.83, and 0.79, respectively. The content of carbon in char calculated by these equations increased with the increase of the pyrolysis temperature, while the contents of hydrogen and oxygen decreased. The element composition of coal tar were calculated by the element conservation of the whole pyrolysis unit. The content of carbon, hydrogen, and oxygen in the 500°C coal tar of three coals were around 85%, 9.8%, and 4.5%, respectively. 27 coal tar model compounds were consisted of 4 phenolic compounds, 13 hydrocarbons, 5 aromatic compounds, 2 oxy-compounds and 3 asphaltenes. And the 500°C tar component of the three coals were quantitatively analysized by the model. The yields of phenolic compound, aromatic compound, hydrocarbon, oxy-compound, asphaltene were around 24%, 20%, 30%, and 20%, respectively. While the yield of oxy-compound was between 4.3%-4.7%.