Pyrolysis Study Of Coal-Based Model Compounds

The pyrolysis mechanism study of coal is very important to develop its new thermal conversion technologies. The study will be simpler and the conclusions will be clearer when using coal-based model compounds as the research objects to represent the characteristic structures of coal. The pyrolysis behaviors of many coal-based model compounds were studied on a fixed-bed reactor and a pulsed-injection pyrolysis reactor in this dissertation. The bond dissociation energies (BDEs) were calculated by using quantum theoretical calculation. Through the analysis of pyrolysis behaviours of these model compounds, the work will provide the basis for understanding the characteristics of typical bonds in coal during the pyrolysis and developing the technology of coal clean utilization. The major contents and results in this work are summarized as follows:(1) The pyrolysis behaviors of three phenyl ether model compounds (anisole, phenyl ethyl ether (PEE), benzyloxybenzene (BOB)) was studied on the fixed-bed reactor at 500, 600 and 700℃ . Experimental results and theoretical calculations show that the Caliph-O bond dissociation is an initial radical step of these model compounds and phenol is the main liquid product. The conjugate function generated from the two benzene rings in BOB weakens the BDE. Therefore, the conversion of BOB is higher than that of anisole and PEE. The existence ofβ-H allows the PEE to form phenols through non-radical reactions. Therefore, the conversion and phenol yield of PEE is higher than those of anisole. A large number of C2H4 product for PEE pyrolysis at 700℃ is the evidence for the non-radical reactions.(2) The pyrolysis of three diarylalkanes (biphenyl, diphenyLMethane, bibenzyl) was studied on the fixed-bed reactor at 500,600,700 and 750 ℃. The sequence of the BDE in descending order is Caryl-Caryl bond in biphenyl (452.39 kJ/mol)> Caiiph-Caryi bond in diphenylmethane (335.08 kJ/mol)> CaliPh-Caliph bond in bibenyzl (232.10 kJ/mol), while the sequence of conversion in ascending order is biphenyl< diphenyhnethane< bibenzyl. With the increase of the bridge-bond length, the BDE and dissociation temperature decrease while the pyrolysis conversion increase. Compared with bibenzyl, the existence of O atom in BOB weakens the BDE and improves the conversion.(3) Heteroatom containing coal-based model compounds were also investigated on the fixed-bed reactor. The results show that O-containing ethers are easy to cleavage, while ketone and phenol are thermal stable. The pyrolysis of thiophene is very difficult while that of disulfide ether is relatively easy. The main gas product during the pyrolysis of disulfide ether is H2S. Due to the high stability of N-contianing heterocyclic ring and high BDEs, indole and quinoline are hard to decompose under our experiment conditions.(4) The effect of heating rate to the pyrolysis of the coal-based model compounds were studied on the fixed-bed reactor. The pyrolysis process of the coal-based model compounds will be enhanced with improving the heating rate. Meanwhile, the secondary reactions will be limited because of the shorter heating time.(5) The effect of additives to the pyrolysis of BOB and bibenzyl were investigated on a pulsed-injection pyrolysis reactor. The results show that the pyrolysis of BOB and bibenzyl are significantly influenced by tetrahydronaphthalene (THN) as a hydrogen-donor solvent. THN will supply hydrogen radicals to stabilize the radicals generated from the pyrolysis of BOB or bibenzyl during the co-pyrolysis process. Thus, the pyrolysis process of BOB or bibenzyl will be enhanced and their pyrolysis conversion will be improved. The existence of hydrogen radicals will restrict the secondary reactions and polycondensation. As a result, the yields of liquid product increase while the char yields decrease.