Pyrolysis Behavior Of Weakly Reductive Coals From Northwest China

Abudant Jurassic coalfields exist in the Northwest China and play an important role in China's energy supply. Most Jurrassic coals were subjected to strong oxidative but weakly reductive effects during accumulation process; therefore, these Jurassic coals were named as weakly reductive coals from the reducibility of colification environment. The weakly reductive coals exhibit different physicochemical properties from common coals for their particular coalification process, but the information about their thermal behavior was rarely reported for they were only exploited recently. Coal pyrolysis is an initial process of all major coal conversion process, such as gasification, liquefaction, combustion and carbonization; therefore, to rationally and effectively utilize these weakly reductive coals, it is essential to understand their pyrolysis behavior. In this paper, three typical weakly reductive coals, Shengdong (SD) coal from Shaanxi province, Lingwu (LW) coal from Ningxia province and Hami (HM) coal from Xinjiang province, and their macerals were selected to study the effect of reducibility on the pyrolysis behavior by TG-MS and in a fixed bed reactor, and one reductive Pingshuo(PS) coal from Shanxi province and its maceals, were also studied for comparison. The main results in this work are as follows:1. The weakly reductive coals are characterized with low ash content, low sulfur content, low H/C and ash fusion, but high moisture, oxygen, inertinite and alkaline earth metal contents. Compared with PS coal, the weakly reductive coals exhibit lower pyrolysis reactivity, lower tar yield, lower C1-C4 light hydrocarbons and C7-C8 aromatic hydrocarbons evolution intensity; but higher CO2 evolution inteinsity. On the basis of the FT-IR and 1H NMR analyses, the tars of the weakly reductive coals C contain more oxygen-containing groups and aliphatic hydrogen at carbon atoms bonded to other aliphatic carbon atoms but lower aliphatic hydrogen adjacent to aromatic alkene groups than that of PS coal.2. The effect of inherent mineral matter, three added mineral matter (CaO, Fe2O3 and Al2O3) and ion-exchange on the pyrolyis behavior of three weakly reductive coals and PS coal were investigated by a thermogravimetric analyzer and in a fixed bed reactor. The results showed that the inherent mineral matter has no evident effect on the reactivity except for the decompostion of itself, but can change gas and liquid product distribution, and the inherent mineral matter of the west weakly reductive coals is benefit for the production of tar. Fe2O3 and Al2O3 have also little effect on the reactivity of coal pyrolysis, but can facilitate volatile matter into gas. CaO has obvious effect on the reactivity and product distribution, but the effect on the weakly reductive coals and PS coal is different. The presence of Ca2+ and Mg2+ restricts the escape of tar molecules but have catalytic effect on LW coal pyrolysis at high temperature region.3. TG-MS analysis showed that the inertinite has higher thermal stability than the vitrinite from same parent coal, and the pyrolysis reactivity of the vitrinite and the inertinite from PS coal is better than those from the weakly reductive coals. The pyrolysis process of the macerals studied can be described by three independent first order reaction kinetics models, and the activated energy obtained is between 87 and 225 kJ/mol. The vitrinite and inertinite from the weakly reductive coals have lower C1-C4 light hydrocarbon evolution intensity but higher CO2 evolution intensity than those from PS coal, respectively. In the fixed bed reactor, the volatile matter of PS macerals is inclined to evolve as tar, but that of the macerals from the weakly reductive coals is inclined to release as gas during pyrolysis. TG/DTG analysis of model coals prepared by mixing different maceral components implied the synergistic effect between the vitrinite and the inertinite is not obvious.4. The sulfur-containing gases emission results of the vitrinite and the inertinite from PS coal indicated that, except for the evolution peak originating from decomposition or reaction of pyrite, the evolution intensity of sulfur-containing gases from the inertinite is lower than that from vitrinite, and H2S is mainly caused by two kinds elementary reactions, while other sulfur-containing gases due to five kinds elementary reactions. The total sulfur removal of both the vitrinite and the inertinite increases with temperature, and is about 60% at 650℃, while the organic sulfur in the inertinite is more stabe than that in the vitrinite.5. The combustion reactivities of the maceral chars obtained from west weakly reductive coal and reductive coal were performed on a thermogravimetric analyzer. At the same pyrolysis temperature, the vitrinite char of reductive coal shows better combustion reactivity than its inertinite char, while the vitrinite char of west weakly reductive coal has lower or similar combustion reactivity compared with its inertinite char. The inherent mineral matter of vitrinites reduces the ignition temperature of the vitrinte chars, while the inherent mineral matter of the inertinites shows obvious inhibition effct on the combustion reaction of their chars. The presence of three added mineral matter obviously inhibit the combustion reaction of the maceral chars from reductive coal, and the ralative inhibition sequence could be described as follows:Al2O3>CaO>Fe2O3; CaO has a little catalytic effct on the ignition of the maceral chars from west weakly reductive coal, and the inhibition effect of Al2O3 and Fe2O3 on the combustion reaction of maceral chars from the west weakly reductive coal is lower than them on that of the maceral chars from reductive coal.