Fundamental Study On Thermal Chemical Conversion Characteristics Of Weak Reducibility Coal In Western China

At present, the development of the west regions has been a key of China's economic development. Coal is the pillar industry in China's western regions's development, so it has important theoretical significance and practical utilization value to study the structure characteristics and thermal chemical conversion properties on the coals from that region.Three coals which have high inertinite content from Xinjiang Hami, Ningxia Lingwu and Shengdong mines in China's western regions were selected as experimental coal samples. Their structure characteristics and pyrolysis, gasification and combustion reactivities were detailedly researched and compared with that of Pingshuo coal from China's middle region. In addition, intrinsic minerals in these coals were eliminated by deionized water, single acid and compound acid washing and the influences of these intrinsic and added minerals on the thermal chemical conversion properties of these coals were also researched. Main conclusions were obtained as follows:1) Three coals from China's western ragions demonstrate the properties of the weak reducibility and Pingshuo coal has the strong reducibility properties. Weak reducibility coals have high inertinite and O content, low volatile, ash and H/C atomic ratio; C content in inertinite is higher and H/C and O/C atomic ratio are lower than that in vitrinite; and for strong reducibility coal, C content and H/C atomic ratio in inertinite are lower and O/C atomic ratio is higher than that in vitrinite. The ashes in the weak reducibility coals have higher Fe2O3, CaO, MgO and Na2O, lower SiO2 and Al2O3, and that the ash in strong reducibility Pingshuo coal has higher SiO2 and Al2O3. Most of the coals in china's western belong to non-coking or weak-coking coal, and that Pingshuo coal belongs to coking coal. Aromaticity and mean ring condensation number increase with inertinite content increase in coal. The aromatic crystallite unit is bigger, aromatic layer structure is more regularity and mutual directional degree is superior in inertinite to that in vitrinite. As heating treatment temperature increase coal crystallite structure becomes gradually regularity.2) The experimental relation expression has been put forward to estimate the reducibility of coal: k = I%×0.8 + O% + Rz - H%×10 to provide a basis for quantitative analysis of the relative intensity of the coal reducibility.3) The more inertinite in coal, the weaker coal pyrolysis reactivity is. The secondary pyrolysis reactivity at higher temperature on weak reducibility coals is higher than that of strong reducibility Pingshuo coal, which leads to the more realese of containing-O gases. The more containing-O gases and the lesser containing-H gases are released during pyrolysis of weak reducibility coal and its inertinite than that of strong reducibility coal and its inertinite. The yields of containing-O gases from vitrinite pyrolysis of weak reducibility coal are higher than that from strong reducibility coal vitrinite. The yields of containing-O and containing-H gases from coal pyrolysis are also consistent with O and H content in coal. With pyrolysis temperature increasing, the yields of containing-O and containing-H gases released present a regular change trends and the optimal release temperature regions are CO 300-900℃, CO2 200-800℃, H2 500-900℃, CH4 400-800℃and C2-4 hydrocarbon 400-600℃.4) The gasification reactivity of weak reducibility coal char is higher than that of strong reducibility coal char; the gasification reactivity of weak reducibility coal inertinite char is higher than that of vitrinite char; and it is just reverse to strong reducibility coal maceral char. The gasification reactivity of coal has to do with the ultrastructures of the coal. The more fusinite and semifusinite content in inertinite, the higher coal char gasification reactivity is; and that the more telocollinite and desmocollinite content in vitrinite, the weaker coal char gasification reactivity is. There are synergy action between inertinite and vitrinite and appropriate inertinite content could lead to higher gasification reactivity of char. More H2 and CO are released from weak reducibility coal gasification, but the yield of CH4 is smaller than that from strong reducibility coal gasification. Unreacted shrinking core model is fit for the mechanism description of coal char gasification process and the gasification reactivity of char could be generally reflected by activation energy calculated from the model.5) The more inertinite content in coal, the higher coal combustion reactivity is. The combustion reactivity of weak reducibility coal is higher than that of strong reducibility coal. The combustion reaction rate increases with heating rate increasing. The ignitibility of coal decreases with O/C quality ratio decreasing. As the dosage of coal sample increases, the maximum combustion rate temperature and burnout temperature increase, the maximum combustion rate decreases, but the ignite temperature almost unchanges. As the particle size of coal sample decreases, the maximum combustion rate increases, but the last weight loss is almost unchanged. Unreacted shrinking core model is also fit for the mechanism description of coal combustion process and the combustion properties of these coals can be reflected by activation energy calculated from this model.6) Aluminosilicate restrains coal pyrolysis process. Alkaline earth salts and some transition metal salts accelerate, and some water-solubility salts restrain the pyrolysis of weak reducibility coals, but the latter two salts hardly affect the pyrolysis of strong reducibility Pingshuo coal. Alkaline earth salts and transition metal salts can accelerate the formation of containing-O gases during coal pyrolysis, hold back the release of containing-H gases during weak reducibility coal pyrolysis, but accelerate that from strong reducibility coal pyrolysis. Aluminosilicate accelerate weak reducibility coal pyrolysis to release containing-O gases, and restrain to produce containing-H gases, but it is just reverse for strong reducibility coal. Added Ca can accelerate coal pyrolysis to release more H2, CO and CO2. The effect of added Na on coal pyrolysis has connection with types of coal and Na content. It can accelerate coal pyrolysis to produce H2 and CO2, restrain weak reducibility coal pyrolysis to give birth to CH4 and CO, accelerate strong reducibility coal pyrolysis to release CO and haradly influence CH4 release.The effect of minerals on coal gasification and combustion is closely interrelated with coal characteristics. It can restrain the gasification and combustion process of strong reducibility coal char; alkaline earth salts, transition metal salts and aluminosilicate can accelerate weak reducibility coal char gasification, but the effect of minerals on the combustion of the weak reducibility coal has to do with types of coal. Added Ca and Na can promote coal char gasification and combustion. Added Na can accelerate remarkably coal combustion and result that the secondary maximum combustion rate of weak reducibility coals is significantly higher than that of the strong reducibility Pingshuo coal.