Basic Research On Typical Low-quality Coal Upgrading And Poly-generation System Based On The Cascade Utilization Of Coal

The increasing national economy demand for energy and the depletion of high-quality coals (HQCs) have necessitated the exploitation of low-quality coals (LQCs). Given that LQCs may become significant energy sources and chemical feedstock, studies on the utilization of LQCs are imperative. Due to their inherent limitations, the upgrading process will be the key technology to the utilization of LQCs. Microwave irradiation (MI) and hydrothermal dewatering (HTD) are the two typical upgrading processes, which can eliminate defects of LQCs to a certain extent, and improve their qualities and values, thus the LQCs can make up the shortage of HQCs.On the premise of power generation guarantee and the basis of LQCs upgrading techlonogies, poly-generation system based on the cascade utilization of coal is in accordance with the national conditions. Based on the philosophy that coal has the dual concepts of energy and resource, poly-generation system consists of power generation, chemical production and construction industry. In this system, different components of coal are cascade utilized and classified converted according to the various energy and resource value as well as their properties and chemical activity. The maximize extraction of high value-added chemical resources in coal can be realized, as well as electric, chemicals, gas and petroleum products, ash utilization, less emissions of pollutants.In this paper, five typical LQCs were dewatered and upgraded by MI and HTD. Microscopic physicochemical properties including coal composition, morphology, pore structure and chemical structure of raw and upgraded coals were detailed studied, with emphasis on the change of coal rank. Results showed that after upgrading, the moisture decreased, calorific value and density increased, pore structure extended, reactive groups decomposed and converted, overall gel structure destroyed, unstable composition decreased, stable composition increased. All these result in the increment of coal rank. Based on the analysis of thermogravimetric curves, characteristic parameters, and dynamics calculations, macroscopic physicochemical properties including pyrolysis, gasification and combustion characteristics of raw and upgraded coals were detailed studied by thermogravimetric. Results showed that after upgrading, the pyrolysis activity decreased, the thermal stability of coal structure was enhanced, pyrolysis processes were delayed toward the high-temperature region, exhibiting weak pyrolysis characteristics similar to bituminous coal. Releasing amounts of active micro-molecules in the pyrolysis gas products decreased, while releasing amounts of aromatic macro-molecules increased. After MI upgrading, the gasification activity increased, but after HTD upgrading, it decreased. This phenomenon was ascribed to the difference of pore structure changes and the difference on the method mechanism. Similar to the pyrolysis characteristics, due to the increment of stable composition and coal rank after upgrading, the combustion reaction processes were delayed to the high-temperature region, exhibiting weak combustion characteristics close to bituminous coal.This paper aims to provide the basic experimental data for the large-scale utilization of LQCs after MI and HTD upgrading process. According to the results, shortages of the LQCs are modified after upgrading and their qualities are improved. Although these lead to the decrease of chemical reaction activities during thermal conversion processes, the decrement is not remarkable and some characteristics even shown better reactivity than raw coal. Compared with bituminous coal, the upgraded coal keep the advantages of raw coal and still have the good reaction activities. All these are very useful for the large-scale utilization of Chinese LQCs after upgrading processes.Finally, poly-generation system based on the cascade utilization of coal was described and studied through theoretical thermodynamic calculations and preliminary experiments. The feasibility of the system was verified by thermodynamic calculations, by which the whole system operation structure can be established, and material flow and energy flow of each unit can be informed. The preliminary experimental results showed that excessively small coal particles or high pyrolysis temperature had a high volatile matter conversion ratio, a weak chemical reactivity of the semi-char also emerged. On the other hand, although excessively large coal particles or low pyrolysis temperature resulted in better reactivity, a low conversion ratio of chemically active components also emerged. When the particle size was smaller than0.3mm, conversion was similar, and when the pyrolysis temperature reached more than1000℃, the ratio did not increase further. Therefore, the appropriate conditions for the novel poly-generation system were a pyrolysis temperature of1000℃and a coal particle size of0.15-0.3mm.