Translated Abstract
Coal plays the most important role in the energy consumption in China. However, relatively inefficient utilization of coal not only brings tremendous waste of resources, but also causes serious environmental pollution. Supercritical water gasification of coal(SCWGC) has great development prospects, which can realize clean and efficient utilization of coal without generating pollutants such as SOx, NOx and particulate matters. It is necessary to find a low-cost and high activity catalyst for the large-scale SCWGC process to reduce the gasification temperature. At present, there is a need of high-efficient systems for catalysts evaluation in supercritical water, and the understanding of the control process in gasification and the catalytic gasification mechanism are insufficient. In this dissertation, systematic study on these subjects was carried out experimentally and theoretically.
First of all, a high-thoughput gasification system was designed to realize efficient and accurate evaluation of catalysts used in SCWGC. The system has a high heating rate (50-70 ℃•min-1). It is easy to realize the collection and analysis of gasification products. Six sets of experiments can be conducted parallelly to improve the experimental efficiency and facilitates the precise comparison of serial experiments.
Secondly, experiments on coal and model compounds gasification were conducted in high-throughput reactors for the problems of insufficient understanding of the control process in gasification and consequent inability to implement targeted enhancement on that process. The results show that the temperature is the key factor in the SCWGC process. Increasing the temperature can promote the gasification reaction significantly. By analyzing the gasification products, it is considered that increasing the steam reforming reaction on coal surfaces to decompose various intermediate products before dehydrocondensing to form difficult-to-gasify complex polymers might be the most important way to increase the gasification rate.
Thirdly, the activities of different low-cost catalysts were systematically investigated from the aspect of promoting the steam reforming reaction of coal in supercritical water, and the catalyst with the highest activity was obtained. The catalytic properties of the catalyst were further studied. The results show that the heterogeneous catalysts such as Ni-metallic oxide catalysts, Raney-Ni catalysts and natural ore catalysts have relatively high efficiency in heterogeneous catalysis, which can accelerate the reactions between gas products. However, the improvement of gasification efficiency is limited due to the limited efficiency of mass transfer at the interface between the catalysts and coal particles when it comes to the steam reforming of coal. The homogeneous catalysts such as alkali or alkali salt catalysts can be uniformly adsorbed on the surface of coal particles, even into the internal pore structure, which greatly enhances the contacts between the active centers and the coal particles and effectively promotes the steam reforming reaction. The catalytic efficiency of eutectic alkali salt has not been further improved on the basis of alkali salt since the eutectic structure might be demolished by the reaction between alkali salt and aluminosilicates in coal in supercritical water. K2CO3 is a suitable catalyst for SCWGC from the aspects of catalytic activity, stability and economy. 10 wt% K2CO3 can greatly reduce the reaction activation energy of SCWGC down to 59.47 ± 4.87 kJ•mol-1. The complete gasification of 5 wt% Hami lignite and the nearly complete gasification of 2 wt% Hongliulin bitumite can be achieved at 750 ℃, with the gasification efficiency of 99.81% and 97.12%, respectively.
Finally, on the basis of above studies and further understanding of the characteristics of coal pyrolysis and steam reforming reaction, the reaction path of SCWGC was proposed, and the kinetics model for catalytic SCWGC was established. The kinetics parameters of each reaction in coal gasification were obtained. The results show that the steam reforming reaction is the key factor that affects the gasification efficiency, while water-gas shift reaction is an important way to change the molar composition of gaseous products. These two reactions are the primary source of H2. K2CO3 mainly promotes the heterogeneous steam reforming reaction, water-gas shift reaction, homogeneous steam reforming reaction and pyrolysis reaction. With 10 wt% K2CO3, the reaction rates of these four reactions are 12.5, 10, 5, 1.67 times of that without catalysts, which restrains the dehydrocondensation of intermediate products. This study is expected to provide the basis for reactor design and production control optimization in SCWGC.
Translated Keyword
[Catalyst, Coal, Gasification, Kinetics, Supercritical water]
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