Comparative analysis of the selectivity of liquefaction processes for different coal types! ! !
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This post was last edited by jordan569 on 2013-1-6 22:41. Comparative analysis of the selectivity of liquefaction processes regarding coal types. The selectivity of coal-based indirect liquefaction processes for different coal types is essentially the same as that of the gasification processes suitable for those coal types. The purpose of gasification is to obtain gas mainly composed of syngas (CO+H2) as much as possible. The most advanced gasification process currently recognized is the dry coal powder fluidized bed pressurized gasification process, and the typical commercially implemented process is Shell’s SCGP process in the Netherlands. The fluidized bed pressure gasification of dry coal powder is theoretically highly adaptable to various feedstocks; it can gasify a wide range of coals, from anthracite to lignite, as well as solid fuels such as petroleum coke. No specific requirements are imposed on the activity of the coal, and it can handle a broad range of ash fusion temperatures. It is also suitable for coals with high ash content, high moisture content, and high sulfur content. However, from a techno-economic perspective, lignite and low-graded, highly active bituminous coal are more suitable. Generally, the coal used as feedstock for the furnace should have a ash fusion flow temperature (FT) of less than 1400°C; if this temperature is higher, a flux needs to be added ； Ash content is less than 20% ； Dry coal powder is dried to a moisture content of less than 2% before being fed into the furnace, in order to prevent phenomena such as bridging, cavitation, and blockages in the dry coal powder transfer tanks and pipelines. ? The properties of the feed coal have a decisive impact on all hydroliquefaction processes. Practice has shown that as the degree of coalification of the raw coal increases, the hydrogenation reactivity of the coal remains relatively stable at first, but drops sharply after bitcoals with a moderate degree of metamorphism ； Among the microscopic components of coal, vitrinite and stable components are hydrogenation-active components, while inert components are non-hydrogenation-active components ； Pyrite in the raw coal serves as an excellent hydrogenation catalyst, while the alkaline substances in the minerals (such as MgO, Na2O, K2O) are detrimental to liquefaction ； Gas production is high when the oxygen content is high, while the liquid production is relatively low. Based on the extensive experimental research on hydroliquefaction, it is believed that the feed coal should generally meet the following conditions: bituminous coals and hard lignites with high volatiles and low degree of metamorphism, and a carbon content ranging approximately between 77% and 82% ； The content of the inert phase is less than 15% ； Ash content is less than 10% ； High-sulfur coal is beneficial for hydroliquefaction, and high-sulfur coal should be used as much as possible. China is rich in coal varieties. Research shows that there are numerous coal types suitable for gasifier-based gasification in the country, as well as an even larger number of coal types suitable for hydro liquefaction. Moreover, the higher-quality coal types suitable for hydro liquefaction are mostly found in areas where oil supply is relatively tight. . Note $ # , $ $
Coal liquefaction is a current focus in the coal chemical industry, and many coal mines are eager to give it a try. What they don’t realize is that coal liquefaction has certain requirements regarding coal quality – not all types of coal can be liquefied. Coal liquefaction is divided into direct liquefaction and indirect liquefaction. These two liquefaction methods have different requirements regarding coal quality. 1) Basic requirements for coal quality in direct liquefaction (1) The ash content in coal should be low, generally less than 5%; therefore, raw coal needs to be washed and processed to produce refined coal suitable for liquefaction. High ash content in coal affects oil yield and the proper operation of the system. The ash composition of coal also has an impact on the liquefaction process. Elements such as Fe, Co, and Mo in the ash are conducive to liquefaction and act as catalysts for it; whereas elements like Si, Al, Ca, and Mg in the ash are detrimental to liquefaction. They tend to cause scaling, which affects heat transfer and hinders normal operation. They can also lead to blockages and wear in the piping system, reducing the service life of the equipment. (2) The coal should have good grindability. Because for the direct liquefaction of coal, it is first necessary to grind the coal into coal powder with a particle size of around 200 mesh, and dry it to a moisture content of less than 2%, thereby forming an oil-coal slurry; this slurry is then subjected to hydrogenation reactions under high temperature and pressure. If the grindability is poor, energy consumption is high, equipment wear is severe, and there is heavy usage of spare parts and materials, all of which increase production costs. At the same time, the moisture content of coal is required to be low. High moisture levels are unfavorable for grinding and for preparing oil-containing coal slurry, thereby increasing investment and production costs. (3) The higher the hydrogen content in coal and the lower the oxygen content, the better; this can reduce the amount of gas required for hydrogenation as well as the amount of wastewater generated, thereby improving economic efficiency. (4) The lower the content of impurity atoms such as sulfur and nitrogen in coal, the better, in order to reduce the costs associated with oil processing and upgrading. (5) The composition of coal rocks is also a key indicator for liquefaction. The higher the silkiness component, the better the liquefaction properties of the coal; a high vitrinite content results in poor liquefaction activity. Therefore, the coals that can be used for direct liquefaction are generally younger coal types such as lignite and bituminous coal, and not all of these coal grades can be directly liquefied. Shenhua’s non-stick coal, long-flame coal, and Yunnan Xianfeng’s lignite are all good coal types for direct liquefaction. The indirect liquefaction of coal involves gasifying the coal to produce a feed gas containing H2CO, which is then combined with a catalyst under certain pressure and temperature conditions to yield liquid oil; therefore, the requirements regarding the quality of the coal are relatively lower. 2) Requirements for coal quality in indirect liquefaction: (1) The ash content of the coal must be below 15%. Of course, the lower it is, the more favorable it is for gasification as well as for liquefaction. (2) The coal should have good grindability and low moisture content. Regardless of the gasification process used, powder production is an important step. (3) For the process of producing gas from water-coal slurry, the coal must have good slurry-forming properties. The solid concentration of water-coal slurry should be above 60%. (4) Requirements for the ash fusion point of coal. Fixed-bed gasification requires that the ash fusion temperature of coal be as high as possible, with an ST value generally not less than 1250°C; fluidized-bed gasification requires that the ash fusion temperature ST of coal be less than 1300°C. Although indirect liquefaction has a wider range of applicability to coals, and different gasification methods need to be chosen for various types of coal, it is necessary to wash and process the raw coal in order to reduce its ash and sulfur content.