Method of forming a feed for coal gassification

#The invention relates to improved methods of forming a feedstock, comprising a wax-impregnated coal, for coal gasification.

#The gasification of solid fuels such as coal is well known. Several methods have been proposed for feeding the coal into the gasifier. In one method, the coal is ground to a fine powder and fed to the gas generator as a suspension in steam or a free oxygen-containing gas. This method is unsatisfactory as it is difficult to control the amount and rate of the coal fed to the gas generator and, in the case of a free oxygen-containing gas, care must be taken to maintain the velocity of the suspension above the rate of flame propagation to avoid a dangerous and damaging backflash.

#Newer methods have been developed to overcome the drawbacks of the dry, ground coal feedstock. One method is the production of a coal-water slurry in which the coal is ground to a particle size, mixed in water or organic liquids, and injected into the gasifier. The coal is ground to a fine particle size to ensure that almost complete conversion of carbon to oxides takes place during the residence time in the gasification zone of the gasifier. To properly feed such a slurry into the gasification zone, the slurry must be conveyed from the point at which it is generated to the gasifier. The slurry must not be too viscous to be pumped from its starting point to its destination but, simultaneously, cannot be diluted to a level that will cause incomplete or inefficient conversion to gas in a gasifier. The total water content of the slurry must therefore be kept, preferably, close to 30-40%.

#FIG. 1 shows a schematic of a coal gasification system operated in conjunction with a Fischer-Tropsch reactor and an IGCC (Integrated Gasification/Combined-Cycle) gas turbine unit.

#FIG. 2 shows a schematic of a method of preparing a wax-impregnated coal feedstock for a fixed-bed gasifier.

#FIG. 3 shows a schematic of another method of preparing a wax-impregnated coal feedstock for a slurry-fed gasifier.

#There is an enormous amount of coal available as an energy supply. Indeed, it is estimated that world wide there is more energy available in coal than in petroleum, natural gas, and oil shale combined. Coal is a useful energy source for gasification. High-grade coals have historically been preferred because of their high energy content, which makes the gasification process economically attractive. Considerable lower-grade coal reserves exist, which have a higher water content and lower energy content. Therefore, one embodiment of the present invention provides a new method of producing a feedstock for coal gasification in which low-rank coals are mixed with a wax that has been heated to a temperature above its melting temperature to form a wax-impregnated feedstock. The wax-impregnated coal is then introduced as briquettes to a fixed-bed gasifier or as a slurry to a conventional gasifier. This method of preparing a feedstock has the advantage of increasing the energy content of the coal prior to introduction into the gasifier. The energy content is increased two ways. First, the wax reduces the amount of water necessary to produce a pumpable slurry. Second, the relatively high energy content of the wax (typically greater than 19,000 Btu/lb) increases the overall energy content of the feedstock.

#Low-rank coals generally have an energy content of less than 7,000 Btu/lb, making them unattractive as a feedstock for conversion to synthesis gas via gasification using conventional technology. While the present invention is useful with any type of coal, it is particularly beneficial with low-rank coals such as sub-bituminous, lignite, and brown coal.

#One aspect of the present invention involves mixing coal with a wax. Waxes are relatively heavy paraffinic hydrocarbon compounds, typically having a carbon number in excess of twenty. These waxes exist as solids at ambient temperatures. Preferred waxes for the present invention include waxes produced within the mixture of paraffinic hydrocarbon compounds produced in the conversion of hydrogen and carbon monoxide on a powdered catalyst to liquid hydrocarbons (the Fischer-Tropsch reaction described below). The wax formed by the Fischer-Tropsch reactor is a polymethylene-type wax formed by the polymerization of carbon monoxide. The Fischer-Tropsch wax typically has a melting point ranging from 45° C. to 104° C. The hydrocarbons drawn from a Fischer-Tropsch reactor must be maintained above the wax melting temperature to prevent solidification of the wax, which results in a heavy solid that fouls and plugs the separation and transport machinery. This is an energy-intensive solution, and therefore it is imperative to use the wax near its source to prevent its transport from becoming prohibitively expensive. Depending on the catalyst and the Fischer-Tropsch reaction conditions, the liquid hydrocarbon phase drawn from a Fischer-Tropsch reactor typically has a composition resembling a highly paraffinic crude oil containing, for example, ranges of 10 to 40% naphtha, 20 to 40% distillate, and 20 to 60% wax compounds by volume. The naphtha recovered in this process may be mixed with or separated from the wax.