This invention relates to a new and improved method for refining coal, particularly coal containing sulfur.
Various methods have been proposed for removal of sulfur from coal. The methods are generally tailored to remove a particular form of sulfur which may be an elemental, pyritic, organic or sulfate form of sulfur. Because of the extremely complex composition of coal and the radical variance in composition from seam to seam, and even within a single seam, the problems in effectively removing substantially all sulfur and sulfur compounds are significant.
It is believed, that the goal of reducing sulfur emissions from the combustion of coal can best be achieved prior to pyrolysis using efficient refining methods. In this manner combustion systems can be designed to a maximized efficiency through fueling with a uniform, high Btu, low ash and low-sulfur feed stock. Use of efficient combustion systems and high grade refined coal will have a substantial impact on acid raid caused by sulfur dioxide emissions from coal-fired energy and processing plants.
It is a primary object of this invention to produce a moisture free, fine particle material that can be used, as is, in a blow-fired or fluidized bed combustor; formed into a briquette or slug, for a grate-type combustor; or, mixed in an slurry or emulsion for transport and firing in a liquid spray as an oil substitute.
It has become apparent from technology trends that future transportation methods and combustion methods will utilize finely ground coal, often below 200 mesh. In certain instances where emulsions with oil or water or mixtures thereof are formulated, coal is ground to powders 350-400 mesh and below. While it may be advantageous to finely grind coal to this size to liberate the carbonaceous materials from the inert ash and chemically undesirable compounds as pyrite, such finely ground particles present substantial problems in applying conventional separation procedures.
In conventional systems for cleaning coal, it is customary to remove substantial portions of the pyrites and ash using largely mechanical separation means. Coal is crushed to a uniform size and specific gravity techniques are used to separate the lighter weight carbon and hydrocarbon materials from the heavier pyrite and ash waste materials. One effective method of separation is with a water cyclone. A water cyclone or hydrocyclone is a concentrator for the wet processing, of finely divided materials. The cyclone has no moving parts, and utilizes the principles of centrifugal forces to separate materials of different specific gravity by directing a high velocity slurry at a steep surface of a conical bowl, which provides for an overflow carrying lighter weight materials, and an underflow carrying heavier weight materials. By careful design of the geometry of the surface, materials can be separated at select median specific gravities or cut points. For coal, complex multiple angle surfaces have been devised to provide a full range of selectable cut points adaptable to the variety of different coals. Such compound water cyclones are customerily used in multiples with a recirculating of middling material to improve a clear separation of lights and heavies in desired proportions.
The composition of coal renders the specific gravity separation of constituent materials an imprecise art. Coal is an amalgamation of different materials which is not consistently amorphous and is not strictly stratified. Liberating particles of the constituent materials by crushing improves the potential ability to separate at desired specific gravities. However, it has been found that there is a limit for grinding using conventional cyclone systems because fine particles in the range of 200 mesh and below tend to naturally agglomerate in the cyclone feedwater system, thereby inhibiting particle differentiation and select separation in the cyclone. While the cause of such balling is not wholly understood, it is believed that the substantial increase in surface area in a mass of extremely fine particles allow exposed hydrophobic hydrocarbons of the carbonaceous material, which have a natural mutual affinity, to cause a coalescence of particles into an agglomerate. While the use of selective surfactants can encourage this agglomeration process and lead to methods for select separation of carbonaceous and non-carbonaceous compositions, such methods are somewhat slower than cycloning yet are recognized as competitive processes.
The method of refining coal here devised solves the problem of undersirable agglomeration and provides additional benefits improving the resultant efficiency of the refining operation and the quality of the refined product.