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A new scheme is proposed which would extend our present reserves of coal by making use of what is now a waste product.
Studies show that up to 25% of the volume of coal is lost as fine coal particles in the coal mining and handling process. These fines, considered to be nontransportable and useless may be concentrated as thick "deposits' in tailings ponds.
Actually, if the fines can be transported (as in the case of local slurry pipelines directly to the user), they are preferred, for most coal-burning electrical power plants inject the coal into their boilers as dust (-200 mesh). This allows for greater burning efficiency. Unfortunately, since fines are rarely available, utility companies must grind all coal received. Such grinding is very expensive, frequently costing as much as the coal itself.
At present, the concept of agglomeration of coal fines into coherent pellets strong and stable enough to allow transportation is being considered, and several pilot-scale operations have been undertaken. These have shown that while pelletization can be done with relative ease, the economics of the process is at best marginal. The binders considered to date include bentonite, various oils and asphalts, and organic waste. Obviously a binder which burns is preferable to one which contributes to the ash. Unfortunately, in each case, the coal pellets must be reground to dust to be injected into the furnace.
If, however, a binder is employed which is both combustible and contains a small amount of water, the expensive grinding stage can be eliminated. When a pellet held together by a water-soluble polymer or other water-based binder is introduced into a hot environment (a pre-heating chamber or the boiler itself), the vapor contained in the binder vaporizes and undergoes a rapid volume increase, causing a dramatic pressure increase inside the pellet. Meanwhile, the tensile strength of the pellet is being lowered by the degradation of the binder. Once the internal pressure exceeds the ability of the pellet to contain it, the pellet bursts. This "explosion" reduces the pellet once again to dust, since any pellet fragments would likewise burst. No grinding is necessary, and the economic pict re of the process improves dramatically.
Application of this process finds breadth when one considers the potential sources of coal fines. As environmental regulations tighten, coal-cleaning standards rise. To effectively remove organic sulfur from coal, the coal must first be crushed, and conceivably large supplies of coal dust would be available. Pelletizing might also find application between the end of a slurry pipeline and the ultimate user. Another potential source is in-situ comminution of thin, deep coal seams, which reduces the coal to small fragments before pumping it to the surface.
To be sure, coal agglomeration will find widespread use in the near future, and the use of a water-based binder will make the process economically feasible. Indeed, the self-bursting concept may revolutionize the burning industry.
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