Coal Derived and Related Fossil Fuels

As noted earlier, there are a variety of fuels derived from coal. These include coke, char, and liquids and a wide array of gases from coal gasification, including coke oven gas, blast furnace gas, water gas, and producer gas. There are also several new gasification processes as sources of synthetic natural gas that are currently under development.

There is variety of coal liquefaction processes that are under development. The three basic processes used for liquefaction are direct and indirect liquefaction and pyrol-ysis. In the direct liquefaction process, coal is slurried in a process-derived oil and then reacted with hydrogen at high pressure and moderate temperature to form liquid hydrocarbons. Ash and unreacted coal are removed from the product and the unused coal is gasified to produce the required hydrogen. In the indirect process, coal is first gasified to a synthetic gas and then catalytically converted to methanol or other liquid hydrocarbons. The direct method has received more development focus because it offers higher conversion efficiencies (about 65 to 70%). Pyrolysis requires heat to drive off volatile portions of the coal, producing gases, liquids, and solid products.

The ability to transform coal to useful gases and liquids has a great appeal in that these by-products can be more easily used in various combustion applications and can be used with reduced pollution control expense. While some of the synthetic gases are already commercially used, other products may become increasingly prominent in the future, depending on the status and market price of natural gas reserves.

In addition to the vast global reserve of coal, other fossil fuels in large reserve include shale and tar sands. Shale and tar sands are amenable to producing oil. Based on current technologies and oil prices, no extensive commercial production of shale oil is expected for many years. Tar sands, however, have already been economically produced and remain a potentially valuable energy source, depending on technology advances and world energy prices.

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