Coal is a heterogeneous rock that is infiltrated by substances derived from natural organic materials derived from millions of years ago. The organic part contains elements of Carbon, Hydrogen, Oxygen, Sulfur, and Nitrogen. Coal is not a mixture of chemical compounds such as petroleum but rather a mixture of various types of substances determined by the source of organic material.
Based on the geochemical time of its formation, coal can be classified into (from the youngest): peat, brown coal, lignite, sub-bituminous, bituminous, and anthracite. The carbon content of older coal can increase up to 93.7% and calorific value will also increase. This increase in calorific value will affect the economic value of coal. The economic value of coal will increase with increasing calorific value.
Although the consumption of coal is tremendous, coal utilization nowadays mostly limited to power plant fuel use. This is due to its solid form, which is not practical to be used at a consumer level, even an ordinary stove. To make it easier to use for the consumer, coal should be converted into more commonly used forms, such as liquid fuels. There are two methods of converting coal to liquid: gasification and direct liquefaction. The gasification process is already available commercially. However, its process requires more steps since the coal need to be converted into gaseous substances first then it will be transformed into liquid form.
Meanwhile, direct coal liquefaction offers lesser step. However, the process has its own difficulties such as slurry handling and process efficiency. This direct coal liquefaction process has been studied since 1920 in Germany. During World War II, liquefaction plants using the Pott-Broche (solvent extraction) and Bergius process (catalysis hydrogenation) were commercially operated. And after 1950 there has been a lot of coal liquefaction processes like Solvent Refined Coal (SRC), Exxon Donor Solvent (EDS), and H-Coal process.
The liquefaction process of coal is generally divided into two kinds: extraction with a solvent and catalytic hydrogenation. The process of SRC and EDS is an example of the process with solvent extraction while H-Coal is an example of catalytic hydrogenation process.