Extraction with solvent and catalytic hydrogenation are the general division of coal liquefaction process. Each process can be described as following.
In the extraction process with the solvent, the coal is extracted and dissolved at a temperature of 625 – 725 K with a hydrogen donor solvent. During the dissolution process Hydrogen from the donor-solvent reacts with the coal molecule to form a hydrocarbon liquid. Other reactions such as depolymerization, hydrocracking, and sulfur removal also occur during dissolution. This donor solvent will be in various coal liquid fractions but more concentrated in the fraction with the highest boiling point. The product formed is then separated by using a solid filter. This process has disadvantages, especially in the dissolved process of coal separation
The catalytic hydrogenation process was initially very uneconomical because it used two reactors with high pressure (up to 600 bar) and high temperatures. In this process, coal is reacted with hydrogen gas at high pressure and temperature using hydrogen donor solvents and catalysts (usually Cobalt-Molybdenum type). Coal will be dissolved at high temperatures and at high hydrogen gas pressure, coal will form free radicals that will react with hydrogen gas.
This process produces more lightweight hydrocarbon products than solvent extraction processes because the principle is similar to the fluidized catalytic cracking process in petroleum. This process can use bituminous, sub-bituminous, lignite and brown coal coated with recycle oil.
The mechanism of coal liquefaction processes in both processes is difficult to describe compared to other reactions because as mentioned above, coal is a complex structure. The mechanism will depend on the type of coal itself, even in the same class. But generally, it will involve hydrogenation, hydrogenolysis, hydrocracking, and recombination reaction. To add the complexity, the reactor is operated in fluidized bed catalytic reactor where transport process also has impact on the reaction process.