The Purpose of Coal Liquefaction and its Benefits

Coal is a blackish-brownish sedimentary rock that is combustible in nature and takes the form of rock strata known as coal seams. It consists primarily of carbon and hydrocarbons with some amounts of other elements, including nitrogen, hydrogen, oxygen, and sulfur.

Coal is formed by the breakdown of dying plant matter that decays into peat and then transforms into coal after undergoing pressure and heat for more than millions of years. Due to the longer time span, taken for the coal to form, it is classified as a non-renewable energy source.

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The purpose of coal liquefaction is to convert coal into liquid hydrocarbons, liquid fuels, and petrochemicals similar to crude oil. The conversion process is called Coal to X or Carbon to X, in which X stands for several different hydrocarbon-based products.

The two processes that are heavily employed are carbonization, which means heating oil without any presence of air, and hydrogenation, which makes coal react with hydrogen at elevated pressures, commonly with a catalyst. It is a process that fulfils the demand for consuming liquids in various energy applications.

The liquefaction process is completed by several methods, but two of them are the most common processes, including the indirect and direct routes.

In the indirect route, there are two steps: coal is first converted into gas using oxygen and steam, producing a synthesis gas (syngas). The obtained syngas is then cleansed to eliminate tar, dust, and acid gases. In the second step, the gas is reacted with a catalyst using the Fischer-Tropsch process that transforms the gas into a wide range of hydrocarbon fuels like diesel, gasoline, and methanol.

In the direct route, coal undergoes pulverisation and is then reacted with a catalyst. Next, under high temperatures and pressures, hydrogen is added with a solvent present. This method results in raw liquid fuel that is further refined to obtain fuels that can be used for transportation.

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The coal liquefaction process for obtaining liquid fuels is more expensive than the actual cost of refining crude oil; the technology is pursued mainly owing to the relative price of the raw feedstocks. The technology is a source that is aided by the sizeable recoverable coal resources all over the world.

The process was developed after significant research and development (R&D) that began in the early 1970s, especially in the United States of America, the United Kingdom and Japan, as a result of many oil price shocks. Since then, the process has been dismissed except in South Africa. Currently, the liquefaction plant in South Africa is the only one in operation. It contains substantial coal reserves but no gas or oil.

Coal liquefaction has considerable benefits as it is a widely proven technology for obtaining valuable liquid products. The process can produce transportation fuels using coal that is present in abundance. Also, it can counter the problem of oil stock depletion and oil supply constraints.

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