Coal liquefaction

Coal liquefaction is a chemical process that converts solid coal into liquid fuels such as synthetic crude oil, diesel, or other hydrocarbons. The process enables coal—an abundant fossil fuel—to be used as a substitute for petroleum in the production of transport fuels, lubricants, and chemical feedstocks. Coal liquefaction can be achieved through two primary methods: direct liquefaction and indirect liquefaction, both of which break down coal’s complex carbon structure into liquid hydrocarbons suitable for refining and utilisation.

Background and Historical Development

The idea of converting coal into liquid fuels dates back to the early twentieth century, driven by the need for energy independence and fuel diversification. The first large-scale coal liquefaction technology was developed in Germany during the 1920s by Friedrich Bergius, who received the Nobel Prize in Chemistry in 1931 for his pioneering work on high-pressure hydrogenation of coal.
During World War II, Germany used coal liquefaction extensively to produce synthetic fuels when access to petroleum was restricted. Later, in the 1970s oil crises, countries with large coal reserves—such as South Africa, China, and the United States—revived interest in coal liquefaction to reduce reliance on imported oil.
Today, while global focus has shifted towards cleaner renewable energy sources, coal liquefaction remains strategically important for nations with abundant coal resources and limited crude oil reserves.

Types of Coal Liquefaction

Coal liquefaction can be categorised into two principal methods depending on the route of conversion:

1. Direct Coal Liquefaction (DCL)
   • In this method, coal is directly converted into liquid hydrocarbons by breaking down its macromolecular structure using hydrogen at high temperature and pressure.
   • Catalysts (such as iron or molybdenum compounds) are used to promote hydrogenation, converting solid coal into a mixture of liquid hydrocarbons and gases.
   • The liquid product resembles crude oil and can be further refined into fuels like petrol, diesel, and kerosene.
   • The key chemical process involves hydrogen addition, making it a hydrogenation-based method.
   • Example: The Bergius process is a classical example of direct liquefaction.
2. Indirect Coal Liquefaction (ICL)
   • In indirect liquefaction, coal is first gasified to produce syngas—a mixture of carbon monoxide (CO) and hydrogen (H₂).
   • This syngas is then converted into liquid hydrocarbons using the Fischer–Tropsch synthesis or related catalytic processes.
   • Indirect methods are more flexible as syngas can also be used for producing chemicals like methanol and ammonia.
   • Example: The Fischer–Tropsch process, developed in Germany in the 1920s, and used widely in South Africa by Sasol, is a notable ICL technique.

Process Overview

The general stages of coal liquefaction include:

1. Coal Preparation – Coal is crushed and dried to remove moisture and impurities.
2. Reaction Stage –
   • In DCL: Coal is reacted with hydrogen and solvents at high temperature (around 400–500°C) and pressure (about 200–700 atm).
   • In ICL: Coal undergoes gasification at around 1,200°C with limited oxygen and steam to produce syngas.
3. Catalytic Conversion – Catalysts convert the coal-derived compounds or syngas into longer-chain hydrocarbons.
4. Upgrading and Refining – The resulting liquid hydrocarbons are refined into usable fuels and chemicals by cracking, distillation, or hydroprocessing.
5. By-Product Recovery – Valuable by-products such as gases, tars, and chemicals (phenols, sulphur, and ammonia) are recovered and processed.

Major Technologies and Examples

• Bergius Process – Direct hydrogenation of coal using catalysts under high pressure.
• Fischer–Tropsch Process – Catalytic conversion of syngas to liquid hydrocarbons.
• Sasol Technology (South Africa) – Large-scale ICL plants producing fuels and chemicals from coal since the mid-twentieth century.
• Shenhua Group (China) – Operates one of the world’s largest DCL plants using modern catalytic methods.

Advantages of Coal Liquefaction

• Energy Security – Provides an alternative source of liquid fuels for countries with abundant coal and limited oil reserves.
• Fuel Diversity – Expands the range of energy options available for transportation and industry.
• Economic Utilisation of Coal – Enables the conversion of low-grade coals into higher-value liquid fuels.
• By-Product Generation – Produces useful chemicals and gases that can be monetised.
• Stable Supply – Reduces dependence on volatile global oil markets.

Disadvantages and Environmental Concerns

• High Carbon Emissions – Coal liquefaction produces significantly more CO₂ compared to conventional petroleum refining, contributing to climate change.
• High Water Consumption – Both direct and indirect processes require substantial water for gasification, cooling, and hydrogen production.
• Expensive Infrastructure – High capital and operational costs make liquefaction economically viable only under certain conditions.
• Complex Process – Requires advanced technology, specialised equipment, and continuous maintenance.
• Pollution Risks – Disposal of solid and liquid residues poses environmental hazards if not managed properly.

Modern Developments and Innovations

Recent advances aim to improve the efficiency and environmental performance of coal liquefaction:

• Carbon Capture and Storage (CCS) – Integration with CCS technologies to reduce CO₂ emissions.
• Co-liquefaction – Combining coal with biomass or waste plastics to reduce carbon intensity and improve hydrogen balance.
• Catalyst Development – Use of nanocatalysts to enhance reaction rates and lower operating temperatures.
• Hydrogen Integration – Using green hydrogen (from renewable sources) instead of conventional hydrogen to reduce the carbon footprint.

Global Perspective

• South Africa remains the global leader in commercial coal-to-liquids (CTL) technology through Sasol’s operations.
• China has invested heavily in both direct and indirect liquefaction projects as part of its national energy security strategy.
• The United States and India have conducted extensive research into coal liquefaction, though large-scale deployment has been limited due to economic and environmental factors.