Coal To Liquid Fuels
In this page, the term “CTL” will stand for “Coal To Liquid Fuels” only.
“CTL” is also often used as a general expression for all liquid products produced from coal, including chemicals which is a conversion dealt with on a specific page.
Several processes are available to convert coal to liquid fuels. The ones which today are the most frequently applied and studied are (i) the “indirect route” and (ii) the “direct route”.
1) Indirect route
It is called “indirect” because it is composed of two clearly separated steps.
Coal is first gasified with steam and oxygen to produce a synthetic gas or “syngas”, composed of carbon monoxide (CO), hydrogen (H2) and to a lesser extent carbon dioxide (CO2) and methane (CH4) as well as impurities.
The [H2]/[CO] ratio in the syngas generally needs to be increased, which is achieved in a “water-gas shift” reaction, where CO and water are converted to CO2 and H2. Syngas is then cleaned to eliminate dust, tar and acid gases.
The second step can consist in either one of the two following processes:
- The “Fischer-Tropsch” catalytic synthesis, which has benefitted from numerous experiences and is used in most demonstration plants and projects;
- The “Methanol to Gasoline” or “MTG” process: methanol (CH3OH) is first produced from syngas by a catalytic reaction of H2 and CO. Methanol is then dehydrated to give first DME (dimethyl ether, CH3OCH3) and then gasoline.
2) Direct route
In the “direct” route, coal is pulverized and mixed in a recycled slurry in which hydrogen is added under pressure. The mix reacts in an ebullated bed reactor to produce hydrocarbons which are refined under conventional refinery processes.
Both indirect and direct routes have respective advantages, in terms of versatility and quality of outputs (naphtha and diesel respectively). Most projects today are based on indirect processes, mainly thanks to the higher level of knowledge accumulated by experience and research so far.
Use of biomass
Liquid fuels can be produced from any hydrocarbon including biomass.
The main advantage is in decreasing the environmental footprint, as biomass is carbon-neutral.
Challenges come from the chemical nature of biomass, with lower calorific value, higher moisture content and tar formation, as well as seasonal supply variations.
Combining coal and biomass presents major advantages in terms of reaction temperature, as well as regular supply and the environmental footprint.
Operations and projects
Today, CTL is commercially operated in South Africa.
Four demonstration plants are in operation in China, with capacities from 7,000 to 20,000 barrels per day.
Several projects are in progress in China. Other projects exist in Australia, Botswana, India, Mongolia, USA and Russia.