The processes for converting lignite into chemical and petrochemical raw materials are known as coal-to-liquid (CtL) or coal-to-gas (CtG) processes. In the first step, raw lignite, which has a water content of around 50 %, is dried until its residual water content is around 12 %. It is then converted in a gasifier at high temperatures with a substoichiometric amount of oxygen, so that the coal does not burn but instead produces a gas mixture known as synthesis gas. The main components of this gas mixture are hydrogen, carbon monoxide, carbon dioxide and small amounts of hydrogen sulphide.
Methanol, ammonia, naphtha: synthesis creates lots of different products
In addition to the removal of CO2 and H2S, a subsequent gas treatment step must be taken to adjust the ratio of carbon monoxide to hydrogen, depending on the desired product. In the final step, the synthesis is carried out, creating the product, be it methanol, ammonia, naphtha (raw material used for example in the plastics industry) or methane (a major component in natural gas). There are two key technologies behind the use of Rhenish lignite in CtL/CtG processes: coal drying and coal gasification. RWE has decades of experience in both technologies from numerous projects, pilot facilities and demonstration plants. The company has also developed its own process for drying lignite, known as “fluidised bed drying with internal waste heat utilisation” (WTA®).