In some countries without petroleum resources of their own the production of synthetic petrol from coal or from gases containing carbon is considered important but even in countries which are well supplied with petroleum, a certain amount of synthetic petrol is usually added to the natural product. Synthetic petrol is manufactured either from coal or from natural gas or from petroleum fraction. The designation synthetic is appropriate only if the initial substance is gaseous or of low volatility. Crude oil residues and other residue rich in carbon can be converted into petrol such petrol, too can be called synthetic.

Petrol is a high energy mixture of hydrocarbons consisting of molecules containing 5 to 12 carbon atoms and 12 to 26 hydrogen atoms.

Coal contains only very little hydrogen. To convert it into petrol it is therefore necessary to introduce hydrogen into the compounds. This is done under high pressure and introduce pressure and temperature and in the presence of active catalysts. A great deal of petrol used to be produced by so called catalytic high pressure hydration.

In the process it took 4 tons of coal to produce 1 ton of petrol, pressure up to 700 atm and temperatures of 410° - 460°C being employed. The coal was dried, pulvering and mixed with heavy oil to form thick slurry. Catalysts were added, and about 70,000 cu. Ft. of hydrogen gas per ton of coal was forced in. the hydrogen was produced from coal and water, the carbon monoxide formed in this process being utilized as fuel gas or converted.

Synthetic petrol is also manufactured from water and coal by a process in which carbon monoxide and hydrogen are produced form coke, raw brown coal or brown coal briquettes and, after careful cleaning, are passed over catalysts at low pressure. Solid hydrocarbons, in addition to petrol and other products are formed in this process. In a more recently developed process, gases containing carbon monoxide are conducted, together with water over suitable catalysts.

The resulting reactions produce petrol, as well as acids, alcohols and other substances. Also, petrol is produced from unsaturated hydrocarbon gases with aid of catalysts. However, since such synthetic petrol are more expensive to produce than petrol from petroleum in western Europe synthetic petrol is nowadays importance only as a additive for natural petrol so as to adjust their properties to meet the exacting requirements of modern internal combustion engines. For purpose, synthetic petrol have a high octane number are particularly valuable.