Fuels are substances that when combined with oxygen produce heat and are used either industrially or domestically. Fuels, depending on their condition, are divided into:

✓  Solid fuels: wood, charcoal, lignite, coal, coke, etc.

✓  Liquid fuels: lighting oil, diesel oil, heavy oil or fuel oil, motor or heating oil, alcohol, petrol etc.

✓  Gaseous fuels: propane, butane, coal gas, natural gas, etc.

The most common physicochemical analyzes carried out depending on the type of fuel are summarized in the following table: