Fossil fuels are called hydrocarbons because they are in dominance composed by Carbon and Hydrogen.

When a  fossil fuel has the right conditions of temperature and pressure can be oxydated by the oxygen so the CH chemical link can break down in water H₂O and CO₂. That happens naturally in case of an ideal combustion, a real combustion has particles like Nitorgen Oxydated (NOx), fuels unburned (HC), Sulfure oxydes (SO, SO₂,...)  

The combustion is prepared with the mass of fuel in ratio with the air: stechiometric ratio (lean combustion, oxygen is just enough to burn the overall) or air excess ratio. Air gets heated during the compbustion hence his volume increases: the heat can than be transformed in work.

The heat can be used as it is for processes where there is no need to work trasnformation:

syderurgy to produce iron and alloys, cement, tiles, glass, ....

The heat can be transformed in work when there is teh need of mechanical energy: combustion engines (diesel, gasoline, gas, hydraulic pomps, steam turbine).

The mechanical energy can be transformed in electricity: by using generators ....

Here after a list of the most common fuel used to produce heat that.

When using a fossil fuel we need to evaluate four main points:

1. the amount of CO2 eq / MJ
2. the amount of H2O / MJ
3. the risk of Green House Gas effect of the fuel itself if leaked in the athmosphere
4. The natural tendency of the fuel to have a clean combustion

Hydrogen is the combustible that doesn't have any CO2 emission at the combustion, naturally its production has to be green: not produced with processes underlining processes that prodcue CO2.

Methane has a low CO2 emission but the leakage in athmosphere of 1 molecole of CH4 values ~ 40 molecoles of CO2

MORE TO COME ...