The fuels commonly used in boiler houses in Poland include hard coal, heating oil, high-methane natural gas (GZ-50) and liquefied gases (propane and propane-butane technical).
Fuel type selection, which the boiler room will be powered by, is the first and most important issue, which determines the selection of devices at the design stage and the conditions, in which they will function during the subsequent long-term operation. Even before the project is completed, it is necessary to consider the rationale for choosing a specific type of fuel, taking into account the investment costs, operating costs, forecasts for the fuel market, local conditions and, and, in fact, most of all, investor expectations and preferences, for which the project is carried out.
Each type of fuel has its own specific properties and characteristics, which give rise to its advantages and disadvantages. For each of them, the combustion process is slightly different. Getting to know the specificity of fuels allows you to better understand the course of energy conversion processes and learn about the principles of operation of individual devices in boiler rooms.

In general, fossil fuels may contain the following components:
- elemental carbon (C),
- hydrogen (H),
- sulfur (S),
- oxygen (O),
- nitrogen (N),
- mineral substances (A) - ash,
- total moisture (Wc),
- hygroscopic moisture (Wh) - related to the structure of coal,
- external moisture (In) - otherwise transient - removable when drying the coal in the open air.

In hard coal, the worst quality among the fuels in question, all ingredients are present: palne, non-flammable, mineral and both types of moisture. In heating oil, moisture is practically absent, while minerals - in trace amounts. Natural and liquid gases now contain only flammable and non-flammable components, Thus, they are among the four fuels in terms of quality.

The value of the basic fuel parameter - the calorific value - depends on the content of individual components in the fuel.
Opal value (Hi) it is the amount of heat obtained with complete and complete combustion of a mass unit; fuel and cooling the combustion products to the initial temperature, without taking into account the condensation heat of the water vapor it contains.
Another parameter is related to the calorific value, namely, the heat of combustion (Hs). It is defined as the amount of heat obtained with complete and complete combustion of a unit of fuel mass and cooling of the combustion products to the initial temperature of air and fuel and condensation of water vapor contained in them..
The relationship between the heat of combustion and the calorific value is as follows
Hs = Hi + latent heat (condensation)

The calorific value is therefore a component of the heat of combustion and is always lower than Hi < Hs