Wood as a form of fuel is a natural source of energy. By proper burning it makes only a small amount of ash and smoke. Fuel contains relatively a lot of water. For example a new-cut tree can contain up to 50 % water.
The amount of ash in wood is less than
0,8 %, in low quality coal it can be up to 40 %.
By combustion of fuels energy is created which is contained in its mass. All kinds of energy are measured in units which we call Joules. The influence of heat in a period of time is called thermal power. The unit of any power is Watt. The relation between power and energy is 1 Watt = 1 Joule / sec, resp. 1 Ws = 1 Joule. In praxis we use multipliers of these units: 1 kW = 1 000 W, 1 kWh = 3 600 kJ, resp. 860 kcal.
We get a better view on fuel energy if we will measure it in the general unit kWh. 1 kWh = 3 600 kJ, resp. 3,6 MJ. For example, the energy in 1kg of dry wood is 15 MJ. In kWh it is: 15 000 / 3 600 = 4,2 kWh.
Let us remember that the output of one vacuum cleaner is 1 kW. That means that the energy contained in 1kg of wood can drive the vacuum cleaner for more than four hours!
Fire is a chemical reaction, where the flamable particles of the fuel react with oxygen in air. The requirements for combustion are enough heat and oxygen. The products of combustion is the heat and gas waste in form of carbon dioxide, nitrogen, sulfur and water steam. Local heating provides easy tempering of warmth in houses especially before and after winter. The combination of central heating and local heating has an advantage in cases when the supply of gas is interrupted. We can hear very often from media about this situation. The cons of imported energy are its costs which are high and will remain so.