school energy&environment

Thermochemical processes

It is well known that to light and feed a fire you need materials that burn and these, in technical terms, are known as fuel. Fuel alone is not enough for a fire to  burn, another element is required: the combustive agent. The most common combustive agent is oxygen, which, in a combustion reaction, has the task of “oxidizing” the fuel, with the consequent release of energy in the form of heat and light. Fire therefore is simply the visible manifestation of a chemical reaction, combustion, that  takes place between two different substances: the fuel and the combustive agent. There are numerous combustible substances and materials. Initially man burnt wood, then coal. Today the most common fuels are the fossil fuels: petroleum, methane gas and coal or fossilized carbon.
Combustion is the most ancient method to obtain energy from the biomass. Antique fireplaces, chimneys and stoves have now been replaced by modern and efficient heaters that are able to exploit the hidden energy of wood and its derivatives. Fungi and bacteria prefer humid protein substances,  but fire feeds best on dry materials that are rich in cellulose. Cellulose is a complex molecule. It is very resistant and consists of long glucose chains, the most simple of sugars. Plants are made of cellulose,   so are wood, leaves, paper and cotton.  There are various systems to obtain energy from wood, that are classified according to the combustion temperature and the type of physical and chemical transformation that is obtained. Firstly it must be pointed out that these systems use crushed wood. The chips of wood can be used just as they are, or compacted in small blocks or pellets. Wood pellets increase the efficiency of the heaters and leave them cleaner. Wood transformed in this way can be burnt at extremely high temperatures (about 1000°C), and it turns into a mixture of gases that can move turbines and produce electric power.  When it is burnt at lower temperatures, (from 400 to 800°C) wood separates into gases, liquids and solids. The solid component, coal, can still be used as a fuel, and the liquid component, pyrolysis oil can be used as a fuel for engines or can be used as the base in the synthesis of other products.