The definition of biomass which is taken from Directive 2001/77/CE, and acknowl­edged at a national level from the Legislative Decree no. 387 of 29 December 2003, reunites a wide class of materials of vegetable and animal origin that also includes rubbish. More simply, the biomasses which are appropriate for energy transformation, if it happens directly using the biomass or prior to changing of the same in a solid, liquid or gaseous fuel, can be divided based on their source into the following compartments [2]:

• forest and agro-forest compartment: forest-cultural or agro-forest activities and operations residuals, use of the coppice, etc.

• agricultural compartment: farming residuals which come from the agricultural activities and the dedicated lignocelluloses species cultures, oil bearing plants for the extraction of oils and their transformation into bio-diesel, alcohol-producing plants for bio-ethanol production;

• zoo technique compartment: livestock sewage wastes for the production of biogas;

• industrial compartment: coming from wood or wood product industries and paper industries, as well as agricultural and food industry residuals;

• urban rubbish: maintenance of the public green and urban solid rubbish maintenance operations.

The term biomass groups materials that differ from each other in terms of chemical and physical characteristics. They can have multiple uses on the energy production front. Generally, as we will see hereafter, it is possible to group trans­formation processes into different categories: the processes of biochemical conversion, which allows the gain of energy through chemical reactions due to the presence of enzymes, fungi and other micro-organisms that form, in particular conditions where the biomass is held; the processes of thermochemical conversion has, as it basis, the heat action that allows the development of chemical reactions which are necessary to transform matter into energy. The factors that favour the choice of one of the two processes are the carbon/nitrogen (C/N) ratio and the grade of humidity at the time of collection: when the C/N ratio is lower than 30 and the humidity content exceeds 30%, biochemical processes are generally used; on the contrary, thermochemical processes are more suitable [2-5].