The process of pyrolysis consists of a thermochemical conversion that allows transforming the organic substance into final fuel products (solid, liquid, gase­ous). Pyrolysis takes place in the absence of oxidizing agents, or with a limited presence of these agents so that the oxidation reactions can be neglected. The heat required for the evolution of the process can be indirectly supplied through the reactors walls (transport of heat for convention and irradiation) or directly by recirculating a heating tool in the bed (heat transport for conduction) [44].

The products of pyrolysis, although they differ depending on the feed material, can include the following [44]:

• a fuel gas having a medium calorific power (13-21 MJ/N m3), mainly made of CO, CO2 (if oxygen is present in the basic material), H2 and light hydrocarbons (both saturated and unsaturated);

• a liquid product (obtainable from the condensation of the vapour phase) that is separated into two phases: an aqueous phase containing low molecular weight organic species that are soluble and a non-aqueous phase that is mainly made of organic molecules and oils with high molecular weight, called tar or bio-oil;

• a solid carbon product (char) and the cinders.

The most common modalities for execution of the pyrolysis process are [30, 44]:

• ‘Carbonization’, the most ancient and well-known pyrolysis process, which takes place at temperatures between 300°C and 500°C. From this process, only the solid fraction (vegetable carbon) is obtained and therefore the other fractions can be minimized.

• ‘Conventional pyrolysis’, which takes place at temperatures lower than 600°C, with moderate reaction times. From this process three fractions in about the same proportions are obtained.

• ‘Fast pyrolysis’, which takes place at temperatures between 500°C and 650°C, with brief reaction times. This process favours the production of a liquid fraction up to 70-80 % of the feed biomass weight.

• ‘Flash pyrolysis’, similar to fast pyrolysis but which take place at temperatures higher than 700°C and have reaction times that are lower than the former. This process allows the production of a liquid fraction up to 80% of the feed biomass weight, but with a composition variation that is more restricted than that of the fraction obtained by fast pyrolysis.

The main parameters that influence the process are [44]:

• temperature and pressure;

• speed of feed heating;

• dimensions and shape of the biomass to be treated;

• presence of additional catalysts;

• residence times of the solid phase and volatile phase in the reactor.

The products of pyrolysis can be used for the following purposes [44]:

• The gas: It can be burnt to give heat to the reactor involved in the pyrolysis or it can be applied as a fuel in turbo-gas or internal fuel engines.

• The tar: In most of cases, it is not directly applicable as a fuel because of its high viscosity and acidity due to the presence of oxygenated organic compounds. Before combustion it is necessary subject them to catalytic hydrogenation (upgrading) which involves, practically, the removal of the oxygen present. Recent studies have evaluated the possibility of using bio-oils for the production of H2 by catalytic reforming for application in combustible cells [30, 45];

• The aqueous solution: This fraction is derived from the pyrolysis of the feed’s humidity. It helps in the dissolution of the organic oxygenated species that orig­inate from the pyrolysis as organic acids, aldehydes, ketones, phenols, which are otherwise difficult to dispose.

• The char: These solid carbonaceous residuals can be used as fuel or find application in the chemical industry.

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