Pyrolysis is defined as the thermochemical decomposition of organic materials in the absence of oxygen or other reactants executed under specific parameter conditions (Table 7.1). Several types of pyrolysis have been developed: vacuum pyrolysis (Roy 2000), pressurized pyrolysis (Tomeczek and Stanislaw 2003), fast pyrolysis (Bridgwater and Peacocke 2000), flash pyrolysis (Demirbas and Arin

2002) , torrefaction pyrolysis (Prins et al. 2006) and slow pyrolysis (Antal and Gr0nli

2003) . The processes differ from each other according to the conditions maintained in the pyrolysis reactor (Table 7.1). Fast pyrolysis leads to a high yield of bio­oil, while vacuum and slow pyrolysis offer a good compromise for the production of char and bio-oil, providing relatively high yields of both as well as providing superior quality char products (Bridgewater 2011; Bridgwater and Peacocke 2000; Chen et al. 2003). Bio-oil represents a valuable liquid fuel for boilers, while chemicals, nutritional supplements and/or pharmaceutical products may be isolated from it, provided that the challenging separation of these compounds can be achieved efficiently. The char represents a good feedstock both for boiler fuel and for the production of activated carbon. In more recent years char has also become an option for soil improvement and carbon sequestration (Antal and Gr0nli 2003).

Fast pyrolysis thermal decomposition of a feedstock using a relatively high heating rate can yield liquids (collectively termed bio-oil) of up to70-75 % of the weight of the starting material (Bridgwater 2011; Butler et al. 2011). One of the main advantages of fast pyrolysis lies in the fact that it is an effective method for densification of voluminous biomass. Different fast pyrolysis technologies exist, namely ablative, cyclonic, rotating cone, entrained flow, bubbling fluidised bed (BFB), auger, circulating fluid bed (CFB), transported bed, screw and auger kiln and wire-mesh reactor fast pyrolysis (Hoekstra et al. 2012; Bridgwater 2011; Butler et al. 2011). Currently, BFB and screw kiln reactors can be used for commercial — scale production of biofuel. Because of its properties relating to aging, instability, corrosion and viscosity, fast pyrolysis bio-oil may be upgraded physically, chemi­cally or catalytically (Bridgwater 2011). Bio-oil can be a substitute for fuel oil used for electricity generation and biorefinery.

Slow and vacuum pyrolysis processes deliver higher char and gas yields, limiting their energy applications to electricity and heat production. Slow pyrolysis follows the conventional carbonisation process of charcoal or biochar production, with recent improvements in recycling of gaseous/liquid products providing much of the heat required by the process, increasing overall energetic efficiency. The main difference between vacuum and slow pyrolysis lies in the method of removing gaseous vapours from the reaction zone. In vacuum pyrolysis, a vacuum is created which serves the same function as purge gas used in slow pyrolysis. Because of the lower pressures applied, aerosols tend to evaporate more easily. This removes them from the reaction zone and results in a significantly reduced vapour residence time of 2-3 s for vacuum pyrolysis, relative to the 165-170 s required for slow pyrolysis. Vacuum pyrolysis is therefore a modified slow pyrolysis resulting in improved quality of both liquid and char products (Carrier et al. 2011).

Torrefaction is classified as a mild pyrolysis technique, because it takes place in an inert atmosphere at relatively low temperatures (between 200 and 300 °C (Uslu et al. 2008)). The technology is less sophisticated than fast, flash, slow and vacuum pyrolysis technologies and can be seen as something in-between the combustion of dried biomass and of pyrolysis products. From a chemical point of view, torrefaction removes oxygen from the original biomass resulting in a solid product which has a lower O/C molar ratio (Van der Stelt et al. 2011). Torrefied biomass has potential for application in various industries: as raw minerals for pellet production, as reducer for smelters in the steel industry, for the manufacturing of charcoal or activated carbon, for use in gasification and for co-firing during boiler operation.