Pyrolysis is a physical-chemical process in which biomass is heated to between 400°C and 800°C, resulting in the production of a solid phase rich in carbon and a volatile phase

composed of gases and condensable organic vapors (Mesa-Perez et al., 2005). These organic vapors condensate in two different phases: bio-oil and acid extract (Beenackers and Bridgwater, 1989).

Through pyrolysis, carbon-carbon bonds are broken, forming carbon-oxygen bonds. It is a redox process in which part of the biomass is reduced to carbon (coal) while the other part is oxidized and hydrolyzed yielding phenols, carbohydrate, aldehydes, ketones, and carboxylic acids, which combine to form more complex molecules such as esters and polymers (Rocha et al., 2004).

Due to the extreme conditions to which biomass is submitted, many simultaneous reac­tions occur, resulting in gaseous, liquid, and solid products:

1. Gas phase. Consists primarily of low-weight products that have moderate vapor pressure at room temperature and do not vaporize at pyrolysis temperature.

2. Liquid phase. Further subdivided into two other phases determined by density differences:

• Bio-oil, which is a mixture of many compounds with high molecular weight that became vapors at pyrolysis temperature but condense at room temperature.

• Acid extract (or aqueous extract), which consists of an aqueous phase with numerous soluble and/or suspended substances.

3. Solid phase. Also known as biochar, the solid phase is composed of an extremely porous matrix, very similar to charcoal (DalmasNeto, 2012).

Pyrolysis conditions can be manipulated to produce preferably one phase or the other. Residence time is one of the factors that most influence the final result. To produce incondensable gases, high residence time at high temperature is generally used; higher yields of solids are generally achieved by very high residence time at low temper­atures (allowing polymerization reactions) (Sanchez, 2003). For preferential production of the liquid phase, fast pyrolysis is often chosen. Table 7.1 summarizes the conditions and main effects of residence time and temperature in gaseous, liquid, and solid product generation. Other pyrolysis technologies and their characteristics are presented in Table 7.2.

Due to its tendency to preferentially form bio-oil, coupled with high-speed reaction and greater productivity, fast pyrolysis is the best model for the production of biofuels from algae.