The term bio-oil is used mainly to refer to liquid fuels. When biomass is processed under high temperature in the absence of oxygen, products are produced in three phases: the vapor phase, the liquid phase, and the solid phase. The liquid phase is a complex mixture called bio-oil. The compositions of bio-oils vary significantly with the types of feedstock and processing conditions.

Thermochemical conversion is a process through which biomass in the absence of oxygen and at high temperature can be converted into various fuels including char, oil, and gas. The resulting bio-oils present an alternative to liquid biofuels with similarities to petroleum oil (Kishimoto et al. 1994). The process can be subdivided into pyrolysis and thermochemical liquefaction (Demirbas 2000).

Bio-oils are liquid or gaseous fuels made from biomass materials, such as agri­cultural crops, municipal wastes, and agricultural and forestry byproducts, via bio­chemical or thermochemical processes.

Bio-oil has a higher energy density than biomass and can be obtained by quick heating of dried biomass in a fluidized bed followed by cooling. The byproduct char and gases can be combusted to heat the reactor. For utilization of biomass in remote locations, it is more economical to convert the biomass into bio-oil and then transport the bio-oil. Bio-oil can be used in vehicle engines — either totally or partially in a blend.

Biomass is dried and then converted into an oily product known as bio-oil by very quick exposure to heated particles in a fluidized bed. The char and gases produced are combusted to supply heat to the reactor, while the product oils are cooled and condensed. The bio-oil is then shipped by truck from these locations to the hydrogen production facility. It is more economical to produce bio-oil at remote locations and then ship the oil, since the energy density of bio-oil is higher than biomass. For this analysis, it was assumed that the bio-oil would be produced at several smaller plants that are closer to the sources of biomass so that lower-cost feedstocks could be obtained.

The feasibility of producing liquid fuel or bio-oil via pyrolysis or thermochemical liquefaction of microalgae has been demonstrated for a range of microalgae (Dote et al. 1994; Sawayama et al. 1999; Peng et al. 2000,2001; Tsukahara and Sawayama 2005; Demirbas 2006).

Five moss samples (Polytrichum commune, Dicranum scoparium, Thuidium tamarascinum, Sphagnum palustre, Drepanocladus revolvens), one alga sample (Cladophora fracta), and one microalga sample (Chlorella protothecoides) were used in the earlier work (Demirbas 2006). The yields of bio-oil from the samples via pyrolysis are presented as a function of temperature (K) in Figure 5.3. The yield of bio-oil from pyrolysis of the samples increased with temperature, as expected. The yields were increased up to 750 K in order to reach the plateau values at 775 K. The maximum yields were 39.1, 34.3, 33.6, 37.0, 35.4, 48.2, and 55.3% of the sample for Polytrichum commune, Dicranum scoparium, Thuidium tamarascinum, Sphag-

num palustre, Drepanocladus revolvens, Cladophorafracta and Chlorella protothe — coides, respectively.