Bio-oil production can be achieved along two alternative approaches: biomass pyrolysis or biomass thermochemical liquefaction, as explained in this section.

The pyrolysis process is basically an anaerobic heating process carried out at high temper­atures (between 200 °C and 750 °C). Pyrolysis may take place quickly or slowly; the former produces bio-oil (19-58% of the final product) and biochar (Miao et al., 2004; Grierson, Strezov et al., 2009). On the other hand, slow pyrolysis results in gas and biochar, with methane and CO2 accounting for most of the gaseous product.

Bio-oil produced from microalgal spent biomass is more stable than that produced from traditional crops (e. g., wood), although it is not as stable as fossil fuel (Mohan et al., 2006). Such bio-oil is composed mainly of aliphatic and aromatic hydrocarbons, phenols, long-chain fatty acids, and nitrogenous compounds (Du, Li et al., 2011). During pyrolysis 10-25% of biomass is converted into char (i. e., solid porous carbon particles), whereas 10-30% becomes a (noncondensable) gas (Grierson, Strezov et al., 2009; Ross, Biller et al., 2010).

An alternative fuel gas is synthesis gas (syngas), a gas mixture that comprises carbon mon­oxide (CO) and dioxide as well as hydrogen. It can be obtained by gasification of algal bio­mass via a process consisting of reaction of carbonaceous compounds with atmospheric air, steam, or oxygen at high temperature (ranging from 200 ° C to 700 ° C) in a gasifier (Suali and Sarbatly, 2012). As a result, one obtains clean H2 with yields from 5-56%, and CO with yields ranging from 9-52% (Abuadala, Dincer et al., 2010). Methane can be considered a coproduct since it is produced only to low levels, 2-25% (Suali and Sarbatly, 2012). The hydrocarbon products of gasification can be further processed to produce methanol: at 1000 °C, methanol production is 64% (w/w), on a biomass weight basis.

Another method for bio-oil production is thermochemical liquefaction of biomass. This requires heating the biomass at temperatures between 200 °C and 500 °C, under pressures above 20 bar in the presence of a catalyst. This process leads to bio-oil yields of 9-72%, together with a gaseous mixture (containing, for example, H2) ranging from 6-20% (Ross, Biller et al., 2010; Suali and Sarbatly, 2012). The remaining ash ranges in term from 0.2-0.5%. The product of biomass liquefaction is somewhat comparable to crude fuel: most biomass feedstock characterized a ratio of solid to water of 1:10 lead to a bio-oil yields of ca. 37% (Zou, Wu et al., 2010).

The profile of products is mainly affected by the biomass composition and the processing conditions of temperature, pressure, residence time, and catalyst. The bio-oil yield can be 5-25% higher than the lipid content of the original microalgae, depending on the composition in other compounds such as carbohydrates (Biller and Ross, 2011). For instance, Dunaliella tertiolecta is mainly composed of crude protein (63.6%) and fat (20.5%) and produces a bio-oil yield of ca. 37% on an organic basis (Minowa, Yokoyama et al., 1995); on the other hand, Spirulina sp. (a well-known food supplement, owing to its protein content) was reported to produce a bio-oil yield of up to 54% (Matsui, Nishihara et al., 1997). Microcystis viridis, which is composed of 46% carbon, 7.3% hydrogen, and 9.5% nitrogen, was able to lead to up to 33% bio-oil (Yang, Feng et al., 2004).

The aqueous coproduct of biomass liquefaction can be recycled to the microalgal culture; it is indeed rich in nitrogen, phosphorus, and potassium. The growth rate of microalgae cultured in a medium containing 0.1% aqueous coproduct was found to be one-half of that in microalgae cultured with established media, e. g., BG11 (Jena, Vaidyanathan et al., 2011).