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energy is 80% derived from fossil fuels, and with current consumption, fossil fuel
reserves are projected to be depleted within the next few decades [2, 3]. Action
plans have been taken to transition into alternative renewable and sustainable energy
sources. Among these sources, biomass such as dedicated energy crops, agricultural
wastes, forest products, aquatic plants, and municipal wastes present a viable feedstock because these materials can sequester CO 2 through photosynthesis and they
can be transformed into liquid or gaseous fuels. Biomass-derived fuels are not foodbased, and they do not interfere with the food supply chain unlike corn ethanol or
soybean biodiesel. Some areas of the world lack fossil reserves, but they have abundant supply of biomass which can be capitalized to achieve energy security [4].
Nonfood biomass is also referred as lignocellulosic feedstock. The fuel (“biofuel”)
derived from biomass has a net energy ratio (i.e., lower heating value embodied in
the biomass versus fossil energy requirement in producing the fuel) and has lower
greenhouse gas emissions relative to petroleum [5].
Algae are especially appealing because they have high biomass yields and high
photosynthetic efficiency [6–8]. They grow approximately ten times faster than terrestrial plants, have low footprint, and require marginal land and water for cultivation. These characteristics make algae-based biofuel suitable to address
environmental, land-use, and food issues associated with food crop biomass [9, 10].
Microalgae are unicellular algae cultivated in fresh, brackish, or marine water
requiring less arable land [11] and are measured in micrometers. In contrast, macroalgae are multicellular organisms capable of growing tens of meters in fresh,
brackish, or marine water. This chapter focuses on microalgae as biomass feedstock.
Conversion methods for microalgae into biofuels are classified into two categories: (1) biochemical (e.g., alcohol fermentation, anaerobic digestion, hydrogen
production) and (2) thermochemical (combustion, carbonization, liquefaction, gasification, hydrothermal, pyrolysis) [2, 12]. Figure 1 shows all conversion technologies employed in microalgae. Out of those methods, hydrothermal liquefaction
(HTL) is the most promising because it utilizes microalgae cells (presence or
absence of lipids) without dewatering or drying (as wet biomass), therefore significantly lowering fossil energy costs [14–18]. Unlike HTL, gasification (dry) and
pyrolysis demand the reduction of microalgae’s water content [19]. In the case of
direct combustion, water level must be lower than 10 wt% to be effective [6, 7, 20].
HTL entails the reaction of biomass in water at high temperature (250–350 °C) and
pressure (5–15 MPa) in the presence of solvent (water or alcohol) yielding biocrude,
a dark viscous liquid immiscible in water [8]. The process can occur with or without
a catalyst. Biocrude is very viscous, rich in oxygen and nitrogen, and easily deteriorates when stored for prolonged periods of time. The high operating pressure maintains water at liquid or supercritical state. The biocrude has higher heating value and
lower oxygen content than pyrolysis bio-oil, the product of other microalgae conversion methods. Despite these desirable characteristics, biocrude requires further
upgrading to be adapted as drop-in transportation fuels (i.e., direct fuel substitute to
petrol) due to its low hydrogen-to-carbon (H/C) ratio, high heteroatom content, and
substantial high-boiler fractions (i.e., components with relatively high boiling
points). The most common method of upgrade is through hydrotreating in which the
E. P. Resurreccion and S. Kumar
energy is 80% derived from fossil fuels, and with current consumption, fossil fuel
reserves are projected to be depleted within the next few decades [2, 3]. Action
plans have been taken to transition into alternative renewable and sustainable energy
sources. Among these sources, biomass such as dedicated energy crops, agricultural
wastes, forest products, aquatic plants, and municipal wastes present a viable feedstock because these materials can sequester CO 2 through photosynthesis and they
can be transformed into liquid or gaseous fuels. Biomass-derived fuels are not foodbased, and they do not interfere with the food supply chain unlike corn ethanol or
soybean biodiesel. Some areas of the world lack fossil reserves, but they have abundant supply of biomass which can be capitalized to achieve energy security [4].
Nonfood biomass is also referred as lignocellulosic feedstock. The fuel (“biofuel”)
derived from biomass has a net energy ratio (i.e., lower heating value embodied in
the biomass versus fossil energy requirement in producing the fuel) and has lower
greenhouse gas emissions relative to petroleum [5].
Algae are especially appealing because they have high biomass yields and high
photosynthetic efficiency [6–8]. They grow approximately ten times faster than terrestrial plants, have low footprint, and require marginal land and water for cultivation. These characteristics make algae-based biofuel suitable to address
environmental, land-use, and food issues associated with food crop biomass [9, 10].
Microalgae are unicellular algae cultivated in fresh, brackish, or marine water
requiring less arable land [11] and are measured in micrometers. In contrast, macroalgae are multicellular organisms capable of growing tens of meters in fresh,
brackish, or marine water. This chapter focuses on microalgae as biomass feedstock.
Conversion methods for microalgae into biofuels are classified into two categories: (1) biochemical (e.g., alcohol fermentation, anaerobic digestion, hydrogen
production) and (2) thermochemical (combustion, carbonization, liquefaction, gasification, hydrothermal, pyrolysis) [2, 12]. Figure 1 shows all conversion technologies employed in microalgae. Out of those methods, hydrothermal liquefaction
(HTL) is the most promising because it utilizes microalgae cells (presence or
absence of lipids) without dewatering or drying (as wet biomass), therefore significantly lowering fossil energy costs [14–18]. Unlike HTL, gasification (dry) and
pyrolysis demand the reduction of microalgae’s water content [19]. In the case of
direct combustion, water level must be lower than 10 wt% to be effective [6, 7, 20].
HTL entails the reaction of biomass in water at high temperature (250–350 °C) and
pressure (5–15 MPa) in the presence of solvent (water or alcohol) yielding biocrude,
a dark viscous liquid immiscible in water [8]. The process can occur with or without
a catalyst. Biocrude is very viscous, rich in oxygen and nitrogen, and easily deteriorates when stored for prolonged periods of time. The high operating pressure maintains water at liquid or supercritical state. The biocrude has higher heating value and
lower oxygen content than pyrolysis bio-oil, the product of other microalgae conversion methods. Despite these desirable characteristics, biocrude requires further
upgrading to be adapted as drop-in transportation fuels (i.e., direct fuel substitute to
petrol) due to its low hydrogen-to-carbon (H/C) ratio, high heteroatom content, and
substantial high-boiler fractions (i.e., components with relatively high boiling
points). The most common method of upgrade is through hydrotreating in which the
E. P. Resurreccion and S. Kumar