Lower Plants - Bioenergy Crop Plants - page 654

Methane production from macroalgal biomass, especially using the kelps Macrocystis and
Saccharina ( Laminaria) , the green alga Ulva , and the rhodophytes Hypnea and Gracilaria , has
been studied by several authors (Hansson 1983; Habig et al. 1984; Schramm and Lehnberg 1984;
Østergaard et al. 1993) and in the 1970s and 1980s extensive research was carried out on methane
production, mainly from Macrocystis pyrifera , by the Marine Biomass Program in the United States
(Flowers and Bird 1990; Chynoweth 2002).
The best anaerobic digestion systems for macroalgal biomass appear to be vertical flow reactors,
especially when operated as an upflow solids reactor in which feed is added to the bottom of the reactor
and effluent removed from the top of a nonmixed vessel. This system produces approximately 0.35 m 3
methane/kg volatile solids added at loading rates of 3.2 kg/m 3 per day (Chynoweth et al. 1987). The
efficiency of the process can be improved using a two-stage system. In the first stage, digester marine
algal hydrolysis and acidification occurs, but not conversion of volatile acids to methane. In the second
stage, digester methanogenic bacteria convert the volatile acids to methane (Chynoweth et al. 1987). In
the digestion of Macrocystis biomass, the methane yields are highly correlated with the mannitol and
algin content of the biomass, with mannitol yielding approximately 75% more methane than algin.
Similarly, methane yield in the red alga Gracilaria is closely correlated with the carbohydrate content
or protein and carbohydrate content (Habig et al. 1984). On the other hand, the brown alga Sargassum
is a poor feedstock, apparently because of the low mannitol content and an unidentified ”fiber-like”
component (Flowers and Bird 1990). More recently, trials on methane production using Laminaria
digitata in Europe (Morand et al. 1991) and beach-cast Laminaria and Ulva in Japan (Koike et al.
2005) have been conducted. In the latter test, the maximum methane yield was 22 m 3 /t biomass.
The anaerobic digestion of microalgal biomass to produce methane has also been examined by
several workers since the original study of Golueke et al. (1957). Algae harvested from wastewater
treatment ponds (Chen 1987; Chen and Oswald 1998; Yen and Brune 2007) and unialgal laboratory
cultures of Chlorella , Dunaliella , Tetraselmis , Scenedesmus , and Spirulina (Asinari Di San
Marzano et al. 1982; Samson and Leduy 1982; Sanchez and Travieso 1993; Munoz et al. 2005) have
been used as feed biomass. These studies have reported methane yields of 0.09-0.45 L/g volatile
solids. High temperatures (>40°C) enhance methane conversion. De Schlamphelaire and Verstaete
(2009) have developed a closed-loop system combining an algal growth unit for biomass production,
an aerobic digestion unit to convert the biomass to biogas (methane), and a microbial fuel cell to
polish the effluent from the digester. This system resulted in a power plant with a potential capacity
of 9 kW/ha of solar reactor.
Microalgae generally have a high nitrogen (protein) content and therefore a low carbon-to-
nitrogen ratio (C/N). This affects the performance of the anaerobic digester and can result in a
significant release of ammonia during anaerobic digestion (Golueke et al. 1957; Samson and Leduy
1986); however, methanogenic bacteria can acclimate to high concentrations of ammonium (Koster
and Lettinga 1984). Co-digestion with a high C/N material such as waste paper can result in a
significant increase in methane production (Yen and Brune 2007).
The anaerobic digestion of marine microalgae also requires the use of salt-adapted microorganisms,
which can tolerate the high salinities (Chen et al. 2008). Methane can also be produced from microalgal
biomass by hydrothermal gasification at high temperatures (~350-400°C) and pressure in the presence
of a nickel catalyst to produce a synthetic natural gas (Minowa and Sawayama 1999; Haiduc et al. 2009).
Sialve et al. (2009) have suggested that anaerobic digestion of microalgal biomass is the opti-
mal strategy, on an energy-balance basis, for the energetic recovery from microalgal biomass.
Furthermore, the nutrient-rich effluent of the digestion potentially can be recycled into new algal
growth medium (Phang et al. 2000).
26.3 ethanol and Butanol
The sugars and carbohydrates of algae may be fermented to produce ethanol or possibly butanol, both
of which can be blended with petrol to produce a renewable transport fuel. For example, the brown