Environmental Engineering Reference
In-Depth Information
kelp
Saccharina latissima
(
Laminaria saccharina
) contains approximately 25% mannitol and 30%
laminaran, a linear polysaccharide of (1→3)-β-d-glucopyranose, with the chains terminated by
d-mannitol. The bacterium
Zymobacter palmae
has been shown to be able to ferment the mannitol
to ethanol (Horn et al. 2000a), and the yeast
Pichia angophorae
can use the mannitol and the
laminaran; however, the ethanol yields are still low and the process requires further optimization
(Horn et al. 2000b). Laminaran can also be fermented by the yeast S
accharomyces cerevisiae
when used in combination with the enzyme laminarase (Adams et al. 2008). Recently, a process
for ethanol production from seaweeds has been patented (Kim et al. 2008). The sugars produced
by microalgae can, of course, also be fermented to produce ethanol (Nakas et al. 1983; Ueda et al.
1996). Microalgae such as
Chlorella
have a high starch content (~30-40% of dry weight) and an
up to 65% ethanol conversion efficiency has been reported (Anonymous 1995; Hirano et al. 1997).
Ueno et al. (1998) have also produced ethanol from the marine green alga
Chlorococcum littorale
in a dark fermentation process. Ethanol-producing cyanobacteria have also been developed (Fu and
Dexter 2007; Lee 2008), and the ethanol can be recovered from the airspace above the medium in
which the algae grow (Lee 2008; Woods et al. 2008).
Butanol is produced by anaerobic fermentation using solventogenic clostridia bacteria, such
as
Clostridium acetobutylicum
and
C. beijerinckii
, and other bacteria, such as
Butyribacterium
methylotrophicum
and
Hyperthermus butylicus
(Dürre 2007). The clostridia secrete a wide range
of enzymes that break down polymeric carbohydrates to various monosaccharides that are then
taken up by the cells and metabolized. The current state of production of biobutanol has recently
been reviewed by Ezeji et al. (2007). Butanol has several advantages over ethanol as a transport fuel
in that it relatively less polar than ethanol and more similar to gasoline, making it easier to blend
with gasoline. Algae are clearly a potential source of renewable biomass for biobutanol production;
however, the only published study so far is on the production of butanol from algae (together with
ethanol and 1,3-propanediol) is using glycerol-producing algae, such as
Dunaliella
spp.
(Nakas
et al. 1983).
26.4 PyrolysIs
Pyrolysis is a thermochemical decomposition process in the virtual absence of oxygen. The
pyrolysis of biomass produces char, a crude “bio-oil” and a noncondensable gas, which contains
hydrogen, methane, and higher hydrocarbons. In “fast pyrolysis” the biomass is rapidly heated
(in ~5-10 s) to between 400 and 500°C. In “slow pyrolysis,” the biomass is heated slower to less
than approximately 400°C (Grierson et al. 2009). Fast pyrolysis produces more bio-oil than slow
pyrolysis. The application of pyrolysis to produce liquid fuel from microalgae was first proposed by
Ginzburg (1993) using
Dunaliella
biomass. Pyrolysis of microalgal biomass from the green algae
Chlorella protothecoides
and
Cladophora fracta
and the cyanobacterium
Microcystis aeruginosa
have given oil yields of up to 57.9% of the biomass dry weight (Peng and Wu 2000; Miao and Wu
2004a; 2004b; Demirbas 2006). Slow pyrolysis trials, which gave good bio-oil yields, have also
been carried out with a range of microalgae species (Grierson et al. 2009). However, pyrolysis oils
will require upgrading because they are acidic, unstable, viscous, and contain solids and chemically
dissolved water (Demirbas 2001; Chiaramonti et al. 2007).
Pyrolysis of dried biomass of the coccolithophores,
Emiliania huxleyi
and
Gephyrocapsa
oceanica
, at 300°C produced a high yield of liquid-saturated hydrocarbons, the major components
of which were normal alkanes in a series ranging from
n
C
11
to
n
C
35
(Wu et al. 1999a), and increasing
temperature to 400°C resulted a decrease in the liquid saturates and an increase in hydrocarbon
gases, mainly methane (Wu et al. 1999b). Coccolithophorid algae seem to be particularly well
suited to pyrolysis as they have a high lipid and hydrocarbon content including long-chain (C
37
-C
39
)
alkenones and alkeonates (Volkman et al. 1995; Bell and Pond 1996; Pond and Harris 1996). There
is also good evidence that the yield of hydrocarbon gases is dependent on the lipid content of the
biomass (Wu et al. 1996).
