Page 253 - Synthetic Fuels Handbook

FUELS FROM BIOMASS 239

components of wood. When the yield of acetic acid originating from the cellulose, hemicel-
luloses, and lignin is taken into account, the total is considerably less than the yield from the
wood itself (Wenzl et al., 1970). Acetic acid comes from the elimination of acetyl groups,
originally linked to the xylose unit.
If wood is completely pyrolyzed, resulting products are about what would be expected
by pyrolyzing the three major components separately. The hemicelluloses would break down
first, at temperatures of 200 to 250°C. Cellulose follows in the temperature range 240 to
350°C, with lignin being the last component to pyrolyze at temperatures of 280 to 500°C.
A wide variety of organic compounds occur in the pyrolysis liquid fractions given in the
literature (Beaumont, 1985). Degradation of xylan yields eight main products: water, metha-
nol, formic, acetic and propionic acids, 1-hydroxy-2-propanone, 1-hydroxy-2-butanone, and
2-furfuraldeyde. The methoxy phenol concentration decreases with increasing temperature,
while phenols and alkylated phenols increases. The formation of both methoxy phenol and
acetic acid was possibly as a result of the Diels-Alder cycloaddition of a conjugated diene and
unsaturated furanone or butyrolactone.
The chemical structure of the xylan as the 4-methyl-3-acetylglucoronoxylan has been
described (Timell, 1967). Furthermore, it has been reporte that the pyrolysis of the pyro-
ligneous acid produces 50% methanol, 18% acetone, 7% esters, 6% aldehydes, 0.5% ethyl
alcohol, 18.5% water, and small amounts of furfural (Demirbas, 2000). The composition of
the water soluble products was not ascertained but it has been reported to be composed of
hydrolysis and oxidation products of glucose such as acetic acid, acetone, simple alcohols,
aldehydes, and sugars (Sasaki et al., 1998).

Biophotolysis. The photosynthetic production of gas (e.g., hydrogen) employs micro-
organisms such as Cyanobacteria, which have been genetically modified to produce pure
hydrogen rather than the metabolically relevant substances (notably NADPH ). The con-
2
version efficiency from sunlight to hydrogen is very small, usually under 0.1 percent, indi-
cating the need for very large collection areas.
The current thinking favors ocean locations of the bioreactors. They have to float on the
surface (due to rapidly decreasing solar radiation as function of depth) and they have to be
closed entities with a transparent surface (e.g., glass), in order that the hydrogen produced
is retained and in order for sunlight to reach the bacteria. Because hydrogen buildup hinders
further production, there further has to be a continuous removal of the hydrogen produced,
by pipelines to, for example, a shore location, where gas treatment and purification can take
place. These requirements make it little likely that equipment cost can be kept so low that
the very low efficiency can be tolerated.
A further problem is that if the bacteria are modified to produce maximum hydrogen,
their own growth and reproduction is quenched. Presumably, there has to be made a com-
promise between the requirements of the organism and the amount of hydrogen produced
for export, so that replacement of organisms (produced at some central biofactory) does
not have to be made at frequent intervals. The implication of this is probably an overall
efficiency lower than 0.05 percent.
In a life cycle assessment of biohydrogen produced by photosynthesis, the impacts from
equipment manufacture are likely substantial. To this, one should add the risks involved in pro-
duction of large amounts of genetically modified organisms. In conventional agriculture, it is
claimed that such negative impacts can be limited, because of slow spreading of genetically mod-
ified organisms to new locations (by wind or by vectors such as insects, birds, or other animals).
In the case of ocean biohydrogen farming, the unavoidable breaking of some of the
glass- or transparent-plastic-covered panels will allow the genetically modified organisms
to spread over the ocean involved and ultimately the entire biosphere. A quantitative discus-
sion of such risks is difficult, but the negative cost prospects of the biohydrogen scheme
probably rule out any practical use anyway.