Page 246 - Synthetic Fuels Handbook

232 CHAPTER EIGHT

agents, from the microbes decomposing and hydrolyzing plant material, over the acidophilic
bacteria dissolving the biomass in aquatic solution, and to the strictly anaerobic methane bac-
teria responsible for the gas formation. Operating a biogas plant for a period of some months
usually makes the bacterial composition stabilize in a way suitable for obtaining high conver-
sion efficiency (typically above 60 percent, the theoretical limit being near 100 percent), and
it is found important not to vary the feedstock compositions abruptly, if optimal operation is
to be maintained. Operating temperatures for the bacterial processes are only slightly above
ambient temperatures, for example, in the mesophilic region around 30°C.
A straightforward (but not necessarily economically optimal) route to hydrogen pro-
duction would be to subject the methane generated to conventional steam reforming. The
ensuing biomass-to-hydrogen conversion efficiency would in practice be about 45 percent.
This scheme could be operated with present technology and thus forms a reference case for
assessing proposed alternative hydrogen production routes.
One method is to select bacteria that produce hydrogen directly. Candidates would
include Clostridium and Rhodobacter species. The best reactor-operating temperatures are
often in the thermophilic interval or slightly above (50–80°C). Typical yields are 2 mol
of hydrogen per mole of glucose, corresponding to 17 percent conversion efficiency. The
theoretical maximum efficiency is around 35 percent, but there are also acetic or butanoic
acids formed, which could be used to produce methane and thus additional energy, although
not necessarily additional hydrogen.
Operation of this type of gas-producing plant would require pure feedstock biomass
(here sugar), because of the specific bacteria needed for hydrogen production, and because
contamination can cause decreased yields. Even the hydrogen produced has this negative
effect and must therefore be removed continually.
Gasification. Gasification occurs through the thermal decomposition of biomass with the
help of an oxidant such as pure oxygen or oxygen-enriched air to yield a combustible gas
such as synthesis gas (syngas) rich in carbon monoxide and hydrogen (Albertazzi et al.,
2007). The synthesis gas is posttreated, by steam-reforming or partial oxidation, to convert
the hydrocarbons produced by gasification into hydrogen and carbon monoxide. The car-
bon monoxide is then put through the shift process to obtain a higher fraction of hydrogen,
by carbon dioxide removal and methanation or by pressure swing adsorption (Mokhatab
et al., 2007; Speight 2007).
In the gasification process, one or more reactants, such as oxygen, steam, or hydrogen,
are introduced into the system. These chemical reactants combine with solid carbon at
the higher gasification temperatures, so increasing the gas yield while consuming char. The
amount of char by-product remaining on gasification is in fact essentially zero with bio-
mass materials, while the small quantity of tars and oils evolved may be recycled to extinc-
tion. The introduced reactants also enter into gas phase reactions which, together with the shift
in equilibrium and the change in relative reaction rates at the higher temperatures, results in a
significantly better-quality gas than that obtained on pyrolysis. Important distinctions between
pyrolysis and gasification are therefore the improved gas yield and the elimination of solid and
liquid by-products.
One major advantage with gasification is the wide range of biomass resources avail-
able, ranging from agricultural crops, and dedicated energy crops to residues and organic
wastes. The feedstock might have a highly various quality, but still the produced gas is quite
standardized and produces a homogeneous product. This makes it possible to choose the
feedstock that is the most available and economic at all times (Prins, 2005).
There are various types of gasifiers which, although already discussed (Chap. 5), deserve
mention here.
The air-blown direct gasifiers operated at atmospheric pressure and used in power
generation—fixed bed updraft and downdraft (Fig. 8.3) and fluidized bed bubbling