Environmental Engineering Reference
In-Depth Information
thus producing bio-based syngas [139-141]. Similar technological advances for
traditional fossil-derived syngas to chemicals are directly applicable to the
bio-based equivalent as the fundamental constituents of syngas (CO and H
2
) are
the same. However, purification of the crude syngas following gasification is
required prior to further processing, removing any particulates, ash, sulphur com-
pounds, nitrogen compounds and tars [142, 143]. A key advantage of the biomass
gasification route, and the subsequent syngas platform, is that any carbonaceous
material can be used to produce a similar composition of CO and H
2
, thus allowing
lower-value fractions such as lignin to be converted to platform molecules. Of the
chemicals produced from syngas (Figure 4.14), MeOH is dominant in terms of
quantity and is usually generated by a high-temperature and -pressure exothermic
reaction over various copper catalysts [144]. The yields of MeOH from synthesis
gas depend on the ratio of CO/CO
2
and sulphur content; it is therefore desirable
for gasification methodologies to focus on reducing CO
2
and sulphur impurities
levels if MeOH is the target product [145].
Methanol itself can then be viewed as an important platform molecule when
derived from bio-syngas, with a wide array of derivative chemicals accessible
from it [144]. Olefins and gasoline can be produced from MeOH over zeolite cata-
lysts (the Mobil process) at temperatures in excess of 400°C, via the intermediate
dimethyl ether (DME) [146]. Ether products of MeOH are themselves important
fuels as well as potential bio-based solvents, the two main products being DME
and the fuel oxygenate methyl
tert
-butyl ether (MTBE), the latter formed from the
reaction of MeOH with isobutene (also derivable from syngas) [147]. DME, a
promising diesel fuel alternative [148, 149] and low boiling solvent [150], can be
formed from the reaction of MeOH over an alumina or zeolite catalysts [151] and
can also be synthesised from syngas directly using copper and zinc oxide doped
alumina [152]. Esterification or transesterification (i.e. of fatty acids) are obvious
additional uses of MeOH, and particularly important to the biorefinery when
considering how many of the platform molecules contain carboxylic acid groups.
Other major products from MeOH include formaldehyde (used for production of
urea, resins, plastics, 1,4-butanediol and (di)isocyanates) and acetic acid via
oxidation and carbonylation (using CO from syngas), respectively [144, 153,
154]. Routes for the production of ethanol via the reaction of MeOH with syngas
are also under investigation [155]. However, both ethanol and isobutanol can be
formed directly from syngas, without the need to isolate the intermediate MeOH,
obtained via heterogeneously catalysed routes and are both useful fuel and
platform molecules themselves [156-161]. Along with the various catalytic routes
from syngas to alcohols and hydrocarbons, technology is under development for
the use of syngas in acetogen fermentations, potentially producing ethanol,
butanol and 2,3-butanediol [162]. The classic Fischer-Tropsch process can also
be applied to bio-syngas for the production of bio-based drop-ins for diesel,
gasoline, olefins and waxes [144, 163]. Another important application of syngas
within the biorefinery will be for use in hydroformylation and reduction, both
