Partnerships, Future, and Emerging Technologies - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 407

Thermochemical
A schematic of a representative thermochemical conversion process for cellulosic
ethanol is shown in Fig. 19.3 . This conversion route is based upon utilization of
catalytic fuel synthesis for ethanol production. An alternative to catalytic fuel
synthesis would be fermentation of the syngas to ethanol [ 32 ]. The catalytic fuel
synthesis option will be covered here primarily because better public data exists for
this option.
Similarly to the biochemical conversion process, the thermochemical ethanol
conversion process can essentially be categorized into main components: the
gasification of the biomass to syngas (H 2 and CO) and then the catalytic conversion
of this syngas to ethanol. Biomass gasification is an early-stage commercial tech-
nology that has been deployed at a few locations [ 33 ]. Biomass gasification
technologies cover a fairly broad gamut of simple to sophisticated approaches
with, as would be intuitively expected, the sophisticated approaches having higher
costs, both from an operating and capital perspective. The simplest biomass gasi-
fication technologies, updraft or downdraft air-blown approaches, produce a syngas
highly diluted with N 2 that is generally not well suited for catalytic conversion to
fuels. Oxygen blown or direct gasification and indirect gasification do not introduce
air into the gasification process and hence do not have the N 2 dilution issue; thus,
they are the two best gasification technology choices for producing a syngas
suitable for catalytic fuel synthesis. Phillips, Dutta, and coauthors did a series of
studies [ 34 - 36 ] where they looked at dry ash and slagging direct biomass gasifica-
tion approaches compared to indirect biomass gasification for mixed alcohol fuel
synthesis and concluded that for the scales of biomass (2,000 tonnes/day), indirect
gasification was the preferred route on both a cost and an efficiency basis.
A drawback of indirect biomass gasification is the amount of tars and light
hydrocarbons produced during the gasification process [ 37 ]. Light hydrocarbons
and tars are problematic since they represent a carbon and hence an efficiency loss,
Fig. 19.3 Thermochemical conversion process schematic