Environmental Engineering Reference
In-Depth Information
catalysts are mainly based on iron and cobalt [89, 103, 104, 107, 108]. Cobalt cata-
lysts have the advantage of a higher conversion rate and they are also more reactive
in hydrogenation, producing less unsaturated hydrocarbons and alcohols compared
to iron catalysts that produce higher alkenes and oxygenates content [89, 103, 104].
The process using biomass as feedstock is currently under development. In
theory, there are no restrictions in the type of biomass that can be used as feed-
stock. Woody and grassy materials and agricultural and forestry residues have been
investigated in the process [107-109].
Pilot production facilities for Fischer-Tropsch liquids from biomass are currently
in operation in Germany (e.g. Lurgi and Choren) and the Netherlands [88].
8.2.2.3 Biofuels Produced by Biological Conversion
Bioalcohols
Bioethanol
. Second generation bioethanol is usually produced from a range of alter-
native readily abundant and inexpensive cellulosic biomass feedstocks including
woody biomass, grasses, forestry and agricultural waste [110, 111]. Very interesting
reviews about the progress in bioethanol and lignocellulosic processing have been
recently reported [51, 112, 113].
An overview of the production routes of second generation bioethanol is included
in Fig. 8.12.
The process (Fig. 8.12, right hand side) is identical to that described in the
production of first generation bioethanol: decomposition of the material into fer-
mentable sugars (hydrolysis) and transformation of the sugars into bioethanol
(fermentation).
The main changes are the processing technologies and the feedstocks that usu-
ally account for the majority of the plant cost. Cellulosic biomass comprises of
two main components. Cellulose and hemicellulose (complex carbohydrate poly-
mers), accounting roughly for about a 70-75 wt% of the lignocellulose. A mixture
of enzymes (cellulases and hemicellases) different from those of the first genera-
tion bioethanol production are employed in the hydrolysis step. In the particular
case of lignocellulosic (woody) materials, lignin is obtained as by-product of the
process. Lignin can be burned to produce heat and power for the processing plant
and potentially for surrounding homes and businesses and it is to be hoped that
Fig. 8.12
Thermochemical
and biological routes to
second generation bioethanol
Source: Sustainable biofuels:
prospects and challenges, RS
Policy document 01/08, ISBN
978 0 85403 662 2.
Reproduced with permission
of the Royal Society.
