Environmental Engineering Reference
In-Depth Information
Biofuels obtained via HydroThermal Upgrading (HTU)
. HTU-diesel can be pro-
duced from various feedstocks including dry (wood and lignocellulose) [99] and wet
(beet pulp, sludge or bagasse) biomass [100-102]. The methodology involves the
hydrothermal treatment of biomass that is converted into a mixture of hydrocarbons
at relatively low temperatures (250-350
◦
C) and moderate (autogenous pressure)
[99] to high (120-180 bar) pressures [100, 101].
The biocrude obtained is a heavy organic liquid immiscible with water that
contains a wide range of hydrocarbons including acids (e.g. acetic acid), alcohols
(e.g. isopropyl alcohol) and phenolic derivatives (in the particular case of lignocel-
lulosic materials) [99-102]. Often, the hydrocarbon mixture obtained needs further
processing [via catalytic hydro-de-oxygenation (HDO)] to yields a liquid biofuel
similar to fossil diesel that can be blended with fossil diesel in any proportion with-
out the necessity of engine or infrastructure modifications [102]. HTU research has
been mainly performed in The Netherlands, with an HTU demonstration plan in
Amsterdam that is able to generate over 12,000 tonnes of biocrude (including ash)
per year [100, 101].
Biofuels obtained via Fischer-Tropsch Synthesis (FTS)
. The Fischer-Tropsch
(FT) process is one of the advanced biofuels conversion technologies. It has been
known since 1923 when German scientists Franz Fisher and Hans Tropsch aimed
to synthesize long-chain hydrocarbons from a CO and H
2
gas mixture, but it was
mainly used in the past for the production of liquid fuels from coal or natural gas
[103, 104].
Prior to the FTS, the gasification of biomass feedstocks takes place in a similar
way that described for the production of bio-SNG (Fig. 8.9). Then, a cleaning and
conditioning step of the produced syngas is normally performed to remove all the
impurities present prior to the catalytic reaction to minimise the poisoning of the
catalyst [105, 106].
The FTS process is then carried out. It comprises of various steps described by
the set of equations in Fig. 8.11, where x is the average length of the hydrocarbon
chain and y is the number of H
2
atoms per carbon.
The first step involves the reaction of CO with H
2
in the presence of a Co or Fe
catalyst (Fig. 8.10, top reaction) to afford a hydrocarbon chain extension (-CH
2
-)
that is a building block for the formation of longer hydrocarbons. Typical opera-
tion conditions are temperatures between 200 and 400
◦
C and 15-40 bar pressures,
depending on the process [89, 103, 104].
All reactions are exothermic and the product is a sulphur free mixture of different
predominantly linear hydrocarbons (primarily alkanes and alkenes) that frequently
undergoes upgrade and refining steps to be turned into automotive fuels, namely
FT-diesel (main product) and gasoline-like biofuels (by-products) [89]. The FT
x
CO
2
x
H
2
(-CH
2
-)
+
x
H
2
O
+
+
2(
x
+1
)
H
2
C
x
H
2
x
+1
x
CO
+
x
H
2
O
Fig. 8.11
FTS reactions for
the production of linear
long-chain hydrocarbons
x
CO
+
(
x
+
y
/2)H
2
C
x
H
y
+
x
H
2
O
