Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 395

catalyst at high pressure but at moderate temperature. The hydrogen for this
reaction can come from biomass gasification.

Catalyst

N 2 1 3H 2 ! 2NH 3 (11.19)

Catalysts play an important role in this reaction. Iron catalysts (FeO,
Fe 2 O 3 ) with added promoters like oxides of aluminum and calcium, potas-
sium, silicon, and magnesium are used (Reed, 2002, p. III-250).
Syngas contains both CO and H 2 . So, for production of ammonia, the
syngas must first be stripped of its CO through the shift reaction
( Eq. (11.2) ). As mentioned earlier, the shift conversion is aided by commer-
cial catalysts, such as iron oxide and chromium oxide, that work in a high-
temperature range (350 475 C); zinc oxide and copper oxide catalysts work

well in a low-temperature range (200 250 C).

In a typical ammonia synthesis process, the syngas is first passed through
the shift reactor, where CO is converted into H 2 and CO 2 following the shift
reaction. Then the gas is passed through a CO 2 scrubber, where a scrubbing
liquid absorbs the CO 2 ; this liquid is passed to a regenerator for regeneration
by stripping the CO 2 from it. The cleaned gas then goes through a methana-
tion reactor to remove any residual CO or CO 2 by converting it into CH 4 .
The pure mixture of hydrogen obtained is mixed with pure nitrogen and is
then compressed to the required high pressure of the ammonia synthesis. The
product, a blend of ammonia and unconverted gas, is condensed, and the
unconverted syngas is recycled to the ammonia converter.

11.4.4 Glycerol Synthesis
Biodiesel, that is produced from fat or oil, generates a large amount (about
10%) of glycerol (HOCH 2 CH[OH]CH 2 OH) as a by-product. Large-scale
commercial production of biodiesel can therefore bring a huge amount of
glycerol into the market. For example, for every kilogram of biodiesel,
0.1 kg of glycerol is produced (i.e., 86% FAME, 9% glycerol, 4% alcohol,
and 1% fertilizer) ( www.biodiesel.org ). If produced in the required purity
( 99%), glycerol may be sold for cosmetic and pharmaceutical production,

but the market for them is not large enough to absorb it all. Therefore, alter-
native commercial uses need to be explored. They include:

.

Catalytic conversion of glycerol into biogas (C 8 C 16 range) (Hoang

et al., 2007).

Liquid-phase or gas-phase reforming to produce hydrogen (Xu et al.,
1996).

A large number of other chemicals may potentially come from glycerol.
Zhou et al. (2008) reviewed several approaches for a range of chemicals
and fuels. Through processes like oxidation, transesterification, esterification,