Environmental Engineering Reference
In-Depth Information
temperature, pressure, and residence time as well as the catalyst that is used in
the process. One of the advantages of syngas production is the wide range of
feedstocks or raw materials that it can accommodate. When syngas is produced
from a biomass precursor it is called bio-syngas in order to distinguish it from
syngas made from fossil raw materials. The raw syngas may be contaminated
with sulfur compounds (e.g. H
2
S), nitrogen compounds (e.g. HCN, NH
3
), hal-
ides (e.g. HCl), and heavy organic compounds that are known collectively as
“tar.” Unpurified gas produced by gasification may also contain benzene, tolu-
ene, and xylenes (mixture of these three aromatic hydrocarbons known as BTX)
[43]. These contaminants may inactivate the catalysts. The raw syngas must
therefore often be subjected to a gas cleaning step prior to conversion into bio-
fuels such as DME or methanol. Tar separation is a very complex and expensive
process involving a series of absorption steps, but it can remove some contami-
nants. There are also several other methods for tar removal such as thermal
cracking, catalytic cracking and scrubbing methods as discussed by Hamelinck
et al
. [43].
Diesel is a mixture of aliphatic hydrocarbons (alkanes) with carbon chains of
different lengths (C
12
-C
22
) that contains no aromatic hydrocarbons. A generalized
illustration of the process by which carbon in biomass is converted into fuels such
as methane and other alkanes by the FT method is shown in the following:
(
)
→
LurgiprocessC HOSteam
: +
Heat
CO H
+
2
2
Syngas
HFe
Heat,Pressure
,
FT synthesis:
CO H
+
→
AlkaneHO
+
2
2
2
or
CO H
,
→
2
HNi
Heat,Pressure
2
+
CH HO
+
2
4
2
The process requires high temperatures (200-350°C) and pressures (10-40 bar).
The carbon source for FT diesel has varied throughout the years; during World
War II, the Germans used coal to produce liquid fuel. During the apartheid era,
South Africa was boycotted by most of the oil-producing countries and had to
produce fuels and chemicals from coal by FT synthesis for several years. South
Africa also produces various compounds such as ethane and propene for the man-
ufacture of polyethylene (PE), polyvinyl chloride (PVC), polypropylene (PP), and
acrylonitrile by FT processes [42]. Nowadays, natural gas is used to produce syn-
thetic gas. Biomass has not yet been used commercially as a feedstock for the
production of synthetic gas; it is an appealing feedstock however because it is
CO
2
-neutral source whereas fossil fuel precursors are not. A range of biomass
materials have been used in the laboratory-scale production of FT diesel but sev-
eral limitations remain to be addressed before these processes will be suitable for
commercial use, including the need for gas cleaning steps and the development of
