Environmental Engineering Reference
In-Depth Information
Cu-Zn catalysts require very low H
2
S concentrations (<60 ppbv) and the Co-Mo catalyst tolerates
very high H
2
S concentrations (several thousands of parts per million by volume), perhaps the best
catalyst for WGS reactions for biomass synthesis gas is the Fe-Cr catalyst, which can tolerate mod-
erate H
2
S levels of 50-100 ppmv. At this high temperature, remaining tars might continue in the
vapor rather than deposit on the catalyst surface.
8.3.1.4 co hydrogenation reaction
The final step in production of fuels and chemicals from biomass is the CO hydrogenation reaction.
Conversion to hydrocarbons, alcohols, and other organic compounds is dependent on catalyst
type, CO/H
2
ratio, and other reaction conditions. Fischer-Tropsch synthesis (FTS) converts
CO and H
2
to gaseous and liquid hydrocarbons through a highly exothermic polymerization
mechanism. FTS reaction begins with adsorption and dissociation of CO and H
2
on active sites on
the catalyst surface (NSF 2008). A single C atom and multiple H atoms combine on a single active
site to initiate chain growth of the hydrocarbon. Further chain growth occurs when adjacent
intermediates combine in a C-C bond. Chain termination occurs when an adjacent adsorbed H
combines with the growing chain to form a terminal C-H bond. The likelihood of chain growth
is governed by the Anderson-Schultz-Flory chain growth probability, α, which is dependent
on catalyst type, promoters present in the catalyst, and reaction conditions (Bartholomew and
Farrauto 2006). Products ranging from C
1
to C
60
+ can be achieved, and the actual distribution of
carbon number in the hydrocarbon products is dependent on α, with high values of α > 0.9 being
preferred for liquid hydrocarbon products.
Common catalysts for FTS include those based on Fe and Co. Iron-based FTS catalysts are
favored for converting low H
2
/CO (0.6-1), non-WGS synthesis gas from biomass because these
catalysts also exhibit significant WGS activity. Cobalt-based catalysts are used on higher H
2
/CO
(2.0-2.2) synthesis gas from WGS biomass synthesis gas. Co catalysts achieve much higher activities
compared to Fe (5-10 times higher), and Co catalysts are more selective to higher-molecular-weight
hydrocarbons (NSF 2008).
Gasoline products are favored in the high-temperature (HT) FTS range of 300-350°C, whereas
diesel and jet fuel are prominent in the low-temperature (LT) FTS at 200-250°C. In a large
biorefinery based on FTS, LT and HT reactors would be in operation along with refinery-type
FTS liquid and wax upgrading steps (oligomerization, catalytic reforming, hydrotreating, and
hydrocracking/hydroisomerization) and separations. Furthermore, aromatic content of the FTS jet
fraction must be increased to provide desired properties, such as low freeze point (-47°C).
Because of the exothermic FTS reaction, the reactor design must remove heat effectively to avoid
catalyst thermal degradation and maintain product selectivity. A thorough review of FTS reactor
configurations is available in Bartholomew and Farrouto (2006), but the choices are between tubular
fixed-bed reactors (TFBRs), fluidized bed reactors, and slurry bubble column reactors (SBCRs).
SBCRs are reported to have advantages over TFBRs because of the simplicity, lower cost, higher
volumetric productivity, heat removal efficiency, and more favorable catalyst productivities and
handling. However, recent improvements of TFBR designs achieve performance similar to large
SBCRs, as reported by Hoek and Kersten (2004).
Interest in alcohols as transportation fuels stems from their favorable engine performance and
lower emissions compared with hydrocarbon fuels (Verbeek and Van der Weide 1997; Phillips and
Reader 1998; NSF 2008). A blend of 10% ethanol in petroleum gasoline decreases GHG emissions,
lowers CO and particulate emissions, and increases octane rating (NSF 2008). Dimethyl ether
(DME) produced from methanol is reported to decrease particulate and NO
x
emissions (Sorenson
and Mikkelsen 1995), and DME can easily form blends with petroleum diesel.
Methanol production from synthesis gas is practiced commercially and occurs at high pressure
(50-80 atm) and at 225-250°C in the presence of Cu-Zn catalyst with nearly 100% selectivity,
but with low conversions, and with a requirement of syngas recycling (Bartholomew and Farrauto
2006). Other catalysts are also used (silicoaluminophosphate) at similar temperatures with similarly
