Environmental Engineering Reference
In-Depth Information
high selectivity (Williams et al. 1995; Shonnard et al. 2006). Presence of a small amount of CO
2
(4%
vol.) helps productivity because the reaction mechanism is thought to involve hydrogenation of CO
2
.
Cu-Zn catalysts are very susceptible to poisoning by sulfur and arsine in syngas, thus requiring
gas clean-up. Methanol synthesis is carried out in various reactor configurations, as discussed in
Bartholomew and Farrouto (2006).
Higher alcohols are also produced by catalytic conversion of syngas; for example, producing the
octane enhancer methyl tertbutylether (MTBE) from methanol and isobutanol. One alcohol of interest
is ethanol, the direct synthesis of which from synthesis gas can be accomplished using a rhodium-
or copper-based catalyst. Hu et al. (2007) report ethanol selectivity of more than 50% using an Rh
catalyst at high pressure but with low conversion. Side reaction byproducts include methane, C
2
-C
5
alkanes and alkenes, and low-molecular-weight oxygenated organics. A few pilot-scale processes for
higher alcohol synthesis are in place, but no commercial facilities currently exist (NSF 2008).
Although catalytic CO hydrogenation reactions have been investigated in the past, and there is
commercial production of select products, there is a need for further catalyst and reactor innovation.
A comprehensive list of research recommendations leading to improved commercial production
from catalytic CO hydrogenation is included in a recent benchmark study (NSF 2008).
8.3.2 p
yrolySiS
-B
aSEd
c
onvErSion
of
p
lant
B
iomaSS
Pyrolysis is a thermal depolymerization and molecular fragmentation process carried out in
the absence of oxygen (or air) and at moderate temperature (~450-700°C) (Mohan et al. 2006).
These thermal reactions occur in stages, each corresponding to higher reaction severity, in which
increasing severity refers to longer reactor residence times and higher reaction temperatures.
Primary reactions at low severity yield gases (CO
2
, CO, H
2
O), organic vapors, and liquid products.
Secondary reactions act on primary products decomposing larger molecules into low-molecular-
weight gaseous and liquid species as well as char (a carbonaceous solid). Tertiary reactions further
the degradation process to produce synthesis gas (CO
2
, CO, H
2
O, H
2
) and soot (NSF 2008). Three
primary co-products are present at the reactor exit: synthesis gas (CO, CO
2
, H
2
O), bio-oil, and char.
The proportion of each of these is dependent on reaction severity.
A process flow diagram showing pyrolysis-based conversion of woody biomass into liquid
transportation fuel is shown in Figure 8.5. In a sand fluidized bed pyrolysis reactor (a common
configuration), char and gaseous co-product generated in the pyrolysis reaction are combusted in
an integrated recycle vessel to maintain the sand at the required pyrolysis reactor temperature.
Liquid bio-oil from the pyrolysis unit is then subjected to hydrotreating and hydrocracking
catalytic reactions. Gaseous products from the hydrotreater can be converted to hydrogen and CO
2
using a steam reformer and gas shift reactor. Heavy oil product from the hydrotreater will feed a
hydrocracker reactor to generate additional light oil components. The light oil products are then
separated into different product blends such as gasoline, diesel, and, potentially, aviation fuel.
The following sections describe the processing steps in more detail and provide information on
the properties and stability of pyrolysis bio-oil.
8.3.2.1 Pyrolysis reactions
Pyrolysis reactions on biomass feedstocks have been carried out under various temperature regimes
and in the presence of different solvents (see Mohan et al. 2006, Table 8.5). Carbonization occurs
when biomass is heated slowly over several days at approximately 400°C, yielding charcoal as the
main product, but also significant amounts of gas (CO
2
, H
2
O, CO). Conventional pyrolysis features
biomass residence times of between 5 and 30 min, a slow heating rate, and 600°C temperature in
which the products include synthesis gas, bio-oil, and char. Fast pyrolysis produces mostly bio-oil
by using high heating rates, short residence times of 0.5-5 s, and temperatures of 425-650°C and
it produces mostly bio-oil, but small amounts of synthesis gas and char. Ultrapyrolysis produces
mostly gaseous products and chemicals through very high heating rates, high temperatures of
