Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 386

400 C range). In the absence of a suitable catalyst,
the actual yield is very low, so catalysts based on Zn, Cu, Al, and Cr are
used.
Syngas, which is the feedstock for methanol production, can be produced
from biomass through either thermal or hydrothermal gasification. One of
the most commonly used commercial methods use natural gas (CH 4 ) as the
feedstock. This process uses the steam reforming of methane as shown in the
equation below:

temperature (in the 240

CH 4 1 H 2 O - CO 3H 2 1 206 kJ = mol (11.5)

1

We note from this that for every mole of CO produced, 3 mol of H 2 are
produced, but the methanol synthesis reaction ( Eq. (11.4) ) requires only
2 mol of hydrogen for every mole of carbon monoxide. Thus, there is an
extra hydrogen molecule for every mole of methanol. In such a situation,
carbon dioxide, if available, may be used in the following reaction to pro-
duce an additional methanol molecule utilizing the excess hydrogen:

CO 2 1 3H 2 - CH 3 OH 1 H 2 O 2 50 kJ = mol (11.6)

Methanol synthesis reaction (Reed, 2002, p. III-225) can take place at
both high pressure (

30 MPa, 300 400 C) and low pressure (5 10 MPa,

B

350 C).
In the high-pressure process, the syngas is first compressed. The pressur-
ized syngas is then fed into either a fixed- or a fluidized-bed reactor for syn-
thesis in the presence of a catalyst at 300

220

400 C.
A fluidized bed has the advantage of continuous catalyst regeneration and
efficient removal of the generated heat. The catalyst used is an oxide of Zn
and Cr.
The product is next cooled to condense the methanol. Since the conver-
sion is generally small, the unconverted syngas is recycled to the reactor to
be further converted. Today, the most widely used catalyst is a mixture of
copper, zinc oxide, and alumina.
The low-pressure process is similar to the high-pressure process, except
that it uses low pressure and low temperature. In one of the several varia-
tions, a fixed bed of Cu/Zn/Al catalyst

350 atm and at 300

is used at 5 10 MPa and at

290 C (Reed, 2002, p. II-225).
Liquid-phase synthesis is another option, but it is in the development
stage. This option could potentially give a much higher level of conversion
(

220

90%) compared to 20% for the high-pressure process (Chu et al., 2002).
Here, the syngas is fed into the slurry of the catalysts in an appropriate sol-
vent. The compressed syngas is mixed with recycled gas and then heated in
a heat exchanger to the desired reactor inlet temperature, which is usually
about 220

B

230 C. In a cold-quench operation, only about two-thirds of the
feed gas is preheated; the rest is used to cool the product gas between the
individual catalyst layers.