Production of Synthetic Fuels and Chemicals from Biomass - Biomass Gasification, Pyrolysis and Torrefaction - page 377

11.2.3 Production of Syngas
Gasification is the preferred route for the production of syngas from coal or
biomass. The current (2013) low price of natural gas is favorable to the pro-
duction of syngas from methane, but the situation may change in future
when the gas's price rises.
Steam reformation reaction that is widely used for bulk production of
hydrogen is a popular method for production of syngas from natural gas.

11.2.3.1 Steam Reforming of Methane
In the steam reforming method, natural gas (CH 4 ) reacts with steam at high
temperatures (700

1100 C) in the presence of a metal-based catalyst (nickel).

Catalyst

CH 4 1 H 2 O ! CO 3H 2 1 206 kJ = mol (11.1)

1

If hydrogen production is the main goal, the carbon monoxide produced
is further subjected to the shift reaction ( Eq. (11.3) ) to produce additional
hydrogen and carbon dioxide.
The ratio of hydrogen and carbon monoxide in the gasification product
gas is a critical parameter in the synthesis of the reactant gases into desired
products such as gasoline, methanol, and methane. The product desired
determines that ratio. For example, gasoline may need the H 2 /CO ratio to be
0.5

2.0 (Probstein and Hicks,
2006, p. 124). In a commercial gasifier, the H 2 /CO ratio of the product gas is
typically less than 1.0, so the shift reaction ( Eq. (11.3) ) is necessary to
increase this ratio by increasing the hydrogen content at the expense of CO.
The shift reaction often takes place in a separate reactor, as the temperature
and other conditions in the main gasifier may not be conducive to it.

1.0, while methanol may need it to be

B

11.2.3.2 Partial Oxidation of Natural Gas
Steam reforming of natural gas is a highly endothermic reaction. An alterna-
tive approach for production of syngas from CH 4 is partial oxidation, which
is slightly exothermic instead of being highly endothermic like the steam
reforming reaction.

CH 4 1

1

2 O 2 5

CO 1 2H 2 22 : 1kJ = mol (11.2)

A comparison between Eq. 11.1 and 11.2 shows that the latter produces
less hydrogen. A lower H 2 /CO ratio (2:1) in the partial oxidation reaction is
favorable for use of the syngas for FTS. The selectivities toward CO or H 2,
however, are influenced by simultaneous occurrence of total combustion of
methane and secondary oxidation reactions of CO and H 2 . Potential catalysts
for this reaction are Ni and Rh. Though Ni shows high conversion and selec-
tivity, it suffers from catalyst deactivation (Smet, 2000).