Page 35 - Synthetic Fuels Handbook

FUEL SOURCES 23

Thermal decomposition in hydrogen atmosphere, (hydrocracking or hydropyrolysis) can
increase the yield of distillable products because the attendant hydrogenation diminishes the
tendency for the formation of higher molecular weigh products (tar and coke).
For the solid feedstocks, particle size is known to influence product yield. Larger particles
heat up more slowly, so the average particle temperatures will be lower, and hence volatile
yields may be expected to be less. In addition, the time taken for the thermal products to
diffuse out of the larger particle (i.e., longer residence time in the hot zone) also contributes
to product distribution. However, if the particle size is sufficiently small, the feedstock will be
heated relatively uniformly, and, with rapid diffusion of the products from the hot zone, a different
product slate can be anticipated.

1.4.2 Gasification

Gasification is the conversion of a solid or liquid into a gas at high temperature in a con-
trolled amount of oxygen. In a broad sense it includes evaporation by heating, although the
term is generally reserved for processes involving chemical change. For example, the term
coal gasification refers to the overall process of converting coal to a product gas, including
the initial pyrolysis and subsequent gas thermal upgrading steps. The resulting gas mixture
(synthesis gas, syngas) is a fuel.
Gasification is a very efficient method for extracting energy from many different types
of organic materials, and also has applications in waste disposal. The syngas combusts
cleanly into water vapor and carbon dioxide. Alternatively, syngas may be converted effi-
ciently to methane via the Sabatier process or gasoline/diesel-like synthetic fuels via the
Fischer-Tropsch process (Sec. 1.4.3).
The Sabatier process involves the reaction of hydrogen with carbon dioxide at elevated
temperatures and pressures in the presence of a nickel- or ruthenium-containing catalyst to
produce methane and water:
CO + 4H → CH + 2H O
2 2 4 2
Usually, the nickel or ruthenium is supported on alumina.
Inorganic components of the input material, such as metals and minerals, are trapped
in the char and may or may not be environmentally safe because of the potential for the
inorganic constituents to leach (caused by rain, melting snow, or acid rain) into the sur-
rounding environment.
The advantage of gasification is that using the syngas is more efficient than direct com-
bustion of the original fuel; more of the energy contained in the fuel is extracted. The
syngas may be burned directly in internal combustion engines, used to produce methanol
and hydrogen or converted via the Fischer-Tropsch process (Sec. 1.4.3) into synthetic fuel.
Gasification can also begin with materials that are not otherwise useful fuels, such as bio-
mass or organic waste.
Gasification of coal and petroleum is currently used on a wide scale to generate elec-
tricity. However, almost any type of organic material can be used as the raw material for
gasification, such as biomass, wood, or even waste plastic.
However, gasification relies on chemical processes at elevated temperatures (>700°C),
which distinguishes it from biologic processes such as anaerobic digestion that produce
biogas.
Regardless of the final fuel form, gasification itself and subsequent processing
neither emits nor traps greenhouse gases such as carbon dioxide. Combustion of synthesis
or derived fuels does of course emit carbon dioxide. However, biomass gasification is per-
ceived to play a role in a renewable energy economy because biomass production removes