Page 324 - Synthetic Fuels Handbook

310 CHAPTER TEN

The hydrocarbon fraction consists of methane to high-boiling tar. The composition of this
fraction can be influenced by many parameters, such as particle size of the biomass, tempera-
ture, pressure, heating rate, residence time, and catalysts. The operative reactions are:
C + CO → 2CO
2
C + H O → H + CO
2
2
2C + 2H O → 2H + CO + CO
2
2
2
C + 2H → CH 4
2
CO + H O → CO + H 2
2
2
CO + 3H → CH + H O
2
2
4
The composition of the producer gases varies widely with the properties of the biomass,
the gasifying agent, and the process conditions. Depending on the nature of the raw solid
feedstock and the process conditions, the char formed from pyrolysis contains 20 to 60 percent
of the energy input. Therefore the gasification of char is an important step for the complete
conversion of the solid biomass into gaseous products and for an efficient utilization of the
energy in the biomass.
The producer gases from the reduction zone rise beyond the reduction zone. When they
come into contact with the cooler biomass, the temperature drops down and the aforemen-
tioned reactions are frozen. The unreacted char further undergoes the oxidation with air in
the lowest zone:
C + O → CO 2
2
As a result, ash is left at the bottom of the reactor. The produced carbon dioxide flows
upward and is involved in the reactions in the reduction zone. The heat released in the oxi-
dation zone drives both the reduction and pyrolysis processes.

10.4.2 Liquid Fuels

As already noted (Sec. 10.4.1), wood can be used to make both liquid and gaseous fuels.
When wood is heated in the absence of air, or with a reduced air supply it is possible to pro-
duce a liquid fuel which can be used in a similar way to conventional oil fuels. It can be used
to run internal combustion engines in vehicles or generators. The gas produced from wood
is a mixture of hydrogen and carbon monoxide (synthesis gas) which is similar to the coal
gas which was made before the arrival of natural gas from the North Sea. This wood gas
can be used in internal combustion engines or in gas turbines which can be used to power
generators. Although the liquid fuels are rarely produced from wood at present, wood gas
is important in other countries for producing electricity in more remote areas.
Processes for making liquid fuels from wood have been understood and available for
longer than such fuels have been used in vehicles for transportation. However, the eco-
nomics of liquid fuels from wood as compared to liquid fuels from fossil fuels have been
unfavorable. Nevertheless, the recent phenomenal increases in the cost of gasoline and
diesel fuel, that seem to bear little relationship to the increases in petroleum process, has
caused a renewed interest in liquid fuels from sources other than petroleum. Wood is a fuel
source of interest!
The three approaches that are most promising for making liquid fuels from wood
are methanol, ethanol, and diesel fuel, but other liquid fuels from wood are possible