Page 254 - Synthetic Fuels Handbook

240 CHAPTER EIGHT

Biogas contains methane and can be recovered in industrial anaerobic digesters and
mechanical-biological treatment systems. Landfill gas is a less clean form of biogas which
is produced in landfills through naturally occurring anaerobic digestion. Paradoxically if
this gas is allowed to escape into the atmosphere it is a potent greenhouse gas.
When biomass is heated with no oxygen or only about one-third the oxygen needed for
efficient combustion (amount of oxygen and other conditions determine if biomass gasifies
or pyrolyzes), it gasifies to a mixture of carbon monoxide and hydrogen (synthesis gas,
syngas).
Combustion is a function of the mixture of oxygen with the hydrocarbon fuel. Gaseous
fuels mix with oxygen more easily than liquid fuels, which in turn mix more easily than
solid fuels. Syngas therefore inherently burns more efficiently and cleanly than the solid
biomass from which it was made. Biomass gasification can thus improve the efficiency of
large-scale biomass power facilities such as those for forest industry residues and special-
ized facilities such as black liquor recovery boilers of the pulp and paper industry, both
major sources of biomass power. Like natural gas, syngas can also be burned in gas tur-
bines, a more efficient electricity generation technology than steam boilers to which solid
biomass and fossil fuels are limited.
Most electrical generation systems are relatively inefficient, losing half to two-thirds of
the energy as waste heat. If that heat can be used for an industrial process, space heating, or
another purpose, efficiency can be greatly increased. Small modular biopower systems are
more easily used for such “cogeneration” than most large-scale electrical generation.
Just as syngas mixes more readily with oxygen for combustion, it also mixes more read-
ily with chemical catalysts than solid fuels do, greatly enhancing its ability to be converted
to other valuable fuels, chemicals, and materials. The Fischer-Tropsch process converts
syngas to liquid fuels needed for transportation (Chap. 7). The water-gas shift process
converts syngas to more concentrated hydrogen for fuel cells. A variety of other catalytic
processes can turn syngas into a myriad of chemicals or other potential fuels or products.

8.3.2 Environmental Issues
There is a consensus amongst scientists that biomass fuels used in a sustainable manner
result in no net increase in atmospheric carbon dioxide (CO ). Some would even go as far as
2
to declare that sustainable use of biomass will result in a net decrease in atmospheric CO .
2
This is based on the assumption that all the CO given off by the use of biomass fuels was
2
recently taken in from the atmosphere by photosynthesis. Increased substitution of fossil
fuels with biomass-based fuels would therefore help reduce the potential for global warm-
ing, caused by increased atmospheric concentrations of carbon dioxide.
Unfortunately, things may not be as simple as has been assumed above. Currently, bio-
mass is being used all over the world in a very unsustainable manner, and the long-term
effects of biomass energy plantations have not been proven. As well, the natural humus and
dead organic matter in the forest soils is a large reservoir of carbon. Conversion of natural
ecosystems to managed energy plantations could result in a release of carbon from the soil
as a result of the accelerated decay of organic matter.
An ever increasing number of people on this planet are faced with hunger and starva-
tion. It has been argued that the use of land to grow fuel crops will increase this problem.
Hunger in developing countries, however, is more complex than just a lack of agricultural
land. Many countries in the world today, such as the United States, have food surpluses.
Much fertile agricultural land is also used to grow tobacco, flowers, food for domestic
pets, and other “luxury” items, rather than staple foods. Similarly, a significant proportion
of agricultural land is used to grow feed for animals to support the highly wasteful, meat-
centered diet of the industrialized world. By feeding grain to livestock we end up with