Page 280 - Synthetic Fuels Handbook

266 CHAPTER NINE

fuel over some other renewables since it is not an intermittent resource—it can be supplied
on a continuous basis to fuel base load plants.
Regularly coppiced plantations will actually absorb more carbon dioxide than mature
trees—since carbon dioxide absorption slows once a tree has grown. Growing crops for
fuel, particularly wood coppice, offers very promising developments for the future. Short
rotation arable coppicing, using fast growing willows, is currently seen as an important
source of fuel for electricity generation. The overall process involves several stages—growing
over 2 or 3 years, cutting and converting to wood chip, storage and drying, transport to a
power plant for combustion. And the combustion process can be very efficient, given the
development of advanced cogeneration techniques.
Energy crop fuel contains almost no sulfur and has significantly less nitrogen than fossil
fuels; therefore reductions in pollutants causing acid rain (SO ) and smog (NO ) may be
x
2
realized. For example, the use of energy crops will greatly reduce greenhouse gas emis-
sions. Burning fossil fuels removes carbon that is stored underground and transfers it to the
atmosphere. Burning energy crops, on the other hand, releases carbon dioxide but as their
growth requires carbon dioxide there is no net release of carbon into the atmosphere, that is,
it creates a closed carbon cycle. Furthermore, where energy crops are gasified there is a net
reduction of carbon dioxide. In addition, substantial quantities of carbon can be captured in
the soil through energy crop root structures, creating a net carbon sink.
An additional environmental benefit is in water quality, as energy crop fuel contains less
mercury than coal. Also, energy crop farms using environmentally proactive designs will
create water quality filtration zones, uptaking and sequestering pollutants such as phospho-
rus from soils that leach into water bodies.
Also, growing energy crops on agricultural land that might otherwise be converted to
residential or industrial use will reduce erosion/chemical runoff and enhance wildlife habitat.
This will give energy producers and a renewable energy option with uniquely desirable char-
acteristics. For example, energy crops differ from other sources of renewable energy in virtue
of the fact that they can be grown to meet the needs of the market whereas other renewable
resources (e.g., wind and wave power) must be harnessed where and when they occur.
Moreover, the security of energy supply will be significantly enhanced for many crop-
growing countries. Energy crop production and use could reduce dependency on imported
oil, although compete energy independence will remain a possible future event.

9.1 ENERGY CROPS

Markets for biomass crops have been slow to develop. The concept of co-firing biomass in
existing power plants continues to show promise. Electric utilities are interested in co-firing
biomass as a way to meet the renewable energy mandate, should it become law. In addition,
there are air quality benefits from biomass fuels. Since biomass fuels are low in sulfur and
nitrogen, they lower the smokestack emissions of sulfur oxides and nitrogen oxides—SO x
causes acid rain and NO contributes to smog—when burned with coal.
x
There are many opportunities for producing fuel from agricultural crops and crop
residues. Possible end products include ethanol, vegetable oil, and solid cellulosic fuels.
Ethanol can be produced from any grain, root, fruit, or juice crop containing fermentable
carbohydrates. It also can be made from crops, residues, or woods that contain cellulose or
other long-chain carbohydrates which can be hydrolyzed to fermentable sugars.
Vegetable oils are produced from numerous oil seed crops. Some of these oils have been
evaluated in other laboratories as substitutes for diesel fuel. While all vegetable oils have
high-energy content, most require some processing to assure safe use in internal combus-
tion engines.