Page 281 - Synthetic Fuels Handbook

FUELS FROM CROPS 267

The simplest form of agricultural biomass energy use involves direct combustion of
cellulosic crops or residues, such as hay, straw, or corn fodder, to heat space or produce
steam. Such fuels are useful for heating farm buildings and small commercial buildings
in rural areas and for drying crops. Ideally, energy crops should be produced on land not
needed for food production. This use should not increase the erosion hazard or cause other
environmental damage.
On the other hand, a variety of crops can be grown specifically to provide sources
of energy and, once established, a stand of perennial biomass/energy crop is expected to
remain productive for a period of 6 years or more.
Thus, perennial crops that regenerate annually from buds at the base of the plant offer
the greatest potential for energy-efficient production. These include: (a) short rota-
tion coppice, (b) miscanthus, (c) reed canary grass, (d) cordgrass, (e) switchgrass, (f) reed
plants, (g) Jerusalem artichoke, and (h) sorghum.

9.1.1 Short Rotation Coppice
Short rotation coppice (or SRC) refers to fast-growing deciduous trees which are grown as
energy crops, such as willow and poplar trees. The species of short rotation coppice that are
most suitable, and therefore most popular, for use as energy crops are poplar and willow
(and possibly also birch) because they both require deep, moisture-retentive soils for proper
growth. Willow, in particular, is able to endure periods of water logging and is therefore
better suited to wetter soils.
Short rotation coppice is harvested on a 2- to 5-year cycle, although commonly every
3 years. Willow and poplar are well adapted to cool climates and tolerate temporary wet
conditions better than most of other species.
The harvesting of the wood takes place at the vegetative rest during the winter months.
At this time of the year the wood has a water content of around 50 percent. Harvesting can
be carried out using three main methods: (a) the coppice is cut and bundled, (b) the coppice
is cut and chipped in a single operation, then blown into a trailer, and (c) an intermediate
system where the coppice is cut into billets and blown into a trailer. After harvesting, the
product may be stored for a few weeks in order to reduce its moisture content to a satis-
factory level for use in energy production. Dry short rotation coppice can then be burnt
under controlled conditions to produce other fuels, gas, or liquid, which are then used for
electricity generation.
A plantation could be viable for up to 30 years before replanting becomes necessary,
although the yield decreases after 5 to 6 rotations.

9.1.2 Miscanthus

Miscanthus is a hardy perennial rhizomatous grass that produces a crop of bamboo-like
cane up to 4 m tall. Rhizomatous implies that it spreads naturally by means of underground
storage organs (rhizomes). Miscanthus can grow up to 10 ft tall and theoretically can give
an annual harvest of up to 12 tons per acre. Like other bioenergy crops, the harvested stems
of miscanthus may be used as fuel for production of heat and electric power, or for conver-
sion to other useful products such as ethanol. Miscanthus is high in lignin and lignocel-
lulose fiber and uses the C4 pathway. It can be grown in a cool climate and on many types
of arable land. Miscanthus does not require a big input of fertilizers due to its capability to
recycle large amounts of nutrients.
Miscanthus is also well equipped for high productivity under relatively cool tempera-
tures and may require substantial amounts of water for maximal growth (its growth could