Page 332 - Synthetic Fuels Handbook

318 CHAPTER TEN

Portable steel kilns can be made from oil drums, and can be used both in horizontal and
vertical position. They generally have a short lifetime. When used in the horizontal position,
an opening is made in the side, through which the wood is loaded. For the vertical kiln the
top is cut out and used as a lid.
The production of briquettes (briquetting or densification) is used to improve character-
istics of materials for transport and use as energy source. Raw materials include sawdust,
loose crop residues, and charcoal fines. The material is compacted under pressure, and
depending on the material, the pressure, and the speed of densification, additional binders
may be needed to bind the material. The two main briquetting technologies are the piston
press and the screw press. In the piston press the material is punched into a die by a ram
with a high pressure. In the screw press, the material is compacted continuously by a screw.
With the screw press, generally briquettes of higher quality can be produced.
Logs and Wood Chips. Log-fired heating conjures up images of open fires and log stoves
for many people, but there are also sophisticated, controllable log-boilers available which
can provide central heating and hot water. Logs are readily available in many areas where
there are existing markets for domestic fuel wood. Logs should be well seasoned before
burning to ensure the most efficient combustion, and should ideally be stored under cover
in a well ventilated log store for a year or more.
Logs are ideal for providing heat for domestic buildings and are suitable for heating
loads of up to around 50 kW. Log-heating systems do require manual stoking once a day,
which makes logs less convenient than more automated-heating systems such as woodchips
and pellets. Log-fired systems are available with outputs greater than 50 kW to heat larger
building such as village halls; however, these larger systems will require more frequent
fuelling.
Wood chips are made from whole trees, branch wood, or coppice products which have
been mechanically shredded by a chipping machine. For some types of boilers, the wood
needs to have been air-dried before chipping, or the chips dried before burning. Wood chips
are a bulky fuel and sufficient storage and delivery access needs to be considered when
designing a heating system. Transport costs can be high, but if wood chip is sourced within
20 miles it can be a very cost-effective fuel.
The potential for woodchip heating in many countries is high. The use of a timber
resource for local woodchip heating would provide a valuable economic return and stimu-
late the rural economy. Woodchip systems can provide automated, clean, and convenient
heating for larger domestic properties with outputs of 20 to 30 kW, up to large-scale sys-
tems for hospitals, factories, schools, and district-heating schemes with heating loads in
the megawatt range. Woodchips can also be used to fuel combine heat and power plants in
which the heat produced during electricity generation is used to provide hot water, and is
not lost as in conventional power stations.
Pellets. Pellets made of compressed sawdust or wood shavings have been available in
many parts of the world for more than several decades. Because they are compressed, pel-
lets offer a more concentrated form of fuel than wood chips. Consequently they need less
storage space and are easier to handle. The manufacture of wood pellets requires more
energy than woodchips and the capital cost for production plant is high; pellets are therefore
more expensive than chips.
Pellets can be used to fuel a variety of appliances and heating systems. The small-
est are pellet stoves with outputs of up to around 9 kW which are suitable for heating
individual rooms. These stoves are electronically controlled and can deliver regulated
heat output and only need fuelling once every few days. Pellet boilers are available in a
wide range of outputs from small domestic scale to large industrial scale to heat schools
or hospitals.