Page 334 - Synthetic Fuels Handbook

320 CHAPTER TEN

permits. The question of the temperature of the carbonization is important; charcoal
produced made at 300°C is brown, soft and friable, and readily inflames at 380°C while
charcoal, made at higher temperatures it is hard and brittle, and does not fire until heated
to about 700°C.
Commercial charcoal is found in either lump, briquette, or extruded forms: (a) lump
charcoal, which is made directly from hardwood material and usually produces far less
ash than briquettes, (b) briquettes, which are produced by compressing charcoal, typically
made from sawdust and other wood by-products, with a binder (usually starch) and other
additives, (c) extruded charcoal, which is made by extruding either raw ground wood or
carbonized wood into logs without the need for a binder since the heat and pressure of the
extruding process hold the charcoal together.
Animal charcoal or bone black is the carbonaceous residue obtained by the dry distil-
lation of bones; it contains only about 10 percent carbon, the remainder being calcium
and magnesium phosphates (80 percent) and other inorganic material originally present
in the bones. It is generally manufactured from the residues obtained in the glue and
gelatin industries.
Charcoal fines have a much lower purity than lump charcoal and, in addition to char-
coal, contain fragments, mineral sand, and clay picked up from the earth and the surface of
the wood and its bark. The fine powdered charcoal (produced from bark, twigs, and leaves)
has a higher ash content than normal wood charcoal. Most of this undesired high-ash mate-
rial can be separated by screening the fines and rejecting undersize material passing, say,
a 2- to 4-mm screen.
Fines cannot be burned by the usual simple charcoal burning methods but if fines could
be fully used, overall charcoal production would rise by 10 to 20 percent. Briquetting—
turning fines into lumps of charcoal—seems an obvious answer.
Briquetting requires a binder to be mixed with the charcoal fines, a press to form the
mixture into a cake or briquette which is then passed through a drying oven to cure or set it
by drying out the water so that the briquette is strong enough to be used in the same burning
apparatus as normal lump charcoal.
Charcoal needs addition of a sticking or agglomerating material to enable a briquette to
be formed. The binder should preferably be combustible, though a noncombustible binder
effective at low concentrations can be suitable. Starch is preferred as a binder though it is
usually expensive. Tar and pitch from coal distillation or from charcoal retorts have been
used for special purpose briquettes but they must be carbonized again before use to form a
properly bonded briquette.
The binders which have been tried are many but, as stated, the most common effective
binder is starch. About 4 to 8 percent of starch made into paste with hot water is adequate.
First, the fines are dried and screened. Undersized fines are rejected and oversized hammer-
milled. This powder is blended with the starch paste and fed to the briquetting press and the
resulting briquettes are dried in a continuous oven at about 80°C. The starch sets through
loss of water, binding the charcoal into a briquette which can be handled and burned like
ordinary lump charcoal in domestic stoves and grates. Briquettes bonded with tar or pitch
and subsequently carbonized in charcoal furnaces to produce a metallurgic charcoal bri-
quette of adequate crushing strength are needed.
Charcoal fines when available in large quantities do have industrial uses such as in
metallurgic and calcining operations. For example, in charcoal iron making fine charcoal
can be injected at the base of the blast furnace with the air blast. Fine charcoal is excellent
for producing sinter, partially reduced iron ore, to provide a high grade feed to the blast
furnace. This is one of the best ways to use charcoal fines as the amount which can be used
is not limited to a percentage of the total as is the case of injection into the base of the blast
furnace. Pulverized fine and lump charcoal can be burned in rotary furnaces producing
cement clinker and calcium bauxite.