Mineral Facts and Problems: 1960 Edition Page: 451
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LIGNITE AND PEAT
the revenue from tar must absorb the cost of
the carbonization process.
The concept of using lignite char as a source
for storable high-B.t.u. briquets led, in 1928, to
the construction of commercial plants at Dick-
inson, N. Dak., and in Saskatchewan, Canada.
These two plants used the Lurgi-Spiligas in-
ternally heated carbonization process, developed
in Germany in the early 1920 s, for the carbon-
ization of noncoking coals to produce char and
tar.
In the North Dakota plant, as-mined lignite
is crushed to a size of 4 inches or less and passed
over a vibrating screen. Undersize material,
consisting of approximately minus l/4-inch
particles, goes to the power plant, while the
oversize is discharged to the carbonizer. Mois-
ture is removed from the lignite in the upper
section of the retort, with the dried material
descending into the lower section, where carbon-
ization occurs. The char residue settles at the
bottom of the carbonizer, where it is cooled by
recycled carbonization gas and discharged. A
gas mixture consisting of carbonization gas, tar
vapors, and inert products of combustion is
drawn off at the top of the carbonization
chamber. The gas leaving the carbonization
section is washed to remove dust and tars, and
the clean gas is returned to the carbonizer to
cool and char and supply the heat required
for drying and carbonization. The Lurgi car-
bonizers installed at the Dickinson plant had a
daily capacity of approximately 100 to 150 tons
of lignite as mined. The tar and light oils,
after separation from water condensate, are
distilled into a liquid fraction and a pitch resi-
due. The distillate oil was sold for use as a
wood preservative, and the pitch, supplemented
by petroleum asphalt, served as part of the
binder for manufacturing briquets from the
char.
Work on low-temperature carbonization of
lignite, using new techniques, was reactivated in
the cooperative effort by the Texas Power &
Light Co. and the Bureau of Mines. In the
process developed, carbonization of fine-par-
ticle coal in the fluidized state is used to obtain
high heat transfer rates, high capacities, and
low operating costs. Feed size of the coal is
minus 1/4 inch; residence time of the particles
in the carbonizer is 10 to 15 minutes. The
process essentially consists of two steps. Lig-
nite, as mined, is crushed to minus 1/4 inch and
dried in the entrained state in a stream of hot
gases. The dried coal is then transferred to the
fluidized carbonization retort, where process
heat is furnished, in part by combustion within
the retort through limited addition of air and
in part by external heating. The mixture of
char, gases, and tar vapors leaving the retortat the top is separated in conventional dust-re-
moval equipment and an especially designed
hot-electrostatic precipitator. Nonagglomerat-
ing coals can be carbonized readily in the en-
trained and fluidized state by this technique.
Char obtained from normal carbonization of
U.S. lignites is of a loose, noncoherent nature,
not suitable physically for blast-furnace use.
Efforts have been made to improve the coking
characteristics of lignite and noncoking coals
in studies of the influence of a number of vari-
ables on carbonization and the products of
carbonization.
The temperature of carbonization influences
the characteristics of tar and light oil produced
from coal. The chief products from low-tem-
perature carbonization of lignite are char, tar,
and light oil and gas. Char is the most im-
portant product on a weight basis, representing
approximately half of the as-mined lignite.
The char contains about 80 percent of the origi-
nal heating value of the lignite. The most logi-
cal large-scale outlet for this char is utilization
as boiler fuel for large power plants or indus-
tries that consume large quantities of processed
steam. To make utiliaztion of char as a boiler
fuel attractive, the cost per million B.t.u. at the
point of consumption must be equal to or less
than that of competitive industrial fuels.
Where the power plant is close to the mine,
char must compete with as-mined lignite. Since
the char contains only 90 percent of the heating
value of the lignite, more lignite is initially
required to make available equal amounts of
B.t.u. than would be necessary when raw lignite
is fired directly. Tar credit must pay for the
cost of carbonization and for the additional raw
material requirement. Therefore, the develop-
ment of the low-temperature carbonization in-
dustry has not advanced beyond the pilot plant
stage, with large-scale exploitation hinging on
the realization of a market for the low-tempera-
ture tar products.
Char from brown coal, which is somewhat
similar to that from lignite, has been used ex-
tensively in Germany to produce synthesis gas
and hydrogen. The utilization of lignite or its
char in such applications is more a question of
cost than of technology. Lignite tar is a suit-
able source material for manufacturing aro-
matic hydrocarbons and liquid fuels by the
hydrogenation process. Hydrogenation of tar
is simpler and requires less hydrogen than di-
rect hydrogenation of coal. But here again, cost
considerations make such piocessing economical
only for specialty products. Tar acids recov-
ered from processing low-temperature tar are
currently being tested for suitability as wood
preservatives. Tar products also have poten-
tial applications as raw materials for low-cost451
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United States. Bureau of Mines. Mineral Facts and Problems: 1960 Edition, report, 1960; Washington D.C.. (https://digital.library.unt.edu/ark:/67531/metadc38790/m1/459/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.