Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to the beneficiation of low-rank coals by a
simultar)eous thermal treatb~g and surFace modiFying process to improve their
calorific value and to prevent autogenous heating. More particularly, it
relates to a process wherein low-rarlk coals are immersed in a heated residuum
of crude oil having a so-ftening point above 80C, which residuurn Forrns a hardcoating on the particles at ambient temperature.
The most abundant coal resource in western North America are the
low rank coals, including sub-bituminous and lignite. Many deposits of these
coals are relatively inexpensive to mine compared to higher-rank coals in
eastern North America, but their economic value is reduced because they
contain large amounts oF oxygen in combined form and moisture. These
compounds are non-combustible and merely add to the weight of the coal,
increasing transportation costs significantly. A more dangerous problem with
low~rank coals is that they can undergo autogenous heating9 especially when
wetted by rain or by being placed on wet ground; this sometimes leads to
spontaneous combustion. Furthermore, the poor characteristics of low rank
coals results in a derating of sorne boilers that have been designed specifically
~or better-burning coals.
The moisture content of low-rank coals can be decreased by drying
the coal in a hot inert gas. However, the product can still reabsorb moisture
under wet conditions, and be heated up to a temperature at which spontaneous
combustion occurs by the combined heats o-F absorption o-f the moisture and
oxidation of material in the coal. In U.S. Patent 4,192,650, Seit~er disclosed
the prevention of such autogenous heating by rehydrating the dried coal with
steam at 100C to 115C to yield a moisture content of 2% to 1û%. I<indig et
al, in U.S.P. 3,961,914, disclosed coating dried coal particles with silicon
dioxide by introducing silicon tetrachloride gas and reacting it with water to
produce a silicon dioxide Film on the sur-face of the coal. Johnson et al, in
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U.S.P. 3,985,516, disclosed the coating of sub-bituminous and lignite coal
particles with crude oil residuurn after drying. The residuum could
advantageously be diluted with a lighter oil to improve the uniformity of the
coating. Kromrey disclosed in U.S.P. 4,214,875 a coating composition to be
5 applied to a pile of coal exposed to the weather in order to exclude rain and
air by forming a continuous covering over the entire pile~ The composition
was normally thixothropic and included wax, tar or pitch or a polyrner which
provided a covering from one-quarter inch to one inch thick. It was necessary
to break the covering in order to transfer or utilize the coal. Berkowitz, in
10 Canadian Patent 959,783, described a rnethod oF treating low-rank coals which
included heating the coal to a temperature above the decomposition
temperature by immersion in a liquid medium. At and above the
decornposition temperature, pyrolytic material began to diffuse out from the
interior to the surFace of the coal particles and plugged the fine pores of low
15 rank coals to exclude moisture reabsorption.
The aforementioned problems have been overcome by the present
invention, which provides a method oF improving the calorific value of low-
rank coal comprising immersing said coal in a distillation residuum of
petroleum crude oil3 said residuum being at a temperature between
20 substantially 240C and the decomposition temperature of the coal, said
decomposition temperature being defined as the temperature for a particular
coal at which its rate of weight loss upon heating first reaches a maximum
value, said residuum having a softening point o-F at least substantially 80C.
The present invention also provides a method of improving the
25 caloriFic value of low-rank coals further comprising, after the immersion step,
allowing excess residuum to drain from said coal to yield a beneficiated coal
product containing a coating of said residuum comprising from 2% to 15% by
weight of said coal product.
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The present invention also consists in a beneficiated particulate
coal product of improved calorific value comprising low-rank coal and a
residuum coating having a softening point of at least substantially 80C, said
residuum coating being applied by immersing said coal in said residuum heatecl
5 to a temperature between substantially 2~0C and the decomposition
temperature o-f the coal, said decomposition temperature being definecl as the
temp~rature -For a particular coal at which its rate oF weight loss upon heating
first reaches a maxirnum value.
Low rank, high moisture content coals can be beneficiated by the
10 process o-F the invention, -for example sub-bituminous and lignitic coals. As
mined, these coals have moisture contents of up to 25% or more. The process
can also be used with low-rank coals that have undergone drying, for example
air drying to e~uilibriurn, or even with fully dried coals.
The residuum can be any atmospheric or vacuum residuum, for
15 example tar, pitch or asphalt, rnade from conventional or heavy crude oils or
from tar sands bitumen. Also suitable are the vacuum residua of upgraded
heavy crudes and bitumens, the upgrading being done, for example, by
hydrocracking with hydrogen or hydrogen donor material. Asphalt or oxidized
asphalt can also advantageously be used as the immersion medium, as can
20 mixtures of any of the aforementioned residua. Suitable immersion media
must, however, have a softening point of at least 80C, and preferably at least
85C, because the residuum is not washed off the coal particles in the process
of the invention, and thus a coating of residuum remains on the surface of the
particles. During transportation and outdoor storage, coal can under brlght
25 sunlight reach temperatures in the range of 70C to 80C; the coating
materials of the invention remain non-tacky at those temperatures and impede
agglomeration of the coated coal particles. Because tackiness is inversely
correlated with softening point, and tackiness measurements are not common
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in the coal industry, soFtening point is usecl in this specification as a measure
of suitability of a residuum For coating coal particles.
As the coal particles are heated in the immersion step, water in
the coal is evaporated to steam and driven o-f f the particles. Thus no separate
drying step is necessary to rel-nove the moisture from the coal particles prior
to applying the coating material. The minimum temperature o-f the residuurn
at the time oF immersion is about 24[ oC; lower temperatures render the
residuum too viscous to apply the required thin coating. The maximum
temperature of the residuum at the time of immersion of the coal particles is
10 the decomposition temperature oF the coal. The decornposition temperature is
defined as the te-nperature at which the rate of weight loss o-F the coal -first
reaches a maximum value when the coal is heated in an inert atmosphere. For
many Western sub-bituminous coals and lignites, the decomposition
temperature is in the range of 340C to 350C. A preferred temperature of
15 heating is about 290C to about the decomposition temperature~ and a most
preferred temperature range is from about 320C to 340C. Degradation of
the coal particle size and loss of combustible volatile constituents oF the coal
during immersion also become unacceptably high above the decornposition
temperature of the coal. Oxygen occurs substantially entirely in combined
20 form in low-rank coals, principally in carboxylic, phenolic and ether forms.
Carboxylic oxygen is both abundant and less thermally stable than the other
forms of oxygen in low rank coals9 and is reduced rnore readily than ths
phenolic and etheric nxygen.
The immersion time can be from about 5 minutes to 60 minutes,
25 preferably from 5 minutes to 30 minutes. The coal particle size can be from
about 0.1 cm, iOe. No. 18 screen, to about 3 cm. Preferably, the coal particles
are in the size range from 0.5 cm to 2 cm.
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Optionally, the method include(3 the further step of allowiny excess
hot residuum to drain -from the coal particles to yield a beneficiated coal
product with a residuum coating which comprises -frorn 2% to 15% by mass of
the product. The draining can be accomplished by well-known methods, for
5 example placing the coat0d coal particles on a heated screen which is
retentive of the desired size o-f coal particles, but which allows the hot
residuum to drip through. The proportior) o-f residuum coating can be
controlled by adjusting the draining conditions, particularly the draining time7
the temperature of the hot screen and the amounL of cooling provided by the
10 surrounding gaseous medium. The medium is usually air, which does not
oxidize the coal product significantly during the draining step. The maximum
proportion of residuum coating on the procluct coal particles is about 15% by
mass of product. The maximum proportion of residuum coating allowable
commercially is influenced by, among other -factors, the allowable sulphur
15 content of the finished product when it is to be burned in steam boilers. The
typical sulphur content of most petroleum residua is greater than the typical
sulphur content of western low-rank coals, and thus it can be seen that a
limited sulphur content in the beneficiated coal product dictates a limited
residuurn coating content. For example, a 0.3%-sulphur coal with a coating o-f
20 6%-sulphur residuum amounting to 7% by mass on the beneficiated coal
particles has a final overall sulphur content of about 0.7%, which is acceptable
under most existing sulphur emission regulations. A preferred coating
proportion is from 5% to 7% by mass.
The invention will be further described by the following examples,
25 which illustrate preferred embodiments o-f the invention.
EXAMPLES I-II
Samples of Wabamun coal, a sub-bituminous coal originatiny from
Alberta province were classified into size ranges of 2.4 mm to 6.3 mm and
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6.3 mm to 9.5 mm. The as-received samples had been partially dried, having a
moisture cnntent of 9.0%. Aliquots of approximately 10U grams were placed
in a dip bucket made -from No. 40 screening material and inserted into a 2-litre
bealcer approximately two-thirds full oF l-eated DRB vacuurn resicluum. This
5 material was obtained from the upgrading o-f the vacuum resicluum of heavy oil
from the Pelican field in Alberta by the Donor Refined Bitumen (DRB) process
described in Canadian Patent 1,122,914, issued on May 4th, 1982. The DRB
vacuum residuum had a soFtening point of 91C, zero penetration at 25C and
a viscosity of 37000 centistokes at 150C. The time and temperature of
10 immersion were as indicated in Table 1. After removal from the beaker, the
coal product in the dip bucket was allowed to drain -for about 10 minutes in the
hot air above the beal<er~ and then to cool to ambient temperature, and the
beneficiated coal particles were poured into product bottles for testing.
Test procedures throughout were the applicable A.S.T.M. methods
15 except for the equilibrium moisture determination; because the coating on the
product coal particles would be broken by crushing, a modified ASTM D1412-
74 test method was used wherein the particles were tested directly without
crushing. For consistency, the same method was used to measure equilibrium
moisture of the raw coals.
20 EXAMPLE_
A similar process to that described in Examples I and II was carried
out using a 40-yram sample of partially dried sub-bituminous coal from the
Atlas mine in the Drumheller area of Alberta province, having a particle size
from 2.4 mm to 6.3 mm; the results are described in Table 1.
25 EXAMPLES IV AND V
Samples of the Wabamun coal described above were irnmersed in
210 grade asphalt, a material having a softening point of 101C, a penetration
of 9 x 10-4 m at 25C and a viscosity of 1875 centistokes at 177C, with
results described in Table 2.
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TABLF 1: BENEFICIATION OF LOW R~l\IK COALS
EXAMPLES
~
11 111
Coal l ype Wabamun Wabamun Atlas
Particle mm 2.4-6.3 2.4-6.3 2.4-6.3
10 Residuum Type [~RB Vac. DRB Vac.DRB Vac.
Residuum Softening Point 91C 91C 91C
Residuum Sulphur Content 7.0% 7.0% 7.0%
Temperature 315C 330C 330C
Immersion Time, min. 30 30 15
15 Raw Coal Properties
Calori fic Value,
Joules/kg x 106 23.2 23.2 22.4
Actual Moisture Content 9.0% 9.0% 14.9
Equilibrium Moisture 13.9% 13.9% 16.8%
Sulphur Content 0.14% 0.14% 0.43%
Product Prnperties
Calorific Value,
Joules/kg x 106 27.5 27.8 27.5
Actual Moisture Content 0% 0.31% n%
Equilibrium Moisture 8.6% 7.4% 8.1%
Sulphur Content 1.0% 0.79% 1.0%
Residuum Content 10.4% 8.1% 7.1%
EXAMPLE VI
High-moisture samples of Wabamun coal were prepared by
immersing samples of the raw material of Example V in distilled water for 72
hours and drying for 20 minutes to remove surface moisture. The resulting
moisture content was 20.9%. The particles were treated similarly to those in
Example Il and after treatment, the beneficiated coal showed a moisture
35 content of 0% and a coating weight o-f 8.4% by mass based on the finished
product. Specific conditions oF treatment and results are shown in Table 2.
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T Bl E 2: BENEFICIATION OF LOW-RA_K COALS
EXAMPLES
S ~
IV V VI
Coal Type Wabarnun Wabamun Wabamun
Particle Size, mm 2.4-6.3 6.3-9.5 6.3 -9.5
10Residuum Type 210 Asphalt 210 Asphalt DRB Vac.
Residuum Softening Point ]01C 101C 91C
Residuum Sulphur Content 4.0% 4.0% 7.0%
Temperature 320C 320C 330C
Immersion Time, min. 30 30 30
15Raw Coal Properties
Calori-Fic Value9
Joules/l<g x 106 23.2 23.1 20.3
Actual Moisture Content 9.0% 9.0% 20.9%
Equilibrium Moisture 13.9% 13.9% 13.9%
20 Sulphur Content 0.14% 0.14% 0.12%
Product Properties
Calorific Value,
Joules/kg x 106 27 6 28.3 28.9
Accual Moisture Content 0% 0% 0%
25 Equilibrium Moisture 4.4% 3.3% 7.6%
Sulphur Content 0.52% 0.45% 0.90%
Residuum Content 8.4% 7.4% 8.4%
COMPA~ISON TEST
For comparison, a sample of Wabamun coal having a particle size
from 6.3 mm to 9.5 mm was treated by the method of Example I by immersion
in propane-precipitated asphalt residuum at a temperature of 330C for 15
minutes. The asphalt residuum, which was made from a standard mix of
Interprovincial Pipeline crudes, had a softening point oF 74C, a viscosity of
35 851 centistokes at 150C and a penetration of 3 x 10-4 m at 25C. After
immersion and draining, the coated coal particles were sticky and tended to
agglomerate, being physically unsuited to normal transportation and storage
procedures. It was clear that the so-ftening point of this residuum was below
the required range.
The beneficiated coal product in all Examples equalled or exceeded
the calori-Fic value of a typical bituminous coal, which is about 26.3 x 106
Joules/kg. After cooling in the dip bucket, the particles oF beneficiated coal
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product prepared by the rnethocl of the invention exhibited a slight adhesion toeach other caused by their being in contact during cooling, but a light tap on
the bucket was sufficient to break up the mass oF particles, which remained
pourable thereafter and exhibited a hard, glossy surFace.
Roughly correlated to a cnal's equilibrium moisture content is its
prGpenslty towards autogenous heating and hence, spontaneous combustion.
An advantage of the process o-F the invention is that the signiFicantly lowered
equilibrium moisture content of the coal product o-f the invention, as compared
to the raw material, is indicative of a much lower tendency to undergo
autogenous heating and consequent spontaneous combustion. The process o-F
the invention has the signi-Ficant advantage that it can use a residuum which isotherwise of low economic value and which is expected to be in surplus in the
future. The immersion media of the invention, being distillation residua, have
very low volatility and thus unlike light oil coatings they are not subject to
evaporation losses during product transport and storage. The method o-f the
invention produces a bena-Ficiated coal product with a heating value equivalent
to most bituminous coals because of reduced water and oxygen content.
Because of its high calorific value, the beneficiated coal produced by the
method of the invention will produce a required thermal output in a furnace
with significantly less transportation cost where the furnace is located at
a site remote from the coal mine.
