Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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FIELD OF THE INVE~TION
The present invention relates to an improved process
for making coal-oil-water fuel mixtures.
DESCRIPTION OF PRIOR ART
As is well known, the energv required by the present
industrialized societies is largely obtained from the
combustion of fossil fuels, particularly liquid fuels derived
from petroleum. Recent substantial increases in the price of
petroleum, the prospect of further such increases and actual
or threatened shortages of petroleum, have led to increasing
interest in alternative fuels, derived in whole or in part
from sources other than liquid petroleum. Many industrialized
countries, including Canada, still have substantial reserves
of available coal, but the use of coal in solid form, as an
a~ternative fuel, is attended by many problems, not least of
which ie the fact that much existing physical plant is
designed to use liquid fuel.
One approach to this problem, is to utilize a
composite fuel, comprising a mixture of oil and coal, or a
mixture of oil, coal and water, in order to reduce the
quantity of petroleum-derived fuel which must be consumed to
produce a given quantity of energy. At the present time, such
composite fuels are contemplated primarily as substitutes for
heavy industrial fuel oils, such as are consumed in thermal
electric generating plants, but it is, of course, possible
that coal-containing liquid fuels may find wider applications
in the future.
Since coal is not soluble in fuel oil or water,
composite fuels of the type contemplated, are in the nature of
mechanical mixtures of finely divided coal in oil or in a
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water-oil emulsion. See, for example, United States Patent
B No. 3,941j~ (Cottell, 2 March, 1976). A principal problem
associated with such composite fuels is lack of stability;
i.e. the tendency of the coal component to settle out of the
mixture during storage and handling.
A number of different techniques have been tried to
optimize the properties of such fuels. Obviously, the coal
must be finely pulverized to enable it to remain in suspension
in the fuel and not settle in the bottom of the tank in which
the fuel is stored. Various grinding processes and apparatus
have been experimentally used. Various emulsifying agents
have been employed. The ratios of coal-to-water-to-oil have
been experimentally varied. The objective throughout has been
to obtain a fuel that burns cleanly and efficiently, does not
clog supply lines or burners, produces little or no solid ash
residue, and maintains its consistency during storage. The
sonic or ultrasonic treatment of coal particles in suspension
in a liquid medium has been previously proposed by Eric C.
Cottell in the aforementioned U.S. patent, and has achieved
some measure of success in meeting the aforementioned
objective. However, the Cottell technology has been found
insufficient per se to provide a completely satisfactory fuel.
Too much solid ash tends to be produced, and the stabilization
process is unreliable.
A quite separate and distinct technology has been
developed by Capes et al. and is described, for example, in
Canadian Patents No. 1,039,059 (Capes et al., 26 September,
1978) and No. 1,020,~80 (Capes et al., 15 November, 1977).
Capes et al. have discovered how to clean particulate coal by
the technique referred to as agglomeration. Agglomeration
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is a process whereby very fine particles are combined together
in a liquid suspension to form larger, generally spherical
granules. Using appropriate liquids, the coal particulate
materials have a tendency to agglomerate which facilitates the
separation of non-agglomerated ash particles out of the liquid
medium by means of screening techniques.
Before the present invention, the focus of the Capes
agglomeration technique was on cleaning the coal. The focus
of the Cottell sonic treatment was on improving the stability
of the emulsion. Since agglomeration tends to bond particles
together, whereas sonic treatment tends to break up and
separate particles, the two types of treatment did not prima
facie appear to be compatible with one another.
SUMMARY OF THE I~VE~TIO~
I have found that, surprisingly, agglomeration
improves the susceptibility of a coal-oil-water mixture to
emulsification by sonic treatment.
A preferred method for producing a coal-oil-water
mixture for use as a fuel according to the invention comprises
the steps of
(1) grinding coal to a fine particle size;
(2) mixing the coal particles with water (this may
be done in the grinding stage);
(3) adding distillate oil and mixing to form
coal-oil-water agglomerates (this too may be done in the
grinding stage);
(4) separating out the agglomerates;
(5) adding a controlled amount of water to the
agglomerates to form a slurry;
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(6) reagglomerating with a residual oil;
(7) dewatering the agglomerates with a screen;
(8) adding a controlled amount of residual oil
(steps 6, 7 and 8 establish the desired coal-oil-water ratio;
(9) mixing; and
(10) adding sonic energy to cavitate the mixture,
so as to break down the agglomerates to form a relatively
stable coal-oil-water mixture.
BRIEF DESCRIPTIO~ OF THE DRAWING
The drawing is a flow diagram illustrating a
preferred application of the method of the present invention;
DETAILED DESCRIPTIO~ OF THE DRAWI~G
As shown in the drawing, coal is ground and mixed
with water. (These two stages may be combined in a suitable
grinder. Depending upon starting materials and equipment
choices, other stages may be combined also, and the following
discussion is not intended to exclude such combinations).
Distillate oil is added to promote the formation of spherical
agglomeration. Additional water is then added as required to
obtain a mixture having a solids content of less than 30%.
The resulting mixture is then mixed to further promote
agglomeration. The mixture is then passed over a screen
having a mesh size less than the size of the agglomerates.
Residual oil is then added to the mixture together with
additional water (preferably hot water). Typically, the
residual oil will be number 6 oil. The addition of the
residual oil also assists in displacing some of the water
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thereby further reducing the water content of the mixture.
The mixture is then mixed and screened again for dewatering
purposes. The dewatered mixture is passed through a final
mixing stage where additional residual oil is added as
required. Finally, the mixture is pumped to a suitable
sonic cavitation device as will be further described below
for carrying out the final stage of the method.
This method is particularly useful where the coal
to be used has a relatively high ash content. The inter-
mediate stages of this method serve not only to promote the
mixing of coal, oil and water but also serve to reduce the
ash content of the coal by separating the coal-oil-water
agglomerates from the ash and excess water by screening.
The coal may also or instead be passed through additional
preliminary cleaning stages to reduce the ash content of the
coal prior to processing by the method of this invention.
Apparatus suitable to carry out the method of this
invention is already known and commercially available. A
variety of ultra-fine, wet mills may be used to grind the
coal in the presence of water. In particular, a horizontal
rotating pinmill has been found suitable. A large hammer
mill or ball mill could also be employed for this purpose.
The distillate oil can be added before or after grinding.
The subsequent first mixing and screening stages employ a
high shear mixer and a slotted screen. The slotted screen
is inclined and is used to effect coarse filtration. The
excess water and non-agglomerated residue such as ash tend
to fall through the screen. The water can be re-cycled in
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the system and the recovPred ash can be used to manufacture
bricks and the like.
After the agglomerates have passed over the
slotted screen, they enter a surge tank where the residual
oil and
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additional water are added. The surge tank provides a
relatively stable flow pattern to the mixture. The mixture is
then introduced into a low shear mixer; a low shear mixer
being desirable at this stage since it is not desired to
break up the larger coal-oil-water agglomerates that have
formed. The mixture is subsequently passed over a vibratory
screen (such as the one sold under the trade mark SWEC0). As
the screen is vibrated, excess water and non-agglomerated
particulate matter such as ash tend to pass through the
10 screen. The mixture is then directed to a second surge tank
and finally to a ribbon mixer or static mixer where additional
residual oil is added.
Although considerable variation in composition of
such mixtures is possible, a mixture of 40 to 65~ coal, 25 to
45% oil, and 2 to 20% water roughly represents the useful
ranges of constituents. A mixture of 55~ coal, 33% oil, 12%
water has been found satisfactory. It is preferred to use, as
the oil component, as much residual oil and as little
distillate oil as possible, because of the relatively lower
price of residual oil. However, at least some distillate oil
has been found desirable to form agglomerates which can then
be screened to separate out any finely divided ash particles.
The agglomerates in suspension then may be cavitated.
Distillate oil may comprise from about 5 to 50% of the total
oil in the coal-oil-water mixture. The percentages expressed
above and in the claims are by weight.
A suitable distillate oil is No. 2 oil and a
suitable residual oil is No. 6 oil.
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