Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR MANU~ACTURING SMOKELESS AGGLOMERA'rE FUELS,
SMOKELESS AGGLOMERA~E FUELS ~IIIJS PREPARED AND OVFN FOR
USE IN THIS PROCESS
This invention relates to a process for manufacturing smoke-
less agglomerate fuels from agglomerates of carbonaceous material,
in particular coal and/or coke particles and a bituminous binder, to
smokeless agglomerate fuels thus prepared and to an oven fur use
in this process.
Air pollution from various fuels increasingly becomes the
object of attention in several countries, and for instance the
British "Clean Air Act" has laid down requirements for fuels
for domestic heating as regards their smoking properties.
Agglomerates of carbonaceous particles and bituminous binder,
e.g., "green briquettes", may burn with evolution of smoke,
depending upon the nature and the amount of volatile matter.
Therefore they are usually subjected to some kind of heat treat-
ment, e.g., a dry distillation, optionally preceded by an
oxidizing treatment. Thus, it is known from French Patent
1,047,584 and its additions 63,415, 66,133 and 67,980 to render
smokeless by oxidation small agglomerates of carbonaceous sub-
stance. That process comprises two heating stages at different
temperatures~ followed by a cooling stage. In this process the
reaction rate is mainly controlled by the oxygen content and/or
the temperature of the treating gases, thus requiring complicated
gas dosing means. Moreo~er, it is not mentioned to which degree
the carbonaceous substance is rendered smokeless.
n tihe ~reSeQt invention the problem of controlling the
reaction rate has been solved by recognizing that - given a
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mass of agglomerates - three variables affect the reaction rate:
the speed, the temperature, and the oxygen content of the treat-
ing gas. By maintaining a certain temperature and oxygen content,
the gas speed, which is easily regulated by electric fans,
governs the reaction rate. Moreover, it has been found that a
special sequence of three or more oxidizing stages results in a
great flexibility of the process. In addition to considerably low-
ering the tar content the mechanical strength, expressed in, e.g.,
crushing strength, abrasion resistance and resistance to dropping
of the agglomerates is substantially improved.
According to this invention, smokeless agglomerate fuels
are manufactured from agglomerates of carbonaceous material, in
particular coal and/or coke particles and a bituminous binder by
subjecting the agglomerates to an oxidizing heat treatment in at
least three stages using hot gas and then cooling the agglomerates,
the first stage being a heating and drying step, the second stage
being an oxidation step, and the third stage being a finishing
oxidation step in which third stage the tar content is reduced to
less than 1.4% by weight. By smokeless is meant a tar content be-
low 1.4%w, which corresponds with the requirements laid down inthe British Clean Air Act for domestic heating.
The process of this invention is now described in more
detail.
The agglomerates ofl e.g., coal and/or coke particles and
bituminous binder may be in the form of briquettes, extrudates,
pellets, etc.,such as the agglomerates disclosed in British Patent
Specification 1,498,494 and in French Patent Specification
7,538,325. It is possible to use coal duff, coke breeze, pulver-
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ized anthracite, or any coal fine, carbonaceous product or waste,
pure or mixed with other matter. The binder may be any convention-
al bituminous material, such as coal tar pitch, petroleum bitumen
or pitch, ethylene cracker residual pitch and so forth. Preferred
is a petroleum bitumen, in particular a hard bitumen, penetration,
e.g., 1-15 dmm, softening point 80-95C, such as a high vacuum
bitumen or a cracked bitumen.
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Semi-blown or fully blown precipitation, in particular propane
bitumen, which may contain a fluxing oil, can also be used.
In the first sta~e of the heat treatment the agglomerates
are heated up to, e.g., 250 C. ~hey lose their residual water
and an exothermic reaction, i.e., the oxidation of the binder,
starts. The temperature of the gas just before it strikes the
agglomerates is adjusted to about 250-350C, preferably 250-300C,
but a lower or higher setting is also possible, provided the
residual water is lost and the exothermic reaction is started,
respectively no spontaneous combustion of agglomerates occur~.
~he purpose of the gas flow in the first stage is primarily ~o
deliver the necessary calories.
Second stage: because of the exothermic reaction the temper-
ature of the agglomerates rises, and exceeds the gas inlet
temperature. To prevent the agglomerates from catching fire,
it is necessary to moderate the conditions, e.g., by lowering
the gas inlet temperature or preferably by water-spraying. The
purpose of the gas flow is now to take away heat created inside
the agglomerates. Accordingly, the gas outlet temperature becomes
higher than the inlet temperature. Usually the gas inlet temper-
ature is set between 5 and 50C lower, preferably 20C lower
than in the first stage, but it is possible to maintain the
temperature and moderate the reaction rate by a higher gas speed,
a lower oxygen content and/or the addition of water, as will be
explained.
~ or every oxygen concentration of the gas there is an
e~uilibrium gas speed, where the heat created by the exothermic
reaction is carried away by the gas. If the gas speed is lower
than this equilibrium speed the exothermic reaction will run out
of control and the agglomerates will start burning, while too
high a gas speed will result in the agglomerates not being cor-
rectly desmoked within a given time, as their core temperature
is not sufficiently high. Il is ~so possi~le, and some'imes
advisable, to inject water and/or air to slow down the reaction
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rate. In the case of water its evaporation lowers the temper-
ature and secondly the resulting steam lowers the partial oxygen
concentration, thus slowing the reaction rate in two ways.
The third stage of the heat treatment is meant to desmoke
the remaining unreacted parts of the agglomerates. As the
exothermic reaction in the second stage proceeds, the rate
slows, because the concentration of unreacted material decreases.
Likely the outer layers of the agglomerates already have reacted
with oxygen, while the cores yet have to react, because the
diffusion of oxygen takes some time. In order to complete the
reaction quickly, the inlet gas temperature is raised again~
e.g.,to the same value as in the first stage. Any other te~per-
ature is possible, in principle, but the same remarks about this
as in the second stage apply.
Sometimes it is necessary to add a fourth, fifth, etc. stage
of thermal treatment, to remove or convert the last remnants of
potentially smoke-producing matter. The temperature of the last
stage will almost always be higher than in the penultimate stage,
for reasons discussed above in the third stage. The total treatment
time is normally less than 2 hours, usually about 80 minutes, e.g.,
four stages of 20 minutes each.
Suitable layer thicknesses of the agglomerates are 10 to
50 cm
Preferably the gas speed and oxygen content of the hot gas
are such that the core temperature of the agglomerates does not
exceed 420 C in any of the s-tages, and does not drop below 200 C
in the second or any following stage~ Thus, the oxidizing heat
treatment may be carried out in a travelling grate oven, using
air and/or fumes as a vehicle to exchange calories with agglomer-
ates. In the stages2 and higher the exothermic reaction has tobe controlled by a minimum gas speed through the layer to prevent
the agglomerates from catching fire. The gas speed leaving a
40 cm thick layer of egg-shaped br quettes of 30 g must, e.g.,
exceed 1.6 m/s for an oxygen content of 17 to 18%v. Oxygen
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contentshigher than 10%v are preferred.
It is obvious that the fumes evolved during the heat treat-
ment can be combusted, condensed or recycled to avoid pollution
of the environment.
Finally, a cooling step is required for the safe handling,
storage and use of the agglomerates. In this way the exothermic
reaction is stopped, and catching fire of hot agglomerates is
prevented. It is feasible to cool the agglomerates with air and/or
water. Air cooling is time- and space-consuming. Spraying with
water is also possible, but by far -the quickest and most space-
saving method is immersion in a water tank. The temperature of
the water and the immersion time can be varied to control the
final temperature and the water content of the agglomerates.
When the agglomerates are allowed to cool further in the air,
part of the water that was taken up during the immersion will
evaporate. Typical but not limitative values are: temperature
of hot agglomerates: 350 C, after 7 minutes of water cooling:
110C, temperature of the water tank: 80-85 C.
The apparatus in which the heat treatment is carried out
can be derived from any known furnace or oven, operating batchwise
or continuously and may be a circulating furnace, a travelling
grate oven, a tunnel furnace, etc. A travelling grate oven is
preferred. The oven comprises at least 3 zones, preferably 3
separated compartments, being regulated separately with burners,
water nozzles, etc., pre~erably in fixed position. A fluidized
bed type oven may also be used. If the hot gas is flowing
through a horizontal layer of agglomerates from below, it is
observed that the agglomerates in the upper layer reach the
highest temperatures, because of the generation of heat in the
lower l~yers. Of course, this is the reverse of the first
stage where the gas is delivering heat and the upper layer will
be cooler initially.
~ The resulting agglomerates are particularly suitable for
; domestic use.
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Examples
EXAMPLE I (comparative Example)
Ground and dried anthracite was fed into a heated mixer and
heated to 70-80 C, then 5.5%w of ~ 80/go bitumen binder
(so~tening point 80-goc, penetration 6-15 d~m) at 220C was
added to the coal. After mixing ~or 10 minutes at 80 C, the
mix-ture was pressed. 46 kg of small briquettes (a~out ~ cm3,
11.2 g, 4.3%w water) were produced. The desmoking was carried
out in a two-zone pilot oven without atmosphere control, i.e.,
in air. The briquettes were put in a metallic basket
(70 x 70 x 15 cm), bed thickness was 14 cm. They were heat-
treated for 20 minutes in zone 1 at 250C and for 40 minutes in
zone 2 at 230C (gas inlet temperatures). The upper layer reached
315 C at the end of the treatment. After forced cooling in air
the briquettes were analyzed; weight 10.8 g, water content nil,
', residual tar content 1.6%w and thus do not meet the re~uirement
o~ the British Clean Air Act as indicated hereinbefore.
EXAMPLE II
The experiment of Example I was repeated, but the heat
20 treatment consisted of three stages: 20 minutes at 250C,
20 minutes at 230C and 20 minutes at 285 C. The upper layer
~ reached 360C at the end of the treatment. The residual tar
i content was 1.2%w.
The rise in temperature at the end of the heat treatment
in Example II apparently served to remove 25% more tar in the
same time.
EXAMPLE III
The process of the invention was carried out under the
conditions tabulated. The mechanical properties of the "green"
vs. the heat-treated briquettes were measured. Quenching was
l~ by immersion in water.
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Example III A ~xample III B
Composition
Binder content, parts per
hundred 5,5 6.1
r~ype of binder 85/2 bitumen H 80/go bitumen
5 r~ype of coiql, %w anthracite 60anthracite 100
meager coal 15
steam coal 15
fettlings 10
Pro~erties of ~een briquettes
lO Water content, %w 4 5 4.7
Average crushing strength,kg 74 40
Standard deviation
~30 briquettes) kg 7.2 9.8
Resistance to dropping
% passing 5 mm sieve after
1 x 5 m 5.8 8.7
2 x 5 m 14.2 21.4
3 x 5 m 22.5 31.5
Resistance to abrasion,
after 100 revolutions
% passing 5 mm sieve 8.8 9.1
r~ar content, %w 3.18 3.39
Weight volume density
Conditions of treatmeNt
Number of zones 3 4
Gas inlet temperature, C 290-270-310 300-280-300-320
Ox~gen content, %v 17 19.5
Residence time in zones, min. 40-20-20 20-20-20-20
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Example III A Example III B
(cont'd) (cont'd)
Properties treated briquettes
Weight, g 44 . 4 33, 1
Volume, ml 34 . 2 26 . 7
Apparent density, g/ml1.30 1.24
Water content, %w - ~
Average crushing strength, kg 129 79
Standard deviation ( 30
briquettes) kg 12. 7 1 5 . 7
Resistance to dropping
% passing 5 mm sieve after
1 x 5 m 1.8 3.0
2 x 5 m 4.5 7.4
3 x 5 m 8.7 11.8
Resis~ance to abrasion,
after 100 revolutions
% passing 5 mm sieve 4.8 5.0
Tar content, %w 1.0 1.0
These briquettes are virtually smokeless, and show improved
mechanical properties.
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