Note: Descriptions are shown in the official language in which they were submitted.
The invention relates to a method and an apparatus for the combus-
tion of finely-ground coal of the kind which is slow to react.
When coal of this kind is burned, difficulties are often caused by
extinction of the flame and/or a decline in combustion temperature, leading
to incomplete combustion or interruption of the combustion process.
It is therefore the purpose of the invention to obviate or mitigate
these difficulties which are caused by the fact that the particles of fuel
take too long to reach their ignition temperature after they enter the com-
bustion chamber. There may be many reasons for this, for example a high
moisture content.
According to the invention, this purpose is achieved by preheating
the coal before it enters the combustion chamber of the burner such that as
it enters the combustion chamber the coal is at a temperature below its
ignition temperature and in the range 100 - 200C. This preheating may be
carried out very advantageously by cooling the burner with air and using the
heated cooling air to preheat the coal.
According to another aspect of the invention, there is provided
apparatus for burning finely-ground, slow-reacting coal, comprising a burner
having a combustion chamber surrounded by a casing through which cooling air
is passed, an air outlet from the casing connected to a coal bunker and to
a feed inlet of the combustion chamber whereby coal in the bunker is pre-
heated and carried by the air from the air outlet into the combustion chamber.
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The invention will now be described in greater detail with reference
to the accompanying drawings, in which:
Figure 1 is a diagrammatic general layout of a burner according to
the invention;
Figures 2 and 3 show details of the burner of Figure l;
Figures 4 and 5 show alternative coal-feeds for the burner of
Figures 1 - 3.
The burner illustrated has an output of 5 MW, which lies in the
middle of the 0.5-10 MW range for such burners.
The burner has a cylindrical, double-walled steel casing 1, the
lower end 2 of which is open, while the upper end 3 is closed. At upper
end 3, the casing is equipped with two cold-air connections 4. During
operation, cold air enters casing 1 through connection 4, passes down adja-
cent the inner surface of outer casing 5 to lower end 2, and then up adjacent
the outer surface of inner casing 6 to outlet 7. A guide-plate 8, between
outer casing 5 and inner casing 6, prevents a short-circuit flow of air from
inlet 4 to outlet 7. In addition to guide-plate 8, steel casing 1 contains
guide-plates 9, 10. Guide-plate 9 runs spirally around the inside of outer
casing 5 and is secured thereto. Guide-plate 10 also runs spirally, but
around the inside of guide-plate 8, to which it is secured. Guide-plates
9 and 10 impart a spiral flow to the air passing through double-walled steel
casing 1. The opposing pitches of the two guide-plates 9, 10 permits a
constant direction of twist in the flow of air in the space between outer
casing 5 and guide-plate 8 and in the space between inner casing 6 and guide-
plate 8. Reversal of the direction of twist would result in a considerable
reduction in the flow. Guide-plates 9, 10 form single-start spirals, but
multiple-start spirals may also be used.
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Outlet connectioll 7 is an e~tension of inner casing 6 and guide-
plate 8 beyond upper end 3.
The steel casing, with connections 4, casings5, 6, outlet connection
7 and guide-plates 9, 10 is a welded structure, with an internal lining 11
of ceramic. The cylindrical cavity, enclosed by the ceramic lining and extend-
ing to the upper end, is the combustion chamber. The ceramic lining is made in
part of aluminum oxide or silicon carbide. If silicon carbide is used, the
proportion must be at least 20 and at most 95%, but other substances may be used
instead. The lining may also be other than ceramic. Any lining must, however,
have an average heat capacity of 0.2 - 0.3 kcal/kg/K or 0.22 to 0.35
watt/kg/K. In the specific embodiment described the heat capacity is 0.25
kcal/kg/K. The coefficient of thermal conduction is between 1 and 20
watt/m/K. If the ceramic lining is between 10 and 50 mm thick, and if the
cooling of the burner casing absorbs between 200 and 300 watt/m2/K, the
wall of the burner has a high heat capacity and heat conductivity, with
no danger of heat accumulation.
The burner, consisting of steel casing 1 and ceramic lining 11, has
a diameter of 1200 mm and a length of 2000 mm. This gives a burner
performance ratio of 0.5 MW per cubic metre of combustion chamber. This
is within the permissible range of 0.3 to 0.7 MW/m3. This range provides
relatively low combustion-chamber loading and results in stable firing.
Unstable firing may definitely be expected at a loading of 2 ~/m3 and above.
All of the combustion-chamber loadings mentioned above are related to a
pressure of 1 bar for 1 hour.
According to Figure 1, ceramic lining 11 is arched at upper end 3 of
the steel casing for production reasons. The shape of lining ll illustrated
in Figure 1 is achieved by means of a cored mould. The ceramic provided for
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the lining is initially plastic, i.e. the substance is filled into the
space between the mould-core and the steel casing in the form of a slip or
mould sand; or the substance may be placed in the steel casing and the excess
expelled by introducing the mould-core. It is essential to ensure uniform
distribution of the substance during the moulding process, since this
produces a homogeneous lining 11 which, in turn ensures uniform heat conductivity
and heat accumulation.
The initially plastic material achieves its final ceramic strength
at least by hardening when the burner is in operation.
A firm union between steel casing 1 and ceramic lining 11 is obtained
by pins, mats or wires around which the material is laid during the moulding
of lining 11. Satisfactory results are obtained with radial pins which
produce a strong joint only in the axial direction of cylindrical steel
casing 1 and which slide radially in ceramic lining 11, thus preventing
stresses in the said lining produced by differential thermal expansion
between ceramic lining 11 and steel casing 1. Axial expansion of the said
steel casing is compensated for by overlapping joints.
When the unit is in operation, the cold air flowing through steel
casing 1 provides a specific amount of cooling. This cooling may also be
obtained by using water or some other liquid coolant.
The air emerging from outlet 7, after flowing through steel casing 1,
reaches a distributor cover 12, which is screwed to the outlet connection
and consists of two star-shaped plates 13, 14, the lower plate being shown
in Figure 3 which is a section along the line III-III in Figure 2. In the
area enclosed by outlet connection 7, plates 13, 14 have a plurality of
uniformly distributed recesses 15. At the edge of each recess, the spaces
between plates 13 and 14 are sealed off with webs 16. The space between the
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outer edges of plates 13, 14 is also sealed of, with webs 17. Plates 13, 14
and webs 16, 17 are a welded structure. Recesses 15 in welded distributor
cover 12 provide access to space 18 between cover 12 and upper end 3 of the
steel casing. Recesses 15 may be circular instead of diamond-shaped, the
former having considerable production advantages. The same applies if dis-
tributor cover 12 is provided with a circular edge instead of the star-shaped
edge shown in Figure 3. On the other hand, the points of the star-shaped
edge may be used as funnels, as shown in Figure 3 where each point has a
passage 19 which connects the interior of distributor cover 12 with a
welded-on pipe connection 12.
Each pipe connection continues as a pipeline 20 containing an
adjusting valve 21. Adjusting valves are provided for burners which do not
operate constantly. In the case of constant operation, the air is distributed
through passages with fixed cross sections. All pipelines 20 run outside
steel casing 1, and each has, at the end remote from distributor cover 12, a
bend 23 terminating in a nozzle 24 located just in front of lower end 2 of
steel casing 1. Nozzles 24 are arranged radially of the longitudinal
axis of the steel casing and at an angle of 70 thereto, an angle which is
within the permissible range of 60 to 80. If nozzles 24 are aimed accurately
at the longitudinal axis and centreline of steel casing 1, the angle between
the centreline of the said nozzles and the radius passing through the centre-
line and longitudinal axis of the steel casing, and through the centres of the
nozzle, is 0. If required, however, nozzles 24 may be aimed past the longi-
tudinal axis and centreline of steel casing 1. In this case, an angle of 30
between the centreline of the nozzles and the radius intersecting the nozzle
centre is permissible. Pipelines 20 are secured to steel casing 1 by webs 25
and a burner flange 26 which also surrounds steel casing 1. Flange 26 is used
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to mount the burner on a boiler, not shown. Upper end 2 of the burner then
projects into the boiler combustion chamber as far as flange 26. In contrast
to other burners, conventional front or top burners having fuel inlets sub-
stantially in the plane of the wall of the boiler, the burner according to the
invention possesses, within the steel casing and the ceramic lining, a combus-
tion chamber or flame tunnel which is protected and closed off from the usual
boiler combustion chamber.
In the case of the burners according to Figures 1 to 3, the fuel is
introduced into the combustion chamber, in the form of a mixture of dùst and
air, through a central pipeline 27 opening into the upper end of steel casing
1. Above distributor cover 12, pipeline 27 is enclosed in another pipeline
28, which is spaced therefrom and is welded to the distributor cover. Below
- distributor cover 12, pipelines 27 is enclosed in a third pipeline 29 which
is spaced still farther away from it and is welded to the upper end of steel
casing 1. This arrangement provides, between pipelines 27 and 29, an annular
passage 30 leading into the combustion chamber and connected to distributor
cover 12. An annular passage 31 is also located above the distributor cover,
being formed by pipelines 27,28 and, like annular line 30, being connected to
the distributor cover. This means that the air emerging from outlet 7 is
distributed in different ways through cover 12, in that it passes into pipe-
lines 20 which open into nozzles 24, into annular passage 30, and into
annular passage 31. The feeding of air in~o annular passages 30, 31 is
facilitated in that the recesses 15 are each arranged in the distributor
cover in such a manner that each of the apertures 32, between two adjacent
recesses 15, faces a passage 19, with webs 16 in recesses 15 forming funnels.
Thus the funnels pertaining to passages 19 and apertures 32 lie exactly
opposite each other, as shown in Figure 3. Thus the air emerging from outlet
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7 is passed optimally into passages 19 and apertures 32
The distribution in cover 12 takes place in specific proportions.
Between 10 and 30% of the air at outlet 7 passes into annular passage or line
31, between 25 and 50% of this total amount of air passes into pipelines 20
A and emerging at nozzles 24. The remainder passes through annular passage or
line 30 to the combustion chamber. This distribution is controlled by ele-
ments such as flaps or valves in the various lines, for example adjusting
` valves 21. However, the range of control should be as narrow as possible,
since wide control ranges result in flow losses for design reasons. For this
reason, a maximum control range of 1 to 2,5 is provided. Furthermore, the
control of the flow of air, as shown in Figures 1 to 3, should take place as
far as possible in one line-area, at valves 21 in pipeline 20, for example.
Control is effected by measuring the flow velocity in pipelines 20. To this
end, measuring orifices 33 are incorporated into the said pipelines, and
communicate with indicating devices 34. Any deviation from predetermined
nominal values is corrected by manual adjustment of valves 21. Moreover,
like all other valves and flaps provided for controlling the flow of air, the
adjusting valves must remain constant while the burner is in actual operation
and when it is being started up. In the case of the embodiment illustrated,
the flow velocity in pipelines 20, during the combustion of anthracite dust
at a rate of 600 kg/h, should amount to 80 m/sec.. This velocity corresponds
to a volume of air equal to 35% of the total volume of air. With 15% of the
air in annular line 31 and 50% in annular line 30, the air velocity in pipe-
line 27 is 15 m/sec. and in annular line 30 it is 50 m/sec.. This is within
the 10 to 20 m/sec. range found satisfactory for pipeline 27, within the
range of two to four times this velocity in annular line 30, and within the
range of five to seven times this velocity in pipelines20. The maximum air
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velocity is therefore about 100 m/sec..
The temperature of the air entering distributor cover 12, with the
burner operating at just below the maximum permissible temperature of 1350 C,
and with an inlet temperature of 20C, is of the order of 400C. The hot air
flows at this temperature through annular line 31 into pipelines 36 running
to bunkers 35 (Figure 4). Pipelines 36 are secured to flanges welded lateral-
ly to pipeline 28. The air flowing in pipelines 36 is to be regarded as
primary air. Added to this primary air at 400C, before it reaches each
bunker 35, in a ratio of 7 : 3, is primary air at 20C, the primary air at
20C being the larger amount. The 20 primary air is passed by a blower 38,
through a control valve 39 and a connection 37, into respective pipeline 36.
The necessary proportion of cold air is controlled by valve 39. Blower 38
ensures an adequate supply of air. This means that the supply of air is, of
necessity, as at connection 4 on steel casing 1. Blower 40 associated with
connection 4 is shown diagrammatically in Figure 1. If desired, blowers 38
and 40 may be replaced by a common blower followed by an appropriate air
distributor.
The supply of cooler air to pipe]ine 36 cools the 400 primary air
to such an extent that by the time it encounters the coal-dust reaching pipe-
line 36, through a control device 41, from bunkers 35, it is at a temperature
below the ignition temperature of the coal-dust. This temperature should be
just above the desired temperature at which the mixture of coal-dust and air
enters pipeline 27. This is achieved by locating coal bunkers 35 close to
pipeline 27, in order to prevent heat losses, and/or by heat-insulating pipe-
line 36. The outlet temperature from pipeline 36, which is secured to pipe-
lines 27 and 28, and the temperature of the mixture of coal-dust and air at
the inlet to pipeline 27, is 160C, and is thus within the permissible range
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of 100 to 200C.
Control device 41 meters the fuel from the surface bunker 35 into
: pipeline 36. It is adjusted to produce a degree of dust saturation of the
coal-dust and air mixture corresponding to about 900 gr/m3.
A simple flow-control valve may be used, if necessary, as the
control device 41 for the desired admixture of coal-dust. If more accurate
metering is desired, mechanical conveyors may be provided to deliver quanti-
ties of coal~dust positively from bunker 35 to pipeline 36. As shown in
Pigure 5, the metering device may be in the form of a vane-wheel 42 located
at the lower, hopper-shaped end of a bunker, the amount of coal-dust carried
between its vanes, downwardly into pipeline 36, being predetermined by the
r.p.m. of the vane-wheel which can be adjusted.
The mixture of coal-dust and air emerging from pipeline 27 into the
combustion chamber is surrounded by an annular flow of air emerging from
annular line 30 and hereinafter referred to as secondary air. Like the air
flowing in annular line 31, this secondary air is at a temperature of 400C
and is caused by a spinner 43 (Figure 2) to move along a spiral path around
; the emerging mixture of coal-dust and air. Spinner 43 consists of a plurality
of deflectors 44 distributed uniformly in annular line 30 and mounted pivot-
ably in pipelines 27 and 29, and of a locking means 45. The shape of deflec-
tors 44 is adapted to the cross section of annular line 30, and the deflectors
may be pivoted in such a manner as to impart to the secondary air in annular
line 33 a twist of between 10 and ~0, depending on the setting of the
deflectors. It is desirable to provide the smallest possible control range
for deflectors 44, so that they fill annular line 30 as far as possible.
Locking means 45 may, for instance, be in the form of rods arranged
rotatably at pipeline 29, one of which is inserted into a bore chosen from a
plurality of bores in a collar seated upon each deflector shaft to give a
specific locking position. This occurs easily through recesses 15 in distri-
butor cover 12. In order to be able to adjust spinner 43 in spite of the
heat and danger of combustion during operation, the deflector shafts may be
fitted, if desired with sprocket wheels. They may then be adjusted from a
safe location by means of a chain drive passing through recesses 15. The
chain drive may also be replaced by other mechanical transmissions.
Spinner 43 may also be replaced by a series of nozzles arranged
spirally around pipeline 27.
The flow of coal-dust and air emerging from pipeline 27, which is
also guided be deflectors 46 distributed uniformly around the inside of pipe-
line 27 and running in the longitudinal direction thereof, is linear and free
from twist. In contrast to this, the flow of secondary air emcrging from
annular line 30 follows a more or less tight spiral, depending upon the
setting of the spinner. If the flow of secondary air, and the flow of coal-
dust and air, are regarded as a total flow, the spiral flow amounts to at
least 30% and at most 90%. This twist may be achieved without spinning the
flow of secondary air, for example by spinning the mixture of coal-dust and
air. However, the solution used in the particular embodiment described above
is a particularly satisfactory arrangement. The spinning secondary air moves
the lighter particles of the flow of coal-dust and air along a tight spiral,
while its effect upon the heavier particles is less pronounced. Centrifugal
force carries the lighter particles of coal-dust outwardly towards ceramic
lining 11. In the vicinity of this lining, the particles of coal-dust are
exposed to a considerable amount of heat which has collected there during the
combustion process. This causes the coal-dust to burst into flames when the
flame breaks away from the fuel inlet into the combustion chamber. This effect
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is amplified by the effect produced by the particles of coal-dust burning in
the combustion chamber over a longer period of time, and by the preheating
of the coal. This longer period of time provides some assurance that the
flame in the steel-casing combustian chamber and lining will not die out
entirely if the flame should break away from the fuel inlet, as a result, for
example, of an interuption in the fuel feed. In this case, the flame still
in the combustion chamber flashes back to the fuel inlet. The advantageous
effect of the longer period of residence in the combustion chamber increases
with time, and this is dependent upon the angle of twist of the flow of
secondary air. However, an increased angle of twist produces considerable
flow losses. The advantages of a longer period of residence must therefore
be balanced against the disadvantages of flow loss.
In addition to the period of residence and the heating effect of
the lining, which, in order to maintain the operating temperature of about
1350C, must not exceed a certain value, and therefore requires cooling with
cold air flowing through the steel casing and absorbing between 200 and 300
watt/m2/K, uniform preheating of the fuel with hot primary air contributes
to combustion in that only relatively little preheating is required to bring
the coal to its ignition temperature.
The first stage of a two-stage combustion process takes place in the
combustion chamber. This first stage is determined by the adjustment of the
secondary air, the primary air, and the incoming mixture of coal-dust and air,
and by imparting a spin to the secondary air. In other words, the spin and
the period of residence are to be adapted to this combustion stage. In the
first combustion stage, as far as the outlet from steel casing l and lining 11,
a degree of combustion c~= 0.4 - 0.8 is to take place. C~ = 1 would represent
100% combustion. In the case of the particular embodiment described, the
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degree of combustion selected is at the upper permissible limit and is
achieved by less than stoichiometrical combustion. In combustion of this
kind, the amount of oxygen provided is less than that corresponding to the
molecular ratio of the fuel. This results in incomplete combustion, since the
particles of coal-dust do not burn consecutively but uniformly. This causes
delayed combustion which inhibits the formation of nitric oxide (NOX).
Since the combustion temperature does not exceed 1350C, there is no
melting in the first combustion stage. Instead, the combustion process is dry.
The partly burned particles of coal-dust are soft and cause much less erosion
of the burner lining, as compared with melted and solidified slag particles.
In the second stage, tertiary air is passed through pipelines 20
outside the chamber enclosed by steel casing 1 and lining 11. This tertiary
air is to produce over-stoichiometric combustion, i.e. an excess of air is
to be produced. This excess of air is to achieve the best economic compro-
mise between the minimal airflow required for complete combustion of the
particles of coal and the advantageous effect of a large excess of air upon
subsequent heat recovery by means of heat exchangers located in the flow of
waste gas. Whereas an excess of flue gas facilitates heat recovery with sub-
sequent heat exchangers, the provision of the amount of air required to pro-
duce the required amount of flue gas becomes more difficult as the volumeincreases. One of the reasons for this is the disproportionate increase in
flow resistance with increasing flow velocity. This applies in particular to
flow conditions in steel casing 1, where it is caused by, among other things,
the flow cross sections and the degree of air deflection induced by deflectors
9 and 10. In this connection it should be noted that, as the pitch of these
deflectors decreases, and with constant flow velocity, the air has a longer
period of residence in steel casing 1 and therefore undergoes additional heat-
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ing. The deflectors also serve to force air into all peripheral areas of thesteel casings, in order to prevent spot- or strip-overheating.
The flame in front of steel casing 1 may be adapted as required to
the geometry of the relevant boiler by means of nozzles 24 and a flow of air
of a different configuration, if necessary. Adaptation means, for example,
lengthening, widening, or deflecting.
If nozzles 24 are aimed past the longitudinal axis and centreline
of steel casing 1, steps are taken to ensure that the emerging tertiary air
enters the spirally flowing mixture of coal-dust and air in the direction of
movement thereof, and not in the opposite direction. This avoids destruction
of kinetic energy. Furthermore, the resulting mixing with the flow of
tertiary air is sufficient to ensure complete combustion of the particles of
coal.
The embodiment described uses 600 kg/h of anthracite dust with a
calorific value of 30,000 GJ/kg. 95% of the particles are less than 0.02 mm
in diameter, far below the permissible 0.05 mm. In the raw state, the pro-
portion of volatile components in anthracite dust is 8%. The water content
in the raw state is 1%, which is half the permissible water content. The
ash content is 15%. This is normal, slow-reacting anthracite coal, the
ignition temperature of which is about 400C, depending upon the proportion
of volatile components. Instead of anthracite coal it is possible to use
any other lean coal, the physical characteristics of which have been adapted
to those of anthracite.
In order to start up the burner with anthracite coal, an igniting
burner or lance 47 (Figure 2) is inserted into the combustion chamber through
a central tube 48 in pipeline 27. The igniting burner burns oil or gas and
has an ignition power equal to 10% of the burner power. After the burner has
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been duly preheated, the emerging mixture of coal-dust and air is ignited
with burner 47. When ignition takes place, the distributing device, i.e. the
metering of tlle fuel, is readjusted to the stoichiometrical conditions in the
secondary range, i.e. in the secondary combustion stage. The burner may then
be opened up, preferably with the same air-volume distribution as in continu-
ous operation. This greatly facilitates burner operation.
Part-loading is dealt with by metering the coal-dust and controlling
the flow of tertiary air.
The burner is also operated in this way if the anthracite coal with
a volatile content of 10% is replaced by coke. It is even easier to operate
with coke, since dust containing particles of up to 0.15 mm in diameter may
be used.
The burner according to the invention is particularly suitable for
industrial boilers, remote heating, combustion chambers in the cement industry
and other industrial kilns. It may be used as a top or front burner. It is
particularly suitable for short combustion chambersbecause it provides some
control of flame geometry. Moreover, since the burner according to the
invention has its own cooling, it may also be used in uncooled combustion
chambers, and it is particularly suitable for modular designs~
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