CA1111315A

CA1111315A - Coal combustion

Info

Publication number: CA1111315A
Application number: CA325,748A
Authority: CA
Inventors: Heinz-Werner Bucker; Horst S. Greulich; Jorn P. Jacobs; Aloys Schlutter; Jorg Schmalfeld
Original assignee: Ruhrkohle AG
Current assignee: RAG AG
Priority date: 1978-04-18
Filing date: 1979-04-18
Publication date: 1981-10-27
Also published as: EP0006974A2; AU4612979A; DE2960677D1; AU533315B2; EP0006974A3; DE2816674A1; BR7902331A; ZA791839B; EP0006974B1; DE2816674B2; DE2816674C3

Abstract

Abstract of the Disclosure For the combustion of finely-ground coal which is slow to ignite, the coal is preheated before it enters the combustion chamber. The preheating can be carried out by passing cold air around the outside of the combustion chamber and then using this heated air to preheat the coal in bunkers. For this purpose the combustion chamber is provided with a hollow casing surround-ing the combustion chamber proper and a partition in the hollow casing pro-vides an annular outer space and a concentric annular inner space. A
helical baffle arrangement is also provided in the inner and outer spaces to guide the cold air down the outer space and up the inner space. The coal is burned below the melting point of the coal slag and this produces a soft ash rather than molten material. The ash is cleaner and cannot bake onto burner housing. Heat is stored in the combustion chamber wall and the time of residence of the coal in the combustion chamber is increased by impart-ing a spiral motion to the coal and/or air supplied to the combustion chamber.
The layer residence ensures more complete combustion.

Description

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.

. ., 3~S

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.

' ! ' ~ _ _ ~, ' :
, 3~.S

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 ..~, ' 3~5 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 3iL~

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 .. . ..

J~

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 3:~5 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 : . . :.. , .: ., :

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 . .

~L~h~3~5 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 ! . ` . : ' L13~l~
r 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 ~s '' 4~ ~ A

3~
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-3~5 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 .:

.
. .

3~

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~

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

l. A method of burning finely-ground, slow-reacting coal, in a combustion chamber in which the coal used is preheated such that as it enters the combustion chamber it is at a temperature below the ignition temperature of the coal and in the range 100-200°C.
2. A method according to claim 1 in which the preheating is achieved by hot air obtained by passing cold air over the exterior of the combustion chamber.
3. A method according to claim 2, in which the hot air produced by passing cold air over the combustion chamber is subsequently cooled down to a temperature below the ignition temperature of the coal by an admixture of cold air.
4. A method according to claim 3 in which the cold air passed over the combustion chamber is thereby heated to substantially 400°C and in which the cold air admixed is at a temperature of substantially 20°C, the ratio of 20°C air to 400°C air being substantially 7 : 3.
5. 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 preheated and carried by the air from the air outlet into the combustion chamber.
6. A device according to claim 5, in which a cold-air source is connected to the air outlet from the casing.