GB2110356A

GB2110356A - Furnace and method of operating the furnace

Info

Publication number: GB2110356A
Application number: GB08233249A
Authority: GB
Inventors: Karl Strauss
Original assignee: Steag GmbH
Current assignee: Steag GmbH
Priority date: 1981-11-27
Filing date: 1982-11-22
Publication date: 1983-06-15
Also published as: ZA827918B; JPS5899612A; DE3147083A1

Abstract

It is known how to operate coal- fired furnaces with at least one pulveriser with intermediate bunkering of the pulverised fuel or with at least one pulveriser with direct injection of the pulverised fuel. Both types of installations have certain disadvantages. According to the invention, there is provided, in a coal- fired furnace, a first pulveriser 20 with coal dust burners 7, 9 connected to the outlet end, into which the ground coal dust is directly injected, and at least one second coal pulveriser 15, supplied with hot flue gas, and an intermediate bunker 35 for the pulverised coal dust from the second pulveriser 15. A second group of coal dust burners is connected to the outlet end of the dust bunker via at least one controllable feeder system 37. <IMAGE>

Description

SPECIFICATION Furnace and method of operating the furnace This invention relates to a furnace having at least one raw coal bunker, a first coal pulveriser supplied with warm pulverising air with a grader connected to the outlet end thereof, a first group of coal dust burners connected to the outlet end of the first grader, the ground coal dust being blown directly into these burners, and with pilot burners attached to the coal dust burners.

Gas, oil or ignition coal dust can be considered as fuel for the pilot burners, which differs in its consistency and/or composition from the coal dust which is fed to the principal coal dust burners. In furnaces with direct injection, the combustion chamber of the furnace must be brought to a relatively high combustion temperature by means of pilot burners, for example to a temperature of 350or within a period of two hours, until with a relatively high minimum capacity of the pulveriser of around 30%, the coal dust burners can be put into operation. With direct injection, the individual principal coal dust burner can only be operated with a partial load of below 60%.With a furnace with several pulverisers and principal coal dust burners with direct injection connected at the outlet end, when operating with a partial load, the partial load is therefore preferably provided by operating individual pulverisers. The minimum pulveriser capacity is determined not only by the method of operation of the coal pulveriser but also by a minimum rotational speed for the bunker feeder attached to the pulveriser.

Furthermore, furnaces with intermediate storage are known in which a coal pulveriser is supplied with hot flue gas and in which a dust bunker with at least one filter is connected at the outlet end to the mill sifter, the ground dust from the coal pulveriser being intermediately stored in this dust bunker. Coal dust burners are connected to the dust bunker at the outlet end via a controllable feeder device. As in a furnace with direct injection, pilot burners are attached to these coal dust burners (principal burners). When operating the known furnaces with intermediate storage and several burner levels, after preheating a connection of all the principal coal dust burners of one level takes place, that is, the combustion chamber temperature must be raised so high that the main burner is sure to be ignited.When there is a change in load, disconnection of the whole level is provided for.

The object of the present invention is to produce a furnace of the type described above which has improved qualities with regard to operating and partial load.

According to the invention there is provided a furnace comprising at least one raw coal bunker, a first coal pulveriser supplied with warm pulverising air with a grader connected to the outlet end thereof, a first group of coal dust burners connected to the outlet end of the first grader, the ground coal dust being blown directly into these burners, and with pilot burners attached to the coal dust burners, at least one second coal pulveriser, which is to be supplied with hot flue gas, provided with a grader connected to the outlet end thereof, of least one dust bunker with at least one filter connected to the outlet end of the second grader, and a second group of coal dust burners connected to the outlet end of the dust bunker via at least one controllable feeder device, pilot burners also being attached to these coal dust burners.

The invention also provides a method of operating such a furnace wherein, when starting the burner, the pilot burner is first ignited, and, after obtaining a specified combustion chamber temperature unrelated to the operation and output of the second pulveriser, at least one burner of the second group is supplied with intermediately stored dust via the feeder device, and after obtaining a specified higher combustion temperature, the first pulveriser is put into operation and thereby the burners of the first group are supplied with dust.

Contrary to expectations, up till now no boiler plant has been known in which pulverisers with direct injection of the ground fuel dust have been used together with pulverisers with intermediate storage of the ground fuel dust. Experts are opposed to thesimultaneous use of direct injection and intermediate storage, although in unexpected ways considerable operational advantages have been achieved for power plants with variable load ratios.

Pilot burners are also attached to the principal coal dust burners in the second group. (One pilot burner can be attached to each of the principal coal dust burners of the two groups; however, it is also possible for fewer pilot burners than principal coal dust burners to be provided. However, it is preferable for an individual pilot burner to be attached to each principal coal dust burner of the second group.) In the furnace according to the invention, all coal dust burners of the second group can be connected at the outlet end to an individual controllable feeder device. In this minimum case, it is still possible to load the principal coal dust burner independently of the operation and performance of the second pulveriser, as long as dust can be brought out from the bunker.In the optimum case, a controllable feeder device is attached to each of the principal coal dust burners of the second group, so that after starting up the furnace after obtaining a low combustion chamber temperature, the coal dust burners of the second group can be connected one after the other or consecutively in subgroups. By this means the consumption of pilot burner fuel, for example oil, is reduced, since this connection of the individual burners or subgroups of burners can take place at a lower combustion air temperature.

It is particularly appropriate if the first grader is a mill sifter with fixed separation and the second grader is a sifter with adjustable separation, since then the grain size of the coal dust used for operating the principal coal dust burner can be varied. If, for example, the first grader operates with a residue of 25% to 90 ym, then it is appropriate for the adjustable sifter to be controlled in such a way that a range of 5 to 25% at 90 ,um is achieved for the residue. When using a centrifugal or impeller sifter, the sifting can be varied by adjusting the rotational speed of the drive.

This type of formation of the second sifter is particularly advantageous if the gas outlet of the filter attached to the dust bunker has a dust filter connected to it at the outlet end, and the dust outlet of the dust filter is connected to at least one fine dust bunker, which for its part is connected via at least one feeder device to at least one of the principal coal dust burners or to at least one coal dust pilot burner arranged in one of the principal coal dust burners, or near to it. In this case, by adjusting the sifting in the sifter attached to the second pulveriser, the distribution of dust to the dust bunker and to the fine dust bunker can be determined, for example, preferably 8090% weight to the main dust bunker and 2010% weight to the fine dust bunker as fine dust, particularly ignition dust.Thereby, the fine dust preferably has a residue of less than 10% at 90 mum.

In the preparation of fine dust as fuel dust in the principal burners, by connecting the main burners after preheating the combustion chamber, the combustion air temperature necessary for switching on can be still further reduced.

After starting up the furnace, a supporting firing of the focal burners by direct injection can become necessary during operation. For this purpose, the principal coal dust burners of the second group can be operated in their entirety, preferably in subgroups and more preferably individually with the ground main fuel dust, the ignition fuel dust or mixtures of the two.

It has proved that satisfactory operation of the furnace according to the invention can also be achieved if several pulverisers with direct injection are attached to the second pulveriser with intermediate storage. By arranging the burners at one level, however, care should be taken that the burner level of the second group lies between two burner levels with direct injection.

Two embodiments of the invention are described in further detail below, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of an embodiment of the furnace, and Figure 2 is a diagrammatic perspective view of the boiler of a second embodiment of the furnace, to illustrate the attachment of the individual burner groups.

Four burner groups 3, 4, 5 and 6, each with eight coal dust burners, are attached to the combustion chamber 1 of a boiler 2. The coal dust burners in groups 3, 4 and 6 are given the reference numeral 7, and the coal dust burners in group 5 are given the reference numeral 8. As can be seen in Figure 2, the burners 7 and 8 in groups 3, 4, 6 and 5 are arranged one above another at different levels, whereby one half of the burners of a group are arranged on the front wall 2a of the boiler 2 and the other half are arranged on the back wall 2b of the boiler. The level of the burner group 5 lies between the levels of the burner groups 4 and 6.

Each of the burners 7 and 8 has a pilot burner 9 in the form of an oil burner.

The flue gases from the combustion chamber 1 are dispersed across a superheater 10, a hot gas valve 11, and an electrostatic precipitator 12 by means of an induced-draught blower 1 3. Some of the hot flue gases at a temperature of, e.g., 4200C are drawn off between the superheater 10 and hot gas valve 11 by means of a branch circuit 14, and are supplied as gas for mill-drying and deactivation to a coal pulveriser 1 5 with a centrifugal sifter 1 6 connected at the outlet end.

The pulveriser 1 5 is supplied with coal from a raw coal bunker 1 7 by a feeder system 18.

A coal pulveriser 20 with a mill sifter 21 connected at the outlet end is supplied with coal from the same raw coal bunker 1 7 or from another bunker 17' with a feeder system connected at the outlet end.

The pulveriser 20 is supplied with mill-drying air in the following way: fresh air is drawn in by a fresh air blower 22 and supplied to a booster blower 24 by a duct 23. Part of the air current is supplied directly to the pulveriser 20 by a duct 25, while another air current, after preheating in a preheater 26 and further heating in the hot gas valve 11, is fed into the duct 25 by a duct 27. The proportion of cold fresh air and warmed fresh air is so adjusted that the heat content of the milldrying air fed into the pulveriser corresponds to the amount of the coal to be pulverised in the pulveriser. The temperature in the duct 27 can amount to e.g. 3200C.

The dust outlet from the sifter 21 leads to a two-output junction box 28, the outlets of which are connected respectively, by means of fouroutput junction boxes 29 and 29', to the burners 7 of burner group 3. In Figure 2, for simplicity, the distributors are omitted and the burners 7 connected directly to the pulveriser 20. The burner groups 6 and 4 are supplied with fuel dust in the same way from coal pulverisers 30 or 31, as shown diagrammatically in Figures 1 and 2. With pulverisers 20, 30 and 31 it is therefore a matter of pulverisers with direct injection.

The dust outlet from the centrifugal sifter 1 6 is connected to four dust separators via a fouroutput junction box 32, only one dust separator 33 being shown for simplicity. (If one separator can deal with the resulting amount of dust alone, the other separators are omitted.) The dust outlet of the separator 33 is connected to a main dust bunker 35 via a bucket wheel valve 34 for the maintenance of the deactivation of the dust bunker.

In order to obtain the optimum performance of the furnace according to the invention, the bunker is furnished with a number of outlets 36 which correspond to the number of coal dust burners 8 in burner group 5, i.e. with eight burners in burner group 5 eight outlets 36 are required. (These details apply only to Figure 1.) These outlets 36 are connected in each case via a feeder system 37 to a primary air line 38 leading to a single burner 8. As seen in Figure 1, four primary air lines 38 are in each case supplied with fresh air from a primary air blower 39.

Figure 2 shows an embodiment of a lower performance level, but one that also falls within the invention, in which two burners of the burner group 5 are in each case connected to the main dust bunker 35 via a feeder system, so that in contrast to Figure 1 with single control of the burners 8, Figure 2 describes a furnace with control of the burners 8 in pairs.

A duct 40 branches off between the bucket wheel valve 34 and the main dust bunker 35, and this leads to a dust store bunker 41.

The gas outlet of the separator 33 is connected to a gas filter 43 via a pressure blower 42, the gas filter having two dust outlets as shown. The dust outlets are connected via a duct 44 to a fine dust bunker 45. The fine dust bunker 45 is connected via ducts 46, feeder system 47 and in each case a two-output junction box 47' to the two primary air lines 38. The number of feeder systems 47 may correspond to the number of feeder systems 37, or more than two primary air lines 38 may be connected to the outlet end of a feeder system 47.

Furthermore, the feeder systems 47 may be connected, via the different ducts 38' leading from the primary air lines 38, to pilot burners 9' adapted as coal-dust pilot burners, the fine dust preferably being introduced as fuel dust in a separate section in burners 8 (integral coal dust pilot burner); an example of this is shown in Figure 1.Furthermore it is possible to conduct the fine dust to coal dust pilot burners which are not integral with the coal dust pilot burners. (Integral and separate coal dust pilot burners are known in themselves.) The filtered gas leaves the gas filter 43 with a temperature of e.g. 1 O00C via a duct 48 and is mixed with the hot flue gas in duct 14, i.e. the temperature of the mill-dry gases fed into the pulveriser 1 5 can, with the aid of these gases, be adjusted to the required amount between 4200C and 100c. If necessary, part of the gas leaving the filter 43 is led via a branchpipe 49 to a gas burner, not shown. It is also possible to direct the gas to one of the burners from the groups 3, 4, 5 and 6.

For the deactivation of the main dust bunker 35 and the dust store bunker 41, the warm e.g.

1 300C flue gas downstream of the induceddraught blower 1 3 is drawn off via an inert gas pipe 50 connected to an induced-draught blower 51.

Furnace air is supplied via a duct 52 which is connected to the fresh air blower 22, a preheater 53 and the hot-gas valve 11 to the combustion chamber 1 or the burner groups 3, 4, 5 and 6. The corresponding ducts are not shown, for simplicity.

The sifter 1 6 is a centrifugal sifter with a drive motor 1 6', so that the sifting of the centrifugal sifter 1 6 is adjustable by changing the rotational speed. For example, the sifting action of the sifter 1 6 is to be adjustable, so that at 90 Lm the residue is adjustable between 5 and 25%, The sifting action of the pulveriser sifter 21 should preferably operate with a residue of 25% at 90 Mm. This is a constant size, as the sifting function of pulveriser plus sifter alters in relation to load, composition of the coal and wear of the pulveriser.

A preferred adjustment of the centrifugal sifter 1 6 is such that 80% of the coal put in is fed into the main dust bunker and/or dust store bunker as main dust, and 20% is fed into the fine dust bunker 45 as fine dust.

The furnace described above offers several possible starting methods. It is firstly possible to raise the temperature of the combustion chamber 1 with the aid of the pilot burners 9 (e.g. oil) to such a level that by the operation of one of the feeder systems 47 the fine dust supplied to one of the main burners 8 can be ignited. The temperature of the combustion chamber is thereby raised and the other burners of group 5 can be connected one after the other. With sufficiently good ignition quality of the dust supplied in the main dust bunker 35, the burners 8 of the burner group 5 could also be connected singly, one after the other, at respectively higher temperatures of the combustion chamber, without the fine dust having to be used. Furthermore it is possible to load the primary air lines 38 with mixtures of fine dust and main dust.

The supply of main dust, fine dust or a mixture thereof is independent of the working and/or efficiency of the pulveriser 1 5.

Operation of the pulveriser 1 5 begins when the necessary amount of flue gas is available at the necessary temperature.

Preferably after further raising of the temperature of the combustion chamber by the burners of group 5, the pulveriser 30 is first put into operation, which provides the direct injection of coal dust into the burners 7 of the burner group 6. The pulverisers 20 and 31 for the burner groups 3 or 4 are subsequently connected, whereby the pulverisers 20, 30 and 31 are driven with preferably 20% mill-air.

In the embodiment shown in Figure 2, the burners 8 of the group 5 are connected in pairs.

Regardless of whether the burners 8 of the burner group 5 receive dust via, in each case, an associated feeder system 37 (maximum) or whether they are supplied commonly via one single feeder system 37 (minimum), this can take place independently of the operation and efficiency of the pulveriser 1 5, provided that the dust bunker contains dust.

Where there is a load change from full-ioad to partial-load in the burners of group 5, the adjustment of the pulveriser output to the burner output must first take place, if the coarse dust bunker 35 is filled, and, where a dust store bunker is provided, this bunker is filled.

The furnace according to the invention, with at least one pulveriser 1 5 with intermediate bunkering and at least one pulveriser 20, 30 or 31 with direct injection, also offers considerable advantages in the operation of the boiler with support firing from the burners of group 5.

The main burners cannot be run under 60% of their efficiency, so that in the case of a firing with solely direct injection whole levels of burners must be disconnected if the necessary load change falls below 60% with respect to the single burner and one level. By means of the pulveriser 1 5 and the intermediate bunkering in the coarse dust bunker 35 and the supply, independent of the pulveriser, of the burners 8, support firing can be provided by single or subgroup control of the group 5, without all burners 8 of the group 5 having to be disconnected.

The direction of the gas currents at junction points of ducts, e.g., 14/48 and 25/27, takes place preferably by means of regulating valves (not shown) associated with the respective lines.

It is naturally also possible for more than one pulveriser 1 5 to be connected together with pulverisers with direct injection.

Claims

1. A furnace comprising at least one raw coal bunker, a first coal pulveriser supplied with warm pulverising air with a grader connected to the outlet end thereof, a first group of coal dust burners connected to the outlet end of the first grader, the ground coal dust being blown directly into these burners, and with pilot burners attached to the coal dust burners, at least one second coal pulveriser, which is to be supplied with hot flue gas, provided with a grader connected to the outlet end thereof, at least one dust bunker with at least one filter connected to the outlet end of the second grader, and a second group of coal dust burners connected to the outlet end of the dust bunker via at least one controllable feeder device, pilot burners also being attached to these coal dust burners.

2. A furnace according to claim 1, wherein at least two controllable feeder devices are connected to the outlet end of the dust bunker, a predetermined subgroup of the second group of coal dust burners being connected to the outlet ends of these feeder devices.

3. A furnace according to claim 1 or 2, wherein a controllable feeder device is attached to each coal dust burner in the second group of coal dust burners.

4. A furnace according to any one of claims 1 to 3, wherein the first grader is a mill sifter with fixed separation, and the second grader is a sifter with adjustable separation.

5. A furnace according to claim 4, wherein the second grader has a sifting element which can be rotated by means of a drive and the rotational speed of the drive can be varied.

6. A furnace according to any one of claims 1 to 5, wherein a dust filter is connected to the gas outlet of the filter attached to the dust bunker, and the dust outlet of the dust filter connected to at least one fine dust bunker, which is in turn connected by at least one feeder device to at least one of the coal dust burners or to at least one coal dust pilot burner arranged in said coal dust burner or arranged adjacent the coal dust burner.

7. A furnace according to any one of claims 1 to 6, wherein the coal dust pilot burner is arranged in the principal burner of the second burner group.

8. A furnace according to any one of claims 1 to 7, wherein a dust store bunker is arranged in parallel with the dust bunker.

9. A furnace according to any one of claims 1 to 8 wherein further pulverisers and groups of coal dust burners are provided with direct injection.

10. A furnace according to any one of claims 1 to 9, wherein further pulverisers and attached burner groups are provided with intermediate storage.

11. A furnace according to any one of claims 1 to 10, wherein the burners of one group are arranged at one level, and the burners of the second group are arranged at a level between the level of the first burner group with direct injection and the level of a further burner group with direct injection.

12. A furnace according to any one of claims 1 to 5, wherein the gases leaving the filter can be conducted to the second pulveriser or to a gas burner.

13. A method of operating a furnace according to any one of claims 1 to 12, wherein, when starting the furnace, the pilot burner is first ignited, and, after obtaining a specified combustion chamber temperature unrelated to the operation and output of the second pulveriser, at least one burner of the second group is supplied with intermediately stored dust via the feeder device, and after obtaining a specified higher combustion temperature, the first pulveriser is put into operation and thereby the burners of the first group are supplied with dust.

14. A method according to claim 13, wherein the burners of the second group are put into operation individually one after the other or in subgroups of a specified number by means of the attached feeder devices.

1 5. A method according to claim 13 or 14, wherein a supporting firing is produced with at least one burner of the second burner group in operation with at least two directly loaded burner groups.

1 6. A method according to any one of claims 13 to 1 5, wherein the burners of the second burner group are supplied with coarse dust, fine dust or a mixture of the two, drawn off from the first pulveriser.

1 7. A furnace substantially as hereinbefore described with reference to the accompanying drawings.

1 8. A method of operating a furnace substantially as hereinbefore described with reference to the accompanying drawings.