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(11) | EP 0 225 157 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Method and apparatus for reduced NOx emissions from coal furnaces |
| (57) The present invention discloses a method of, and apparatus for, reducing NOx emissions
from black coal fired updraught furnaces (20). The pulverised coal is entrapped in
the "primary air" and separated into two streams, one fuel rich (13) and one fuel
lean (15). These streams are introduced into the furnace (20) via a corresponding
burner(s) (14,16) with the burner(s) (16) for the fuel lean stream (15) being located
at an elevation above the burner(s) (14) for the fuel rich stream (13). The apparatus comprises a concentrator (3) positioned between the substantially conventional mill (1) and furnace (20) and arranged to split the primary air and entrapped fuel into the two abovementioned streams. The concentrator preferably includes a classifier (8) and takes the form of a swirl generator (5) and radially positioned outlets which make use of the action of centrifugal force on the "primary air" and entrapped coal particles to bring about the desired separation. |
[0001] The present invention relates to a method and apparatus .for reducing the various nitric oxide (NOx) emissions from furnaces.
[0002] The invention in particular relates to black coal furnaces of the type which are commonly used for generation of electric power. It is to be understood that the term "black coal" as used herein covers a range of coals including anathraicite, bituminous coals, and lignite. Typically such coals generally have a moisture content less than approximately 30-40%.
[0003] The coal for such a furnace is pulverized in a mill and the pulverized coal is then entrapped in a primary gaseous transport medium for injection into the furnace. The primary gaseous transport medium is loosely termed "primary air" but in addition to air often includes substantial quantities of flue gas and the like. Typically the mills for such furnaces operate with a ratio of approximatley 30 to 45 cubic feet of primary gaseous transport medium, or primary air, for each pound of the black coal (2 to 3.5 cubic metres air/kg coal).
[0004] Whilst this ratio has been found to be a convenient one for the general operation of the furnace and its auxiliary equipment, it has been found that this ratio is not one which results in low NOx emissions. In fact, studies of this subject indicate that NOx emissions could be reduced provided that combustion takes place at an primary air: fuel ratio with either additional air, or less air.
[0005] It is known as a result of the abovementioned studies to supply a mixture of pulverized fuel and "primary air" to an updraught furnace via an upwardly extending conduit, that conduit being provided with an elbow so that the fuel mixture is injected horizontally into the furnace. Under these circumstances, the centrifugal forces experienced by the fuel particles as they pass around the elbow result in the upper portions of the primary air/fuel mixture stream being rich in fuel, whilst the lower portions are relatively lean. These portions are then supplied to separate burners.
[0006] This arrangement has the disadvantage that the fuel rich portion is at the top and the fuel lean portion is at the bottom and, for an updraught furnace, the opposite is more desirable since if these opposite circumstances could be created, the residence time of the fuel particles within the furnace would then be substantially proportional to fuel particle size. Also the lean portion burning in a higher region of the furnace, acts as a region which reduces the formation of NOx emissions since it is less flame intense. Therefore if these conditions could be brought about, improved NOx emissions could be obtained. However, it would be difficult to invert the above described elbow arrangement without incurring substantial expenses in relation to additional conduits, supporting structures, and the like.
[0007] It is known in the brown coal updraught furnace art, where separate drying gases are required to reduce the very high moisture content (up to 70%) of the brown coal fuel, to separate the drying gases from the mixture of "primary air" and the pulverized fuel in a swirl concentrator. The outer layers of the output from the concentrator which are rich in pulverized fuel are then taken to one burner or set of burners at a low elevation, whilst the centre gases which comprise a relatively lean fuel mixture, go to an inerts burner of set of burners at a higher elevation. This arrangement has been adopted in the brown coal furnace art, in order to give improved flame stability since the high moisture content of the brown coal fuel creates special combustion problems not experienced with black coal, because of the natural tendency of the moisture vapour to extinguish combustion. The improvement of flame stability in such updraught brown coal furnaces, comes about because the draught of the drying gases from the inerts burner(.s) located above the fuel rich burner(s), does not destabilize the burning of the fuel rich mixture of pulverized fuel and "primary air".
[0008] It is the object of the present invention to provide both a method and apparatus whereby the NOx emissions from black coal updraught furnaces can be reduced.
[0009] According to a first aspect of the present invention there is disclosed a method of reducing NOx emissions in black coal fired updraught furnaces, said method comprising the steps of entraining pulverized black coal in a primary gaseous transport medium to produce a resulting gaseous flow, separating said resulting gaseous flow into two streams, one fuel rich and one fuel lean, and introducing said streams into said furnace via a corresponding burner or a corresponding set of burners, the burner or set of burners for the fuel lean stream being located at an elevation higher than that of the fuel rich burner or set of burners. Preferably the separation is also accompanied by a classification in which oversize particles of coal are extracted from the primary air and returned to the mill.
[0010] According to a second aspect of the present invention there is disclosed apparatus when used to carry out the above described method, said apparatus comprising a substantially conventional mill and a concentrater located above said mill, said concentrater being arranged to split the primary gaseous transport medium and pulverized fuel mixture from said mill into two streams, the first of which is fuel rich and is introduced into said furnace via a first conduit at a first elevation, and a second of which is fuel lean and is introduced into said furnace via a second conduit located at a second, higher elevation.
[0011] According to a third aspect of the present invention there is disclosed a concentrator/classifier for connection above the output of a substantially conventional pulverized black coal mill, said concentrator/ classifier comprising a swirl generator means to impart a swirling motion to a stream of primary gaseous transport medium including entrapped pulverized coal particles issuing from said mill, whereby under the action of centrifugal force, the larger of said particles are directed to the outer region of said stream as it passes through an output conduit of said swirl generator means; an opening located in the periphery of said output conduit and leading back to said mill; and a separator means located downstream of said output conduit to divide said stream into a fuel rich stream and a fuel lean stream.
[0012] One embodiment of the present invention will now be described with reference to the drawings in which:
Fig. 1 is a schematic vertical cross-section through the apparatus of the preferred embodiment,
Fig. 2 is a side elevation of the concentrator/ classifier portion of the apparatus of Fig. 1,
Fig. 3 is a vertical cross-section through the upper portions of the concentrartor/classifier along the line III-III of Fig. 1, and
Figs. 4 to 6 are each views similar to Fig. 3 but illustrating a different arrangement.
[0013] As seen in Fig. 1, the arrangement of the preferred embodiment comprises a substantially conventional mill 1 which is provided with a side entry chute 2 through which black coal drops onto the grinding surfaces of the mill 1. The mill 1 entraps pulverized black coal particles in a stream of primary gaseous transport medium, or "primary air" which is exhausted upwardly into a swirl concentrator 3.
[0014] The swirl concentrater 3 is formed from a centrally positioned column 4 on which are mounted a set of swirl blades 5 which preferably have a variable pitch. The swirl blades 5 impart a swirling motion to the primary air/pulverized fuel mixture and, under the action of centrifugal forces, the fuel particles are moved outwardly so that the outer portion of the output from the concentrator 3 is fuel rich, whilst the inner portion is lean.
[0015] In addition, arranged in the periphery of the outlet of the swirl concentrater 3 is an annular skimmer slot 6 above which is located a number of vent openings 7. Both the vent openings 7 and skimmer slot 6 communicate with a cylindrical settling chamber 8 which includes a number of guide vanes 9. From the settling chamber 8 a number of return finger shutes 10 are provided, each of which at its lower end has a flap valve 11.
[0016] It will be apparent that the oversized skimmer slot 6 permits oversized fuel particles to pass, under the action of centrifugal forces, into the settling chamber 8 where they drop downwardly under the influence of gravity. In this way, classification takes place so that the oversized fuel particles can be returned to the mill 1 via the return finger chutes 10.
[0017] Those fine particles of pulverized fuel which may pass through the oversized skimmer slot 6, are returned to the concentrator 3 via the vent openings 7. The number of, and location of, the return finger chutes 10 can be selected in such a manner as to tailor the size and location of the returned components to best suit the configuration of the mill 1.
[0018] From the concentrator 3, the outer, fuel rich portion passes into a spiral outlet 12 which feeds a first conduit 13 which conducts the fuel rich mixture to multiple burner slots 14 of a black coal updraft furnace 20. However, the lean primary air/fuel mixture passes into a second conduit 15 and thence into multiple burner slots 16. It will be apparent that the second conduit 15 and its associated burner slots 16 lie at an elevation higher than that of the first conduit 13 and its associated burner slots 14. Preferably the second conduit 15 is located directly above the first conduit 13. Apart from the arrangement of the burner slots 14, 16 the furnace 20 is substantially conventional and therefore is only schematically illstrated.
[0019] In.an alternative arrangement illustrated by broken lines in Fig. 1, instead of the mill 1 having a side entry chute 3, a central feed chute 17 can be provided.
[0020] Fig. 3 illustrates the detail of the classifier arrangement of Fig. 1. Here a skimmer bar 30 scoops the heavy particle carrying component of the swirling primary air and deflects it outwardly into the settling chamber 8. A hood 31 further deflects the oversize. particles downwardly. The gases and not so heavy particles pass over the guide vanes 9 and through the vent openings 7 to return to the primary air. The oversize particles settle within the chamber 8 and fall into the return finger chutes 10. The guide vanes 9 can be rotated about an axis 32 to provide a further mixture control.
[0021] Further classifier embodiments are illustrated in Figs. 4 to 6. In Fig. 4 the skimmer 130 takes the form of a ring which is secured to both the hood 131 and the fixed guide vanes 191. In Fig. 5 the skimmer 231 is tilted towards the centre to adjust the size of the skimmer slot. In Fig. 6 the hood 131 has an adjustable vane 33 to vary the degree of classification by varying the downward momentum of the return material.
[0022] It will be apparent that the above described arrangement enables the lean fuel mixture to be fed into the furnace above the fuel rich mixture so as to achieve the desirable result sought in order to reduce NOx emissions. The interposing between the mill outlet and the burners, of a variable rate concentrator enables the splitting of the "primary air" and pulverized fuel into two fractions. The division of the fuel within these fractions is now determined by the design of the concentrator 3 and thus it is within the designer's choice to select the conditions for combustion to be completed with the minimum of NOx formation.
[0023] The remainder of the combustion air, termed the secondary air, is fed (in conventional manner) into the furnace 20 through separate ports or wind boxes (not illustrated). Some of the secondary air is fed into the furnace 20 adjacent the burners 14,16. However, in addition secondary air is fed into the furnace 20 at a location 21 above the upper burner(s) 16. This secondary air is arranged to intersect the fuel stream from the burner slots 14,16 after the volatile matter has been burned. The volatile fraction is thus first liberated and burnt within the limits of the primary air available. The flue gas/char stream is intersected by the secondary air so that the combustion is completed. This delay in the burning of the fraction of the fuel will reduce the Nox emissions to a minimum.
[0024] The above described arrangement also provides two additional advantages. The first is the flexibility in the selection of operating conditions for the furnace, especially at the minimum low range at which the furnace can operate without the requirement for supporting flame. This arises because the air/fuel ratio of the concentrator output can be maintained at a desired level. With prior art arrangements, the minimum low load able to be supported by a pulverized coal fired furnace (without supporting flame) was dependent upon the minimum air flow at which the mill could operate.
[0025] The second advantage of the above described arrangement is that the auxiliary power required for the mill and concentrator is substantially reduced because of the classifying action of the apparatus. In a conventional double cone classifier, there are changes in both direction and velocity of the primary gaseous transport medium which results in a pressure drop in the classifier of approximately 10 to 14 inches water gauge (2.5 to 3.5 kPa). With the above described concentrator/classifier, because such changes in direction and velocity are avoided, there is a substantially lower pressure drop of the order of approximately 4 inches water gauge (1 kPa).
1. A method of reducing NOx emissions in black coal fired updraught furnaces, said
method comprising the steps of entraining pulverized black coal in a primary gaseous
transport medium to produce a resulting gaseous flow, separating said resulting gaseous
flow into two streams (13,15), one fuel rich and one fuel lean, and introducing said
streams into said furnace (20) via a corresponding burner (14,16) or a corresponding
set of burners, the burner or set of burners for the fuel lean stream being located
at an elevation higher than that of the fuel rich burner or set of burners.
2. A method as claimed in claim 1 wherein said resulting gaseous flow is classified
by extraction of oversize coal particles from said flow.
3. A method as claimed in claim 1 or 2 wherein said separation into streams is carried
out by the application of centrifugal force to the entrained pulverized black coal.
4. A method as claimed in claim 3 wherein said fuel rich stream comprises larger,
heavier particles of said entrained pulverized black coal and said fuel lean stream
comprises smaller lighter particles of said entrained pulverized black coal.
5. A method as claimed in claim 1 or 2 wherein a secondary gaseous stream comprising
at least some of the combustion gases of said furnace other than said primary gaseous
transport medium is introduced into said furnace (20) at an elevation (21) above said
burners, (14,16) or sets of burners, and spaced therefrom a distance sufficient to
permit the volatile fraction of said coal fraction to be liberated and burnt within
the limits of the volume of said primary gaseous transport medium available whereby
combustion of said volatile fraction is substantially completed in the combustion
gases of said secondary gaseous stream.
6. Apparatus when used to carry out the method of claim 1, said apparatus comprising
a substantially conventional mill (1) and a concentrator (3) located above said mill,
said concentrater being arranged to split the primary gaseous transport medium and
pulverized fuel mixture from said mill into two streams (13, 15), the first of which
is fuel rich and is introduced into said furnace via a first conduit (13) at a first
elevation, and a second of which is fuel lean and is introduced into said furnace
via a second conduit (15) located at a second, higher elevation.
7. Apparatus as claimed in claim 6 wherein said concentrator (3) operates by the application
of centrifugal force on said pulverized fuel.
8. A concentator/classifier for connection above the output of a substantially conventional
pulverized black coal mill, said concentrator/classifier comprising a swirl generator
means (5) to impart a swirling notion to a stream of primary gaseous transport medium
including entrapped pulverized coal particles issuing from said mill, whereby under
the action of centrifugal force, the larger of said particles are directed to the
outer region of said stream as it passes through an output conduit of said swirl generator
means; an opening ( ) located in the periphery of said output conduit and leading
back to said mill; and a separator means (12) ocated downstream of said output conduit
to divide said stream into a fuel rich stream (13) and a fuel lean stream(15).
9. A concentrator/classifier as claimed in claim 8 wherein said opening leads to a
settling chamber (8), a deflection means (30,31) is located in said opening to deflect
entrained particles passing through said opening, and said settling chamber has a
return path (7) leading into said stream for gases of said stream which pass through
said opening.
10. A concentrator/classifier as claimed in claim 9 wherein said settling chamber
has a return chute (10) leading to said mill.
11. A concentrator/classifier as claimed in claim 9 or 10 wherein said deflection
means comprises a skimmer (30,130,231) and hood (31,131) and a plurality of guide
vanes (9,191) are mounted in said return path.
12. A concentrator/classifier as claimed in claim 11 wherein said guide vanes (9)
are pivotally mounted.
13. A concentrator/classifier as claimed in claim 8 wherein said swirl generator as
a plurality of variable pitch swirl blades (15) located in said mill output.