Over fire arrangement and method

The present invention relates to an over fire air arrangement for a furnace (1), the furnace (1) having opposing first wall (4) and second wall (6) and opposing first side wall (5) and second side wall (7) between the first and second walls (4, 6) for forming a furnace enclosure (2). The over fire air arrangement comprising at least one first over fire air port (20) provided to the first wall (4) for supplying a first over fire air flow (40) into the furnace (1) and at least one first additional over fire air port (50) provided to at least one of the first and second side walls (5, 7) in the vicinity of the first wall (4), the at least one first additional over fire air port (50) being arranged to supplying a first additional over fire air flow (60) into the furnace (1) transversely to the first over fire air flow (40).

This application is the U.S. national phase of International Application No. PCT/FI2017/050488 filed 29 Jun. 2017, which designated the U.S. and claims priority to FI Patent Application No. 20165555 filed 1 Jul. 2016, the entire contents of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to an over fire air arrangement and more particularly to an over fire air arrangement according to the preamble of claim1. The present invention also relates to a method for supplying over fire air into a furnace, and more particularly to a method according to the preamble of claim12.

BACKGROUND OF THE INVENTION

Nitrogen oxides (NOx) are unwanted byproducts of combustion processes, such as combusting fossil fuels. NOx emissions from combustion processes have negative effects on environment by causing acid rains. In the prior art NOx emissions have been reduced by staged combustion of fuels in the furnace or staged air supply into the furnace. This means that the fuel is burned in stages. In principle, increasing the staging of the combustion process the NOx emissions may be decreased. However, increasing the staging of the combustion process increases the amount of unburns, including carbon monoxide (CO). Increased unburns decrease the combustion efficiency and also cause emissions. The increase of unburns may be prevented by using over fire air arrangement in which air is supplied to the upper furnace for good burn-out of the fuel after staged combustion.

In prior art over fire air arrangements there are usually one or two levels of over fire air ports provided to furnace walls in upper furnace and in vertical direction over the lower furnace in which the staged combustion is carried out. In these prior art over fire air arrangements the over fire air is supplied from one furnace wall or two opposing furnace walls towards the centre of the furnace. Thus the over fire air improves burn-out of fuel when the staged combustion is used.

However, one of the disadvantages of prior art over fire air arrangements is that the mixing of the over fire air and coverage of the whole upper furnace is inadequate for achieving good burn-out of fuel in the whole upper furnace area. This compromises the combustion efficiency and also generates unwanted unburn emissions.

BRIEF DESCRIPTION OF THE INVENTION

An object of the present invention is therefore to provide an over fire air arrangement and method for supplying over fire air into a furnace so as to overcome or at least alleviate the prior art disadvantages. The objects of the invention are achieved by an over fire arrangement according to the characterizing portion of claim1. The objects of the invention are further achieved by a method according to the characterizing portion of claim12.

The invention is based on the idea of providing an over fire air arrangement for a furnace having opposing first wall and second wall and opposing first side wall and second side wall between the first and second walls for forming a furnace enclosure. The over fire air arrangement comprises at least one first over fire air port provided to the first wall for supplying a first over fire air flow into the furnace and at least one first additional over fire air port provided to at least one of the first and second side walls in the vicinity of the first wall, the at least one first additional over fire air port being arranged to supplying a first additional over fire air flow into the furnace transversely to the first over fire air flow.

In an alternative embodiment the over fire air arrangement further comprises at least one second over fire air port provided to the second wall for supplying a second over fire air flow into the furnace and at least one second additional over fire air port provided to at least one of the first and second side walls in the vicinity of the second wall, the at least one second additional over fire air port being arranged to supplying a second additional over fire air flow into the furnace transversely to the second over fire air flow.

In one embodiment of the invention the first and second additional over fire air ports are arranged to supply the first and second additional over fire air flows along and in vicinity of the first and second walls respectively.

In one embodiment of the invention the invention the first and second additional over fire air ports are arranged to supply the first and second additional over fire air flows parallel the first and second walls respectively, or parallel and along the first and second walls, respectively.

In yet another embodiment of the invention the invention the first and second additional over fire air ports are arranged to supply the first and second additional over fire air flows perpendicularly to the first and second over fire air flows, or parallel and along the first and second walls, respectively, and perpendicularly to the first and second additional over fire air flows.

The first and second over fire air ports provided to the first and second wall, respectively, are arranged to provide together with the first and second additional over fire air ports a crossflow of over fire air into the furnace.

The invention is further based on the idea of supplying over fire air into a furnace, the furnace having opposing first wall and second wall and opposing first side wall and second side wall between the first and second walls for forming a furnace enclosure. The method comprises supplying a first over fire air flow from the first wall into the furnace and supplying a first additional over fire air flow from at least one of the first and second side walls in the vicinity of the first wall and transversely to the first over fire air flow.

In one embodiment of the invention the method further comprises supplying a second over fire air flow from the second wall into the furnace and supplying a second additional over fire air flow from at least one of the first and second side walls in the vicinity of the second wall and transversely to the second over fire air flow.

When air is supplied from the over fire air ports provided to the first and second opposing walls of the furnace, the over fire air flows are directed towards the centre or opposite wall of the furnace. Thus an oxygen rich area is formed in the centre of the furnace or towards the opposite wall of the furnace. At the same time on oxygen lean area is formed in the vicinity of the first and second walls via which over fire air is supplied in to the furnace. This is due to the fact that the over fire air flows flow away from the first and second walls via which they are supplied and start to rise upwards in the furnace. According to the present invention additional over fire flows are supplied from at least one of the side walls of the furnace in the vicinity of the first and second walls and substantially along the first and second wall. Thus additional over fire air is supplied into the oxygen lean area.

The additional over fire air flows provide oxygen to the oxygen lean area in vicinity of the first and second walls via which the over fire air is supplied. The additional over fire air enhances the burn-out of fuel and thus decreases the amount of unburns in the combustion process. The additional over fire air flows may also provide shield flows and decrease corrosion tendency of the first and second walls of the furnace.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1shows a prior art furnace1having a bottom3and top9, and opposing front wall4and rear wall6as well as opposing first and second side wall5,7(shown inFIG. 2B). The front wall4and rear wall6as well as opposing first and second side wall5,7form together an enclosure2of the furnace1, as shown inFIG. 2B. The furnace1comprises burners14,12,10,15,13,11provided to the opposing first and second walls4,6of the furnace for combusting fuel. In this embodiment the furnace comprises at least one first primary burner14provided to the first wall4and at least one second primary burner15provided to the second wall6for forming a first level of burners and first combustion stage. The furnace1further comprises at least one first secondary burner14provided to the first wall4and at least one second secondary burner15provided to the second wall6, the secondary burners being arranged above the primary burners14,15in vertical direction for staged combustion of fuel. The secondary burners12,13form a second level of burners and second combustion stage. The combustion products of the burners14,15,12,13flow towards the centre of the furnace1as shown by the arrows16. Thus the opposing burners14,15,12,13form a staged combustion zone18at the lower furnace100.

It should be noted that the furnace1may comprise at least one primary burner14,15and at least one secondary burner12,13in vertical direction above the at least one primary burner14,15for providing staged combustion. The furnace1may also comprise tertiary or further burner levels for forming more than two combustion stages. Furthermore, burners may also be provided only on one wall of the furnace1.

The furnace further comprises an over fire air arrangement comprises one or more over fire air ports20,21provided to the upper furnace110above the lower furnace100. In the embodiment ofFIG. 1the furnace comprises at least one first over fire air port20, provided to the first wall4and at least one second over fire air port21provided to the second wall6. It should be noted that the first or second over fire air ports20,21may also be omitted such that over fire air is supplied only from one wall of the furnace1.

In the context of this application the over fire air ports20,21may be provided on the same wall with the burners14,12,15,13or alternatively on adjacent walls. Therefore, the arrangement of the over fire air ports20,21is not dependent on the arrangement of burners14,12,15,13combustion air ports provided to the lower furnace100.

The over fire air ports20,21supply over fire air flows in the direction of arrows26such that on oxygen rich over fire air area42is formed in the centre of the upper furnace110.FIG. 2Ashows this in more detail as a side view of the upper furnace110. When the oxygen rich over fire air area42is formed, first and second oxygen lean over fire air area44,46is formed in the vicinity of the first and second walls4,6, respectively, as the over fire air flows towards the vertical centre line A-A′ of the furnace1.

FIG. 2Bshows a top view of the upper furnace110. The first and second over fire air ports20and21are arranged overlapped to the first and second walls4,6towards each other, such that the first and second over fire air flows40,41produced by the first and second over fire air ports20,21overlap at the centre of the furnace1, as shown inFIG. 2B. Thus an oxygen rich over fire air area42is formed in the centre of the furnace1and first and second oxygen lean over fire air area44,46is formed in the vicinity of the first and second walls4,6, respectively.

FIG. 3shows a furnace1and over fire air arrangement according to the present invention. The furnace1ofFIG. 3corresponds the furnace ofFIG. 1, and therefore repeating the description of the structure is omitted. The over fire air arrangement of the furnace1ofFIG. 3comprises further additional over air ports50,51for supplying additional over fire air to the upper furnace110, as shown in greater detail inFIG. 4A. The additional over air ports50,51are arranged to supply over fire air to the oxygen lean over fire air areas44,46.

The additional over air ports50,51are provided to the first and second side walls5,7as shown inFIG. 4B. The furnace1of the embodiment ofFIGS. 3, 4A and 4Bmay comprise at least one first additional over fire air port50provided to the first side wall5and second side wall7and at vicinity of the first wall4and arranged to supply first additional over fire air flows60to the first oxygen lean over fire air area44in the vicinity of the first wall4. The furnace1may further comprise at least one second additional over fire air port51provided to the first side wall5and or second side wall7and at vicinity of the second wall6and arranged to supply second additional over fire air flows61to the second oxygen lean over fire air area46in the vicinity of the second wall4. Accordingly the first and second additional over fire ports50,51are arranged to supply the first and second additional over fire air flows60,61into the furnace1transversely to the first and second over fire air flow40,41and to the first and second oxygen lean over fire air area44,46. It should be noted that there may also be only one first additional over fire air port50and only one second additional over fire air port51provided to the first or second side wall5,7.

FIG. 5shows a modification of the furnace1ofFIG. 3. The furnace1ofFIG. 5comprises only first burners14,12provided to the first wall4and only first over furnace air ports20provided also to the first wall4. Thus the furnace1provides a single wall combustion furnace. The over fire air arrangement of this embodiment also comprises only at least one first additional over fire air ports50provided to the first side wall5and second side wall7and at vicinity of the first wall4and arranged to supply first additional over fire air flows60to the first oxygen lean over fire air area44in the vicinity of the first wall4, as shown inFIGS. 6A and 6B.FIG. 6Ashows, that in this embodiment the oxygen rich over fire air area42is formed in the centre of the upper furnace110and also in the vicinity of the second wall6in the upper furnace110. This due to the fact that there is not opposing over air supply ports and thus the first over fire air flows40supplied from the first over fire air ports20flow towards to second wall6via the centre of the furnace1. Accordingly, the first oxygen lean over fire air area44is formed in the vicinity of the first wall4, but not in the vicinity of the second wall6. The first additional over air ports50are provided to the first and second side walls5,7as shown inFIG. 6B. The furnace1of the embodiment ofFIGS. 5, 6A and 6Bmay comprise at least one first additional over fire air port50provided to the first side wall5and second side wall7and at vicinity of the first wall4and arranged to supply first additional over fire air flows60to the first oxygen lean over fire air area44in the vicinity of the first wall4. It should be noted that there may also be only one first additional over fire air port50provided to the first or second side wall5,7.

It should be noted that also in single wall combustion furnace the over fire air ports may be provided on two opposite walls of furnace in the same way as shown in context ofFIG. 3.

Furthermore, it should be noted that the present invention is not restricted to any special type on furnace, but the over fire arrangement according to the present invention may be utilized in any type of furnaces. For example, the furnace may be single wall combustion furnace, opposite wall fired furnace, corner fired furnace, a grate furnace, bubbling fluidized bed furnace or even a circulating fluidized bed furnace. The present invention is not restricted to any particular fuel, but fuel used in the furnace may be any known fuel.

According to the above mentioned, the over fire air arrangement of the present invention comprises at least one first over fire air port20on the first wall4and at least one first additional over fire air port50provided to at least one of the first and second side walls5,7in the vicinity of the first wall4. The at least one first additional over fire air port50is arranged to supplying the first additional over fire air flow60into the furnace1transversely to the first over fire air flow40of the at least one first over fire air port2. Alternatively the upper furnace110may also comprise at least one second over fire air port21provided to the second wall6and at least one second additional over fire air port51provided to at least one of the first and second side walls5,7in the vicinity of the second wall6. The at least one second additional over fire air port51being arranged to supplying a second additional over fire air flow61into the furnace1transversely to the second over fire air flow41of the at least one second over fire air port21.

As described above and shown inFIGS. 3 to 6B, above the additional over fire air ports50,51may be arranged to supply the first and second additional over fire air flows60,61along the first and second walls4,6respectively. In one detailed embodiment the additional over fire air ports50,51may be arranged to supply the first and second additional over fire air flows60,61parallel the first and second walls4,6respectively. In another detailed embodiment the additional over fire air ports50,51may be arranged to supply the first and second additional over fire air flows60,61perpendicularly to the first and second over fire air flows40,41. Accordingly, the additional over fire air ports50,51are arranged to provide a crossflow of over fire air to the furnace1and especially to the to the oxygen lean over fire air areas44,46.

FIGS. 7A, 7B, 7C and 7Dshow different kinds of over fire air arrangement in which the additional over fire air ports50,51are arranged differently. All the alternatives are in the scope of the present invention.

the first and second additional over fire air ports (50,51) are arranged such that:

As described above the first and second additional over fire air ports50,51are arranged in the vicinity of the first and second walls4,6, respectively. The term in the vicinity means that the first and second additional over fire air ports50,51are arranged such that the a first distance z between the first and second additional air port50,51and the first and second wall4,6, respectively, is smaller than a second distance y between the first and second additional air port50,51and a vertical centre line A-A′ of the furnace enclosure2, as shown in allFIGS. 7A, 7B, 7C and 7D.

In one embodiment the first and second additional over fire air ports50,51are arranged such that first distance z between the first and second additional air port50,51and the first and second wall4,6, respectively, is smaller than or equal to one third of a third distance x between the a vertical centre line A-A′ of the furnace enclosure2and the first and second wall4,6, respectively.

In an alternative embodiment the first and second additional over fire air ports50,51are arranged such that the first distance z between the first and second additional air port50,51and the first and second wall4,6, respectively, is smaller than a second distance y between the first and second additional air port50,51and a vertical centre line A-A′ of the furnace enclosure2, and that the first distance z is smaller than 2.0 m, or preferably the first distance z is between 0.3-1.0 m.

According to the present invention the first and second additional over fire air ports50,51are arranged above the over fire air ports20,21in the vertical direction of the furnace1. The first and second additional over fire air ports50,51may also be arranged even below the over fire air ports20,21in the vertical direction of the furnace1.

FIG. 7Ashows one embodiment of the present invention in which the first and second additional over fire air ports50,51are arranged above the over fire air ports20,21in the vertical direction of the furnace1. In one embodiment the first and second additional over fire air ports50,51may be in vertical direction less than 2.0 m, preferably 0.3-1.0 m, above the first and second over fire air ports20,21. In an alternative embodiment shown inFIG. 7B, the first and second additional over fire air ports50,51are in vertical direction below the first and second over fire air ports20,21.

The over fire air arrangement may also comprise at least one first primary over fire air port20and at least one first secondary over fire air port22provided to the first wall4. The first secondary over fire air port22is arranged in vertical direction above the first primary over fire air port20.

FIGS. 7C and 7Dshow an embodiment of the over fire air arrangement having at least one first primary over fire air port20and at least one first secondary over fire air port22provided to the first wall4and at least one second primary over fire air port21and at least one second secondary over fire air port23provided to the second wall6. The first secondary over fire air port22is arranged in vertical direction above the first primary over fire air port20and the second secondary over fire air port23is arranged in vertical direction above the second primary over fire air port21.

In the embodiment ofFIG. 7Cthe first and second additional over fire air ports50,51are arranged such that the first and second additional over fire air ports50,51are in vertical direction above the first and second secondary over fire air ports22,23. In one embodiment the first and second additional over fire air ports50,51may in vertical direction less than 2.0 m, preferably between 0.3-1.0 m, above the first and second secondary over fire air ports22,23.

In the embodiment ofFIG. 7Dthe first and second additional over fire air ports50,51are arranged such that the first and second additional over fire air ports50,51are in vertical direction between the first and second primary over fire air ports20,21and the first and second secondary over fire air ports22,23. In one embodiment the first and second additional over fire air ports50,51may be in vertical direction between the first and second primary over fire air ports20,21and the first and second secondary over fire air ports22,23and less than 2.0 m, preferably 0.3-1.0 m, above the first and second primary over fire air ports22,23.

The over fire air arrangement may also comprise at least one first primary additional over fire air port50and at least one first secondary additional over fire air port52. The at least one first secondary additional air port52is arranged in vertical direction above the at least one first primary additional air port50.

FIG. 8shows an embodiment of the present invention in which the over fire air arrangement comprises at least one first primary additional over fire air port50and at least one first secondary over fire air port52, as well as at least one second primary additional over fire air port51and at least one second secondary over fire air port53. The at least one first secondary additional air port52is arranged in vertical direction above the at least one first primary additional air port50and the at least one second secondary additional air port53is arranged in vertical direction above the at least one second primary additional air port51. The at least one first and second secondary over fire air ports52,53supply first and second secondary additional over fire air flows62,63to the oxygen lean areas44,46.

The furnace ofFIG. 8, is grate furnace having grate70on to which fuel is supplied through fuel supply ports72. Primary air ports71are provided under the grate70and secondary and tertiary air ports72,73and74,75are provided to the first and second wall4,6respectively. The tertiary air ports76,75are provided in vertical direction above the secondary air ports72,73. The furnace could also be bubbling fluidized bed furnace in which the grate70is replaced with fluidized bed.

It should be noted that in some embodiments of a grate furnace, the secondary and tertiary air ports72,73and74,75may be omitted and there is only primary air ports72and the over fire air arrangement.

This configuration of the over fire air arrangement may be utilized when there are at least one first primary over fire air port20and at least one first secondary over fire air port22provided to the first wall4and at least one second primary over fire air port21and at least one second secondary over fire air port23provided to the second wall6. This enables increasing the height of the additional over fire air supply to the first and second oxygen lean over fire air areas44,46, as shown inFIGS. 9A, and 9B.

The first primary and secondary and second primary and secondary additional over fire air ports50,51,52,53may be arranged such that both the first primary and secondary and second primary and secondary additional over fire air ports50,51,52,53are in vertical direction above the first and second over fire air ports20,21,22,23(not shown) or below the first and second secondary over fire air ports22,23, as inFIG. 10C. In the embodiment ofFIG. 10C, the first and second primary additional over fire air ports50,51are at the first distance z from the first and second side walls4,6, respectively. The first and second secondary additional over fire air ports52,53are at the fourth distance q from the first and second side walls4,6, respectively. The fourth distance q is greater than the distance z and both the first distance z and fourth distance q are smaller than the fifth distance w between the vertical centre line A-A′ of the furnace1and the first and second secondary additional over fire air ports52,53.

The a fourth distance q between the first and second secondary additional air port52,53and the first and second wall4,6, respectively, is smaller than or equal to one third of a third distance x between the a vertical centre line A-A′ of the furnace enclosure2and the first and second wall4,6, respectively. In an alternative embodiment the first and second secondary additional over fire air ports52,53are arranged such that the fourth distance q between the first and second secondary additional air port52,53and the first and second wall4,6, respectively, is smaller than 2.0 m, or preferably the first distance z is between 0.3-1.0 m.

FIGS. 10A and 10Bshow different kinds of over fire air arrangement according to the over fire arrangement ofFIGS. 8, 9A and 9B. All the alternatives are in the scope of the present invention.

FIG. 10Ashows an embodiment in which the first and second primary additional over fire air ports50,51are in vertical direction above the first and second over fire air ports20,21and between the first and second over fire air ports20,21and the first and second secondary over fire air ports22,23. The first and second secondary additional over fire air ports52,53are in vertical direction above the first and second secondary over fire air ports22,23.

FIG. 10Bshows an embodiment of the over fire air arrangement in which the first and second primary additional over fire air ports50,51are in vertical direction below the first and second primary over fire air ports20,21, and the first and second secondary additional over fire air ports52,53are in vertical direction between the first and second primary over fire air ports20,21and the first and second secondary over fire air ports22,23.

In an alternative embodiment the both the first primary and secondary and second primary and secondary additional over fire air ports50,51,52,53may be in vertical direction between the first and second primary over fire air ports20,21and the first and second secondary over fire air ports22,23. In another alternative embodiment the first and second primary additional over fire air ports50,51may be in vertical direction between the first and second primary over fire air ports20,21and the first and second secondary over fire air ports22,23, and the first and second secondary additional over fire air ports52,53are in vertical direction above the first and second secondary over fire air ports22,23.

When the over fire air arrangement comprises the first primary and secondary and second primary and secondary additional over fire air ports50,51,52,53, the momentum of the additional over fire air flows62,63of the first and second secondary over fire air ports52,53is lower than the momentum of the additional over fire air flows60,61of the first and second secondary over fire air ports50,51, arranged below the first and second secondary over fire air ports52,53. Momentum means product of mass flow of the air and air velocity from the additional over fire air ports.

The present invention further provides a furnace1for combustion of fuels. The furnace comprises a bottom3, first and second opposing walls4,6extending in vertical direction from the bottom3and first and second opposing side walls5,7between the first and second opposing walls4,6. The first and second opposing walls4,6form together with the first and second opposing side walls5,7a furnace enclosure2. The furnace1further comprises a lower furnace100comprising

a) at least one primary combustion air supply port71and at least one secondary combustion air supply port74,73in vertical direction above the at least one primary combustion air supply port71for providing staged combustion, the at least one primary combustion air supply port71and the at least one secondary combustion air supply port74,73being arranged to the first wall4or the first and second wall4,6, or

b) at least one primary burner14,15and at least one secondary burner12,13in vertical direction above the at least one primary burner14,15for providing staged combustion, the at least one primary burner14,15and the at least one secondary burner12,13being arranged to the first wall4or the first and second wall4,6.

The furnace1further comprises an the over fire arrangement according to present invention. The furnace1may be a bubbling fluidized bed furnace, a grate furnace or a pulverized fuel furnace, such as pulverized coal or peat furnace.

The present invention also provides a method for supplying over fire air into a furnace1. The method comprises supplying a first over fire air flow40from the first wall4into the furnace1and supplying a first additional over fire air flow60from at least one of the first and second side walls5,7in the vicinity of the first wall4and transversely to the first over fire air flow40.

In an alternative embodiment the method further comprises supplying a second over fire air flow41from the second wall6into the furnace1and supplying a second additional over fire air flow61from at least one of the first and second side walls5,7in the vicinity of the second wall4and transversely to the second over fire air flow41.

The first and second additional over fire air flows60,61may be supplied along the first and second walls4,6respectively, and/or parallel the first and second walls4,6respectively. In one embodiment the first and second additional over fire air flows60,61may be supplied perpendicularly to the first and second over fire air flows40,41. In another embodiment the first and second additional over fire air flows60,61from the first and second side walls5,7, respectively, at a first distance z from the first and second walls4,6, respectively. The first distance z is smaller than a second distance y between the first and second additional over fire air flows60,61and a vertical centre line A-A′ of the furnace enclosure2.

The method also comprises supplying 10-35%, preferably 20 to 35%, of total amount of over fire air with the first and second additional over fire air flows60,61.