EP1116915A1

EP1116915A1 - Boiler and method for conducting gases in a boiler

Info

Publication number: EP1116915A1
Application number: EP01660005A
Authority: EP
Inventors: Raimo Airola
Original assignee: Kaukora Oy
Current assignee: Kaukora Oy
Priority date: 2000-01-14
Filing date: 2001-01-12
Publication date: 2001-07-18
Also published as: FI20000071A0

Abstract

The present invention relates to a boiler (1), especially, to a boiler with undergrate air firing having a water space (4) for water to be heated and a furnace in indirect heat exchange contact therewith, divided in the flow direction of the combustion gases by a partition wall (12) into a primary (2) and a secondary (3) combustion chamber. The primary combustion chamber (2) is provided with a feeding opening (8) for primary air and a filling opening (7) for wood, and for secondary air an inlet opening (17, 20, 46) is provided at a location (15), where the primary and secondary combustion chambers (2 and 3) are joined at their lower parts. According to the invention at said junction (15) at the bottom level (10) of the primary combustion chamber (2), a trough (16) transverse to the flow direction of the combustion gases is provided. The partition wall (12) extends down to the bottom level (10). The invention also relates to a method for conducting the gases in a boiler (1) according to the invention.

Description

The present invention relates to a boiler and, especially, to a boiler of the type having a water space and a furnace, which is divided by a partition wall into a primary and a secondary combustion chamber in the flow direction of the combustion gases, the primary combustion chamber thereby having a feeding opening for primary air, and a filling opening for fuel, and for secondary air at least one inlet opening is provided at a location where the primary and secondary combustion chambers are joined at their lower parts. This type of boiler is generally referred to as boiler with undergrate air firing. The invention also relates to a method for conducting the gases in a boiler.
In boilers with undergrate air firing, as in boilers in general, as good a combustion result as possible is attempted to be attained, as well as minimizing the amount of carbon monoxide forming e.g. soot contained in the exiting flue gases. The combustion process has been tried make more effective by feeding supplementary air, so-called secondary air into the furnace, and especially to the junction of the primary and secondary combustion chambers in the bottom of the furnace. It has become apparent, however, that the selection the feeding point for secondary air must be done very carefully, and that even slight changes in the conditions may call for shifting the feeding point, which practically, nevertheless, is neither easy nor even always feasible.
For clean and effective combustion, it is necessary that the secondary and primary gases be mixed as thoroughly as possible. In prior art boilers, however, the problem is often in ensuring a turbulent flow necessary for intense mixing.
From the Danish patent publication DK 148087, a boiler is known where the gases exiting from the primary combustion chamber are conducted along a partition wall formed as a gutter transverse to the flow direction of the gases.
The object of such proceeding is to increase the heat transfer to the secondary gas to be heated, flowing on the other side of the partition wall.
It also is known to lead gases coming from the primary combustion chamber, by arranging a trough in the flow direction thereof, first away from the secondary combustion chamber, and then from beneath the trough into the secondary combustion chamber. The purpose of this solution is to intensify the mixing of secondary combustion air with combustion gases.
Thus, the object of the present invention is to provide a boiler of the type mentioned in the preamble, i.e. a so-called boiler with undergrate air firing, in which mixing of supplementary air, i.e. secondary combustion air, with the combustion gases is more effective than previously, and in which the feeding point of secondary combustion air is no way critical but, in the contrary, insensitive to changes of the conditions.
The object is especially to provide a boiler with undergrate air firing having a trough for guiding the combustion gases from the primary combustion chamber of the boiler with undergrate air firing to the secondary combustion chamber of the same so that, under all conditions, the supplementary air is efficiently mixed with the combustion gases. In particular, the object is to effect sufficient turbulence in the gas flow in the boiler.
The drawbacks mentioned above are eliminated or minimized, and the objects defined above are achieved by a boiler and a method according to the invention, characterized in what is defined in the preamble of the independent claims presented hereinafter.
Thus, in a boiler according to the invention, in the junction of the primary and secondary combustion chambers, at the bottom level of the primary combustion chamber, a trough transverse to the flow direction of the combustion gases is provided directing gases flowing from the primary combustion chamber at first downwards, and immediately thereafter upwards into the secondary combustion chamber. By this rapid and abrupt change of direction, a strong turbulence is created intensely mixing the combustion gases and the secondary air supplied thereto, so that the carbon monoxide present in the combustion gases is combusted in the secondary combustion chamber. Thus, the feeding point of secondary air is in no way critical, and a good combustion result also is achieved even under varying combustion conditions.
In a boiler according to the invention, the wall between the primary and secondary combustion chambers extends down to the bottom level or grate level of the primary combustion chamber. In this case the trough is parallel with the partition wall directly below it, e.g., half on each side of the partition wall. Such a partition wall extending down to the bottom level of the primary combustion chamber or below it, exposes gases flowing in the trough or the like to turbulence necessary for their mixing.
In a method according to the invention for conducting the gases in a boiler, the combustion gases passing through the inventive trough are directed to flow below the bottom level of the primary combustion chamber. The wall between the primary and secondary combustion chambers forces the gases to pass through the trough thereby intensifying mixing of the gases. The method according to the invention can, of course, be applied and varied in accordance with the various embodiments of the boiler as mentioned in the text.
Secondary air can be fed in different ways directly into the trough. In a preferred embodiment of the invention, the section of the trough beneath the primary combustion chamber is provided with holes or openings, and under the bottom or grate of the primary combustion chamber, a feeding channel for secondary air is provided for secondary air to flow through the holes or openings in its partition walls into the trough. In an alternative embodiment of the invention, the feeding pipe for secondary air opens on to the trough or a grate section thereabove. The feeding pipe for secondary air can be led to the trough or the grate above it, e.g., through a wall or the bottom of the boiler. There may be several feeding pipes. Alternatively, one substantially horizontal pipe may be used having in the longitudinal direction thereof spaced holes or openings allowing an equal feeding of secondary air into the trough along the whole length thereof. Such a secondary air feeding pipe may also simultaneously function as a part of the grate preventing solid fuel from passing through the opening of the grate.
The supply of air into the furnace is advantageously intensified by a fan allowing efficient feeding of primary and secondary air. Alternatively or in addition, the removal of flues gases from the secondary combustion chamber may be intensified by an exhaust fan arranged in the discharge opening thereof. When using a fan and/or an exhaust fan, the size of the secondary air feeding opening or openings may be reduced, as a sufficient air flow now can be ensured even through a smaller opening. At the same time the turbulence in the trough is intensified, which in turn improves the combustion result.
One half of the curved wall of the trough used in the boiler and, in this case, in the boiler with undergrate air firing, is advantageously perforated allowing secondary air to flow through the holes to the combustion gases flowing into the secondary combustion chamber. The trough may be replaceable being thereby advantageously closed at the ends by plates supporting and carrying the trough. This solution results in a self-bearing construction easily removable and arranged in a boiler with undergrate air firing. In this case the trough has advantageously a semicircular shaped cross-section, and the edges of the plates may be bent in order to stiffen the construction. In the immediate vicinity of the other upper edge of the trough, the holes are preferably successively. The trough is advantageously made of steel or ceramics, or some other material resistant to the temperatures prevailing in the boiler.
In the boiler, rails or corresponding fastening means of the trough have been mounted advantageously for fastening the trough according to the invention thereon. Such fastening means have been advantageously arranged to allow an easy detachment of the trough from the boiler. Thus, the trough according to the invention may be used as an ash pan allowing an easy removal of ashes from the boiler accumulated in the boiler. The trough can be removed, e.g., through a hatch in the front wall or sidewall of the boiler. By the easy removal of the trough from the boiler, maintenance and checking of the condition of the boiler are facilitated.
The invention is disclosed hereinafter exemplary with reference to the accompanying drawings of which
Fig. 1 shows in cross-section an elevational view of a preferred embodiment of the invention seen from one side,
Fig. 2 shows a perspective view of the trough construction in a boiler with undergrate air firing, seen from Fig. 1,
Fig. 3 shows a sectional elevation of an alternative embodiment of the invention,
Fig. 4 shows a similar view of a third embodiment of the invention,
Fig. 5 shows a sectional elevation of the lower part of the boiler in a fourth preferred embodiment of the invention,
Fig. 6 shows a section A-A of Fig. 5, and
Fig. 7 shows a sectional elevation of the lower part of a further boiler according to the invention.
A boiler with so-called undergrate air firing generally indicated by the reference number 1, is shown in Fig. 1. It consists of a furnace 2, 3, and a water space enclosing it in a jacket-like manner. The furnace 2, 3, and the water space 4 enclosing it in a jacket-manner are in a heat exchange contact, so that heat is conducted indirectly to the water space 4 by the combustion gases in the furnace. Additionally, seen in the flow direction of the combustion gases, the furnace is divided in to two successive sections, namely a primary combustion chamber 2 and a secondary combustion chamber 3, separated from each other by a partition wall 12 extending from the upper wall 26 of the primary combustion chamber 2 down to the bottom 10 level of the primary combustion chamber. In the rear part of the bottom 10, a grate 11 is provided for allowing the hot combustion gases to exit from the primary combustion chamber 2 into the secondary combustion chamber 3.
The water space 4 is heat insulated by a heat insulation layer 25, and in the upper part of the water space are outlet 5 for heated water, and at a lower level an inlet 6 for water to be heated provided. The wall in the lower part of the secondary combustion chamber 3, facing the water space, is lined with masonry 29 so as to prevent hot combustion gases from getting in direct heat exchange contact with the water space wall. Thus, excessive heat transfer from combustion gases to water is prevented.
For example, hot water exiting from boiler 1 can be directed to a hot water accumulator, wherefrom water is further led, e.g., to a radiator network. It also is possible to lead hot water from boiler 1 directly to the radiator network. In the situations mentioned above, the water to be heated entering boiler 1 is recirculated from said network or accumulator.
In the upper front part of the primary combustion chamber 2, a hatch 7 is provided which can be opened, for the adding of solid fuel like wood, briquettes, peat, coal or anthracite into the primary combustion chamber. Furthermore, the filling hatch 7 is provided with a damper 8 for combustion air. In the lower front part of the primary combustion chamber 2, there is yet another hatch, a so-called poking hatch 33 likewise provided with a damper 9 for supplementary, i.e., secondary air. Via the poking hatch 33 it is possible to poke the fire in the primary combustion chamber 2, to clean the grate 11 as well as the bottom 10 of the primary combustion chamber 2, and to remove ashes.
Through the damper 9 of the poking hatch 33, combustion air can flow into the primary combustion chamber 2 via passage 27 between the bottom 10 thereof and the poking hatch 33, to be used as primary air, and into the channel 14 below the bottom 10 as well, to be used as secondary combustion air.
According to the present invention, in the passage 15 below the partition wall 12 between the primary 2 and secondary combustion chambers 3, a transverse trough 16 is provided for reversing the downwards flow of combustion gases from the primary combustion chamber 2 through the opening of grate 11, i.e. to be an upwards flow directed to the secondary combustion chamber 3. Due to this rapid and abrupt change of direction, a highly intense turbulence is created in the trough 16. In the section of the trough 16 facing the secondary air duct 14, holes 17 are provided allowing supplementary air, that flowed in through damper 9, to flow into the trough 16, and, due to the intense turbulence therein, to be intensely mixed with combustion gases coming from the primary combustion chamber 2, so that the residual carbon monoxide still contained therein to combust to carbon dioxide in the secondary combustion chamber 3, the flue gases being removed through a discharge opening 13 in the upper part thereof. According to the solution of the invention, these flue gases only contain very little carbon monoxide (CO).
Thus, the trough 16 according to the invention is located at the junction between the primary 2 and secondary combustion chambers 3 in the passage 15, at the bottom 10 level of the primary combustion chamber 2 transverse to the flow direction of the gases, and approximately half on each side of the partition wall 12 above it.
The trough 16 used in the boiler with undergrate air firing is shown in Fig. 2, and it can be seen therefrom that the trough construction 21 consists of a trough having a substantially semicircular shaped cross-section, the ends of which being provided with rectangular plates 22 supporting and carrying the trough 16. The upper edges 24 of the plates 22 are bent so as to stiffen the trough construction 21. In the vicinity of one upper edge 23 of the curved trough 16, holes 17 are provided in a horizontal plane successively, for allowing secondary air to join combustion gases flowing from the primary combustion chamber into the secondary combustion chamber, and, which combustion gases are in a very turbulent state due to the trough, thereby secondary air is intensely mixed therewith. The plates 22 are not necessarily rectangular in form, and in the trough, e.g., in its end plate 22, a hatch or an opening may be formed for cleaning of the trough, e.g., for the removal of ashes.
The trough 16, however, can be completely unperforated, secondary air being thereby led into the trough 16 via the pipe 20 arranged through the sidewall 30 of the boiler with undergrate air firing, as can be seen in greater detail from Fig. 3. The pipe 20 opens on to a location above the trough between the bottom 10 of primary combustion chamber 2 and the partition wall 12. At the same time, such a pipe 20 can function as a part of the grate preventing fuel from falling into the trough 16. In order to intensify the flow of combustion gases at this location, the dampers for supplementary air have been replaced by an opening 28 in the front wall of the primary combustion chamber 2, and by an exhaust fan 19 arranged in a discharge opening 13 in the upper part of the secondary combustion chamber 3, in which case the diameter of the pipe 20 can be smaller than when using natural draught, since by means of the exhaust fan 19, a higher pressure difference is achieved between the combustion air inlet opening 28 and the flue gas discharge opening 13. For maintenance and cleaning reasons, the pipe 20 has advantageously been arranged to be detachable.
The embodiment shown in Fig. 4 differs from the previous one in so far as the exhaust fan in the discharge opening 13 of the secondary combustion chamber 3 has been replaced by a fan 18 arranged in the inlet opening 28 of combustion air. Via the pipe 32, secondary air is led from the pressure chamber 31 of the fan outside the boiler and through the sidewall 30 into the pipe 20. Broken lines illustrate the pipe 32.
In an advantageous embodiment shown in Figs. 5 and 6, in the lower part of the boiler with undergrate air firing 1 according to the invention, substantially horizontal rails 40 have been arranged, on which the trough 16 has been mounted. The bent upper edges 24 of the side plates 22 have been arranged in the grooves of the rails 40 allowing the trough to be moved parallel with the rails. The trough 16 can thus be easily removed from the boiler 1, e.g., for the removal of ashes and maintenance of the inner parts of the boiler. In Fig. 5 broken lines illustrate the trough 16 as having been moved between the different positions. In the example of the figures the rails 40 are mounted on the upper edge of the secondary air duct 14, that is, in the masonry of the bottom 10 of the primary combustion chamber, in the partition wall 12 and in the rear wall of the secondary combustion chamber 3. The trough 16 is removed from the boiler through the poking hatch 33. A dismantling opening of the trough also could be made in the sidewall 30 of the boiler. In the example of Fig. 5, the rails 40 have been provided with a groove 42 for detachable fastening of the edge of trough 16. By such fastening means, fixing of the trough 16 is ensured when operating the boiler 1. Secondary air is led from the secondary air duct 14 into the trough 16 via slot 46 remaining between its bent front edge 23 and the masonry of the bottom 10 of primary combustion chamber.
As can be seen in Fig. 6, the bent upper edges 24 of the trough fit within the rails 40. In Fig. 6 also the bent front edge 23 of the trough, the edges 35 of the passage 27 between the bottom 10 and the poking hatch 33, as well as the foot 37 of the boiler are visible.
Fig. 7 shows an advantageous boiler according to the invention mainly corresponding, as far the structure is concerned, to the boiler illustrated in Figs. 5 and 6. Compared with the boiler of Fig. 5, the difference is that the gap between partition wall 12 and the bottom 10, from which gap the primary gases flow into the trough 16, has been moved at a distance from the partition wall 12 by providing the partition wall 12 with additional piece of brickwork 44. Furthermore, compared with the boiler of Figs. 5 and 6, the trough 16 has been stretched a distance corresponding to the length of the additional piece 44.
It is clear that the invention may be widely varied within the scope defined by the appended claims. For example, secondary air could also be introduced into the boiler via other than the routes described above, for example, through the lower bottom of the boiler, or channels formed in the bottom masonry. It is essential, however, that the combustion gases at the junction of the primary and secondary combustion chambers are exposed to intense turbulence, additional air being simultaneously introduced into the combustion gases. The turbulence is achieved by a trough located at said junction transverse to the flow direction of the gases, exposing the mixture of combustion gases and supplementary air to an instant and abrupt change of direction having a strong mixing effect. Thus, the feeding point of supplementary air is neither critical nor dependent on the combustion conditions.

Claims

A boiler (1) having a water space (4) for water to be heated, and a furnace in indirect heat exchange contact therewith, divided in the flow direction of the combustion gases by a partition wall (12) into a primary (2) and a secondary (3) combustion chamber, said water space (4) having an inlet opening (5) for water and an outlet opening (6) for water, which primary combustion chamber (2) of said furnace is provided with one or more feeding openings (8) for primary air and a filling opening (7) for fuel, and for secondary air, at least one inlet opening (17, 20, 46) is provided at a location (15), where the primary and secondary combustion chambers (2 and 3) are joined at their lower parts, said secondary combustion chamber (3) additionally having a discharge opening (13) for flue gases, and at said junction (15) of the primary and secondary combustion chambers (2 and 3), at the bottom level (10) of the primary combustion chamber, a trough (16), gutter or a corresponding recess transverse to the flow direction of the combustion gases is provided, characterized in that the partition wall (12) extends down to the bottom level (10) of the primary combustion chamber (2).
A boiler according to claim 1, characterized in that said trough (16) is disposed in parallel with, and below the partition wall (12) and advantageously approximately equally on each side of the primary (2) and the secondary combustion chambers (3).
A boiler according to claim 1 or 2, characterized in that the inlet opening or openings (17, 20, 46) for secondary air open on to said trough (16).
A boiler according to claim 3, characterized in that the wall section of the trough (16) below the primary combustion chamber (2) is provided with openings (17) allowing the trough to be connected with a secondary air feeding channel (14) below the bottom (10) of the primary combustion chamber.
A boiler according to claim 3, characterized by a secondary air feeding pipe (20) extending to the trough (16) or to a grate (11) provided possibly above the trough.
A boiler according to any of the previous claims, characterized by a fan (18) for blowing primary and possibly secondary air into the furnace.
A boiler according to any of the previous claims, characterized by a suction device (19) in the flue gas discharge opening (13) for removing the flue gases from the secondary combustion chamber (3).
A boiler according to any of the previous claims, characterized by that fastening means (40, 42) of the trough are provided in said boiler (1), the trough (16) being detachably arranged on the fastening means (40, 42).
A boiler according to claim 8, characterized by that the fastening means (40, 42) of the trough comprise rails (40) or the like allowing the trough (16) to be moved from and into the boiler so as to enable maintenance of the trough (16) and the inner part of the boiler (1).
A method for conducting gases in a boiler (1) divided in the flow direction of the combustion gases by a partition wall (12) into a primary (2) and a secondary combustion chamber (3), which are joined at their lower parts at a location (15), at which location (15) a trough (16), gutter or a corresponding recess transverse to the flow direction of the combustion gases is provided through which trough or the like combustion gases are conducted from the primary combustion chamber (2) into the secondary combustion chamber (3), and at which junction (15) of the primary and secondary combustion chambers (2 and 3) secondary air is mixed with primary gases, characterized in that the combustion gases passing through the trough (16) are directed to flow below the bottom level (10) of the primary combustion chamber (2).