WO2014083504A2

WO2014083504A2 - Post combustor

Info

Publication number: WO2014083504A2
Application number: PCT/IB2013/060425
Authority: WO
Inventors: Cesar ROSAS ZUMELZU; Valeri BUBNOVICH; Roberto SANTANDER MOYA; Nelson MORAGA BENAVIDES
Original assignee: Universidad De Santiago De Chile
Priority date: 2012-11-30
Filing date: 2013-11-26
Publication date: 2014-06-05
Also published as: WO2014083504A3; CL2012003372A1

Abstract

A post combustor (1) which allows increasing the thermal efficiency and reducing the emission of pollutants in firewood heaters or combusting equipment (19) having a combustion chamber or primary chamber (22) for burning firewood (21), thus generating combustion gases (17). Said post combustor (1) comprising: a plurality of layers (2) consisting of solid particles which generate a porous medium wherein said plurality of layers (2) of solid particles that generate said porous medium are contained in an inner container (5). The inner container (5) has at least one lower opening allowing the passage of entering gases (6) which have already been burned in the combustion chamber or primary chamber (22) and has at least one upper opening allowing the passage of exhaust gases (7) which have already been burned in the post combustor (1). The inner container (5) is surrounded by a tubular duct (10) creating an annular space (16) between the outer wall (18) of said inner container (5) and the tubular duct (10), said annular space (16) having a lower annular portion (11) for the combustion gases (17) to enter, having on the upper part of the annular space (16) a gas shut-off valve (12) which is handled by regulation means (13). On the upper portion of the annular space (16) is found a conical cap (14) to which the gas exhaust duct (15) is connected.

Description

POST COMBUSTOR

TECHNICAL FIELD OF THE INVENTION

The present invention relates to a porous post combustor to be incorporated in firewood burning stoves or heaters which allow burning the gases generated in the primary combustion, which in a conventional combustor are directly released to the atmosphere. The aforementioned allows reducing pollutants such as particulate matter and carbon monoxide and an increase with regard to the thermal efficiency. Basically, the post combustor is formed by a plurality of layers consisting of solid particles which generate an inert porous medium wherein the diameter of particles changes from one layer to the other from the lower end towards the upper end.

This post combustor is installed in the first portion of the outlet duct or else, directly on the combustion chamber in such a way that the particles are heated and the gas passing through the interstices generated by said particles, ends up being burned.

BACKGROUND OF THE INVENTION

The environmental pollution is one of the main concerns at a global level. In emerging and developing countries, the firewood is used as fuel for home heating. However, this implies a high level of impact with regard to the environmental pollution, mainly due to the particulate matter being released to the atmosphere as well as the incomplete combustion generated in those heaters which only have primary combustion chambers.

For example, the firewood consumption in Chile since 2008 to date has increased from 12 to 15 million cubic meters per year, its participation in the primary energy matrix has resulted in an increase from 1 6% to 20%. Among the types of fuels used for home heating, firewood is the cheapest one.

Also, the environmental pollution episodes due to the firewood burning, which are normal in winter season in many regions of Chile, make necessary to adopt measures in order to solve this problem, among which undoubtedly is the use of more efficient and less polluting technologies which allow a sustainable use of this energy source, which due to its low cost is used by a high number of people, particularly, of low socioeconomic status, both from the urban and rural areas.

There have been several attempts in the state of art in order to improve the firewood combustion by using secondary combustion chambers or post combustors.

Thus, for instance, document CA 1 198641 discloses a stove with a catalytic wood combustor having a combustor ignitor plate which produces turbulence and a mixture of combustion gases and air from a primary air supply to feed a completely mixed gas to the catalytic combustor of the firebox at a low temperature within a range of 204 °C (400 °F) to 260 °C (500 °F), but operate the combustor at a high temperature above 538 °C (1000°F) to 649^0 (1200°F) or higher.

Document EP 0483878 discloses a combustion device for wood and coal, consisting of a front wall with fuel charging opening, two side walls, a rear wall, a bottom wall, a top wall, a door for the fuel charging opening, a control element for controlling a fresh-air inlet opening, an inner partition which divides the inner device into a precombustion chamber and a combustion chamber having a passage opening between the two chambers, and a grating on the upper portion of the post combustion chamber and a smoke gas take-off. In this connection, the partition is horizontally arranged and inserted towards its passage opening in a sealed manner wherein there is a shaft pipe surrounded by an annular space which is connected to the air-fresh inlet opening. The grating lies on the shaft pipe. The device disclosed in this document, has a contractive simple design and at the same time optimal combustion conditions.

The partition consists essentially of a grating and a circumferential duct pipe with an inner annular space. To form the grating, tubes can be used, which are connected to the annular space and have slots which are laterally directed, possibly towards the neighboring pipes.

Document US 4319556 discloses a wood burning stove having a primary and secondary combustion chamber with a catalytic converter therein. The primary air is introduced into said primary chamber and the secondary air is introduced into said secondary chamber, using completely separated and independent control means for said primary air and secondary air, being located below the fuel support level (wood).

Means are provided for preheating both the secondary air and the catalytic converter.

Document US 4363785 discloses a wood burning stove which is formed by double front and rear walls of heat conductive metal spaced apart by heat conductive spacer fins and providing air passageways by which room air is heated by conduction from the walls which are heated by the wood burner deposited on a firebox floor supported in heat conducting relationship with the inner side walls.

A catalytic converter is disposed over the fire area in the upper portion of the stove, and is arranged to receive preheated fresh secondary air which mixes with hot, incompletely combusted compounds from the fire and, in the presence of the catalyst, induces a secondary combustion of the substances. This mixture is channeled into a heat extraction chamber where the secondary combustion is completed and the resultant heat is transferred to the metal body of the stove. An exhaust passageway is provided for releasing the products of complete combustion into the atmosphere.

Document CA 1231014 discloses a heating stove comprising: a combustion chamber, a grate therein, upper and lower combustion air inlets to provide a bottom air feed upwardly through fuel from below the grate, and downwardly through fuel from above the fuel, a heat exchanger consisting of a series of vertically oriented horizontally spaced metal plates interconnected so as to form two sets of flow passages, one vertical set for the products of combustion the other horizontal set for conducting a flow of air to be heated. Passageways above and below said series of plates, each passageway connected to the combustion chamber, two flow control adjustable baffles above the plates in top passageway, one centrally of the series of plates, the other controlling flow from the combustion chamber, an outlet connecting the rear end of the top passageway to a gas duct or chimney. The previously described documents disclose several secondary chambers, many of them having diverse constitutive elements and control systems which generate expensive and not very competitive solutions for the current demanding conditions of the market.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a porous post combustor to be incorporated in firewood burning stoves or heaters which allow burning gases generated in the primary combustion of firewood.

The post combustor consists of a porous inert material which is located in a steel container resisting the temperatures generated in a firewood heater.

The porous material is formed by solid particles gathered in layers generating interstices such as the porous medium.

The porosity, the diameter of the particles in the porous medium, the entering velocity of gases, the entering temperature of gases and the percentage of surplus air used in the secondary combustion vary according to the layers of solid particles.

The present invention allows that the use of an inert porous medium to perform the post combustion of gases generated by the primary combustion of firewood and derivative compounds thereof makes possible the increase of thermal efficiency and the reduction of the emission of pollutants in firewood combusting equipment which have this device incorporated therein with respect to the conventional technology.

Among the different materials and available geometries for the porous medium, spheres are considered as an appropriate material (from 3 to 6 mm in diameter), or alumina sponges. To configure the present invention, different types of materials are considered for the body of the combustor.

The materials to be used must be capable of resisting high temperatures such as the ones developed in this kind of combustors of around of 1 100°C.

BRIEF DESCRIPTION OF THE FIGURES

The enclosed figures are included in order to provide a better understanding of the invention, they constitute part of this specification and also illustrate part of the previous art and some of the preferred embodiments to explain the principles of this invention.

Figure 1 shows a schematic perspective view of one of the embodiments of the post combustor of a firewood heater.

Figure 2 shows a schematic perspective view of one of the embodiments of the post combustor of a firewood heater and the container thereof.

Figure 3 shows a frontal view of the inner container and cylindrical duct which generate an annular space for the circulation of air in the post combustor of a firewood heater.

Figure 4 shows a frontal cut-away view of the inner container and outer cylindrical duct which generate an annular space for the air circulation in the post combustor of a firewood heater.

Figure 5 shows a frontal cut-away view of the inner container and cylindrical duct, circulating combustion gases through the annular ring in the post combustor of the firewood heater in the start-up stage o in the pre-heating stage of the post combustor.

Figure 6 shows a frontal cut-away view of the inner container and cylindrical duct, circulating combustion gases through the porous medium in the post combustor of the firewood heater during the performance in normal operating conditions.

Figure 7 shows a sectional schematic view of a first embodiment of the post combustor, installed in a firewood heater. Figure 8 shows a sectional schematic view of a second embodiment of the post combustor, installed in a firewood heater.

DESCRIPTION OF THE INVENTION

The present invention relates to a porous post combustor to be incorporated in firewood burning stoves, combustors or heaters which allow combusting gases generated in the primary combustion of firewood.

The post combustor (1 ) is formed by a plurality of layers (2) consisting of solid particles which generate an inert porous medium wherein the diameter of particles changes from one layer to the other from the lower end (3) of the post combustor (1 ) towards the upper end (4) of the post combustor (1 ).

The diameter and porosity of the particles in the porous medium, the entering velocity of gases, the entering temperature of gases and the percentage of surplus air used in the secondary combustion vary according to the layers of solid particles.

These parameters should be determined, as appropriate, when considering the case and design of the firewood heater or combustor, and they could be even determined through experimentation.

The porous medium can be formed by different materials and available geometries and as an appropriate material are considered spheres (3 to 6 mm in diameter) or alumina sponges. Different types of materials are considered for the body of the combustor. Among the materials to be used is, for example, rolled steel, said materials must be capable of resisting the high temperatures normally developed in this type of combustors which are in the range of 1 100°C.

The plurality of layers (2) formed by solid particles generating an inert porous medium are contained in an inner container (5) which can be made of iron, steel or another similar material that resists the temperatures generated within the combustion chamber.

This inner container (5) has at least one lower opening which allows the passage of entering gases (6) that have already been burned in the combustion chamber or primary chamber.

Similarly, the inner container (5) has at least one upper opening allowing the passage of exhaust gases (7) which were post burned in the post combustor (1 ).

According to what has been shown in figures 3 and 4, the post combustor (1 ) is contained within the inner container (5) which has lower perforated fastening means (8) and upper perforated means (9), which can comprise perforated steel sheets

The inner container (5) is surrounded by a tubular duct (10) creating an annular space (1 6) between the outer wall (1 8) of said inner container (5) and the tubular duct (10). The lower annular portion (1 1 ) of the annular space (1 6) is open for the combustion gases to enter (17). On the upper part of the annular space (1 6) a shut-off valve for the passage of gases is installed, formed preferably by a pair of perforated rings one of which is fixed and the other is movable.

The opening, regulation or closing of the air shut-off valve (12) is handled by regulation means (13) and the function of which is defining the flow path of gases.

According to what has been shown in figure 5, when the gas shut-off valve (12) is open, the combustion gas flow (17) enters through the lower annular portion (1 1 ) of the annular space (1 6) to heat the post combustor (1 ), thus continuing towards the upper portion of the annular space (1 6) through the interstices (not shown) of the gas valve (12) towards the upper part wherein there is a conical cap (14) to which the gas exhaust duct (15) is connected so as said gases can be released to the atmosphere.

It should be observed that when combustion gases (17) pass through the annular space (16) they have not been purified nor filtrated yet.

According to what has been shown in figure 6, when the gas shut-off valve (12) is closed, the gas flow (17) cannot circulate through the annular space (1 6) and forcedly must pass through the porous layers (2) of the post combustor (1 ), thus getting a second combustion wherein the particles contained in the gas flow (17) are filtrated and said gas flow (17) gets out purified through the gas exhaust duct (15).

The passage of the combustion gas flow (17) through the annular space (1 6) allows during the start-up stage of the heater or combustor, heating the post combustor (1 ) until it reaches the necessary functioning temperature in normal operating conditions which is in the range of 1 100 °C.

This temperature can be monitored with a thermometer or, in a practical way, the gas shut-off valve (12) can be closed when performing the second firewood loading (21 ). When said temperature is reached, the gas shut-off valve (12) is closed and the combustion gas flow (17) passes through the post combustor (1 ), thus achieving a second gas combustion (17) and, therefore also the performance, in operation, of the firewood combustor or heater.

In a first embodiment, as it is shown in figure 7, the post combustor (1 ) is installed in a firewood heater or combustor (19). The air enters through outlet conducts (20) through the lower part of the firewood combustor or heater (19) which allows to burn the firewood (21 ) within the combustion chamber or primary chamber (22). The combustion gases (17) enter to the post combustor (1 ) that is installed on the lower portion (23) of the chimney or gas exhaust duct (15).

The heat generated in the combustion chamber (22) and by the same hot combustion gases (17) within said chamber, allows the plurality of solid particle layers (2) of the post combustor (1 ) to be heated with which the gases passing through the interstices generated among said particles of the plurality of layers (2) end up getting burned, thus generating exhaust gases (24) towards the atmosphere with a non-existent or minimal quantity of pollutant products.

In a second embodiment, as it is shown in figure 8, the post combustor

(1 ) is installed in a firewood heater or combustor (19). The air enters through inlet conducts (20) on the lower part of the heater or combustor (19) which allows the burning of the firewood (21 ) in a combustion chamber or primary chamber (22). The combustion gases (13) enter to the post combustor (1 ) which in this embodiment is located over the combustion chamber or primary chamber (22). To that end, the post combustor (1 ) is hold by supporting means (25) which also have the function of allowing the combustion gases (17) to pass, during the start-up phase, through the annular space (1 6) and once the post combustor (1 ) is heated, said gasses can pass through it.

In this embodiment, it is also necessary to have a gas shut-off valve (12) in order to direct the flow either through the annular space (16) or the post combustor (1 ).The heat produced in the combustion chamber (22) and by the same hot combustion gases (17) allows the plurality of solid particle layers (2) to be heated with which the gases passing through the interstices generated among the particles of the plurality of layers (2) end up getting burned, thus producing exhaust gases (24) having a non-existent or minimal quantity of pollutant products. Thus, the combustion gases (17) enter to the post combustor (1 ) which is installed over the primary chamber (22) in such a way that post burned gases are directed towards the lower portion (23) of the chimney or gas exhaust duct (15) in such a way that on the upper part thereof, said gases are released to the atmosphere with a more complete combustion in comparison to the heaters of the previous art.

With the purpose of simplifying the functioning of the post combustor (1 ) when the natural draft is not enough to defeat the resistance imposed by the porous material (2) to the gas flow (17) over said post combustor (1 ), an extractor can be installed (not shown) in order to force the gas flow.

COMPARATIVE EXAMPLE OF THE RESULTS

In the following table are presented the results obtained for a heater with a post combustor (1 ) as the one of the present invention in comparison to a conventional heater taken as a basis for the study and a pellet heater of cutting edge technology.

Table:

Pollutant emissions and thermal efficiency of the conventional heater with a combustor as the one of the present invention and a pellet stove.

The analysis of the values obtained for the pollutant emissions and thermal efficiency among the heaters of the previous art and a heater with a post combustor as the one of the present invention allows inferring that: a) The heater with a porous post combustor with respect to the conventional heater reduces in a 25% the CO emission, reduces in a 13% the particulate matter emission expressed in gr/hr, reduces in a 62% the concentration of the particulate matter expressed in gr/m3 of emitted gases and increases in a 24% the thermal efficiency.

b) The pellet heater with respect to a conventional heater reduces in a 87% the CO emission, reduces in a 62.3 % the particulate matter emission expressed in gr/hr and increases in a 25% the thermal efficiency.

CONCLUSIONS:

a) The pellet heater is the one that presents the best performance with regard to all the considered parameters.

b) The heater with a porous post combustor in comparison with the conventional firewood stove presents a noticeable improvement in all the considered parameters, in which it is worth mentioning:

· -The concentration of the particulate matter expressed in gr/m3 of emitted gases that is a more representative indicator than the emission of the particulate matter expressed in gr/hr.

• -The thermal efficiency, which reaches almost the same value as the one of the pellet stove.

As a conclusion, it can be mentioned that the stove having a porous post combustor presents a more than satisfactory performance in all functioning parameters in comparison with a conventional heater.

With regard to the pellet stove, the thermal efficiency is very similar and although the pollutant emissions do not reach the same levels, it must be considered that the pellet stove has a much more complex technology and therefore it is quite expensive in comparison with the present invention.

Claims

1 . - A post combustor (1 ) which allows increasing the thermal efficiency and reducing the emission of pollutants in firewood heaters or combusting equipment (19) having a combustion chamber or primary chamber (22) for burning firewood (21 ), thus generating combustion gases (17) CHARACTERIZED in that said post combustor (1 ) comprises:

a plurality of layers (2) consisting of solid particles which generate a porous medium wherein the diameter of particles changes from one layer to the other from the lower end (3) of the post combustor (1 ) towards the upper end (4) of the post combustor (1 ), wherein said plurality of layers (2) of solid particles generating said porous medium are contained in an inner container (5);

wherein said inner container (5) has at least one lower opening allowing the passage of entering gases (6) which have already been burned in the combustion chamber or primary chamber (22) and has at least one upper opening allowing the passage of exhaust gases (7) which have already been burned in the post combustor (1 ), said inner container (5) having lower perforated fastening means (8) and upper perforated means (9) among which there is said plurality of layers (2);

wherein the inner container (5) is surrounded by a tubular duct (10) creating an annular space (1 6) between the outer wall (18) of said inner container (5) and the tubular duct (10), said annular space (1 6) having a lower annular portion (1 1 ) for the combustion gases to enter (17), having on the upper part of the annular space (16) a gas shut-off valve (12) which is handled by regulation means (13); and

wherein on the upper portion of the annular space (16) is located a conical cap (14) to which the gas exhaust duct (15) is connected.

2. - The post combustor (1 ) according to claim 1 CHARACTERIZED in that said gas shut-off valve (12) is formed by a pair of perforated rings, one of them being fixed and the other movable.

3. - The post combustor (1 ) according to claim 1 CHARACTERIZED in that said porous medium is formed by different materials and geometries such as spheres and alumina sponges.

4. - The post combustor (1 ) according to claim 3 CHARACTERIZED in that said spheres or alumina sponges have a diameter of 3.6 mm.

5. - The post combustor (1 ) according to any of the precedent claims CHARACTERIZED in that said materials resist temperatures in the range of 1400 °C.

6. - The post combustor (1 ) according to any of the precedent claims CHARACTERIZED in that said post combustor (1 ) is installed on the lower portion

(23) of the chimney or gas exhaust duct (15) of the heater of combustor (19).

7. - The post combustor (1 ) according to any of the precedent claims 1 to 5 CHARACTERIZED in that said post combustor (1 ) is installed over the combustion chamber or primary chamber (22) of the heater or combustor (19), which is hold by supporting means (25).

8. - The post combustor (1 ) according to any of the precedent claims CHARACTERIZED in that over said post combustor (1 ) can be installed an extractor.

9. - A method for operating a post combustor (1 ) which allows increasing the thermal efficiency and reducing the emission of pollutants in firewood heaters or combusting equipment (19) having a combustion chamber or primary chamber (22) for burning firewood (21 ), thus generating combustion gases (17) according to the post combustor (1 ) described in any of claims 1 to 8 CHARACTERIZED in that said post combustor (1 ) comprises the following stages:

(a) opening the gas shut-off valve (12) for the combustion gas flow

(17) to enter through the lower annular portion (1 1 ) of the annular space (1 6) to heat the post combustor (1 ), thus continuing towards the upper portion of the annular space (1 6) in direction towards the gas valve (12) and then towards the upper part wherein there is a conical cap (14) to which the gas exhaust duct (15) is connected so as said gases can be released to the atmosphere. (b) waiting until the post combustor reaches a temperature in the range of 1 100 ^QC; and

(c) closing the gas shut-off valve (12) so as the gas flow (17) can circulate through the porous layers (2) of the post combustor (1 ) thus reaching a second combustion and filtrating the particles contained in the gas flow (17) with which said gas flow (17) gets purified by the gas exhaust duct (15).

10.- The method for operating a post combustor (1 ) according to claim 9 CHARACTERIZED in that in stage (b) the temperature is monitored with a thermometer.

1 1 .- The method for operating a post combustor (1 ) according to claim

9 CHARACTERIZED in that the gas shut-off valve (12) is closed when the second firewood (21 ) loading is performed.