EP0204704A1

EP0204704A1 - Combustion apparatus for solid fuel.

Info

Publication number: EP0204704A1
Application number: EP85901118A
Authority: EP
Inventors: Tore Lennart Danielsson
Original assignee: Individual
Current assignee: Individual
Priority date: 1984-02-16
Filing date: 1985-02-15
Publication date: 1986-12-17
Also published as: DE3571496D1; FI84856C; SE8400836L; WO1985003762A1; EP0204704B1; FI84856B; FI863279A0; FI863279A; NO854103L; SE8400836D0

Abstract

Dans un appareil de combustion pour combustible solide, par exemple un poêle à porte frontale, au moins un conduit pour l'alimentation en air de combustion amène à une chambre de combustion (1). Le ou les conduits (8a-8c) débouchent dans la chambre de combustion au-dessus du foyer, alors que des dispositifs de distribution (9a-c, 10a-c, 11a-c) sont utilisés pour amener l'air de combustion à descendre vers le foyer. Les dispositifs de distribution sont situés de préférence à différents niveaux au-dessus du foyer et dirigent l'air essentiellement en direction du centre du foyer, de préférence par un écoulement d'air turbulent.In a combustion appliance for solid fuel, for example a stove with a front door, at least one duct for supplying combustion air leads to a combustion chamber (1). The duct (s) (8a-8c) open into the combustion chamber above the hearth, while distribution devices (9a-c, 10a-c, 11a-c) are used to bring the combustion air to go down to the foyer. The distribution devices are preferably located at different levels above the hearth and direct the air essentially towards the center of the hearth, preferably by a turbulent air flow.

Description

Combustion apparatus for solid fuel and a method of firing therein.

Technical Field The present invention relates to the technical field of combustion or firing apparatuses for solid fuel, e.g. wood, coal and briquettes. Particularly, but not exclusively, the invention relates to combustion apparatuses of such types as fire-places, stoves and the like. More specifically, the invention relates to an improved combustion apparatus for solid fuel and a new method of firing solid fuel therein.

Background of the Invention

Combustion apparatuses of such types as open fire-places, door-fronted stoves and the like have become more and more usual in houses, leisure dwellings and flats, not in the least due to the heavily rising prices in recent years for the conventional energy sources oil and electricity. Door-fronted stoves have become a supplement to the conventional heating systems as reserve heating sources or for energy saving, but also as a congeniality factor. The heavy increase of such solid fuel-burning combustion apparatuses has caused air pollution problems, perhaps primarily in thickly populated areas. In many countries the authorities have therefore put strict requirements on flue gas discharges from such apparatuses and also on heating boilers, or are expected to make such requirements. In Sweden, for example, attention is concentrated on the tar content in the discharged flue gases, decrease in the tar content being sought for. With a lower tar content the content of the most important remaining pollutants also decreases.

The traditional door-fronted stove comprises a hearth which may be closed by doors, through which fuel is also supplied. The doors, and possibly several of the side wails of the stove are often transparent so that the congeniality of an open fire will be felt. The stove may be insulated, provided with a heat accumulator, fan system, combustion air preheating etc. for increasing the heating efficiency.

In such traditional stoves the combustion air is often taken through a special duct from below and directly up into the fire, through a grate on which the fuel is placed. When the stove doors are closed, the hearth becomes substantially sealed off from the surroundings, excepting for the mentioned supply of combustion air from below.

Although these known stoves have many advantages, it has been found to be very difficult, if not impossible, to satisfy applicable regulations or recommendations with respect to the discharge of pollutants. For a modern door- fronted stove of the kind in question and which is common on the market, the tar content in the effluent gases is of the order of magnitude 100 mg tar per kJ.

Object of the Invention A chief object of the present invention is to improve the combustion in solid fuel-fired combustion apparatuses, particularly stoves and door-fronted stoves so that the amount of discharged pollutants is reduced in comparison with conventional such apparatuses with combustion air supplied from below.

Another object of the invention Is to increase the efficiency in iring this type of combustion apparatuses.

Other objects of the invention will be apparent from the following description.

Short Description of the Invention What primarily characterizes the invention is disclosed in the_. accom¬ panying claims and is explained in the following description of an embodiment preferred at present. A salient feature of the combustion apparatus in accordance with the invention is that the combustion air is supplied to the fire from above/obliquely from above, in contradistinction to the conventional door-fronted stoves described hereinbefore, where combustion air is supplied from below. The combustion air is supplied in a controlled way via special supply ducts. After an initial heating-up period, the combustion chamber of the apparatus is kept substantially entirely closed off, so that the sole supply of air takes place via the special supply ducts.

The air supply duct or ducts thus open out in the combustion chamber at least at a level, and preferably at several different levels, above the hearth, spreader devices directing the supplied air down towards the hearth, preferably in a turbulent flow directed substantially towards the center of the hearth. The spreader devices comprise preferably at least two substantially horizontal . spreader ducts situated at different heights and provided with a plurality of blowing apertures with coacting deflection plates, which direct the combustion air flowing out from the blowing apertures towards the hearth.

During operation, i.e. after a short initial heating-up period with an open draught door, the air supply continues by self induction. The flow rate depends, inter alia, on the firing temperature and the design of the combustion apparatus or hearth, and it is primarily regulated by varying the flow area and flow resistance of the supply ducts (as well as adjusting the flue damper associated with the apparatus). It is essential for the invention that this regulation is carried out so that there is a balance between the downwardly directed combustion air and the upwardly directed gases from the hearth. When combustion on the hearth has been once adjusted in this respect, the flows will be self-balancing/self- reguiating during the course of combustion. For example, a smaller flame leads to a reduced intake of combustion air, which in turn gives the desired lesser degree of cooling from the combustion air. Short Description of Drawings

Fig. 1 is a schematic front view, partly sectioned, of a presently preferred embodiment, illustrating a combustion apparatus in accordance with the invention.

Fig. 2 is a schematic side view of the apparatus in Fig. 1 during firing. Description of Preferred Embodiments

The invention will now be described in detail with reference to the embodiment illustrated on the drawings, but it should be emphasized that many modifications and variations are possible within the scope of the accompanying claims. In the Figures the reference numeral 1 denotes the combustion chamber itself, there being a plinth 2 supporting the hearth, and the flue gases are led out via a lue or a flue connection 3. In Fig. 2 the combustion apparatus is illustrated during firing solid fuel *, e.g. wood, coal or briquettes, which can be kept in place with the aid of a support 5. Flames from the fire are denoted by the numeral 6. The combustion chamber of the apparatus is suitably and conventionally surrounded with heat-insulating and/or heat accumulating mate¬ rial 7. It deserves to be emphasized that the bottom of the hearth (on which the fuel is placed) is impervious, in contradistinction to conventional stoves with air supply from below. The apparatus is, of course, conventionally provided with draught doors (not shown) through which fuel is also supplied.

In accordance with the invention, the combustion apparatus is provided with inlet ducts 8a, 8b, 8c for the supply of combustion air, these ducts leading the air to substantially horizontal blowing ducts 9a, 9b, 9c, situated at different levels in the combustion chamber 1, all above the hearth. The flow of supplied combustion air is denoted on the drawings by arrowed full lines, while the flow from the hot gases from the fire are schematically denoted by arrowed dashed lines. The blowing or spreader ducts 9a, 9b, 9c are provided with blowing apertures 10a, 10b, 10c in the form of slits or, preferably a plurality of separate holes distributed along the respective duct. The blowing apertures 10a, 10b, 10c coact with deflection plates 11a, l ib, lie directing the air flow from the respective aperture downward towards the fire, preferably substantially towards its centre.

In the illustrated embodiment, there are two outer supply ducts 8a and 8c and a central duct 8b. The central duct 8b leads air only to the first spreader or blowing duct 9a, while the outer supply ducts 8a and 8c divide their flow between the ducts 9b and 9c (and possibly also 9a). In the drawing the two undermost ducts 9a and 9b are illustrated as extending across the entire combustion chamber 1, while the upper duct 9c is illustrated as being divided into two smaller ducts 9c.

A further air supply duct 9d is also illustrated in the Figures, with its corresponding deflection plate lOd, both situated above the blowing duct 9c. Figure 2 also depicts a guide plate 12. The combustion apparatus is finally equipped with a regulatable, schematically illustrated damper 13 and a flame catcher I*.

When a combustion apparatus in accordance with the invention is installed, it is first adjusted so that a balanced flow of downwardly directed combustion air (arrowed full lines) and upwardly directed flue gases (arrowed dashed lines) is obtained, primarily by adjusting the flow area and flow resistance in the ducts or the conduits for supplying combustion air to the combustion chamber. For example, the inlet openings of the risers 8a-8c can be suitably obstructed to obtain this effect. An important advantage of the design in accordance with the invention in this respect is that it is adjusted once and for all on installation, so that nobody later on can cause incorrect adjustment by mistake.

The damper 13 is also adjusted to suit the installation in question, primarily with regard to the draught properties of the flue used. If so desired, and to advantage, an extended flow path for the flue gases may be arranged in connection with the damper 13 for increasing the post combustion zone. It has been found to be particularly advantagenous to allow the flue gases to pass the rear side of the air inlet unit in heat-exchanging contact with it for further preheating the combustion air, as illustrated by the arrowed dashed line P in Fig. 2. Combustion will be more effective and the flue gas temperature higher, due to the preheating of the combustion air and the extended path of the flue gases.

The combustion apparatus thus installed is operated and functions in the following manner. Firing is started in a conventional manner, and the draught door is closed when the fire has got well under way, i.e. when the apparatus has got a sufficiently high temperature to function as a self-inducting pump for taking in combustion air via the risers 8a-8c. The combustion air flows according to the arrowed full lines in the drawing, and from the lower spreader duct 9a there is a strong flow of air towards the centre of the fire, which then gets a very high temperature at its centre (e.g. in the order of magnitude of 1000°C). This results in that the smoke is drawn in towards the centre, resulting in that surrounding wood gets a poor supply of oxygen and combustible gases are formed. These gases rise and are combusted in the counter-directed air from the upper spreader ducts. Due to combustion air blowing in a direction downwards, heavier particles are taken back towards the hearth for burning once again, the total result being that very effective and complete combustion is obtained, which is reflected, inter alia, in very low pollutant contents and most often smoke-free flue gases in the flue, as well as a higher efficiency. It may be said that combustion takes place in four combustion zones A, B, C and D, where the zone D may be regarded as a post combustion zone without extra oxygen supply.

Since the air supply takes place with a turbulent flow, the combustion zones may also be designated as whirl zones. Combustion will be practically complete and free from soot. Measurements that have been carried out show that in firing with wood in a door-fronted stove designed in accordance with the drawing Figures there are only obtained tar contents in the order of magnitude 10-15 mg/kJ and below, i.e. only approximately a tenth of the values for stoves with conventional air supply under the hearth.

The efficiency of the combustion apparatus can be improved further by providing the flame catcher 1*, which is situated at a level in the combustion chamber such that it is in contact with the flames 6, suitably between the spreader ducts 11a and lib. The flame catcher 1 is made from perforated plate, netting, grating or the like, which allows air and flue gases to pass through and which breaks up the flames into many small flames, thus increasing the utilizable oxygenation surface. The flame catcher 14 also functions as an ash barrier. It is particularly preferred, as in the illustrated embodiment, to combine the upper spreader duct 9c and its associated deflection plate l ie with the guide plate 12 and one or more reflector plates 15. The guide plate 12 guides the flue gases to the combustion zone C and the reflector plate/plates 15 also function as heat reflectors by reflecting heat to the combustion zone C. The air flow from the blowing apertures lOd also serves as a flame brake.

The flow from the different spreader ducts can be varied within the scope of the invention, inter alia with respect to their number and placing, the number, size and placing of the blowing apertures, etc. In general it is preferred to have the heaviest flow from the lowest spreader duct (9a). The combustion apparatus in accordance with the invention can be manufactured as an entire unit, but it may also be implemented as a special insert for building into existing fireplaces or stoves.

Claims

1. Combustion apparatus in which solid fuel (4) is fired in a combustion chamber (1) to which there leads at least one duct for the supply of combustion air, characterized in that the duct or ducts (8a-8c) for the supply of combustion air open out in the combustion chamber (1) at at least one level above the hearth thereof, and in that spreader devices (9a-c, lOa-c, Ila-c) blow the supplied combustion air down towards the hearth.

2. Combustion apparatus as claimed in claim 1, characterized in that it is provided with spreader devices situated at at least two different levels above the hearth, and in that spreader means direct the flow of air substantially towards the centre of the hearth, preferably with a turbulent air flow.

3. Combustion apparatus as claimed in claim 1 or 2, characterized in that the supply duct or ducts include at least one substantially vertical riser pipe (8a-8c) from which combustion air is pumped into the heated- up combustion chamber (1) by self-induction.

4» Combustion apparatus as claimed in any of claims 1-3, characterized in that the spreader devices include substantially horizontal spreader ducts (9a-9c) provided with a plurality of blowing apertures (lOa-lOc) and also with deflection plates (lla-c) coaeting therewith, which direct the combustion air flowing out from the blowing apertures towards the hearth.

5. Combustion apparatus as claimed in any of claims 1-4, characterized in that it further includes a perforated flame catcher (14) which is placed substantially horizontally in the combustion chamber at a level such that during firing it is impinged on by the flames to break them up into a plurality of smaller flames.

6. Combustion apparatus as claimed in claim 5, characterized in that it is provided with at least one spreader device (9, 10, 1 la and 9, 10, lib) above, as well as below, the flame catcher (14).

7. Combustion apparatus as claimed in any of the preceding claims, characterized in that it is a so-called door-fronted stove.

8. Method of firing with such combustion apparatus where solid fuel is combusted in a combustion chamber under the controlled supply of combustion air, characterized in that the combustion air is blown into the combustion chamber with a turbulent, downward flow directed towards the hearth of the apparatus.

9. Method as claimed in claim 8, characterized in that air is arranged to flow into the combustion chamber at at least two different levels, all above the hearth.

10. Method as claimed in claim 8 or 9, characterized in that the intake of combustion air is achieved by self-induction generated and controlled by the firing heat.