WO1998031968A1

WO1998031968A1 - Burner for solid fuel and method of controlling the supply of air to a burner

Info

Publication number: WO1998031968A1
Application number: PCT/SE1998/000087
Authority: WO
Inventors: Jan Ericson
Original assignee: J-TEC
Current assignee: J-TEC
Priority date: 1997-01-21
Filing date: 1998-01-21
Publication date: 1998-07-23
Anticipated expiration: 1999-07-21
Also published as: SE9700166D0; SE508546C2; SE9700166L

Abstract

The present invention relates to a method for controlling, in a burner (1) for solid fuel, such as pellets, wood chips, etc., the supply of air necessary for the combustion, whereby primary air is introduced into an inner burner tube (2) through openings (10, 11) in the wall of the tube, said inner burner tube forming a combustion chamber (2a) for the gasification of the fuel, and whereby secondary air for the combustion of the formed gases is supplied through a space (5a) formed between the inner burner tube and an outer burner tube (5) and through a secondary air opening (8) formed in the region of free ends of the inner and outer burner tubes. According to the invention the temperature of the combustion of the formed gases is continuously detected and the supply of primary and secondary air is controlled proportionally, directly depending upon the detected temperature.

Description

BURNER FOR SOLID FUEL AND METHOD OF CONTROLLING THE SUPPLY OF AIR TO A BURNER

The present invention relates generally to burners or burner heads for solid fuels, such as pellets, wood chips, etc., and more specifically to a method and a burner for controlling the supply of air necessary for the combustion, and of the kind indicated in the preamble of patent claims 1 and 4 respectively.

Burners of the above indicated, general kind have lately attracted more and more attention since they have been improved in many respects and have been adapted so as to be used instead of oil burners in existing boiler designs as well as in specifically developed heating boilers. Although the burning of solid fuels is an ancient technique there are, however, still several shortcomings in the technique that has been developed, much so due to the fact that primarily pellets and shavings, but also wood chips are refined solid fuels that require specific conditions for enabling the achievement of optimized combustion, but also due to the ever increasing environmental concern that sets the condition that the combustion must be optimized with regard to the amount of pollutants influencing the environment by the combustion.

In the last mentioned respect it is of specific importance that the combustion takes place within a relatively narrowly, delimited area of temperatures of approximately 850-1050°C, since there will be problems related to the emission of carbon monoxide if the combustion takes place below this temperature area and since there will be problems related to the emission of polynuclear aromatic hydrocarbons and so forth at even lower temperatures. On the other hand, larger and larger amounts of nitrogen oxides are formed at combustion temperatures above approximately 1050°C. From the above it is clear that in order to lower the emission it is essential to maintain the combustion, i.e. the flame temperature, at least within the specified area.

In view of the above the present technique is inferior in one important respect, namely the control of the air supply which is so essential for providing optimized combustion. To simplify, it may be said that in the burners of the kind in question, which are presently on the market, the control of the air supply is mainly utilized in order to control the effect of the burner in combination with the supplied quantity of fuel. There are indeed many suggestions for solutions with respect to the control of the air supply, see for instance SE-B-8207193-7, SE- C-501 416, FR 2 571 474, FR 2 671 166 and SE 8504437-8, but these previous solutions which are known to us, are unsatisfactory due to the fact that in each case they provide coarse or insensitive, slow and inexact control of the supply or combustion air for achieving the sought optimized combustion.

In most cases these known structures do also cause the speed of the flow of supply air to be to high past the glow bed in the combustion chamber, which especially at higher power outputs, results in an increased danger of blowing out unburned particles to the heating boiler.

The basic object of the present invention is therefore to improve the known technique by providing a method and a burner of the kind indicated in the introduction, by means of which conditions are provided for an optimized combustion by removing the above discussed shortcomings.

This basic object of the invention is achieved by means of a method and a burner of the above- indicated kind, which comprise the characterizing features indicated in the enclosed patent claims 1 and 4 respectively.

Preferred further developments of the basic inventive idea are stated in the dependent sub- claims.

The invention and its specific objects, features and advantages will be clearly described in the following detailed description of an embodiment of the invention. In the single enclosed, very schematical drawing a presently preferred embodiment of the burner or burner head according to the invention is illustrated in a longitudinal section. As indicated above the invention is based on the knowledge that for achieving optimized combustion both with regard to the efficiency and to environmental concerns, it is essential that the flame temperature lies at least within the interval of 850-1050°C, and in practice lies in the narrower interval of 900-1000°C.

According to the invention this object is now achieved by maintaining the flame temperature within the desired interval in connection with a burner for solid fuels, and more specifically by providing a very quick and exact control of the supply of air necessary for the combustion. To be precise it is suggested that during the combustion of the gases formed by the gasification of the fuel, the flame temperature is detected continuously and that the supply of primary and secondary air is controlled proportionally in direct dependence upon the detected temperature.

The very quick and exact control of the supply air required in accordance with the invention can not, without extensive and costly control measures, be achieved through a direct controlling of the fan, since a conventional stepwise control thereof provides a far too slow and indefinite control of the actual supply of air. Thus, according to a further development of the basic inventive idea, a preferred embodiment of the method is applied to a burner specifically developed for this purpose, and that embodiment will become clear in connection with the following description of said burner.

It is clearly indicated in the drawing figures that in the presently preferred embodiment the burner 1 according to the invention basically consists of an inner burner tube 2 with a rotatably journalled screw conveyor 3 therein, for controlled supply of fuel B from a generally funnel- shaped storage 2b, being connected to the inner tube 2, and to a combustion chamber 2a having an ignition zone in the forward region of the inner tube 2. The fiinnel-shaped storage 2b is in this connection only illustrated in order to indicate a fuel supply and it should be emphasized that the feed or supply of fuel may in practice be designed in many alternative ways, and preferably as a closed feed from a larger store or supply, thereby eliminating the danger of self-ignition. It should likewise be obvious that the feed of the screw 3 is controlled in a manner known in itself within this technique and not specifically described herein, in order to control the effect or power of the burner in dependence on the existing energy requirement.

At its upper side the inner burner tube 2 is further provided with at least one primary air opening 10 for the supply of air to the fuel bed 5, i.e. the glow bed, supplied by the screw 3 and ignitable by means of ignition means, in the illustrated embodiment heating-wires, in the combustion chamber 2a. At the lower side or bottom side of the inner tube 2, in the region of the glow bed, openings 11, known per se, are furthermore provided for so called blast air (injection).

An outer burner tube 5 surrounds the inner burner tube 2 at a distance therefrom, whereby a secondary air duct 6 is formed between the inner and outer tubes. The inner and outer tubes 2 and 5 respectively are thereby preferably circular in cross-section, such that it should be possible in principle, with proper dimensioning, to fit the burner 1 instead of an oil burner in an existing heating boiler 15 as indicated in the drawing figure. Obviously the burner 1 could, within the scope of the invention, also be designed having other cross- sectional shapes. The secondary air duct 6 opens into an annular secondary air opening 8 formed between a tapered outer end portion 4 of the inner tube 2 and a likewise tapered portion 7 of the outer tube 5. The cone angle of the tapered portions 4 and 7 respectively is thereby substantially identical, for reasons that will be made clear below.

Turbulence generating members 20, which are only indicated on the drawing figure and which may for instance be in the shape of a thread, are preferably provided at the outer side of the tapered portion 4 of the inner tube 2 as well as at the inner side of the tapered portion 7 of the outer tube 5, and preferably also at the inner side of an end portion 5 a of the outer tube, said end portion protruding past the outer free end of the inner tube. Such turbulence generating members serve the purpose of producing a very intense mixing of the secondary air and the gases formed when the fuel 5 is gasified, in order to thereby obtain a very efficient combustion in the flame. Similar turbulence generating members 20a may also, in certain cases, be provided in the secondary air duct 6 in order to further intensify the turbulence.

Primary air as well as secondary air is supplied by a schematically indicated fan 18, the outlet of which is connected to an inlet tube 9 in the outer burner tube 5. The inlet tube 9 is in turn connected to the area of the space formed between the inner and outer burner tubes, which is illustrated to the right in the drawing figure, and does therefore supply primary as well as secondary air to the burner 1. Especially by burners for larger heating boilers it may also be suitable to provide separate fans for the primary and secondary air.

As far as the burner 1 has been described so far it does in principle correspond to the prior art designs. However, according to the invention the inner 2 and outer 5 burner tubes are provided displaceable relative to each other for obtaining the very fast and exact controlling of the supply of air necessary for the combustion, which is the object of the invention. More specifically, the relative displacement provides for a simultaneous and proportionate variation of the flow area of the secondary air opening 8 and of the at least one primary air opening 10. It is obvious that the size of the secondary air opening 8 is adjusted by the mutual displacement of the tapered portions 4 and 7. The size of the primary air opening 10 is on the other hand adjusted through a screening member 12 which is attached to the inner wall of the outer tube 5 and which, through the relative displacement of the tubes 2, 5 changes the size of the primary air opening 10 by screening or covering said opening to a varying extent.

The relative displacement is preferably performed in the manner that is clear from the illustrated embodiment, i.e. with the outer burner tube 5 firmly connected to the heating boiler 15 indicated in the drawing figure and by displaceably supporting the inner burner tube 2 in the outer burner tube 5 by means of suitable bearings and seals, one is indicated in the drawing figure and is identified by the reference number 13. Within the scope of the basic principles of the invention it is quite possible to have the inner tube 2 firmly connected to the heating boiler 5 and to support the outer tube 5 so that it is displaceable thereon, although such a design would cause certain problems and would make the design more complex. As a result of the described design a relative displacement of the inner and outer tubes 2 and 5 will therefore cause a change of the flow area of the primary air opening 10 and of the secondary air opening 8, said adjustments being mutually proportional as well as proportional to the displacement. The displacement of the inner burner tube 2 should be completely stepless and a drive therefore may be provided in different manners. A presently preferred solution for the drive is to utilize a proportional control magnet (solenoid) 16 - illustrated in principle in the drawing figure - of the kind employed for displacing valve members in a hydraulic valve, which through a symbolically drawn wire 22 and from a control unit 23, which is not specifically illustrated but which is designed in accordance with known control technique, receives an energizing signal from a temperature sensor 17 provided in the combustion chamber 15a of the heating boiler 15 for continuous detection of the temperature of the flame which is not illustrated.

The burner suggested in accordance with the invention is employed in the following manner. In a start sequence a specified amount of fuel is fed to the combustion chamber and is ignited by means of the ignition means 14, and during this ignition phase the fan 18 is operated, likewise by the control system 23 and through a symbolically illustrated connection 24, at a low power level with the inner burner tube 2 simultaneously being displaced as far as possible to the left in the drawing figure, such that the secondary air outlet 8 is practically closed, while the primary air opening 10 is partially covered or screened. Fixed time control according to a fixed program is preferably employed to control the ignition sequence through the control system 23.

When the start sequence has been performed the fan 18 is switched to a higher power level corresponding to the fuel feed determined through the control system depending upon the existing energy requirement, i.e. in a burner 1 utilized in a heating boiler 15 for heating a building, the energy requirement is determined by the temperature control system of the building and the power output from the burner 1 is controlled depending thereupon. Thus, in a conventional manner, the burner effect is thereby controlled by controlling the quantity of fed fuel and the quantity of fed air. During the continuous operation, i.e. after completion of the ignition sequence, the invention then takes effect for securing the above discussed optimized combustion through extremely fast and exact control of the primary and secondary air, independent of the chosen power level of the fan 18 and the chosen fuel feed, but directly depending upon the continuously detected temperature of the flame.

As soon as the temperature sensor 17 detects a temperature in the flame which lies outside the above indicated area or any other determined area, the inner burner tube 2 will therefore, by the drive member 16 and controlled through the control unit 23, be displaced in the direction corresponding to the limit of the temperature area which has been exceeded and underpassed respectively and with a distance corresponding to the size of the detected deviation. The restriction and increase respectively of the primary air opening 10 and the secondary air outlet 8 which is achieved through the displacement of the inner burner tube 2 by a certain distance, and the distance with which the inner burner tube is displaced corresponding to a certain temperature deviation should in this connection naturally be "tuned" to each other.

Through these measures it is therefore possible, at each instance and independent on the power output of the burner and thereby independent on the quantity of air as such that is fed by the fan, to obtain the control of the primary and secondary air supplied to the combustion process providing the desired steady and fine, maintained combustion, and simultaneously a relatively low air speed is achieved in the combustion chamber with the associated, above- mentioned advantages.

Although the invention has been described above with specific reference to an embodiment of the burner installed in a heating boiler for heating a building, it should be emphasized that the invention also covers such changes and variations thereof that are obvious to a man skilled in the art. The scope of the invention should therefore only be delimited by the enclosed patent claims.

Claims

1. A method of controlling, in a burner (1) for solid fuels, such as pellets, wood chips, etc., the supply of air necessary for the combustion, whereby primary air is introduced into an inner burner tube (2) through openings (10, 11) in the tube wall, said inner burner tube forming a combustion chamber (2a) for the gasification of the fuel, and whereby secondary air for the combustion of the formed gases are supplied through a space (5 a) formed between the inner burner tube and an outer burner tube (5) and through a secondary air opening (8) formed in the region of free ends of the inner and outer burner tubes, whereby the temperature by the combustion of the formed gases is detected continuously and the supply of primary and secondary air is controlled proportionally, directly depending upon the detected temperature, characterized in that the control of the supply of primary and secondary air is performed through relative displacement of the inner (2) and outer (5) burner tubes and the resulting change of the flow area of the secondary opening (8) and at least one (10) of the primary air openings (10, 11).

2. A method according to claim 1 with a burner (1) introduced into a heating boiler (15), characterized in that the inner tube (2) is displaced relative to the outer tube (5) which is firmly connected relative to the heating boiler (15).

3. Burner (1) for solid fuels, such as pellets, having an inner burner tube (2) forming a combustion chamber (2a) in communication with a fuel feed (3, 2b), and being provided with primary air openings (10, 11), having an outer burner tube (5) surrounding the inner burning tube and having a secondary air duct (6) formed between the inner and outer burner tubes, said secondary air duct opening into a secondary air opening (8) formed in the region of free ends of the inner and outer burner tubes, characterized in that the inner (2) and outer (5) burner tubes are provided displaceable relative to each other for simultaneous and proportional variation of the flow area of the secondary air opening (8) and of at least one (10) of the primary air openings (10, 11).

4. Burner according to claim 5 having inner (2) and outer (5) burner tubes of substantially circular cross-section at least in the region of the secondary air duct (6) and of the secondary air opening (8), characterized in that the secondary air opening (8) is formed by a tapered end portion (4) of the inner burner tube and a tapered portion (7) of the outer burner tube, each having substantially the same cone angle and in that the at least one primary air opening (10) of the inner burner tube cooperates with a screening member (12) provided at the inner surface of the outer burner tube.

5. Burner according to claim 3 or 4 for fitting to a heating boiler (15), characterized in that the outer burner tube (5) is adapted for fixed attachment to the heating boiler and in that the inner burner tube (2) is connected to a drive member (16) for displacing the inner burner tube relative to the outer tube.

6. Burner according to claim 5, characterized in that the drive member (16) is connected to a temperature sensor (17) provided in the heating boiler and is controlled proportional to a combustion temperature detected by the temperature sensor.

7. Burner according to claim 6, characterized in that the drive member (16) is a proportional magnet.

8. Burner according to any of claims 3-7, characterized by a common fan (18) for the supply of primary as well as secondary air, the outlet of which is connected to an air intake tube (9) in the outer burner tube (5), in flow communication with the secondary air duct (6).

9. Burner according to any of claims 3-8, characterized in that turbulence generating members (20) are provided at least in the area upstream and downstream of the secondary air opening (8).