STOKER FOR BURNING SOLID FUEL

Abstract: A stoker for intermittent burning of solid fuel, comprising an upwards open grate (14). This grate is connected to a fan for supply of combustion air, feeding means (11, 12) for controlled feeding of fuel from a reservoir (15, 17) to the grate (14). There are also means (34) for ignition of the fuel in the grate or of gases which are generated by the fuel. The mouth of the grate (14) can be closed by means of a lid apparatus (25) which may be moved between a closed position and an open position, in order to interrupt combustion in the grate in said closed position.

Description: TITLE Stoker for burning solid fuel TECHNICAL FIELD The present invention relates to a stoker for intermittent burning of solid fuel, comprising an upwards open grate which is connected to a fan for supply of combustion air, feeding means for controlled feeding of fuel from a reservoir to the grate, and means for ignition of the fuel in the grate or of gases which are generated by the fuel . STATE OF THE ART Combustion of non-fossil solid fuel, like pellets, firewood, wood chips, etc., is normally performed inside a grate. The solid fuel is slowly gasified within the grate under limited supply of air. The obtained gas is then burned in a secondary step under abundant supply of air. The process results in a minimum discharge of environmental polluting gases, in comparison with most of the other types of combustion. The residual product is a fine ash which contains matter having a pH-increasing effect and being in shortage in acidificated ground. According to prior art technique such a stoker may be switched between a nominal power mode of operation and a low power mode of operation, wherein thermostat regulators are used for controlling the switching from one mode to the other with regard to the energy requirement. Normally, the grate is optimised with regard to the maximal efficiency and with regard to environmental protection, in order to be operated in the nominal power mode of operation. It is CONFIRMATION COPY known that operation of the grate in the lower power mode of operation will involve that the combustion takes place under much less efficient conditions. It would be desirable to be able to operate the stoker in the nominal power mode of operation only, with the best possible efficiency and discharge conditions. So far this has not been possible. OBJECT OF THE INVENTION One object of the present invention is therefore to produce a stoker for burning solid fuel, which may be intermittently operated with nominal power and apart from that has a shut off combustion. SUMMARY OF THE INVENTION For this object, the stoker according to the invention is characterized in that the mouth of the grate can be closed by means of a lid apparatus which may be moved between a closed position and an open position, in order to interrupt combustion in the grate in said closed position. Because of the closing of the mouth of the grate by means of the lid apparatus, the supply of oxygen to the grate is choked to such an extent, that the combustion is shut off. SUMMARY OF THE INVENTION The invention will now be described with reference to an embodiment which is shown in the accompanying drawings, in which FIG. 1 is a side view schematically showing a stoker which is designed in accordance with the invention, FIG. 2 shows in a larger scale the stoker in Fig. 1 in a broken view from the side, and FIG. 3 shows the burner in view from the left in Fig. 1. DETAILED DESCRIPTION OF THE INVENTION The stoker according to the invention is arranged to be mounted directly into the furnace opening which is used for mounting an oil burner at a common house heating furnace. For this object, the stoker is provided with a mounting plate 10 with not shown screw holes for mounting screws. A thick tube 11 is rigidly connected to the mounting plate 10 extending transversely through this plate. The tube 11 forms a housing for a feeder screw 12, which is driven intermittently with a very low speed by means of a stepping motor 13. The motor 13 drives the screw 12, so that its thread feeds in the direction towards the at the opposite end of the tube 11 located, cup shaped grate 14. A primary fuel storage chamber 15 is located with its bottom at an opening in the tube 11, in such a way that fuel, e.g. cylindrical oblong pellets, may fall down into the tube 11 from the chamber 15 and is fed in the direction towards the grate 14. The primary chamber 15 is connected via a hose 16 (or a pipe) to a secondary fuel storage 17, which is shown in reduced scale in Fig. 1 and is provided with a worm screw 18 which is operating obliquely upwards and is driven by a motor 19. This fuel feeding is previously described in more detail in SE Patent 9304289-3. An air distribution chamber 20 is located at the mounting plate 10 and is supplied with air via a fan 21. The chamber 20 is connected to the grate 14 via a primary air pipe 22 which distributes air around the inside of the grate cup 14 via slits in a cast steel ring 23. A secondary air pipe 24 is connected via a flexible pipe to a lid apparatus 25, which is pivotally mounted in a pivot point 26 via a double-armed lever 27. At the opposite side of the pivot point 26, the free end of the lever forms a point of action for a push rod 29 which is connected to an actuator 28 (see Fig. 1) As Fig. 1 shows, the lid apparatus 25 is pivoting between the position shown with unbroken lines and the position shown with dash dotted lines. In the first mentioned position, the lid apparatus closes the mouth of the grate cup 14 via a circular flange section 30 which rests upon a surface of the ring 23 which is conically inclined in the direction towards the middle of the grate cup. In this position, the heat generating, in Fig. 1 left part of the stoker may be entered into the furnace opening of a conventional central heater. In the second position, the lid apparatus 25 is open and forming a wide hood above the grate mouth with inclined side wall elements 31, which hood has the effect that gas that rise upward during gasifying of fuel in the grate 14 are retarded and brought to a turbulent mixing of the gas within the combustion zone. The secondary air pipe 24 connects to a space 32 inside the lid which extends out into the side wall elements 31 to distribute secondary air via openings 33 which are directed in toward the middle of the space between the side wall elements 31 of the hood, so that a complete final combustion of the gas takes place inside the hood. The exhausts stream out of the hood and further on through the central heater in a conventional manner. The rotatable ring 23 is provided with gear tooth segments, which are actuated by a finger (not shown in the drawings) which projects out from the end of the screw shaft 12. Each time the finger sweeps against the gear tooth segments of the outer ring, the ring 23 is turned a small amount forward. By driving around the ring in this manner, a levelling of the contents of the grate cup 14 is achieved, without any concentration of said contents, so that accumulation of cinders at the mouth of the grate may be avoided. A glow electrode 34 is mounted adjacent the grate mouth at the outside of the ring flange 30. When the lid arrangement 25 is opened, e.g. by the command of a control thermostat, also the feeder screw 12 and the fan 21 are started and the glow electrode 34 is supplied with electric current. Now an ignition of the fuel will take place within a few seconds, whereafter the glow electrode may be switched off and combustion takes place in the conventional manner until the control thermostat gives a command that production of heating energy may be stopped. This command stops the feeder screw 12 and the fan 21 and the lid apparatus 25 is closed, so that combustion is stopped. A detector tube, which extends from the air distribution chamber 20 through the mounting plate 10, is used in the prior art way in order to register when combustion takes place in the grate 14, so that the glow electrode may be switched off. Because of the arrangement of the lid apparatus at the mouth of the grate, combustion in the grate may be stopped immediately at a command from the control thermostat. This involves that the consumption of fuel may be reduced, at the same time as the combustion takes place under optimum conditions, which increases the degree of efficiency of the stoker considerably. Furthermore, the lid apparatus in its open position of use causes the final combustion of the gases from the grate to take place in a controlled manner, which also increases the degree of efficiency of the stoker at the same time as leakage of environmentally hazardous hydrocarbons is prevented. The invention is not limited to the above described embodiment, but several variants are conceivable within the scope of the accompanying claims. For example, the grate 14 does not require a circular mouth, but the mouth may for example have a rectangular shape .

Claims: CLAIMS 1. A stoker for intermittent burning of solid fuel, comprising an upwards open grate (14) which is connected to a fan for supply of combustion air, feeding means (11,12) for controlled feeding of fuel from a reservoir (15,17) to the grate (14), and means (34) for ignition of the fuel in the grate or of gases which are generated by the fuel , c h a r a c t e r i z e d i n , that the mouth of the grate (14) can be closed by means of a lid apparatus (25) which may be moved between a closed position and an open position, in order to interrupt combustion in the grate in said closed position. 2. A stoker according to claim 1, c h a r a c t e r i z e d in that the lid apparatus (25) is pivotally mounted in a pivot point (26) beside the grate mouth. 3. A stoker according to claim 1 or 2 , c h a r a c t e r i z e d in that the lid apparatus (25) is provided with an internal channel (32) for supply of secondary air to a combustion area adjacent the grate mouth. 4. A stoker according to claim 3, c h a r a c t e r i z e d in that the lid apparatus (25) comprises side wall elements (31) , which together form a hood above the mouth of the grate, in the open position of the lid apparatus. 5. A stoker according to any one of the preceding claims, c h a r a c t e r i z e d in that the grate forms a round cup which is provided with an outer ring (23) which is rotatably mounted on the upper edge of the cup and which rotated for avoiding accumulation of cinders at the mouth of the grate. 6. A stoker according to claim 5, c h a r a c t e r i z e d in that a primary air pipe (22) is connected to the grate (14) via around the inside of the grate bowl distributed slits.

Inventors: DANIELSSON BJOERN, DANIELSSON, BJOERN
Applicants: DANIELSSON BJOERN, E K TEKNIK UTVECKLING AB, DANIELSSON, BJOERN, E.K. TEKNIK UTVECKLING AB
Application year: 1997
Main classification: F23Q25
IPC codes: F23K3, F23L1, F23L9