FURNACE

Abstract: A furnace has a solid fuel hopper 35 above a combustion chamber 10, an ash outlet 11 below said combustion chamber, a combustion gas outlet 11 at or below the level of the combustion chamber, and an auger 22 in an ash chamber 20 below the ash opening 11 to regulate the size of an ash pile 21 below the combustion chamber 10 and hence control the escape of ash or fuel or non-combustibles through the ash opening 11. This enables the combustion gas outlet 11 to be covered by hot coals 53 within the combustion chamber 10 so that any combustion gas from the combustion chamber or any pre-combustion zone must pass through a filter of hot coals 53 before reaching the combustion gas outlet 11.

Description: This invention relates to a downdraft or sidedraft furnace and to a method of operating the same. By a downdraft or sidedraft furnace (whether a large commercial furnace for industrial heating or the combustion of waste materials, or a small domestic stove or furnace) is meant one in which solid fuel or combustible waste material moves downward to a combustion zone and combustion air is introduced either laterally or in the same direction as the fuel, in contrast to an updraft furnace in which combustion air is introduced in the direction opposite to that in which the fuel is moving. An advantage of a downdraft or sidedraft furnace over an updraft furnace is that in the last stages of combustion the combustion gases pass through a filter of hot coals and are "burnt clean" before leaving the combustion chamber. However existing downdraft or sidedraft furnaces have required the provision of a lower grate to hold the burning mass of fuel in place. This is disadvantageous in that the grate becomes clogged with clinker. Although it does not relate to a downdraft or sidedraft furnace British Patent Specification GB-A-913 067 suggests as an alternative to a fixed lower grate a movable fuel support plate 5 which is positioned below and adjacent to the ash outlet. However even with this arrangement clinker will become baked and fused onto the plate and will be almost impossible to remove. An object of the present invention is to avoid the problems of previous proposals for a downdraft or sidedraft furnace in that, in accordance with one aspect of the invention, the ash outlet is unobstructed by a grate or like means for supporting the burning fuel and in accordance with another aspect of the invention an ash pile is produced and maintained under the ash outlet of the furnace and a bed of hot coals is built and maintained supported on the ash pile. The following is a description of preferred embodiments of the invention given by way of example only, with reference to the accompanying drawings, in which: Figure 1: illustrates a first embodiment of the apparatus of this invention in a sectional view from a front elevation. Figure 2: illustrates a partial view of a modified form of the apparatus of figure 1. Figure 3: illustrates a schematic view showing further modifications to the apparatus of figure 1. Figure 4: illustrates a partial view showing another modified form of the apparatus of figure 1. Figure 5: illustrates a second embodiment of the invention in sectional view from a side elevation. Figure 6: illustrates a third embodiment of the invention in sectional view from a side elevation. FIRST EMBODIMENT - Figures 1 to 4 A downdraft furnace in accordance with this invention has a combustion chamber 10 (which is preferably in the form of a trough and may be of any length), an ash outlet 11 which runs the length of the bottom of the combustion chamber, a combustion gas outlet (which in this case is also the ash outlet 11), a lower chamber 20, and means for regulating the relationship between (a) an ash pile 21 within chamber 20 and (b) the ash outlet 11. A solid fuel supply is provided above the chamber 10 and is preferably in the form of a hopper 35 (only part of which is shown in figure 1) although other means of conveying fuel to the combustion chamber can be used. There is a main or secondary combustion zone in the lower part of chamber 10 (which in use will maintain a bed of hot coals) and there may also be a primary or pre-combustion zone in the upper part of chamber 10 (and depending upon design this may also occur in the lower part of the hopper). In addition to the chambers 10, 20, there may be a number of heat exchanging and filtering chambers, and air ducts. The design of the furnace enables it to be of modular construction as the combustion chamber can be of any length and multiples of the furnace can be positioned side by side as required. Referring to figure 1, solid fuel 52 is supplied to the upper combustion chamber 10 through the open top 11A. A rotating shaft 12 (set with pins) may be used to rake the fuel in the chamber 10, to prevent the fuel bridging across the mouth of the chamber 10 and blocking it. A slide gate 13 may be provided to close off the supply of fuel to the chamber when the furnace is not in use, or when the body of the furnace is removed, eg. for repairs. Air is supplied to the upper part of chamber 10 through vents 14, leading from side chambers 15. This primary air may be blown in or drawn in by fans, or pulled in by the suction created by combustion and any fans used to draw the combustion gases into chamber 20. More primary air is admitted than secondary air as the primary combustion zone at the top of chamber 10 is of greater area than the secondary combustion zone towards the bottom of chamber 10. Optionally, liquid fuel may be supplied from the rotating shaft 12, which may be hollow as shown in figure 1, and provided with apertures for this purpose. The liquid fuel is heated as it passes along the shaft 12 and as it falls into the chamber, by combustion in the chamber 10, and will in normal use ignite upon contact with air from the vents 14, and be fully consumed before filtering to the bottom of chamber 10. Optionally, gaseous fuel may be provided under pressure, from manifolds 16 through chambers 15 and into combustion chamber 10. Water might also be added to the air in the side chambers 15, to control the temperature of the combustion, and thereby prevent it from reaching a level where nitrous oxides are formed. The ash outlet 11 at the bottom of the upper combustion chamber 10 is open during use of the furnace, although optionally a slide gate 17 may be provided to prevent escape of the fuel when the furnace is not in use, i.e. when the ash pile is not present. Ashes from the combustion chamber 10 will fall through ash outlet 11, and clean combustion gases created will also pass out of the chamber 10 by this route. Smoke will however be held in the chamber 10 and recombusted. Unburnt solid matter (e.g. rocks or metal rubbish ) falls onto the top of a pile of material in the lower chamber 20, which in use will be a pile of ashes 21. Although at cold start up, the material may be provided by a pile of sand or other non-combustible material. The top of this pile 21 thus provides a controlled "floor" 51 for the combustion chamber 10. If this "floor" 51 falls below the bottom of chamber 10 a combustion gas outlet can be provided around the top of the pile 21. A bed of hot coals 53 can be maintained within the combustion chamber 10 such that combustion can be promoted by secondary air from side chambers 18, through vents 19, and the general air flow out of chamber 10 into chamber 20. Fans or blowers (not shown) may be used to blow or draw air into the combustion chamber or alternatively to withdraw combustion gases from chamber 20. A tapered auger 22, set on a hollow shaft at the base of chamber 20, runs the length of the combustion chamber and draws ashes away from the bottom of the pile and ejects them, thereby regulating the level of the ash pile 21 such that any unburned or partially burned material at the top of the pile 21 remains in the airstream. It will be generally convenient to regulate the height of the ash pile so that its height remains constant at the level shown in figure 1. Weighted or spring-loaded flaps (not shown) may be provided over the openings into chamber 18, which automatically reduce the area of the opening as draft through the furnace decreases, will change the ratio of air in the primary combustion area to air in the secondary combustion area, to optimise this ratio under different burning rates. Preferably such a flap would never completely close off the openings such that more air is always available to the primary air inlets than the secondary air inlets and that air is always available to both combustion zones under any burning conditions. Additional air may be supplied for combustion of carbon in the ashes, from the hollow shaft of the auger, through vents 23. This air also cools the auger, and stirs through the ashes, aiding the movement process. The taper of the auger allows it to create an even distribution of ash along the length of the chamber 20, such that the top of the pile is at the same height all the way along the chamber. As shown in figure 4 additional augers 40 might be provided to either side of the tapered auger 22, acting to remove ash significantly faster than the auger 22, so as to remove any ash coming into contact with them effectively at once - in this way the height of the ash pyramid is controlled, by preventing extension of the width of the pyramid base beyond a set limit. The central tapered auger 22 in such apparatus acts to remove any large or heavy lumps of non-combusting material which sink directly down through the ash pyramid rather than sliding down the sides, and also aerates the ash and ensures a complete and continuous turnover. The auger 22 in such apparatus would consequently rotate at a slower speed than that in apparatus such as that shown in figure 1, as it does not need to keep the ash level constant, and does not therefore need to remove as much from the bottom of the pile as is added to the top of it. Air and combustion gases leave the chamber 20 through vents 24 which lead to heat exchangers and/or filters 25. Preferably the filters 25 are limestone filters, which remove sulphur oxides from the combustion gases by reacting with them to form solid calcium sulphate. Limestone is fed into the filter box 26 by an auger 27 at the top, and the calcium sulphate is removed by another auger 28 at the bottom. Combustion gases leave through vents 29 in the sides. Combustion gases leaving the furnace may be expected in normal use to be relatively 'clean', having had the soot burned out by recombustion in the lower part of chamber 10, and the sulphur oxides removed by filtration. Preferably the furnace has double walls, with a space between the inner 30 and the outer 31. Cold air enters the furnace through vents 32 in the underside and passes along the space between walls 30 and 31 before passing into the side chambers 15 and 18. While moving between the walls the air is heated, and therefore is already hot when it enters the inner chambers, thereby speeding combustion. As shown in figure 2, an additional grate 33 might be provided in the upper primary combustion chamber 10 (or the lower part of the fuel hopper), this comprising a number of relatively widely spaced bars, with air channels therethrough having apertures by which air may be directly provided to fuel resting on the grate 33. These additional air inlets allow for a pre-combustion zone near the top of the combustion chamber. This grate may hold large pieces of fuel for pre-combustion, and provide additional aeration to fuel in the upper chamber. It is preferred that this grate has a grate spacing less than or equal to the size of the ash opening so that in use solid waste may be partially combusted on the grate of the pre-combustion zone and any large incombustible material prevented from reaching the combustion zone by the presence of this grate. Similarly ribs could be provided running down the tapered walls of the upper chamber 10, to support the fuel off the walls and allow circulation of the air around and past the fuel. As with the bars of the grate 33, such ribs might have vented air channels running through them, but these are probably not necessary, as air is already provided to these parts of the chamber 10 through the vents 14. The entire burner may be mounted on bearings in tracks 34 such that it may slide in and out from under a fuel-holding bin or hopper 35 for ease in cleaning. The hopper may be provided with rakes mounted on rotating shafts 36 to push the fuel forward and ensure a continuous supply to the furnace. This will however only be required with some types of fuel, and in some applications, and may not be necessary with all furnaces. The apparatus of figure 3 includes a sliding block 37 or other conveyor means at the base of a fuel hopper 35, which in use may slide forward to push a load of fuel through a door 38 into the combustion chamber, and then slide back to allow another load of fuel to fall down in front of it. As long as the level of fuel is maintained above the level of the conveying means 37 at all times, substantially no air or smoke can escape by this route, and functioning of the apparatus may proceed normally. If the motive power of the block 37 is strong enough, and the doorway both strong enough and of an appropriate size, large pieces of fuel may be made of a size more suitable for combustion by crushing at the doorway. This may be required with some types of fuel, and in some applications, but will not be necessary with all furnaces. Similarly the movable, track mounted furnace system is not necessarily suited to all applications and features of the invented apparatus could be used with an emplaced furnace or small stove or the like, within the scope of the invention. It is generally preferable for the upper portion of chamber 10 to taper towards the base, to provide slow release of partially consumed fuel to the lower chamber. Alternatively however, the upper portion of chamber 10 could be straight walled, and the lower portion of smaller diameter to achieve a similar result. The vents and ducts need not be positioned as shown, and alternative arrangements will be apparent to those skilled in the art. The relative sizes and shapes of the various chambers and ducts might also be altered, although at present the proportions illustrated appear preferable. SECOND EMBODIMENT - Figure 5 A larger furnace might be made as illustrated in figure 5, for the use in burning of rubbish and similar mixed and unsorted fuels, possibly with a relatively high proportion of non-combustible materials included with it. Some details of the combustion chamber 10 and associated vents are not visible in the side elevation of Figure 5 but it will be appreciated that this furnace is of similar construction to the combustion chamber of Figure 1 and has a similar arrangement of air inlets which are not shown, as well as further air inlets which are shown. As in the embodiment illustrated in figure 2, a grille composed of hollow bars 33 is provided, (one of which is shown in side view rather than end view) preferably of box or rectangular section rather than round, and being partly filled with water 41, or some other coolant, to prevent warping under heat and weight stress. (Similar water filled bars are shown in end view in figure 6). Pre-combustion occurs in the zone just above the bars 33 and secondary combustion occurs both in the zone at the lower part of chamber 10 and in zone 42. Solid fuel 43 is fed into the chamber 10, where it is caught on the bars 33 and burned. A moving pusher block 44 moves across the upper surface of the bars 33 to shift the mass of fuel 43, and cause small and combusted pieces of fuel to fall between the bars 33 into the lower part of the chamber 10, while large pieces of non-combusting material, which will not fall through, are pushed to the end of the bars 33, and fall off into the chamber 42, into which the non-combusting scrap falls. In addition to the air flow system including the air inlets described with reference to Figure 1 and not shown in Figure 5, in this second embodiment air also enters chamber 42 either through simple vents 45 in the wall, or through vents 46 in the hollow shaft of an auger 47, the auger 47 being the conveyor means by which the non-combusting scrap is discharged from the chamber 42. Air entering chamber 42 is heated by hot scrap before entering the top of combustion chamber 10, either directly through the opening 48, or indirectly through the interior of the hollow bars 33 and vents 49 in said bars 33. The air and combustion gases exit through the secondary chamber 20, as detailed above, as does air provided for combustion of the hot coals on top of the ash pile. THIRD EMBODIMENT - Figure 6 A smaller furnace 60 has a combined hopper and combustion chamber and has an upper lid 61 for access to the hopper. The upper hopper portion contains a reservoir of un-burnt sold fuel 63 which moves downwardly towards a pre-combustion zone 64 at an upper grate 65 provided by widely spaced box section bars 66 containing water 67 and air tubes 68 having upper air vents to aerate the pre-combustion zone. Larger pieces of fuel will be trapped on this upper grate 65 and burnt to the stage where they are small enough to fall between the bars 66 and drop onto the bed of hot coals 75 burning in the combustion chamber 70. The combustion chamber 70 has air inlets 71 around its sides and supplied by fans 72, with some of these inlets being around the combustion gas outlet 74 which is provided by a plurality of apertures in the vertical face of the furnace. Although not shown in this way the air inlets may also be interspersed between the apertures of the combustion gas outlet. A chimney (not shown) can be added to the combustion gas outlet. An ash outlet 76 is provided at the bottom of the combustion chamber 70 and allows ash to fall onto an ash pile 77 in chamber 78. The size of this ash pile is regulated by an auger 80 or other conveying means. By varying the ratio of air supplied to the pre-combustion and combustion zones it is possible to regulate the rate at which the bed of hot coals is built up or consumed. ADVANTAGES By using a pile of removable material (such as ash) below the ash outlet it is possible to regulate the combustion and removal of material from the combustion chamber as well as avoiding the build up of clinker or non-combustible material (such as stones or metal rubbish) which occurs in prior art downdraft furnaces on the grates situated below their combustion chambers. The design of the furnace of this invention with its combustion gas outlet at or below the level of the combustion chamber also allows for a "clean-burning" operation as any smoke or combustion gases from the cooler pre-combustion zone must pass through a filter of hot coals in the combustion chamber before reaching the combustion gas outlet. My designs, all of them in this application, have the advantage of completing the oxidation of all smoke and combustible gases while they are still inside the hot filter material because the secondary air is supplied uniformly in the exit zone of this filter of hot coals. VARIATIONS It is preferred that a central auger runs the length of the combustion chamber (making it a simple matter to design furnaces of different lengths) but other arrangements are possible, e.g. one or more augers or other conveying means may run at right angles to the ash outlet. Instead of a central auger any other means may be used to regulate the relationship between the ash outlet and its associated ash pile. For example the lower chamber might have a sloping base to generally bias the ash pile toward an exit at one end of the chamber. This is however a less preferable system, as rocks or slag failing to slide down this slope could cause difficulties. Vibrating apparatus acting on the sloped base could lessen the likelihood of such difficulties. As further alternatives reciprocating pushers or scraper chain conveyors could be used, or one or more relatively high speed augers could be used with an on/off switching system, such that when the ash pile reaches a particular height, the auger or augers are switched on, and act to reduce the height of the pile until it falls below a predetermined level, whereupon the auger or augers may be switched off. Any of a number of switching systems might be used for this. One such system could use a heat sensor placed in the airflow passage out of the upper chamber into the lower chamber 20 (of figure 1) such that when the level of ash rises to a point where air flow is significantly blocked, and the combustion rate lowers, the switch will be activated and the ash level lowered.

Claims: A downdraft or sidedraft furnace having a combustion zone in a combustion chamber (10, 70), an ash outlet (11, 76) below the combustion zone, an ash chamber (21, 78) below the ash outlet (11, 76), combustion gas outlet(s) (11, 74) at or below the level of the combustion zone in the combustion chamber (10, 70) solid fuel supply (35, 62) to supply solid fuel to the combustion zone, and air inlet(s) (14,19,48,49,68,71) opening into the combustion zone positioned at or above the level of the combustion gas outlet(s) (11, 74), at least one said air inlet (19,71) directed in close proximity to said ash outlet (11,76) to promote hot coal formation and combustion at said outlet (11,76), characterised in that the ash outlet (11,76) is unobstructed, and there is ash removal means comprising one or more movable impeller elements (22,40,47,80) located in the ash chamber (20,78). A downdraft or sidedraft furnace as claimed in claim 1, characterised in that coolant ducts (41,66) lead into or around the combustion chamber (10,70). A downdraft or sidedraft furnace as claimed in either preceding claim, characterised in that the ash removal means includes one or more augers (22,40,47,80). A downdraft or sidedraft furnace as claimed in any one of the preceding claims, characterised in that the combustion gas outlet (11) is also the ash outlet. A downdraft or sidedraft furnace as claimed in any one of claims 1 - 3, characterised in that combustion gas outlets are provided by a plurality of apertures (74) in at least one wall of the combustion chamber (70). A downdraft or sidedraft furnace as claimed in any one of claims 1 to 5, characterised in that a moving pusher block (44) is mounted above the surface of a grate comprising bars (33) provided in the upper part of the combustion chamber (10). A downdraft or sidedraft furnace as claimed in any one of claims 1 to 5, characterised in that a grate (65) is provided in the upper part of the combustion chamber (70) and that a pre-combustion zone (64) is provided above the grate (65), said grate (65) comprising bars (66) having air inlet channels therein and having a spacing between the bars (66) less than or equal to the width of the ash outlet (76). A downdraft or sidedraft furnace as claimed in any one of the preceding claims, characterised in that the air inlets include primary air inlets (14,33,68) and secondary air inlets (19,71) directed into the combustion zone, with the primary air inlets (14,48,49,68) directed into an upper part of the combustion zone, and the total cross-sectional area of the primary air inlets (14,48,49,68) is greater than that of the secondary air inlets (19,71). A method of operating a downdraft or sidedraft furnace according to any of the preceding claims 1 to 8 characterised in that the method comprises the steps of burning solid fuel in air in the combustion zone of the combustion chamber (10,70) in a downdraft or sidedraft mode, producing and maintaining an ash pile (20,21,77) under the ash outlet (11,76) and building and maintaining a bed of hot coals supported on the ash pile (20,21,77) to cover the combustion gas outlet(s) (11,74) within the combustion chamber (10,70) so that all the combustion gases pass through a filter of hot coals before leaving the combustion chamber (10,70) through the combustion gas outlet(s) (11,74).

Inventors: PIKE, CLINTON BADGER
Applicants: PIKE, CLINTON BADGER
Application year: 1987
Main classification: F23B7
IPC codes: F23G5, F23H13, F23B99, F23L9, F23H1, F27D23, F27D3, F23H15, F27D15, F23J1, F27D7, F24B1