EP1859011A1 - Gasifiers - Google Patents

Abstract

A gasifier (10) has a processing chamber (11) within which combustible material is reduced at an elevated temperature to yield producer gas. Material is introduced at the top of the chamber (11) and undergoes successive processing steps of drying, pyrolysis, gasification and reduction. Below the reduction zone (15) there is a screw auger (57) provided within a horizontally (5) extending tube (56) having an inlet opening (55) communicating with the bottom of the processing chamber so that ash and other waste material may fall into the screw auger tube (56). The outlet (62) from the tube (56) leads to an inclined further screw auger (63) to transfer the ash to a suitable receptacle.

Description

GASIFIERS

This invention relates to gasifiers and in particular to an improved gasifier to facilitate the removal of waste burnt matter (and primarily ash) therefrom.

Gasifiers are well known for the generation of producer gas (a mixture of about 30% carbon monoxide and 70% nitrogen, though other gases may be present) by burning combustible materials under carefully controlled conditions usually at a temperature in excess of 1000⁰C in a gasifier chamber. Many products may be processed in a gasifier, but of particular (though not exclusive) interest here are waste organic substances of various kinds such as dried sewage sludge and vehicle tyres, which otherwise can be difficult to dispose of in an environmentally acceptable manner.

In a gasifier chamber, a combustible material is initially dried (if needed), pyrolysed and gasified to produce a bed of hot carbon. Provided the required combustion conditions are set properly, the oxygen in air introduced into the chamber combines with the hot carbon to form carbon monoxide. Other gases such as methane or hydrogen may also be produced, depending upon the chemical composition of the combustible material.

A known form of gasifier such as that described in our International Patent Publication No. WO 99/61561 has a fuel inlet at or near the top of the combustion chamber, through which the combustible material is introduced into the chamber. The material gradually descends down through the gasifier, in succession through a drying zone, a pyrolysis zone, a gasification zone and a reduction zone, the temperature of the material increasing in the downward direction. A grate assembly is provided at or near the bottom of the chamber to support the column of material within the gasifier chamber, and to permit the removal of waste solid products of combustion, without allowing an inrush of air when those waste solid products are removed.

The principal solid product of the combustion process in a gasifier is ash; for certain fuels, this can be removed from the bottom of the combustion chamber relatively easily. The ash falls through the grate assembly and can then be extracted from the gasifier chamber through an airlock, so that there is no inrush of air through the grate assembly when ash is being extracted. The airlock can instead be provided by a sump which receives the ash falling from the grate assembly, the ash being withdrawn periodically from the sump by a screw conveyor comprising a screw-auger arranged within a tube. With other fuels, there may be produced more solid matter such as hard lumps of slag which can cause blockages in the grate assembly. If too much slag accumulates on the grate assembly, it may be necessary to shut down operation of the gasifier in order to remove the blockages. A particular problem arises if worn vehicle tyres are being processed; in this case, the steel wires incorporated in the tyre casing may combine with ash and other products of combustion to form large and hard lumps of residue, which cannot pass through the grate assembly. Consequently, only relatively small volumes of tyres may be gasified before the gasifier must be shut down for cleaning out the residues.

It is a principal aim of this invention to provide a gasifier which does not suffer from the problems associated with the production of hard lumps of slag and other residues which are likely to block the operation of the grate assembly and so restrict the period over which the gasifier may be operated. According to this invention, there is provided a gasifier including a processing chamber within which combustible matter is reduced at an elevated temperature to yield producer gas, there being a material inlet at an upper part of the chamber and an opening at a lower part of the chamber for the discharge therefrom of burnt matter which opening is in direct communication with a reduction zone of the chamber, and there being removal means to remove burnt matter from the opening which removal means comprises a screw-auger contained within a tube, the tube having an orifice in register with and sealed to said opening at the lower part of the chamber and a discharge opening remote from said orifice, and power-drive means to effect rotation of the auger thereby to draw towards the discharge opening waste matter entering the tube through the orifice therein. The gasifier of this invention does not use a grate, as such, for supporting the column of material to be processed within the gasifier and allowing the separation of ash and other waste solid products of combustion. Rather, the gasifier utilises a screw-auger arrangement to remove ash and waste products from the bottom of the combustion chamber, that auger serving directly to support the column of material to be processed. Apart from simplifying the construction and obviating the need for a grate assembly typically including a shaking mechanism, there is the significant advantage that the design is very much less prone to blockages caused by gasifying materials including significant solid content or chemicals which will solidify into large lumps of slag. In turn, this leads to the possibility of operating the gasifier for longer periods between shutdowns than is currently possible with a gasifier having a complex grate assembly.

A further advantage of the use of a screw-auger is that the material within the auger will serve to effect a seal to a containing tube for the auger. In this way, the entry of air to the gasifier through the removal means for the burnt material will be restricted. Despite this, for operational safety it may be advantageous to provide a valve arrangement at the outlet of the screw-auger, to prevent air moving along the auger and then entering the combustion chamber. In this case, an airlock arrangement with two valves may be provided so that the waste material may be moved from the auger on a batch basis while maintaining the integrity of the airtight seal provided by the valve arrangement.

In one embodiment of this invention, the means to remove the burnt matter includes a pair of screw-augers arranged side by side with their axes substantially parallel to one another. In this case, the augers preferably have threads of opposite hands and the associated power drive means then effects contra-rotation of the two augers, to convey the material to the discharge opening.

Preferably, the auger discharges the material to a further conveyor which lifts the material to a height suitable for discharge into a receptacle. That further conveyor may also be a screw conveyor. - A -

By way of example only, one specific embodiment of gasifier of this invention and a modification to that embodiment will now be described in detail, reference being made to the accompanying drawings in which:-

Figure 1 is a diagrammatic side view, mostly in section, of the embodiment of gasifier;

Figure 2 is a detail view on an enlarged scale of the central region of the gasifier of Figure 1 ;

Figure 3 is an end view on the lower portion of the gasifier of Figure 1 ;

Figures 4, 5 and 6 show alternative inlet arrangements for the gasifier; and

Figure 7 is a view corresponding to Figure 3, but of a modified form of gasifier.

Referring initially to Figure 1 there is shown a gasifier 10 having a combustion chamber 11 within which a combustible fuel is processed under controlled conditions to yield producer gas. When in operation, there are certain distinct zones within the combustion chamber but which of course merge one into the next zone. From the top, there is a drying zone 12, a pyrolysis zone 13, a gasification zone 14 and a reduction zone 15, the producer gas being formed in the reduction zone. Below the reduction zone there is provided an outlet for ash and other waste material, which also serves to support the column of combustible material being processed in the chamber, following its introduction to the gasifier chamber in the drying zone 12.

The upper part 16 of the combustion chamber is of generally conical form, of increasing diameter in the downward direction. At the top of that upper part there is provided a removable lid 17 on which is supported a spear 18 operated by a pneumatic ram (not shown) arranged externally of the lid. The spear may be reciprocated vertically when required as shown by arrow A, to break up any bridging of combustible material which may occur within the combustion chamber and so allow that material to descend downwardly as the combustion process continues. Near the top of the upper part of the combustion chamber, there is provided an inlet arrangement 19 for combustible material to be processed. This inlet arrangement is configured to permit the introduction of the combustible material during operation of the gasifier, without allowing an inrush of atmospheric air. Figure 4 illustrates one possible form of inlet arrangement, utilising a receptor 20 within which is provided a screw-auger 21 driven by an electric motor 22 through a belt 23. The auger continues into an inlet pipe 24 of the gasifier and seals closely to the internal walls of that pipe. Provided that combustible material is in the receptor 20 and along the thread of the auger 21 , that material forms an effective seal to the walls of the pipe 24 and so prevents both outflow of gas from the combustion chamber and inflow of air into the chamber. When more fuel is to be introduced, the auger 21 is driven by the motor 22 simultaneously with the introduction of more combustible material into the receptor 20, so driving that material along pipe 24 and into the gasifier chamber. To increase the seal, a valve arrangement may be provided on the inlet to the receptor 20, or in pipe 24. In the latter case, the valve is opened only when the motor 23 is operated.

Figure 5 shows an alternative inlet arrangement including an airlock receptor 26 fitted with inlet and outlet slide valves 27,28 powered by respective motors 29,30. In this alternative example, each slide valve is provided with a screw-threaded rod 30,31 , the respective motor driving a nut threaded on the associated rod thereby opening or closing the slide valve, depending upon the sense of rotation of the nut, driven by the motor. Combustible material is introduced by closing outlet valve 28 and then opening inlet valve 27; the airlock receptor 26 is then packed with combustible material whereafter valve 27 is closed and valve 28 opened. A pusher arrangement (not shown) may then be operated to drive that combustible material through inlet pipe 24 into the gasifier, and the process repeated as required.

The combustion chamber 11 is fitted with an access port 34 through which a probe may be inserted to initiate the combustion process, and also for observing and monitoring the combustion or breaking up a solid mass of combustible material which might form within the pyrolysis zone chamber, where the material is subjected to heat but substantially without oxygen. Nearer the bottom of the conical upper part 16 of the combustion chamber, further ports 35 are provided, for the insertion of temperature probes. The lower end of the conical upper part 16 is secured to a cylindrical central part 36 (see particularly Figure 2), a generally conical transverse support 37 being provided within the upper part 16 immediately above the junction with the central part, that conical support having a central opening 38. Below the conical support 37 is a further conical wall 39 also having a central opening 40, but of a smaller diameter than the opening 38. Mounted in the further conical wall 39 is a plurality of tuyeres 41 arranged on a common pitch circle, to direct air into the gasifier chamber, in the region of the central opening 38 of the conical support.

A tube 43 extends downwardly from the further conical wall 39, concentrically with the central opening 40. A radial wall 44 extends from the inner periphery of the lower part 36 to the tube 43, thereby defining a plenum 45 for inlet air, within the central part 36. That air is supplied under pressure to fitting 46 communicating with the plenum. One or more closed openings 47 give access to the plenum 45, for example for servicing the tuyeres 41. Suspended below the tube 43 is a sleeve 48, which may be adjusted axially of the tube 43 to a required position. For this purpose, the sleeve 48 is provided with axially extending slots (not shown) through which bolts 49 extend and are threaded into the tube 43. In this way, the operation of the gasifier may be tuned to suit particular fuels and operating conditions, by adjusting the relative position of the sleeve 48 with respect to the tube 43. The outer wall of the sleeve 48 is perforated as shown at 50, for a purpose to be described below.

The cylindrical central part 36 of the combustion chamber is connected to a lower part 52 of the same diameter as the central part but having a rectangular base plate provided with legs 53 (Figure 3) to support the gasifier. The lower part 52 defines the reduction zone 15 wherein the producer gas is generated, in conjunction with the perforated sleeve 48. At the bottom of the lower part 52, there are provided inclined end walls 54 leading to a slot-like orifice 55 in a tube 56 extending horizontally below the lower part 52. Within the tube 56 there is provided a screw-auger 57 comprising a central shaft 58 with a continuous helically formed plate 59 extending therearound and closely fitting to the tube, the plate 59 defining a coarse thread. A motor drive 60 is provided for the auger 57 such that when operated in the correct sense, the auger 57 will transfer material from the orifice 55 to the further end of the tube 56 whereat there is provided a discharge opening 62. Though not shown in the drawings, in order to ensure no air moves back along the auger, especially when relatively low density ash is being discharged or during start-up conditions, a valve may be provided at the discharge opening 62, which valve is opened only when matter is to be discharged. It would also be possible to provide an airlock arrangement including slide-valves, such as has been described with reference to Figure 5, effectively to seal the discharge opening 62 from incoming air, at all times.

A further screw conveyor 63 (Figure 3) of a similar construction to the tube 56 and auger 57, and also having a motor drive 64, is arranged to receive material from the discharge opening 62 and to convey that material to a discharge chute 65. The screw conveyor 63 is mounted with its axis inclined upwardly from the auger 57 and has an appropriate length and angle of inclination to ensure that the lower end of the discharge chute 65 is sufficiently high for the release of waste burnt material into a suitable receptacle.

Figure 6 shows a modified arrangement for the upper end of the combustion chamber 11 , giving a choice of mechanisms for introducing combustible material into the gasifier. A lid 70 is releasably secured to the upper end of the chamber 16 whereby access may be gained to the interior of the chamber for cleaning, servicing and so on. Mounted centrally on the lid is a double valve arrangement 71 including an inlet slide valve 72, an outlet slide valve 73 and a receptor 74 for material to be introduced into the gasifier. In normal operation, valves 72 and 73 are closed but when a new charge is to be introduced, valve 72 is opened and material is fed into the receptor 74. When full, valve 72 is closed and then valve 73 is opened so allowing the material to fall under gravity into the chamber. When receptor 74 is empty, valve 73 is closed again. A conveyor may be provided to lift material to the top of receptor 74 and a power arrangement provided for opening and closing the valves 72 and 73 so obviating the need for human access when the gasifier is in operation.

In addition to the mechanism described above, the gasifier may also be provided with essentially the same charging arrangement as has been described above with reference to Figure 4. The same reference numbers are employed to identify components the same as those of Figure 4 and those components will not be described again here.

In use, combustible material to be processed is introduced into the gasifier until a sufficient quantity has been built up on the conical support plate 37. Combustion is started by means of a probe introduced through access port 34, while air is blown into the plenum 45 through fitting 46. Suction is applied to pipe 67 communicating with the reduction zone 15, to draw out the gaseous products of combustion. Once combustion is underway, further combustible material may be added and the temperature built up within the chamber 11 , until a steady state is reached where fresh combustible material is dried in zone 12, the material is then pyrolysed in pyrolysis zone 13, is gasified in gasification zone 14 and then is reduced in reduction zone 15 to yield producer gas. Once this steady state operation has been achieved, following drying the material is heated essentially in the absence of oxygen in the pyrolysis zone and then is reduced to carbon in the gasification zone by air introduced through the tuyeres 41.

The hot carbon (typically at about 1000⁰C) descends through tube 43 and into the sleeve 48, where the producer gas is generated by the oxygen of the incoming air which also descends through tube 43 and sleeve 48, to combine with the carbon to form carbon monoxide. That gas leaves the sleeve 48 through the perforations 50 and then is drawn out of the gasifier through the gas outlet pipe 67. Suction is applied to that fitting for example by a centrifugal pump (not shown), to draw the producer gas out of the lower part 52 of the chamber, in this way ensuring that the air from the tuyeres is drawn downwardly through tube 43. When steady state operation has been achieved, ash will build up in the lower part, supported solely by the auger 57. The ash building up between the threads of the auger form a seal to the tube 56 and so prevent air being drawn into the gasifier chamber, from the discharge opening 62. Periodically, as and when required, the motor drive 60 is operated to turn the auger 57 to draw waste material from the orifice 55 and convey that material to the discharge opening 62. Operation of the screw conveyor 63 then takes that material to the discharge chute 65, to be received in a suitable receptacle (such as a waste skip) for subsequent disposal.

It will be appreciated that this embodiment of gasifier does not employ any kind of grate to support the column of material being processed within the chamber. Rather, the auger 57 together with its tube 56 and the end walls 54 of the lower part 52 of the chamber all support the column of material. In the region of the auger 57, the waste material typically takes up an angle of repose as shown in broken lines 68 whereby the column of material is supported through the tube 43. This arrangement has the advantage that the auger will tend to break up lumps of material which might form as a consequence of the particular fuel being gasified, which lumps in a conventional gasifier might cause blockages. Since steady state operation takes a significant time to build up and establish full gasification, the process is best operated on a continuous basis. If it is necessary to remove blockages with a conventional gasifier having a grate, a complete shutdown with a relatively long period out of service is required.

Figure 7 shows a modification of the gasifier design described above. Here, there are provided two screw-threaded augers 75,76 arranged side by side with their axes parallel and extending in a common oval tube. The augers 75,76 have helical plates of opposite hands and the motor drive is arranged to cause contra-rotation of the two augers. The diameter of each auger is such that the periphery of one is closely adjacent the periphery of the other, in order to optimise material transfer and minimise gas leakage, once the tube containing the two augers is full of waste material.

The gasifier of Figure 7 operates in essentially the same manner as has been described above. However, by providing two augers side by side within the same tube, it is possible to break up larger lumps of material and transfer larger pieces to the discharge chute than is the case with the embodiment of Figures 1 to 6.

Claims

1. A gasifier including a processing chamber within which combustible matter is reduced at an elevated temperature to yield producer gas, there being a material inlet at an upper part of the chamber and an opening at a lower part of the chamber for the discharge therefrom of burnt matter which opening is in direct communication with a reduction zone of the chamber, and there being removal means to remove burnt matter from the opening which removal means comprises a screw-auger contained within a tube, the tube having an orifice in register with and sealed to said opening at the lower part of the chamber and a discharge opening remote from said orifice, and power-drive means to effect rotation of the auger thereby to draw towards the discharge opening waste matter entering the tube through the orifice therein.

2. A gasifier as claimed in claim 1 , wherein the axis of the tube is arranged substantially horizontally, to extend below the gasifier chamber.

3. A gasifier as claimed in claim 1 or claim 2, wherein said means comprises a pair of screw-augers arranged side-by-side within a common tube.

4. A gasifier as claimed in claim 3, wherein the pair of screw-augers are arranged with their axes substantially parallel to one another.

5. A gasifier as claimed in claim 3 or claim 4, wherein the screw-augers have threads of the opposite hands and the power-drive means is arranged to effect contra-rotation of the two screw-augers, so that both augers draw waste matter to the discharge opening.

6. A gasifier as claimed in any of the preceding claims, wherein there is provided a further screw-auger contained within a further tube having an inlet opening in communication with said discharge opening whereby waste matter leaving the discharge opening falls into the further tube and is then moved by the further screw-auger on operation thereof.

7. A gasifier as claimed in claim 6, wherein the axis of the further tube is disposed at an angle to the horizontal and inclined upwardly from the inlet opening of said further tube.

8. A gasifier as claimed in any of the preceding claims, wherein there processing chamber includes an intermediate transverse wall below which is formed the reduction zone, the transverse wall being generally of conical shape with a central opening through which matter to be reduced passes.

9. A gasifier as claimed in claim 8, wherein the reduction zone is defined by a tube extending downwardly from the transverse wall towards to the opening for waste matter.

10. A gasifier as claimed in claim 9, wherein the tube is perforate whereby the producer gas may leave the reduction zone through the perforations in the tube.

11. A gasifier as claimed in any one of claims 8 to 10, wherein air is introduced into the processing chamber through tuyeres provided in the transverse wall and leading to the interior of the processing chamber.

12. A gasifier as claimed in any of the preceding claims, wherein a valve arrangement is provided at the discharge opening from the screw-auger.

13. A gasifier as claimed in claim 12, wherein an airlock arrangement having two valves is provided at the discharge opening from the screw-auger.

14. A gasifier as claimed in any of the preceding claims, wherein the material inlet at the upper part of the processing chamber is provided with an airlock arrangement to prevent the ingress of air when combustible matter is fed into the processing chamber.