GB2231131A - Apparatus and method for combustion of solid fuel - Google Patents

Abstract

Apparatus for the combustion of solid fuel, e.g. in the form of discrete pieces of rubber, comprises a combustion chamber 1, delivery means for delivering fuel into said chamber, means 12 for controlling the amount and bulk density of fuel delivered to said chamber, means for supplying said chamber with combustion medium for combustion of fuel therein, means for modulating the ratio of said combustion medium to fuel in said chamber, means for providing steam or compressed hot air in said chamber, means for controlling the amount of steam or compressed hot air in said chamber, means for the outlet of combustion products from said chamber, and means for discharging ash from said chamber. <IMAGE>

Description

TITLE: APPARATUS AND METHOD FOR COMBUSTION OF SOLID FUEL DESCRIPTION This invention relates to apparatus and a method for the combustion of solid fuel of varying bulk density and calorific value, e.g. rubber and particularly, but not exclusively, of scrap car and truck tyres. For convenience, references are made hereinafter to the combustion of rubber. However, it should be understood that in the context of this specification the term "rubber" includes other solid fuels of varying bulk density and calorific value.

Hitherto, fuels such as coal or oil have been used to fuel conventional combustion chambers. However the cost of these fuels can make the provision of a heating system very expensive. Moreover, financial planning by establishments which depend upon a continuous and flexible heat supply system throughout the year is often frustrated by the fluctuating prices of solid fuels.

Consequently, it is evident that, the provision of an alternative, inexpensive fuel would represent a significant economic advantage.

Hydrocarbons are efficient combustible fuels and recently a number of hydrocarbon based fuels have been considered as alternative energy sources and attention has been focused upon waste products such as wood, paper and, more recently, rubber waste, such as scrap tyres.

Rubber is a particularly attractive candidate as an alternative fuel source because in economic terms it has a negative market value. Furthermore, it is also an abundant commodity; it has been estimated that there is waste arising to approximately 2 x 108 kg of rubber tyres per year in the U.K. alone. Moreover, the use of rubber as a fuel solves the problems associated with its waste disposal whilst displacing the need to use coal or oil as a fuel. Currently, methods of rubber disposal involve burying the material, preferably after shredding it. The shredding process is beneficial as it reduces the bulk density of the rubber thus facilitating its burial and furthermore it enhances the stability of the land fill process.

Hitherto, it has not been possible to use rubber as a fuel due to the technological deficiencies associated with burning rubber in existing combustion plants. The principle problem associated with the combustion of rubber involves the unacceptable production of levels of black smoke and associated pollutants.

It is known that combustion is dependent upon: the time, temperature and turbulence within the combustion chamber. However, the residence time of the products of combustion within the combustion chamber is minimal.

Hence, in a typical combustion chamber, such as a shell furnace, the duration of combustion is insufficient to burn the evolved toluenes, benzenes and carboniferous particulate of rubber, without substantial modification to the furnace. A further difficulty associated with burning shredded rubber is the fact that it is characterised by a variable bulk density which is partially related to particle size and cleanness of chop of the rubber pieces. A further difficulty which affects bulk density is represented by the matting of rubber pieces due to exposed reinforcing wire. These difficulties result in a need to balance the oxygen/fuel ratio, which may be achieved through oxygen sensing devices which are in communication with combustion gases and a control system which increases or decreases the rate of fuel feed according to the oxygen level.

Consequently the effective combustion of rubber is aided by optimising the conditions within a combustion chamber and also by optimising the fuel density fed to the combustion chamber.

It is an aim of this invention to provide a safe and efficient means of rubber combustion which is achieved by overcoming or mitigating problems which hinder the complete combustion of rubber.

In the invention as hereinafter defined conditions within the chamber are adapted to aid combustion by the provision of at least one jet of steam or compressed hot air. It has been established that toxic emissions which are characterised by a dark smoke are largely neutralised by the introduction of steam or compressed hot air.

Hence the dark smoke is burnt within the combustion chamber although the exact chemical reactions will not be described herein. Suffice it to say that, in terms of known toxic compounds, clean combustion occurs. In addition, the fuel supply is carefully metered to control the bulk density of the fuel fed into the combustion chamber.

The invention enables the adaption of currently used combustion chambers with a conventional solid fuel grate and thus provides for a comparatively inexpensive means of combusting rubber fuel. However, the invention is also capable of application in specially built combustion chambers.

It will be understood -that the steam supply may be derived from water jets which are converted into steam as a result of the radiant heat present within the combustion chamber.

According to a first aspect of the invention there is provided an apparatus for the combustion of fuel in the form of discrete pieces of rubber or other material of varying calorific value and bulk density, which apparatus comprises: a combustion chamber; delivery means for delivering fuel into said chamber; means for controlling the amount and reducing the bulk density variance of fuel delivered to said chamber; means for supplying said chamber with combustion medium (comprising air) for combustion of fuel therein; means for modulating the ratio of said combustion medium to fuel in said chamber; means for providing steam or compressed hot air in said chamber; means for controlling the amount of steam or compressed hot air in said chamber in relation to the output therefrom; means for the outlet of combustion products from said chamber; and means for discharging ash from said chamber.

Preferably, a movable grate is provided for conveying fuel in said chamber. The combustion chamber may comprise a shell and tube boiler, a water tube boiler or a tunnel furnace.

Preferably, said means for providing steam comprise at least one jet for supplying steam or water. Each said jet is preferably arranged to provide a stream of steam travelling in a direction concurrent but angled downwards to the direction of travel of rubber fuel on said movable grate. The jet or jets may be provided in the upper part of the chamber.

In a preferred arrangement, said means for supplying said chamber with combustion medium comprise guide means for directing air to issue therefrom underneath said grate.

Preferably, said chamber is adapted to contain a deep fuel bed, which may desirably be between 150 and 200 mm in depth, whereby grate damage is avoided or minimised.

Said ash discharging means may comprise a water-filled conveyor, which may be located below ground level and discharge ash to an outside tip.

Preferably, screening means are provided for screening the fuel, so that only discrete pieces thereof not exceeding a predetermined maximum size are delivered to said chamber. Thus, a recourse to more extensive fuel handling modifications is made unnecessary. Suitably, the screening means may comprise a filter with holes no larger than 6.35 cm.

Preferably, a surge hopper is provided adapted to meter predetermined volumes of rubber fuel to said chamber.

It is preferred that said delivery means comprises conveyor means which are adapted to supply fuel to said hopper according to the amount of fuel in the hopper.

The hopper is preferably provided with sensing means for monitoring the amount of fuel contained therein. It is preferred that means are provided operatively connecting the sensing means and the conveyor means in such manner that when the hopper is filled to a predetermined extent the conveyor means is deactivated and when the fuel supply in the hopper falls below a predetermined amount the conveyor means is activated and thus conveys a replenishing amount of fuel to the hopper.

Preferably, said hopper is provided with agitator means which facilitates the metered delivery of a predetermined amount of fuel to said chamber.

According to a second aspect of the invention there is provided a method for the combustion of rubber fuel in the form of discrete pieces of rubber or other material of varying calorific value and bulk density, which method comprises: delivering fuel to a combustion chamber; controlling the amount and bulk density of fuel delivered to the chamber; supplying combustion medium (comprising air) and steam or compressed hot air to said chamber; controlling the amount of combustion medium and steam or compressed hot air supplied to said chamber to optimise conditions for combustion of the fuel; and discharging from said chamber combustion products and ash produced as a result of combustion of the fuel.

Preferably, the delivery means is a positive delivery means.

Preferably, fuel is transported through said chamber on a movable grate. It is preferred that the steam travels in the chamber in a direction concurrent with the direction of travel of the fuel on the grate. Desirably, the steam or water for producing steam may be introduced into the chamber at a level above the top of the fuel bed therein.

Preferably, the steam is supplied through one or more jets modulated so that the output thereof is related to the thermal output of the chamber. The steam pressure at said jet or jets may be substantially 7 kg per square cm. Preferably, the energy contained in the steam from said jet or jets is controlled at 3 - 4% of the thermal output of said chamber.

Preferably, the fuel is screened, so that only discrete pieces thereof not exceeding a predetermined maximum size are delivered to said chamber. Said maximum size may be 6.35 cm, pieces of greater size being discarded.

It is preferred that fuel in the chamber is controlled by sensing the amount of said fuel through analysis of outlet flue gases and communicating the information thus derived to the fuel feed whereby the operational status of the conveying means and thus delivery of fuel to the hopper are varied accordingly.

It is preferred that the fuel in a supply hopper feeding the chamber is agitated to facilitate the delivery of a metered amount of fuel to said chamber.

Preferably, the depth of a bed of fuel in said chamber is limited at a predetermined level. The depth of said bed may suitably be between 150 and 200 mm.

Preferably, combustion conditions in said chamber are such that the combustion products contain less than 100 ppm carbon monoxide. Preferably also the combustion products contain between 10% and 14% carbon dioxide.

It is preferred that said combustion medium is introduced below a grate supporting a bed of fuel in said chamber.

The combustion medium will normally comprise air.

Specific embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, wherein: Fig. 1 is a partial sectional side view of a "coking design" low ram stoker combustion chamber adapted in accordance with the invention; Fig. 2 is a partial end view of the ram stoker combustion chamber of Fig. 1; Fig. 3 is a sectional plan view of the ram stoker combustion chamber of Fig. 1, the conventional coking plates are removed; and Fig. 4 is a schematic perspective view of a chain grate stoker combustion chamber adapted in accordance with the invention.

The invention as illustrated in the accompanying drawings lies in the way in which the combustion chambers are adapted so that they comprise apparatus in accordance with the invention and so that they function in a way which is in accordance with the invention. Accordingly, although a number of features are illustrated in the drawings, only those features which are pertinent to an explanation of the embodiments of the invention are referenced and described.

The combustion chamber 1 shown in Fig. 1 comprises a tunnel furnace 2. The furnace 2 is of a conventional size and comprises a grate 5, on which the fuel travels.

The grate 5 is clearly represented in Fig. 3 and comprises a plurality of fire bars 6.

Located within the furnace 2 is a fire bed (not shown) which provides the ignition source for the burning of the fuel introduced into the combustion chamber 1.

Towards the proximal end of the combustion chamber 1 is located a delivery site 3. The delivery site comprises a hopper 8 which is of a conventional form except in that the lower end adjacent to an entrance 9 to the tunnel furnace 2 has been adapted by the provision of an agitator means 12 comprising a reciprocating fuel feed assembly.

Referring to Fig. 2, the entrance 9 to the tunnel furnace 2 can be seen in more detail as hopper 8 has been removed and its former position is represented by dotted lines. Entrance 9 is defined by aperture 13.

Immediately behind the entrance is a front refractory wall 15 in which there are located steam jet assemblies 16.

The assemblies 16 comprise pipes (not shown) leading from a steam generator/boiler suitably rated for the furnace or boiler output. In order to facilitate adaption of the apparatus such that it functions in accordance with the invention, the assemblies 16 can be fitted in existing "spy holes" which are located on either side and within the refractory arch 15 in certain furnace designs. The assemblies 16 also comprise jets 17 which are made of durable steel and have orifices which can be adjusted in a conventional manner. The jets 17 direct steam into the tunnel furnace 2. Instead of using a separate boiler, steam may be supplied from water heated by the operation of the furnace. It will be understood that the location of the jets 17 will determine the direction of the steam issuing therefrom.

Thus, in the embodiment illustrated, steam issuing from the jets 17 will travel in a direction concurrent with the direction of transportation of the fuel in the furnace 2. Conversely, location of the jets 17 in the wall opposite the refractory arch 15, that is in the rear wall of the combustion chamber, would result in the issuance of steam in a direction countercurrent to the direction of travel of the fuel within the chamber. It is also possible that the jets may be arranged so that steam issues tangentially of the furnace or in any other feasible direction relative to the location of the fuel in the chamber. In the preferred embodiment, as illustrated steam issues from the jets in a direction concurrent with the direction of transportation of the fuel in the furnace.

The hopper 8 is also equipped with sensing means 20 which detects the level of fuel within the hopper.

Sensing means 20 is connected via cabling 25 to detector means 22 located on a fuel conveyor means 23 which conveys fuel to the hopper 8. Sensing means 20 functions in such a way that when the level of fuel falls below a predetermined level, the sensing means 20 is activated and thus activates the detector 22 which in turn activates the conveyor 23 so that fuel is delivered to the hopper 8. In this manner the hopper 8 is regularly supplied with fuel according to its fuel status.

Alternatively, delivery of rubber fuel may be controlled by sensing the amount of fuel through outlet gas analysis and temperature measurement and conveying the information to the conveyor means. The grate speed is suitably controlled.

In operation, chopped rubber fuel is firstly screened, by conventional means, so that portion sizes greater than 6.35 cm are rejected. The screened rubber is then transported via the conveyor means 23 to the hopper 8. Activation of the agitator means 12 results in the metering of a predetermined volume of rubber into the furnace 2. The precise metering of rubber contributes significantly towards the success of the invention as the propensity of rubber to aggregate tends to result in the bulk density of the fuel varying uncontrollably. The accurate metering of the rubber fuel coupled with a fuel feed back sensor (sensing 02 in the outlet flue) and a variable speed grate drive overcomes this problem and facilitates the successful combustion of the fuel.

Rubber fuel located in the tunnel 2 is transported through the combustion chamber 1 in a conventional manner. The fire bed provides the heat required to ignite the rubber and efficient combustion is obtained by the provision of the steam assemblies 16. Thus, steam issuing from the jets 17 in a direction concurrent with the direction of travel of the fuel through the combustion chamber optimises the combustion conditions such that effective combustion can take place. In addition, the air supply within the combustion chamber 1 is also adapted to optimise the combustion conditions, the air supply being modulated by means of air supplied via a conduit 26 which directs the air into the furnace 2 from below the fuel bed therein. The fuel bed depth is between 150 and 200 mm such that flame stability is ensured.

Gases produced as a result of combustion are discharged in a conventional manner and ash produced is discharged and disposed of in a conventional manner.

Referring now to Fig. 4, the combustion chamber comprises a tunnel furnace 2A, a delivery site 3A and an ash discharge site 4A. The tunnel furnace 2A is of conventional size and comprises a continuous grate 5A which moves over sprockets 6A in a conventional manner so that the grate 5A circulates continuously. The grate 5A comprises a plurality of fire bars 6A.

The ash discharge site comprises an ash tube 7A which communicates with a water-filled, below-ground conveyor (not shown) for the efficient disposal of ash waste.

Located within the furnace 2A is a fire bed (not shown) which provides the ignition source for the burning of the fuel introduced into the furnace 2A.

Towards the proximal end of the furnace 2A is located a delivery site 3A comprising a hopper 8A which is adapted by providing an agitator means 12A comprising a reciprocating fuel feed assembly which facilitates the metered delivery of a predetermined volume of rubber fuel.

The entrance 9A to the tunnel furnace 2A is defined by the aperture 13A. The aperture 13A may be adjusted by movement of a suspended guillotine plate (not shown), conventional in the art, adjacent to the refractory wall 15A.

Steam jet assemblies 16A with jets 17A is mounted in refractory wall 15A. The assemblies 16A comprise pipes (not shown) connected to a separate steam boiler.

Alternatively, where steam is supplied as a result of the operation of the chain grate stoker, a separate steam boiler may be dispensed with and water can be heated to form the steam as a result of the operation of the stoker.

Jets 17A are made of durable steel and comprise adjustable orifices.

The location of the jets 17A will determine the direction of steam issuing therefrom. Thus in the embodiment illustrated in Fig. 4 steam travels in a direction concurrent with the direction of travel of the circulating grate 5A and thus concurrent with the direction of transportation of fuel.

Conversely, location of the jets 17A in rear tunnel furnace wall 19A would result in the issuance of steam in a direction countercurrent to the direction of travel of the fuel within the chamber. Alternatively, the jets 17A may be located so that steam issues in a direction which is tangential to the direction of travel of the fuel. In the preferred embodiment steam issues in a direction concurrent with the direction of transportation of the fuel.

The hopper 8A is also equipped with sensing means 20A which detects the level of fuel within the hopper.

The sensing means 20A is connected via cabling 25A to detector means 22A located on a conveyor means 23A which conveys fuel to the hopper 8A. Sensing means 20A functions in such a way that when the level of fuel falls below a predetermined level, the sensing means 20A is activated and thus activates the detector means 22A which in turn activates the conveyor 23A so that fuel is delivered to the hopper 8A. In this manner the hopper 8A is regularly supplied with fuel according to its fuel status.

In operation, chopped rubber fuel is firstly screened, by conventional means, so that portion sizes greater than 6.35 cm are rejected. The screened rubber is then transported via conveyor means 23A to hopper 8A.

Activation of the agitator means 12A results in the metering of a predetermined volume of rubber into the furnace 2A. The precise metering of rubber contributes significantly towards the success of the invention as the propensity of rubber to aggregate tends to result in the bulk density and hence the weight of fuel varying uncontrollably. The accurate metering of rubber overcomes this problem and facilitates the successful combustion of the fuel.

Rubber fuel located in the tunnel furnace 2A is transported through the combustion chamber on the circulating (or otherwise moving) grate 5A in a conventional manner. The fire bed provides the heat required to ignite the rubber and efficient combustion is obtained by the provision of steam assemblies 16A. Thus, steam issuing from the jets 17A in a direction concurrent with the direction of travel of the fuel through the combustion chamber optimises the combustion conditions so that effective combustion can take place. In addition, the air supply within the furnace 2A is also adapted to optimise the combustion conditions and the air supply is modulated by means of air supplied via a conduit 26A which directs air into the furnace 2A below the grate 5A.

The fuel bed depth is between 150 - 200 mm so that flame stability is ensured.

Gases produced as a result of combustion are dealt with in a conventional manner and ash produced is discharged and disposed of in a conventional manner.

(Pneumatic methods are not suitable due to the metallic clinker).

Consequently, it can be seen that adapting conventional combustion chambers in a manner in accordance with the invention provides for a safe and efficient means of combusting rubber materials.

The hot combustion gases produced by burning the rubber fuel may be used to generate steam, e.g. for materials drying or space heating, in a boiler of which the combustion chamber forms part or in a separate boiler. Alternatively, the hot gases may be used to provide heat for other purposes.

Used motor vehicle tyres are a very plentiful and suitable source of the rubber fuel.

Claims (40)

1. Apparatus for the combustion of fuel in the form of discrete pieces of rubber or other material of varying calorific value and bulk density, which apparatus comprises: a combustion chamber; delivery means for delivering fuel into said chamber; means for controlling the feeding of fuel to compensate for variances in bulk density delivered to said chamber; means for supplying said chamber with combustion medium for combustion of fuel therein; means for modulating the ratio of said combustion medium to fuel in said chamber; means for providing steam or compressed hot air in said chamber; means for controlling the amount of steam or compressed hot air in said chamber in relation to modulation of the aforesaid ratio; means for the outlet of combustion products from said chamber; and means for discharging ash from said chamber.

2. Apparatus according to claim 1 wherein a movable grate is provided for conveying fuel in said chamber.

3. Apparatus according to claim 2 comprising a shell and tube boiler.

4. Apparatus according to claim 2 comprising a water tube boiler.

5. Apparatus according to claim 2 comprising a tunnel furnace.

6. Apparatus according to any of claims 2 to 5 wherein said means for providing steam comprise at least one jet.

7. Apparatus according to claim 6 comprising at least one steam jet.

8. Apparatus according to claim 6 comprising at least one water jet.

9. Apparatus according to claim 7 or claim 8 wherein each said jet is arranged to provide a stream of steam travelling in a direction concurrent but angled downwards to the direction of travel of rubber fuel on said movable grate.

10. Apparatus according to any of claims 6 to 9 wherein said jet or jets is or are disposed in an upper part of said chamber.

11. Apparatus according to any preceding claim wherein said means for supplying said chamber with combustion medium comprise guide means for directing air to issue therefrom underneath said grate.

12. Apparatus according to any preceding claim wherein said chamber is adapted to contain a fuel bed between 150 and 200 mm in depth.

13. Apparatus according to any preceding claim wherein said ash discharging means comprises a water-filled conveyor.

14. Apparatus according to any preceding claim wherein screening means are provided for screening the fuel, so that only discrete pieces thereof not exceeding a predetermined maximum size are delivered to said chamber.

15. Apparatus according to claim 14 wherein the screening means comprises a filter with holes no larger than 6.35 cm.

16. Apparatus according to any preceding claim wherein a surge hopper is provided adapted to meter predetermined volumes of fuel to said chamber.

17. Apparatus according to claim 16 wherein said delivery means comprises conveyor means which are adapted to supply fuel to said hopper according to the amount of fuel in the hopper.

18. Apparatus according to claim 17 wherein the hopper is provided with sensing means for monitoring the amount of fuel contained therein.

19. Apparatus according to claim 18 wherein means are provided operatively connecting the sensing means and the conveyor means in such manner that when the hopper is filled to a predetermined extent the conveyor means is deactivated and when the fuel supply in the hopper falls below a predetermined amount the conveyor means is activated and thus conveys a replenishing amount of fuel to the hopper.

20. Apparatus according to any of claims 16 to 19 wherein said hopper is provided with agitator means which facilitate the metered delivery of a predetermined amount of fuel to said chamber.

21. Apparatus for the combustion of rubber fuel in the form of discrete pieces of rubber, said apparatus being substantially as hereinbefore described with reference to the accompanying drawings.

22. A method for the combustion of rubber fuel in the form of discrete pieces of rubber or other material of varying calorific value and bulk density, which method comprises: delivering fuel to a combustion chamber; controlling the amount and bulk density of fuel delivered to the chamber; supplying combustion medium including steam or compressed hot air to said chamber; controlling the amount of combustion medium and steam or compressed hot air supplied to said chamber to optimise conditions for combustion of the fuel; and discharging from said chamber combustion products and ash produced as a result of combustion of the fuel.

23. A method according to claim 22 wherein the fuel is chopped or shredded rubber.

24. A method according to claim 22 or claim 23 wherein fuel is transported through said chamber on a movable grate.

25. A method according to claim 24 wherein the steam travels in the chamber in a direction concurrent with the direction of travel of the fuel on the grate.

26. A method according to any of claims 22 to 25 wherein the steam or water for producing steam is introduced to the chamber at a level above the top of the fuel bed therein.

27. A method according to any of claims 22 to 26 wherein the steam is supplied through one or more jets modulated so that the output thereof is related to the thermal output of the chamber.

28. A method according to claim 27 wherein the steam pressure at said jet or jets is substantially 7 kg per square cm.

29. A method according to claim 27 or claim 28 wherein the energy contained in the steam from said jet or jets is controlled at 3 - 4% of the thermal output of said chamber.

30. A method according to any of claims 22 to 29 wherein the fuel is screened, so that only discrete pieces thereof not exceeding a predetermined maximum size are delivered to said chamber.

31. A method according to claim 30 wherein, when the fuel is screened, pieces of a size greater than 6.35 cm are discarded.

32. A method according to any of claims 22 to 31 wherein fuel in the chamber is controlled by sensing the amount of said fuel through analysis of outlet flue gases and communicating the information thus derived to the fuel feed whereby the operational status of the conveying means and thus delivery of fuel to the hopper are varied accordingly.

33. A method according to any of claims 22 to 32 wherein fuel in a supply hopper feeding the chamber is agitated to facilitate the delivery of a metered amount of fuel to said chamber.

34. A method according to any of claims 22 to 33 wherein the depth of a bed of fuel in said chamber is limited at a predetermined level.

35. A method according to claim 34 wherein the depth of said bed is between 150 and 200 mm.

36. A method according to any of claims 22 to 35 wherein combustion conditions in said chamber are such that the combustion products contain less than 100 ppm carbon monoxide.

37. A method according to claim 36 wherein the combustion products contain between 10% and 14% carbon dioxide.

38. A method according to any of claims 22 to 37 wherein said combustion medium is introduced below a grate supporting a bed of fuel in said chamber.

39. A method according to any of claims 22 to 38 wherein said combustion medium comprises air.

40. A method for the combustion of fuel in the form of discrete pieces of rubber, said method being substantially as hereinbefore described with reference to the accompanying drawings.