GB2070212A - Combustion apparatus and process - Google Patents

Abstract

Substantially smokeless combustion of refuse or other liquid or solid fuel may be effected in apparatus comprising a sub-stoichiometric combustioner chamber (1) in which the fuel may be burnt sub- stoichiometrically to produce a stream of dirty combustible gas, a final combustion chamber (26) that includes a tubular portion in which there is a circumferential slot (8) through which air may be drawn into the chamber by gas flowing along the portion, and a duct (22) for leading the stream of dirty combustion gas to the tubular portion and chamber (26). The chamber (26) is preferably horizontal and either or both sides of the slot are preferably defined by flanges (29 and 30). There may be a heat exchanger (10) for collecting the heat. <IMAGE>

Description

SPECIFICATION Combustion apparatus and process There is an urgent need for simple and reliable apparatus capable of burning refuse in such a manner as to give substantially no smoke or other pollution.

Combustible material can be burnt moderately satisfactorily by burning it initially in a combustion apparatus under sub-stoichiometric conditions (i.e. insufficient oxygen is supplied for complete combustion) thereby forming a combustible gas stream which, if mixed with oxygen (e.g. air) and ignited will burn to completion. Desirably the addition of oxygen is effected simply and combustion is such as to give no smoke. Processes that aim at these objectives are known but they tend to be complicated and to give inferior combustion, for example resulting in smoke pollution.

Various devices are known for purifying exhaust gases, for instance from car engines or industrial stacks, by passing the gases with additional oxygen to an after-burner. However this is of no help in the practical design of combustion apparatus for refuse.

Apparatus according to the invention for the substantially smokeless combustion of liquid or solid fuel comprises a sub-stoichiometric combustion chamber in which the fuel may be burnt sub-stoichiometrically to produce a stream of dirty combustible gas, a final combustion chamber having at least one tubular portion in which there is a circumferential slot through which air may be drawn into the chamber by gas flowing along the portion, and a duct for leading the stream of dirty combustible gas to the tubular portion, or one of the tubular portions, of the final chamber.

A method according to the invention comprises burning fuel in the sub-stoichiometric combustion chamber to produce a stream of dirty combustible gas, drawing excess oxgyen for smokeless combustion of the stream into the final combustion chamber through the one or more slots and completing smokeless combustion of the gas in the final combustion chamber.

The fuel is normally refuse which may be solid or liquid, for instance a suspension of solids in a liquid fuel. The sub-stoichiometric combustion chamber should therefore be designed for the sub-stoichiometric combustion of such fuel, and generally is designed so as to burn the fuel while in bulk form. Thus the chamber may have, for instance, a door through which solids can be charged. Air or other oxygen containing gas required for the sub-stoichiometric combustion is preferably introduced at or near the base. The method and rate of introduction should be such that fluidisation of the fuel does not occur. The chamber may include means for igniting the charge or ignition may be spontaneous. The amount of oxygen introduced into the chamber is insufficient for complete combustion.

The sub-stoichiometric combustion results in a gas stream comprising volatiles, and sometimes a little entrained char, being driven off from the chamber. It is often desirable for some combustion of the volatiles iri the gas stream to be effected in the duct, before the gas stream enters the tubular portion of the second chamber. To promote controlled combustion of volatiles in the duct a small amount of oxygen (e.g. air or other oxygen containing gas) may be introduced at the top of the first chamber or in the duct, normally in the part of the duct close to the first chamber. This introduction of the air may be effected by pumping air through one or several orifices distributed around the duct. The amount of air is of course insufficient for complete combustion.

Preferably an ignitor is provided for igniting the mixture of combustible gas and oxygen containing gas. Conveniently this ignitor is a single burner positioned to direct its flame along the tubular portion, and generally along the final part at least of the duct. Preferably the flame is substantially coaxial with the duct and the tubular portion but if this is not convenient it may be at an angle of below 90 to the direction of gas flow. If the gas flow is substantially horizontal then the flame is preferably also directed substantially horizontally, generally with a slight downwards inclination.

An essential characteristic of the invention is the simplicity of the apparatus, and preferably nothing is provided in the duct between the sub-stoichiometric chamber and the slot other than the optional inlets for oxygen, mentioned above, and the single ignitor burner. Further, the part of the duct through which the combustible gases pass in travelling to the slot may even be free of both these features. Thus the oxygen inlets may be absent or may be provided in the top of the substoichiometric chamber, and the ignitor burner may be positioned in an extension of the duct that extends behind (i.e. distant from the slot) the point at which the combustible gases enter the duct, the flame from the ignitor extending along the duct and on the slot side of the point where the gases enter the duct.

Means may be provided for recovering heat from the burnt gas mixture by providing a heat exchanger for removing heat either from the second combustion chamber or from a subsequent duct through which the hot gases pass.

The apparatus may be designed primarily for the combustion of refuse, for instance domestic refuse, with heat recovery being optional but generally heat recovery is important and sometimes good quality fuel may be used to generate the heat, instead of refuse. If heat recovery is important then the ignitor which is preferably provided for igniting the mixture of combustible gas and oxygen may itself be a burner for the sub-stoichiometric combustion of gas or oil, since then this burner can be operated to provide the gas for combustion whenever it is necessary to supplement the amount of gas generated by the sub-stoichiometric combustion chamber. As a result, even if this first chamber operates on a batch feed it is possible, as a result of supplying oil or gas to the ignitor burner, to maintain a continuous supply of combustible gas and thus heat.

The final combustion chamber has a tubular, generally cylindrical, portion that serves as an inlet and is of sufficient length to ensure that gas is drawn in through the slot as described below and generally the entire second chamber is tubular and is often of constant diameter.

Part (and generally all) of the final combustion chamber is generally positioned horizontally. The chamber may lead to a vertical stack, generally after any heat exchanger, and means may be provided for promoting the updraught in this stack. For instance there may be a blower at the base of the stack or compressed air may be forced into the stack through an annular slot under conditions such that, by virtue of the Coanda effect, the updraught is increased. Combustion may be completed in the second chamber or burning gases from the second chamber may be used for burning other combustible material e.g.

organic solids, in a subsequent combustion apparatus.

An important feature of the process and apparatus of the invention is that air or other oxygen containing gas is drawn into the stream of combustible gases entering and passing along the tubular inlet. In order that gas is drawn in at an adequate rate for achieving complete combustion it is necessary for the slot to have appropriate dimensions.

An important aspect of the invention is that it has been found that the combustion within the second combustion chamber can be controlled extensively and accurately by appropriate design of the slot (or each slot).

Generally the slot is exposed to air at substantially atmospheric pressure. Thus it may be exposed merely to the atmosphere but for safety reasons it is sometimes desirable to enclose the area surrounding the slot in a protective casing. If it is so surrounded this casing may be open to the atmosphere or may be provided with forced draught, for instance by a fan, such as to create an ambient pressure of substantially atmospheric pressure.

The amount of air or other oxygen containing gas flowing through the slot is thus preferably controlled primarily by the dimension and shape of the slot and the gas flow through the tubular portion of the chamber and is preferably not controlled by throttling or increasing the supply of air to around the slot.

The nature of combustion within the final chamber, and in particular the shape of the flame, can be controlled by appropriate selection of the angle of the air stream entering the combustion chamber through the slot. Generally the slot is constructed so that the air stream enters at an angle of from 45 to 90 to the axis of the tubular portion.

Generally the slot (or each slot) is an entire circumferential slot but it can instead extend around parts only of the circumference, provided the parts account for the major proportion of the circumference and are distributed substantially uniformly around the circumference.

If the sides of the slot (or each slot) are of the conventionai thickness required or structural reasons in such parts, e.g. 0.5 cm. less satisfactory results are obtained than if at least one of the walls of the slot has a radial length (i.e. the distance between the outer and inner edges of the wall) having a much greater length, generally at least 1 cm, normally at least 2 cm and most usually at least 3 or 4 cm, around a substantial part at least of the circumference. Thus preferably at least one slot wall has this radial length around at least half of the circumference and preferably around the entire circumference.It is often preferred that both walls defining the sides of the slot should have such a radial length around substantially their entire circumference but in some apparatus optimum results are obtained if one wall has such a radial length around all or substantially all the circumference while the other wall has such a radial length over a significant part only, but not all, of the circumference, e.g. about half of the circumference.

When a wall of the first slot is defined by the tubular portion and has an elongated radial length this wall is preferably provided mainly exterior of the tubular inlet, i.e. as an outwardly directed flange. When the other side of that slot is an elongated wall it also may be mainly exterior of the duct, i.e. as an outwardly directed flange around the duct, or it may be within the duct. Thus it may be a restriction in the outlet from the duct, which is thus a nozzle.

In the simplest aspect, the sides of the first slot are defined by circumferential flanges around the outside of the duct and/or tubular portion, and the duct and tubular portion may have substantially the same diameter and flanges of substantially the same dimensions.

Such flanges may be perpendicular to the axis of the duct and tubular portion. The radial length of the flanges (including the thickness of the member to which they are attached) is preferably from 3 to 5 cm, this being especially suitable for ducts having a diameter of around 50 cm. The radial length of the flange is often at least 1/20 of the diameter of the duct. The optimum width of the slot can be determined by routine experiment, by observing the flame quality in the second combustion chamber with different slot widths, but is generally between 1/5 and 1/20 of the diameter of the duct, often around 1/10 of the diameter of the duct.

By providing substantially perpendicular flanges, with at least one preferably extending around the entire circumference, as described above it appears that very turbulent and rapid mixing of the air or other oxygen containing gas with the combustible gas occurs, this in turn leading to a short flame and smokeless combustion. In some instances it is desirable to have a much longer and harder flame, also without smoke, and this can be achieved by creating a venturi effect at the slot. For instance the duct outlet may have a diameter, at its outlet point, less than the internal diameter of the tubular portion and preferably the duct outlet reduces in diameter near the slot as a result of having a conically inwardly tapered end. This inwardly tapered end may itself serve as one of the aforementioned slot walls having a radial length of e.g. 3 cm or more.

Additionally the tubular portion may also define such a wall, generally extending outwardly. Thus the tubular portion may at its entrance have a larger diameter than over the remainder of its length and may then be inwardly conically tapered towards its normal diameter, i.e. leading to a substantially cylindrical throat. The throat normally has a diameter greater than the diameter of the outlet end of the nozzle. This arrangement creates a venturi effect and it is possible to calculate and control very accurately the relative rates of flow of gases through the nozzle and through the slot around the nozzle. It will be appreciated that with such an arrangement the slot may resemble an annular slot rather than a true circumferential slot.

Instead of defining one side of the slot by a conically tapered end portion of the duct it can be desirable to define it by an end of the duct that has a step-wise reduction in diameter.

As mentioned, the second chamber is preferably substantially tubular but beyond the tubular, and generally cylindrical, inlet portion it may lead to an outwardly conically tapered diffuser portion, especially when it is designed to create a venturi effect as discussed above.

Depending upon the purpose of the apparatus the included angle of the diffuser may be 12" or less in order to reduce pressure loss to a minimum or may be above 12" so as to retain turbulence near the walls of the combustion chamber.

Additional oxygen may be supplied to the final chamber in a controlled amount, for instance by pumping controlled amounts of air or other oxygen containing gas into the tubular portion, for instance along the duct.

However at least half and generally substantially all, for instance above 90%, of the oxygen fed to the final chamber is drawn in through the described slot or slots.

It is of course necessary that the final chamber should be sufficiently long to give adequate residence time for complete combustion and that it should be possible to maintain a sufficiently high combustion temperature or a sufficient amount of oxygen containing gas for complete combustion, or both, along the entire length of the chamber. It may therefore sometimes be desirable for the combustion chamber to comprise a plurality of combustion zones with air or other oxygen containing gas being drawn through a slot in the described manner at the start of each combustion zone.

For instance the chamber may comprise the tubular portion into which the duct leads from the sub-stoichiometric chamber, a first combustion zone, a second tubular portion into which the gases from the first zone are lead and in which there is a circumferential slot through which air may be drawn into that tubular portion, and a second combustion zone. A supplementary burner may be positioned to direct its flame along the tubular portion leading into the second combustion zone, and into any subsequent combustion zones. When the final combustion chamber is to be particularly long, for instance when it is to comprise a plurality of combustion zones, it may be desirable for the chamber to be horizontal and to be folded back upon itself, for instance with the second zone being horizontal and substantially parallel to the first zone.

When the final combustion chamber is to include more than one slot each slot may be constructed as described above.

The invention is illustrated in the accompanying drawings in which Figs. 1, 4 and 6 are diagrammatic side views of three forms of apparatus according to the invention while Figs. 2, 3 and 5 are diagrammatic side views of parts of apparatus according to the invention.

The apparatus shown in Fig. 1 comprises a first combustion chamber 1 provided with a base feed 2 for air supplied in predetermined amounts by blower 3. The burner 1 is charged batchwise by, for instance, baled refuse through door 4. A starter burner 5, e.g.

fired by oil or gas, may be provided at any convenient location to ignite the refuse, particularly if the refuse is wet. The chamber is preferably cylindrical in cross-section and is filled, for instance three-quarters full, before initiating combustion.

The resultant combustible gases leave the burner 1 at outlet 6 and pass through duct 22 into a tubular portion 7 that serves as the inlet to a second combustion chamber 26, the portion 7 being separated from duct 22 by a circumferential slot 8. An ignitor 9, for instance an atomising oil burner, directs a flame coaxially (or slightly downwards) with duct 22 and the passage of hot combustible gas along duct 22 and past slot 8 draws air in through the slot and combustion occurs within the chamber 26. Suitable heat exchangers 10 are provided along part or all of the tubular combustion chamber to recover heat of combustion from the burning mixture and the burnt gases are then vented to the atmosphere through a vertical stack 11.

To increase gas flow along the combustion chamber 26 and up the stack 11 compressed air may be pumped to a device having an annular inlet chamber 1 2 and a diffuser portion 1 3 into which air is foreced through a narrow peripheral slit 23, thereby generating an upwardly directed air stream which induces upflow by the Coanda effect.

As a result of blowing air or other oxygen containing gas into the base of the chamber 1, optionally combined with ignition using ignitor 5, there is partial combustion and the formation of a combustible gas stream containing volatiles that leaves through outlet 6.

Some combustion of this stream preferably occurs between the outlet 6 and the slot 8, in the duct 22. To promote this combustion air may be supplied to a manifold 27, e.g. from the blower 3 by means not shown, and hence through a number of orifices 28.

In the apparatus shown in Fig. 1 the slot 8 is defined by circumferential flanges 29 (on the end of the duct 22) and 30 (on the tubular inlet 7). These flanges extend around the entire circumference of the duct and the tubular inlet.

In Fig. 2 a modification is shown, Fig. 2b being a broken away part of the apparatus in Fig. 1 and Fig. 2a being a section BB in Fig.

2b and Fig. 2c being a section on CC in Fig.

2b. In this modification the flange distant from the inlet 6 which leads to the duct 22 extends only around the upper half of the inlet 7. This modification appears to improve combustion and reduce smoke. When the diameter of the inlet is around 50 cm the flanges 29 and 30 are preferably each about 5 cm wide and the width of the slot preferably is about 5 cm. Such an apparatus can conveniently burn about 1 50 kilograms fuel per hour in the chamber 1.

Preferably the rate of blowing through the blower 2 and 3 is maintained fairly low, so as to prevent particulate non-combustible material being carried off with the combustible gas stream.

Conveniently the chamber 4 is charged batchwise and when the volatiles from the first batch have substantially all been driven off, leaving only combustible char, the chamber is recharged, and the process repeated several times, and then eventually air feed through the blowers 2 and 3 continued so as to burn off all the char, e.g. overnight.

In order to facilitate control of the amount of air drawn into the combustion chamber a modification of this apparatus is shown in Fig.

3. The duct 22 leads to an inwardly conically tapered nozzle 1 4 which directs the combustion gases into the open end of the combustion chamber 7, this open end preferably being inwardly conically tapered as shown at 1 5. The nozzle 1 4 and inlet 1 5 together define an annular opening 25, although of course the nozzle can terminate before the inlet. By controlling the dimensions of this and the rate of flow through the nozzle it is possible to obtain accurate control of the rate of entry of air through the annulus 25 and thus of combustion, thereby maximising heat and minimising pollution.

Preferred apparatus is illustrated in Fig. 4 and comprises a primary sub-stoichiometric combustion chamber 1, generally as described above fitted with an outlet 6 and duct 22 for leading combustible gases from the chamber towards a conically tapered nozzle 14. An ignitor 9 for igniting atomised oil or other hydrocarbon fuel in a secondary sub-stoichiometric combustion chamber 9a is provided coaxial with the nozzle 14. The nozzle 14 directs the combustible gases, either from the primary burner 1 or the secondary burner 9 or both into a tubular combustion chamber 1 6 comprising a conically tapered inlet end 15, a cylindrical throat portion 1 7 and an outwardly conically tapered diffuser portion 18.Gases from the diffuser portion then pass through a tubular section 1 9 fitted with an appropriate heat exchanger 10 and discharge through a nozzle 20 into the base of a stack 11. To promote updraught in the stack blower 21 may be provided instead of the device shown in Fig. 1.

Shielding 24 may be provided around the nozzle 14 and inlet 1 5 but must be constructed such that substantially atmospheric pressure prevails within the shielding.

In Fig. 5 the apparatus is designed so that the final combustion chamber gives a sufficiently long residence time, e.g. 2 seconds or more, to enable combustion of gases such as those obtained from the incineration of chemical wastes. This chamber is horizontal and non-linear and consists of two horizontal portions 31 and 32 connected by adjoining portion 33 that is shown as being vertical. Alternatively portions 31 and 32 may be vertical and in side-by-side relationship. To improve combustion the chamber is constructed so that portion 32 serves as a first combustion zone and portion 31 serves as a second combustion zone with a substantially circumferential slot 35 between the two zones. The side walls of slot 35 may be defined by flanges 34 similar to the flanges 29 and 30 discussed previously for defining the side walls of the slot 8.

To increase the temperature in the later parts of the final combustion chamber a sup plementary burner 36 may be provided near the inlet to the second combustion zone. It may operate in broadly the same manner as burner 9 and may direct its flame along the tubular part 37 and the tubular combustion zone 31.

The apparatus illustrated in Fig. 6 is a compact apparatus having a relatively short combustion chamber 26 in particular illustrates a preferred form of the duct 22. The duct has an initial internal diameter D, along part of its length but the outlet 38 from the duct has a much reduced internal diameter D2. A flange 29 extending at least to a diameter D,, and preferably extending beyond that (as illustrated), extends around part at least of the outlet 38. The duct 22 may include a tubular portion 39 having the internal diameter D2. The reduction in diameter from D, to D2 may occur solely by a conically tapered portion 40 or may include also a stepwise reduction as shown at 41.

Although the provision of a horizontal combustion chamber fitted with a heat exchanger is very desirable for many purposes, in some instances, and especially when heat recovery is not necessary, it can be convenient for the final combustion chamber to be vertical. Thus such an apparatus might comprise a substoichiometric chamber 1, as in Fig. 1, with a duct 22 leading vertically upwards from it and in turn leading to a final tubular vertical combustion chamber 7, with the gases leaving the chamber either being vented direct to the atmosphere or through a vertical stack. A suitable arrangement for such a duct and final chamber 7 may be as shown in Fig. 3. When it is desired to provide a burner 9 this may be fitted into the side of the duct 22 so as to direct its flame into an upwardly along the duct.

Depending upon the nature of the combustible material it may sometimes be desirable to provide a fine water spray at the top of the stack 11, so as to decrease the visible effluent from the stack.

When it is necessary, for safety or other reasons, to provide a casing 24 around the slot, as shown in Fig. 4, it is desirable for this casing to include means for maintaining the pressure in the casing at or only slightly above atmospheric pressure. A suitable casing is a solid cylindrical casing having one or more inlets at one end and one or more outlets at the other, generally being positioned so that the air flows from the inlet towards the outlet in the same direction as the gas passing towards the final combustion chamber. The inlet or inlets may be connected to a fan or other means for driving the air into the cylindrical casing and is preferably such that it automatically stalls if pressure within the casing rises significantly, for instance more than a few millimetres above atmospheric pressure.

The outlet or outlets should be constructed so as to permit transit of air across them, but so as to prevent flashing in the event of unwanted combustion. Suitably the outlet or outlets are filled with mesh, for instance, of wire, in which event the pressure drop across she outlets can be controlled by appropriate selection of the length of mesh through which the air has to pass and the void ratio of the mesh.

Claims (22)

1. Apparatus for the substantially smokeless combustion of liquid or solid fuel comprising a sub-stoichiometric combustion chamber in which the fuel may be burnt substoichiometrically to produce a stream of dirty combustible gas, a final combustion chamber having at least one tubular portion in which there is a circumferential slot through which air may be drawn into the chamber by gas flowing along the portion, and a duct for leading the stream of dirty combustible gas to the tubular portion, or one of the tubular portions, of the final chamber.

2. Apparatus according to claim 1 in which the slot or each slot is exposed to air at substantially atmospheric pressure.

3. Apparatus according to claim 1 or claim 2 in which there is a sub-stoichiometric ignitor burner positioned to direct its flame along the tubular portion of the final chamber.

4. Apparatus according to any preceding claim including means for supplying controlled amounts of oxygen to the duct or to the part of the sub-stoichiometric chamber close to the duct.

5. Apparatus according to any preceding claim in which the final chamber is substantially horizontal.

6. Apparatus according to any preceding claim including means for recovering heat from the final chamber.

7. Apparatus according to any preceding claim in which the sub-stoichiometric combustion chamber is designed for burning bulk liquid or solid refuse.

8. Apparatus according to any preceding claim in which the air supply to the substoichiometric chamber is a base feed and is such that fluidisation of the fuel does not occur in the chamber.

9. Apparatus according to any preceding claim in which the slot is defined on one side at least by a surface having a radial length of at least 1 cm around a substantial part at least of the circumference of the slot.

1 0. Apparatus according to claim 9 in which the radial length is at least 3 cm.

11. Apparatus according to claim 9 or claim 10 in which the at least one surface is provided by an external flange.

1 2. Apparatus according to any preceding claim in which the slot is so constructed that air that enters through the slot enters at an angle of from 45 to 90 to the axis of the tubular portion.

1 3. Apparatus according to any preceding claim in which one side of the slot in the tubular portion into which the duct leads is defined by the end of the duct.

1 4. Apparatus according to claim 1 3 in which the end is inwardly conically tapered.

1 5. Apparatus according to claim 1 3 in which the end has a step-wise reduction in diameter.

1 6. Apparatus according to any preceding claim in which the final combustion chamber comprises the tubular portion into which the duct leads, a first combustion zone, a second tubular portion into which the gases from the first zone are lead and having a circumferential slot through which may be drawn, and a second combustion zone, and in which there is a supplementary burner positioned to direct its flame along the tubular portion leading in to the second combustion zone.

1 7. Apparatus according to claim 1 6 in which the second zone is substantially parallel to the first zone and both zones are substantially horizontal.

18. Apparatus according to any preceding claim in which there is a vertical stack into which gases from the final chamber are lead.

1 9. Apparatus according to claim 1 substantially as herein described with reference to any of the accompanying drawings.

20. A process for the substantially smokeless combustion of a solid or liquid fuel in apparatus according to any preceding claim comprising burning sub-stoichiometrically the fuel in the sub-stoichiometric combustion chamber to produce the stream of dirty combustible gas, drawing excess air for smokeless combustion of this stream into the final combustion chamber through the one or more slots and completing smokeless combustion in the final combustion chamber.

21. A process according to claim 20 in which the fuel is liquid or solid refuse charged in bulk into the sub-stoichiometric combustion chamber and the oxygen for combustion in this chamber is supplied to the base of the chamber without fluidising the fuel.

22. A process according to claim 20 or claim 21 in which the apparatus includes a heat exchanger for collecting heat from the final combustion chamber and at least one sub-stoichiometric burner positioned to direct its flame along the axis of the chamber and in which this at least one burner is ignited whenever necessary to-maintain the heat output of the apparatus.