WO2003012339A1

WO2003012339A1 - Turbulent mixing of fluids

Info

Publication number: WO2003012339A1
Application number: PCT/GB2002/003516
Authority: WO
Inventors: Stuart Jonathan Pratt; Gerry Booth
Original assignee: INVECTOMENT Ltd
Current assignee: INVECTOMENT Ltd
Priority date: 2001-07-31
Filing date: 2002-07-31
Publication date: 2003-02-13
Anticipated expiration: 2004-01-31
Also published as: GB0118623D0

Abstract

Apparatus for turbulent mixing of fluids in, combustion apparatus. Conduits extend into a combustion chamber in a direction transverse to the direction of travel of a first fluid stream. Each conduit has a first outlet for injecting a jet of a second fluid, such as air, into the first fluid stream in said transverse direction. The jets are directed such that they are offset from said axis. The jets combine so as to form a first swirling stream of second fluid around said axis. The first and second fluid streams combine to form a first spiral flow path along the chamber. The wall of the chamber may have at least one passage therethrough extending in a direction transverse to the direction of flow of the first fluid stream. The second fluid passes into the chamber through said passage and combines with said first fluid stream.

Description

TURBULENT MIXING OF FLUIDS

The present invention relates to the turbulent mixing of fluids (e.g. liquid or gases) and in particular to the mixing of gases in combustion, gasification, pyrolysis or incineration apparatus for waste material such as, for example, biomass, household and commercial waste, rubber tyres, mixed plastics.

When materials such as those referred to above are subjected to high temperatures for sufficient time in an oxygen starved gasification or combustion chamber they are broken down into constituents such as a synthetic producer gas and a solid residue such as a carbon char material. The producer gas is usually of such a calorific value that it can be combusted in a separate secondary combustion process or used in a reciprocating engine or gas turbine.

The disposal of waste material in this manner is an environmentally attractive proposition as energy can be recovered from the combustion gases. However, it is difficult to achieve reasonable efficiency whilst restricting the emission of pollutants to atmosphere to an acceptable level. Although gasification processes have been shown to reduce emissions heavy metals and nitrogen oxides it has proved difficult to control the process effectively.

One example of secondary combustion apparatus is described in EP 0444122. In order to reduce or eliminate noxious exhaust emissions from a combustion chamber an outlet stack is lined with a secondary air inlet in the form of a perforated sleeve and a central distributor in the form of an elongated perforated housing. Compressed air is fed to the sleeve and the housing such that rising gases from the main combustion chamber passes over the perforations to draw air from the sleeve and housing so as to effect secondary combustion.

Although the apparatus described above works effectively there is a constant requirement to increase combustion efficiency by improving the thoroughness of the mixing of the combustion gas and the air.

It is an object of the present invention to provide for an improved fluid mixing method so as to increase the efficiency of combustion in apparatus of the kind described above. It is also an object of the present invention to provide for combustion apparatus with improved longevity.

The term "gasification" is used to refer to the thermal degradation of material with an amount of oxygen insufficient for stoichiometric combustion (also known as starved air combustion).

According to a first aspect of the present invention there is provided apparatus for turbulent mixing of fluids comprising a mixing chamber having a wall, an inlet, an outlet and an axis extending between said inlet and outlet, means for injecting a first fluid into said chamber so that it travels from the inlet to the outlet in a first fluid stream, a plurality of conduits extending into said chamber in a direction transverse to the direction of travel of the first fluid stream and each having a first outlet for injecting a jet of a second fluid into the first fluid stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are substantially parallel to a tangent to a common imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of second fluid around said axis, the streams of first and second fluid combining to form a first spiral flow path along the chamber.

The conduits preferably extend towards a central area of the chamber proximal said axis to induce the spiral flow path around said axis.

The first outlet of each conduit is defined by an opening in an end thereof. In one preferred embodiment each conduit has a second outlet defined by an aperture in a side wall thereof, said aperture facing a direction substantially perpendicular to that faced by the outlet so as to direct a further jet of the second fluid into the chamber in a direction substantially perpendicular to the first jet of the second fluid. The further jets of second fluid combine to form a second swirling stream of the second fluid around the axis radially outboard of the first swirling stream, the first and second swirling streams combining. The apertures may be in the form of elongate slots along the length of the conduits.

The chamber is preferably generally cylindrical, said axis being a longitudinal axis thereof. The conduits are ideally offset from radial lines that intersect said axis so as to provide said offset for the outlets. In one preferred embodiment there are provided four conduits spaced around chamber and occupying substantially the same plane through the chamber. The conduits may be substantially equi-angularly spaced.

The outlets are ideally arranged to inject said second fluid into the first fluid stream in a direction perpendicular thereto.

There may be provided multiple conduit groups, said groups being spaced along axis of the chamber. Each group preferably occupies substantially the same plane and may comprise four conduits. In one preferred embodiment there is provided a multiple conduit group near to the outlet whose apertures face in opposite directions to those of the other conduit groups so as to provide a counter-swirl to the second swirling stream.

In one preferred embodiment the wall of the chamber has at least one passage therethrough extending in a direction transverse to the direction of flow of the first fluid stream, the passage defines an inlet for the second fluid and has a longitudinal axis that is a tangent to a further imaginary circle drawn around the axis of the chamber, the second fluid passing into the chamber through said passage and combining with said first fluid stream. The inlet is preferably disposed so as to direct the second fluid close to the wall of the chamber. The (or each) passage may be radially offset from the axis of the chamber so as to form a spiral stream close to the chamber wall.

Ideally there is a plurality of such passages around the chamber wall so as to generate a third swirling stream of second fluid radially outboard of said first and second swirling streams. The passages may be arranged such that they are radially and axially offset from neighbouring passages. Ideally, the passages are arranged in a helical configuration along the chamber wall, the third swirling stream of second fluid combining with said first fluid stream so as to produce a helical swirling flow path progressing from the chamber inlet to the outlet and disposed radially outboard of said first spiral flow path.

Preferably some of the passages are inclined towards the outlet of the chamber and some are inclined towards the inlet of the chamber. The passages inclined towards the inlet are preferably disposed so as to inject said second fluid in an opposing direction to the helical swirling flow path so as to cause the flow within the helix to twist about itself

The (or each) axis of the passage occupies a plane that is inclined relative to a plane that is perpendicular to the first stream of fluid.

One or more passages nearest to the outlet may be reversed in direction so as to provide a counter swirl of second fluid.

According to a second aspect of the present invention there is provided a method for turbulent mixing of fluids in a mixing chamber having an inlet, an outlet and a central axis extending between said inlet and outlet, the method comprising injecting a first fluid through said inlet and into said chamber so that it travels to the outlet in a first fluid stream, using a plurality of conduits extending into said chamber in a direction transverse to the direction of the first fluid stream for injecting a plurality of jets of a second fluid into the first fluid stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are each substantially parallel to a tangent to an imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of second fluid around said axis, the streams of the first and second fluid combining to form a first spiral flow path along the chamber.

According to a third aspect of the present invention there provided combustion apparatus comprising a primary gasification chamber for effecting partial primary combustion of combustible material charged thereinto, an exhaust conduit for producer gases emitted from partial primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the producer gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the producer gas passes from the inlet to the outlet in a gas stream, a plurality of conduits extending into said chamber in a direction transverse to the direction of the producer gas stream and having first outlets for injecting a plurality of jets of air into the producer gas stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are each substantially parallel to a tangent to an imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of air around said axis, the gas and air streams combining to form a first spiral flow path along the chamber.

The primary gasification chamber provides for partial combustion of the material so as to generate what is known as a "producer gas" having energy that can be exhausted by further combustion in the secondary combustion chamber.

The conduits preferably extend towards a central area of the secondary combustion chamber proximal to said axis to induce the spiral flow path around said axis.

The first outlet of each conduit is defined by an opening in an end thereof. In one preferred embodiment each conduit has a second outlet defined by an aperture in a side wall thereof, said aperture facing a direction substantially perpendicular to that faced by the outlet so as to direct a further stream of air into the chamber in a direction substantially perpendicular to the air jet emanating from the outlet. The further streams of air combine to form a second swirling stream of air around the axis radially outboard of the first swirling stream; the first and second swirling streams combining. The apertures may be in the form of elongate slots along the length of the conduits.

The chamber is preferably generally cylindrical, said axis being a longitudinal axis thereof. The conduits are ideally offset from radial lines that intersect said axis so as to provide said offset for the outlets.

In one preferred embodiment there are provided four conduits spaced around chamber and occupying substantially the same plane through the chamber. The conduits may be substantially equi-angularly spaced

The outlets are ideally arranged to inject said air into the producer gas stream in a direction perpendicular thereto.

There may be provided multiple conduit groups, said groups being spaced along axis of the chamber. Each group preferably occupies substantially the same plane and may comprise four conduits. In one preferred embodiment there is provided a multiple conduit group near to the outlet whose apertures face in opposite directions to those of the other conduit groups so as to provide a counter-swirl to the second swirling stream. There may be further provided an outer housing surrounding the secondary combustion chamber and defining a sealed clearance between it and the chamber wall, an inlet to said clearance and means for supplying the clearance with a gas, the conduits in said secondary combustion chamber being connected to a supply conduit disposed in the clearance.

During operation of the combustion apparatus the gas within the clearance is heated by radiation from and/or conduction through the chamber wall so as to pre-heat the gas in the supply conduit.

The supply conduit may comprise a main ring conduit around the clearance, a plurality of branch conduits along the length of the clearance, and a plurality of transverse supply conduits in communication with said conduits in the chamber.

The gas supplied to the clearance is preferably air but may be oxygen or an oxidising gas.

In one preferred embodiment the wall of the secondary combustion chamber has at least one passage therethrough extending in a direction transverse to the direction of flow of the producer gas, the passage defining an air inlet that has a longitudinal axis that is a tangent to a further imaginary circle drawn around the axis of the chamber, the air passing into the chamber through said passage and combining with said producer gas stream. The inlet is preferably disposed so as to direct the air close to the wall of the chamber. The (or each) passage may be radially offset from the axis of the chamber so as to form a spiral stream close to the chamber wall.

Ideally there is a plurality of such passages around the chamber wall so as to generate a third swirling stream of air radially outboard of said first and second swirling streams. The passages may be arranged such that they are radially and axially offset from neighbouring passages. Ideally, the passages are arranged in a helical configuration along the chamber wall, the third swirling stream of air combining with said producer gas stream so as to produce a helical swirling flow path progressing from the chamber inlet to the outlet and disposed radially outboard of said first spiral flow path.

Preferably, some of the passages are inclined towards the outlet of the chamber and some are inclined towards the inlet of the chamber. The passages inclined towards the chamber inlet are preferably disposed so as to inject said air in an opposing direction to the helical swirling flow path so as to cause the flow within the helix to twist about itself

The (or each) axis of the passage occupies a plane that is inclined relative to a plane that is perpendicular to the producer gas stream.

One or more passages nearest to the outlet may be reversed in direction so as to provide a counter-swirl of air.

According to a fourth aspect of the present invention provided combustion apparatus comprising a primary gasification chamber for effecting primary combustion of combustible material charged thereinto, an exhaust conduit for producer gases emitted from primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the producer gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the gas passes from the inlet to the outlet in a gas stream, a plurality of passages through said wall and extending in a direction transverse to the direction of flow of the gas stream, each passage defining an air inlet that has a longitudinal axis that is a tangent to an imaginary circle drawn around the axis of the chamber, the plurality of passages being arranged in a helix along at least part of the length of the secondary combustion chamber wall, the air being directed into the chamber through said passage so as to combine with said gas stream.

Preferably there is further provided an outer housing surrounding the secondary combustion chamber and defining a sealed clearance between it and the chamber wall, an air inlet to said clearance and means for supplying the clearance with compressed air, the passages in the chamber wall providing communication between the clearance and the secondary combustion chamber.

The air inlet is preferably disposed so as to direct the air in a stream close to the wall of the chamber. Each passage may be radially offset from the axis of the chamber so as to form a swirling stream close to the chamber wall.

According to a fifth aspect of the present invention there is provided combustion apparatus comprising a primary gasification chamber for effecting primary combustion of combustible material charged thereinto, an exhaust conduit for producer gases emitted from primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the exhaust gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the gas passes from the inlet to the outlet in a gas stream, an outer housing surrounding the chamber and defining a sealed clearance between it and the chamber wall, an air inlet to said clearance and means for supplying the clearance with compressed air, the chamber wall having openings providing communication between the chamber and the clearance so as to permit the air to pass into the chamber.

This arrangement provides a structure that allows pre-heating of the air in the clearance by the heat that is conducted through and radiated from the secondary combustion chamber wall.

A specific embodiment of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which:

Figure 1 is a schematic representation of combustion apparatus in accordance with the present invention;

Figure 2 is a longitudinal sectioned view of the secondary combustion chamber of the apparatus of figure 1;

Figure 3 is a cross-section through the secondary combustion chamber of figure 2 with some features removed to illustrate clearly one air inlet port that combines with other such air inlet ports to create an outer gas stream;

Figure 4 is a cross-section through the secondary combustion chamber of figure 3 with some feature removed to illustrate clearly air injection nozzles that combine to create an inner gas stream;

Figure 5 is a representation showing the inner and outer gas streams in the secondary combustion chamber.

Referring now to figure 1 of the drawings, the exemplary combustion apparatus comprises a primary gasification chamber 1 and an exhaust conduit 2 in the form of a stack. In order to reduce or eliminate noxious exhaust emissions, the apparatus has a secondary combustion zone 3 defined in the stack.

Material 4 to be burned is supplied to the primary gasification chamber 1 and is supported on a grate 5 that may be rotationally mounted or translatable (e.g. in the form of a conveyor). An example of the latter design is described in our co-pending PCT application no. GB02/002991. A controlled amount of air is supplied to the primary gasification chamber via air inlet ports 6 as is conventional. Pilot burners 7 fuelled with propane or the like are disposed in the primary gasification chamber above the grate 5.

The partially combusted gases given off from the material 4 in the primary gasification chamber rise up into the stack 2 where they are combusted further in a secondary combustion chamber of the secondary combustion zone 3 as described below.

The secondary combustion zone, shown in detail in figures 2, 3 and 4, comprises a generally cylindrical sealed outer casing 10 of metal formed in two vertically stacked portions that are fixed together at a flange 11. The casing may have a thin lining of ceramic (not shown) on its inner surface so as to reduce heat losses. The chamber 12 itself is defined by a cylindrical chamber wall 13 of ceramic based material that is concentrically disposed in the outer casing 10 and has an outside diameter that is smaller than the inside diameter of the casing so as to define an annular clearance 14. The outer surface of the wall 13 is covered with a stainless steel support sleeve 15. Partially combusted ("producer") gas from the primary gasification chamber enters at the lower end 16, is mixed with air in the secondary combustion chamber 12 and the fully combusted gas exits at the top end 17. The exiting combusted gas may then be used in a heat exchanger to recover energy.

Air is supplied to the secondary combustion chamber 12 through the wall 13 from two sources. First, a pair of inlet conduits 18 supplies compressed air at a relatively low pressure from a fan or compressor to the annular clearance 14 between the wall 13 and the casing 10. This air passes through openings in the wall to the chamber. Second, the annular clearance 14 houses a network of conduits 19 that inject air into the chamber at relatively high pressure. Each of these air supplies will be described in turn below.

The ceramic wall 13 is perforated by a plurality of ports 20 that are arranged along the full length of the cylinder in a helical configuration. Thus each port 20 is vertically and radially offset from the previous port in the helix. Each port 20 is inclined upwardly or downwardly relative to the horizontal (see figure 2) so as to point towards the inlet 16 or outlet 17. The majority are inclined downwardly at an angle of approximately twenty-five degrees to the horizontal and the others are inclined upwardly by the same angle. The sectioned view of figure 3 illustrates that the ports 20 extend in a direction offset from a radial plane i.e. along a line that does not intersect the longitudinal axis of the chamber 12 and which approaches a tangent. Air supplied to the annular clearance 14 through the inlets 18 emerges into the secondary combustion chamber 12 through the ports 20. The helical arrangement and radial offset of the ports 20 provides an outer column comprising a spiralling vortex flow of air (and producer gas) 30 that progresses up the chamber 12 close to the inner surface of the chamber wall 13 as represented by the dashed line in figure 5. By passing close to the chamber wall 12 the air cools the ceramic slightly so as to reduce the tendency for thermal degradation. The downwardly directed ports 20 induce the air to twist about the general direction of flow in a rotational movement as is illustrated by the wave-like arrows 31 in figure 5. This combined spiralling and twisting movement gives the air flow a tortuous path that increases its residence time in the combustion chamber and ensures turbulent mixing with the producer gas in the radially outer regions of the combustion zone.

The direction of entry of final few ports 20 into the chamber in a horizontal plane (figure 3) is reversed so as to provide a counter flow 32 that increases the final turbulence so as to mix air with any remaining non-combusted gas.

The network of conduits 19 in the annular clearance 14 comprises two identical structures, one being located in each portion of the outer casing 10. A main ring conduit 21 is disposed at the bottom of each portion of the casing and is connected to a single air inlet 22 and four equi-angularly spaced vertical branches 23 that extend up the annular clearance 14. Each branch 23 has three vertically spaced perpendicular outlet conduits 24 that extend, in a roughly radial direction, through the ceramic wall 13 and into the chamber interior where they are covered with a ceramic coating or sleeve. Thus, at six separate vertical locations within the secondary combustion chamber 12 there are four equi-angularly spaced outlet conduits 24 occupying roughly the same horizontal plane as shown in figure 4. Each outlet conduit 24 has two openings 25, 26 from which air may egress. A first opening 25 is an orifice at the end of the conduit and the second is an elongated slot 26 in a side wall of the conduit. The outlet conduits 24 are radially offset such that their longitudinal axes do not intersect the perpendicular longitudinal axis of the combustion chamber 12. This arrangement ensures that the air jets egressing the orifices are not in direct opposition to one another but instead they combine to form a swirling anti-clockwise rotational flow 33 (in the view of figure 4) that intermingles with the gas flow to form a helical spiral 34 that passes up centre of the chamber. The air flow from the slots produces a similar anti-clockwise flow 35 (figure 4) radially outboard of the chamber centre and reinforces the spiral flow path. The result is a central column of helical flow 34 within the outer column 30 generated by the air from the ports as can be seen in figure 5. The air mixes with the producer gas as it passes up the combustion chamber 12. The volume of air fed into the chamber through the air injectors 24 is significantly less in volume than that supplied through the air ports 20 but is at a considerably higher pressure so that it travels at a higher velocity. The spiralling rotation of the central column of air/gas 34 serves to drags the outer air/gas column 30 with it so as to increase its rotational velocity.

The last four air outlet branches 24a nearest to the outlet 17 of the secondary combustion zone 3 have slots that are oriented in a reverse direction so as to create a clockwise flow spiral. This increases the turbulence of the mixing gas immediately before the outlet so as to encourage combustion of any remaining non-combusted gas.

After a start-up procedure the combustion apparatus has reached an operational temperature and the ceramic wall 13 radiates a certain amount of heat into the annular clearance 14. This serves to increase the temperature of the air in the clearance before it passes into the chamber 12. Preheating the air in this way means that less energy and time is spent in the combustion process in raising the temperature of the air in the chamber.

The tortuous air flow paths 30, 31, 34 are designed for efficient mixing of the air with the gases. Since the length of the air paths is increased significantly in comparison to a direct flow the air has a longer residence time in the chamber thereby giving it more opportunity to mix with the combustion gases and to reach a higher temperature. This improves the combustion efficiency in that the apparatus is able to operate at higher temperatures thereby reducing particulate emission levels and less air is wasted so that the air to fuel ratio approaches stoichiometric values The improvement in efficiency reduces the production of noxious substances such as nitrous oxides and carbon monoxide. By injecting air into the centre of the chamber the candlestick effect (in which air introduced into the secondary combustion chamber merely flows along the inner wall of the chamber without adequately mixing with the waste gases flowing along the central core) is eliminated. The structure of the air injectors means that only parts of the outlet conduits of the air injection conduit network are exposed to conditions in the combustion chamber and thus vulnerable to deterioration under the constant high temperatures. The provision of the ceramic sleeves or coating reduces the effect of the temperatures on the outlets.

Recycled outlet flue gas may be added to inlet of the fans supplying air to the chamber so as to improve the efficiency of the combustion operation and further reduce nitrous oxides.

It is to be understood that the term "fluid" includes liquid or gas. It is also to be understood that oxygen or any oxidising gas can be used in place of air to effect combustion or gasification.

It will be appreciated that numerous modifications to the above described design may be made without departing from the scope of the invention as defined in the appended claims. For example, the secondary combustion zone described may be used in any orientation between and including horizontal or vertical. Mixing can be improved further by circumferentially staggering the four outlet conduits of one horizontal plane from those in an adjacent plane. It is also to be understood that the arrangement of the helical flow paths may have application to other fields of technology where the efficient mixing of any gases or liquid is important.

Claims

1. Apparatus for turbulent mixing of fluids comprising a mixing chamber having a wall, an inlet, an outlet and an axis extending between said inlet and outlet, means for injecting a first fluid into said chamber so that it travels from the inlet to the outlet in a first fluid stream, a plurality of conduits extending into said chamber in a direction transverse to the direction of travel of the first fluid stream and each having a first outlet for injecting a jet of a second fluid into the first fluid stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are substantially parallel to a tangent to a common imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of second fluid around said axis, the streams of first and second fluid combining to form a first spiral flow path along the chamber.

2. Apparatus according to claim 1, wherein the conduits extend towards a central area of the chamber proximal said axis so as to induce the spiral flow path around said axis.

3. Apparatus according to claim 1 or 2, wherein the first outlet of each conduit is defined by an opening in an end thereof.

4. Apparatus according to claim 3, wherein each conduit has a second outlet defined by an aperture in a side wall thereof, said aperture facing a direction substantially perpendicular to that faced by the outlet so as to direct a further jet of the second fluid into the chamber in a direction substantially perpendicular to the first jet of the second fluid.

5. Apparatus according to claim 4, wherein outlets are arranged so that the further jets of second fluid combine to form a second swirling stream of the second fluid around the axis radially outboard of the first swirling stream, the first and second swirling streams of second fluid combining.

6. Apparatus according to claim 5, wherein the apertures are in the form of elongate slots along the length of the conduits.

7. Apparatus according to any preceding claim, wherein the chamber is substantially cylindrical, said axis being a longitudinal axis thereof.

8. Apparatus according to claim 7, wherein the conduits are offset from radial lines that intersect said axis so as to provide said offset for the outlets.

9. Apparatus according to any preceding claim, the conduits occupy substantially the same plane through the chamber.

10. Apparatus according to any preceding claim wherein there are four conduits spaced around chamber and occupying substantially the same plane through the chamber.

11. Apparatus according to claim 9 or 10, wherein the conduits are substantially equi-angularly spaced.

12. Apparatus according to any preceding claim, wherein the outlets are arranged to inject said second fluid into the first fluid stream in a direction perpendicular thereto.

13. Apparatus according to any preceding claim, wherein there are provided multiple conduit groups, said groups being spaced along axis of the chamber, each group occupying substantially the same plane.

14. Apparatus according to claim 13, in which there is provided a multiple conduit group near to the chamber outlet, the apertures of said conduits facing in opposite directions to those of the other conduit groups so as to provide a counter-swirl to the second swirling stream.

15. Apparatus according to any preceding claim, wherein the wall of the chamber has at least one passage therethrough extending in a direction transverse to the direction of flow of the first fluid stream, the passage defines an inlet for the second fluid and has a longitudinal axis that is a tangent to a further imaginary circle drawn around the axis of the chamber, the second fluid passing into the chamber through said passage and combining with said first fluid stream.

16. Apparatus according to claim 15, wherein the inlet is disposed so as to direct the second fluid close to the wall of the chamber.

17. Apparatus according to claim 16, wherein the (or each) passage is radially offset from the axis of the chamber so as to form a spiral stream close to chamber wall.

18. Apparatus according to claim 17, wherein there is a plurality of such passages around the chamber wall so as to generate a third swirling stream of second fluid radially outboard of said first and second swirling streams.

19. Apparatus according to claim 18, wherein the passages are arranged such that they are radially and axially offset from neighbouring passages.

20. Apparatus according to claim 18 or 19, wherein the passages are arranged in a helical configuration along chamber wall, the third swirling stream of second fluid combining with said first fluid stream so as to produce a helical swirling flow path progressing from the chamber inlet to the outlet and disposed radially outboard of said first spiral flow path.

21. Apparatus according to any one of claims 18, 19 or 20, wherein some of the passages are inclined towards the outlet of the chamber and some are inclined towards the inlet of the chamber.

22. Apparatus according to claim 21, wherein the passages inclined towards the inlet are disposed so as to inject said second fluid in an opposing direction to the helical swirling flow path so as to cause the flow within the helix to twist about itself.

23. Apparatus according to any one of claims 15 to 22, wherem the axis of the (or each) passage occupies a plane that is inclined relative to a plane that is perpendicular to the first stream of fluid.

24. Apparatus according to any one of claims 18 to 23, wherein one or more passages nearest to the outlet may be reversed in direction so as to provide a counter swirl of second fluid.

25. A method for turbulent mixing of fluids in a mixing chamber having an inlet, an outlet and a central axis extending between said inlet and outlet, the method comprising injecting a first fluid through said inlet and into said chamber so that it travels to the outlet in a first fluid stream, using a plurality of conduits extending into said chamber in a direction transverse to the direction of the first fluid stream for injecting a plurality of jets of a second fluid into the first fluid stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are each substantially parallel to a tangent to an imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of second fluid around said axis, the streams of the first and second fluid combining to form a first spiral flow path along the chamber.

26. A method according to claim 25, wherein a further stream of the second fluid is directed into the chamber in a direction substantially peφendicular to the first stream of the second fluid.

27. A method according to claim 26, wherein the further streams of second fluid combine to form a second swirling stream of the second fluid around the axis radially outboard of the first swirling stream, the first and second swirling streams of second fluid combining.

28. A method according to claim 27, wherein further streams of second fluid are produced by apertures are in the form of elongate slots along the length of the conduits.

29. A method according to any one of claims 25 to 28, wherein the chamber is substantially cylindrical, said axis being a longitudinal axis thereof, wherein the jets of second fluid being offset from radial lines that intersect said axis.

30. A method according to any preceding claim, wherein the jets of second fluid occupy substantially the same plane through the chamber.

31. A method according to any one of claims 25 to 30, wherein the second fluid is injected into the first fluid stream in a direction peφendicular thereto.

32. A method according to any one of 25 to 31, wherein there are a plurality of swirling streams of second fluid, the streams being spaced along axis of the chamber, each stream occupying substantially the same plane.

33. A method according to claim 32, wherein a swirling stream of second fluid near to the chamber outlet, the stream swirling in an opposite direction to the other swirling streams so as to provide a counter-swirl to the second swirling stream.

34. A method according to any one of claims 25 to 33, wherein second fluid is additionally directed through one or more passages through the secondary chamber wall, the (or each) passage producing a flow that has a longitudinal axis that is a tangent to a further imaginary circle drawn around the axis of the chamber, the second fluid passing into the chamber through said passage and combining with said first fluid stream.

35. A method according to claim 34, wherein the passage is disposed so as to direct the second fluid close to the wall of the chamber.

36. A method according to claim 35, wherein the (or each) flow of second fluid through said passage is radially offset from the axis of the chamber so as to form a spiral stream close to chamber wall.

37. A method according to claim 36, wherein there is a plurality of such passages around the chamber wall so as to generate a third swirling stream of second fluid radially outboard of said first and second swirling streams.

38. A method according to claim 37, wherein the passages are arranged such that they are radially and axially offset from neighbouring passages.

39. A method according to claim 37 or 38, wherein the passages are arranged in a helical configuration along chamber wall, the third swirling stream of second fluid combining with said first fluid stream so as to produce a helical swirling flow path progressing from the chamber inlet to the outlet and disposed radially outboard of said first spiral flow path.

40. A method according to any one of claims 35 to 39, wherein some of the passages are inclined towards the outlet of the chamber and some are inclined towards the inlet of the chamber.

41. A method according to claim 40, wherein the passages inclined towards the inlet are disposed so as to inject said second fluid in an opposing direction to the helical swirling flow path so as to cause the flow within the helix to twist about itself.

42. A method according to any one of claims 34 to 41, wherein the axis of the (or each) passage occupies a plane that is inclined relative to a plane that is peφendicular to the first stream of fluid.

43. A method according to anyone of claims 34 to 42, wherein one or more passages nearest to the outlet may be reversed in direction so as to provide a counter-swirl of second fluid.

44. Combustion apparatus comprising a primary gasification chamber for effecting partial primary combustion of combustible material charged thereinto, an exhaust conduit for a producer gas emitted from primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the producer gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the producer gas passes from the inlet to the outlet in a gas stream, a plurality of conduits extending into said chamber in a direction transverse to the direction of the producer gas stream and having first outlets for injecting a plurality of jets of air into the producer gas stream in said transverse direction, the outlets being directed such that the jets are offset from said axis and are each substantially parallel to a tangent to an imaginary circle whose centre is on said axis, the jets combining so as to form a first swirling stream of air around said axis, the gas and air streams combining to form a first spiral flow path along the chamber.

45. Combustion apparatus according to claim 44, wherein the conduits extend towards a central area of the secondary combustion chamber proximal to said axis to induce the spiral flow path around said axis.

46. Combustion apparatus according to claim 44 or 45, wherein the first outlet of each conduit is defined by an opening in an end thereof.

47. Combustion apparatus according to claim 44, 45 or 46, wherein each conduit has a second outlet defined by an aperture in a side wall thereof, said aperture facing a direction substantially peφendicular to that faced by the outlet so as to direct a further stream of air into the chamber in a direction substantially peφendicular to the air jet emanating from the outlet.

48. Combustion apparatus according to claim 47, wherein the further streams of air combine to form a second swirling stream of air around the axis radially outboard of the first swirling stream, the first and second swirling streams combining.

49. Combustion apparatus according to claim 47 or 48, wherein the apertures are in the form of elongate slots along the length of the conduits.

50. Combustion apparatus according to any one of claims 44 to 49, wherein the chamber is cylindrical, said axis being a longitudinal axis thereof.

51. Combustion apparatus according to claim 50, wherein the conduits are offset from radial lines that intersect said axis so as to provide said offset for the outlets.

52. Combustion apparatus according to any one of claims 44 to 51, wherein there are provided four conduits spaced around chamber and occupying substantially the same plane through the chamber.

53. Combustion apparatus according to claim 52, wherein the conduits are substantially equi-angularly spaced.

54. Combustion apparatus according to any one of claims 44 to 53, wherein the outlets are arranged to inject said air into the producer gas stream in a direction peφendicular thereto.

55. Combustion apparatus according to any one of claims 44 to 54, wherein there are provided multiple conduit groups, said groups being spaced along axis of the chamber.

56. Combustion apparatus according to claim 55, wherein each group preferably occupies substantially the same plane.

57. Combustion apparatus according to claim 56, wherein each group comprises four conduits.

58. Combustion apparatus according to claim 55, 56 or 57, wherein there is provided a multiple conduit group near to the outlet whose apertures face in opposite directions to those of the other conduit groups so as to provide a counter-swirl to the second swirling stream.

59. Combustion apparatus according to any one of claims 44 to 58, further comprising an outer housing surrounding the secondary combustion chamber and defining a sealed clearance between it and the chamber wall, an inlet to said clearance and means for supplying the clearance with a gas, the conduits in said secondary combustion chamber being connected to a supply conduit disposed in the clearance.

60. Combustion apparatus according to claim 59, wherein the supply conduit comprises a main ring conduit around the clearance, a plurality of branch conduits along the length of the clearance, and a plurality of transverse supply conduits in communication with said conduits in the chamber.

61. Combustion apparatus according to claim 60, wherein the gas supplied to the clearance is air.

62. Combustion apparatus according to any one of claims 44 to 61, wherein the wall of the secondary combustion chamber has at least one passage therethrough extending in a direction transverse to the direction of flow of the producer gas, the passage defining an air inlet that has a longitudinal axis that is a tangent to a further imaginary circle drawn around the axis of the chamber, the air passing into the chamber through said passage and combining with said exhaust gas stream.

63. Combustion apparatus according to claim 62, wherein the inlet is disposed so as to direct the air close to the wall of the chamber.

64. Combustion apparatus according to claim 62 or 63, wherein the (or each) passage is radially offset from the axis of the chamber so as to form a spiral stream close to chamber wall.

65. Combustion apparatus according to any one of claims 62 to 64, wherein there is a plurality of such passages around the chamber wall so as to generate a third swirling stream of air radially outboard of said first and second swirling streams.

66. Combustion apparatus according to claim 65, wherein the passages are arranged such that they are radially and axially offset from neighbouring passages.

67. Combustion apparatus according to claim 66, wherein the passages are arranged in a helical configuration along chamber wall, the third swirling stream of air combining with said producer gas stream so as to produce a helical swirling flow path progressing from the chamber inlet to the outlet and disposed radially outboard of said first spiral flow path.

68. Combustion apparatus according to claim 65, 66 or 67, wherein some of the passages are inclined towards the outlet of the chamber and some are inclined towards the inlet of the chamber.

69. Combustion apparatus according to claim 68, wherein the passages inclined towards the chamber inlet are disposed so as to inject said air in an opposing direction to the helical swirling flow path so as to cause the flow within the helix to twist about itself.

70. Combustion apparatus according to any one of claims 65 to 69, wherein the (or each) axis of the passage occupies a plane that is inclined relative to a plane that is peφendicular to the producer gas stream.

71. Combustion apparatus according to any one of claims 65 to 70, wherein one or more passages nearest to the outlet are reversed in direction so as to provide a counter swirl of air.

72. Combustion apparatus comprising a primary gasification chamber for effecting partial primary combustion of combustible material charged thereinto, an exhaust conduit for a producer gas emitted from primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the producer gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the gas passes from the inlet to the outlet in a gas stream, a plurality of passages through said wall and extending in a direction transverse to the direction of flow of the gas stream, each passage defining an air inlet that has a longitudinal axis that is a tangent to an imaginary circle drawn around the axis of the chamber, the plurality of passages being arranged in a helix along at least part of the length of the secondary combustion chamber wall, the air being directed into the chamber through said passage so as to combine with said gas stream.

73. Combustion apparatus according to claim 72, further comprising an outer housing surrounding the secondary combustion chamber and defining a sealed clearance between it and the chamber wall, an air inlet to said clearance and means for supplying the clearance with compressed air, the passages in the chamber wall providing communication between the clearance and the secondary combustion chamber.

74. Combustion apparatus according to claim 73, wherein the air inlet is disposed so as to direct the air in a stream close to proximal the wall of the chamber.

75. Combustion apparatus according to any one of claims 72 to 74, wherein each passage is radially offset from the axis of the chamber so as to form a swirling stream close to the chamber wall.

76. Combustion apparatus comprising a primary gasification chamber for effecting primary combustion of combustible material charged thereinto, an exhaust conduit for a producer gas emitted from primary combustion and a secondary combustion chamber disposed in said exhaust conduit for turbulent mixing of air with the exhaust gas so as to effect secondary combustion, the secondary combustion chamber having an inlet, an outlet and a central axis extending between said inlet and outlet and arranged so that, in use, the gas passes from the inlet to the outlet in a gas stream, an outer housing surrounding the chamber and defining a sealed clearance between it and the chamber wall, an air inlet to said clearance and means for supplying the clearance with compressed air, the chamber wall having openings providing communication between the chamber and the clearance so as to permit the air to pass into the chamber.