Multiple hearth furnace improvements

Abstract

A multiple hearth furnace in which in a gas space above at least one hearth an annular baffle is provided above the rabble arms of that hearth. The annular baffle modifies gas flow in the gas space, in particular gas residence times above the hearth. This in turn can enhance performance of the furnace, for example in respect of carbon monoxide emissions.

Claims

1. A multiple hearth furnace for contacting a feed material with a gas, the multiple hearth furnace comprising: a plurality of circular hearths vertically spaced apart from each other within a cylindrical external casing, the uppermost hearth being vertically spaced from a roof structure of the external casing so that directly above each hearth lies an associated gas space, each hearth being either an in hearth configured to enable downward movement of feed material and upward movement of gas through a central drop hole or an out hearth configured to enable downward movement of feed material and upward movement of gas through a plurality of peripherally located drop holes; a shaft assembly extending upwardly and centrally through the gas spaces and rotatable about an upright axis; rabble arms, within each gas space, secured to the shaft assembly to rotate therewith and each of the rabble arms having rabbles adapted to move feed material inwardly where the hearth associated with the gas space is an in hearth or outwardly where the hearth associated with the gas space is an out hearth; and an annular baffle comprising a wall that is cylindrical and has an upright axis, located in the gas space above a specific circular hearth of the plurality of circular hearths, the annular baffle secured to and extending downwardly from the roof structure or hearth above the specific circular hearth to a lower edge that, in operation of the furnace, is clear of the rabble arms associated with the specific circular hearth so as divide the gas space above the lower edge into inner and outer annular spaces, and wherein the annular baffle surrounds the rotatable central shaft assembly and lies everywhere at a greater radius from the axis than the central drop hole where the specific hearth is an in hearth or everywhere at a lesser radius than the peripheral drop holes where the specific hearth is an out hearth.

2. The multiple hearth furnace of claim 1 wherein the upright axis of the annular baffle is coaxial with the central shaft assembly.

3. The multiple hearth furnace of claim 1 wherein the specific hearth is an in hearth so that feed material leaves the specific hearth and gas enters the gas space directly above the specific hearth through the central drop hole of the specific hearth, the gas subsequently passing outwardly under the annular baffle.

4. The multiple hearth furnace of claim 3 wherein the specific hearth is an in hearth and is the uppermost one of the hearths, and wherein gas passing outwardly under the annular baffle leaves the gas space directly above the specific hearth through a gas outlet opening that extends through one of the roof structure or the external casing and outside the annular baffle.

5. The multiple hearth furnace of claim 1 wherein there is further provided in the gas space directly above the specific hearth, and below the roof structure or hearth above the specific hearth, a baffle element that: extends partway circumferentially around the shaft assembly and is located adjacent to and clear of the annular baffle; is supported by and rotates with a circumferentially adjacent pair of the rabble arms in the said gas space; extends downwardly into a gap between the circumferentially adjacent pair of the rabble arms in the said gas space, whereby to partially restrict gas flow under the annular baffle.

6. A method for subjecting feed materials to contact with a gas in a multiple hearth furnace, the multiple hearth furnace comprising: a plurality of circular hearths vertically spaced apart from each other within a cylindrical external casing and the uppermost hearth being vertically spaced from a roof structure of the external casing so that directly above each hearth lies an associated gas space, each hearth being either an in hearth having for downward movement of feed material and upward movement of gas a central drop hole or an out hearth having for downward movement of feed material and upward movement of gas a plurality of peripherally located drop holes; a shaft assembly extending upwardly and centrally through the gas spaces and rotatable about an upright axis; and within each gas space rabble arms secured to the shaft assembly to rotate therewith and each having rabbles adapted to move feed material inwardly where the hearth associated with the gas space is an in hearth or outwardly where the hearth associated with the gas space is an out hearth, the method comprising the steps of: providing, in the gas space above a specific circular hearth of the plurality of circular hearths, an annular baffle that comprises a wall that is cylindrical and has an upright axis and that is secured to and extends downwardly from the roof structure or hearth above the specific circular hearth to a lower edge that, in operation of the furnace, is clear of the rabble arms associated with the specific circular hearth so as to divide the gas space above the lower edge into inner and outer annular spaces, the annular baffle surrounding the rotatable central shaft assembly and lying everywhere at a greater radius from the axis than the central drop hole where the specific hearth is an in hearth or everywhere at a lesser radius than the peripheral drop holes where the specific hearth is an out hearth; feeding feed material onto the uppermost hearth; passing the feed material over each hearth by means of the rabbles and downwardly from the uppermost hearth to successively lower hearths; simultaneously passing a gas upwardly through the furnace through the drop holes of each out hearth and through the drop hole of each in hearth and through the gas spaces directly above the hearths.

7. The method of claim 6 wherein the upright axis of the annular baffle is coaxial with the central shaft assembly.

8. The method of claim 6 wherein the specific hearth is the uppermost one of the hearths.

9. The method of claim 8 wherein the specific hearth is an in hearth.

10. The method of claim 6 wherein at least one hearth is within a combustion zone of the multiple hearth furnace in which combustion zone at least a proportion of the feed material is combusted; the specific hearth is within a preheating zone of the multiple hearth furnace; the feed material passes firstly through the preheating zone and thereafter into the combustion zone.

11. The method of claim 6 wherein volatiles liberated from the feed material are at least partially oxidized in the gas that enters the gas space above the specific hearth.

12. The method of claim 6 wherein the feed material comprises biosolids derived from treatment of municipal sewage.

13. A method for modifying a multiple hearth furnace to increase gas residence times therein, the multiple hearth furnace comprising: a plurality of circular hearths vertically spaced apart from each other within a cylindrical external casing and the uppermost hearth being vertically spaced from a roof structure of the external casing so that directly above each hearth lies an associated gas space, each hearth being either an in hearth having for downward movement of feed material and upward movement of gas a central drop hole or an out hearth having for downward movement of feed material and upward movement of gas a plurality of peripherally located drop holes; a shaft assembly extending upwardly and centrally through the gas spaces and rotatable about an upright axis; and within each gas space rabble arms secured to the shaft assembly to rotate therewith and each having rabbles adapted to move feed material inwardly where the hearth associated with the gas space is an in hearth or outwardly where the hearth associated with the gas space is an out hearth, the method comprising the step of: providing, in the gas space above a specific circular hearth of the plurality of circular hearths, an annular baffle that comprising a wall that is cylindrical and has an upright axis and that is secured to and extends downwardly from the roof structure or hearth above the specific circular hearth to a lower edge that, in operation of the furnace, is clear of the rabble arms associated with the specific circular hearth so as to divide the gas space above the lower edge into inner and outer annular spaces, the annular baffle surrounding the rotatable central shaft assembly and lying everywhere at a greater radius from the axis than the central drop hole where the specific hearth is an in hearth or everywhere at a lesser radius than the peripheral drop holes where the specific hearth is an out hearth.

Description

BRIEF DESCRIPTIONS OF THE DRAWINGS

(1) FIG. 1 is a schematic view of a prior art multiple hearth furnace, with some detail omitted, shown in a vertical cross-section whose plane includes the upright axis of rotation of a central shaft assembly;

(2) FIG. 2 is a schematic view of a portion of a multiple hearth furnace having an annular baffle according to the invention, with some detail omitted, shown in a vertical cross-section whose plane includes the upright axis of rotation of a central shaft assembly;

(3) FIG. 3 is a cross sectional view of the multiple hearth furnace portion shown in FIG. 2, the section being taken at station 3-3 of FIG. 2;

(4) FIG. 4 is a schematic view of a further portion of a multiple hearth furnace having an annular baffle according to the invention, with some detail omitted, shown in a vertical cross-section whose plane includes the upright axis of rotation of a central shaft assembly;

(5) FIG. 5 is a cross sectional view of the multiple hearth furnace portion shown in FIG. 4, the section being taken at station 5-5 of FIG. 4;

(6) FIG. 6 is a perspective schematic sketch showing a first typical gas streamline in the gas space of a multiple hearth furnace having an annular baffle according to the invention as shown in FIG. 2;

(7) FIG. 7 is a perspective schematic sketch showing a first typical gas streamline in the gas space of a multiple hearth furnace having an annular baffle according to the invention as shown in FIG. 2;

(8) FIG. 8 is a perspective schematic sketch showing two typical gas streamlines in the gas space of a multiple hearth furnace having an annular baffle according to the invention as shown in FIG. 4;

(9) FIG. 9 is a perspective schematic sketch showing two typical gas streamlines in the gas space of a multiple hearth furnace as shown in FIG. 4 but with the annular baffle shown in FIG. 4 no longer present;

(10) FIG. 10 is a perspective view showing one hearth of the furnace as shown in FIGS. 2 and 3, rabble arms thereon and baffles according with an embodiment of the invention.

DETAILED DESCRIPTION

(11) The invention will be described by reference to modification of an existing multiple hearth furnace used for incinerating biosolids derived from the treatment of sewage. However, it is to be understood that the invention is equally applicable to new manufacture of multiple hearth furnaces and that there is potential for application of the inventive concept or concepts to applications of multiple hearth furnaces other than those intended for biosolids incineration. Multiple hearth furnaces are used for many applications, and on a very wide range of materials, but many of the basic elements described below for incineration are also present in furnaces used in other applications and for other materials.

(12) FIG. 1 shows the basic elements of a typical multiple hearth furnace 10 (for convenience only described below simply as a furnace) 10 as used for incineration of a stream of feed material comprising a proportion of solids that are biosolids derived from treatment of sewage and generally some moisture. In its number of hearths and general proportions, the furnace 10 is representative of the existing furnace that was modeled in the simulations described below.

(13) The furnace 10 comprises a cylindrical housing 11, a roof structure 12, and a bottom structure 13 all typically formed of steel and having a lining of refractory material 14 able to withstand the temperatures generated within the furnace 10. Inside the housing 11 there are eight circular hearth structures, numbered 1 to 8, vertically spaced apart from each other. In the cylindrical housing 11, the refractory lining 14 defines on an inwardly facing inner wall surface 23 that is also cylindrical. The hearth structures 1 to 8 comprise interlocking refractory elements (not shown individually). Upper surfaces of the hearth structures 1 to 8 constitute hearths (numbered 1a to 8a) which are traversed by feed material (not shown) passing through the furnace 10. Above each of the hearths 1a to 8a is a gas space (these being numbered 1b to 9b in FIG. 1), which are traversed by gas passing generally upwardly through the furnace 10.

(14) A shaft assembly 15 extends vertically through the hearth structures 1 to 8 and in operation of the furnace 10 rotates about its axis 16, being driven by a drive mechanism 17. Secured to the shaft assembly 15 and rotated thereby are radially extending rabble arms 18, each provided with depending rabbles 19 (also known as rabble teeth) which in use contact feed material on the hearths so as to move the feed material progressively inwards on some hearths (often referred to in the art as in hearths) and outward on other hearths (often referred to in the art as out hearths). In FIG. 1, hearths 1a, 3a, 5a and 7a are in hearths and hearths 2a, 4a, 6a, and 8a are out hearths. Rabbles 19 in general also agitate the feed material for enhanced contact and reaction with gas flowing over the hearths. For clarity, only one rabble arm 18 is shown above each of hearths 1a to 8a (and bottom structure 13) in FIG. 1; however in practice a furnace such as furnace 10 would be provided with two, three or four rabble arms for each hearth, circumferentially spaced apart.

(15) The rabble arms 18 have internal passages (not shown) along their length through which is passed cooling gas, delivered by first internal gas passages (not shown) in the shaft assembly 15. Gas passing through each rabble arm is returned to second internal gas passages also, in shaft assembly 15. Means for supplying gas to the shaft assembly are provided but not shown in FIG. 1.

(16) Feed material to be treated in furnace 10 enters through one or more feed inlets 20. Typically all material passing through a feed inlet 20 will fall on to the uppermost hearth 1a close to its periphery, however it is possible to provide feed inlets (not shown) for placement of feed material on a hearth other than (or additional to) the uppermost hearth 1a.

(17) Feed material flows downwardly through the furnace 10 as follows. Each of the in hearths 1a, 3a, 5a and 7a has a central drop hole (these being numbered 211, 213, 215, 217 respectively) with a diameter larger than that of shaft assembly 15, and feed material is moved radially inward by the rabble arms 18 and rabbles 19 on each of these hearths and on reaching the central drop hole (211, 213, 215, or 217) falls downward to the out hearth below.

(18) Each of the out hearths 2a, 4a, 6a, 8a has several peripheral drop holes 222, 224, 226, 228 respectively, adjacent to the inner wall surface 23 and feed material falling onto each out hearth through the central drop hole (for example, 211) of the in hearth immediately above is moved radially outwards by its associated rabble arms 18 and rabbles 19 and falls through the peripheral drop holes (for example 222) onto the in hearth below. In the case of the lowermost out hearth 8a, feed material reaching its peripheral drop holes falls onto bottom structure 13 and a rabble arm 18 acting thereon moves the feed material to a feed outlet 24 (which may be one of several such feed outlets 24) and thus out of furnace 10.

(19) As feed material passes downwardly through the furnace, gas with which that solid material is to be contacted, also passes through the furnace, in a generally upward direction (i.e. opposite to the feed material). As represented by unnumbered arrows in the right half of FIG. 1, the gas passes through the central drop holes 211, 213, 215 and 217 and peripheral drop holes 222, 224, 226 and 228 so as to pass successively through gas spaces 8b to 1b and radially over each of hearths 8a to 1a. The gas leaves furnace 10 through a gas outlet opening 25b in the wall surface 23 and then gas outlet duct 25 laterally through the cylindrical casing 11. (Furnace 10 has a single gas outlet duct 25, but it would also be possible (though this is not shown) to provide multiple gas outlets in the roof structure 12 or cylindrical housing 11.)

(20) At entry to furnace 10 the gas comprises air, introduced into gas space 8b through several air inlets 26 (of which only one is shown) in the cylindrical housing 11. Two burners 27 are provided over each of hearths 5, 6, 7 and 8. Additional air is supplied to furnace 10 via ducts containing the burners 27. On its way upward through furnace 10 to gas outlet duct 25 the gas's composition changes progressively due to combustion of the feed material, liberation of any volatile material comprised in the feed material, combustion of such volatile material, and liberation of water from the feed material. Some of the gas passing through gas outlet opening 25b and/or gas emerging from shaft assembly 15 heated by its passage through the rabble arms 18, may be ducted back (by ducts not shown in FIG. 1) to one or more of the lower hearths to augment the flow of air introduced at air inlets 26 and burners 27. This can be a useful fuel-saving measure in incineration applications.

(21) Furnace 10 would in practice be part of a system including other elements, not shown. For example the feed material is typically partially dewatered by suitable plant so as to enter the furnace 10 as a moist and friable cake. For further example, ducts external to the furnace for recirculating outlet gas and/or cooling gas have been mentioned, and are often provided. Importantly, plant to condition gas exiting the system including furnace 10 before its discharge to the atmosphere may be provided.

(22) In normal operation of furnace 10 for incineration, the feed material passes through three distinct zones, each containing at least one of the hearths 1a to 8a. Feed material entering at feed inlet 20 passes through firstly a preheating zone including one or more of the uppermost hearths, then secondly a combustion zone comprising one or more of the hearths below the hearths of the preheating zone, where substantial combustion of the biosolids occurs, and thirdly a cooling zone in which solid material, now essentially ash, is cooled before discharge from furnace 10 thus preheating the gas rising to the combustion zone. Burners 27 are used to initiate combustion during startup, but some or all may subsequently be able to be turned off with combustion of the feed material being self-sustaining (this being known as autogenous operation). The three zones are not numbered in FIG. 1 as their boundaries can vary, but users often seek to make at least the preheating zone correspond to one or more of the uppermost hearths, above which there is no visible flame.

(23) In the preheating zone, gas which has passed through the combustion zone is cooled by evaporation of moisture from the feed material, and heat is also transferred to the feed material. Some volatile material may be liberated from the feed material in the preheating zone and at least partially combusted there. However, some may reach gas outlet opening 25b without having been adequately combusted. This can lead to emissions such as total hydrocarbons and carbon monoxide (CO) that are environmentally unacceptable. There may also be problems with particulates in gas at gas outlet opening 25b.

(24) Some multiple hearth furnaces have been fitted with external afterburners (not shown) to condition outlet gas through additional combustion so that emissions are acceptable. Afterburners generally require a fuel supply leading to extra operating cost. The problem of emissions can be exacerbated by excess or unsteady feed material flow. Also, where a multiple hearth furnace is required to handle feed material throughputs that are greater than were contemplated in their original design, unacceptable emission performance can result.

(25) However, as an alternative approach to alleviating emission problems in existing furnaces, or as a new option in the original design of multiple hearth furnaces, the invention provides for inclusion of a baffle in one gas space (or more) which acts to increase average gas residence times therein. It is believed that, particularly when applied to preheating zone hearths, such baffles can allow more complete chemical reactions to occur with resulting benefits for emissions.

(26) FIGS. 2 and 3 show an upper portion of furnace 10, now modified to accord with the invention by provision of annular baffle 30 in the gas space 1b above hearth 1a, which is an in hearth. As in FIG. 1, only one of the rabble arms 18 operating on hearth 1a is shown, for clarity. Annular baffle 30 is secured to roof structure 12 and depends therefrom. As best seen in the horizontal cross-section of FIG. 3, baffle 30 is circular, and coaxial with the shaft assembly 15. Baffle 30 has a bottom edge 31 that lies close to rabble arm 18 so that in operation there is no contact between the rabble arm 18 and the baffle 30 and that part of gas space 1b above the rabble arms 18 operating on hearth 1a is substantially divided by baffle 30 into inner and outer concentric annular spaces 32 and 33. The mean diameter of baffle 30 is shown as being 65% of the diameter of the inner surface 23 of the casing, however, other diameters may be found suitable.

(27) Baffle 30 has a diameter that is larger than the diameter of the central drop hole 211.

(28) In furnace 10, gas outlet duct 25 is of rectangular cross-section and extends horizontally through housing 11, substantially all gas entering gas space 1b leaving it through gas outlet opening 25b and duct 25. This arrangement is typical of multiple hearth furnace practice, where it is usual to provide for the uppermost hearth to be an in hearth, and for gas outlet opening (25b in furnace 10) to be at a greater radius than the central drop hole (211 in furnace 10) of that hearth, to ensure the gas traverses hearth 1a outwardly and to avoid low average gas residence time in gas space 1b above the uppermost hearth.

(29) Based on simulations, discussed below, it is believed that provision of annular baffle 30 modifies the flow in gas space 1b so that average gas residence times in gas space 1b are greater than when baffle 30 is omitted.

(30) An annular baffle may also be provided above a different (or additional) hearth of furnace 10. FIGS. 4 and 5 show another portion of furnace 10, now modified to accord with the invention by provision of annular baffle 40 in the gas space 3b above hearth 3a, which like uppermost hearth 1a is an in hearth. As in FIG. 3, only one of the rabble arms 18 operating on hearth 3a is shown, for clarity. Annular baffle 40 is secured to hearth structure 2 and depends therefrom, having a bottom edge 41 that lies close to the rabble arm 18 operating on hearth 3a so that in operation there is no contact between the rabble arm 18 and the baffle 30. The mean diameter of baffle 30 is shown as being 65% of the diameter of the inner surface 23 of the casing, however, other diameters may be found suitable. Baffle 40 divides that part of space 3b into inner and annular spaces 46 and 47 respectively. As with baffle 30, the diameter of baffle 40 is made larger than that of the central drop hole 213 in hearth 3a.

(31) In contrast to gas flow in gas space 1b, gas entering gas space 3b leaves it by passing through the peripheral drop holes 222 that pass through hearth structure 2. Although not shown, the furnace 10 has 20 peripheral drop holes (such as 222) per hearth, a value typical in practice. Accordingly, given that the peripheral drop holes are equispaced around the out hearths, the flow above hearth 3a when baffle 40 is present is more nearly radial. In this case, too, it has been found in flow simulations that baffle 40 is beneficial in increasing mean and median gas residence times in gas space 3b.

(32) It is possible also (or alternatively) to provide an annular baffle (not shown) in one or more out hearths, for example hearth 2a. Taking hearth 2a as an example, gas would enter gas space 2b through peripheral drop holes 222 of hearth 2a and move towards the central drop hole 211 of the in hearth structure 1. It is believed that a baffle above hearth 2a similar to that shown for hearth 3a in FIG. 4 would partially obstruct the generally radial inward gas flow, increasing mean and median gas residence times in gas space 2.

(33) Based on simulations. FIGS. 6 and 7 are approximate sketches of two representative streamlines of gas flow within the gas space 1b above hearth 1a when baffle 30 is present. In these Figures, the only parts of furnace 10 shown are hearth 1a, baffle 30, shaft assembly 15 and central drop hole 211, and gas outlet opening 25b is shown only in outline, with other parts of the furnace 10 being omitted for clarity. Streamline 34 illustrates gas rising through central drop hole 211 in a plane including axis 16 and gas outlet opening 25b, flowing into space 32 within baffle 30, then under the lower edge 31 and out through gas outlet opening 25b. If baffle 30 were not present, gas rising through central drop hole 211 circumferentially close to gas outlet 25 would be expected to stream along a more direct path to gas outlet opening 25b with a shorter residence time in gas space 1b.

(34) Streamline 35 (FIG. 7) shows gas rising through central drop hole 211 at a point more distant circumferentially from gas outlet opening 25b than streamline 34. If baffle 30 were not present, gas rising at this point would be expected to steam upward and around shaft assembly 15 in a more direct path to gas outlet opening 25b than that of streamline 35. Streamline 35 passes upward into annular space 32, under edge 31 and around the exterior of baffle 30, with a swirling motion, with a longer residence time in gas space 1b.

(35) FIGS. 8 and 9 are also based on simulations, and are approximate sketches of two representative streamlines of gas flow within gas space 3b above hearth 3a when baffle 40 is present (FIG. 8) and absent (FIG. 9) respectively. In these Figures, the only parts of furnace 10 shown are hearth 1a, baffle 30, shaft assembly 15 and central drop hole 213, and peripheral drop holes 222 are shown only in outline, with other parts of the furnace 10 being omitted for clarity. Streamlines 42 and 43 (FIG. 9) illustrate gas rising through the central drop hole 213 in hearth 3a and, with baffle 40 not present, moving radially and upwardly to leave gas space 3b through the peripheral drop holes 222 of heath structure 2. Streamlines 44 and 45 (FIG. 8) illustrate the effect of providing annular baffle 40, where gas rises through central drop holes 213 into inner annular space 46, under edge 41 and outwardly and upwardly to peripheral drop holes 222. Some swirl may be developed where the flow is not perfectly radial, as shown by streamline 45.

(36) It is noted that streamlines 34, 35, 42, 43, 44 and 45 are sketches made based on simulations described below, but are not themselves calculated individual streamlines or the results of physical trials.

(37) As mentioned above, the furnace 10, as shown in FIG. 1, is closely representative of an existing furnace in use in incineration of sewage-derived biosolids. As part of an investigation of possible modifications to improve emissions performance, increase throughput and enhance efficiency of this existing furnace, it was modeled by computer simulation for a range of operating conditions and with and without a range of modifications, including annular hearths according to the invention. Physical trials of annular baffles according to the invention have not been carried out.

(38) A 3D CAD (Computer Aided Design) model of the complete furnace 10 was developed and its steady state operation simulated using the commercially available Star CCM+ software package, marketed in the USA by CD-adapco of Melville, N.Y. This package has comprehensive computational fluid dynamics (CFD) capabilities, including flows involving combustion, heat transfer, and chemical reaction all of which were relevant to determination of the gas flows including residence times, temperature distribution, and composition, including outlet O.sub.2, CO.sub.2 and CO levels. Also, movement of the feed material (including rabbling) was simulated using discrete element modelling (DEM), based on representation of the feed material as individual spherical particles with a range of diameters), a further capability of the software. Models of heating, drying and devolatilisation specific to the particular feed material were developed, based on measured feed material samples using procedures known in the art and the technical literature.

(39) A key quantity of interest in the simulations was carbon monoxide (CO) emissions from furnace 10. Opacity of the emissions was also of interest, and was known to be closely correlated to CO emissions.

(40) The furnace 10 was instrumented and data sets for two steady state operating conditions were obtained and used for validation of the modeling. Detailed measurements were also taken of feed material characteristics and used in the modelling. From the validation work, it was concluded that CO emissions reductions greater than 8% (from a base case) could be considered reliable estimates of what would happen in practice.

(41) The geometry of those parts of furnace 10 as shown in FIGS. 2 to 5 inclusive is substantially representative of the actual furnace modeled. The diameter of the cylindrical inner wall surface was 6 m. The height of the gas space 1b above hearth 1a (measured at its periphery) was 1.8 m and the height of the gas space 3b above hearth 3a (also at its periphery) was 0.9 m. The diameter of the central drop holes 211 to 217 was 2.19 m and the nominal diameter of the shaft assembly was 1 m. There were 20 peripheral drop holes 224 to 228 on each out hearth each having an area of 0.16 square metres.

(42) The bottom edges 31 and 41 of annular baffles 30 and 40 were each located about 50 mm above the rabble arms 18 operating on their respective hearths. Both annular baffles had an inner diameter of 3.8 m and an outer diameter of 4 m, so that their radial thickness was 100 mm. There was assumed to be no heat conduction through the baffles, consistent with their comprising refractory material or surfaces.

(43) Although the gas flow simulations took account of the presence of the rabble arms 18, their (low) rotational speed was taken to be zero, so as to decouple the gas flow and feed material movement computations. Effects on gas flow of movement of feed material through the gas (i.e. in the central and peripheral drop holes 211, 213, 215, 217, 222, 224, 226 and 228) and between the feed material inlet 20 and feed material on hearths 1a, 3a were neglected as insignificant. The simulations included recirculation of shaft cooling air.

(44) The giving of these dimensions of the furnace 10 as simulated is not intended to be a limitation on the spirit or scope of the invention, which a person of ordinary skill in the art may adapt for application in multiple hearth furnaces of other sizes and proportions and in different applications, wherever it is found (for example by actual trial or by simulation) to be suitable.

(45) Of the simulations carried out, the following were relevant to the present invention. All simulations were for steady state operation.

(46) Simulation D1This provided a base case, and was representative of normal operation of furnace 10 without any annular baffles fitted. Feed rate was set at 41 tonnes/day, fed to hearth 1a. The moist feed material comprised 34.4% solids, and the excess (over stoichiometric) air supply was 43.6%. The combustion zone was confined to hearths 2a and 3a. Steady state autogenous combustion (i.e. burners not in operation) was simulated.

(47) Simulation D2This was the same as base case D1 in all respects, except that the feed rate was made 50 tonnes/day.

(48) Simulation E12This was the same as base case D1 in all respects, including the feed rate and characteristics, but with provision of an annular baffle over hearth 1a (only) as shown in FIGS. 2 and 3.

(49) Simulation E13This was the same as simulation E12 in all respects, except that the feed rate was set at 50 tonnes/day.

(50) Simulations E12 and E13 both showed reductions of furnace CO emissions by at least about 80% relative to the base cases D1 and D2 respectively. More precision cannot be given due to the uncertainty in resolvable reductions mentioned above.

(51) Some further simulations were made involving a baffle above hearth 3a (only). These were as follows:

(52) Simulation O5This simulation was carried out to test the effect of providing an annular baffle (as shown in FIGS. 4 and 5) above hearth 3a (only) and introducing all feed material at hearth 3a, rather than hearth 1a. The feed rate was 41 tonnes/day, feed material, excess air 43.6% over stoichiometric. The combustion zone was confined to hearth 5a. Again, autogenous combustion (i.e. burners not in operation) was simulated. (Note: The feed material was slightly drier than in simulations D1, D2, E12 and E13, with solids content of 39.4%.)

(53) Simulation O6this was the same as O5 in all respects, except feed rate was increased to 50 tonnes/day. Autogenous combustion (i.e. burners not in operation) was simulated.

(54) Simulations O5 and O6 also showed reductions of furnace CO omissions of at least about 80% relative to base cases D1 and D2 respectively. While the drier feed material and its introduction at hearth 3 prevent the effect of the baffle alone being isolated in these results, they do suggest that baffles above lower hearths can, at the least, also be consistent with enhanced furnace emission performance. Mean gas particle residence times for the hearth having an annular baffle in these simulations were increased by comparison to the relevant base cases.

(55) While the simulations were detailed and complex in principle, they were within the current state of the art, as applied to furnaces of various types and other equipment and phenomena. However, it was noted that increased gas particle mean residence times in gas spaces with annular baffles were evidently the key to the effects of those baffles. Moreover, it will be apparent from FIGS. 6 to 9 that flow patterns were affected by the baffles and lead to increased mean gas particle residence times. Accordingly, the potential applicability of annular baffles where increased gas particle residence times could be expected to enhance performance, is considered clear. This may include multiple hearth furnaces of different sizes or proportions, provision of annular baffles above more than one hearth of a furnace, and applications involving other feed materials and types of treatment than those simulated in detail.

(56) Similarly, the invention may be applied by providing annular baffles in the gas spaces above out hearths instead of (or in addition to) in hearths.

(57) A modified version of the invention will now be described by reference to FIG. 10. FIG. 10 shows the uppermost hearth 1a of furnace 10, portions of two of the rabble arms 18 operating on hearth 1a, and annular baffle 30. For clarity the inner wall surface 23 of the casing 11 is shown by chain-dotted lines only. Although the uppermost hearth 1a is shown in FIG. 10, and referred to in the explanation below, the modification can be applied to any hearth of a multiple hearth furnace.

(58) The requirement for clearance for baffle 30 above rabble arms 18 means that between bottom edge 31 of baffle 30 and feed material on hearth 1a, there may be a considerable gap. In some circumstances it may be desirable to reduce that gap somewhat, despite the presence of rabble arms 18. This can be achieved by providing additional baffle elements 50 that are secured to and/or supported on rabble arms 18 so as to revolve around axis 16, and that extend downward between circumferentially adjacent rabble arms 18. Each baffle element 50, of which one is provided per rabble arm 18, is arcuate with a radius about axis 16 similar to but not the same as the radius of baffle 30 and located radially so as to clear baffle 30. Each baffle element 50 is supported on two adjacent rabble arms 18. Although the need for clearance from baffle 30 means that baffle elements 50 are not sealingly connected to baffle 30, they do provide a disruption additional to that of baffle 30 to gas flow from central drop hole (for example, 211 in the case of hearth 1a) to the gas outlet 25.

(59) Each baffle element 50 may be secured to a rabble arm 18 by a pin (not shown) to the rabble arm 18 at its leading end (based on the direction of rotation) and simply sit on top of the rabble arm 18 at its trailing end, so that there is a degree of freedom of movement of the baffle elements 50. This allows for the fact that rabble arms 18 may sag or otherwise move in use.