GB2074889A - Heat treatment of foodstuff - Google Patents

Abstract

A method of heat treatment of foodstuff in which a stream of oxygen- containing gas and fuel is passed over one or more catalysts to raise the temperature of the gas stream and the resulting hot gas stream is contacted with foodstuff; in which a proportion of the total fuel may be injected initially and a further proportion or proportions of fuel is subsequently injected preceding passage of the gas and fuel stream over a catalyst and the gas and fuel stream may be passed over catalysts in parallel or in series or so that a portion is continually divided off from the main stream flowing to a main catalyst and passed over a subsidiary catalyst and returned to a point in the mainstream upstream of where it was divided off.

Description

SPECIFICATION Heat treatment of foodstuff This invention relates to a method of heat treatment of foodstuff and in particular to a process in which the foodstuff is for human consumption.

When a carbon containing fuel is burnt by flame combustion the hot gas stream produced unsatisfactorily contains pollutants such as carbon monoxide, hydrocarbons and nitrogen oxides which may be harmful in baking. The gas stream can be heated indirectly by being passed through a heat exchanger in which, for example, heat from the gas produced by the flame combustion of the carbon containing fuel is transferred free of pollutants to the gas stream; however, heat is lost in such a transfer. Direct heating of a gas stream is less expensive because less fuel is used and the heat exchange equipment and associated fans and pumps are not required.

A system for heating a gas stream directly should be capable of working at a reduced throughput compared with the maximum throughput. The 'turndown ratio' is the fraction of full throughput to which the actual throughput can be reduced without loss of continuity. For conventional systems using flame combustion a turndown ratio of 4/5 :1 is usual.

It is an object of the present invention to provide a process for the heat treatment of foodstuff particularly for human consumption in which a directly heated gas stream having a low level of pollutants can be employed.

According to a first aspect of the present invention, a process for the heat treatment of foodstuff comprises supplying a stream of oxygen-containing gas and fuel, passing the gas and fuel stream over catalyst to raise the temperature of the gas stream, and contacting the resulting hot gas stream with foodstuff.

According to a second aspect of the present invention a process for the heat treatment of foodstuff material comprises the following steps:- (a) passage of air stream to a mixing chamber; (b) combining the said stream in the said mixing chamber with a second air stream pre-heated by contact with a pilot burner operating by thermal combustion or an electrical heating element; (c) injecting into the streams combined at stage (b) a further portion of the total fuel requirement for heating;; (d) passage of the combined streams plus injected fuel through one or more catalytic combustor sections each comprising a temperature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the combined fuel-air streams and optionally means for further injection of fuel such that catalytic combustion of substantially the remainder of the uncombusted fuel is initiated; (e) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof, and (f) contacting the said foodstuff material with resulting gas stream.

According to a third aspect of the present invention a process for the heat treatment of foodstuff material comprises the following steps:- (a) passage of an air stream to a first mixing chamber; (b) combining the said stream in the said mixing chamber with a second air stream preheated by contact with a pilot burner operating by thermal combustion or an electrical heating element; (c) subdivision of the said combined streams into portions by means of one or more dividers enabling the passage of each portion of the stream through a separate injection and catalytic combustion section; (d) separately injecting into each portion of the divided stream a further portion of the total fuel requirement for that portion of the stream;; (e) passage of the stream portions plus injected fuel through one or more catalytic combustor sections comprising a temperature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the said stream portions plus injected fuel such that catalytic combustion of substantially the remainder of the uncombusted fuel in each portion of the stream is initiated; (f) recombining the resulting streams from each catalytic combustor section in a second mixing chamber; (g) optionally further stages (c), (d), (e) and (f) may be repeated one or more times using the remainder of the fuel; (h) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof, and (i) contacting the said foodstuff material with the resulting gas stream.

According to a fourth aspect of the present invention an apparatus for the heat treatment of foodstuff material comprises: (a) a fan or other means for supplying a stream of air; (b) a pilot burner, fuelled by a fuel injector, or an electrical heating element operating to pre-heat a stream of oxygen-containing gas; (c) a mixing chamber for combining the stream of gas pre-heated by the pilot burner or the electrical heating element with the stream of air produced by the fan at stage (a); (d) an injector system for injecting a further portion of the fuel requirement into the hot gases from chamber (c);; (e) a catalytic combustor section comprising one or more temperature stable and oxidation resistant monoliths, said monoliths providing catalytic channels for contact with and passage therethrough of the air-fuel streams combined with means for further injection of fuel such that catalytic combustion of substantially the remainder of the fuel is initiated; (f) optionally means for diluting the hot combustion gases from stage (e) and (g) means for contacting the resulting hot gases with the said foodstuff material.

Preferably the oxygen-containing gas is air. The fuel used may be either gaseous or liquid.

According to a fifth aspect of the present invention a process for the heat treatment of foodstuff material comprises the following steps:- (a) passage of an air stream preheated by contact with a pilot burner operating by thermal combustion or an electrical heating element to a mixing chamber:: (b) injecting into the said stream a further portion of the total fuel requirement for heating; (c) passage of the stream plus injected fuel through one or more catalytic combustor sections each comprising a temperature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the combined fuel-air streams such that catalytic combustion of substantially the remainder of the uncombusted fuel is initiated; (d) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof; (e) optionally further stages (b), (c) and (e) may be repeated one or more times using the remainder of the fuel, and (f) contacting the said foodstuff material with the resulting gas stream.

By using the method according to the invention it is possible to directly heat a gas stream and to obtain a gas stream having a low pollutant content. Also, direct heating is less expensive than indirect heating since less fuel is used, because heat production is efficient, and additional requirement such as a heat exchanger is not needed.

Several embodiments of the present invention will now be described by way of example and with reference to the accompanying drawings, in which: Fig. 1 is a schematic diagram of a first embodiment according to the invention; Fig. 2 is a schematic diagram of a second embodiment according to the invention; Fig. 3 is a schematic diagram of a third embodiment according to the invention; Fig. 4 is a schematic diagram of a fourth embodiment according to the invention; Fig. 5 is a flow diagram of a fifth embodiment according to the invention; and Fig. 6 is a schematic diagram of the fifth embodiment according to the invention.

A first embodiment is illustrated in Fig. 1. A fan A1 supplies a stream of air or other oxygen containing gas, F1 which is mixed with a pre-heated stream of oxygen-containing gas e.g. air F2 to form a stream of heated gas F4 at a temperature of 500--5000C. A fan A2 and an igniter L1, fuelled by a fuel injector 11, comprising a pilot burner, supplies the second stream F2. In a mixing chamber M1, a set of four dividers D1-D4 separate F4 into five streams and ten injectors l3l12, two in each stream inject the remaining fuel available to the system.A supported catalyst is similarly divided into five sections C1-C5, each section being fed with each air stream plus fuel supplied by the two respective injectors e.g. C1 is supplied by 13 and 14, and C3 by 17 and i8. The hot streams of air from each catalyst section are recombined into a stream F6 which flows out of the chamber. F5 is diluted if necessary with a stream of air F6 from fan A4 to produce a final stream of air F7 at the temperature required.

A second embodiment is shown in Fig. 2, which is a modification of the first embodiment, operating on two fuel sources so that if for some reason there is a shortage of one fuel, the second fuel can be used without stopping the process. A second pilot burner comprising an igniter L2 fuelled by a fuel injector 12 is supplied with air by a fan A3. In the mixing chamber M1 there are ten further fuel injectors 113122 located upstream of the injectors l3l12. The first fuel may be a gas, such as natural gas, and the second fuel may be a liquid fuel.

As before, in the mixing chamber four dividers D1-D4 divide the air stream F4 into 5 separate streams and four injectors, two for each fuel, inject the remaining fuel available into each stream.

The catalysti may be whole or segmented as shown in Figure 2. With more than one combustion section the total quantity of fuel supplied can be adjusted. One or more of the catalysts need not be used and the temperature of the gas stream can therefore be varied.

A third embodiment is shown in Fig. 3. A supply of air F1 provided by a fan A1 is mixed with a preheated stream of air heated by an igniter burner L1, fuelled by a fuel injector 11 to form a stream of air F4 in a mixing chamber M5. The remainder of the fuel is injected by fuel injectors 13 and 14 before the gas stream flows through a catalyst C1. After flowing through C1 into a mixing chamber M6, a further supply of air F10 is mixed with the gas stream to form a stream of gas F1l which flows through a second catalyst C2. The resulting hot gas stream F5 is mixed in a mixing chamber M7 as required with a further stream of air F6 by a fan A4 to produce a final gas stream F7.

The gas stream F4 should not contain sufficient oxygen for the complete combustion of the fuel when it passes over the first catalyst C1. Teh gas stream F11 flowing over the second catalyst C2, however, has an excess of oxygen present due to the addition of the air, F 10 The gas stream F4 passing over the first catalyst should only contain 3565% of the oxygen required for complete combustion of the fuel present.

This apparatus can be used with fuels containing bound nitrogen; such fuels on burning normally form exhaust gases containing nitrogen oxides. By using catalytic combustion the amount of nitrogen oxides in the exhaust gas is considerably reduced.

In the three embodiments described, the pilot burner consists essentially of a fuel injector and an ignitor. The burner can be placed directly in the main gas stream F1 so that a proportion of this stream passes through the burner.

The amount of fuel supplied to the pilot burner is between 0.0115% volume by evaporate and preferably 0.0110% of the total fuel available. The temperature of the gas stream flowing into the combustion chamber, that is the inlet temperature of the gas flowing through the catalyst, is controlled by adjusting the fuel supply to the pilot burner. During the initial period of operation the inlet temperature is 200--5000C, depending on the fuel used. Once combustion of the fuel over the catalyst has started the inlet temperature can be lowered to between SOC and 2500C by reducing the amount of fuel supplied to the pilot burner.

The gas stream flowing over the catalyst, in the first and second embodiments, and over the second catalyst in the third embodiment should have an excess of air for the complete combustion of the fuel present. The percentage of fuel present in a gas stream containing air and fuel is 0.0120% volume by evaporate and preferably between 0.0110% of the total fuel available. The concentration of a gaseous fuel should be below the lower inflammability limit.

A fourth embodiment of the invention is shown in Figure 4. A fan A30 supplies a stream of air F1 which passes over an electrical heater S raising the temperature of the air to 200--5000C. A proportion of the total fuel available to the system, 0.1-10%, is added to the stream of air F2 by a fuel injector 130 before the air stream F2 passes through a catalyst C30. The temperature of the stream of air F3 leaving the catalyst C30 is between 5000C and 1 2000C. A stream of air F4 from a second fan A31 is mixed with the air stream F3 to form a stream of air F8 at a temperature of 200-5000C and a further proportion of the fuel is then added from injector 131.

The stream of air F6 then passes over a catalyst C31. The stream of air F6 leaves the catalyst C31 at a temperature 500-1 2000C and is mixed with another stream of air F7 from a third fan A32 to form an air stream F8 at 200--5000C. The remainder of the total fuel available is added by a fuel injector 132 before the air stream F8 passes through a catalyst C32. The stream of air F8 resulting at 500-1 2000C from the catalyst C32 is mixed with an air stream F10 from a fourth fan A33 to produce a final stream of air F11.

Thus, the stream of air containing a proportion of fuel, should be at a temperature of 200-5000C as it flows through each catalyst. The temperature of the gas stream as it leaves each catalyst is increased and lies in the range 500-1 2000C and a further stream of air, at a lower temperature, supplied by a fan or similar is added and the temperature of the resulting stream of air is 200-5000C.

A proportion of fuel is injected into the stream of air prior to its passage through each catalyst. The number of catalysts and associated fans and fuel injectors may be two or more.

Each successive catalyst, downstream of the first catalyst, is larger than the previous catalyst and the amount of fuel injected before each catalyst is increased with the size of each said successive catalyst.

Only one fan need be used for the exiting streams of air, being separated into a number of air streams and conveyed to the required areas by tubes or ducts.

To commence operation, air from the fan A30 is heated by the heater S to about 4000C before any fuel is added via the fuel injector 130. Once the first catalyst C30 has been passed, the other catalysts (31 and 32) may be successively brought into operation. To decrease the output, the fuel supply to the last catalyst C32 and the fan associated with that catalyst are stopped and to further decrease the output, the fuel supply to the catalysts C30 and C31 is turned off, C31 first.

A fifth embodiment of the invention will be described with reference to Figures 5 and 6. A stream of air F1 produced by a fan A34, or similar apparatus, is passed over an electrical heater S1 and the resulting stream F2 is passed through another fan A35. The stream of air F4 may be divided into two streams F8 and F9. The stream of air F9 flows into a mixing chamber M8 where fuel from an injector 133 is added to air stream F10 before this contacts a supported catalyst C33 and finally energes as a heated stream of air F11 for contact with foodstuff.

The electrical heater S1 heats the stream of air F2 to 200-6000C which is above the light-off temperature of the catalysts. The second stream of air F8 is recirculated via the mixing chamber M9 and heated by the combustion of fuel from injector 134 over the catalyst C34. The heated air stream F7 at a temperature of 550-1 0000C then passes back into the main air stream F2. The electricity supply to S can be reduced or switched off as desired since the subsidiary catalyst C34 is used to heat the stream of air for the main catalyst C33. A ratio-regulation controller is used to adjust the flow of air and the injection of fuel into the apparatus using valves T1, T2 and T3.

The sizes of the catalysts, heater and fans will be dependent on the required output of the system.

Turndown of the system is affected by reducing both the quantity of fuel injected into and the amount of air in the air stream F10 by the injector 133. A turndown ratio of 1 5:1 can be achieved during general running and a ratio 20:1 during start-up. The space velocity of the gas flowing through the catalyst i.e.

the total volume of gas flowing per hour/volume of chamber can be up to 60,000 hr-1.

In all the embodiments the velocity of the gas stream flowing through the combustion chamber depends on the fuel but it must be greater than the flame propagation velocity to prevent the risk of flame spreading.

The catalysts are each supported on a metallic or ceramic substrate with an intermediate layer (the "washcoat") of a refractory metal oxide which covers at least part of the substrate. The substrate is preferably a monolith i.e. a unitary body with channels running through it in the direction of gas flow; a structure of knitted or woven wire-may be used as the substrate. Metals or alloys used in the fabrication of the substrate should be oxidation resistant and thermally stable up to at least 1 0000C at high temperature zones.

The temperatures of the resulting gas streams will depend on the nature of the fuel and the catalyst, the deactivation temperature of the catalyst, the temperature stability of the support material and other factors.

Suitable base metal alloys are Ni and Cr alloys, having an aggregate nickel plus chromium content greater than 20% by weight of the total weight, and alloys of iron, including at least one of the elements in % by weight 340% Cr, 1-10% Al, trace-5% Co, trace-72% Ni and trace-0.5% carbon. Such substrates are described in DOS 2450664.

Other examples of base metal alloys capable of withstanding the rigorous conditions required are iron-aluminium-chromium alloys, which may also contain yttrium. These latter alloys may contain in weight % 0.512% Al, 0.1-3.0% Y, 0.20% Cr and balance Fe; these are described in U.S. Patent No. 3,298,826. Another range of Fe-CR-AI-Y alloys contain by weight % 0.54% Al, 0.5-3.0% Y, 20.095% Cr and balance Fe and these are described in U.S. Patent No. 3,027,252.

Alternatively, the base metal alloys may have low corrosion resistance e.g., mild steel and a protective coating composition covering the surface of the substrate is employed as described in our copending British Patent Application GB 2,013,517A.

Suitable ceramic materials for use as the substrate are mullite, zircon, mullite, cordierite, silicon carbide, silicon nitride and zirconia. The material must have sufficient thermal shock resistance to withstand being heated from room temperature to 10000C for at least a period of 30 seconds.

The washcoat layer preferably contains, in oxide form, one or more of Mg, Ca, Sr, Ba, Si, Y, the lanthanides, Ti, Zr, Hf, Th, V, Cr, Mn, Co, Ni, B, Al, Si and Sn. Preferred washcoat material is Al2O3 or alumina hydrates but stabilising oxides such as barium oxide and oxides promoting catalytic activity such as TiO2, ZrO2, HfO2, ThO2, Cr2O3 and NiO may also be present. The preferred washcoat loading is between 0.006 and 0.1 g cm-3 monolith.

The catalytic metal is selected from Rh, Pd, Ir, Pt, Os, Cu, Co, Ni, W and the lanthanides and mixtures, alloys and inter-metallic compounds of these metals disposed upon the surface of or throughout the refractory metal oxide layer. The preferred metals are Ce, Ni, Ba, Co and W. The loading of the catalytic metal is between 700 and 8,800 gm-3 monolith.

It will be appreciated that other combustible gases will be suitable for use, instead of natural gas, such as propane or butane.

The following example shows the effectiveness of heating a gas stream directly according to the invention. In a test rig, a stream of air was heated to between 200-6000C by an electrical heater.

Natural gas was injected into the air stream and the resulting gaseous stream was mixed in a mixing chamber before passing through a first catalyst and a second catalyst. The temperature of the gaseous stream before and after the catalysts was measured. The concentration of natural gas, nitrogen oxides and carbon monoxide was measured in the outlet gaseous stream from the catalysts.

The catalysts used were supported in a 62 cell cm-2 metallic monolith fabricated from FeCr alloy having a diameter of 48 mm and a length of 50.8 mm. The support was coated with a washcoat of alumina containing barium dried and fired at 9500C for 1 hour. The washcoated support was impregnated with a solution containing cerium and a solution containing platinum or platinum and palladium, dried and fired at 9500C for 1 hour. The washcoat loading was 0.12 g cm-3 and the total platinum group metal loading was 4260 gm-3 with a palladium to platinum ratio of 3 :1.

The proportion of natural gas used was 2% of the air stream. The effect of inlet temperature on the conversion of methane was studied. To produce a heated air stream containing little or no pollutantathe conversion of methane should be at least 99%. The results of the tests were as follows:- I) Catalyst: Platinum Conversion of natural gas 99.8% i.e. > 99% inlet temperature 5400C Outlet temperature 900-9500C Outlet gas contained NOx < 1 ppm CO < 100 ppm II) Catalyst: Platinum/Palladium Conversion of natural gas 99.8% i.e. > 99% Inlet temperature 500"C Outlet temperature 900-9500C Outlet gas contained NOx < 1 ppm CO < 100 ppm It will thus be seen that according to the present invention a hot gas stream is produced which has very low pollutant content and which is therefore suitable for use in the heat treatment of foodstuff i.e.

in baking and the like. Also, the method according to the invention has a much improved turndown ratio over conventional systems.

Claims (27)

1. A method of heat treatment of foodstuff comprising supplying a stream of oxygen-containing gas and fuel, passing the gas and fuel stream over a catalyst to raise the temperature of the gas stream, and contacting the resulting hot gas stream with foodstuff.

2. A method according to Claim 1 wherein a stream of oxygen-containing gas is initially supplied and fuel is injected into the path of the gas stream.

3. A method according to Claim 1 or 2, wherein the oxygen-containing gas is supplied via a fan.

4. A method according to Claims 1, 2 or 3 wherein the stream of oxygen-containing gas is combined with one or more preheated gas and fuel streams.

5. A method according to Claim 3 or 4 wherein one or more of the preheated gas and fuel streams is preheated by a pilot burner comprising a fuel injector and a fuel igniter.

6. A method according to any preceding claim wherein one or more of the gas and fuel streams is heated by an electrical heating element.

7. A method according to any preceding claim wherein one or more fuel injectors is located upstream of one or more catalysts.

8. A method according to any preceding claim wherein the hot gas resulting from passage of one or more catalysts is diluted by an oxygen containing gas stream at a lower temperature.

9. A method according to Claim 1 wherein the gas and fuel stream is divided and the catalyst is divided so that the portions of the gas and fuel stream are passed in parallel through separate catalytic combustion sections.

1 0. A method according to Claim 9, wherein the hot gas streams resulting from the separate catalytic combustion sections are recombined for contact with the foodstuff.

11. A method according to Claim 1 wherein the gas and fuel stream is passed through two or more catalysts in series.

12. A method according to Claim 1 , wherein the gas and fuel stream is passed through two or more catalysts and divided so that a portion of the gas and fuel passes over a first catalyst and a second portion passes over a second catalyst at least once.

13. A method according to Claim 12 wherein the second divided off portion is divided off from the main gas and fuel stream before the second catalyst and returned to the main gas and fuel stream upstream of where it was divided off.

14. A method according to any preceding claim in which the oxygen-containing gas is air.

1 5. A method according to any preceding claim, in which the fuel is natural gas, butane or propane.

16. A method according to any preceding claim, in which the catalyst is supported on a metallic or ceramic substrate with an intermediate layer of a refractory metal oxide.

17. A method according to any preceding claim wherein the base metal is NiCr alloy, Fe alloy S1, FeAICrY alloy, or mild steel.

1 8. A method according to any preceding claim wherein the ceramic substrate is mullite, zircon mullite, cordierite, silicon corbide, silicon nitride or zirconia.

1 9. A method according to any preceding claim wherein the washcoat is in the form of the oxide of Mg, Ca, Sr, Ba, Xi, Y, Mn, Co, Ti, Zr, Hf, Th, Cr, Ni, Al, any of the lanthanide group or alumina hydrate.

20. A method according to any preceding claim wherein the catalytic metal is Rh, Pd, Ir, Pt, Os, Cu, Co, Ni, W, Ce or Ba, the lanthanides and mixtures, alloys and intermetallic compounds thereof.

21. A method according to any preceding claim wherein the system has turndown ratio in the range 15-20:1.

22. A method for the heat treatment of foodstuff material comprising the following steps:- (a) passage of air stream to a mixing chamber; (b) combining the said stream in the said mixing chamber with a second air stream pre-heated by contact with a pilot burner operating by thermal combustion or an electrical heating element; (c) injecting into the streams combined at stage (b) further portion of the total fuel requirement for heating; (d) passage of the combined streams plus injected fuel through one or more catalytic combustor sections each comprising a temperature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the combined fuel-air streams and optionally means for further injection of fuel such that catalytic combustion of substantially the remainder of the uncombusted fuel is initiated;; (e) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof, and (f) contacting the said foodstuff material with resulting gas stream.

23. A method for the heat treatment of food stuff material comprising the following steps:- (a) a passage of an air stream to a first mixing chamber; (b) combining the said stream in the said mixing chamber with a second air stream preheated by contact with a pilot burner operating by thermal combustion or an electrical heating element; (c) subdivision of the said combined streams into portions by means of one or more dividers enabling the passage of each portion of the stream through a separate injection and catalytic combustion section; (d) separately injecting into each portion of the divided stream a further portion of the total fuel requirement of that portion of the stream;; (e) passage of the stream portions plus injected fuel through one or more catalytic combustor sections comprising a temprature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the said stream portions plus injected fuel such that catalytic combustion of substantially the remainder of the uncombusted fuel in each portion of the stream is initiated; (f) recombining the resulting streams from each catalytic combustor section in a second mixing chamber; (g) optionally further stages (c), (d), (e) and (f) may be repeated one or more times using the remainder of the fuel; (h) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof, and (i) contacting the said foodstuff material with the resulting gas stream.

24. A method for the heat treatment of foodstuff material comprising the following steps:- (a) A fan or other means for supplying'a stream of air; (b) a pilot burner, fuelled by a fuel injector, or an electrical heating element operating to pre-heat a stream of oxygen-containing gas; (c) a mixing chamber for combining the stream of gas pre-heated by the pilot burner or the electrical heating element with the stream of air produced by the fan at stage (a); (d) an injector system for injecting a further portion of the fuel requirement into the hot gases from chamber (c);; (e) a catalytic combustor section comprising one or more temperature stable and oxidation resistant monoliths, said monoliths providing catalytic channels for contact with and passage therethrough of the air-fuel streams combined with means for further injection of fuel such that catalytic combustion of substantially the remainder of the fuel is initiated; (f) optionally means for diluting the hot combustion gases from stage (e) and (g) means for contacting the resulting hot gases with the said foodstuff material.

25. A method for the heat treatment of foodstuff material comprising the following steps:- (a) a passage of an air stream preheated by contact with a pilot burner operating by thermal combustion of an electrical heating element to a mixing chamber; (b) injecting into the said stream a further portion of the total fuel requirement for heating.

(c) passage of the stream plus injected fuel through one or more catalytic combustor sections each comprising a temperature stable and oxidation resistant monolith providing catalytic channels for contact with and passage therethrough of the combined fuel-air streams such that catalytic combustion of substantially the remainder of the uncombusted fuel is initiated; (d) optionally further dilution of the resulting hot gas stream if necessary to adjust the temperature thereof; (e) optionally stages (b), (c) and (e) may be repeated one or more times using the remainder of the fuel, and (f) contacting the said foodstuff material with the resulting gas stream.

26. A method as described herein with reference to and as illustrated in Figs. 1 to 6 of the drawings.

27. A method as described herein with reference to and as illustrated in the example.