GB2039775A

GB2039775A - Furnaces Having Catalytic Heaters

Info

Publication number: GB2039775A
Application number: GB7944002A
Authority: GB
Original assignee: BTU Engineering Corp
Current assignee: BTU Engineering Corp
Priority date: 1978-12-22
Filing date: 1979-12-20
Publication date: 1980-08-20
Also published as: CA1144364A; GB2039775B; NL7909288A

Abstract

A sintering or other precision furnace comprises a pre-heat part 10 in which hydrocarbon effluent gases are present, a flue chamber 26 containing a catalytic heater 36, an opening in the furnace chamber 16 and an exhaust 28 external of the furnace. An oxidising gas such as CO2 is introduced into the flue for combination with the effluent gases being drawn out of the chamber. As the contaminating gases move through the flue, they pass over the heated catalytic surfaces of the heater 36 and react with the oxidising gas to form water and carbon monoxide, all of which is drawn out through the flue and exhausted therefrom or burned off. The draft or negative pressure within the flue is closely controlled by an air operated Venturi pump 37, the air of which assists in burning exhaust gases at the exhaust 28. Alternatively, the catalytic heater can be disposed in a gas barrier 14 located between the pre-heat part 10 and high-heat or sintering part 12 of the furnace chamber 16. <IMAGE>

Description

SPECIFICATION Furnaces having Catalytic Heaters This invention relates to furnaces and more particularly to sintering and other precision furnaces having catalytic heaters for removal of hydrocarbon effluents.

In the heat processing of a product in a precision furnace, the process in many instances produces hydrocarbon effluents which can contaminate the furnace and seriously affect its operation. In sintering furnaces, for example, products often contain organic binders and lubricants which can include natural and synthetic waxes, starches, sugars, gums, and zinc and other metal stearates. These organic materials are often removed by out-gasing in a heated reducing atmosphere but this results in major problems.

The gases condense in cooler parts of the furnace to form deposits of pasty and hard carbonaceous substances which can build up to the point of interfering which the heat process being performed within the furnace. In addition, the gases can react with the furnace lining causing deterioration and can also fall onto the product within the furnace with consequent contamination or degradation of the product.

Metal organic substances present in the binders or lubricants can recombine to form hard oxides and carbides which can block an exhaust of the furnace and otherwise interfere with furnace operation. High humidity and the presence of oxygen in the furnace atmosphere can promote breakup and elimination of hydrocarbons.

However such a furnace atmosphere is not always feasible or compatible with the process being performed or the intended quality of the product being processed.

The use of catalytic elements in a furnace or oven exhaust for the purpose of preventing contamination of the atmosphere is generally known as shown for example in U.S. patents 2,658,742 and 3,130,961. Other furnace exhaust systems are shown in U.S. patents 3,708,157; 3,940,237 and 3,963,416.

According to the present invention, a furnace in which hydrocarbon effluents are present during use comprises: a chamber having an entrance for receiving a product to be transported through the furnace: a gas outlet extending from an opening in the chamber to an exhaust external of the furnace; a catalytic heater disposed within the gas outlet and providing catalytic surfaces over which hydrocarbon effluent gases must pass as they are exhausted from the furnace; means for introducing an oxidizing gas into said opening of the gas outlet for mixture with the effluent gases passing over the catalytic surfaces; means for applying electrical energy to said catalytic heater to heat said catalytic surfaces; and means for drawing gases in a controlled manner through the gas outlet and out of the exhaust.

In use, as the contaminating hydrocarbon effluent gases move through the gas outlet, they pass over the heated catalytic surfaces and also mix with the oxidizing gas. The oxidizing gas, such as carbon dioxide (Co2), is reduced to release oxygen which is then present to oxidise the hydrocarbons to form water and carbon monoxide, all of which is drawn out in a controlled manner through the gas outlet and exhausted therefrom or burned off. The oxidizing gas is not allowed to enter the furnace chamber where it could cause oxidation and decarburization of the product therein.

The catalytic heater preferably includes a ribbon formed in a serpentine path to provide a relatively large surface area while also providing open spaces through which the gases can be drawn within the gas outlet. The ribbon typically is a nickel-containing metal such as Nichrome (RTM) or Inconel (RTM). The ribbon serves both as a catalyst and as a heater and since it is confined to a relatively narrow space, between the walls of the gas outlet, it does not materially affect the time-temperature curve of the furnace.

The present invention is especially suited to sintering furnaces having the gas outlet formed as a flue opening into a pre-heat part of the furnace chamber in which hydrocarbon effluent gases are present from an out-gasing operation conducted before the product has been transported to a high-heat part of the furnace chamber. The present invention is also especially suited to furnaces wherein hydrocarbon gases such as methane are added to carburize ferrous products or to prevent decarburization of products. In an alternative embodiment of the present invention, said opening of the gas outlet opens into the furnace chamber at the location of a gas barrier separating pre-heat and high-heat parts of the furnace chamber to trap any stray gases and prevent such gases from migrating into the highheat part, where they are undesirable.

A furnace in accordance with the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a fragmentary partly sectioned elevation of a furnace according to the present invention; Figure 2 is a section taken along the line 2-2 of Figure 1; and Figure 3 is a section taken along the line )3-3 of Figure 1.

Referring to the drawings, there is shown a sintering furnace having a pre-heat or burnout part 10 and a high-heat or sintering part 12 separated by a gas barrier 14. The furnace walls are constructed of ceramic brick or other suitable refractory material, and a chamber 1 6 is provided along the entire length of the furnace through which a product 1 8 is conveyed by an appropriate transport mechanism 20, such as moving belt or a walking beam. The furnace can itself be of any known construction suitable to the particular heat process being carried out, and thus no details of the furnace structure need be provided herein.

A gas outlet in the form of a flue is provided at the entrance end of the pre-heat part of the chamber 1 6 and is composed of two parallel walls 22 and 24 extending substantially across the entire width of the furnace and in height extending above the furnace. The walls are spaced apart to define a flue chamber 26 which at its lower end communicates with the pre-heat part 10 and at its upper end joins an exhaust 28 which can include a flame curtain 30 including a pilot burner and igniter. The flue walls are lined with respective ceramic or other electrically insulating plates 32 and 34 which are grooved to retain a catalytic heater 36 within the flue. A gas supply pipe has a nozzle end 38, disposed where the flue opens into the chamber 16, for introduction of carbon dioxide or other oxidizing gas from a suitable supply (not shown).A Venturi exhaust pump 37 maintains a closely controlled negative pressure in the flue by means of an air jet supplied from a suitable source (not shown). The air assists in burning the exhausted gases at the flame curtain 30.

The catalytic heater 36 is formed of a ribbon of nickel-containing metal such as Nichrome (RTM) or Inconel (RTM), the ribbon being formed in a serpentine or convoluted path across the width of the furnace, as illustrated more particularly in Figures 2 and 3. The heater ribbon is vertically disposed, so that its minor surfaces lie in horizontal planes and its major surfaces confront each other, and extends in a plurality of horizontal courses 39 which are vertically stacked within the flue. Each course 39 of the convoluted ribbon is supported within respective confronting grooves 40 of the ceramic plates 32 and 34. The ends of the courses of the ribbon heater are electrically connected in the manner illustrated to form one continuous heater.The courses can be interconnected by extensions 42 of the ribbon itself, or separate electrical connector strips can be welded to the ribbon ends being connected.

The ribbon heater is connected to electrical terminals 44 by means of leads 46 each being connected to a respective end of the heater ribbons. The catalytic heater is energized from a suitable electrical energy source (not shown). The serpentine path of the ribbon heater not only provides channels within the flue through which the mixture of oxidizing gas and contaminating gases must pass, but also provides a relatively large catalytic surface area and thus efficient catalytic action.

In operation, gases within the pre-heat part of the chamber 1 6 tend to move towards the entrance of the furnace and are drawn into the flue for mixture with the oxidizing gas introduced by the nozzle end 38 and passage over the heated catalytic surfaces of the ribbon 36. When the mixture passes over the hot catalytic ribbon, the carbon dioxide is reduced to carbon monoxide and free oxygen, which oxidizes the hydrocarbons to form water and carbon monoxide, all of which is drawn out through the flue by the Venturi pump 37 and exhausted or burned.

The catalytic heater is operated typically at a temperature between 700 to 1000 C and does not materially affect the time-temperature curve of the furnace or product conveyed therethrough since the ribbon heater is confined to a narrow space between the insulated walls of the flue. As an example, a product moving through the furnace at a rate of 12 inches per minute would be exposed to heat from the catalytic heater radiating from the bottom of the flue for only about ten seconds, for a typical flue width of two inches, which is not sufficient time to influence significantly the time-temperature curve.

The present invention can also be implemented by employing as its gas outlet a passage or chimney which opens into the chamber 1 6 at the location of the gas barrier 14 for removing the gases of said barrier. The purpose of the gas barrier 14 is to trap any stray gases and prevent their entry into the high-heat or sintering part 12 where such gases are particularly undesirable.

The gas barrier 14 itself can be of known construction, a preferred barrier being described in U.S. patent 3,041,056. The catalytic heater can be the same or similar to the ribbon heater described above and can be disposed, as shown in Figure 1 by reference numeral 41, in a passage through the wall of the furnace for removing the gases of the gas barrier from the furnace.

Claims

1. A furnace in which hydrocarbon effluents are present during use comprising: a chamber having an entrance for receiving a product to be transported through the furnace; a gas outlet extending from an opening in the chamber to an exhaust external of the furnace; a catalytic heater disposed within the gas outlet and providing catalytic surfaces over which hydrocarbon effluent gases must pass as they are exhausted from the furnace; means for introducing an oxidizing gas into said opening of the gas outlet for mixture with the effluent gases passing over the catalytic surfaces; means for applying electrical energy to said catalytic heater to heat said catalytic surfaces; and means for drawing gases in a controlled manner through the gas outlet and out of the exhaust.

2. A furnace according to claim 1, wherein said catalytic heater includes a ribbon of nickelcontaining metal formed in a serpentine path within the gas outlet to provide a relatively large catalytic surface area over which exhausted gases must pass.

3. A furnace according to claim 1 or claim 2, wherein said gas outlet includes a pair of spaced walls extending substantially across the width of the furnace; a pair of electrically insulating plates disposed within the gas outlet and spaced apart to define a gas outlet chamber, said plates having a plurality of confronting grooves extending thereacross; and said catalytic heater including a ribbon formed in a serpentine path and supported within confronting grooves of said plates.

4. A furnace according to claim 3, wherein said catalytic heater ribbon has a plurality of stacked courses electrically connected to form one continuous heater.

5. A furnace according to claim 3 or claim 4, wherein said catalytic heater ribbon has its minor surfaces in horizontal planes and its major surfaces confronting each other to define channels through which effluent gases pass during their removal from the chamber.

6. A furnace according to any one of claims 1 to 5, wherein said opening of the gas outlet opens into said chamber at the location of a pre-heat part of said chamber.

7. A furnace according to any one of claims 1 to 5, wherein said opening of the gas outlet opens into said chamber at the location of a gas barrier separating a pre-heat part of said chamber from a high-heat part of said chamber.

8. A furnace according to any preceding claim, wherein said gas drawing means includes: a Venturi exhaust pump having a nozzle in the gas outlet for providing an air jet therein.

9. A furnace according to claim 6 and claim 8, further including means for burning off gases exhausted from the gas outlet.

10. A furnace according to claim 9, wherein said turning off means includes a flame curtain.

11. A furnace according to claim 1 and substantially as hereinbefore described with reference to the accompanying drawings.