EP1141421B1

EP1141421B1 - Method and apparatus for the direct reduction of iron oxides

Info

Publication number: EP1141421B1
Application number: EP99956288A
Authority: EP
Inventors: Henry Rafael Bueno; David Rabascall; Gianpietro Benedetti
Original assignee: Danieli and C Officine Meccaniche SpA
Current assignee: Danieli and C Officine Meccaniche SpA
Priority date: 1998-12-11
Filing date: 1999-12-06
Publication date: 2003-03-26
Anticipated expiration: 2019-12-06
Also published as: DE69906377D1; DE69906377T2; EP1141421A1; ATE235563T1

Description

FIELD OF THE INVENTION

This invention concerns a process to produce metallic iron starting from mineral iron, wherein the iron is present in the form of oxides, and the relative apparatus which comprises a reduction furnace which may have one or more inlets for the reducing gas and inside which the process of direct reduction of the iron (DRI) is carried out. The reducing gas is obtained by mixing a part of the process gas, which emerges from the reduction furnace, with additional gas arriving from an outside reforming circuit.

BACKGROUND OF THE INVENTION

The state of the art includes processes of direct reduction which use the injection of hydrocarbons into the current of reducing gas to allow the reaction of reforming the methane in the furnace with the H₂O and CO₂ in the gas; there are also known processes of direct reduction which use the injection of hydrocarbons with C>5 directly into the furnace in the zone between the injection of the reducing gas and the outlet from above of the burnt gas.

From the following patent documents other processes are known for the direct reduction of mineral iron:

US-A-2,189,260,	US-A-3,601,381,	US-A-3,748,120,
US-A-3,749,386,	US-A-3,764,123,	US-A-3,770,421,
US-A-4,054,444,	US-A-4,173,465,	US-A-4,188,022,
US-A-4,234,169,	US-A-4,201,571,	US-A-4,270,739,
US-A-4,374,585,	US-A-4,528,030,	US-A-4,556,417,
US-A-4,720,299,	US-A-4,900,356,	US-A-5,064,467,
US-A-5,078,788,	US-A-5,387,274, and	US-A-5,407,460.

The state of the art also includes processes wherein the hot metallic iron is produced in a reduction furnace of the shaft type, with a vertical and gravitational flow of the material, which is subsequently sent to the melting furnace by means of a closed pneumatic transport system in an inert atmosphere.
Document EP-A-0262353 discloses a method and apparatus for producing hot sponge iron in a vertical shaft reactor, wherein the reducing gas to reduce the ore is generated in a reformer unit by the catalytic conversion of a mixture of steam and natural gas to a reducing gas largely composed of CO and H₂. The spent or process gas, leaving the reduction zone of the reactor, flows to a quench cooler wherein it is cooled and de-watered, and to a carbon dioxide unit, wherein the CO₂ is removed. A back pressure regulator allows to withdraw the exceeding gas and to maintain the reactor and associated equipment to a desired pressure. Therefore, the process gas is substantially solely "cleaned" from H₂O and CO₂ in order to recover a part of the active gas (H₂ and CO) which is contained in the spent gas. What it happens therein is simply that something that it is no more necessary (i.e. H₂O and CO₂) is withdrawn from the process gas, while the remaining part is reintroduced in the main gas supply pipe, wherein flows the reducing gas produced by the steam reformer unit.

SUMMARY OF THE INVENTION

The method to produce metallic iron by the direct reduction of iron oxides and the relative apparatus according to the invention are set forth and characterised in the respective main claims, while the dependent claims describe other innovative features of the invention.
The method according to the invention consists in bringing into contact the mineral iron, of various granulometry, with a feed gas in a reduction furnace of the shaft type, wherein both the gas and the material are fed continuously, so that a vertical and gravitational flow of material is created and the direct reduction of the mineral is achieved. The material may be discharged from the reactor either cold or preferably hot to be sent subsequently to a melting furnace or so that it may be converted into hot briquette iron (HBI) or cooled and converted into direct reduction iron (DRI).
The reduction furnace is equipped with means to feed the mineral iron and means to discharge the reduced metallic iron; it is equipped with at least one inlet collector to inject the reducing gas in correspondence with a reduction zone or reactor inside the furnace.
The reducing gas sent into the reactor contains hydrocarbons injected into the current after the partial combustion of the hydrogen and carbon monoxide with the oxygen and is obtained by mixing a part of the process gas, which exits from the reduction furnace, with additional gas arriving from an outside reforming circuit.
In a variant, the hydrocarbons are injected before the partial combustion is achieved, with the purpose of raising the temperature of the gas introduced into the reactor.
According to another variant, the hydrocarbons are at least partly injected into a zone between the reduction zone and the zone where the reduced material is discharged.
In all cases, the injected hydrocarbons cooperate in reducing the iron oxide (FeO) to metallic iron, generating more H₂ and CO.
The direct reduction of the iron oxides is achieved in two different continuous stages inside the reduction reactor.
In a particular embodiment, the furnace is provided with a first stage, defined as the pre-heating and pre-reduction stage, where the fresh iron oxides, that is, those just introduced into the furnace, come into contact with a mixture of reducing gas, consisting of partly burnt gas, arriving from the underlying part of the furnace and of fresh hot gas, that is, gas introduced from outside, arriving from a collector which brings fresh reducing gas and possibly CH₄ or other natural gas. This first stage takes place in a corresponding first zone arranged in the upper part of the furnace.
In the second stage, the reduction stage proper, the complete reduction of the iron oxides is achieved, due to the action on the oxides, already partly reduced in the first stage, of a mixture of reducing gas based on H₂ and CO and at least a hydrocarbon, preferably natural gas, injected in the median zone of the reduction reactor. This second stage takes place in a corresponding second zone arranged below the first zone.
The two inlets to the furnace through which the gas is introduced can be independently regulated both in the flow of fresh reducing gas and in the addition of natural gas in the current introduced.
Moreover, the inlet temperature of the two currents of reducing gas can be independently regulated by injecting O₂ before they enter the reduction reactor.
The oxidation reaction needed to raise the temperature of the gas leads to a change in the level of oxidation of the gas, from normal values of 0.04-0.08 to 0.06-0.15.
The following ratio is intended for the level of oxidation of the reducing gas: Nox = (H₂O+CO₂)/(H₂O+CO₂+H₂+CO)
In the second reaction zone of the furnace, wherein the reduction of the iron oxides is completed, a gas is generated with a high content of H₂ and CO and with an oxidation level of between 0.15 and 0.25 due to the reduction reactions of the iron oxides with H₂, CO and CH₄.
Once this gas has left the second reaction zone, it enters the first reaction zone, located higher up, and mixes with the hot gas injected into the first zone to pre-heat and pre-reduce the iron oxides.
The gas emerging from the reduction reactor is partly recircled and partly used as fuel.
The recircled gas has a volume composition within the following fields:
H₂=20-41%, CO=15-28%, CO₂=15-25%, CH₄=3-10%,
N₂=0-8%, H₂O=2-7%.
According to one characteristic of the invention, the gas feeding the reduction reactor consists of a mixture of natural gas, recircled gas, also known as process gas or top gas, which exits from the reactor itself, and reformed gas; the recircled gas is pre-heated to a temperature of between 650°C and 950°C; the gas emerging from the pre-heater is in turn mixed with fresh reformed gas and subsequently with air, or air enriched with oxygen, or pure oxygen, to carry out a partial combustion of the H₂ and CO in the reducing gas in order to raise the temperature to values of between 800°C and 1150°C, preferably between 1000°C and 1150°C; and the oxidation level of the resulting gas feeding the furnace is between 0.06 and 0.15.
The methane represents between 6 and 20% in volume of the mixture of reducing gas.
When the feed gas comes into contact in the reduction zone with the hot, partly reduced material, which therefore consists partly of metallic iron and partly of iron oxides, a highly endothermic reaction is produced.
There is a also an endothermic reaction in the pre-heating and pre-reducing zone when the gas comes into contact with the iron oxide.
One advantage of this invention is that the first pre-heating and pre-reducing zone is extended, which allows to start the transformation of the Ematite (Fe₂O₃) into Wustite (FeO) more quickly.
The whole reactor works at a higher average temperature and above all which is constant along both zones, both the pre-reduction and reduction zones, encouraging a higher reaction speed, with a consequent effect of reducing consumption and increasing productivity.
In the case where the furnace has two inlets for the introduction of the reducing gas, the first inlet is located at a set distance (x) with respect to the second inlet, which is located in the median part of the furnace, in correspondence with the second reduction zone. This distance (x) is suitably between 1 and 6 meters, preferably between 2 and 4 metres, to encourage the reactions in the most suitable zone between the reducing gas and the iron oxides.
The first gas inlet also has the function of pushing the gases arriving from the second reduction zone towards the centre of the furnace so as to create a uniform distribution of the gas in the section of the reactor.
According to a variant, there are multiple, or more than two, inlets for the reducing gas into the furnace. The first current of reducing gas is injected into the middle of the reactor, into the reduction zone proper, while the other currents are introduced into the zone between the injection of the first current of gas and the outlet of the burnt gas, in the upper part of the furnace. This intermediate zone will be called the pre-heating and pre-reducing zone for the iron oxide based material.
The flow of gas into the reactor thus composed allows to have the whole reduction and pre-reduction zone at as constant a temperature as possible, and to have a gas inside the furnace which always has a high reducing power, encouraging a greater productivity and a lower consumption of gas; this also allows to improve the final metalisation of the product.
In this way, moreover, the iron oxides arrive at the reduction zone already partly reduced, thus encouraging the completion of the final reduction reaction from FeO to Fe.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other characteristics of the invention will become clear from the following description of some preferred forms of embodiment, given as a non-restrictive example with the aid of the attached Figures wherein:

Fig. 1a: shows in diagram form an apparatus for the direct reduction of iron oxides according to the invention in a first form of embodiment;
Fig. 1b: shows in diagram form an apparatus for the direct reduction of iron oxides according to the invention in a second form of embodiment;
Fig. 2: is a first variant of a furnace employed in the apparatus in Fig. 1a;
Fig. 3: is a diagram showing the temperature inside the furnace shown in Figs. 1a and 2;
Fig. 4: shows a second variant of a furnace employed in the apparatus in Fig. 1a;
Fig. 5: is a diagram showing the temperature inside the furnace shown in Fig. 4; and

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

With reference to Fig. 1a, an apparatus for the direct reduction of iron oxides according to the invention comprises a reduction furnace of the shaft type or reduction reactor 10, comprising in turn an upper mouth 11 for feeding from above, through which the mineral (iron oxides) is able to be introduced, a first pre-heating and pre-reduction zone 12, a second zone, or median zone 14 wherein the final reduction reaction of the iron oxides takes place, and a lower zone, or discharge zone 15, shaped like a truncated cone, terminating at the bottom in a lower aperture 16 through which the iron is discharged.
The iron-based metal oxides are introduced into the reactor 10 in the form of pellets or crude mineral in the appropriate sizes; the iron contained therein is usually between 63% and 68% in weight.
At the end of the process according to the invention, the iron contained in the reduced material exiting from the reactor 10 is normally between 80% and 90% in weight.
In correspondence with the two zones 12 and 14 of the reactor 10 there are two independent inlets 17, respectively 18, through which a mixture of gas is suitable to be introduced, as will be described in greater detail hereafter.
In its upper part, above zone 12, the reactor 10 is provided with an aperture 19 through which the burnt gas or process gas exits. This gas normally has the following characteristics: composition: H₂=20-41%, CO=15-28%, CO₂=12-25%, CH₄=2-10%, N₂=0-8%, H₂O=2-15%; temperature between 500°C and 700°C; oxidation level between 0.3 and 0.50, preferably between 0.40 and 0.45; and a reduction ratio R of between 1 and 1.8 where reduction ratio is taken as: R= (H₂+CO)/(H₂O+CO₂).
In the embodiment shown in Fig. 1b, the furnace 10 comprises only one reaction zone 14 and only one inlet 18 through which the reducing gas is injected into the furnace.
In both versions, that is, the one shown in Fig. 1a and the one shown in Fig. 1b, the burnt gas emerging from the reactor 10 is sent through a pipe 20 to a cooling unit 21, suitable to recover the heat which can be given up; from the cooling unit 21, through another pipe 22, it arrives at a cooling and condensing unit 24. In this unit 24 the burnt gas is washed in water at a temperature of between 40°C and 65°C and the quantity of water present in the gas itself is partly removed. The percentage of water remaining in the gas at outlet from the unit 24 is between 2% and 7%.
The gas at outlet from the unit 24 is sent through a pipe 30 partly to a pre-heater 36, partly to a catalytic reformer 44, to be used as fuel, and partly to a compressor 26.
The gas emerging from the compressor 26 is in turn used partly as a recircling gas and sent, through a pipe 28, inside the unit 21, and partly, through a pipe 46, mixed with a natural gas comprising methane (CH₄), or pure methane, arriving from a pipe 34 in a proportion of about 4:1 (that is to say, for every part of natural gas there are about four parts of gas arriving from the pipe 46) and introduced into the reformer 44 so that the reforming reaction of the methane (CH₄) with H20 and CO₂ can begin.
The part of gas which is sent to the unit 21 through the pipe 28 is pre-heated, and is then sent through a pipe 32 to the pre-heater 36, where it is further pre-heated to a temperature of between 650°C and 950°C. CH₄ may also be injected into the pipe 32.
The gas emerging from the pre-heater 36, which has a delivery rate of between 600 Nm³/ton DRI and 1500 Nm³/ton DRI, is mixed in a pipe 38 with the gas arriving from the reformer 44 through a pipe 50.
The gas resulting from this mixture is divided into two parts and distributed into two pipes 40 and 41, connected to the inlets 17 and 18 of the furnace 10. The delivery of reducing gas is controlled in each zone 12, 14 by means of regulation valves 55 and 56.
Into each pipe 40 and 41 air is injected, or air enriched with oxygen or pure oxygen and natural gas in variable percentages, in order to achieve a partial combustion of the CO and the H₂ and raise the temperature of the gas.
A current of CH₄ or natural gas is injected into the gas before it is introduced into the reactor.
In a variant, shown by a line of dashes in Figs. 1a and 1b, the CH₄ is injected before achieving the partial combustion, with the purpose of raising the temperature of the gas introduced into the reactor.
The CH₄ may also be introduced in a zone between the reduction zone 14 and the discharge cone of the material, through a pipe 81. In this case, before entering into the zone 14 where the reduction reactions are carried out, the CH₄ injected partially cools the reduced iron, before the latter is discharged.
Valves V1-V11 are located in correspondence with the different conduits of the plant so that the flow can be selectively controlled.
The resulting mixtures are then introduced into the reduction zone 14 and optionally into the pre-heating and pre-reduction zone 12.
With regard to the furnace 10 with two inlets (Fig. 1a), for each zone 12 and 14 the corresponding mixture of gas is regulated in an autonomous and independent manner.
To be more exact, the flow of gas in the first zone 12 is between 500 Nm³/ton DRI and 800 Nm³/ton DRI and enters the reduction reactor 10 with a temperature of between 800°C and 1150°C, preferably between 1000°C and 1150°C, while the flow of gas in the second zone 14 is between 1000 Nm³/ton DRI and 1500 Nm³/ton DRI and also enters the reduction reactor 10 with a temperature of between 800°C and 1150°C, preferably between 1000°C and 1150°C.
The consumption of oxygen, which is necessary to raise the temperature of the reducing gas from 650°C-950°C to 800°C-1150°C, intended as pure oxygen plus that contained in the air, if air is also injected, is between 8 Nm³/ton DRI and 60 Nm³/ton DRI, preferably between 20 and 60 Nm³/ton DRI.
The consumption of CH₄ is between 50 and 120 Nm³/ton DRI, preferably between 90 and 110 Nm³/ton DRI.
In volume the CH₄ represents between 6 and 20% of the mixture of 'reducing gas introduced into the reactor.
The reactions involved in the reduction zone 14 are as follows; FeO + CH₄ = Fe + 2H₂ + CO
Simultaneously, in the same zone 14, the following reduction reactions take place with hydrogen and carbon monoxide: FeO + H₂ = Fe + H₂O FeO + CO = Fe + CO₂
The consequence of these endothermic reactions is that the temperature of the gas in the reduction zone decreases from 800°C-1150°C to 700°C-900°C, yet still maintains the reaction temperature higher than in furnaces in the state of the art, and the gas leaving the reduction zone 14 has an oxidation level of between 0.15 and 0.35 and a reducing power of between 1.1 and 2.8.
The reactions involved in the pre-reduction zone 12 are as follows: Fe₂O₃ + H₂ = 2FeO + H₂O Fe₂O₃ + CO = 2FeO + CO₂
In the lower zone 15, shaped like a truncated cone, it is also possible to introduce gas containing natural gas to control the final carbon in the hot reduced iron to values of between 1.5% and 3.0%.
In a variant as shown in Fig. 2, instead of having a single lower part shaped like a truncated cone, the furnace 10 has at least two, and preferably three or four lower ends, shaped like a cone or truncated cone 15a, 15b and 15c, through which the reduced metallic iron is discharged in a controlled and independent manner. In this case the CH₄ may also be introduced by means of devices located on the zone of intersection of the truncated cone ends 15a, 15b and 15c, thus exploiting the geometric conformation of the system.
The development of the temperature inside the furnace 10, both in the version shown in Fig. 1a and also in the variant shown in Fig. 2, is shown in Fig. 3, from which it can be seen how the temperature is higher and more constant in the segment affected by the two zones 12 and 14.
According to another variant shown in Fig. 4, instead of having two inlets to introduce reducing gas, the furnace 10 is provided with a plurality of inlets, more than two. In this case a first current of gas is introduced into the lower inlet 18 through the pipe 41, a second current of gas is introduced into the inlet 17 through the pipe 40 and other currents of gas, each of which can be autonomously regulated, are introduced through pipes 42 and corresponding inlets 43 arranged between the inlet 17 and the upper aperture 19.
The development of the temperature inside the furnace 10, in the variant shown in Fig. 4, is shown in the diagram in Fig. 5, from which it can be seen how the temperature is higher and more constant in the whole segment affected by the pipes 40, 41 and 42.
Obviously, it is possible to make modifications and additions to the method for the direct reduction of mineral iron and the relative apparatus as described heretofore, but these will remain within the scope of the invention.

Claims

Method for the direct reduction of mineral iron inside a vertical reduction furnace (10) of the type with a gravitational load, comprising the following steps: feeding the mineral iron from above into the furnace (10), injecting a mixture of high temperature reducing gas based on H₂ and CO into said furnace (10), said reducing gas flowing in counterflow with respect to the said mineral iron introduced into the furnace, the process gas or top gas is removed from above from said furnace (10), and the reduced mineral is removed from the lower part of said furnace (10), wherein said reducing gas is obtained by mixing, in variable parts depending on the requirements of the iron reducing process, at least a part of said process gas and additional gas arriving from an outside reforming circuit (44), the method being characterised in that said process gas is split in two parts, of which a first part is directed to a pre-heater (36) for being subsequently re-introduced into said furnace (10) and a second part is directed to said reforming circuit (44) for being reformed and then sent to said furnace (10), the gas emerging from said pre-heater (36) and from said reforming circuit (44) being mixed together to form a mixed gas before the injection thereof into said furnace (10).
Method as in Claim 1, characterised in that, before being split in two parts, said process gas is washed, cooled and subjected to a substantial reduction of the quantity of water contained therein.
Method as in Claim 1, characterised in that said pre-heater (36) increases the temperature of said first part of process gas to a value comprised between 650 and 950°C.
Method as in Claim 1, characterised in that, before being introduced into the furnace, said mixed gas is further mixed with O₂ or air enriched with O₂ in order to achieve the partial combustion of the H2 and the CO present in the reducing gas and thus raise the temperature in the reduction zone of the furnace to values of between 800°C and 1150°C, advantageously between 1000 and 1150°C.
Method as in Claim 1, characterised in that at least a hydrocarbon is added to said mixed gas before its injection in the reduction zone of the furnace.
Method as in Claim 5, characterised in that said hydrocarbon consists of CH₄, in order to cooperate in the reduction of the iron oxide to metal iron, at the same time generating further H2 and CO.
Method as in Claim 1, characterised in that said reducing gas is introduced in at least two zones (12, 14) of said furnace (10) arranged one above the other so as to achieve, in a controlled manner, a first stage of pre-heating and pre-reduction in the upper part (12) of the furnace (10) and a second stage of final reduction in the lower part (14) of the furnace (10).
Method as in Claim 6, characterised in that said methane represents between 6 and 20% in volume of said mixture of reducing gas.
Method as in Claim 7, characterised in that the delivery, in different percentages, of said mixed gas is controlled in the different reduction zones (12, 14) along the length of said furnace (10).
Method as in Claims 5 and 7, characterised in that said hydrocarbon consists preferably of natural gas and in that said hydrocarbon in said mixed gas is proportioned and controlled independently in the different reduction zones (12, 14) along the length of said furnace (10).
Method as in claim 7, characterised in that said mixed gas is heated independently in the different zones (12, 14) along the length of said furnace (10).
Method as in Claim 1, characterised in that said mixed gas injected into said reactor has an oxidation level of between 0.06 and 0.25.
Method as in Claim 7, characterised in that further CH₄ is partly injected into said furnace (10) in a zone between said lower part (14) and an underlying discharge zone (15).
Apparatus for the direct reduction of mineral iron comprising a vertical reduction furnace (10) of the type with a gravitational load to achieve therein reactions of reduction of the iron mineral, feeding means (11) to feed the mineral iron into said furnace (10) from above, first injection means (17, 18) to inject a mixture of high temperature gas, consisting of reducing gas based on H2 and CO, first means (19) to remove the process gas or top gas from the top of said furnace (10), and second means (16) to remove the reduced mineral from the lower part of said furnace (10), wherein first mixing means are provided to mix, in variable parts according to the requirements of the iron reducing process, at least a part of said process gas with additional gas arriving from a reforming circuit (44) disposed outside said furnace (10) to obtain said reducing gas, the apparatus being characterised in that split means (28, V₈, 46, V₉) are provided to split said process gas in two parts, of which a first part is able to be directed to a pre-heater (36) for being subsequently re-introduced into said furnace (10) and a second part is able to be directed to said reforming circuit (44) for being reformed and then sent to said furnace (10), and said first mixing means (38, 50) are provided to mix together the gas emerging from said pre-heater (36) and the gas emerging from said reforming circuit (44) to form a mixed gas before the injection thereof into said furnace (10) .
Apparatus as in Claim 14, characterised in that washing and cooling means (21, 24) are provided upstream of said split means, so that said process gas is able to be washed, cooled and subjected to a substantial reduction of the quantity of water contained therein.
Apparatus as in Claim 14, characterised in that said pre-heater (36) is able to pre-heat said process gas to a temperature of between 650 and 950°C, before it is mixed with said additional gas.
Apparatus as in Claim 14, characterised in that second mixing means (V₂, V₄) are provided to further mix said mixed gas with O₂ or air enriched with O₂ in order to achieve the partial combustion of the H₂ and the CO present in the reducing gas and thus raise the temperature in the reduction zone of the furnace to values of between 800°C and 1150°C, advantageously between 1000 and 1150°C.
Apparatus as in Claim 14, characterised in that means are provided to inject at least a hydrocarbon into said reducing gas or together therewith in the reduction zone of the furnace.
Apparatus as in Claim 18, characterised in that said hydrocarbon consists of CH₄, able to cooperate in the reduction of the iron oxide to metal iron and, at the same time to generate further H2 and CO.
Apparatus as in Claim 14, characterised in that means (17, 18) are provided to introduce said reducing gas in at least two reduction zones (12, 14) of said furnace (10) arranged one above the other so as to achieve, in a controlled manner, a first stage of pre-heating and pre-reduction in the upper part (12) of the furnace (10) and a second stage of final reduction in the lower part (14) of said furnace (10).
Apparatus as in Claim 20, characterised in that regulating means (55, 56) are provided to regulate the introduction, also in different percentages, of said reducing gas in the different reduction zones (12, 14) along the length of said furnace (10).
Apparatus as in Claims 18 and 20, characterised in that at least two elements to mix the reducing gas and hydrocarbon are provided upstream of the inlets to said furnace (10) in correspondence with said reduction zones (12, 14), in order to supply a mixture wherein the hydrocarbon is proportioned and controlled independently and autonomously in each of said zones (12, 14).
Apparatus as in any claim from 14 to 22 inclusive, characterised in that said second removal means (15) comprise at least two ends (15a-15c) shaped like a cone or a truncated cone.
Apparatus as in Claim 23, characterised in that said ends (15a-15c) shaped like a cone or a truncated cone taper downwards and each is provided with a corresponding lower aperture (16a-16c) through which said reduced metallic iron can be selectively discharged in a controlled and independent manner.
Apparatus as in any claim from 20 to 24 inclusive, characterised in that second injection means (81) are provided to inject at least partly CH₄ into said furnace (10) in a zone between said second removal means (15) and the lower of said reaction zones (14).
Apparatus as in Claims 23 and 25, characterised in that said second injection means (81) are arranged in a zone of intersection between said ends (15a-15c) shaped like a cone or a truncated cone.