METHOD OF PREPARING A MELT FOR THE PRODUCTION OF MINERAL WOOL AND A SHAFT FURNACE FOR CARRYING OUT SAID METHOD

Abstract: A method of preparing a melt for the production of mineral wool wherein raw material for the melt is heated in a shaft furnace by combustion of coke while introducing oxygen-containing air through the tuyeres of the shaft furnace, and wherein part of the heat required is provided by introducing and combusting a mixture of oxygen and fuel in the central portion of the shaft furnace. A shaft furnace for carrying out the method comprises a burner pipe for introducing and combusting a mixture of oxygen and fuel in the central portion of the furnace chamber.

Description: Method of preparing a melt for the production of mineral wool and a shaft furnace for carrying out said method. The present invention relates to a method of preparing a melt for the production of mineral wool wherein a raw material having a composition suitable for the preparation of the melt and a carbonaceous material are introduced into the upper portion of a shaft furnace and oxygen-containing gas is introduced into the lower portion of the shaft furnace to effect a combustion of the carbonaceous material and to melt the raw material, and wherein a melt is discharged from the bottom of the shaft furnace. In the prior art methods of the above mentioned type the carbonaceous material used typically consists of coke and the oxygen- containing gas is normally oxygen-enriched air which is suitably preheated to a temperature of about 500°C. A known type of shaft furnace for the preparation of a melt for the production of mineral wool is a cupola furnace. Such a furnace normally comprises four zones i.e. a melting bath, an oxidation zone, a reduction zone, and a preheating zone. The lower portion of the cupola constitutes the melting bath and comprises the melt formed in the cupola which melt is located in the space between the pieces of coke which are resting on the bottom of the cupola and which support the located thereabove. The temperature in the melting bath is typically in the range of 1500-1550°C. The oxidation zone is located above the melting bath and the Tower portion of said zone is provided with air intake nozzles, the so-called tuyeres, through which the preheated air is introduced into the furnace. The actual combustion of coke takes place during the movement of the preheated air up through the oxidation zone, and the gas temperature rises from about 500°C to about 2000°C thus causing the material which moves down through the oxidation zone to be heated as a result of the melting of the raw material. The melt formed flows down into the melting bath where the temperature, as mentioned, typically is 1500-1550°C. The vertical extension of the oxidation zone is determined by the amount of oxygen introduced as the reduction zone starts at the level where the oxygen introduced through the tuyeres has been consumed for the combustion of coke. The quality of the mineral fibres prepared from the melt is determined to a significant degree by the viscosity of the melt. As the viscosity i.a. depends on the temperature of the melt, it is essential that the temperature of the melt can be quickly changed. It is not possible to obtain a quick adjustment of the temperature in the shaft furnace with the prior art methods of the type mentioned in the introductory part as a change in temperature may by and large only be brought about by altering the relative amount of carbonaceous material, the so-called coke percentage, and as the effect of a change of the coke percentage ordinarily does not show until after a period of 2-3 hours. Furthermore, the sensitivity of such an adjustment is relatively poor. In practice, it has been found that in order to adjust the temperature of the melt by 1%, a 5% change of the coke percentage is required. A further drawback of the known methods is that an increase in the coke percentage in addition to reducing the melting capacity results in increased amounts of smoke gas which may cause environmental problems, especially in connection with the use of sulphurous coke. An attempt has been made to provide part of the amount of heat required for the melting of the starting material by combustion of gas in gas burners mounted in the furnace wall in the oxidation zone. This attempt failed and this is partly due to the very high temperatures which are generated in the oxidation zone and particularly in the zones contiguous to the furnace wall and which also cause relatively high temperatures to be generated in the reduction zone. This is unfortunate since at high temperatures, i.e. temperatures between 1000 and 1500°C, in the reduction zone, coke reacts with the C0« formed in connection with the combustion of coke in the oxidation zone so as to form CO in an amount which is double the amount of consumed C0 2 based on volume. This reaction is heat consuming and causes 20-25% of the energy released by the combustion in the oxidation zone to be lost as latent heat in the smoke gas, when the CO-content is 6-10%. Even though this amount of heat may be recovered in a secondary combustion, it will be desirable to avoid the formation of CO i.a. because a secondary combustion requires relatively high plant investments, and the efficiency of such a secondary combustion is relatively low. The reduction, i.e. the formation of CO, strongly depends on temperature in the temperature range above 1000°C as it e.g. is increased with a factor of 10 when the temperature rises from 1000° to 1200°C. On the other hand, the reaction rate for the conversion of C0 2 into CO is so low at temperatures below 1000°C that no appreciable reduction takes places in the preheating zone located above the reduction zone wherein the materials introduced at the top of the furnace are heated from the ambient temperature to about 1000°C. The object of the invention is to avoid the above mentioned drawbacks of the prior art methods of the type defined above. This object is obtained with the method according to the invention which method is characterized in that part of the heat required to melt the raw material is generated by introducing and combusting a mixture of oxygen and fuel in the central portion of the shaft furnace. By supplementing the combustion of coke with the combustion of the mixture of oxygen and fuel quick temperature changes in the interior of the furnace can be produced by adjusting the feed rate of said mixture. The fact that combustion takes place in two zones i.e. in the zone contiguous to the furnace wall and in the central portion of the furnace permits the maintenance of more uniform temperatures at a given level than in the case when the combustion is solely based on coke and oxygen-containing air introduced through the tuyeres in the furnace wall. The reduced temperature variation results in a more uniform melt and the possibility of maintaining the temperature in the reduction zone below 1000°C is increased. A further advantage of the method according to the invention is that it permits a reduction of the coke percent so as to improve the melting capacity and reduce the sulphur emission. A reduction of the coke percent makes it possible to limit the introduction of oxygen-containing air through the tuyeres and thus to reduce the amount Of smoke gas. A particularly efficient way of maintaining the temperature at the desired level consists in introducing a solid particulate material in admixture with oxygen and fuel. By varying the amount of such a material it is thus possible to change the temperature in the zone in which said mixture is introduced and combusted. An example of such a solid particulate material is waste material from mineral wool production. Thus, the possibility is obtained of using said material which otherwise often has to be removed. Attempts have been made to introduce the waste material together with the raw material when charging a shaft furnace, but the relatively fine waste material has an unfavourable effect on the movement of the smoke gasses up through the furnace. Therefore, the waste material is normally introduced in the form of preformed briquettes. By introducing waste material together with the mixture of oxygen and fuel into the central portion of the furnace, the waste material is brought directly into the melting zone without affecting the stream of smoke gasses moving up through the furnace. The solid particulate material may also be an agent for quickly changing the properties of the melt, such as a way of changing the viscosity of the melt. An example of such an agent is calcium oxide. The above mentioned mixture of oxygen and fuel preferably consists of pure oxygen and a hydrocarbon, and the amounts of the two components should normally be adjusted so as to obtain a complete combustion of the fuel component. Instead of pure oxygen mixtures of oxygen and other gasses may be used, e.g. atmospheric air, and the fuel component may consist of other carbonaceous materials than hydrocarbon, such as coal dust and/or oil vapour. In a preferred embodiment of the method according to the invention the mixture of oxygen and fuel are introduced into the central portion of the furnace through a pipe extending parallel with the longitudinal axis of the furnace so as to avoid an unfavourable effect on the.movement of the raw material and coke down through the furnace chamber. The invention also relates to a shaft furnace for carrying out the method described above. The shaft furnace according to the invention comprises a cylindrical, essentially vertically mounted furnace chamber surrounded by a furnace wall in which there are mounted tuyeres for introducing oxygen-containing gas into the furnace chamber, means for introducing a fusible raw material and carbonaceous material into the upper end of the furnace chamber, and means for discharging melt at the lower end of the furnace chamber, and the furnace is characterized in that it additionally comprises a burner pipe for introducing and combusting a mixture of oxygen and fuel in the central portion of the furnace chamber. In a particularly preferred embodiment the burner pipe extends parallel with the longitudinal axis of the furnace chamber in order, as previously mentioned, not to obstruct the movement of the raw material and coke down through the furnace chamber. In said embodiment the burner pipe is preferably placed in such a manner that it extends down into the furnace chamber from its top. The lenght of the portion of the burner pipe located in the furnace chamber is preferably adjustable so as to obtain the possibility of concentrating the heat development at a desired level in the furnace chamber and thus to obtain a desired temperature distribution. The burner pipe which may be provided with cooling devices preferably comprises separate passages for supplying oxygen and fuel to the free end of the pipe. Furthermore, tiie pipe may be provided with devices for introducing fine waste material into the furnace chamber. The invention will now be described in further detail with reference to the drawings wherein Fig. 1 shows a vertical cross-sectional view of a shaft furnace according to the invention and Fig. 2 shows a perspective view of a burner pipe for use in a shaft furnace according to the invention. The shaft furnace shown in the drawings comprises a cylindrical furnace chamber 1 surrounded by a water-cooled furnace wall 2. At the upper end of the shaft furnace there is a feed hopper 3, which communicates with a feed vessel 4 having a bottom which is formed by an axially displaceable cwie 5. At t e upper end of the shaft furnace there is also provided an outlet 6 for smoke gasses. The shaft furnace has a plane bottom 7 and in a suitable distance above the bottom 7 there is provided a melt outlet 8. A number of tuyeres 9 communicating with an annular air inlet pipe 10 is mounted in the furnace wall some distance above the level of the melt outlet 8. The furnace also comprises a burner pipe 11 extending down through the feed vessel 4 and terminating in the lower portion of the furnace chamber. The burner pipe is provided with a cooling jacket having a pipe stub 12 for the supply of cooling medium and a further pipe stub 13 for the discharge of spent cooling medium. The position of the lower end of the burner pipe 11 in the furnace chamber is adjustable, the burner pipe 11 being connected with mechanical or hydraulic 0 devices (not shown) for longitudinally displacing the burner pipe 11. In addition to the pipe stubs 12 and 13 for the supply and discharge of cooling medium respectively, the burner pipe shown in Fig. 2 comprises three more stubs 14, 15, and 16, the pipe stubs 14 and 15 serving to introduce oxygen and fuel respectively and the pipe stub I * _) __!__* 16 to introduce fine waste material obtained from mineral wool production. In the burner pipe there are provided three pairs of longitudinally extending passages 17, 18 and 19, the passages 17 being connected with the pipe stub 14 and the passages 18 being connected with the pipe stub 15. Finally, the passages 19 are 0 connected with the pipe stub 16 for introducing waste material. The raw material for preparing the desired melt and coke are introduced into the shaft furnace through the feed hopper 3 and may be introduced in the actual furnace chamber after the cone 5 has been displaced from the closing position. Thus, said materials reach the 5 preheating zone of the furnace in which they are preheated by the ascending smoke gasses. The melting of the raw material in the lower portion of the furnace and the collection of the melt on the bottom of the furnace from which it is discharged through the outlet 8 takes place concurrently with the descend of the preheated materials down 0 through the furnace chamber 1 and their entry into the oxidation zone of the furnace. In the oxidation zone the materials are heated partly by the heat produced by the combustion of coke and partly by the combustion of fuel introduced through the burner pipe 11. The burner pipe may e.g. have a capacity of up to about 10 megawatt and by 5 adjusting the supply rate for the mixture of oxygen and fuel it is thus possible to effect a quick adjustment of the temperature in the furnace chamber when necessary. The supply of said mixture may optionally be controlled automatically as a function of the temperature of the melt formed in the furnace. During normal operation about 50% of the heat may derive from the combustion of coke and the remaining 50% from the combustion of the mixture of 5 oxygen and fuel introduced through the burner pipe. By using the burner pipe a reduction of the amount of coke from the normal 14% down to about 6% is obtainable. By using the burner pipe shown in Fig. 2 it is possible to introduce fine waste material through the central passage 19 of the ^° burner pipe 11 concurrently with the introduction of a mixture of oxygen and fuel. Thus, the possibility is obtained of reusing said material without adversely affecting the movement of the smoke gasses up through the furnace. When the furnace is empty the burner pipe 11 may optionally be used to burn away carbon which shows a tendency to '5 accumulate in the lower portion of the furnace and to form a coating on the bottom of the furnace. 20 25 30 35

Claims: C l a i m s 1. A method of preparing a melt for the production of mineral wool wherein a raw material having a composition suitable for the production of the melt and a carbonaceous material are introduced into the upper portion of a shaft furnace and oxygen-containing gas is introduced into the lower portion of the shaft furnace to effect a combustion of the carbonaceous material and to melt the raw material, and wherein a melt is discharged from the bottom of the shaft furnace c h a r a c t e r i z e d in that part of the heat required to melt the raw material is generated by introducing and combusting a mixture of oxygen and fuel in the central portion of the shaft furnace. 2. A method according to claim 1 c h a r a c t e r i z e d in using a mixture consisting of pure oxygen and hydrocarbon gas. 3. A method according to claim 1 or 2 c ti a r a c t e r i z - e d in that the mixture is introduced through a pipe extending parallel with the longitudinal axis of the furnace chamber. 4. A method according to claim 3 c tι a r a c t e r i z e d in using a pipe which extends from the top of the furnace down into the furnace chamber. 5. A method according to any of the claims 1-4 c h a r a c ¬ t e r i z e d in that solid particulate material is introduced into the central portion of the furnace concurrently with the introduction of said mixture. 6. A method according to claim δ c h a r a c t e r i z e d in using particulate waste material obtained from mineral wool production. 7. A shaft furnace for carrying out the method according to claim 1 which shaft furnace comprises a cylindrical, essentially vertically mounted furnace chamber surrounded by a furnace wall in which there are mounted tuyeres for introducing oxygen-containing gas into the furnace chamber, means for introducing a fusible raw material and carbonaceous material into the upper end of the furnace chamber, and means for discharging the melt at the lower end of furnace chamber c h a r a c t e r , z e d " in that it additionally comprises a burner pipe for introducing and combusting a mixture of oxygen and fuel in the central portion of the furnace chamber. 8. A shaft furnace according to claim 7 c h a r a c t e r i - z e d in the burner pipe extends parallel with the longitudinal axis, of the furnace chamber. 9. A shaft furnace according to claim 8 c h a r a c t e r i - z i d in the burner pipe extends from the top of the furnace down into the furnace chamber. 10. A shaft furnace according to claim 7, 8 or 9 c h a r a c- t e r i z e d in that the burner pipe has separate passages for the introduction of oxygen and fuel. 11. A shaft furnace according to any of claims 7-10 c h a r a c t e r i z e d in that the burner pipe also comprises a passage for the introduction of a solid particulate material into the central portion of the furnace chamber. 12. A shaft furnace according to any of claims 8-11 c h a r a c t e r i z e d in that the length of the part of the burner pipe which extends into the furnace chamber is adjustable.

Inventors: HANSEN JENS KRISTIAN GAMBORG, JENSEN LEIF M LLER, HANSEN, JENS, KRISTIAN, GAMBORG, JENSEN, LEIF, MLLER
Applicants: ROCKWOOL INT, ROCKWOOL INTERNATIONAL A/S
Application year: 1987
Main classification: C03B5
IPC codes: C03B3, C03B5