SUBMERGED COMBUSTION MELTER AND METHOD

Abstract

The present invention relates to a melter assembly (1) for melting solid raw batch material, which comprises a submerged combustion melter section (3) and an afterburner section (5), wherein the submerged combustion melter section is designed to contain the melt bath (4) at a maximum melt level (4) and comprises at least one submerged combustion burner (21) and a melt outlet (9), and wherein the afterburner section (5) is designed as a space contiguous with, and in continuity of, the internal space defined by the submerged combustion melter section (3), and arranged over the maximum melt level (4) of the submerged combustion melter section (3). The afterburner section (5) is dimensioned such that the gases escaping from the melt bath (4) remain for at least 2 seconds at a temperature of at least 850 C. in said afterburner section (5), prior to being evacuated to the environment. The invention also relates to a process making use of such melter assembly (1).

Claims

1. Melter assembly for melting solid raw batch material, which comprises a submerged combustion melter section (3) and an afterburner section (5), wherein the submerged combustion melter section (3) is designed to contain the melt bath (4) at a maximum melt level (4) and comprises at least one submerged combustion burner (21,24) and a melt outlet (9), and wherein the afterburner section (5) is designed as a space contiguous with, and in continuity of, the internal space defined by the submerged combustion melter section (3), and arranged over the maximum melt level (4) of the submerged combustion melter section (3), and is dimensioned such that under normal operating conditions, the gases escaping from the melt bath contained in the submerged combustion melter section remain for at least 1 second at a temperature of at least 750 C. in said afterburner section (5), said afterburner section being connected to an exhaust gas outlet (6).

2. The melter assembly of claim 1 wherein the afterburner section (5) is dimensioned such that the gases escaping from the melt bath (4) contained in the submerged combustion melter section (3) remain for at least 2 seconds at a temperature of at least 800 C., preferably at least 850 C., in said afterburner section (5).

3. The melter assembly of claim 1 wherein the afterburner section (5) shows a transversal cross-section substantially equivalent to the substantially horizontal cross-section of the submerged combustion melter section.

4. The melter assembly of claim 1 wherein the raw material feeder (10) is connected to the afterburner section (5).

5. The melter assembly of claim 1 wherein the raw material feeder is a submerged feeder connected to the submerged combustion melter section (3).

6. The melter assembly of claim 1 wherein the afterburner section (5) comprises baffles (7).

7. The melter assembly of claim 1 wherein the afterburner section comprises additional combustion means and/or reagent injection means, such as devices for injection of oxygen containing gas.

8. The melter assembly of claim 1 wherein the afterburner section (5) comprises heat recovery means.

9. The melter assembly of claim 1 wherein the submerged combustion melter section (3) shows a polygonal, preferably trapezoidal or rectangular, horizontal cross-section.

10. The melter assembly of claim 1 wherein the submerged combustion melter section (3) shows a substantially circular (19) or ovoidal horizontal cross-section.

11. The melter assembly of claim 1 wherein the submerged combustion melter section (3) shows an essentially trapezoidal or rectangular vertical cross-section.

12. The melter assembly of claim 1 wherein the submerged combustion melter section (3) comprises a substantially centrally arranged submerged combustion burner or at least three submerged combustion burners (21,22,23,24,25,26), preferably at least 5 submerged combustion burners, and wherein said submerged combustion burners are arranged on a substantially annular or ovoidal burner line (27), the said submerged combustion burners being arranged through the bottom of said submerged combustion melter section (3).

13. The melter assembly of claim 1 wherein the submerged combustion melter section walls comprise double steel walls separated by circulating cooling liquid, preferably water.

14. The melter assembly of claim 1 wherein the afterburner section walls comprise double steel walls separated by circulating cooling liquid, preferably water.

15. Production line for flat glass, hollow glass, glass fibers, glass wool or stone wool, comprising the melter assembly (1) of claim 1.

16. The production line of claim 15 comprising energy recovery means and means for use of the recovered energy in relevant equipments of the production line, such as preheating of raw material, preheating of fuel and/or oxidant adduced to the melter burners, heating of ovens mounted downstream of the forming equipment.

17. Process for the production of flat glass, hollow glass, continuous fibers or mineral wool fibers, glass wool or stone wool, comprising melting vitrifiable raw material in the melter assembly (1) of claim 1 and withdrawing glass melt from said melter assembly for downstream processing into glass fibers, glass wool or stone wool.

18. The process of claim 17 wherein the submerged burners are controlled such that the volume of the aerated melt is at least 8%, preferably at least 10%, more preferably at least 15% higher than the volume the melt would have at the same temperature without any burner firing.

Description

[0037] An embodiment of the present invention will be described in more details below, with reference to the appended drawings of which:

[0038] FIG. 1 schematically shows a vertical section through a melter assembly;

[0039] FIG. 2 is a schematic representation of a burner layout; and

[0040] FIGS. 3a and 3b are representations of a toroidal flow pattern generated by computer simulation.

[0041] FIG. 1 shows a melter assembly 1 according to the invention comprising a submerged combustion melter section 3 and an afterburner section 5. The submerged combustion melter section 3 is designed to contain the melt bath 4 at a maximum melt level 4 and comprises at least one submerged combustion burner 21,24 and a melt outlet 9. In the exemplified embodiment, the outlet 9 is arranged near the bottom of the submerged combustion melter section and may be controlled by a piston for batchwise unload of melt contained in the submerged combustion melter section 3. The afterburner section 5 is designed as a space contiguous with, and in continuity of, the internal space defined by the submerged combustion melter section 3, and arranged over the maximum melt level 4 of the submerged combustion melter section 3. The horizontal or transversal cross-section of the afterburner section is substantially equivalent to that of the submerged combustion melter section 3. The afterburner section 5 is dimensioned such that, under normal operating conditions, the gases escaping from the melt bath 4 contained in the submerged combustion melter section 3 remain for about 2 seconds at a temperature of at least 850 C. in the space delimited by the said afterburner section. Hence the afterburner section is designed to comply with the general understanding of an afterburner. The afterburner section 5 is further connected to an exhaust gas outlet 6. It may further comprise baffles 7 in order to avoid that melt projections generated by the heavy turbulence of the melt 4 contained in the submerged combustion melter section 3 are entrained in the effluent gas flow with all disadvantages that may entail with respect to deposition of solidifying or solidified melt projections and clogging of certain elements of the exhaust system. In the exemplified embodiment, a raw material feeder 10 is arranged in the afterburner section and is designed to discharge vitrifiable raw material over the top of the agitated melt bath 4. As described hereabove, a raw material feeder may also be provided for at the submerged combustion melter section; such a raw material feeder then preferably consists in a submerged feeder and may be provided for as an alternative to or in addition to the feeder 10 described hereabove.

[0042] As can be seen in FIG. 2, the melter assembly 3 has a circular transversal cross-section 19. The melter assembly comprising melter section and afterburner section represented in the figures is advantageously substantially cylindrical. Submerged combustion generates high stress components that act on the melter walls and the melter is subjected to heavy vibrations. These may be significantly reduced in the case of a cylindrical melting chamber. If so desired, the melter may further be mounted on dampers which are designed to absorb most of the vibrational movements. Six burners 21,22,23,24,25,26 are arranged on a circular burner line 27. For the sake of clarity, the design represented in the figures has a preferred layout with six submerged burners distributed around the burner line. Different layouts are possible depending on the dimensions of the melter, the viscosity of the melt and the characteristics of the burners.

[0043] The walls, including bottom and side walls, of the submerged combustion melter section 3 consist in double steel walls. Cooling fluid, preferably water, is circulated between said walls in order to cool. Similarly, the side walls of the afterburner section also consist in double steel walls cooled by circulating cooling fluid, preferably water. Cooling water connections are provided for on the external walls. Such connections are known per se and should be calculated to allow a flow of cooling fluid sufficient to withdraw energy from the inside wall such that melt can solidify on the internal wall at about 150 C. and the cooling liquid, here water, does not boil

[0044] The submerged combustion burners 21,22,23,24,25,26 may comprise concentric tube burners (also known as tube in tube burners) operated at gas flow or speed in the melt of 100 to 200 m/s, preferably 110 to 160 m/s. The burners preferably are designed such as to generate combustion of fuel gas and air and/or oxygen within the melt. The combustion and combustion gases generate high mixing within the melt before they escape into the afterburner section 5 and then through the chimney 6. These hot gases may be used to preheat the raw material and/or the fuel gas and/or oxidant (air and/or oxygen) used in the burners. The fumes generally are filtered prior to release to the environment. Where filtering needs to occur at reduced temperatures, prior dilution of fumes with cooler ambient air may be used.

[0045] In normal operation, submerged burners 21,22,23,34,35,26 inject high pressure jets of combustion products of oxygen and fuel, preferably propane gas, into the melt sufficient to overcome the liquid pressure in the melt and to create forced upward travel of the flame and combustion products. The speed of the combustion and/or combustible gases, notably at the exit from the burner nozzle(s), may be 60 m/s, 100 m/s or 120 m/s and/or 350 m/s, 330 m/s, 300 or 200 m/s. Preferably the speed of the combustion gases is in the range of about 60 to 300 m/s, preferably 100 to 200, more preferably 110 to 160 m/s. The combustion gases hence generate high turbulence in the melt bath which ensures suitable energy transfer between the hot gases and the melt and/or fresh raw material absorbed in it. The high turbulence thereby ensures a homogenous melt in terms of composition and temperature.

[0046] The submerged burners are controlled such that the injected combustion gas and the convection flows within the melt generate a turbulence such that the volume of the aerated melt is about 30-50% higher than the volume the melt would have without any submerged burner firing. Preferably, the submerged burners are controlled such as not to generate any significant foam layer or no foam layer at all at the top of the melt.

[0047] Furthermore, submerged burners 21,22,23,24,25,26 are arranged in the cylindrical submerged combustion melter section and controlled in such a way that a toroidal melt flow pattern is generated in the melter section. The distance between burners is comprised between 250 and 900 mm, preferably about 300-800. Further, the burners are arranged at a suitable distance of about 150-750 mm from the side wall of said submerged combustion melter section 3; this favors the flow described above and avoids flame attraction to the melting chamber side walls. Too small a distance between burners and side wall may damage or unnecessarily stress the side wall. Reference is made to FIGS. 3a and 3b. Melt follows an ascending direction close to submerged burners arranged on a substantially circular burner line, flows inwardly towards the center of the relevant circle line, at the melt surface, and then downwards again, in proximity of the said center. Such toroidal flow ensures good stirring of the melt and absorption of fresh raw material. The skilled person will need to adapt the distance between burners and between burner and wall, for given burner designs, such as to avoid the burner flames fuse or are attracted to the wall or otherwise diverted from the central burner axis

[0048] As will be understood, additional flow circulations may take place. Melt may flow between burners and side wall. Other flows may take place between burners. These are not necessarily disadvantageous and, to the contrary, may even be desirable.

[0049] The toroidal flow pattern has been generated by computer simulation, taking into consideration common Eulerian, multi-phase fluid dynamics modeling techniques familiar to those skilled in the art. The computational fluid dynamics code selected for this exercise advantageously is ANSYS R14.5. The model advantageously takes into consideration the multi-phase flow field spanning the full range of mixture fractions from dispersed gas bubbles in liquid to distributed solid particles or liquid droplets in gas, with the solid phase batch undergoing a multi-phase, thermo-chemical conversion reaction to produce liquid phase melt and gas phase species. The system utilizes submerged combustion of fuel and oxygen gas phase species to produce carbon dioxide and water vapor. In addition, the melt viscosity is highly temperature dependent. The complex batch-to-melt conversion process may be modeled with the reaction step following an Arrhenius rate law

Batch.sub.solid+H.sub.r>Melt.sub.liquid+0.074CO.sub.2+0.093H.sub.2O

with Arrhenius reaction rate k=AT.sup.2e.sup.(E/T)
the Arrhenius rate constants being taken from the literature (see A Ungan and R Viskanta, Melting behavior of continuously charged loose batch blankets in glass melting furnaces, Glastech. Ber. 59 (1986) Nr. 10, p. 279-291). The molar ratios of the batch gases in this reaction are consistent with the production of 0.0503 kg CO.sub.2 and 0.0258 kg H.sub.2O from 1 kg batch. The heat of reaction accounts for all energy required to convert batch into liquid phase melt and gas species, including both chemical conversion and phase change heat requirements. Physical properties for the batch and melt may be taken from literature as far as available and/or may be determined by methods known per se. Radiation heat exchange is simulated using the Discrete Ordinates Radiation model, with the gas phase absorption coefficient estimated using the Weighted Sum of the Gray Gases model, the melt absorption coefficient specified (to a high value of 300 l/m) and the batch absorption coefficient advantageously specified so as to render it opaque relative to the other fluids. While the melt is assigned as the primary fluid phase and the gases are assigned as the secondary fluid phase having uniform bubbled diameter of 5 mm. Momentum exchange among the liquid and gas phases above the anticipated bath height is artificially suppressed.

[0050] In an example of a melter assembly of the invention, the illustrated submerged combustion melter section 3 shows a diameter of about 2.0 m. The submerged burners are arranged on a circular burner line concentric with the burner axis and having a diameter of about 1.4 m. The distance between submerged combustion burner and wall is about 0.10-0.20 m. The temperature within the melt may be comprised between 1100 C. and 1600 C. or 1650 C., or 1200 C. and 1500 C., or 1200 C. and 1450 C., preferably 1250 C. and 1400 C., depending on the composition of the melt, desired viscosity and other parameters. For computational fluid dynamics modeling of the melter described above (see flow pattern shown in FIG. 1), a batch inlet of 0.833 kg/s at inlet temperature of 27 C. was set in the model, consistent with a 72 T/day production rate. Burner inlet was set as follows: firing rate=5.2 WM (based on LHV); mass flow rate of 0.109 kg/s per burner; molar composition=0.11 C.sub.3H.sub.8, 0.89 O.sub.2; inlet temperature of 15 C. The walls were modeled by specifying a uniform surface temperature of 152 C. behind a thickness of solidified glass which serves as an insulator. The thermal conductivity specified for the glass is 1 W/mK. The glass thickness, nominally 15 mm, is varied to achieve an average heat flux of from 50 to 70 kW/m.sup.2.

[0051] It has been found that the exhaust system of the invention melter assembly may be designed with either a reduced or simplified afterburner or that the afterburner may be completely omitted in the exhaust system mounted downstream of the melter assembly. In any of the above situations, the capital investment required for the construction of a glass melter is significantly reduced. Concomitantly, the energy requirements for operating a submerged combustion melter are further reduced as well, hence improving the energy efficiency, because of a reduced afterburner or omission thereof in the exhaust system mounted downstream of the melter assembly. It has been determined that the exemplified invention melter assembly with integrated afterburner no longer requires any afterburner known per se for arrangement in the exhaust system downstream of the melter assembly.

[0052] A melter according to the invention is particularly advantageous in a glass fiber, glass wool or stone wool production line or a production line for flat glass or hollow glass, because it is particularly efficient leading to reduced energy consumption and increased flexibility which allows for easy changes of raw material composition. Ease of maintenance and low production costs of the invention melter assembly are of major interest in building a production line comprising such a melter. It is well within the experience and skill of the person skilled in the art to design a production line as described above which comprises suitable energy recovery means and means for use of the recovered energy in relevant equipments in the production line, such as for example preheating of raw material, preheating of fuel and/or oxidant adduced to the burners, heating of ovens mounted downstream of the forming equipment etc.

[0053] Flat glass, hollow glass, continuous fibers or mineral wool fibers, glass wool or stone wool may be produced in an advantageous manner by melting suitably selected vitrifiable raw material in a melter assembly as described above, and withdrawing glass melt from said melter assembly for downstream processing into flat glass, hollow glass, such as glass containers, glass fibers, glass wool or stone wool. The process may be adapted to the final product by the choice of appropriate downstream processing.