Patent Application
20190047891
The present invention relates to glass fibre manufacturing plant comprising a forehearth forming a passage for conveying a glass melt, said forehearth being heated by a series of burners, in particular oxy-burners, for maintaining the glass melt at a desired temperature. In particular, it concerns a cooling unit for keeping an oxy-burner at a temperature below a safety threshold use temperature, regardless of whether the burner or oxy-burner is operational of not.
Glass fibre production includes melting a composition of raw materials in a furnace or melter to form a molten glass. The molten glass thus formed is transferred to various fiberization units, generally bushings, through channels forming a forehearth. In the forehearth, the molten glass is conditioned and its temperature and pressure controlled before it reaches the bushings where it is formed into fibres and cooled. A forehearth is formed by one or more passages, which are defined by a first and second opposite longitudinal walls, a ceiling, a bottom floor and an end wall. Bushings are disposed along the bottom floor of the one or more passages.
In order to yield and control a uniform temperature of the molten glass in the forehearth, burners are aligned along the first and second opposite longitudinal walls and usually also at the end wall. Oxygen-gas burners, oxygen-fuel burners or, more simply, oxy-burners form a new generation of burners forming a flame from a mixture of oxygen and gaseous fuel, and gradually replace conventional burners using a mixture of fuel and air, as they yield much improved furnace and forehearth energy efficiency and productivity. It is estimated that oxy-burners significantly reduce natural gas consumption in the overall glass manufacturing process, lowering forehearth energy usage by up to 70%, compared with conventional air-burners using air-gas firing system. Oxy-burners are described e.g., in
It is claimed that oxy-burners can simply replace conventional air-burners in existing forehearths. This statement is arguable, because there are at least two differences changing the thermal conditions surrounding an oxy-burner compared with a conventional air-burner. First, the higher flame temperature of an oxy-burner versus a conventional burner must be considered. The flame temperature of a conventional air-gas mixture burner is about 1920-1950° C. By contrast, the flame temperature of an oxy-burner is much higher and can reach values of the order of 2500-2900° C. For this reason, the refractory material of the burner block in which a burner is mounted must be optimized, ensuring that it does not degrade when exposed to higher temperatures. Another issue is the degradation of the burners exposed to higher thermal radiations. In use, the oxygen and fuel flowing through an oxy-burner and mixed at a tip of the oxy-burner act as cooling medium, ensuring that the burner's temperature remains below a safety threshold. If for any reason an oxy-burner stops functioning and no more oxygen and/or fuel flows through the burner, the tip of the burner, exposed to the high temperatures reigning in the interior of the forehearth and of the burner block, may rapidly heat up above a safety threshold temperature and be irreversibly damaged if exposed too long to such temperatures. Considering that a forehearth may count hundreds of oxy-burners, even if they are controlled at regular intervals, it is nearly impossible to identify a defective oxy-burner before it got irreversibly thermally degraded.
Second, another decisive difference between oxy-burners and conventional air-burners is that oxygen and fuel flow rates in oxy-burners are generally lower in oxy-burners than the corresponding air and fuel flow rates in conventional air-burners. Since air contains about 20% oxygen, the oxygen flowrate in oxy-burners is about five times lower than in corresponding air and fuel burners. It results that the temperature of the downstream end of an oxy-burner is higher than the one of a conventional air-burner. We have seen supra that if the temperature of the downstream end exceeds a threshold temperature, the burner can be irreversibly damaged. Even without exceeding said threshold temperature, the higher temperature of the downstream end of an oxy-burner can have negative effect on the functioning and service life of an oxy-burner. Indeed, a higher temperature, even if lower than the threshold temperature, may provoke cracking (or degradation) of carbon in the hydrocarbon gaseous fuel. The cracked carbon deposits at the tip of the downstream end of the oxy-burner, thus deviating the trajectory of the flame. A deviation of the flame is certainly undesired, as it changes the thermal conditions in the passage and also heats up the refractory of the burner block supporting the oxy-burner, with corresponding risks of degradation. When this happens, the oxy-burners must be removed from the burner blocks supporting them and the tip of the downstream ends must be cleaned.
Heating of the forehearth in a glass manufacturing plant differs from the heating of the furnace of the glass manufacturing plant. Oxy-burners of smaller dimensions are used in forehearth than in furnaces, and their number is substantially higher. The volume of the forehearth is much smaller than the volume of a furnace and the radiating heat to which the oxy-burners are exposed is substantially higher, as radiating heat decreases strongly with distance. Furthermore, oxy-burners are distributed along two opposite walls distant from one another by a couple of metres only, and generating a flame in the general direction of the opposite wall, thus further increasing the heat the oxy-burners are exposed to.
The present invention proposes a solution for preventing thermal degradation or dysfunctioning of the oxy-burners mounted in a forehearth. These and other objects of this invention will be evident when viewed in light of the drawings, detailed description, and appended claims.
The present invention is defined in the appended independent claims. Preferred embodiments are defined in the dependent claims. In particular, the present invention concerns a glass fibre manufacturing plant comprising a forehearth forming a passage for conveying molten glass and defined by a first and second opposite longitudinal walls, a ceiling, a bottom floor and an end wall, each longitudinal wall being provided with at least one burner assembly comprising:
In one embodiment of the present invention, the cooling plate is massive and the cooling wall is formed by a portion of the first main surface. In this embodiment, the cooling channel preferably comprises an annular cavity portion extending along the burner body axis, Xb, from
The annular cavity portion may define a prismatic passage, which extends along the burner body axis, Xb, and is contiguous with the aperture of the cooling plate. The prismatic passage is preferably so dimensioned as to accommodate a portion of the body of the oxy-burner, with or without contact therewith, and with the downstream end of the oxy-burner in thermal contact with the aperture wall of the cooling plate.
In an alternative embodiment, the cooling plate is hollow, forming a cavity surrounding the aperture and forming part of the cooling channel, said cavity being in fluid communication with both inlet and outlet.
The cooling plate must be made of a material having good thermal conduction. For example, an austenite nickel-chromium-based superalloy, such as Inconel, could be used for the cooling plate.
In a preferred embodiment, the through passage of the burner block comprises three portions:
In this embodiment, the cooling plate is encased in the burner portion, adjacent to the joining portion.
The burner block is preferably made of a refractory material composed of at least 90 wt. % alumina, preferably of at least 95 wt. % alumina, more preferably of at least 99 wt. %.
For an easier insertion into the masonry of the longitudinal wall, the burner block preferably comprises a hot cuboid portion comprising the hot surface, and a cold cuboid portion comprising the cold surface, wherein the cross-sectional area normal to the passage axis, Xp, of the hot cuboid portion is smaller than the one of the cold cuboid portion. This way, it is impossible for the burner block to protrude out of the longitudinal wall beyond the predefined design restrictions.
The flame portion and the joining portion of the burner block through-passage can have a geometry of revolution around the passage axis, Xp. On the other hand, the geometry of the cross-section normal to the passage axis, Xp, of the burner portion should not be of revolution, in order to prevent a burner from rotating about the passage axis, Xp. The burner portion cross-section may have a polyhedral geometry, preferably rectangular or square. Alternatively, the overall geometry of the burner portion cross-section can be of revolution with respect to xp, but comprising a protrusion or a recess, matching a corresponding recess or protrusion in the oxy-burner or the cooling unit.
In a preferred embodiment, each longitudinal wall of the forehearth comprises at least two burner assemblies aligned horizontally, the at least two burner assemblies of the first longitudinal wall preferably facing the at least two burner assemblies of the second longitudinal wall in a staggered arrangement. The end wall may also be provided with a burner assembly as discussed supra. In practice, each longitudinal wall is provided with at least 10, preferably at least 20, more preferably at least 30 burner assemblies, facing the corresponding burner assemblies of the second longitudinal wall in a staggered arrangement.
The present invention also concerns the use of a cooling unit as defined supra for cooling a downstream end of an oxy-burner mounted in a forehearth of a glass fibre manufacturing plant.
For a fuller understanding of the nature of the present invention, reference is made to the following detailed description taken in conjunction with the accompanying drawings in which:
As shown in
Oxy-burners are mounted in burner blocks (20) included in the masonry of the longitudinal walls and distributed along said longitudinal walls of the passage. Typically, a forehearth comprises hundreds of burner blocks aligned along the longitudinal axis, X1, of the longitudinal walls and each loaded with an oxy-burner. Examples of burner blocks (20) suitable for the present invention are represented in
The flame portion and the joining portion of the burner block through-passage preferably have a geometry of revolution around the passage axis, Xp, the geometry of the cross-section normal to the passage axis, Xp, of the burner portion is not of revolution, and either is polyhedral, preferably rectangular or square, or has an overall geometry of revolution with respect to xp, comprising a protrusion or a recess.
As shown in
A burner block is preferably made of a refractory material composed of at least 90 wt. % alumina, preferably of at least 95 wt. % alumina, more preferably of at least 99 wt. %. The refractory material of the support block preferably comprises less than 10 wt. % silica, more preferably comprises less than 5 wt. % silica, most preferably comprises less than 2 wt. % silica. The bricks surrounding the cavity may be made of mullite, preferably of alumina enriched mullite.
As shown in the top views along a plane (X1, X2) of
In a preferred embodiment illustrated in
Because the longitudinal walls of a forehearth must be substantially impervious to gases, to prevent hot air flowing out of the passage, the gap formed between the burner block and the cavity must be filled with a resilient material (29), preferably provided in the form of a fibrous sheet, which is wrapped around external surfaces of the support block. The resilient material is preferably made of refractory fibres, typically alumina fibres, or fibres comprising at least 90 wt. %, more preferably at least 95 wt. %, most preferably at least 99 wt. % alumina. The resilient fibres preferably do not comprise more than 2 wt. % silica, and more preferably comprise no silica, as silica may form an eutectic with oxides of adjacent refractory bricks, which could contaminate the glass melt by sublimation at high temperatures. An example of resilient material supplied as a sheet which is suitable for the present invention comprises ceramic fibre paper such as supplied by Morgan Thermal Ceramics.
As illustrated in
Burners and, in particular, oxy-burners, are loaded into a burner block from the opposite side of the longitudinal walls from the passage, which defines a cold longitudinal wall surface. As discussed before, a burner can be mounted in the burner portion (21B) of the through-passage of a burner block with the tip of the downstream end of the burner protruding slightly in the joining portion (21j). The tips of the burners are therefore exposed directly to the temperature reigning in the flame portion (21F) of the through-passage and to the service temperature, hT, reigning in the passage beyond the opening of the flame portion at the hot longitudinal wall surface. Theoretically, this is not a problem because the fuel and oxygen flowing through the body and converging at the tip of a burner act as refrigerants and cool the downstream end of the burner, protecting it from over-heating. In case, however, of a low flow rate of oxygen and/or fuel, or in case of malfunction of one of the oxygen or fuel lines (1Ox, 1F) the downstream end of a burner might not be cooled down sufficiently and the temperature thereof may raise above cracking temperature of the fuel or even above a threshold limit beyond which the burner is damaged. The present invention provides a solution to this problem, and also to the prolongation of the service life of burners.
As illustrated in
The refrigerating fluid can be water, air, nitrogen or any refrigerating fluid known in the art. It flows into the cooling unit at a first temperature, T1, through the inlet (3U) of the cooling channel, contacts the cooling wall (5W) whence it extracts heat from the cooling plate, and flow out of the cooling unit at a temperature, T2>T1, through the outlet (3D) of the cooling channel.
For example, with burners having a conical or trunco-conical downstream end, the geometry of the aperture wall of the cooling plate is likely preferably trunco-conical, to match perfectly the geometry of the downstream portion and thus form an optimal thermal contact therewith. A cooling plate suitable for the present invention is preferably made of a highly thermally conductive material, such as an austenite nickel-chromium-based superalloy, for example, Inconel.
As illustrated in
The cooling channel may comprise an annular cavity surrounding the body of the oxy-burner. A thermal contact between the cooling channel and the body of the oxy-burner is possible but not essential, since it is the downstream end of the oxy-burner which is exposed to the highest temperatures. The annular cavity portion extends along the burner body axis, Xb, from,
If the passage defined by the annular cavity portion for receiving the body of the oxy-burner is contiguous with the aperture of the cooling plate, than the whole area of the first main surface of the cooling plate can form the cooling wall (5W).
One difference between the cooling units of the
As shown in
In an alternative embodiment, the cooling plate (5) is not massive but hollow, as shown in
Both an assembly comprising a sub-assembly, and a kit-of-parts are described herein, said sub-assembly and kit-of-parts comprising an oxy-burner (1) and a cooling unit (3) as defined above. In the sub-assembly, the downstream end (1D) of the oxy-burner is in thermal contact with the aperture wall of the cooling plate (5), as shown in
In an alternative embodiment, a cooling unit can be permanently fixed in the burner portion of the burner block, e.g., with mortar, to ensure an optimal and reproducible positioning thereof. An oxy-burner can then be loaded into, or unloaded out of the thus sealed cooling unit.
As illustrated in
Both longitudinal walls are provided with at least one burner assembly comprising an oxy-burner sub-assembly encased in a burner block as discussed supra, with a hot surface (20H) of the burner block forming a portion of the corresponding longitudinal walls (31L). It is clear that several burner assemblies are required in each longitudinal wall (31L) to maintain a controlled temperature in the passage, which can be several dozens of metres long. Consequently, as illustrated in
The bricks adjacent to and surrounding the burner blocks preferably contain less than 5 wt. % of SiO2, more preferably less than 1 wt. % SiO2, most preferably less than 0.2 wt. % SiO2, or no SiO2. For example, the adjacent bricks of the longitudinal wall may be made of alumina enriched mullite. Alternatively or additionally, a sheet of refractory non-woven fibres (29), poor in SiO2 as discussed with respect to exchangeable burner blocks, as illustrated in
The use of a cooling unit (3) as discussed supra for cooling a downstream end (1D) of an oxy-burner (1) mounted in a forehearth (31) of a glass fibre manufacturing plant has several advantages. First, in case of low gas flow rates, as can be common with oxy-burners, a cracking temperature of the fuel is not reached, preventing carbon deposition at the tips of the oxy-burners. It has been observed in preliminary tests on a full scale glass fibre production line, that oxy-burners provided with a cooling unit according to the present invention required three to ten times (3 to 10×) less frequent cleaning operations than the same oxy-burners used in the same conditions, absent a cooling unit. Second, in case of accidental interruption of the flow of the oxygen or the fuel in an oxy-burner, the downstream end (1D) of the oxy-burner equipped with a cooling unit does not heat up above a threshold temperature which would degrade the oxy-burner. This means that the range of flow rates an oxy-burner can be used is increased, since it must not be above a certain flow rate value required for ensuring the cooling of the downstream end of the oxy-burner. Third, although oxy-burners are designed for working at relatively high temperatures (several hundreds of degrees Celsius), their service lifetime is increased when working at lower temperatures. A cooling unit according to the present invention permits to work with oxy-burners maintained at a lower temperature. Fourth, the handling of a thus cooled oxy-burner mounted in a burner block by an operator is facilitated if the temperature of the oxy-burner body is lower as allowed by the cooling unit.
| Number | Date | Country | Kind |
|---|---|---|---|
| 16156639.3 | Feb 2016 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2017/053420 | 2/15/2017 | WO | 00 |
