1. Field of the Invention
The present invention relates to a method of melting of glass for the production of flat glass, and to an apparatus for use in that method.
2. Description of the Prior Art
The batch constituents used for making glass are melted by means of burners which may be disposed in a variety of configurations. Probably the most common burner arrangement is one in which fossil fuel fired burners are disposed opposite one another along the length of a melting area of the furnace. For many years, the fuel has been combusted in air. More recently, however, so-called xe2x80x98oxy-fuelxe2x80x99 firing has been employed, primarily in container glass furnaces. In such a system the fuel is supplied together with oxygen (rather than air). In one particular arrangement that is known, the fuel and oxygen are supplied through co-axial conduits.
In oxy-fuel firing, the oxygen and the fuel are mixed as soon as they issue from their feed conduits into the furnace and, as a result, the oxygen-fuel mixture burns with an intense, concentrated, well-defined flame. Such a system has been primarily used in container glass furnaces, that is to say, furnaces used for melting glass for the production of shaped articles such as bottles, and is claimed to have number of advantages, including reduced energy requirements, reduced volume of waste gases and reduced capital costs.
Combustion of the fuel in oxygen as opposed to air results in an increase in the proportion of water (steam) in the furnace atmosphere, leading to the formation of a foam lying on the surface of the glass. An increase in the proportion of gas dissolved in the molten glass is also observed. These are not such serious problems in container glass furnaces, because the molten glass in such furnaces is extracted from the furnace from below the surface glass, and in any event, a significant proportion of bubble can generally be tolerated in container glasses. However, it is unacceptable in flat glass production, where the molten glass is drawn off from the surface of the melt and the acceptable bubble limits are usually about two orders of magnitude below those acceptable in container glass production.
We have now found that the problem can be overcome, to the extent of allowing flat including float glass of commercially acceptable quality to be produced using oxy-fuel melting, by directing a diffuse, luminescent flame on the surface of the molten glass, downstream of the oxy-fuel firing giving rise to the foam to be dissipated. Surprisingly, the use of this flame not only substantially destroys the foam, but leads to a reduction of the bubble within the glass to a level acceptable for float glass production.
According to the present invention there is provided a method of producing flat glass in which the glass batch is melted using burners fired by fuel and oxygen resulting in the formation of foam on the surface of the molten glass, and a burner downstream of such burners producing foam to be dispersed produces a flame which is diffuse, luminescent and impinges on the surface of the molten glass dispersing said foam.
By using such a flame, we are able to remove most if not all of the foam and reduce the bubbles in the resultant glass to commercially acceptable levels for general glazing or even automotive glazing. Use of the invention permits oxy-fuel firing to be used as the sole heat source in a flat glass furnace rather than, as at present, only for boost heating in combination with other firing techniques.
In conventional oxy-fuel burners, as used in container glass and glass fibre furnaces, the oxygen and fuel are introduced through co-axial or otherwise adjacent conduits and their mixing is almost instantaneous; a very turbulent gas mixture is formed which leads to almost instantaneous and substantially complete combustion with a clean (i.e. low luminosity), well-defined flame being produced. Similar burners may be used to melt the glass batch in oxy-fuel fired flat glass furnaces according to the invention, but result in the production of foam as described above. However, to achieve the diffuse, luminescent flame required by the present invention to disperse the foam resulting from such burners, we prefer to use ports for the fuel and oxygen which are spaced apart from each other such that delayed mixing of the fuel and oxygen occurs in a region remote from the ports, and takes place over a larger area. This leads to the production of flames which are considerably more diffuse and luminescent than conventional oxy-fuel flames and generally cooler; in fact, a flame dispersing the foam will generally be cooler than the surface of the molten glass it contacts. The flame impinges on the surface of the glass and destroys foam which has built up thereon. The exact mechanism by which the flame disperses the foam is not understood; it is possible that, because the flame is luminescent, it contains small particles of carbon which physically burst the foam.
According to a further aspect of the present invention, there is provided a flat glass making furnace including a melting zone into which glass-making batch components are fed and a plurality of firing zones including burners for melting the components, wherein at least one of the firing zones includes a first port through which a fuel is introduced into the furnace and a second port through which oxygen is introduced into the furnace, the first and second ports being spaced apart from one another such that mixing of the fuel and of the oxygen occurs in a region remote from the ports and the flame produced by the mixture of the fuel and the oxygen is diffuse, luminescent and impinges on the surface of the glass.
It is preferred that the fuel and oxygen ports extend parallel to one another. Alternatively or additionally, one or both ports may be directed downwardly at an angle to the horizontal.
It is customary in side-fired furnace for firing zones to be provided in pairs disposed on opposed sides of the furnace substantially opposite one another. In oxy-fuel furnaces used for container glass or glass fibre production, the zones in each pair are usually slightly staggered with respect to one another in the longitudinal direction of the furnace. In the case of the present invention, it is preferred if at least one pair of spaced-apart oxygen and fuel ports are provided on each side of the furnace.
If desired, more than one pair of oxygen and fuel ports providing diffuse, luminescent flames directed on to the glass may be provided on each side of the furnace. However, because the chief reason for the provision of such ports is to attack the foam, it is not particularly useful to have such burner arrangements in regions where the foam is not a problem. Accordingly, we do not envisage more than two pairs of such burner arrangements on each side of the furnace and, normally, only one pair (i.e. one burner each side) will be used. It is desirable if at least one such burner arrangement on each side of the furnace form either the final or penultimate firing zone. However, one or more additional firing burners or pair of firing burners may be provided downstream of any burners dispersing the foam provided such burner or burners do not give rise to unacceptable foam. Thus it is preferred if a pair of burners dispersing the foam form the penultimate firing zone such that the final firing zone can be used for control purposes. Preferably, the fuel is a fuel oil, optionally a light fuel oil of the diesel type, or natural gas. Desirably, the fuel and oxygen are caused to enter the furnace in flow streams which extend substantially parallel to one another. Alternatively or additionally, one or both flow streams are directed downwardly towards the surface of the glass at a slight angle to the horizontal.