MELTING PROCESS USING THE COMBUSTION OF LIQUID AND GASEOUS FUELS

The present invention relates to a process for melting a charge by means of combustion burners that burn liquid fuel and gaseous fuel.

In an industrial furnace, the choice of the nature of the fuel used, for example between liquid fuel and natural gas, depends on several parameters. The first parameter is the operating cost of the fuel used. Another parameter is the transfer of energy to the charge, a natural gas flame being known to be less emissive than a liquid fuel flame. Another parameter is the concentration of water vapor created by the combustion above the charge, which may have an influence on the quality of the product. Finally, the polluting emissions, such as NO _x or SO _x must be taken into account, these generally being greater in the case of liquid fuel than in the case of natural gas.

Hybrid operation using both types of fuel will make it possible to maximize the benefit of the advantages of each type of fuel and to limit their drawbacks. Many studies have thus recently addressed various methods for increasing the emissivity of a natural gas flame. For example, AJ. Faber and M. Van Kersbergen in Glass Technology Vol. 46, No. 2, April 2005 have shown that the emissivity of a natural gas flame could be increased in the visible and near-infrared range either by adding a small amount of diesel or by injecting gases conducive to the formation of soot, such as propylene or acetylene. However, the amount of diesel injected remains less than 20% of the total flow of fuel. One method of increasing the proportion of liquid fuel relative to that of natural gas is to develop injectors using natural gas as atomizing gas. L. S. Messias, M. M. Dos Santos and H. F. D. Schettini in Proceedings of Clean Air 2005, April 2005 proposed for example an atomizer injecting 50% liquid fuel and 50% natural gas and show, for an air-fuel flame, a substantial reduction in polluting emissions, such as nitrogen oxides and sulfur oxides, thanks to the use of such a system. This type of dual-fuel atomizer also exists in the case of burners using oxygen as fuel: B. Leroux, F. Lacas, P. Recourt and O. Delabroy, Proceedings of AFRC - JFRC International Symposium, Hawaii, September 2001 , describe external-spray injectors that can use natural gas as atomizing gas. However, for these types of injector dedicated to oxycombustion, the range of possible flow rates of natural gas remains narrow: the mass flow rate of natural gas is generally between 15% and 30% of the mass flow rate of liquid fuel. Below 15%, the quality of

the atomizing becomes overly mediocre and there is incomplete combustion. Conversely, above 30%, the flame may become unstable because of excessively high velocities of the atomizing gas.

One purpose of the present invention is to find a solution to this lack of flexibility of current hybrid oxy-fuel systems.

For this purpose, the invention relates to a process for melting a charge by means of oxy-fuel combustion burners in a furnace that includes at least one upstream flue gas outlet, in which furnace, in the charge melting zone:

- the burners placed close to the upstream flue gas outlet burn a liquid fuel; and

- at least one of the burners placed far from the upstream flue gas outlet burns a gaseous fuel.

Other features and advantages of the invention will become apparent on reading the following description. Embodiments and methods of implementation of the invention are given by way of nonlimiting examples and illustrated by the appended drawings in which:

- figure 1 is a schematic view of a furnace in which the process according to the invention is carried out; - figure 2 is an alternative method of implementing the process according to the invention; and

- figure 3 is another alternative way of implementing the process according to the invention.

The process according to the invention involves heating a charge by means of oxy-fuel combustion burners, that is to say burners using an oxidant consisting of an oxygen-rich gas. The term "oxygen-rich gas" is understood to mean a gas having an oxygen content of greater than 70%. The process is implemented in a furnace comprising at least one combustion flue gas outlet in the upstream part of the furnace. In this text, the term "upstream" is understood to mean that part of the furnace in which the charge to be heated is introduced and the term "downstream" is understood to mean that part of the furnace in which the heated charge is discharged. The flue gas outlet may be located at the end of the furnace or on one of its lateral sides. In general, the upstream flue gas outlet is located in the first quarter of the length of the furnace. The upstream flue gas outlet or outlets are preferably located on one or more lateral sides of the furnace. The furnace may include one or more upstream flue gas outlets. In the case of several flue gas

outlets, these may be placed at the same height over the length of the furnace or may be staggered. The principle of how the burners according to the invention are arranged applies relative to all the upstream outlets. The process of the invention relates to placement of the oxy-fuel burners in the furnace according to the nature of the fuel that they burn. Thus, in the melting zone of the furnace:

- the burners placed close to the upstream flue gas outlet burn a liquid fuel. In the present context, the expression "burners placed close to an element or a zone" of the furnace is understood to mean burners that are less than 4 m from said element or from said zone. Preferably, "burners placed close to an element or a zone" are considered to be burners placed less than 8 m from said element or said zone. In practice, the skilled person will typically choose 4 or 8 m in function of the total length of the furnace; and

- at least one of the burners placed far from the upstream flue gas outlet burns a gaseous fuel. The expression "burners placed far from an element or a zone" is understood to mean burners that are not placed close to said element or said zone, that is to say burners placed at least 4 m, or even at least 8 m, from said element or said zone, the selection being typically, as mentioned above, made in function of the total length of the furnace.

The furnace comprises three zones: - the melting zone of the furnace, which is defined as the zone lying upstream of the line demarking the solid material from the molten material. In general, for melting glass, this melting zone extends over the upstream zone representing the first third to the first half of the furnace;

- the heating zone, which is defined as the zone which is covered by the burner flames and which includes part of the melting zone and the zone lying directly downstream of the melting zone; and

- the refining zone, which is defined as the zone lying downstream of the heating zone and which is subjected to no heating.

According to a variant of the process, in the heating zone, the burners placed closest to the refining zone may burn a liquid fuel. This variant makes it possible to reduce the water vapor concentration levels near the refining zone.

Furthermore, in the case in which the furnace includes at least one downstream flue gas outlet:

- the burners placed close to the downstream flue gas outlet burn a liquid fuel; and

- at least one of the burners placed far from the downstream flue gas outlets burn a gaseous fuel.

Preferably, the downstream flue gas outlet is located in the last third of the length of the furnace.

The process according to the invention applies preferentially to furnaces in which the charge is glass or enamel. The process of the invention applies in particular to glass furnaces having a length of at least 20 m, preferably at least 30 m.

Figures 1 , 2 and 3 illustrate the device and the process according to the invention.

In figure 1 , the furnace 1 is fitted: - with chargers 2, for introducing the charge to be melted;

- with two upstream flue gas outlets 3; and

- with eight oxy-fuel burners 41 , 42, 43, 44, 51 , 52, 53, 54 placed either facing one another or in a staggered fashion.

According to the invention, the burners 41 , 42, 43, 44 placed close to the upstream flue gas outlet 3 burn a liquid fuel, whereas the burners 51 , 52, 53, 54 placed far from the upstream flue gas outlet 3 burn a gaseous fuel.

In figure 2, the furnace 1 is fitted:

- with chargers 2, for introducing the charge to be melted;

- with two upstream flue gas outlets 3; and - with nine oxy-fuel burners 41 , 42, 43, 44, 45, 46, 51 , 52, 53 placed either facing one another or in a staggered fashion.

According to the invention, the burners 41 , 42, 43, 44 placed close to the upstream flue gas outlet 3 burn a liquid fuel, whereas the burners 51 , 52, 53 placed far from the upstream flue gas outlet 3 burn a gaseous fuel. Furthermore, the burners 45 and 46 located near the refining zone 8 burn a liquid fuel. In figure 3, the furnace 1 is fitted:

- with chargers 2, for introducing the charge to be melted;

- with two upstream flue gas outlets 3;

- with two downstream flue gas outlets 7; and - with nine oxy-fuel burners 41 , 42, 43, 44, 45, 46, 51 , 52, 53 placed either facing one another or in a staggered fashion.

According to the invention, the burners 41 , 42, 43, 44 placed close to the upstream flue gas outlet 3 burn a liquid fuel, whereas the burners 51 , 52, 53 placed far from the upstream flue gas outlet 3 and far from the downstream flue gas outlet 7 burn a gaseous fuel. Furthermore, the burners 45 and 46 located close to the downstream flue gas outlet 7 burn a liquid fuel.

By implementing the process as described above, it becomes possible to

combine the advantages of flames using liquid and gaseous fuels in an optimum manner. Thus, the greater inertia of liquid fuel flames guarantees their stability, despite the proximity of the flue gas outlets. Moreover, since their transfer to the charge is greater, the flue gases from these flames have a lower temperature and the energy balance is better than if gaseous fuel burners had been installed. The way in which the gaseous fuel burners, for their part, are positioned ensures a longer residence time in the furnace for the flue gases from these flames so as to guarantee complete transfer to the charge.