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Electric Melting and Boosting for Glass Quality Improvement (Part 1)

In the first part of his article, Richard Stormont looks at how electric boosting can help improve the quality of glass being produced.

Each year new electric boost systems are incorporated into new fuel- or oxy-fuel-fired glass melting furnaces, and a much greater number of boost systems are added, reinstalled or enlarged in the course of furnace repairs. In many of these cases the boost system is still seen primarily as a means of increasing furnace output, to be used only when necessary. But this is to under estimate the potential of electric boosting to influence overall furnace operation, energy consumption and glass quality, as well as just adding energy to the melting process and increasing output.

Figure 1: The process relies on achieving flame and superstructure temperatures higher than the glass temperature, and transmitting heat to the glass by radiation and conduction.

Glass melting is an energy intensive process. The net melting energy needed to convert mixed raw material into fully melted and refined glass can be taken as about 520 Kilocalories per Kilogram, or in electrical energy terms, about 0.6 Kilowatt-hours per Kilogram (kWh per kg) of glass. This assumes about 20% cullet, and varies to some extent according to both cullet percentage and glass type. The thermal efficiency of glass melting furnaces varies according to furnace design and glass type, but the best fuel-fired furnaces might have a thermal efficiency of only around 45%, and many have thermal efficiencies of significantly less than this. In this illustration I have assumed a figure of 40%. Clearly in all fuel-fired furnaces more energy is released into the environment as heat loss than is used to actually convert the raw materials to molten glass. To save energy in glass melting, we must therefore focus on ways to reduce these heat losses.

The reason for most of this inefficiency is in the basic concept of all fuel-fired or oxy-fuel-fired furnaces. Some level of loss from the bottom and sides of the tank is inevitable, although they can be minimised by good insulation. However as illustrated in Figure 1, the process relies on achieving high flame and super structure temperatures through oil or gas combustion, with temperatures that must by definition be higher than the glass temperature, and trans­mitting heat to the glass by radiation and some conduction. The result of this indirect transfer of energy from fuel to glass is high heat losses from the superstructure of the furnace and high losses in the residual waste gases, even if heat recovery systems such as regenerators or recuperators are used.

DIRECT MELTING ENERGY

So how can we get melting energy more directly into the glass? The most effective method is by the use of immersed electrodes in the form of either electric boosting or all-electric melting. With immersed electrodes in the glass connected to a suitable power supply and transformer, we can pass an electric current through the glass, releasing heat energy directly into the glass itself, with no significant losses in the process.

Figure 2: The typical net melting energy and the energy losses in a 200 tpd unboosted fuel-fired furnace.

Of course we have to ensure that the heat released into the glass by electrodes does not adversely affect other aspects of the furnace performance, for instance by creating convection currents in the wrong places or causing short cut flow paths that reduce effective residence time in the furnace. Selection of the optimum number, positions, spacing, size, immersion and connection arrangements for boosting electrodes all contribute directly to the difference between an efficient and an inefficient boost system design.

In electrical energy terms, again at about 20% cullet levels, the theoretical net melting energy requirement is about 0.6 kWh per kg of additional glass melted by the boosting system. In many boost systems the actual figure is substantially higher, however with today’s best boosting technology we regularly see actual boost system energy requirements of 0.48 kWh per kg, or 20 Kilowatts of continuous power input per additional tonne per day (tpd) of glass. This apparent ‘super­ efficiency’ is entirely due to the fact that a well-designed boost system can actually improve the operational efficiency of the fuel-firing heat transfer through beneficial convection currents and increasing the effective minimum residence time, as well as adding melting energy to the process.

So electric boosting can be a Highly effective way to reduce overall energy consumption. In Figure 2 we see the typical net melting energy and the energy losses in a 200 tpd unboosted fuel-fired furnace. In this example we have a total energy consumption of about 1300 KiloCalories per kg of glass. If we then compare that with an electrically boosted furnace in which 75% of the glass is melted by the fuel-firing and 25% by a modern Convection Current Control boosting system, as illustrated in Figure 3, then due entirely to the efficiency of the boosting we find that total energy consumption might fall to around 1080 Kilocalories per kg, an energy saving of 17%.

Figure 3: An electrically boosted furnace gives efficient boosting where total energy consumption might fall to 1080 kilocalories per kg, an energy saving of 17%

Of course a comparison of melting energy costs in this example will depend on the relative costs of fuel (gas or oil) and electricity for the boost system. In capital cost terms it can be significantly cheaper to build, for instance, a furnace with an unboosted output of 150 tpd and install a 50 tpd boosting system, giving a total output of 200 tpd, rather than to build a furnace with an unboosted capacity of 200 tpd.

CONTRIBUTING TO GLASS QUALITY

Just as there are often substantial differences in the energy efficiencies of different designs of boost system, so there are big differences in their contribution to glass quality – the two are closely linked. The key is in the electrode arrangement and the energy release pattern that the electrodes create in the furnace, which in turn directly affects temperature profiles, convection currents, flow paths and residence time. Understanding the distribution of energy release from immersed electrode systems is crucial in boost system design.

Any electrode system releases energy directly into the glass, making the application of boost energy fundamentally much more efficient than the indirect heat transfer from the top firing. Whatever steps are taken to create the desired furnace temperature profiles by means of the top firing, radiant heat transfer from flames and superstructure is multi-directional, resulting in imprecise temperature profile creation in the glass. By contrast, electric boost system energy release can be highly focused.

Boost system design, and in particular electrode positioning, is therefore extremely important in determining conditions in the glass bath. A poorly designed boost system will create adverse convection currents, perhaps conflicting with hot spot convection created by the top firing. It may also result in reduced residence time by promoting forward convection currents, affecting glass quality. It will certainly require an excessive amount of electrical energy in relation to the additional glass being produced, simply to overcome the design limitations of the system itself.

A well designed boost system creates convection currents that co-operate with the actual or desired effects of the top firing and uses the electrical energy release pattern to increase the minimum residence time, improving glass quality in terms of seed, stone losses and homogeneity. Most importantly, by releasing the boost energy in a way that actually helps the top firing to perform its function better, today’s best boost systems can require significantly less electrical energy input than is theoretically required for the additional glass produced.

CONVECTION CURRENT CONTROL

The Convection Current Control (CCC) boosting concept was developed in the 1980s and has been continuously refined since. Consistent with the approach of designing the boost system to co-operate with the top firing, most or all of the boost energy is applied by means of electrodes in the hot spot area of the furnace. The object is to create or reinforce hotspot convection, promoting long-range convection currents. The action of the thermal barrier effectively prevents un-melted material from reaching the throat and increases minimum residence time by preventing the short-cut movement of glass along the surface. The reverse convection current also helps to keep the batch back and the slow circulation of glass improves heat transfer from the top firing.

Figure 4: A boost system redesign

One case history that demonstrates the effect of boost design on both energy efficiency and glass quality is a 90m2 UVA green container glass furnace which was originally equipped with an electric boost system supplied by the furnace designer. After about two years of operation Electroglass was invited to review the boost system design and operation and subsequently to completely redesign the system. The original system was designed to apply the majority of the boost energy in the melting end of the furnace. The Electroglass recommendation was to abandon the melting end electrodes and to install additional hot spot area electrodes to create a CCC configuration. The result was an immediate reduction in boost energy consumption of almost 20% and a 94% reduction in seed count (as illustrated in Figure 4), combined with a 60°C reduction in bottom temperature.

So, to make a real impact on glass quality as well as energy consumption and environmental protection in the energy-intensive process of glass melting, we should be making better use of technologies already available in the electric boosting of fuel-fired furnaces. In Part 2, I will look at all-electric melting and electric forehearths and their contribution to glass product quality, as well as energy conservation and the environment.

ABOUT THE AUTHOR:
Richard Stormont is Managing Director of Electroglass Ltd

19/12/2023

As published in Glass Worldwide magazine.

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