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Electric Melting and Boosting for Glass Quality Improvement (Part 2)

In the concluding part of his article, Richard Stormont investigates all-electric melting and electric forehearths.

The first part of this article explored the beneficial effects that electric boost can have on convection currents, heat transfer, temperature homogeneity
and residence times – all of which contribute to glass quality and energy efficiency. The logical next step, then, is to consider all-electric melting.

Figure 1: Most such furnaces operate “cold top”, with the raw material being distributed evenly over the melting surface of the glass, forming an insulating batch blanket

In all-electric furnaces the entire melting energy is applied by means of immersed electrodes, with gas being used only for the initial heat-up of the furnace, or as an emergency heat source in case of a prolonged power cut. Most such furnaces operate ‘cold top’, with the raw material being distributed evenly over the melting surface of the glass, forming an insulating batch blanket (as shown in Figure 1). Melting and refining essentially take place in one continuous vertical process, with glass being drawn through a throat at the bottom of a relatively deep melting tank.

The energy efficiency of such furnaces can be very much higher than fuel-fired furnaces because, while there are still some losses from the substructure, there are very low
losses from the superstructure. Due to the insulating properties of the batch blanket in a well-designed all­-electric furnace, the superstructure temperature can typically be little more than 100° C (212° F).The result is a thermal efficiency that can be over 70% even in a small electric furnace of 10 tonnes per day (tpd), and can reach 85% in a large electric furnace.

RELEVANCE TO SMALL FURNACES

Figure 2: As furnace size decreases, the energy efficiency of electric furnaces remains very high, whereas the efficiency of fuel-fired furnaces drops dramatically and can be less than 20%in small furnaces

The difference in energy efficiency between fuel-fired and electric furnaces is particularly important in the case of relatively small furnaces. Figure 2 shows that as furnace size decreases, the energy efficiency of electric furnaces remains very high, whereas the efficiency of fuel-fired furnaces drops dramatically and can be less than 20% in small furnaces.
The batch blanket in an all-electric melter does much more than reduce heat losses, leading to improved energy consumption. A key advantage of the cold-top electric melter is that everything that goes into the batch stays in the glass, apart from the gases released from the melting process, which permeate out through the batch blanket. Losses of batch constituents such as fluorine, boron, lead, various relatively volatile refining agents and other constituents are virtually eliminated.

In an Electroglass fluoride opal electric melter in Germany, the loss of fluorine between batch and glass is just 3%. This not only reduces batch costs, as the losses do not have to be compensated for by increasing the batch quantities, but has a direct effect on glass quality as glass composition is predictable and consistent. So all-electric melting can lead to energy efficiencies of up to 85%, consistent glass quality, with no environmental pollution and furnaces that are silent in operation.

Figure 3: A 250 tonnes per day Electroglass all-electric furnace, the largest in the world, with a thermal efficiency of 85%when melting flint glass

WIDENING THE PRODUCT RANGE

Electric glass melting furnaces have been used mostly for special glasses, and particularly glasses with significant volatile constituents such as fluoride opal glasses, borosilicates and lead crystal. However as world energy costs recently moved against gas and oil and in favour of electricity, there is greatly increased interest in large all-electric furnaces for conventional glasses for containers and other products. Figure 3 shows a 250 tpd Electroglass all­-electric furnace which, when melting flint container glass, requires just 710 Kilowatt-Hours of electricity per tonne of glass, equivalent to a thermal efficiency of 85%.

Energy consumption, combustion gas emissions and glass quality are other benefits of forehearths used in many sectors of the glassmaking industry – remarkably, while most glassmakers know how much energy they are using in their melting furnaces, a surprising number take little notice of the energy used in their forehearths.
As with electric melting furnaces, well-designed all-electric forehearths can have exceptionally high energy efficiencies. In a typical container glass application under normal operating conditions, a modern electric forehearth may only require a total power input of 15 to 25 Kilowatts to properly temperature control and condition 90 tpd of glass.

Figure 4: In 2008 a container glass producer in Korea installed two Electroglass all-electric forehearths

SIGNIFICANT REDUCTIONS

In 2008 Hite Industries, a Korean container glass producer, installed two Electroglass all-electric forehearths, one with a 36-inch and one with a 48-inch channel (see Figure 4), as direct replacements for gas-fired forehearths and reduced its forehearth energy costs by over 70%_ Electric forehearths are also completely silent in operation and emit no polluting combustion gases. Radiant profile heating and radiant centre-line cooling allow excellent temperature control and response, eliminating the effects that combustion gases can have on the glass and eliminating forced air cooling, which can introduce impurities as well as creating undesirable surface chilling effects.

So to make a real impact on glass quality, energy consumption and environmental protection in the energy-illtensive process of glass melting and conditioning, perhaps we should be making better use of technologies already available: electric boosting of fuel-fired furnaces, all-electric melting and electric forehearths.

Glass product quality, as well as the world’s energy supplies and the environment, will benefit from this responsible approach to glass production.

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

21/12/2023

As published in Glass Worldwide magazine.

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