There has been a common theme during discussions with customers, at conferences, and at exhibitions during 2024; – reducing reliance on fossil fuels and moving to a more sustainable and therefore greener energy policy.

For many the key to meeting these two goals in our industry lies in the melting stage of the glass making process, a high proportion of which takes place in fuel fired furnaces with thermal efficiencies of 50% or less. In other words, over half of the melting energy applied is simply lost, despite the industry’s efforts to minimise both the losses and their environmental impact.

While concentrating on this area of the glass making process will no doubt bring further medium- and long-term improvements, there are solutions elsewhere within the overall process that can offer major benefits, both in terms of carbon reduction and operating cost savings, using proven technology and in a much shorter timescale. One of these areas is the conditioning of the glass within the distributor and forehearth system.

The majority of distributor channels and forehearths used by container glass manufacturers are gas heated. In terms of energy usage, these are inefficient. Total heat energy input can be many times higher than needed in an equivalent well designed all-electric system, in many cases eight to twelve times higher. Even allowing for sometimes significant differences in the unit energy cost between electricity and gas, such savings translate directly to major energy cost savings as well as eliminating all combustion gas emissions.

The gas heated approach requires the application of often complex, special shaped roof blocks and superstructure designs to attempt to control the distribution of heat release from the combustion process, allow the evacuation of waste gases and contain any forced air cooling in order to achieve acceptable thermal homogeneity. Whilst many designs have been successful in this quest, they remain thermally inefficient and ultimately rely on non-renewable, polluting fossil fuels.

From soda-lime container glass to aggressive specialist glasses such as borosilicate and fluoride opal for tableware, cookware and containers, all-electric forehearth systems have demonstrated proven performance in terms of both energy usage, cost reductions and simplified, low maintenance operation.

In the last decade there has been an increased interest in high and very high-capacity all-electric distributor and forehearths systems, particularly for the cooling and conditioning of container glass. This is of little surprise when three of our most recent projects offered operating COST savings ranging from 71% for 3 x 48” high-capacity forehearths, 75% for 2 x 36” forehearths and 86% for 3 x 52” very high-capacity forehearths.

For many, the move to all-electric conditioning has relied on modified gas heated forehearth designs, where gas burner systems are maintained for warm up and emergency use, with electrical energy input directly into the glass by means of some form of molybdenum electrode. These design compromises are not optimised in terms of insulation, heat loss, or energy input and are likely to result in much higher than necessary energy usage, risk of glass reboil due to localised heating around the many high-power electrodes often used and, in many cases, where certain dry electrode designs are employed, a risk of oxygen blister generation.

To truly benefit from a move to all-electric conditioning a different approach is needed, – an approach that offers a purpose-built design without compromises carried over from other gas or electric forehearth design types. Key to this approach is the selection and use of low thermal mass insulating materials to significantly reduce losses and the application of radiant heating elements above the glass surface to apply heat where needed.

Element type and zoning is an important part of our approach when designing our Electroflex Forehearths for container and other non-volatile glasses. The use of profile heating elements to apply heat along the channel sides where the glass is coldest and the ability to offer independent side to side heating where needed enables Thermal Homogeneity Index figures higher than similar gas heated designs, up to and over 98%.

When producing dark or low transmission glasses it is often advantageous to provide additional thermal homogeneity security by applying specially designed low power dry electrodes in the conditioning or equalising zone. These will typically be operating at powers less than 6kW for the entire zone and give the ability to control the power independently to each channel side with automatic temperature setpoint control from tri-level thermocouples placed ahead of the spout entrance.

Dry electrode design is critical to ensure satisfactory long-term operation and to prevent glass defects and refractory erosion. In their simplest form dry electrodes can be a section of G.M.E grade molybdenum connected via a thread to a piece of stainless steel or Inconel. This concept relies on the junction between the two dissimilar materials being at a point where the glass temperature is low enough to create a cold glass seal thereby preventing oxidisation of the molybdenum. However, there is significant risk of galvanic reaction at the junction of the two materials leading to the generation of small DC voltages which can cause the glass to disassociate creating bubbles of pure oxygen.

These bubbles will of course impact production yield, but a more serious and often overlooked problem is the risk of oxidisation of the molybdenum electrode at its junction with the stainless steel which if left unchecked will lead to electrode failure, increased localised heating, and accelerated refractory wear.

The approach used in our dry electrode design is different and ensures that the entire current path from electrical connection to glass contact is through the molybdenum. The protective stainless-steel sheath of our design is electrically isolated from the molybdenum ensuring no dissimilar metal contact in the current path. Their use is not limited to our own systems, and they are widely used by glass makers looking for a better dry electrode solution in their own and other suppliers’ systems.

CONVERSION FROM GAS TO ELECTROFLEX ALL-ELECTRIC.

Whether planned for the cold repair of an existing distributor or forehearth system, or for a new build, it is a very quick process for us to calculate energy consumptions, operating COST savings, capital costs and associated payback times of adopting the Electroglass all-electric solution. Energy cost savings of between 60% and 90% are typical.

On one current project for example, it was quickly shown that savings of 90% in overall operating cost would be achieved. As in almost every such conversion project from gas to all-electric conditioning we will maintain the widths and lengths of the existing gas heated system and will reuse existing support steel and casings.

In this particular case the calculated energy cost savings of £750,000.00 over a typical campaign are for just one forehearth and if so desired can have the conversion carried out during furnace operation offering immediate savings whilst totally removing the use of fossil fuels.

These savings are for a single forehearth, imagine the savings that could be realised by converting the distributor and all other forehearths.

NOT ALL ALL-ELECTRIC FOREHEARTH DESIGNS ARE EQUAL.

The concept of electric heating of distributors and forehearths is far from new, – Electroglass has been developing, designing, and supplying this technology to all sectors of the glass industry for over 45 years. There are however important differences in the concepts that various designers have  used which can significantly affect operating cost, energy consumption, thermal homogeneity and operating stability.  

As an example, a recent comparison showed that when comparing an Electroflex All-Electric design to another all-electric design the energy inputted could be as much as 550% higher!

CONVERT NOW, – NO NEED TO WAIT FOR A MAJOR REPAIR.

Where converting from gas to all-electric makes financial and operational sense it can be done relatively easily during furnace operation without the need to wait for a major furnace stoppage or repair. Most conversions can maintain the existing steel support structure, casings, substructure, and glass contact material. Work to replace the superstructure with a special low thermal mass design, install the heating element zones and damper systems can be carried out following a controlled cool down, with production restored in a matter of weeks.

This means that the many benefits including energy cost savings, improved homogeneity, simplified operation, and minimal maintenance requirements can be realised now.

ABOUT THE AUTHOR:
Grahame Stuart is Technical Sales Manager at Electroglass Ltd.