Grahame Stuart* discusses the role all-electric forehearths could play in the transition to Net Zero glass manufacturing by replacing gas heated systems. He explains how this will not only reduce carbon emissions, but operating costs too.

Much has been written and presented about the melting stage of the glass making process and how shifts in technology and the application of hydrogen and hybrid melting will help the glass industries to meet its goals of achieving Net Zero and using natural resources more sparingly. Both approaches could theoretically reduce reliance on fossil fuels and other non-renewable fuel sources, however they both still rely heavily on modified fuel fired furnace designs where thermal efficiencies of 50% or less are commonplace. In other words, regardless of fuel type, over half of the energy applied is not used for melting, but simply lost as waste heat and structural thermal losses.

Concentrating on furnace design and in particular a move away from fuel fired melting to high efficiency cold-top all-electric melting will no doubt bring medium to long-term improvements. There are however solutions elsewhere within the overall process that can offer major benefits, both in terms of carbon reduction and operating cost savings now. One such area is the conditioning of the glass within the distributor and forehearth system where proven technology already exists and can be employed in a considerably shorter timeframe.

Distributor channels and forehearths used by container glass manufacturers have traditionally been gas heated and in terms of energy usage are extremely 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.

Gas heated forehearth systems require 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 highly inefficient and ultimately rely on non-renewable, polluting fossil fuels.

For many years certain designs of all-electric distributors and forehearths have demonstrated proven performance in terms of both energy usage and cost reductions, as well as improvements in thermal homogeneity and simplification in operation. Applications have ranged from soda-lime container glass to aggressive specialist glasses such as borosilicate and fluoride opal for tableware, cookware, and containers.

In recent times the target of reduction or elimination of fossil fuel use has driven a greatly increased interest in high and very high-capacity all-electric distributor and forehearth 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.

Care must be taken when considering a switch to all-electric forehearths in the same way caution is required when looking to hybrid melting as simply modifying existing fuel fired systems is unlikely to give the results envisaged or required.

For many the move to all-electric 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 optimal 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 the 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 special profile heating elements to apply heat along the channel sides where the glass is typically coolest 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.

As already mentioned, 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.

Although it has often been considered that the replacement of a forehearth design or concept is something to be tackled at a major furnace repair or shutdown, views are now changing as the cost savings and environmental benefits cannot be ignored or delayed. On one current forehearth undergoing study, it was quickly shown that savings of 90% in overall operating cost would be achieved which equates to savings of £750,000.00 over a typical campaign. 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.

These savings are for a single forehearth; it is easy to imagine the savings to be realised by converting the distributor and all other forehearths.
Where, as above, the cost savings are significant and the forehearth design allows, it is possible to install our design of Electroflex forehearth during a short shutdown of individual forehearths, without waiting for a major repair. Such an operation typically requires a forehearth stoppage of as little as 14 days.
In the week or so leading up to the stoppage, new power and control equipment will have been installed and the required thermocouple and power cables run. The Electroglass power and control system is a modular design whereby all control and comms are run via network cables from the power racks to the control panel. An internal main isolator and busbar systems mean only a single set of incoming power cables will be required. For single forehearths a blind interface allows the system to be viewed, monitored, and controlled over the factory network via a web browser or where multiple forehearths are being added, or will be added in the future a new SCADA control system can be included. There is also the option of monitoring and controlling the forehearth on the customers’ existing factory computer system.

Once the power and control equipment has been successfully installed and tested the forehearth can then be stopped. The existing superstructure, insulation, glass contact and substructure material are then removed. The steel casings and spout remain in place and our new design will maintain the existing gob drop point. In some cases, it may not be essential to replace the glass contact and substructure refractories.

Certain modifications are then made to the casings to accept any superstructure steel, damper mechanisms and Temptrim electrodes required for the new build.
New substructure (where applicable) and superstructure materials of Electroglass design are then installed. Superstructure bracings, damper assemblies, and busbars are then added.

Lastly, heating elements, thermocouples and safety guarding is installed making the forehearth ready for heat-up. Heat-up times vary depending on glass contact material, but typically range from between 4 and 7 days.

Once back in operation an Electroglass engineer will carry out final commissioning, customer training and remain present for several days whilst the forehearth is brought into full, normal operation and the customer can begin benefiting from the significant energy cost savings, improved thermal homogeneity, simplified operation, and minimal maintenance requirements of our Electroflex design as well as removing reliance on fossil fuels in this important area of their operation.

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 between an Electroflex All-Electric design and an alternative all-electric design showed the energy required for the alternative could be more than 5 times higher! The role that converting to well-designed all-electric distributors and forehearths has to play in the future of a Net Zero glass manufacturing industry is clear, and in most cases this will come with the additional benefit of hugely reduced operating costs.

ABOUT THE AUTHOR:

Grahame Stuart is Project Sales Engineer at Electroglass Ltd.

19/12/2023

As published in Glass International.