Solutions for 'green' glassmaking

In the Glass International February 2019 issue (p.46-47), Dr. Michel Gaubil* and Dr. Diane Nicklas** highlight the opportunities that refractories provide for environmentally conscious glassmaking.

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In addition to innovation and enhanced performance at optimised costs, the glass industry is also striving for an ecological set-up and footprint for glass melting furnaces.

The environmental importance of lower emissions is steadily increasing. In Europe, as well as the rest of the world, environmental regulations for CO₂ and NOx are becoming tighter.

Additionally, there is a global focus on the ‘after life of refractories’ when the furnace’s lifecycle has reached its end. Some materials must be classified as hazardous waste.

Refractory solutions actively support these ecologically responsible trends, and offer various possibilities for the modern glass industry.

CO₂ and NOx emissions

Several well-known furnace construction concepts and technologies contribute to reduced CO₂ or NOx emissions.
Electrical boosting, greater insulation at both glass contact and superstructure, oxy combustion technology and high performance regenerators are some of the concepts and technologies that have led to higher refractory benchmarks.
Better thermal insulation results in reduced thermal losses and a better efficiency of the furnace, but also can result in higher running temperatures, increased exudation and increased corrosion.
There are several highly qualified products in the refractory portfolio that meet all those requirements. The use of low or extra-low exudation fused cast AZS in combination with high alumina and/or high zirconia fused cast materials has proven to perfectly match with high temperature furnace conditions (Fig 1).
The deep understanding of the specific corrosion processes allows the design of superior refractory shape and material combinations. (Fig 2):
The latest solutions for tuckstones will be composite ceramics with high compression resistance and low thermal conductivity (Fig 3a and b). They contribute to avoid thermal losses in the cooled area and are much less sensitive to cracking than conventional material. Consequentially the stability of the superstructure and also the thermal protection of the below located soldier blocks increases and contributes to a longer furnace lifetime.
Electrical boosting directly results in a higher temperature at the bottom of a glass furnace, in parallel with an increased convection flow rate of the melt. Using fused cast tiles is the well-known answer to those challenges. The application of a complete refractory solution system is the advised progression.
Void free fused cast tiles with extra tight joints (>0.5mm) offer a higher quality of the assembly and better corrosion resistance than other solutions.
In combination with the adapted mortars, and insulation tiles underneath, this solution becomes predominant for the application under higher thermal threats (Fig 4).

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Harmonised dilation

A harmonised dilatation of the various layers of tiles and mortars results in a secured solution to avoid unexpected glass infiltration.

Specific glasses with high electrical resistivity, or in the case of high current density, may necessitate the use of fused cast refractory solutions particularly designed for those extreme conditions: doped high zirconia. In combination with fused cast AZS for the bottom, these materials are the premium choice for electrodes.

Glass furnaces with regenerative technology recover a substantial part of their fume energy. Layout and material combination of the regenerators can obviously drive furnace efficiency to excellence and contribute to reduced CO₂ or NOx emissions.

The refractory industry has a range of sintered or fused cast products to realize various channel shapes and chemical compositions of the checker packs. The choice depends on running conditions, fumes and air flow as well as the fuel type and consumption.

All those parameters influence multiple corrosion processes which are complex and vary from top to bottom of the furnace, following the thermal gradient and fume cooling temperature: from vapor corrosion at the top of a regenerator, over slag condensation in the middle range to solid dusts in the bottom zone. Refractories for the regenerators have to be carefully chosen to optimize the design. This can be tailor-made to the specific conditions. The right choice is facilitated by thermal studies offered on the market. Specialized software allows the simulation of alternative configurations (Fig 5). This service is usually completed with a detailed interpretation and consulting for the glassmaker. This helps understand, in the end, what is necessary to obtain the best regenerative performance, adapted to the individual situation of the furnace.

Oxyfuel combustion also leads to a lower carbon CO₂ footprint and low NOx emissions. This technology induces comparably high running temperatures combined with high water vapours and alkaline concentration in the fumes. Refractories must withstand these conditions, particularly in the crown.

The first choice for crowns in oxyfuel combustion are fused cast refractory solutions based on low exudation-AZS materials in combination with fused cast high alumina (Fig 6). An assembly with tight specifications ensures the required corrosion and creep resistance properties of the furnace crown.

The ‘after-life’ of refractories

All types of glass furnaces share one phenomenon for their refractories: at the end of the production lifecycle a high quality product becomes waste, in some cases even hazardous waste.

Over the years there are several established providers in the market offering demolishing and waste evacuation, and removal services. Some of them offer the revalorization of waste materials, that are then transformed and recycled into new raw materials.

The ecological awareness and responsibility of the glass industry for the ‘after-life’ of their process materials becomes evident if refractories might be classified as hazardous waste at the shut down time of the furnace, as it might be the case for materials that contain chrome oxides.

The conscientious exposure with the questions of what happens to those materials does not stop after they are evacuated and removed from the site. Many sustainability charters of the modern glass include the treatment of waste material and drive the ecological responsibility further forward.

Those questions become particularly sensitive in countries where legislation holds the furnace owner responsible even beyond the evacuation of the waste materials. In those areas it becomes crucial to find a service provider that grants an approved utilisation.

Conclusion

As with many other industries, the glass industry is going through a transition with an environmental focus. Furnace efficiency and the consequent CO₂ and NOx emissions are in focus even more than in the past. A conscious treatment of waste materials also plays an increasing role in the sustainability strategies for many companies.

Refractories and refractory services can assist these trends and support them with the latest technologies and innovative developments.

*SEFPRO, Le Pontet, France
**SEPR Keramik, Germany