Glass LCA: cullet ratio, furnace campaigns and the transport paradox

In glass production the melting furnace accounts for the overwhelming majority of energy consumption. A batch of silica sand, soda ash and limestone is melted at around 1500 °C, and the furnace runs continuously for years. That continuity is both the sector's strength and its constraint: stopping and restarting is expensive, so efficiency improvements are usually timed to furnace rebuild campaigns.

Cullet: the single strongest lever

Cullet is recycled glass added to the batch, and it delivers on two fronts. First, cullet melts at a lower temperature than raw batch; each ten per cent increase in cullet saves roughly two to three per cent of melting energy. Second, and more importantly, using cullet reduces soda ash and limestone consumption.

That second point is often missed. Soda ash and limestone calcine during melting and release process CO₂ — a chemical emission independent of fuel. It can only be reduced by lowering the raw material share in the batch, not by furnace efficiency. Cullet targets that line directly.

Container glass versus flat glass

In container glass, cullet can exceed seventy per cent, and higher still for coloured bottles because colour tolerance is wide. In flat glass, optical quality requirements limit external cullet; usually only internal process scrap is fed back. That structurally separates the footprints of the two sub-sectors.

• Container glass — high cullet, lower melting energy, dependent on the collection system
• Flat glass — low external cullet, quality constrained; coatings add process burden
• Colour separation — mixed-colour cullet is unusable outside green glass
• Contamination — ceramic, stone and porcelain residue renders cullet unusable
• Collection rate is regional and directly determines the result

The transport paradox

Glass is heavy, and that is a cost across the whole life cycle. Moving a litre of beverage in a glass bottle takes markedly more fuel than the same beverage in plastic or carton. The environmental case for glass rests on recyclability and refillability — both of which depend on local infrastructure.

In refillable systems the equation changes entirely. If a bottle is refilled twenty times, production impact is divided by twenty and glass wins almost every comparison. But that requires collection, washing and short-distance distribution infrastructure to exist.