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Natural gas turns the lights on Glashütte Limburg manufactures exclusive interior luminaires with natural gas

For industrial enterprises, natural gas is often an indispensable source of heat delivered in exactly the right volume. The glassmakers at the glassworks in Limburg in Hessen have also opted for natural gas for heating the smelting furnaces and for cooling. For natural gas provides exactly the amount of heat required, which means it can be used very cost-efficiently compared to other sources of energy, and has benefits for the entire production.

Glashütte Limburg has been making high-grade glass from sand, potash, lime and soda for almost 70 years. The glassmakers mould designer luminaires of high quality from these materials. The manufacture’s recipe for success is based on traditional craftsmanship, patience and highly efficient technology.

Glass production at 1,520 degrees

The manufacture of glass is a very complex process – and barely imaginable without natural gas. The glassworks consumes about 30 million kilowatt hours of this energy a year. Around 400 tons of sand from the Rhine valley are smelted every year in a melting tank using natural gas. The process requires extremely high temperatures, which can be generated the fastest and most cost-effectively with natural gas. The mixture of raw materials added to the furnace via a funnel reacts at 1,520 degrees Celsius. It disintegrates into a fluid glass mass that flows into the working end through a floor channel. There it cools down to about 1,200 degrees Celsius and becomes viscous.

The glassworks consumes about 30 million kilowatt hours of this energy a year.

Around 400 tons of sand from the Rhine valley are smelted every year in a melting tank using natural gas.

The mixture of raw materials added to the furnace via a funnel reacts at 1,520 degrees Celsius.

Gruelling craftsmanship

Now the really hard work by the glassmakers begins. Around 40 craftsmen work round the hot smelting furnaces, either as glassblowers or in the mechanical glass manufacturing operations. The manufacture of opal glass – a speciality of Glashütte Limburg that requires high-precision handiwork – is both complex and fascinating at the same time.

The glassmaker winds a certain amount of the viscous crystal glass from the working end around the blowing iron. Then he blows the glowing mass into a glass body. With the addition of air, the glass blank enlarges, reduces its thickness and cools down to about 600 degrees Celsius.

Then the pipe is dipped in opal glass, which is wrapped round the crystal glass and distributed evenly with the help of a wet shaping block. This raises the temperature of the glass again to about 840 degrees Celsius.

In the last stage the glass blank is given another clear glass coating and its final shape. The glassmakers have to concentrate very hard here because they only have a few seconds for this stage of the process. The hot and still shapeable glass is quickly put into an iron mould and blown by mouth and with compressed air. In the cast-iron mould, the glass cools down to 600 degrees Celsius and can be removed from the mould in the solidified form. 

1. Around 40 craftsmen work round the hot smelting furnaces, either as glassblowers or in the mechanical glass manufacturing operations.

2. The glassmaker takes the liquid glass mass from the working end and winds this by rotating the blowing iron. This is very strenuous and physically demanding work.

3. By blowing the glass body the glass blank enlarges and cools down to around 600 degrees.

4. The glassmakers use wet shaping blocks to shape the glass.

5. If the breath of the glassmaker is not enough, the glass is blown with compressed air. The use of compressed air is particularly vital for larger products.

Clean combustion with natural gas

The smelting furnace is the centrepiece of the production facility and runs 365 days a year round the clock. Shutting down the furnace would be fatal because then the liquid glass mass would cool down and solidify in the melting tank.

“That would make the melting tank unusable and it would have to be replaced. That would be pretty costly, especially with the production losses incurred. But with natural gas we can maintain the high temperatures constantly,” explains Harald Merz, glass operations manager at Glashütte Limburg.

Natural gas cooling for the hot glass body

The glassworks also take advantage of the benefits of natural gas for cooling the solidified glass bodies. Since cooling down the material abruptly would cause stress fractures, once it has been shaped by the glassmakers, the glass is slowly cooled down to about 40 degrees Celsius. To do so it is placed on cooling belts.

The glass bodies now run on a conveyor belt through a furnace where there are various temperature zones. The provision of heat in the individual combustion chambers in exactly the right amount is important for the quality of the product and this natural gas can perform this task. The glass is carefully cooled down to the right temperature. Depending on the thickness of the glass, this process takes up to six hours. Then the glass is inspected, sorted into types of product, and, following another visual check, is sent to the assembly department where the staff mount the luminaires.

“Natural gas has clear economic benefits“ Interview with Harald Merz, works manager

What challenges do you face in glass production?
Producing glass is a complex and energy-intensive process. The melting tank is heated 365 days a year. That´s why constant heat supply is so important. Besides the right temperature, skilful craftsmanship is required. A speciality of Glashütte Limburg is the three-ply cased opal glass (the opal glass is cased between two plies of crystal glass). The production of this valuable glass is very challenging for the glassmakers and requires precise working under time pressure.

Where do you use natural gas during the manufacturing process?
Natural gas is used at different pressure stages during the manufacturing process, mainly in the melting tank. Thanks to the pin-point heat of natural gas, we are able to melt the raw material mix of sand, potassium carbonate, and lime at 1,520 °C into a molten substance. The working ends, from where the glassmakers take the liquid glass to start forming it, are also natural gas-heated. In addition, we use natural gas to run our annealing lehrs to slowly cool down the hot glass. But we also use natural gas the traditional way: to generate heat energy and for process heating.

What are the advantages of natural gas in manufacturing processes?
The advantage is that burning natural gas in the place of other fossil fuels does not emit nearly any harmful pollutants. That´s why our furnaces have low coke rates. Besides, there are clear economic benefits: natural gas is significantly cheaper than other energy sources. And, since we are directly connected to the energy grid, we don´t have any storage costs.

Are there any other ways you use natural gas?
Yes. In summer, we use the waste heat of our melting tanks and working ends to feed our heating system and to cover our needs for process heat.  Normally, we use a gas-heated boiler for these purposes, with a steam output of 1.4 metric tons per hour. During summer, however, the boiler runs in stand-by mode because the waste heat passing through preheater and heat exchanger is converted into steam. This helps improve boiler efficiency and save money. During summer time, we are practically self-sufficient in heating.