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Hearth furnace

Process description

Frits are prepared mainly by melting raw materials in a tank melting furnace, at high temperatures of 1350 up to 1550 ºC. The material is charged in the rear wall by a screw charger forming a pile, melted and then quenched in water, thus turned into a solid, insoluble, fragmented material. In comparison to the glass industry are these tanks rather small 8-15m² producing app. 20 t/d in average. The combustion air has a low recuperative air preheat of 350 - 500 °C.

Gas application

In addition to traditional natural air-gas combustion, a significant number of furnaces use oxy-fuel combustion (mainly in Italy and recently in Turkey), accounting for about 30% of the total furnaces in Europe. The enrichment of combustion air with variable amounts of oxygen, in order to provide a higher temperature in the melting furnace and, eventually, increase the production rate, is widely used particularly in Spain. Oxy-fuel fired furnaces show lower energetically values. Due to decreasing pollution levels most of the new furnaces are operated by oxyfuel. Reasons for oxygen enhanced combustion are:

  • The nitrogen is eliminated from the combustion process
  • Exhaust gas volume reduced by 75%
  • Less heat loss through the exhaust system
  • Less batch loss by the lower exhaust velocity, less dust emission
  • Flue gas channel area can be reduced
  • Higher flame temperature
  • Better heat transfer by the 3 atom molecules taking place in the combustion
  • Better heat distribution
  • Higher pull rate (up to >200%)

Oxygen enrichment

In the combustion process of the hearth furnace, the supplied oxidizing agent can be enriched with oxygen, whereby the flow rate of oxygen as a function of exhaust gas temperature and / or other process parameters are specifically controlled.

Oxyfuel combustion

Air-fuel burners are replaced by oxyfuel burners using pure oxygen. Additionally oxyfuel burners are used to regulate the heat of the flow stream. Converting these furnaces to oxy-fuel results in:

  • Substantial capacity increase (up to double)
  • Better product quality
  • Lower emission values


Examples for the enhancements of capacity and the reductions of emissions in the case of a hearth furnace are shown in the table below. The data is derived from a frit production unit in which recuperative air-fuel burners were replaced by oxyfuel burners of the Oxipyr-type.

Results of the replacement of air-fuel by oxyfuel technology in a hearth furnace.

Oxipyr Burners:

  • Oxipyr - F
  • Oxipyr - Flex3
  • Oxipyr - P / P LON

Oxipyr - Burner Regulation:

  • Oxipyr - Advanced regulation
  • Oxipyr - Basic+, Basic regulation
  • Oxipyr - 400 regulation