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.
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%)
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.
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 - F
- Oxipyr - Flex3
- Oxipyr - P / P LON
- Oxipyr - Advanced regulation
- Oxipyr - Basic+, Basic regulation
- Oxipyr - 400 regulation