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The high waste gas temperatures of glass furnaces lead to high energy losses. The batch and cullet (b & c) preheating (system Nienburger Glas) is a well-tried way to reduce these energy losses and thus the corresponding emissions of CO2 and NOx.
The first plant was installed at the end of 1987 at a 300 t/d green glass tank in Nienburg (furnace No. 4) and operated until October 1999. After the cold repair of the furnace and some improvements the preheater started again, but now with a capacity of 350 t/d.
Further units were installed for flint glass including an endfired furnace. Early in 1997 the first b & c preheater for an oxy-fuel flint glass tank went into service.
The following table shows the installations and the operating data.

In addition to energy savings there are significant environmental benefits associated with b & c preheating.
-The savings in fuel lead to a reduction of the NOx and CO2 mass flow rates.
-Part of the SOx, HCl and HF in the waste gas is separated as a result of the direct contact between the batch and the flue gas.
-There is about 80 % recovery of selenium which is used as a decolorizer in flint glass production.

The hot flue gases from the glass furnace travel cross-counter-flow from the bottom to the top of the preheater. A part of the heat content of the flue gases is transferred to the slowly downward flowing glass raw material which is dried and then subsequent heated from ambient temperature to about 300 °C. It is then charged into the glass tank from the bottom part of the preheater via conventional batch chargers.
The flue gases flow through the preheater within ducts open at the bottom, allowing direct contact with the batch. The ducts are arranged in multiple layers and connected inside in the vessel. The slowly downward moving batch presents a constantly renewed surface to the flue gases, thereby ensuring a good heat and mass transfer. The heat is directly utilised in the glass melting process without intermediate heat transfer and any efficiency losses.
Due to reactions with the alkali components in the batch the concentration of acid impurities like SOx, HCl, HF will be distinctly reduced. Also the consumption of special batch components like selenium for decolorizing for flint glass production can be decreased. 
The cooled flue gas will be dedusted in an electrostatic precipitator and then released to the atmosphere via the existing stack. 
3.3 Energy saving
The energy saving rates are in the range of up to 20 %. The following diagram shows the specific energy consumption from the beginning with furnace No. 4 in Nienburger Glas (NG 4) until 2000 for all furnaces. 
One can see that furnace No. 4 started with 3,600 kJ/kg glass. The specific energy consumption has increased continuously during the 12 years operation period due to the aging of the furnace. After repair of the furnace the energy consumption is again 3,600 kJ/kg glass. 
The oxy fuel furnace No. 2 in Gerresheim (GX 2) shows the lowest energy consumption. Always at the end of the year there was a production stop or a reduced melting rate as it can be recognised by the peaks. 
For an oxy fuel furnace there is also an operation cost saving due to the reduced oxygen demand. 
In the case of green and also of amber glass it is possible to reduce the electrical boosting by using a b & c preheater, resulting in even higher reduction of operation costs and, subsequently an increased overall saving rate.
3.4 Environmental aspects 
Dust and SOx 
B & c preheating leads to a certain amount of dust carry-over because of the direct contact between the flue gas and the batch. This is separated without any problems in electrostatic precipitators and recycled to the melting process. Electrostatic precipitators were installed on each furnace in the Nienburger plants. 
Due to the direct contact between flue gas and batch material a part of the SOx is neutralized by the alkali of the batch material in the preheater. 
CO2 and NOx reductions 
B & c preheaters reduce the consumption of primary energy. The emission of CO2 and NOx are reduced in direct proportion to the reductions in energy consumption. Furthermore our clients have observed NOx reductions in the range of 10 to 30 % by using the b & c preheater. 
The combination of the b & c preheater with an oxy fuel furnace shows a dramatic reduction in NOx emissions. In the case of Gerresheimer Glas, the specific NOx emission was reduced from 4.6 kg/t glass to 0.25 kg/t glass. This figure is equivalent to 170 mg/mN3 dry with 8 % O2 content for a conventional regenerative furnace. 
The first preheater was in operation for 12 years. After repair of the furnace and some modifications of the b & c preheater the furnace started again in December 1999 with the same energy consumption as in 1987. 
Besides reducing energy consumption and CO2 and NOx emissions the Nienburger Glas b & c preheater process offers a number of other benefits: 
-Removal of SO, HCl and HF from the waste gases. 
-Recovery of selenium with flint glass production. 
-Reduction of the oxygen requirements with oxy fuel furnace. 
Modern glass furnaces using b & c preheating are exemplary regarding and economical glass production. 
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