Investigations into the Numerical Modelling of Oxy-Fuel Combustion in Glass Melting Furnaces

Authored by: Jörg Leicher and Anne Giese

Corresponding Author: Jörg Leicher

Gas- und Wärme- Institut Essen e.V.
Industrial Combustion Technology
Hafenstrasse 101
45356 Essen
Germany

A mail can be sent to the Corresponding Author at leicher@gwi-essen.de

The glass industry has an interest in reducing NOx formation in furnaces.  Given that secondary measures to filter NOx from the flue gas are expensive, oxy-fuel is one possible approach.  The article describes a recent research project in which Gas- und Wärme-Institut Essen e. V used oxy-fuel combustion in a glass melting furnace to obtain improved design criteria for purpose-built oxy-fuel glass melting furnaces.   The broader question being asked is whether a standard CFD code with reaction packages validated under air-firing can still give good results for oxygen-firing.  Five burners were tested in a semi-industrial scale furnace.  The major gas species (CO, CO2, O2, NO, NO2) were measured on the burner plane in the furnace giving detailed profiles of gas concentrations.  These measurements are compared to the results of CFD simulations using a standard package and six reaction schemes.  Whilst comparison of the simulation and measurement results shows that many combustion models commonly used in CFD codes seem to be unable to adequately describe oxy-fuel combustion and that further research is necessary, the article tracks significant progress in this direction.  The praxis of industrial combustion is very well served by critical investigations like this one.

Key Words:

Oxy-fuel combustion, glass melting furnace, numerical modelling, CFD

Summary:
Melting glass requires a lot of energy, which means that process temperatures of 1600 °C and more are necessary in a glass melting furnace, depending on glass quality.  In conventional plants, these high temperatures are usually obtained by the combustion of natural gas with strongly pre-heated air.  Air pre-heat temperatures may reach up to 1400 °C in regenerator furnaces which, in combination with near-stoichiometric combustion in the furnace, often lead to high emissions of nitrous oxides (NOx).  The glass industry is therefore very much interested in technologies which reduce NOx formation in the furnace itself, as secondary measures to filter NOx from the flue gas are expensive.

Oxy-fuel is one possible approach to achieve this: air is substituted by almost pure oxygen as the oxidizer, thus providing high flame temperatures while eliminating the main source of nitrogen in the system.  This is different from oxy-fuel combustion in power plants where the main focus is to facilitate CCS by replacing combustion air with a mixture of recirculated flue gas and oxygen.

There are many oxy-fuel glass melting furnaces already in operation, but most of these are based on conventional air-fired furnace designs.  In the course of a recent research project, Gas- und Wärme-Institut Essen e. V. (GWI) investigated the use of oxy-fuel combustion in such furnaces in order to provide improved design criteria for purpose-built oxy-fuel glass melting furnaces by both experimental and numerical means.  The comparison of the simulation and measurement results shows, however, that many combustion models commonly used in CFD codes seem to be unable to adequately describe oxy-fuel combustion.  Only a numerically expensive EDC approach yields a reasonable agreement with the experiments, though at a significantly increased numerical effort.  Further research is necessary to provide a means with which oxy-fuel combustion processes can be modelled accurately at reasonable numerical cost.

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