Melting glass is a very energy intensive process, with process temperatures of more than 1600°C required to melt the raw materials in the furnace. Such high temperatures are usually achieved by intensive air preheating and near stoichiometric conditions. This leads to a significant production of nitrous oxides (NOX). As the emissions of nitrous oxides are regulated be increasingly stringent environmental legislation, the glass industry is very interested in combustion techniques which reduce NOX emissions without resorting to expensive flue gas treatment while maintaining the glass quality.
In the steel industry, the so-called flameless oxidation (FLOX) combustion concept is firmly established as a state-of-the-art primary technique to reduce NOX formation in furnaces. This technology uses high momentum jets of fuel and oxidizer to generate an intense recirculation of hot, but chemically inert flue gas into the reaction zone. By mixing flue gas into the reaction zone, its shape changes from a quasi-twodimensional flame front into a three-dimensional reaction volume. A much more homogeneous temperature distribution is obtained while the formation of hot spots can be avoided, thus significantly reducing thermal NOX emissions. The name “Flameless Oxidation” derives from the fact that no visible flame can be observed with the naked eye since the local OH concentrations are very low due to the large amounts of recirculated flue gas. Experience from the steel industry shows great promise for the introduction of this technology into other industrial sectors as a means to reduce nitrous oxide emissions.
In the course of a German research project, the Gaswärme-Institut e.V. Essen (GWI) in cooperation with several industrial partners investigated how to best introduce the flameless oxidation technique to glass melting furnaces equipped with recuperative burners, so-called unit melters. A furnace of a project partner, producing glass for compact fluorescent lamps, was chosen for conversion to FLOX burners.
Initially, there was some skepticism with regards to the applicability of a combustion process without a visible flame in a glass furnace, as normally, a slow, highly luminous flame is considered desirable in such furnaces. Also, the high gas velocities in the fuel and oxidizer jets carry the risk of blowing dust from the batch into the central recuperator. Thus, a careful design of both the new burner system as well as their positions in the furnace was necessary to avoid high gas velocities immediately above the glass bath.
In a first step, a FLOX burner system for recuperative glass melting furnaces was developed and optimized at GWI, using CFD simulations. This prototype was then tested at GWI’s semi-industrial test rig in order to verify that the new design was able to comply with the required NOX emissions limit. Compared to the burner originally mounted in the glass furnace, a reduction of almost 60 per cent was achieved.
In order to reduce the downtime of the furnace to a minimum, the exchange of the burners was planned using CFD simulations. Different configurations were simulated in order determine potential problems and an optimum burner set up was found, which was subsequently implemented on the site.
The retrofitted plant has been in operation for five years now still maintaining to produce the same glass quality as before the retrofit. The NOX emissions, on the other hand, were reduced by about 50 per cent. In addition the energy consumption of the process was reduced because an optimized burner positioning and more stable combustion allows for lower air ratios in the furnace, thus reducing fuel consumption.
