Einfluß der Prozeßbedingungen auf die Verdampfung bei Kalk-Natron- und Borosilicatgläsern

The evaporation of sodium and boron species from the melts in industrial glass furnaces leads to emissions of particulates (dust) and to furnace atmospheres containing reactive evaporation products. These reactive species, especially alkali vapors, can react with the superstructure refractories causing attack on or corrosion of these materials. The vapors form condensation products during the cooling process in the flue gas channels, recuperators or in regenerators, causing deposition of salt. In some cases, this deposition leads to reduced heat transfer in these heat exchangers or blockage of the flue gas channels. The evaporation processes at the glass melt surface include: - direct evaporation of glass components; - evaporation of compounds formed by a reaction of different glass components in the melt; - evaporation of compounds formed by a reaction of a glass component with gases in the furnace atmosphere. In the last case, the composition of the furnace atmosphere will influence the vapor pressures of the volatilized compounds. Diffusion of gases from the melt into bubbles or evaporation into gas bubbles may take place, especially during the primary fining process. This hardly contributes to the total evaporation losses of boron and alkali species, but it is essential for the SO2 release from the melt during fining. Under industrial conditions, gas flows above the melt may be high. Experimental investigations show that evaporation kinetics are enhanced by higher gas flows especially for evaporating components having high concentration levels in the melt. For minor but volatile components in the melt, diffusion from the bulk of the melt to the surface may limit the evaporation rates, depending on diffusion coefficient and convection flows. Evaporation is drastically enhanced by high glass melt surface temperatures, high gas velocities and high activity coefficient values of components in the melt. The presence of both boron and alkali in melt gives high activity coefficient values for alkali borates in the melt, which leads to high vapor pressures of these compounds. Increasing water vapor pressures will enhance evaporation of HBO2 and NaOH or KOH but will hardly have an impact on KBO2 or NaBO2 evaporation. This means that high water vapor pressures in the furnace atmosphere, as encountered with oxygen firing, will increase NaOH, KOH or HBO2 vapor pressures, however for sodium borosilicate glasses, NaBO2 is the major evaporating species. NaBO2 evaporation is not directly influenced by water vapor. In oxygen-fired furnaces combustion gas volume flows are much less than in air-fired furnaces, the lower gas velocities will suppress the evaporation process. Initially, a high evaporation rate may lead to depletion of volatile components at the surface of the melt. This depletion will slow down the evaporation process from stagnant melts as time proceeds. Thus there may be compensating effects for NaOH, KOH and HBO2 evaporation, but for NaBO2 or KBO2 volatilization, oxygen firing will lead to lower specific (per unit quantity of molten glass) evaporation losses of these components

Evaporation in industrial glass melt furnaces

Process conditions in glass furnaces, especially the settings of the combustion system, determine the rate of the evaporation processes at the glass melt surface. Reduction of volatilization to lower dust and heavy metal emissions, to minimize the refractory attack by the aggressive volatile components, and to limit depletion of volatile glass components at the glass melt surface is of great technological importance. This can be achieved by changes in burner design and burner positioning, optimizing combustion control and avoiding extreme glass surface temperatures. Such high local temperatures at the glass surface may lead to very high concentrations of PbO, NaOH or KOH vapors attacking the crown materials of the furnace. Evaporation model studies show the potential of process measures for the reduction of evaporation and minimizing depletion of alkali, lead or boron compounds at the glass melt surface. Sodium depletion down to 80% of the original content can take place at the glass melt surface, in lead silicate melting processes depletion may reduce the lead concentrations by more than 50%. Industrial tests support the results of modeling studies and show the effects of the settings of the combustion processes on glass-melt evaporation kinetics

Evaporation in industrial glass melt furnaces

Process conditions in glass furnaces, especially the settings of the combustion system, determine the rate of the evaporation processes at the glass melt surface. Reduction of volatilization to lower dust and heavy metal emissions, to minimize the refractory attack by the aggressive volatile components, and to limit depletion of volatile glass components at the glass melt surface is of great technological importance. This can be achieved by changes in burner design and burner positioning, optimizing combustion control and avoiding extreme glass surface temperatures. Such high local temperatures at the glass surface may lead to very high concentrations of PbO, NaOH or KOH vapors attacking the crown materials of the furnace. Evaporation model studies show the potential of process measures for the reduction of evaporation and minimizing depletion of alkali, lead or boron compounds at the glass melt surface. Sodium depletion down to 80% of the original content can take place at the glass melt surface, in lead silicate melting processes depletion may reduce the lead concentrations by more than 50%. Industrial tests support the results of modeling studies and show the effects of the settings of the combustion processes on glass-melt evaporation kinetics

Evaporation experiments and modelling for glass melts

A laboratory test facility has been developed to measure evaporation rates of different volatile components from commercial and model glass compositions. In the set-up the furnace atmosphere, temperature level, gas velocity and batch composition are controlled. Evaporation rates have been measured for sodium, potassium, boron and chloride species released from different glass types. From mass transfer relations, derived from computational fluid dynamics (CFD) modelling and experiments, and the measured volatilisation rates, information on the chemical activities of the volatile glass components can be derived. The measured evaporation rates or adapted mass transfer relations and chemical activities can be applied in mathematical models of evaporation. Such models have been developed to simulate evaporation processes in industrial glass furnaces and to estimate the resulting emissions. Besides evaporation and emission rates, depletion of volatile components at the glass melt surface has been investigated. The models enable evaluation of changes in combustion chamber design or the effect of burner type on the evaporation rates and emissions of particulates and boron compounds

Future energy-efficient and low-emissions glass melting processes

All over the world, there is an increasing drive to develop new technologies or concepts for industrial glass melting furnaces, with the main aim to increase the energy efficiency, tabilize production and reduce emissions. The application of new process sensors, improved furnace design, intelligent control strategy and waste gas heat recovery systems support the glass manufacturers to achieve ever increasing requirements concerning energy efficiency and emission limits. Glass industries are searching for breakthrough innovations (revolution), but introduction of such major changes in industry is hampered by large risks and investments. Therefore most companies prefer a stepwise improvement (evolution) based on existing concepts, such a regenerative glass furnaces. When different technologies are combined, it will even be possible, in specific cases, to avoid the use of flue gas scrubbers or DeNOx systems. An overview of requirements for industrial glass furnaces is given concerning melting characteristics and performance of the furnaces. A short overview of evolutionary and revolutionary developments in industrial glass melting processes will be presented. In this paper, an example of an optimized, regenerative end-port fired glass furnace concept, based on a combination of current (BAT) technologies, will be given in more detail. Basic innovative elements of such a furnace will be shown and its expected performance

Mass transfer relations for transpiration evaporation experiments

Transpiration evaporation experiments are often used to study evaporation kinetics from liquids or melts. The mass transport of volatile species in a transpiration experiment depends among others on the flow conditions of the carrier gas in the tube and on the geometrical configuration. For a transpiration test set-up, CFD modelling showed to be an excellent tool to predict the mass transport of volatile species into a carrier gas and to understand the fluid dynamics in the gas phase and distribution of volatile species in this phase. Relatively simple mass transport relations were obtained for a fixed geometry of the transpiration test set-up. These relations proved to be also applicable for different volatile species and different temperature ranges