Correlating the melting rates of feeds in electric melters with results of simple laboratory experiments can help evaluate melter feed additives and their effects on melting rate, and support the feed scheduling and plant operation. A recently proposed melting rate correlation (MRC) equation, relating the melting rate to melt viscosity, feed‐to‐glass conversion heat, and cold‐cap bottom temperature, was tested using data from experiments covering various feed compositions and melter operating parameters. The MRC equation is shown to reasonably represent the measured data and thus can be used to quantify how individual variables (melt viscosity, cold‐cap bottom temperature, conversion heat, melter operating temperature, and bubbling flux) affect the glass production rate.
Conversion of melter feed to glass occurs in the cold cap that floats on the melt pool in a nuclear waste glass melter. The conversion rate (the melting rate or the glass production rate) is controlled by the heat flux delivered to the cold cap from the molten glass. In an attempt to analyze the intricate relationship between the rate of heating, the feed foaming response, and the rate of melting, we measured the change in feed volume at different heating rates by using several melter feeds known to exhibit a wide range of melting rates under identical melter operating conditions. As expected, the maximum foam porosity increased as the heating rate increased.However, contrary to expectation, the temperature at which the foam reached maximum volume either decreased or increased with the heating rate, depending on the feed composition. A change in maximum foam temperature from the feed volume expansion test indicates a similar change of the cold-cap bottom temperature, which influences the heat flow to the cold cap, and thus the rate of melting.
K E Y W O R D Sfeed-to-glass conversion, foaming, heat flux, melting rate 402 | LEE Et aL.
Reductants, such as sucrose (C12H22O11), are added to nuclear waste melter feeds containing high fractions of nitrates and nitrites to reduce excessive foaming during feed‐to‐glass conversion, decrease sulfate segregation, and increase technetium retention. The effect of sucrose on foaming and melting reactions during the conversion was examined using the feed volume expansion test, thermogravimetric analysis, evolved gas analysis, x‐ray diffraction, and scanning electron microscopy with energy dispersive x‐ray spectrometry. Different amounts of sucrose were added to vary the carbon to nitrogen (C/N) ratio in the melter feed. As the C/N ratio increased, the extent of foaming decreased, and the N2/NO ratio increased in the evolved gas. Significant foam suppression, rapid gas release at approximately 250°C, and reduction in transition metal oxides were observed at C/N > 1.1.
In electric melters, the conversion heat is transferred through the foam layer at the cold‐cap bottom. Understanding cold‐cap foaming is thus important for enhancing the efficiency of both commercial and waste glass melters as well as for the development of advanced batch‐to‐glass conversion models. Observing foam behavior is still impossible “in situ,” that is, directly, in glass melters. To investigate the feed foaming behavior in laboratory conditions, we employed the feed volume expansion test, evolved gas analysis, and thermogravimetry. Combining these techniques helps assess the cold‐cap bottom temperature that directly influences the temperature gradient at the melt/cold‐cap interface, and thus the rate of melting. We also discuss the behavior of cavities formed by coalescing primary foam bubbles and ascending secondary bubbles.
Mathematical models of glass melting furnaces are incomplete in the sense that they do not estimate the rate of glass production (the rate of melting). Instead, they attempt to optimize melter efficiency and product quality for a specified production rate with other experimentally measured data. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] The melting rate correlation (MRC) attempts to bypass this
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