The emissivity of
a substance is a measure of how effectively its surface emits radiation, compared to an
ideal radiator. The emissivity of ash deposits is an important property in determining the
chemical composition of an ash deposit. The figure below was obtained through the use of
an FTIR-emission-spectroscopy diagnostic.
This graph plots the emissivity of black liquor deposits as a function of wavenumber and deposit thickness and accumulation time. This portion of the emissivity spectrum indicates the presence of two sodium salts, sodium sulfate and sodium carbonate, that dominate the deposit composition. Gas-phase water interference is evident in one of the broad peaks.