Melter Off-Gas Flammability Assessment for DWPF Alternate Reductant Flowsheet Options

Abstract

EXECUTIVE SUMMARYGlycolic acid and sugar are being considered as potential candidates to substitute for much of the formic acid currently being added to the DWPF melter feed as a reductant. A series of small-scale melter tests were conducted at the Vitreous State Laboratory (VSL) in January 2011 to collect necessary data for the scoping assessment of the impact of these alternate reductants on the melter off-gas flammability. The resulting data were analyzed in this work with the aid of modeling and re-baselined to reflect the DWPF melter operating conditions. Based on the results of these steady state analyses presented in this report, the following conclusions can be drawn:1. The baseline formic acid flowsheet feed has ~3X higher melter off-gas flammability potential than either the glycolic acid or sugar-based flowsheet feed at measured vapor space temperatures ≤ 500 o C.2. The melter off-gas flammability potentials of the glycolic and sugar-based flowsheet feeds are similar at measured vapor space temperatures ≤ 500 o C.3. The 3X difference in melter off-gas flammability potential between the baseline and either alternate reductant feeds is expected to decrease significantly at typical vapor space operating temperatures  500 o C for both bubbled and non-bubbled operations due to increased kinetics of H 2 and CO oxidation.4. Bubbling increases the melter off-gas flammability potential likely due to increased feed rate at a relatively constant air purge rate.5. It appears that over 50% of the glycolate detected by IC in the glycolic flowsheet feed remained un-dissociated and exited the melter as such, thus having no significant impact on either the glass redox or melter off-gas flammability potential. The condensed glycolic acid will then have to be neutralized and recycled back to the Tank Farms.

Material Compatibility Evaluation for DWPF Nitric-Glycolic Acid - Literature Review

“…SRNL has conducted process testing to support the change in the alternate reductant from formic acid to glycolic acid [4,7], evaluated the impact on some downstream facilities' processes [8,9,32], as well as modeled the melter off gas stream for destruction of glycolic acid [10]. From these investigations the following points were highlighted for further evaluation:…”

Literature Review on Impact of Glycolate on the 2h Evaporator and the Effluent Treatment Facility

“…They indicated Y(OH) 3 and Zr(OH) 4 precipitate in the absence of glycolate. Wang et al 48 used sodium glycolate to dissolve antimony(III) acetate.…”

“…The ion exchange resin in the previous step is regenerated (i.e, cesium and strontium removal from the resin) to allow it to be reused using 2M NaNO 3 .…”

Impact Of Melter Internal Design On Off-Gas Flammability

“…The DM10-SB6 data were actually analyzed earlier to support the down-select and reanalyzed here with an emphasis on the impact of melter design on the data. 27 Efforts were again made to achieve as low a vapor space temperature as practically possible, as was done during the DM10-A2 run. However, instead of overfeeding, the DM10 vapor space was cooled this time by introducing ambient air directly into the vapor space, while maintaining a constant feed rate; therefore, it was presumed to be non-disruptive to the steady state operation.…”

“…The melter off-gas flammability assessment performed earlier in support of the down-select process indicated that a significant fraction of glycolic acid added might remain undissociated and volatilize upon entering the melter, ending up in the condensate recycle to the Tank Farms. 2 Hence, it is also of vital importance to analyze the off-gas condensate produced during the test for the presence of glycolate or any other organic species that may pose operational difficulties on the downstream processes.…”