(66c) Steam Reforming Technology Demonstration on Actual Savannah River Site Tank 48H Waste

This presentation describes the Bench-scale Steam Reformer (BSR) that produces the solid and gas phase reactions of a THOR Treatment Technologies Fluidized Bed Steam Reformer (FBSR), but which is small enough to fit into a radioactive cell and can be operated using cell manipulators. It compares the results between the FBSR and BSR as well as the results between the simulant and actual waste.

Sodium tetraphenylborate (Na-TPB) was added to approximately 240,000 gallons of salt waste in Tank 48H in order to remove the cesium as Cs-TPB by cross-flow filtration. However, the DOE decided to discontinue the process due to excessive benzene off-gassing from the solution.

The original process included an acid hydrolysis step to breakdown the TPB into benzene which would be disposed of by incineration. The DOE now wishes to remove the organics from the solution without the need for incineration. One candidate technology for organic removal is fluidized bed steam reforming.

Fluidized bed steam reforming destroys organics by the water gas shift reactions at temperatures above 600oC. Any carbon bearing species is converted to CO and CO2 as H2O becomes H2.

C(s) + H2O(g) -> CO(g) + H2(g)

H2O(g) + CO(g) -> H2(g) + CO2

In addition to destroying the organics, the CO and H2 rich atmosphere then promote the de-nitration of the salt solution.

CO + NO2 -> CO2 + NO

2CO + 2NO -> 2CO2 + N2

H2 + NO2 -> H2O + NO

2H2 + 2NO -> 2H2O + N2

Typically, the H2 concentration was controlled to about 2% on a dry basis. It was controlled by adding air to create steam and heat.

2H2 (g) + O2(g) -> 2H2O(g) + heat

Off-gases from the steam reformer still included some organics so a second reactor operated at 925oC with excess air to oxidize the remaining organics fully to CO2.

The steam reformer creates a solid carbonate product which is readily soluble in water for further processing. One representative carbonation reaction is the conversion of sodium nitrate to sodium carbonate.

5C + 4NaNO3 -> 3CO2 + 2Na2CO3 + 2 N2

Note that these reactions represent the overall chemistry, but are not all inclusive.

Full scale steam reforming is performed in a Fluidized Bed Steam Reformer(FBSR). Tests performed in the pilot scale FBSR at the Hazen Facility in Golden, Colorado proved the technology using simulants. With the technology proven on simulants, the DOE then requested SRNL to prove the technology on actual Tank 48 waste.

Due to the radioactivity of the actual waste, the demonstration had to be performed in the SRNL high level cells. The dimension of the smallest proven FBSR far exceeds the space available within the cells, so an alternative approach had to be taken. Thus the Bench-scale Steam Reformer (BSR) was created.

The BSR is a two reactor system that produces the solid and gas phase reactions of a FBSR and the secondary off-gas oxidizer. The complete reaction unit, which is located inside the radioactive cell, fits on a 4'x 3' pan and requires less than 5' of head space above the pan surface. The control and data acquisition system fits on a 30" x 45" table outside of the cell and two mass spectrometers used for measuring off-gases fit into a 6' x 2' hood behind the cell.

The same simulant used in the Hazen FBSR was run in the BSR. The BSR also ran the actual tank 48 waste. Comparisons are made between the FBSR and the BSR using simulant. Comparisons are made between the actual waste and the simulant waste in the BSR.