DWPF COAL-CARBON WASTE ACCEPTANCE CRITERIA LIMIT EVALUATION BASED ON EXPERIMENTAL WORK (TANK 48 IMPACT STUDY)

Lambert, D. ; Choi, A.

Savannah River Site (S.C.)

关键词: Flammability;    Hydrogen Production;    Testing;    Storage Facilities;    Nitric Acid;   

DOI  :  10.2172/991225
RP-ID  :  SRNL-STI-2010-00589
RP-ID  :  DE-AC09-08SR22470
RP-ID  :  991225

美国|英语

来源: UNT Digital Library

【 摘 要 】

This report summarizes the results of both experimental and modeling studies performed using Sludge Batch 10 (SB10) simulants and FBSR product from Tank 48 simulant testing in order to develop higher levels of coal-carbon that can be managed by DWPF. Once the Fluidized Bed Steam Reforming (FBSR) process starts up for treatment of Tank 48 legacy waste, the FBSR product stream will contribute higher levels of coal-carbon in the sludge batch for processing at DWPF. Coal-carbon is added into the FBSR process as a reductant and some of it will be present in the FBSR product as unreacted coal. The FBSR product will be slurried in water, transferred to Tank Farm and will be combined with sludge and washed to produce the sludge batch that DWPF will process. The FBSR product is high in both water soluble sodium carbonate and unreacted coal-carbon. Most of the sodium carbonate is removed during washing but all of the coal-carbon will remain and become part of the DWPF sludge batch. A paper study was performed earlier to assess the impact of FBSR coal-carbon on the DWPF Chemical Processing Cell (CPC) operation and melter off-gas flammability by combining it with SB10-SB13. The results of the paper study are documented in Ref. 7 and the key findings included that SB10 would be the most difficult batch to process with the FBSR coal present and up to 5,000 mg/kg of coal-carbon could be fed to the melter without exceeding the off-gas flammability safety basis limits. In the present study, a bench-scale demonstration of the DWPF CPC processing was performed using SB10 simulants spiked with varying amounts of coal, and the resulting seven CPC products were fed to the DWPF melter cold cap and off-gas dynamics models to determine the maximum coal that can be processed through the melter without exceeding the off-gas flammability safety basis limits. Based on the results of these experimental and modeling studies, the presence of coal-carbon in the sludge feed to DWPF is found to have both positive (+) and negative (-) impact as summarized below: (-) Coal-carbon is a melter reductant. If excess coal-carbon is present, the resulting melter feed may be too reducing, potentially shortening the melter life. During this study, the Reduction/Oxidation Potential (REDOX) of the melter could be controlled by varying the ratio of nitric and formic acid. (-) The addition of coal-carbon increases the amount of nitric acid added and decreases the amount of formic acid added to control melter REDOX. This means that the CPC with the FBSR product is much more oxidizing than current CPC processing. In this study, adequate formic acid was present in all experiments to reduce mercury and manganese, two of the main goals of CPC processing. (-) Coal-carbon will be oxidized to carbon dioxide or carbon monoxide in the melter. The addition of coal-carbon to the FBSR product will lead to approximately 55% higher offgas production from formate, nitrate and carbon due to the decomposition of the carbon at the maximum levels in this testing. Higher offgas production could lead to higher cold cap coverage or melter foaming which could decrease melt rate. No testing was performed to evaluate the impact of the higher melter offgas flow. (+) The hydrogen production is greatly reduced in testing with coal as less formic acid is added in CPC processing. In the high acid run without coal, the peak hydrogen generation was 15 times higher than in the high acid run with added coal-carbon. (+) Coal-carbon is a less problematic reducing agent than formic acid, since the content of both carbon and hydrogen are important in evaluating the flammability of the melter offgas. Processing with coal-carbon decreases the amount of formic acid added in the CPC, leading to a lower flammability risk in processing with coal-carbon compared to the current DWPF flowsheet. (+) The seven SB10 formulations which were tested during the bench-scale CPC demonstration were all determined to be within the off-gas flammability safety basis limits during the 9X/5X off-gas surge for normal bubbled melter operation. The concentration of coal-carbon in these baseline melter feeds varied widely from 0 to 17,863 ppm, depending on the acid addition strategy used and the extent to which the required reductant (formic acid) was replaced with coal-carbon. All baseline feeds were redox-adjusted and three of them contained TOC higher than the current theoretical TSR limit of 18,900 ppm. (-) Additional coal-carbon was then added to each baseline feed until the calculated off-gas flammability equaled the safety basis limit of 60% of the LFL at the peak of off-gas surge ('max-coal'). In doing so, however, no counterbalancing nitrate was added, thus simulating the scenario where slugs of coal enter the melter as a result of uneven distribution of coal in the slurry.