CARBON BED MERCURY EMISSIONS CONTROL FOR MIXED WASTE TREATMENT
Mercury has had various uses in nuclear fuel reprocessing and other nuclear processes, and so is often present in radioactive and mixed (both radioactive and hazardous according tohe Resource Conservation and Recovery Act) wastes. Depending on regulatory requirements, the mercury in the off-gas must be controlled with sometimes very high efficiencies. Compliance to the Hazardous Waste Combustor (HWC) Maximum Achievable Control Technology (MACT) standards can require off-gas mercury removal efficiencies up to 99.999% for thermally treating some mixed waste streams. Several test programs have demonstrated this level of off-gas mercury control using fixed beds of granular sulfur-impregnated activated carbon. Other results of these tests include: (a) The depth of the mercury control mass transfer zone was less than 15-30 cm for the operating conditions of these tests, (b) MERSORB® carbon can sorb Hg up to 19 wt% of the carbon mass, and (c) the spent carbon retained almost all (98 – 99.99%) of the Hg; but when even a small fraction of the total Hg dissolves, the spent carbon can fail the TCLP test when the spent carbon contains high Hg concentrations. Localized areas in a carbon bed that become heated through heat of adsorption, to temperatures where oxidation occurs, are referred to as “bed hot spots.” Carbon bed hot spots must be avoided in processes that treat radioactive and mixed waste. Key to carbon bed hot spot mitigation are (a) designing for sufficient gas velocity, for avoiding gas flow maldistribution, and for sufficient but not excessive bed depth, (b) monitoring and control of inlet gas flowrate, temperature, and composition, (c) monitoring and control of in-bed and bed outlet gas temperatures, and (d) most important, monitoring of bed outlet CO concentrations. An increase of CO levels in the off-gas downstream of the carbon bed to levels about 50-100 ppm higher than the inlet CO concentration indicate CO formation in the bed, caused by carbon bed hot spots. Corrective actions must be implemented quickly if bed hot spots are detected, using a graded approach and sequence starting with corrective actions that are simple, quick, cause the least impact to the process, and are easiest to recover from. Multiple high and high-high alarm levels should be used, with appropriate corrective actions for each level.
- Research Organization:
- Idaho National Lab. (INL), Idaho Falls, ID (United States)
- Sponsoring Organization:
- DOE - EM
- DOE Contract Number:
- DE-AC07-05ID14517
- OSTI ID:
- 993863
- Report Number(s):
- INL/JOU-09-15828; JAWAEB; TRN: US1008161
- Journal Information:
- Journal of the Air and Waste Management Association, Vol. 60; ISSN 1047-3289
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ACTIVATED CARBON
ADSORPTION HEAT
CARBON
COMBUSTORS
COMPLIANCE
GAS FLOW
HOT SPOTS
MASS TRANSFER
MERCURY
MITIGATION
MONITORING
NUCLEAR FUELS
OXIDATION
PACKED BEDS
REMOVAL
REPROCESSING
RESOURCE CONSERVATION
WASTE PROCESSING
WASTES
mercury
mixed waste treatment
nuclear fuel reprocessing