Abstract
The use of a Flue Gas Desulphurisation (FGD)-gypsum as material for selenium removal in re-circulated waters from a wet limestone FGD system with water re-circulation to the scrubber and with use of an Al-additive, to increase SO2 emission abatement efficiencies, has been evaluated by adsorption studies. Potentiometric titration experiments for FGD-gypsum reveal that the acidic conditions of the aqueous phase of gypsum slurry, induced by the Al-additive, result in the protonation of the FGD-gypsum surface. The adsorption isotherms of Se onto FGD-gypsum are appropriately described by the Langmuir model suggesting that Se is adsorbed by the protons adhered on FGD-gypsum surface and forms a monolayer. The removal of Se from FGD waters by the employment of FGD-gypsum is significant as prevention measure based on the management on FGD-gypsum and water streams before their production and the subsequent disposal in landfills and/or in application scenarios.
Highlights
Because coal-combustion processes are well evaluated, general trends in trace element behaviour during combustion have been identified [1,2,3,4,5,6,7,8,9]
The correlation of Langmuir adsorption isotherm within the studied temperature suggests that Se is adsorbed by the protons adhered on Flue Gas Desulphurisation (FGD)-gypsum surface and forms a monolayer
The different operational parameters observed during the process of investigations reveal that the protonation of the FGDgypsum surface is the cause of Se retention on FGD-gypsum
Summary
Because coal-combustion processes are well evaluated, general trends in trace element behaviour during combustion have been identified [1,2,3,4,5,6,7,8,9]. It is common knowledge that many of the trace elements from coal are partially or totally vaporised during combustion. The degree of vaporisation determines how each element is partitioned between various solid residues and the flue gas [1]. Selenium during Pulverised Coal Combustion (PCC) can be released as gas SeO2 and/ or as particulate Se [7,8,9]. When the flue gas cools down some of gas SeO2 may condense on the finest particle surfaces of Fly Ashes (FAs). Once in FGD, gas SeO2 and particulate Se may be dissolved in the aqueous phase of absorber slurries and/or concentrated in the gypsum sludge [7,8,9]. Depending on the abatement capacity of FGDs, a given proportion of Se can be released in a gaseous and/or PM form [8,9]
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