NOx Removal From Flue Gas by an Integrated Physicochemical Absorption and Biological Denitrification Process

An integrated physicochemical and biological technique for NOx removal from flue gas, the so-called BiODeNOx process, combines the principles of wet absorption of NO in an aqueous Fe(II)EDTA2- solution with biological reduction of the sorbed NO in a bioreactor. The biological reduction of NO to di-nitrogen gas (N2) takes place under thermophilic conditions (55°C). This study demonstrates the technical feasibility of this BioDeNOx concept in a bench-scale installation with a continuous flue gas flow of 650 l.h-1 (70-500 ppm NO; 0.8-3.3% O2). Stable NO removal with an efficiency of at least 70% was obtained in case the artificial flue gas contained 300 ppm NO and 1% O2 when the bioreactor was inoculated with a denitrifying sludge. An increase of the O2 concentration of only 0.3% resulted in a rapid elevation of the redox potential (ORP) in the bioreactor, accompanied by a drastic decline of the NO removal efficiency. This was not due to a limitation or inhibition of the NO reduction, but to a limited biological iron reduction capacity. The latter leads to a depletion of the NO absorption capacity of the scrubber liquor, and thus to a poor NO removal efficiency. Bioaugmentation of the reactor mixed liquor with an anaerobic granular sludge with a high Fe(III) reduction capacity successfully improved the bioreactor efficiency and enabled to treat a flue gas containing at least 3.3% O2 and 500 ppm NO with an NO removal efficiency of over 80%. The ORP in the bioreactor was found to be a proper parameter for the control of the ethanol supply, needed as electron donor for the biological regeneration process. The NO removal efficiency as well as the Fe(III)EDTA - reduction rate were found to decline at ORP values higher than -140 mV (pH 7.0). For stable BioDeNOx operation, the supply of electron donor (ethanol) can be used to control the ORP below that critical value