Nitrogen oxide removal using seawater electrolysis in an undivided cell for ocean-going vessels

Abstract

As massive nitrogen oxide (NO_x) emissions from marine slow speed diesel engines have caused serious health and environmental problems, NO_x removal using electro-generated chlorine under seawater electrolysis was studied in a lab-scale scrubbing reactor. Electro-generated chlorine was prepared by seawater electrolysis in an undivided cell. Results showed that active chlorine concentration increased linearly with the increase of current density and electrolysis time. Energy consumption decreased from 83 to 6.1 kW h per (kg [Cl₂]) with the salinity varying from 3.2 to 37.8 ppt after 5 min of electrolysis. Onsite generation of active chlorine using concentrated seawater from desalination plants operated on a ship was suggested to be a cost-effective solution with less energy consumption. Then, effects of dilution ratio (seawater/electrolyte), inlet NO and SO₂ concentration, and initial pH value of scrubbing solution on NO_x removal efficiency were further investigated. NO_x removal efficiency of electro-generated chlorine at pH value of 7 decreased from 98.9% to 20.2% with the dilution ratio increasing from 2.4 to 96. The NO absorption rate by HOCl was proved to be higher than that by OCl⁻. NO absorption rate increased linearly as inlet NO concentration increased. When SO₂ concentration increased from 207 to 814 ppm, NO_x removal efficiency decreased slightly, but SO₂ removal efficiency was almost kept at 100%. The possible reaction mechanism and pathways were discussed by evaluating the pH-dependant NO_x removal efficiency using electro-generated chlorine. The proposed method of wet scrubbing using electro-generated chlorine was demonstrated to be a potential after treatment strategy to control NO_x emissions from large marine diesel engines.