Comparison of various chemical compounds for the removal of SO2 and NOx with wet scrubbing for marine diesel engines

Terence Chin,Ivan Ck Tam,Chun-Yang Yin

doi:10.1007/s11356-021-16155-9

Terence Chin, Ivan Ck Tam + Show 1 more

Open Access

https://doi.org/10.1007/s11356-021-16155-9

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Abstract

Seawater, NaOH, NaClO, NaClO2, H2O2, and KMnO4 were used as scrubbing liquids to react with SOx and NOx separately in a customized wet scrubber. The absorption of SO2 in the aqueous phase was influenced by three factors: pH, ionic concentration, and oxidation potential. For NOx removal, the effectiveness of various chemical compounds can be ranked from least to most effective as follows: seawater, NaOH, H2O2 < NaClO < KMnO4 < NaClO2. This effectiveness was influenced by the chemical compound’s ability to oxidize NO to NO2, absorb the NO2 that was formed, and retaining the nitrogen in the aqueous phase. High oxidation potential promoted the oxidation of NO to NO2 but hindered the absorption of NO2. NaClO2 was superior compared to NaClO in all three categories of oxidizing, absorption and retention. NaClO could not retain a significant amount of NO2 which it absorbed in the aqueous phase. The pH around 8 provided a good balance between oxidation versus absorption/retention and reactant utilization for the chlorine-based oxidants. KMnO4 had the lowest reactant consumption rate; only half a mole was consumed for every mole of NO removed, compared to around 2–3 mol of chlorite or 3–5 mol of hypochlorite.

Highlights

The combustion of fossil fuels in power plants, boilers, and diesel engines are known to generate large amounts of air pollutants in the form of sulfur oxides (SOx) and nitrogen oxides (NOx), among other polluting substances
These series of equations can roughly be grouped into two categories, the first being the absorption of SO2 into bisulfite or sulfite in the aqueous phase (Eqs. 3 and 4), followed by oxidation to sulfate, its most stable aqueous form (Eqs. 5 and 6)
It can be seen that the absorption of 1 mol of SO2 in the aqueous phase results in the release of 1 to 2 mol of protons, thereby causing the pH to reduce over time

Summary

Introduction

The combustion of fossil fuels in power plants, boilers, and diesel engines are known to generate large amounts of air pollutants in the form of sulfur oxides (SOx) and nitrogen oxides (NOx), among other polluting substances Control of these pollutants are more challenging in the marine sector where ocean-plying ships with large diesel engines depend on lowcost heavy fuel oil for propulsion (Deng et al 2021; Marine 2015). The simplest solution for complying with the SOx cap set in “Annex VI—Prevention of Air Pollution from Ships” of the International Convention for the Prevention of Pollution from Ships (MARPOL) would be to switch to low-sulfur fuel (Lloyd’s Register Marine 2015). Once considered a fringe solution, the installation of wet scrubbers onboard vessels for SOx removal is fast becoming mainstream (ABS Advisory On Exhaust Gas Scrubber Systems 2018)

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