Marine Exhaust Gas Treatment Systems for Compliance with the IMO 2020 Global Sulfur Cap and Tier III NOx Limits: A Review

Abstract

In the present work, the contemporary exhaust gas treatment systems (EGTS) used for SOx, PM, and NOx emission mitigation from shipping are reviewed. Specifically, after-treatment technologies such as wet scrubbers with seawater and freshwater solution with NaOH, hybrid wet scrubbers, wet scrubbers integrated in exhaust gas recirculation (EGR) installations, dry scrubbers, inert gas wet scrubbers and selective catalytic reduction (SCR) systems are analyzed. The operational principles and the construction specifications, the performance characteristics and the investment and operation of the reviewed shipping EGTS are thoroughly elaborated. The SCR technology is comparatively evaluated with alternative techniques such as LNG, internal engine modifications (IEM), direct water injection (DWI) and humid air motor (HAM) to assess the individual NOx emission reduction potential of each technology. Detailed real data for the time several cargo vessels spent in shipyards for seawater scrubber installation, and actual data for the purchase cost and the installation cost of seawater scrubbers in shipyards are demonstrated. From the examination of the constructional, operational, environmental and economic parameters of the examined EGTS, it can be concluded that the most effective SOx emission abatement system is the closed-loop wet scrubbers with NaOH solution which can practically eliminate ship SOx emissions, whereas the most effective NOx emission mitigation system is the SCR which cannot only offer compliance of a vessel with the IMO Tier III limits but can also practically eliminate ship NOx emissions.

Mitigation of Ship Emissions: Overview of Recent Trends

“…In the former, more or less basic aqueous solutions are used as scrubbing media, whereas in the latter, the exhaust gases react exothermically with solid granular chemicals. Post-combustion NO X reduction is achieved by selective catalytic reduction (SCR), which is a relatively mature technology with an efficiency of 80%–90% . Compatibility of SO X scrubbers with waste heat recovery units (WHRUs), exhaust gas recirculation systems (EGRS), and ship SCR systems requires special attention during integration. , In addition, the function of EGCSs is also to significantly reduce the level of particulate matter (PM 2.5 , PM 10 ) from combustion…”

“…The main reactions taking place in caustic soda scrubbers are listed below, following the hydration of SO 2 (reaction ) and SO 3 (reaction ): ,, NaOH + H 2 SO 3 → NaHSO 3 + H 2 O goodbreak0em2em⁣ (sodium bisulfite) NaOH + NaHSO 3 → Na 2 SO 3 + H 2 O goodbreak0em2em⁣ (sodium sulfite) NaOH + H 2 SO 4 → NaHSO 4 + H 2 O goodbreak0em2em⁣ (sodium bisulfate) NaOH + NaHSO 4 → Na 2 SO 4 + H 2 O goodbreak0em2em⁣ (sodium sulfate) …”

“…The main reactions taking place in caustic soda scrubbers are listed below, following the hydration of SO 2 (reaction ) and SO 3 (reaction ): ,, …”

“…18 Compatibility of SO X scrubbers with waste heat recovery units (WHRUs), exhaust gas recirculation systems (EGRS), and ship SCR systems requires special attention during integration. 11,18 In addition, the function of EGCSs is also to significantly reduce the level of particulate matter (PM 2.5 , PM 10 ) from combustion. 19 2.1.…”

Combined removal experiment of NOx, SO2 and PM from marine diesel exhaust gas with SCR-DryEGCS system

“…Due to the specific nature of ship power units, ships mostly use wet EGCS, [33][34][35] and the most effective SOx emission reduction system is a closed-loop wet scrubber using NaOH solution. 36 Few research has been carried out on dry EGCS for marine applications, the most representative dry EGCS is developed by the Couple System company, 37 for SOx removal trial operations with dry scrubbers has proved up to 99%. Zhang 38 developed a PESTEL analytical model to provide a comprehensive comparison of various SOx emission control technologies in terms of 13 indicators in six areas.…”