# Green Shipping--Multifunctional Marine Scrubbers for Emission Control: Silencing Effect
## 1 Introduction  
Although maritime transport accounts for approximately 10-15% of global sulphur (SOx) and nitrogen (NOx) oxide emissions, the volume of seaborne trade has increased by about 3% annually over the past 50 years or so, accounting for about 80-85% of world trade by volume [1]. This has led to increasing concerns about the global impact of maritime emissions, and, consistently, the IMO (International Maritime Organization) has restricted the limits imposed by MARPOL 73/78 Regulations [2, 3] on ship emissions, specifically those of SOx [3] and NOx [2]. EGR (exhaust gas recirculation) and SCR (selective catalytic reduction) systems are recognized as effective technologies for the control of marine NOx emissions [1, 4]. However, the former solution increases particulate matter emissions and fuel consumption by about 4% [5], making SCR the preferred option. Different strategies allow for SOx emissions to be reduced [6]: alternative fuels (e.g., LNG--liquefied natural gas), alternative energy sources (e.g., fuel cells), and conventional fuels with low a sulphur content (e.g., VLSFO--very low sulphur fuel oil). Since January 2020, for ships using conventional fuels, the above-quoted regulations have made the use of VLSFO mandatory, as its sulphur content is lower than 0.5 wt%. Moreover, to sail in ECAs (emission control areas), ULSFO (ultra low sulphur fuel oil) must be used, as the sulphur content limit in fuels has been pushed down to 0.1 wt% in these areas. Whereas the adoption of alternative fuels or onboard energy sources represents, besides the costs, the burden of entire propulsion system refitting, the use of low sulphur content combustibles significantly impacts the total operating costs of a ship, with over 60% of them being associated with the fuel. Despite COVID-19 resulting in an extreme reduction in oil prices, prices of USD 365, USD 472, USD 515, and USD 520 per ton were reported for HSFO (high sulphur fuel oil), VLSFO, ULSFO, and MGO (marine gas oil), respectively, at the Rotterdam/Antwerp hub on 22 February 2021 [7], indicating that use of HSFO may represent higher savings compared to other fuels. Accordingly, an alternative solution is to install a scrubber as an emission abatement device, ensuring compliance with SOx regulations. Scrubbers represent a viable solution [8] and show a lower climate impact than low sulphur fuels [9]. The installation of both SCR and scrubber systems onboard ships could represent a reliable technical solution to satisfy the limits imposed on both NOx and SOx emissions; however, this solution is hardly applicable because of space limitations, especially for existing ships [10]. Indeed, pollution control devices are usually installed in the funnel and are often large in size, so integrating such systems with other exhaust line components, such as silicers, becomes mandatory in order to save space [1]. What is also remarkable is the importance of controlling and minimizing exhausted gas noise. Several regulations [11; 12] limit the perceived noise level at ship decks and have prescribed distances from vessels. As such, the integration of pollution control devices and silencers can represent a smart way to satisfy these regulations by saving space along the exhaust line. Concerning the acoustic performances of pollution control devices such as scrubbers, in the literature, only a notable example of an SCR converter [1] is present, and a patent for a marine scrubber with acoustic properties is deposited [13]. On the contrary, the acoustic performance of mufflers has been deeply studied [14; 15; 16; 17; 18]. The present work is a part of a research project (Project ABE: see funding) oriented to integrate different functionalities within emission control systems and intending to develop a methodology to achieve compact, efficient, and cost-effective scrubber-based marine engine exhaust emission systems.