Particulate Mass and Nonvolatile Particle Number Emissions from

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Energy and the Environment

Particulate mass and non-volatile particle number emissions from marine engines using low-sulfur fuels, natural gas or scrubbers Kati Lehtoranta, Päivi Tellervo Aakko-Saksa, Timo Murtonen, Hannu Vesala, Leonidas Ntziachristos, Topi Rönkkö, Panu Karjalainen, Niina Kuittinen, and Hilkka Timonen Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b05555 • Publication Date (Web): 19 Feb 2019 Downloaded from http://pubs.acs.org on February 27, 2019

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Environmental Science & Technology

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Particulate mass and non-volatile particle number

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emissions from marine engines using low-sulfur

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fuels, natural gas or scrubbers

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Kati Lehtoranta1*, Päivi Aakko-Saksa1, Timo Murtonen1, Hannu Vesala1, Leonidas

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Ntziachristos2, Topi Rönkkö2, Panu Karjalainen2, Niina Kuittinen2, Hilkka Timonen3

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1VTT

Technical Research Centre of Finland, P.O.Box 1000, FI-02044 VTT, Finland, 2Tampere

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University of Technology, P.O. Box 692, FI-33101, Finland, 3Finnish Meteorological Institute,

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P.O.Box 503, FI-00101 Helsinki, Finland

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ABSTRACT

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In order to meet stringent fuel sulfur limits, ships are increasingly utilizing new fuels or,

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alternatively, scrubbers to reduce sulfur emissions from the combustion of sulfur-rich heavy fuel

13

oil. The effects of these methods on particle emissions are important, since particle emissions from

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shipping traffic are known to have both climatic and health effects. In this study, the effects of

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lower sulfur level liquid fuels, natural gas (NG), and exhaust scrubbers on particulate mass (PM)

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and non-volatile particle number (PN greater than 23 nm) emissions were studied by measurements

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in laboratory tests and in-use. The fuel change to lower sulfur level fuels or to NG and the use of

ACS Paragon Plus Environment

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scrubbers significantly decreased the PM emissions. However, this was not directly linked with

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non-volatile PN emission reduction, which should be taken into consideration when discussing the

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health effects of emitted particles. The lowest PM and PN emissions were measured when utilizing

21

NG as fuel, indicating that the use of NG could be one way to comply with up-coming regulations

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for inland waterway vessels. Low PN levels were associated with low elemental carbon. However,

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a simultaneously observed methane slip should be taken into consideration when evaluating the

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climatic impacts of NG-fueled engines.

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Abstract Art

26 27

INTRODUCTION

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Shipping is a very efficient way of transporting goods, and currently the majority of global

29

trade volumes is transported by sea. Although significant reductions in emissions of terrestrial

30

vehicle engines have been achieved due to a series of environmental regulations and subsequent

31

implementation of emission control techniques over the years, marine vessel emissions have not

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been thoroughly addressed. As a result, their relative contribution to air pollution has increased

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during recent years 1.


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Container ships, bulk carriers and oil tankers, with slow-speed 2-stroke engines, consumed

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approx. 74% of all marine fuels in 2015, whereas ships having medium-speed 4-stroke engines

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(like most cruise ships) consumed approx. 23%2. However, compared to containers and cargo

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vessels, cruise ship routes are generally nearer to coastlines, and these ships may spend more

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time at anchor or berthing, which can have a significant effect on local air quality.

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Around 15% of global anthropogenic NOx and 5-8% of global SOx emissions can be attributed

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to shipping operations 3–5. In response, the International Maritime Organization (IMO) has

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introduced specific regulations to reduce shipping emissions of both pollutants. NOx limits have

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been set at a global level with Tier I and II standards, whereas lower limits are applicable to

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ships constructed after 2016 (Tier III) in NOx emission control areas (NECA). Tier III leads to an

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increase in the use of emission control devices, the most important of which is selective catalytic

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reduction (SCR). The SCR system utilizes a catalyst and externally fed ammonia to reduce NOx

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emissions. Catalysts in ship applications are usually vanadium-based, and an aqueous solution of

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urea is utilized as a source of ammonia (see, for example 6,7).

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In order to address SOx emissions, a global cap in fuel sulfur content (FSC) of marine fuels has

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been set at 0.5% m/m by 2020, compared to earlier permitted FSC levels of up to 3.5% m/m. In

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SOx emission control areas (SECA), the FSC is further limited to 0.1 %. Higher sulfur levels are

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applicable only if an on-board sulfur scrubber is used. Different types of SOx scrubbers use fresh

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water (closed loop), seawater (open loop), a hybrid technology (either fresh or sea water), or dry

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removal agents. Alternatively, distillate fuels (ISO 8217), alcohols and biofuels, or gaseous fuels

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– primarily liquefied natural gas (LNG) with inherently low FSC, may be used for compliance

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with sulfur regulations. Of the distillate fuels, marine gas oil (MGO, free from residual fuel) is

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primarily used in marine engines with displacement below 5 liters per cylinder, whereas marine


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diesel oil (MDO, may have traces of residual fuel) is typically used in engines with over 5 liters

58

per cylinder. In addition, low sulfur residual marine fuel oils (also called hybrid fuels) may be

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utilized in ships operating in SECAs.

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The IMO has not defined explicit emission limits for PM, despite its environmental and health

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implications. Shipping PM emissions are important especially in Asia, with China having 7 of the

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10 largest ports in the world in terms of volume of transported goods8. Liu et al.9 recently showed

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that during ship plume-influenced periods, shipping emissions contributed up to 20–30% of total

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PM2.5 within tens of kilometers of the coastal and riverside Shanghai regions. Chen et al.10

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estimated that the contribution of shipping emissions to ambient PM2.5 concentrations over the

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urban area of Qingdao could exceed 20%. Corbett et al.5 estimated that on a global scale, shipping-

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related PM emissions are annually responsible for approximately 60 000 premature deaths related

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to cardiopulmonary and lung cancer diseases. Sofiev et al.11 recently estimated that total premature

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mortality due to shipping is much higher, and that even low-sulfur marine fuels still account for

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approximately 250 000 deaths annually. SOx regulations also have an implicit impact on PM, since

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they lead to decreased particulate sulfate emissions. A recent study by Sofiev et al.11 showed that

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low-sulfur fuels may provide pollution health benefits but with a climate trade-off, since the

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reduction in sulfate in PM decreases the climatic cooling effects of the exhaust aerosol. Although

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sulfate aerosol has a cooling effect, absorbent black carbon (BC) emissions warm the atmosphere

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and are not explicitly dependent on the FSC12. BC deposition on ice and subsequent albedo change

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is especially important in the Arctic area, which has recently experienced an increase in shipping

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activity13.

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The existing evidence discussed above suggests that global shipping PM emissions have a clear

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effect on climate, air pollution and public health. PM emission levels are also significantly


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influenced by regulations addressing other pollutants. The present study provides evidence on how

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different options enabling ships to operate within SECAs affect the PM emissions of marine

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engines. The options examined include lower sulfur level fuels (liquid and gaseous) as well as on-

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board scrubbers. Both the PM and non-volatile PN emissions are studied in experiments conducted

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on board two different ships and in a marine engine laboratory.

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EXPERIMENTAL

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Engines in laboratory

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Experiments with low-sulfur liquid fuels and natural gas were conducted in the marine engine

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laboratory. The first engine was a Wärtsilä Vasa 4R32, a four-cylinder medium-speed 4-stroke

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marine engine that was retrofitted to enable operation with natural gas in dual fuel (DF) mode. In

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this mode, a small quantity of liquid fuel is first injected to pilot combustion, which is then

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sustained by delivery of the main quantity of natural gas. The maximum power of this engine

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was 1400 kW. The engine was modified to run on natural gas in 2016, using state-of-the-art

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components of current commercial natural gas marine engines. The modification of the engine

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included, for example, installing gas admission valves on the cylinder heads and replacing the

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cylinder heads as well as the liners, pistons, camshaft and turbocharger. In addition, the engine

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control system was updated and a pilot fuel line with a high-pressure pilot fuel pump was added.

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This engine was tested with the 40% and 85% loads that are considered representative of in-

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harbor maneuvering and open-seas operation, respectively. Emissions of PM and PN were also

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characterized from a second DF engine on the test bed. This was a 2017 production series


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Wärtsilä 20DF (medium speed, 4-stroke engine), which is of similar size to the 4R32 engine and

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delivers a maximum power output of 1000 kW.

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Engines and scrubbers on board

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Emission measurements were also made on board two ships during regular cruising conditions.

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A modern cruise ship equipped with SCR and a hybrid sulfur scrubber was first tested. The

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hybrid scrubber used closed loop during harbor operation, whereas sea water scrubbing was used

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on the open sea. PM and PN emissions were measured from two main engines (E1, E2) at

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constant engine loads of 40% and 75%. Both engines were medium-speed, 4-stroke, modern

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engines. For E1, the measurements were made upstream (engine outlet) and downstream of the

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scrubber. The E2 engine was equipped with the SCR placed upstream of the scrubber, so the

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measurement points were downstream of the scrubber and downstream of the SCR, before the

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scrubber. The second ship was a RoPax vessel (built for freight vehicle transport along with

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passenger accommodation) equipped with a diesel oxidation catalyst (DOC) and an open loop

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seawater-operating scrubber. PM and PN from one of its main engines (a medium-speed, 4-

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stroke engine, E3) were measured upstream of the scrubber (thus downstream of the DOC) and

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downstream of the scrubber. The ship operated normally and the engine load varied during the

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cruise mainly in the range of 63-66% of maximum load. A schematic layout of all tested engines

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and related after-treatment is presented in supporting information (Figure S1).

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Fuels

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Table 1 contains the main specifications of all liquid fuels used in the measurement campaigns.

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Two different liquid fuels (MDO, MGO) were used in the laboratory experiments with the

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Wärtsilä Vasa 4R32 engine when operated in liquid-fuel mode. The MDO had an FSC of


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0.0822% and thus fulfilled the SECA requirements (limit 0.1% S), whereas the MGO was of

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even higher quality (on-road diesel quality, EN 590), with a very low sulfur level. High quality

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MGO with a very low sulfur level is not widely utilized in ships today, but it was selected here

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based more on its role as a pilot in DF mode. In DF mode, the main fuel was compressed natural

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gas (CNG) and the pilot injection was with MGO according to the engine manufacturer’s

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instruction. The CNG had a high methane content (volumetric composition 96.4% methane, 2.3

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% ethane, 0.35 % propane, 0.15% other hydrocarbons, 0.57% nitrogen and 0.21% carbon

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dioxide). In DF mode, the pilot fuel quantity was adjusted to 1.2 % and 3.8 % of the total fuel

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mass flow for 85% and 40% loads, respectively. LNG was used as the main fuel in the 20DF

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engine, since there was no natural gas distribution system available for that engine. The LNG

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used contained 90% methane and approximately 9% ethane. MGO was also used as pilot in this

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case, with an amount of approximately 2% of the total fuel mass flow for 75% engine load.

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The cruise ship, equipped with a scrubber to meet SECA requirements, operated on heavy-fuel

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oil (HFO) with 0.65% sulfur content as a fuel (in all its engines). For comparison, the E2 engine

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was also operated on 0.10% FSC MGO for a period of nine hours with 40% load to conduct

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emission measurements. The RoPax ship operated on 1.9% FSC HFO.

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Table 1 Main specifications of liquid fuels used in the measurement campaigns.

Parameter

Unit

Use Engine Density (15°C) Viscosity (40°C) Heating value. lower Flash point

kg/m3 mm2/s MJ/kg °C

MGO

MDO

HFO

MGO

HFO

< 0.001 % S

< 0.1 % S

< 0.7 % S

< 0.1 % S

23nm concentrations

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were determined with an Airmodus A23 Condensation Particle Counter (CPC). The DR setting

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in the case of non-volatile PN sampling is much less critical than in the case of PM mass, due to

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the elimination of condensation dynamics in the former case. The relatively high DR, which is

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recommended for road vehicles, was also adopted in this study to decrease coagulation dynamics


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and the corresponding PN concentration sensitivity to residence time before counting 19. The DR

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setting would be even less relevant if non-volatile PM rather than PN was to be characterized, as

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in the case of jet exhaust sampling, where DRs as low as 10 are being implemented 20.

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The concentrations of NOx (NO and NO2) in exhaust emissions were measured with a

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chemiluminescence detector (CLD), and carbon monoxide (CO) and carbon dioxide (CO2) with

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a non-dispersive infrared (NDIR) analyzer. Light hydrocarbon components were speciated with a

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gas chromatograph when natural gas was used as a fuel. A Flame Ionization Detector (FID) was

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utilized in some of the tests to measure the total hydrocarbon (THC) content in the exhaust.

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Concentrations of sulfur dioxide (SO2) and water were measured with FTIR (Fourier Transform

191

Infrared spectroscopy).

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RESULTS AND DISCUSSION

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PM and PN emissions

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Liquid fuels

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HFO was the fuel of choice for the majority of shipping operations before the global sulfur cap

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enforcement. In addition to high FSC, this fuel may contain large amounts of metals, such as V

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and Ni, and other ash components, compared to distillate fuels that contain practically no metals

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or ash (Table 1). Earlier experiments performed on the same Vasa 4R32 engine used in the

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current study demonstrated that marine engine PM emissions depend heavily on fuel quality. At

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75% load, PM decreased from 850 mg/kWh, when using 2.5% FSC HFO, to 280 mg/kWh with

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1% FSC HFO12, and down to 60 mg/kWh when using a light fuel oil (similar to MGO in the

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present study)14.


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The present results confirm previous evidence. Figure 1a presents PM emissions of the two

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laboratory engines fueled with NG, MDO and MGO on the test bed. PM with MGO at 85% load

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was at a similar level to what has been measured previously at 75% load14. Earlier evidence

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using HFO has shown that sulfate and associated water comprise more than half of total PM12,21.

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With the lighter fuels used in the present study, sulfate accounted for only a minor fraction of

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total PM (Figure 1b); even in the case of MDO the sulfate comprised only 4% of the total PM in

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the 85% load case. EC (18-19% of total PM) and OC (60-63% of PM) comprised the majority of

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PM for MDO and MGO. The ‘rest’ PM fractions were calculated by subtracting the EC, OC and

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sulfate amounts from the total PM. This ‘rest’ fraction was not further specified in the present

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study, but typically it is comprised of water as well as fuel and lube-derived ash.

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Figure 1 a) PM and c) non-volatile PN>23nm emissions measured from 40% load mode and 85%

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load mode with the 4R32 DF engine utilizing MDO, MGO and NG fuels and from 75% load with

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the 20DF engine utilizing MGO and LNG fuels (marked load 75% p). Error bars show the standard

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deviation of a minimum of five measurements; b) the PM composition of exhaust of the 4R32 DF

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engine fueled with MDO, MGO and NG at 85% load.


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The particle emission results also show that the MGO produced 17-25% less PM than the

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MDO, but without any measurable effects on PN>23nm (see Figure 1c). Other studies (e.g. 21–24)

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concluded that liquid fuel change has the potential to decrease particle emissions from ships,

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although they also concluded that such reductions are not only due to FSC reduction.

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Additionally, even if reduced sulfur content of the fuel efficiently reduced the PM emission, the

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BC emissions were not necessarily reduced12. Khan et al.22 and Zetterdahl et al.23 both compared

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emissions of HFO (higher S) to a lower sulfur level fuel. They both observed significant

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reductions in PM mass when changing to lower sulfur content fuel. Zetterdahl et al. observed

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67% reduction in PM mass and smaller average particle size when changing to low sulfur level

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fuel, but the total particle number was found to be on similar levels for both fuels. In the present

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study, we also showed a PM decrease with fuel sulfur decrease, although no difference was

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found in the PN level.

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Effect of engine load

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Engine load was found to affect the work-specific PM emissions for the laboratory engine 4R32.

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PM emissions at low load (40%) were higher than when the load was increased, with all of the

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fuels used. Emission rates at low load were also more variable than at higher load, as indicated by

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the error bars in Figure 1a. In DF operation, the quantity of liquid fuel used per unit of energy was

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higher at low load (3.8 % of main fuel) compared to high load (1.2 % of main fuel), which may

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have contributed to the higher energy-specific PM emissions at low load. Marine engines are

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generally optimized for loads in the 75-90% range, which is the typical operation range for open-

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seas sailing. In ports, low loads are required for manoeuvring and for berthing. Although in-port

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emissions make only a small contribution to the total emissions of any vessel, they are important

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for local air quality and associated health effects on populations in the vicinity of ports 25. This


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means that any future regulation initiatives on PM emissions control from ships will require that

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emissions are also regulated at low load operation.

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Natural gas

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DF operation (with NG as main fuel) resulted in the lowest PM levels for both engines, at a

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level 63-69% lower than the MGO and 72-75% lower than the MDO. When shifting to DF

247

operation, only traces of EC were observed and OC accounted for 83% of PM (Figure 1b). The

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impact of NG use was actually magnified when non-volatile PN>23nm was considered, with PN

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levels 98-99% lower than when using liquid fuels (Figure 1c). Particles in this size range are

250

often assumed to represent the soot mode that accounts for the major part of EC. The observed

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low PN>23nm for NG is consistent with the very low levels of EC measured for this particular fuel.

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The low soot emissions with NG also explain why changes in the load when in DF mode (85%

253

load compared to 40% load with the 4R32 engine, Figure 1c) had practically no effect on the

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non-volatile particle number, which is different from what is observed with liquid fuels.

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Differences under DF mode are observed when comparing the retrofitted engine with the

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production-series engines; the latter were significantly lower in terms of both PM and PN>23nm

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emissions than the retrofitted engine. In addition to the impact of engine design, load level, and

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NG composition differences between the two engines, the lubricating oil might be a significant

259

contributor to the observed differences, as observed in previous studies26–29. The two engines

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operated with lubricating oils of different specifications and, presumably, different consumption

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rates, although lubrication oil consumption was not measured in any of the engines. There were

262

also differences in the pilot fuel amounts (i.e. 2% pilot fuel injection at 75% load of the

263

production series engines compared to the 1.2% ratio used at 85% load of the retrofitted engine),


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but these do not provide an explanation for the observed lower PN>23nm emissions of the

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production engine.

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PN>23nm discussion

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The level of PN>23nm emissions from marine engines is important in view of the upcoming EU

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regulations for inland waterway vessels. The Stage V regulation, applicable from 2020 on, has

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introduced the requirement of PN>23nm remaining below 1012 kWh1 over a testing procedure

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involving several steady-state modes and weighing factors. Although we have not tested the

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complete range of conditions required by the regulation, the orders of magnitude of PN collected

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are indicative of the potential of different technology options.

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PN>23nm emissions were well beyond the limit with all of the tested liquid fuels, almost two

274

orders of magnitude or higher than the limit, with both engines tested. The retrofitted engine

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(with NG as the main fuel) produced 1.1×1012 kWh1 and 1.0×1012 kWh1 at 85% and 40% load,

276

respectively, thus being within the range of regulatory requirements. The PN>23nm emissions of

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the production-series engine at 75% load with LNG was 1.3×1011 kWh1, which was actually

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much lower than the limit (1012 kWh1). These results indicate that the use of natural gas as a fuel

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for marine and inland waterway vessels could be one way to comply with upcoming regulations.

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The low non-volatile PN>23nm level is also associated with low EC concentration (seen Figure 1).

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This strongly indicates that the regulation of PN>23nm may also be effective in controlling EC

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(and, correspondingly, BC) emissions of vessels and may thus contribute to decreasing the

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shipping impact on climate forcing. Figure 1c shows that light liquid fuel effects on PN>23nm

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were more marginal.


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The observations related to fuel effects on PN>23nm emissions in this study refer to non-volatile

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particles above 23 nm in size and could be different if smaller particles and/or semi-volatile

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particles were included in the analysis. Anderson et al. 28 studied total particle emissions from an

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LNG-powered ship and also observed that LNG resulted in significantly lower PN numbers than

289

when MGO was used. However, they also found that a significant fraction of particles were in

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the sub-23 nm size range. Alanen et al. 29 observed a remarkable amount of sub-23 nm

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nanoparticles with diameters down to only a few nm when using NG. Marine exhaust PN

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conclusions should therefore be observed carefully in relation to the particle population to which

293

they refer.

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Scrubbers on board

295

The results of PM and PN>23nm measurements conducted on board the two different ships are

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presented in Figure 2. The PM level from the E3 engine (RoPax ship) during regular vessel

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operation with the engine load varying between 63% and 66% was the highest. In this case, the

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FSC of the HFO used was significantly higher than that of the HFO used on the cruise ship

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(engines E1 and E2). Operation of the scrubber was found to decrease the PM level by 21-45%

300

in the high load cases (E1, E2, E3, loads 63-66% and 75%) and 8-17% at low load (40%, E1 and

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E2). The PM composition analysis (Figure 2b) showed that the organic carbon was reduced by

302

the scrubber. Interestingly, sulfate aerosol was at the same level both upstream and downstream

303

of the scrubber, whereas EC seemed to be decreased by passage over the scrubber. The ‘rest’ part

304

of PM (consisting typically of sulfate-associated water and ash) was almost halved by the

305

scrubber.


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Figure 2 a) PM and c) non-volatile PN>23nm emissions measured on board two different ships (i.e.

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engines E1 and E2 on a cruise ship, E3 on a RoPax); b) PM composition from measurements made

309

on engine E1 at 75% load. Note: In the case of engine E3 the HFO differed from the HFO utilized

310

with engines E2 and E1 (see Table 1).

311

The impact of the scrubber on PM emitted from the E1 and E2 engines was different, despite

312

the fact that both engines were on the same ship and utilizing same scrubber. One reason for this

313

is that sampling from the E2 was conducted downstream of the SCR to which the engine was

314

connected. First, the SCR by itself was most probably having a positive impact on PM emissions.

315

The SCR system has been shown to have an impact on PM e.g. by decreasing the organic

316

fraction6. The additional benefit that the scrubber can offer is therefore lower than the

317

improvement it offers over engine-out PM emissions.

318

Even though PM clearly decreased over the scrubber, its effect on the non-volatile PN was less

319

clear. In the case of E1, the PM decrease was 41% in 75% load mode, whereas there was

320

practically no change in the PN>23nm level. On the other hand, in the case of E2 and 75% load, the


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PM decrease was 21% and the PN>23nm decrease was observed to be 30%. However, bearing in

322

mind the fact that the standard deviation for PN measurements made in the laboratory (with

323

MDO fuel) was ±15-18%, this PN decrease is considered to be minor.

324

Use of MGO in engine E2 led to lower PM and PN>23nm concentrations than were observed

325

downstream of the scrubber with HFO fuel. However, this is only one example (one MGO fuel,

326

one engine, one load mode) and cannot be presented as a general conclusion.

327

Fridell & Salo30 also studied the particle emissions from a marine engine equipped with a

328

scrubber (open-loop wet scrubber using seawater). They found the total number of particles to be

329

reduced by 92% in the scrubber, while the solid fraction was reduced by 48%. The PM was also

330

significantly decreased, i.e. about 75% of the total PM was captured by the scrubber. These are

331

all higher reduction values than in the present study and there might be several reasons for this,

332

e.g. differences in engines, fuels and/or scrubbers. Moreover, one big difference is the particle

333

measurement method. Fridell & Salo measured the PN with an Engine Exhaust Particle Sizer

334

with a size range of 5.6 - 560 nm therefore also covering particles below 23 nm size, which were

335

not measured in the present study. Several studies performed with liquid fuels report particle size

336

distributions from marine engines suggesting that a share of non-volatile PN resides below the 23

337

nm level28,31–33. In addition to the PN 23 nm limit, the results of Ntziachristos et al.14,21 show that

338

even when conducted following the ISO 8178 protocol, the dilution system can have a

339

significant effect on the measured PM result. At minimum, the dilution ratio range allowed

340

should be more strictly defined in order to obtain comparable results.

341

To the authors’ knowledge, the standard PM and non-volatile PN measurement methods were

342

utilized for the first time in the present study in studying ship emissions with different fuels and


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after-treatment systems. Furthermore, the dilution conditions were strictly controlled in order to

344

make comparison of the results of different measurement campaigns possible both on board and

345

in engine laboratories.

346

Gaseous exhaust emissions

347

Liquid fuels and natural gas

348

With the laboratory engine (4R32), gaseous emissions were also studied (Table 2). Lower

349

levels of NOx were measured with NG compared to MGO or MDO. This was most probably due

350

to the homogenous premixing of the gas with air and the lower combustion temperature of NG

351

(in lean burn conditions). As expected, no SO2 was detected in the exhaust gas when utilizing

352

either NG or MGO (23) measurement system

425 426

AUTHOR INFORMATION

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Corresponding Author

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*E-mail [email protected]; phone +358 407236703

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Author Contributions

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The manuscript was written through contributions of all authors. All authors have given approval

431

to the final version of the manuscript.

432

ACKNOWLEDGMENT

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This study was part of several projects: HERE, SEA-EFFECTS BC and INTENS, funded by

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Business Finland and several Finnish companies, and Hercules-2 with funding from the

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European Union’s Horizon 2020 research and innovation programme under grant agreement No.

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634135.

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Particulate Mass and Nonvolatile Particle Number Emissions from

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