Introduction
Gaseous and particulate pollutants emitted from vessels operating in the open
ocean as well as in coastal areas and inland waterways have significant
adverse impacts on human health, air quality, and climate change (Cappa et
al., 2014; Righi et al., 2011; Marmer and Langmann, 2005; Winebrake et al.,
2009). It has been estimated that 87 000 premature deaths occurred in 2012
due to burning of marine fuels with high sulfur content. Shipping-related
particulate matter (PM) emissions have been reported to be responsible for
approximately 60 000 cardiopulmonary and lung cancer deaths annually, with
most cases occurring near coastlines in Europe (Viana et al., 2014), East
Asia, and South Asia (Corbett et al., 2007). Approximately 9200 and
5200 t year-1 of PM are emitted from oceangoing and coastal ships,
respectively, in the USA (Corbett and Fischbeck, 2000), most of which are fine or even
ultrafine aerosols (Viana et al., 2009; Saxe and Larsen, 2004). Globally,
about 15 % of nitrogen oxides (NOx) and 5–8 % of sulfur oxides
(SOx) emissions are attributable to oceangoing ships (Corbett and Fischbeck, 2000).
Shipping emissions affect acid deposition and ozone concentrations,
contributing more than 200 mg S m-2 year-1 over the
southwestern British Isles and Brittany as well as additional 6 ppb surface
ozone during the summer over Ireland (Derwent et al., 2005). Moreover,
aerosol emissions from international shipping also greatly impact the Earth's
radiation budget, directly by scattering and absorbing solar radiation and
indirectly by altering cloud properties (Righi et al., 2011). Besides,
according to estimates from IMO (2014), total shipping emissions were
approximately 938 million tonnes CO2 and 961 million tonnes
CO2e
(CO2 equivalent) for GHGs combining CO2, CH4 and N2O for
the year 2012. International shipping emission accounts for approximately 2.2
and 2.1 % of global CO2 and GHG emissions on a CO2 equivalent
(CO2e) basis, respectively. Because nearly 70 % of ship emissions
are estimated to occur within 400 km of land (Endresen, 2003), ships have
the potential to contribute significantly to air quality degradation in
coastal areas. In addition, ports are always the most concentrated areas for
ships to berth at, emission reduction measures such as switching from heavy fuels
to cleaner fuels are required when ships are close to ports or offshore
areas, but not all of them can obey the regulations (De Meyer et al., 2008),
which results in significant influence on atmospheric environment of port
cities and regions.
Rapid developments of ports, international trade, and the shipbuilding
industry in China have negatively affected the ambient air quality of the
coastal zone due to shipping emissions. It was estimated that 8.4 % of
SO2 and 11.3 % of NOx were emitted from ships in China in 2013
with port cities being the worst effect areas
(http://news.xinhuanet.com/politics/2015-06/08/c_127890195.htm). In
2013, there were 0.18 million water transport vessels (Ministry of
Transportation, 2013) active in Chinese waters, 8 ports in China were listed
among the world's largest 10 ports, and 11 container ports were listed among
the world's largest 20 container ports. The number of ports with cargo
handling capacity of more than 200 million t year-1 grew to 16
(Ministry of Transportation, 2010). Rapid development of ports in China has
resulted in increasingly serious pollution of ambient air, particularly in
coastal zones and near ports. Only a few studies have focused on pollution
from shipping emissions in China. Rough estimates of the influence of
shipping emissions on ambient air in the port of Shanghai, the largest port
in China (Zhao et al., 2013) and in the Bohai Rim (Zhang et al., 2014);
these estimates have been generated using empirical formulas. One case study of real-world
emissions of inland vessels on the Grand Canal of China has been conducted
(Fu et al., 2013), and another study focused on inland ships and several
offshore vessels resulted in some rough emission data (Song, 2015). Other studies
also have developed to the inventories in large ports or delta regions (Zheng
et al., 2011; Zheng et al., 2009) by using EFs (emission factors) obtained from other countries or areas.
However, there are no systematic studies of vessel emissions in the coastal
zone or in ports, nor accurate estimates of shipping emissions to ambient air
based on measured emission factors. Conditions in China differ
substantially from those in other countries, such as in vessel types (more
small motor vessels, and the type composition of offshore vessels is shown in
Table S1 in the Supplement, more light-tonnage vessels, e. g. with 79.3 %
less than 3000 t in offshore area of Yangtze River Delta that is shown in
Table S2), different fuel standards compared with other countries (the GB/T 17411-2012 standard with sulfur content of less than
3.5 % m / m;
however, the ISO 8217-2010 international standard has the maximum sulfur
content according to the relevant statutory requirements that always have
lower values, such as less than 0.1 % in emission control areas; besides,
a large percentage of diesel fuel was used in China, especially in offshore
areas, seen in Table S3). The age of vessels is also important (Chinese commercial vessels have an
average age of 19.2 years compared with 8.0 years and 8.9 years for Japan and
Germany, respectively). However, a large part of previous studies focused on emission of large-tonnage vessels such as cargo ships (Moldanova
et al., 2009; Celo et al., 2015), large marine ships (Khan et al., 2013;
Sippula et al., 2014), tankers (Agrawal et al., 2008b; Winnes and Fridell,
2010) and so on, whose fuel types were typically of the heavy oil variety, and most of them
had engines less than 10 years. Thus, experimentally determined EFs for
vessels in other countries cannot be used directly to estimate shipping
emissions and their contribution to ambient air quality in China, especially
in offshore areas. Systematical measurement EFs for different kinds of
vessels in China is essential.
Numerous studies of shipping emissions based on experimental measurements
have been conducted since the International Maritime Organization (IMO) first
began to address air pollution from vessels in 1996, particularly in
developed countries. Most of these studies have been carried out by
performing tests on board the vessel from the exhaust pipe (Agrawal et al., 2008b; Murphy
et al., 2009; Fridell et al., 2008; Juwono et al., 2013; Moldanová et
al., 2013) or by taking measurements within the exhaust plumes (Sinha et al.,
2003; Chen et al., 2005; Lack et al., 2009; Murphy et al., 2009; Berg et al.,
2012; Pirjola et al., 2014; Petzold et al., 2008). NOx, carbon monoxide
(CO), sulfur dioxide (SO2), and PM are the main constituents of shipping
emissions (Moldanova et al., 2009; Williams et al., 2009; Agrawal et al.,
2008a; Poplawski et al., 2011; Endresen, 2003) that have been quantified. In
addition, black carbon (BC) (Lack and Corbett, 2012; Sinha et al., 2003;
Moldanova et al., 2009; Corbett et al., 2010), and cloud condensation nuclei
(CCN) (Sinha et al., 2003; Lack et al., 2011) also have been reported in some
studies. Reported emission factors for CO, SO2, NOx, PM, and BC
are in the ranges of 0.5–16, 2.9–44, 22–109, 0.3–7.6, and
0.13–0.18 g kg-1 fuel, respectively, and
0.2–6.2 × 1016 particles kg-1 fuel for CCN. Besides,
characteristics of gaseous species and PM have attracted more attention
recently (Anderson et al., 2015; Celo et al., 2015; Mueller et al., 2015;
Reda et al., 2015).
The IMO has set the emission limits for NOx and SOx in the revised
MARPOL (Maritime Agreement Regarding Oil Pollution) Annex VI rules (IMO,
1998). Ships operating in the emission control areas (ECAs) (the Baltic Sea,
the North Sea, the North America and the Caribbean) should use fuels
with a sulfur content of less than 0.1 % m / m since January 2015. Even more
stringent limits have been laid down in some national or regional
regulations. For example, in some EU ports, seagoing ships at berth are
required to switch into fuels of under 0.1 % m / m sulfur since
2010 (The Council of the European Union, 1999); both marine gas oil and
marine diesel oil used in water area within 24 nautical miles of coastline in
California should have a sulfur content of less than 0.1 % m / m since
2014 (California Code of Regulation Titles 13 and 17, 2016). Emission standard of
Tier II for NOx set by MARPOL VI has been executed since January 2011 in
ECAs, and more stringent rules of Tier III will be executed from January 2016.
However, in China, no specific policy or limit for shipping emissions
has been implemented except in Hong Kong, which is making legislation about
the limit of 0.5 % sulfur content fuel used when berth in the port from
2015. But because of the serious air pollution currently in China, emission
limits for the main sources such as vehicle exhaust, coal combustion, biomass
combustion and fugitive dust have become more and more stringent. A draft
aimed at limiting the emissions from marine engines set by Ministry of
Environmental Protection is on soliciting opinions. It has set the limits of
CO, HC, NOx and PM for different kinds of vessels, which are mainly
based on the Directive 97/68/EC set by EU (European Union, 2012) and 40 CFR part 1042 set by EPA (US Environmental Protection Agency, 2009). In
addition, an implementation plan has been released by the Ministry of
Transport of the People's Republic of China in December 2015 aiming to set
shipping emission control areas to reduce SO2 emissions in China
(Ministry of Transport of the People's Republic of China, 2015). All the
regulations were set mostly based on other directives and regulations.
Detailed measurement data will assist with further policy, making it more
relevant to current situations of vessels.
Average EFs are often used for shipping emissions inventories on large scales
or in regional areas (Tzannatos, 2010; Eyring et al., 2005). However, to evaluate
the effects of shipping emissions on air pollution in local areas such as
near ports, various ship speeds and operating modes should be considered,
including docking, berthing, and departing from ports etc. Previous studies
have confirmed that EFs are significantly different under various load
conditions (Petzold et al., 2010) or in different operating modes (Fu et al.,
2013; Winnes and Fridell, 2010) for individual vessels. Therefore, more
detailed measurements of EFs in different operating modes are necessary to
better estimate the impacts of shipping emissions on the environment.
In this study, experimental data for three different diesel-engine-powered
vessels were collected. All pollutants were measured directly in the stack.
Gaseous emissions and PM from the diesel engines were the main targets,
including CO, carbon dioxide (CO2), SO2, NOx, total volatile
organic compounds (TVOCs), and total suspended particulates (TSPs).
Fuel-based EFs for the three vessels were calculated using the carbon
balance method under different operating conditions. In addition, fuel-based
average EFs as well as power-based average EFs to values reported in other
studies and for other vessels were compared. Finally, the impacts of engine
speed on the EFs of NOx were evaluated.
Technical parameters of test vessels.
Vessel
Vessel
Displacement
Ship length
Engine
Vessel
Rated
Fuel
ID
type
(ton)
× width
power
age
speed
consumption
(m)
(kw)
(year)
(rpm)
rate (g KWh-1)
HH
Engineering vessel
307
44 × 13
350 × 2
4
1200
200
DFH
Research vessel
3235
96 × 15
1600 × 2
18
900
200
XYH
Research vessel
602
55 × 9
600
5
1000
200
Experimental
Test vessels and fuel types
Initially, it was hoped that the choice of measurement ships would reflect
the shipping fleet in general, i.e. in terms of engine type (engine speed
and power output), fuel used, engine age and mode of operation, with more
than 10 vessels planned to test. However, consideration was given to the
practicalities involved with the measurements, i.e. installation of sampling
systems, external conditions, etc. Besides, time and economic constraints
weighed heavily and only several shipowners willing to participate in the
project. Thus, the chosen vessels of different engine powers with diesel
used represent a compromise.
Three different diesel-engine-powered offshore vessels, including one
engineering vessel, Haohai 0007 (HH), with low-power and high-speed engine,
one large research vessel, Dongfanghong 2 (DFH), with high-power and medium-speed engine, and another research vessel, Xiangyanghong 08 (XYH), with
medium-power and medium-speed engine were selected for this study; their technical parameters are shown in Table 1. High-speed and medium-speed
engines are the predominant engines used in vessels of offshore and inland
rivers in China, which always use light diesel as fuel. Two of the test
vessels were small motor, light-tonnage vessels (HH and XYH), and another one
was a medium-speed engine vessel with an 18 year-old engine. Engineering
vessels are designed for construction activities such as building docks in
port areas or waterways, dredging, etc. They are common vessels in coastal
areas of China because of the heavy demand for oilfield construction and port
expansion. The maintenance of engineering vessels is typically poorer than
for other types of vessels and as a result, they may have relatively high
emissions. On the other hand, research vessels of DFH and XYH from
universities and research institutes are generally well maintained and use
high-quality diesel fuel but with different engine powers, which might have
relatively low emission factors for pollution. Therefore, these research
vessels can reflect the impact of engine power on emissions and also can
represent the lower end of expected EFs for Chinese vessels. The test vessels
in the present study could account for 34.7 % of the total vessels according
to the distribution of vessels through gross tonnage in China (seen in
Table S2), which could have a certain degree of representation. Everything considered, a
general range of EFs for gaseous and PM pollutants emitted from different
offshore vessels of China and their influence factors could be given through
the on-board measurement.
The fuels used in all test vessels were common diesel fuels obtained from
fuelling stations near the ports. According to statistical data, the total oil
consumption of vessels in China was 20.99 million tons in 2011, including
10.99 million tons bonded oil and 5.93 million tons domestic trade oil, with
light fuel oil accounting for 40 % of the domestic trade oil and 25 %
of the total consumption (shown in Table S3) (Zhu, 2013). The test vessels in
the present study could reflect the emission condition of diesel vessels in
China, especially in offshore areas where diesel oil always been used as
fuel. Results of fuel analyses are presented in Table 2. All of these fuels
had relatively low sulfur content (≤ 0.13 % m) and low metals
concentrations (V, Al, Si, Pb, Zn, Mn, etc.).
Results from the fuel analysis (diesels).
Units
HH
DFH
XYH
Total calorific value
MJ kg-1
45.44
45.40
45.50
Net calorific value
MJ kg-1
42.51
42.48
42.55
Ash content
% m
0.001
< 0.001
< 0.001
Sulfur (S)
% m
0.0798
0.0458
0.130
Carbon (C)
% m
86.66
86.40
86.49
Hydrogen (H)
% m
13.32
13.22
13.44
Nitrogen (N)
% m
< 0.2
< 0.2
< 0.2
Oxygen (O)
% m
< 0.4
< 0.4
< 0.4
Test operating modes
EFs are significantly different under differing load conditions and operating
modes. Vessel speed is also an important influence factor for emissions; it was reported by Starcrest Consulting Group, LLC (Starcrest Consulting Group,
2012) that 15–20 % of fuel consumption could be reduced by 10 % of the vessel speed. In this study, vessel operating modes were
classified according to actual sailing conditions. There were six modes of
HH: low speed (4 knots), medium speed (8 knots), high speed (11 knots),
acceleration process, moderating process, and idling; four modes of DFH:
cruise (10 knots, medium speed for DFH), acceleration process, moderating
process, and idling; and five modes of XYH: low speed (3 knots), high speed
(10 knots), acceleration process, moderating process, and idling. Three to
five sets of replicate samples were collected for each operating mode.
Emissions measurement system and chemical analysis of particulate matter
A combined on-board emissions test system (Fig. 1) was used to measure
emissions from the coastal vessels under actual operating conditions. There
was no dilution in this test system with all the species measured directly
from the exhaust and there were four main components of the system: a flue
gas analyser, three particulate samplers, an eight-stage particulate sampler,
and a TVOCs analyser. (see Supplement for more details). All analytes are
also shown in Fig. 1: the flue gas analyser (Photon II) is aimed to test
instantaneous emissions of gaseous pollution, including O2, NO2,
NO, N2O, CO, CO2, and SO2 (detection parameters for the gaseous
matter are shown in Table S4). Three particulate samplers are installed to
collect PM using different filters at the same time, including a quartz fiber
filter, glass filter, and polytetrafluoroethylene filter to analyse different
chemical components of PM. And the portable TVOCs analyser is used to monitor
the concentration of total VOCs with isobutylene as correction coefficient
gas. Besides, a temperature sensor is installed near the smoke outlet to test
the flue gas temperature. A total of 33 sets of samples for HH, 20 sets for
DFH, and 23 sets for XYH were collected, with 3 to 5 sets for each operating
mode.
On-board emissions test system and measured analytes.
The OC (organic carbon) and EC (elemental carbon) were measured on a 0.544 cm2 quartz filter punched from
each filter by thermal optical reflectance (TOR) following the IMPROVE
protocol with a DRI Model 2001 Thermal/Optical Carbon Analyzer (Atmoslytic
Inc., Calabasas, CA). The measuring range of TOR was from 0.05 to
750 µg C cm-2 with an error of less than 10 %.
Concentrations of water-soluble ions in PM2.5, such as Na+,
NH4+, K+, Mg2+, Ca2+, Cl-, NO3- and
SO42-, were determined by ion chromatography (Dionex ICS3000, Dionex
Ltd. America) based on the measurement method of Shahsavani et al. (2012).
The detection limit was 10 ng mL-1 with an error of less than 5 %,
and 1 mL RbBr with concentration of 200 ppm was put in the solution as
internal standard before sampling. The concentrations of 33 inorganic
elements in PM2.5 were estimated using a inductively coupled plasma
mass spectrometer (ICP-MS of ELAN DRC II type, PerkinElmer Ltd.
Hong Kong) following the standard method (Wang et al., 2006). The resolution
of ICP-MS ranged from 0.3 to 3.0 amu with a detection limit lower than
0.01 ng mL-1, and the error was less than 5 %.
Data analysis
Carbon balance formula was used to calculate the EFs for all exhaust gas
components. It was assumed that all carbon in the fuel was emitted as
carbon-containing gases (CO, CO2, and TVOC) and carbon-containing
particulate matter. So there was a certain equilibrium relationship between
the carbon in the fuel and in the exhaust:
CF=RFG×(c(CCO)+c(CCO2)+c(CPM)+c(CTVOC)),
where CF represents the mass of C in per kg diesel fuel
(g C kg-1 fuel); RFG represents the flue gas emissions rate
(m3 kg-1 fuel); and c(CCO), c(CCO2),
c(CPM), and c(CTVOC) represent the mass
concentrations of carbon as CO, CO2, PM, and TVOC (g C m-3) in
the flue gas, respectively.
The EF for CO2 was calculated as follows:
EFCO2=RFG⋅c(CO2)⋅MCO2,
where EFCO2 is the EF for CO2
(g kg-1 fuel), c(CO2) is the molar concentration of
CO2 (mol m-3), and MCO2 is the molecular weight
of CO2 (44 g mol-1).
The remaining EFs were calculated as follows:
EFx=ΔXΔCO2⋅MXMCO2⋅EFCO2,
where EFx is the EF for species X (g kg-1 fuel), ΔX and
ΔCO2 represent the concentrations of X and CO2 with
the background concentrations subtracted (mol m-3), and MX
represents the molecular weight of species X (g mol-1).
In addition, average EFs for each vessel were calculated based on actual
operating conditions as follows:
EFX,A=∑X,iEFi×Pi,
where EFX,A is the average EF for species X, EFi is the EF
for operating mode i for species X, and Pi is the percentage of time
spent in operating mode i during the shipping cycle.
Power-based emission factors and fuel-based emission factors could be
interconverted with the formula as following:
EFX,P=EFx⋅FCR,
where EFX,P is the power-based emission factor for species X
(g kW h-1), and FCR is fuel consumption rate for each vessel
(kg fuel (kW h)-1).
Results and discussion
Concentrations in shipping emissions
Concentrations of CO, NOx, SO2, TVOC, and PM from the three vessels
are shown in Fig. S1 in the Supplement. Nearly all of the concentrations measured in the
exhaust of low-engine-power vessel HH were higher than those of the two
higher-engine-power vessels. Concentrations of CO, SO2, and NOx
from HH were 10.7–756, 5.34–33.1, and 87.8–1295 ppm, respectively, and
14.3–59.5 mg m-3 PM. In contrast, concentrations of CO, SO2,
NOx, and PM were 50.1–141, 5.27–16.9, 169–800 ppm, and
7.06–21.8 mg m-3, respectively, for DFH and 36.0–224, 0.49–35.9,
and 235–578 ppm and 0.56–6.31 mg m-3, respectively, for XYH.
A previous study demonstrated that concentrations of CO primarily depend on
engine power, with higher CO emissions resulting from vessel engines with
lower power (Sinha et al., 2003). There was a similar trend in this study
with generally higher concentrations for HH and lower concentrations for
DFH. The CO concentrations in the present study were similar but slightly
lower than those of inland vessels (Fu et al., 2013), except in the idling
mode of HH. In different operating modes, CO concentrations were
significantly different. For example, the maximum value was observed in
idling mode and the minimum value in medium-speed mode for HH. All three
ships had the lowest CO concentrations at their economic speeds (medium
speed for HH, cruise mode for DFH, and high speed for XYH), demonstrating
that their engines are optimized for the most common operating mode.
More than 80 % of the NOx was NO in this study, with NO2 and
N2O accounting for < 20 % in all operating modes (Fig. S1).
Again, nearly all of these concentrations were higher in the exhaust gas of
HH than in that of the two vessels. In high-speed modes, all of the vessels
had high concentrations of NOx. NOx emissions mainly depend on the
combustion temperature of the engines. More powerful combustion systems
operate at higher temperatures, thereby producing more NOx (Corbett et al.,
1999). However, the NOx emissions were much lower than for the inland
vessels studied by Fu et al. (2013), particularly in cruise mode (NOx
concentrations of ∼ 1000 ppm).
SO2 concentrations in the exhaust gas depend on the sulfur content of
the fuel and the flow rate of the flue gas. There were significant
differences among the three vessels in their flow rates, which could account
for the different concentrations of one vessel in different operating modes.
But because of the low-sulfur fuels used in these vessels, the SO2
concentrations were low compared with those in other studies (Williams et
al., 2009; Berg et al., 2012).
EFs for the typical pollutants in different operating modes.
Much lower concentrations of PM in the exhaust gas were observed in the
present study compared to those of inland ships in China (Fu et al., 2013).
However, they were similar to those from ships at berth reported by Cooper et
al (Cooper, 2003). HH had higher PM concentrations than the two vessels in
the exhaust gas. There were significant differences among the different
operating modes because of changes in the injection point of the engines
(Sippula et al., 2014; Li et al., 2014).
Fuel-based emission factors
Fuel-based EFs for the gaseous species CO2, CO, NO, NO2, N2O,
and TVOCs and for PM based on the carbon balance method were determined. In
addition, SO2 was calculated based on the sulfur content of the fuels.
Fuel-based EFs for the typical pollutants such as CO, PM and nitrogen oxides
in different operating modes are shown in Fig. 2 (detailed EFs for all the
gaseous pollutants are shown in Table S5 and detailed EFs for PM and its
chemical composition are shown in Table S6).
CO2 emissions from vessels primarily depend on the carbon content of the
fuel (Carlton et al., 1995). Accordingly, the EFs for CO2 in the present
study should theoretically be 3177, 3168, and 3171 g kg-1 fuel for
complete combustion. Under actual conditions, CO2 emissions were
2940–3106, 3121–3160, and 3102–3162 g kg-1 fuel for HH, DFH, and
XYH, respectively, which means they had combustion efficiencies with
92.5–97.8, 98.5–99.7, and 97.8–99.7 % in terms of CO2 for these
three vessels.
CO emissions of HH were much higher than of XYH, followed by DFH. The power
of their respective engines was 350, 600, and 1600 kW. In addition, there
were large differences in CO emissions among different modes. All of these three
vessels had relatively high EFs for CO while accelerating compared with other
modes, but the highest EFs were during the idling modes of HH and DFH, as
well as during the low-speed mode of XYH. Because CO emissions in diesel engines primarily
depend on the excess air ratio (which determines the fuel–air mixture),
combustion temperature, and uniformity of the fuel–air mixture in the
combustion chamber (Doug, 2004), ship engines with lower power generally have
higher CO emissions (Carlton et al., 1995). Localized hypoxia and incomplete
combustion in the cylinder were the main reasons for CO emission of diesel
engine. CO emissions always had positive relationships with the air-to-fuel
ratio. There was lower air-to-fuel ratio when in low engine load, which resulted
in lower CO emission, and vice versa (Ni, 1999).
Much higher NOx EFs were observed for HH than for the other two vessels.
These results were inconsistent with those of Sinha et al. (2003), in which emissions of NOx increased with the power of the ship
engine. With increasing vessel speed, NOx EFs for HH first increased and
then decreased. XYH had lower EFs when operating at high speed than at low
speed. Nitrogen oxides included NO, NO2, and N2O in the present
study. More than 70 % of the NOx was in the form of NO for all
vessels, because most of the NOx emissions were generated through
thermal NO formation (Haglind, 2008). The primary reasons that slow diesel
engines such as the one in HH have higher NOx emissions include higher
peak flame temperatures and the NO formation reactions being closer to their
equilibrium state than in other engines (Haglind, 2008). NOx emissions
from vessels are temperature-dependent (Sinha et al., 2003) and also are
influenced by the oxygen concentration in the engine cylinder (Ni, 1999). In
larger engines, the running speed is generally slower and the combustion
process more adiabatic, resulting in higher combustion temperatures and more
NOx. Besides, with the increasing of air-to-fuel ratios, concentration of
NOx showed a tendency first to increase, then to decrease, which always
had the maximum value in the operating mode that close to full load of engine
because of the high temperature and oxygen in the engine cylinder (Ni, 1999).
Furthermore, there were always higher EF values in acceleration process and
lower in moderating process in this study. When the engines were in transient
operating conditions, such as acceleration process or moderating process,
concentrations of NOx always had corresponding changes in the cylinder.
Studies about diesel engines showed that when the rotational speed had a
sudden increase, there would be a first increasing, then decreasing and last
stable tendency for the NOx concentrations, and vice versa (Tan et al.,
2012).
TVOCs emissions from HH were much higher than from the other two vessels; the
lowest emissions were observed for DFH. Previous studies (Sinha et al., 2003)
have reported that hydrocarbon emissions from vessels depend on engine power,
with low-power engines emitting more hydrocarbons. The present results were
partially consistent with these previous studies. Besides, hydrocarbon
emissions also depend on the percentage utilization of engine power (Sinha et
al., 2003). As for various operating modes, TVOCs EFs had large differences.
For example, HH had the highest TVOCs emissions in accelerating mode, which
was almost 3 times the height of the lowest value in medium–speed mode.
The EFs for SO2 depended solely on the sulfur content of the fuels and
were 1.6, 0.9, and 2.6 g kg-1 fuel for HH, DFH, and XYH, respectively
in this study. Hydrocarbon could be generated because of the incomplete
combustion. For example, in diesel cylinders, there will always be air present in wall
regions and crevices; this is also the case when scavenging occurred during the
aeration, which could cause the uneven mixing of air and fuel (Ni, 1999).
Fuel-based EFs for PM and its chemical components were shown in Table S6. OC
and EC were the main components of PM, followed by SO42-,
NH4+, and NO3-. Metals such as V, Ni, Cr, Fe, As, and Cd made
up a proportionately small part of the total PM mass. However, other rare
elements such as Tb, Er, Yb, and Lu had higher values than did some of the
common elements. PM was an in-process product during the combustion in
a cylinder; the forming process included the molecular cracking,
decomposition, and polymerization, which resulted in lack of oxygen. High temperature
and oxygen deficiency were the main reasons for the formation in diesel
engines, which always had high concentration values in high load operating
modes (Ni, 1999). HH had much higher PM emission factors than the other two
vessels, the engine type was considered to be the most significant influence
factor, which had a good agreement with NOx emission factors.
EFs for OC and EC and the ratios between them are shown in Fig. 3. EFs for OC
and EC for HH were higher than for the other two vessels. Organic matter (OM)
is generally calculated as OC × 1.2 (Petzold et al., 2008) to
account for the mass of elements other than carbon in the emitted molecules.
OM EFs for individual vessels mainly depend on the engine type and the amount
of unburned fuel, i.e. the efficiency of combustion (Moldanová et al.,
2013). BC emissions also depend heavily on the engine type (Lack et al.,
2009). Therefore, the different types of engines and their levels of
maintenance could account for the large differences in OC and EC EFs observed
among the three vessels in this study. The ratios of OC-to-EC in the present
study were much lower than those for large diesel ships reported previously
(OC / EC = 12) (Moldanova et al., 2009) and also lower than that
reported for a medium-speed vessel (Petzold et al., 2010). The usage of
non-dilution sampling in this study was one possible reason for the lower OC
to EC ratio. Besides, TOR was used to measure OC and EC in PM, which always
had a lower OC content compared with other methods (such as TOT) because of
the different definitions of OC and EC (Khan et al., 2012). Compared with
other diesel engines, the ratios of OC to EC in this study were higher than
that of automobile diesel soot, in which EC comprises 75–80 wt % of the
total PM (Clague et al., 1999), and also higher than heavy heavy-duty diesel
trucks (HHDDTs) with OC to EC ratios below unit for cruse and transient modes
even though higher in cold-start/idle and creep modes (Shah et al., 2004).
EFs for OC and EC and the ratios between them.
Studies have shown that SO42- formed from vessel-emitted SO2 is
a major contributor to CCN and ship track formation (Schreier et al., 2006;
Lauer et al., 2007). Sulfate is also an important component of PM emitted
from vessels. In the present study, EFs for SO42- were much lower
than previously reported (Petzold et al., 2008; Agrawal et al., 2008a), but
similar to those detected by a high-resolution time-of-flight aerosol mass
spectrometer in a previous study (Lack et al., 2009). This may be because EFs
for SO42- are mainly related to the sulfur content of the fuel;
SO42- is not generally emitted directly from the engines, but forms
after release from the stack (Lack et al., 2009). Because PM was collected
directly from engine emissions in the present study, the sulfur-to-sulfate
ratios were low (< 0.6 % for vessels). Other ions such as NO3-
and NH4+ accounted for a small percentage of the PM emitted from the
vessels compared with SO42-, consistent with previous studies (Lack
et al., 2009). SO2 is more easily oxidized to SO3 in catalytic
reaction cycles with metals commonly present in the exhaust gas (V, Ni),
while hydroxyl radicals are additional needed to convert NOx to
NO3- (Moldanova et al., 2009).
Na+ and Cl- were considered to originate from marine air. Their
concentrations were highly correlated (r2 = 0.78); the differing air
demands of the engines under different conditions might have caused observed
variations in the EFs relative to the fuel demand.
Fuel-based average EFs in the present study and previous studies
(g kg-1 fuel).
Vessel ID
CO2
CO
NO
NO2
N2O
NOx
TVOCs
PM
SO2
S content (% m)
HH
3071 ± 1565
30.2 ± 16.2
98.2 ± 37.2
15.5 ± 5.45
1.28 ± 1.70
115 ± 44.3
23.7 ± 21.0
9.40 ± 2.13
1.60
0.08
DFH
3153 ± 176
6.93 ± 1.00
30.2 ± 1.60
5.09 ± 0.42
0.38 ± 0.18
35.7 ± 2.20
1.24 ± 0.04
0.72 ± 0.33
0.92
0.05
XYH
3151 ± 175
9.20 ± 2.95
26.6 ± 1.63
4.71 ± 0.42
0.30 ± 0.15
31.6 ± 2.20
4.18 ± 0.15
0.16 ± 0.07
2.60
0.13
Commercial vessel (Williams et al., 2009)
3170
7–16
–
–
–
60–87
–
–
6–30
Cargo vessel (Moldanova et al.,2009)
3441
2.17
–
–
–
73.4
–
5.3
39.3
1.9
Diesel engine (Haglind, 2008)
–
7.4
–
–
–
87
–
7.6
54
2.7
Ocean-going ships (Sinha et al., 2003)
3135
19.5
–
–
–
22.3
–
–
2.9
0.1
Ocean-going ships (Sinha et al., 2003)
3176
3.0
–
–
–
65.5
–
–
52.2
2.4
Cargo and passengerships (Endresen, 2003)
3170
7.4
–
–
0.08
57–87
2.4
1.2–7.6
10–54
0.5–2.7
Ships operating in
–
–
42–72
–
–
65–86
–
–
4.6–9.8
NONE
harbour areas (Pirjola et al., 2014)
–
–
16–49
–
–
25–79
–
–
5.4–17.0
SCR
Ships operating in Port (Diesch et al., 2013)
–
–
16
37
–
53
–
–
7.7
NONE = No treatment of emissions, SCR = selective catalytic reduction.
The elemental composition of PM in the present study differed from previous
studies showing high elemental content of S, Ca, V, Fe, Cu, Ni, and Al
(Agrawal et al., 2008a; Moldanova et al., 2009). V and Ni are typically
associated with combustion of heavy fuel oil (Almeida et al., 2005). In the
present study, the high-quality fuels resulted in low EFs for V and Ni. In
our previous study, PM from shipping emissions was estimated to account for
2.94 % of the total PM2.5 at Tuoji Island in China, using V as a
tracer of shipping emissions (Zhang et al., 2014). Reconsidering the former
results based on the EFs obtained in the present study, we determined that
the contribution of vessels near Tuoji Island had been underestimated,
because the estimate should have included both heavy and other types of
fuels. However, some rare elements such as Tb, Er, Yb, and Lu had relatively
high EFs compared with those of other elements in the present study, which
may be related to the source of the fuels.
Fuel-based average emission factors
Based on actual operating conditions (Table S7), average EFs for the three
vessels in the present study (according to Eq. 4) along with EFs from
previous studies are shown in Table 3. EFs for all of the pollutants except
SO2 were significantly higher for HH than for the other two vessels,
potentially due to poor combustion conditions. Most of the EFs for DFH and
XYH were within the range of emissions for other vessels due to having well
maintained engines and the high quality of the fuels used. The EFs for
NOx, PM, and SO2 were much lower than reported in previous studies
(other than NOx for ocean-going vessels). All the sulfur of the fuels in
the present study were significantly below the emissions limit of 3.50 %
established by IMO in the revised MARPOL Annex VI rules, applicable since
2012 (IMO, 1998).
The IMO Tier I emissions limit for NOx is
45.0 × n-0.2 g kWh-1 (n, rated speed,
130 < n < 2000 rpm). The rated speed and fuel consumption rates for
each vessel are shown in Table 1. Thus, the emissions limits for HH, DFH, and
XYH would be 54.5, 57.5, and 56.5 g kg-1 fuel, respectively,
calculated combined with Eq. (5). The average fuel-based EFs for NOx of
ship HH was more than 100 % above the IMO standard, while those of the
other two ships were below the IMO standard (Table 3). PM emissions for HH
were also higher than previously reported, but those for the two research
vessels were much lower (Table 3). Fuel type is one of the most important
influence factors on pollutant emissions, for example, sulfur content in the
fuel not only influence the SO2 emission directly, but also had impact
on PM formation in the flue gas stack with low sulfur content in fuels
reduces PM formation (Lack et al., 2011). Vessels with higher sulfur content
always had relatively higher PM emissions, which were also shown in Table 3.
In addition, different engines and levels of maintenance have a significant
impact on all combustion-dependent emissions. Emission reduction measures
have been used in some vessels. For example, NOx emissions can be
reduced by measures such as selective catalytic reduction (SCR) and direct
water injection (DWI), which had been implemented on some vessels previously
studied in a harbour in Finland (Pirjola et al., 2014). The results showed
that SCR effectively reduced NOx emissions, while vessels with DWI had
high PM emissions. The engine type might be an important cause of the
different emissions, such as HH had much higher pollutants emissions with an
engine produced in China and yet DFH's engine produced in Germany. Besides,
emission tests for a high-speed marine diesel engine with different kinds of
diesels showed that diesel type had limited influence on emissions such as
NOx, CO and CH, but a significant impact on PM emission
(28.9–41.5 %) because of the different sulfur content in fuel (Xu,
2008).
Power-based EFs in the present study and previous studies (g kWh-1).
Vessel
CO2
CO
NO
NO2
N2O
NOx
TVOCs
PM
SO2
Engine power
Engine
Fuel type and
ID
(kW)
speed
sulfur content
(rmp)
(wt %)
HH
699 ± 352
7.38 ± 3.76
22.0 ± 8.41
3.45 ± 1.24
0.30 ± 0.39
25.8 ± 10.0
5.44 ± 4.84
2.09 ± 0.48
0.36
350
1200
DO, 0.0798
DFH
631 ± 35.2
1.39 ± 0.20
6.04 ± 0.32
1.02 ± 0.08
0.08 ± 0.04
7.14 ± 0.44
0.17 ± 0.01
0.14 ± 0.07
0.18
1600
900
DO, 0.0458
XYH
697 ± 38.5
2.01 ± 0.65
5.87 ± 0.36
1.04 ± 0.09
0.07 ± 0.03
6.97 ± 0.48
0.92 ± –
0.04 ± 0.01
0.57
600
1000
DO, 0.130
Tanker (Winnes andFridell, 2010)
–
1.61
–
–
–
7.82
–
0.58
–
4500
600
HFO, 1.6
Berthed ships (Cooper,
653–768
0.33–0.97
–
–
–
14.2–20.2
–
0.14–0.45
0.26–5.3
AE, 720–1270
720–1800
MGO, 0.06–1.2
2003)
691–803
0.77–1.71
12.9–17.5
0.48–0.67
2.5–9.6
1270–2675
720–750
RO, 0.53–2.2
691–694
0.92–0.98
9.6–9.9
0.17–0.19
1.0
1480
720
MDO, 0.23
Crude oil tanker(Agrawal et al., 2008b)
588–660
0.77–1.78
–
–
–
15.8–21.0
–
1.10–1.78
7.66–8.60
15 750
90
HFO, 2.85
Cruise ships (Poplawskiet al., 2011)
–
–
–
14.0
–
–
–
2.91
4.20
US EPA
621
1.4
–
–
–
18.1
–
1.31
10.3
Marine engine (Sippula et
–
1.2–11.4
11.3–29.5
–
–
11.4–30.9
0–9.5
0.83–6.36
–
1500
HFO, 2.7
al., 2014)
0–88
5.69–25.8
5.84–33.9
0.83–19.7
0.15–0.93
DF
Large marine ships (Khan et al., 2013)
533–612
0.35–0.60
–
–
–
16.6–20.6
–
0.91–2.19
7.2–11.4
36 740–68 530
97–102
HFO, 2.15–3.14
Ocean going containervessel (Agrawal et al.,2008a)
588–660
0.77–1.81
–
–
–
15.8–21.0
–
1.09–1.76
7.66–8.60
50 270
104
HFO, 2.05
Large cargo vessel(Moldanova et al., 2009)
667
0.42
–
–
–
14.22
–
1.03
10.3
20 200
97
HFO, 1.9
Ocean going cargo vessel
614 ± 1
0.83 ± 0.01
–
–
–
16.3 ± 0.2
–
1.51 ± 0.07
8.7 ± 0.1
128
IFO180
(Celo et al., 2015)
626 ± 7
0.26 ± 0.01
11.4 ± 0.1
0.81 ± 0.02
5.8 ± 0.07
525
IFO180
628 ± 9
0.30 ± 0.01
11.3 ± 0.1
0.94 ± 0.02
8.7 ± 0.1
450
IFO180
628 ± 1
0.81 ± 0.03
12.2 ± 0.01
0.83 ± 0.01
6.4 ± 0.1
450
IFO180
609 ± 1
1.31 ± 0.02
–
–
–
8.4 ± 0.03
–
0.37 ± 0.01
4.7 ± 0.01
440
IFO60
605 ± 1
0.00
–
–
–
16.7 ± 0.1
–
2.2 ± 0.2
10.3 ± 0.03
116
IFO380
622 ± 1
1.22 ± 0.02
–
–
–
10.7 ± 0.04
–
0.30 ± 0.03
0.47 ± 0.1
450
MDO
AE, auxiliary engine.
DO, diesel oil; HFO, heavy fuel oil; MGO, marine gas oil; RO, residual oil;
MDO, marine diesel oil; DF, diesel fuel; IFO, intermediate fuel oil.
Power-based emission factors
Based on the engine power and fuel consumption rates of the vessels,
power-based EFs were calculated (according to Eq. 5) and compared to results
from previous studies (Table 4). The EFs for HH were much higher than those
for the other two vessels, except for SO2. HH also had significantly
higher EFs for NOx than previously reported values, while EFs for
NOx of DFH and XYH were within the range of previously reported results.
Engine type was considered to be a significant influence factor for NOx
emissions, with lower engine speed having higher NOx emission factors
(Celo et al., 2015). In addition, compared to other vessels with a similar
engine type and diesel fuel, HH still had relatively higher NOx EF (seen
in Table 4), which could reflect the impact of engine condition (engine
quality and maintenance level) on shipping emissions. CO EFs for the test
vessels in the present study were higher than previous studies, which
produced similar results to those of inland ships and other test vessels in China (Fu et al.,
2013; Song, 2015). In spite of the influence of engine type on CO emissions
that with the higher engine speed having higher CO EFs (Celo et al., 2015),
engine condition combined with fuel quality might have significant influence.
All of the EFs for SO2 in the present study were lower than those in
previous studies, because of the low sulfur content of the present fuels.
Generally, PM emissions from marine diesel fuels are dependent on the fuel
(sulfate and metal oxide ash constituents) and on combustion conditions
(unburned hydrocarbons and carbon residue constituents) (Cooper, 2003). HH
had the highest PM emissions among the test vessels, although there were
almost no differences among the fuels (Table S6). Besides, HH had even higher
PM EFs than previously reported vessels with HFO fuel, and XYH had much lower
PM EFs than all the other vessels with even lower sulfur content fuel.
Therefore, combustion conditions were likely the determining factor for the
differences. It can be seen from Table 4 that most previous studies focused on the heavy fuel oil of shipping emissions. Compared with diesel fuels,
heavy fuel oil always had relatively low CO emission factors and high PM emission
factors. And among the heavy-fuel-oil-using vessels, engine type (engine speed
and engine power level) always played an important role on emissions such as
NOx and CO, which with lower engine speed having higher NOx EFs and lower CO
EFs.
Combined with other emission data of test ships in China (Fu et al., 2013;
Song, 2015), it could be seen that inland and some test offshore ships in
China always had higher NOx, CO, and PM emissions compared with other
test vessels in previous studies. And among the test vessels in China, there
also were differences for different engine types and ship types. In addition,
emission factors that were used for calculation of ship inventories in China
always came from other countries and areas. However, there seemed to be significant differences between the reference and test data, such as 10.0 to
13.2 g kW h-1 of NOx EF and 1.1 to 1.7 g kW h-1 of CO EF used for inland ships for ship inventory calculation (Zhu et al., 2015),
10.0 to 18.1 g kW h-1 of NOx EF and 1.1 to 1.5 g kW h-1 of
CO EF for harbour ships (Yang et al., 2015), compared to 15 to
17.3 g kW h-1 of NOx EF and 4.6 to 10.3 g kW h-1 of CO
EF from test inland ships (convert the fuel-based EF to power-based EF with
a factor of 200 g kW h-1) (Song, 2015), and 6.97 to
25.8 g kW h-1 of NOx EF and 1.39 to 7.38 g kW h-1 of CO
EF in the present study (Yang et al., 2015). Besides, whether there are obvious
differences of EFs between other types of vessels in China (such as low-speed
engine vessels with heavy fuel oil) and previous studies is still unclear.
Therefore, much more measurement data for different vessels in China are
still in urgent need for more accurate assessment of shipping emissions.
Impact of engine speed on NOx emission factors
NOx is formed in the combustion chamber by a combination of atmospheric
nitrogen and oxygen under high-pressure and high-temperature conditions. Many
factors affect NOx formation, including engine temperature, injection
point, and fuel quality. The IMO emissions limit for NOx is determined
by the rated speed of the engine; however, other factors must also be
considered to reduce NOx emissions.
The NOx EFs for the test vessels at various engine speeds are shown in
Fig. 4. The rated speeds of the vessels were 1200, 900, and 1000 rpm for HH,
DFH, and XYH, respectively. The actual engine speeds of HH were much lower
than the rated speed, while the two larger-engine-power vessels operated
close to their rated speeds, except during one operating mode of DFH. The
NOx EFs for HH differed significantly in different operating modes,
ranging from 39.1 to 143 g kg-1 of fuel. The NOx EF was highest
when the engine speed reached ∼ 750 rpm (Fig. 4). At lower engine
speeds, the NOx EFs had fluctuating but lower values. At higher engine
speeds closer to the rated speed of 1200 rpm, the NOx EFs were much
lower. The NOx EFs for the two larger-engine-power vessels changed
slightly with engine speed, but also had lowest values when their engine
speeds approached their rated speeds. Combined with the diesel propulsion
characteristic curve, there were large increases in the fuel consumption rate
when the engine speed increased. Therefore, a best-fit engine speed should be
determined based on both EFs and economic costs.
Emission factors for NOx at different engine speeds.
Engineering approaches for reducing the NOx emissions of marine engines
may be applied before, during, or after the combustion process (Verschaeren
et al., 2014; Habib et al., 2014). In the present study, the NOx EFs of
the two research vessels were below the IMO Tier I emissions limits. However,
for EMS, measures should be taken to meet the IMO emissions limit, including
increasing the engine speed and applying engineering technologies during or
after combustion, such as exhaust gas recirculation (EGR), selective
non-catalytic reduction (SNCR), or SCR.
Conclusions
Three offshore vessels with different engine power sources were chosen in this study
to collect measured data of gaseous species and particulate matter, including
NO2, NO, N2O, CO, CO2, TVOCs, SO2, and the total suspended
particulate. Besides, chemical composition of the PM were also analysed to
give detailed EFs for OC, EC, water-soluble ions and metal elements.
Concentrations, fuel-based EFs, fuel-based average EFs as well as power-based
average EFs for species of offshore vessels in China were presented. Furthermore,
impact of engine speed on NOx EFs was also discussed.
There were higher concentrations of pollutants for low-engine-power vessel HH
than for the other two vessels. CO concentrations for offshore vessels were
slightly lower than inland vessels in China, and all the three vessels had
the lowest CO concentrations at their economic speeds (the speed of the least
vessel operating expenditures during one voyage, they were high-speed mode,
cruise mode, and high-speed mode for HH, DFH and XYH, respectively). More than
80 % of the NOx was NO, and all the offshore vessels had higher
NOx concentrations in high-speed modes. Because of the low-sulfur fuels
used in this study, SO2 concentrations of these three offshore vessels
were lower than that in the literatures. And the PM concentrations were much
lower than inland vessels while showing significant differences among
different operating modes.
Fuel-based EFs for gaseous species and PM were presented based on the carbon
balance method. EFs for CO2 were 2940–3106, 3121–3160, and
3102–3162 g kg-1 fuel for HH, DFH and XYH. Because of the combustion
conditions such as excess air ratio, combustion temperature and uniformity of
the fuel–air mixture, EFs for CO showed high values in idling mode, but low
values in economic speed. All the offshore vessels had higher NOx EFs in
low speed than in high speed, but showed higher values when in acceleration
process. EFs for SO2 were 1.6, 0.9 and 2.6 g kg-1 fuel for HH,
DFH and XYH based on sulfur content of the fuels. OC and EC were the main
components of PM, with low OC to EC ratios that were lower than 0.1, followed
by SO42-, NH4+, and NO3-. Metals such as V, Ni, Cr,
Fe, As, and Cd made up a proportionately small part of the total PM mass.
Fuel-based average EFs as well as power-based EFs for the three different vessels of differing engine power were presented. EFs for most gaseous species and PM of HH
were much higher compared with the other higher-engine-power vessels, which
was also > 100 % above the IMO standard for NOx. Average PM EF of
the low-engine-power vessel HH was also much higher than that in the
literatures. However, average EFs for most species of the two larger-engine-power vessels were within the range of previously reported results. Engine
type was inferred as one of the most influence factors for the differences of
emission factors. Inland and some offshore ships in China always had higher
NOx, CO and PM emissions compared with other test vessels in previous
studies. In addition, emission factors that used for calculation of ship
inventories in China always had lower values than test vessels.
The impact of engine speed on EFs for NOx showed that when the engine
speed was close to the rated speed, there would be lower NOx EFs
values. However, combined with the high fuel consumption rate, an optimal
engine speed should be determined based on both EFs and economic costs.
Emission reduction measures for NOx for some of the offshore vessels in
China are still essential to meet the IMO emission limit.
Given the limits of vessel types and numbers, this study substantially gives
the EFs for gaseous species and PM of three different diesel-engine-powered
offshore vessels. However, as the development of ports in China, emissions
from cargo ships and container ships with large engine power have become
one of the most significant air pollution sources in port cities and regions.
Systematical measurement EFs of all kinds of offshore vessels in China are
essential in order to present the accurate emission inventory of ships.
Information about the Supplement
Supplementary information includes the details of the real-world measurement
system for vessels (Fig. S1), the concentrations of main gaseous matter and
PM of shipping emissions (Fig. S2), the types composition of offshore vessels
in China (Table S1), the distribution of vessels through gross tonnage in
2014 in offshore area of Yangtze River Delta (Table S2), the Chinese market
consumption of marine oil in 2011 (Table S3), the detection parameters for
gaseous matter (Table S4), the fuel-based EFs for the gaseous pollutants
(Table S5), PM and the chemical composition in PM for different operating
(Table S6) modes, and the actual operating conditions of vessels (Table S7).