Woodyard D. (ed.) Pounders Marine diesel engines and Gas Turbines
Подождите немного. Документ загружается.
78 Exhaust Emissions and Control
cut NOx reductions by over 90 per cent. An SCR installation thus gives engine
designers greater scope to pursue primary in-cylinder measures without increas-
ing fuel consumption.
In an SCR system the exhaust gas is mixed with ammonia (preferably in
the form of a 40 per cent solution of urea in water) before passing through a
layer of special catalyst at a temperature between 290°C and 450°C. The lower
limit is mainly determined by the sulphur content of the fuel: at temperatures
below 270°C ammonia and SOx will react and deposit as ammonium sulphate.
At excessive temperatures, the catalyst will be degraded depending on the
exhaust gas temperature; therefore, reliable SCR operation calls for fuels with
low sulphur contents.
Urea decomposes into ammonia and carbon dioxide on injection into the
hot exhaust gas stream. The ammonia reduces NOx to harmless gaseous waste
products—water and nitrogen; and parts of the soot and HC in the exhaust are
also removed by oxidation in the SCR process reactor.
Among the desirable qualities of a catalyst are: a low inertia, which means
that the ammonia slip (the quantity slipping past the catalyst) is extremely low,
even during transient operations; a low-pressure drop and a short heating-up
time; and a low fouling tendency, ensuring minimal deterioration in perform-
ance over time.
The catalytic conversion rate of SCR systems is highly dependent on the
amount of urea dose: increased dosage yields increased conversion. However,
excessive urea causes ammonia slip downstream of the reactor, which is detri-
mental to both the process and operational economy. Urea dosing must there-
fore be very accurate under different load conditions. The set point for dosing is
derived primarily from the engine speed and load; in addition, control is adjusted
by measuring the residual NOx level after the reactor. The NOx measurement
data are used for tuning the stoichiometric ammonia/NOx ratio and maintaining
the ammonia slip at a constant level.
The temperature requirement of the process generally dictates that the SCR
reactor serving a low-speed two-stroke engine is installed before the turbo-
charger; the post-turbocharger exhaust gas temperature of a four-stroke engine,
however, is sufficient for the catalytic process. If an exhaust gas boiler is speci-
fied, it should be installed after the SCR unit. The temperature window has
been the subject of catalyst development.
The impressive de-NOxing efficiency of SCR technology has been demon-
strated in deep sea and coastal vessel installations since early 1990. NOx emis-
sion reductions of up to 95 per cent are yielded by pioneering marine SCR plant
serving the MAN B&W 6S50MC low-speed engines of bulk carriers on a dedi-
cated trade between Korea and California where strict environmental regulations
have to be satisfied. The amount of ammonia injected into the exhaust gas duct
is controlled by a process computer, which arranges dosing in proportion to the
NOx produced by the engine as a function of engine load (Figure 3.11).
Typically, a ship on a regular trade to California will bypass the SCR reac-
tor on the ocean leg of the voyage, complying with the IMO emission rules by
using primary methods for controlling NOx. Approaching the Californian-
regulated waters, the engine feed is switched from heavy fuel to fuel comply-
ing with California Air Resources Board rules (gas oil). The exhaust gas is
then gradually passed through the SCR reactor, and, when the temperature has
been raised to the right level, ammonia dosing is started to effect near-complete
NOx control.
An early SCR installation serving medium-speed marine engines was com-
missioned in 1992 on a diesel–electric ferry plying short crossings between
Denmark and Sweden. Its initial NOx reduction efficiency of 96 per cent was
increased to 98 per cent after 12 000 running hours.
The number of SCR systems in service has proliferated as ship owners
anticipate tougher regulations and seek NOx emission levels of 2 g/kW h or
lower from the main and auxiliary engines of diverse tonnage, including fast
car/passenger ferries. Applications in newbuilding and retrofit projects have
selective catalytic reduction 79
Preheating and
sealing air
Engine
Orifice
Air
High-efficiency
turbocharger
1
3
2
Static
mixer
NOx and O
2
analysers
Support
Deck
Exhaust gas outlet
Air outlet Air intake
SCR reactor
Evaporator
Ammonia
tank
Air
Process
computer
FigurE 3.11 schematic layout of sCr system for a low-speed diesel (MAN Diesel)
80 Exhaust Emissions and Control
been facilitated by the efforts of system designers to reduce reactor space
requirements and offer compact solutions. The reactors can now be installed
separately in the engineroom or integrated in the exhaust manifolds of both
four-stroke and two-stroke engines, doubling as an efficient silencer.
Siemens’ SINOx SCR system is designed to reduce the emissions of dif-
ferent exhaust gas pollutants simultaneously—notably NOx, HC and soot—as
well as to achieve a sound attenuation effect. The following performance can
be expected, according to the German designer:
l NOx reduction—90 to 99 per cent at MCR
l HC/CO reduction—80 to 90 per cent at MCR
l Soot reduction—30 to 40 per cent at MCR
l Noise reduction—30 to 35 dB(A).
A system investment cost of US$40–70/kW and an operating cost of
US$3–4/MW h were quoted in 2002, with some 15–20 L/h/MW of 40 per cent
urea solution consumed.
Important factors in designing such a system are the exhaust gas temperature
and the amount, composition and required purity level of the cleaned gas. The
control system ensures optimum process supervision and correct dosing of the
reducing agent; a special algorithm secures a fully automatic supply of the cor-
rect amount of aqueous urea solution in line with the NOx emission levels from
the engine. Reducing agent consumption is thus minimized and emissions are
reliably reduced below the specified limits. The exhaust gas emissions are meas-
ured during plant commissioning at different engine loads and the derived values
programmed into the control system to ensure correct dosing at various loads.
The SINOx system is based on a fully ceramic, fine-celled honeycomb cata-
lyst made of titanium dioxide and containing vanadium pentoxide as the active
substance. The honeycomb elements are packed in prefabricated steel modules
and can be replaced quickly and individually. The catalytic converter is designed
chemically and physically to suit the particular conditions of a given installation,
the catalyst lifespan varying from 10 000 h to 40 000 h depending on the applica-
tion. The converters are characterized by high catalytic activity, high selectivity,
high resistance to erosion and to chemicals such as sulphur, as well as insensitiv-
ity to particulate deposits, and high mechanical and thermal loads.
R&D by SCR system designers targets increased compactness, low weight,
no waste products, extended catalyst lifetimes and lower operating and main-
tenance costs. Catalysts themselves remain the subject of continuing develop-
ment to improve performance and longevity.
Results from a research programme on a Sulzer 6RTA38 low-speed engine
led Wärtsilä to the following conclusions on SCR systems:
l NOx reductions 90 per cent can be achieved commercially.
l The catalyst housing (reactor) including insulation has a volume of
2–5 m
3
/MW of engine power, depending on the make of catalyst; the size
of the housing is more or less independent of the input NOx concentration.
l The exhaust gas back pressure imposed by an SCR plant is typically
between 15 mbar and 25 mbar.
l The reactor can be designed in such a way that it serves as a silencer
with achievable noise reductions of more than 25 dB(A).
l Some 30 L of 40 per cent urea solution (corresponding to 15 kg of urea
granulates) are needed per megawatt hour.
l If the SCR plant is only for temporary use, a burner is absolutely neces-
sary to heat the catalyst before the engine is started; otherwise, the catalyst
will inevitably become clogged by ammonium sulphates.
The schematic arrangement of an SCR system installed to serve the
Wärtsilä (Sulzer) 7RTA52U two-stroke engine of a RoRo paper carrier is
shown in Figure 3.12; the SCR converter is installed before the turbine of the
turbocharger.
PArTiCulATEs, sooT AND sMokE
Particulate emissions are considered a contributory factor in causing asthma,
allergies and various other human health problems. Studies have shown that
smaller particulates show a stronger correlation between ambient concentrations
and health symptoms than larger particulates. Smaller particulates are thought
more likely to penetrate deep into the human lungs, the most minute of them
perhaps even moving into the blood stream; and the chemical composition of
the particulate may significantly contribute to the biological effect.
Particulates are made up of randomly agglomerated carbonaceous spher-
ules, which build up into a highly branched three-dimensional structure. Various
Particulates, soot and smoke 81
Static mixers
SCR
Turbocharger
Engine exhaust receiver
Urea injection
Flow dresser
FigurE 3.12 schematic arrangement of sCr system installed to serve a Wärtsilä
(sulzer) low-speed engine
82 Exhaust Emissions and Control
HC, ash and sulphur compounds are all associated with the structure. The
complexity of the particulate makes it impossible to give a satisfactory general
definition because the characterization depends largely on the measurement
method. It can be defined, for example, in terms of opacity, filter blackening,
particle number, size or mass.
A study carried out by Wärtsilä found that typically between 50 per cent
and 70 per cent of the particulate composition comprises compounds that are
related directly to the quality of the residual fuel oil (notably its sulphur and
ash contents) and cannot be reduced by improved combustion. Consequently,
only around 30–50 per cent of the particulate composition can be affected.
Even a significant improvement in engine combustion will thus not necessarily
result in any major reduction of particulate emissions.
After formation in the cylinder, the nanometre-sized primary particles coag-
ulate to create larger particulates to which HC and sulphate are attached as the
exhaust gas cools. Secondary particulates can be formed outside the combustion
chamber as a result of the absorption and condensation processes. This means
that the particulate composition and size distribution in the engine exhaust duct
is completely different from that reaching the human respiratory tract.
Wärtsilä advises that abatement measures for diesel particulates can
be divided into three categories: improvement of fuel and lube oil quality,
improvement of the engine combustion process, and exhaust gas cleaning.
Examples of engine measures for improving combustion include advanced fuel
injection properties (such as rate shaping with common rail technology) and
improved combustion chamber geometry (including swirl and squish).
Traps and oxidation catalysts, used for exhaust gas cleaning in truck
engines, are unsuitable for applications involving large residual fuel-operated
diesel engines due to the high sulphur and ash content of the fuel. For engines
running on such fuels, however, an electrostatic precipitator is a viable option,
but its size makes marine applications impractical. Exhaust gas scrubbers are
among potential options for the future.
Whatever measures are favoured for lowering particulate emissions, meas-
urement systems are an inevitable legislative requirement. Various methods are
approved by different administrative bodies, but the results are often not com-
parable because regulators define particulates in different ways.
The ISO 8178 method (with dilution) is the standard applied method to
most marine emission regulations, although it should not be used in conjunc-
tion with high sulphur fuel. According to ISO, the upper sulphur limit for
applying this method is 0.8 per cent, whereas a CIMAC recommendation is
0.05 per cent. Dilution conditions are crucial to the measurement result.
Wärtsilä reports that reproducibility of measurement results with the ISO
8178 standard when operating on typical marine fuel is often poor. Furthermore,
an improper choice of dilution and measurement settings within the permitted
requirements of the standard makes it possible to manipulate the results signifi-
cantly. To achieve repeatable results and enable comparison with land-based
industrial sources, Wärtsilä recommends using direct measurement methods.
MAN Diesel explains that particulate generation in diesel engines is a
complex process depending on numerous factors, such as engine type, speed,
engine setting, operating mode, load, fuel and even weather conditions.
Particulates comprise all solid and liquid exhaust gas components, which,
after cooling by exhaust gas dilution with filtered particulate-free ambient air to
a temperature below 51.7°C, are collected on specified filters (dilution tube sam-
pling). The so-called PM fraction represents a broad mixture of partly burned or
unburned HC, sulphate bound water, sulphates, ash and elemental carbon (soot).
Soot and ash are therefore only part of the total particulates. At high loads,
ash and soot might contribute 20 per cent to the particulates, but at low loads
and idling, this percentage can be much higher.
The specific mass of all particulates from modern MAN medium-speed
diesel engines averages around 0.6 g/kW h at maximum continuous rating,
assuming heavy fuel oil with a 2 per cent sulphur content is burned. Lower
particulate values can be achieved, however, the company citing a PM value of
0.2 g/kW h from the four 18V48/60 engines of a floating barge.
Much of the elemental carbon formed in a diesel engine is oxidized dur-
ing combustion, and only the remainder leaves the combustion space with the
exhaust gas as soot, which becomes visible as a dark smoke plume emanating
from the funnel. There is a clear correlation between the level of soot forma-
tion and the type of fuel used; heavy fuel oil combustion generates substan-
tially greater volumes of particles (and soot) than the burning of cleaner fuels,
such as marine diesel and marine gas oils. MAN Diesel notes that although
soot particles themselves are not toxic, the potential hazard posed by the build-
up of liquid HC onto them is viewed critically by many.
At very high engine loads, combustion in a state-of-the-art medium-speed
diesel engine can be modelled to give invisible smoke (IS). At low service
loads, however, and especially during rapid start-up manoeuvres and load
changes, the turbochargers deliver less intake air than the engine needs for
complete combustion and the engine ‘smokes’.
Smoke as the visible manifestation of soot production is highly undesir-
able in all types of ships, but particularly in passenger ferries and cruise lin-
ers. In sensitive waters, such as the glacier regions and bays of Alaska or
the Galapagos Islands, a vessel producing excessive amounts of visible soot
(smoke) can even be banned from cruising there. Achieving zero emissions
may dictate shutting down all engines and boilers during a port stay, and plug-
ging into a land-based electrical supply system.
MAN Diesel designed and successfully field-tested a package of meas-
ures to suppress the formation of soot in highly turbocharged medium-speed
engines, even during long periods of slow steaming. The full package for these
IS near-zero soot engines with low NOx emissions comprises the following:
l Turbocharger optimized for part load, with a waste gate
l Charge air bypass below 65 per cent engine load
l Charge air preheating (80°C) below 20 per cent load
Particulates, soot and smoke 83
84 Exhaust Emissions and Control
l Smaller injection bores
l Nozzles with short sac holes
l Auxiliary blower for operation below 20 per cent load
1
l
Fuel–water emulsification (15–20 per cent water)
2
l
Retarded fuel injection below 80 per cent engine load.
With smoke readings of 0.3 maximum on the Bosch scale, at very low
engine (MAN L/V 48/60IS) loads and even idling, soot production at steady
operating conditions is drastically reduced and kept at an invisible level over
the entire operating range. Smoke reduction was by a factor of seven at idling
and by a factor of five at 10 per cent load, respectively. The mass of soot pro-
duced during diesel combustion is reduced by even higher factors than that due
to the non-linear correlation between smoke emission and soot mass.
Elements of the IS package have become standard for all MAN medium-
speed engines; the other measures are specified only when extremely strict
requirements on soot and NOx emissions have to be met. The package is avail-
able for new engines and by retrofit to existing engines.
A different approach to smokeless medium-speed engines was adopted by
Wärtsilä, which explains that smoke in a large diesel engine can be formed
in two different ways: if the fuel spray touches a metal surface and there is
not enough remaining combustion time to burn away the soot formed (this is
consequently a low load problem); and when more fuel is injected than can be
burned in the air amount in the cylinder (this can easily happen at load varia-
tions with a conventional injection system).
Wärtsilä asserts that a common rail fuel system can cure both of these
problems, because it is possible to maintain high injection pressures independ-
ently of engine load and thus ensure good spray atomization even at very low
loads. If the combustion space is also optimized, there will be no risk of low
load smoke. The risk of over-injection of fuel can also be avoided by a com-
mon rail system because its computer—supplied with air temperature, pressure
and rotational speed data—can calculate the amount of air trapped in the cylin-
der. Based on empirical maps, the computer can decide how much fuel can be
injected. A common rail fuel-injected engine can have a faster load response
than a conventionally injected engine and without smoke. (The common rail
system for Wärtsilä medium-speed engines is detailed in Chapter 8.)
Wärtsilä’s EnviroEngine concept for its W46 and W32 medium-speed designs
exploits an electronically controlled common rail fuel injection system to achieve
‘smokeless’ performance (Figure 3.13). The package embraces the following:
l Camshaft-driven high-pressure pumps
l Accumulators for eliminating pressure waves
1
Can be dispensed with if load below 20 per cent is not required.
2
For NOx cycle values between 6.7 g/kW h and 8 g/kW h; can be dispensed with if NOx
emission levels meeting the IMO limit curve are sufficient.
l Hot box enclosure of all elements for maximum safety
l Engine-driven control oil pump for easy ‘black’ start
l Suction control of fuel flow for high efficiency
l Low cam load for high reliability.
Such a system offers the freedom to choose the fuel injection pressure and
timing completely independent of the engine load, whereas computerized con-
trol allows several key engine parameters to be considered and the injection
and combustion optimized for each load condition. The ability to maintain fuel
injection pressures sufficiently high at all engine loads and speeds (even at the
lowest levels and during starting and transient load changes) contributes to
clean combustion with no visible smoke emissions. Four 16V46 EnviroEngines
were specified as the diesel element of the CODAG-electric propulsion plant of
Cunard’s Queen Mary 2.
Wärtsilä notes that there is limited scope to significantly reduce particu-
late emissions by improving combustion, but methods of removing particulates
from the exhaust gas are available. Compact ceramic filters can be used with
some success on smaller engines but appear not to work when operating with
heavy fuel-burning engines because they become clogged by the metal matter.
Bag filters work with certain efficiency, but most applications are too bulky
and hence impracticable. Electrostatic precipitators are efficient in removing
particulates, but they are also extremely bulky and too expensive.
Conventional scrubbers have proved to be quite inefficient for the task
because the surface contact between the exhaust gas and the water is limited.
Promising results were reported by Wärtsilä, however, for a scrubber based on
ultra-fine water droplets created by Hi-Fog nozzles, which yield a much greater
surface area between gas and liquid.
Particulates, soot and smoke 85
Conventional
Common rail
2
1.5
1
0.5
0
0 20 40 60
Load %
80 100
Smoke (FSN)
Smoke comparison between conventional and common rail FIE at 750 rpm
1
2
5
10
20
50
Engine size dependent visible smoke limit MW
FigurE 3.13 smoke test results from a Wärtsilä 32 medium-speed engine with and
without common rail fuel injection
86 Exhaust Emissions and Control
suMMAry oF oPTioNs
Summing up the various options for noxious exhaust emission reduction,
Wärtsilä suggests:
l The first choice is engine tuning modifications that can achieve up to
39 per cent reductions in NOx emission levels compared with those of
standard engines in 1990.
l For further NOx reductions, separate water injection is considered the
most appropriate solution; the technique has proved its ability on the
test bed to reduce emission levels by some 60 per cent compared with
today’s standard engines.
l Exhaust gas after-treatment by SCR has proved an effective solution in
reducing NOx by 90 per cent or more, despite the special difficulties
imposed by using high sulphur fuel oils.
l Requirements for lower emissions of SOx are most favourably met by
using fuel oils with reduced sulphur contents. Although SOx reduction by
exhaust gas after-treatment is technically feasible, the de-sulphurization
process inevitably imposes a disposal problem, which may not be
acceptable for shipping.
l Carbon monoxide and HC can, if necessary, be reduced by an oxidation
catalyst housed within an SCR reactor.
l Particulate reduction, with the engine running on heavy fuel, poses a
challenge. Technical solutions are available (e.g. electrostatic precipi-
tators) but involve either great space requirements or great expense.
Particulate emissions are reduced by 50–90 per cent, however, through a
switch to distillate fuel oils.
Acknowledgements to MAN Diesel, Wärtsilä Corporation and Siemens.
87
C h a p t e r | f o u r
Fuels and Lubes:
Chemistry and
Treatment
Fuel remains one of the highest single cost factors in running a ship and also
the source of the most disruptive operating problems. New refining tech-
niques—fluid catalytic cracking (FCC) and visbreaking, introduced as a result
of political developments in the Middle East in 1973/1974—yield a more
concentrated residual fuel of very poor quality. This residual fuel is the heavy
fuel oil (HFO) traditionally supplied to ships as bunkers and used in the main
engines of most deep-sea and some coastal vessels. Despite the high cost of
these poor quality residual fuels, owners generally have no alternative but to
burn them, though some still prefer to use more expensive intermediate grades
produced as a result of mixing residual fuel oil with distillate.
Residual fuel or bunker oil is essentially a refinery by-product blended to
satisfy market demand for a cheap source of energy, some of which is used
in shipping. The main driver in the refining industry is the production of light
and middle distillate grades, which are used to formulate motor gasoline, jet
fuel, automotive diesel fuel and chemical feedstock. Over the past 20 years the
worldwide demand for distillate grades has continued to rise. Refineries have
thus incorporated more sophisticated processes to extract as much distillate
feedstock as possible from a barrel of crude oil.
The net effect is a reduction in the percentage of the crude oil barrel end-
ing up as residual fuel oil. In 1990, for example, approximately 20 per cent of
a barrel was processed into residual fuel compared with 14 per cent in early
2006. The downward trend is expected to continue.
ReFineRy pRoCesses
Crude oil is a complex mixture of hydrocarbons that must be processed to
provide both the products and the quantity of each product required to satisfy
worldwide demand. No two crude oils are alike: there are significant variations
in density, viscosity, sulphur and vanadium contents, and in the yield of each
product that can be produced by a refinery.