CC BY
32
Section 6. Technical science
Kuropyatnyk Oleksiy Andriiovych, graduate student, National University Odessa Maritime Academy E-mail: [email protected]
REDUCTION OF NOX EMISSION IN THE EXHAUST GASES OF LOW-SPEED MARINE DIESEL ENGINES
Abstract: The article analyzes the effect of the exhaust gas recirculation (EGR) system on the emission of NOX in the exhaust gases of marine low-speed diesel engines. Tests have proven that EGR in the range of 4.7...18.8% provides reduction of NOX emission to 13.3...3.3 gNOX/(kW h) accordingly, depending on the crankshaft rotation speed and engine load. It is shown that the decrease in NOX concentration in exhaust gases (in comparison with NOX concentration without the use of EGR) can reach 37.9.53.5%. The use of the EGR system as the main way of reducing the NOX emission level is proposed, which provides the needed decrease in emission level in accordance with the requirements of international organizations.
Keywords: low-speed marine diesel, emission of nitrogen oxides in the exhaust gases, exhaust gas recirculation system.
Introduction
Shipbuilding industry today shows increase in the deadweight tonnage ofvirtually all types of ships that is most pronounced in container carriers, bulk carriers and tankers. In order to avoid loss of speed of the ship and to ensure proper performance of shipboard operations it is necessary to the increase propulsive output proportionally to the increase in the deadweight tonnage. The output of the main engines (power of which transmits to the ship propeller that is essential for movement of the ship) of the leading diesel-building companies such as Mitsubishi, MAN Diesel, Wartsila-Sulzer goes up to 56000.81000 kW, and auxiliary engines (which are driven by ship electric generators) - to 2500.4200 kW. The main engine daily fuel consumption, with an average specific fuel rate of 175.188 g/(kW h), can be approximately 235.350 ton/day, and for auxiliary
engines (up to 4 engines on the modern ships, up to 3 of which can work simultaneously), up to 8.10 ton/day. And 16.24 ton/h of exhaust gases, which contain toxic components, are emitted into the atmosphere. All the toxic components that are formed in ship engines can be divided into two groups. The first group includes products of incomplete combustion of fuel - carbon monoxide, hydrocarbons, aldehydes, soot. The second group is the toxic components formed as a result of complete oxidation of the chemical elements that are mixture of fuel and air - these are nitrogen oxides (NOX) and sulfur oxides (SOX).
Presently, a large number of techniques that reduce emissions of NOX in exhaust gases exist. Depending on the type of diesel engines (structure and operating conditions), strictly defined options are considered to be optimal for the purpose of mini-
mizing of NOX emission. For example, for marine diesel engines with a capacity of more than 10000 kW, Mitsubishi and MAN Diesel recommend to use EGR system. Still, further research required to determine the optimum rate of gas recirculation, as well as the effect of EGR system on the main parameters of the diesel. Taking this into account, the purpose of this research was to determine the effect of the rate of recirculation of exhaust gases (as one of the methods for reducing NOX emissions) of a marine low-speed diesel engine on its environmental, economic and energy parameters.
Relevance of research
The most harmful emissions in almost all modes of operation of the diesel engines (regardless of their type, class size and structural differences) are, obviously, nitrogen oxides. The rates of nitrogen oxides in total emissions is 30...80% by mass and is 60...95% by equivalent toxicity. Nitrogen oxides released to the atmosphere along with aerosols and organochlorine compounds destroy the ozone layer, which is located at an altitude of 25 km and absorbs 99% of the sun and ultraviolet rays. NO and NO2 that are emitted in the operation of marine engines and plants are most important to ensuring environmental standards.
Nitrogen oxides are the only pollutants that cannot be eliminated by changing the grade of fuel (as it is possible to reduce SOX emissions this way), since they are most often formed by the combination of nitrogen (which is necessarily in the liquid fuel composition) with oxygen (contained in the air in the diesel cylinder). Marine diesels, which are in operation, require constant search for effective ways to reduce the toxicity of exhaust gases, most important of which are emissions of nitrogen oxides. Reduction of NOX concentration in exhaust gases is achieved through the use ofvarious technological and design solutions, one of which is exhaust gas recirculation (EGR). This determines the relevance of research to determine the optimum operating conditions for exhaust gas recirculation systems [1].
Research objective
The amount of nitrogen oxides in the exhaust gases of marine diesel engines is regulated by the requirements of the MARPOL International Convention, Annex VI. EGR systems received wide spread in both stationary and ship engines in the last decade. These systems are developed and installed for new ships. Extensive experience in the technical operation of these systems does not currently exist, and recommendations for their use are mainly based on theoretical calculations and modeling of the processes occurring in this process. Therefore, the research objective was to determine the influence of the exhaust gas recirculation system on the environmental, energy and economic performance of the 7UEC60LS low-speed marine diesel engine of Mitsubishi with a regular ERG system.
One of the features of the operation of marine vessels is their accountability to international classification societies (for example, Lloyd's Register of Shipping - England, Bureau Veritas - France, Det Norske Veritas & Germanischer Lloyd - Germany, The American bureau of shipping - USA). And one marine vessel can be under the supervision of several societies at once. On the basis of empirical experience, statistical accounting and scientific research, these organizations develop their own Rules for the Classification of Marine Vessels and maintain a system of continuous monitoring of compliance with these rules on classified ships. This limits the ability to perform structural changes in the engines ofvessels (both thermal and mechanical engines, and systems that ensure their operation) without appropriate coordination with these supervisory authorities. The crew of ship, in the performance of its functional duties, is deprived ofthe possibility ofindependently re-equipping both the power plant itself and the systems that serve it. Therefore, the task ofship mechanics and representatives of research organizations is to determine the optimal operating conditions for the ship's power plant without introducing any improvements and upgrades of its structure.
Research results
Tests of the effect of the exhaust gas recirculation system on the environmental, energy and economic performance of the internal combustion engine were performed on a Mitsubishi 7UEC60LS marine low-speed diesel engine, operated by a two-stroke cycle with a regular ERG system.
The main characteristics of the diesel engine: - diameter of the cylinder - 600 mm;
- piston-stroke - 2400 mm;
- number of cylinders - 7;
- nominal power - 12600 kW;
- rotation speed corresponding to the nominal power - 82 rpm.
Schematic diagram of the Mitsubishi 7UEC60LS marine diesel engine with the exhaust gas recirculation system on which the tests were performed is shown in (Fig. 1).
Figure 1. Schematic diagram of EGR system for a marine low-speed diesel engine: 1, 5 - turbocharger; 2 - outlet manifold; 3 - EGR valve; 4 - scrubber; 6 - water pump; 7 - water tank; 8 - electric gas turbocharger; 9, 11 - air cooler; 10 - air manifold T, C - gas turbine and turbocharger compressor
The exhaust gases from the diesel cylinders enter the common exhaust manifold 2 and then to the gas turbochargers 1 and 5, after which they are vented to the atmosphere through the gas outlet pipe. The gas turbochargers take air from the engine room and, after compression, direct it through the coolers 9 and 11 in air manifold 10. The gas turbocharger 5 is equipped with an exhaust gas recirculation system that consists of a control valve 3, a gas scrubber 4, a gas turbocharger 8, tank 7 and water pump 6. In the case of the exhaust gas recirculation system, the quantity is regulated by the valve 3. The exhaust gases are cleaned and pre-cooled in the scrubber 4, they are fed for mixing with air by the additional turbocharger 8 (coming from the exhaust gas turbocharger 5) and fed to the intercooler 9, the air receiver 10 and further to the
cylinder of a diesel engine. A fan with constant flow geometry is used as a gas turbocharger 8 [2].
In experimental research conducted onboard the vessel, NOx values in exhaust gases were measured, Specific fuel oil consumption (SFOC), and effective diesel engine power (N) for various rates of exhaust gas recirculation.
Portable gas analyzers are most expedient in monitoring the concentration of harmful components [4]. German gas analyzer Testo350XL was used in this research to determine the concentration of NOX (as well as the O2 concentration) in the exhaust gases, which can determine the concentrations of the following substances: CO, O2, N2, NOX, CH4, SO2, as well as measure temperature, humidity, speed and the differential pressure of the medium.
The Testo 350XL gas analyzer provides the specified parameters in the temperature range of -40... 1200 °C, which completely covers the interval of the exhaust gas temperatures of the diesel engine at all modes of its operation. Using the Testo 350XL gas analyzer, it is possible to determine the content of nitrogen oxides (NOX), as well as oxygen (O2 Gas) and nitrogen (N2 Gas) in outgoing gases in the range of0.3000 mln-1 with an accuracy of 1 mln-1.
The rate of recirculation of exhaust gases during the tests varied in the following values: EGR = 4.7%, EGR = 9.8%, EGR = 14.6%, EGR = 18.8%, and was calculated using the following formula:
EGR = ^EGR
a
where a is rate of excess air without EGR system operating;
a EGR is rate of excess air with EGR system. To determine the EGR rates of excess air, a and a EGR were determined in consideration with the volume concentrations of oxygen and nitrogen in the exhaust gases when the diesel engine is operating without EGR - O , N and with EGR -
2,Gas 2,Gas
oEGR NEGR
O2,Gas' N2,Gas
1
1
a =
O2 G 1 - 3,76—
N2, Gas
1 - 3,76-
O
'n
EGR 2, Gas EGR 2, Gas
Figure 2. Change in SFOC, g/(kW h) of the Mitsubishi marine diesel engine 7UEC60LS, depending on the crankshaft rotation speed n, rpm and EGR rate,%: 0 - without circulation (EGR = 0%;) 1 - EGR = 4.7%; 2 - EGR = 9.8%; 3 - EGR = 14.6%; 4 - EGR = 18.8%
The tests were performed on the following operation modes of diesel: 55, 65, 75 and 80 rpm, which corresponded to the following values of the relative power of the diesel: 0.3N , 0.5N , 0.77N and
enom enom enom
0.93N . N stands for the nominal power corre-
enom enom
sponding to its excess air ratio - a. The ship diagnostic system Doctor was used to determine the power of the diesel. In addition, for each operation mode of the diesel engine, the hourly fuel consumption was measured [3].
The results, which reflect the change in the specific effective fuel consumption be, nitrogen oxide concentration NOX in the exhaust gases and the relative decrease in the effective power
N.
2EGR
N
100,
of the Mitsubishi marine diesel engine 7UEC60LS, depending on the crankshaft rotation speed n, and the EGR rate are shown in (Fig. 2-4).
Figure 3. Change in the concentration of nitrogen oxides NOX, g/(kW h) in the exhaust gases of the Mitsubishi marine diesel engine 7UEC60LS depending on the crankshaft rotation speed n, rpm and EGR rate,%: 0 - without circulation (EGR = 0%;) 1 - EGR = 4.7%; 2 - EGR = 9.8%; 3 - EGR = 14.6%; 4 - EGR = 18.8%
Figure 4. Relative reduction in the effective power NeEGR -100, %, of the Mitsubishi marine diesel
Ne
engine 7UEC60LS depending on the crankshaft rotation speed n, rpm and EGR rate: 1 - EGR = 4.7%;
2 - EGR = 9.8%; 3 - EGR = 14.6%; 4 - EGR = 18.8%
Conclusion use of additional technological solutions. One such In order to meet the required environmental solution is the addition to marine diesel engines of
performance of marine diesel engines (most impor- EGR systems, which ensure that the part of exhaust
tantly, emissions of NOX in exhaust gases) forces the gas passes through the cylinder. EGR system reduces
the amount of air intended for fuel combustion, so the amount of exhaust gases returning to the diesel cylinder maintains a reliable autoignition and subsequent combustion of the fuel.
The use of EGR system improves the environmental performance of the marine diesel, in particular, reduces the level of NOX emissions. It has been experimentally established that a change in the rate of recirculation of exhaust gases in the range 4.7.18.8% ensures a reduction in the NOx concentration in exhaust gases to 13.3.3.3 gNOX/(kW h) respectively, depending on the rotational speed and diesel load, which in experiments varied in the interval n ,=(0.67.0.975) n and N ,=(0.3.0.93)
work v ' nom ework v '
N . Decrease in NO concentration in exhaust
ework X
gases is in the range of 19.5.48.8%, with large values corresponding to the load interval (0.77.0.93) of (0.77.0.93) N , which is the most common
v ' enom'
operating modes of the diesel engine.
The use of the exhaust gas recirculation system helps to reduce the energy and economic performance of the low-speed marine diesel engine. Conducted tests established the following:
Specific fuel oil consumption (SFOC), which characterizes the economy of the diesel engine, increases in proportion to the increase in the rate of recirculation of exhaust gases and for different speeds of the diesel engine in percentage terms is 0.85.2.01% for EGR = 4.7% and 2.16.4.34% for EGR = 18.8%;
The effective power of the diesel engine, which characterizes its energy performance, decreases as the rate of recirculation of the exhaust gases increases. In modes close to the nominal load (for example for n , =0.975n , when N , =0.93N ,
work nom ework enomy
the drop in the effective power value is 1.2%, and for n =0.67n , when N =0.3N modes it is
work nom ework enom
3.43%.
Despite the deterioration in the economic and energy performance of diesel, the use of the EGR method on ships has broad prospects, because its use ensures the fulfillment of international requirements for protecting the ambient air from pollution and contributes to the maintenance of environmental safety of ship power plants.
References:
1. Sagin S. V., Kuropyatnyk O. A. The Use of Exhaust Gas Recirculation for Ensuring the Environmental Performance ofMarine Diesel Engines // Nase more, 2018.- Vol 65.- Iss. 2.- РР. 78-86. DOI 10.17818/ NM/2018/2.3
2. Sagin S. V., Kuropyatnik A. A. Application of the system of recirculation of exhaust gases for the reduction of the concentration of nitric oxides in the exhaust gases of the ship diesels // American Scientific Journal, 2017.- No. 15.- Iss. 2.- P. 67-71.
3. Куропятник А. А. Снижение концентрации оксидов азота в выпускных газах судовых дизелей // Universum: Технические науки. 2018.- Вып. 3(48).- С. 67-71.
4. Yoo J., Prikhodko V., Parks J. E., Partridge W. P., Perfetto A., Geckler S. High-speed multiplexed spatiotemporally resolved measurements of exhaust gas recirculation dynamics in a multi-cylinder engine using laser absorption spectroscopy // Applied spectroscopy.2016.- Vol. 70.- Iss. 4.- P. 572-584. DOI: 10.1177/0003702816636802.
