CC BY
33
https://doi.org/10.29013/AJT-21-7.8-36-43
Sagin Sergii Victorovych, Doctor of Technical Sciences National University Odessa Maritime Academy
Odessa, Ukraine E-mail: [email protected] Kuropyatnyk Oleksiy Andriiovych, Doctor of Philosophy National University Odessa Maritime Academy
Odessa, Ukraine E-mail: [email protected]
USING EXHAUST GAS BYPASS FOR ACHIEVING THE ENVIRONMENTAL PERFORMANCE OF MARINE DIESEL ENGINES
Abstract. The article analyses ways of meeting the Annex VI MARPOL requirements for nitrogen oxide emission limits for marine diesel engines. The system of exhaust gas bypassing of 6L26 Wartsila marine diesel engine is examined. It was found experimentally that exhaust bypassing in a range of 2...10% ensures NOx emission reduction by 3.33...15.42%.
Keywords: marine diesel engine, exhaust gases, nitrogen oxide emissions.
Heat engines that provide road, rail, river and include toxic components: carbon dioxide CO, hy-sea transport are sources of harmful emissions to drocarbons C H , carbon black C, nitrogen oxides
r n m' ' o
the environment [1]. In marine and river vessels, NOX, sulfur oxides SOX, and compounds of heavy
the most common heat engine is the diesel engine metals, which are in the fuel. Nitrogen oxides NOX
[2]. Its efficiency is higher and the specific effective are one of the most toxic components of exhaust
fuel consumption is less in comparison with other gases [3]. Norms of nitrogen oxides emissions by
types of power plants (gas and steam turbines also diesel engines of sea and river vessels are regulated
used in the sea transport). Implementation of the by requirements ofAnnex VI MARPOL and depend
thermodynamic cycle of the diesel engine is impos- on year of ship construction and speed of diesel en-
sible without the formation of exhaust gases, which gine shaft - (table 1).
Table 1. - Limit emissions of nitrogen oxides, g/(kWh),
Year of building Standard Speed range, min-1
n < 130 130 < n < 2000 n > 2000
2000 TierI 17 45n-°'2 9.8
2011 TierII 14.4 44 n-°'23 7.7
2016 TierIII 3.4 9n 02 2.0
The requirements of Tierl and TierII standards changing fuel parameters (including the use of gas fuel can be achieved by regulating diesels (by changing and fuel blends), as well as by selecting the optimal the phase of fuel supply, exhaust and blowdown), by modes of operation [4; 5]. Tier III requirements can
be achieved only by using additional equipment which ensures purification of exhaust gases from nitrogen oxides (methods of selective catalytic reduction - SCR, exhaust gas recirculation - EGR, gas electrolysis - CS--NOX, air humidification - HAM). All the methods aimed at reducing the emission of nitrogen oxides, degrade the fuel combustion process and worsen the efficiency of diesel engine operation (increasing the specific effective fuel consumption) [6; 7].
One additional method which must be used to comply with the Tier II exhaust gas emission standard is the exhaust gas wastegate (EWG). Wartsila recommends EWG primarily for limiting charge air pressure and avoiding surge phenomena at high loads and as an additional function for reducing NOx emissions [8].
The research objective was determination of optimum operation modes of marine diesel engine EWG
system providing maximum reduction of nitrogen oxides concentration in exhaust gases and minimum increasing of specific effective fuel consumption.
Research was conducted on three Wartsila 6L26 medium-speed diesel engines of the same type with electronic fuel supply, air and gas distribution phases control system. Nominal power of diesels was N =1200 kW at rpm 1000 min-1. The diesels were
enom
part of the power plant of the vessel and transmitted their power to the electric generator. The diesels had the same service life and were operated under equal loads. The diesels were fitted with EWG system for exhaust gas recirculation. According to the design documentation, the EWG system ensures gas bypass in the range of 0...10%. The schematic diagram of the EWG system for the Wartsila 6L26 marine diesel engine is shown in (Figure 1).
Figure 1. Schematic diagram of a medium-speed marine diesel engine 6L26 Wartsila with EWG exhaust gas control system: 1, 9 - gas flow control points; 2 - NOX concentration control point; 3, 6 - main gas flow; 4 - wastegate; 5 - gas bypass; 7 - controller; 8 - wastegate servomotor
In the course of the research, parameters were monitored and measured to determine NOX emission, specific effective fuel consumption b, as well as effective power of the diesel engine [9-11]. Degree of exhaust gas bypass EWG varied in the range 0.10% and was determined as follows
^EWG
G
wg
G
100%,
where Gwg - quantity of exhaust gases that passed through the wastegate, kg/s (measured at point 9 using MT100S flow meter) [12-14];
Gs - total quantity of exhaust gases entering the exhaust system from gas turbocharger at completely closed wastegate, kg/s (measured at point 1 with flow meter MT100S) [15-17].
NOx concentration in exhaust gases was determined in point 2 (Fig. 1) with a Testo350XL analyzer which provides fast response time of30.. .90 s and makes it possible to obtain both current and average (up to 1000 measurements) values. In addition, the
exhaust gas temperature (both for individual cylinders and its deviation from the average value) was monitored. Accuracy in measuring flow rate of gases determined by flow meter MT100S didn't exceed ± ± 0.5%, error in measuring NOx emission in exhaust gases with gas analyzer Testo350XL was ±3 .5%, error in determining of specific effective flow rate didn't exceed ± 2.5% [18]. The experimental results are given in (Table 2).
Table 2.- Variation of 6L26 Wartsila marine diesel engine parameters for different experimental conditions (at different load and different degree of exhaust gas bypass)
Load,% Degree of exhaust gas bypass, dEWG,%
0 2 4 6 8 10
NOv emission, g/(kWxh)
55 7.41 7.36 7.31 7.22 7.18 7.13
65 7.62 7.52 7.38 7.29 7.21 7.18
75 7.93 7.77 7.53 7.45 7.32 7.23
85 8.46 8.10 8.01 7.68 7.57 7.31
specific eff :ective fuel consumption, be, g/(kWxh)
55 196.6 203.0 204.0 204.7 204.8 205.3
65 195.3 200.5 202.7 203.2 203.3 203.4
75 193.7 196.8 197.3 197.5 197.6 197.7
85 189.2 190.6 190.9 191.2 191.3 191.4
exhaust gas temperature tg, °C
55 283 285 291 b 298 308 315
65 276 277 280 285 298 301
75 276 278 279 283 288 297
85 273 275 277 281 287 292
c)
d)
Figure 2. Variation of NOX emissions as a result of the degree of 5EWG при bypass at different loads of the 6L26 Wartsila diesel engine: а - 55%; b - 65%; с - 75%; d- 85%
The reduction in NOX emissions using the EWG system is shown in (Figure 2).
The efficiency of using the EWG system for each of the diesel operation modes was evaluated by the under the curve NOx = f (SE ), which was defined
by the method of trapezoids as a specific integral of
b h
j f (x)dx * 2[f (x0) + ff M + ... + ff (Vl) + f (xB)] ,(l)
a
wheref(x) is an analogue function with argument x; a, b - limits of integration (a < x < b); h = {b - f step of integration; x0, x: ..., xnxn- value of argument with increment step h in integration interval [a, b]; n - number of areas [19-20]. Based on NOX, values given in (Table 2), the following values of the integral (1) determining the area
under the curve NOx = f (8d ): S0N
NO,
SNO^T - 1.80; S,
0.65N„
NOx 0.75N„
■EWG'- "0.55Ne 1.38;
- 3.00; Sn°xn - 5.39. The area
under the curve NOx=f (S dEWG) can be taken as a criterion of ecological efficiency of EWG system application. An increase in this area indicates a higher eco-efficiency of the bypass mode. Consequently, increasing the degree of exhaust gas bypassing in all modes of diesel engine operation contributes to increasing its eco-efficiency [21-22].
Ecological efficiency of modes of EWG system can also be estimated by the margin of ecological stability of diesel engine operation ANO^,%, of the chosen mode of operation, which is determined by the following equation
ANOX =
NOXier - NOrk
NO
Tier
100%
(2)
X
where NO^161 is the limit value of nitrogen oxides concentration in exhaust gases (determined for the relevant Tier standard depending on the diesel parameters), g/(kWxh);
NOW°rk is the concentration of nitrogen oxides in exhaust gases at the selected mode of operation of the diesel engine, g/(kWxh).
Given that the value NO^16r in (Table 1) is calculated as follows:
NOX1er -NO™1 -44n 023 - 44 •1000-0f3 - 8.98 g(kWh),
for the corresponding values in (Table 2) by formula (2), we define the values of ecological stability of diesel engine operation ANOX, which, depending on the load and the degree of bypass, are summarized in (Table 3).
Table 3. - Environmental sustainability of the 6L26 Wartsila diesel engine,%, for different experimental conditions
Load,% Degree of exhaust gas bypass, $EWG,%
0 2 4 6 8 10
55 17.5 18,0 18.6 19.6 20.0 20.6
65 15.1 16.3 17.8 18.8 19.7 20.0
75 11.7 13.5 16.1 17.0 18.5 19.5
85 5.8 9.8 10.8 14.5 15.7 18.6
Figure 3. Environmental sustainability of the 6L26 Wartsila diesel engine:
1 - without EWG; 2 - ô
EWG
10%
A nomograhic chart showing the eco-sustainabil-ity values of the 6L26 Wartsila diesel for different operating conditions is shown in (Figure 3).
The use of the EWG system reduces the amount of exhaust gases entering the turbocharger. In doing so, the amount of air entering the diesel cylinders is proportionally reduced, resulting in worse combustion and an increase in specific effective fuel consumption (Table 2). This increase can be defined as
Ab -
bbWG - b e_i_e_
•100%,
(3)
where beBWG is the specific effective fuel consumption at the selected diesel operation mode, g/(kW x h);
be - is the specific effective fuel consumption without the EWG system, g/(kW x h).
The relative increase in specific fuel consumption Ab for different diesel operation modes is shown in (Table 4).
Table 4.- Increase in specific fuel consumption Ab,%, when using exhaust gas bypass system
Load,% Degree of exhaust gas bypass,
2 4 6 8 10
55 3.25 3.78 4.12 4.17 4.43
65 2.66 3.78 4.05 4.08 4.15
75 1.62 1.88 1.95 1.98 2.07
85 0.75 0.88 1.07 1.12 1.17
Figure 4. Range of increase in specific fuel consumptionDbe for the different operating modes of the 6L26 Wartsila diesel: 1 - 5 _ = 2°%; 2 - 5 FWG = 10%
A nomograhic chart showing the range of increase in specific fuel consumption Abe s a function of diesel load for various degrees of 8ШС bypass is shown in (Figure 4).
When using the EWG system, it is necessary to additionally monitor the temperature stress of the diesel, which increases as the combustion process deteriorates and the specific effective fuel consumption increases. For a 6L26 Wartsila diesel engine the value of the exhaust gas temperature (the main indicator of thermal stress of a diesel engine) must not exceed 300 ° C according to the requirements of the operating manual. The analysis of experimental data presented in (table 2) shows that the operation mode: load -55%, the degree ofbypass of exhaust gases - 10% does not meet the required temperature tension.
Приведенные результаты позволяют сделать следующие выводы.
1. Exhaust gas bypassing contributes to improvement of environmental performance of me-
dium-speed marine diesel engines, in particular in the range of operating loads (0.55.0.85) Nenom by 3.33.15.42% decreases level of NO emission in
X
exhaust gases.
2. Use of EWG system reduces the amount of exhaust gases entering the gas turbine, this leads to decrease in turbocharger capacity, decrease in the amount of air entering a diesel cylinder, and growth of specific effective fuel consumption. Thus for modes corresponding to 55 ... 65% loading growth of this parameter increases in proportion to a degree of overflow of exhaust gas SEWG and amounts to 2.66.4.43% (in range of 5EWG = 2... 10%). For loads close to nominal power - 0.85N increase in spe-
J- enom J-
cific fuel consumption does not exceed 1.2% (for the maximum value of bypass SEWG = 10%).
3. Assessment of EWG performance, as a way to meet Annex VI MARPOL NOX emission limits, must be carried out by a comprehensive evaluation of the following diesel performance parameters: the
amount of nitrogen oxides in exhaust gases, the in- reduction of NOX emission with a minimum increase
crease in specific effective fuel consumption, exhaust in fuel consumption and at the same time maintaingas temperature. The optimal degree of gas bypassing ing the exhaust gas temperature within the limits not
should be the values corresponding to the maximum exceeding the allowable level of thermal stress.
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