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Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate
international journal of hydrogen energy xxx (xxxx) xxx
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Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate Gesheng Li a,b,*, Yangxiang Long b, Zunhua Zhang a,b, Junjie Liang b, Xiaowu Zhang b, Xintang Zhang a,b, Zhongjun Wang a,b a
Key Laboratory of High Performance Ship Technology, Wuhan University of Technology, Ministry of Education, Wuhan, Hubei, 430063, PR China b School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei, 430063, PR China
highlights
graphical abstract
The application of REGR in marine NG engines can achieve low NOx emissions. The equivalent BSFC decreases with the increase of Rre at medium and low loads. The increase of Rre leads to low THC
emissions
and
high
CO
emissions.
article info
abstract
Article history:
The exhaust gas-fuel reforming technique known as reformed exhaust gas recirculation
Received 2 June 2019
(REGR) can generate on-board hydrogen-rich gas mixture (i.e., reformate) by catalytic
Received in revised form
reforming of the exhaust gas and fuel added into the reformer and then recirculate the
1 October 2019
reformate into the engine cylinder, which can realize the combination of hydrogen-rich
Accepted 4 October 2019
lean combustion and exhaust gas recirculation. The REGR technique can be employed to
Available online xxx
achieve efficient and stable lean-burn combustion for the marine engine fueled with natural gas (i.e., marine NG engine) and it is considered as an effective way to meet the
Keywords:
stringent ship emissions regulations. In the present study, an experimental investigation
Reformed exhaust gas recirculation
into the effects of reformate addition ratio (Rre) and excess air ratio (l) on the combustion
Hydrogen-rich reformate
and emissions characteristics of a marine NG engine under various loads was conducted,
Marine engine
and the potential of applying the REGR technique in a marine NG engine to achieve low
Liquefied natural gas
emissions (i.e., International Maritime Organization Tier III emissions legislations for in-
Combustion and emissions
ternational ships) was discussed. The results indicate that the addition of the hydrogen-
Thermal efficiency
rich reformate gases can extend lean-burn limit. For a given l, the flame development duration and rapid combustion duration decrease with the increase of Rre, and the combustion efficiency is improved. The brake specific NOx emissions first increase and then
* Corresponding author. School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei, 430063, PR China. E-mail address: [email protected] (G. Li). https://doi.org/10.1016/j.ijhydene.2019.10.007 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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decrease with the increase of Rre due to the competition between the combustion phase and total heat release value. The brake specific THC emissions decline with the increase of Rre, while the reverse holds for the brake specific CO emissions, and the behavior tends to be obvious under large l. It is demonstrated that the combination of REGR and the leanburn combustion strategy can improve the trade-off relationship between the NOx emissions and brake specific fuel consumption of the marine NG engine to meet the IMO Tier III NOx emissions legislations and maintain relatively low brake specific fuel consumption. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nomenclature Abbreviations Stoichiometric air-fuel ratio AFst BMEP Brake mean effective pressure BSFC Brake specific fuel consumption CA Crank angle CA 0-10 Crank angle interval from the crank angle of 0% mass fraction burned to the crank angle of 10% mass fraction burned CA 10-90 Crank angle interval from the crank angle of 10% mass fraction burned to the crank angle of 90% mass fraction burned CO Carbon monoxide Carbon dioxide CO2 ECAs Emission control areas EGR Exhaust gas recirculation GDI Gasoline direct injection HCCI Homogeneous charge combustion ignition IMO International Maritime Organization LHV Low heating value LNG Liquefied natural gas Nitrogen oxides NOx PM Particulate matter REGR Reformed exhaust gas recirculation SI Spark ignition Sulfur oxides SOx THC Total hydrocarbon
international ships are implemented. Liquefied natural gas (LNG) has the advantages of low sulfur content and carbon/ hydrogen ratio, using the LNG as the fuel of a marine engine can reduce the emissions of sulfur oxides (SOx), carbon dioxide (CO2) and particulate matter (PM). In addition, it is reported [2,3] that the low nitrogen oxides (NOx) emissions and high thermal efficiency can be achieved for the marine engine fueled with natural gas (i.e., the marine NG engine) by employing the lean-burn strategy. Therefore, LNG as an alternative fuel for marine engines has received extensive attention. Generally, high excess air ratio may lead to the misfire and the increase of unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions, and meanwhile the retarded combustion phase reduces the thermal efficiency of the lean-burn spark-ignition (SI) engine due to the low flame speed of natural gas [4e6]. Many studies have shown that the addition of hydrogen is beneficial to the low-temperature lean-burn combustion of the natural gas engine, which can reduce the misfire rate, extend the lean operation area and promote the complete combustion of the natural gas, and therefore the reduction of the HC and CO emissions [7e9]. Unfortunately, the cost and risk of producing and transporting hydrogen are relatively high [10], which restricts the application of on-board H2 addition in the optimization of the combustion of the natural gas engine. Recently, reformed exhaust gas recirculation (REGR) technique [11,12], which generates on-board hydrogen-rich gas
Symbols Reformate addition ratio Rre l Excess air ratio Combustion efficiency hc
Introduction Emissions from marine engines account for a large portion of the air pollutant emission inventories, especially for the coastal areas. The International Maritime Organization (IMO) established the emission control areas (ECAs, such as Baltic Sea and North Sea) where the stricter emissions regulations (IMO Tier III emissions legislations [1], shown in Fig. 1) for
Fig. 1 e IMO Emission Legislations (n ¼ engine speed).
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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(i.e., reformate) via catalytic reforming of fuel and engine exhaust gas in an exhaust gas-fuel reformer and recirculates the reformate into the cylinder for improving combustion, is regarded as an effective way to achieve the combustion with on-board H2 addition and exhaust gas recirculation. The REGR technique can achieve stable lean-burn combustion, and then reduce the NOx emissions and improve the thermal efficiency [13e15]. Compared with the vehicle engine, the operating conditions of the marine engine usually are more stable and the fluctuations in the temperature and component of exhaust gas are smaller, which is conducive to the stable hydrogen production of the exhaust gas-fuel reformer and the regulation between the reformer and the marine NG engine. Our previous numerical studies [16,17] showed that the onboard hydrogen can be generated by catalytic reforming of natural gas and partial exhaust gas from a marine NG engine via the exhaust reforming reaction tube. In the last few years, several investigations on the exhaust gas reforming of various fuels including diesel [18e21], gasoline [22e24], ethanol [25,26] and natural gas [27,28], etc. Have been reported, and the studies on the application of exhaust gas-fuel reforming were mostly related to the diesel and gasoline engine. For instance, Tsolakis and his co-workers [18,22,23] carried out a series of studies to investigate the potential of REGR technique in energy-saving and emissions reduction of the vehicle engine. They experimentally investigated the effect of the reformate generated from a dieselexhaust gas reformer on the performance of diesel engine. The results showed that the addition of reformate reduced the NOx and PM emissions and led to an increase in the thermal efficiency [18]. After that, they generated a reformate-like mixture by adding bottled H2/CO to conventional EGR to study the effect of reformate addition on the efficiency and emissions performance of a multi-cylinder GDI engine, and the results showed that the thermal efficiency was increased, and both the gaseous pollutants and PM emissions were reduced [22]. Additionally, they compared the effect of EGR and REGR on PM emissions in a GDI engine and found that REGR could reduce the diameter and concentration of the particle emission [23]. Moreover, Yap et al. [28] found that REGR could expand the low-load operating range of the natural gas HCCI engine. As the quantity of reformate added was increased, NOx emissions decreased while HC and CO emissions increased, respectively. The above studies indicated that the application of REGR technique could improve the fuel economy and emissions characteristics of the engine fueled with various types of fuels. To the best of the authors’ knowledge, the previous research on the REGR technique mostly focused on vehicle engines, while little is known about the marine NG engine with REGR. Therefore, the objective of the present study is to carry out relevant research to obtain the influence of REGR on the performance of a marine NG engine, and the emphasis is to investigate the potential of REGR technique on reducing pollution emissions from a marine NG engine to meet more stringent ship emission regulations (i.e., IMO Tier III emissions legislations). In the present study, the effects of reformate addition ratio (Rre) and excess air ratio (l) on the combustion and emissions characteristics of the marine NG engine under different loads were analyzed, and then the appropriate way
to apply REGR technique in the marine NG engine for meeting the Tier III emissions legislations was presented. The present study is of great importance for the application the RGER technique to the control of air pollutant emission from the ships powered with LNG.
Experimental setup and procedure Engine specification and instruments The engine used in the present study was a six-cylinder, water cooled, turbo-charged, premixed SI marine NG engine produced by Yuchai Co. (YC6MK 200N). The specifications of the test engine are listed in Table 1 and the schematic diagram of the experimental system is presented in Fig. 2 and the specifications of test instruments and equipment are listed in Table 2. The electronic eddy current dynamometer was connected to the engine to automatically adjust the engine torque and speed. A Coriolis mass flowmeter was employed to measure the mass flow rate of natural gas, and Table 3 shows the composition of the natural gas used in the present test. An exhaust gas analyzer was used to analyze the exhaust gas emissions. The concentrations of CO, CO2, HC, NOx and O2 and fuel consumption rate were recorded and averaged for calculating the brake specific emissions and fuel consumption under specific experimental points, which was based on the measurement method listed in the legislative documents [29]. The pressure sensor was installed in the sixth cylinder head for recording the instantaneous in-cylinder pressure. At each test point, the in-cylinder pressures were recorded for 200 cycles at 0.1 crank angle ( CA) by the shaft encoder, and then the pressures were averaged and post-processed for combustion process analysis. With respect to the hydrogen-rich reformate gas addition, an independent gas supply system was designed.
Hydrogen-rich reformate The objective of the present study is to investigate the effects of the reformate gas addition on the performance and emissions characteristics of a marine NG engine. Due to the complex reforming reactions, we have to, first of all, define the composition of the reformate gas mixture to generate the hydrogen-rich reformate gas for the engine bench test. It is known that methane is the main constituent of natural gas, and thus the exhaust gas-fuel reforming process of the natural gas mainly involves in the partial oxidation methane reforming reaction (CH4 þ 0.5O2 <¼> CO þ 2H2), steam methane
Table 1 e Engine specifications. Parameters Bore (mm) Stroke (mm) Displacement (L) Compression ratio Power (kW) Rated speed (r/min) Piston bowl
Values 123 145 10.338 11.5:1 147 1500 Shallow bowl
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 2 e Schematic of the experiment system.
Table 2 e The specifications of instruments and equipment. Instruments
Type
Dynamometer
Y880
Coriolis mass flowmeter Emission analyzer Mass flow controller Pressure sensor
DFM-1-3A CAI 600 Red-y Kistler 7013C
Accuracy Torque: ±0.2% of full scale Speed: ±5 rpm ±0.5% of full scale ±1.0% of full scale ±0.3% of full scale ±0.7% of full scale
reforming reaction (CH4 þ H2O <¼> CO þ 3H2), water gas shift reaction (CO þ H2O <¼> CO2 þ H2), etc. [17,30]. Therefore, a typical reformate generated from the methane and exhaust gas reforming mainly consists of H2, CO, CH4, H2O, CO2 and N2 [31,32]. The existence of CH4 in the reformate is because of the incomplete catalytic reforming of methane. Actually, the unreacted CH4 is recirculated into the cylinder and combusted as fuel, and meanwhile most of the H2O steam in the reformate will be condensed and removed after passing through the REGR cooler. Thus, both the CH4 and H2O in the reformate are neglected during the process of hydrogen-rich reformate generation in the present study. In addition, considering that both N2 and CO2 are dilution gases and the fraction of N2 is larger than that of CO2, thus N2 is retained as the main dilution gas and CO2 is neglected during the process of hydrogen-rich reformate generation. As a result, the reformate gas is simplified as a mixture of H2, CO and N2 in the present study. Usually, there is a variation in the composition of reformate. However, the composition of the reformate will only change within a small range by adjusting the boundary conditions (i.e., gas hour space velocity, O2/CH4 ratio) during the exhaust gas-fuel reforming process [10]. The minor change in the composition of the reformate has little influence on the combustion and emissions characteristics of the engine. Therefore, we temporarily ignored the effect of the variation
Table 3 e Composition of the natural gas. Item
CH4
C2H6
C3H8
N2
Others
Volumetric fraction (%)
99.03
0.63
0.049
0.29
0.001
Manufacturer Qidong, China Shouke, China California, USA € gtlin, Switzerland Vo Kistler, Switzerland
in the composition of reformate in the present study. Then, a hydrogen-rich reformate gas with fixed composition of 24% H2, 12% CO, and 64% N2 in mole fraction was generated to carry out the present experiment study based on the methane/exhaust gas reforming experiments from previous research [33,34] and our methane reforming experiments over nickel- and rhodium-based Catalysts. In the present study, a hydrogen-rich reformate gas supply system was designed. Three mass flow controllers connected with compressed gas tanks of H2, CO and N2 were used to generate the hydrogen-rich reformate with the specific composition and mass flow rate. The hydrogen-rich reformate gas mixed with fresh air in the mixer installed at the intake manifold before the turbo charger, and then entered the cylinder. The purities of H2, CO and N2 are 99.999%, 99.99% and 99.99%, respectively.
Experimental conditions and definitions In the present study, the performance and emissions of the marine NG engine with various hydrogen-rich reformate addition ratio (Rre) and excess air ratio (l) were compared under three different engine loads, 110.0 kw & 1366 rpm, 73.5 kw & 1163 rpm and 37.0 kw & 969 rpm, respectively. Before the experiment, the engine was warmed up, and the measurements started when the coolant reached 85 ± 5 C. During the test, the spark ignition timing was fixed at the specific value according to the original ignition timing data from the prototype engine, and the position of the throttle was adjusted to maintain the engine torque with the variation of Rre and l at a specific engine speed. The excess air ratio (l) is defined as:
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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mair l¼ mNG AFst;NG þ mH2 AFst;H2 þ mCO AFst;CO
(1)
where mair, mNG, mH2 and mCO represent the consumption rate (g/h) of the intake air, natural gas, H2 and CO, respectively. AFst,NG, AFst,H2 and AFst,CO represent the stoichiometric airfuel ratio of the natural gas, H2 and CO, respectively. And meanwhile, the intake gas flow rate was measured to ensure the amount of hydrogen-rich reformate added into the intake manifold at two different ratios of 3% and 5% of the total volumetric intake gas flow rate at different engine loads. The reformate addition ratio (Rre) can be written as: Rre ¼
Vre Vintake þ Vre
(2)
where Vre and Vintake represent the volumetric flow rate (m3/h) of the reformate and intake air in the inlet pipe, respectively. The specific experimental points are listed in Table 4. The equivalent brake specific fuel consumption (BSFC) is used to evaluate the fuel economy of the engine with hydrogen-rich reformate addition in the previous studies [35,36], and it can be written as: equivalent BSFC ¼
1 LHVH2 LHVCO mNG þ mH2 þ mCO W LHVNG LHVNG (3)
where W is the engine power (kW) and LHVNG, LHVH2 and LHVCO are the low heat value (kJ/g) of natural gas, H2 and CO, respectively. The combustion efficiency (hc) is used to evaluate the quality of in-cylinder combustion, and the total hydrocarbon (THC) and CO emissions are counted to calculate the heat loss due to the imperfect combustion. Since the main component of THC emissions from the NG engine is CH4 [37], the heat loss due to THC emissions is evaluate by the heat value of CH4 in the present study. Therefore, the combustion efficiency (hc) can be defined as: hc ¼ 1
LHVCH4 ETHC þ LHVCO ECO LHVNG mNG
(4)
where LHVCH4 is the low heat value (kJ/g) of CH4, and ETHC and ECO are THC and CO emissions (g/h), respectively.
Results Effects of reformate addition on the combustion and efficiency Combustion phase Fig. 3 shows the flame propagation duration (CA 0e10, crank angle interval from the crank angle of 0% mass fraction
burned to the crank angle of 10% mass fraction burned) and rapid combustion duration (CA 10e90, crank angle interval from the crank angle of 10% mass fraction burned to the crank angle of 90% mass fraction burned) [38] versus l at different Rre. As can be seen, CA 0e10 increases with the increase of l, and the reason can be ascribed to the dilution effect of air on the combustion. The lean fuel-air mixture leads to the delay of ignition and the decline in the formation and development of the flame kernel. At a specific l, the addition of the hydrogen-rich reformate shortens the CA 0e10 compared with the engine fueled with natural gas. With the addition of hydrogen-rich reformate, H2 and CO are introduced into the cylinder, and then the production of free radicals like OH, O and H are boosted during the combustion process [39,40]. Since the promotion effect of H2 and CO on the combustion is stronger than the dilution effect of N2 introduced into the cylinder with the addition of reformate, the CA 0-10 decreases dramatically. Moreover, the increase of Rre can raise the concentrations of H2 and CO in cylinder and advance the combustion phase. The trend of CA 10e90 versus l is similar to that of CA 0e10, and the increase in l leads to the increase of CA 10e90. It is because that the extending air dilution rate decreases the combustion speed and leads to the long combustion duration. However, when the hydrogen-rich reformate is added into the cylinder, the CA 10-90 declines at a specific l due to the high flame speed of hydrogen. As the Rre increases up to 5%, the volume percentage of H2 and CO in the fuel mixture reaches 21% and the laminar flame speed of the fuel-air mixture increases by 18% [41,42]. Therefore, the CA 10e90 is shortened.
Combustion efficiency Fig. 4 illustrates the combustion efficiency (hc) versus l at different Rre. With the increase of l, the combustion efficiency decreases gradually. The hc decreases by 1% when l increases from 1.3 to 1.5 in the case of both natural gas combustion and natural gas-reformate combustion. As discussed in the combustion phase, the effect of air dilution on combustion leads to an increase in the misfire rate and THC and CO emissions (more details will be discussed later), which reduces the total heat release value and then the hc decreases. Moreover, this behavior will be remarkable at large l due to the worse combustion performance. For a given l, hc increases with the addition of reformate. For example, the improvement in hc can reaches 0.5% when Rre increases from 0% to 5% at the l of 1.55 under high load. On one aspect, with the addition of reformate, H2 and CO introduced into cylinder can promote more active free radicals during the combustion process [43], which enhances the oxidation of the fuel mixture. The combustion quality is improved and therefore hc increases. On another aspect, the addition of reformate may increase the CO emissions and
Table 4 e Experimental points. Load 75% 50% 25%
Power & Speed 110.0 kW & 1366 rpm 73.5 kW & 1163 rpm 37.0 kW & 969 rpm
SOI
l (-)
Rre
26 CA BTDC 25 CA BTDC 23 CA BTDC
1.31, 1.40, 1.50 and 1.55 1.35, 1.40, 1.45, 1.50 and 1.55 1.30, 1.35, 1.40, 1.45 and 1.50
0%, 3% and 5% 0%, 3% and 5% 0%, 3% and 5%
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 3 e CA 0e10 and CA 10e90 versus excess air ratio at different Rre.
decrease the hc. In the present study, the effect of the increase in CO emission on hc is weaker than that of the improvement in the combustion quality, and thus the hc increases with the increase of Rre. Therefore, compared with the natural gas combustion, the addition of reformate can enable the marine NG engine to operate at large l for reducing the NOx emissions while maintaining the equal fuel consumption.
Fuel consumption Fig. 5 illustrates the equivalent brake specific fuel consumption (BSFC) versus l at different Rre. With the increase of l, equivalent BSFC first decreases and then increases under various engine loads. This could be contributed to the following factors. First, with the increase of l, the intake manifold pressure is boosted with the increase of throttle opening angle to maintain the engine load and the peak combustion temperature decreases, and thus the pumping work and heat losses through the combustion chamber walls are reduced. Meanwhile, higher intake pressure also increases the heat capacity ratio of the fuel-air mixture in the cylinder, which promotes the thermal efficiency of the engine cycle. Therefore, the equivalent BSFC decreases. However, when l is extended to 1.5, high air dilution rate leads to the misfire and low constant-volume combustion (shown in Fig. 3), resulting in the reduction of thermal efficiency and the increase of equivalent BSFC. At high load, the equivalent BSFC increases with the increase of Rre and it is higher than the baseline of natural gas combustion, primarily due to the increase in negative work
caused by the advance in combustion phase. As it is seen from Fig. 3a, when Rre reaches 5%, both the CA 0e10 and CA 10e90 are advanced by about 4 CA and 6 CA, respectively. Nevertheless, the equivalent BSFC tends to decrease with the rise of Rre and is lower than the baseline of natural gas combustion under medium and low loads. According to the combustion phase analysis, the crank angle corresponding to the maximum heat release rate is gradually far away from top dead center with the decrease of load. Since the addition of hydrogen-rich reformate increases the combustion speed, the combustion process tends to be the constant-volume combustion and the thermal efficiency is improved. The above analyses imply that the addition of reformate will be more beneficial to improve the BSFC of the lean-burn marine NG engine under medium and low loads.
Effects of reformate addition on the emissions NOx emissions Fig. 6 gives the brake specific NOx emissions versus l at different Rre. The results clearly show that NOx emissions gradually decrease with the increase of l under various engine loads due to the effect of air dilution on combustion. As the fuel-air mixture in cylinder becomes leaner, the increase in combustion duration and the decrease in the total heat release value reduce the peak combustion temperature. Since the formation of thermal NOx is quite sensitive to the incylinder temperature [44,45], the rate of NOx formation decreases dramatically. It can be seen from Fig. 6 that relatively
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 4 e Combustion efficiency versus l at different Rre.
low NOx emissions (meet the Tier III emissions legislations) can be achieved when l extents to 1.55 at high and medium load and 1.5 at low load, respectively. As far as Rre is concerned. The NOx emissions first increase and then decrease with the increase of Rre at a specific l. From one aspect, the combustion phase analysis shows that the combustion duration is shortened with the increase of Rre, the heat release rate turns to be concentrated and thus the incylinder temperature is raised. From another aspect, with the addition of the hydrogen-rich reformate, the concentrations of H2 and CO in the fuel mixture are increased and the dilution effect of N2 is enhanced, decreasing the total heat release value in the cylinder. Therefore, the in-cylinder average temperature is reduced, which decreases the rate of NOx formation. With the addition of the reformate, the advance in combustion phase first leads to the increase of NOx emissions, and then the decrease in total heat release value inhibits the formation of NOx emissions and turns to dominate. Finally, the NOx emissions first increase and then decrease with the raise of Rre.
THC emissions Fig. 7 gives the brake specific THC emissions versus l at different Rre. THC emissions linearly increase with the raise of l, and show a similar trend at different Rre. In detail, the increase of THC emissions can be explained as the following reasons. First, the in-cylinder CH4 concentration and
temperature decline with the increase of l, which leads to the increase of unburned HC near the cylinder wall due to the quench effect [46,47]. Moreover, the lean fuel-air mixture in cylinder may cause misfire, resulting in the increase of unburned HC during the combustion process. Compared with the effect of l on the THC emissions, the THC emissions decrease with increase of Rre under various engine loads. This can be explained by the following interpretations. First, The CH4 concentration in the cylinder decreases with the increase of Rre at a specific l, and thus the in-cylinder unburned methane and the methane slip during the valve overlap period are decreased. Then, with the addition of hydrogen-rich reformate, the quenching distance of natural gas is reduced and the flame can propagate into the gap between the piston and the liner, reducing the unburned hydrocarbon from those regions. Although the in-cylinder combustion temperature slightly decreases with the addition of reformate and consequently limits the post-oxidize process of the hydrocarbon fuel, the decrease in the incylinder HC emissions are dominant with the addition of hydrogen-rich reformate. Finally, the overall brake specific THC emissions decrease with the increase of Rre.
CO emissions Fig. 8 gives the brake specific CO emissions versus l at different Rre, which shows a similar trend with that of THC at various Rre. There are two reasons to explain this
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 5 e Equivalent BSFC versus l at different Rre.
phenomenon. First, the lean fuel-air mixture increases the misfire rate, which promotes the formation of CO during the combustion process. Second, the decline in the exhaust gas temperature inhibits the oxidation process of the CO due to the air dilution, resulting in the increase of the CO emissions. For a given l, the brake specific CO emissions increase with the increase of Rre under various engine loads, and the difference between two specific Rre is remarkable at large l. Generally, the CO emissions from the NG engine with REGR mainly consist of three parts [48], i.e., CO from the incomplete combustion of the methane, CO from the unburned reformate and CO from the leakage during the valve overlap period. As discussed before, the concentration of CH4 in the fuel mixture decreases with the increase of Rre and consequently reduces the quantity of CO emissions from the incomplete combustion of the CH4. Then, with the increase of Rre, the mass of unburned fuel-air mixture in the crevice zone nearly keeps constant due to the interruption of flame while the mass concentration of CO in the crevice zone increases linearly at the end of combustion process, and thus the CO from the unburned reformate is increased. In addition, the CO from the leakage increases with the increase of Rre due to the unchanged leakage mass and the linear increase of CO concentration in the leakage during the valve overlap period. Based on the above analysis, since the
increase in the CO from the unburned reformate and the leakage is higher than the decrease in the CO from the incomplete combustion of the methane, the brake specific CO emissions increase with the increase of Rre. For this phenomenon, previous studies [49,50] showed that the improvement in the selectivity of H2 can be realized to generate the reformate with low CO concentration by controlling water-carbon ratio and oxygen-carbon ratio during the reforming process. In other words, through improving the reforming process, the CO emissions for natural gasreformate combustion case will be lower than those for natural gas combustion case which is treated as the baseline for comparison.
Discussion Both the NOx emissions and the brake specific fuel consumption (BSFC) are the major indices of performance optimization of the internal combustion engine. Generally, there is a “trade-off” relationship between NOx emissions and BSFC and it is realistic to find a balance between the two performance indices to meet the requirement of engineering applications. Thus, how to effectively reduce the NOx emissions and the BSFC simultaneously has been one of the research
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 6 e Brake specific NOx emissions versus l at different Rre.
Fig. 7 e Brake specific THC emissions versus l at different Rre. Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 8 e Brake specific CO emissions versus l at different Rre.
Fig. 9 e Relationship between the NOx emissions and equivalent BSFC (the green dash lines in the figures show the Tier III limits for NOx emissions at a specific engine speed). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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hotspots in the performance optimization of internal combustion engines. In the present study, the potential of the REGR technique in emissions reduction and energy saving of the marine leanburn NG engine was presented. Fig. 9 illustrates the relationship between the brake specific NOx emissions and equivalent BSFC versus l at different Rre. As the l increases, the brake specific NOx emissions decrease while the equivalent BSFC increases, approximately presenting a “trade-off” relationship. However, with the increase of Rre, the curves of the brake specific NOx emissions and equivalent BSFC present different relative positional relationships under various engine loads. More specifically, the curves of brake specific NOx emissions and equivalent BSFC move up with the increase of reformate addition ratio at high load as shown in Fig. 9 (a), while the curves move down with the increase of Rre at low and medium loads in Fig. 9 (b) and (c). This indicates that it is possible to simultaneously reduce NOx emissions and equivalent BSFC by optimizing Rre and l under different engine loads. More specifically, when the NOx emissions meet the Tier III limits, the optimization of Rre and l are: Rre of 3% and the l of 1.55 at the engine speed of 1363 rpm and BMEP of 0.940 MP, Rre of 5% and l of 1.55 at the engine speed of 1190 rpm and BMEP of 0.718 MPa and Rre of 5% and the l of 1.5 at engine speed of 945 rpm and BMEP of 0.452 MP. Then performance and emissions of the marine NG engine can get the reasonable values.
Conclusions An experimental investigation on the influences of the reformate addition ratio (Rre) and excess air ratio (l) on the combustion and emissions characteristics of a marine NG engine with reformed exhaust gas recirculation was conducted, and the potential of REGR technique in reducing pollution emissions from the marine NG engine to meet more stringent ship emission regulations was discussed. The major conclusions are as follows: (1) Both the flame propagation and rapid combustion duration decrease with the increase of hydrogen-rich reformate addition ratio, mainly due to the increase in the flame propagation speed. And then, the combustion efficiency is promoted for the addition of reformate. (2) The equivalent brake specific fuel consumption (BSFC) first decreases and then increases with the increase of l. At high load, the equivalent BSFC increases with the increase of Rre due to the advance in combustion phase at a specific l, while the reverse holds for the medium and low loads as a result of the high constant-volume combustion. (3) With the increase of l, the brake specific NOx emissions decrease while the brake specific THC and CO emissions increase. At a specified l, the NOx emissions first increase and then decrease with the increase of Rre due to the competition between the combustion phase and total heat release value. The increase of Rre lead to low brake specific THC emissions and high brake specific CO
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emissions, and the difference in THC or CO emissions between various Rre is remarkable at high l. (4) The combination of reformate addition and lean-burn combustion can improve the “trade-off” relationship between the brake specific NOx emissions and equivalent BSFC, and it would be appreciated to meet Tier III emissions legislations and maintain a relatively low fuel consumption with Rre of 3% and l of 1.55 at high load, Rre of 5% and l of 1.55 at medium load and Rre of 5% and l of 1.5 at low load, respectively.
Acknowledgments This work was financially supported by the National Key Research and Development Program of China under Grant 2016YFC0205600 and National Natural Science Foundation of China under Grant 51979212.
references
[1] Imperato M, Kaario O, Sarjovaara T, Larmi M. Split fuel injection and Miller cycle in a large-bore engine. Appl Energy 2016;162:289e97. [2] Mavrelos C, Theotokatos G. Numerical investigation of a premixed combustion large marine two-stroke dual fuel engine for optimising engine settings via parametric runs. Energy Convers Manag 2018;160:48e59. [3] Thomson H, Corbett JJ, Winebrake JJ. Natural gas as a marine fuel. Energy Policy 2015;87:153e67. [4] Æsøy V, Einang PM, Stenersen D, Hennie E, Valberg I. LNGfuelled engines and fuel systems for medium-speed engines in maritime applications. 2011. SAE technical paper 2011-011998. [5] Korakianitis T, Namasivayam AM, Crookes RJ. Natural-gas fueled Spark-Ignition (SI) and Compression-Ignition (CI) engine performance and emissions. Prog Energy Combust Sci 2011;37:89e112. [6] Ma F, Wang M, Jiang L, Chen R, Deng J, Naeve N, Zhao S. Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio. Int J Hydrogen Energy 2010;35:6438e47. [7] Sun Z, Liu F, Liu X, Sun B, Sun D. Research and development of hydrogen fuelled engines in China. Int J Hydrogen Energy 2012;37:664e81. guez PM, Urroz JC, Marcelino-Sa daba S, Pe rez-Ezcurdia A, [8] Die inz D, Gandı´a LM. Experimental study Benito-Amurrio M, Sa of the performance and emission characteristics of an adapted commercial four-cylinder spark ignition engine running on hydrogenemethane mixtures. Appl Energy 2014;113:1068e76. € z B, Celik C, Soyhan HS, Go € kalp B, Karabag B. Emission [9] Ac¸ıkgo characteristics of a hydrogeneCH4 fuelled spark ignition engine. Fuel 2015;159:298e307. [10] Tartakovsky L, Sheintuch M. Fuel reforming in internal combustion engines. Prog Energy Combust Sci 2018;67:88e114. ndez J, Golunski S. [11] Leung P, Tsolakis A, Rodrı´guez-Ferna Raising the fuel heating value and recovering exhaust heat
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
12
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
international journal of hydrogen energy xxx (xxxx) xxx
by on-board oxidative reforming of bioethanol. Energy Environ Sci 2010;3:780e8. Zhang B, Ji C, Wang S. Performance of a hydrogen-enriched ethanol engine at unthrottled and lean conditions. Energy Convers Manag 2016;114:68e74. Zheng Z, Liu C, Zhang X. Numerical study of effects of Reformed Exhaust Gas Recirculation (REGR) on dimethyl ether HCCI combustion. Int J Hydrogen Energy 2014;39:8106e17. Park C, Choi Y, Kim C, Oh S, Lim G, Moriyoshi Y. Performance and exhaust emission characteristics of a spark ignition engine using ethanol and ethanol-reformed gas. Fuel 2010;89:2118e25. Poran A, Tartakovsky L. Energy efficiency of a directinjection internal combustion engine with high-pressure methanol steam reforming. Energy 2015;88:506e14. Zhang Z, Jia P, Zhong G, Liang J, Li G. Numerical study of exhaust reforming characteristics on hydrogen production for a marine LNG engine fueled with LNG. Appl Therm Eng 2017;124:241e9. Zhang Z, Jia P, Feng S, Liang J, Long Y, Li G. Numerical simulation of exhaust reforming characteristics in catalytic fixed-bed reactors for a natural gas engine. Chem Eng Sci 2018;191:200e7. Tsolakis A, Megaritis A. Partially premixed charge compression ignition engine with on-board H2 production by exhaust gas fuel reforming of diesel and biodiesel. Int J Hydrogen Energy 2005;30:731e45. Fennell D, Herreros JM, Tsolakis A, Wyszynski M, Cockle K, Pignon J, Millington P. On-board thermochemical energy recovery technology for low carbon clean gasoline direct injection engine powered vehicles. Proc Inst Mech Eng - Part D J Automob Eng 2018;232:1079e91. Chuahy FDF, Kokjohn SL. High efficiency dual-fuel combustion through thermochemical recovery and diesel reforming. Appl Energy 2017;195:503e22. Chintala V, Benaerjee D, Ghodke PK, Porpathamb E. Hydrogen rich exhaust gas recirculation (H2EGR) for performance improvement and emissions reduction of a compression ignition engine. Int J Hydrogen Energy 2019;44:18545e58. Fennell D, Herreros J, Tsolakis A. Improving gasoline direct injection (GDI) engine efficiency and emissions with hydrogen from exhaust gas fuel reforming. Int J Hydrogen Energy 2014;39:5153e62. Bogarra M, Herreros JM, Tsolakis A, York APE, Millington PJ, Martos FJ. Impact of exhaust gas fuel reforming and exhaust gas recirculation on particulate matter morphology in gasoline direct injection engine. J Aerosol Sci 2017;103:1e14. Bartley G, Gukelberger R, Henderson R, Henry C. Fuel reforming and catalyst deactivation investigated in real exhaust environment. 2019. SAE Technical Paper 2019-010315. Catapan RC, Cancino LR, Oliveira AAM, Schwarz CO, Nitschke H, Frank T. Potential for onboard hydrogen production in an direct injection ethanol fueled spark ignition engine with EGR. Fuel 2018;234:441e6. Nguyen DK, Sileghem L, Verhelst S. Exploring the potential of reformed-exhaust gas recirculation (R-EGR) for increased efficiency of methanol fueled SI engines. Fuel 2019;236:778e91. Zhang Z, Xie Q, Liang J, Li G. Numerical study of combustion characteristics of a natural gas HCCI engine with closed loop exhaust-gas fuel reforming. Appl Therm Eng 2017;119:430e7. Yap D, Peucheret S, Megaritis A, Wyszynski M, Xu H. Natural gas HCCI engine operation with exhaust gas fuel reforming. Int J Hydrogen Energy 2006;31:587e95.
[29] IMO. Resolution MEPC.177(58), Amendments to the technical code on control of emission of nitrogen oxides from marine diesel engines (NOx Technical Code 2008)vol. 1; 2008. MEPC 58/23/Add. [30] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrogen Energy 2015;40:11094e111. [31] He Z, Gao Z, Zhu L, Li S, Li A. Effects of H2 and CO enrichment on the combustion, emission and performance characteristics of spark ignition natural gas engine. Fuel 2016;183:230e7. [32] Halabi MH, Croon MHJMD, Schaaf JVD, Cobden PD, Schouten JC. Modeling and analysis of autothermal reforming of methane to hydrogen in a fixed bed reformer. Chem Eng Sci 2008;137:568e78. [33] Hamedi MR, Tsolakis A, Lau CS. Biogas upgrading for onboard hydrogen production: reforming process CFD modelling. Int J Hydrogen Energy 2014;39:12532e40. [34] Lau CS, Allen D, Tsolakis A, Golunski SE, Wyszynskia ML. Biogas upgrade to syngas through thermochemical recovery using exhaust gas reforming. Biomass Bioenergy 2012;40:86e95. [35] Li G, Zhang Z, You F, Pan Z, Zhang X, Dong J. A novel strategy for hydrous-ethanol utilization: demonstration of a sparkignition engine fueled with hydrogen-rich fuel from an onboard ethanol/steam reformer. Int J Hydrogen Energy 2013;38:5936e48. [36] Wang Y, Zhao Y, Xiao F, Li D. Combustion and emission characteristics of a diesel engine with DME as port premixing fuel under different injection timing. Energy Convers Manag 2014;77:52e60. [37] Balki MK, Sayin C, Canakci M. The effect of different alcohol fuels on the performance, emission and combustion characteristics of a gasoline engine. Fuel 2014;115:901e6. [38] Heywood JB. Internal combustion engine fundamentals. United states of American: McGraw-Hill; 1988. p. 389. [39] Sun Z. Experimental studies on the explosion indices in turbulent stoichiometric H2/CH4/air mixtures. Int J Hydrogen Energy 2019;44:469e76. [40] Li G, Zhang Z, Liang J, Dong F, Li Y, Gao X. Effects of hydrogen addition on the premixed laminar-flames of ethanoleair gaseous mixtures: an experimental study. Int J Hydrogen Energy 2012;37:4490e501. [41] Liu J, Zhang X, Wang T, Hou X, Zhang J, Zheng S. Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture. Int J Hydrogen Energy 2015;40:8475e83. [42] Ennetta R, Alaya M, Said R. Numerical study of laminar flame velocity of hydrogen-enriched methane flames using several detailed reaction mechanisms. Arabian J Sci Eng 2017;42:1707e13. [43] Sun Z. Structure of turbulent rich hydrogen-air premixed flames. Int J Energy Res 2018;42:2845e58. [44] Masum BM, Masjuki HH, Kalam MA, Fattah IR, Palash SM, Abedin MJ. Effect of ethanolegasoline blend on NOx emission in SI engine. Renew Sustain Energy Rev 2013;24:209e22. [45] Cho HM, He BQ. Spark ignition natural gas engines-A review. Energy Convers Manag 2007;48:608e18. [46] Park C, Kim C, Choi Y, Lee J. Operating strategy for exhaust gas reduction and performance improvement in a heavyduty hydrogen-natural gas blend engine. Energy 2013;50:262e9. [47] Verma G, Prasad RK, Agarwal RA, Jain S, Agarwal AK. Experimental investigations of combustion, performance and emission characteristics of a hydrogen enriched natural gas fuelled prototype spark ignition engine. Fuel 2016;178:209e17.
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
international journal of hydrogen energy xxx (xxxx) xxx
[48] Long Y, Li G, Zhang Z, Liang J, Mao L, Li Y. Effects of reformed exhaust gas recirculation on the HC and CO emissions of a spark-ignition engine fueled with LNG. Int J Hydrogen Energy 2018;43:21070e8. [49] Figen HE, Baykara SZ. Effect of ruthenium addition on molybdenum catalysts for syngas production via catalytic
13
partial oxidation of methane in a monolithic reactor. Int J Hydrogen Energy 2017;43:1129e38. [50] Loukou A, Mendes MAA, Frenzel I, Pereira JMC, Ray S, Pereira JCF, Trimis D. Experimental and numerical investigation of methane thermal partial oxidation in a small-scale porous media reformer. Int J Hydrogen Energy 2017;42:652e63.
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
Available online at www.sciencedirect.com
ScienceDirect journal homepage: www.elsevier.com/locate/he
Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate Gesheng Li a,b,*, Yangxiang Long b, Zunhua Zhang a,b, Junjie Liang b, Xiaowu Zhang b, Xintang Zhang a,b, Zhongjun Wang a,b a
Key Laboratory of High Performance Ship Technology, Wuhan University of Technology, Ministry of Education, Wuhan, Hubei, 430063, PR China b School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei, 430063, PR China
highlights
graphical abstract
The application of REGR in marine NG engines can achieve low NOx emissions. The equivalent BSFC decreases with the increase of Rre at medium and low loads. The increase of Rre leads to low THC
emissions
and
high
CO
emissions.
article info
abstract
Article history:
The exhaust gas-fuel reforming technique known as reformed exhaust gas recirculation
Received 2 June 2019
(REGR) can generate on-board hydrogen-rich gas mixture (i.e., reformate) by catalytic
Received in revised form
reforming of the exhaust gas and fuel added into the reformer and then recirculate the
1 October 2019
reformate into the engine cylinder, which can realize the combination of hydrogen-rich
Accepted 4 October 2019
lean combustion and exhaust gas recirculation. The REGR technique can be employed to
Available online xxx
achieve efficient and stable lean-burn combustion for the marine engine fueled with natural gas (i.e., marine NG engine) and it is considered as an effective way to meet the
Keywords:
stringent ship emissions regulations. In the present study, an experimental investigation
Reformed exhaust gas recirculation
into the effects of reformate addition ratio (Rre) and excess air ratio (l) on the combustion
Hydrogen-rich reformate
and emissions characteristics of a marine NG engine under various loads was conducted,
Marine engine
and the potential of applying the REGR technique in a marine NG engine to achieve low
Liquefied natural gas
emissions (i.e., International Maritime Organization Tier III emissions legislations for in-
Combustion and emissions
ternational ships) was discussed. The results indicate that the addition of the hydrogen-
Thermal efficiency
rich reformate gases can extend lean-burn limit. For a given l, the flame development duration and rapid combustion duration decrease with the increase of Rre, and the combustion efficiency is improved. The brake specific NOx emissions first increase and then
* Corresponding author. School of Energy and Power Engineering, Wuhan University of Technology, Wuhan, Hubei, 430063, PR China. E-mail address: [email protected] (G. Li). https://doi.org/10.1016/j.ijhydene.2019.10.007 0360-3199/© 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
2
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decrease with the increase of Rre due to the competition between the combustion phase and total heat release value. The brake specific THC emissions decline with the increase of Rre, while the reverse holds for the brake specific CO emissions, and the behavior tends to be obvious under large l. It is demonstrated that the combination of REGR and the leanburn combustion strategy can improve the trade-off relationship between the NOx emissions and brake specific fuel consumption of the marine NG engine to meet the IMO Tier III NOx emissions legislations and maintain relatively low brake specific fuel consumption. © 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Nomenclature Abbreviations Stoichiometric air-fuel ratio AFst BMEP Brake mean effective pressure BSFC Brake specific fuel consumption CA Crank angle CA 0-10 Crank angle interval from the crank angle of 0% mass fraction burned to the crank angle of 10% mass fraction burned CA 10-90 Crank angle interval from the crank angle of 10% mass fraction burned to the crank angle of 90% mass fraction burned CO Carbon monoxide Carbon dioxide CO2 ECAs Emission control areas EGR Exhaust gas recirculation GDI Gasoline direct injection HCCI Homogeneous charge combustion ignition IMO International Maritime Organization LHV Low heating value LNG Liquefied natural gas Nitrogen oxides NOx PM Particulate matter REGR Reformed exhaust gas recirculation SI Spark ignition Sulfur oxides SOx THC Total hydrocarbon
international ships are implemented. Liquefied natural gas (LNG) has the advantages of low sulfur content and carbon/ hydrogen ratio, using the LNG as the fuel of a marine engine can reduce the emissions of sulfur oxides (SOx), carbon dioxide (CO2) and particulate matter (PM). In addition, it is reported [2,3] that the low nitrogen oxides (NOx) emissions and high thermal efficiency can be achieved for the marine engine fueled with natural gas (i.e., the marine NG engine) by employing the lean-burn strategy. Therefore, LNG as an alternative fuel for marine engines has received extensive attention. Generally, high excess air ratio may lead to the misfire and the increase of unburned hydrocarbon (UHC) and carbon monoxide (CO) emissions, and meanwhile the retarded combustion phase reduces the thermal efficiency of the lean-burn spark-ignition (SI) engine due to the low flame speed of natural gas [4e6]. Many studies have shown that the addition of hydrogen is beneficial to the low-temperature lean-burn combustion of the natural gas engine, which can reduce the misfire rate, extend the lean operation area and promote the complete combustion of the natural gas, and therefore the reduction of the HC and CO emissions [7e9]. Unfortunately, the cost and risk of producing and transporting hydrogen are relatively high [10], which restricts the application of on-board H2 addition in the optimization of the combustion of the natural gas engine. Recently, reformed exhaust gas recirculation (REGR) technique [11,12], which generates on-board hydrogen-rich gas
Symbols Reformate addition ratio Rre l Excess air ratio Combustion efficiency hc
Introduction Emissions from marine engines account for a large portion of the air pollutant emission inventories, especially for the coastal areas. The International Maritime Organization (IMO) established the emission control areas (ECAs, such as Baltic Sea and North Sea) where the stricter emissions regulations (IMO Tier III emissions legislations [1], shown in Fig. 1) for
Fig. 1 e IMO Emission Legislations (n ¼ engine speed).
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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(i.e., reformate) via catalytic reforming of fuel and engine exhaust gas in an exhaust gas-fuel reformer and recirculates the reformate into the cylinder for improving combustion, is regarded as an effective way to achieve the combustion with on-board H2 addition and exhaust gas recirculation. The REGR technique can achieve stable lean-burn combustion, and then reduce the NOx emissions and improve the thermal efficiency [13e15]. Compared with the vehicle engine, the operating conditions of the marine engine usually are more stable and the fluctuations in the temperature and component of exhaust gas are smaller, which is conducive to the stable hydrogen production of the exhaust gas-fuel reformer and the regulation between the reformer and the marine NG engine. Our previous numerical studies [16,17] showed that the onboard hydrogen can be generated by catalytic reforming of natural gas and partial exhaust gas from a marine NG engine via the exhaust reforming reaction tube. In the last few years, several investigations on the exhaust gas reforming of various fuels including diesel [18e21], gasoline [22e24], ethanol [25,26] and natural gas [27,28], etc. Have been reported, and the studies on the application of exhaust gas-fuel reforming were mostly related to the diesel and gasoline engine. For instance, Tsolakis and his co-workers [18,22,23] carried out a series of studies to investigate the potential of REGR technique in energy-saving and emissions reduction of the vehicle engine. They experimentally investigated the effect of the reformate generated from a dieselexhaust gas reformer on the performance of diesel engine. The results showed that the addition of reformate reduced the NOx and PM emissions and led to an increase in the thermal efficiency [18]. After that, they generated a reformate-like mixture by adding bottled H2/CO to conventional EGR to study the effect of reformate addition on the efficiency and emissions performance of a multi-cylinder GDI engine, and the results showed that the thermal efficiency was increased, and both the gaseous pollutants and PM emissions were reduced [22]. Additionally, they compared the effect of EGR and REGR on PM emissions in a GDI engine and found that REGR could reduce the diameter and concentration of the particle emission [23]. Moreover, Yap et al. [28] found that REGR could expand the low-load operating range of the natural gas HCCI engine. As the quantity of reformate added was increased, NOx emissions decreased while HC and CO emissions increased, respectively. The above studies indicated that the application of REGR technique could improve the fuel economy and emissions characteristics of the engine fueled with various types of fuels. To the best of the authors’ knowledge, the previous research on the REGR technique mostly focused on vehicle engines, while little is known about the marine NG engine with REGR. Therefore, the objective of the present study is to carry out relevant research to obtain the influence of REGR on the performance of a marine NG engine, and the emphasis is to investigate the potential of REGR technique on reducing pollution emissions from a marine NG engine to meet more stringent ship emission regulations (i.e., IMO Tier III emissions legislations). In the present study, the effects of reformate addition ratio (Rre) and excess air ratio (l) on the combustion and emissions characteristics of the marine NG engine under different loads were analyzed, and then the appropriate way
to apply REGR technique in the marine NG engine for meeting the Tier III emissions legislations was presented. The present study is of great importance for the application the RGER technique to the control of air pollutant emission from the ships powered with LNG.
Experimental setup and procedure Engine specification and instruments The engine used in the present study was a six-cylinder, water cooled, turbo-charged, premixed SI marine NG engine produced by Yuchai Co. (YC6MK 200N). The specifications of the test engine are listed in Table 1 and the schematic diagram of the experimental system is presented in Fig. 2 and the specifications of test instruments and equipment are listed in Table 2. The electronic eddy current dynamometer was connected to the engine to automatically adjust the engine torque and speed. A Coriolis mass flowmeter was employed to measure the mass flow rate of natural gas, and Table 3 shows the composition of the natural gas used in the present test. An exhaust gas analyzer was used to analyze the exhaust gas emissions. The concentrations of CO, CO2, HC, NOx and O2 and fuel consumption rate were recorded and averaged for calculating the brake specific emissions and fuel consumption under specific experimental points, which was based on the measurement method listed in the legislative documents [29]. The pressure sensor was installed in the sixth cylinder head for recording the instantaneous in-cylinder pressure. At each test point, the in-cylinder pressures were recorded for 200 cycles at 0.1 crank angle ( CA) by the shaft encoder, and then the pressures were averaged and post-processed for combustion process analysis. With respect to the hydrogen-rich reformate gas addition, an independent gas supply system was designed.
Hydrogen-rich reformate The objective of the present study is to investigate the effects of the reformate gas addition on the performance and emissions characteristics of a marine NG engine. Due to the complex reforming reactions, we have to, first of all, define the composition of the reformate gas mixture to generate the hydrogen-rich reformate gas for the engine bench test. It is known that methane is the main constituent of natural gas, and thus the exhaust gas-fuel reforming process of the natural gas mainly involves in the partial oxidation methane reforming reaction (CH4 þ 0.5O2 <¼> CO þ 2H2), steam methane
Table 1 e Engine specifications. Parameters Bore (mm) Stroke (mm) Displacement (L) Compression ratio Power (kW) Rated speed (r/min) Piston bowl
Values 123 145 10.338 11.5:1 147 1500 Shallow bowl
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 2 e Schematic of the experiment system.
Table 2 e The specifications of instruments and equipment. Instruments
Type
Dynamometer
Y880
Coriolis mass flowmeter Emission analyzer Mass flow controller Pressure sensor
DFM-1-3A CAI 600 Red-y Kistler 7013C
Accuracy Torque: ±0.2% of full scale Speed: ±5 rpm ±0.5% of full scale ±1.0% of full scale ±0.3% of full scale ±0.7% of full scale
reforming reaction (CH4 þ H2O <¼> CO þ 3H2), water gas shift reaction (CO þ H2O <¼> CO2 þ H2), etc. [17,30]. Therefore, a typical reformate generated from the methane and exhaust gas reforming mainly consists of H2, CO, CH4, H2O, CO2 and N2 [31,32]. The existence of CH4 in the reformate is because of the incomplete catalytic reforming of methane. Actually, the unreacted CH4 is recirculated into the cylinder and combusted as fuel, and meanwhile most of the H2O steam in the reformate will be condensed and removed after passing through the REGR cooler. Thus, both the CH4 and H2O in the reformate are neglected during the process of hydrogen-rich reformate generation in the present study. In addition, considering that both N2 and CO2 are dilution gases and the fraction of N2 is larger than that of CO2, thus N2 is retained as the main dilution gas and CO2 is neglected during the process of hydrogen-rich reformate generation. As a result, the reformate gas is simplified as a mixture of H2, CO and N2 in the present study. Usually, there is a variation in the composition of reformate. However, the composition of the reformate will only change within a small range by adjusting the boundary conditions (i.e., gas hour space velocity, O2/CH4 ratio) during the exhaust gas-fuel reforming process [10]. The minor change in the composition of the reformate has little influence on the combustion and emissions characteristics of the engine. Therefore, we temporarily ignored the effect of the variation
Table 3 e Composition of the natural gas. Item
CH4
C2H6
C3H8
N2
Others
Volumetric fraction (%)
99.03
0.63
0.049
0.29
0.001
Manufacturer Qidong, China Shouke, China California, USA € gtlin, Switzerland Vo Kistler, Switzerland
in the composition of reformate in the present study. Then, a hydrogen-rich reformate gas with fixed composition of 24% H2, 12% CO, and 64% N2 in mole fraction was generated to carry out the present experiment study based on the methane/exhaust gas reforming experiments from previous research [33,34] and our methane reforming experiments over nickel- and rhodium-based Catalysts. In the present study, a hydrogen-rich reformate gas supply system was designed. Three mass flow controllers connected with compressed gas tanks of H2, CO and N2 were used to generate the hydrogen-rich reformate with the specific composition and mass flow rate. The hydrogen-rich reformate gas mixed with fresh air in the mixer installed at the intake manifold before the turbo charger, and then entered the cylinder. The purities of H2, CO and N2 are 99.999%, 99.99% and 99.99%, respectively.
Experimental conditions and definitions In the present study, the performance and emissions of the marine NG engine with various hydrogen-rich reformate addition ratio (Rre) and excess air ratio (l) were compared under three different engine loads, 110.0 kw & 1366 rpm, 73.5 kw & 1163 rpm and 37.0 kw & 969 rpm, respectively. Before the experiment, the engine was warmed up, and the measurements started when the coolant reached 85 ± 5 C. During the test, the spark ignition timing was fixed at the specific value according to the original ignition timing data from the prototype engine, and the position of the throttle was adjusted to maintain the engine torque with the variation of Rre and l at a specific engine speed. The excess air ratio (l) is defined as:
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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mair l¼ mNG AFst;NG þ mH2 AFst;H2 þ mCO AFst;CO
(1)
where mair, mNG, mH2 and mCO represent the consumption rate (g/h) of the intake air, natural gas, H2 and CO, respectively. AFst,NG, AFst,H2 and AFst,CO represent the stoichiometric airfuel ratio of the natural gas, H2 and CO, respectively. And meanwhile, the intake gas flow rate was measured to ensure the amount of hydrogen-rich reformate added into the intake manifold at two different ratios of 3% and 5% of the total volumetric intake gas flow rate at different engine loads. The reformate addition ratio (Rre) can be written as: Rre ¼
Vre Vintake þ Vre
(2)
where Vre and Vintake represent the volumetric flow rate (m3/h) of the reformate and intake air in the inlet pipe, respectively. The specific experimental points are listed in Table 4. The equivalent brake specific fuel consumption (BSFC) is used to evaluate the fuel economy of the engine with hydrogen-rich reformate addition in the previous studies [35,36], and it can be written as: equivalent BSFC ¼
1 LHVH2 LHVCO mNG þ mH2 þ mCO W LHVNG LHVNG (3)
where W is the engine power (kW) and LHVNG, LHVH2 and LHVCO are the low heat value (kJ/g) of natural gas, H2 and CO, respectively. The combustion efficiency (hc) is used to evaluate the quality of in-cylinder combustion, and the total hydrocarbon (THC) and CO emissions are counted to calculate the heat loss due to the imperfect combustion. Since the main component of THC emissions from the NG engine is CH4 [37], the heat loss due to THC emissions is evaluate by the heat value of CH4 in the present study. Therefore, the combustion efficiency (hc) can be defined as: hc ¼ 1
LHVCH4 ETHC þ LHVCO ECO LHVNG mNG
(4)
where LHVCH4 is the low heat value (kJ/g) of CH4, and ETHC and ECO are THC and CO emissions (g/h), respectively.
Results Effects of reformate addition on the combustion and efficiency Combustion phase Fig. 3 shows the flame propagation duration (CA 0e10, crank angle interval from the crank angle of 0% mass fraction
burned to the crank angle of 10% mass fraction burned) and rapid combustion duration (CA 10e90, crank angle interval from the crank angle of 10% mass fraction burned to the crank angle of 90% mass fraction burned) [38] versus l at different Rre. As can be seen, CA 0e10 increases with the increase of l, and the reason can be ascribed to the dilution effect of air on the combustion. The lean fuel-air mixture leads to the delay of ignition and the decline in the formation and development of the flame kernel. At a specific l, the addition of the hydrogen-rich reformate shortens the CA 0e10 compared with the engine fueled with natural gas. With the addition of hydrogen-rich reformate, H2 and CO are introduced into the cylinder, and then the production of free radicals like OH, O and H are boosted during the combustion process [39,40]. Since the promotion effect of H2 and CO on the combustion is stronger than the dilution effect of N2 introduced into the cylinder with the addition of reformate, the CA 0-10 decreases dramatically. Moreover, the increase of Rre can raise the concentrations of H2 and CO in cylinder and advance the combustion phase. The trend of CA 10e90 versus l is similar to that of CA 0e10, and the increase in l leads to the increase of CA 10e90. It is because that the extending air dilution rate decreases the combustion speed and leads to the long combustion duration. However, when the hydrogen-rich reformate is added into the cylinder, the CA 10-90 declines at a specific l due to the high flame speed of hydrogen. As the Rre increases up to 5%, the volume percentage of H2 and CO in the fuel mixture reaches 21% and the laminar flame speed of the fuel-air mixture increases by 18% [41,42]. Therefore, the CA 10e90 is shortened.
Combustion efficiency Fig. 4 illustrates the combustion efficiency (hc) versus l at different Rre. With the increase of l, the combustion efficiency decreases gradually. The hc decreases by 1% when l increases from 1.3 to 1.5 in the case of both natural gas combustion and natural gas-reformate combustion. As discussed in the combustion phase, the effect of air dilution on combustion leads to an increase in the misfire rate and THC and CO emissions (more details will be discussed later), which reduces the total heat release value and then the hc decreases. Moreover, this behavior will be remarkable at large l due to the worse combustion performance. For a given l, hc increases with the addition of reformate. For example, the improvement in hc can reaches 0.5% when Rre increases from 0% to 5% at the l of 1.55 under high load. On one aspect, with the addition of reformate, H2 and CO introduced into cylinder can promote more active free radicals during the combustion process [43], which enhances the oxidation of the fuel mixture. The combustion quality is improved and therefore hc increases. On another aspect, the addition of reformate may increase the CO emissions and
Table 4 e Experimental points. Load 75% 50% 25%
Power & Speed 110.0 kW & 1366 rpm 73.5 kW & 1163 rpm 37.0 kW & 969 rpm
SOI
l (-)
Rre
26 CA BTDC 25 CA BTDC 23 CA BTDC
1.31, 1.40, 1.50 and 1.55 1.35, 1.40, 1.45, 1.50 and 1.55 1.30, 1.35, 1.40, 1.45 and 1.50
0%, 3% and 5% 0%, 3% and 5% 0%, 3% and 5%
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 3 e CA 0e10 and CA 10e90 versus excess air ratio at different Rre.
decrease the hc. In the present study, the effect of the increase in CO emission on hc is weaker than that of the improvement in the combustion quality, and thus the hc increases with the increase of Rre. Therefore, compared with the natural gas combustion, the addition of reformate can enable the marine NG engine to operate at large l for reducing the NOx emissions while maintaining the equal fuel consumption.
Fuel consumption Fig. 5 illustrates the equivalent brake specific fuel consumption (BSFC) versus l at different Rre. With the increase of l, equivalent BSFC first decreases and then increases under various engine loads. This could be contributed to the following factors. First, with the increase of l, the intake manifold pressure is boosted with the increase of throttle opening angle to maintain the engine load and the peak combustion temperature decreases, and thus the pumping work and heat losses through the combustion chamber walls are reduced. Meanwhile, higher intake pressure also increases the heat capacity ratio of the fuel-air mixture in the cylinder, which promotes the thermal efficiency of the engine cycle. Therefore, the equivalent BSFC decreases. However, when l is extended to 1.5, high air dilution rate leads to the misfire and low constant-volume combustion (shown in Fig. 3), resulting in the reduction of thermal efficiency and the increase of equivalent BSFC. At high load, the equivalent BSFC increases with the increase of Rre and it is higher than the baseline of natural gas combustion, primarily due to the increase in negative work
caused by the advance in combustion phase. As it is seen from Fig. 3a, when Rre reaches 5%, both the CA 0e10 and CA 10e90 are advanced by about 4 CA and 6 CA, respectively. Nevertheless, the equivalent BSFC tends to decrease with the rise of Rre and is lower than the baseline of natural gas combustion under medium and low loads. According to the combustion phase analysis, the crank angle corresponding to the maximum heat release rate is gradually far away from top dead center with the decrease of load. Since the addition of hydrogen-rich reformate increases the combustion speed, the combustion process tends to be the constant-volume combustion and the thermal efficiency is improved. The above analyses imply that the addition of reformate will be more beneficial to improve the BSFC of the lean-burn marine NG engine under medium and low loads.
Effects of reformate addition on the emissions NOx emissions Fig. 6 gives the brake specific NOx emissions versus l at different Rre. The results clearly show that NOx emissions gradually decrease with the increase of l under various engine loads due to the effect of air dilution on combustion. As the fuel-air mixture in cylinder becomes leaner, the increase in combustion duration and the decrease in the total heat release value reduce the peak combustion temperature. Since the formation of thermal NOx is quite sensitive to the incylinder temperature [44,45], the rate of NOx formation decreases dramatically. It can be seen from Fig. 6 that relatively
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 4 e Combustion efficiency versus l at different Rre.
low NOx emissions (meet the Tier III emissions legislations) can be achieved when l extents to 1.55 at high and medium load and 1.5 at low load, respectively. As far as Rre is concerned. The NOx emissions first increase and then decrease with the increase of Rre at a specific l. From one aspect, the combustion phase analysis shows that the combustion duration is shortened with the increase of Rre, the heat release rate turns to be concentrated and thus the incylinder temperature is raised. From another aspect, with the addition of the hydrogen-rich reformate, the concentrations of H2 and CO in the fuel mixture are increased and the dilution effect of N2 is enhanced, decreasing the total heat release value in the cylinder. Therefore, the in-cylinder average temperature is reduced, which decreases the rate of NOx formation. With the addition of the reformate, the advance in combustion phase first leads to the increase of NOx emissions, and then the decrease in total heat release value inhibits the formation of NOx emissions and turns to dominate. Finally, the NOx emissions first increase and then decrease with the raise of Rre.
THC emissions Fig. 7 gives the brake specific THC emissions versus l at different Rre. THC emissions linearly increase with the raise of l, and show a similar trend at different Rre. In detail, the increase of THC emissions can be explained as the following reasons. First, the in-cylinder CH4 concentration and
temperature decline with the increase of l, which leads to the increase of unburned HC near the cylinder wall due to the quench effect [46,47]. Moreover, the lean fuel-air mixture in cylinder may cause misfire, resulting in the increase of unburned HC during the combustion process. Compared with the effect of l on the THC emissions, the THC emissions decrease with increase of Rre under various engine loads. This can be explained by the following interpretations. First, The CH4 concentration in the cylinder decreases with the increase of Rre at a specific l, and thus the in-cylinder unburned methane and the methane slip during the valve overlap period are decreased. Then, with the addition of hydrogen-rich reformate, the quenching distance of natural gas is reduced and the flame can propagate into the gap between the piston and the liner, reducing the unburned hydrocarbon from those regions. Although the in-cylinder combustion temperature slightly decreases with the addition of reformate and consequently limits the post-oxidize process of the hydrocarbon fuel, the decrease in the incylinder HC emissions are dominant with the addition of hydrogen-rich reformate. Finally, the overall brake specific THC emissions decrease with the increase of Rre.
CO emissions Fig. 8 gives the brake specific CO emissions versus l at different Rre, which shows a similar trend with that of THC at various Rre. There are two reasons to explain this
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 5 e Equivalent BSFC versus l at different Rre.
phenomenon. First, the lean fuel-air mixture increases the misfire rate, which promotes the formation of CO during the combustion process. Second, the decline in the exhaust gas temperature inhibits the oxidation process of the CO due to the air dilution, resulting in the increase of the CO emissions. For a given l, the brake specific CO emissions increase with the increase of Rre under various engine loads, and the difference between two specific Rre is remarkable at large l. Generally, the CO emissions from the NG engine with REGR mainly consist of three parts [48], i.e., CO from the incomplete combustion of the methane, CO from the unburned reformate and CO from the leakage during the valve overlap period. As discussed before, the concentration of CH4 in the fuel mixture decreases with the increase of Rre and consequently reduces the quantity of CO emissions from the incomplete combustion of the CH4. Then, with the increase of Rre, the mass of unburned fuel-air mixture in the crevice zone nearly keeps constant due to the interruption of flame while the mass concentration of CO in the crevice zone increases linearly at the end of combustion process, and thus the CO from the unburned reformate is increased. In addition, the CO from the leakage increases with the increase of Rre due to the unchanged leakage mass and the linear increase of CO concentration in the leakage during the valve overlap period. Based on the above analysis, since the
increase in the CO from the unburned reformate and the leakage is higher than the decrease in the CO from the incomplete combustion of the methane, the brake specific CO emissions increase with the increase of Rre. For this phenomenon, previous studies [49,50] showed that the improvement in the selectivity of H2 can be realized to generate the reformate with low CO concentration by controlling water-carbon ratio and oxygen-carbon ratio during the reforming process. In other words, through improving the reforming process, the CO emissions for natural gasreformate combustion case will be lower than those for natural gas combustion case which is treated as the baseline for comparison.
Discussion Both the NOx emissions and the brake specific fuel consumption (BSFC) are the major indices of performance optimization of the internal combustion engine. Generally, there is a “trade-off” relationship between NOx emissions and BSFC and it is realistic to find a balance between the two performance indices to meet the requirement of engineering applications. Thus, how to effectively reduce the NOx emissions and the BSFC simultaneously has been one of the research
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 6 e Brake specific NOx emissions versus l at different Rre.
Fig. 7 e Brake specific THC emissions versus l at different Rre. Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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Fig. 8 e Brake specific CO emissions versus l at different Rre.
Fig. 9 e Relationship between the NOx emissions and equivalent BSFC (the green dash lines in the figures show the Tier III limits for NOx emissions at a specific engine speed). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
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hotspots in the performance optimization of internal combustion engines. In the present study, the potential of the REGR technique in emissions reduction and energy saving of the marine leanburn NG engine was presented. Fig. 9 illustrates the relationship between the brake specific NOx emissions and equivalent BSFC versus l at different Rre. As the l increases, the brake specific NOx emissions decrease while the equivalent BSFC increases, approximately presenting a “trade-off” relationship. However, with the increase of Rre, the curves of the brake specific NOx emissions and equivalent BSFC present different relative positional relationships under various engine loads. More specifically, the curves of brake specific NOx emissions and equivalent BSFC move up with the increase of reformate addition ratio at high load as shown in Fig. 9 (a), while the curves move down with the increase of Rre at low and medium loads in Fig. 9 (b) and (c). This indicates that it is possible to simultaneously reduce NOx emissions and equivalent BSFC by optimizing Rre and l under different engine loads. More specifically, when the NOx emissions meet the Tier III limits, the optimization of Rre and l are: Rre of 3% and the l of 1.55 at the engine speed of 1363 rpm and BMEP of 0.940 MP, Rre of 5% and l of 1.55 at the engine speed of 1190 rpm and BMEP of 0.718 MPa and Rre of 5% and the l of 1.5 at engine speed of 945 rpm and BMEP of 0.452 MP. Then performance and emissions of the marine NG engine can get the reasonable values.
Conclusions An experimental investigation on the influences of the reformate addition ratio (Rre) and excess air ratio (l) on the combustion and emissions characteristics of a marine NG engine with reformed exhaust gas recirculation was conducted, and the potential of REGR technique in reducing pollution emissions from the marine NG engine to meet more stringent ship emission regulations was discussed. The major conclusions are as follows: (1) Both the flame propagation and rapid combustion duration decrease with the increase of hydrogen-rich reformate addition ratio, mainly due to the increase in the flame propagation speed. And then, the combustion efficiency is promoted for the addition of reformate. (2) The equivalent brake specific fuel consumption (BSFC) first decreases and then increases with the increase of l. At high load, the equivalent BSFC increases with the increase of Rre due to the advance in combustion phase at a specific l, while the reverse holds for the medium and low loads as a result of the high constant-volume combustion. (3) With the increase of l, the brake specific NOx emissions decrease while the brake specific THC and CO emissions increase. At a specified l, the NOx emissions first increase and then decrease with the increase of Rre due to the competition between the combustion phase and total heat release value. The increase of Rre lead to low brake specific THC emissions and high brake specific CO
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emissions, and the difference in THC or CO emissions between various Rre is remarkable at high l. (4) The combination of reformate addition and lean-burn combustion can improve the “trade-off” relationship between the brake specific NOx emissions and equivalent BSFC, and it would be appreciated to meet Tier III emissions legislations and maintain a relatively low fuel consumption with Rre of 3% and l of 1.55 at high load, Rre of 5% and l of 1.55 at medium load and Rre of 5% and l of 1.5 at low load, respectively.
Acknowledgments This work was financially supported by the National Key Research and Development Program of China under Grant 2016YFC0205600 and National Natural Science Foundation of China under Grant 51979212.
references
[1] Imperato M, Kaario O, Sarjovaara T, Larmi M. Split fuel injection and Miller cycle in a large-bore engine. Appl Energy 2016;162:289e97. [2] Mavrelos C, Theotokatos G. Numerical investigation of a premixed combustion large marine two-stroke dual fuel engine for optimising engine settings via parametric runs. Energy Convers Manag 2018;160:48e59. [3] Thomson H, Corbett JJ, Winebrake JJ. Natural gas as a marine fuel. Energy Policy 2015;87:153e67. [4] Æsøy V, Einang PM, Stenersen D, Hennie E, Valberg I. LNGfuelled engines and fuel systems for medium-speed engines in maritime applications. 2011. SAE technical paper 2011-011998. [5] Korakianitis T, Namasivayam AM, Crookes RJ. Natural-gas fueled Spark-Ignition (SI) and Compression-Ignition (CI) engine performance and emissions. Prog Energy Combust Sci 2011;37:89e112. [6] Ma F, Wang M, Jiang L, Chen R, Deng J, Naeve N, Zhao S. Performance and emission characteristics of a turbocharged CNG engine fueled by hydrogen-enriched compressed natural gas with high hydrogen ratio. Int J Hydrogen Energy 2010;35:6438e47. [7] Sun Z, Liu F, Liu X, Sun B, Sun D. Research and development of hydrogen fuelled engines in China. Int J Hydrogen Energy 2012;37:664e81. guez PM, Urroz JC, Marcelino-Sa daba S, Pe rez-Ezcurdia A, [8] Die inz D, Gandı´a LM. Experimental study Benito-Amurrio M, Sa of the performance and emission characteristics of an adapted commercial four-cylinder spark ignition engine running on hydrogenemethane mixtures. Appl Energy 2014;113:1068e76. € z B, Celik C, Soyhan HS, Go € kalp B, Karabag B. Emission [9] Ac¸ıkgo characteristics of a hydrogeneCH4 fuelled spark ignition engine. Fuel 2015;159:298e307. [10] Tartakovsky L, Sheintuch M. Fuel reforming in internal combustion engines. Prog Energy Combust Sci 2018;67:88e114. ndez J, Golunski S. [11] Leung P, Tsolakis A, Rodrı´guez-Ferna Raising the fuel heating value and recovering exhaust heat
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
12
[12]
[13]
[14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
international journal of hydrogen energy xxx (xxxx) xxx
by on-board oxidative reforming of bioethanol. Energy Environ Sci 2010;3:780e8. Zhang B, Ji C, Wang S. Performance of a hydrogen-enriched ethanol engine at unthrottled and lean conditions. Energy Convers Manag 2016;114:68e74. Zheng Z, Liu C, Zhang X. Numerical study of effects of Reformed Exhaust Gas Recirculation (REGR) on dimethyl ether HCCI combustion. Int J Hydrogen Energy 2014;39:8106e17. Park C, Choi Y, Kim C, Oh S, Lim G, Moriyoshi Y. Performance and exhaust emission characteristics of a spark ignition engine using ethanol and ethanol-reformed gas. Fuel 2010;89:2118e25. Poran A, Tartakovsky L. Energy efficiency of a directinjection internal combustion engine with high-pressure methanol steam reforming. Energy 2015;88:506e14. Zhang Z, Jia P, Zhong G, Liang J, Li G. Numerical study of exhaust reforming characteristics on hydrogen production for a marine LNG engine fueled with LNG. Appl Therm Eng 2017;124:241e9. Zhang Z, Jia P, Feng S, Liang J, Long Y, Li G. Numerical simulation of exhaust reforming characteristics in catalytic fixed-bed reactors for a natural gas engine. Chem Eng Sci 2018;191:200e7. Tsolakis A, Megaritis A. Partially premixed charge compression ignition engine with on-board H2 production by exhaust gas fuel reforming of diesel and biodiesel. Int J Hydrogen Energy 2005;30:731e45. Fennell D, Herreros JM, Tsolakis A, Wyszynski M, Cockle K, Pignon J, Millington P. On-board thermochemical energy recovery technology for low carbon clean gasoline direct injection engine powered vehicles. Proc Inst Mech Eng - Part D J Automob Eng 2018;232:1079e91. Chuahy FDF, Kokjohn SL. High efficiency dual-fuel combustion through thermochemical recovery and diesel reforming. Appl Energy 2017;195:503e22. Chintala V, Benaerjee D, Ghodke PK, Porpathamb E. Hydrogen rich exhaust gas recirculation (H2EGR) for performance improvement and emissions reduction of a compression ignition engine. Int J Hydrogen Energy 2019;44:18545e58. Fennell D, Herreros J, Tsolakis A. Improving gasoline direct injection (GDI) engine efficiency and emissions with hydrogen from exhaust gas fuel reforming. Int J Hydrogen Energy 2014;39:5153e62. Bogarra M, Herreros JM, Tsolakis A, York APE, Millington PJ, Martos FJ. Impact of exhaust gas fuel reforming and exhaust gas recirculation on particulate matter morphology in gasoline direct injection engine. J Aerosol Sci 2017;103:1e14. Bartley G, Gukelberger R, Henderson R, Henry C. Fuel reforming and catalyst deactivation investigated in real exhaust environment. 2019. SAE Technical Paper 2019-010315. Catapan RC, Cancino LR, Oliveira AAM, Schwarz CO, Nitschke H, Frank T. Potential for onboard hydrogen production in an direct injection ethanol fueled spark ignition engine with EGR. Fuel 2018;234:441e6. Nguyen DK, Sileghem L, Verhelst S. Exploring the potential of reformed-exhaust gas recirculation (R-EGR) for increased efficiency of methanol fueled SI engines. Fuel 2019;236:778e91. Zhang Z, Xie Q, Liang J, Li G. Numerical study of combustion characteristics of a natural gas HCCI engine with closed loop exhaust-gas fuel reforming. Appl Therm Eng 2017;119:430e7. Yap D, Peucheret S, Megaritis A, Wyszynski M, Xu H. Natural gas HCCI engine operation with exhaust gas fuel reforming. Int J Hydrogen Energy 2006;31:587e95.
[29] IMO. Resolution MEPC.177(58), Amendments to the technical code on control of emission of nitrogen oxides from marine diesel engines (NOx Technical Code 2008)vol. 1; 2008. MEPC 58/23/Add. [30] Dincer I, Acar C. Review and evaluation of hydrogen production methods for better sustainability. Int J Hydrogen Energy 2015;40:11094e111. [31] He Z, Gao Z, Zhu L, Li S, Li A. Effects of H2 and CO enrichment on the combustion, emission and performance characteristics of spark ignition natural gas engine. Fuel 2016;183:230e7. [32] Halabi MH, Croon MHJMD, Schaaf JVD, Cobden PD, Schouten JC. Modeling and analysis of autothermal reforming of methane to hydrogen in a fixed bed reformer. Chem Eng Sci 2008;137:568e78. [33] Hamedi MR, Tsolakis A, Lau CS. Biogas upgrading for onboard hydrogen production: reforming process CFD modelling. Int J Hydrogen Energy 2014;39:12532e40. [34] Lau CS, Allen D, Tsolakis A, Golunski SE, Wyszynskia ML. Biogas upgrade to syngas through thermochemical recovery using exhaust gas reforming. Biomass Bioenergy 2012;40:86e95. [35] Li G, Zhang Z, You F, Pan Z, Zhang X, Dong J. A novel strategy for hydrous-ethanol utilization: demonstration of a sparkignition engine fueled with hydrogen-rich fuel from an onboard ethanol/steam reformer. Int J Hydrogen Energy 2013;38:5936e48. [36] Wang Y, Zhao Y, Xiao F, Li D. Combustion and emission characteristics of a diesel engine with DME as port premixing fuel under different injection timing. Energy Convers Manag 2014;77:52e60. [37] Balki MK, Sayin C, Canakci M. The effect of different alcohol fuels on the performance, emission and combustion characteristics of a gasoline engine. Fuel 2014;115:901e6. [38] Heywood JB. Internal combustion engine fundamentals. United states of American: McGraw-Hill; 1988. p. 389. [39] Sun Z. Experimental studies on the explosion indices in turbulent stoichiometric H2/CH4/air mixtures. Int J Hydrogen Energy 2019;44:469e76. [40] Li G, Zhang Z, Liang J, Dong F, Li Y, Gao X. Effects of hydrogen addition on the premixed laminar-flames of ethanoleair gaseous mixtures: an experimental study. Int J Hydrogen Energy 2012;37:4490e501. [41] Liu J, Zhang X, Wang T, Hou X, Zhang J, Zheng S. Numerical study of the chemical, thermal and diffusion effects of H2 and CO addition on the laminar flame speeds of methaneeair mixture. Int J Hydrogen Energy 2015;40:8475e83. [42] Ennetta R, Alaya M, Said R. Numerical study of laminar flame velocity of hydrogen-enriched methane flames using several detailed reaction mechanisms. Arabian J Sci Eng 2017;42:1707e13. [43] Sun Z. Structure of turbulent rich hydrogen-air premixed flames. Int J Energy Res 2018;42:2845e58. [44] Masum BM, Masjuki HH, Kalam MA, Fattah IR, Palash SM, Abedin MJ. Effect of ethanolegasoline blend on NOx emission in SI engine. Renew Sustain Energy Rev 2013;24:209e22. [45] Cho HM, He BQ. Spark ignition natural gas engines-A review. Energy Convers Manag 2007;48:608e18. [46] Park C, Kim C, Choi Y, Lee J. Operating strategy for exhaust gas reduction and performance improvement in a heavyduty hydrogen-natural gas blend engine. Energy 2013;50:262e9. [47] Verma G, Prasad RK, Agarwal RA, Jain S, Agarwal AK. Experimental investigations of combustion, performance and emission characteristics of a hydrogen enriched natural gas fuelled prototype spark ignition engine. Fuel 2016;178:209e17.
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007
international journal of hydrogen energy xxx (xxxx) xxx
[48] Long Y, Li G, Zhang Z, Liang J, Mao L, Li Y. Effects of reformed exhaust gas recirculation on the HC and CO emissions of a spark-ignition engine fueled with LNG. Int J Hydrogen Energy 2018;43:21070e8. [49] Figen HE, Baykara SZ. Effect of ruthenium addition on molybdenum catalysts for syngas production via catalytic
13
partial oxidation of methane in a monolithic reactor. Int J Hydrogen Energy 2017;43:1129e38. [50] Loukou A, Mendes MAA, Frenzel I, Pereira JMC, Ray S, Pereira JCF, Trimis D. Experimental and numerical investigation of methane thermal partial oxidation in a small-scale porous media reformer. Int J Hydrogen Energy 2017;42:652e63.
Please cite this article as: Li G et al., Performance and emissions characteristics of a lean-burn marine natural gas engine with the addition of hydrogen-rich reformate, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2019.10.007