Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends

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Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends V.E. Kozlov a,b, I.V. Chechet b, S.G. Matveev b, N.S. Titova a,b, A.M. Starik a,b,* a

Central Institute of Aviation Motors, Scientific Educational Centre “Physical and Chemical Kinetics and Combustion”, Moscow 111116, Russia b Samara State Aerospace University, Samara 443086, Russia

article info

abstract

Article history:

Comprehensive analysis of combustion and emission characteristics of HCCI engine

Received 19 August 2015

operating on alternative fuels, including methane, syngas, methane/hydrogen and

Received in revised form

methane/syngas blends is conducted on the basis of CFD simulation with the use of

2 December 2015

detailed reaction mechanism. It is shown that the replacement of methane to methane/

Accepted 4 December 2015

hydrogen blend with 50% H2 volume content makes it possible to increase the output

Available online xxx

energy during HCCI combustion and simultaneously to reduce the NO and CO emissions. The use of methane/syngas blend as a fuel also leads to an improvement of HCCI perfor-

Keywords:

mance and to reducing the pollutant emissions, including the emission of CO2.

Hydrogen rich fuel blends

Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

HCCI Combustion Emission Numerical simulation

Introduction Nowadays, a great deal of interest is expressed to the analysis of potentialities of the usage of alternative fuels for the engines of various transportation systems [1e3]. Different alternative fuels such as hydrogen, natural gas, propane/ butane blend, biofuels (ethanol, dimethyl ether, biodiesel) and etc. are considered for the applications. Among such fuels, hydrogen rich fuel blends and synthesis gas (syngas)

composed of hydrogen and carbon monoxide attracts a special attention. As is known, hydrogen possesses very promising combustion characteristics compared with heavy hydrocarbons usually applied as a fuel for internal combustion (IC) and gas turbine engines, and even with natural gas, which also can be utilized as a fuel for different systems. Hydrogen has extremely high flame speed and wide flammability limits. The admixture of hydrogen to methane and other hydrocarbons: ethane, propane, n-hepthane, i-octane,

* Corresponding author. Aviamotornaya st. 2, Moscow 111116, Russia. Tel.: þ7 495 3616468. E-mail address: [email protected] (A.M. Starik). http://dx.doi.org/10.1016/j.ijhydene.2015.12.078 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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n-decane improves notably the combustion characterizes of hydrocarbons: decreases the ignition delay in the certain temperature range [4e9], extends the flammability limits [10], increases the laminar and turbulent burning velocities [11e15] and improves the flame stability [13,16]. Addition of hydrogen to hydrocarbons can also decrease the emission of environmentally harmful gases, such as CO, CO2, NOx and unburned hydrocarbons (HC) as well as particulate matter [10,14,17e19]. The effect of syngas addition on the combustion characteristics of hydrocarbons was not investigated in such a detailed manner. However, it was shown [8] that the change in the ignition characteristics of i-octane/syngas/air mixture was determined by the interaction of i-C8H18 and H2 chemistries, whereas CO adding seemed to have a negligible effect. The combustion of hydrogen/hydrocarbon blends in IC engines has been extensively studied in past few years [20e34]. It was shown that H2 addition could essentially improve the performance of IC engines: increase the burning rate, reduce the cycle-to-cycle variations, extend the lean burning limit and raise the engine lean burning ability [21,23,25e28,32e37]. Moreover, burning of hydrogen/hydrocarbon blends in IC engines can provide smaller emissions of CO, HC and soot [25e28,32e37]. In respect with emission of NOx, it can be also decreased with optimal combination of spark timing and exhaust gas recirculation (EGR) rate at low loads [21,25,26,32,33]. As well, few works were focused on the analysis of performance and emission of IC engine operating on hydrocarbon/syngas blend [29,31,38]. The experiments showed that the addition of syngas to hydrocarbon fuel improved the thermal efficiency and shortened the combustion duration. As well, HC emission is decreased with the rise of syngas volume fraction in the blended fuel, but CO emission is increased in this case. The change in the NOx emission upon using the fuel blend enriched by syngas depends on engine operating regime. Among IC engines, the devices with compression ignition of premixed homogeneous fuel-air charge (HCCI engine) are considered, nowadays, as the most promising ones [39e41]. The ignition, in this case, is determined by kinetics of chain mechanism development in the gas mixture, and combustion occurs in the whole working volume of cylinder without arising of the flame front. HCCI engines possess improved emission characteristics compared to conventional IC engines with spark ignition and diesel engines. So, they produce smaller amounts of NO and CO, and there are practically no soot particles in their combustion exhaust [40]. However, HCCI engines have some disadvantages. They can ensure stable operation in a rather narrow operating range and produce smaller output energy compared to traditional IC engines. In addition, at low load regime, such engines produce notable amounts of carbon monoxide and unburned hydrocarbons [39]. The usage of hydrogen rich fuel blends can help to overcome these disadvantages. The effect of hydrogen addition on HCCI combustion was studied previously elsewhere [42e48]. It was demonstrated that the influence of addition of hydrogen or reformed gas (RG) (it is a mixture of hydrogen, carbon monoxide and some diluents, such as CO2, H2O and HC, that form during reforming of primary hydrocarbon) depended on the octane number of the basic fuel. For the fuels with low octane number (for example, n-heptane),

the addition of RG retarded combustion phasing, and, for the fuels with high octane number (iso-octane), the effect of RG addition depends on the intake temperature. It was also shown that the rise of RG concentration in the basic fuel results in the increase of NOx emission and the decrease of HC and CO emissions. The most attractive hydrocarbon for the application as a fuel is methane, because of its cleaner combustion with respect to carbon emitted species. Methane and natural gas, composed mostly of methane, have a great octane number and, therefore, requires fairly high initial temperature for the ignition. In HCCI engine, such level of temperature can be achieved by preheating the charge or due to rise of compression ratio. The influence of hydrogen addition on natural gasfueled HCCI combustion was investigated in Refs. [49,50]. So, Yap et al. [49] showed that addition of hydrogen enhanced the ignition and advanced the start of combustion (SOC). This enables one to lower the value of intake temperature required for the auto-ignition, especially at low loads. It was also demonstrated that low NOx emission and high rate of heat release, typical for HCCI combustion, were retained upon hydrogen addition. The similar effects for natural gas-fueled HCCI combustion were observed when RG was used as additive [46,51e53]. As well, numerical simulations [53] showed that the chemical effect, provided by RG addition, was stronger than the simple dilution or thermal effects caused by RG admixture. The intensity of chemical effect was mainly dependent on H2 content in RG. It turned out that a significant influence of RG addition on SOC was achieved when H2 mass fraction in RG was lower than 10%. However, after some saturated level of H2 content, the increase of CO mass fraction significantly altered SOC. Hosseini and Checkel [46] concluded that the practical effectiveness of RG addition to methane or natural gas depends on the composition of RG produced in a local fuel processor that consumes the same basic fuel as the engine. The general goal of this paper is the comparative analysis of energy and emission characteristics of typical HCCI engine operating on methane-based fuel blends comprising hydrogen or syngas as additives.

Kinetic model Because the ignition and combustion in HCCI cycle is determined generally by features of chemical kinetics, it is of great importance to choose an appropriate reaction mechanism, which can reproduce with rather high accuracy the ignition and combustion characteristics not only for the basic fuel (methane), but also for fuel blends composed of methane and hydrogen or syngas. For this purpose, the reaction mechanism developed previously by Starik et al. [54,55] was applied. The general features of this kinetic mechanism are that it involves the sub-mechanisms of hydrogen and syngas oxidation as well as the principal mechanisms of nitric oxides (thermal, prompt and N2O-mechanism) and other N-containing species (CN, HCN, CNO, CCN, C2N2) formation. It includes 403 reversible reactions and 44 species. This mechanism was carefully validated over a vast set of experimental data on ignition delay and evolution of H, O

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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atoms and OH radicals measured behind reflected shock waves using shock tube technique, laminar flame speed and evolution of different species along the flame front in the H2air, H2eCO-air CH4eO2-Ar and CH4-air mixtures (see, for example, [55e57]). As well, this reaction mechanism reproduces with rather high accuracy the measurements of temperature and CO, NO and OH profiles in Bunzen burner [58]. In order to validate this reaction mechanism additionally for the mixtures comprising methane and hydrogen, we performed the comparison of the measured in Ref. [5] and predicted values of ignition delay tin for the H2/CH4/O2/Ar mixture with different content of H2. The comparison is shown in Fig. 1 for the fuel-lean mixture with the fuel/oxidizer equivalence ratio f ¼ 0.5. Because the measurements [5] were conducted behind the shock wave, reflected from the end wall of the shock tube, upon the computations we used the volumeconstant stirred reactor approximation. It should be emphasized that ignition in such experiments is controlled by chemical kinetics only. Fig. 1 depicts the values of tin predicted in the present work and obtained in Ref. [5]. Taking into

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account that the declared value of uncertainty in the measurements of ignition delay is 28 ms [5], one can conclude that the applied kinetic mechanism describes the experimental data with reasonable accuracy. Some discrepancy between the predictions and measurements is observed for the oxidation of CH4/H2 blend with 40 and 60% H2 content in the temperature range T0 ¼ 1400e1700 K for P0 ¼ 5 bar. The predictions slightly overestimate the measurements. One can see that both experimental data and computations demonstrate that admixture of hydrogen to methane reduces the ignition delay (induction time). So, at 20% H2 volume content (it corresponds only to 3% H2 content per mass) in the CH4/H2 blend, the decrease in the tin value is not substantial and does not exceed a factor of 2. The increase of H2 volume content up to 60% (16% H2 content per mass) leads to the decrease of the ignition delay by an order of magnitude and even greater. In the experiments [11,59e61] it was observed that addition of H2 to methane increases the flame speed Un. Our reaction mechanism [54,55] reproduces this experimental data for fuellean and stoichiometric mixtures, quiet well. Only for fuelrich CH4/H2/air mixture at f > 1.2, the predictions

Fig. 1 e Induction time tin as a function of initial temperature T0 for the mixtures CH4/H2/O2/Ar with 0, 20, 40, 60, 80 and 100% of H2 in the fuel at f ¼ 0.5 and P0 ¼ 5 (a), 10 (b) and 20 (c) atm. Symbols are experimental data [5] (the value of the experimental uncertainty in ignition delay is depicted by vertical error bars), curves are calculation results of the model [54,55]. Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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overestimate the experimental data. So, at f ¼ 1.4, the difference in Un value achieves a factor of 2. This is seen from the plots shown in Fig. 2. From these examples one can conclude that the applied reaction mechanism can be successfully used for the computations of the characteristics of HCCI combustion with CH4/H2 and CH4/syngas fuel blends for fuel-lean and stoichiometric mixtures.

Methodology Today, for modeling the ignition and combustion in HCCI engine, different approaches are applied. They are zerodimensional (0D) single zone thermochemical model, reactor net model taking into account the combustion features in near wall and crevice regions, and more accurate 2D (or even 3D) CFD model based on Favre averaged non-stationary NaviereStokes equations for reacting mixture coupled with respective turbulent model [39]. It is known that 0D single zone model, even taking into account heat losses through the cylinder walls, cannot reproduce properly the concentrations of NO and CO in the engine exhaust, and the application of non-stationary 2D CFD model is more preferable. In the present work, the combustion in HCCI cylinder was simulated by using 2D CFD and 0D single zone models. The 0D single zone model was used mostly only for the evaluation of inaccuracies, which this model can produce in the predictions of energy and emission characteristics of HCCI engine operating on alternative hydrogen rich fuels. Upon the computations with 0D single zone model the CHEMKIN program package was utilized [62]. CFD calculations were performed with the usage of the program FLUENT from the ANSYS-CFD software package [63]. The geometry of HCCI cylinder and parameters of HCCI engine were taken from the guideline of ANSYS program package and presented in Fig. 3 and in Table 1. The calculations ran from the crank angle b ¼ 142 , corresponding to Inlet Valve Closing (IVC), to crank angle b ¼ 115 , relating to Exhaust Valve Opening (EVO). It was supposed that the walls of the cylinder and the top surface of

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U n , cm/s

Fig. 3 e Geometry of HCCI cylinder and computational dynamic grid for 2D simulation at IVC b ¼ ¡142 (5480 cells) and at the top dead point b ¼ 0 (480 cells).

piston were cooled, and the temperature at the cooling surfaces (boundaries) was equal to 510 K as it was realized for water-cooled metal cylinder in the experimental study of Joelsson et al. [64]. The quasi-laminar combustion model and realizable k-ε turbulence model were chosen for the simulation. It was assumed for simplicity, that all mixture parameters and its composition were uniform at IVC (crank angle b ¼ 142 ). This assumption is rather strong and not similar to reality. However, for comparative analysis of combustion and emission characteristics of HCCI engine operating on different fuels it seems quite acceptable. The swirl ratio was equal to 3. EGR was assumed to be absent. Concrete analysis was carried out for following alternative fuels: pure methane, methane/hydrogen blend with the composition CH4/H2 ¼ 50/50 and 20/80, syngas with the composition CO/H2 ¼ 25/75 and methane/syngas blend with CH4/CO/H2 ¼ 100/25/75 (herein and hereafter the composition is given in volume fractions). For all compositions of fuel/air mixture, the initial temperature of the mixture was chosen by

20% H2 Table 1 e Parameters of HCCI engine.

40 0% H2

30 20 10 0.5

0.7

0.9

1.1

1.3

φ

Fig. 2 e Laminar flame velocity Un as a function of f for the mixture CH4/H2/air with 0% and 20% H2 in the fuel at T0 ¼ 300 K, P0 ¼ 1 atm. Symbols are experimental data: -, , e [11], , e [59], , e [60], curves are calculation results of the model [54,55].

Engine speed, rpm Cylinder bore diameter, mm Cylinder clearance volume, cm3 Crank radius, mm Connecting rod length, mm Compression ratio Inlet valve closing (IVC), degree Exhaust valve opening (EVO), degree Equivalence ratio, f P0, atm T 0, K CH4 CH4/H2 ¼ 50/50 CH4/H2 ¼ 20/80 CO/H2 ¼ 25/75 CH4/CO/H2 ¼ 100/25/75

1000 100 67.3 66.4 232.4 16.5 142 115 0.385 1.065 471 416 397 382 422

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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40 , dergee

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1 0

40 , degree

2, 3 80

Fig. 4 e Traces of volume-averaged static pressure Pev and mass-averaged mass fraction of CO, CCO, in the HCCI cylinder for the CH4-air mixture at T0 ¼ 471 K computed with coarse grid 1 comprising 5480 cells, middle grid 2 (22014 cells) and fine grid 3 (87680 cells) (curves 1e3).

Fig. 5 e Temperature fields in the cylinder of HCCI engine operating on methane at different values of crank angle b (T0 ¼ 471 K). Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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Fig. 4 depicts the traces of volume-averaged static pressure Pev and mass-averaged mass fraction of CO, CCO, computed with different grids for the CH4-air mixture. One can see that the pressure traces, obtained with grids 1, 2 and 3 are identical. It is also seen that the values of CO concentration computed with the use of coarse grid 1 differ notably from those obtained with the use of grids 2 and 3. At the same time, the computations with grids 2 and 3 predict practically the same values of CO concentration in the combustion exhaust. It means that the convergence was achieved for the grid 2 with 22,014 cells, and precisely this grid was used for the calculations. The further increase of the cell number did not lead to notable rise in the accuracy of computations.

such a way to provide the maximal value of mass-averaged temperature in the cylinder at b ~ 9e10 . The values of initial temperature for different fuels, chosen in according to this condition, are presented in Table 1. In order to achieve a reasonable accuracy in the CFD calculations of gas parameters in the cylinder and averaged values of the concentrations of pollutants in the engine exhaust, the series of computations of combustion of methane-air mixture in the cylinder with different grids were preliminary performed. Primarily, the dynamic grid comprising 5480 cells at IVC (b ¼ 142 ) and 480 cells at top dead center (TDC) with b ¼ 0 were used for the simulation (grid 1). The size of cell side, in this case, was approximately 1 mm. Fig. 3 shows this computational grid at b ¼ 142 and 0 . To make a certain convergence, the numerical runs for the CH4 combustion were tested for the grids comprising 22,014 cells at b ¼ 142 (4 times greater amount) with the cell dimension of ~0.5 mm (grid 2) and 87,680 cells (16 times greater amount) with the cell dimension of ~0.25 mm (grid 3).

Results and discussion At first, let us consider the variation of temperature fields in the cylinder of HCCI engine operating on methane. Results of 2D CFD modeling are shown in Fig. 5 at different values of b. Note that, in this case, the initial temperature, which can ensure the achievement of the maximal value of T at b ~ 10 , is equal to 471 K. One can see that during the compression stroke, the mixture temperature is somewhat higher in the central region of cylinder than that in near-wall one, where the energy losses through the walls take place (see Fig. 5 with the temperature field at b ¼ 1 ). It is worth noting that the region with maximal temperature does not locate at the cylinder axis, because of the generation of the particular vortex, which carries the colder gas to the near axis region from the zone adjoining to the cooled cylinder head (see Fig. 6). The ignition occurs at b ~ 2.5 in the zone with maximal temperature. The region with high temperature broadens, successively spreading to the mixture layers (with a somewhat smaller temperature) adjacent to the hot region, and, at b ¼ 8 , the mixture is burnt in the entire space over the piston (see Fig. 5 for b ¼ 8 ). The estimated ‘visible speed’ of the propagation of the boundary of high temperature region (T ~ 2000 K) proved to be equal to ~30 m/s. Note that the laminar flame speed Un in the CH4/air mixture with fuel/air equivalence ratio f ¼ 0.385, T0 ¼ 1180 K and P0 ¼ 38 atm (such parameters are achieved in the cylinder just before the ignition) is

Fig. 6 e Field of instantaneous streamlines in the cylinder of HCCI engine operating on methane (T0 ¼ 471 K) at b ¼ 1 .

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Fig. 7 e Temperature (a) and pressure (b) traces in the cylinder of HCCI engine operating on methane predicted during 2D CFD modeling (maximal local static temperature Tmax, mass-averaged static temperature Tav and volume averaged static pressure are depicted) and by 0D single zone model. Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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substantially smaller, Un ~ 4 m/s. Therefore, one can conclude that the combustion inside the cylinder is determined by the subsequent ignition of the regions with various temperatures, but not by the flame propagation through the cylinder volume. Compare the temperature and pressure traces computed with the use of 2D non-stationary CFD model and 0D single zone thermochemical model. Such a comparison is shown in Fig. 7. Note that 0D single zone model predicts the maximal temperature value at b ¼ 10 for T0 ¼ 452 K. One can see that CFD modeling predicts the ignition (the ignition event is determined as a point with maximal value of temperature gradient) at smaller value of b compared to the predictions of 0D single zone model, and the maximal value of local static temperature Tmax achieved after ignition is greater, in this case, though the maximal magnitude of the mass-averaged static temperature Tav is smaller than that predicted by 0D single zone model. The difference in the pressure traces computed with two models under study also takes place. Let us compare now the values of output energy released during HCCI combustion. This energy can be determined as Z V2 PdV, where V1 and V2 are the values of volume availE¼ V1

able for combustion in the cylinder at IVC (b ¼ 142 ) and EVO (b ¼ 115 ), respectively. Despite the fact that pressure and temperature traces predicted by 2D CFD model and 0D single zone model differ notably, the values of E calculated with the usage of these two models turned out to be very close to each other. For the CH4/air mixture the difference in E values is as small as 1%. It should be emphasized that it does not exceed

10% for other alternative fuels under study. This means that for the estimation of output energy for HCCI engine one can use rather simple 0D single zone thermochemical model [62] that allows applying detailed reaction mechanisms. Consider how the replacement of methane by other alternative fuels changes the energetic characteristics of HCCI engine. Fig. 8 shows the temperature and pressure traces upon burning of considered fuels. It is seen that the major value of pressure after ignition is observed when syngas is used as a fuel, and minor one upon burning of methane. This means that the major value of output energy occurs upon burning of syngas, and combustion of pure methane provides the minor value of E. This is clearly seen from the diagram depicted in Fig. 9a. The output energy E, reached during the syngas combustion, is by a factor of 1.4 higher than that for methane burning. Addition of hydrogen or syngas to methane leads to an increase in output energy. The greater H2 content in the fuel is, the higher is the value of E. However, if we consider the specific output energy released during combustion of unit mass of the fuel, the other tendency takes place: the major specific output energy Em is achieved upon burning of CH4/H2 ¼ 20/80 blend and the minor one is observed for burning of syngas (see Fig. 9b). It is also seen that Em value for pure methane is by a factor of 1.7 higher than that for syngas with CO/H2 ¼ 25/75, but it is by 1.5 times smaller than that for the CH4/H2 ¼ 20/80 blend. It is worth noting that the mass of syngas supplied into the cylinder is by a factor of about 2.3 higher than the mass of methane, and in spite of the maximal value (among considered fuels) of output

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Fig. 8 e Mass-average static temperature Tev and volume-average static pressure Pev traces upon burning of different alternative fuels. Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

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energy achieved during syngas combustion, the specific output energy for syngas turned out to be the smallest. Compare now the emission characteristics of HCCI engine operating on different alternative fuels. As is known, upon burning of fuel-lean CH4-air mixture in the HCCI cylinder, thermal mechanism contributes mostly to the NO formation, while the N2O and prompt mechanisms make much smaller contribution [55]. Therefore, one can expect that the greater concentration of NO in the HCCI exhaust will take place for the fuel providing the higher temperature after ignition. Computations proved this conclusion. As was shown above (see Fig. 8), the highest value of temperature is achieved upon burning of syngas with the composition CO/H2 ¼ 25/75, and HCCI combustion of this fuel provides the greatest value of NO mole fraction in the engine exhaust. This is clearly seen from Fig. 10 where NO mass fraction in the exhaust gases and the maximal value of mass-average static temperature are depicted. It should be emphasized that the value of mass fraction of NO in the combustion exhaust, achieved upon burning of considered alternative fuels, correlates very well

E, J

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CH42/H2 = CH43/H2 = CO/H 4 2 = CH4/CO/H2= 50/50 20/80 25/75 100/25/75

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E m *10 , J/kg 3

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CH4 /H2 = CH4 /H2 = CO/H2 = CH4/CO/H2= 50/50 20/80 25/75 100/25/75

Fig. 9 e The values of output energy E (a) and specific output energy Em (b) released during HCCI combustion of different alternative fuels. The mass of the fuels supplied into the cylinder for all considered fuels are also indicated.

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CH4 /H2 = CH4 /H2 = CO/H2 = CH4/CO/H2= 50/50 20/80 25/75 100/25/75

1900

Fig. 10 e Mass-average mass fraction of NO and maximal value of mass-average static temperature max(Tev) in the combustion exhaust of HCCI engine operating on different alternative fuels.

with the maximal value of mass-average temperature in the cylinder. This means that formation of NO occurs mostly via thermal mechanism. Fig. 11 depicts the NO mass fraction fields in the cylinder at EVO (b ¼ 115 ) for different fuels under study. One can see that the patterns of NO mole fraction inside the cylinder are similar. The formation of NO occurs in the central region of the cylinder with high temperature. Near the cooled cylinder walls, NO concentration is small due to low temperature in this region. For syngas combustion, the temperature in the central zone is by 100e150 K higher than that for other considered fuels that results in essentially higher concentration of NO in this region for syngas-fueled engine. NO emission index EINO (its value is determined as an amount of NO in grams per 1 kg of the fuel) strongly depends on fuel mass. Burning of CH4/H2 ¼ 50/50 blend ensures minor value of EINO (EINO ¼ 25 g/kg). Somewhat higher NO emission index (EINO ¼ 27 g/kg) is achieved upon burning of the blend composed of methane and syngas (CH4/CO/H2 ¼ 100/25/75), and major value of EINO is achieved upon combustion of hydrogen rich blend (CH4/H2 ¼ 20/80) and syngas (CO/H2 ¼ 25/ 75) (see Fig. 12a). The same tendency is observed for NOx emission index EINOx (see Fig. 12b). The NOx emission index m 2 EINO þ EINO2 , was calculated using the formula EINOX ¼ mNO NO where mNO2 and mNO are the molar masses of NO2 and NO. It is worth noting that 0D single zone model predicts essentially smaller (by 2e3 times) values of EINO and EINOx. This is caused by the fact that this model provides lower maximal temperature and shorter residence time in the hot region inside the cylinder compared to those predicted by 2D CFD model (see Fig. 7a). Emission of CO is due to the existence of low temperature region near the cold wall of the cylinder, where carbon oxide cannot convert to CO2. The location of the region with maximal CO concentration is identical for different fuels. This is clearly seen from the CO mass fraction fields shown in Fig. 13. The region with maximal CO concentration is preferably allocated near the wall in the peripheral zone of the

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

Fig. 11 e NO mass fraction fields in the cylinder at EVO (b ¼ 115 ) for different fuels under study.

Fig. 13 e CO mass fraction fields in the cylinder at EVO (b ¼ 115 ) for different fuels under study.

a EINO, g/kg

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b EINOx, g/kg

50 80

40 60

30

40

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20

10 0

CH4

CH4 /H2 = CH4 /H2 = CO/H2 = CH4/CO/H 2= 50/50 20/80 25/75 100/25/75

0

CH4

CH4 /H2 =

CH4 /H2 = CO/H2 = CH4/CO/H 2= 100/25/75 20/80 25/75

Fig. 12 e NO (a) and NOx (b) emission indices of HCCI engine operating on different alternative fuels. Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

a

b EICO2 , g/kg

EICO, g/kg 10

2500

8

2000

6

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4

1000

2 0

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CH4/CO/H2= CH4 /H 2 2 = CH43/H2 = CO/H 4 2= 5 50/50 20/80 25/75 100/25/75

0

CH14

CH /CO/H = CH4 /H 2 2 = CH43/H2 = CO/H 4 2= 4 5 2 50/50 20/80 25/75 100/25:75

Fig. 14 e CO (a) and CO2 (b) emission indices, EICO and EICO2, of HCCI engine operating on different alternative fuels.

piston bowl. In this zone, there exists the vortex (see Fig. 6), which permanently sucks the cooler gas from near-wall zone. It results in smaller temperature in this region at expansion stroke and, as a consequence, larger CO concentration. Some portion of CO leaves this region due to motion of the mixture during the expansion stroke. Fig. 14a depicts the values of CO emission index, EICO, for different alternative fuels under study. One can see that the smallest values of EICO are observed for the CH4/H2 ¼ 20/80 blend and syngas, and the major one e for methane. Note that the magnitude of EICO predicted by 0D single zone model is by a factor of 103e104 smaller than that predicted by 2D CFD model. This is due to the fact that 0D single zone model cannot resolve the cool near-wall region. It is very interesting to evaluate also the emission of CO2 for HCCI engine operating on considered alternative fuels. The results of the calculations of EICO2 are shown in Fig. 14b. The minor value of EICO2 is achieved upon burning of syngas, and the major value of CO2 emission takes place during combustion of methane. The difference in the magnitudes of EICO2 for these fuels is as large as a factor of 2. Thus, one can conclude that in viewpoint of ensuring the appropriate levels of CO and CO2 emissions in HCCI engine, the most promising fuels are syngas and CH4/H2 ¼ 20/80 blend. Note that if the combustible mixture ignites in the engine cylinder, EICO2 is determined by mass fraction of carbon in the fuel only and doesn't depend on the computational model.

Conclusions Analysis of the combustion and pollutant formation in the cylinder of HCCI engine operating on alternative fuels such as methane, methane/hydrogen blend with 50 and 80% H2 content, syngas with the composition CO/H2 ¼ 25/75 and methane/syngas blend with 50% syngas content was conducted with the use of 2D CFD and 0D single zone thermochemical models. Computations showed that simple 0D single zone model can predict properly the output energy, released during HCCI combustion, and emission of CO2. However, it underestimates notably the emission of NO (by a factor of 2e3)

and, especially, CO (by a factor of 103) for the engine with cooled cylinder walls. It was shown that the usage of methane-hydrogen blend with 80% H2 content as a fuel in HCCI engine allows one to increase the specific output energy Em by 50% and, simultaneously, to decrease the CO emission by a factor of 4.4 compared to pure methane. The minor value of NO emission index (EINO ¼ 25 g/kg) is achieved upon the usage of CH4/ H2 ¼ 50/50 blend and somewhat higher one upon burning of methane/syngas blend (EINO ¼ 27 g/kg). The increase of H2 content in the CH4/H2 blend leads to the rise of EINO value. Thus, the most promising fuel for HCCI engine among considered ones, providing both rather high output energy and appropriate emission of NO and CO, are the fuel blends with the composition: CH4/H2 ¼ 50/50 and CH4/CO/H2 ¼ 100/ 25/75. The minor value of CO2 emission can be achieved by using syngas or CH4/H2 ¼ 20/80 as a fuel.

Acknowledgment This work was supported by the Russian Foundation for Basic Research (grants 14-08-00743-a, 14-08-90034-Bel-a) and by the Council of President of Russian Federation for support of Young Russian Scientists and Leading Scientific Schools (grant SS-960.2014.8).

references

[1] White CM, Steeper RR, Lutz AE. The hydrogen-fueled internal combustion engine: a technical review. Int J Hydrogen Energy 2006;31:1292e305. [2] Verhelst S, Wallner T. Hydrogen-fueled internal combustion engines. Prog Energy Combust Sci 2009;35:490e527. [3] Blakey S, Rye L, Wilson CW. Aviation gas turbine alternative fuels: a review. Proc Combust Inst 2011;33:2863e85. [4] Herzler J, Naumann C. Shock-tube study of the ignition of methane/ethane/hydrogen mixtures with hydrogen contents from 0% to 100% at different pressures. Proc Combust Inst 2009;32:213e20.

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

[5] Zhang Y, Huang Z, Wei L, Zhang J, Law CK. Experimental and modeling study on ignition delays of lean mixtures of methane, hydrogen, oxygen, and argon at elevated pressures. Combust Flame 2012;159:918e31. [6] Man X, Tang C, Wei L, Pan L, Huang Z. Measurements and kinetic study on ignition delay times of propane/hydrogen in argon diluted oxygen. Int J Hydrogen Energy 2013;38:2523e30. [7] Frolov SM, Medvedev SN, Basevich VYa, Frolov FS. Selfignition of hydrocarbon-hydrogen-air mixtures. Int J Hydrogen Energy 2013;38:4177e84. [8] Jain S, Li D, Aggarwal SK. Effect of hydrogen and syngas addition on the ignition of iso-octane/air mixtures. Int J Hydrogen Energy 2013;38:4163e76. [9] Titova NS, Kuleshov PS, Favorskii ON, Starik AM. The features of ignition and combustion of composite propanehydrogen fuel: modeling study. Int J Hydrogen Energy 2014;39:6764e73. [10] Guo H, Smallwood GJ, Liu F, Ju Y, Gulder OL. The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counter flow CH4/air premixed flames. Proc Combust Inst 2005;30:303e11. [11] Yu G, Law CK, Wu CK. Laminar flame speeds of hydrocarbonþair mixtures with hydrogen addition. Combust Flame 1986;63:339e47. € kalp I. [12] Halter F, Chauveau C, Djebaı¨li-Chaumeix N, Go Characterization of the effects of pressure and hydrogen concentration on laminar burning velocities of methanehydrogen-air mixtures. Proc Combust Inst 2005;30:201e8. [13] Zhang Y, Wu J, Ishizuka S. Hydrogen addition effect on laminar burning velocity, flame temperature and flame stability of a planar and a curved CH4eH2eair premixed flame. Int J Hydrogen Energy 2009;34:519e27. [14] Hui X, Zhang C, Xia M, Sung C-J. Effects of hydrogen addition on combustion characteristics of n-decane/air mixtures. Combust Flame 2014;161:2252e62. [15] Shy SS, Chen YC, Yang CH, Liu CC, Huang CM. Effects of H2 or CO2 addition, equivalence ratio, and turbulent straining on turbulent burning velocities for lean premixed methane combustion. Combust Flame 2008;153:510e24. [16] Schefer RW. Hydrogen enrichment for improved lean flame stability. Int J Hydrogen Energy 2003;28:1131e41. [17] Naha S, Aggarwal SK. Fuel effects on NOx emissions in partially premixed flames. Combust Flame 2004;139:90e105. [18] Naha S, Briones AM, Aggarwal SK. Effect of fuel blends on pollutants emissions in flames. Combust Sci Technol 2005;177:183e220. [19] Guo H, Neill WS. A numerical study on the effect of hydrogen/reformate gas addition on flame temperature and NO formation in strained methane/air diffusion flames. Combust Flame 2009;156:477e83. [20] Bauer CG, Forest TW. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: effect on SI engine performance. Int J Hydrogen Energy 2001;26:55e70. [21] Dimopoulos P, Bach C, Soltic P, Boulouchos K. Hydrogenenatural gas blends fuelling passenger car engines: combustion, emissions and well-to-wheels assessment. Int J Hydrogen Energy 2008;33:7224e36. [22] Ma F, Wang Y. Study on the extension of lean operation limit through hydrogen enrichment in a natural gas spark-ignition engine. Int J Hydrogen Energy 2008;33:1416e24. [23] Shirk MG, McGuire TP, Neal GL, Haworth DC. Investigation of a hydrogen-assisted combustion system for a light-duty diesel vehicle. Int J Hydrogen Energy 2008;33:7237e44. [24] Ma F, Ding S, Wang Y, Wang Y, Wang J, Zhao S. Study on combustion behaviors and cycle-by-cycle variations in a turbocharged lean burn natural gas S.I. engine with

[25]

[26]

[27]

[28]

[29]

[30]

[31]

[32]

[33]

[34]

[35]

[36]

[37]

[38]

[39]

[40]

[41]

11

hydrogen enrichment. Int J Hydrogen Energy 2008;33:7245e55. Ma F, Wang M, Jiang L, Chen R, Deng J, Naeve N, et al. 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. Ji C, Wang S. Combustion and emissions performance of a hybrid hydrogen-gasoline engine at idle and lean conditions. Int J Hydrogen Energy 2010;35:346e55. Ji C, Wang S, Zhang B. Combustion and emissions characteristics of a hybrid hydrogen-gasoline engine under various loads and lean conditions. Int J Hydrogen Energy 2010;35:5714e22. Deng J, Ma F, Li S, He Y, Wang M, Jiang L, et al. Experimental study on combustion and emission characteristics of a hydrogen-enriched compressed natural gas engine under idling condition. Int J Hydrogen Energy 2011;36:13150e7. Ji C, Dai X, Ju B, Wang S, Zhang B, Liang C, et al. Improving the performance of a spark-ignited gasoline engine with the addition of syngas produced by onboard ethanol steaming reforming. Int J Hydrogen Energy 2012;37:7860e8. Wang S, Ji C. Cyclic variation in a hydrogen-enriched spark ignition gasoline engine under various operating conditions. Int J Hydrogen Energy 2012;37:1112e9. Ji C, Dai X, Wang S, Liang C, Ju B, Liu X. Experimental study on combustion and emissions performance of a hybrid syngas-gasoline engine. Int J Hydrogen Energy 2013;38:11169e73. € z Y, Sandalcı T, Yu¨ksek L, Dalkılıc¸ AS. Engine Karago performance and emission effects of diesel burns enriched by hydrogen on different engine loads. Int J Hydrogen Energy 2015;40:6702e13. Zhang B, Ji C, Wang S. Combustion analysis and emissions characteristics of a hydrogen-blended methanol engine at various spark timings. Int J Hydrogen Energy 2015;40:4707e16. Gharehghani A, Hosseini R, Mirsalim M, Yusaf TF. A computational study of operating range extension in a natural gas SI engine with the use of hydrogen. Int J Hydrogen Energy 2015;40:5966e75. € z S, Akansu SO, Kahraman N, Malkoc¸ Y. Effects of Tango compression ratio on performance and emissions of a modified diesel engine fueled by HCNG. Int J Hydrogen Energy 2015;40:15374e80. http://dx.doi.org/10.1016/j. ijhydene.2015.02.058. Deb M, Sastry GRK, Bose PK, Banerjee R. An experimental study on combustion, performance and emission analysis of a single cylinder, 4-stroke DI-diesel engine using hydrogen in dual fuel mode of operation. Int J Hydrogen Energy 2015;40:8586e98. € z Y, Sandalcı T, Dalkılıc¸ AS. Effects of hydrogen and Karago oxygen enrichment on performance and emissions of an SI engine under idle operating condition. Int J Hydrogen Energy 2015;40:8607e19. Dai X, Ji C, Wang S, Liang C, Liu X, Ju B. Effect of syngas addition on performance of a spark-ignited gasoline engine at lean conditions. Int J Hydrogen Energy 2012;37:14624e31. Dec JE. Advanced compression-ignition enginesunderstanding the in-cylinder processes. Proc Combust Inst 2009;32:2727e42. Yao M, Zheng Z, Liu H. Progress and recent trends in homogeneous charge compression ignition (HCCI) engines. Prog Energy Combust Sci 2009;35:398e437. Saxena S, Bedoya ID. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078

12

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e1 2

and strategies for extending these limits. Prog Energy Combust Sci 2013;39:457e88. Shudo T, Ono Y, Takahashi T. Influence of hydrogen and carbon monoxide on HCCI combustion of dimethyl ether. SAE Paper; 2002. 2002-01-2828. Shudo T, Ono Y, Takahashi T. Ignition control by DME reformed gas in HCCI combustion of DME. SAE Paper; 2003. 2003-01-1824. Hosseini V, Checkel MD. Effect of reformer gas on HCCI combustion e part II: low octane fuels. SAE Paper; 2007. 200701-0206. Hosseini V, Checkel MD. Effect of reformer gas on HCCI combustion e part I: high octane fuels. SAE Paper; 2007. 2007-01-0208. Hosseini V, Checkel MD. Reformer gas composition effect on HCCI combustion of n-heptane, iso-octane, and natural gas. SAE Paper; 2008. 2008-01-0049. Guo H, Hosseini V, Neill WS, Chippior WL, Dumitrescu CE. An experimental study on the effect of hydrogen enrichment on diesel fueled HCCI combustion. Int J Hydrogen Energy 2011;36:13820e30. Guo H, Neill WS. The effect of hydrogen addition on combustion and emission characteristics of an n-heptane fuelled HCCI engine. Int J Hydrogen Energy 2013;38:11429e37. Yap D, Megaritis A, Peucheret S, Wyszynski ML, Xu H. Effects of hydrogen addition on natural gas HCCI combustion. SAE Paper; 2004. 2004-01-1972. Elkelawy M, Yu-Sheng Z, El-Din HA, Jing-zhou Y. A comprehensive modeling study of natural gas (HCCI) engine combustion enhancement by using hydrogen addition. SAE Paper; 2008. 2008-01-1706. Yap D, Peucheret SM, Megaritis A, Wyszynski ML, Xu H. Natural gas HCCI engine operation with exhaust gas fuel reforming. Int J Hydrogen Energy 2006;31:587e95. Konsereeparp P, Checkel MD. Investigating the effects of reformed fuel blending in a methane- or heptanes-HCCI engine using a multi-zone model. SAE Paper; 2007. 2007-010205. Voshtani S, Reyhanian M, Ehteram M, Hosseini V. Investigating various effects of reformer gas enrichment on a

[54]

[55]

[56]

[57]

[58]

[59]

[60]

[61]

[62]

[63] [64]

natural gas-fueled HCCI combustion engine. Int J Hydrogen Energy 2014;39:19799e809. Starik AM, Kozlov VE, Titova NS. On the influence of singlet oxygen molecules on the speed of flame propagation in methane-air mixture. Combust Flame 2010;157:313e27. Starik AM, Kozlov VE, Titova NS. On the influence of singlet oxygen molecules on characteristics of HCCI combustion: a numerical study. Combust Theory Model 2013;17:579e609. Starik AM, Titova NS, Sharipov AS, Kozlov VE. Syngas oxidation mechanism. Combust Explos Shock Waves 2010;46:491e506. Bezgin LV, Kopchenov VI, Sharipov AS, Titova NS, Starik AM. Evaluation of prediction ability of detailed reaction mechanisms in the combustion performance in hydrogenair supersonic flows. Combust Sci Technol 2013;185:62e94. Kozlov VE, Starik AM, Titova NS, Vedishchev IYu. On mechanisms of formation of environmentally harmful compounds in homogeneous combustors. Combust Explos Shock Waves 2013;49:520e35. Tanoue K, Goto S, Shimada F, Hamatake T. Effects of hydrogen addition on stretched premixed laminar methane flames: 1st report, effects on laminar burning velocity. Trans Jpn Soc Mech Eng B 2003;69:162e8. Huang Z, Zhang Y, Zeng K, Liu B, Wang Q, Jiang D. Measurements of laminar burning velocities for natural gasehydrogeneair mixtures. Combust Flame 2006;146:302e11. Hu E, Huang Z, He J, Jin C, Zheng J. Experimental and numerical study on laminar burning characteristics of premixed methaneehydrogeneair flames. Int J Hydrogen Energy 2009;34:4876e88. Kee RJ, Rupley FM, Miller JA, Coltrin ME, Grcar JF, Meeks E, et al. CHEMKIN release 4.0. San Diego, CA: Reaction Design, Inc; 2004. ANSYS FLUENT. User's guide, version 12. ANSYS Inc.; January 2009. Joelsson T. Large eddy simulation of turbulent reactive flows under HCCI engine conditions. Thesis for the Degree of Doctor of Philosophy in Engineering. Lund, SWEDEN: Division of Fluid Mechanics, Department of Energy Sciences, Faculty of EngineeringeLTH, Lund University; 2011.

Please cite this article in press as: Kozlov VE, et al., Modeling study of combustion and pollutant formation in HCCI engine operating on hydrogen rich fuel blends, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2015.12.078