Figure 1.
Flow configuration for a diffusion flame [17].
Citation: Patrick Wanjiru, Nancy Karuri, Paul Wanyeki, Paul Kioni, Josephat Tanui. Numerical simulation of the effect of diluents on NOx formation in methane and methyl formate fuels in counter flow diffusion flame[J]. AIMS Environmental Science, 2020, 7(2): 140-152. doi: 10.3934/environsci.2020008
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Annotations
λ: Thermal conductivity (W m-1 K-1); μ: Coefficient of dynamic viscosity (Ns m-2); ρ: Mass density (kg m-3); cp: Mixture’s frozen specific heat capacity at constant pressure (Jkg-1K-1); cpi: Specific heat capacity at constant pressure for species i (J kg-1 K-1); hi: Specific enthalpy of species i (J kg-1); L: Lewis number; N: Total number of chemical species present; P: Stress tensor (Pa); p: Hydrostatic pressure (Pa); Pr: Prandtl number; q: Heat flux vector (W m-1); qR: Radiant heat flux vector (W m-1); R: Relating to Radiant heat; R: Universal gas constant (J mol-1 K-1); s: Specific entropy (J kg-1 K-1); T: Temperature (K); t: Time (s); v: Velocity vector (m s-1); Vi: Diffusion velocity of species i in y direction (m s-1); Vx: Velocity component in x direction (m s-1); Vy: Velocity component in y direction (m s-1); Wi: Molecular weight of species i (kg mol-1); wi: Rate of production of species i by chemical reactions (kg m-3 s-1); Y: Mass fraction; y: Spatial coordinate (m)
The main challenges associated with fossil fuels are energy demand increase and environmental pollution. In the last two decades, the use of biodiesel fuel as alternative fossil fuels has increased considerably. Biodiesel fuels lead to lower emissions and reduce dependencies on fossil fuels [1,2]. The use of biodiesel gives higher thermal efficiencies at various engine speeds [3]. Biodiesel fuels typically contain about 10% or higher oxygen content by mass. The presence of oxygen atom in the structure of the biodiesel fuels makes the fuel burn more efficiently with reduced emissions of soot, unburnt hydrocarbons and carbon monoxide [4]. However, combustion of biodiesel fuels produces more oxides of nitrogen (NOx) compared to fossil fuels [2,5].
Chemical kinetic mechanisms for the combustion methyl formate, a major component of biodiesel have been identified and further developed to capture the production processes of pollutants (NOx) [6]. These models were then improved with the addition of but research output has been low since these first studies [7]. This is particularly important given that kinetics of combustion of methyl esters have not received much attention in the existing literature. The formation of NOx has been shown to be high in the methyl esters combusted in a diesel engine system. Among these fuels; methane, methanol and methyl formate, the highest amount of NOx was obtained in methane and the lowest obtained in methyl formate. These studies were carried out at 1 bar [8,9]. The same trends were observed but NOx was higher at 50 bars than at 1 bar [10].
The effect of exhaust gas recirculation on diesel engine nitrogen oxide reduction operating with methyl ester derived from jojoba, where the recirculation was done up to 15% and showed a considerable reduction in NOx formation [11]. The effect of a sustainable biofuel–n-octanol–on the combustion, performance, and emissions of a DI diesel engine under naturally aspirated and exhaust gas recirculation (EGR) modes [12]. Investigation and exploration of the mid-load potential of ethanol-diesel dual-fuel combustion with and without EGR showed a massive reduction of NO formation in the fuels [13]. The effect of high EGR causes low efficiency of a diesel engine and an increase of soot and low combustion of fuels [14]. Hence, there should be more investigations on the optimum and the right content of exhaust gas recirculated. The study of EGR effect on methyl esters has not gained much interest in research; hence, this research on the effect of diluents using an EGR system on oxygenated fuels. There is a gap in knowledge in reduction of NOx in methyl ester fuels. In this report this gap knowledge is addressed by studies which provide an optimal EGR rate for minimal NOx formation in combustion of methyl esters in a model system.
Understanding the physical and chemical processes that occur during the combustion of methyl-ester fuels, especially those that greatly influence the formation of NO, is required for the development of better alternatives and design of efficient burners. The four main routes of NO formation in combustion processes are Zeldovich mechanism (thermo NO), N2O route, prompt NO and Fuel Bound Nitrogen (FBN).
It is formed at high temperature flame regions through extended Zeldovich mechanism which is shown by Eqs: [15]
| O+N2→NO+N | (1) |
| N+O2→NO+O | (2) |
| N+OH→NO+H | (3) |
It is formed at low temperature flame front regions due to the presence of CH radicals. The CH radical reacts with molecular nitrogen according to Eq 4. The nitrogen generated from this mechanism proceeds to form NO through Eqs [16]
| CH+N2→HCN+N | (4) |
| N+O2→NO+O | (5) |
| N+OH→NO+H | (6) |
NO production through N2O route occurs through the free body recombination reactions, Eq 7 under the conditions of high pressures. The N2O formed in Eq 7 reacts with O to form NO through Eq 8. [16]
| N2+O+M→N2O+M | (7) |
Where M is an energy carrier
| N2O+O→NO+NO | (8) |
Combustion of fuels that contain nitrogen results in formation of NOx. In most of the fuels, including biodiesel fuels used in internal combustion engines, contain a significantly small amount of Fuel Bound Nitrogen and hence Fuel NOx is neglected in kinetic mechanisms in this research.
The flow configuration considered is as shown in Figure 1, with the fuel and airside, positioned at the left and right-hand side, respectively. The temperature of both the fuel and air stream was taken as 300 K. The flames were computed at a constant pressure of 1 bar and a strain rate of 50 s-1. At the fuel nozzle, fuel (mole fraction of 1) was specified. The diluents (CO2) was recirculated through the fuel side in mole fraction of 0% to 29.5% of the exhaust CO2 and the H2O is cooled at the EGR cooling system and release as water droplets. At the air nozzle, the mole fractions concentration of O2 and N2 were specified using the air standard composition of 0.21 and 0.79 respectively.
The flame was assumed to be embedded in a thin boundary-layer formed by stagnating flow. Hence, the computation required the use of the boundary layer theory approximations. Though the flame is two dimensional, under this assumption the equations of conservation reduced to one-dimensional forms. Also, the thermodynamic part of the pressure was assumed to be spatially uniform. The effect of viscous dissipation, the Soret and Dufour, and the body forces were neglected. Thus, the governing equations, as derived in [18] are as follows.
| L1(1)=0 | (9) |
| L1(G)=∂∂y(μ∂G∂y)−ρG2+P′(t) | (10) |
| cpL1(T)=∂∂y(λ∂T∂y)−∂T∂yN∑i=1cpiρYiVi−N∑i=1hiWi+dpdt−∂qR∂y | (11) |
| L1(Yi)=−∂∂y(ρYiVi)+wii=1,…,N | (12) |
Here L1 is the accumulative-convective operator given by:
| L1(Φ)=∂(ρΦ)∂t+∂(ρVyΦ)∂y+ρGΦ | (13) |
The thermodynamic properties (the frozen specific heat capacities at a constant pressure of the pure species i, cpi, the frozen specific heat capacity at a constant pressure of the gas mixture, cp, and the enthalpy, hi, for pure species i) were calculated using polynomial curve fits of NASA type [19]. The transport properties required are the dynamic viscosity, μ, and the thermal conductivity, λ, of the gas mixture, which are calculated using the method illustrated in [18].
Exhausts consist mainly of CO2, N2 and water vapor. Through experimental analysis it was found out that the most efficient diluent is CO2 which decreased NOx emissions by 64% while Ar and N2 had an effect of 30% and 58% respectively [20]. When a part of this exhaust gas is re-circulated to the cylinder, it acts as a diluent to the combusting mixture. This also reduces the concentration of O2 in the chamber of combustion. The EGR's specific heat is much higher than fresh air. Hence EGR increases the heat capacity (specific heat) of the intake charge, thereby reducing the temperature increase in the combustion for the same heat release [21]. The percentage of recirculation can be shown in Eq 14.
| EGR%= Volume of EGR Total intake charge into the cylinder ×100 | (14) |
The flames were numerically simulated using the RUN1DL code in the software package [22]. The simulation of methyl formate fuel was done using a developed chemical kinetic mechanism which captures the processes of pollutants [7], while methane fuels used GRI mechanism 2.0 [23]. Conservation equations were discretized with a finite difference method. Both central and one-sided difference (upwind) schemes are adopted in the discretization of first-order derivatives, while the central difference scheme is adopted for the second-order derivatives. For the case of upwind scheme applied to first-order derivatives, the first-order accuracy was achieved. When second-order accurate central difference scheme was applied to a second-order derivative in a non-uniform grid, one order of accuracy was lost. Numerical accuracy was enhanced by having many grid points in the region of high gradient and having the grids points which were approximately equally spaced. This has been achieved by applying the adaptive selection of grid point technique. Modified Newton method was applied to the resulting nonlinear differential-algebraic equations.
It was found out that the recirculation can only be up to 29.5% of EGR system. The system could only run when the EGR percentage is strictly less than 30%. The software predicted an error in running EGR from 30% and above. This shows that the recirculation cannot exceed 30% of exhaust gas which can hinder combustion of the fuel.
The production NOx in combustion is favoured by high combustion temperature [24]. Based on this the temperature profiles for methyl formate/air diffusion flame was simulated at different percentages of varying EGR. The results were compared with that of methane/air diffusion flame. Figure 2 represents temperature profiles of methyl formate/air and methane/air combustion, respectively. The maximum temperature for combustion without recirculation was found to be 2100 K for methyl formate/air and 2050 K for methane/air combustion. Figure 2Bshows that with an increase of EGR from 0% to 29.5%, the methane/air diffusion flame, had a reduction in temperature of approximately 58 K while the methyl formate/air diffusion flame had a reduction of 55 K. As EGR was increased the methane/air peak values shifted to the fuel side while the peak values of methyl formate/air did not show a shift only a reduction. Differences in the influence of EGR on the diffusion flame temperature in methyl formate/air compared to methane/air suggest that there are differences in NOx production in these two different systems.
In addition to temperature, radicals of N, O, OH, and CH plays a significant role in prompt-NO and N2O route in NOx formation. Hence, the reduction of these species may result in a reduction in the formation of NOx in the diffusion flames. Figure 3 shows how the mole fractions of N radicals compares to that of methane/air diffusion flame for different amount of EGR. Through reactions: N+O→NO+H and N+O2→NO+O shows that N has a significant impact in formation of NO and with its reduction will have a highly significant reduction of nitrogen oxides. An increase in EGR results in a reduction of N radicals in both methyl formate/air and methane/air diffusion flames, but this decrease is more significant in methane than in methyl formate. The reduction in peak N radical values in methyl formate/air is approximately 0.03 ppm compared to that of methane/air of approximately 0.447 ppm. Similarly there is a shift in both methyl formate/air and methane/air diffusion flames with increasing EGR towards the fuel side. In methyl formate/air diffusion flame peak N radical values were less than those of the methane/air diffusion flame and decreased with increasing EGR showing less NOx production by EGR recirculation.
Another important radical in NOx formation is the OH radical. Figure 4 shows the effect of EGR on mole fractions of OH radicals in parts per million (ppm) in methyl formate/air and methane/air diffusion flames. From the reaction, N+OH→NO+H show that OH radical is critical in the formation of NO which is in high quantity; hence, its reduction will reduce the formation of NO. As EGR was increased in methyl formate/air diffusion flame (Figure 4A), the amount of OH radicals decreased. This was the same as the control methane/air diffusion flame. However, methyl formate/air showed a higher reduction in OH radical of approximately 1699 ppm compared to the control methane/air of approximately 962 ppm. The peak values in methyl formate/air diffusion flame shifted towards the fuel side with increasing EGR. This behavior was also observed in methane/air diffusion flame indicating delayed combustion and higher oxygen consumption with increasing EGR. Analysis of OH radicals supports previous observation with N radicals that increasing EGR decreases NOx formation in methyl formate/air diffusion flame.
A similar relationship between OH radicals and EGR was observed between O radicals and EGR in methyl formate/air flame. NO formation can also be through N2+O→NO+N. Figure 5 shows how mole fractions of O radicals changed with EGR in methyl formate/air diffusion flame and methane/air diffusion flame. Similar to findings with OH radicals an increase of EGR results in a reduction of O radicals in both methyl formate/air and methane/air diffusion flames, but this decrease is more significant in methyl formate than in methane. The reduction in methyl formate/air is approximately 400 ppm compared to that of methane/air of approximately 300 ppm. Similarly there is a shift in both methyl formate/air and methane/air diffusion flames with increase in EGR towards the fuel side. However, peak values are different between methyl formate/air and methane/air diffusion flame. In methyl formate/air diffusion flame peak O radical values at maximum EGR were higher than those of methane/air diffusion flame. The results demonstrate that NOx in methyl formate/air diffusion flame can be effectively reduced by EGR. However, the effects of EGR on NOx are different to that of the widely characterized methane/air system.
One of the reasons for use of EGR in combustion is reduction in NOx emissions [25,26]. In this research, it was found that with the incorporation of different percentages of EGR the system has a significant reduction in NOx formation in methyl formate/air system. Figure 6 shows the amount of NO in methyl formate/air and methane/air diffusion flames respectively. There was less NO formed in methyl formate/air compared to that of methane/air; approximately 39 ppm versus 200 ppm respectively without EGR. Similarly with increase in EGR, the amount of NO decreases in both systems but the reduction in peak values was higher in methane/air compared to methyl fomate/air; 56 ppm versus 8 ppm. The approximate reduction in peak values from 0% to 29.5% EGR was 26% and 14% in methane/air and methyl formate/air respectively. The difference between methane and methyl formate may be related to decrease in temperatures, O radicals, and OH radicals. Since the flame temperatures were below 2500 K, it is likely that prompt NO was the dominant kinetic mechanism for NO formation.
Numerical simulations of NOx production and reduction in methane/air and methyl formate/air diffusion flame configurations have been investigated. The temperatures attained by diffusion flames favour the production of NO mainly through the prompt NO route [16]. The reduction of the temperature causes a reduced formation of all the species, which in the end reduces the formation of NO, O and OH radicals are produced in high quantities during combustion; hence their reduction has been established as a significant effect of reduction of NO formation. The other minor species investigated and not reported here; N, and CH have an impact on NO formation, and they all reduced significantly. The reduction of temperature and species associated with NOx suggests a reduction in NOx formation. Therefore EGR can significantly reduce NOx in methyl formate combustion.
The authors gratefully acknowledge the financial support of the Dedan Kimathi University of Technology, whose Combustion Simulation Laboratory Software package (COSILAB) was used to carry out the Flame simulation.
All authors declare no conflicts of interest in this research paper.
| [1] |
Basha SA, KR Gopal, S Jebaraj (2009) A review on biodiesel production, combustion, emissions and performance. Renew Sus energ Rev 13: 1628-1634. doi: 10.1016/j.rser.2008.09.031
|
| [2] | Abed K, M Gad, A El Morsi, et al. (2019) Effect of biodiesel fuels on diesel engine emissions. Egypt J Petrol 34:198-223. |
| [3] | Liu HP, S Strank, M Werst, et al. (2010) Combustion emissions modeling and testing of neat biodiesel fuels. in ASME 2010 4th International Conference on Energy Sustainability. ASME Digit Collect 2010: 131-140. |
| [4] | Ekarong S (2013) Synergistic effects of alcohol-based renewable fuels: fuel properties and emissions. Ph. D. thesis, University of Birmingham. |
| [5] | Graboski MS, RL McCormick, TL Alleman, et al. (1999) Effect of biodiesel composition on NOx and PM emissions from a DDC Series 60 engine. Report for National Renewable Energy Laboratory. |
| [6] | Dooley S, Chaos M, Burke MP, et al. (2009) An experimental and kinetic modeling study of methyl formate oxidation. Proceedings of the European Combustion Meeting. |
| [7] |
Dooley S, Dryer FL, Yang B, et al. (2011) An experimental and kinetic modeling study of methyl formate low-pressure flames. Combust Flame 158: 732-741. doi: 10.1016/j.combustflame.2010.11.003
|
| [8] | Kioni PN, Tanui JK, Gitahi A (2013) Numerical simulations of nitric oxide (NO) formation in methane, methanol and methyl formate in different flow configurations. J Clean Energ Tech 1. |
| [9] | Tanui J, PN Kioni, A Gitahi (2014) Numerical Simulation of Nitric Oxide (NO) Formation in Methane, Methanol and Methyl Formate in a Homogeneous System. J Sust Res Engin 1. |
| [10] | Ngugi JM, Kioni PN, Tanui JK (2018) Numerical Study of Nitrogen Oxides (NOx) Formation in Homogenous System of Methane, Methanol and Methyl Formate at High Pressures. J Clean Energ Tech 6. |
| [11] |
Saleh H (2009) Effect of exhaust gas recirculation on diesel engine nitrogen oxide reduction operating with jojoba methyl ester. Renew Energ 34: 2178-2186. doi: 10.1016/j.renene.2009.03.024
|
| [12] |
Kumar BR, S Saravanan, D Rana, et al. (2016) Effect of a sustainable biofuel-n-octanol-on the combustion, performance and emissions of a DI diesel engine under naturally aspirated and exhaust gas recirculation (EGR) modes. Energ Convers Manage 118: 275-286. doi: 10.1016/j.enconman.2016.04.001
|
| [13] |
Pedrozo VB, I May, H Zhao (2017) Exploring the mid-load potential of ethanol-diesel dual-fuel combustion with and without EGR. Appl Energ 193: 263-275. doi: 10.1016/j.apenergy.2017.02.043
|
| [14] |
Shi X, B Liu, C Zhang, et al. (2017) A study on combined effect of high EGR rate and biodiesel on combustion and emission performance of a diesel engine. Appl Therm Eng 125: 1272-1279. doi: 10.1016/j.applthermaleng.2017.07.083
|
| [15] |
Drake MC, RJ Blint (1991) Calculations of NOx formation pathways in propagating laminar, high pressure premixed CH4/air flames. Combust Sci Tech 75: 261-285. doi: 10.1080/00102209108924092
|
| [16] | Pillier L, P Desgroux, B Lefort, et al. (2006) NO prediction in natural gas flames using GDF-Kin® 3.0 mechanism NCN and HCN contribution to prompt-NO formation. Fuel 85: 896-909. |
| [17] |
Tsuji H (1982) Counterflow diffusion flames. Prog Energ Combust Sci 8: 93-119. doi: 10.1016/0360-1285(82)90015-6
|
| [18] |
Barlow R, A Karpetis, J Frank, et al. (2001) Scalar profiles and NO formation in laminar opposed-flow partially premixed methane/air flames. Combust Flame 127: 2102-2118. doi: 10.1016/S0010-2180(01)00313-3
|
| [19] | Goos E, A Burcat, B Ruscic (2012) Extended third millennium thermodynamic database for combustion and air-pollution use with updates from active thermochemical tables. Burcat und B. Ruscic, Third Millennium Ideal Gas Condensed Phase Thermochemical Database for Combustion with Updates from Active Thermochemical Tables, Joint Report: ANL-05/20, Argonne National Laboratory, Argonne, IL, USA, TAE. 960. |
| [20] |
Li W, Z Liu, Z Wang, et al. (2014) Experimental investigation of the thermal and diluent effects of EGR components on combustion and NOx emissions of a turbocharged natural gas SI engine. Energ Conve Man 88: 1041-1050. doi: 10.1016/j.enconman.2014.09.051
|
| [21] | Zelenka P, H Aufinger, W Reczek, et al. (1998) Cooled EGR-a key technology for future efficient HD diesels. SAE Technical Paper. |
| [22] | Version C (2009) 3, Rotexo-Cosilab GmbH & Co. KG, Bad Zwischenahn, Germany. |
| [23] |
Hughes K, T Turányi, A Clague, et al. (2001) Development and testing of a comprehensive chemical mechanism for the oxidation of methane. Int J Chem Kinet 33: 513-538. doi: 10.1002/kin.1048
|
| [24] | Hu B, H Yong (2011) Theoretical analysis of lowest limits of NOx formation of methane-air mixtures. in Asia-Pacific Power and Energy Engineering Conference. IEEE. |
| [25] | Gomaa M, A Alimin, K Kamaruddin (2011) The effect of EGR rates on NOX and smoke emissions of an IDI diesel engine fuelled with Jatropha biodiesel blends. Int J Energ Environ 2: 477-490. |
| [26] |
Maiboom A, X Tauzia (2011) NOx and PM emissions reduction on an automotive HSDI Diesel engine with water-in-diesel emulsion and EGR: An experimental study. Fuel 90: 3179-3192. doi: 10.1016/j.fuel.2011.06.014
|
| 1. | Adelaida O. Conza, Álvaro L. de Bortoli, Modeling and simulation of diffusion flames of $${\text {H}}_{2}$$ and methyl formate using the REDIM method, 2022, 60, 0259-9791, 562, 10.1007/s10910-021-01319-2 |
Figures(6)
Patrick Wanjiru, Nancy Karuri, Paul Wanyeki, Paul Kioni, Josephat Tanui. Numerical simulation of the effect of diluents on NOx formation in methane and methyl formate fuels in counter flow diffusion flame[J]. AIMS Environmental Science, 2020, 7(2): 140-152. doi: 10.3934/environsci.2020008
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