Kinetic models of natural gas combustion in an internal combustion engine

Journal of Natural Gas Chemistry 19(2010)6–14

Kinetic models of natural gas combustion in an internal combustion engine M. Mansha1∗ ,

A. R Saleemi1 ,

Badar M. Ghauri2

1. Department of Chemical Engineering, University of Engineering & Technology, Lahore 54570, Pakistan; 2. SUPARCO, HQs, Karachi 49430, Pakistan [ Received May 9, 2009; Revised June 4, 2009; Available online December 24, 2009 ]

Abstract In this study, combustion of methane was simulated using four kinetic models of methane in CHEMKIN 4.1.1 for 0-D closed internal combustion (IC) engine reactor. Two detailed (GRIMECH3.0 & UBC MECH2.0) and two reduced (One step & Four steps) models were examined for various IC engine designs. The detailed models (GRIMECH3.0, & UBC MECH2.0) and 4-step models successfully predicted the combustion while global model was unable to predict any combustion reaction. This study illustrated that the detailed model showed good concordances in the prediction of chamber pressure, temperature and major combustion species profiles. The detailed models also exhibited the capabilities to predict the pollutants formation in an IC engine while the reduced schemes showed failure in the prediction of pollutants emissions. Although, there are discrepancies among the profiles of four considered model, the detailed models (GRIMECH3.0 & UBC MECH2.0) produced the acceptable agreement in the species prediction and formation of pollutants. Key words kinetic models; detailed models; reduced models; combustion; methane; IC engine

1. Introduction The combustion of hydrocarbon fuel removes O2 from the atmosphere and releases equivalent amount of H2 O and CO2 inevitably mingled with trace amounts of numerous other compounds including hydrocarbons (CH4 , C2 H2 , C6 H6 , CH2 , CHO, etc.), carbon monoxide (CO), nitrogen oxides (NO, N2 O) and reduced nitrogen (NH3 and HCN), sulfur gases (SO2 , OCS, CS2 ), halo-carbons (CHCI and CH3 Br), and particles [1]. Heywood [2] described that in light duty 4-stroke engine, a premixed fuel-air mixture (CNG-Air) enters the engine during the intake stroke, which starts when the piston is at top center (TC) and ends with the piston at bottom center (BC), and draws fresh mixture into the cylinder. To increase the mass inducted, the inlet valve opens shortly before the intake stroke starts and closes after it ends. The mixture is compressed to small fraction of its volume in the compression stroke. Towards the end of the compression stroke, combustion is initiated by the spark plug typically. As the combustion progresses, the cylinder pressure rises more rapidly when both intake and exhaust valves are closed. The power stroke, or expansion stroke starts with the piston at TC and ends at ∗

BC as high temperature and high pressure gases push the piston down and force the crank to rotate. During an exhaust stroke, the remaining burned gases exit the cylinder; first because the cylinder pressure may be substantially higher than the exhaust pressure; then as they are swept out by the piston as it moves towards TC. When the piston approaches the TC, the inlet valve opens. Just after TC, the exhaust valve closes and the cycle starts again. An illustrative example of possible trend of measured cylinder pressure is given in Figure 1. Because of the unique tetrahedral molecular structure with large C–H bond energies, methane exhibits some unique combustion characteristics. For example it has high ignition temperature, low flame speed and it is essentially unreactive in photochemical smog chemistry. Chemical kinetics of methane is the most widely studied topic. Kaufman [6], in a review of combustion kinetics indicated that the methane combustion models evolved in the period of 1970−1982 from less than 15 elementary steps with 12 species to 75 elementary steps, plus the 75 reverse reactions, with 25 species. Recently, several research groups have collaborated in the creation of an optimized methane kinetic model [7]. This mechanism designated GRI MECH3.0 is based on the optimization techniques of Frenklach et al. [8]. Various detailed reaction models are

Corresponding author. Tel: +92-333-4847327; E-mail: muhammad [email protected]

Copyright©2010, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. All rights reserved. doi:10.1016/S1003-9953(09)60024-4

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reported in the literature [9]. They can be divided into full models, skeletal models, and reduced models. These models differ with respect to the considered species and reactions [10]. In literature, several models of methane combustion exist. For example: • Detailed models include those of Westbrook [11], Glarborg et al. [12], Miller and Bowman [13], Konnov [14], Hughes et al. [15], and the standard GRI MECH3.0 [16]. • Reduced models include Westbrook and Dryer [17], Duterque et al. [18] (1 to 2 global reactions), Peters [19], Hautman et al., [20], Jones and Lindstedt [21] (more than 2 global reaction), Edelman and Fortune [22], and Edelman and Harsha [23]-one step global reaction with many elementary reactions; (these models are called quasi-global models). All chemical models used in combustion share the same description of elementary chemical reactions, based on an Arrhenius law, leading to a rate coefficient expressed as k = AT β e−E a /RT . The values of A, E a (or T a = Ea /R) and the temperature-dependent coefficient β are thus reaction dependent. Based on this expression, different levels of approximation can be defined to describe the kinetics.

Figure 1. Measured cylinder pressure for ten cycles in single cylinder SI engine operating at 1500 rpm, φ = 1.0, Pin = 0.7 atm (Taken from Heywood [2])

Oxidation models of methane combustion, reported in the literatures [6−25], were used to study methane combustion/burning in furnaces, burners, bunsen flame burner etc.

Simulation of the CH4 combustion in an internal combustion engine is very important to the design of engine and the control of air pollutants derived from the exhaust. One of the key objectives is to establish a kinetic model, in which the pressure and temperature profile in the engine, and the important reactants and products can be simulated. In current study, consequences of the selected (detailed & reduced) models for the profiles of temperature, pressure and major species produced are discussed. An appropriate model which predicts combustion species like NOx , CO, CO2 , and H2 O etc. in engine combustion chamber is identified. The simple criteria of comparing simulation results (profiles) of detailed and reduced models as followed in the study was described in Ref[26]. 2. Materials and methods CHEMKIN is a powerful set of software tools for solving complex chemical kinetics problems. It is used to study reacting flows, such as those found in combustion, catalysis, chemical vapour deposition, and plasma etching. CHEMKIN consists of rigorous gas-phase and gas-surface chemical kinetics in a variety of reactor models that can be used to represent the specific set of systems of interest. It provides a broad capability that addresses needs of both non-expert and expert users [27]. The IC model is for 0-D closed system, the simulation is only valid within the time period when both intake and exhaust valves are closed. Conventionally, engine cylinder events are expressed in crank rotation angle relative to the top dead center (TDC). The intake valve close (IVC) time of our test engine is 142 degrees (crank angle) before TDC (BTDC). In this study, we set our simulation starting crank angle to −142 degrees in the software input. Other simulation parameters we used in the software simulation were cycles end time as 0.043 sec or for 257 degrees crank angle to 115 degrees after TDC. The gas mixture pressure and temperature at IVC are 107911 Pa (or 1.065 atm) and 550 K, respectively. The following four mechanisms were investigated for methane combustion in internal combustion engines as given in Table 1.

Table 1. Tested models of methane combustion Sr. No 1 2

3 4

Kinetic model type

Reactions

Global one-step reaction [18] Four step reaction models of Jones and Lindstedt [21]

Arrhenius parameters A β 1.50E+13 0.0 4.40E+14 0.0 3.00E+1 0.0 2.50E+1 −1.0 2.75E+12 0.0

CH4 +2O=CO2 +2H2 O (i) CH4 +1/2O2 = CO+2H2 (ii) CH4 +H2 O = CO+3H2 (iii) H2 +1/2O2 = H2 O (iv) CO+H2 O = CO2 +H2 GRIMECH 3.0 (53 species & 325 reactions) (available on Internet) [7] UBC MECH2.0 Kinetic mechanism available on Internet at http://kbspc.mech.ubc.ca/kinetics.html

Where, A, β, T a (Ta = Ea /T ) are the parameters for Ar−Ea rhenius Law defined below: k = AT β e RT . Two of these model are the detailed such as GRIMECH 3.0 & UBC MECH2.0 and other two are reduced such as Duterque

(Global One Step) Two types of data tion of CHEMKIN (ii) thermodynamic

T a (K) 20000.0 24000.0 24000.0 32000.0 16000.0

& Jones and Lindstedt (Four Steps). files are required to input for execumodule; (i) Mechanism data files and data files. The details of mechanism

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and thermodynamics files can be found in references at [7,18,28]. Some important species and reaction (pressure

dependent) of both detailed models are given in Table 2 to Table 4.

Table 2. Important species considered in UBC MECH 2.0 and GRI MECH 3.0 kinetic models Detailed kinetic model UBC MECH2.0

No. of species 54

No. of reactions 277

GRI3.0

53

325

Important reacting species/intermediates (radicals) H2 , H, O, O2 , OH, H2 O, HO2 , H2 O2 , C, CH, CH2 , CH2 (S), CH3 , CH4 , CO2 , CO, CH2 O, CH2 OH, CH3 O, CH3 OH, C2 H, C2 H2 , C2 H3 , C2 H4 , C2 H5 , C2 H6 , HCCO, CH2 CO, HCCOH, N2 , AR, CH3 O2 , CH3 O2 H, C2 H5 O, C2 H5 O2 , C2 H5 O2 H, CH3 CO, CH3 CHO, C2 H4 O, C2 H3 O, C3 H8 , nC3 H7 , iC3 H7 , nC3 H7 O2 , iC3 H7 O2 , nC3 H7 O2 H, iC3 H7 O2 H, nC3 H7 O, iC3 H7 O, C3 H6 , C3 H5 , C3 H4 , C2 H4 O2 H O, O2 , OH, H2 O, HO2 , H2 O2 , C, CH, CH2 , CH2 (S), CH3 , CH4 , CO, CO2 , HCO, CH2 O, CH2 OH, CH3 O, CH3 OH, C2 H, C2 H2 , C2 H3 , C2 H4 , C2 H5 , C2 H6 , HCCO, CH2 CO, HCCOH, N, NH, NH2 , NH3 , NNH, NO, NO2 , N2 O, HNO, CN, HCN, H2 CN, HCNN, HCNO, HOCN, HNCO, NCO, N2 , AR, C3 H7 , C3 H8 , CH2 CHO

Table 3. Some important reactions of GRI MECH 3.0 mechanism (pressure & temperature dependent reactions are listed) Sr. No

Reactions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

O+H2 ⇐⇒ H+OH O+H2 O2 ⇐⇒ OH+HO2 O+CH4 ⇐⇒ OH+CH3 O+CH3 OH ⇐⇒ OH+CH2 OH O+CH3 OH ⇐⇒ OH+CH3 O O+C2 H2 ⇐⇒ H+HCCO O+C2 H2 ⇐⇒ OH+C2 H O+C2 H2 ⇐⇒ CO+CH2 O+C2 H4 ⇐⇒ CH3 +HCO O+C2 H6 ⇐⇒ OH+C2 H5 H+O2 ⇐⇒ O+OH H+H2 O2 ⇐⇒ HO2 +H2 H+CH3 (+M) ⇐⇒ CH4 (+M) H+CH4 ⇐⇒ CH3 +H2 H+HCO(+M) ⇐⇒ CH2 O(+M) H+CH2 O(+M) ⇐⇒ CH2 OH(+M) H+CH2 O(+M) ⇐⇒ CH3 O(+M) H+CH2 O ⇐⇒ HCO+H2 H+CH2 OH(+M) ⇐⇒ CH3 OH(+M) H+CH2 OH ⇐⇒ OH+CH3 H+CH2 OH ⇐⇒ CH2 (S)+H2 O H+CH3 O(+M) ⇐⇒ CH3 OH(+M) H+CH3 O ⇐⇒ H+CH2 OH H+CH3 O ⇐⇒ OH+CH3 H+CH3 O ⇐⇒ CH2 (S)+H2 O H+CH3 OH ⇐⇒ CH2 OH+H2 H+CH3 OH ⇐⇒ CH3 O+H2 H+C2 H3 (+M) ⇐⇒ C2 H4 (+M) H+C2 H4 (+M) ⇐⇒ C2 H5 (+M) H+C2 H4 ⇐⇒ C2 H3 +H2 H+C2 H5 (+M) ⇐⇒ C2 H6 (+M) H+C2 H6 ⇐⇒ C2 H5 +H2 H2 +CO(+M) ⇐⇒ CH2 O(+M) OH+H2 ⇐⇒ H+H2 O 2OH ⇐⇒ O+H2 O OH+CH3 (+M) ⇐⇒ CH3 OH(+M) OH+CH3 ⇐⇒ CH2 +H2 O OH+CH3 ⇐⇒ CH2 (S)+H2 O OH+CH4 ⇐⇒ CH3 +H2 O OH+CO ⇐⇒ H+CO2 CH3 +CH2 O ⇐⇒ HCO+CH4 CH3 +CH3 OH ⇐⇒ CH2 OH+CH4 CH3 +CH3 OH ⇐⇒ CH3 O+CH4 CH3 +C2 H4 ⇐⇒ C2 H3 +CH4 CH3 +C2 H6 ⇐⇒ C2 H5 +CH4 HCO+H2 O ⇐⇒ H+CO+H2 O HCO+M ⇐⇒ H+CO+M

k = A T β exp(−E/RT ) A (mol·cm·sec·K) β 3.87E+04 2.7 9.63E+06 2.0 1.02E+09 1.5 3.88E+05 2.5 1.30E+05 2.5 1.35E+07 2.0 4.60E+19 −1.4 6.94E+06 2.0 1.25E+07 1.8 8.98E+07 1.9 2.65E+16 −0.7 1.21E+07 2.0 1.39E+16 −0.5 6.60E+08 1.6 1.09E+12 0.5 5.40E+11 0.5 5.40E+11 0.5 5.74E+07 1.9 1.06E+12 0.5 1.65E+11 0.7 3.28E+13 −0.1 2.43E+12 0.5 4.15E+07 1.6 1.50E+12 0.5 2.62E+14 −0.2 1.70E+07 2.1 4.20E+06 2.1 6.08E+12 0.3 5.40E+11 0.5 1.32E+06 2.5 5.21E+17 −1.0 1.15E+08 1.9 4.30E+07 1.5 2.16E+08 1.5 3.57E+04 2.4 2.79E+18 −1.4 5.60E+07 1.6 6.44E+17 −1.3 1.00E+08 1.6 4.76E+07 1.2 3.32E+03 2.8 3.00E+07 1.5 1.00E+07 1.5 2.27E+05 2.0 6.14E+06 1.7 1.50E+18 −1.0 1.87E+17 −1.0

E (cal/mol) 6260 4000 8600 3100 5000 1900 28950 1900 220 5690 17041 5200 536 10840 −260 3600 2600 2742 86 −284 610 50 1924 −110 1070 4870 4870 280 1820 12240 1580 7530 79600 3430 −2110 1330 5420 1417 3120 70 5860 9940 9940 9200 10450 17000 17000

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Sr. No

Reactions

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

CH3 O+O2 ⇐⇒ HO2 +CH2 O C2 H+H2 ⇐⇒ H+C2 H2 C2 H3 +O2 ⇐⇒ HCO+CH2 O C2 H4 (+M) ⇐⇒ H2 +C2 H2 (+M) N+O2 ⇐⇒ NO+O NH+O2 ⇐⇒ NO+OH NH2 +OH ⇐⇒ NH+H2 O NNH+M ⇐⇒ N2 +H+M H+NO+M ⇐⇒ HNO+M HNO+H ⇐⇒ H2 +NO HNO+OH ⇐⇒ NO+H2 O CN+H2 ⇐⇒ HCN+H NCO+NO ⇐⇒ N2 O+CO NCO+NO ⇐⇒ N2 +CO2 HCN+M ⇐⇒ H+CN+M HCN+O ⇐⇒ NCO+H HCN+O ⇐⇒ NH+CO HCN+O ⇐⇒ CN+OH HCN+OH ⇐⇒ HOCN+H HCN+OH ⇐⇒ HNCO+H HCN+OH ⇐⇒ NH2 +CO CH+N2 ⇐⇒ HCN+N CH2 +NO ⇐⇒ H+HNCO CH2 +NO ⇐⇒ OH+HCN CH2 +NO ⇐⇒ H+HCNO CH2 (S)+NO ⇐⇒ H+HNCO CH2 (S)+NO ⇐⇒ OH+HCN CH2 (S)+NO ⇐⇒ H+HCNO HNCO+O ⇐⇒ NH+CO2 HNCO+O ⇐⇒ HNO+CO HNCO+O ⇐⇒ NCO+OH HNCO+H ⇐⇒ NH2 +CO HNCO+H ⇐⇒ H2 +NCO HNCO+OH ⇐⇒ NCO+H2 O HNCO+OH ⇐⇒ NH2 +CO2 HCNO+H ⇐⇒ H+HNCO HCNO+H ⇐⇒ OH+HCN HCNO+H ⇐⇒ NH2 +CO HOCN+H ⇐⇒ H+HNCO

Table 3. Continue k = A T β exp(−E/RT ) A (mol·cm·sec·K) β 4.28E-13 7.6 5.68E+10 0.9 4.58E+16 −1.4 8.00E+12 0.4 9.00E+09 1.0 1.28E+06 1.5 9.00E+07 1.5 1.30E+14 −0.1 4.48E+19 −1.3 9.00E+11 0.7 1.30E+07 1.9 2.95E+05 2.5 1.90E+17 −1.5 3.80E+18 −2.0 1.04E+29 −3.3 2.03E+04 2.6 5.07E+03 2.6 3.91E+09 1.6 1.10E+06 2.0 4.40E+03 2.3 1.60E+02 2.6 3.12E+09 0.9 3.10E+17 −1.4 2.90E+14 −0.7 3.80E+13 −0.4 3.10E+17 −1.4 2.90E+14 −0.7 3.80E+13 −0.4 9.80E+07 1.4 1.50E+08 1.6 2.20E+06 2.1 2.25E+07 1.7 1.05E+05 2.5 3.30E+07 1.5 3.30E+06 1.5 2.10E+15 −0.7 2.70E+11 0.2 1.70E+14 −0.8 2.00E+07 2.0

E (cal/mol) −3530 1993 1015 86770 6500 100 −460 4980 740 660 −950 2240 740 800 126600 4980 4980 26600 13370 6400 9000 20130 1270 760 580 1270 760 580 8500 44000 11400 3800 13300 3600 3600 2850 2120 2890 2000

Table 4. Some important reactions of UBC MECH2.0 mechanism (only pressure & temperature dependent reactions are listed) Sr. No

Reactions

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

O+H2 ⇐⇒ H+OH O+H2 O2 ⇐⇒ OH+HO2 O+CH4 ⇐⇒ OH+CH3 O+CH3 OH ⇐⇒ OH+CH2 OH O+CH3 OH ⇐⇒ OH+CH3 O O+C2 H2 ⇐⇒ H+HCCO O+C2 H2 ⇐⇒ OH+C2 H O+C2 H2 ⇐⇒ CO+CH2 O+C2 H4 ⇐⇒ CH3 +HCO O+C2 H6 ⇐⇒ OH+C2 H5 H+CH3 (+M) ⇐⇒ CH4 (+M) H+CH4 ⇐⇒ CH3 +H2 H+HCO(+M) ⇐⇒ CH2 O(+M) H+CH2 O(+M) ⇐⇒ CH2 OH(+M) H+CH2 O(+M) ⇐⇒ CH3 O(+M) H+CH2 O ⇐⇒ HCO+H2 H+CH3 OH ⇐⇒ CH2 OH+H2 H+CH3 OH ⇐⇒ CH3 O+H2 H+C2 H3 (+M) ⇐⇒ C2 H4 (+M) H+C2 H4 (+M) ⇐⇒ C2 H5 (+M)

A (mol·cm·sec·K) 5.00E+04 9.63E+06 1.02E+09 3.88E+05 1.30E+05 1.02E+07 4.60E+19 1.02E+07 1.92E+07 8.98E+07 1.27E+16 6.60E+08 1.09E+12 5.40E+11 5.40E+11 2.30E+10 1.70E+07 4.20E+06 6.08E+12 1.08E+12

k = A T β exp(−E/RT ) β 2.7 2.0 1.5 2.5 2.5 2.0 −1.4 2.0 1.8 1.9 −0.6 1.6 0.5 0.5 0.5 1.1 2.1 2.1 0.3 0.5

E (cal/mol) 6290 4000 8600 3100 5000 1900 28950 1900 220 5690 383 10840 −260 3600 2600 3275 4870 4870 280 1820

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Table 4. Continue Sr. No

Reactions

21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H+C2 H4 ⇐⇒ C2 H3 +H2 H+C2 H5 (+M) ⇐⇒ C2 H6 (+M) H+C2 H6 ⇐⇒ C2 H5 +H2 OH+H2 ⇐⇒ H+H2 O OH+CH2 ⇐⇒ CH+H2 O OH+CH3 ⇐⇒ CH2 +H2 O OH+CH4 ⇐⇒ CH3 +H2 O OH+CO ⇐⇒ H+CO2 OH+CH2 O ⇐⇒ HCO+H2 O OH+CH3 OH ⇐⇒ CH2 OH+H2 O OH+CH3 OH ⇐⇒ CH3 O+H2 O OH+C2 H2 ⇐⇒ H+CH2 CO OH+C2 H2 ⇐⇒ H+HCCOH OH+C2 H2 ⇐⇒ C2 H+H2 O OH+C2 H2 ⇐⇒ CH3 +CO OH+C2 H4 ⇐⇒ C2 H3 +H2 O OH+C2 H6 ⇐⇒ C2 H5 +H2 O CH+H2 ⇐⇒ H+CH2 CH2 +H2 ⇐⇒ H+CH3 CH2 +CH4 ⇐⇒ 2CH3 CH2 +CO(+M) ⇐⇒ CH2 CO(+M) 2CH3 (+M) ⇐⇒ C2 H6 (+M) 2CH3 ⇐⇒ H+C2 H5 CH3 +CH2 O ⇐⇒ HCO+CH4 CH3 +CH3 OH ⇐⇒ CH2 OH+CH4 CH3 +CH3 OH ⇐⇒ CH3 O+CH4 CH3 +C2 H4 ⇐⇒ C2 H3 +CH4 CH3 +C2 H6 ⇐⇒ C2 H5 +CH4 HCO+H2 O ⇐⇒ H+CO+H2 O HCO+M ⇐⇒ H+CO+M CH3 O+O2 ⇐⇒ HO2 +CH2 O C2 H+H2 ⇐⇒ H+C2 H2 C2 H4 (+M) ⇐⇒ H2 +C2 H2 (+M) CH3 +O2 ⇐⇒ CH3 O2 C2 H5 +O2 ⇐⇒ C2 H5 O2 C2 H5 +O2 ⇐⇒ C2 H5 O+O C2 H5 +O2 ⇐⇒ CH3 CHO+OH C2 H3 +O2 ⇐⇒ C2 H3 O+O C2 H3 +O2 ⇐⇒ C2 H2 +HO2 OH+C3 H8 ⇐⇒ iC3 H7 +H2 O

A (mol·cm·sec·K) 1.32E+06 5.21E+17 1.15E+08 2.16E+08 1.13E+07 5.60E+07 1.00E+08 4.76E+07 3.43E+09 1.44E+06 6.30E+06 2.18E-04 5.04E+05 3.37E+07 4.83E-04 3.60E+06 3.54E+06 1.11E+08 5.00E+05 2.46E+06 8.10E+11 2.12E+16 4.99E+12 3.32E+03 3.00E+07 1.00E+07 2.27E+05 6.14E+06 2.24E+18 1.87E+17 4.28E-13 4.07E+05 8.00E+12 8.52E+58 1.10E+47 1.10E+13 1.60E+14 6.61E+06 8.40E+05 7.08E+06

Table 2 shows some common species and intermediates (or radicals) present in the reacting mixture for both detailed kinetic models and these models have about 44 common reactions. In Table 3 & 4, only the pressure dependent reactions are listed as apparent from value of Arrhenius parameter “β”. We have simulated the combustion of methane with four models (Table 1) with different engine specifications (An example of engine specifications is given in Table 5). More details about the engine specifications, used in this simulation, exist in studies cited at References [5,27,29]. The other simulation inputs to the CHEMKIN software are given in Table 6 and adopted from Heywood [2]. Table 5. Example of test engines specifications used in simulation of methane combustion Parameters Compression ratio Cylinder clearance volume (cm3 ) Engine speed (rpm) Connecting rod to crank radius ratio Cylinder bore diameter (mm) Displacement (cm)

Values 10.0 1530 1000 2.97729 102 −

k = A T β exp(−E/RT ) β 2.5 −1.0 1.9 1.5 2.0 1.6 1.6 1.2 1.2 2.0 2.0 4.5 2.3 2.0 4.0 2.0 2.1 1.8 2.0 2.0 0.5 −1.0 0.1 2.8 1.5 1.5 2.0 1.7 −1.0 −1.0 7.6 2.4 0.4 −15.0 −10.6 −0.2 −1.2 1.9 1.9 1.9

E (cal/mol) 12240 1580 7530 3430 3000 5420 3120 70 −447 −840 1500 −1000 13500 14000 −2000 2500 870 1670 7230 8270 4510 620 10600 5860 9940 9940 9200 10450 17000 17000 −3530 200 88770 17018 14830 27926 10392 979 2246 158.9

Table 6. General input parameters Parameters Heat transfer correlation coefficients

Woschni correlation coefficients

Coefficient a Coefficient b Coefficient c C11 C12 C2

Wall temperature (K)

Values 0.035 0.71 0.0 2.28 0.308 3.24 400

The composition of the initial gas mixture is a combination of natural gas, air, and exhaust gas recirculation (EGR) gas as given in Table 7. Table 7. Composition (mole fraction) of initial gas mixture Species CH4 C2 H6 C3 H8 CO2 N2

Mole fraction 0.8709 0.105 0.0027 0.0205 0.072

Journal of Natural Gas Chemistry Vol. 19 No. 1 2010

3. Results and discussion The combustion of methane in engine cylinder was simulated with four kinetic model schemes and we used various input parameters. In this section, we focus more on the consequences of the used four kinetic reaction schemes (models) of methane oxidation for the predicted pressure, temperature profiles and major combustion species including gaseous pollutants. The predicting capabilities of theses models under the similar simulation conditions were also discussed and an appropriate reaction scheme (detailed & reduced) was identified simply based on the simulation results. Figure 2 &3 shows the pressure and temperature profiles respectively of four models for the adiabatic and stoichiomet-

11

ric conditions (Initial inlet temperature, Tini = 447 K, initial inlet pressure, Pini = 1.07 bar and φ = 1.0). As shown in the Figures, the reduced model (Duterque, Jones and Lindstedt) predicts the earlier combustion as the detailed models. The reason of this deviation in delay is that the species and temperature reach their end values very sharply. Each pressure profile clearly shows that the peak cylinder pressure occurs close to TC (top-center). At TC, this pressure built up is closely related to the rate of burning of the premixed fuel mixture. There is an early built up of pressure with the reduced model (Duterque, Jones and Lindstedt) than the detailed reaction schemes (UBC MECH2.0 & GRIMECH 3.0). The detailed models predict the maximum cylinder pressure and temperature of approximately of 40 atm and 2000 K, respectively. In case of reduced models, the predicted pressure and temperature significantly deviate.

Figure 2. Predicted pressure profiles for equivalence ratio of φ = 1.0 (Tini = 447 K, Pini = 1.07 bar)

Figure 3. Predicted temperature profiles for equivalence ratio of φ = 1.0 (Tini = 447 K, Pini = 1.07 bar)

These deviations in the prediction of pressure and temperature occur due to reaction paths for the detailed and reduced models.

Figure 4 demonstrates the main combustion species profiles of fuel (methane, CH4 ), carbon dioxide (CO2 ), and water vapours (H2 O) at stoichiometric conditions. Obviously,

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the 4-step model better predicts the early consumption of fuel than both the detailed models. If we look at the profiles of the produced species (CO2 and H2 O), 4-step model indicates that these species are formed at the earlier stage very rapidly and later on, these are consumed at the intermediate steps (which indicate the pyrolysis of fuel) and then produced. These intermediates then further are oxidized to CO which is then ox-

idized to CO2 . It is clear from Figure 4 that, both detailed models (GRIMECH 3.0 & UBC MECH2.0) and 4-steps model predict CO emissions and one step global model fails in this regard because there is no CO pathway in the model. In each graph of CO and NOx (as NO2 ), the reduced model shows the earlier formation than the detailed models.

Figure 4. Major species profiles (a) CH4 , (b) CO2 , (c) H2 O for equivalence ratio of φ = 1.0 (Tini = 447 K, Pini = 1.07 bar)

Journal of Natural Gas Chemistry Vol. 19 No. 1 2010

Each profile of N2 O graph illustrates that N2 O is formed immediately during the combustion and then its fraction is decreased as shown in Figure 5. The reason for this production of N2 O production is the oxidation of N2 with O2 and the fur-

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ther conversion into NO2 and NO. There is more rapid formation of NO than NO2 on the whole, the reduced models (only the 4-steps mechanism) predict the higher fractions than the detailed reaction schemes.

Figure 5. NOx (a) and CO (b) emissions for equivalence ratio of φ = 1.0 (Tini = 447 K, Pini = 1.07 bar). NOx is used as collective term for NO2 & N2 O

In the light of the above simulation results, detailed models are more appropriate in prediction of combustion species and pollutants formation in IC engine chamber. The results (Figures 2−5) of present study predict that GRIMECH3.0 model could be utilized in practical design on an IC engine for low emissions. 4. Conclusions Combustion in an IC engine was simulated using four reaction models (two detailed and two reduced). The effect of these reaction schemes on the pressure profiles, temperature profiles and major species profiles was compared under various simulation conditions (equivalence ratios, engine parameters keeping initial gas composition constant). The detailed models showed the encouraging results but the computational cost of a simulation is high. The comparison of the predicted temperature profiles, major species profiles and pollutants

emission for detailed models identified some of the discrepancies but on the whole the detailed models (GRIMECH3.0 & UBC MECH2.0) can be superseded over the reduced model (4-steps model of Jones and Lindstedt) in prediction of pollutants emissions of CO & oxides of nitrogen (NO, N2 O & NO2 ). Acknowledgements Authors are thankful to the Higher Education Commission (HEC) for financial support for this study and Mr. Jamal Gul for his continuous technical support during simulation experiments for keeping the system operational.

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