# Experimental Study of Explosion Characteristics and General Correlations of Lean H2/CO/CH4/Air Mixtures in Quiescence

Authors: Minh Tien Nguyen; Van Ga Bui; Xuan Bao Nguyen; Le Chau Thanh Nguyen; Minh Man Pham; Thi Hoa Truong; Minh Tung Phung; Van Hung Bui
Journal: International Journal of Energy Research
 (2023)

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## Abstract

A comprehensive understanding of the combustion characteristics of complex and changeable synthesis gas (syngas) compositions is essential for the further development of syngas-fueled engines with high thermal efficiency and low emissions. This study investigated explosion duration properties and laminar burning velocity (SL) of multicomponent syngas/air using the pressure-rise method. The lean multicomponent syngas/air mixtures at various equivalence ratios (ϕ=0.6, 0.7, and 0.8) containing different volumetric fractions of H2 (10–30%), CO (10–30%), and CH4 (40–60%) were conducted in a cylindrical constant volume combustion chamber using a dual-coil car ignition system. The explosion duration was characterized by the flame development time (FDT) and flame rising time (FRT), where FDT (FRT) is the time between spark ignition and 10% (10% and 90%) of the total heat release. The results showed that the values of FDT were approximately twofold of the FRT values, and they decreased with increasing H2 or CO content in the mixture. The leaner the mixture, the faster would be the reduction in the FRT and FDT as increasing H2 and/or CO content. Moreover, FDT and FRT were the shortest at a (H2/(H2+CO)) ratio of 0.5, at which the CH4 content was the lowest. Beyond this ratio, FDT and FRT increased, suggesting a significant effect of CH4 concentration. The shorter the FRT value, the higher would be the SL value. The current SL values, together with previous syngas data, could be well represented by a modified correlation with a slight discrepancy. This suggests a possible self-similar propagation of syngas/air laminar flames, regardless of the equivalence ratio, constituent fraction, temperature, pressure, and dilution. Additionally, a small value of mean absolute percentage error (~10.8%) revealed a better correlation for predicting SL values of syngas fuels.


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## Body

## 1. Introduction

As a renewable fuel, synthesis gas (syngas) is a promising alternative to traditional fossil fuels and plays a vital role in developing lean-burn internal combustion engines (ICEs) with high thermal efficiency and low emissions. Syngas, which mainly contains hydrogen (10~30%), carbon monoxide (14~30%), methane (1~10%), nitrogen (40~50%), and carbon dioxide (3~20%) [1], is produced directly through the gasification of coal, biomass, or waste, providing a more excellent opportunity to use low-cost and renewable fuels [2, 3]. Compared with traditional fossil fuels, syngas can extend the lean limit of spark ignition (SI) engines and produce lower emissions [4, 5]. Lean combustion is a promising method to boost the thermal efficiency of spark ignition (SI) engines by increasing the specific heat ratio [6] and reducing heat-transfer losses due to the lower combustion temperature [7, 8]. However, the slower kernel development in the lean condition and, thus, longer explosion duration interfere with the optimal combustion phasing [9] in SI engines, causing lower engine performance. In addition, the cycle-to-cycle variation in the early stage of combustion owing to misfiring and/or partial burning may restrict the applicability of lean-burn technology. In particular, the variation of complex constituents in the syngas mixtures is the most challenging when applying syngas fuels to SI engines, depending on the gasification process and the feedstock [3]. Therefore, comprehensive studies of the fundamental combustion characteristics of multicomponent syngas are necessary for further developing high-thermal-efficiency SI engines.The explosion duration and in-chamber pressure are the critical physicochemical properties [10, 11] of combustible fuel/air mixtures and are essential for designing better syngas-fueled applications [12]. Available data in the literature [4, 8, 12–14] indicated that the explosion duration was experimentally quantified by the flame development time (0-10% burned mass fraction) and flame rising time (10-90% burned mass fraction). Despite numerous studies on the explosion duration and in-chamber pressure available in the literature, most of them were performed on n-pentane, propylene, methane, nitromethane, and ethylene ([15–19] and references therein) and quite a few on syngas [4, 12, 20, 21]. Ran et al. [4] found that the flame development time (FDT) and flame rising time (FRT) were significantly reduced when increasing the equivalence ratio of syngas (C0.41H1.181O0.41)/air from 0.3 to 0.8. Sun [12] found that increasing the hydrogen fraction in the “simulated” syngas (H2/CO)/air mixtures could result in higher pressure peaks and shorten the FDT/FRT owing to the high H2 reactivity. However, the “simulated” syngas fuels may not fully characterize the real syngas fuels because other syngas constituents, such as CH4, significantly affect the reactivity of the mixture [22, 23]. For example, Kishore et al. [22] reported that as 12% of CH4 replaces the same fraction of CO, the increase in H2 content has less effect on the laminar flame speed and so does the explosion duration. Similarly, Wang et al. [24] reported that the addition of CH4 decreased the H and O concentrations, resulting in a decrease in the laminar burning velocity of H2/CO/N2/CH4/air mixtures. Moreover, the results in [20, 21] revealed that the explosion duration and in-chamber pressure are sensitive to syngas constituents. However, methane enrichment of syngas (MES) could improve the low calorific value of syngas fuel and thus be a better replacement for CNG in direct injection spark-ignition engines as reported by Hagos et al. ([25] and the references therein). Hence, considerable effort is required to comprehensively understand the explosion duration characteristics and pressure rise of multicomponent syngas fuels.The laminar burning velocity (SL) is widely used to examine the fuel burning rate, validate the detailed/simplified mechanism, and/or develop a complex turbulent combustion model [26]. Despite numerous studies on SL for syngas ([5, 24, 26–30] and references therein), there are significant discrepancies between these data, even under atmospheric conditions, as reported by Han et al. [26]. Therefore, measuring the accurate SL data for syngas/air mixtures under varying conditions is still needed. Furthermore, the complex and changeable composition of syngas requires many more studies to obtain comprehensive SL data. As a computationally convenient method, the value of SL can also be estimated from general correlations developed using experimental data or multistep chemistry models [28]. Therefore, seeking a better general correlation for SL is highly interesting for studying the syngas laminar combustion characteristics. Although many correlations have been proposed in the past decade, most of them have worked properly for hydrocarbon fuels, such as CH4 [31, 32], H2 [28, 33], and gasoline and its blending [34–36], while the correlations for syngas are scarce. Ravi and Petersen [28] proposed a simple correlation for different H2/CO mixtures at a high temperature and pressure while considering water dilution. They reported that the syngas correlation based on H2 chemistry only agreed within ±12% of the kinetic model prediction, including CO chemistry. However, shall Ravi’s correlation work properly for other types of syngas with added CH4? This question was not addressed in Ravi and Petersen’s report [28]. Recently, Lapalme and Seers [29] established a correlation for H2/CO/CH4/CO2 under atmospheric conditions and for 0.7<ϕ<2.2 of less than 5%-CH4 concentration. They found excellent agreement between the experimental data and the correlated results, with a minor discrepancy of 10%. The correlation also worked for high-temperature data up to 450 K but for a limited equivalence ratio between 0.8 and 1.2. Finding a better correlation for syngas covering the effect of constituent fraction (λ), temperature (T), pressure (p), equivalence ratio (ϕ), and dilution is thus still an open issue.It is beyond the scope of this study to discuss all correlations ofSL in the literature. Thus, we focus on two recent correlations proposed by Ravi and Petersen [28] and Lapalme and Seers [29] and then propose a new correlation for syngas fuels. Ravi and Petersen [28] suggested the following correlation for H2/CO/H2O mixtures:
(1)SL,uϕ,T,p,λ=k1expm1λhydrogen+k2expm2λwaterSL,hydrogenc,where λhydrogen and λwater are the volumetric fractions of hydrogen and water, respectively; SL,hydrogen is the laminar burning velocity of H2 at different pressures, temperatures, and equivalence ratios; and k1, k2, m1, m2, and c are the empirical coefficients. Lapalme and Seers [29] proposed a correlation for H2/CO/CH4/CO2 mixtures as follows:
(2)SL,u=SL,max−Bϕ−ϕM×1−cλCO2T300a,where SL,max, B, and ϕM are functions of λCH4 and the ratio λH2/λH2+λCO2. B is the reduction ratio between SL and its maximum values. ϕM is the equivalence ratio at SL,max. Parameters a and c are functions of ϕ and λCH4. To determine the parameters mentioned above, the reader can refer to Eqs. (4)–(8) in Ref. [29]. In addition, because of the significant effect of CH4 on the reactivity of the syngas mixtures, we rescaled Eq. (1) with λmethane instead of λwater in the following form:
(3)SL,uϕ,T,p,λ=k1expm1λhydrogen+k2expm2λmethaneSL,hydrogenc.To determine how well these three correlations could be applied to estimate the reliable value ofSL, the mean absolute percentage error (MAPE) [37, 38] was calculated using Eq. (4) to assess the accuracy of these correlations, where the lower the value of MAPE, the more accurate is the correlation.
(4)MAPE%=∑i=1nxexp,i−xcorr,i/xexp,in×100.In summary, the complex and changeable constituents of syngas may cause a lack of data on the laminar burning velocity and explosion duration. In addition, the simulated models and/or the general correlations could be inaccurate when predicting the laminar combustion characteristic of syngas fuels. In this study, the multicomponent syngas/air mixtures atϕ=0.6, 0.7, and 0.8 were thus conducted in a cylindrical constant-volume combustion chamber to obtain more beneficial information on the explosion duration and laminar burning velocity. The volume fractions of H2 and CO varied from 10% to 30%, and the volume fraction of CH4 varied from 40% to 60%. The current SL data together with previous data for H2/CO [28–30], H2/CO/H2O [28], H2/CO/CH4 [29], H2/CO/CO2 [29], and CH4/H2 [39] were used to validate the general correlations. Besides, uncertainty analysis of the new correlation is discussed and compared with other correlations in [28, 29] by using a mean absolute percentage error (MAPE).


## 2. Experimental Method

The lean multicomponent syngas (H2/CO/CH4)/air mixtures at ϕ=0.6, 0.7, and 0.8 having different volumetric fractions of H2 (10%-30%), CO (10%-30%), and CH4 (40%-60%) (please see Table 1) were conducted in a cylindrical constant volume combustion chamber (CVCC) under atmospheric conditions. It should be noted that the high fraction of CH4 was selected in this study to improve the low calorific value of syngas. Detailed information on the CVCC setup is presented in Ref. [39]. For completeness, Figure 1 shows a simplified sketch of a cylindrical vessel with an inner diameter of 160 mm and length of 300 mm, along with the ignition circuit and fuel supply system. The nominal purity of the syngas constituents was more than 99.9%. Each component was introduced into the vacuumed CVCC using the partial pressure method. The initial pressure of the multicomponent syngas–air mixtures was monitored using a pressure transducer P1 (CSR1 model, ±0.5% FS). The combustion pressure history was detected using a high-frequency pressure transducer P2 (JC960 model, 20 kHz, ±0.25% FS) and digital oscilloscope (Gwinstek GDS-1104B, 100 MHz). It was noted that dual-coil car ignition could induce multispark channels, which may enhance the ignitability and kernel propagation in the early stage of lean multicomponent syngas fuels.Table 1
Composition of the mixtures (volumetric fractions).
LabelH2/(H2+CO) ratioH2COCH4H10C30M600.25103060H20C30M500.4203050H30C30M400.5303040H30C20M500.6302050H30C10M600.75301060Figure 1
Simplified sketch of a constant volume combustion chamber alongside the fuel supply and ignition system.According to the recorded in-chamber pressure history, the laminar burning velocity (SL) was obtained using a least-squares fit to Eq. (5), as suggested by Matsugi et al. [40]:
(5)p=pt=t0+∫t0t3SLpe−p0R1−pe−ppe−p0p0p1/γ2/3pp0cdt,where R is the cylinder radius; p0 and p are the initial pressure and instantaneous pressure of the mixture, respectively; pe is the equilibrium pressure under an adiabatic constant volume condition [40] determined by the GasEq program, γ is the specific heat ratio of the unburned mixture, and c is the pressure-temperature dependence coefficient. To mitigate the effect of spark ignition and flame-wall interaction, the SL should be calculated within the specific pressure data points in the range 10–90% of the total heat release in which the flame radius Rf≤0.9R [41]. Figure 2(a) shows three examples of pressure rise for the H10C30M60, H20C30M50, and H30C30M40 fuels at ϕ=0.6. Using a least-squares fit method (as indicated by the bold solid lines), the laminar burning velocities were obtained at 9.9 cm s-1, 13.8 cm s-1, and 15.5 cm s-1.Figure 2
(a) The recorded pressure rise in CVCC (square symbols and fitting lines), and (b) determination of FDT and FRT based on NCHR curves for three fuels (H30C30M40, H20C30M50, and H10C30M60) atϕ=0.6.
(a)(b)The explosion duration was characterized by the flame development time (FDT) and flame rising time (FRT). FDT was defined as the time duration from the start ignition command to 10% of the total normalized cumulative heat release (NCHR), and FRT was defined as the time duration from 10% to 90% of the total NCHR [14] (Figure 2(b)). The value of the NCHR is defined by the following equation:
(6)NCHR=∫t0tRHRRdt∫t0tendRHRRdt,where RHRR is the representative heat release rate and defined by directly taking the time differentiation on pressure profile (dp_in−chamber/dt) and tend is the time at the maximum pressure.


## 3. Result and Discussion

### 3.1. In-Chamber Pressure Rise History

To analyze the laminar combustion characteristics (i.e., FDT, FRT, andSL), we first recorded the pressure-time profiles during the combustion process of multicomponent syngas at ϕ=0.6, 0.7, and 0.8, as shown in Figure 3. The pressure is almost constant after discharging within a specific time and then rapidly approaches its peak value (ppeak). Owing to the effect of the cold chamber wall, the increase in heat loss resulted in a rapid decrease in the pressure curves. The value of ppeak, together with its leftward movement, increased as the equivalence ratio was increased from 0.6 to 0.8, indicating a boost explosion with a higher heat release rate. Moreover, increasing H2/CO and decreasing CH4 in the mixtures lead to a faster combustion process with a higher value of ppeak owing to the benefits of H, O, and OH radicals [12, 23, 42]. However, these impacts weaken as the equivalence ratio increases. For instance, Figures 3(a)–3(c) show that the value of ppeak (tend) rises (shortens) by 18% (48%) with increasing H2 content from 10% to 30% and decreasing CH4 content from 60% to 40% at ϕ=0.6, whereas ppeak and tend are insignificantly changed at ϕ=0.8. This could be attributed to the higher number of C atoms, resulting in a lower effect of hydrogen on the reactivity of methane-rich mixtures, as concluded by Stylianidis et al. [5]. A similar result is found in Figures 3(d)–3(f) as the CO content increased from 10% to 30%.Figure 3
In-chamber pressure profiles for multicomponent syngas H2/CO/CH4 at three equivalence ratios ϕ=0.6, 0.7, and 0.8.


### 3.2. Flame Developing Time (FDT), Flame Rising Time (FRT), and Laminar Burning Velocity (SL)

For successful flame development, FDT and FRT effectively represent the physicochemical properties of fuel/air mixtures [10, 43–45]. Figure 4 shows the influence of the H2 and CO contents on the FDT and FRT of H2/CO/CH4/air mixtures at ϕ=0.6, 0.7, and 0.8. Each experimental condition was repeated at least three times using high ignition energy (Eig≥20mJ) to mitigate the impact of Eig on the FDT in the early flame propagation. For the H10C30M60, H20C30M50, and H30C30M40 mixtures, Figures 4(a) and 4(b) show that the value of FDT (FRT) decreases by 29% (70%), 22% (33%), and 14% (11%) when the H2 volumetric fraction increases from 10% to 30% at ϕ=0.6, 0.7, and 0.8, respectively. The leaner the mixture, the more profound the falling slope. It was also found that longer FDT and FRT were observed in the leaner mixture, probably because of the larger critical flame radius for successful flame development [46, 47] and/or slow flame propagation. Similarly, Figures 4(c) and 4(d) present the effect of CO concentration on the FDT and FRT of H2/CO/CH4/air mixtures with a 30%-H2 (i.e., H30C10M60, H30C20M50, and H30C30M40). An increase in the CO volumetric fraction could reduce the FDT and FRT; however, the falling slope was less than that in the case of increasing the H2 volumetric fraction. Especially at ϕ=0.6, increasing the volumetric fraction of CO from 10% to 20% and decreasing 10%-CH4, the values of FDT and FRT are almost unchanged, suggesting the insignificant effect of low concentrations of CO. When 30%-CO replaced the same fraction of CH4, FDT and FRT rapidly decreased, indicating faster reactivity owing to the lower CH4 content in the fuel mixtures [22, 23].Figure 4
Effect of H2 and CO volume fraction on FDT and FRT of H2/CO/CH4/air mixtures.
(a)(b)(c)(d)Figure5 shows the FDT and FRT as functions of the H2/(H2+CO) ratio using the same data depicted in Figure 4. As can be seen, the values of FDT and FRT are the shortest at the H2/(H2+CO) ratio of 0.5 for the H30C30M40 mixture; beyond this ratio, the FDT and FRT increase. When the H2/(H2+CO) ratio increases from 0.2 to 0.5, FDT and FRT quickly decrease with an increase in the H2 fraction and a decrease in the CH4 fraction for the promoted chemical reaction process by more H, O, and OH radicals. When the H2/(H2+CO) ratio was greater than 0.5, the FDT and FRT gradually increased with a decrease in the CO fraction and an increase in the CH4 fraction to reduce the reactivity of the mixtures. Furthermore, the variation in the H2/(H2+CO) ratio had an insignificant (or negligible) influence on the values of FDT and FRT in the richer mixture (i.e., ϕ=0.8).Figure 5
FDT and FRT as a function of the H2/(H2+CO) ratio using the same data presented in Figure 4.
(a)(b)Figure6 presents the SL values of H2/CO/CH4/air with different constituent fractions at ϕ=0.6, 0.7, and 0.8. The results indicate that increasing the H2 and/or CO fraction promotes the laminar burning velocity. Figure 6(a) reveals that when increasing the H2 fraction from 10 to 30% and adding 30%-CO, the values of SL increased by approximately 57, 22, and 27% at ϕ=0.6, 0.7, and 0.8, respectively. With the addition of 30%-H2 and increasing the CO fraction from 10 to 30% (see Figure 6(b)), the value of SL increased by approximately 29, 15, and 18% at ϕ=0.6, 0.7, and 0.8, respectively.Figure 6
The laminar burning velocity of multicomponent syngas (H2/CO/CH4)/air mixtures at ϕ=0.6, 0.7, and 0.8 having different constituent fractions.


### 3.3. Syngas General Correlation

The currentSL data, together with the previous data for different syngas-type fuels such as H2/CO/CH4 [29], H2/CO [28–30], H2/CO/H2O [28], H2/CO/CO2 [29], and CH4/H2 [39], were used to validate the three correlations (Eqs. (1)–(3)). Figure 7 compares the experimental SL data and the correlated results obtained using Eqs. (1) and (2). Although Ravi and Petersen [28] did not consider the effect of CH4, all scattering SL data of syngas mixtures (see Tables 1 and 2) could be fitted reasonably well onto a linear curve (y=x) with acceptable goodness of R2=0.896, as shown in Figure 7(a). Using Lapalme and Seer’s correlation [29], Figure 7(b) shows a large discrepancy between the experimental SL values and the correlated values when adding high-pressure/-temperature data (i.e., H2/CO and H2/CO/H2O at ϕ=0.6÷0.8 [28]) and H2/CH4 [39]. The linear fitting is in the form of y=0.81x, with lower goodness of fit (R2=0.76).Figure 7
Comparison ofSL data between the experiments and estimates by using (a) Ravi’s correlation Eq. (1) [28] and (b) Lapalme’s correlation Eq. (2) [29].Table 2
Range of validity of the correlation.
Base fuelϕp (bar)T (K)Volumetric fraction (%)Refs.SL,H2 at ϕ, T, p (cm/s)H2COCH4CO2H2OH2/CO0.6/0.7/0.812983070———[30]80/110/150 [26]5050———7030———H2/CO0.7/0.812982575———[29]110/150 [26]5050———7525———H2/CO/CH40.8/0.9/1/1.1/1.2/1.31298303040——[29]150/180/210/230/240/260 [26]404020——47.547.55——H2/CO0.6/0.812984545—10—[29]80/150 [26]4040—20—3535—30—3030—40—H2/CO0.6/0.753235050———[28]95/130 [28]0.6/0.7/0.810423595———105/145.3/206.6 [28]0.6/0.7/0.81037346.2546.25——7.572/100/135 [28]0.6/0.7/0.8142342.542.5——15180/230/280 [28]H2/CH40.6/0.7/0.813000—100——[39]80/110/150 [26]20—80——30—70——40—60——50—50——Figure8 compares the experimental SL to the results estimated by the new modified correlation in the form of SL,correlationϕ,T,p,λ=0.017∗exp3.68∗λH2+0.108∗exp−1.67∗λCH4∗SL,H21.164, showing good agreement with a very high goodness of fit (R2=0.97). A small discrepancy mostly occurs when the value of SL is greater than 40 cm/s, probably because of the difference in the SL determination method and the facility used in Refs. [28–30, 39]. These results indicate that a better correlation for syngas fuels must contain at least H2 and CH4 components. In addition, the good fitting also suggests a self-similar flame propagation of the syngas mixtures, regardless of ϕ, p, T, λ, and dilutions.Figure 8
Comparison ofSL data between experiments and estimates by the modified correlation (Eq. (3)), SL,correlationϕ,T,p,λ=0.017∗exp3.68∗λH2+0.108∗exp−1.67∗λCH4∗SL,H21.164.To determine how well these three correlations could be applied to estimate reliableSL values, an uncertainty analysis was performed using MAPE. Compared to the goodness of fit R2, the MAPE should be a better statistical value because R2 strongly depends on the validated data volume. Among the three correlations, the modified correlation in Eq. (3) is the best, with the lowest MAPE of 10.8%, regardless of ϕ, p, T, λ, and dilution. The other two general correlations Eqs. (1) and (2) have MAPE values of 23% and 39%, respectively.


## 3.1. In-Chamber Pressure Rise History

To analyze the laminar combustion characteristics (i.e., FDT, FRT, andSL), we first recorded the pressure-time profiles during the combustion process of multicomponent syngas at ϕ=0.6, 0.7, and 0.8, as shown in Figure 3. The pressure is almost constant after discharging within a specific time and then rapidly approaches its peak value (ppeak). Owing to the effect of the cold chamber wall, the increase in heat loss resulted in a rapid decrease in the pressure curves. The value of ppeak, together with its leftward movement, increased as the equivalence ratio was increased from 0.6 to 0.8, indicating a boost explosion with a higher heat release rate. Moreover, increasing H2/CO and decreasing CH4 in the mixtures lead to a faster combustion process with a higher value of ppeak owing to the benefits of H, O, and OH radicals [12, 23, 42]. However, these impacts weaken as the equivalence ratio increases. For instance, Figures 3(a)–3(c) show that the value of ppeak (tend) rises (shortens) by 18% (48%) with increasing H2 content from 10% to 30% and decreasing CH4 content from 60% to 40% at ϕ=0.6, whereas ppeak and tend are insignificantly changed at ϕ=0.8. This could be attributed to the higher number of C atoms, resulting in a lower effect of hydrogen on the reactivity of methane-rich mixtures, as concluded by Stylianidis et al. [5]. A similar result is found in Figures 3(d)–3(f) as the CO content increased from 10% to 30%.Figure 3
In-chamber pressure profiles for multicomponent syngas H2/CO/CH4 at three equivalence ratios ϕ=0.6, 0.7, and 0.8.


## 3.2. Flame Developing Time (FDT), Flame Rising Time (FRT), and Laminar Burning Velocity (SL)

For successful flame development, FDT and FRT effectively represent the physicochemical properties of fuel/air mixtures [10, 43–45]. Figure 4 shows the influence of the H2 and CO contents on the FDT and FRT of H2/CO/CH4/air mixtures at ϕ=0.6, 0.7, and 0.8. Each experimental condition was repeated at least three times using high ignition energy (Eig≥20mJ) to mitigate the impact of Eig on the FDT in the early flame propagation. For the H10C30M60, H20C30M50, and H30C30M40 mixtures, Figures 4(a) and 4(b) show that the value of FDT (FRT) decreases by 29% (70%), 22% (33%), and 14% (11%) when the H2 volumetric fraction increases from 10% to 30% at ϕ=0.6, 0.7, and 0.8, respectively. The leaner the mixture, the more profound the falling slope. It was also found that longer FDT and FRT were observed in the leaner mixture, probably because of the larger critical flame radius for successful flame development [46, 47] and/or slow flame propagation. Similarly, Figures 4(c) and 4(d) present the effect of CO concentration on the FDT and FRT of H2/CO/CH4/air mixtures with a 30%-H2 (i.e., H30C10M60, H30C20M50, and H30C30M40). An increase in the CO volumetric fraction could reduce the FDT and FRT; however, the falling slope was less than that in the case of increasing the H2 volumetric fraction. Especially at ϕ=0.6, increasing the volumetric fraction of CO from 10% to 20% and decreasing 10%-CH4, the values of FDT and FRT are almost unchanged, suggesting the insignificant effect of low concentrations of CO. When 30%-CO replaced the same fraction of CH4, FDT and FRT rapidly decreased, indicating faster reactivity owing to the lower CH4 content in the fuel mixtures [22, 23].Figure 4
Effect of H2 and CO volume fraction on FDT and FRT of H2/CO/CH4/air mixtures.
(a)(b)(c)(d)Figure5 shows the FDT and FRT as functions of the H2/(H2+CO) ratio using the same data depicted in Figure 4. As can be seen, the values of FDT and FRT are the shortest at the H2/(H2+CO) ratio of 0.5 for the H30C30M40 mixture; beyond this ratio, the FDT and FRT increase. When the H2/(H2+CO) ratio increases from 0.2 to 0.5, FDT and FRT quickly decrease with an increase in the H2 fraction and a decrease in the CH4 fraction for the promoted chemical reaction process by more H, O, and OH radicals. When the H2/(H2+CO) ratio was greater than 0.5, the FDT and FRT gradually increased with a decrease in the CO fraction and an increase in the CH4 fraction to reduce the reactivity of the mixtures. Furthermore, the variation in the H2/(H2+CO) ratio had an insignificant (or negligible) influence on the values of FDT and FRT in the richer mixture (i.e., ϕ=0.8).Figure 5
FDT and FRT as a function of the H2/(H2+CO) ratio using the same data presented in Figure 4.
(a)(b)Figure6 presents the SL values of H2/CO/CH4/air with different constituent fractions at ϕ=0.6, 0.7, and 0.8. The results indicate that increasing the H2 and/or CO fraction promotes the laminar burning velocity. Figure 6(a) reveals that when increasing the H2 fraction from 10 to 30% and adding 30%-CO, the values of SL increased by approximately 57, 22, and 27% at ϕ=0.6, 0.7, and 0.8, respectively. With the addition of 30%-H2 and increasing the CO fraction from 10 to 30% (see Figure 6(b)), the value of SL increased by approximately 29, 15, and 18% at ϕ=0.6, 0.7, and 0.8, respectively.Figure 6
The laminar burning velocity of multicomponent syngas (H2/CO/CH4)/air mixtures at ϕ=0.6, 0.7, and 0.8 having different constituent fractions.


## 3.3. Syngas General Correlation

The currentSL data, together with the previous data for different syngas-type fuels such as H2/CO/CH4 [29], H2/CO [28–30], H2/CO/H2O [28], H2/CO/CO2 [29], and CH4/H2 [39], were used to validate the three correlations (Eqs. (1)–(3)). Figure 7 compares the experimental SL data and the correlated results obtained using Eqs. (1) and (2). Although Ravi and Petersen [28] did not consider the effect of CH4, all scattering SL data of syngas mixtures (see Tables 1 and 2) could be fitted reasonably well onto a linear curve (y=x) with acceptable goodness of R2=0.896, as shown in Figure 7(a). Using Lapalme and Seer’s correlation [29], Figure 7(b) shows a large discrepancy between the experimental SL values and the correlated values when adding high-pressure/-temperature data (i.e., H2/CO and H2/CO/H2O at ϕ=0.6÷0.8 [28]) and H2/CH4 [39]. The linear fitting is in the form of y=0.81x, with lower goodness of fit (R2=0.76).Figure 7
Comparison ofSL data between the experiments and estimates by using (a) Ravi’s correlation Eq. (1) [28] and (b) Lapalme’s correlation Eq. (2) [29].Table 2
Range of validity of the correlation.
Base fuelϕp (bar)T (K)Volumetric fraction (%)Refs.SL,H2 at ϕ, T, p (cm/s)H2COCH4CO2H2OH2/CO0.6/0.7/0.812983070———[30]80/110/150 [26]5050———7030———H2/CO0.7/0.812982575———[29]110/150 [26]5050———7525———H2/CO/CH40.8/0.9/1/1.1/1.2/1.31298303040——[29]150/180/210/230/240/260 [26]404020——47.547.55——H2/CO0.6/0.812984545—10—[29]80/150 [26]4040—20—3535—30—3030—40—H2/CO0.6/0.753235050———[28]95/130 [28]0.6/0.7/0.810423595———105/145.3/206.6 [28]0.6/0.7/0.81037346.2546.25——7.572/100/135 [28]0.6/0.7/0.8142342.542.5——15180/230/280 [28]H2/CH40.6/0.7/0.813000—100——[39]80/110/150 [26]20—80——30—70——40—60——50—50——Figure8 compares the experimental SL to the results estimated by the new modified correlation in the form of SL,correlationϕ,T,p,λ=0.017∗exp3.68∗λH2+0.108∗exp−1.67∗λCH4∗SL,H21.164, showing good agreement with a very high goodness of fit (R2=0.97). A small discrepancy mostly occurs when the value of SL is greater than 40 cm/s, probably because of the difference in the SL determination method and the facility used in Refs. [28–30, 39]. These results indicate that a better correlation for syngas fuels must contain at least H2 and CH4 components. In addition, the good fitting also suggests a self-similar flame propagation of the syngas mixtures, regardless of ϕ, p, T, λ, and dilutions.Figure 8
Comparison ofSL data between experiments and estimates by the modified correlation (Eq. (3)), SL,correlationϕ,T,p,λ=0.017∗exp3.68∗λH2+0.108∗exp−1.67∗λCH4∗SL,H21.164.To determine how well these three correlations could be applied to estimate reliableSL values, an uncertainty analysis was performed using MAPE. Compared to the goodness of fit R2, the MAPE should be a better statistical value because R2 strongly depends on the validated data volume. Among the three correlations, the modified correlation in Eq. (3) is the best, with the lowest MAPE of 10.8%, regardless of ϕ, p, T, λ, and dilution. The other two general correlations Eqs. (1) and (2) have MAPE values of 23% and 39%, respectively.


## 4. Conclusion

In this paper, the laminar combustion characteristics of the multicomponent syngas (H2/CO/CH4)/air mixtures at ϕ=0.6, 0.7, and 0.8 and at various volumetric fractions of constituents were studied in a constant volume combustion vessel. The main results are summarized as follows:
(1)
The values of FDT and FRT decreased with increasing volumetric fractions of H2 or CO. Compared to H2, CO had less effect on FDT, FRT, and SL. Moreover, the increasing CH4 content could reduce the reactivity of multicomponent syngas mixtures by increasing the C atom in the mixture(2)
All scatteringSL data together with the previous data of various syngas fuel types (i.e., H2/CO, H2/CO/CH4, H2/CO/CO2, H2/CO/H2O, and CH4/H2) could be well represented by a modified correlation, Eq. (3), regardless of different ϕ, p, T, λ, and dilution, suggesting self-similar propagation of syngas mixtures(3)
Compared to the previous correlations, Eqs. (1) and (2), the modified correlation in Eq. (3) worked the best, owing to its low MAPE of 10.8%Beneficial information on the explosion duration characteristics is essential for designing and optimizing syngas-fueled engine systems (i.e., ignition system, advanced timing control, fuel supply, etc.). Furthermore, a better correlation could conveniently provide an accurate and reliable value ofSL at a given condition, which is used to validate the detailed/simplified mechanism and/or develop a complex turbulent combustion model.



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