Thermal and Chemical Effects of Water Addition on Laminar Burning

Apr 14, 2014 - laminar burning velocity of humid air combustion of syngas for different ... flame propagation were recorded by a high speed camera (Ph...
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Thermal and Chemical Effects of Water Addition on Laminar Burning Velocity of Syngas Yongliang Xie, Jinhua Wang,* Nan Xu, Senbin Yu, Meng Zhang, and Zuohua Huang* State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: An experimental and numerical study on thermal and chemical effects of water vapor addition on the laminar burning velocities of syngas was conducted using a constant-volume chamber and CHEMKIN package. The experimental conditions in the present study for CO/H2/air/H2O mixtures with hydrogen fraction in syngas were from 5% to 50%, initial temperature of 373 K, pressures of 0.1 and 0.5 MPa and water dilution ratios from 0% to 30%. The measured laminar burning velocity data were compared with simulations with three mechanisms, the San Diego mechanism and those of Davis et al. and Li et al. The experimental data showed a reasonable agreement with the calculated values at low and high pressures when the content of H2 in the fuel was low. However, when H2/CO ratio in the fuel was higher (75/25 and 95/05), all three mechanisms overpredicated the laminar burning velocity for fuel-rich mixtures. Sensitivity analysis was performed to identify the possible sources of discrepancy between the experimental data and calculated results. Furthermore, chemical effects of H2O on the laminar burning velocities at various CO/H2 ratios when water was added into the mixtures were studied. For syngas with CO/ H2 ratio of 50/50, water had a weak inhibiting effect on the chemical reaction of the mixtures. For the higher CO/H2 ratios, water addition accelerated the chemical reaction and this positive effect became more significant for the syngas with the increased CO/H2 ratio. Different trends were explained using the consumption path analysis.

1. INTRODUCTION IGCC (Integrated Gasification Combined Cycle) is one of the most promising technologies for the coming clean energy system. For IGCC power plant, the coal or other fuel resources are gasified and the derived gas is called syngas, with the main species being CO, H2, CO2, N2, and H2O. In addition, combustible constituents such as CH4 also exist in the syngas. The principal advantage of IGCC is ability to use diverse fuels and lower emissions. Though IGCC can reduce the pollutant emissions greatly, the NOx emission is still a challenge because NOx mainly comes from the high-temperature reaction of N2, which is the main component of the air. Several methods have been proposed to reduce the NOx emission and dilution has been proved to be an effective one. Water vapor is widely used as the diluent and the previous investigation1 on methane/air flame proved that water had the most noticeable effect on NOx emission reduction among a selection of dilution gases. Meanwhile, IGHAT (Integrated Gasification Humid Air Turbine Systems) has been intensively studied as a clean and effective energy system. HAT (Humid Air Turbine) is a novel concept turbine cycle, in which air is humidified by the residual heat of the system. The residual heat can be utilized and the IGHAT is highly efficient, environmentally friendly, and economical. The major issue of water-diluted IGCC and IGHAT is the humid air combustion of syngas, in which, syngas combustion will be even complicated by water dilution and this is much different from that of traditional hydrocarbon fuels. Though studies on IGCC2 and IGHAT3 have been carried out extensively, most of them focused on the system optimization, and few studies on fundamental flame characteristics of the humid air combustion of syngas were conducted. Laminar burning velocity is a fundamental parameter of the mixture, which is helpful for the better understanding of flame behaviors, such as extinction, flame instability, and flashback.4 © 2014 American Chemical Society

Moreover, it can also be employed to validate the chemical reaction mechanisms. The kinetics of CO and H2 are the base of hierarchical hydrocarbons combustion chemistry.5 Laminar burning velocities of syngas6−8 have been widely investigated. However, most of them focused on the CO/H2/air mixture, and few studies were conducted on the measurement of the laminar burning velocity of moist syngas. As summarized in Table 1, Singh et al.9 reported the laminar burning velocity of CO/H2/ air/H2O mixture using a spherically expanding flame configuration for a specific temperature (400 K), pressure (1 atm), and equivalence ratio. Das et al.10 used the counterflow twin-flame configuration to measure the laminar burning velocities of humid air combustion of syngas under atmospheric pressure and temperature, just for fuel lean mixtures. Using the expanding flame configuration, Krejci et al.11 experimentally measured the laminar burning velocity of humid air combustion of syngas for different moisture contents (0−15% by volume), temperatures (323−423 K), and pressures (0.1−1.0 MPa). It should be noted that a DOE (Design-of-Experiments) methodology, which reduces the experimental data greatly, was used to explore the entire range of variables. Conclusions can be drawn that the laminar burning velocity data are still scarce especially for fuelrich mixtures and high-pressure conditions. Syngas composition varies significantly according to different gasification and postprocessing conditions. This variability has a significant influence on the combustion process and is a big challenge for combustor design. When water is added into the syngas mixtures in the combustion chamber of IGCC or IGHAT, the syngas combustion process becomes more complex and Received: October 13, 2013 Revised: April 14, 2014 Published: April 14, 2014 3391

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Table 1. Sources of Experimental Data of Laminar Flame Speeds authors

P (atm)

T (K)

H2/CO

dilution ratio (%)

ϕ

measurement method

ref.

Singh et al. Das et al. Krejci et al. Xie et al. (present work)

1 1 1, 5, 10 1, 5

400 323 323, 373, 423 373

5/95 50/50 5/95 to 100/0 50/50 5/95 50/50 75/25 95/5

0−40 0−35 0−15 0−30

1.0 0.3−0.9 0.4−5.0 0.6−3.0

spherical flame counterflow spherical flame spherical flame

9 10 11

flame propagation were recorded by a high speed camera (Phantom V611) operating at 10 000 frames/s. For an outwardly spherical propagating flame, the stretched flame velocity, Sn, is calculated by13,14

needs further investigation. Thus, in this study, the laminar burning velocity data of CO/H2/air/H2O mixture over a wide range of hydrogen fractions and water dilution ratios at different pressures were reported for the development and validation of the chemical kinetic mechanism of the moist syngas. Sensitivity analysis of the laminar burning velocity is conducted to identify the possible sources of discrepancy between the experimental data and predicted ones. Moreover, the chemical effect of water addition on the syngas combustion under different CO/H2 ratios was also studied. A consumption path analysis was conducted to further identify the detailed chemical effects of water addition on the laminar burning velocity.

Sn =

dru dt

(1)

where ru (m) is the inner radius of spherical flame front. The overall stretch rate of spherical flame, α can be obtained through α=

d(ln A) 1 dA 2 dru 2 = = = Sn dt A dt ru dt ru

(2)

For moderate stretch rates, the unstretched flame propagation speed, Sb, can be obtained by the following relationship,13,15,16

2. EXPERIMENTAL AND CALCULATION METHOD

S b − Sn = L bα

Detailed description of the experimental apparatus has been given elsewhere.12 Here, a brief description is given. Compared to other experimental setups, a constant-volume combustion bomb is more convenient to obtain the laminar burning velocity at higher pressures. As shown in Figure 1, it consisted of a constant-volume combustion

(3)

where Sb is obtained as the intercept value at α = 0 in the plot of Sn against α (1/s). The negative value of the slope of Sn−α curve is defined as the burnt gas Markstein length Lb (m). Then the laminar burning velocity SL is calculated through the following continuity equation of the flame front, ρu SL = ρb Sb

(4)

where ρu (g/cm ) and ρb (g/cm ) are the densities of unburned and burned mixture assumed to be in equilibrium state, respectively. Usually, flame images with the radius between 5 mm and 25 mm were chosen to obtain laminar burning velocities by synthetically considering the effects of ignition energy and pressure change in the combustion chamber.17 Flame images with the radius less than 5 mm were discarded to avoid the possible effect from spark−ignition disturbance. Flame images with the radius of 25 mm were selected as the upper limit because the flame propagation process could be regarded as constant pressure when flame radius were lower than 25 mm. In this study, initial pressures were set as 0.1 and 0.5 MPa, temperature was 373 ± 3K considering the evaporation of water. Air was replaced by a mixture with 21% O2 and 79% N2 by volume. Purities of hydrogen, carbon monoxide, oxygen, and nitrogen were 99.995%, 99.9%, 99.995%, and 99.995%, respectively. For suppressing the intrinsic flame instability and acquiring the accurate laminar burning velocity, Helium (He) was used as the diluent to reduce thermal-diffusive instabilities at 0.5 MPa and the ratio of helium and oxygen is 7:1 to produce the similar adiabatic flame temperature with that of the flame with N2 as the diluent.11 Water dilution ratio is defined as ZH2O = (XH2O)/(XH2O + XCO + XH2). Here, X refers to mole fraction of the specific species in the mixtures. One-dimensional freely propagating plane flame was simulated with PREMIX code18 combined with CHEMKIN-II19 and the chemical mechanism. The mechanism of Davis et al.20 involved 6 species and 38 elemental reactions and was developed based on optimization of the CO−H2 kinetic rate constants and available combustion data. The mechanism of Li et al.21 was built on the work of Mueller et al.22 and had 21 species and 84 reactions. It has been confirmed against a wide range of experimental conditions. The latest San Diego Mechanism (version 20120907)23 had 50 species and 244 reactions. A 21-step chemical scheme for hydrogen combustion24 was developed by the research group around F. A. Williams, and it is being updated all the time. For the present calculation, the multicomponent transport model was used to evaluate the transport properties and the Soret effect was also included in the calculation. Both GRAD and CURV values were set at 0.04 to ensure the grid independent solution. 3

Figure 1. Experimental apparatus. chamber, heating system, ignition electrodes, and high-speed Schlieren photography and data acquisition system. The combustion chamber was cylindrical with inner length of 210 mm and inner diameter of 180 mm, and it contained two quartz windows with diameter of 80 mm. The windows were separately located on the two sides of the vessel for the optical access. In experiment, the combustion chamber was heated with the heating tape wrapped around the chamber, and the temperature was measured and monitored by a thermocouple with an uncertainty of ±3 K. The centrally located electrodes are used to ignite the combustible mixture. The gases were introduced into the chamber in sequence according to their partial pressures. Ten minutes were needed before ignition to ensure the complete mixing and attainment of quiescent condition. Then, the mixture was ignited and schlieren images of the 3392

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ϕ = 1.85 and peak laminar burning velocity of CO with small amount of H2 occurs at ϕ = 2.630 Comparison of experimental and calculated results shows that all three mechanisms underestimate the laminar burning velocity at equivalence ratios less than 2.0 but overestimate the laminar burning velocity at equivalence ratios larger than 2.0. Predications of the mechanism of Li et al. are higher than those of the mechanism of Davis et al. and the San Diego Mechanism. Thus, the mechanism of Li et al. is the best in predicting the laminar burning velocity under conditions at ϕ 2. This conclusion is consistent with that of Das et al. that the mechanism of Li et al. is better in predicting the laminar burning velocity at fuel-lean conditions.10 The discrepancy between prediction and experiment at equivalence ratios larger than 2.0 increases with the decrease of H2 fraction in the mixture. Mechanism must be revised in order to predict better the laminar burning velocity at fuel-rich and high H2-content conditions. Figure 4 shows the influence of H2O on laminar burning velocities of the mixture for the 50/50 CO/H2 case at pressures

3. RESULTS AND DISCUSSION 3.1. Experimental System Validation. The accuracy of the present experimental apparatus has been tested in previous studies.25,26 Here, the experimental system was validated again to guarantee its accuracy for syngas combustion. Figure 2 gives the comparison between experimental results and the data in the previous studies. It is found that the present

Figure 2. Comparison of laminar burning velocities of syngas of present study and previous researches (CO/H2 = 50/50, T = 298 ± 3 K, P = 0.1 MPa, ZH2O = 0%).

results show good agreement with the data of Sun.27 As for the data of Bouvet28 and Parthap,29 the present results show a little discrepancy with them. The comparison between the present results and previous data sets shows reasonable agreement and this indicates that the apparatus and methodology are applicable to the measurement of the laminar burning velocity for syngas mixture. 3.2. Laminar Burning Velocities of CO/H2/air/H2O Mixtures. Figure 3 indicates the experimentally measured

Figure 4. Laminar burning velocities of CO/H2/air/H2O mixture at various pressures: (a) 0.1 MPa; (b) 0.5 MPa (CO/H2 = 50/50, T = 373 ± 3 K).

Figure 3. Laminar burning velocities of CO/H2/air/H2O mixtures at various CO/H2 ratios (T = 373 ± 3 K, P = 0.1 MPa, ZH2O = 20%).

laminar burning velocities of CO/H2/air/H2O mixtures at different CO/H2 ratios. The calculated data with chemical mechanisms of Davis et al.20 and Li et al.21 and the San Diego Mechanism23 are also plotted for comparison. Laminar burning velocities increase with the increase of the H2 fraction in the fuel for the tested equivalence ratios (ϕ = 0.6−3.0). Laminar burning velocities reach their peak values at ϕ = 2.0. It is worthy to note that laminar burning velocity of pure H2 reaches its maximum at

of 0.1 and 0.5 MPa. There was a little difference between calculated and experimental results for mixture with CO/H2 ratio of 50/50 at lower and higher pressures. Laminar burning velocities decrease with the increase of water dilution ratio under all equivalence ratios. In addition, the equivalence ratio of peak burning velocity shifts toward smaller values with the increase of water dilution ratio. This shifting of peak velocity position will be analyzed qualitatively in the following discussion. 3393

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According to the classical laminar flame theory31 (SL ∝ (αRR)1/2), laminar burning velocity is proportional to the thermal diffusivity,α, and the reaction rate, RR, which has a positive correlation with the adiabatic flame temperature. Figure 5 gives the thermal diffusivity (calculated using STANJAN32)

Figure 5. Thermal diffusivity and adiabatic flame temperature at various water dilution ratios (CO/H2 = 50/50, T = 373 ± 3 K, P = 0.1 MPa).

and the adiabatic flame temperature (calculated using EQUIL33) for the mixture with CO/H2 ratio of 50/50. Thermochemical and transport properties were primarily provided by the CHEMKIN thermodynamic database.34 The adiabatic flame temperature always reaches its peak value at around ϕ = 1 regardless of water addition. However, thermal diffusivity monotonically increases with equivalence ratio from ϕ = 0.6 to ϕ = 3.0. This indicates that the peak value of laminar burning velocity will not occur at around ϕ = 1 but at ϕ > 1. With the increase of water dilution ratio, thermal diffusivity increases more modestly with equivalence ratio and this leads to the light shift of peak laminar burning velocity. Moreover, both adiabatic flame temperature and thermal diffusivity decrease with water addition at all equivalence ratios. This leads to the decrease of laminar burning velocity as water is added. Discrepancies between experimental and calculated results may be due to the inaccuracy of reaction rates of certain reactions. Motivated by this, logarithmic reaction rate sensitivity coefficients of the laminar mass burning flux, ∂(In m0)/∂(In Ai) are calculated using the PREMIX code to highlight the most important chemical reactions that affect laminar burning velocities of moist syngas mixtures with various H2/CO ratios. It should be noted that the laminar mass burning flux sensitivity coefficient is directly related to the laminar burning velocity sensitivity coefficient.12 In consideration that the mechanism of Davis et al. is better in predicting the laminar burning velocity of moist syngas mixture at fuel-rich condition, it was used for all simulations described henceforth. These results of sensitivity analyses calculated with the mechanism of Davis is presented in Figure 6. Generally, the chain-branching reaction of H + O2 = OH + O and reaction CO + OH = CO2 + H are the two most important reactions for the moist syngas mixture. With the increase of H2/CO ratio in the mixture, the reaction H + O2 = OH + O becomes insignificant and the reaction CO + OH = CO2 + H becomes more and more important. The reaction CO + OH = CO2 + H is of critical importance in the combustion of syngas mixtures and moist CO mixtures.35 It is the main pathway for CO conversion to CO2 and also responsible for a major fraction of heat release. Sensitivity analysis36 shows the reaction CO + OH = CO2 + H has little effect on predictions of ignition delay times. However, it strongly affects the calculations of laminar burning

Figure 6. Logarithmic sensitivity coefficients to laminar burning velocity: (a) P = 0.1 MPa; (b) P = 0.5 MPa.

velocities. The rate constant has been widely studied, and Das et al.10 concluded that there was less room for further rate constant refinement. In fact, there are still different rate constants of reaction CO + OH = CO2 + H in various mechanisms. In the mechanism of Davis et al.,20 the rate constant for CO + OH = CO2 + H was reanalyzed and refitted by the sum of two modified Arrhenius expressions. In the mechanism of Li et al.,21 the method of weighted least-squares was used to fit the available experimental data and obtain the modified Arrhenius expression. As for the San Diego Mechanism, the rate constant of reaction CO + OH = CO2 + H employed an expression taken from previous research.37 The chain-branching reaction of H + O2 = OH + O is particularly important for high-temperature oxidation reactions of hydrocarbon fuels and has also been studied extensively. The rate coefficient in the Davis mechanism is adopted from the mechanism of GRI.38 In the mechanism of Li et al, the rate coefficient of H + O2 = OH + O is calculated by Hessler.39 In the San Diego Mechanism, the value reported by Masten et al.40 was employed. It is worth mentioning that Hong et al.41 recently used tunable diode laser absorption behind reflected shock waves to determine the rate coefficient of the reaction H + O2 = OH + O over the temperature range 1100− 1530 K. Results show excellent agreement with that of Masten et al.,40 and the new rate coefficient expression was proposed combining those experimental results. Except for those two important elementary reactions, the reaction OH + H2 = H + 3394

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different trends at different CO/H2 ratios. Specifically, laminar burning velocities decrease with the increase of water dilution ratio for the syngas with CO/H2 ratio of 50/50 and 75/25. However, the effect of water addition on laminar burning velocity presents a nonmonotonic behavior for the 95/5 CO/H2 case. Laminar burning velocity increases with the increase of water dilution ratio until it reaches its maximum and then decreases. Generally, the influence of H2O on the laminar burning velocity can be accounted for through the following mechanisms:50 chemical effect, thermal, transport effect, and radiation effect. The chemical effect will be examined in the present paper. A fictitious H2O species,50,51 FH2O, is introduced to study the chemical effect of water addition. It has the same thermal and transport properties with H2O but does not directly participate in any chemical reaction. It acts as a third body and is assigned the identical third-body collision efficiency as H2O. If discrepancies exist with H2O and FH2O, this implies that H2O directly participates in chemical reaction. In other words, H2O has a chemical effect on the syngas flame. Moreover, the other fictitious H2O that never directly participates in any chemical reaction nor acts as a third body is also introduced for the investigation of the effect of third-body reaction. As can be observed in Figure 7, the chemical effect of H2O presents different trends at different CO/H2 ratios. For the CO/H2 50/50 case, the calculated laminar burning velocities with FH2O are slightly higher than those of H2O. This indicates that H2O has a weak inhibiting effect on the chemical reaction of syngas. When the CO/H2 ratio in the fuel is greater, an opposite behavior appears. The existence of H2O accelerates the chemical reaction and this positive effect becomes more obvious for the syngas mixture with higher CO/H2 ratio. In addition, the nonmonotonic behavior of water addition on laminar burning velocity with CO/H2 ratio of 95/5 can be attributed to the competitiveness between the chemical effect and thermal effect. Specifically, the positive chemical effect is higher than the negative thermal effect up to ZH2O that corresponds to peak laminar burning velocity. When three-body reactions are not taken into consideration, laminar burning velocities are apparently higher than those with three-body reactions, and the discrepancy increases with increasing water dilution ratio. This reflects that the three-body reactions of water have a significant inhibiting effect on the chemical reaction of syngas. Further analysis of chemical effect of H2O is provided in the following paragraph. Figure 8 gives the integral reaction consumption pathway for the mixture at different dilution and CO/H2 ratios. Rate of production (ROP) of different species is computed with CHEMKIN-II19 and the final consumption of a specific species from the individual reactions is calculated by the spatial integration of the ROP of the elementary reaction within the whole reaction zone. Consumption of a specific species from the individual reaction is calculated by the spatial integration of ROP of the elementary reaction within the whole reaction zone.10,52 Sensitivity analysis in the previous work9,53 has shown that the most important elementary reactions that affect the laminar burning velocity are all involved in free radicals. In addition, the laminar burning velocity and concentrations of H and OH radicals in the reaction zone have a strong positive correlation.9,54 Thus, we can identify the effect of H2O on the laminar burning velocity by studying the change of the important radicals. For the mixture with CO/H2 ratio of 95/5, consumption percentage of H2 due to chain branching reaction H2 + O = H + OH changes from 73% to near 100% as water is added. Generally speaking,

H2O and the chain-propagation reaction H + O2 (+M) = HO2 + (M) are also very important for the combustion of the moist syngas. The reaction OH + H2 = H + H2O is a very important chain-propagating reaction for the consumption of H2 and heat release rate. As noted in Figure 6, this reaction has a totally different effect on the prediction of the laminar burning velocity at various CO/H2 ratios. For the CO/H2 50/50 case, this reaction has a large and positive sensitivity coefficient. With the increase of CO/H2 ratio, this sensitivity coefficient gradually becomes a negative value. The phenomenon was also observed by Das et al.10 and Singh et al.,9 and it was concluded that this critical reaction has a great influence on the prediction of the laminar burning velocity at various CO/H2 ratios. Li et al.21 adopted the rate constant measured by Michael et al.42 in their C1 mechanism. Davis et al.20 chose the rate constant of reaction OH + H2 = H + H2O in GRI 3.0 and reduced it slightly. In the mechanism of San Diego, the rate constant previously adopted by Smooke43 was employed. This reaction has been widely studied but there are comparatively larger error limits for its reaction rate constant10.44 Hence, Lam et al.45 recently used UV laser absorption of OH radicals behind reflected shock waves to study the rate constant for the reaction of OH + H2 = H + H2O over a temperature range 902−1518 K at pressures of 1.15−1.52 and estimated the overall uncertainties to be ±17% at 972 and 1228 K. As shown in Figure 6, the chain propagation reaction H + O2 (+M) = HO2 + (M) is pressure-dependent and becomes more important at elevated pressures. This reaction has also been studied extensively and many rate coefficients were proposed. Davis et al.20 chose the rate constant of reaction H + O2 (+M) = HO2 + (M) in GRI 3.0 and increase it by about 10%. The highpressure limiting constant measured by Cobos et al.46 was used in the mechanism of Li et al. Besides, Li et al. employed lowpressure limit rate constant of Cribb et al.47 for M = N2 and that of Michael et al.48 for M = Ar or He. As for the San Diego Mechanism, the high- and low-pressure limiting rate constants proposed by Troe49 were employed. Rate constants of those important elementary reactions can help to predicate the laminar burning velocities accurately. 3.3. Effect of Water Addition on Syngas Combustion. Figure 7 shows the measured and calculated laminar burning velocities at different CO/H2 and water dilution ratios. It is observed that the effect of water addition on the syngas presents

Figure 7. Laminar burning velocities of CO/H2/air/H2O mixtures at various CO/H2 and water dilution ratios, M refers to the third body (T = 373 ± 3 K, P = 0.1 MPa, ϕ = 1.6). 3395

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Figure 8. Integrated species consumption paths of CO/H2/air/H2O mixtures at various CO/H2 and water dilution ratios: (a) CO/H2 = 50/50, ZH2O = 0%; (b) CO/H2 = 50/50, ZH2O = 30%; (c) CO/H2 = 50/50, ZFH2O = 30%; (d) CO/H2 = 95/5, ZH2O = 0%; (e) CO/H2 = 95/5, ZH2O = 30%); (f) CO/H2 = 95/5, ZFH2O = 30% (T = 373 ± 3 K, P = 0.1 MPa, ϕ = 1.6).

+ O = OH decreases from 19% to 10%. This indicates the increased production of OH and decreased consumption of H since the reaction H + O = OH consumes the H radical. The consumption percentage of CO via the reaction CO + OH = CO2 + H increases from 82% to 91%, and this increases the production of H radicals. The conclusion agrees with that in Das et al.10 that H and OH radicals increase with water addition. When the chemical effect of H2O is not considered, the

consumption of H2 is mainly through two reactions: H2 + O = H + OH and H2 + OH = H + H2O. Thus, the production of H radical is increased because the reaction H2 + OH = H + H2O consumes the OH radical. Moreover, the consumption pathway of O radical is also noteworthy. With water addition, consumption percentage of O through reaction of H2O + O = OH + OH increases remarkably from 10% to 34% due to the high fraction of H2O in the mixture and that through the reaction of H 3396

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consumption percentage of H2 through H2 + O = H + OH decreases from 100% to 69%. This will decrease the radical pool concentration of H and OH, as discussed above. In addition, the consumption percentage of CO through CO + OH = CO2 + H reduces from 91% to 85%. This indicates more CO is converted into CO2 through CO + O = CO2. It will reduce the consumption of OH radical. In general, the radical pool concentration of H + OH decreases greatly when H2O does not directly participate in the reaction. When the CO/H2 ratio changes from 95/5 to 50/ 50, a different phenomenon is demonstrated. Production of OH radical is increased through the reaction H2O + O = OH + OH with water dilution. However, this does not lead to the ultimate increase of OH radical pool. With water addition, the consumption percentage of H2 via reactions H2 + O = H + OH and H2 + OH = H + H2O decreases by 4% and increases by 4%, respectively. This demonstrates that the production of OH decreases with water addition. The consumption percentage that varies greatly with water addition is the reaction of H + O2 = HO2. It changes from 25% to 30%, showing increased consumption of H radical. In general, the comprehensive effect of water addition on different elementary reactions leads to the reduced concentrations of the H and OH radicals. In addition, the elementary reaction H + O2 = O + OH, via which the percentage consumption of H decreases from 33% to 28%, is noteworthy, although the value of H + OH will not change. When the chemical effect of H2O is not considered, the elementary reaction which changes most is H2O + O = OH + OH. It changes from 12% to 2% when H2O does not directly participate in the reaction. This will decrease the concentration of OH radical. On the whole, the concentration of H + OH is almost unchanged, since other important elementary reactions are not changed much.

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +86 29 82668789. E-mail: [email protected]. *Fax: +86 29 82668789. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This study is supported by National Natural Science Foundation of China (No. 51376004, 51006080) and the National Basic Research Program (2013CB228406). Jinhua Wang acknowledges the Japan Society for the Promotion of Science for a JSPS Postdoctoral Fellowship grant.

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4. CONCLUSIONS Laminar burning velocities of CO/H2/air/H2O mixture at different CO/H2 ratios, water dilution ratios, and pressures were measured using the outwardly propagating spherical flame method. Effects of water addition on humid air combustion of syngas were analyzed. The main conclusions are summarized as follows: (1) The mechanism of Li et al. is closest in predicting the laminar burning velocity at ϕ < 1.6 and the mechanism of Davis et al. and the San Diego Mechanism are better at ϕ > 2.0. This conclusion is consistent with that of Das et al. (2) Reaction mechanisms can well predict the laminar burning velocity with H2/CO in the ratio of 50/50 at low and high pressures. When the H2 content in the fuel increases, all three mechanisms overestimate the laminar burning velocity at ϕ > 2.0. Mechanisms are needed in order to revise in order to predicate well the laminar burning velocity at fuel-rich and high H2-content condition. (3) The chemical effect of water at different CO/H2 ratios is presented. At low CO/H2 ratio, H2O has a weak inhibiting effect on the chemical reaction of CO/H2/air/H2O mixtures. With the increase of CO/H2 ratio, the existence of H2O accelerates the chemical reaction gradually and this positive effect becomes more pronounced at higher CO/ H2 ratios. Chemical kinetic analysis shows that elementary reactions H2 + O2 = OH + H and CO + OH = CO2 + H are the key positive reactions. The three-body reactions have a high inhibiting effect on laminar burning velocity. 3397

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