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Auto-ignition of biomass synthesis gas in shock tube at elevated temperature and pressure
Sci. Bull. (2015) 60(22):1935–1946 DOI 10.1007/s11434-015-0935-4
www.scibull.com www.springer.com/scp
Article
Engineering Sciences
Auto-ignition of biomass synthesis gas in shock tube at elevated temperature and pressure Linqi Ouyang • Hua Li • Shuzhou Sun Xiaole Wang • Xingcai Lu
•
Received: 21 August 2015 / Accepted: 15 October 2015 / Published online: 16 November 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract Ignition delay times of multi-component biomass synthesis gas (bio-syngas) diluted in argon were measured in a shock tube at elevated pressure (5, 10 and 15 bar, 1 bar = 105 Pa), wide temperature ranges (1,100–1,700 K) and various equivalence ratios (0.5, 1.0, 2.0). Additionally, the effects of the variations of main constituents (H2:CO = 0.125–8) on ignition delays were investigated. The experimental results indicated that the ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa. The ignition delays were also found to rise as CO concentration increases, which is in good agreement with the literature. In addition, the ignition delays of bio-syngas were found increasing as the equivalence ratio rises. This behavior was primarily discussed in present work. Experimental results were also compared with numerical predictions of multiple chemical kinetic mechanisms and Li’s mechanism was found having the best accuracy. The logarithmic ignition delays were found nonlinearly decrease with the H2 concentration under various conditions, and the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable. However, the effect of pressure is relatively smaller under current conditions. Sensitivity analysis and reaction pathway analysis of methane showed that R1 (H ? O2 = O ? OH) is the most sensitive reaction promoting ignition and R13 (H ? O2 (?M) = HO2 (?M)), R53
Electronic supplementary material The online version of this article (doi:10.1007/s11434-015-0935-4) contains supplementary material, which is available to authorized users. L. Ouyang H. Li S. Sun X. Wang X. Lu (&) Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China e-mail: [email protected]
(CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) as well as R56 (CH4 ? OH = CH3 ? H2O) are key reactions prohibiting ignition under current experimental conditions. Among them, R53 (CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) have the largest positive sensitivities and the high contribution rate in rich mixture. The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases sharply as the mixture turns rich. Therefore, the ignition delays become longer as the equivalence ratio increases. Keywords Shock tube Biomass synthesis gas Ignition delay time Sensitivity analysis Reaction pathway analysis
1 Introduction Biomass synthesis gas with carbon-neutral property has been paid extensive attention because of the severe pollution in some countries at present. Biomass synthesis gas can be utilized in various energy power devices— it not only can be used in biomass gasification power generation and combined cooling, heating and power (CCHP), but also used as clean alternative fuels in internal combustion engines, fuel cells and boilers [1, 2]. It can be promising that biomass synthesis gas will occupy a critical position in the future energy fields. Generally, the feedstocks which can be utilized to produce synthesis gas include natural gas, coal and biomass (such as feces of animals, straw and other organic wastes) [3, 4]. In China, due to the emission caused by the burning of crop straw [5], it is absolutely a promising method to make use of straw by turning them into synthesis gas. The synthesis gas derived from biomass is called biomass-derived syngas or bio-syngas. Recently, the bio-syngas has aroused great
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interest among researchers because of the diversity and availability of feedstocks [6–12]. Practically, the main components of bio-syngas are CO and H2. Other components (including methane, nitrogen, carbon dioxide, water [9, 12]) differ a lot due to the diversity of biomass feedstocks and production procedures of bio-syngas. The combustion characteristics will alter since the components in bio-syngas vary. Therefore, the power output and emission of internal combustion engines and gas turbines will be affected, which could be adverse to the design of engines and control strategies [13]. No matter if bio-syngas is used in internal combustion engines or in gas turbines, the ignition properties and flame properties of bio-syngas could have great effects on the efficiency and emission of the whole combustion system. Mittal et al. [14] studied the ignition delay times of 80 % CO ? 20 % H2 mixtures in rapid compression machine (RCM) at temperature of 950–1,050 K and pressure of 15, 30 and 50 bar. Gersen et al. [15] measured the ignition delay times of stoichiometric and lean H2, H2 ? CO, CH4, CH4 ? CO, CH4 ? H2 and CH4 ? CO ? H2 mixtures at pressures ranging from 20 to 80 bar and in the temperature range 900–1,100 K in a RCM. The effects of the content of CO (\50 %) and CH4 on the ignition delays of hydrogen were investigated. Walton et al. [16] also investigated the ignition of simulated syngas mixtures in a rapid compression facility at spanned pressures ranging from 7.1 to 26.4 atm, temperatures from 855 to 1,051 K, equivalence ratios from 0.1 to 1.0, oxygen content from 15 % to 20 % and H2:CO from 0.25 to 4.0. Mansfield and Wooldridge [17] investigated the effects of chemical impurities (CH4, TMS) on the combustion of syngas in a rapid compression facility. Twostep ignition behavior was found at higher pressures, and the ignition delays were affected differently by the addition of impurities. Thi et al. [18] measured the ignition delay times of syngas in a shock tube at temperature ranging from 757 to 1,280 K, equivalence ratios ranging from 0.3 to 1.0 and pressure of 2 and 10 bar. Sivaramakrishnan et al. [19] measured ignition delay times of dilute CO/H2 mixtures in a high-pressure shock tube. They also analyzed variation of some stable species (CO, O2, CO2) using GC and GC/MS. Petersen et al. [20] studied the ignition delays of CO/H2/CO2 mixtures using shock tube and flow reactor. Results were compared with previous work of Peschke and Spadaccini [21] indicating that their variations of ignition delay times over time agreed well, but there are deviations between experimental results and predictions. Mathieu et al. [22, 23] studied the ignition delay times of syngas derived from coal and biomass in a shock tube. The effects of other components (CH4, CO2, H2O, NH3, H2S) other than CO and H2 on the ignition delays were investigated. In terms of the combustion flame characteristics of syngas, Wang et al. [24] measured the transient front of
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turbulent premixed flame of biomass syngas (CO/H2/CO2/ air) by using PLIF. Prathatp et al. [25] studied the effect of dilution with CO2 on the laminar burning velocity and flame stability of syngas (50 % hydrogen ? 50 % CO) in a constant-pressure spherical diffusion flame. The results showed that peak burning velocity shifts because of dilution with CO2. CO2 shows a stronger inhibiting effect on laminar burning velocity than that of nitrogen possibly due to the participation of CO2 in the chemical reactions. As for the chemical effects of CO2, Liu et al. [26] numerically studied the chemical effects of CO2 replacement of N2 in air on the burning velocity of CH4/O2/N2/CO2 and H2/O2/ N2/CO2 mixtures. Bouvet et al. [27] particularly measured the spherical flame velocity of syngas at spanned H2/CO ratios ranging from 5 %/95 % to 50 %/50 % and equivalence ratios ranging from 0.4 to 5.0. Natarajan et al. [28] investigated the burning velocity of H2/CO/CO2 syngas mixtures using conical Bunsen flame and one-dimensional stagnation flame at a wide range of fuel fractions (fraction of H2 ? CO spanning from 5 % to 95 %, fraction of CO2 is less than 40 %). Dong et al. [29] experimentally measured the laminar flame velocity of H2/CO mixtures (volume fraction of H2 ranging from 0 to 100 %) in a Bunsen burner. Goswami et al. [30] measured the laminar flame speed of H2/CO and H2/N2 mixtures in O2/He. Krejci et al. [31] studied the laminar flame speed of the neat H2 and CO/H2 mixtures. Hydrogen, as a reactive component in syngas, was also experimentally and numerically studied with air in a closed combustion vessel [32]. Tinaut et al. [33] studied the combustion velocity of biomass producer gas (CO/CO2/CH4/H2/N2) with various gas concentrations in a constant-volume combustion bomb. Multitudes of scholars have developed the chemical kinetics of CO/H2 mixtures due to the importance of chemical kinetic characteristics of bio-syngas [34–37]. In addition, Wang et al. [38] developed a detailed chemical kinetic mechanism of mixtures containing CO, H2 and other species. Dryer and Chaos [39] investigated the homogeneous gas-phase combustion kinetics of syngas at low temperatures and high pressures (T \ 1,000 K, 10 \ P \ 30 atm). Boivin et al. [40] developed a four-step reduced chemical kinetic model based on CO ? H2 biosyngas, specifically for combustion in gas turbine. Petersen et al. [20] performed an investigation on the ignition of syngas at real conditions, and the predictions of previous chemical kinetics were found to be deviating sharply from results at real conditions. The major reason is that the main components in syngas are CO, H2, CO2 and other gases; however, the widely studied H2/CO mechanisms lack investigation at high pressure and low temperature. Ke´romne`s et al. [41] measured the ignition delay times of H2/ CO/O2/N2/Ar mixture in shock tubes and two rapid compression machines at pressures ranging from 1 to 70 bar,
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temperature from 914 to 2,220 K and equivalence ratio from 0.1 to 4, and a detailed chemical kinetic mechanism was established. Experimental results indicated the ignition delays are strongly dependent on the compression pressures and temperatures and will decrease with increasing pressure, temperature and equivalence ratio. The whole reactivity of syngas is dominated by the chemistry of hydrogen (CO lower than 50 %). However, the inhibiting effect of CO on ignition becomes obvious when the fuel fraction of CO is higher than 50 %. As for bio-syngas or marsh gas, their major constituents, like CH4, CO and H2, have very high octane numbers and also a high proportion of CO2 or N2. The octane number of marsh gas is about 130 while that of bio-syngas is around 100–105, and the auto-ignition temperatures for both are higher than 898 K. Consequently, it is difficult for them to be ignited by compression. Therefore, spark-ignition combustion is preferable. Tsiakmakis et al. [42] coupled a singlecylinder spark-ignition engine to fluidized bed in a portable cogeneration cycle unit and evaluated bio-syngas derived from three kinds of feedstocks blending with propane in the engine. The heat release rate and cylinder pressure in the engine were analyzed. The power output loss was found owing to the lower heat value. In addition, the in-cylinder peak pressure and heat release rate decreased and the combustion stability was also affected. Arroyo et al. [43] studied two kinds of bio-syngas in a spark-ignition engine, and the results were compared with the performances of engines fueled with gasoline, methane and biogas. The investigation indicated that the increasing content of hydrogen in syngas would increase the cylinder pressure. Although there are other diluents in syngas, high engine speed and lean conditions lead to higher efficiency than those obtained with gasoline. The burning velocity and heat release rates were strongly affected by the compositions of diluents and the fraction of hydrogen. CO and CO2 in syngas would increase some pollutants in exhaust gas with decreasing HC emission. NOx probably would increase as the fraction of hydrogen in syngas increases, but it was mainly influenced by CO2 and equivalence ratio. Papagiannakis and Zannis [44] studied the wood gas in a heavy-duty spark-ignition engine. Previous work indicated the critical issues of burning wood gas lie in its lower thermal efficiency than natural gas and higher NO and CO emissions. Chen et al. [45] investigated the combustion characteristics of gasoline engine fueled with biosyngas (H2–CO) diluted by CO2. BMEP was discovered to decrease, and the cyclical variation of IMEP was less than 10 % as the dilution rate of CO2 rose. However, the effects of CO2 were in contrast with those of hydrogen. The CO2 addition would substantially decrease NOx emissions while the thermal efficiency would not be affected. Although there are many researches about syngas currently, the major investigations were just based on
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CO ? H2 binary fuels. Additionally, the effects of equivalence ratio on the ignition delay of multi-component biosyngas were less experimentally studied by scholars before, and few researches were conducted on multi-component bio-syngas from the aspect of chemical kinetics. Therefore, in the present work, the effects of operation conditions, including pressure, temperature as well as equivalence ratio and the variation of H2 and CO compositions as well as methane on the ignition delays of multi-component biosyngas, were investigated. To further interpret experimental results in terms of chemical kinetics, multiple chemical kinetic mechanisms are utilized to perform simulations and sensitivity analyses, and reaction pathway analyses are fulfilled. Generally, the major compositions of bio-syngas include H2, CO, CO2, N2, CH4, H2O and other impurities. Among them, the fractions of CO2 ? N2 ? H2O can be as high as over 50 %. As mentioned earlier, the compositions and gas concentrations in bio-syngas could be different because of the raw materials and production process. In present work, the mixtures of H2, CO, CH4 and N2 are adopted as the simulated multi-component bio-syngas. The volume fraction ratio between H2 ? CO ? CH4 and N2 is around 50:50. The volume fraction of H2 and CO ranges from 5 % to 40 %. The volume fraction of CH4 is about 5 %. Large amounts of argon are added in the mixtures to avoid non-ideal combustion phenomena in shock tube experiment [20].The ignition delay times of multi-component biomass synthesis gas diluted in 90 %–95 % Ar (mol.%) were measured by using reflected shock wave at elevated pressure (P = 5, 10, 15 bar), elevated temperature (T = 1,100–1,700 K) and various equivalence ratios (u = 0.5, 1.0, 2.0).
2 Experimental system A single-diaphragm stainless steel shock tube is used to measure the ignition delay times behind reflected shock wave. Previous studies [46, 47] have given a detailed introduction of the apparatus and validated the reliability and repeatability of it. The apparatus is primarily composed by a 6-m-long driving section (inner diameter of 90 mm), a 5-m-long driven section (inner diameter of 90 mm) and a diaphragm section (the material of diaphragms is polyethylene terephthalate). There are five pressure transducers (interval between each sensor is 333 mm, all types are PCB 113B26) and an OH* sensor at the end of the driven section. The pressure transducers are employed to record the arrival time of shock waves, and the last one (20 mm distant from the end wall) is to record the arrival time of reflected shock wave. The OH* signal is detected by a PMT (HamamatsuR928), which is installed at 20 mm distance from the end wall. All signals are recorded
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and saved by an oscilloscope. The definition of ignition delay time in present experiment is the time between the arrival of reflected shock wave at the last pressure transducer and the intersection of lines drawn along the steepest slope of OH* emission profile and the OH* signal baseline. It is shown in Fig. 1. The temperatures and pressures behind reflected shock wave are calculated according to shock wave equation. As the main uncertainty of ignition delays in the experiment is the determination of temperature behind reflected shock wave, it is crucial to minimize temperature errors. The total uncertainty of ignition delays in present study is less than 10 % at lower pressure (5, 10 bar) and 20 % at higher pressure (15 bar). Mixtures are prepared manometrically in a stainless steel tank with a volume of 210 L. The mixtures are allowed to stand about 2 h for ample premix before use. Details of mixtures’ compositions are summarized in Table 1. The purity of all gases in test mixtures is: 99.95 % for CO, 99.99 % for CH4 and H2, 99.999 % for N2, O2 and Ar. In view of the fact that CO may suffer possible contamination by Fe, thus forming FeCO5, which has inhibiting effect on combustion of CO, the vessel containing CO is made of aluminum alloy to minimize the possible formation of FeCO5. All sections of shock tube are evacuated to ultra-low vacuum (0.03 Pa) by vacuum pumps before each experiment followed by introduction of test mixtures into the driven section. High purity of helium (99.999 %) is introduced into driver section to rupture the diaphragm then produce shock waves. Under the compression of shock waves, the mixtures will be ignited. The whole process is recorded by the pressure transducers and an OH radical sensor. The experimental data are available in the supplemental material.
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3 Ignition delay times of bio-syngas 3.1 Experimental results The effects of major constituents on ignition delay times were primarily investigated in the present work. The ignition delay times of five test mixtures at equivalent ratio of 0.5 and pressure of 10 bar are compared in Fig. 2. It is shown that increasing fraction of CO (the fraction of hydrogen decreases) will gradually lengthen the ignition delay times of mixtures. It indicates that CO could inhibit the ignition process, which is in accordance with previous studies performed by Ke´romne`s et al. [41] and Walton et al. [16]. Additionally, the inhibition effects are negligible when the CO concentrations are low. Explanations from Ke´romne`s et al. [41] are that the reactivity of H2 ? CO mixtures is mainly dominated by the chemistry characteristics of hydrogen when the fraction of carbon monoxide in mixtures is low. As the CO concentrations rise, the chemistry characteristic of CO outweighs that of hydrogen leading to the extension of ignition delay times. The ignition delay times of syngas3 were measured at different pressure (5, 10, 15 bar) and stoichiometric equivalence ratio, as shown in Fig. 3. The ignition delay times increase as temperature decreases. Additionally, due to the competition between the reaction H ? O2 = OH ? O and the pressure-dependence reaction H ? O2 (?M) = HO2 (?M), the ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa. The effects of pressure on the reactivity of syngas3 are similar to that of pure hydrogen oxidation in Ref. [41]. Figure 4 shows the ignition delay times of syngas3 at pressure of 15 bar and equivalence ratios of 0.5, 1.0 and 2.0. It is very interesting to notice that the ignition delay times of syngas3 go up when the equivalence ratio increases. This behavior will be primarily discussed in Sect. 4. 3.2 Comparisons between experimental results and kinetic simulation
Fig. 1 Definition of ignition delay time
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As mentioned above, there are multiple chemical kinetic mechanisms developed for the ignition and combustion of syngas all over the world. Four representatively detailed mechanisms were chosen here, including C1 mechanism from Li et al. [35], C1–C2 mechanisms from Metcalfe et al. [48], C0–C4 mechanism from Wang et al. [38] and the latestgeneration combustion mechanism of methane supported by Gas Research Institute (GRI-Mech 3.0) (available from: http://www.me.berkeley.edu/gri_mech/). Zero-dimensional, adiabatic and constant-volume reactor in ChemkinPro software is used to simulate and analyze the combustion
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Table 1 Compositions of syngas mixtures and operating conditions Mixtures
/
P (bar)
H2 (%)
CO (%)
CH4 (%)
N2 (%)
O2 (%)
Ar (%)
Syngas1
0.5
10
0.1515
1.2121
0.1515
1.5152
1.9697
95
Syngas2
0.5
10
0.3030
1.0606
0.1515
1.5152
1.9697
95
Syngas3
0.5
10
0.6061
0.7576
0.1515
1.5152
1.9697
95
Syngas3
0.5
15
1.2121
1.5152
0.3030
3.0303
3.9394
90
Syngas3
1
5
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
1
10
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
1
15
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
2
15
1.7204
2.1505
0.4301
4.3011
1.3978
90
Syngas4
0.5
10
0.9091
0.4545
0.1515
1.5152
1.9697
95
Syngas5
0.5
10
1.2121
0.1515
0.1515
1.5152
1.9697
95
Fig. 2 Influence of bio-syngas composition on the ignition delay times (lines: fitting curves; symbols: experimental data; syngas1: 0.1515 %H2/1.2121 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/1.5152 %N2/ 1.9697 %O2/95 %Ar; syngas3: 0.6061 %H2/0.7576 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/ 0.4545 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas5: 1.2121 %H2/0.1515 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
of bio-syngas mixtures in the present study. The experimental results and predictions of above mechanisms are compared in Figs. 5 and 6. Figure 5 shows the comparisons between the experimental results of syngas3 and predictions of chemical kinetic mechanisms at different pressures and equivalence ratios. As can be seen from Fig. 5, all mechanisms can well predict the trends of variations of ignition delay times. Under all the test conditions, Li’s mechanism and Wang’s mechanism have the best agreement with experimental results. However, Metcalfe’s mechanism overpredicts the results, especially in low temperature range (T \ 1,450 K). GRIMech 3.0 also overpredicts the results in most temperature range, yet experimental results and predictions gradually approach when temperatures are lower than certain
Fig. 3 Influence of pressure on the ignition delay times (lines: fitting curves; symbols: experimental data; syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar)
Fig. 4 Influence of equivalent ratio on the ignition delay times (lines: fitting curves; symbols: experimental data; P = 15 bar)
temperature. GRI-Mech 3.0 and Metcalfe’s mechanism have the identical accuracy when temperatures are higher than 1,450 K.
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Fig. 6 Comparisons between chemical kinetic models in the literature and ignition delay times measured in the present work (P = 10 bar; syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/1.5152 %N2/ 1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/0.4545 %CO/0.1515 % CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
Fig. 5 Comparisons between chemical kinetic models in the literature and ignition delay times measured in the present work (syngas3)
Figure 6 shows the comparisons between experimental ignition delay times of different mixtures and predictions of chemical kinetic mechanisms. Li’s mechanism still has the best predictability among all the mechanisms followed by Wang’s mechanism, which is predicted well at temperature above 1,300 K yet a bit under-predicted results at temperatures below 1,300 K. As can be seen in Fig. 6a, Metcalfe’s mechanism relatively over-predicted the results
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for syngas2. GRI-Mech 3.0 predicts well at high temperature (T [ 1,450 K) as well as when hydrogen concentration is low. We can see from Fig. 6b that when the fraction of hydrogen is high (syngas4) and the chemistry of hydrogen dominates ignition, the deviation between experimental results and predictions by GRI-Mech 3.0 at low temperature is relatively large. As summarized in Figs. 5 and 6, the C1 mechanism from Li et al. [35] has the greatest accuracy over the whole range of test conditions. Li’s mechanism [35] is a revised comprehensive kinetic mechanism for CO/H2O/H2/O2, CH2O and CH3OH oxidation consisting of 84 reversible elementary reactions among 18 species. The mechanism compared against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay and flow reactor species time history data. Excellent agreement of the model prediction with all the above experimental data was found. Due to its accuracy over other models in present work, the
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mechanism will be utilized as a basis to analyze the chemical kinetic characteristics of test mixtures in Sect. 4.
4 Analysis and discussion In order to quantitatively interpret the effects of each parameter (pressure, temperature, equivalence ratio and H2/CO ratio, or H2 concentration) on the ignition delays, comparative analyses were conducted with Li’s mechanism. Figure 7 shows the trends of ignition delay time with H2 concentration in gaseous fuel at various temperature, pressure and equivalence ratio. The logarithmic ignition delays nonlinearly decrease with the H2 concentration under various conditions. A new parameter, changing rate (CR), is defined here to compare the level of effects of experimental conditions on the ignition delays, CR ¼
jsmax sj 100 %; smax
where smax indicates the maximum ignition delay time of fuel within the range of studied parameter and s indicates the ignition delay time of fuel at current value of parameter. The variation of CR of ignition delays with pressure, temperature, equivalence ratio and H2 concentration is exhibited in Fig. 8. It shows that, under present experimental conditions, the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable with maximum CR of over 80 %. However, the effect of pressure is relatively smaller, with maximum CR of nearly 54 %, because of the current small-scale pressure range (5–15 bar). Sensitivity analyses of the ignition delay times of test mixtures were performed to get insight into the primary elementary reactions in the chemical kinetics of bio-syngas. The normalized first-order sensitivity coefficients of the OH radical of top 12 most sensitive reactions at various temperatures, pressures, equivalence ratios and compositions are shown in Fig. 9a–d. The normalized first-order sensitivity coefficients of the OH radical are calculated by Chemkin-Pro software, and the definition can be seen from Andrae et al. [49]. Negative sensitivity coefficients indicate the reaction promotes ignition and vice versa. Figure 9a shows the most sensitive reactions of syngas3 at pressure of 10 bar, stoichiometric equivalence ratio and various temperatures (1,100, 1,250 and 1,400 K). It indicates R1 (H ? O2 = O ? OH), as the uppermost chain branching reaction, has the largest negative sensitivity coefficient under all temperatures, and it is one of the most critical reactions in hydrogen auto-ignition. R2 (O ? H2 = H ? OH), R3 (H2 ? OH = H2O ? H) and R29 (CO ? OH = CO2 ? H) also promote ignition for their formation of H
Fig. 7 The variation of ignition delay time with H2 concentration in gaseous fuels at various temperature (a), pressure (b) and equivalence ratio (c)
radical which is provided to R1. R51 (CH3 ? HO2 = CH3O ? OH) consumes weakly active radicals CH3 and HO2, forming strongly active OH radical. Additionally, R15 (HO2 ? H = OH ? OH) promotes ignition as it consumes H radical to form two OH radicals. On the contrary, R13 (H ? O2 (?M) = HO2 (?M)) has the largest positive
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Fig. 8 The variation of changing rate (CR) of ignition delay time with temperature (a), pressure (b), equivalence ratio (c) and H2 concentration (d)
sensitivity coefficient under current conditions since it competes with R1 for H radical, producing weakly active HO2 [41]. Moreover, R54 (CH4 ? H = CH3 ? H2) and R56 (CH4 ? OH = CH3 ? H2O) consume strongly active OH and H radicals, forming weakly active CH3. As the temperature rises, R54 becomes relatively crucial. R53 (CH3 ? H (?M) = CH4 (?M)) is a chain-termination reaction for consuming CH3 and H radicals then forming stable CH4. It is noted that the number of sensitive reactions, which affect ignition delay times, decreases as temperature goes up. Figure 9b shows the most sensitive reactions of syngas3 at temperature of 1,400 K, stoichiometric equivalence ratio and various pressures (5, 10, 15 bar). As can be seen, R1 and R54 are the most negatively and positively sensitive reactions, respectively, and the sensitivity of both reactions increases as the pressure increases. Additionally, R13 becomes more sensitive with the increasing pressure, and the sensitivity of R13 is still lower than that of R54 under higher pressure (15 bar). This indicates CH4 successfully competes with O2 for the reaction with H radical under high temperature, which shows the great influences of CH4 on the ignition characteristics of mixtures. Figure 9c shows the most sensitive reactions of syngas3 at temperature of
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1,250 K, pressure of 15 bar and various equivalence ratios of 0.5, 1.0, and 2.0. R1 is dominant in promoting ignition under all equivalence ratios. As shown in Fig. 4, the ignition delay times of lean mixture are shorter than those of richer mixture. Nevertheless, Walton et al. [16] found that the ignition delays are getting shorter as the equivalence ratios turn higher in the low-intermediate temperature range. In our opinion, the main reasons of the difference are firstly the different temperature range and secondly the CH4 in the mixtures. We can see the sensitivity of R53 and R54 (those reactions only exist during the auto-ignition of CH4) becomes higher while R13 becomes less sensitive as the mixture turns rich. Figure 9d shows the most sensitive reactions of syngas2 and syngas4 at temperature of 1,400 K and pressure of 10 bar. R1, R13 and R54 are the most crucial reactions. As the content of CO goes up, R13 wins over R54 becoming the most sensitive reaction prohibiting ignition. We also find that R29 has a large negative sensitivity coefficient as one of the primary exothermal reactions in CO oxidation, especially for syngas2. It can be explained that the CO concentration in syngas2 is higher than that in synga4; consequently, R29 will be promoted by CO and form active H radical that promotes ignition. Even though the H2/CO
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Fig. 9 a Normalized first-order sensitivity coefficients of ignition delay times of syngas3, at pressure of 10 bar, stoichiometric ratio with Li’s mechanism (syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar). b Normalized first-order sensitivity coefficients of ignition delay times of syngas3, at temperature of 1400 K, stoichiometric ratio with Li’s mechanism (syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar). c Normalized first-order sensitivity coefficients of ignition delay time of syngas3, at pressure of 15 bar and temperature of 1,250 K with Li mechanism. d Normalized first-order sensitivity coefficients of ignition delay time of syngas2 and syngas4, at pressure of 10 bar, temperature 1400 K, equivalence ratio of 0.5 (syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/ 1.5152 %N2/1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/0.4545 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
Table 2 CH4 reaction pathway in syngas3 at temperature of 1250 K and pressure of 15 bar with 20 % fuel consumption
=0.5
ratio would have influence on the sensitivity of reactions promoting ignition, its influence on that of most sensitive reactions prohibiting ignition is more obvious.
=1.0
=2.0
The sensitivity analyses above indicate that R1 is the dominant reaction promoting ignition under all current conditions. As for reactions prohibiting ignition, R13 has
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R1 was found as the most sensitive reaction promoting ignition under all current conditions. The OH rate of production (ROP) of R1 (H ? O2 = OH ? O) was studied at various equivalence ratios, as shown in Fig. 10. As can be clearly seen in the figure, the time when the peak value of OH ROP of R1 was reached gets longer with the increasing equivalence ratio, and the OH ROP decreases remarkably in rich mixture because of the insufficient oxygen. That also explains the longer ignition delays in rich mixture.
5 Conclusions Fig. 10 OH rate of production (ROP) of R1 (H ? O2 = OH ? O) versus time at equivalence ratios of 0.5, 1.0 and 2.0
the largest positive sensitivity at relatively low temperature when the mixtures are lean or H2/CO ratio of mixtures is low. Additionally, although CH4 concentration in mixtures accounts for only a small portion, the H-abstraction reactions are important reactions in prohibiting ignition, especially the H-abstraction reactions with H, R54 and with OH, R56. Although the sensitivity of R13 becomes larger as the pressure increases, the sensitivity of R54 still wins over R13. R53, as a chain-termination reaction, is the most positively sensitive reaction in rich mixtures. In total, R1 is the most sensitive reaction promoting ignition and R13, R53, R54 as well as R56 are important reactions prohibiting ignition under current conditions, which indicates the importance of CH4 in inhibiting ignition. To deeply interpret the behavior of ignition delays over equivalence ratios and in view of the importance of CH4 in inhibiting ignition in the multi-component mixtures, the reaction pathway of CH4 in syngas3 was analyzed. The reaction pathway at temperature of 1,250 K, pressure of 15 bar, various equivalence ratios and fuel consumption of 20 % is shown in Table 2. The reaction pathway will show us the key reactions in the chemical kinetics of methane. It can be seen that the H-abstraction of methane producing CH3 is realized by H, OH and O radicals. The contribution rate of H radical increases as the equivalence ratio increases, while that of other radicals (O, OH) decrease. Meanwhile, note that the combination reaction (R53) of CH3 and H with third-body participation has the peak contribution rate in the consumption of CH3 in stoichiometric mixtures. However, the contribution rate of R53 is still high (32.52 %, just a bit lower than the combination reaction of dual CH3 radicals) in rich mixtures. As founded earlier, R53 and R54 have the largest positive sensitivities in rich mixtures. So these two reactions can be the cause of the behavior of ignition delays over equivalence ratios in the reactions inhibiting ignition.
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Simulated multi-component bio-syngas was studied in a shock tube, and the comparison between experimental results and multiple simulation results indicated the Li’s mechanism has the best accuracy. The ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa due to the competition between the reaction H ? O2 = OH ? O and the pressure-dependence reaction H ? O2 (?M) = HO2 (?M). Ignition delay times will also increase as CO concentration in mixtures increases, which indicates that CO has inhibiting effect on fuel ignition. It is also necessary to point out that the inhibiting effect is conspicuous only if the CO concentration is high enough. The above behaviors are in accordance with the literature. Moreover, ignition delay times are found going up with increasing equivalence ratios, which is different from the past discovery. The reasons were concluded in chemical kinetic methods. Comparative analyses were conducted with Li’s mechanism to quantitatively interpret the effects of each parameter on the ignition delays. The logarithmic ignition delays were found nonlinearly decreasing with the H2 concentration under various conditions. Under present experimental conditions, the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable with maximum CR of over 80 %. However, the effect of pressure is relatively smaller, with maximum CR of nearly 54 %. Sensitivity analyses indicated R1 (H ? O2 = O ? OH) is undoubtedly the most sensitive reaction promoting ignition and R13 (H ? O2 (?M) = HO2 (?M)), R53 (CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) as well as R56 (CH4 ? OH = CH3 ? H2O) are key reactions prohibiting ignition under current experimental conditions. Among them, R53 (CH3 ? H (?M) = CH4 (?M)) and R54 (CH4 ? H = CH3 ? H2) have the largest positive sensitivities in rich mixture. Moreover, reaction pathway analyses on CH4 also discover the high contribution rate of R53 and R54 in prohibiting ignition in rich mixture. The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases
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sharply as the mixture turns rich. As a result, the ignition delays become longer as the equivalence ratio increases. Acknowledgments This work was supported by the Key Fundamental Research Projects of Science and Technology Commission of Shanghai (14JC1403000). Conflict of interest of interest.
The authors declare that they have no conflict
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Article
Engineering Sciences
Auto-ignition of biomass synthesis gas in shock tube at elevated temperature and pressure Linqi Ouyang • Hua Li • Shuzhou Sun Xiaole Wang • Xingcai Lu
•
Received: 21 August 2015 / Accepted: 15 October 2015 / Published online: 16 November 2015 Ó Science China Press and Springer-Verlag Berlin Heidelberg 2015
Abstract Ignition delay times of multi-component biomass synthesis gas (bio-syngas) diluted in argon were measured in a shock tube at elevated pressure (5, 10 and 15 bar, 1 bar = 105 Pa), wide temperature ranges (1,100–1,700 K) and various equivalence ratios (0.5, 1.0, 2.0). Additionally, the effects of the variations of main constituents (H2:CO = 0.125–8) on ignition delays were investigated. The experimental results indicated that the ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa. The ignition delays were also found to rise as CO concentration increases, which is in good agreement with the literature. In addition, the ignition delays of bio-syngas were found increasing as the equivalence ratio rises. This behavior was primarily discussed in present work. Experimental results were also compared with numerical predictions of multiple chemical kinetic mechanisms and Li’s mechanism was found having the best accuracy. The logarithmic ignition delays were found nonlinearly decrease with the H2 concentration under various conditions, and the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable. However, the effect of pressure is relatively smaller under current conditions. Sensitivity analysis and reaction pathway analysis of methane showed that R1 (H ? O2 = O ? OH) is the most sensitive reaction promoting ignition and R13 (H ? O2 (?M) = HO2 (?M)), R53
Electronic supplementary material The online version of this article (doi:10.1007/s11434-015-0935-4) contains supplementary material, which is available to authorized users. L. Ouyang H. Li S. Sun X. Wang X. Lu (&) Laboratory for Power Machinery and Engineering of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China e-mail: [email protected]
(CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) as well as R56 (CH4 ? OH = CH3 ? H2O) are key reactions prohibiting ignition under current experimental conditions. Among them, R53 (CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) have the largest positive sensitivities and the high contribution rate in rich mixture. The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases sharply as the mixture turns rich. Therefore, the ignition delays become longer as the equivalence ratio increases. Keywords Shock tube Biomass synthesis gas Ignition delay time Sensitivity analysis Reaction pathway analysis
1 Introduction Biomass synthesis gas with carbon-neutral property has been paid extensive attention because of the severe pollution in some countries at present. Biomass synthesis gas can be utilized in various energy power devices— it not only can be used in biomass gasification power generation and combined cooling, heating and power (CCHP), but also used as clean alternative fuels in internal combustion engines, fuel cells and boilers [1, 2]. It can be promising that biomass synthesis gas will occupy a critical position in the future energy fields. Generally, the feedstocks which can be utilized to produce synthesis gas include natural gas, coal and biomass (such as feces of animals, straw and other organic wastes) [3, 4]. In China, due to the emission caused by the burning of crop straw [5], it is absolutely a promising method to make use of straw by turning them into synthesis gas. The synthesis gas derived from biomass is called biomass-derived syngas or bio-syngas. Recently, the bio-syngas has aroused great
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interest among researchers because of the diversity and availability of feedstocks [6–12]. Practically, the main components of bio-syngas are CO and H2. Other components (including methane, nitrogen, carbon dioxide, water [9, 12]) differ a lot due to the diversity of biomass feedstocks and production procedures of bio-syngas. The combustion characteristics will alter since the components in bio-syngas vary. Therefore, the power output and emission of internal combustion engines and gas turbines will be affected, which could be adverse to the design of engines and control strategies [13]. No matter if bio-syngas is used in internal combustion engines or in gas turbines, the ignition properties and flame properties of bio-syngas could have great effects on the efficiency and emission of the whole combustion system. Mittal et al. [14] studied the ignition delay times of 80 % CO ? 20 % H2 mixtures in rapid compression machine (RCM) at temperature of 950–1,050 K and pressure of 15, 30 and 50 bar. Gersen et al. [15] measured the ignition delay times of stoichiometric and lean H2, H2 ? CO, CH4, CH4 ? CO, CH4 ? H2 and CH4 ? CO ? H2 mixtures at pressures ranging from 20 to 80 bar and in the temperature range 900–1,100 K in a RCM. The effects of the content of CO (\50 %) and CH4 on the ignition delays of hydrogen were investigated. Walton et al. [16] also investigated the ignition of simulated syngas mixtures in a rapid compression facility at spanned pressures ranging from 7.1 to 26.4 atm, temperatures from 855 to 1,051 K, equivalence ratios from 0.1 to 1.0, oxygen content from 15 % to 20 % and H2:CO from 0.25 to 4.0. Mansfield and Wooldridge [17] investigated the effects of chemical impurities (CH4, TMS) on the combustion of syngas in a rapid compression facility. Twostep ignition behavior was found at higher pressures, and the ignition delays were affected differently by the addition of impurities. Thi et al. [18] measured the ignition delay times of syngas in a shock tube at temperature ranging from 757 to 1,280 K, equivalence ratios ranging from 0.3 to 1.0 and pressure of 2 and 10 bar. Sivaramakrishnan et al. [19] measured ignition delay times of dilute CO/H2 mixtures in a high-pressure shock tube. They also analyzed variation of some stable species (CO, O2, CO2) using GC and GC/MS. Petersen et al. [20] studied the ignition delays of CO/H2/CO2 mixtures using shock tube and flow reactor. Results were compared with previous work of Peschke and Spadaccini [21] indicating that their variations of ignition delay times over time agreed well, but there are deviations between experimental results and predictions. Mathieu et al. [22, 23] studied the ignition delay times of syngas derived from coal and biomass in a shock tube. The effects of other components (CH4, CO2, H2O, NH3, H2S) other than CO and H2 on the ignition delays were investigated. In terms of the combustion flame characteristics of syngas, Wang et al. [24] measured the transient front of
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turbulent premixed flame of biomass syngas (CO/H2/CO2/ air) by using PLIF. Prathatp et al. [25] studied the effect of dilution with CO2 on the laminar burning velocity and flame stability of syngas (50 % hydrogen ? 50 % CO) in a constant-pressure spherical diffusion flame. The results showed that peak burning velocity shifts because of dilution with CO2. CO2 shows a stronger inhibiting effect on laminar burning velocity than that of nitrogen possibly due to the participation of CO2 in the chemical reactions. As for the chemical effects of CO2, Liu et al. [26] numerically studied the chemical effects of CO2 replacement of N2 in air on the burning velocity of CH4/O2/N2/CO2 and H2/O2/ N2/CO2 mixtures. Bouvet et al. [27] particularly measured the spherical flame velocity of syngas at spanned H2/CO ratios ranging from 5 %/95 % to 50 %/50 % and equivalence ratios ranging from 0.4 to 5.0. Natarajan et al. [28] investigated the burning velocity of H2/CO/CO2 syngas mixtures using conical Bunsen flame and one-dimensional stagnation flame at a wide range of fuel fractions (fraction of H2 ? CO spanning from 5 % to 95 %, fraction of CO2 is less than 40 %). Dong et al. [29] experimentally measured the laminar flame velocity of H2/CO mixtures (volume fraction of H2 ranging from 0 to 100 %) in a Bunsen burner. Goswami et al. [30] measured the laminar flame speed of H2/CO and H2/N2 mixtures in O2/He. Krejci et al. [31] studied the laminar flame speed of the neat H2 and CO/H2 mixtures. Hydrogen, as a reactive component in syngas, was also experimentally and numerically studied with air in a closed combustion vessel [32]. Tinaut et al. [33] studied the combustion velocity of biomass producer gas (CO/CO2/CH4/H2/N2) with various gas concentrations in a constant-volume combustion bomb. Multitudes of scholars have developed the chemical kinetics of CO/H2 mixtures due to the importance of chemical kinetic characteristics of bio-syngas [34–37]. In addition, Wang et al. [38] developed a detailed chemical kinetic mechanism of mixtures containing CO, H2 and other species. Dryer and Chaos [39] investigated the homogeneous gas-phase combustion kinetics of syngas at low temperatures and high pressures (T \ 1,000 K, 10 \ P \ 30 atm). Boivin et al. [40] developed a four-step reduced chemical kinetic model based on CO ? H2 biosyngas, specifically for combustion in gas turbine. Petersen et al. [20] performed an investigation on the ignition of syngas at real conditions, and the predictions of previous chemical kinetics were found to be deviating sharply from results at real conditions. The major reason is that the main components in syngas are CO, H2, CO2 and other gases; however, the widely studied H2/CO mechanisms lack investigation at high pressure and low temperature. Ke´romne`s et al. [41] measured the ignition delay times of H2/ CO/O2/N2/Ar mixture in shock tubes and two rapid compression machines at pressures ranging from 1 to 70 bar,
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temperature from 914 to 2,220 K and equivalence ratio from 0.1 to 4, and a detailed chemical kinetic mechanism was established. Experimental results indicated the ignition delays are strongly dependent on the compression pressures and temperatures and will decrease with increasing pressure, temperature and equivalence ratio. The whole reactivity of syngas is dominated by the chemistry of hydrogen (CO lower than 50 %). However, the inhibiting effect of CO on ignition becomes obvious when the fuel fraction of CO is higher than 50 %. As for bio-syngas or marsh gas, their major constituents, like CH4, CO and H2, have very high octane numbers and also a high proportion of CO2 or N2. The octane number of marsh gas is about 130 while that of bio-syngas is around 100–105, and the auto-ignition temperatures for both are higher than 898 K. Consequently, it is difficult for them to be ignited by compression. Therefore, spark-ignition combustion is preferable. Tsiakmakis et al. [42] coupled a singlecylinder spark-ignition engine to fluidized bed in a portable cogeneration cycle unit and evaluated bio-syngas derived from three kinds of feedstocks blending with propane in the engine. The heat release rate and cylinder pressure in the engine were analyzed. The power output loss was found owing to the lower heat value. In addition, the in-cylinder peak pressure and heat release rate decreased and the combustion stability was also affected. Arroyo et al. [43] studied two kinds of bio-syngas in a spark-ignition engine, and the results were compared with the performances of engines fueled with gasoline, methane and biogas. The investigation indicated that the increasing content of hydrogen in syngas would increase the cylinder pressure. Although there are other diluents in syngas, high engine speed and lean conditions lead to higher efficiency than those obtained with gasoline. The burning velocity and heat release rates were strongly affected by the compositions of diluents and the fraction of hydrogen. CO and CO2 in syngas would increase some pollutants in exhaust gas with decreasing HC emission. NOx probably would increase as the fraction of hydrogen in syngas increases, but it was mainly influenced by CO2 and equivalence ratio. Papagiannakis and Zannis [44] studied the wood gas in a heavy-duty spark-ignition engine. Previous work indicated the critical issues of burning wood gas lie in its lower thermal efficiency than natural gas and higher NO and CO emissions. Chen et al. [45] investigated the combustion characteristics of gasoline engine fueled with biosyngas (H2–CO) diluted by CO2. BMEP was discovered to decrease, and the cyclical variation of IMEP was less than 10 % as the dilution rate of CO2 rose. However, the effects of CO2 were in contrast with those of hydrogen. The CO2 addition would substantially decrease NOx emissions while the thermal efficiency would not be affected. Although there are many researches about syngas currently, the major investigations were just based on
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CO ? H2 binary fuels. Additionally, the effects of equivalence ratio on the ignition delay of multi-component biosyngas were less experimentally studied by scholars before, and few researches were conducted on multi-component bio-syngas from the aspect of chemical kinetics. Therefore, in the present work, the effects of operation conditions, including pressure, temperature as well as equivalence ratio and the variation of H2 and CO compositions as well as methane on the ignition delays of multi-component biosyngas, were investigated. To further interpret experimental results in terms of chemical kinetics, multiple chemical kinetic mechanisms are utilized to perform simulations and sensitivity analyses, and reaction pathway analyses are fulfilled. Generally, the major compositions of bio-syngas include H2, CO, CO2, N2, CH4, H2O and other impurities. Among them, the fractions of CO2 ? N2 ? H2O can be as high as over 50 %. As mentioned earlier, the compositions and gas concentrations in bio-syngas could be different because of the raw materials and production process. In present work, the mixtures of H2, CO, CH4 and N2 are adopted as the simulated multi-component bio-syngas. The volume fraction ratio between H2 ? CO ? CH4 and N2 is around 50:50. The volume fraction of H2 and CO ranges from 5 % to 40 %. The volume fraction of CH4 is about 5 %. Large amounts of argon are added in the mixtures to avoid non-ideal combustion phenomena in shock tube experiment [20].The ignition delay times of multi-component biomass synthesis gas diluted in 90 %–95 % Ar (mol.%) were measured by using reflected shock wave at elevated pressure (P = 5, 10, 15 bar), elevated temperature (T = 1,100–1,700 K) and various equivalence ratios (u = 0.5, 1.0, 2.0).
2 Experimental system A single-diaphragm stainless steel shock tube is used to measure the ignition delay times behind reflected shock wave. Previous studies [46, 47] have given a detailed introduction of the apparatus and validated the reliability and repeatability of it. The apparatus is primarily composed by a 6-m-long driving section (inner diameter of 90 mm), a 5-m-long driven section (inner diameter of 90 mm) and a diaphragm section (the material of diaphragms is polyethylene terephthalate). There are five pressure transducers (interval between each sensor is 333 mm, all types are PCB 113B26) and an OH* sensor at the end of the driven section. The pressure transducers are employed to record the arrival time of shock waves, and the last one (20 mm distant from the end wall) is to record the arrival time of reflected shock wave. The OH* signal is detected by a PMT (HamamatsuR928), which is installed at 20 mm distance from the end wall. All signals are recorded
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and saved by an oscilloscope. The definition of ignition delay time in present experiment is the time between the arrival of reflected shock wave at the last pressure transducer and the intersection of lines drawn along the steepest slope of OH* emission profile and the OH* signal baseline. It is shown in Fig. 1. The temperatures and pressures behind reflected shock wave are calculated according to shock wave equation. As the main uncertainty of ignition delays in the experiment is the determination of temperature behind reflected shock wave, it is crucial to minimize temperature errors. The total uncertainty of ignition delays in present study is less than 10 % at lower pressure (5, 10 bar) and 20 % at higher pressure (15 bar). Mixtures are prepared manometrically in a stainless steel tank with a volume of 210 L. The mixtures are allowed to stand about 2 h for ample premix before use. Details of mixtures’ compositions are summarized in Table 1. The purity of all gases in test mixtures is: 99.95 % for CO, 99.99 % for CH4 and H2, 99.999 % for N2, O2 and Ar. In view of the fact that CO may suffer possible contamination by Fe, thus forming FeCO5, which has inhibiting effect on combustion of CO, the vessel containing CO is made of aluminum alloy to minimize the possible formation of FeCO5. All sections of shock tube are evacuated to ultra-low vacuum (0.03 Pa) by vacuum pumps before each experiment followed by introduction of test mixtures into the driven section. High purity of helium (99.999 %) is introduced into driver section to rupture the diaphragm then produce shock waves. Under the compression of shock waves, the mixtures will be ignited. The whole process is recorded by the pressure transducers and an OH radical sensor. The experimental data are available in the supplemental material.
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3 Ignition delay times of bio-syngas 3.1 Experimental results The effects of major constituents on ignition delay times were primarily investigated in the present work. The ignition delay times of five test mixtures at equivalent ratio of 0.5 and pressure of 10 bar are compared in Fig. 2. It is shown that increasing fraction of CO (the fraction of hydrogen decreases) will gradually lengthen the ignition delay times of mixtures. It indicates that CO could inhibit the ignition process, which is in accordance with previous studies performed by Ke´romne`s et al. [41] and Walton et al. [16]. Additionally, the inhibition effects are negligible when the CO concentrations are low. Explanations from Ke´romne`s et al. [41] are that the reactivity of H2 ? CO mixtures is mainly dominated by the chemistry characteristics of hydrogen when the fraction of carbon monoxide in mixtures is low. As the CO concentrations rise, the chemistry characteristic of CO outweighs that of hydrogen leading to the extension of ignition delay times. The ignition delay times of syngas3 were measured at different pressure (5, 10, 15 bar) and stoichiometric equivalence ratio, as shown in Fig. 3. The ignition delay times increase as temperature decreases. Additionally, due to the competition between the reaction H ? O2 = OH ? O and the pressure-dependence reaction H ? O2 (?M) = HO2 (?M), the ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa. The effects of pressure on the reactivity of syngas3 are similar to that of pure hydrogen oxidation in Ref. [41]. Figure 4 shows the ignition delay times of syngas3 at pressure of 15 bar and equivalence ratios of 0.5, 1.0 and 2.0. It is very interesting to notice that the ignition delay times of syngas3 go up when the equivalence ratio increases. This behavior will be primarily discussed in Sect. 4. 3.2 Comparisons between experimental results and kinetic simulation
Fig. 1 Definition of ignition delay time
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As mentioned above, there are multiple chemical kinetic mechanisms developed for the ignition and combustion of syngas all over the world. Four representatively detailed mechanisms were chosen here, including C1 mechanism from Li et al. [35], C1–C2 mechanisms from Metcalfe et al. [48], C0–C4 mechanism from Wang et al. [38] and the latestgeneration combustion mechanism of methane supported by Gas Research Institute (GRI-Mech 3.0) (available from: http://www.me.berkeley.edu/gri_mech/). Zero-dimensional, adiabatic and constant-volume reactor in ChemkinPro software is used to simulate and analyze the combustion
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Table 1 Compositions of syngas mixtures and operating conditions Mixtures
/
P (bar)
H2 (%)
CO (%)
CH4 (%)
N2 (%)
O2 (%)
Ar (%)
Syngas1
0.5
10
0.1515
1.2121
0.1515
1.5152
1.9697
95
Syngas2
0.5
10
0.3030
1.0606
0.1515
1.5152
1.9697
95
Syngas3
0.5
10
0.6061
0.7576
0.1515
1.5152
1.9697
95
Syngas3
0.5
15
1.2121
1.5152
0.3030
3.0303
3.9394
90
Syngas3
1
5
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
1
10
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
1
15
1.5094
1.8868
0.3774
3.7736
2.4528
90
Syngas3
2
15
1.7204
2.1505
0.4301
4.3011
1.3978
90
Syngas4
0.5
10
0.9091
0.4545
0.1515
1.5152
1.9697
95
Syngas5
0.5
10
1.2121
0.1515
0.1515
1.5152
1.9697
95
Fig. 2 Influence of bio-syngas composition on the ignition delay times (lines: fitting curves; symbols: experimental data; syngas1: 0.1515 %H2/1.2121 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/1.5152 %N2/ 1.9697 %O2/95 %Ar; syngas3: 0.6061 %H2/0.7576 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/ 0.4545 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar; syngas5: 1.2121 %H2/0.1515 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
of bio-syngas mixtures in the present study. The experimental results and predictions of above mechanisms are compared in Figs. 5 and 6. Figure 5 shows the comparisons between the experimental results of syngas3 and predictions of chemical kinetic mechanisms at different pressures and equivalence ratios. As can be seen from Fig. 5, all mechanisms can well predict the trends of variations of ignition delay times. Under all the test conditions, Li’s mechanism and Wang’s mechanism have the best agreement with experimental results. However, Metcalfe’s mechanism overpredicts the results, especially in low temperature range (T \ 1,450 K). GRIMech 3.0 also overpredicts the results in most temperature range, yet experimental results and predictions gradually approach when temperatures are lower than certain
Fig. 3 Influence of pressure on the ignition delay times (lines: fitting curves; symbols: experimental data; syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar)
Fig. 4 Influence of equivalent ratio on the ignition delay times (lines: fitting curves; symbols: experimental data; P = 15 bar)
temperature. GRI-Mech 3.0 and Metcalfe’s mechanism have the identical accuracy when temperatures are higher than 1,450 K.
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Fig. 6 Comparisons between chemical kinetic models in the literature and ignition delay times measured in the present work (P = 10 bar; syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/1.5152 %N2/ 1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/0.4545 %CO/0.1515 % CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
Fig. 5 Comparisons between chemical kinetic models in the literature and ignition delay times measured in the present work (syngas3)
Figure 6 shows the comparisons between experimental ignition delay times of different mixtures and predictions of chemical kinetic mechanisms. Li’s mechanism still has the best predictability among all the mechanisms followed by Wang’s mechanism, which is predicted well at temperature above 1,300 K yet a bit under-predicted results at temperatures below 1,300 K. As can be seen in Fig. 6a, Metcalfe’s mechanism relatively over-predicted the results
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for syngas2. GRI-Mech 3.0 predicts well at high temperature (T [ 1,450 K) as well as when hydrogen concentration is low. We can see from Fig. 6b that when the fraction of hydrogen is high (syngas4) and the chemistry of hydrogen dominates ignition, the deviation between experimental results and predictions by GRI-Mech 3.0 at low temperature is relatively large. As summarized in Figs. 5 and 6, the C1 mechanism from Li et al. [35] has the greatest accuracy over the whole range of test conditions. Li’s mechanism [35] is a revised comprehensive kinetic mechanism for CO/H2O/H2/O2, CH2O and CH3OH oxidation consisting of 84 reversible elementary reactions among 18 species. The mechanism compared against a wide range of experimental conditions for laminar premixed flame speed, shock tube ignition delay and flow reactor species time history data. Excellent agreement of the model prediction with all the above experimental data was found. Due to its accuracy over other models in present work, the
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mechanism will be utilized as a basis to analyze the chemical kinetic characteristics of test mixtures in Sect. 4.
4 Analysis and discussion In order to quantitatively interpret the effects of each parameter (pressure, temperature, equivalence ratio and H2/CO ratio, or H2 concentration) on the ignition delays, comparative analyses were conducted with Li’s mechanism. Figure 7 shows the trends of ignition delay time with H2 concentration in gaseous fuel at various temperature, pressure and equivalence ratio. The logarithmic ignition delays nonlinearly decrease with the H2 concentration under various conditions. A new parameter, changing rate (CR), is defined here to compare the level of effects of experimental conditions on the ignition delays, CR ¼
jsmax sj 100 %; smax
where smax indicates the maximum ignition delay time of fuel within the range of studied parameter and s indicates the ignition delay time of fuel at current value of parameter. The variation of CR of ignition delays with pressure, temperature, equivalence ratio and H2 concentration is exhibited in Fig. 8. It shows that, under present experimental conditions, the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable with maximum CR of over 80 %. However, the effect of pressure is relatively smaller, with maximum CR of nearly 54 %, because of the current small-scale pressure range (5–15 bar). Sensitivity analyses of the ignition delay times of test mixtures were performed to get insight into the primary elementary reactions in the chemical kinetics of bio-syngas. The normalized first-order sensitivity coefficients of the OH radical of top 12 most sensitive reactions at various temperatures, pressures, equivalence ratios and compositions are shown in Fig. 9a–d. The normalized first-order sensitivity coefficients of the OH radical are calculated by Chemkin-Pro software, and the definition can be seen from Andrae et al. [49]. Negative sensitivity coefficients indicate the reaction promotes ignition and vice versa. Figure 9a shows the most sensitive reactions of syngas3 at pressure of 10 bar, stoichiometric equivalence ratio and various temperatures (1,100, 1,250 and 1,400 K). It indicates R1 (H ? O2 = O ? OH), as the uppermost chain branching reaction, has the largest negative sensitivity coefficient under all temperatures, and it is one of the most critical reactions in hydrogen auto-ignition. R2 (O ? H2 = H ? OH), R3 (H2 ? OH = H2O ? H) and R29 (CO ? OH = CO2 ? H) also promote ignition for their formation of H
Fig. 7 The variation of ignition delay time with H2 concentration in gaseous fuels at various temperature (a), pressure (b) and equivalence ratio (c)
radical which is provided to R1. R51 (CH3 ? HO2 = CH3O ? OH) consumes weakly active radicals CH3 and HO2, forming strongly active OH radical. Additionally, R15 (HO2 ? H = OH ? OH) promotes ignition as it consumes H radical to form two OH radicals. On the contrary, R13 (H ? O2 (?M) = HO2 (?M)) has the largest positive
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Fig. 8 The variation of changing rate (CR) of ignition delay time with temperature (a), pressure (b), equivalence ratio (c) and H2 concentration (d)
sensitivity coefficient under current conditions since it competes with R1 for H radical, producing weakly active HO2 [41]. Moreover, R54 (CH4 ? H = CH3 ? H2) and R56 (CH4 ? OH = CH3 ? H2O) consume strongly active OH and H radicals, forming weakly active CH3. As the temperature rises, R54 becomes relatively crucial. R53 (CH3 ? H (?M) = CH4 (?M)) is a chain-termination reaction for consuming CH3 and H radicals then forming stable CH4. It is noted that the number of sensitive reactions, which affect ignition delay times, decreases as temperature goes up. Figure 9b shows the most sensitive reactions of syngas3 at temperature of 1,400 K, stoichiometric equivalence ratio and various pressures (5, 10, 15 bar). As can be seen, R1 and R54 are the most negatively and positively sensitive reactions, respectively, and the sensitivity of both reactions increases as the pressure increases. Additionally, R13 becomes more sensitive with the increasing pressure, and the sensitivity of R13 is still lower than that of R54 under higher pressure (15 bar). This indicates CH4 successfully competes with O2 for the reaction with H radical under high temperature, which shows the great influences of CH4 on the ignition characteristics of mixtures. Figure 9c shows the most sensitive reactions of syngas3 at temperature of
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1,250 K, pressure of 15 bar and various equivalence ratios of 0.5, 1.0, and 2.0. R1 is dominant in promoting ignition under all equivalence ratios. As shown in Fig. 4, the ignition delay times of lean mixture are shorter than those of richer mixture. Nevertheless, Walton et al. [16] found that the ignition delays are getting shorter as the equivalence ratios turn higher in the low-intermediate temperature range. In our opinion, the main reasons of the difference are firstly the different temperature range and secondly the CH4 in the mixtures. We can see the sensitivity of R53 and R54 (those reactions only exist during the auto-ignition of CH4) becomes higher while R13 becomes less sensitive as the mixture turns rich. Figure 9d shows the most sensitive reactions of syngas2 and syngas4 at temperature of 1,400 K and pressure of 10 bar. R1, R13 and R54 are the most crucial reactions. As the content of CO goes up, R13 wins over R54 becoming the most sensitive reaction prohibiting ignition. We also find that R29 has a large negative sensitivity coefficient as one of the primary exothermal reactions in CO oxidation, especially for syngas2. It can be explained that the CO concentration in syngas2 is higher than that in synga4; consequently, R29 will be promoted by CO and form active H radical that promotes ignition. Even though the H2/CO
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Fig. 9 a Normalized first-order sensitivity coefficients of ignition delay times of syngas3, at pressure of 10 bar, stoichiometric ratio with Li’s mechanism (syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar). b Normalized first-order sensitivity coefficients of ignition delay times of syngas3, at temperature of 1400 K, stoichiometric ratio with Li’s mechanism (syngas3: 1.5094 %H2/1.8868 %CO/0.3774 %CH4/3.7736 %N2/2.4528 %O2/90 %Ar). c Normalized first-order sensitivity coefficients of ignition delay time of syngas3, at pressure of 15 bar and temperature of 1,250 K with Li mechanism. d Normalized first-order sensitivity coefficients of ignition delay time of syngas2 and syngas4, at pressure of 10 bar, temperature 1400 K, equivalence ratio of 0.5 (syngas2: 0.303 %H2/1.0606 %CO/0.1515 %CH4/ 1.5152 %N2/1.9697 %O2/95 %Ar; syngas4: 0.9091 %H2/0.4545 %CO/0.1515 %CH4/1.5152 %N2/1.9697 %O2/95 %Ar)
Table 2 CH4 reaction pathway in syngas3 at temperature of 1250 K and pressure of 15 bar with 20 % fuel consumption
=0.5
ratio would have influence on the sensitivity of reactions promoting ignition, its influence on that of most sensitive reactions prohibiting ignition is more obvious.
=1.0
=2.0
The sensitivity analyses above indicate that R1 is the dominant reaction promoting ignition under all current conditions. As for reactions prohibiting ignition, R13 has
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R1 was found as the most sensitive reaction promoting ignition under all current conditions. The OH rate of production (ROP) of R1 (H ? O2 = OH ? O) was studied at various equivalence ratios, as shown in Fig. 10. As can be clearly seen in the figure, the time when the peak value of OH ROP of R1 was reached gets longer with the increasing equivalence ratio, and the OH ROP decreases remarkably in rich mixture because of the insufficient oxygen. That also explains the longer ignition delays in rich mixture.
5 Conclusions Fig. 10 OH rate of production (ROP) of R1 (H ? O2 = OH ? O) versus time at equivalence ratios of 0.5, 1.0 and 2.0
the largest positive sensitivity at relatively low temperature when the mixtures are lean or H2/CO ratio of mixtures is low. Additionally, although CH4 concentration in mixtures accounts for only a small portion, the H-abstraction reactions are important reactions in prohibiting ignition, especially the H-abstraction reactions with H, R54 and with OH, R56. Although the sensitivity of R13 becomes larger as the pressure increases, the sensitivity of R54 still wins over R13. R53, as a chain-termination reaction, is the most positively sensitive reaction in rich mixtures. In total, R1 is the most sensitive reaction promoting ignition and R13, R53, R54 as well as R56 are important reactions prohibiting ignition under current conditions, which indicates the importance of CH4 in inhibiting ignition. To deeply interpret the behavior of ignition delays over equivalence ratios and in view of the importance of CH4 in inhibiting ignition in the multi-component mixtures, the reaction pathway of CH4 in syngas3 was analyzed. The reaction pathway at temperature of 1,250 K, pressure of 15 bar, various equivalence ratios and fuel consumption of 20 % is shown in Table 2. The reaction pathway will show us the key reactions in the chemical kinetics of methane. It can be seen that the H-abstraction of methane producing CH3 is realized by H, OH and O radicals. The contribution rate of H radical increases as the equivalence ratio increases, while that of other radicals (O, OH) decrease. Meanwhile, note that the combination reaction (R53) of CH3 and H with third-body participation has the peak contribution rate in the consumption of CH3 in stoichiometric mixtures. However, the contribution rate of R53 is still high (32.52 %, just a bit lower than the combination reaction of dual CH3 radicals) in rich mixtures. As founded earlier, R53 and R54 have the largest positive sensitivities in rich mixtures. So these two reactions can be the cause of the behavior of ignition delays over equivalence ratios in the reactions inhibiting ignition.
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Simulated multi-component bio-syngas was studied in a shock tube, and the comparison between experimental results and multiple simulation results indicated the Li’s mechanism has the best accuracy. The ignition delay decreases as the pressure increases above certain temperature (around 1,200 K) and vice versa due to the competition between the reaction H ? O2 = OH ? O and the pressure-dependence reaction H ? O2 (?M) = HO2 (?M). Ignition delay times will also increase as CO concentration in mixtures increases, which indicates that CO has inhibiting effect on fuel ignition. It is also necessary to point out that the inhibiting effect is conspicuous only if the CO concentration is high enough. The above behaviors are in accordance with the literature. Moreover, ignition delay times are found going up with increasing equivalence ratios, which is different from the past discovery. The reasons were concluded in chemical kinetic methods. Comparative analyses were conducted with Li’s mechanism to quantitatively interpret the effects of each parameter on the ignition delays. The logarithmic ignition delays were found nonlinearly decreasing with the H2 concentration under various conditions. Under present experimental conditions, the effects of temperature, equivalence ratio and H2 concentration on the ignition delays are all remarkable with maximum CR of over 80 %. However, the effect of pressure is relatively smaller, with maximum CR of nearly 54 %. Sensitivity analyses indicated R1 (H ? O2 = O ? OH) is undoubtedly the most sensitive reaction promoting ignition and R13 (H ? O2 (?M) = HO2 (?M)), R53 (CH3 ? H (?M) = CH4 (?M)), R54 (CH4 ? H = CH3 ? H2) as well as R56 (CH4 ? OH = CH3 ? H2O) are key reactions prohibiting ignition under current experimental conditions. Among them, R53 (CH3 ? H (?M) = CH4 (?M)) and R54 (CH4 ? H = CH3 ? H2) have the largest positive sensitivities in rich mixture. Moreover, reaction pathway analyses on CH4 also discover the high contribution rate of R53 and R54 in prohibiting ignition in rich mixture. The rate of production (ROP) of OH of R1 showed that OH ROP of R1 decreases
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sharply as the mixture turns rich. As a result, the ignition delays become longer as the equivalence ratio increases. Acknowledgments This work was supported by the Key Fundamental Research Projects of Science and Technology Commission of Shanghai (14JC1403000). Conflict of interest of interest.
The authors declare that they have no conflict
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