Shock tube pyrolysis of thiophene

The kinetics of the thermal decomposition of thiophene diluted in argon have been studied behind reflected shock waves in a single pulse shock tube over the temperature range 1598–2022 K and pressures between 2.5 and 3.44 bar. Product yields and composition were determined using capillary column gas chromatography with flame ionization detection and flame photometric sulphur selective detection. The principal hydrocarbon product at all temperatures was ethyne. Ethanethiol was found to be the major sulphur product together with H₂S formed in significant concentrations at lower temperatures. Carbon disulphide was also formed at higher temperatures. Additional reaction products were CH₄, C₂H₄, C₃H₄, C₄H₃, C₄H₆, C₄H₄, C6H6 and C₄H₂ with some traces were found of C₅ and C₆H₅ species. It was concluded that pyrolysis of thiophene is initiated by C–S bond fission to form the C₄H₄S radical which reacts to give C₄H₃ + SH together with the reaction giving C₃H₄ + CS. The rate expression obtained for the pyrolysis reaction was k _(C4H4S)=2.2×1011 exp (270 kJ mol⁻¹) s⁻¹. Copyright © 2002 John Wiley & Sons, Ltd.     

Shock tube study of ethylamine pyrolysis and oxidation

Ethylamine (CH₃CH₂NH₂) pyrolysis and oxidation were studied using laser absorption behind reflected shock waves. For ethylamine pyrolysis, NH₂ time-histories were measured in 2000 ppm ethylamine/argon mixtures. For ethylamine oxidation, ignition delay times, and NH₂ and OH time-histories were measured in ethylamine/O₂/argon mixtures. Measurements covered the temperature range of 1200–1448 K, with pressures near 0.85, 1.35 and 2 atm, and fuel mixtures with equivalence ratios of 0.75, 1 and 1.25 in 0.2%, 0.8% and 4% O₂/argon. Simulations using the recent Li et al. mechanism gave significantly shorter ignition delay times and higher intermediate radical species concentrations than the experimental results. The reaction rate constants for the two major ethylamine decomposition pathways were modified in the Li et al. mechanism to improve the prediction of the time-histories of NH₂ and OH in ethylamine pyrolysis. In addition, recommendations from recent studies of ethylamine + OH reactions were implemented. With these modifications, the Modified Li et al. mechanism provides significantly improved agreement with the species time-history measurements and the ignition delay time data.     

Shock tube study of methanol,methyl formate pyrolysis: CH3OH and CO time-history measurements

Methanol and methyl formate pyrolysis were studied by measuring CH₃OH and CO concentration time-histories behind reflected shock waves. In the study of methanol pyrolysis, experimental conditions covered temperatures of 1266–1707 K, pressures of 1.1–2.5 atm, and initial fuel concentrations of 1% and 0.2% with argon as the bath gas. Detailed comparisons of CH₃OH and CO concentration profiles with the predictions of the detailed kinetic mechanism of Li et al. (2007) [8] were made. Such comparisons combined with sensitivity analysis identified the need to include an additional methanol decomposition channel, CH₃OH ? CH₂(S) + H₂O, into the mechanism. Pathway and sensitivity analyses for methanol decomposition were performed, leading to rate constant recommendations both for CH₃OH unimolecular decomposition and H-abstraction reactions with improved model performance. In the study of methyl formate pyrolysis, methanol concentration time-histories were measured at temperatures over the range of 1261–1524 K, pressures near 1.5 atm, and initial fuel concentrations of 1% with argon as the bath gas. Our current work, and CO time-histories from previous work, indicates that the Dooley et al. (2010) [3] model is able to accurately simulate most species concentrations in shock tube experiments at early times. However, model improvement is still needed to match the CH₃OH and CO time-histories at later times. Incorporation of the modified rate constants in the methanol sub-mechanism leads to good predictions of the full methanol time-histories at all temperatures. The kinetic implications of some aspects of the CO time-histories and suggestions for further improving the predictive capabilities of these mechanisms are discussed. The current results are the first quantitative measurements of CH₃OH time-histories in shock tube experiments, and hence are a critical step toward understanding of the chemical kinetics of oxygenates.     

Shock tube measurements of 3-pentanone pyrolysis and oxidation

High-temperature 3-pentanone pyrolysis and oxidation studies were performed behind reflected shock waves using laser-based species time-history measurements (3-pentanone, CH₃, CO, C₂H₄, OH and H₂O) and ignition delay time measurements. The overall 3-pentanone decomposition rate coefficient was inferred from the measured 3-pentanone and CH₃ time-histories during pyrolysis at temperatures of 1070–1530 K and a pressure of 1.6 atm., and yielded a mathematical expression for k_tot = 4.383 × 10⁴⁹ T^?10 exp(?44,780/T) s^?1 with an uncertainty of ±35% over 1070–1330 K. The measured species time-histories and ignition delay times were also compared to simulations from a detailed kinetic mechanism of Serinyel et al. (2010) [14]. The measured k_tot was approximately 3.5 times faster than the value used by Serinyel et al. Additionally, the absence of a methyl ketene decomposition reaction was identified as the cause of a deficiency in the O-atom balance of the measured 3-pentanone and CO time-histories. Using the revised overall 3-pentanone decomposition rate coefficient and an additional methyl ketene decomposition pathway, the modified mechanism was able to successfully simulate all six species time-histories, and showed a significant improvement in the predictions of ignition delay times. Finally, a comparison of ignition delay times and OH species time-histories during 3-pentanone, 2-pentanone and acetone oxidation found that 3-pentanone was the most reactive of the three ketones.     

Shock tube study of the pressure dependence of monomethylhydrazine pyrolysis

Monomethylhydrazine (MMH, CH₃NHNH₂) pyrolysis was studied behind reflected shock waves using a laser absorption method. NH₂ concentration time-histories in MMH/argon mixtures were measured over the temperature range of 1100–1400 K and pressures 0.3–5 atm. The MMH pyrolysis mechanism developed by Sun et al. (2009) [9], with the update by Cook et al. (2011) [11], was used to simulate the NH₂ time-histories and to compare with the experiment. The rate constant of the reaction: MMH = CH₃N·H + NH₂(1a) was determined based on the NH₂ time-history measurements. Pressure dependence of k_1a was observed at 0.3–5 atm. The measured reaction rate constants follow a pressure dependence trend close to the theoretical results by Zhang et al. (2011) [10] based on transition state theory master equation analysis, and reducing their theoretical results by ∼40% leads to a close match with the current data. With the experimentally determined k_1a, the simulation results match closely with the NH₂ time-histories at early times. Utilizing the later times of the NH₂ time-histories, we find that a good fit is achieved with the reaction rate constant of k₂ = 1.5 × 10¹⁴ exp (−755/T) cm³/mol/s for the reaction: NHNH₂ + H = NH₂ + NH₂ (2). Simulation results using the modified mechanism, with the updated k_1a and k₂, match well with the measured NH₂ time-histories in the current study.     

Shock tube measurements of species time-histories in monomethyl hydrazine pyrolysis

The first measurements of NH₂ and NH₃ time-histories in monomethyl hydrazine (MMH) pyrolysis were performed behind reflected shock waves in a shock tube using laser absorption techniques. An improved measurement of MMH using IR laser absorption is also presented. MMH concentrations of ∼1% in Ar were employed, over the temperature range 941–1252 K, at pressures near 2 atm. NH₂ was measured at the peak of the overlapping doublet lines at 16739.90 cm⁻¹ (597.4 nm). NH₃ and MMH were measured using direct absorption of CO₂ laser lines at 9.22 and 10.22 μm, respectively. These measurements were then compared to a current comprehensive MMH pyrolysis mechanism based on the work of Sun et al. (2009) and Zhang et al. (2010). Based on the measurements of NH₂ and NH₃, it was possible to measure rate coefficients for two key reactions in the MMH pyrolysis system:(1)CH₃NHNH₂→CH₃NH+NH₂(2)CH₃NHNH₂+NH₂→CH₃NNH₂+NH₃These rates combined with the measured overall MMH decomposition rate strongly imply that Reaction (1) is the dominant MMH decomposition channel. The following rate coefficients (2 atm, 900–1300 K) were uniquely determined:Based on the MMH measurement, the value of the CH₃ decomposition channel is 0–20% of the NH₂ channel, and a value of 1.64 × 10⁵⁸ * T^−12.84 exp(−39580/T) s⁻¹ is recommended for the overall unimolecular decomposition of MMH. Further analysis of the NH₂ measurements indicate that the rate of the following reaction used in the Princeton mechanism should also be significantly increased:(4)CH₃NNH+NH₂→CH₃NN+NH₃The changes to the MMH pyrolysis mechanism recommended in this work result in greatly improved agreement between measured and modeled NH₂, NH₃, and MMH time-histories over the entire range of the study.     

Experimental and kinetic modeling study of methyl butanoate and methyl butanoate/methanol flames at different equivalence ratios and C/O ratios

Premixed laminar methyl butanoate/oxygen/argon and methyl butanoate/methanol/oxygen/argon flames were studied with tunable synchrotron vacuum ultraviolet (VUV) photoionization and molecular-beam sampling mass spectrometry at 30 torr (4.0 kPa). Three flames were investigated in the experiment: MB (methyl butanoate) flame F1.54 (? = 1.54, C/O = 0.479), MB flame F1.67 (? = 1.67, C/O = 0.511) and MB/methanol flame F1.67M (? = 1.67, C/O = 0.479). By measuring the signal intensities at different distances from the burner surface, the mole fraction profiles of intermediates are derived. Experimental results show that the flame front shifts downstream and peak mole fractions of intermediates increase remarkably with the increase of equivalence ratio for pure MB fuel. When methanol is added, the peak mole fractions of most intermediates including those of soot precursors decrease remarkably at the same equivalence ratio, while peaks of soot precursors vary little (only slightly decreasing) at same C/O ratio. It is concluded that the formation of soot precursors is more sensitive to C/O ratio than to equivalence ratio. Besides, more CO₂ is produced near the burner surface in MB flame than that in MB/methanol flame, and this validates an early production of CO₂ in methyl ester oxidation. In addition, a modified MB detailed mechanism is used to model flame structure, and improved agreements between the experimental and predicted results are realized. Based on the simulation results, reaction flux and sensitivity are analyzed for CO₂ and C₃H₃, respectively.     

The shock tube pyrolysis of pyridine

Fossil fuels such as coal and heavy fuel oils contain up to about 2 per cent by weight of fuel nitrogen, most of this being present in pyridine or pyrolic aromatic structures. Under pyrolytic conditions these ring structures decompose to give HCN and CH₃CN. For present day computer modelling of NO_x formation in flames it is necessary to know the mechanism and rates of reaction. A number of previous studies of pyridine pyrolysis have been undertaken using shock tubes or flow reactors. In this present study a shock tube was used to obtain the kinetics of pyridine decomposition in the range 1590–2335 K and with pressures between 2.2 and 3.4 atm. Two independent sets of data were obtained. One set of results was found to be represented by an Arrhenius rate constant k=10^9.7±0.5 exp (−220.0 kJ mol⁻¹ RT⁻¹)s⁻¹. For the other work k= 10^9.8±0.5 exp (−228.191±18.42 kJ mol⁻¹ RT⁻¹)s⁻¹. In addition, the pyrolysis mixtures of pyridine plus toluene have also been studied to understand the synergistic effects. The results indicated the strong involvement initiated by fission of the pyridine ring system. Copyright © 2000 John Wiley & Sons, Ltd.     

Combustion characteristics of biodiesel saturated with pyrolysis oil for power generation in gas turbines

There is a perceived need for multi-fuel burner geometries capable of operating with variable composition fuels from diverse sources to achieve fuel flexibility in gas turbines. The objective of the research covered herein is a comparison study between two liquid fuels, a biodiesel (in a pure form) and the biodiesel as a saturated mixture with a pyrolysis by-product; these two fuels were compared against a standard kerosene as a baseline. The research methodology involved two stages: firstly atomization patterns and injection regimes were obtained using a high speed imaging method, secondly a combustion test campaign was undertaken using a swirl burner to quantify the operational behaviour, species production and exhaust gas compositions of the fuels. Emissions, flame stability trends and power outputs were measured at gas turbine relevant equivalence ratios. Excess oxygen and atomization trends in the biodiesel seem to be playing a major role in the production of emissions and flame stability when compared to kerosene. Also, heavy organics seem to be acting as catalytic substances for OH production close to the burner mouth. In terms of stability and combustion, it is proposed that the saturated blend would be a viable candidate for power generation.     

Shock tube studies on ignition delay and combustion characteristics of oxygenated fuels under high temperature

A comparative study on ignition delay time and combustion characteristics of four typical oxygenated fuel/air mixtures of dimethyl ether (DME), diethyl ether (DEE), ethanol and E92 ethanol gasoline was conducted through the chemical shock tube. The fuel/air mixtures were measured under the ignition temperature of 1100 to 1800 K, initial pressure of 0.3 MPa and the equivalence ratios of 0.5, 1.0 and 1.5. The experimental results show that the ignition delay time of these four oxygenated fuels satisfies the Arrhenius relation. The reaction H + O₂ = OH + O has a high sensitivity in four fuel/air mixtures during high-temperature ignition, which makes the ignition delay lengthen with the increase of the equivalence ratios. By comparing the ignition delay of four fuels, ether fuels have excellent ignition performance and ether functional group has better ignition promotion than hydroxyl group. Moreover, the carbon chain length also significantly promotes the ignition. Due to the accumulation of a large number of active intermediates and free radicals during the long ignition delay time before ignition, the four fuels all have intense deflagration and generate the highest combustion peak pressure at the relatively low ignition temperature (1150-1300 K). For DME, DEE and ethanol, due to the high content of oxygen in their molecules, the combustion peak pressure and luminous intensity increased with the equivalence ratio, and the combustion is intense after ignition. E92 ethanol gasoline with low oxygen content has a lower combustion peak pressure and a longer combustion duration than the other three fuels, and its highest combustion peak pressure appears in the stoichiometric ratio. The combustion process of E92 ethanol gasoline is more oxygen-dependent than the other three fuels.     

Experimental and chemical kinetic modeling study of small methyl esters oxidation: Methyl (E)-2-butenoate and methyl butanoate

This study examines the effect of unsaturation on the combustion of fatty acid methyl esters (FAME). New experimental results were obtained for the oxidation of methyl (E)-2-butenoate (MC, unsaturated C₄ FAME) and methyl butanoate (MB, saturated C₄ FAME) in a jet-stirred reactor (JSR) at atmospheric pressure under dilute conditions over the temperature range 850-1400 K, and two equivalence ratios (Φ=0.375,0.75) with a residence time of 0.07 s. The results consist of concentration profiles of the reactants, stable intermediates, and final products, measured by probe sampling followed by on-line and off-line gas chromatography analyses. The oxidation of MC and MB in the JSR and under counterflow diffusion flame conditions was modeled using a new detailed chemical kinetic reaction mechanism (301 species and 1516 reactions) derived from previous schemes proposed in the literature. The laminar counterflow flame and JSR (for ?=1.13) experimental results used were from a previous study on the comparison of the combustion of both compounds. Sensitivity analyses and reaction path analyses, based on rates of reaction, were used to interpret the results. The data and the model show that MC has reaction pathways analogous to that of MB under the present conditions. The model of MC oxidation provides a better understanding of the effect of the ester function on combustion, and the effect of unsaturation on the combustion of fatty acid methyl ester compounds typically found in biodiesel.     

New trends in biodiesel production: Chemical interesterification of sunflower oil with methyl acetate

In this study, the chemical interesterification of oil with methyl acetate using potassium hydroxide, methoxide and polyethylene glycolate as catalysts was investigated. The reactions were performed at 50 °C using the range methyl acetate 12 mol mol⁻¹ of refined sunflower oil to a value of 100 mol mol⁻¹ and the range of potassium methanolate to oil was 100-500 mmol mol⁻¹. The effect of methanol and water on the catalyst and reagents was to reduce the yield of triacetin, forming diacetin, monoacetin and glycerol instead. A compromise between the product yield, reaction kinetics and methyl acetate recovery was achieved with the conditions methyl acetate to oil 50 mol mol⁻¹ and catalyst to oil 100 mmol mol⁻¹ when potassium methoxide was used as a catalyst and the reagents were dehydrated. Under these conditions, equilibrium was reached within the first 15 min of the reaction. The mass fractions of fatty acid methyl esters (FAME) and triacetin in the product were 76.7% and 17.2%, respectively, with a mass fraction of 4.7% for the intermediate diacetinmonoglyceride. Diacetin, monoacetin and glycerol were also found at a mass fraction of 1.2%.     

Techniques for studying and modeling coal pyrolysis and their relevance to biomass and wastes

Pyrolysis is the first step in the thermochemical conversion of any organic material. It plays a key role in coal conversion, where processes like combustion, gasification and hydrogenation include pyrolytic reactions as a starting step, and is the basic process in cokemaking. In the long history of industrial use of thermochemical coal conversion, various methods have been developed for feedstock characterization and for the main chemical reactions. Furthermore, approaches have been established to describe pyrolysis behavior quantitatively and to provide tools for reactor modeling. The main experimental techniques are reviewed and their relevance for the treatment of other solid feedstocks like biomass and wastes is discussed.     

Parametric studies of tube in tube heat exchanger coupled to a panel of collectors

This paper deals with the transient analysis of a one pass, tube in tube heat exchanger coupled to a panel of flat plate collectors under both cocurrent and counter-current modes of flow. Explicit expressions for the temperatures of the cold and hot fluid streams have been developed as a function of the time and space coordinates for both modes of flow. To make a quantitative assessment of the analytical results, numerical calculations have been carried out for a typical winter day, i.e., 13 February 1985 at Delhi. The effects of several parameters, viz. length of the heat exchanger, mass flow rate, radii of heat exchanger tube, etc., have been studied in detail.     

Fast pyrolysis of cellulose in a single pulse shock tube

A shock tube technique was employed to study the thermal decomposition of cellulose in an inert argon gas under the conditions of high temperature, high heating rate, and short reaction times. The influence of temperature and reaction times on product yields and their distribution were investigated. A clean, tar and char free gas consisting mainly of CO, CO₂, C₂H₂, C₂H₄ and CH₄ were produced throughout the course of this investigation. A mass conversion of cellulose to gas exceeding 90 wt% has been realized between the temperatures 700 and 2200°C for the reaction times examined. Carbon monoxide is the major product and attains a yield in excess of 65 wt% for temperatures above 1300°C. Global kinetic parameters for the decomposition of cellulose and its principal gas products were obtained by fitting the experimental data to a single, first order kinetic model. The energy of activation for the decomposition of cellulose was found to be 130.5 kJ/mol. The material balances made for the total mass, carbon and oxygen are good.     

A shock tube study of pyrolysis of tetrahydrothiophene at elevated temperatures

A single pulse shock tube has been used to study the pyrolysis of a hydrogenated sulphur compound, tetrahydrothiophene over the temperature range 1686–1885 K and pressures between 2.4 and 3.5 bars. Product yield and composition was determined using capillary column gas‐chromatography with flame ionization detection and flame photometric sulphur selective detection. The principal hydrocarbon products at all temperatures were C₂H₄ and C₂H₂. Other hydrocarbon reaction products were CH₄, C₂H₆, C₃H₄, C₃H₆, C₄H₃, C₄H₆, C₄H₁₀, C₄H₄, C₆H₆, C₄H₂ and some traces of C₅ and C₆H₅ species. The sulphur compounds identified were hydrogen sulphide, carbon disulphide, thiophene and traces of ethyl mercapton. The pyrolysis experiments indicated that at lower temperatures the hydrogenated thiophene molecule reacts in two unimolecular channels to form C₂H₄+(CH₂)2–S in the major faster channel which may be the route for other products. However, a second lower route may be the formation of C₃H₆+CH₂S. The rate constant obtained for tetrahydrothiophene pyrolysis calculated for this study was k_dis(C4H8S)=1.26×10¹³ exp (316.9 kJ mol^?1) s^?1. Copyright © 2004 John Wiley & Sons, Ltd.     

Shock tube measurements of high temperature rate constants for OH with cycloalkanes and methylcycloalkanes

High temperature experiments were performed with the reflected shock tube technique using multi-pass absorption spectrometric detection of OH radicals at 308 nm. The present experiments span a wide T-range, 801-1347 K, and represent the first direct measurements of the title rate constants at T>500 K for cyclopentane and cyclohexane and the only high temperature measurements for the corresponding methyl derivatives. The present work utilized 48 optical passes corresponding to a total path length ∼4.2 m. As a result of this increased path length, the high [OH] detection sensitivity permitted unambiguous analyses for measuring the title rate constants. The experimental rate constants in units, cm³ molecule⁻¹ s⁻¹, can be expressed in Arrhenius form as