Importance of pyrolysis and catalytic decomposition for the direct utilization of methanol in solid oxide fuel cells

There is interest in developing solid oxide fuel cells (SOFC) operated directly with liquid fuels such as methanol. This mode of operation increases the complexity of the anodic processes, since thermal and catalytic decomposition reactions are relevant. In this study, the pyrolysis and catalytic decomposition of methanol are investigated experimentally for conditions typical of SOFC. The results are compared to the thermodynamic equilibrium values and also to the predictions of a kinetics model. The main species of the thermal decomposition of methanol are H₂, CO, and HCHO; soot formation is relevant below 973 K. The presence of a catalyst allows the gas-phase composition to reach equilibrium. However, the catalysts tested – Ni/YSZ, Ni/CeO₂, Cu/CeO₂ and Cu–Co/CeO₂ – deactivate by coking so that the gas-phase composition reverts to that of pyrolysis alone. The results presented reveal part of the complex dynamics occurring within the anode compartment during the direct utilization of methanol.     

Production of synthesis gas by partial oxidation and steam reforming of biomass pyrolysis oils

As the lowest cost biomass-derived liquids, pyrolysis oils (also called bio-oils) represent a promising vector for biomass to fuels conversion. However, bio-oils require upgrading to interface with existing infrastructure. A potential pathway for producing fuels from pyrolysis oils proceeds through gasification, the conversion to synthesis gas. In this work, the conversion of bio-oils to syngas via catalytic partial oxidation over Rh–Ce is evaluated using two reactor configurations. In one instance, pyrolysis oils are oxidized in excess steam in a freeboard and passed over the catalyst in a second zone. In the second instance, bio-oils are introduced directly to the catalyst. Coke formation is avoided in both configurations due to rapid oxidation. H₂ and CO can be produced autothermally over Rh–Ce catalysts with millisecond contact times. Co-processing of bio-oil with methane or methanol improved the reactor operation stability.     

Effect of oxygen-rich combustion on soot formation in laminar co-flow propane diffusion flames

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. In this work, the effects of oxygen-rich combustion on soot formation in the propane/(O₂+N₂) laminar flow coaxial jets diffusion flame were numerically investigated by using the detailed gas-phase chemical reaction model with the mechanism of tetracyclic aromatic hydrocarbons and the complex thermodynamic properties and transport characteristics parameters. Soot surface growth follows the hydrogen-abstraction-carbon-addition (HACA) model. A hybrid gas-phase mechanism was adopted, which contains a DLR-based polycyclic aromatic hydrocarbons (PAHs) formation, growth model and a gas-phase model. Results show that the oxygen-rich combustion has a great influence on the flame temperature, especially the high temperature region. With the increase of oxygen concentration, the soot formation region of flame broadens and the maximum of soot volume fraction increase from 3.95 ppm to 10.87 ppm. The extra oxygen makes PAHs increased around the nozzle, leading to larger rate in early soot nucleation and surface growth, eventually more soot yield.     

Investigation and improvement of the kinetic mechanism for methanol pyrolysis

Due to the shortage of fossil energy and its pollutants emission, the utilization of alcohol fuel by pyrolysis and combustion has attracted increasing attention. However, there is still a lack of good understanding of the kinetic mechanism for accurate prediction of its combustion and pyrolysis process. In the paper, to improve the kinetic mechanism of methanol pyrolysis and combustion, an experiment was conducted to investigate methanol decomposition using a plug flow reactor (with temperature of 873–1273 K and volume flow rate of 4.4–34.2 ml/s) with on-line gas detection system, the reaction paths were analyzed and the intrinsic reaction kinetics of methanol decomposition was calculated with CHEMKIN. Then, the process of methanol pyrolysis was optimized, and an improved kinetic mechanism was proposed by modified the rate constant of methanol decomposition reaction, hydrogen abstraction reaction of methanol and formaldehyde based on the experimental results, and verified by the combination of calculated and experimental data. The results show that methanol starts decomposing at 910 K and reaches complete conversion at 1150 K, with major products of hydrogen, methane and carbon monoxide. Also, formaldehyde is an important intermediate species, and two dominant paths for methanol decomposition have been proposed: CH₃OH → CH₂O → CO and CH₃OH → CH₃ → CH₄, which are in good agreement with that proposed by previous studies. Meanwhile, the much accurate rate constant k of methanol decomposition can be expressed as $k=8.323\times {10}^3\left({\text{s}}^{?1}\right)\exp \left[?64.05\left(\text{J}{\text{mol}}^{?1}\right)/\text{RT}\right]$. Moreover, mechanism verification indicated that the calculated result with improved mechanism is highly consistent with that of experiment detected in shock tube, especially for predicting methanol consumption and carbon monoxide fraction.     

A wide range kinetic modeling study of the pyrolysis and combustion of naphthenes

The aim of this paper is to analyze and discuss the kinetics of the pyrolysis and combustion of naphthenes. The primary propagation reactions of cyclohexane and methylcyclohexane are presented to extend the validity of a semi-detailed kinetic model for the pyrolysis and oxidation of hydrocarbons. Naphthenes are relevant species as reference components in liquid fuels and surrogate blends. A lumped approach is used to reduce the complexity of the overall scheme in terms of species and reactions. Particular attention is devoted to the role of the isomerization or internal abstraction of H atoms in competition with β−decomposition ones. Primary oxidation and decomposition reactions of the cyclohexyl radical are discussed to explain and justify this lumping procedure. The modeling predictions are compared with different sets of measurements. The validation of the low temperature oxidation mechanism of cyclohexane is based on the ignition delay times obtained both in the rapid compression machine at Lille and in closed vessels. Jet-stirred reactors at different pressures and stoichiometric ratios also confirm the reliability of the overall mechanism of oxidation. The comparisons between the model’s predictions and the measurements relating to the pyrolysis and oxidation of methylcyclohexane in the Princeton turbulent flow reactor further support this extension of the kinetic scheme to naphthenes. Finally, the agreement with the oxidation experiments using mixtures of toluene + methylcyclohexane is a primary and simple example of the model’s ability to deal with the combustion of real fuels or surrogate blends.     

ReaxFF molecular dynamics simulations of n-eicosane reaction mechanisms during pyrolysis and combustion

The pyrolysis and combustion mechanism of the hydrocarbon fuel has important scientific and practical significance. However, it is difficult to detect the whole intermediates and products using traditional methods, which brings trouble to the analysis of the reaction process. In this paper, the microscopic reaction mechanism and the main products of n-eicosane (C₂₀H₄₂) were simulated based on the reactive force field molecular dynamics (ReaxFF-MD). The effects of temperature (2000–3500 K) and oxygen on the initial decomposition, the distribution of main products, and the reactive pathways of C₂₀H₄₂ fuel were studied to determine its reaction mechanism. The initial decomposition of C₂₀H₄₂ was mainly initiated by small alkyl radicals in pyrolysis, and by the oxygen-containing radicals in combustion. The participation of oxygen had a greater effect on accelerating the decomposition reaction. The reactions involving oxygen of C₂₀H₄₂ initial decomposition accounted for 87.5% of the total reactions at 2000 K. Moreover, the detailed distribution and formation pathways of the main products of H₂, C₂H₄, CH₄, H₂O, CO, and CO₂ were depicted to construct the overall reaction mechanism of C₂₀H₄₂. •H radical formed from the composition of C₂H₄ was exactly consistent with the •H radical consumed by the generation of CH₄ and H₂ in the pyrolysis stage. The feasibility of the simulation method was verified by the result of thermal analysis. The results are helpful for further research on the reaction mechanism of hydrocarbon fuels.     

Numerical modeling and TGA/FTIR/GCMS investigation of fibrous residue combustion

Numerical modeling results of combustion of fibrous sludge are presented and validated in a series of experiments. Combustion experiments were conducted in a thermogravimetric coupled with Fourier transform infrared spectrometer and gas chromatograph mass spectrometer. Sludge material (open matrix of lignocellulosic fibers with inorganic fillers) was generated in pulp and a paper mill during the de-inking process. Mathematical models were developed for solid- and gas-phase combustion. The mathematical model for the decomposition of solidiphase is based on the following assumptions: (1) rate of combustion determined by oxygen mass transfer, (2) laminar gas flow, and (3) negligible radiation. The combustion of aromatic hydrocarbons formed/released during the combustion process is formulated taking the following assumptions: (1) reaction rates of methyl-naphthalene and naphthalene are relatively fast and thereby constitute the driving force for the initiation of combustion; and (2) kinetics rate data for the oxidation of methyl-naphthalene and naphthalene are equal to those of benzene. Numerical computations compare well with measurements and provide good predictions of the reactivity of the material during the combustion process. Mass fraction remaining at the end of the simulation period was predicted within 2% accuracy. Flue gas combustion simulations have shown acceptable results, however the computed overall reaction rate was over-predicted. Predictions of the behavior of major gaseous species (CO₂, O₂, CO and PAH) were reasonable. Simulations also revealed the mechanism of solid biomass combustion to start at the center of the sample and then propagate toward the surface. Such information could not be obtained from experimental data. It was also shown that indenyl may play an important role in the pulp and paper biomass combustion and may be considered as a catalyst for ignition.     

Detailed chemical kinetic mechanism for surrogates of alternative jet fuels

Blends of n- and iso-alkane components are employed as surrogates for Fischer–Tropsch (F–T) and biomass-derived jet fuels. The composition of the blends has been determined based on data available for two F–T fuel samples obtained from different sources, using a systematic optimization approach. A detailed chemical kinetic mechanism for combustion of the surrogate blends has been assembled. The mechanism has been validated against fundamental experimental data. While drawing initially from other studies in the literature, the mechanism has been improved by enforcing self-consistency of the kinetic and thermodynamic data for the various surrogate-fuel components represented by the mechanism. These improvements have led to more accurate predictions of flame propagation, flame extinction, and NO_x emissions. As part of the validation process, simulations were performed for a wide variety of experimental configurations, as well as for a wide range of temperatures and equivalence ratios for fuel/air mixtures. Comparison of the model predictions to the available literature data confirms the accuracy of the mechanism as well as of the approach for selecting the surrogate blends.     

Reaction of oxygenated biomass pyrolysis model compounds over a ZSM-5 catalyst

The application of zeolite catalysis to the upgrading of biomass-derived pyrolysis vapours has received increasing interest in recent years. It represents a potential route for the production of hydrocarbon products which can be used as substitutes for traditional fossil fuels. The complex nature of the pyrolysis oils means that there is only a very limited understanding of the reactions which take place during zeolite catalysis. Therefore, the upgrading of individual oxygenated compounds which are present in the pyrolysis oils can help in developing an understanding of the overall catalysis process. In this work, four oxygenated compounds: methanol, furfural, anisole and cyclopentanone were passed over the zeolite catalyst ZSM-5 in its hydrogen form at different temperatures varying from 300 to 500°C. The results show that methanol can be catalytically converted to hydrocarbon products at relatively low catalysis temperatures of 300–350°C, whereas the other oxygenated feedstocks require higher catalysis temperatures. The formation of polycyclic aromatic hydrocarbons (PAHs) was found to vary depending on the oxygenated feedstock. The presence of PAHs is undesirable due to their toxic nature.     

Characterization and prediction of biomass pyrolysis products

In this study some literature data on the pyrolysis characteristics of biomass under inert atmosphere were structured and analyzed, constituting a guide to the conversion behavior of a fuel particle within the temperature range of 200-1000 °C. Data is presented for both pyrolytic product distribution (yields of char, total liquids, water, total gas and individual gas species) and properties (elemental composition and heating value) showing clear dependencies on peak temperature. Empirical relationships are derived from the collected data, over a wide range of pyrolysis conditions and considering a variety of fuels, including relations between the yields of gas-phase volatiles and thermochemical properties of char, tar and gas. An empirical model for the stoichiometry of biomass pyrolysis is presented, where empirical parameters are introduced to close the conservation equations describing the process. The composition of pyrolytic volatiles is described by means of a relevant number of species: H₂O, tar, CO₂, CO, H₂, CH₄ and other light hydrocarbons. The model is here primarily used as a tool in the analysis of the general trends of biomass pyrolysis, enabling also to verify the consistency of the collected data. Comparison of model results with the literature data shows that the information on product properties is well correlated with the one on product distribution. The prediction capability of the model is briefly addressed, with the results showing that the yields of volatiles released from a specific biomass are predicted with a reasonable accuracy. Particle models of the type presented in this study can be useful as a submodel in comprehensive reactor models simulating pyrolysis, gasification or combustion processes.     

Investigations on performance and emission characteristics of an industrial low swirl burner while burning natural gas,methane, hydrogen-enriched natural gas and hydrogen as fuels

Although many detailed chemical reaction mechanisms, skeletal mechanisms and reduced mechanisms are available in the literature to modeling the natural gas, they are computational expensive, required high power computing especially for three dimensional complex geometries with intense meshes. For example, though the DRM19 reduced mechanism does not include NO and NO₂ species, it includes 19 species and 84 reactions. On the other hand, Eddy Dissipation combustion model in which the overall rate of reaction is mainly controlled by turbulent mixing can be utilized as a practical approach for fast burning and fast reaction fuels such as natural gas. Unlike fossil fuels, hydrogen is a renewable energy and quite clean in terms of carbon monoxide and carbon dioxide emissions. However, numerical and experimental studies on hydrogen combustion in burners are very restricted. In this study, the combustion of natural gas in an industrial low swirl burner–boiler system has been experimentally investigated. The results obtained from the experimental setup have been utilized as boundary conditions for CFD simulations. With the use of Eddy Dissipation method, methane-air-2-step reaction mechanism is used for modeling of natural gas as methane gas and the reaction mechanism has been modified for natural gas considering the natural gas properties to reveal the similarities and differences of both fuels in modeling. In addition, the combustion performances of natural gas with the use of full and periodic models, which are geometric models of the burner–boiler pair, are compared. Moreover, in order to reveal the effect of the hydrogen-enriched natural gas and pure hydrogen on the performance of low swirl burner–boiler considering the combustion emissions, four various gas contents (thermal load ratio: 75%NG + 25%H₂, 50%NG + 50%H₂, 25%NG + 75%H₂, 100%H₂) at the same thermal load have been investigated. The turbulent flames of the industrial low swirl burner have been studied numerically using ANSYS Fluent 16.0 for the solution of governing equations. The results obtained in this study show that with the utilizing Eddy Dissipation method, natural gas can be modeled as methane gas with well-known methane-air-2step reaction mechanism or as natural gas with modified methane-air-2step reaction mechanism with approximate results. Additionally, the use of periodic boundary condition, which enables studying with 1/4 of geometric model, gives satisfactory results with less number of meshes when compared to the full model. Furthermore, in the case of using hydrogen-enriched natural gas or pure hydrogen instead of natural gas as the fuel, the combustion emissions of the burner–boiler such as CO and CO₂ are remarkably decreasing compared to the natural gas. However, the NOx emissions are significantly increasing especially due to thermal NO.     

Hierarchical and comparative kinetic modeling of laminar flame speeds of hydrocarbon and oxygenated fuels

The primary objective of the present endeavor is to collect, consolidate, and review the vast amount of experimental data on the laminar flame speeds of hydrocarbon and oxygenated fuels that have been reported in recent years, analyze them by using a detailed kinetic mechanism for the pyrolysis and combustion of a large variety of fuels at high temperature conditions, and thereby identify aspects of the mechanism that require further revision. The review and assessment was hierarchically conducted, in the sequence of the foundational C₀–C₄ species; the reference fuels of alkanes (n-heptane, iso-octane, n-decane, n-dodecane), cyclo-alkanes (cyclohexane and methyl-cyclo-hexane) and the aromatics (benzene, toluene, xylene and ethylbenzene); and the oxygenated fuels of alcohols, C₃H₆O isomers, ethers (dimethyl ether and ethyl tertiary butyl ether), and methyl esters up to methyl decanoate. Mixtures of some of these fuels, including those with hydrogen, were also considered. The comprehensive nature of the present mechanism and effort is emphasized.     

Modeling of autoignition and NO sensitization for the oxidation of IC engine surrogate fuels

This paper presents an approach for modeling with one single kinetic mechanism the chemistry of the autoignition and combustion processes inside an internal combustion engine, as well as the chemical kinetics governing the postoxidation of unburned hydrocarbons in engine exhaust gases. Therefore a new kinetic model was developed, valid over a wide range of temperatures including the negative temperature coefficient regime. The model simulates the autoignition and the oxidation of engine surrogate fuels composed of n-heptane, iso-octane, and toluene, which are sensitized by the presence of nitric oxides. The new model was obtained from previously published mechanisms for the oxidation of alkanes and toluene where the coupling reactions describing interactions between hydrocarbons and NO_x were added. The mechanism was validated against a wide range of experimental data obtained in jet-stirred reactors, rapid compression machines, shock tubes, and homogeneous charge compression ignition engines. Flow rate and sensitivity analysis were performed in order to explain the low temperature chemical kinetics, especially the impact of NO_x on hydrocarbon oxidation.     

Homogeneous combustion of fuel-lean H2/O2/N2 mixtures over platinum at elevated pressures and preheats

The gas-phase combustion of H₂/O₂/N₂ mixtures over platinum was investigated experimentally and numerically at fuel-lean equivalence ratios up to 0.30, pressures up to 15 bar and preheats up to 790 K. In situ 1-D spontaneous Raman measurements of major species concentrations and 2-D laser induced fluorescence (LIF) of the OH radical were applied in an optically accessible channel-flow catalytic reactor, leading to the assessment of the underlying heterogeneous (catalytic) and homogeneous (gas-phase) combustion processes. Simulations were carried out with a 2-D elliptic code that included elementary hetero-/homogeneous chemical reaction schemes and detailed transport. Measurements and predictions have shown that as pressure increased above 10 bar the preheat requirements for significant gas-phase hydrogen conversion raised appreciably, and for p = 15 bar (a pressure relevant for gas turbines) even the highest investigated preheats were inadequate to initiate considerable gas-phase conversion. Simulations in channels with practical geometrical confinements of 1 mm indicated that gas-phase combustion was altogether suppressed at atmospheric pressure, wall temperatures as high as 1350 K and preheats up to 773 K. While homogeneous ignition chemistry controlled gaseous combustion at atmospheric pressure, flame propagation characteristics dictated the strength of homogeneous combustion at the highest investigated pressures. The decrease in laminar burning rates for p ? 8 bar led to a push of the gaseous reaction zone close to the channel wall, to a subsequent leakage of hydrogen through the gaseous reaction zone, and finally to catalytic conversion of the escaped fuel at the channel walls. Parametric studies delineated the operating conditions and geometrical confinements under which gas-phase conversion of hydrogen could not be ignored in numerical modeling of catalytic combustion.     

A small detailed chemical-kinetic mechanism for hydrocarbon combustion

A chemical-kinetic mechanism is presented that is designed to be used for autoignition, deflagrations, detonations, and diffusion flames of a number of different fuels. To keep the mechanism small, attention is restricted to pressures below about 100 atm, temperatures above about 1000 K, and equivalence ratios less than about 3 for the premixed systems, thereby excluding soot formation and low-temperature fuel-peroxide chemistry. Under these restrictions, hydrogen combustion is included with 21 steps among 8 chemical species, combustion of carbon monoxide with 30 steps among 11 species, methane, methanol, ethane, ethylene, and acetylene combustion with 134 steps among 30 species, and propane, propene, allene, and propyne combustion with 177 steps among 37 species. The mechanism has been extensively tested previously for all of these fuels except propane, propene, allene, and propyne. Tests are reported here for these last four fuels through comparisons with experiments and with predictions of other mechanisms for deflagration velocities and shock-tube ignition.     

Performance and exhaust emission of turpentine oil powered direct injection diesel engine

This paper presents the results of experimental work carried out to evaluate the combustion performance and exhaust emission characteristics of turpentine oil fuel (TPOF) blended with conventional diesel fuel (DF) fueled in a diesel engine. Turpentine oil derived from pyrolysis mechanism or resin obtained from pine tree dissolved in a volatile liquid can be used as a bio-fuel due to its properties. The test engine was fully instrumented to provide all the required measurements for determination of the needed combustion, performance and exhaust emission variables. The physical and chemical properties of the test fuels were earlier determined in accordance to the ASTM standards.ResultsIndicated that the engine operating on turpentine oil fuel at manufacture's injection pressure – time setting (20.5 MPa and 23° BTDC) had lower carbon monoxide (CO), unburned hydrocarbons (HC), oxides of nitrogen (NO_x), smoke level and particulate matter. Further the results showed that the addition of 30% TPOF with DF produced higher brake power and net heat release rate with a net reduction in exhaust emissions such as CO, HC, NO_x, smoke and particulate matter. Above 30% TPOF blends, such as 40% and 50% TPOF blends, developed lower brake power and net heat release rate were noted due to the fuels lower calorific value; nevertheless, reduced emissions were still noted.