René Fournet

Combustion chemistry and flame structure of furan group biofuels using molecular-beam mass spectrometry and gas chromatography – Part I: Furan

Fuels of the furan family, i.e. furan itself, 2-methylfuran (MF), and 2,5-dimethylfuran (DMF) are being proposed as alternatives to hydrocarbon fuels and are potentially accessible from cellulosic biomass. While some experiments and modeling results are becoming available for each of these fuels, a comprehensive experimental and modeling analysis of the three fuels under the same conditions, simulated using the same chemical reaction model, has - to the best of our knowledge - not been attempted before. The present series of three papers, detailing the results obtained in flat flames for each of the three fuels separately, reports experimental data and explores their combustion chemistry using kinetic modeling. The first part of this series focuses on the chemistry of low-pressure furan flames. Two laminar premixed low-pressure (20 and 40 mbar) flat argon-diluted (50%) flames of furan were studied at two equivalence ratios (φ=1.0 and 1.7) using an analytical combination of high-resolution electron-ionization molecular-beam mass spectrometry (EI-MBMS) in Bielefeld and gas chromatography (GC) in Nancy. The time-of-flight MBMS with its high mass resolution enables the detection of both stable and reactive species, while the gas chromatograph permits the separation of isomers. Mole fractions of reactants, products, and stable and radical intermediates were measured as a function of the distance to the burner. A single kinetic model was used to predict the flame structure of the three fuels: furan (in this paper), 2-methylfuran (in Part II), and 2,5-dimethylfuran (in Part III). A refined sub-mechanism for furan combustion, based on the work of Tian et al. [Combustion and Flame 158 (2011) 756-773] was developed which was then compared to the present experimental results. Overall, the agreement is encouraging. The main reaction pathways involved in furan combustion were delineated computing the rates of formation and consumption of all species. It is seen that the predominant furan consumption pathway is initiated by H-addition on the carbon atom neighboring the O-atom with acetylene as one of the dominant products.

Unimolecular decomposition of 2,5-dimethylfuran: a theoretical chemical kinetic study

The unimolecular decomposition of 2,5-dimethylfuran (DMF), a promising next-generation biofuel, was studied at the CBS-QB3 level of theory. As most of its decomposition routes remain unknown, a large number of pathways were explored: initial C-H bond fission, biradical ring opening, H-atom and CH(3)-group transfers involving carbene intermediates. Based on the computed potential energy surfaces, thermochemical data and high-pressure limit rate constants were determined and included in a small detailed kinetic mechanism for DMF unimolecular decomposition. Simulations performed under the conditions of recent experimental data focusing on the initial steps in DMF thermal decomposition showed that the unimolecular decomposition of DMF is dominated by two product channels. For typical combustion conditions, the major product channel (~70% at 1500 K and 1 bar) is reached by a 3,2-hydrogen transfer in DMF leading to a carbene intermediate that rearranges to hexa-3,4-dien-2-one which in turn decomposes into CH(3)CO and C(4)H(5) by initial C-C bond fission. In contrast with previous studies, the initial C-H bond fission in DMF has not been found to be the major decomposition channel (~30% at 1500 K and 1 bar). Pressure effects were probed using a semi-quantitative approach and were found to be negligible above 1 bar. Below atmospheric pressure, initial C-H bond cleavage yields increase while CH(3)CO + C(4)H(5) branching ratios decrease. This study brings a new understanding of the nature of the initial radicals and molecules created upon thermal activation of DMF.

Quantum Chemical Study of the Thermochemical Properties of Organophosphorous Compounds.

Organophosphorous compounds are involved in many toxic compounds such as fungicides, pesticides, or chemical warfare nerve agents. The understanding of the decomposition chemistry of these compounds in the environment is largely limited by the scarcity of thermochemical data. Because of the high toxicity of many of these molecules, experimental determination of their thermochemical properties is very difficult. In this work, standard gas-phase thermodynamic data, i.e., enthalpies of formation (ΔfH298°), standard entropies (S298°), and heat capacities (Cp°(T)), were determined using quantum chemical calculations and more specifically the CBS-QB3 composite method, which was found to be the best compromise between precision and calculation time among high accuracy composite methods. A large number of molecules was theoretically investigated, involving trivalent and pentavalent phosphorus atoms, and C, H, O, N, S, and F atoms. These data were used to propose 83 original groups, used in the semiempirical group contribution method proposed by Benson. Thanks to these latter group values, thermochemical properties of several nerve agents, common pesticides and herbicides have been evaluated. Bond dissociations energies (BDE), useful for the analysis the thermal stability of the compounds, were also determined in several molecules of interest.

Rich methane premixed laminar flames doped with light unsaturated hydrocarbons

The structure of three laminar premixed rich flames has been investigated: a pure methane flame and two methane flames doped by allene and propyne, respectively. The gases of the three flames contain 20.9% (molar) of methane and 33.4% of oxygen, corresponding to an equivalence ratio of 1.25 for the pure methane flame. In both doped flames, 2.49% of C3H4 was added, corresponding to a ratio C3H4/CH4 of 12% and an equivalence ratio of 1.55. The three flames have been stabilized on a burner at a pressure of 6.7 kPa using argon as dilutant, with a gas velocity at the burner of 36 cm/s at 333 K. The concentration profiles of stable species were measured by gas chromatography after sampling with a quartz microprobe. Quantified species included carbon monoxide and dioxide, methane, oxygen, hydrogen, ethane, ethylene, acetylene, propyne, allene, propene, propane, 1,2-butadiene, 1,3-butadiene, 1-butene, isobutene, 1-butyne, vinylacetylene, and benzene. The temperature was measured using a PtRh (6%)–PtRh (30%) thermocouple settled inside the enclosure and ranged from 700 K close to the burner up to 1850 K. In order to model these new results, some improvements have been made to a mechanism previously developed in our laboratory for the reactions of C3–C4 unsaturated hydrocarbons. The main reaction pathways of consumption of allene and propyne and of formation of C6 aromatic species have been derived from flow rate analyses.

Prediction of Auto-Ignition Temperatures and Delays for Gas Turbine Applications

Gas turbines burn a large variety of gaseous fuels under elevated pressure and temperature conditions. During transient operations like maintenance, start-ups or fuel transfers, variable gas/air mixtures are involved in the gas piping system. Therefore, in order to predict the risk of auto-ignition events and ensure a safe and optimal operation of gas turbines, it is of the essence to know the lowest temperature at which spontaneous ignition of fuels may happen. Experimental auto-ignition data of hydrocarbon-air mixtures at elevated pressures are scarce and often not applicable in specific industrial conditions. AIT data correspond to temperature ranges in which fuels display an incipient reactivity, with time scales amounting in seconds or even in minutes instead of milliseconds in flames. In these conditions, the critical reactions are most often different from the ones governing the reactivity in a flame or in high temperature ignition. Some of the critical paths for AIT, especially those involving peroxy radicals, are similar to those encountered in slow oxidation. Therefore, the main available kinetic models that have been developed for fast combustion, are unfortunately unable to represent properly these low temperature processes.In this context, a numerical approach addressing the influence of process conditions on the minimum auto-ignition temperature of different fuel/air mixtures has been developed. For that purpose, several chemical models available in the literature have been tested, in order to identify the most robust ones. Based on previous works of our group, a model covering a large temperature range has been developed, which offers a fair reconciliation between experimental and calculated AIT data through a wide range of fuel compositions. This model has been validated against experimental auto-ignition delay times (AID) corresponding to high temperature in order to ensure its relevance not only for AIT aspects but also for the reactivity of gaseous fuels over the wide range of gas turbine operation conditions. In addition, the AITs of methane, of pure light alkanes and of various blends representative of several natural gas and process-derived fuels were extensively covered. In particular, among alternative gas turbine fuels, hydrogen-rich gases are called to play an increasing part in the future so that their ignition characteristics have been addressed with particular care. Natural gas enriched with hydrogen, and different CO/H2 syngas fuels originating from a blast furnace (BFG) have namely been studied. Globally, the individual species covered are: H2, CO, CO2, N2, CH4, C2H6, C3H8, C4H10, and C5H12. AIT values have been evaluated in function of the equivalence ratio and pressure. All the results obtained have been fitted by means of a practical mathematical expression. The overall study leads to a simple correlation of AIT versus equivalence ratio/pressure that may be of fruitful use for the engineering community.Copyright

Combustion and Oxidation Kinetics of Alternative Gas Turbines Fuels

Heavy duty gas turbines are very flexible combustion tools that accommodate a wide variety of gaseous and liquid fuels ranging from natural gas to heavy oils, including syngas, LPG, petrochemical streams (propene, butane…), hydrogen-rich refinery by-products; naphtha; ethanol, biodiesel, aromatic gasoline and gasoil, etc. The contemporaneous quest for an increasing panel of primary energies leads manufacturers and operators to explore an ever larger segment of unconventional power generation fuels. In this moving context, there is a need to fully characterize the combustion features of these novel fuels in the specific pressure, temperature and equivalence ratio conditions of gas turbine combustors using e.g. methane as reference molecule and to cover the safety aspects of their utilization.A numerical investigation of the combustion of a representative cluster of alternative fuels has been performed in the gas phase, namely two natural gas fuels of different compositions, including some ethane, a process gas with a high content of butene, oxygenated compounds including methanol, ethanol, and DME (dimethyl ether). Sub-mechanisms have specifically been developed to include the reactions of C4 species. Major combustion parameters, such as auto-ignition temperature (AIT), ignition delay times (AID), laminar burning velocities of premixed flames, adiabatic flame temperatures, and CO and NOx emissions have then been investigated. Finally, the data have been compared with those calculated for methane flames.These simulations show that the behaviors of alternative fuels markedly differ from that of conventional ones. Especially, DME and the process gases appear to be highly reactive with significant impacts on the auto-ignition temperature and flame speed data, which justifies burner design studies within premixed combustion schemes and proper safety considerations. The behaviors of alcohols (especially methanol) display some commonalities with those of conventional fuels. In contrast, DME and process gas fuels develop substantially different flame temperature and NOx generation rates than methane. Resorting to lean premix conditions is likely to achieve lower NOx emission performances. This review of gas turbine fuels shows for instance that the use of methanol as a gas turbine fuel is possible with very limited combustor modifications.Copyright

Combustion and Pyrolysis Kinetics of Chloropicrin

Chloropicrin (CCl3NO2) is widely used in agriculture as a pesticide, weed-killer, fungicide or nematicide. It has also been used as a chemical agent during World War I. The precise understanding of its combustion chemistry for destruction processes or in the event of accidental fire of stored reserves is a major safety issue. A detailed chemical kinetic model for the combustion and pyrolysis of chloropicrin is proposed for the first time. A large number of thermo-kinetic parameters were calculated using quantum chemistry and reaction rate theory. The model was validated against experimental pyrolysis data available in the literature. It was shown that the degradation of chloropicrin is ruled by the breaking of the C-N bond followed by the oxidation of the trichloromethyl radical by NO2 through the formation of the adduct CCl3ONO, which can decompose to NO, chlorine atom, and phosgene. Phosgene is much more stable than chloropicrin and its decomposition starts at much higher temperatures. Combustion and pyrolysis simulations were also compared and demonstrated that the addition of oxygen has very little effect on the reactivity or product distribution due to the absence of hydrogen atoms in chloropicrin.