Marina Braun-Unkhoff

Reduced Reaction Mechanisms for Methane and Syngas Combustion in Gas Turbines

Two reduced reaction mechanisms were established that predict reliably for pressures up to about 20 bar the heat release for different syngas mixtures including initial concentrations of methane. The mechanisms were validated on the base of laminar flame speed data covering a wide range of preheat temperature, pressure, and fuel-air mixtures. Additionally, a global reduced mechanism for syngas, which comprises only two steps, was developed and validated, too. This global reduced and validated mechanism can be incorporated into CFD codes for modeling turbulent combustion in stationary gas turbines.

Power generation based on biomass by combined fermentation and gasification – A new concept derived from experiments and modelling

A new concept is proposed for combined fermentation (two-stage high-load fermenter) and gasification (two-stage fluidised bed gasifier with CO2 separation) of sewage sludge and wood, and the subsequent utilisation of the biogenic gases in a hybrid power plant, consisting of a solid oxide fuel cell and a gas turbine. The development and optimisation of the important processes of the new concept (fermentation, gasification, utilisation) are reported in detail. For the gas production, process parameters were experimentally and numerically investigated to achieve high conversion rates of biomass. For the product gas utilisation, important combustion properties (laminar flame speed, ignition delay time) were analysed numerically to evaluate machinery operation (reliability, emissions). Furthermore, the coupling of the processes was numerically analysed and optimised by means of integration of heat and mass flows. The high, simulated electrical efficiency of 42% including the conversion of raw biomass is promising for future power generation by biomass.

Quantitative laser diagnostic and modeling study of C2 and CH chemistry in combustion.

Quantitative concentration measurements of CH and C(2) have been performed in laminar, premixed, flat flames of propene and cyclopentene with varying stoichiometry. A combination of cavity ring-down (CRD) spectroscopy and laser-induced fluorescence (LIF) was used to enable sensitive detection of these species with high spatial resolution. Previously, CH and C(2) chemistry had been studied, predominantly in methane flames, to understand potential correlations of their formation and consumption. For flames of larger hydrocarbon fuels, however, quantitative information on these small intermediates is scarce, especially under fuel-rich conditions. Also, the combustion chemistry of C(2) in particular has not been studied in detail, and although it has often been observed, its role in potential build-up reactions of higher hydrocarbon species is not well understood. The quantitative measurements performed here are the first to detect both species with good spatial resolution and high sensitivity in the same experiment in flames of C(3) and C(5) fuels. The experimental profiles were compared with results of combustion modeling to reveal details of the formation and consumption of these important combustion molecules, and the investigation was devoted to assist the further understanding of the role of C(2) and of its potential chemical interdependences with CH and other small radicals.

An Experimental and Modeling Study of Laminar Flame Speeds of Alternative Aviation Fuels

The present work reports on measurements of burning velocities of synthetic fuel air mixtures exploiting the cone-angle method, as part of the EU project ALFA-BIRD. The GtL (Gas-to-Liquid)-air mixtures — (i) 100% GtL and (ii) GtL+20% hexanol, respectively — were studied at atmospheric pressure, with values of the equivalence ratio φ ranging between φ ∼ 1.0 and φ ∼ 1.3, at preheat temperatures To = 423 K (GtL+20% hexanol) as well as To = 473 K (for 100% GtL and GtL+20% hexanol). A comparison between experimentally obtained burning velocities and predicted values of laminar flame speed is presented, too. In general, good agreement was found between predicted and measured data for the range of conditions considered in the present study. The predictive capability of the detailed reaction model consisting of 3479 reactions involving 490 species will be discussed focusing on the laminar flame speed and the combustion of the components (n-decane, iso-octane, and 1-hexanol) of the surrogate used.Copyright

Formation of H-atoms in the Pyrolysis of 1,3-butadiene and 2-butyne: A Shock Tube and Modelling Study

Abstract For investigating the pyrolysis of 1,3-butadiene (1,3-C4H6) and 2-butyne (2-C4H6), reactive gas mixtures highly diluted with argon as bath gas were prepared. The experiments were carried out in a high purity shock tube device over a temperature range of about 1500–1800 K at total pressures between 1.2 and 1.9 bar. The time-dependent formation of H-atoms was measured behind reflected shock waves by using the very sensitive method of atomic resonance absorption spectrometry (ARAS). A detailed chemical kinetic reaction mechanism consisting of 33 elementary reactions and 26 species was used to model the experimentally obtained H-atom profiles. From kinetic modelling, with help of sensitivity and reaction flux analysis, it was concluded that reaction R 1 2-C4H6 ↔ 2-C4H5 + H is crucial for the observed formation of H-atoms during the thermal decomposition of both investigated species and within the investigated range of temperatures and pressures. Moreover, at temperatures above about 1650 K, the decay of propargyl radicals (C3H3) turns out to contribute significantly to the amount of produced H-atoms. The following rate expressions were obtained for three reactions (R 1–R 3) – among them the isomerisation from 2-butyne to 1,3-butadiene – important with respect to the formation of H-atoms within the investigated parameter range. The uncertainties are estimated to be ±30%: 2-C4H6 ↔ 2-C4H5 + H (R 1) 2-C4H6 ↔ 1,2-C4H6 (R 2) 1,3-C4H6 ↔ C2H2 + C2H4 (R 3) k 1=3.8⋅1015exp(-44871K/T)s-1 k 2=6.9⋅1013exp(-32496K/T)s-1 k 3=7.0⋅1012exp(-33768K/T)s-1

A Detailed and Reduced Reaction Mechanism of Biomass-Based Syngas Fuels

In the present work, the elaboration of a reduced kinetic reaction mechanism is described, which predicts reliably fundamental characteristic combustion properties of two biogenic gas mixtures consisting mainly of hydrogen, methane, and carbon monoxide, with small amounts of higher hydrocarbons (ethane and propane) in different proportions. From the in-house detailed chemical kinetic reaction mechanism with about 55 species and 460 reactions, a reduced kinetic reaction mechanism was constructed consisting of 27 species and 130 reactions. Their predictive capability concerning laminar flame speed (measured at To =323 K, 373 K, and 453 K, at p = I bar, 3 bars, and 6 bars for equivalence ratios ϕ between 0.6 and 2.2) and auto ignition data (measured in a shock tube between 1035 K and 1365 K at pressures around 16 bars for ϕ=0.5 and 1.0) are discussed in detail. Good agreement was found between experimental and calculated values within the investigated parameter range.

Skeletal Mechanism for C2H4 Combustion With PAH Formation

A semi-detailed kinetic mechanism with 100 species and 816 reactions for ethylene combustion including PAH formation was elaborated. The model includes the C2 H5 OH sub mechanism combustion as well. This mechanism has in view to be the base of further kinetic schemes of practical fuels (reference fuels). The mechanism was reduced to a skeletal model with 72 species and 580 reactions. The elaborated models were validated on experimental data bases of heat release as well as formation of polyaromatic hydrocarbons and soot in laminar premixed C2 H4 , C2 H4 / C2 H5 OH flames taken from literature. The calculated ignition delay times, laminar flame speeds, as well as temporal profiles of small and large aromatics and also soot particles are in good agreement with experimental data obtained for pressures 1 – 5 bar, temperatures T0 = 1100 – 2300 K, fuel/oxygen equivalence ratio φ = 0.5 – 2.Copyright

An Investigation on Laminar Flame Speed as Part of Needed Combustion Characteristics of Biomass-Based Syngas Fuels

The present work reports on laminar flame speed measurements with biogenic gas mixtures over a wide range of parameters, such as preheat temperatures, pressure, equivalence ratio, and gas compositions. The biogenic gas mixtures were derived from gasification of ethanol and of corn silage as representatives for alternative fuels containing hydrogen and methane as major components (after CO2 -sequestration), light hydrocarbons to some extent, and diluents such as nitrogen and carbon monoxide. In the present work, premixed flames were stabilized in a high pressure burner system at pressures up to 6 bars at preheat temperatures between 323 and 453 K and for equivalence ratios of φ = 0.5–1.6. In addition, a gas mixture of methane and hydrogen burning in air was investigated at atmospheric pressure. Furthermore, different detailed reaction mechanisms were used to predict the measured data. The trends and main features were captured by predictions applying different reaction mechanisms.© 2007 ASME

A Single Pulse Shock Tube Study on the Pyrolysis of 2,5-Dimethylfuran

Abstract The pyrolysis of 2,5-dimethylfuran has been studied in a single pulse shock tube equipped with fast probing device at temperatures between 1175 K and 1450 K and pressures of 8.0±0.5 bar. The initial concentration of 2,5-dimethylfuran diluted in argon (500 ppm) was much lower than in previous studies reported in the literature. Sixteen different product species were quantified by gas chromatography. The product distribution pattern was compared with the prediction of two comprehensive chemical kinetic reaction mechanisms taken from the literature. In general, the predictions of the mechanisms fit the results of the experiments; however, the comparison reveals some differences between the two mechanisms as well as between simulations and experiments.

Enhancement of a Detailed Mechanism of Propene

Propene (C3 H6 ) is an important constituent of practical hydrocarbons fuels and an important intermediate in the combustion of these fuels. Furthermore, synthetic gases such as biogenic gas mixtures not only consist of hydrogen, methane, and carbon monoxide, but also of small amounts of higher hydrocarbons, in different proportions, including propene. In the present work, a detailed propene sub-model was constructed starting from an in-house reaction model (DLR-LS) shown previously to describe major combustion properties including PAH and soot formation for several different fuel air flames. The predictive capability of the detailed propene submodel concerning laminar flame speed and ignition delay time of different propene-oxygen mixtures will be discussed. These data are needed to describe the heat release and to predict the possibility of a flashback. From these comparisons, it is concluded that the extended propene sub-model is capable to predict combustion properties of propene-oxygen gas mixtures.Copyright