PUB - Publikationen an der Universität Bielefeld

The global energy consumption and the related emission of climate gases is one of the most important challenges for society. In the transportation sector, a transition from the combustion of fossil fuels to that of second-generation biofuels is discussed in order to reduce the net carbon emissions.
While the first-generation of biofuels competed with the production of food, second-generation biofuels will avoid this competition and are regarded as potential alternatives to petroleum-based fuels. The combustion of typical biofuels, including alcohols, esters and (cyclic) ethers shows a reduction in the emission of unburnt hydrocarbons, NOx and soot, while the formation of oxygenated compounds is typically increased. In particular small methyl ketones like 2-butanone (methyl ethyl ketone, MEK) are receiving increasing attention because of their availability from cellulosic biomass and their favorable properties for use in spark-ignition engines. However, these biofuels contain at least one oxygen atom in their molecular structure, which leads to a different oxidation behavior from that of conventional petroleum-based fuels. While the oxidation reactions of hydrocarbon has been well studied in the past decades, information on the combustion kinetics of oxygenated biofuels is still relatively scarce.
With the aim to reduce the emissions of volatile compounds, like NOx and soot, current work focusses on engines with a reduced combustion temperature as well as more homogenous mixing of fuel and air. Such homogeneous charge compression ignition engine concepts combine the strengths of both, spark-ignition and diesel engines. However, combustion at lower temperatures reduces the stability of the process, and thus the current knowledge needs to be extended to avoid possible misfire in engines. Similarly, detailed kinetic information and validated mechanisms to simulate the combustion under relevant conditions of pressure, temperature, and fuel-oxidizer mixtures are required to understand conditions that would lead to a sudden stall of the engine.
In this work, the oxidation kinetics of biofuels in defined areas of the phase space (pressure, stoichiometry and temperature) were studied. For this purpose a laminar flow reactor and a jet-stirred reactor were used to measure species concentrations in the low- to intermediate temperature regime (T = 500 - 1100 K) at atmospheric pressures. In addition, the high-temperature oxidation behavior was investigated in a laminar, premixed, low-pressure (40 mbar) flame. Species concentrations were obtained by molecular-beam mass-spectrometry (MBMS), and two different ionization techniques, i. e. electron impact (EI) and photo-ionization (PI), were used.
2-Butanone, a methyl ketone, has recently been identified and discussed as a promising alternative to conventional gasoline. Since available data are lacking for the entire class of small methyl ketone fuels in the literature, regarding the kinetics of ketones, 2-pentanone (methyl propyl ketone, MPK) has also been chosen as an interesting investigation target to study the influence of the carbonyl group. The oxidation of these ketones was investigated from 700 -1100 K in a laminar flow reactor, and for the first time low-temperature-related oxygenated species have been identified. The high-temperature kinetics was investigated in laminar, premixed, low-pressure flames in Bielefeld with EI-MBMS and at the Advanced Light Source (Berkeley, CA, USA) with PI-MBMS enabling the identification of isomers. With the high mass resolution and, relying on two different ionization techniques, a cross-validation helped to minimize the experimental uncertainties for these experiments. These experimental data led to the development and validation of a detailed reaction mechanism emphasizing the importance of low-temperature reaction classes. An older version of the recently published 2-butanone mechanism was used as a base for the 2-pentanone mechanism, which is used and discussed together with the new experimental data for the first time in this work.
The primary reference fuel (PFR) iso-octane was investigated at low temperatures (T = 500 – 700 K) and atmospheric pressure in a jet-stirred reactor coupled to a PI-MBMS experiment at the ALS. Because iso-octane is in general unreactive at these conditions, dimethyl ether (DME) with defined and well-known low-temperature oxidation reactions was added to enhance the reactivity. Two mixing ratios were studied (30:70 and 70:30), and several highly oxygenated compounds up to species related to a third O2-addition were detected. These results are compared to a so-called “horizontally-lumped” kinetic model and are further used to understand the effects of mixtures.