PUB - Publikationen an der Universität Bielefeld

It is well known that emissions from combustion processes are harmful and dangerous for climate, air quality, environment and health. However, a significant increase of anthropogenic CO2, particulate matter, and soot has been measured over the past years. Since more than 80% of the global primary energy is still covered by fossil energy sources, an immediate substitution by renewable energy is not yet possible and efficient and cleaner alternatives are needed for the transition period in the next 10-20~years.
To achieve such cleaner combustion goals, several changes in different fields should be considered, while in engine combustion two main approaches are pursued. These suggested developments include the technical approach of a homogeneous low-temperature combustion, which is supposed to lead to a lower emission of pollutants, as well as the use of alternative fuels (e.g. alcohols, ethers, esters) with a proposed cleaner emission than prototypical fuels. However, due to their different molecular structures including heteroatoms, they often exhibit a very different species distribution in their combustion. The respective chemical composition can lead to the emission of toxic species or pollutants that can have negative influences on human health and the atmosphere by photochemical reactions. Therefore, the combustion behavior of these types of fuels needs to be analyzed in more detail to gain understanding of their complex reaction pathways, especially in the low-temperature regime.
Technical studies often analyze global parameters of combustion as e.g. ignition delay times, flame speeds or the concentration of unburnt hydrocarbons at the tailpipe. However, from the chemical point-of-view, the combustion process is highly complex. Therefore, the aim of this work was to achieve detailed knowledge about specific reaction pathways in the combustion process of different fuels and fuel mixtures to help evaluating the potential of possible alternative fuels and fuel additives. For this purpose, laminar premixed low-pressure flames and a laminar flow reactor were used as model experiments on a laboratory scale to cover a broad range of the relevant phase space including temperature, pressure and stoichiometry. The species distributions in different oxidation processes were analyzed by molecular-beam mass spectrometry serving as a universal technique to measure a multitude of species at the same time. A combination of different ionization techniques covering electron impact ionization, photoionization and photoelectron/photoion coincidence spectroscopy has been used to cross-validate the obtained data and to gain complementary information for a detailed structure analysis of species occuring in the oxidation processes. Therefore measurements at Bielefeld University were combined with several measurements at large-scale setups using synchrotron-generated vacuum-ultraviolet radiation from the Advanced Light Source in Berkeley, USA, the National Synchrotron Radiation Laboratory in Hefei, China and the SOLEIL Synchrotron in Gif-sur-Yvette, France. Furthermore, the experimental data has been complemented by specific and internally consistent reference measurements, theoretical calculations and kinetic modeling as a connection between laboratory-scale experiments and technical processes.
The main focus of this work was the investigation of alternative fuels and their influences on the combustion process of mixtures, as these are already used on the road (e.g. E10, biodiesel). Adding alternative fuels to prototypical compounds can have a significant impact on the reacitivity and the reaction pathways of the oxidation leading to interaction between species rising from the oxidation process. Currently, only little information is available on these mixing effects. Therefore, several pure fuels as well as mixtures of prototypical and alternative fuel candidates have been analyzed in the low- and high-temperature environment to investigate the influence of fuel additives and interactive effects in mixtures. As a fundamental result of this work it could be confirmed that a combination of several experimental techniques together with theoretical calculations and kinetic modeling is very important and necessary to obtain the complex information needed on the combustion process of fuels. The results revealed that the molecular structure of the fuel molecules as well as the oxidation environment are of significant influence for the reaction pathways and therefore the formation of possible pollutants. For the addition of alcohols and ethers very strong and partially contrary influences on the fuels reactivity and the resulting species distribution could be observed for the low- and high-temperature regime. While in a high-temperature environment only small effects and mainly on the formation of soot precursors were found, the reactivity of the mixtures was dramatically influenced in a low-temperature environment leading to a different species distribution, enabling the possibility to influence the combustion process by changing the oxidation environment and a selective addition of specific components. Furthermore, the experimental results of this work have contributed to the further development and validation of several kinetic models by detecting new species and possible reaction pathways that have not been included in simulations before, but can be used to improve the predictability of such mechanisms in the future.