Combustion Characterization of Bio-derived Fuels and Additives

Abstract

Climate change has become a serious concern nowadays. The main reason is believed to be the high emission of greenhouse gases, namely CO2 which is mainly produced from the combustion of fossil fuels. At the same time, energy demand has increased exponentially while the energy supply mainly depends on fossil fuels, especially for transportation. The practical strategy to address such problems in medium term is to increase the efficiency of combustion-propelled energy-production systems, as well as to reduce the net release of CO2 and other harmful pollutants, likely by using nonconventional fuels. Modern internal combustion engines such as Homogeneous Charge Compression Ignition (HCCI) engines are more efficient and fuel-flexible compared to the conventional engines, making opportunities to reduce the release of greenhouse and other polluting gases to the environment. Combustion temperature in modern engines, gas turbines, and industrial burners has been reduced to prevent nitrogen oxides (NOx) formation. Besides that, the pressure has commonly been elevated to promote the efficiency of the systems. Under such conditions, ignition and pollutant formation are determined by reaction kinetic. Alternative fuels may be produced from different sources. If biomass feedstock is used in their production, they have the potential to reduce the net CO2 release to the environment. However, the oxidation chemistry of alternative fuels is less known compared to the conventional fuels. In design/optimization of modern combustionpropelled systems reliable chemical kinetic models are vital while such models are rare for alternative fuels. This knowledge gap has been a challenging factor in utilizing alternative fuels in large scale. This thesis is dedicated to provide characteristic data for fuel oxidation at high pressure and intermediate temperature. Such data provide a detailed insight into the oxidation chemistry and are vital tools in developing chemical kineticmodels. Selected iii fuels for this study, hydrogen, methane, ethane, ethanol, and dimethyl ether (DME), all can be produced from bio-sources. Their reaction kinetics are essential in modeling more complicated bio-derived fuels. Moreover, hydrogen, ethanol, andDME have been considered as additives to improve combustion properties of other fuels. In this work, experiments were carried out in a laminar flow reactor at the temperatures of 450– 900 K and pressures of 20–100 bar. The results provided information about the onset temperature of reaction and the gas composition upon reaction initiation. A wide range of stoichiometry was tested, from very fuel-lean to strong fuel-rich mixtures. For ethanol and DME, further pyrolysis experiments were carried out. The results indicated that the onset temperature of reaction varied considerably among the fuels. DME highly diluted in nitrogen ignited at 525 K, independent of the stoichiometry and much lower compared to the other fuels. Ethane, ethanol, methane, and hydrogen ignited at higher temperatures, subsequently. The effect of doping methane by DME was also investigated and it was found that even small amount of DME can promote the methane oxidation considerably. The flow reactor data have been interpreted in terms of a detailed chemical kinetic model, drawn mostly from earlier work from the same laboratory. The modeling predictions have been in good agreement with the measurements in the flow reactor. The model was further evaluated against high-pressure ignition delays as well as flame speed measurements in literature, and it successfully predicted most of the data. The reaction pathway of different fuels have been discussed, and sensitive reactions have been identified. A few reactions with high sensitivity but with poorly determined rate constants have been identified for further studies. The model was also used to analyze the complex behavior of the ignition of selected fuels against temperature and pressure. This mechanism can be utilized for further studies involving oxidation at high pressures and intermediate temperatures.