Alternative Fuels Based on Biomass: An Experimental and Modeling Study of Ethanol Co-Firing to Natural Gas

Abstract

In response to the limited resources of fossil fuels as well as to their combustion contributing to global warming through CO2 emissions, it is currently discussed to which extent future energy demands can be satisfied by using biomass and biogenic by-products, e.g. by co-firing. However, new concepts and new unconventional fuels for electric power generation require a re-investigation of at least the gas turbine burner if not the gas turbine itself to ensure a safe operation and a maximum range in tolerating fuel variations and combustion conditions. Within this context, alcohols, in particular ethanol, are of high interest as alternative fuel. Presently, the use of ethanol for power generation — in decentralized (micro gas turbines) or centralized gas turbine units, neat, or co-fired with gaseous fuels like natural gas and biogas — is discussed, besides its role within the transport sector. Chemical kinetic modeling has become an important tool for interpreting and understanding the combustion phenomena observed; for example, focusing on heat release (burning velocities) and reactivity (ignition delay times). Furthermore, a chemical kinetic reaction model validated by relevant experiments performed within a large parameter range allows a more sophisticated computer assisted design of burners as well as of combustion chambers, when used within CFD (computational fluid dynamics) codes. Therefore, a detailed experimental and modeling study of ethanol co-firing to natural gas will be presented focusing on two major combustion properties within a relevant parameter range: (i) ignition delay times measured in a shock tube device, at ambient (p = 1 bar) and elevated (p = 4 bar) pressures, for lean (φ = 0.5) and stoichiometric fuel-air mixtures, and (ii) laminar flame speed data at several preheat temperatures, also for ambient and elevated pressure, gathered from literature. Chemical kinetic modeling will be used for an in-depth characterization of ignition delays and flame speeds at technical relevant conditions. An extensive database will be presented identifying the characteristic differences of the combustion properties of natural gas, ethanol, and ethanol co-fired to natural gas.