Kinetic modelling of biofuels: pyrolysis and auto-ignition of aldehydes

Aldehydes are involved in practically all the oxidation processes of solid and liquid fuels, and they are of concern as pollutants from different combustion systems such as conventional spark-ignition gasoline engines and compression-ignition diesel engines. In the case of biofuel combustion, aldehydes are not only pollutants whose emissions have to be controlled, but also, they are primary products of bio-oil, influencing the combustion properties. A similar statement can be applied to biomass combustion and/or gasification processes, where aldehydes (also including aromatic aldehydes) have been highlighted as products of biomass and coal gasification. With the rising interest in biofuels and biomasses for energy applications, there is a need to assess the combustion of aldehydes, in order to better understand the influence of the carbonyl group on the combustion properties of these species. While formaldehyde and acetaldehyde have been studied in detail in the recent and past literature, just a few works have focused on long chain aldehydes. A few recent combustion studies on C3-C5 aldehydes provide new fundamental data for a better understanding of their pyrolysis and combustion behaviour which in turn form the basis for a more reliable extension of the kinetic knowledge towards oxygenated biofuels. A detailed mechanism has been developed at the Combustion Chemistry Centre (NUI Galway, Ireland) and a lumped one developed within the research activity of the CRECK Modeling group (Politecnico di Milano, Italy). Both the models, validated over a wide range of new and previously published data (flame speeds, ignition delay time, pyrolysis speciations), showed good capability to reproduce the main experimental observations and the kinetic analysis highlighted key consumption channels, mainly related to the carbonyl moiety of the molecule. Only the results for the lumped mechanism referring to the auto-ignition behaviour of aldehydes are discussed in this paper. This work constitute a first step towards a better knowledge of the combustion behavior of oxygenated bio-oil and biofuels.