Real reaction kinetics for the production and utilisation of advanced biofuels

The conversion of biomass to liquid transportation fuels is currently being explored as a sustainable method to reduce global reliance on fossil derived fuels and reduce greenhouse gas emissions. One such method is the acid catalysed conversion of lignocellulose derived carbohydrates in ethanol (ethanolysis) to advanced biofuel components. In such processes, the lignocellulose derived carbohydrate is converted to the advanced biofuel component ethyl levulinate and ethanol is also converted to diethyl ether. Thus, the reaction process readily results in mixtures of principally ethyl levulinate, diethyl ether and unreacted ethanol, which could potentially be used as an advanced biofuel mixture. In this Thesis, fundamental theory and quantitative experimental measurements are coupled through chemical reaction kinetic modelling at different levels of detail to develop a comprehensive mechanistic and kinetic understanding of:

(i) The acid catalysed conversion of lignocellulose derived carbohydrates in ethanol to advanced biofuel mixtures.

(ii) The autoignition behaviour of such advanced biofuel mixtures.

Firstly, the acid catalysed reaction system of lignocellulose derived carbohydrates, D-glucose and D-fructose in ethanol is experimentally examined. A simple reaction kinetic model is developed to describe the consumption and formation of key reaction components. Using this model, it is shown that the ethanolysis reaction system can be tailored to produce advanced biofuel mixtures of desired fuel properties, ranging in derived cetane number from 10 to 130.

A mechanistic comprehension of the fundamental processes occurring at a molecular level for the key reactions of D-glucose and hydrogen cation as well as ethanol and hydrogen cation is further pursued using quantum chemical calculations at the G4MP2 level of theory coupled with the universal solvation density model (SMD) at the B3LYP/6-31G(2df,p) level of theory to account for the effects of the solvent. An improved reaction mechanism for the D-glucose and hydrogen cation reaction system is constructed, including key pathways such as isomerisation, protonation, hydrogen cation transfer and decomposition to previously unreported products. The calculated potential energy surface of the ethanol and hydrogen cation reaction system, along with solution phase experimental measurements, is used as a test case to develop a method for constructing solution phase kinetic models from first principles. Through the construction of the first principles model, it is highlighted that the ∆Gsolvation(H+) is the principally defining term of the overall reactivity of the reaction system.

Finally, the autoignition behaviour of advanced biofuel mixtures of ethyl levulinate, diethyl ether and ethanol is characterised using both ignition quality tester and rapid compression machine measurements. Fuel mixtures of comparable autoignition propensity to market gasoline and diesel fuels are identified. A detailed combustion kinetic model is developed to understand the key differing combustion reaction processes between the advanced biofuel mixtures and conventional petroleum derived fuels.