Chemistry of Hydrocarbon and Biofuel Combustion. Reaction Mechanisms

Worldwide, over 80% of the energy produced is by means of a combustion process. The aim of a combustion process is to extract latent chemical energy from a molecule and convert it into heat, which can then be converted into electrical or mechanical energy. The molecules used as raw materials in these processes are currently almost exclusively fossil resources extracted from the subsoil, the easily exploitable quantities of which are limited. This conversion into heat is achieved through a complex set of reactions, the theoretical end products of which are water and carbon dioxide. Emissions of the latter are the main cause of the increase in the planetary greenhouse effect and of the induced climate disruption observed over the last few decades. In addition to these ideal combustion products, other polluting molecules such as nitrogen monoxide, volatile organic compounds (VOCs), nitrogen oxides (NO _x ), polycyclic hydrocarbons and soot degrade local air quality and impact on people's health.

Faced with this dependence on combustion processes, and in the expectation that new technologies will be able to replace them on a massive scale, it is essential to understand precisely how different fuels burn at the chemical level, in order to optimize installations and thus increase their efficiency while limiting pollutant emissions.

Combustion is studied using a variety of techniques involving specific equipment (shock tubes, rapid compression machines, reactors, burners) equipped with diagnostic tools for monitoring or measuring the speed at which the combustion reaction takes place. These experimental data, obtained under controlled temperature and pressure conditions, enable us to identify the reaction sequence leading to the complete oxidation of a hydrocarbon into carbon dioxide. They are then used to test and validate kinetic models.

A kinetic model is a complex, sequential reaction mechanism that associates a rate constant with each reaction. It can be used to describe qualitatively and quantitatively the combustion of a starting molecule as a function of initial temperature, pressure and mixing conditions. When these models are able to reasonably reproduce the experimental data obtained on several pieces of equipment, the model can then be considered validated and used with confidence under more complex conditions, far removed from those used for validation, thus providing a better understanding of pollutant formation and possible sources of combustion optimization in an engine or gas turbine, for example.

With the aim of reducing the environmental footprint of combustion, a great deal of research has been undertaken to advance our knowledge of the combustion kinetics...