Cracking of Exotic Feedstocks: Simplification and Lumping Procedures in the Development of Large Kinetic Schemes

Detailed chemistry of pyrolysis of liquid feedstocks or pyrolysis and oxidation of heavy fuels involves a large number of species and reactions and makes computation expensive. Comprehensive kinetic schemes need to be simplified, maintaining the relevant features of the complete mechanism.

The goal of this paper is to analyse and discuss the lumping procedures applied to the development and validation of the detailed kinetic models of SPYRO program, as well as the simplification rules extensively applied when developing general kinetic scheme of oxidation and combustion of heavy hydrocarbon and oxygenated fuels.

The automatic generation of primary propagation reactions often demands for careful simplifications. It is not of interest to automatically generate detailed mechanisms with several thousands of species and elementary reactions. A compromise has to be found between computation efforts and prediction accuracy. It becomes more practical or even necessary, to lump the chemical species and reactions using defined rules, that satisfy the requirements of the system. Once simplification rules are properly understood and well defined, from a modelling point of view it is convenient to directly link a post-processor to the kinetic generator with the purpose of lumping intermediate and final products into a more limited number of equivalent components.

On these bases, the resulting kinetic model is more consistent and reliable. Advantages and limitations of chemical lumping are analysed using several examples. The automatic generation of detailed kinetic schemes of pyrolysis and combustion of n-heptane and iso-octane are discussed.

These simplifications, useful for liquid feedstocks and heavy fuels, partially limit the capabilities of the semi-detailed kinetic model when it comes to analyse exotic feeds (e.g. treated refinery streams) or unconventional operating conditions. Flexible lumping procedures allow to relax the simplification rules and to extend the prediction capabilities of the kinetic model. Simple examples illustrate this extension for SPYRO program.

The mechanistic nature of detailed and semi-detailed kinetic models, as well as the sound and well established experience in pyrolysis kinetics, makes also easy the analysis of new and exotic feedstocks, such as mixtures containing alcohols and oxygenated species or new fuels with high volumetric energy content.