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Summary

This programs is aimed at determining the fundamental chemical and physical properties of the large organic compounds in fuels related to their combustion. Such information are necessary inputs for the use of the advanced computational tools that are now becoming available for describing the behavior of reactive flow systems. It is a part of an extensive DOD program with the aim of developing the knowledge base necessary for the introduction of alternative fuels for DOD needs. To meet these needs NIST obtain new data through unique experiments in a heated single pulse shock tube on the kinetics of the decomposition of heavy hydrocarbons, determine the diffusion coefficients of such compounds in a newly developed apparatus, interpret results so that fundamentally based structural activity relations can be derived leading to the prediction of the behavior of all the components in real fuel mixtures and examine the existing information so as to build a bridge between old and new results.

Description

The variability of the price of oil, the need for energy efficiency and pollution minimization has led to increasing interest in alternative fuels. These fuels are structurally and compositionally different than the conventional fuels presently used.  Past work on conventional fuels have established on an empirical basis that such differences can lead to drastic changes in the combustion properties of fuels as manifested in pollutant emissions and efficiencies. The differences between alternative fuels and conventional petroleum based fuels are larger It is important to have an information base before their introduction and thus assuring that they can be "drop in" replacements in existing combustion devices.

Progress in the development of Computational Fluid Dynamics codes has led to the possibility of describing the quantitative details of reactive flows. They have applicability to combustion problems dealing with real fuels in real devices. The aim is a computationally based predictive tool that can replace expensive and uncertain physical testing. In order to make use of the computational tools a quantitative understanding of the basic physico-chemical phenomenon is necessary.

The emphasis is on fundamental information on chemical and physical properties that can be used in any environments.  For the former, interest is focused on the quantitative details of the breakdown of the larger hydrocarbons that are components in real fuel mixtures. This is an area that has been neglected in the past. It is important for the present application since the reaction of the breakdown products with active species are responsible for many combustion phenomenon. A single pulse shock tube have been used to generate the large hydrocarbon radicals of interest and observing their branching ratio for decomposition. From this data and available information on related processes at lower temperatures and estimated thermodynamic properties, rate constant for unimolecular decomposition and isomerization are generated under all combustion conditions. Radicals containing up to eight carbon atoms have been studied. Data dealing with the effect methyl substitution have been obtained and is being analyzed. Such information have direct bearing on Fisher-Tropsch fuels. Work on oxygenated radicals (from biofules) will be initiated in the near future. Work on oxidative decomposition have now been initiated. The principal physical property of interest is the diffusion coefficient. For the larger fuel molecules there is no data. CFD modeling led to the conclusion that they can have important effect on the simulation results. A series of experiments to test this hypothesis have been iniiated and preliminary results have been obtained. Finally, existing data on these issues existing in the literature have been analyzed. A database of databases have been constructed. A consistent nomenclature have been developed and detailed comparisons between existing chemical kinetics databases on the same fuel can now be made. Practically all such databases have been on single component fuels. Due to the necessity of fitting global measurements, a mixtures database that is necessary for real fuels cannot be simply a conglomeration of such databases. In the present approach the mixing rules become transparent.

Major Accomplishments

We have developed a new tool for studying combustion kinetics of the breakdown of the various components in a real fuel mixture. A general picture regarding pyrolytic decomposition has now been obtained. Particular attention has been focussed on the pyrolysis of linear hydrocarbons and it is now possible to develop from the data that have been obtained a database that covers all such compounds regardless of their size. Pyrolytic decomposition is of course an important process in its own right. We have found that for many applications the data on these processes may have important implications regarding the behavior of rich mixtures leading to soot formation.

We held this September a highly successful meeting at NIST devoted to fuel issues. As far as we know this is the first such meeting devoted to the combustion properties of fuels. It is clear that the qualification of new fuels that can be used in existing device is an extremely challenging problem. Thus at a recent AICHE meeting it was stated that 50% of current biofuels do not pass the standard ASTM tests required for certification.

ASSOCIATED PUBLICATIONS

Combustion kinetics database through PRIME. This is an exciting new approach that makes use of modern cybertools and seeks to utilize the entire community in building the database and is currently centered at UC Berkeley.

Created March 13, 2009, Updated May 22, 2018