Chemical Functional Group Approach for Low-Temperature Oxidation of Liquid Fuels
Abstract
Liquid fuels, whether petroleum-derived or clean alternatives (such as biofuels), will remain central to defense operations for the foreseeable future. Transitioning to renewable liquid fuels is a significant opportunity to promote sustainability and green operations. A precise control of autoignition behaviors of liquid fuels is the key technology of the advanced unmanned aerial vehicle (UAV) powertrain. Autoigntion characteristics of liquid transportation fuels at the temperature and pressure conditions relevant to UAV powertrain is predominantly governed by so-called low-temperature oxidation reactions. At low temperatures (< 950 K), the fuel molecules (RH) are initially oxidized by hydrogen abstraction reactions principally involving OH and HO2 to form fuel alkyl radicals (R). These fuel alkyl radicals subsequently react with molecular oxygen to form alkylperoxy radicals (R + O2 = RO2). The isomerization of RO2 by internal hydrogen transfer forming alkylhydroperoxy radicals (QOOH) is the Ògateway reactionsÓ to the low temperature degenerate branching reaction mechanism, especially significant for the hydrocarbon distillate fractions utilized in diesel and gas turbine applications. This class of isomerization reactions is of central influence on low temperature global chemistry known to influence autoignition characteristics, also highly dependent on the molecular structure of the fuel. Although the overall reaction pathways at low temperatures have been well-established, their kinetic rates have been rather estimated implicitly rather than determined through quantitative experimental measurements due to the absence of a diagnostic technique that can measure the abundance of these key species (QOOH). Unfortunately, the kinetic rates of their reactions have been a main source of uncertainties for any state-of-the-art chemical kinetic models. A "chemical functional group" is the concept of regarding molecules as group(s) of atoms as a collection yield the chemical (kinetic) behaviors of individual molecules. The chemical functional group approach can be regarded low-dimensional descriptor, which can effectively convert the high dimensional complexity of large molecules into small numbers of key functional groups, which can be accurately quantified by interpreting 1H and 13C Nuclear Magnetic Resonance spectra. The concept of chemical group additivity is the cornerstone of the detailed combustion kinetic models, and it is the key assumption informing the "rate rule" method for reaction rate constant estimation. Therefore, the implementation of chemical functional group approaches in determining chemical kinetic rates can open a novel opportunity to accurately measure and quantify the evolutions of key functionalities at low-temperature reactions. To do this, it is of importance to establish a fundamental foundation that relates the time-evolutions of key functionalities during low-temperature oxidations with the specific molecular structures employed in elementary reaction schemes. To achieve the goals of implementing chemical functional group approach, the proposed research aims to 1) develop a well-defined variable pressure flow reactor (VPFR) that can provide clear initial and boundary conditions as well as providing optical accessibility, 2) develop in-situ and ex-situ configurable-NMR technique to quantify the abundances of key functionality during low-temperature oxidations, and 3) perform fundamental experiments in VPFR to measure time-evolutions of chemical kinetic characteristics of low-temperature oxidation at broad temperature and pressure conditions and establishing the fundamental foundation to related functionalities to the specific molecular structures of key intermediate species.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jun 25, 2021
- Source ID
- W911NF2110306
Entities
People
- Sang Hee Won
Organizations
- Army Contracting Command
- United States Army
- University of South Carolina