60th Annual Report on Research 2015

Background and motivation

The focus of this research project was to understand the ignition and soot formation behavior of furan and gasoline surrogates as well as characteristics of blends of these fuels. The motivation stemmed from the prospects to increase energy sustainability by using biofuels blended with petroleum fuels. A major concern is the cost-effective transformation of biomass into biofuels with favorable thermochemical properties for combustion systems. Recent progress in favorable transformation of biomass into furans necessitates investigation of the fundamental combustion properties of this class of fuels as well as their chemical interactions with conventional gasoline during combustion of their blends. This sponsored project was a useful addition to experimental and modeling studies focusing on furans. The specific aims of this project therefore included: 1) investigating ignition trends among candidate furans for transportation fuels, 2) establishing ignition trends of 2,5-dimethyl furan and iso-octane as a gasoline surrogate, and 3) understanding soot and acetylene formation trends in 2,5-dimethyl furan, iso-octane and blends thereof. The previous report focused on the first and second aspects. Difficulties in attaining the goals stated in 3) opened up the possibility of extending the project to measurement of concentration time histories of interest in model development and validation. Apart from the 2,5-dimethyl furan initially proposed, other furans of interest to combustion have been investigated.

Experimental approach

Experiments were carried out behind reflected shock waves in a newly commissioned shock tube facility. The tube diameter is 10 cm and the preliminary set-up has a driver length of 2.6 m and a test section of approximately 4 m. Temperatures of the reactor were deduced from shock relations using one-dimensional gas dynamics, the initial state variables of the test gas, and the velocity of the incident shock wave. The incident shock velocity was obtained from measurements of shock arrival times using four fast-response pressure transducers mounted 30 cm apart, after accounting for attenuation of the shock by boundary layer and other non-ideal effects. Ignition delay times were obtained from sidewall chemi-luminescence signals referenced to the time of arrival of the reflected shock wave at the observational cross-section. Attempts to measure soot volume fractions using the laser extinction technique proved challenging; the soot formed during fuel-rich ignition was optically transparent to the probing red laser. In lieu of soot measurement, focused was placed on fuel and acetylene concentration measurements using direct laser absorption in the mid-IR range. Fuel concentration measurements were carried out using a He-Ne laser at 3.39 micrometers and photovoltaic detectors; and assessment of an interband cascade laser system (around 3.0 micrometers) was performed for measurement of acetylene, a soot precursor. Using the saturated furan, 2-methyl tetrahydrofuran, fuel concentration measurements were obtained during ignition and pyrolysis and are results are being compared to those of iso-octane.

Recent results

Previous results from this project established the relative reactivity of furan, 2-methyl furan, and 2,5-dimethyl furan through ignition delay time measurements, whereby 2-methyl furan was found to be the easiest to ignite and 2,5-dimethyl furan the most resistant to ignition. Ignition studies of blends of 2,5-dimethyl furan and the gasoline surrogate, iso-octane, were carried out. The results showed that 2,5-dimethyl furan is more resistant to ignition than iso-octane and blends of the two fuels were found to have ignition delay times intermediate between the two pure fuels, albeit closer to iso-octane ignition behavior. A combined chemical kinetic model was assembled from models of the two fuels, with modifications to key reactions identified by sensitivity analysis. Another key finding from this study was the significantly higher reactivity of 2-ethyl furan compared to its isomer, 2,5-dimethyl furan. This is rationalized to stem from the significant differences in the bond strengths of the two fuels, such that H-abstraction and C-C bond cleavages are much easier in the oxidation of 2-ethyl furan.

Focus was shifted to species concentration measurements, in light of the inability of laser extinction to lead to good soot measurement results. Recent results in this direction include fuel concentration measurements during ignition and pyrolysis of 2-methyl tetrahydrofuran and iso-octane. These results will further advance the validation of existing and new chemical kinetic models that will support the development of advanced combustion systems operating on bio-derived furans and conventional gasoline blends.

Impact of support for this project

Support from the ACS PRF DNI program has enabled us to contribute to the growing literature on furan combustion and extending the work to shed light on the chemical interaction between furan and a gasoline surrogate with respect to ignition. It has further enabled us to develop capabilities in shock tube laser absorption spectroscopy. It also provided partial support to 3 PhD students, a master student, and a number of undergraduate researchers working on the project.

The support for undergraduate students has been tremendously helpful as it enabled a number of students to gain research experience, some of whom have now gone on to work for companies in their combustion-related technology branches. The first PhD student is working toward defending his PhD in December 2015. The results from the entire project have been disseminated through two articles in energy journals (Combustion and Flame; Energy and Fuels) with a third article currently under review (Int. J. Chem. Kinetics). The results were also presented at the 35th combustion symposium, the 9th international colloquium on chemical kinetics, college-wide research symposia, and the 2014 RWTH Aachen symposium in Germany on Tailor-Made Fuels from Biomass.