59th Annual Report on Research 2014

The focus of this research project is to understand the ignition and soot formation behavior of furan and gasoline fuel surrogates as well as characteristics of blends of these fuels. The use of biofuels blended with petroleum fuels is viewed as a responsible approach toward improving combustion dynamics in engines and reducing pollutant formation while also increasing transportation fuel sustainability. A major concern is the cost-effective transformation of biomass into biofuels with favorable 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. Hitherto experimental and modeling studies have focused on the individual furans. The specific aims of this project therefore include: 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. This report focuses on the first and second aspects.

Experimental approach

Experiments are 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 are 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 is 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 are obtained from sidewall chemi-luminescence signals referenced to the time of arrival of the reflected shock wave at the observational cross-section. The next phase of the project involves incorporating laser absorption and extinction measurements to quantify acetylene and soot formation. Test mixtures consist of fuel, oxygen, and a diluent, such as argon or nitrogen. Comparable ignition delay times have been observed for combustible mixtures with argon and nitrogen as alternate diluents. For the rest of the studies, argon is used as a diluent because of its faster relaxation times after shock heating.

Recent results

We have compared the ignition behavior of furan, 2-methylfuran, and 2,5-dimethyl furan under similar conditions of equivalence ratio, the ratio of argon to oxygen, and nominal pressure over a range of temperatures. It is shown that 2,5-dimethyl furan has the longest ignition delay times, indicative of reduced chemical reactivity. On the other hand, 2-methyl furan is the most reactive, with the shortest ignition delay times. The ignition behavior of furan lies between those of the other two furans. It is rationalized that double alkylation as in 2,5-dimethyl furan confers greater stability to the furan, thereby making initial attack on the C-H bonds more difficult. This point is pursued further to understand why 2-methyl furan shows much higher reactivity. Existing chemical kinetic models for 2,5-dimethyl and 2-methyl furans confirm the reactivity trends observed in our experiments. However, quantitative agreement is poor, indicating the need for further model improvement.

We have also established that 2,5-dimethyl furan is less reactive than iso-octane, in line with the reactivity trend shown by the global parameter known as the octane number. This is 100 for iso-octane and reported to be 119 for 2,5-dimethyl furan. However, the ignition of 2,5-dimethyl furan is characterized by a weaker global temperature sensitivity compared to iso-octane. These results are based on high-temperature ignition data, where the post-reflected temperatures are higher than 1000 K. Further experiments are needed at lower temperatures to confirm the reduced reactivity of 2,5-dimethyl furan compared to iso-octane. Experiments with blends of 2,5-dimethyl furan and iso-octane in equal liquid volume proportions show that the ignition delay times are intermediate between those of the pure fuels, albeit in closer alignment with those of iso-octane.

Compared to predictions by chemical kinetic models, it is observed that the measured iso-octane ignition delay times are in closer agreement with predictions using an updated iso-octane from the Lawrence Livermore database of chemical kinetic models. A combined chemical kinetic model has been developed in this project by incorporating an iso-octane sub model into a recent model of 2,5-dimethyl furan. It is being improved for better agreement with our observed ignition delay times and other dataset from the literature.

Impact of support for this project

Suppoprt 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 also provided partial support to 3 PhD students and a number of undergraduate researchers working on the project. The results described above have been disseminated through an article in Energy and Fuels, a poster at the combustion symposium, two posters at college-wide research symposia, and a presentation at the 2014 RWTH Aachen symposium in Germany on Tailor-Made Fuels from Biomass.