New Diagnostics Enabling Bio-Fuels in Transportation with Reduced Air Pollutant Emissions and Improved Efficiency

2011-13 New Investigator Award

This project focuses on the development of in-situ sensing technology to measure important radical species, like HO₂, during the combustion of biofuels and to provide insights into the low temperature oxidation kinetics of the combustion process. This is critical to the development of advanced engine technologies that aim at providing reduction in emissions and increase in fuel efficiency through implementation of the low temperature and high pressure oxidation chemistry (< 1000 K) of biofuels.

Existing diagnostic methods are not able to easily measure such key chemical species as HO₂, H₂O₂, CH₂O, and NO_x due to lack of sensitivity or interference from other molecules. Through the interdisciplinary collaboration supported by the Grand Challenges award, it was possible to develop a sensitive in situlaser diagnostic based on Faraday Rotation Spectroscopy (FRS) for measurement of HO₂. Since the low temperature chemistry of biofuel combustion is dominated by the formation of HO₂, this project results in better understanding of this critical combustion regime, which in turn is expected to help with reduction of formaldehyde and NO_x emissions that are of serious environmental concerns.

Before this work, there was no interference-free diagnostic method for in situ measurement of radical species in the combustion process at elevated pressure available to the combustion scientists. The new HO₂ laser diagnostic was implemented in conjunction with already existing instrumentation for measurement of stable non-radical species (i.e. CO,CO₂,H₂O,CH₂O, H₂O₂) to study the oxidation chemistry of a biofuel from an atmospheric flow reactor. The following list summarizes the key accomplishments during the project funding period:

A new mid-IR FRS system to measure HO₂ at sub-ppmv levels generated by oxidation of a biofuel in a flow reactor has been developed and tested.
A theoretical FRS spectral model has been developed to extract HO₂ concentration via non-linear least-squares fitting of the experimental data.
The first direct in situ measurement of HO₂ from an atmospheric flow reactor has been demonstrated.
Implementation of a novel dual-modulation (DM)-FRS sensing technology (see Fig. 1) has been performed to increase the sensitivity of the FRS HO₂measurement.
The HO₂ laser FRS diagnostic was combined with conventional measurement techniques (gas chromatography and mass spectroscopy) to enable real time measurements of stable non-radical products during biofuel oxidation.
The first combined direct measurements of HO₂ and H₂O₂ were successfully carried out under identical experimental conditions using DM-FRS and molecular beam mass spectroscopy (MBMS) respectively (see Fig. 2).
Experiments that focused on validation of chemical kinetic models used to describe biofuel oxidation with experimental observations of HO₂ and H₂O₂have been performed and analyzed.

The successful development of an FRS system for HO₂ measurement paves the way for development of FRS diagnostics for OH and NO_x chemical species (also radicals) with sub-ppmv detection sensitivities. The experimental set-up used for DM-FRS measurement of HO₂ at the exit of a flow reactor is shown in Fig. 3. DM-FRS is implemented using high frequency laser wavelength modulation and two-stage lock-in detection used to demodulate the DM-FRS signal. This DM-FRS system provided near fundamental shot-noise limited performance in quantification of HO₂ at the exit of an atmospheric flow reactor. Additionally to improvements in system sensitivity the DM-FRS eliminates the uncontrollable effects of parasitic electromagnetic interference present in conventional FRS systems. This sensitive radical-selective diagnostics, when combined with the future planned development of sensitive mid-IR absorption spectroscopy of CH₂O and H₂O₂, will provide an unparalleled set of measurement capabilities for combustion chemistry research. As discussed in the “Other Outcomes” and “Future Directions” sections below the future development of additional diagnostics will be supported, and will lead to, additional external funding for studying the fundamental chemical reactions that play a crucial role in understanding the low-temperature oxidation of biofuels.

Fig. 3: Experimental layout used for collection of DM-FRS spectra of HO2. (FXN – function generator, EC-QCL – external-cavity quantum cascade laser, P1&2 – polarizers 1 and 2, HC – Helmholtz coils, FR – flow reactor, PD- photodiode, PA – power amplifier, LIA1&2 – lock-in amplifier 1 and 2, DAQ – digital acquisition)

Educational Impacts

In the year one of the project, PIs Wysocki and Ju have jointly developed a new course entitled “Cleaner transport fuels, combustion sensing and emission control” (ELE/MAE/CEE 428) that has been offered in the Spring semester of 2011. The course was closely linked to the conducted research and it covered fundamental aspects related to the physics of fuel combustion, light-matter interactions in combustion diagnostics, and provided a broad overview of research and development in the areas of green fuels, global environmental impacts, and emission regulations.

In the spring of 2013 Wysocki taught a course ELE555 “Laser Spectroscopy: New Technologies and Applications” which covers broad aspects of spectroscopic sensing that has a significant impact on a wide range of disciplines including environmental and atmospheric science, medicine, energy research and fundamental science. This graduate level course has been enhanced with materials originating from the research conducted within this project and exposed the students to the latest development in laser spectroscopy as reported in the recent literature. The research also helps the teaching of MAE 228 (Energy solutions for the 21^st Century) in quantification of emissions and species from combustion.

Other Outcomes

Peer Reviewed Papers:

B. Brumfield, W. T. Sun, Y. G. Ju, and G. Wysocki, “Direct In Situ Quantification of HO₂ from a Flow Reactor”, J. Phys. Chem. Lett. 4, 872 (2013)
N. Kurimoto, B. Brumfield, X. Yang, T. Wada, P. Dievart, and G. J. Wysocki, Yiguang, “Quantitative Measurements of HO₂/H₂O₂ and Intermediate Species in Low and Intermediate Temperature Oxidation of Dimethyl Ether”, (2013): submitted contribution to 35th International Symposium on Combustion, accepted papers will be published in: Proc. Combst. Inst. 35, (2015)
B. Brumfield, W. Sun, Y. Ju, and G. Wysocki, “Dual Modulation Faraday Rotation Spectroscopy of HO2 from an Atmospheric Flow Reactor”, (2013): Pre-submission manuscript available at: arXiv: 1310.2766

Conference Presentations:

B. Brumfield, W. Sun, Y. Ju, and G. Wysocki, “Faraday Rotation Spectroscopy of Radicals Relevant to Combustion,” presented at the Optical Instrumentation for Energy and the Environmental Applications, Eindhoven, Netherlands, 2012, EM4C.5
B. Brumfield, W. Sun, G. Wysocki, and Y. Ju, “Quantification of HO₂ from an Atmospheric Flow Reactor Using Faraday Rotation Spectroscopy,” in 8th U.S. National Combustion Meeting (Park City, UT, 2013)
B. Brumfield, W. Sun, G. Wysocki, and Y. Ju, “Faraday Rotation Spectroscopy of HO₂ from an Atmospheric Flow Reactor,” presented at the Sixty-Eighth International Symposium on Molecular Spectroscopy, The Ohio State University, Columbus, OH 2013
B. Brumfield, W. Sun, G. Wysocki, and Y. Ju, “Application of Faraday Rotation Spectroscopy for Quantifying HO₂ Radicals in Combustion Processes,” presented at the CLEO: 2013, San Jose, California 2013
B. Brumfield, W. Sun, Y. Ju, and G. Wysocki, “Dual Modulation Faraday Rotation Spectroscopy of HO for Combustion Diagnostics,” presented at the Optical Instrumentation for Energy and Environmental Applications, Tucson, Arizona 2013

New funding obtained as a result of scientific accomplishments from Grand Challenges support:

Andlinger Innovation Project Fund Award, Funding Period 03/2013-06/2014,
Project Title: New multi-species diagnostics and elementary rate constant measurements in biofuel combustion

Future Directions:

The PIs look forward to continuing their interdisciplinary collaborative work on further development of the FRS diagnostics for HO₂ and ON radicals as well as NO_x. Moving forward, sensitive mid-IR absorption diagnostics for important non-radical species in low-temperature oxidation of biofuels (i.e. H₂O₂ and CH₂O) will be pursued. They are currently developing a new flow reactor that will enable fundamental kinetic studies of chemical reactions that are important in combustion and atmospheric chemistry and involve OH and HO2 radicals. The successful outcomes of this research project provided the necessary instrumentation and diagnostics to pursue external funding for future projects through the following programs: