A mid-infrared laser diagnostic for simultaneous detection of furan and nitric oxide

In the context of diminishing fossil fuel resources and a global shift towards more sustainable energy sources, furan-based biofuels have emerged as a promising alternative. Furan (C₄H₄O), a simple ring-structured ether, can be produced from non-food biomass and can be used in spark-ignition engines with minimal modifications. However, a thorough understanding of furan combustion chemistry and the potential for harmful by-product formation is essential for its practical application. This is especially important for its interaction with NO_x as it is considered a model oxygenated reburn fuel for NO reduction. Addressing this need, this work introduces a novel laser-based furan diagnostic in the mid-infrared (MIR) wavelength region. The diagnostic is based on a difference-frequency generation (DFG) process between an external-cavity quantum cascade laser (EC-QCL) and a CO₂ gas laser within a nonlinear orientation-patterned GaAs crystal. This study marks the first instance of laser-based probing of the strongest vibrational band of furan near 13.4 μm, enabling spectral characterization of this band and measurements of its pressure- and temperature-dependent absorption cross-section over 1050 – 1575 K and 1.0 – 2.9 bar at 737.39 cm⁻¹ behind reflected shock waves. The developed diagnostic enabled quantification of furan mole fraction time-histories in both pyrolysis and oxidation processes, which were compared against predictions from existing kinetic models. A furan detection limit of 140 ppm was estimated for representative conditions. Moreover, this work is the first demonstration of DFG-based multispecies detection using, simultaneously, the pump and idler beams. The former and latter were used, respectively, to measure NO and furan time-histories during furan-induced NO reduction behind reflected shock waves. Our approach is not only pivotal for studying the combustion chemistry of furan but also holds significant potential in exploring NO reduction strategies using oxygenated fuels. The findings of this study contribute substantially to the field of furan chemistry, offering new insights into the practical applications of furan-based biofuels.