Measurements of hydrocarbons using laser-induced breakdown spectroscopy

Methods of quickly and rapidly measuring gas composition in combustion systems are of great practical interest. Optical methods such as Raman spectroscopy are quite useful in understanding fluid mixing, optimizing combustion, and minimizing emissions. However, many existing optical methods are limited by the need for some knowledge of the reaction progress, as they measure mole fractions of molecular reactant or product species. Other methods measure condensed-phase (spray) concentrations before combustion, or flame emission directly, to infer composition. Here we describe the use of laser-induced breakdown spectroscopy (LIBS) for direct measurement of atomic species over a wide range of mixture fractions of C3H8, CH4, and CO2 in air. Atomic emission from a laser-induced plasma is observed and ratios of elemental lines present in the spectra are used to infer composition in reactants and in flames. The method has spatial resolution on the order of 1 mm, and equivalence ratio can be determined from the spectra obtained from a single shot of the laser, avoiding time averaging of signals. In this paper we demonstrate that LIBS can be used to obtain quantitative equivalence ratio measurements for propane and methane in air. The C/(N+O) atomic ratio is used to quantify mixture fraction of C3H8 in air, and data from individual breakdown events have a standard deviation of 3% of the mean for mixtures of 0, 1, and 2% propane in air. The strength of the C, O, and N lines in the spectral window 700–800 nm is investigated for binary mixtures of C3H8, CH4, and CO2 in air. The dependence of the atomic emission on the concentration of carbon and hydrogen is investigated in the present paper, as well as the influence of experimental parameters such as the laser power and the temporal gating of the detector.