Chemical effects of hydrogen addition on soot formation in counterflow diffusion flames: Dependence on fuel type and oxidizer composition

Abstract

We experimentally studied and kinetically modeled the effects of hydrogen addition on soot formation in methane and ethylene counterflow diffusion flames (CDFs). To isolate the chemical effects of hydrogen in such flames, we also ran a set of experiments on flames of the same base fuels but with the addition of helium. Specifically, we measured the soot volume fractions of the flames using the planar laser-induced incandescence technique. We simulated detailed sooting structures by coupling the gas-phase chemistry with the polycyclic aromatic hydrocarbon (PAH)-based soot model, using a sectional method to resolve the soot particle dynamics. Our experimental and numerical results show that hydrogen chemically inhibits soot formation in ethylene CDFs. While in methane flames, it is interesting to observe that the difference in soot production between the hydrogen-and helium-doped cases became much smaller when the oxygen concentration in the oxidizer stream (XO) was reduced.This suggests that for methane CDFs the chemical soot-inhibiting effects of hydrogen was highly dependent on oxidizer composition (i.e., XO). Explanations are provided through detailed kinetic analysis concerning the effects of hydrogen addition on the growth of PAH, soot inception, and soot surface growth processes. Our results suggest that hydrogen's chemical role in soot formation not only depends on fuel type (ethylene or methane), it also may be sensitive to oxidizer composition.

Experimental and Numerical Study on the Sooting Behaviors of Furanic Biofuels in Laminar Counterflow Diffusion Flames

“…It is also noticed that the overall SVF profile seems symmetric under the present X F /X O conditions. This situation is slightly different from the previous CDFs [44] (e.g., with X F = 1.0 and X O = 0.24), in which a skewed shape of the SVF profile (towards the fuel side) was observed. This is because the variations in the flame boundary conditions (X F /X O ) will alter the relative location of the flame sheet to the stagnation plane, which, in turn, will notably affect the soot evolution process and result in different axial distributions of SVF.…”

Experimental study of the influence of hydrogen as a fuel additive on the formation of soot precursors and particles in atmospheric laminar premixed flames of methane

“…The impact of H 2 on the formation of soot precursors and soot particles have also been the subject of numerous fundamental studies but mostly for mixtures of H 2 with fuels other than CH 4 [21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37]. By contrast, only a few similar studies, to the best of our knowledge, were addressed to characterize the impact of H 2 on the combustion of CH 4 flames [38][39][40][41]. Liu et al [38] and Xu et al [39] studied the effects of H 2 as a fuel additive (up to 40% comparing to the base fuel) on soot formation in diffusion CH 4 flames and showed that the addition of H 2 strongly decreases the formation of soot.…”

“…By contrast, only a few similar studies, to the best of our knowledge, were addressed to characterize the impact of H 2 on the combustion of CH 4 flames [38][39][40][41]. Liu et al [38] and Xu et al [39] studied the effects of H 2 as a fuel additive (up to 40% comparing to the base fuel) on soot formation in diffusion CH 4 flames and showed that the addition of H 2 strongly decreases the formation of soot. Ezenwajiaku et al [41] also investigated the impact of H 2 (added up to 20% in CH 4 ) on the formation of PAHs also in a diffusion flame.…”

A comparative study of the sooting tendencies of various C5–C8 alkanes, alkenes and cycloalkanes in counterflow diffusion flames

“…Regarding the LII setup, a 10 Hz pulsed Nd-YAG laser with a fundamental emission at 1064 nm was used [ 50 , 59 , 60 ]. The laser beam was manipulated by a series of cylindrical lens to form a laser sheet (8 mm high) at the burner center.…”