Study of Fuel Composition Effects on Flashback Using a Confined Jet Flame Burner

Abstract

Flashback is the main operability issue associated with converting lean, premixed combustion systems from operation on natural gas to operation on high hydrogen content fuels. Most syngas fuels contain some amount of hydrogen (15–100%) depending on the fuel processing scheme. With this variability in the composition of syngas, the question of how fuel composition impacts flashback propensity arises. To address this question, a jet burner configuration was used to develop systematic data for a wide range of compositions under turbulent flow conditions. The burner consisted of a quartz burner tube confined by a larger quartz tube. The use of quartz allowed visualization of the flashback processes occurring. Various fuel compositions of hydrogen, carbon monoxide, and natural gas were premixed with air at equivalence ratios corresponding to constant adiabatic flame temperatures (AFT) of 1700 K and 1900 K. Once a flame was stabilized on the burner, the air flow rate would be gradually reduced while holding the AFT constant via the equivalence ratio until flashback occurred. Schlieren and intensified OH* images captured at high speeds during flashback allowed some additional understanding of what is occurring during the highly dynamic process of flashback. Confined and unconfined flashback data were analyzed by comparing data collected in the present study with existing data in the literature. A statistically designed test matrix was used which allows analysis of variance of the results to be carried out, leading to correlation between fuel composition and flame temperature with (1) critical flashback velocity gradient and (2) burner tip temperature. Using the burner tip temperature as the unburned temperature in the laminar flame speed calculations showed increased correlation of the flashback data and laminar flame speed as opposed to when the actual unburned gas temperature was used.

Experimental investigation of upstream flame propagation during boundary layer flashback of swirl flames

“…Since research on flashback began with the first systematic study by Lewis and von Elbe [1,2], the focus has been on measuring flashback limits in (non-swirling) Bunsen-flame type burners for many years [3][4][5][6][7], including more recent studies carefully testing additional parameters such as confinement, wall temperature and pressure [8][9][10][11][12][13]. High-speed optical diagnostics and advanced simulation tools have been applied more recently to reveal new information about the flame propagation dynamics [14][15][16][17].…”

Turbulent Flame Speed as an Indicator for Flashback Propensity of Hydrogen-Rich Fuel Gases

“…Compared to the laminar case, it is found that the turbulent flashback takes place at a considerably higher average velocity than that prescribed by the gradient principle [5]. In other words, the critical velocity gradients at flashback are significantly increased when the flow conditions are changed from laminar to turbulent [6][7][8][9][10]. Accordingly, the increase in the critical velocity gradient may be attributed to the decreased penetration distance, the increased burning velocity, or the combined effect of both.…”

“…To be compatible with the procedure for the H 2 -rich mixtures, the properties of the syngas flames were evaluated based on the same reaction scheme [34]. Note that since the flashback behavior was considered to be still dominated by the hydrogen content [10,16,17], the Lewis number was again evaluated by dividing the mixture thermal diffusivity with the ordinary multicomponent diffusion coefficient for the H 2 -N 2 pair.…”

“…From the perspective of fuel compositions, it is found that the flashback behavior is dominated by the hydrogen content [10,16,17]. This is attributed to the fact that for the combustible mixtures containing hydrogen, the mass and thermal diffusivities strongly differ among the components.…”

“…Specifically for the hydrogen-containing or hydrogen-rich mixtures, the issue of boundary layer flashback has been investigated in some recent studies [10,[13][14][15]. From the perspective of fuel compositions, it is found that the flashback behavior is dominated by the hydrogen content [10,16,17].…”