Results of an experimental investigation to determine the limits of various hydrogen-carbon dioxide mixtures for a range of elevated initial mixture temperatures of up to 300°C at atmospheric pressure are to be presented. It is to be shown for fuel mixtures containing hydrogen and carbon dioxide, that as the initial mixture is increased, the values of the rich limit and to a lesser extent the corresponding lean limit value, become dependent on the mixture residence time before initiating ignition. The longer this time the narrower becomes the flammable mixture range. Such a behavior is due to slow surface reactions that remove in stoichiometric proportions part of the available oxygen and hydrogen, changing gradually the composition of the test mixture from its initial value. The smooth alloyed steel test vessel surface provides catalysts that increase the rates of the surface oxidation reactions. Gas phase reactions in comparison are negligible under these conditions. Such a behavior is compared with that obtained in an identical apparatus having less catalytically reactive quartz surfaces, under the same initial mixture conditions. The reaction activity with quartz surfaces was much less evident than for the steel and only with mixtures at moderately elevated temperatures. No such surface reaction activity was found with methane-air mixtures for either of the two types of surfaces.
An approach for predicting the flammability limit values of hydrogen-carbon dioxide mixtures in air for different initial mixture temperatures is proposed. Predicted values validate well against those experimentally observed.