Characteristics of propagation of a filtration combustion wave in a low-carbon mixture

Abstract

To date, the characteristics of filtration combustionwith normal (reaction leading) wave propagation, spe-cifically, combustion velocities, product compositions,temperatures, etc., have been studied in a large numberof works [1–4]. The main specific feature of filtrationcombustion during which the gas and solid phasesmove countercurrently is “superadiabatic” heating, i.e.,a significant excess of the temperature in the combus-tion zone over the adiabatic temperature because of theheat concentration within a relatively narrow zone.Unlike any other heterogeneous combustion modes,the maximal temperature in filtration combustion (withair as the oxidant) increases with dilution of fuel (e.g.,carbon) with an inert material down to a concentrationof approximately 20 wt % carbon in the initial mixture.The initial mixture at such fuel content cannot burn inordinary furnaces.In this work, for the first time, we investigated thelower combustion limit, i.e., the lowest fuel (carbon)concentration in the mixture at which combustion isstill possible. It was shown that combustion is possibleat carbon concentrations as low as 2 wt %. The minimalcarbon concentration in filtration combustion turnedout to be many times lower than that in other combus-tion modes. The combustion macrokinetics near thelower concentration limit was explored for the firsttime. It was demonstrated that the process characteris-tics near the limit undergo significant changes unpre-dicted in earlier works [1–3]; namely, oxygen appearsin the gaseous products and there are changes in thedependence of the combustion wave velocity and tem-perature on the carbon concentration in the mixture andits reactivity.The phenomena observed were explained. A physi-cochemical model was formulated to underlie the the-ory and detailed experimental study of the concentra-tion limits and the processes near them.Oxidation and filtration combustion of birch char-coal, activated birch charcoal, carbon–carbon compos-ite, and graphite were investigated (table). Carbon waschosen as a fuel because its chemical composition issimple and the mechanisms of reactions during oxida-tion have been studied quite well. The diversity ofstructural forms of carbon materials, on the one hand,complicates the quantification of the process and, onthe other, allows one to choose structures with differentreactivities and to measure the dependence of the mac-rokinetic characteristics of filtration combustion on thekinetic parameters of the oxidation reaction.The kinetics of carbon oxidation by dry air was stud-ied by measuring the decrease in the weights of samples[4], with the initial weight of carbon material being0.6 g. The carbon particle size was the same as the fuelparticle size in the mixture used in filtration combustionexperiments.Filtration combustion was studied in a 160-mm-long cylindrical reactor 25 mm in diameter made ofporous firebrick. The apparent heat loss coefficient wasα = 8 W/(m