On the combustion of wood-char spheres in O₂/N₂ mixtures-experiments and analysis

Abstract

This paper discusses the results from experiments and analyses of the combustion of wood-char spheresfor varying O₂ in an O₂-N₂ mixture. The experiments show that for particle diameters (d₀) in the range 2-15 mm, the char burn time (t_b) in air is given by t_b~d₀^1.87 at 300 K. At higher ambient temperatures, the exponent on d₀ goes up to 2.0. The results at reduced oxygen fraction show extinction at partial conversion and zero conversion at oxygen mass fractions less than 0.14 at an ambient temperature of 300 K. Regarding flow effects, for Reynolds number (Re) more than 500, conversion is only partially complete for runs at 300 K. The mathematical model consists of a spherically symmetric porous body undergoing heterogeneous conversion with finite-rate chemistry and associated diffusion into the pores. Free and forced convective heat and mass transfer effects outside the sphere are approximately modeled. The results show a near complete agreement on most aspects. The extinction at low ambient oxygen fraction and the extinction after partial conversion at low oxygen fractions or in presence of forced convection are argued to be due to the process of becoming statically unstable. The results of lower ambient oxygen limit, also observed in some sparsely conducted experiments, apparently have implications in planetary evolution studies.