Ignition and combustion of cellulosic dust particles.

Abstract

Observations of the ignition and combustion process and measurements of the ignition delay, volatile combustion, and char combustion times have been made for several hundred nearly-spherical corncob particles having diameters in the range of 100 $\mu$m to 2000 $\mu$m. Experiments were conducted in two stages using different experimental apparatuses, but similar experimental techniques. Initial experiments consisted of volatile and char combustion time measurements for nearly-spherical particles in air at a temperature of 920 K. Analysis of this data focuses on determining the relationship between the burning times and the particle diameter, and shows that the combustion times obey a d-squared law. A study of the effect of particle geometry on the burning times was also conducted using rectangular parallelepiped shaped particles. The results of this study indicate that the volatile combustion time is controlled by heat transfer to the particle. Later experiments consisted of ignition delay, volatile combustion, and char combustion time measurements for several different gas temperatures and oxygen/nitrogen mixtures. Analysis of the volatile and char burning time data focuses on determining the effect of variable particle density on the burning times, and shows that the burning times are proportional to the product of the density with the square of the particle diameter. Analysis of the ignition data focuses on determining the primary ignition mechanism, identifying the conditions for which different ignition/combustion processes occur, and determining the dependence of the ignition delay time on the particle size, particle density, gas temperature, and oxygen concentration. Observations revealed four distinct ignition/combustion processes that involve both the heterogeneous and homogeneous ignition mechanisms. Visual ignition data supports the conclusion that both homogeneous and heterogeneous primary ignition occur with a transition from homogeneous to heterogeneous primary ignition occurring as either the particle size or gas temperature is decreased. Ignition delay times are shown to increase with increasing particle diameter and decreasing gas temperature in accordance with a simple heat transfer model of the ignition process. A more sophisticated model of the ignition process of a pyrolyzing particle is also presented which is shown to agree with the experimental data.