Production of Tar in Pyrolysis of Large Biomass Particles

A paper presents a study of the production of tar in the pyrolysis of spherical biomass particles, with computational simulations performed by using the macroparticle portion of the model described in "Generalized Mathematical Model of Pyrolysis of Plant Biomass" (NPO-20068) elsewhere in this issue of NASA Tech Briefs. The particles were chosen to have sizes of the order of 1 cm, representative of typical waste wood chips. The numerical results indicate that tar formed in primary reactions decomposes in secondary reactions that occur both within particles and in exterior boundary layers; the net amount of tar available for collection is thereby reduced substantially. An analysis of the competing tar-generation and tar-decomposition reactions results in finding reactor-temperature ranges for maximizing tar yields; the range in a given case is a function of the initial particle size and of the efficiency with which pyrolysis products ejected from particles are cooled and the decomposition reactions thereby quenched in the surrounding medium. The tar yield in a given case also depends on the choice of inert carrier gas, primarily via its effect on the heat capacity of the medium. The report concludes by presenting results of a sensitivity study of the influences of the density, thermal conductivity, and heat capacity of the biomass and of the primary heats of reaction.