Fire Safety Science Digital Archive

A two-dimensional, variable property, mathematical model of the transient pyrolysis of dry cellulosic materials, in response to prescribed conditions in a furnace, is presented. Chemical processes account for a reduction in the degree of polymerization of the solid and two competing pathways leading, respectively, to a solid charred residual (char) and low molecular weight gaseous species (gas) and to condensable organic components (tar). Volatile pyrolysis products flow towards both the cold solid and the hot char, with pressure and velocity variations described according to the Darcy law. Radiative, convective and conductive heat transfer interior to the solid and convective and radiative heat losses from the surfaces are also modeled. The pyrolysis of large samples is a wave-like process, controlled by heat transfer for a wide range of heating conditions (furnace temperatures from 600K to 1200K) and affected by the grain structure of the solid. Volatile pyrolysis products flow mainly along the solid grain, carrying thermal energy out from the sample. The total heat transferred to the solid is initially larger along this direction, because of the larger thermal conductivities. However, for long times, the total heat convected out almost equals the total heat conducted in. Larger temperatures along the char layer and a faster advancement of the pyrolysis front are predicted for the cross grain direction. Most of the pyrolysis products are tars with increasing char yields as reaction temperatures are lowered through variations in the sample heating conditions.