A model for primary and heterogeneous secondary reactions of wood pyrolysis

Abstract

A chain growth model for heterogeneous secondary reactions is developed for the pyrolysis of large wood particles and the parameters determined by nonlinear optimization. The model takes both the volatile retention time and cracking and repolymerization reactions of the vapours with the decomposing solid as well as autocatalysis into consideration. The extent of the secondary reactions is strongly influenced by the time and the ratio of the autocatalytic (propagation) reaction rate to noncatalytic (initiation) reaction rate. The wood which has a higher value of the autocatalytic/noncatalytic ratio also has a higher exothermic heat of reaction and yields a higher amount of final char residue. This fact confirms that the heterogeneous secondary reactions lead to carbon enrichment of the final residue and are accompanied with an exothermic heat of reaction. The lower activation energies of the initiation and propagation reactions as compared to primary reactions (competitive reaction model consisting of weight loss and char forming reactions) confirm autocatalysis in large particles. The sealed reactor studies of small quantities of fine wood samples show that heterogeneous secondary reactions and not lower heating rates in large particles are the main source of char formed during the thermal decomposition of large wood particles. The model predictions are in agreement with the weight loss and temperature versus time curves over a wide range of particle size and furnace temperatures.

Influence of maceral composition on the structure, properties and behaviour of chars derived from South African coals

“…Inert atmosphere was provided by a nitrogen gas flowing upwards through the bottom of the reactor at a flow rate of 1.5 dm 3 min À1 . This flow rate was high enough to rapidly remove the volatiles as soon as they were formed, therefore to minimise secondary reactions [28,29]. The samples were separately heated non-isothermally at a rate 20°C min À1 to 1000°C.…”

Density functional theory molecular modelling and experimental particle kinetics for CO2–char gasification

“…The Nitrogen flow rate was 1.5 dm 3 min -1 to ensure a rapid escape of volatile matter in an inert atmosphere, and averting secondary reactions [45][46][47]. The chars were de-ashed by HCl-HF treatments [48][49][50][51][52] and characterized using petrographic, HRTEM, NMR and XRD techniques, similar to Roberts et al [45].…”

Kinetics of scrap tyre pyrolysis under fast heating conditions

“…The particle size of the sample is smaller than 0.2 mm, which has been obtained by grinding the material (subsequent to immersion in liquid nitrogen) in a Retsch ZM100 mill. This size is smaller than that commonly used in the literature, in order to avoid heat and mass restrictions within the particle [48,49], and is the E activation energy, kJ mol À1 k kinetic constant, s À1 k 0 frequency factor, s À1 n reaction order T temperature, K t, t i time and time corresponding to the starting of the isothermal period, s W, W 0 , W 1 sample weight, initial weight and weight at infinite time, respectively, kg X conversion, Eq. (3) X i , X 1 conversion at the beginning of the isothermal period and at infinite time value calculated by Simmons and Gentry [50] for the kinetic study of biomass pyrolysis without physical restrictions.…”