Modeling the coupled effects of heat transfer. thermochemistry, and kinetics during biomass torrefaction

Abstract

Torrefaction is a thermal pretreatment process which improves the energy density, storage, grinding, and handling characteristics of raw biomass. Research efforts to date have focused on empirical measurements of the fuel quality improvements caused by torrefaction at different temperature and residence time conditions. Additionally, these efforts have assumed that torrefaction is a kinetically limited process and do not account for heat transfer limitations present in larger particles. The first component of the present torrefaction model is an analysis of the existing experimental work which results in a detailed thermochemical and grinding energy models. These models enable the prediction of reaction mass, energy, and species balance as well as torrefied product characteristics such as grindability and heating value over a wide range of reactor conditions. Based on this framework, optimal conversion conditions are determined. The second component of the present work is a numerical unsteady one-dimensional single particle model where conservation equations are solved by a custom code in MATLAB. The effects of coupled thermochemical, kinetic, and heat transfer phenomena are analyzed. A comparison of two classes of torrefaction reactors (fixed/moving bed and fluidized bed) is made based on simulation results.