Heat Transfer within Direct Heating Integrated Moving Bed Torrefier by Method of Lines

Bosikawa Mosi Fabrice,Mbuyi Katshiatshia Haddy,Sumuna Temo Vertomene

doi:10.4236/jsbs.2021.111001

Abstract

This work deals with the determination of the temperature profile within a direct heating moving bed torrefier in order to determine its minimum column height. A thermal model based on eulerian-eulerian two-phase solid-gas theory was performed and solved with the method of lines. In addition, this study allows to investigate the effect of the biomass particle size on the minimum torrefier column height. This investigation was performed by changing, simultaneously, the diameter of particles and the minimum fluidization velocity of the bed. Then, the calculations were made for a counter-current torrefaction reactor of 30 cm in diameter and for 5 kg/h of the feed rate of raw sugarcane bagasse. Results show that the height of the reactor column must be at least 30 cm for that are 1 mm in diameter and 108 cm for particles that are 2 mm in diameter.

Highlights

Torrefaction is a thermochemical process in an inert or limited oxygen environment where biomass is slowly heated at a temperature between 200 ̊C and 300 ̊C [1] [2]
On the basis of the heat transfer mechanism, torrefaction technologies fall under two categories including direct and indirect heating
It is noticed that the effect of particles diameter on the minimum column height is not negligible. These temperature profiles indicate that the torrefaction of larger biomass particles require a reactor with a larger minimum column height. This is due to the fact that the heat transfer through the bed depends on biomass particles size

Summary

Introduction

Torrefaction is a thermochemical process in an inert (nitrogen gas is flowed into the reactor) or limited oxygen environment where biomass is slowly heated at a temperature between 200 ̊C and 300 ̊C [1] [2]. For a direct heating integrated torrefier, meaning that torrefaction reactor combines drying and torrefaction steps into one unit, as depicted, heat transfer analysis plays a critical role for successful initial sizing of the reactor. The governing equations are very complex because they are nonlinear, coupled, etc They are very difficult to solve analytically. This is due to the fact that in the method of lines, independent variables are not discretized at the same time such with finite differences or finite element methods This allows to reduce the time as well as the cost of computations [3]. The present work proposes the utilization of the method of lines to solve the equations describing the heat transfer between gas phase and solid phase within a direct heating integrated moving bed torrefier

Methodology

Governing Equations

Initial Conditions

Numerical Calculation

Conclusions