Division of Chemical Reaction Engineering
KTH - Kungl. Tekniska Högskolan
Fixed bed updraft gasification of agricultural residues
Abstract
The goal of the proposed project is to study the gasification of agricultural residues
using a laboratory scale updraft gasifier. The product gas can be used as fuel for engines
for electrical power production, water pumping or in local industries.
The gas can also be used in standard heat appliances such as agricultural dryers and cement,
lime, or brick kilns.
The work includes the optimization of the gasifier. The composition of the product gas and
the temperature profile will be measured. The influence of the type of biomass, particle
size of the biomass and air flow rate in the gasification will be studied.
Fixed-bed pyrolysis tests will also be performed for determination of product yields and
gas composition to establish the role of devolatilization in the gasification
characteristics.
Background
In the updraft gasifier the downward-moving biomass is first dried by the upflowing hot
product gas. After drying, the solid fuel is pyrolysed, giving char which continues to move
down to be gasified, and pyrolysis vapours which are carried upward by the upflowing hot
product gas. The tars in the vapour either condense on the cool descending fuel or are
carried out of the reactor with the product gas, contributing to its high tar content.
The product gas from an updraft gasifier thus contains a significant proportion of tars
and hydrocarbons, which contribute to its high heating value. Usually the gases are
directly used in a closely coupled furnace or boiler. The fuel gas requires substantial
cleanup if further processing is to be performed. There is interest in the cleaning of
the updraft gas for electricity production, as low temperature tars are more reactive and
thus easier to be removed, than the high-temperature tars produced in much lower amounts
by downdraft and fluidized bed gasifiers. The principal advantages of
updraft gasifiers are their simple construction and high thermal efficiency: the sensible
heat of the gas produced is recovered by direct heat exchange with entering feed, which
thus is dried, preheated and pyrolysed before entering the gasification zone.
Updraft gasifiers are suitable for sizes between 2 and 20 MWe. Several industrial updraft
are in operation in Northern Europe for peat as well as straw and wood chips.
Objetives
The goal of the proposed project is to study the gasification of agricultural residues
using a laboratory scale updraft gasifier. The project will focus to the requirements in
the tropical region in Latin America.
The main industrial activities, which generate biomass residues in Latin America, are sugar
cane industry, wood exploitation and some agricultural activities. The most important
residues, considering the availability are sugar cane bagasse, sugar cane straw, wood
residues and various agricultural residues.
Characterisation of the agricultural residues as raw materials for gasification will be
performed. Gasification characteristics can be grouped into: thermochemical (ash content,
volatile products, reactivity of volatile products, etc.), intraparticle rate (thermal
properties, moisture content, size, kinetics and energetics of chemical processes, etc.)
and extra-particle rate (heat transfer from reactor to particle, residence time and mass
transfer conditions depend, in their turn, on the type of gasification unit). This point
is very important because the knowledge of gasification characteristics is a crucial
factor in making a fuel and/or a technology attractive for wide use on industrial and
commercial use.
As in the updraft fixed-bed gasification of biomass the different physical and
chemical processes are stratified along the reactor height in the order of combustion,
gasification, devolatilization and drying (from the bottom), a first set of experiments
will be carried for fixed-bed pyrolysis through a laboratory scale reactor. The main
purpose of this activity is the determination of product yields and gas composition to
establish the role of devolatilization in the gasification characteristics.
Several tests will be carried out on biomass gasification for the optimization of the
plant in relation to gasifier insulation, gas cleaning procedures, operation modalities
(bed height and feeding process) and biomass pretreatment (particle size, presence of
fines and predrying).
The composition of the producer gas and temperature profiles will be determined for
biomass gasification.
The influence of the type of biomass, particle size of the biomass and air flow rate
will be studied. Particle size is an important parameter in biomass gasification because it
determines the bed porosity and thus the fluid-dynamic characteristics of the bed.
Associated with the increase in the air flow rate, there are two counteracting effects on
the temperature profile: (1) the amounts of both biomass burned and heat released increase,
thus favouring the attainment of successively higher temperatures, and (2) the amount of
biomass and air to be brought at a high temperature also increases, which would lead to
lower temperatures.
rolando@ket.kth.se
Last updated: December 2001