Analytical Techniques - Biomass Gasification, Pyrolysis and Torrefaction - page 451

TGA instruments need very accurate measure of mass. Certain models have
an under hanging pan which hangs down from the balance. All models use
computers to accurately record the change in mass.
The temperature scanning rate and the purge gas flow-rate can be chan-
ged. Certain experiments are conducted isothermally, while some samples
are cooled. Software programs are available which plots the first derivative
curve which is an essential tool to determine the point of greatest change on
the mass loss curve.
13.4.2 Differential Thermal Analyzer
Kinetic data can be derived from a differential thermal analyzer. A procedure
is described in Gawz and Reed (1995).
13.4.3 Quartz Wool Matrix Apparatus
This is a simple universal reactor for a wide range of process and reaction. It
works on the same gravimetric principle, as TGA, with an important addi-
tional feature that it can study the influence of particle size, shape, and hydro-
dynamics to some extent. In a typical TGA, the sample is generally ground
and placed on a pan as a fixed bed. It impedes uniform access of gases to all
particles, and the thermal or concentration gradient around one particle affects
those around another. In a quartz wool matrix (QWM) reactor, particles are
dispersed widely on a highly expanded matrix of high temperature inert wool.
This allows individual particles to have equal access to gas and temperature
field. For this reason, it is possible to study the effect of particle size, and
shape if any on a reaction. Furthermore, such a highly expanded bed better
simulates the hydrodynamics of a fluidized bed or entrained bed. It is thus
able to study the effect of mass transfer on a certain reaction.
A typical QWM reactor, originally developed by Chi et al., (1994) is
made of tube that is heated externally by an electric heater. A tubular furnace
controls the furnace temperature. Reactant gases like oxygen, nitrogen, car-
bon dioxide, sulfur dioxide, or others are mixed at desired proportion using a
precision electronic flow meter ( Figure 13.5 ). The mixture, which simulates
the gaseous environment of a reaction, is preheated to the reaction tempera-
ture and is passed into the reactor at desired flow-rates. The volumetric
flow-rates of the individual gases are calculated based on the cross-sectional
area available for flow in the reactor.
The sample fuel ground to the desired size and shape is dispersed on the
inert matrix of quartz wool. The wool is supported on a wire basket, which
in turn is hanged from a microbalance, vertically on the top but outside the
reactor ( Figure 13.5 ). The dispersed location of the sorbent particles closely
resembles that in an actual fluidized bed. Solid samples dispersed on the
matrix allow free access of gas to all sides of the samples. The fuel sample