6th International Freiberg Conference on IGCC & XtL Technologies,
Coal Conversion and Syngas,
19-22 May 2014, Dresden/Radebeul, Germany
The effect of particle size on CO2 reactivity
1
G.H. Coetzee1, H.W.J.P. Neomagusr1, J.R. Bunt1,2, R.E. Everson1
School for Chemical and Minerals engineering, North West University, Potchefstroom, South
Africa,
2
Sasol Technology (PTY) Ltd, Sasolburg, South Africa
email: [email protected], [email protected]
The wide range of coal gasification application has resulted in tremendous growth in research
and extensive development in coal gasification conversion technologies, which include pulverised fuel, fluidised bed and fix bed. The gasification conditions (i.e. temperature, particle heating rate, particle size etc.) vary dramatically within the different gasification technologies. For
example, the particle size range used in pulverised fuel and fixed bed gasification technology is
<100um and <100mm, respectively.
The aim of this study was to investigate the effect of particle size on the reactivity of CO2 gasification, and to establish the link between powder and large coal particle reactivity. A wide range
of particle sizes and temperatures were chosen in order to study the effect of chemical and
mass transfer limitations on the CO2 reactivity. Four particle sizes (38um, 75um, 2mm and 20
mm) and four temperatures (850 C, 900C, 950C and 1000C) were used for this investigation.
The reactivity of CO2 gasification was done in a large particle TGA and a product analysis
reactor, both constructed in-house. The product analysis reactor uses both CO and CO2 IRanalysers and on-line mass spectrometry for product distribution. CO2 and Argon was fed to
both reactors, with a feed composition of 10% CO2.
The transient reaction rate, based on mass carbon, was calculated using the slope and amount
of carbon (in mass) at a set time (ti). The equation used for the transient reaction rate is shown
in eq. (1).
− rA,i =
dg
1
.
dt i g (i ) − ash
(1)
It was found that the initial reaction rate (-rA,i at t=0) was independent of particle size for the
38um, 75um and 2mm particle sizes, over the full range of gasification temperatures. However,
this was not observed for large coal particle gasification (20mm). The transient reaction rate of
the 20mm coal particles increases to as much as 5 times the initial reaction rate. The initial
reaction rate was used to evaluate the Arrhenius plot as well as determining the influence of
particle size on the reactivity of CO2 gasification. This information will be used to develop a
transient CO2 reaction model including mass transfer limitations.