Determination of kinetics of char reactivity with carbon dioxide using thermogravimetry and the distributed activation energy model
Date
2013-07-19
Authors
Vittee, Thinesh
Journal Title
Journal ISSN
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Abstract
A particular field of interest in the development and understanding of current coal-based fuel
and energy production is the determination of reaction kinetics of various materials. A more
efficient and accurate method would improve the design and operation of current technology
used in fuel and energy production.
An established DAEM-based algorithm was further developed to determine the reaction
behaviour of materials reacting in CO2 by means of incorporating the Random Pore Model
(RPM). A method for modifying the algorithm to accurately use any other reaction model (for
both isothermal and non-isothermal cases) was developed. It was found that the multiple
reaction approach characteristic to the DAEM resulted in far more accurate reaction
behaviour predictions than conventional methods that presuppose a single overall reaction.
The novelty in this research was the determination of multiple reactions occurring in RPM
systems. Despite specifying multiple reactions, a single RPM structural parameter (φ=12.2)
was still suitable, and produced accurate data fits. Charred Coal-CO2 reactivity data was
processed with the algorithm and activation energy values of E1=261.7kJ/mol,
E2=246.4kJ/mol and E3=227.6kJ/mol were found. Grouped pre-exponential factor values
were found to be A1
*=1.60E+07s-1.m-1, A2
*=2.08E+06s-1.m-1 and A3
*=2.75E+06s-1.m-1. The
corresponding mass fractions were f0,1=0.31, f0,2=0.32, f0,3=0.37. These proved to predict the
reaction behaviour better than the conventional single reaction approach which found
activation energy and grouped pre-exponential factor of E=254.5kJ/mol and A*=1.0E+07 s-
1.m-1 respectively.
The algorithm was then used to compare reactivities of plain South African coal char and of
the South African coal-pinewood blend char in CO2. This particular combination was found to
exhibit improved reactivity as compared to the plain coal. The coal-pine blend was found to
have a lower structural parameter value (φ=11.7) compared to plain coal char. The change
in the structural parameter value suggests structural changes as a result of the biomass
addition, which improved reactivity.
The findings show that the algorithm can be successfully adapted to process RPM (and
other reaction model) data and produce accurate and reliable results. The biomass-blend
results indicate the potential benefits of co-feeding the two feedstocks commercially; this
must however be investigated further.