A fundamental understanding of the process of devolatilization requires extensive knowledge of
not only the intrinsic properties of the parent coal and its subsequent formed products (tars,
gases and chars), but also its characteristic reaction rate behaviour. Devolatilization behaviour
has been extensively addressed in literature with the use of powdered coal samples, which
normally do not adhere to particle size constraints of coal conversion processes utilizing lump
coal. The aim of this investigation was therefore to assess the devolatilization behaviour (with
respect to product yield and -quality; and reaction rate modelling) of four typical South African
coals (UMZ, INY, G#5 and TSH) confined to the large particle regime. All four coals were found
to be bituminous in rank, with vitrinite contents ranging between 24.4 vol.% and 69.2 vol.%
(mineral matter free basis). Two were inertinite-rich coals (UMZ and INY) and the other two
were vitrinite-rich coals (G#5 and TSH). From thermoplasticity measurements it was evident that
only coal TSH displayed extensive thermoplastic behaviour, while a comparison between
molecular properties confirmed the higher abundance of poly-condensed aromatic structures
(aromaticity of 81%) present in this coal.
Product evolution was evaluated under atmospheric conditions in a self-constructed, large
particle, fixed-bed reactor, on two particle sizes (5 mm and 20 mm) at two isothermal reactor
temperatures (450°C and 750°C) using a combination of both GC and MS techniques for gas
species measurement, while standard gravimetric methods were used to quantify tar- and char
yield respectively. Elucidation of tar- and char structural features involved the use of both
conventional- and advanced analytical techniques. From the results it could be concluded that
temperature was the dominating factor controlling product yield- and quality, with significant
increases in both volatile- and gas yield observed for an increase in temperature. Tar yields
ranged between 3.6 wt.% and 10.1 wt.% and increased in the order UMZ < INY < TSH < G#5,
with higher tar yields obtained for coal G#5, being ascribed to larger abundances of vitrinite and
liptinite present in this coal. For coal TSH, lower tar yields could mainly be attributed to the
higher aromaticity and extensive swelling nature of this coal. Evolved gases were found to be
mainly composed of H2, CH4, CO and CO2, low molecular weight olefins and paraffins; and
some C4 homologues. Advanced analytical techniques (NMR, SEC, GC-MS, XRD, etc.)
revealed the progressive increase of the aromatic nature of both tars and chars with increasing
temperature; as well as subsequent differences in tar composition between the different parent coals. In all cases, an increase in devolatilization temperature led to the evolution of larger
amounts of aromatic compounds such as alkyl-naphthalenes and PAHs, while significant
decreases in the amount of aliphatics and mixed compounds could be observed. From 13C
NMR, HRTEM and XRD carbon crystallite results it was clear that an increase in temperature
led to the formation of progressively larger, more aromatic and structurally orientated polycondensed
carbon structures.
Reaction rate studies involved the use of non-isothermal (5-40 K/min) and isothermal (350-
900°C) thermogravimetry of both powdered (-200 μm) and large particle samples (20 mm) in
order to assess intrinsic kinetics and large particle rate behaviour, respectively. Evaluation of
the intrinsic kinetic parameters of each coal involved the numerical regression of non-isothermal
rate data in MATLAB® 7.1.1 according to a pseudo-component modelling philosophy. Modelling
results indicated that the intrinsic devolatilization behaviour of each coal could be adequately
described by using a total number of eight pseudo-components, while reported activation
energies were found to range between 22.3 kJ/mol and 244.3 kJ/mol. Description of the rate of
large particle devolatilization involved the evaluation of a novel, comprehensive rate model
accounting for derived kinetics, heat and mass transport effects, as well as physical changes
due to particle swelling/shrinkage. Evaluation of the proposed model with the aid of the
COMSOL Multiphysics 4.3 simulation software provided a suitable fit to the experimental data of
all four coals, while simulation studies highlighted the relevant importance of not only the effect
of particle size, but also the importance of including terms affecting for heat losses due to
particle swelling/shrinkage, transport of volatile products through the porous char structure, heat
of reaction and heat of vaporization of water. / Thesis (PhD (Chemical Engineering))--North-West University, Potchefstroom Campus, 2013
| Identifer | oai:union.ndltd.org:NWUBOLOKA1/oai:dspace.nwu.ac.za:10394/8739 |
| Date | January 2012 |
| Creators | Hattingh, Barend Burgert |
| Publisher | North-West University |
| Source Sets | North-West University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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