With increasing energy demand globally and, in particular, in South Africa coupled with
depletion of the earth’s fossil energy resources and growing problem of disposal of nonbiodegradable
waste such as waste tyres, there is a need and effort globally to find alternative
energy from waste material including waste tyres. One possible way of exploiting waste tyre
for energy or chemicals recovery is through gasification for the production of syngas, and this
is what was investigated in this study. The possibility of gasification of waste tyre blended with
coal after pyrolysis was investigated and two Bituminous coals were selected for blending with
the waste tyre in co-gasification. A sample of ground waste tyre / waste tire, WT, a high vitrinite
coal from the Waterberg coalfield (GG coal) and a high inertinite coal from the Highveld
coalfield (SF coal) were used in this investigation.
The waste tyre sample had the highest volatile matter content of 63.8%, followed by GG coal
with 27% and SF coal with 23.8%. SF coal had the highest ash content of 21.6%, GG coal had
12.6% and waste tyre had the lowest of 6.6%. For the chars, SF char still had the highest ash
of 24.8%, but WT char had higher ash, 14.7%, when compared to GG char with 13.9% ash.
The vitrinite content in GG coal was 86.3%, whilst in SF coal it was 25% and SF coal had a
higher inertinite content of 71% when compared to GG coal with 7.7%. SF char had the highest
BET surface area of 126m2/g, followed by GG char with 113m2/g, and WT had the lowest
value of 35.09m2/g. The alkali indices of the SF, WT and GG chars were calculated to be 8.2,
4.2 and 1.7 respectively.
Coal samples were prepared by crushing and milling to particle sizes less than 75μm before
charring in a packed bed balance reactor at temperatures up to 1000oC.Waste tyre samples were
charred at the same conditions before milling to < 75μm particle size. Coal and WT chars were
blended in ratios of 75:25, 50:50 and 25:75 before gasification experimentation. Carbon
dioxide gasification was conducted on the blends and the pure coal and WT chars in a
Thermogravimetric analyser (TGA) at 900oC, 925oC, 950oC and 975oC and ambient pressure.
100% CO2 was used at a flow rate of 2L/min.
Reactivity of the pure char samples was found to be in the order SF > GG > WT, and the
relationship between the coal chars’ reactivities could be explained by the high ash content of
the SF char and low reactivity of the WT char corresponds to its low BET surface area. In
general, the coal/WT char mixtures were less reactive than the respective coal, but more
reactive than the pure WT char, the only exception being the 75% GG char blend which was
initially more reactive than the GG char, and reactivity decreased with increasing WT content.
For all samples reactivity increased with increasing temperature.
The relationship between the reactivities of the GG char and its blends and that of the SF char
and its blends was found to be affected by the amount of WT char added, especially at the
lower temperatures 900oC and 925oC. SF coal is more reactive than GG coal, but at 900oC and
925oC, the reactivity of GG/WT blends improves in relation to the SF/WT blends with an
increase in the ratio of WT in the blends, i.e. the 25% GG char blend is more reactive than the
25% SF char blend. The reactivity of the coal/WT blends was also checked against predicted
conversion rates based on the conversion rates of the pure WT and coal samples. At 900oC and
925oC, the reactivities of the blends of both coal chars with WT char were found to be greater
than the predicted conversion rates, and for the GG/WT blends the deviation increased with
increasing WT ratios, while for the SF/WT blends the deviation increased with increasing SF
ratios. These findings suggest the presence of synergism or enhancement between the coal
chars and WT char in gasification reactions.
The random pore model (RPM) was used to model the gasification results and it was found to
adequately describe the experimental data. Activation energies determined with the RPM were
found to be 205.4kJ/mol, 189.9kJ/mol and 173.9kJ/mol for SF char, WT char and GG char
respectively. The activation energies of the coal/WT blends were found to be lower than those
of both the pure coal and the pure WT chars. For the GG/WT blends the activation energy
decreased with increasing WT char ratio, while for the SF/WT blends the activation energy
decreased with increasing SF char ratio.
The trends of the activation energies and conversion rates of the blends point to synergism or
enhancement between the coal and WT chars in CO2 gasification reactions, and in the GG/WT
blends this enhancement is driven more by the WT char, while in SF/WT blends it is driven by
SF chars. It is possible that enhancement of the reactions is caused by mineral matter catalysis
of the gasification reactions. The ash contents and alkali indices of the pure samples follow the
order SF > WT > GG. / MIng (Chemical Engineering), North-West University, Potchefstroom Campus, 2015
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:nwu/oai:dspace.nwu.ac.za:10394/15278 |
| Date | January 2015 |
| Creators | Gurai, Chaitamwari |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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