An investigation of the recovery of volatiles from Waterberg coal.
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Date
2017
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Abstract
South Africa has large coal reserves in world terms and the coal mining industry serves an
important role in supporting the South African economy. Currently, most of the coal is mined
in Witbank and Highveld Coalfields, where it occurs in relatively thick seams, near the surface.
However, these coalfields are fast becoming depleted and new techniques are required to
utilise the large coal resources still available in South Africa, especially in the Waterberg
region, where a significant number of low-grade seams may be mined but discarded.
The Waterberg region contains approximately 40% of the remaining coal in South Africa.
However, the lithology of most of the coal seams is such that separation of the gangue minerals
by density is difficult. There are many problems associated with mining in the Waterberg
region, which include lack of infrastructure, lack of water sources, environmental issues
(groundwater pollution) and socio-economic issues (Jeffrey, 2005).
The experimental part of the project was aimed at investigating the recovery of chemicals from
Waterberg coal which has a high mineral content. The design and simulation part of the
project utilised the data obtained from laboratory experiments, to determine if a reactor with
counter-current flow of gas and solids could be used to recover the volatile material from the
coal. It was envisaged that the volatile material would be removed in the upper zone of the
reactor, by hot gas generated by combustion of char in the lower zone. It was also envisaged
that the air for combustion would be pre-heated, using heat exchange with the exit gas, as
practiced in power stations. A mass and energy balance calculation was done over the entire
system, to determine the limits of mineral content (expressed as ash content). Since, a large
quantity of energy is required to raise the temperature to the point where volatiles can be
separated, particularly if there is a high mineral content, the proportion of char is important,
to provide sufficient energy to drive the process. It was envisaged that heat exchange would
conserve energy and that it may be possible to use coal containing a relatively high mineral
content, which would normally be discarded.
Laboratory experiments were conducted on single coal pieces, which were first subjected to
pyrolysis in a horizontal tube, which was inserted into a muffle furnace. A stream of nitrogen
was used to transport the volatile material to an external collection vial, submerged in an ice
bath. Some of the volatile material condensed in the cooler portions of the reactor, but it was
recovered by subsequent dissolution in a solvent. The remaining char was then heated in a
furnace with air, to burn the carbon and to determine the mass of ash remaining. The
temperature and time for pyrolysis and combustion were varied, to determine suitable
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operating conditions. The liquid products were analysed using the Karl Fischer method and
GCMS analysis
Four series of experiments were conducted, having a different variables changed. The first
variable that was investigated, was the size of the coal sample. Single coal particles, with
varying mineral content, were used. The mass of the coal piece was about 40g and the
dimensions were about 50mm x 25mm x 30mm) The combustion experiments showed that
these particles were not fully combusted, as they had an unreacted core. The subsequent series
of experiments were conducted on single particles with a mass in the range 7g to 23g.
The pyrolysis experiments show that significant amounts of the liquid product condensed
inside the reactor tube. The combustion experiments showed that the structure of the high ash
samples was maintained, whilst the low ash samples disintegrated. This is an important factor
as the coal dust would cause channelling of the gas flowing up the reactor.
The second variable that was investigated was the effect of time. The results showed that 30
minutes was sufficient to extract the hydrocarbons. It was noted that the coal with a high
mineral content (> 60% ash) contained a relatively large amount of volatiles (17 %), and very
little water (< 2 %). The GCMS analysis showed the presence of large proportion of phenols.
Phenols are an important commodity and have a wide range of industrial applications. Karl
Fischer analysis revealed that the overall water content was low and this made the liquid
product easier to upgrade.
The experimental results were then utilised in a mass and energy balance calculation, to
simulate the proposed counter-current reactor and external heat exchangers. Two scenarios
were investigated; The first was with the pyrolysis temperature set at 600°C and the
combustion zone set at 800°C. The data from samples with varying mineral content was
analysed. (The ash content of the feed ranged from 25.7% to 65.8%). The amount of excess
energy available reduced as the mineral content increased, but the process still worked when
the ash content of the combusted material reached 65.8%. The second scenario was with worst
coal (65.8% ash), the pyrolysis temperature set to 700°C, and the combustion temperature set
at 950°C. The results showed that there was still sufficient energy released by the combustion
of the char to operate the process, but there was a smaller excess of energy. Additional water
was needed to reduce the temperature of the product stream (hydrocarbons).
It should be noted that loss of energy from the high temperature zones by conduction was not
considered and that some of the ‘excess energy’ will be needed.
Description
Master of Science in Chemical Engineering. University of KwaZulu-Natal, Durban 2017.