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Moisture adsorption and desorption characteristics of some South African coals

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Abstract

Synopsis The high final total moisture content of fine coal after mining and processing is one of the major reasons that this resource is not extensively used in subsequent power generation, combustion, or other conversion processes. Some coal products, like export coal, may be subjected to a variety of environmental conditions during transport and storage, such as temperature and humidity. To understand the mechanisms by which moisture is attracted and held on and within fine coal particles, further information is needed regarding the processes occurring at the coal surface. In order to determine the correlation between physical coal properties and its moisture adsorption and desorption characteristics, a series of sorption experiments were conducted on various coal samples under climatically controlled conditions. Equilibrium moisture data was collected while changing the temperature and humidity. This data was correlated to coal properties such as particle size, porosity, maceral composition, and mineral content. All the coals that were studied were medium-rank bituminous coals. It was found that the best predictors for moisture adsorption and desorption were the mineral and inertinite contents. Keywords moisture, adsorption, desorption, fine coal, surface properties.
Introduction
South Africa produces about 250 Mt of coal
annually, of which about 70 Mt are exported,
and the rest used mostly for domestic power
generation and synthetic fuel production
(Department of Minerals and Energy, 2008).
The coal reserves in the country are mainly
bituminous, and are relatively low in sulphur
(typically less than about 1%). Because of the
nature of the mineral matter in the coal, it is
difficult to beneficiate, and therefore some
power station feeds can contain up to 40%
mineral matter (Pinheiro, 1999). Typical of
Gondwana coals, the vitrinite content can vary
from 5% up to 90% for the northernmost
coalfields. The other common maceral group is
inertinite, while liptinite occurrence is typically
less than less than 5% (Falcon and Snyman,
1986).
Investigators have stated that moisture in
coal, especially fine coal, affects the product in
the following ways: the net calorific value
decreases, handling becomes more difficult,
and transport costs increase (Petrick, 1970).
Producers spend up to R3 (about 50 US cents)
per ton of dewatered coal to reduce the final
moisture level to contract levels (De Korte,
2001).
The nature and mechanism of water
adsorption, attachment, and desorption on coal
surfaces and other coal char surfaces had been
studied by many researchers over the years
(Bradley and Rand, 1995; Nishino, 2001;
Pendleton et al., 2002; Prinz and Littke, 2005;
Qi et al., 2000). Many concentrated on the
interaction of clean, homogeneous surfaces
with water under controlled laboratory
conditions (Arenillas et al., 2004). A variety of
adsorption isotherms were determined, and the
types and parameters tended to vary greatly
with the coal type, rank, and source. Isotherms
like the Dubinin-Astakhov Equation [1] have
been used to characterize water adsorption on
microporous carbons with some success
(Bradley and Rand, 1995; Prinz and Littke,
2005).
[1]
where n
ads
is the moles of water adsorbed, P
s
the saturation vapour pressure, and Pthe
partial pressure of water. The fitted parameters
of the model are n0(the maximum adsorption,
m(related to the pore size distribution, and C
(a thermodynamic parameter related to the
energy of adsorption and the temperature).
These results from this project were used
to characterize the chemical and physical
properties of the coal surfaces, contributing
greatly to the understanding of the
fundamentals of water adsorption on coal.
Moisture adsorption and desorption
characteristics of some South African coals
by Q.P. Campbell*, M.D. Barnardo*, and J.R. Bunt*
Synopsis
The high final total moisture content of fine coal after mining and
processing is one of the major reasons that this resource is not
extensively used in subsequent power generation, combustion, or
other conversion processes. Some coal products, like export coal, may
be subjected to a variety of environmental conditions during
transport and storage, such as temperature and humidity. To
understand the mechanisms by which moisture is attracted and held
on and within fine coal particles, further information is needed
regarding the processes occurring at the coal surface. In order to
determine the correlation between physical coal properties and its
moisture adsorption and desorption characteristics, a series of
sorption experiments were conducted on various coal samples under
climatically controlled conditions. Equilibrium moisture data was
collected while changing the temperature and humidity. This data
was correlated to coal properties such as particle size, porosity,
maceral composition, and mineral content. All the coals that were
studied were medium-rank bituminous coals. It was found that the
best predictors for moisture adsorption and desorption were the
mineral and inertinite contents.
Keywords
moisture, adsorption, desorption, fine coal, surface properties.
*School of Chemical and Minerals Engineering,
North-West University, Potchefstroom, South
Africa.
© The Southern African Institute of Mining and
Metallurgy, 2013. ISSN 2225-6253. Paper received
Jan. 2013; revised paper received Apr. 2013.
803
The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 113 NOVEMBER 2013
Moisture adsorption and desorption characteristics of some South African coals
Other researchers found that existing isotherms did not fit
very well, and empirical relationships had to be developed
(McCutcheon et al., 2001).
The research described in this paper was undertaken
specifically to be able to predict the water adsorption
behaviour on the coal product while being stored or
transported. Since the distance between the main producing
coalfields in the Witbank area and the largest export terminal,
at Richards Bay is about 500 km, the coal is exposed to a
variety of environmental conditions over a period of days, or
even weeks. Coal product samples from collieries representing
three coalfields (Waterberg, Witbank, and Free State) were
exposed to simulated environments and their responses
correlated with some of the physical coal properties. Many
coalfields are situated in areas with dry cold winters and hot
rainy summers, while the coal terminals on the coast
generally have high temperatures and relative humidity all
year round.
Experimental
Coal characterization
Coal samples from three major South African coalfields
(identified as Waterberg - GG, Witbank 4 seam – WB, and
Free State - FS) were used in the investigation. Coals from
the GG and WB sources are classified as medium-rank
bituminous, while the FS sample was a medium-rank
bituminous D coal, as determined by vitrinite random
reflectance analysis (Pinheiro, 1999). These sources were
chosen to ensure a variation in mineral type and content, as
well as maceral composition.
Each of the three samples was split into three size
fractions (-0.5 mm, +0.5 mm -1 mm, and +1 mm – 2 mm) by
dry sieving, without any additional intentional size reduction,
in order to ensure that the fractions closely represent the
products broken during normal transport and handling.
Samples were stored in airtight containers under a controlled
conditioned environment (22°C and 40% relative humidity)
prior to use.
A summary of the proximate analyses and other
properties of the samples as received is given in Table I, as
determined using standard SABS / ISO methods. The surface
characteristics of each coal sample were also determined
using a mercury intrusion porosimeter, and a summary of the
results is shown in Table II. Note that the porosity of the -0,5
mm fraction could not be determined due to equipment
failure.
Environment chamber
A controlled environment chamber was developed to test the
bulk moisture adsorption and desorption characteristics of
coal. The apparatus consisted of a 0.5 m3insulated chamber
fitted with a sample holder mounted on a load cell. Air was
circulated through the chamber at 0.005 m3/s by an external
fan. This air could be optionally passed through an air
conditioner for cooling and drying. A standard household
humidifier was installed in the chamber, as well as heating
elements. The on/off status of the air conditioner, humidifier,
and heating elements was controlled based on the
measurements of a thermocouple/humidity sensor (Delta
Ohm HD8508TC150) mounted in the chamber. A
multivariable control strategy was developed to decouple and
control the humidity and temperature independently at
specified set-points. On-line mass measurements were made
while the samples were subjected to various environmental
conditions. The laboratory where the tests were performed is
at an altitude of 1 400 m above sea level, which meant that
the average atmospheric pressure was about 850 kPa.
A series of experiments was conducted for each of the
samples, at three different air temperatures: 15°C, 20°C, and
25°C. The experimental procedure involved placing about
50 g of sample in the chamber, and then setting the humidity
804 NOVEMBER 2013 VOLUME 113 The Journal of The Southern African Institute of Mining and Metallurgy
Table I
Proximate (air-dry basis) and other properties of the coal samples used
Source WB GG FS
Size (mm) -0,5 -1 +0,5 -2 +1 -0,5 -1 +0,5 -2 +1 -0,5 -1 +0,5 -2 +1
CV (MJ/kg) 26,2 26,2 26,8 26,8 29,1 30,1 14,7 16,3 15,5
Ash (%) 15,2 14,7 13,5 16,8 9,8 7,6 42,1 38,0 38,9
Moisture (%) 3,5 3,5 3,5 3,1 3,0 3,1 5,1 5,4 5,5
Volatile matter (%) 28,5 27,4 27,8 32,1 34,4 35,2 24,6 19,4 19,6
Fixed carbon (%) 52,8 54,4 55,2 48,0 52,8 54,1 28,2 37,2 36,0
Total sulphur (%) 0,85 0,68 0,70 1.15 0,94 0,93 0,79 0,67 0,84
Vitrinite (%) 56 94 15
Inertinite (%) 40 4 79
Table II
Results of the mercury intrusion porosimetry tests
Source WB GG FS
Size (mm) -0,5 -1 +0,5 -2 +1 -0,5 -1 +0,5 -2 +1 -0,5 -1 +0,5 -2 +1
Av. pore diameter (nm) - 41,9 23,9 - 132,0 26,6 - 65,7 16,2
Total pore area (m2/g) - 5,45 4,37 - 5,14 4,62 - 10,03 23,51
set-point at 20% relative humidity (RH). While the
equilibrium moisture contents of the coal samples were not
determined by standard methods, the system was allowed to
reach equilibrium, indicated by a constant mass reading, at
this initial relative humidity level. The humidity was then
increased to 40%, 60%, and 80% RH in turn, again allowing
for equilibrium between steps. Thus 80% was the highest
possible humidity level before water started to condense on
the equipment and instruments inside the chamber. After
equilibrium at 80% RH, the set-points were decreased in the
same manner in steps back to 20% to record desorption
responses. The time taken to complete each experiment was
in the order of 36 hours, to ensure equilibrium at each
humidity level. The sample was then removed from the
chamber and analysed immediately for moisture content in a
vacuum oven, using the SANS 5924 standard (SANS, 2009).
This enabled the determination of the mass of adsorbed
moisture per unit mass of coal for each intermediate
equilibrium. Figure 1 shows a typical plot of the data
generated during such a run.
Results and discussion
In order to quantify the adsorption and desorption
parameters, attempts were made to fit the data to known
isotherms normally used in these types of studies. From the
literature, water adsorption on bituminous coal is usually
described as following a Type II isotherm (Mahajan and
Walker, 1971; Unsworth et al., 1989). We attempted to use
the BET equation, but it was not possible to fit this to the
data with sufficient confidence due to the limited partial
pressure range of the experiments. The same applied to the
Langmuir isotherm. The Dubinin-Astakhov Equation [1]
fitted the desorption data fairly well over a limited range, but
for adsorption, none of the equations was found to describe
the data adequately.
Figure 2 shows the desorption data together with the
fitted Dubinin-Astakhov Equation. The n0parameter of the
equation was not used to quantify the maximum adsorption
for monolayer coverage, since this could not be extrapolated
accurately from the data. Instead, the maximum equilibrium
water adsorption (nmax) under the highest relative humidity
condition (about 80% RH) was selected to indicate the
potential of a particular bulk coal to adsorb atmospheric
moisture.
Figure 3 shows this parameter plotted against
temperature for the different particle sizes. It can be seen that
it is largely independent of temperature, as expected
(Ruthven, 1984). The particle size also has little effect, since
the adsorption is primarily dependent on the volume of the
pores, and less on the surface of the particle, which is an
order of magnitude smaller. Other studies (Monazam et al.,
1998) have shown that the particle size dramatically
influenced the rate of hydration but has no effect on the
equilibrium moisture content. Any variation is probably due
to incomplete liberation, and thus varying properties within
the size intervals.
The maximum water adsorption level was then correlated
with selected physical coal properties. Figure 4 shows that
there is no clear relationship between nmax and the total
surface area of the pores. This finding corresponds with that
of Youssef (1974), who stated that the adsorption of water is
related to the surface functional groups rather than to the
extent of the surface. Unsworth et al. (1989) also describe a
poor correlation between these two parameters, due mainly to
the size range of pores that can be measured using mercury
porosimetry.
The strong effect of the mineral content on nmax is shown
in Figure 5. Since clay minerals (illite, kaolinite, and
montmorillonite) form a major constituent of the mineral
Moisture adsorption and desorption characteristics of some South African coals
805
The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 113 NOVEMBER 2013
Figure 1—Typical adsorption-desorption run for 0.05 mm FS coal
Figure 2—Adsorption (hollow markers) and desorption (solid markers)
isotherm data for 0.5 mm FS coal. The solid lines are the fits of the
Dubinin-Astakhov equation
Figure 3—The effect of temperature and particle size on the maximum
water adsorption. Note that the connecting lines are only for enhanced
reading
Moisture adsorption and desorption characteristics of some South African coals
matter in South African coals, especially as finely dissem-
inated syngenetic particles in the Free State coals (Falcon and
Snyman, 1986), the increased water adsorption may be due
to the presence of more clay minerals. This corresponds with
a study on Australian coal where the montmorillonite content
played a major part in water adsorption (McCutcheon and
Barton, 1999). A further study is planned to investigate this
for South African coals
The effect of the maceral composition is illustrated by
Figure 6, showing the variation of nmax versus the vitrinite
content. Inertinite contains more macroporosity (30 nm to
10 μm pore diameter) than vitrinite (Unsworth et al., 1989),
and this demonstrates some agreement with higher inherent
moisture. Also, inertinite tends to contain more oxygen
functional groups, and thus more hydrophilic sites.
The effect of the volatile content on nmax was also
determined, and followed a similar trend to that reported by
Prinz and Littke (2005), where a lower volatile content
yielded higher moisture adsorption, although in that research
the volatile content was used as a rank indicator. Since the
definition of volatile matter is not specific to the species
involved, this correlation is not considered very useful in
terms of moisture adsorption.
Hysteresis between adsorption and desorption was found
in all cases, and is in agreement with most other research
findings. Various explanations for hysteresis are given by
Mahajan and Walker (1971) McCutcheon et al. (2003)
Mitropoulos et al. (1996) and Tarasevich (2001), where it is
stated that low-pressure hysteresis should correlate with
oxygen content, while high-pressure hysteresis is associated
with pores having restricted openings and referred to as ‘ink-
bottle’ type pores. However, no relation between the degree of
hysteresis and the physical coal properties under discussion
here could be found.
Conclusions
The understanding and description of the mechanisms of
moisture adsorption on coal is as complex as the coal itself.
While many researchers have published their findings using
specific types of coal, or even clean carbon surfaces, under
very specific conditions, our research was conducted using
bulk coal samples that included great variability. The
conditions were chosen to emulate the environments that
most coals will be subjected to during normal transport and
storage. While more detailed investigations are required,
these results will contribute to predicting adsorption and
desorption responses for the particular coals studied.
Acknowledgements
The authors wish to thank the Coaltech 2020 Research
Programme and their member organizations for financial
support and permission to publish the results of this investi-
gation. This work is based on the research supported by the
South African Research Chairs Initiative of the Department of
Science and Technology and National Research Foundation of
South Africa. Any opinion, finding, or conclusion or
recommendation expressed in this material is that of the
author(s) and the NRF does not accept any liability in this
regard.
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806 NOVEMBER 2013 VOLUME 113 The Journal of The Southern African Institute of Mining and Metallurgy
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Moisture adsorption and desorption characteristics of some South African coals
The Journal of The Southern African Institute of Mining and Metallurgy VOLUME 113 NOVEMBER 2013 807
... The literature provides average values of physicochemical properties (water content, basic density, volatile matter content, ash content, fixed carbon content and high heating value) of some tropical wood charcoals from plantation wood or sawmill waste (Girard 2002;Syred et al. 2006;Afonso et al. 2015;Mensah, Lamptey Buertey, and Kemausuor 2017;do Rosário da Silva e Silva et al. 2018;Mfomo et al. 2020;Antwi-Boasiako and Glalah 2021). On the other hand, hygroscopicity studies have been more focused on bituminous coals (Campbell, Barnardo, and Bunt 2013;Charrière and Behra 2010;McCutcheon and Barton 1999;McCutcheon, Barton, and Wilson 2001;Švábová, Weishauptová, and Přibyl 2011). ...
... Indeed, the isotherms show a type II sigmoidal behavior according to BET classification system. This result is in agreement with those of authors in the literature who established that water sorption on bituminous coal is generally described according to a type II isotherm (Campbell, Barnardo, and Bunt 2013;Unsworth, Fowler, and Jones 1989). According to some studies, this form of isotherm observed on a carbonaceous material allows to estimate the density and distribution of oxygen-containing functional groups and consequently the sorption sites (Charrière and Behra 2010). ...
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