a
Department of Applied Chemistry, University of Johannesburg, PO Box 17011, Doornfontein 2028, Johannesburg, South Africa
b
ESKOM, Private Bag X40175, Cleveland, South Africa
c
Nanotechnology and Water Sustainability Unit, College of Science, Engineering and Technology, University
of South Africa, Florida Campus, Johannesburg, South Africa
Corresponding author details:
Heena Madhav
ESKOM Private Bag X40175
Cleveland
Johannesburg,South Africa
Copyright:
© 2018 Madhav H, et al. This is
an open-access article distributed under the
terms of the Creative Commons Attribution 4.0
international License, which permits unrestricted
use, distribution, and reproduction in any
medium, provided the original author and source
are credited.
Scaling Potential; Metal-organic Complexes; Visual MINTEQ
Eskom is a major power generating industry in Southern Africa, providing more
than 95% of the electricity consumed in South Africa. In power generation, the basic
thermodynamics are governed by condensation and evaporation. In a power plant, the
cooling water is used to condense the steam by absorbing heat. The water is then cooled
down and can be re-used [1].
The water (raw and cooling (CW)) used in this process is rich in organic and inorganic
mineral ions and these metal ions cause scaling which is indicated by saturation index. The
scaling potential by these free calcium (Ca2+) and magnesium (Mg2+) ions is vital to Eskom’s
business as it has major cost implications. For instance, 1 mm thick scale could add 7.5%
to energy costs, 1.5 mm could add up to 15% and 7 mm thick scale could increase costs by
70% (Roberge 2008). When calculating the scaling potential of the CW it is important to
take into account the metal complexation with dissolved organic carbon as it affects the
scaling potential of the CW.
Figure 1 shows the process involved in a conventional open wet cooling system at
Eskom power stations. The cooling water system is a re-circulating system where warm
water from the condenser (where heat exchange between cooling water and steam from
the turbine takes place) passes through the cooling tower. As the water is cooled, there is
loss of water through evaporation. Raw water is then as make-up water into the cooling
tower pond to replenish water that was lost during evaporation. This water is then recycled
back into the system. The scaling reaction occurs within the condenser tubes carrying the
recycled water. It is to note that these reactions are dependent on the water composition
such as the type of metal present, size of the organic molecules and the physico-chemical
properties of water e.g. the pH and temperature [2,3,4].
Recently, the humic acid concentration on H+
binding has been researched by [5].
The NICA-Donnan model for experimental data carried out by conducting acid-base
titrations with humic acid concentrations of between 20 and 200 mg/L concluding that the
concentration of humic acid in solution affects the binding behavior of the humic substance.
Some researchers explained spectroscopic and thermodynamic equilibrium calculations to
understand the complexation between Cu and NOM their research indicated that stable Cu—NOM complex was formed under acidic conditions due to
steric hindrances [6].
The MINTEQA2 speciation model was used in a study where metalDOM complexes with constant metal (Ca, Mg, Cu) concentrations over
a pH range were investigated [7]. Their modeled results indicated
that in the pH range of 0-6, there was no interaction between the
Ca and DOM, but the concentration of this metal complex increased
at pH > 6 and found that the Cu complex predominates at pH range
between 2.5 and 8 and that the interaction with Mg is relatively small.
Another study by [8] reported on the competitive complexation
of metal ions with humic substances. Their results indicated that at
an increased pH, the complexation of Ni with humic acid increased
and that high concentration of Ca inhibites the complexation of Ni.
The focus of this study is to investigate the gaps of some of the
previous studies using models are EQ 3/6, Geochem, MINTEQA2, NICADonnan, and PHREEQC and WHAM were surveyed to address the
issue of accommodating NOM in equilibrium model [9]. The objective
of the study was to understand the physico-chemical conditions under
which metals complex with DOC as well as individual metal binding
capacities. Appropriate programs will therefore provide a predictive
model that will assist to check the interplay of physicochemical
parameters and thus provide for scaling control protocols for cooling
water recycling in power generating stations.
Figure 1: Conventional Open Wet Cooled System at Eskom Power Stations
Sampling was done at two power stations, Lethabo and Duvha.
The raw and CW water from each station was analysed at Eskom
(RT&D) laboratories. The data was then entered into Visual MINTEQ
to further understand and model how metals complex with DOC.
Figure 2a: The effect of DOC on SI using the CW standard
Figure 2b: The effect of various metals bound to DOC with various concentrations of an artficial anionic poly-electrolyte
Figure 3a: The effect of %M-DOC concentration of CW and SI at Power Station A
Figure 3b: The effect of %M-DOC concentration in the RW and SI (at power station A)
Table 1: Eskom laboratory results for raw water from station A and station B (June sampling)
Figure 3a: The effect of %M-DOC concentration of CW and SI at Power Station A
Figure 4a: The %M-DOC and SI in the CW from power station B (North)
Figure 4b: The %M-DOC and SI in the cooling water from power station B (South)
The results from this study clearly indicate that the concentration
of organics and its speciation in the water play an important role for
scaling potential by Ca2+ ions. The seasonal variation influences the
pH, concentration for divalent ions and DOC and that further affects
the degree of complexation in raw and cooling water. The availability
of free Ca2+ does not influence complex formation with DOC which
depends upon the pH of water that change with season but directly
affects the SI.
This research was supported by Eskom RT&D and Eskom TESP
program for running cost of the project. NRF is acknowledged for
tuition fee to register at University of Johannesburg.
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