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Potential utilization of fly ash for CO2 sequestration and acid mine drainage (AMD) wastewater treatment
Author(s)
Zide, Sibulele
Date Issued
2023
Type
Thesis
Publisher
Cape Peninsula University of Technology
Abstract
The continued use of fossil fuels to meet the worldโs energy demands contributes largely to
the emissions of greenhouse gases. In South Africa, a considerable amount of carbon dioxide
(CO2) is emitted annually from burning coal to produce electricity. To mitigate the adverse
environmental effects arising from the continual increase in CO2 concentration (MtCO2) in the
atmosphere, which in turn gives rise to global warming, this work investigated carbon capture
and storage (CCS) using accelerated mineral carbonation (MC). Accelerated MC is emerging
as a promising technology for the permanent storage of CO2 based on its high storage
potential (> 10 000 Gt of CO2) because of the abundance of natural silicates worldwide. The
accelerated MC process involves reacting captured CO2 from a CO2 emission source with an
alkaline-rich feedstock to produce a mineral carbonate, thereby storing the CO2 permanently.
Despite the high storage potential displayed by MC, the relatively high energy consumption
(MW) and costs ($/tonne CO2) associated with the process continue to hinder its widespread
implementation. Hence, the focus for accelerated MC in this work was the optimization of
several process parameters and the use of coal fly ash (CFA) to improve the carbonation
performance thereby reducing the energy consumption (MW) of the process. A preliminary
study was first conducted to determine the amount of calcium (Ca2+) leachable from the fly
ash (FA). The parameters investigated for the calcium (Ca2+) extraction study were the
temperature (0C), time (min), and particle size (๐๐). Conditions resulting in the maximum
concentration of calcium (Ca2+) extracted were considered the optimum conditions from the
leaching study and these process conditions were subsequently used for the carbonation
process. The optimum temperature was determined to be 70 0C and the optimum reaction
times were 30 min, 90 min, and 120 min due to the lower calcium (Ca2+) concentration obtained
after 60 min, which from the Dixonโs Q-Test proved to be an outlier potentially caused by errors
while conducting the experiment. The particle size (๐๐) was not considered for carbonation
experiments based on the trade-off between the maximum calcium (Ca2+) leached and the
potential additional energy requirement (MW) due to sieving. Following the carbonation
experiments, the carbonation performance was measured through the percentage of CaCO3
formed, the carbonation efficiency CE (%), and the maximum CO2 storage capacity (kg/kg fly
ash). It was found that the maximum % CaCO3 formed after a reaction time of 120 min at 4
Mpa from direct carbonation with AMD wastewater was higher compared to the maximum %
CaCO3 formed of 2.43 % from direct aqueous carbonation under the same conditions of time
(min) and CO2 pressure (Mpa). This was attributed to the lower S/L ratio of 0.2 g/mL used for
experiments involving AMD wastewater, which improved the calcium (Ca2+) extraction, the
additional calcium (Ca2+) concentration of 362.5 ppm from the AMD wastewater, and the
increase in the stirring speed from 100 rpm up to 400 rpm upon process optimization.
The highest CE (%) was 63 % and was achieved from direct carbonation with AMD wastewater
after 120 min of direct carbonation at 4 Mpa, followed by 53.9 % achieved from indirect
carbonation with AMD under the same process conditions. The higher CE (%) through the
direct carbonation route was possibly due to the continued extraction of calcium (Ca2+) from
the fly ash during the direct carbonation reaction. A lower CE (%) was achieved when pure
water was used as the reaction solvent compared to when AMD wastewater was utilized,
again attributed to the lower S/L ratio, the additional calcium (Ca2+) concentration provided by
the AMD wastewater, as well as the higher stirring speed (rpm) used for the AMD carbonation
study. The CE (%) achieved from direct aqueous carbonation was 29.4 % while the CE (%)
achieved from the indirect aqueous route was found to be 35.2 % after 120 min of carbonation
at 4 Mpa. The higher CE (%) achieved from indirect aqueous carbonation compared to direct
aqueous carbonation was due to the higher stirring rate (rpm) used for indirect aqueous
carbonation, which improved calcium (Ca2+) extraction. The maximum CO2 storage capacity
which gave an indication of the maximum CO2 storage potential of the fly ash per 1 kg of fly
ash used, was also measured, and it was found to be 0.026 kg/kg fly ash which was expected
for fly ash material with a lower calcium oxide (CaO wt. %) content of 4.06 wt. % such as the
one used in this study. The study demonstrated a relatively effective storage of CO2
considering the lower CaO (wt. %) in the fly ash used. From material balance considerations,
there was a higher concentration of undissolved CO2 (i.e., ฮฃ๐ถ๐!(#$%) 22.89 g) in the liquid
phase from the total CO2 introduced into the reaction system (i.e., ฮฃ๐ถ๐!('() 36.97 g). The
mass of dissolved CO2 was 14.08 g, and 2.08 g was stored as CaCO3. 0.068 MW was
consumed from the process, and a high amount of the energy from the total energy output
was due to the power for heating, which suggested that lower temperatures (0C) could be
applied for the carbonation process. The energy consumption of 0.068 MW was relatively low
due to the non-pre-treatment (i.e., crushing, grinding, sieving, etc.) of the FA. Effective
neutralization of the AMD was achieved after 120 min of carbonation at a CO2 pressure of 4
Mpa for both direct and indirect carbonation with AMD. A pH of 7.1 was achieved under these
conditions, which was close to a neutral pH of 7. The percentage (%) removal of most toxic
elements was close to 100 % in all cases investigated, which suggests that most of the
concentrations after treatment met the target water quality range (TWQR).
the emissions of greenhouse gases. In South Africa, a considerable amount of carbon dioxide
(CO2) is emitted annually from burning coal to produce electricity. To mitigate the adverse
environmental effects arising from the continual increase in CO2 concentration (MtCO2) in the
atmosphere, which in turn gives rise to global warming, this work investigated carbon capture
and storage (CCS) using accelerated mineral carbonation (MC). Accelerated MC is emerging
as a promising technology for the permanent storage of CO2 based on its high storage
potential (> 10 000 Gt of CO2) because of the abundance of natural silicates worldwide. The
accelerated MC process involves reacting captured CO2 from a CO2 emission source with an
alkaline-rich feedstock to produce a mineral carbonate, thereby storing the CO2 permanently.
Despite the high storage potential displayed by MC, the relatively high energy consumption
(MW) and costs ($/tonne CO2) associated with the process continue to hinder its widespread
implementation. Hence, the focus for accelerated MC in this work was the optimization of
several process parameters and the use of coal fly ash (CFA) to improve the carbonation
performance thereby reducing the energy consumption (MW) of the process. A preliminary
study was first conducted to determine the amount of calcium (Ca2+) leachable from the fly
ash (FA). The parameters investigated for the calcium (Ca2+) extraction study were the
temperature (0C), time (min), and particle size (๐๐). Conditions resulting in the maximum
concentration of calcium (Ca2+) extracted were considered the optimum conditions from the
leaching study and these process conditions were subsequently used for the carbonation
process. The optimum temperature was determined to be 70 0C and the optimum reaction
times were 30 min, 90 min, and 120 min due to the lower calcium (Ca2+) concentration obtained
after 60 min, which from the Dixonโs Q-Test proved to be an outlier potentially caused by errors
while conducting the experiment. The particle size (๐๐) was not considered for carbonation
experiments based on the trade-off between the maximum calcium (Ca2+) leached and the
potential additional energy requirement (MW) due to sieving. Following the carbonation
experiments, the carbonation performance was measured through the percentage of CaCO3
formed, the carbonation efficiency CE (%), and the maximum CO2 storage capacity (kg/kg fly
ash). It was found that the maximum % CaCO3 formed after a reaction time of 120 min at 4
Mpa from direct carbonation with AMD wastewater was higher compared to the maximum %
CaCO3 formed of 2.43 % from direct aqueous carbonation under the same conditions of time
(min) and CO2 pressure (Mpa). This was attributed to the lower S/L ratio of 0.2 g/mL used for
experiments involving AMD wastewater, which improved the calcium (Ca2+) extraction, the
additional calcium (Ca2+) concentration of 362.5 ppm from the AMD wastewater, and the
increase in the stirring speed from 100 rpm up to 400 rpm upon process optimization.
The highest CE (%) was 63 % and was achieved from direct carbonation with AMD wastewater
after 120 min of direct carbonation at 4 Mpa, followed by 53.9 % achieved from indirect
carbonation with AMD under the same process conditions. The higher CE (%) through the
direct carbonation route was possibly due to the continued extraction of calcium (Ca2+) from
the fly ash during the direct carbonation reaction. A lower CE (%) was achieved when pure
water was used as the reaction solvent compared to when AMD wastewater was utilized,
again attributed to the lower S/L ratio, the additional calcium (Ca2+) concentration provided by
the AMD wastewater, as well as the higher stirring speed (rpm) used for the AMD carbonation
study. The CE (%) achieved from direct aqueous carbonation was 29.4 % while the CE (%)
achieved from the indirect aqueous route was found to be 35.2 % after 120 min of carbonation
at 4 Mpa. The higher CE (%) achieved from indirect aqueous carbonation compared to direct
aqueous carbonation was due to the higher stirring rate (rpm) used for indirect aqueous
carbonation, which improved calcium (Ca2+) extraction. The maximum CO2 storage capacity
which gave an indication of the maximum CO2 storage potential of the fly ash per 1 kg of fly
ash used, was also measured, and it was found to be 0.026 kg/kg fly ash which was expected
for fly ash material with a lower calcium oxide (CaO wt. %) content of 4.06 wt. % such as the
one used in this study. The study demonstrated a relatively effective storage of CO2
considering the lower CaO (wt. %) in the fly ash used. From material balance considerations,
there was a higher concentration of undissolved CO2 (i.e., ฮฃ๐ถ๐!(#$%) 22.89 g) in the liquid
phase from the total CO2 introduced into the reaction system (i.e., ฮฃ๐ถ๐!('() 36.97 g). The
mass of dissolved CO2 was 14.08 g, and 2.08 g was stored as CaCO3. 0.068 MW was
consumed from the process, and a high amount of the energy from the total energy output
was due to the power for heating, which suggested that lower temperatures (0C) could be
applied for the carbonation process. The energy consumption of 0.068 MW was relatively low
due to the non-pre-treatment (i.e., crushing, grinding, sieving, etc.) of the FA. Effective
neutralization of the AMD was achieved after 120 min of carbonation at a CO2 pressure of 4
Mpa for both direct and indirect carbonation with AMD. A pH of 7.1 was achieved under these
conditions, which was close to a neutral pH of 7. The percentage (%) removal of most toxic
elements was close to 100 % in all cases investigated, which suggests that most of the
concentrations after treatment met the target water quality range (TWQR).
Additional information
Thesis (MEng (Chemical Engineering))--Cape Peninsula University of Technology, 2023
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