The reactivity of South African limestone of variable quality as potential sorbents in wet flue gas desulphurisation

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The reactivity of South African limestone of

variable quality as potential sorbents in wet flue

gas desulphurisation

PB Swart

orcid.org/0000-0003-1319-6586

Dissertation submitted in fulfilment of the requirements for the

degree

Master of Engineering in Chemical Engineering

at the

North-West University

Supervisor:

Dr DJ Branken

Co-supervisor:

Prof HWJP Neomagus

Graduation ceremony: May 2019

Student number: 21087474

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ABSTRACT

According to the World Health Organisation (WHO) clean air is a basic requirement for the well-being and general health of every human well-being. The results of WHO assessments have shown that annually more than 2 million deaths can be classified as premature and can be linked to outdoor as well as indoor air pollution. Sulphur dioxide (SO2) emissions from coal-fired power plants and other industrial operations form part of the potentially hazardous pollutants, and it is critical that these emissions be reduced where possible.

In 2012 the South African government enforced stricter air quality standards and stipulated that the SO2 emissions from all existing power plants must comply with an SO2 emission limit of 3500 mg/Nm3 by 2015, while the SO2 emissions limit for all new power plants would be limited to 500 mg/Nm3. Therefore, the decision was made to implement the most widely used desulphurisation technology, namely Wet Flue Gas Desulphurisation (WFGD), in the newly built South African power stations. The removal of the SO2 component of the flue gas from a coal-fired power station by means of a WFGD process, involves making use of a sorbent such as limestone to capture the SO2 from the flue gas stream. Limestone consists mainly of CaCO3, is mixed with water to form a slurry and is sprayed into the absorption tower where absorption of SO2 from the flue gas takes place. Further reaction then occurs below the absorber in the reaction tank. Despite the high desulphurisation efficiency offered by WFGD, the continuous improvement of the process efficiency is still required considering environmental legislation that is becoming increasingly stringent, while power plants age and the quality of the coal being consumed degrades over time. A sound understanding of the process and the various steps involved is of significant importance with regards to process and design optimisation. In this regard, the rate of dissolution of the limestone that is used as sorbent in a WFGD is a key step in the process that needs to be further studied to gain an understanding of the influence it has on the overall desulphurisation process.

Maintaining the pH of the aqueous slurry in the WFGD reaction tank is of paramount importance to maintain the optimal reaction conditions for the formation and precipitation of the main WFGD by-product, namely gypsum CaSO4. This can be achieved by matching the rate of fresh limestone addition to the reactor tank with the rate of CaCO3 dissolution (or CaSO4 precipitation). As limestone is a mineral resource that can consist of different constituents in varying concentrations depending on the origin of the limestone, limestone sourced from different origins can have significant differences in CaCO3 content (quality) and therefore the reactivity with respect to WFGD processes. Limestone deposits that are in close proximity of the majority of the South African power stations generally contain limestone of relatively low quality (CaCO3 content of between 75 and 85 %).

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A high-quality limestone source is however located in the Northern Cape. This limestone would have to be transported to the power station where it would be utilised, resulting in an escalated cost. It is therefore valuable to investigate the possibility of utilising sources that are closer by although the quality is lower.

It is therefore prudent to study the rate of dissolution of limestone with varying quality and mineralogical composition as a function of other WFGD process parameters such as pH and reaction temperature. Consequently, four South African limestone samples were selected for this study, namely Limestone A, B, C and D. The quality of these limestone samples based on CaCO3 results determined from XRF analysis were 95 % for Limestone A, 82 % for Limestone B, 89 % for Limestone C, and 71% for Limestone D. Additionally, these limestone samples were crushed and ground and subsequently sieved to obtain three particle size fractions, namely dp < 106 µm, dp < 75 µm and dp < 45 µm. The pH stat method was used to study the dissolution kinetics of the different limestone samples as a function of the particle size, the slurry temperature and the solution pH.

A first-order semi batch model was used to model the experimental measurement results. The rate constant was determined from this model and found to range between 0.07 x 10-6 _{and 4.09 x 10}-6 m/s. It was further found that the rate of dissolution did not correlate with the quality of the limestone, since Limestone D, which had the lowest quality, exhibited the highest rate of dissolution, It was found that the rate of dissolution of the Limestone samples were, in descending order: Limestone D, Limestone A, Limestone B, and Limestone C. It is therefore concluded that other factors such as particle surface area, porosity, degree of crystallinity, and the presence of the dolomite mineral and other impurities also have a significant effect. However, further study is required to elucidate the most influential parameters.

Key words: Limestone, Dissolution kinetics, Wet Flue Gas Desulphurisation, pH stat.

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ACKNOWLEDGEMENTS

I would like to acknowledge and express my thanks to all parties that provided assistance and guidance in all aspects of the study. This includes funding, advice, suggestions and support from the following parties:

Eskom:

• Funding provided through the Eskom Power Plant Engineering Institute (EPPEI).

Eskom business departments that assisted with guidance regarding an industrial point of view, sourcing and analysis of the limestone samples:

• Eskom RT&D

• Production Engineering Integration - Air Pollution Control • Boiler Auxiliaries CoE

North-West University with regards to the following inputs:

• Excellent guidance from my study leaders

• Characterisation analysis conducted on the limestone samples, internally, externally and through external partnerships.

• Students that assisted with the experiments, sample preparation and characterisation analysis.

• Students that are involved in studies relating to FGD that advised accordingly.

A special thanks to Sulandi my loving and supporting fiancé. Without your love and support it would not have been possible to conclude on this dissertation.

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Figure 2-24: Limestone conversion (fraction dissolved) vs. time under varying temperature, while maintaining the solution pH at a constant value of 5.4. Taken from Li et al. (2013). ... 41 Figure 3-1: Splitting and homogenisation process for one limestone sample ... 50 Figure 3-2: Basic functionality of a cone crusher ... 50 Figure 3-3: Composition (wt. %) of the minerals contained in the -45µm sample of the different limestone samples as per XRF analysis. ... 57 Figure 3-4: Composition (wt. %) of the minerals contained in the -45µm sample of the different limestone samples as per QEMSCAN analysis. ... 58 Figure 3-5: SEM Images of Limestone A for particle sizes of -45µm (left), -75µm (middle), and -106 µm (right). ... 59 Figure 4-1: Line diagram of pH stat equipment used for conducting the dissolution experiments. Taken from Carletti et al. (2015). ... 63 Figure 4-2: pH vs. time results for the five repeat experiments at a pH of 5, temperature of 35o_{C and} particle size of -45µm after the pH set point was reached. ... 66 Figure 4-3: Volume vs. time results for the five repeat experiments at a pH of 5, temperature of 35o_C and particle size of -45µm. ... 67 Figure 4-4: Conversion vs. time results for the five repeat experiments at a pH of 5, temperature of 35o_{C and particle size of -45µm. ... 68}

Figure 4-5: Limestone A - Rate of conversion at varying temperature, constant pH of 5 and particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.033 and 0.006 (Table G-1), and the maximum and minimum R2_{values from the parity plots were 1.0 and 0.983} (Figures in Appendix G). ... 69 Figure 4-6: Limestone B: Rate of conversion at varying temperature, constant pH of 5 and particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.021 and 0.007 (Table G-2), and the maximum and minimum R2_{values from the parity plots were 0.999 and 0.993} (Figures in Appendix G). ... 70 Figure 4-7: Limestone C: Rate of conversion at varying temperature, constant pH of 5 and particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.044 and 0.033 (Table G-3), and the maximum and minimum R2_{values from the parity plots were 0.981 and 0.966} (Figures in Appendix G). ... 71 Figure 4-8: Limestone D: Rate of conversion at varying temperature, constant pH of 5 and particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.038 and 0.029 (Table G-4), and the maximum and minimum R2_{values from the parity plots were 0.990 and 0.989} (Figures in Appendix G). ... 71

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Figure 4-9: Limestone A: Rate of conversion at varying particle size, constant pH of 5 and a temperature of 35o_{C. The maximum and minimum RMSE w.r.t. the model fits were 0.039 and 0.006} (Table G-1), and the maximum and minimum R2_{values from the parity plots were 1.0 and 0.961} (Figures in Appendix G). ... 72 Figure 4-10: Limestone B: Rate of conversion at varying particle size, constant pH of 5 and a temperature of 35o_{C. The maximum and minimum RMSE w.r.t. the model fits were 0.032 and 0.021} (Table G-2), and the maximum and minimum R2_{values from the parity plots were 0.993 and 0.942} (Figures in Appendix G). ... 73 Figure 4-11: Limestone C: Rate of conversion at varying particle size, constant pH of 5 and a temperature of 35o_{C. The maximum and minimum RMSE w.r.t. the model fits were 0.045 and 0.033} (Table G-3), and the maximum and minimum R2_{values from the parity plots were 0.980 and 0.951} (Figures in Appendix G). ... 73 Figure 4-12: Limestone D: Rate of conversion at varying particle size, constant pH of 5 and a temperature of 35o_{C. The maximum and minimum RMSE w.r.t. the model fits were 0.046 and 0.017} (Table G-4), and the maximum and minimum R2_{values from the parity plots were 0.994 and 0.972} (Figures in Appendix G). ... 74 Figure 4-13: Limestone A: Rate of conversion at varying pH, constant temperature of 35o_{C and} particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.030 and 0.006 (Table G-1), and the maximum and minimum R2_{values from the parity plots were 1.0 and} 0.989 (Figures in Appendix G). ... 75 Figure 4-14: Limestone B: Rate of conversion at varying pH, constant temperature of 35o_{C and} particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.045 and 0.021 (Table G-2), and the maximum and minimum R2_{values from the parity plots were 0.993} and 0.969 (Figures in Appendix G). ... 76 Figure 4-15: Limestone C: Rate of conversion at varying pH, constant temperature of 35o_{C and} particle size less than 45 µm. The maximum and minimum RMSE w.r.t. the model fits were 0.049 and 0.009 (Table G-3), and the maximum and minimum R2_{values from the parity plots were 0.998} and 0.959 (Figures in Appendix G). ... 76 Figure 4-16: Rate of conversion at varying quality, pH of 5, particle size of less than 45 µm and a temperature of 35o_{C. The maximum and minimum RMSE w.r.t. the model fits were 0.033 and 0.006} (Table G-1 to G-4), and the maximum and minimum R2_{values from the parity plots were 1.0 and} 0.980 (Figures in Appendix G). ... 77 Figure B-1: PSD results of limestone samples with particle size -45µm. ... 94 Figure C-1: Composition (wt. %) of the minerals contained in Limestone A as per XRF analysis. 95 Figure C-2: Composition (wt. %) of the minerals contained in Limestone B as per XRF analysis. 95 Figure C-3: Composition (wt. %) of the minerals contained in Limestone C as per XRF analysis. 96 Figure C-4: Composition (wt. %) of the minerals contained in Limestone D as per XRF analysis. 96

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Figure D-1: Composition (wt. %) of the minerals contained in Limestone A as per QEMSCAN

analysis. ... 97

Figure D-2: Composition (wt. %) of the minerals contained in Limestone B as per QEMSCAN analysis ... 97

Figure D-3: Composition (wt. %) of the minerals contained in Limestone C as per QEMSCAN analysis ... 98

Figure D-4: Composition (wt. %) of the minerals contained in Limestone D as per QEMSCAN analysis ... 98

Figure E-1: SEM Images of Limestone B for particle sizes of -45µm (left), 75 dp < 106 µm (middle), and -106 µm (right). ... 99

Figure E-2: SEM Images of Limestone C for particle sizes of -45µm (left), -75µm (middle), and -106 µm (right). ... 99

Figure E-3: SEM Images of Limestone D for particle sizes of -45µm (left), -75µm (middle), and -106 µm (right). ... 99

Figure F-1: Experimental data Limestone C, temperature 35o_{C, pH 5.5, particle size -45µm and} applied shrinking particle model. ... 100

Figure F-2: Experimental data (Limestone C, pH 5.5) and applied shrinking particle model parity plot. ... 100

Figure F-3: Limestone C, temperature 35o_{C, pH 5.5, particle size -45µm semi bach model parity} plot. ... 101

Figure G-1: Limestone A, temperature 45oC, pH 5, particle size -45µm. ... 102

Figure G-2: Limestone A, temperature 45o_{C, pH 5, particle size -45µm. ... 102}

Figure G-9: Limestone A, temperature 35o_{C, pH 4.5, particle size -45µm. ... 106}

Figure G-12: Limestone A, temperature 35oC, pH 5, particle size -45µm. ... 107

Figure G-15: Limestone B, temperature 45o_{C, pH 5, particle size -45µm. ... 109}

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Figure G-19: Limestone B, temperature 35 C, pH 5, particle size 45 -75µm. ... 111

Figure G-20: Limestone B, temperature 35o_{C, pH 5, particle size 45 -75µm. ... 111}

Figure G-23: Limestone B, temperature 35o_{C, pH 4.5, particle size -45µm. ... 113}

Figure G-24: Limestone B, temperature 35oC, pH 4.5, particle size -45µm. ... 113

Figure G-27: Limestone B, temperature 35o_{C, pH 5.5, particle size -45µm. ... 115}

Figure G-28: Limestone B, temperature 35oC, pH 5.5, particle size -45µm. ... 115

Figure G-29: Limestone C, temperature 45o_{C, pH 5, particle size -45µm. ... 116}

Figure G-37: Limestone C, temperature 35o_{C, pH 4.5, particle size -45µm. ... 120}

Figure G-39: Limestone C, temperature 35oC, pH 5, particle size -45µm. ... 121

Figure G-43: Limestone D, temperature 45o_{C, pH 5, particle size -45µm. ... 123}

Figure G-50: Limestone D, temperature 35oC, pH 5, particle size -45µm. ... 126

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LIST OF TABLES

Table 1-1: Chemical equations involved in the WFGD process adapted from Córdoba (2015). ... 3

Table 1-2: Mineralogical composition of limestone as per previous research (Lamar, 1957, Tokuyama, Kitano & Kaneshima, 1972, Jan, Bilqees, Riaz, Noor, & Younas, 2009, Sdiri et al., 2012) ... 4

Table 1-3: Limestone properties typical to Taiwan. Adapted from Shih et al. (2000). ... 5

Table 1-4: Limestone properties typical to Europe. Adapted from Alkattan et al. (1998). ... 5

Table 1-5: Composition of a limestone that was mined within South Africa. Adapted from Maina & Mbarawa 2012 ... 6

Table 2-1: Ambient Air Quality Standards of selected countries adapted from (Vahlsing and Smith, 2012) ... 12

Table 2-2: Multi-Pollutant processes and the pollutants they remove. ... 16

Table 2-3: Chemical reactions involved in the WFGD process adapted from Córdoba (2015). ... 17

Table 2-4: Typical properties of Oolitic and Freshwater limestone adapted from Török and Ákos (2006). ... 21

Table 2-5: South Africa’s most significant carbonate occurrences, adapted from (Agnello, 2005). 22 Table 2-6: Typical limestone specifications for WFGD application Agnello (2005). ... 24

Table 3-1: Bond work index for the four limestone samples ... 55

Table 3-2: Energy required to obtain one ton of CaCO3 from each limestone sample. ... 56

Table 3-3: Sauter mean diameter of the different limestone samples. ... 56

Table 3-4: Difference between XRF and QEMSCAN results conducted on -45µm samples. ... 58

Table 4-1: Summary of manipulated variables. ... 62

Table 4-2: Total rate constant values obtained under the various experimental conditions for all limestone samples ... 78

Table 4-3: Activation energy for the investigated limestone samples ... 80

Table A-1: Maximum and minimum relative standard error (%) between experimental data sets of Limestone A. ... 90

Table A-2: Maximum and minimum relative standard error (%) between experimental data sets of Limestone B. ... 91

Table A-3: Maximum and minimum relative standard error (%) between experimental data sets of Limestone C. ... 92

Table A-4: Maximum and minimum relative standard error (%) between experimental data sets of Limestone D. ... 93

Table G-1: Maximum and minimum root mean square error between experimental and modelled data of Limestone A. ... 127

Table G-2: Maximum and minimum root mean square error between experimental and modelled data of Limestone B. ... 128

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Table G-3: Maximum and minimum root mean square error between experimental and modelled data of Limestone C. ... 129 Table G-4: Maximum and minimum root mean square error between experimental and modelled data of Limestone D. ... 130

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DEFINITIONS AND ABBREVIATIONS

Abbreviation Meaning

A Pre-exponential factor (units depend on order of reaction) ARP Acid rain program

bflow Proportionality constant for the volumetric flow out BMREQ Bechtel Modified Radian Equilibrium

BWI Bond work index

CCa2+ Calcium ion concentration in the bulk

Ceq Pseudo equilibrium constant (dimensionless) CHNO3 HNO3 concentration

cm2 _{Square centimetre} Cs Solid concentration Cs,0 Initial solid concentration

dp,o,i Measured initial particle diameter E Activation energy

ESP Electrostatic Precipitator FFP Fabric Filter Plant

FGD Flue Gas Desulphurisation kc Mass transfer coefficient ktot Total rate constant MMs Molar mass of the solid

pH Potential of Hydrogen

PSD Particle size distribution

QEMSCAN Quantitative Evaluation of Minerals by SCANning electron microscopy

Qin Inlet volume flow Qout Outlet volume flow R The ideal gas constant

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Re Reynolds number for a single sphere in a liquid RMSE Root mean square error

rs Rate of reaction for the solid

Sc Schmidt number

SE Standard error

Sf,o Initial surface factor (dimensionless)

Sh Sherwood number

SSAreal Specific surface area of real particle

t Time

T _Temperature

USA _{United States of America}

V _Volume

WFGD _{Wet Flue Gas Desulphurisation} WHO World Health Organisation

x Conversion

XRF x-ray fluorescence

zi Mean weight fraction matrix (dimensionless) α Power for the reaction rate equation

ε Mean dissipated energy that is the energy transferred from the impeller to the slurry mixture

μg Micro gram

ρs Solid density

𝜐𝜐 Kinematic viscosity of the liquid

𝐷𝐷𝑎𝑎𝑎𝑎 Diffusion coefficient of hydrogen ion in water

𝐶𝐶_𝐻𝐻∞+ H+_{bulk concentration}

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LIST OF UNITS

Symbol Unit

g Grams – Unit of mass

g/m3 _{Grams per cubic meter – Unit of density}

g/mol Grams per mol – Constant for a given substance J/mol Joule per mol – Unit of energy per amount of material

J/mol.K Jule per mole per Kelvin – Unit of energy per amount of material K Kelvin – Unit of temperature

kg Kilograms – Unit of mass kJ Kilojoule – Unit of energy

kPa Kilopascal – Unit of pressure

kWh Kilowatt hour – Unit of energy l Liter – Unit of volume

l/m2_s _{Liter per square meter per second – Unit of reaction rate} l/s Liter per second – Unit of volume flow

M Molar – Unit of concentration

m/s Meter per second – Unit of speed (reaction rate) m2 _{Square metre – Unit of surface area}

m2_/g _{Squared meter per gram – Unit of surface area} m2_/s _{Squared meter per second - Unit of diffusivity} m3 _{Cubic meter – Unit of volume}

mg Milligram

min Minute – Unit of time

mm Millimetre – Unit of distance

mol/(l s) Mol per liter per second – Unit of reaction rate mol/l Mol per liter – Unit of concentration

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Symbol Unit MW Megawatt – Unit of power

MWe Megawatt (electricity sent out) – Unit of power specifically for electricity generating systems

Nm3 _{Normal Cubic meter – Unit of volume} o_C _{Degrees Celsius – Unit of temperature} ppm Parts per million – Unit of concentration

rpm Revolutions per minute – Unit of rotational speed s Second – Unit of time

W Watt – Unit of power

W/kg Watt per kilogram – Unit of dispersed energy μm Micro meter – Unit of distance

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1. INTRODUCTION

1.1. Background

Presently, coal still serves as the primary energy source in South Africa, despite the drive towards “clean” energy such as solar and wind power (Hancox & Götz, 2014). Between April 2012 and March 2013 South Africa’s principal energy utility, Eskom Hld SOC Ltd., burned 122.95 Mt of coal to supply enough electricity to satisfy the demand national demand (Eskom, 2014). With such a large amount of coal combusted, a considerable amount of gaseous emissions would also have been released into the atmosphere, which includes sulphur dioxide (SO2) (IEA Coal Industry Advisory Board 2013). Coal consists mainly of the elements carbon (the most valuable constituent), hydrogen, oxygen, nitrogen and sulphur, together with ash-forming minerals. Other parameters that influence the value of the coal are the volatile matter content and the amount of ash; these parameters have an influence on the rank of the coal. Quantifying these components is therefore essential for establishing coal rank, and to quantify the concentration of components that contribute to potentially harmful emissions such as SO2 (Van Niekerk et al., 2008).

Cheng et al. (2003) reported that the sulphur contained in coal is distributed between organic sulphur and inorganic sulphur that predominantly occurs as Pyrite (FeS2), both of which are oxidised to form sulphur dioxide (SO2) during the combustion process that takes place within a coal-fired boiler. In this respect, the amount of sulphur dioxide emitted from coal-fired boiler flues is therefore a direct result of the relative sulphur content of the coal feed stock. South African hard coal in general has a typical sulphur content of 0.6 – 1.6 % which is low compared to countries like Australia, China and the USA where the sulphur content ranges from 0.2 % to as high as 4.8 %. Conversely, the sulphur content of coal originating from countries like Colombia and Indonesia range between 0.08 – 1.0 %, and is therefore lower than the sulphur content of South African hard coal as mentioned by Commission of the European Communities, (1992). With this in mind it is understandable that FGD technology will only be implemented within South Africa at this point in time, as the sulphur content is lower and the emissions limit on SO2 is higher and will only be lowered in the near future.

In 2012 it was communicated by government that by 2015 (within 3 years from the date of the communication) all existing Power Plants must comply with an SO2 emission limit of 3500 mg/Nm3 under normal conditions of 10% O2, 273 K and 101.3 kPa. In contrast, all new Power Plants need to conform to a limit of 500 mg/Nm3_{under the same conditions (DEPARTMENT OF ENVIRONMENTAL} AFFAIRS OF SOUTH AFRICA, 2012). Therefore, adherence to legislation was a key factor that was taken into consideration when the decision was made to implement Wet Flue Gas Desulphurisation (WFGD) on newly built Eskom Power Stations (Eskom, 2014). Seeing that wet scrubbers usually

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remove 95 – 98% of the SO2, with the modern WFGD’s capable of removing 99% of the SO2 contained within the flue gas stream as per Carpenter’s report in 2012. The removal of the sulphur dioxide component of the flue gas from a coal-fired power station by means of a Wet Flue Gas Desulphurisation (WFGD) process, involves making use of a sorbent such as CaCO3 to capture the SO2 from the flue gas stream. Despite the high desulphurisation efficiency offered by WFGD, the continuous improvement of the process efficiency is still required in light of environmental legislation that is becoming increasingly stringent, while power plants age and the quality of the coal being consumed degrades over time. A sound understanding of the process and the various steps involved is of significant importance with regards to process and design optimisation. In this regard, the dissolution of the limestone that is used as sorbent in a WFGD is a key step in the process that needs to be further studied to gain an understanding of the influence it has on the overall desulphurisation process using WFGD.

WFGD technology has been used since the early 1970’s in Japan and the United States in the 1980’s for the reduction of SO2 levels in the atmosphere to conform to the U.S. ARP (Acid rain program), from where the technology has since been extended to Europe. WFGD can be characterised by the chemical reactions contained in Table 1-1, note that the CO2 adsorption and desorption will not be mentioned.

pH is very important in the control of the reaction, where a unit change in pH translates into to a tenfold change in the hydrogen ion concentration (Tucker and D’Abramo, 2008). The reason for the importance of pH is that the hydrogen-ion concentration follows from the chemical reaction equations given in Table 1-1. It is clear that up to two hydrogen ions are added to the solution when SO2 dissolves in water, that are then neutralised by the addition of the calcium carbonate, which consumes these hydrogen ions according to Equation (1-4) and Equation (1-5) (Córdoba, 2015). Therefore, the pH may be controlled in an industrial WFGD reaction tank by controlling the rate of fresh limestone addition. In this way, the slurry mixture’s pH in the reaction tank can be maintained at an optimum value such that a fine balance is achieved w.r.t providing favourable conditions for both SO2 absorption (favoured at high pH) and calcium carbonate dissolution (favoured at low pH). For this purpose, the rate of limestone dissolution under the conditions within the WFGD reaction tank must be accurately known.

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Table 1-1: Chemical equations involved in the WFGD process adapted from Córdoba (2015).

Stage Chemical reaction

Equation (#)

SO2 dissolution SO2 (g) ↔ SO2 (aq) 1-1

Hydrolysis of SO2 SO2 (aq) + H2O ↔ H2SO3 (aq) 1-2 Sulphurous acid dissociation H2SO3(aq) ↔ H +_{(aq) + HSO} 3 - _{↔ H}+_{(aq) + SO} 3 2-(aq) 1-3

Limestone dissolution CaCO3(s) + H

+(aq) ↔ Ca2+_{(aq) + HCO}

3

(aq) H2O

(aq) 1-4

Acid-base neutralisation HCO3- (aq) + H+(aq) ↔ CO2 (aq) + H2O (aq) 1-5

Overall reaction gas-to-liquid

CaCO3(s) + SO2 (g) + 2 H2O (aq) → CaSO3 • 2 H2O (aq) + CO2 (g)

1-6

Oxidation and sedimentation

CaSO3 • 2 H2O (aq) + ½ O2(g) → CaSO4 • 2 H2O (s)

1-7

1.2. Motivation

Limestone is a naturally-occurring calcium-based mineral which is found in relative abundance in the earth’s crust. It is a relatively inexpensive material with the transportation thereof playing the main role in determining its cost. (Yu, 2011), and is composed mainly of the minerals as seen in Table 1-2.

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Table 1-2: Mineralogical composition of limestone as per previous research (Lamar, 1957, Tokuyama, Kitano & Kaneshima, 1972, Jan, Bilqees, Riaz, Noor, & Younas, 2009, Sdiri et al., 2012)

Chemical formula

(typical range) Common name

AI2O3 (0 - 3%) Aluminium oxide (alumina)

CaO Calcium oxide (lime)

CaCO3 (75 - 99.7%) Calcium carbonate

CO2 Carbon dioxide

FeO Ferrous oxide (iron oxide)

Fe2O3 (0 - 2%) Ferric oxide (iron oxide) K2O (0 - 1%) Potassium oxide (potash) MgO (0 – 1.2%) Magnesium oxide (magnesia) MgCO3 (0 – 3.2%) Magnesium carbonate

MnO (0 - 0.5%) Manganese oxide

Na2O Sodium oxide

P Phosphorus

P2O5 (0 – 0.2%) Phosphorus pentoxide

R2O3 Oxides of trivalent metals, primarily Fe2O3 and AI2O3

S Sulphur

SO3 (0 - 0.1%) Sulphur trioxide SiO2 (0.3 – 27.3%) Silicon dioxide (silica) TiO2 (0 – 0.3%) Titanium dioxide (titania)

Naturally occurring limestone found in South Africa and internationally varies significantly in the calcium carbonate content, which serves to quantify the quality of limestone samples. Internationally, limestone that is used in the FGD process is usually of high quality, such as the samples used in the experiments conducted by Shih, Lin, & Shiau in 2000 and Alkattan et al. (1998), Oelkers, Dandurand, & Schott in 1998. The composition of the aforementioned limestone samples are summarised in Table 1-3 and Table 1-4.

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Table 1-3: Limestone properties typical to Taiwan. Adapted from Shih et al. (2000).

Table 1-4: Limestone properties typical to Europe. Adapted from Alkattan et al. (1998).

Calcite Quartz Clay minerals Mg-carbonates

Pourtalet calcite 99.6 - - 0.1

St. Maximum

limestone 79 1.6 0.9 1.5

Lavoux limestone 99.1 - 0.2 0.4

In contrast, the quality of South African limestone sources are frequently lower when compared to the limestone sources cited above, with an example from the study of Maina & Mbarawa (2012) shown in Table 1-5. Source CaCO3 (wt. %) MgCO3 (wt. %) 𝒅𝒅 (𝝁𝝁m) 𝑷𝑷𝒃𝒃 (g / cm3₎ _𝒆𝒆 𝑺𝑺𝒈𝒈 (m2 _{/ g)} 𝑷𝑷𝒎𝒎 (mol / cm3₎ Tay-Bair Mt. 94.8 1.1 102.4 2.597 0.05 0.4 0.02478 Her-Pyng 92.7 2.7 112.2 2.497 0.09 0.7 0.02395 Shin-Cherng 95.0 3.2 118.2 2.619 0.04 0.2 0.02586 Chyh-Ke Mt. 77.5 1.1 125.4 2.603 0.05 0.7 0.02051 Dah-Gang Mt. 81.0 0.8 117.5 2.516 0.08 1.6 0.02060 Bann-Pyng Mt. 74.0 1.1 127.6 2.452 0.10 2.5 0.01846

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Table 1-5: Composition of a limestone that was mined within South Africa. Adapted from Maina & Mbarawa 2012

Compound Percentage present

Calcite (CaCO3) 85.14

Quartz (SiO2) 3.80

Dolomite (CaMg(CO3)2) 8.99

Illite ((K, H3O)(Al, Mg, Fe)2 (Si,Al)4O10[(OH)2,

(H2O)]) 2.06

The rate of limestone dissolution in an aqueous medium is influenced by the following factors as previously reported Ukawa et al. (1993), Maldei (1993), Coto et al. (2012).

• The presence of other minerals and trace components (inhibitors), i.e. the limestone quality, • pH of the slurry solution,

• The temperature,

• The limestone particle size,

• Hydrodynamic factors and mass transfer effects, and • The intensity of the agitation.

Whereas the temperature, pH and hydrodynamics can be controlled to a certain extent, the limestone quality is constrained by the feed stock that is used for a specific WFGD unit. Seeing that the limestone feed rate might be governed by equipment such as the pumps, the available calcium of the limestone that is fed into the absorber will play an important part in the reactivity, and the rate at which calcium ions are made available for reaction with absorbed SO2 when using lower quality limestone is therefore important to characterise. The dissolution kinetics of South African limestone sources are poorly studied, and the degree to which WFGD performance will be affected through the implementation of limestone sources with lower qualities are unknown. Studying the influence of limestone quality on dissolution kinetics and the subsequent evaluation of WFGD performance as a function of limestone quality is therefore the focus of this dissertation.

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1.3. Problem Statement

South Africa’s power utility is in the process of constructing a new coal-fired power station, which is the first WFGD-equipped coal-fired power plant in South Africa. It is located near eMalahleni in Mpumalanga. After being fully commissioned, this coal-fired power station consist of six 800 MW units. A sound understanding of this technology is therefore imperative to ensure optimum utilisation of WFGD technology within the context of the South African power generation industry. In this regard, the screening of the various grades of limestone that is available throughout South Africa, and which can be used as potential feedstock to WFGD systems, are required. This need stems from the fact that the influence that limestone sources of varying quality and mineralogical composition will have on WFGD processes, is currently unknown and therefore needs to be quantified.

Since this new coal fired power stations WFGD design has already been finalised, accurate kinetic models of limestone dissolution as a function of limestone quality, pH and temperature, amongst other parameters, are necessary to determine the minimum permissible limestone quality that can be used to achieve the required SO2 removal efficiency. Additionally, limestone utilisation, which is determined by residence time or in other words reactor volume and slurry flow rate, is also an important factor to be considered with respect to overall operational efficiency and costs. Since absorbed SO2 only reacts with (is neutralised by) dissolved calcium carbonate (the major constituent of limestone), the rate of limestone dissolution can be used as a key indicator of WFGD performance for a fixed reactor design. Studying the dissolution kinetics of representative limestone samples under conditions that mimics those found in an industrial WFGD reaction tank as closely as possible are therefore required. The availability of accurate kinetic models would also assist in finding the optimum trade-off between operating costs and SO2 capture efficiency.

1.4. Aim and Objectives

Although WFGD is an established technology it has never been implemented in South Africa. It is therefore essential to be able to predict the behaviour of potential limestone sources under typical process conditions with respect to their reactivity and the resulting process performance and efficiency. The aim of this dissertation is therefore to investigate the effect of different parameters, which could be limiting in terms of the WFDG process or the limestone itself, on the rate of dissolution of the limestone.

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The following objectives have thus been identified to assist in reaching the aim of this study, namely: • To identify and sample limestone samples within South Africa which could potentially be

used within the FGD process.

• To characterise these samples chemically and physically.

• To empirically measures the dissolution rate of the potential limestone samples identified through the screening process and study the influence of temperature, pH, and particle size of the limestone samples on the rate of dissolution. The potential influence of mass transfer effects is also to be evaluated.

• To identify an appropriate kinetic model to mathematically model the results obtained from the empirical measurements by correlating the rate of limestone dissolution with the limestone quality, pH and the other empirical parameters.

• To conclude on the rate-controlling step, i.e. either the rate of mass transfer at the solid-liquid boundary layer or the rate of the surface reaction, and propose a suitable mechanism for the dissolution reaction.

• To advice on the selection criteria with respect to the minimum limestone quality that would be suitable for cost-effective operation of WFGD processes, based solely on the empirical and modelling results.

1.5. The scope of this study

A first point of the investigation was to identify the potential limestone samples for testing. These samples were then prepared by grinding the samples to the correct size for testing purposes, the samples were characterized which gave an understanding of the mineralogical and chemical composition of the selected limestone samples. In this study the dissolution rate of limestone was determined using the pH stat method wherein parameters relevant to WFGD, such as pH, particle size, temperature and limestone composition was investigated. Only the Calcium Carbonate (CaCO3) was considered to be reactive and the dissolution of other components such as the impurities and the dolomite mineral contained within the limestone sample was not considered. Nitric acid (HNO3) was to control the pH. Circumstances that constraint the investigation, included the software of the pH stat equipment that limited the addition rate of nitric acid, and required time to accurately measure the pH during operation. As using nitric acid resulted in formation of a soluble calcium salt, a non-soluble by-product layer, of gypsum for example, around the dissolving particles could not occur, which is possible under plant conditions. It was therefore not possible to investigate this phenomenon, which is known as blinding. CO2 and SO2 absorption was not accounted for in the laboratory-scale experiments, which also takes place on an industrial WFGD. The results were modelled using a simple first-order reaction model that was combined with a semi-batch reactor model to quantitatively compare the reactivity of the different limestone samples. In this regard, the

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calculation of the surface area of the particles was simplified by using the Sauter mean diameter instead of applying the particle size distribution. The accuracy with which the experimental measurements were reproduced by the model varied with the pH, since experiments conducted at a higher pH was modelled more accurately than experiments which were run at a lower pH. Aspects that were not included in the study but identified as the study progressed, are mentioned and recommended for further study.

1.6. Dissertation structure

The structure of this dissertation is as follows: Chapter 1: Introduction

This chapter provides the motivation for why WFGD is being implemented within South Africa. It also states variable parameters associated with the WFGD process and the complexity of the sorbent involved in the process. Finally, the aims and objectives of the study is also defined.

Chapter 2: Literature review

A summary of the effects of elevated ambient SO2 concentrations resulting from emissions on the environment is presented in this chapter. This is followed by an overview of limestone and the different WFGD technologies that are commercially available together with the advantages and disadvantages of each. Finally, various reaction rate models are reviewed.

Chapter 3: Limestone sample preparation and characterisation

The origin of the studied limestone samples are detailed in this chapter together with a discussion on the elemental and mineralogical analysis results. In addition to X-ray Fluorescence, (XRF) and QEMSCAN analysis that were performed on the samples to characterise their respective elemental and mineralogical composition, other analyses included BWI and particle size distribution.

Chapter 4: Limestone dissolution kinetics

The experimental setup and method used to study the dissolution kinetics are presented this chapter. The parameters that were varied are detailed in this chapter together with the measurement results, the degree of repeatability, and the modelling results.

Chapter 5: Conclusions and recommendations

The conclusions of the study are summarised in this chapter, which addressed the objectives of the study, and recommendations are subsequently given that could be of interest in further studies.

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2. LITERATURE REVIEW

2.1. International Ambient Air Quality Standards

According to the World Health Organisation (WHO) (2005) clean air is a basic requirement for the well-being and general health of every human being. The results of WHO assessments have shown that annually more than 2 million deaths can be classified as premature and can be linked to outdoor as well as indoor air pollution.

Among the many types of gaseous and particulate pollutants being emitted from industrial point sources, SO2 is one of the most concerning. Once emitted into the atmosphere, SO2 along with it’s by product sulphuric acid, has the potential to cause great damage to plants. Figures 2-1 to 2-2 are examples of such damaged inflicted onto plants. The changes in colour of the leaves of plants are normally evidence of damaged caused. The negative effect the pollution has on plants can reach levels where the growth of the plant can be affected, reduction on produce in the cases of plant species such as crops can occur. Defoliation which is the loss of leaves could also occur. Normally young and middle-aged plants are most susceptible to damage from SO2.

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Figure 2-2: Dark, brownish coloration on dogwood leaves exposed to SO2.

The distance from the source that emits the SO2 emissions is an important factor. Sikora and Chappelka (2004) reported that damage can be severe to plants located 50 kilometres or less from the source of emission where greatest influence can be observed less than 10 kilometres from the source. These SO2 emitting sources can include but is not limited to electric power plants, oil refineries, copper and iron smelters, chemical factories, and any industry that burns high-sulphur fuels.

As is expected, the concentration and length of exposure to the SO2 gas plays a role in the deterioration of the plants as well. Damage to sensitive plants can be observed by exposing them to 0.5 parts per million (ppm) for 4 hours, or 0.25 ppm for 8 to 24 hours. Plants are most sensitive to SO2 during periods of bright sun, high humidity, and adequate plant moisture during the late spring and early summer when rainfall is expected to be higher (Sikora and Chappelka, 2004). In Sikora and Chappelka’s study, a list of plants most sensitive to SO2 as well as plants that shows some resistance can be found. Effects of other pollutants can be found as well.

WBK & Associates Inc. in 2003 for Alberta Environment, conducted a study on the effect of SO2 on the environment. One key aspect focused on in this study was the effects on livestock and wildlife. It was found that exposure to SO2 produces instant respiratory restriction, narrowing of the airways, greater pulmonary resistance, greater airway reactivity, and deviations in metabolism. Continuous or multiple exposures results in swelling of the mucosal tissues. As with plants, the adverse effects associated with SO2 exposure are noticed to be worse when observed in conditions that are considered to be more humid. As expected, livestock or wildlife that is already suffering from a respiratory condition would have their respiratory condition worsened by exposing such an animal to elevated ambient SO2 concentrations.

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Experiments have been conducted on animals. Immediate effects on smaller animals were observed when exposed to lower SO2 concentrations (between 2.3 and 16 mg/m3). These concentrations where kept constant for up to 4 hours. Symptoms witnessed in such cases included increased respiratory flow resistance, increased production of mucus, irritation to the eyes. This short-term exposure to these lower concentrations did not result in permanent damage to the respiratory system. Exposures of shorter duration (0.5 hours) but slightly elevated concentrations (26

mg/m3_{) proofed to have a bigger impact on the}_respiratory_{system but were usually completely} reversible as soon as exposure was stopped. Exposure to higher (but not defined) concentrations proofed to be lethal. (WBK and Associates Inc., 2003)

In an additional chapter of the same study, they elaborated on the effect that air pollutants have on materials. Building materials are amongst the most effected, this in turn can be an economic burden on a country as buildings could suffer damage to the extent that it needs to be repaired or even certain structures replaced. Annual corrosion losses to building materials in Canada due largely to sulphur compounds in the environment, were estimated at $280 million in 1977. The deterioration of these materials were largely attributed to the formation of acid rain which is responsible for the corrosion of vulnerable materials such as stonework, metals such as carbon steel, zinc and copper, as well as protective coatings such as paints. Aluminium and stainless steel are generally more resistant to corrosion of this nature.

Due to the potential hazards associated with SO2 emissions from industrial activities, the WHO proposed guidelines for air quality standards. The specific guidelines for SO2 states that exposure should not exceed an average of 20 micrograms over a 24 hour period, while short term exposure should not exceed an average of 500 micrograms per cubed meter over ten minutes. Nonetheless, the WHO guidelines are not enforced in any way, and each country has established its own air quality standards, enforced by the local legislation. A brief comparison of the standards from various countries is given in Table 2-1.

Table 2-1: Ambient Air Quality Standards of selected countries adapted from (Vahlsing and Smith, 2012)

Country Name Ambient Air Quality Standard (SO2 in 𝛍𝛍g/m3)

SO2 Year set or revised

Australia 209 (0.08 ppm) 1998

Austria 125 2008

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Brazil 365 1990 Canada 301 (115 ppb) 1998 China 50 2000 France 125 2008 Germany 125 2008 India 80 2009

Russian Federation 50 Unknown

South Africa 125 2004

United States 365 1996

World (WHO, 2006) 20 2005

To facilitate the adherence to the air quality standards, limits for point-source emissions are frequently implemented that apply to specific industrial activities, which include coal combustion for electricity generation (Vahlsing and Smith, 2011). In line with this international trend, South African legislation, which came into effect in 2015, dictates that the amount of SO2 emitted from industrial electricity generating activities should be limited to at least 3500 mg/Nm3_{. The latter emissions limit} applies to existing power plants; however, legislation further requires the same existing power plants to adhere to more stringent emissions limits of only 500 mg/Nm3_{by 2020, which also applies power} stations built before 2020. In all cases, the concentrations to be referenced to a normalised gas volume at 273 K and 101.3 kPa at maximum allowable oxygen (O2) concentration of 10 vol. %. Compliance with the legislation therefore necessitates the implementation of an SO2 abatement technology to reduce the concentration of SO2 in the boiler flue gases before being vented to the atmosphere.

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2.2. Desulphurisation using FGD

In order to remove SO2 from flue gas produced from the coal combustion process FGD technologies can be implemented in order to lower the SO2 content of the set flue gas stream to comply with set legislation.

Several technologies for reducing SO2 emissions are available, they are as follows: • Wet scrubbers

• Dry scrubbers

• Sorbent injection processes • Multi-Pollutant processes

In Sections 2.2.1 to 2.2.4 a more detailed description follows of each technology.

2.2.1. Wet scrubbers

This SO2 removal technology is the most commonly implemented technology within the power generation industry especially limestone wet scrubbers. Other sorbents, such as, sodium carbonate lime, magnesium oxide, and ammonia, are used in wet scrubbing technologies for the removal of SO2. However, limestone is in most countries the most commonly utilised sorbent due to the cost and higher availability. The consumption of water associated with this technology is high, 3.8 to 6.8 L/min per MW, depending on the type of steam generating cycle utilised within the power plant as well as the type of coal used in the combustion process. The latest generation of this technology is capable of removing up to 99% of the SO2 contained within the flue gas stream. Additional benefit to this technology is that it is capable of removing other chemicals from the flue gas stream as well; these include HCl, HF and oxidised mercury. This technology has been installed as part of the process in plants burning both high and low sulphur coals. As this technology’s biggest negative aspect is its high water consumption, a lot of work and time has been spent improving on this by the addition of additional technologies. The removal of water vapour can be achieved by the addition of condensing heat exchangers, membranes that are highly selective towards the recovery of water and liquid desiccant-based dehydration processes. Implementation of these technologies however comes at a cost with each of them having their advantages and disadvantages. Section 2.3 elaborates on the wet FGD process. (Carpenter, 2012)

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2.2.2. Dry scrubbers

In situations where the availability of water is more restricted, a technology that utilises less water should be considered. Dry scrubbers form part of this category. Semi-dry scrubbers are the second most commonly adapted FGD technology within the power generation sector worldwide. This technology has mainly three variants, namely: spray dry scrubbers, duct spray dry process and circulating dry scrubbers. The reagent mainly utilised within these technologies is usually calcium based (Ca(OH)2). It is usually introduced into the process in slurry form, except in the circulating dry scrubbers where it is introduced in the form of a powder. The installed capacity of this technology is about 40,000 MW today, with the most of these plants constructed in the USA on smaller to medium sized (approximately 450 MW) power generating units. This technology is typically implemented where sulphur content in the associated coal is lower. Removal efficiencies of this technology can reach levels of 90-95%, the calcium to sulphur ratio (Ca:S) plays an important role in the removal efficiency, values for this ratio varies with sulphur content in coal and removal efficiency desired. Typical values for this ratio vary between 1.2 and 1.6. This technology also has the capability of removing the same additional harmful chemicals as the wet scrubbers. (Carpenter, 2012)

2.2.3. Sorbent injection processes

Sorbent injection processes were primarily used for SO2 control on smaller units burning lower sulphur coals and where wet FGD systems were not economically feasible. Currently there are about 60 units implementing this technology on a commercial basis within the USA. There are four common variations of this technology namely: furnace sorbent injection, economiser sorbent injection, duct sorbent injection and hybrid systems (injecting into multiple points). These systems generally operate by injecting a dry sorbent into the flue gas stream of furnace (depending on the variation of the technology). This sorbent can be calcium-, magnesium- or sodium-based reagents or even ammonia. After the desired reaction that captures the SO2 the reaction product is captured within the particulate control device implemented on the associated boiler (either ESP’s or FFP’s). In general boilers that make use of bag filter plants, as particulate reduction technology, will be able to remove SO2 more efficiently because of the ash layer (which now includes the sorbent) that forms on the surface of the bag. The temperature, injection location and the type of sorbent has an impact on the efficiency of the technology; a lot of companies have different variations of these technologies available. Sorbent injection technologies are commonly installed on smaller units for the removal of SO2.In cases where the main objective is to remove the SO3 content of the flue gas, this technology was implemented on units up to 1300 MWe. Removal efficiencies in the order of 50-70% can be expected when making use of furnace injection, depending on the sorbent used. Economiser injection’s removal efficiency can vary between 20-80%. When injecting into the duct before the particulate removal technology, a wider variation of sorbents can be used. Expected removal

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efficiencies can range between 50-90% with high efficiencies of SO3 removal as well. The utilisation of ammonia only aids in the removal of SO3 with 80-95% removal expected. (Carpenter, 2012)

2.2.4. Multi-Pollutant processes

Various technologies are available that removes multi pollutants, information on these technologies are freely available, although most of these technologies are still in the development phase. Table 2-2 displays the available multi-pollutant technology along with the pollutants that it removes. (Carpenter, 2012)

Table 2-2: Multi-Pollutant processes and the pollutants they remove.

Technology Pollutants capable of removing

Activated carbon/coke process SO2, SO3, NOx, mercury, HCl, HF and particulates

ReACT™ (Regenerative Activated Coke Technology)

SO2 and SO3, NOx, mercury (both elemental and oxidised) and particulates

Cansolv® technology SO2 and CO2 CEFCO (Clean Energy and Fuel

Company)

trace metals (including mercury), fine particulates, SOx, NOx and CO2 Electron beam process SO2, SO3, NOx and other acid gases, EnviroScrub SO2, SO3, NOx, mercury and particulates

SNOX™ process SOx and NOx

SOx-NOx-Rox Box process (SNRB™) SO2, NOx and particulates

2.3. Wet flue gas desulphurisation (WFGD)

The purpose of a wet flue gas desulphurisation (WFGD) scrubber as with other FGD technologies is to remove SO2 molecules from the flue gas. The WFGD process is based on the principle of gas absorption by an aqueous liquid. In the case of WFGD, SO2 in the flue gas is absorbed by (dissolves in) the aqueous slurry droplets with which the flue gas is contacted. This decreases the pH of the aqueous solution, which in turn drives the dissolution of the limestone. The calcium ions released by the limestone dissolution process then reacts with the absorbed sulphur, which results in the formation of gypsum. Oxygen is also required to complete the formation of gypsum during the neutralisation process, and the net reaction that takes place in the aqueous phase, namely the

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dissolution of calcium carbonate (the major component of limestone) and subsequent reaction with the absorbed SO2 to form calcium sulphate (gypsum), is shown in Equation (2-1). (Walsh et al. 2006): CaCO3 + SO2 + 2 H2O + ½ O2→ CaSO4•(2 H2O) + CO2 (2-1) The reaction takes place in a heterogeneous system however, with many other equilibrium chemical reactions involved, which is summarized in Table 2-3.

Table 2-3: Chemical reactions involved in the WFGD process adapted from Córdoba (2015).

Stage Chemical reaction Equation Eq. (#) pH range: (4.5 – 5.5) Neutralisation by limestone:

CaCO3(s) + SO2 (g) + 2 H2O (aq) → Ca(HSO3)2 + CO2 (g) + H2O (aq)

2-2

H2SO4 (aq) produced: Ca(HSO3)2 + O2(g) + 2 H2O (aq) → CaSO4•2H2O (s) + H2SO4 (aq)

2-3

CO2 desorption and H2SO4 neutralisation:

CaCO3(s) + H2SO4 + H2O (aq) → CaSO4•2 H2O(s) + CO2 (g)

2-4

Stage

pH range: (5 – 6)

Chemical reaction Equation

Eq. (#)

SO2 absorption: SO2 (g) ↔ SO2 (aq) _2-5

Hydrolysis of SO2 : SO2 (aq) + H2O ↔ H2SO3 (aq) 2-6 Sulphurous acid

dissociation:

H2SO3 (aq) ↔ H3O+ (aq) + HSO3

-(aq)↔ H3O+(aq) + SO₃2- (aq)

2-7

Limestone dissolution: CaCO3(s) + H3O+(aq) ↔ Ca2+ (aq) + HCO3

(aq) H2O (l)

2-8

Carbonic acid formation HCO₃- (aq) + H3O+(aq) ↔ H2CO3 (aq) + H2O(l) 2-9

Carbonic acid to CO2 H2CO3(aq) → CO2 (aq) + H2O (l) 2-10

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Overall reaction gas-to-liquid: CaCO3(s) + SO2 (g) + 2 H2O (l) → CaSO3 • 2 H2O (aq) + CO2 (g) 2-12 Oxidation and precipitation:

CaSO3•2 H2O (aq) + ½ O2(g) → CaSO4 • 2 H2O (s) 2-13

The pH, at which the different chemical reactions or dissociations, take place has been debated in literature. One of these reactions is the dissociation of sulphurous acid. Omine et al. (2012) reported equilibrium results for sulfite species in the liquid, where they identified that the concentration of SO3

2-starts to increase at a pH of 5, supporting the equations presented in Table 2-3.

A simplified schematic representation of a WFGD process is shown in Figure 2-3, which gives a basic overview of the various stages involved. From Figure 2-3, it is clear that the limestone is transported into the system as slurry. This requires the limestone to be crushed, which is normally achieved by ball mills. Conventionally, limestone grounded down to a particle size ranging between 5 and 45 µm are used in WFGD, but to achieve such small particle sizes requires a substantial power input to the motors driving the mills. The Bond Work Index (BWI) is an important parameter in this regard, as it provides a measure of the amount of energy input required to grind one ton of limestone to the required particle size. Dündar and Benzer (2015) reported that a BWI value of 11 kWh/ton was typically required to produce crushed limestone with a distribution of 80% passing a 100 µm sieve.

The flow rate of the slurry and thus the preparation of the limestone are governed by the pH of the scrubber that is controlled and kept at a constant level. This is achieved by balancing the opposing effects of fresh limestone introduction, which tends to cause a rise in pH during dissolution (but favoured at lower pH values), and SO2 absorption, which tends to lower the pH (but favoured at higher pH values). The limestone slurry is sprayed into fine droplets and falls to the bottom of the absorber while the flue gas flows counter-currently from the inlet at the bottom of the absorber. The limestone within the droplets subsequently reacts with either HSO₃- or SO₃2- that originate from the hydrolysis of SO2. As shown in Table 2-3, the availability of either the bisulphite (HSO₃-) or sulphite (SO₃-) ions is governed by the pH. While absorbing SO2 from the flue gas the slurry droplets fall to the reaction tank where oxygen is injected. The sulphite species are then oxidised to the corresponding sulphate species Equation (2-13), followed by the precipitation of gypsum Equation (2-13) that is removed from the reaction tank and dried by means of a belt filter to yield a product containing approximately 10% water.

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From the preceding discussion, it is again clear that the amount of SO2 absorbed in the absorption tower, and the rate of limestone (calcium) dissolution collectively determine the pH of the slurry in the reaction tank. The neutralisation reaction to produce gypsum is governed by the availability of calcium ions, and therefore the rate of limestone dissolution. Since the dissolution rate is a function of the conversion when all other influencing parameters do not change, as is the case with many other reactions (Walsh et al. 2006, Córdoba, 2015), therefore the dissolution rate may be adjusted and controlled by adjusting the feed rate of fresh limestone slurry. By controlling the feed rate of the fresh limestone slurry stream, the conversion of limestone within the reaction tank can therefore be maintained to maintain a certain dissolution rate to match the rate of SO2 absorption, and therefore the pH within the whole system. For this reason, accurate kinetic models are required to describe the dissolution kinetics.

Figure 2-3: FGD basic lay out and flow diagram adapted from Córdoba (2015).

2.3.1. Dissolution rate of limestone

The dissolution rate of a particular limestone can be considered as the capacity for providing alkalinity to the FGD process. This is of key importance for the neutralisation reaction that occurs between the calcium ions and the dissolved SO2. Siagi and Mbarawa (2009) reported key factors that have an influence on the rate at which the dissolution of limestone takes place:

The reactivity of South African limestone of variable quality as potential sorbents in wet flue gas desulphurisation