hydration of cao

Transcription

hydration of cao

National Technical University of Athens
School of Chemical Engineering
Laboratory of Inorganic & Analytical Chemistry
HYDRATION OF CaO
PRESENT IN FLY ASHES
Final Edition
Professor Stamatis Tsimas
Professor Aggeliki Moutsatsou
____________________________________________________________
January 2015
HYDRATION OF CAO
PRESENT IN FLY ASHES.
A LITERATURE REVIEW
An Outline
1. From CaO to Ca(OH)2 Reaction kinetics and
mechanisms
2. Hydration at temperatures above 100 oC. Hydration
over vapors. Reduction of size during hydration
3. Hydration in the presence of other gases similar to
flue gases
4. Hydration of CaOf in Fly ashes
5. The case of free CaO in HCFA
6. Application in Bełchatów ashes.
Comments and suggestions.
Conclusions
7. References in Reverse Chronological Order (20151958)
PREFACE
This report has been created to comply with the obligations of the authors according
to the terms of the contract No 2/12/213310/GEKON signed between EKOTECH –IP
Sp. and Prof. Stamatis Tsimas (author).
According to article 1 of the above mentioned contract, the Author undertook:
a report summing up the available knowledge of free lime hydration process
in general and in particular narrowed down to process conditions possibly
close to those found in flue gases carrying the particles of HCFA – at
temperatures in excess of 100oC, in the presence of gaseous atmosphere
similar to flue gases from lignite-burning boilers etc. Of particular importance
is the knowledge of hydration reaction’s kinetics and mechanisms – allowing
for modeling it with mathematical/IT tools.
To bring out the project and to cover bibliographically all the individual parameters
that affect hydration, initially and under the general entry of “calcium oxide
hydration”, summary studied in more than 1000 selected papers. From these the
referred in Chapter 7 (References) more than a hundred (100) have studied in more
details as in a different extend have been evaluated that satisfied the conditions of the
detailed examination of the contract. The obvious data overlap in many references
sought to be minimized when writing the report.
Its structure follows generally the individual topics to be covered. Chapters and
clauses have been selected to such a way in order to be useful to researchers who have
to implement the hydration of calcium oxide in a larger scale according to the Patent.
Chapter one comprises general but detailed information concerning the hydration of
calcium oxide. In this chapter are included: i) the factors that affect the slaking
process insisting in the quality of the water and ii) extended information material
concerning reaction kinetics and mechanisms. Chapter two deals with hydration in
temperatures above 100oC the hydration over vapors and the reduction of size during
hydration. Chapter three deals with the hydration of calcium oxide in presence of
flue gases. The processes of carbonization and flue gas desulfurization with calcium
oxide are also analyzed in details as well the reactivation of CaO. In the same chapter
is included the effect of steam on carbonation and sulfurization of calcium oxide
Chapter four deals with the hydration of CaO f in the presence of other mineral
phases, materials and by products focusing and insisting on the hydration of CaO f
present in fly ashes In the same chapter is also given a more precise approach to the
problem raised through a comparative examination of steam vs. water hydration of
CaOf present in High Calcium Fly Ashes in relation to their sulfurization behavior. In
chapter five except comparative data concerning LCFA and HCFA, is included our
personal experience referring to the hydration of CaOf contained in HCFA in parallel
with its reduction of size Chapter six shows our first efforts and thoughts to adapt the
literature findings (and presented in this report) to Bełchatów ashes. Finally in
Chapter seven are presented in chronological order over 100 references to which this
report was based.
This work aims to contribute to the recording and consolidation of necessary data on
the hydration of the free lime present in HCFA finally pursuing their exploitation
from our Polish colleagues. Therefore the length and content of Chapter 6 is marginal.
The reports attempt to handle with a creative way the review process and publication
as expeditiously as possible within the scheduled deadline.
Table of Contents
HYDRATION OF CAO .......................................................................................... 2
PRESENT IN FLY ASHES..................................................................................... 2
A LITERATURE REVIEW .................................................................................. 2
An Outline ......................................................................................................... 2
Table of Contents .............................................................................................. 5
Index of Figures ................................................................................................. 7
Index of Tables .................................................................................................. 8
1. Introduction. From CaO to Ca(OH)2 Reaction kinetics and mechanisms ............... 1
1.1 General. ................................................................................................... 1
1.2 HydrationofCaO....................................................................................... 2
1.2.1 Introduction .............................................................................................. 2
1.2.2 Factors affecting slaking process ............................................................... 2
A. Type of limestone used in calcination ................................................ 3
B. Calcination process to manufacture CaO ............................................ 3
C. Slaking temperature ........................................................................... 3
D. Lime to water ratio ............................................................................ 4
E. Degree of agitation during slaking ..................................................... 5
F. Viscosity of slurry .............................................................................. 5
G. Slaking time ...................................................................................... 5
H. Air slaking......................................................................................... 5
1.2.3. Water for hydration .................................................................................. 5
Water temperature .................................................................................. 7
1.2.4. Rate of hydration ..................................................................................... 7
1.3 Reaction kinetics and mechanisms ......................................................... 8
1.3.1 Reaction kinetics ....................................................................................... 9
(a)
Reaction rate .......................................................................................... 9
(b)
Effect of rotation speed .......................................................................... 9
(c)
Effect of concentration of Ca2+ or OH- ................................................. 11
(d)
Effect of temperature............................................................................ 11
1.3.2 Mechanism of the slaking reaction .......................................................... 11
(a)
General considerations ......................................................................... 11
The slaking of lime with water ......................................................................... 12
(b)
Effect of temperature............................................................................ 13
1.3.4 Equilibrium and free energy conditions of Ca(OH) 2 ................................ 13
2. Hydration at temperatures above 100oC. Hydration over vapors. Reduction of size
during hydration ...................................................................................................... 16
2.1 Introduction ........................................................................................... 16
2.2 Effects of temperature on the hydration characteristics of free lime ........ 16
2.3 Hydration over vapors ............................................................................ 16
2.3.1. Influence of water vapor pressure ........................................................... 18
2.3.2. Influence of temperature ........................................................................ 18
2.4 Hydration and reduction of size .............................................................. 22
2.5 Particle breakage model ......................................................................... 23
3. Hydration in the presence of other gases similar to flue gases .............................. 26
3.1 Introduction ........................................................................................... 26
3.2. The carbonation of calcium oxide by carbon dioxide ............................. 26
3.2.1 General ................................................................................................... 26
3.2.2. Gas -solid reactions ................................................................................ 27
3.3 Flue gas desulfurization with calcium oxide ........................................... 29
3.3.1 The reactivation of CaO .......................................................................... 30
3.4 Relation between carbonation and hydration of CaO .............................. 30
3.5 The effect of steam on Carbonation and Sulfurization of Calcium oxide . 32
4. Hydration of CaO f in Fly ashes ........................................................................... 34
4.1. Introduction .......................................................................................... 34
4.2.
Hydration of CaO f present in fly ashes ............................................. 34
4.3. The hydration behavior of industrial fly ashes ....................................... 36
4.4 Steam vs. water hydration of high calcium fly ashes in relation to their
sulfurization behavior .................................................................................. 37
5. The case of free CaO in HCFA .......................................................................... 40
5.1 General for Fly ashes. Calcareous and siliceous ashes ............................ 40
5.2 The contribution of fly ash as addition in cement and concrete ............... 41
5.3 Comparison between HCFA and LCFA towards their use in cement and
concrete ....................................................................................................... 42
5.4 Beneficiation tactics of HCFA................................................................ 43
5.5 Facing the increased values of CaOf in HCFA. Attempts towards
hydration of CaO ......................................................................................... 44
6. Application in Bełchatów ashes. Comments and suggestions. Conclusions. ........ 47
6.1 Introduction ........................................................................................... 47
6.2 Bełchatów ash quality ............................................................................ 47
7. References in Reverse Chronological Order (2015-1958) ................................... 50
Appendix................................................................................................................. 64
Index of Figures
Figure 1: pH of Calcium Hydroxide solutions at 25°C .............................................. 2
Figure 2: Solubility of Calcium Hydroxide in Water ................................................. 1
Figure 3: Temperature vs. pH of a saturated Calcium Hydroxide Solution ................ 1
Figure 4: Effect of sulfates on lime slaking ............................................................... 6
Figure 5: Amount of CaO dissolved, at various speeds ............................................. 8
Figure 6: Amount of Calcium oxide dissolved in 0.003 M sodium hydroxide............ 9
Figure 7: Variation of the zero order rates ............................................................... 10
Figure 8: Arrhenius plot for the slaking of lime in water ......................................... 11
Figure 9: Equilibrium pressure (atm) of H2O over Ca(OH) 2. .................................. 14
Figure 10: Weight loss of hydrated sorbent ............................................................. 14
Figure 11: Free energy of the calcium hydroxide decomposition reaction versus
temperature at 1 atmosphere, 10−3 Torr, and 10−5 Torr water partial pressures. ........ 15
Figure 12(a) (b): Influence of water vapor pressure o the hydration ........................ 18
Figure 13: Influence of temperature on hydration of SBL and HBL ........................ 19
Figure 14: Influence of the temperature on the kinetic rate of hydration of SBL CaO
powder with a water vapor pressure of 80 hPa.. ....................................................... 19
Figure 15: Schemes of perpendicular (a) and parallel (b) growth processes. Bold and
dashed arrows indicate the direction of diffusing species and of development of
Ca(OH)2, respectively.............................................................................................. 20
Figure 16: Hydration conversion (XHy) vs. time .................................................... 21
Figure 17: The hydration of CaO readily forms a shell of calcium hydroxide when
exposed to water...................................................................................................... 23
Figure 18: Particle breakage model. ........................................................................ 25
Figure 19: Partial pressure of water vapor over Ca(OH)2. ....................................... 32
Figure 20: Model of water vapor film influence on SO2 and CO2 migration ........... 33
Figure 21: Ashes characterized by an irregular shape and porous uneven surface .... 35
Figure 22: Apparent conversion of CaO to Ca(OH)2 ............................................... 38
Figure 23: From coal particle to fly ash .................................................................. 40
Figure 24: The milling plant equipped with the partial hydrolization system ........... 45
Figure 25: The percentage distribution of the CaOf in production of the mill .......... 46
Figure 26 : CaOf variation (Bełchatów ashes).......................................................... 47
Figure 27: Fineness variation (Bełchatów ashes) .................................................... 48
Index of Tables
Table 1: Lime to Water Ratio settling time ................................................................ 4
Table 2: Mean free lime and Ca(OH)2 contents following steam hydration (expressed
as CaO, wt%) .......................................................................................................... 38
Table 3: SO2 absorption capacity of FA at 90 min sulphurization, mg SO2 g-1 sample
................................................................................................................................ 39
Table 4: Chemical composition of fly ashes ............................................................ 41
Table 5: Comparison of basic properties of LCFA and HCFA-based concrete......... 42
1. Introduction. From CaO to
Ca(OH)2 Reaction kinetics and
mechanisms
1.1 General.
Calcium oxide (CaO), commonly known as quicklime or burnt lime is a widely used
chemical compound. It is a white, caustic, alkaline and crystalline solid at room
temperature. The broadly used term lime connotes calcium-containing inorganic
materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium,
aluminium and iron predominate. By contrast, "quicklime" specifically applies to the
single chemical compound calcium oxide. Calcium oxide which survives processing
without reacting in building products such as cement is called free lime.
Quicklime is relatively inexpensive. Both it and a chemical derivative calcium
hydroxide Ca(OH)2 of which quicklime is the base anhydrite are important commodity
chemicals
Calcium hydroxide, Ca(OH)2 traditionally called slaked lime, is also an inorganic
compound It is a colorless crystal or white powder and is obtained when CaO is
mixed, or "slaked" with water. It has many names including hydrated lime, builders'
lime, slack lime, cal, or pickling lime.
Calcium hydroxide is used in many applications, including food preparation.
Quicklime releases heat energy by the formation of the hydrate, through the following
equation:
CaO + H2 O ↔ Ca(OH)2 + 1135 kJ/kg of CaO
As it hydrates, an exothermic reaction results and the solid puffs up. The hydrate can
be reconverted to quicklime by removing the water by heating it to redness to reverse
the hydration reaction. One litre of water combines with approximately 3.1 kilograms
(6.8 lb) of quicklime to give calcium hydroxide plus 3.54 MJ of energy.
Prof. Stamatis Tsimas, Prof. Angeliki Moutsatsou , School of Chemical Engineering NTUA
CaO (s) + H2O (l)
Ca(OH)2 (aq)
ΔHr = −63.7 kJ/mol of CaO
.
Based on the molecular weights, 56 units of CaO plus 18 units of H 2O results in 74
units of Ca(OH)2. The ratio of hydroxide to CaO is 74 ÷ 56 = 1.32. This means that 1
Kg of CaO and 0.32 Kg of water will produce 1.32 Kg of Ca(OH) 2, this is the
minimum water required for chemical reaction, so calcium hydroxide contains 75.7%
CaO and 24.3% H2O. The process of adding water to calcium oxide to produce
calcium hydroxide is referred to as hydration process or lime slaking. The hydration of
CaO, commercially referred to as quick lime, is an exothermic process releasing a
great quantity of heat.(Mohamad Hassibi)
This hydration process when done with just the right amount of water is called “Dry
Hydration”. In this case the hydrate material is a dry powder. If excess water is used
for hydration, the process is called “Slaking”. In this case, the resultant hydrate is in a
slurry form. Lime manufacturers generally use the dry hydration process for producing
powdered hydrated lime. Our discussion here is limited to lime slaking. The slaking
process is normally done with considerable excess water ranging from 2½ parts water
to 1 part CaO to 6 parts water to 1 part CaO. In Figure 1 is depicted pH of Calcium
Hydroxide solutions at 25°C. Figure 2 shows solubility of calcium hydroxide in water.
Finally Figure 3 depicts temperature vs. pH of a saturated calcium hydroxide solution.
Figure 1: pH of Calcium Hydroxide solutions at 25°C
Solubility of Calcium Hydroxide in Water
0.2
CaO
0.18
0.16
Ca(OH)2
0.14
y = -0.0012x + 0.1872
R² = 0.9991
0.12
0.1
0.08
y = -0.0009x + 0.1416
R² = 0.9991
0.06
0.04
0
20
40
60
80
100
Temperature / °C
Figure 2: Solubility of Calcium Hydroxide in Water
120
Temperature vs. pH
13.5
13
pH
12.5
pH
12
11.5
y = -0.0327x + 13.326
R² = 0.9932
11
0
10
20
30
40
50
60
Temperature / °C
Figure 3: Temperature vs. pH of a saturated Calcium Hydroxide Solution
In next close we will focus on hydration and factors affecting slaking process.
70
1.2 HydrationofCaO
1.2.1 Introduction
A huge number of papers are referred in the literature in the general topic of lime
hydration. The clause and the chapter in the continue refers to papers dealing with the
hydration of Calcium oxide aiming at the production of calcium hydroxide and not to
CaO as part of several by products as slags, fly ashes etc are.
These latter cases will be discussed in Chapter 4.
Research on lime slaking has been done on a limited basis in recent years. Most of
this research has been done under auspices of the National Lime Association. The
information presented in this paper builds upon the research done by others and the
author’s years of hands-on experience in lime slaking.
Because limestone is a naturally occurring mineral its chemical composition and
physical characteristics vary not only from area to area but within veins of limestone
in the same area. This variation in raw material results in variation of quality of the
end product, which is calcium hydroxide. The use of lime in its various forms has
been steadily on the rise with no end in site.
Today, lime is the most important chemical used throughout the world for pollution
control. It is, therefore, imperative that knowledge of handling and processing lime be
well understood by all those who use this chemical.
Hydration or slaking can be described as the process of adding a quantity of water to
lumps of lime causing them to disintegrate to a powder, putty or lime-wash. This
chemical reaction between lime and water results in the development of a
considerable amount of heat. The form into which lime is slaked depends on the use
for which it is required. In the case of lime for use in plasters and mortars, which
could be either in the form of putty or a dry powder, all the implications of using
either one of the two, must be carefully studied before any decision is made.
Qualitatively, the advantage of the use of lime putty over a dry hydrate are that it is
likely to contain a greater portion of fine lime particles and will therefore be more
plastic, a characteristic which is preferred in mortars and plasters. Also, the product is
likely to be more fully slaked and will therefore be less likely to present any of the
typical popping and checking problems that may occur due to the presence of unslaked material. However, more water is required to slake it, so the economic
implications of the availability of water take precedence. In a dry area where distances
to the market are long, it is likely to be preferable to transport and slake quicklime
lumps at sources of water nearer the market than to bring water to the production site
and then transport dry lime hydrate or lime putty over a long distance to the market.
1.2.2 Factors affecting slaking process
The most important single factor that affects the process efficiency of a slaking
system is the specific surface area of the particles of calcium hydroxide. The larger
the specific surface area of the hydrate, the more surface is available for reaction,
therefore, the more efficient the reaction and less consumption of lime. The specific
surface of calcium hydroxide varies a great deal based upon the variables that are
described below. The typical specific surface of calcium hydroxide ranges between
8,000 to 58,000 cm2/gr. Empirical data shows that the relationship between the
particle size of hydrate and specific surface, even though related, is not linear. Except
specific surface area and water (discussed in 1.2.3.) some major comments to other
factors affecting the process are very shortly given in the continue
A. Type of limestone used in calcination
Calcium carbonate deposits are generally not pure. They contain many other
elements, such as magnesium, aluminum oxide, and compounds that affect the
quality of hydrate produced from their limestones. Manufacturers of lime have
no control over the impurities that are interspersed in a vein of limestone.
Magnesian limes are slower in slaking than calcium limes due to the sintering
caused by the over burning of the magnesium carbonate portion of the stone.
Over burnt magnesian limestone or dolomite hydrates very slowly and is just
about impossible to hydrate when impure. Since most of the magnesium oxide
portion remains unslaked when using hand slaking methods, less water will be
required. Searle suggests that up to 20 % less water will be required for
hydration. The wet slaking method described below (hand-slaking) is a
suitable simple means of slaking magnesian limestone quicklimes. The period
in the slaking pits can be extended to one (1) month.
B. Calcination process to manufacture CaO
Proper temperature and residence time during calcination have a great deal of
influence on the quality of hydroxide produced. The most common problem
associated with the calcination process is hard-burned lime. When a lime is
hard-burned, an impervious layer forms on the outside of the CaO particles
making it difficult for water to penetrate and start the slaking process. To slake
a hard-burned lime, the outer layer of the particle must wear off to open up the
pores for water to penetrate. This is done by vigorous agitation that abrades
the outer layer of CaO. This type of lime generally requires more retention
time in the slaker. In practice when using hard-burned lime, the slaker capacity
should be adjusted at 50% to minimize CaO carry over.
C. Slaking temperature
Slaking temperature is the most important factor that affects particle size and
specific surface of hydrate particles. The closer the slaking temperature is to
100°C the finer the particle sizes and greater the specific surface of particles
will be. However, the relationship between temperature, particle size and
specific surface is not linear. In some instances, when slaking at high
temperatures around the boiling point of water, hot spots can develop within
the slurry, which will cause hydrate particles to crystallize and agglomerate
forming larger, flat particles with reduced specific surface. This problem is
more likely to happen in paste slakers since they operate at higher
temperatures and in areas where mixing is not vigorous. Even though from a
theoretical point of view temperatures around 100oC are desirable, from a
practical point of view it is very difficult to slake successfully at these high
temperatures without safety problems or adverse affects due to agglomeration.
In practice slaking in lesser temperatures is more practical for optimum
operation. The release of heat due to the exothermic reaction is different for
different quality limes. A high-reactive, soft-burned lime will produce 1140
kJ/kg (490 BTU /pound) of quicklime. A low reactive lime will produce about
884 kJ/kg (380 BTU /pound) of quicklime. This energy (kJ) will bring the
slurry temperature to a certain degree based on the temperature of dry lime,
temperature of incoming water, and heat losses from the slaker vessel.
As stated before, optimum temperature for slaking varies from job to job
depending on equipment and site conditions. Since temperature is the most
important factor affecting specific surface, temperature control is essential for
a uniform quality product. Controlling a slaking process by lime to water ratio
or slurry consistency is not the best way because of variables such as lime
reactivity, incoming water and lime temperature, which results in a variation
in hydrate quality. The optimum way to control a slaking process is by
controlling the slaking temperature by varying the lime to water ratio as
necessary
D. Lime to water ratio
The water to lime ratio also affects the slaking time by affecting the slaking
temperature (Table 1). The higher the temperature, the shorter the slaking
time is. Controlling a constant lime to water ratio in a slaking process does not
guarantee a constant temperature. The temperature will vary due to the
variation in the water temperature, lime reactivity, and quality of water, thus
requiring operator adjustment frequently. As stated before, a better way to
maintain a correct lime to water ratio is to control the slaking temperature.
Slaking tests performed on the same lime with different water to lime ratios
showed a significant difference in settling rate. In both cases, the samples were
allowed to settle to 50% of their volume.
Table 1: Lime to Water Ratio settling time
Lime to Water Ratio
Settling Time to 50%
in Minutes
Lime slaked with minimum
theoretical amount of water
10
Lime slaked with 10X theoretical water
440
This clearly indicates that an excess amount of water used in slaking will
result in smaller particles, assuming that the slaking temperature was the same.
E. Degree of agitation during slaking
The degree of agitation during the slaking process has an impact on the end
product. Too little agitation will result in an uneven temperature within the
slaking chamber resulting in hot and cold spots. The hot spots will result when
slaking temperatures are over 100oC. Slaking at these temperatures will result
in hexagonal crystals of a large size and reduced surface area; and
agglomeration of particles and cold spots will result in either drowning or unhydrated particles of CaO.
F. Viscosity of slurry
The viscosity of hydroxide slurry can vary greatly from lime to lime as well as
process conditions. Certain changes in the hydration conditions or impurities
in the lime will increase the viscosity of the slurry, thus affecting settling time.
Often times, the viscosity increases at slaking temperatures of 82°C (180°F)
and above. The relationship of viscosity, particle size, specific surface and
settling rate is not completely researched as of now. In general, it is presumed
that the higher viscosity means a smaller particle size of hydrate, greater
specific surface and slower settling rate. Variations of the viscosity of
hydrated lime slurry have been reported between ranges of 45-700 centipoises.
G. Slaking time
Slaking time is the time required to complete hydration. This time varies from
lime to lime. A high-reactive lime will hydrate completely in 2-3 minutes.
Medium reactive limes will hydrate completely in 5-10 minutes. Low reactive
limes, hard burned limes, and magnesium limes will hydrate in 15-30 minutes.
The field results vary a great deal depending on field conditions.
H. Air slaking
Air slaking is caused by hydration of CaO with moisture in the air at ambient
temperatures. The finer the particles of the quicklime the more prone to air
slaking they are due to greater specific surface. Air slaking not only will
produce extremely large particles of hydrate but will also convert calcium
oxide to calcium carbonate due to the absorption of CO2 from the atmosphere.
An air-slaked lime will not yield many kilojoules during slaking and will
increase consumption due to the lack of reactivity.
1.2.3. Water for hydration
The water used in hydration may be drinkable or even brackish borehole water but
water containing a large proportion of organic material can have a bad effect on the
lime hydrate. The water required to slake quicklime to:
A. a dry lime hydrate is around 550 litres per tonne quicklime,
B. a lime putty is around 1300 litres per tonne quicklime depending on
the consistency preferred.
The exact quantities required will vary from one quicklime to another and can best be
determined by trial and error. In general however, highly reactive porous type
quicklime will require a greater proportion of water than dense or over burnt
quicklime. Also, dolomitic lime will normally require less water in hydration since
only small portions of the MgO content, if any, actually hydrate.
Water chemistry is a major factor in the slaking process. Presence of certain
chemicals in the slaking water will accelerate or hinder the slaking process. Water
with high dissolved solids generally causes excessive foaming, which results in
operational problems. Waters containing over 500Mg/l of sulfates or sulfites are
unsuitable for slaking. This is true for paste and slurry-type lime slakers. Ball mill
slakers, because of their ability to grind the particles of lime, are not affected as much
by the presence of sulfates or sulfites in the slaking water. The sulfates or sulfites
cover the surface of the lime pebbles and will not allow water to penetrate the pores;
therefore the slaking is greatly retarded. To slake under these conditions, the lime
particles must be continuously abraded to expose new surfaces to water for slaking.
Figure 4 shows the effects of temperature rise versus time for water containing
sulfates. Some chemicals have an accelerating effect on the slaking process. These are
chlorides and sugars.
Figure 4: Effect of sulfates on lime slaking
Water temperature
The slaking water temperature has a great influence on the slaking process and
specific surface of the hydrate particles. The incoming water temperature and
the water to lime ratio inversely affect the slaking time. Cool slaking water
should not contact the dry lime in the slaker. The water and lime must enter
the slaker apart from each other so that by the time the water comes in contact
with the lime; its temperature is raised to over 65°C (150°F). If cool water and
lime come in contact, a condition called “drowning” takes place. Particles of
hydrate formed under “drowning” conditions are very coarse and not very
reactive.
1.2.4. Rate of hydration
The rate of hydration is determined by the type of stone that is fired to start with, and
the conditions to which it is subjected during firing. Complete hydration can take
place in a matter of a few minutes or continue over a period of months.
The factors which determine the rate of hydration are:
(a) Quicklime with a high MgO content has a slow rate of hydration since
it is normally over burnt when fired at the temperature necessary to
calcine CaCO3.
(b) A pure, high calcium lime hydrates faster than one containing
impurities. Impurities cause the stone to over burn at lower
temperatures which reduces porosity and consequently the rate of
hydration.
(c) Lightly burnt, porous quicklime will hydrate faster than an over burnt,
dense one.
(d) If quicklime is crushed to a size smaller than 25 mm, the rate of
hydration is increased.
(e) The rate of hydration increases with an increase of both the
temperature of the quicklime lumps and of the water used for slaking.
If the quicklime lumps are slaked immediately after they are extracted
from the kiln, i.e. whilst they are still slightly hot, and the heat from the
waste gases is used to heat the water of hydration, the rate can be
increased. With some limes a 10% increase in water temperature could
as much as double the rate of hydration.
(f) The use of an excess amount of water applied to the quicklime at a
rapid rate retards the rate of hydration.
(g) If the quicklime and water mixture is agitated during hydration the rate
is increased.
1.3 Reaction kinetics and mechanisms
Lime is often slaked prior to use, and even when quicklime is added directly to a
solution to, for example, remove carbonate from that solution; it seems very likely
that partial or complete slaking of the lime will occur first. However, despite the
enormous importance of lime slaking, little is known about the reaction kinetics and
almost nothing about the reaction mechanism. Most of the work reported to date is in
the Eastern European literature. Zozulya et al. found that the rate at which quicklime
is hydrated increases with increasing lime surface area and the temperature at which
the lime is slaked, and that the higher the temperature at which quicklime is
manufactured by de-carbonation of calcium carbonate, the less active is the resulting
quicklime. These authors concluded that lime hydration is diffusion controlled and
depends predominantly on the degree of super-saturation of the liquid phase with
calcium hydroxide. Ovechkin et al., using a high calcium lime, found that the rate of
lime slaking increased with temperature but observed little effect of grain size. They
too concluded that the reaction was diffusion controlled, at least in the final stages of
slaking.
Following the classical study of I.M.Ritchie "The kinetics of Lime Slaking"
irrespective of whether the lime slaking reaction was carried out in water or in
solutions containing either calcium nitrate or sodium hydroxide, the slaking rate was
found to be approximately constant at any given disc rotation speed. This can be seen
from the plots of amount of calcium oxide dissolved against time shown in Figure 5
and Figure 6 for the particular cases of the slaking reaction in water, determined
from conductance measurements, and in 0.03 M sodium hydroxide, determined from
analysis of the calcium content of the solution by atomic absorption spectrophotometry.
Figure 5: Amount of CaO dissolved, at various speeds
Figure 6: Amount of Calcium oxide dissolved in 0.003 M sodium hydroxide
1.3.1 Reaction kinetics
(a) Reaction rate. Irrespective of whether the lime slaking reaction was carried
out in water or in solutions containing either calcium nitrate or sodium
hydroxide, the slaking rate was found to be approximately constant at any
given disc rotation speed. This can be seen from the plots of amount to f
calcium oxide dissolved against time shown in Figure 5 and Figure 6 for the
particular cases of the slaking reaction in water, determined from conductance
measurements, and in0.03M sodium hydroxide, determined from analysis of
the calcium content of the solution by atomic absorption spectro-photometry.
It is apparent that when the reaction insufficiently rapid, the rate plots
(e.g.at600r.p.m.) show as light curvature corresponding to small decrease in
reaction rate with time. One possible cause of this slight reduction in rate will
be discussed later. In the meantime, it is sufficient to note that the reaction is
essentially zero order, and the slope of the lines gives the zero order rate
constant, k o .
(b) Effect of rotation speed .It is clear from both Figure 5 and Figure 6 that
the dissolution rate is strongly dependent on disc rotation speed indicating
that the reaction is largely controlled by either the diffusion of some reactant
species to the oxide surface, or the diffusion of some product species away
from the oxide surface.
Figure 7 shows the dependence of the zero order rate constant, ko, on the
square root of the disc rotation speed, to for different concentrations of
sodium hydroxide. It can be seen that at the lowest reaction rates (highest
sodium hydroxide concentrations), ko is directly proportional to the square
root of the disc rotation speed, indicating that the reaction is under diffusion
control.
Figure 7: Variation of the zero order rates
However, at the highest reaction rates (lowest sodium hydroxide
concentrations), ko tends to fall off with increasing rotation sp ee d , suggest in
that the reaction is going from diffusion to chemical control. In the case of
the reaction with water, the rate constant is essentially independent of rotation
speed at the highest rotation speeds investigated (1000r.p.m.), behavior which
is characteristic of chemical control.
Similar results were obtained for their action of calcium oxide with the
solutions containing various concentrations
of calcium nitrates i.e. the
reaction rate constant became progressively smaller and more directly
proportional to the square root of the angular velocity of the disc as the
calcium nitrate concentration was increased.
A rate constant, k , which is independent o f the angular velocity can be
defined by the ratio hoi(J)I. For those systems in which the reaction
becomes partly chemically controlled at high rotation speeds, we define k as
being equal to the tangent to the curve as Co tends to zero i.e. when the
reaction is most likely to be under diffusion control.
(c) Effect of concentration of Ca2 + or OH-. As noted above, the effect to f
increasing the concentration o f Ca2+ or OH- i o n s in the slaking solution is
to reduce the slaking rate. This is shown graphically in Fig.5 in which the
logarithm of the slaking rate constant k[molesCa2+ dissolving in solution
per m2 of CaO disc per second per (radian per second) is plotted against the
logarithm of the concentration, either Ca2+ or OH-. Provided that neither
the Ca2+ nor the OH- concentration isabove10 -2 M, the rate constant k is
essentially that of water, but above this concentration, k decreases and more
rapidly so in the presence of OH-l than in the presence of Ca2+.
(d) Effect of temperature. The effect of temperature o n the kinetics of the
slaking reaction in water was examined, and the results shown in the
Arrhenius plot of Figure 8 were obtained.
The points are a reasonable fit to a straight line, from the slope of which
activation ener gy of 13.6±1.2kJmol-1 was calculated. This low value is
consistent with the effect to f disc rotation speed and indicative of diffusion
control.
Figure 8: Arrhenius plot for the slaking of lime in water
1.3.2 Mechanism of the slaking reaction
(a) General
considerations.Inaformalsense,theslakingreactioncanbeconsideredtoproce
edinthree steps: Step 1,theconversion of calcium oxide to calcium hydroxide
CaO+H2O→ Ca(OH)2 e
followed by Step 2, the dissolution of calcium hydroxide to give calcium
ions and hydroxide ions in solution
Ca(OH)2 →Ca2+ +2OH-
and Step 3, the diffusion of the calcium ions and hydroxide ions into the
bulk of the solution.
Because the reaction shows a strong dependence on disc rotation speed and
low activation energy, it must be diffusion controlled. Step1cannot be
diffusion controlled because the rate constants are too small for a reactant
whose bulk concentration is 56M. On the other hand, Step3 could be rate
controlling and would give a zero order process, as observed.
If the diffusion step governs the rate of the reaction, the dissolution step will
be at equilibrium i.e. there will be a film of solid calcium hydroxide on the
reacting lime surface which is in equilibrium with calcium ions and
hydroxide ions in solution at the reacting surface. The diffusion of calcium
ions and hydroxide ions away from the surface will be described by the
Levich equation.
It is convenient to discuss three special cases: the slaking of lime with water;
the slaking of lime with a high concentration o f calcium ions in solution,
and the slaking of lime with a high concentration o f hydroxide ions in
solution.
The slaking of lime with water
According to the Levich equation, the dissolution rate, 8, for a reaction
controlled by the rate of diffusion of calcium hydroxide away from a surface
at which there is a saturated solution of calcium hydroxide into water is:
S=0.62 D(Ca(OH)2)2/3p-1/6(J)1/2[Ca(OH)2]s Eq. 1
where: D(Ca(OH)2) is the diffusion coefficient of calcium hydroxide in
water, u is the kinematic viscosity of water and [Ca(OH)2]s is the
concentration of a saturated solution of calcium hydroxide at the reacting
surface. Since: ko=8
and k=ko/(J)~,
k=0.62D(Ca(OH)2)2/3v-1/6 [Ca(OH)2]s
Eq. 2
The kinematic viscosity is given in the Handbook of Chemistry and
Physics, and so k can be estimated provided values for D(Ca(OH) 2) and
[Ca(OH)2]s are known.
Hedin has reported that the value of the diffusion coefficient for calcium
hydroxide depends quite strongly on t h e concentration, dropping from:
17.47X10-10m2S-1at1.07X10-3Mto
13.85X10-10m2S-1at1.93X10-2M
this latter concentration being close to the solubility of a standard solution of
calcium hydroxide. In the mechanism suggested here, the calcium hydroxide
is assumed to be saturated at the dissolving surface, and so it seems
reasonable to assume a value of:
D (Ca (OH)2 ) = 14 × 10- l o m 2 s- 1
for the purposes of these calculations.
By approximating activities with concentrations, [Ca(OH) 2] can be estimated
from the solubility product, Ks, via
[Ca(OH)2] = (Ks /4) 1/3
Eq. 3
where Ks= 6.5 × 10 -6 M 3 has been calculated from the average of the log
Ks values listed in Sillen and Martell.
Substituting these numbers into [Ca(OH)2] = (Ks /4) 1/3 Eq. 3 we obtain a value
of:
k equal to 9.2 × 10- 5 tool m- 2 s-1
which is in reasonable agreement with the experimental value of:
(12.0± 2.5) × 10 -5 mol m-2 s- ½
considering the approximations made in the calculations.
(b) Effect of temperature. A theoretical estimate of the activation energy for
the slaking of lime in water can be made by taking logarithms of
k=0.62D(Ca(OH)2)2/3v-1/6 [Ca(OH)2]s Eq. 2 and differentiating
E a =d In S/d(1/T) =d In D(Ca(OH)2)2/3/d(1/T)
+dln~-~/G/d(1/T)+dlnK~/3/d(1/T)
Eq. 4
Literature values for the terms on the right hand side of +dln~~/G/d(1/T)+dlnK~/3/d(1/T) Eq. 4 can now be introduced. According to Levich,
activation energies for diffusion are generally of the order of 12 kJ mol - 1,
while that for viscosity is about -16 kJ mo1-1. The enthalpy for the heat of
solution of calcium hydroxide is 16.3 kJ mo1 -1. Substituting these numbers,
we obtain Ea~ 16 kJ mo1 -1 in reasonable agreement with the measured value
of 13.6±1.2 kJ mol- 1.
1.3.4 Equilibrium and free energy conditions of Ca(OH)2
Figure 9 shows the calculated equilibrium pressure of H2O over Ca(OH)2 for various
temperatures. The calculation indicates 50 vol% steam at 200 °C is enough for CaO
hydration. At 400°C, the equilibrium pressure of H2O over Ca(OH)2 is about 0.1 atm,
i.e., 10 vol%. When the temperature is increased to 500°C, the equilibrium pressure of
H2O is about 1 atm. Figure 10 showed the weight loss curve of hydrated sorbent at
different temperature in TGA in pure N2 atmosphere, this result confirmed that most
of CaO was converted to Ca(OH)2 during sorbent hydration steps. The free energy of
calcium hydroxide decomposition in several conditions is shown in Figure 11.
Figure 9: Equilibrium pressure (atm) of H2O over Ca(OH)2.
Figure 10: Weight loss of hydrated sorbent
Figure 11: Free energy of the calcium hydroxide decomposition reaction versus
temperature at 1 atmosphere, 10−3 Torr, and 10−5 Torr water partial pressures.
2. Hydration at temperatures above 100 oC. Hydration with spraying systems. Hydration over vapors
2. Hydration at temperatures above
100oC. Hydration over vapors.
Reduction of size during hydration
2.1 Introduction
In the previous chapter the kinetics and mechanisms of calcium oxide hydration was
discussed in details. The hydration (except some sub clause) was faced as addition of
the slaking water at ambient temperature. Chapter 2 is focusing on calcium oxide
hydration at temperatures above 100oC including also the hydration over vapors. The
kinetics and mechanisms in this particular case are also discussed. Finally clause 2.4
deals with the reduction of size of CaO which occurs during hydration.
2.2 Effects of temperature on the hydration characteristics of
free lime
The hydration rate of CaO is closely related to the hydration reaction temperature.
Raising the hydration reaction temperature can make the reactant obtain enough
energy to surmount reaction the potential barrier, thus increasing reaction rate
effectively. The effect of the hydration reaction temperature on CaO with high
activation energy and some impurities is more obvious than that on pure CaO. The fCaO in Portland cement clinker forms in coexistence with many other oxides coexists.
Thus, it has higher hydration activation energy, so the ambient temperature has a great
influence on the disappearance and hydration rate of CaO.
2.3 Hydration over vapors
Hydration of lime, discussed in details in the previous chapter, is a reaction of great
commercial importance, both as a method of application of quicklime and in the
manufacture of the commercial product, hydrated lime. Therefore a considerable
amount of work is continuously being reported on the conditions and performance of
the reaction in order to obtain a product with the required characteristics.
Hydration of calcium oxide (lime) by liquid water is a very well-known reaction due
to large domain of applications of hydrated calcium oxide in industry. It is however
surprising to see that only few papers have been published on the “dry” hydration of
CaO or the interaction of water vapor with this oxide in comparison with the recent
growing interest of studies about CO2 interactions on calcium oxides.
D.R. Glasson studied the interaction of water vapor with different kinds of lime with
specific surface areas from 1 to 100 m2 g-1 and he observed the agglomeration of
particles during hydration at room temperature. A theoretical model for this reaction
has been developed with some experiments on CaO pellets at different water vapor
pressure and temperature. They found that the two most important variables were the
water vapor pressure and the calcination temperature. In some recent works on CaO
based sorbents, an anti-Arrhenius behavior was observed and the author linked this
phenomenon to the initial content of CaO. They also studied the CaO hydration and
Ca(OH)2 decomposition over a multitude of cycles with as starting material crushed
and sieved limestone (CaCO3). The hydration rate decreased with increasing number
of cycles.
This reaction is also extremely interesting from the point of view of chemical reaction
engineering because the system involves a reaction of explosive violence between
solid calcium oxide and water leading to the formation of the solid product, calcium
hydroxide.
The speed and exo-thermicity of the reaction may cause unusual temperature profiles
in the solid, breakage and complex phenomena (coalescence and dispersion of small
particles), leading to a complex mathematical description of the reacting system.
However, little effort has been devoted to the study of the kinetic behavior of the
reaction and the design of industrial reactors, and it is very difficult to relate
fundamentals such as the diffusion coefficient and activation energies reported in the
literature for the dissolution of calcium oxide to the technical equipment used in
industry for applied purposes.
Dutta and Shirai have shown the kinetic behavior of the reaction by following the
time-temperature profiles inside a big reacting sphere of pure calcium oxide. The
experimental curves were explained in terms of two different phenomena: the first
corresponding to the solid-liquid interfacial reaction between lime and water and the
second to the gas-solid reaction between water in the vapor form and calcium oxide.
The purpose of this work is the development of a suitable mathematical model for the
description of the solid-liquid reaction between calcium oxide and water from
experiments performed under adiabatic conditions and the evaluation of the kinetic
parameters of the process.
Two main problems are associated with the heterogeneous chemical reaction:
(a) the extremely fast evolution of heat, and
(b) the fact that the volumes of product and reagent are different, leading to
thermal and mechanical stresses inside the solid body, causing it to develop
cracks to various degrees, plugging the initial pores and leading to the
development of new porosity as the reaction proceeds.
Both phenomena take place in the initial stages of the reaction depending on the
liquid-to-solid ratio and physico-chemical properties of the reagent (calcium oxide).
2.3.1. Influence of water vapor pressure
The influence of the water vapor pressure on the kinetic curves is illustrated in Figure
12 . All experiments were realized with the soft burnt limes powder. The temperature
was fixed at 150°C for all tests. According to the experimental α(t) curves, the higher
the pressure, the faster the reaction (Figure 12a). The maximal fractional extent
observed for 80 and 160 hPa reaches mainly 1, which means that SBL powder can be
totally transformed into Ca(OH)2. The rate versus α curve exhibits a maximum for the
higher pressure experiment. It seems that all the curves present such a maximum close
to the initial time.
Figure 12(a) (b): Influence of water vapor pressure o the hydration
2.3.2. Influence of temperature
Figure 13 shows α(t) curves obtained at various temperatures for soft burnt limes and
hard burnt limes samples. It can be seen that for both powders the higher the
temperature, the slower the hydration. This can be observed from the dα/dt(α) curves
shown in Figure 14 for SBL CaO powder. Such a behavior is quite unusual. We
could also note that the maximum of the curves moves from α=0.05 to nearly 0.4
when temperature increased.
Figure 13: Influence of temperature on hydration of SBL and HBL
Figure 14: Influence of the temperature on the kinetic rate of hydration of SBL CaO powder with a
water vapor pressure of 80 hPa..
From the paper of E. Serris et al. we can imagine that the growth of Ca(OH) 2 may
proceed according to two possible mechanisms. Indeed, calcium di-hydroxide
crystallizes in hexagonal structure, which presents a strong anisotropy. So two
directions of growth may be considered: either perpendicular or parallel to the
hexagonal planes of the Ca(OH)2 structure. Figure 15 illustrates schematically the
differences in both possible mechanisms. The so-called perpendicular growth
proceeds quite usual in gas–solid reactions: according to the mechanism described
above and the inward advance of the internal interface. In the parallel mechanism,
after the adsorption, the diffusion of calcium and oxygen ions may proceed from the
internal interface by the side of the hexagonal planes of Ca(OH)2, thus leading to their
possible extension by reacting with hydroxyls and proton ions present at the surface.
Figure 15: Schemes of perpendicular (a) and parallel (b) growth processes. Bold and
dashed arrows indicate the direction of diffusing species and of development of
Ca(OH)2, respectively
This second way of Ca(OH)2 growth better explains the gypsum flower micro
structure observed by SEM in the hydrated samples. Thus the parallel mechanism
seems to be predominant in both kinds of powders. However, the perpendicular one
may occur too, at least in the firsts moments of the reaction, and as long as the
thickness of Ca(OH)2 layer remains low. In such anisotropic crystals, the diffusion in
the direction perpendicular to the layers is generally slow compared to surface
diffusion, especially at the temperatures investigated in this study (lower than 420
°C). In the case of Soft Burnt Limes powder, where numerous open pores exist inside
the aggregates, the reaction takes place in each particle separately, without interaction
with the neighboring particles. Thus the water vapor may easily circulate inside the
porosity, which is in favor of a total conversion of CaO into Ca(OH)2. On the other
hand, since the HBL powder is composed of very compact aggregates, the parallel
growth leads to much more continuous layers of Ca(OH)2, which tend to envelop very
large areas of the aggregates. As a consequence, the access of the reacting gas inside
the aggregates is strongly reduced, and it results in an incomplete transformation, the
perpendicular growth being too sluggish in the temperature range investigated (up to
420°C).
In concluding CaO hydration by water vapor leads to the following remarks
(a) the influence of water vapor pressure enhanced the kinetics of reaction with a
linear dependence of the pressure on the rate
(b) an anti-Arrhenius behavior was observed for the temperature dependence, due
to extremely exothermic reaction combined with a rate determining step
leading to an expression of the rate including all the steps and
(c) a “blocking effect” was also observed for only one of the powders (less inter
aggregate porosity) with “in fine” packed aggregates attributed to the growth
of Ca(OH)2 continuous layers around the aggregates.
In Figure 16 data corresponding to experimental results at 450ºC and four different
partial steam pressures (PH2O 100, 75, 65 and 50kPa), using 3mg samples of
Compostilla limestone (100-200μm), an initial calcination at 800ºC during 10min and
a total gas flow of 7.3×10−6m3 s-1 (STP). As can be seen in this figure after a fast
initial kinetic regime there is a smooth change in the reaction rate to a slower reaction
regime. The experimental results show that complete conversion can be achieved
within 20-30s under high partial steam pressures (i.e. 100 or 75kPa) which are the
most favorable hydration conditions for these experiments.
The fluctuations in hydration conversion between 0.85-1 are attributed to inherent
experimental measurement errors, as no-similar trend has been observed in other
experiments. These experimental results reveal a much faster hydration reaction than
expected when compared to the kinetic results described by other authors in the state
of the art. As pointed out in the experimental section, special care was taken to
minimize diffusional resistances related to the experimental setup, gas solid flows or
sample mass. The absence of these resistances during the kinetic reaction test could
explain why the rate of the hydration reactions in Figure 16 is somewhat higher than
expected
Figure 16: Hydration conversion (XHy) vs. time
2.4 Hydration and reduction of size
Based on Shunji Homma et al. there are not a few cases that the overall particle size is
changed during the reaction unlike the situation assumed in the unreacted shrinking
core model. When the density of the converted solid material is different from that of
the solid reactant, the particle shrinks or swells depending on the density ratio.
Considering this effect, Rehmatand Saxena (1976) and Rehmat et al (1978) obtained
the conversion–time relationship for non-isothermal and non-catalytic reactions.
It is the first to take into account the variation in sizes of both the unreacted core and
the particle itself.
Chemical reaction by which the converted solid material is consumed, also causes the
reduction of the particle size. In this case, both the unreacted core and the particle
itself shrink with different speeds, so that two moving surfaces, reaction fronts, must
be taken into account in the reaction model. In other words, the reaction model will be
a combination of the unreacted shrinking core model and the shrinking particle model.
Although it is a simple idea to represent this situation, the combined reaction model
has been presented quite recently (Braun et al., 2000; Ogata et al., 2004).
According Chung-Yu Cheng et al, water reacts with calcium oxide existing in bigger
ash particles core and produces calcium hydroxide following the well known
chemical equation:
CaO + H2O → Ca(OH)2
Since the molar volume of calcium oxide is 16.9 cm3/mol and that of calcium
hydroxide is 33 cm3/mol, the formation of calcium hydroxide at the sorbent core
causes the volume expansion of the sorbent core and destructs the calcium oxide shell.
The hydration reaction of these ashes includes two stages. In the first stage, the water
molecules diffuse through the shell and react with calcium oxide via an adsorption–
surface reaction mechanism. In the second stage, the calcium hydroxide crystals grow
in the normal and tangential directions on the calcium oxide surface.
Scientists from the University of Leeds are using the UK's national synchrotron to
investigate the efficiency of calcium oxide (CaO) based materials as carbon dioxide
(CO2) sorbents. Their results, published in the journal of Energy & Environmental
Science, provide an explanation for one of the key mechanisms involved. This new
knowledge will inform efforts to improve the efficiency of this economically viable
method of carbon capture and storage. The observations of the scientists suggest a
mechanism for the interaction between CaO and water during hydration.
They found that the stresses in the calcium hydroxide phase when bound to CaO were
more than 20 times higher than its strength, leading to disintegration and the
generation of nano-sized crystallites. Although the generation of a high surface area is
a good thing, mechanical friability needs to be kept in check in order to achieve long
term reliability for these systems. The analysis provides an explanation of the
enhanced capture/disintegration observed in CaO in the presence of steam.
The hydration of CaO readily forms a shell of calcium hydroxide when exposed to
water in the air (right). Due to differences in atomic congurations (Figure 17, top left)
between the oxide and hydroxides, enormous strains develop due to the interface.
These strains of 0.78% lead to stresses 20 times higher than the rupture strength of the
hydroxide leading to rupture and the generation of nano-particles. Deconvolution of
the data generated by Diamond (Figure 17, bottom left) allows the Leeds team to
determine the size and strain in these layers, from the breadth of the peaks (the peaks
from CaOH are far narrower than CaO).
Figure 17: The hydration of CaO readily forms a shell of calcium hydroxide when
exposed to water
2.5 Particle breakage model
Figure 18 shows a pictorial representation of a small section through a particle. The
spent sorbent (CaO) is treated as a sphere with initial radius r0. Owing to the larger
volume of Ca(OH)2 relative to CaO, during hydration the particle’s outer radius
increases to r2, whereas the un-reacted core has a radius r1.
Equation 5 is a mole balance for the conversion of CaO to Ca(OH) 2.
Eq. 5
Here, ρx is the density and εx the envelope porosity of species x.
Equation 6 gives the conversion to Ca(OH)2:
Eq. 6
Substitution of Equation 6 into Equation 5 give Equation 7.
Eq. 7
Now defining α (Equation 8):
Eq. 8
and rearranging, Equation 9 is obtained:
Eq. 9
Equation 9 defines the circumferential strain, Δr/r0, in the particle. The situation is
that the first shell of Ca(OH)2 formed is pushed further and further out as the particle
expands.
The strain can be related to the stress, σ, by Hooke’s law in an orthogonal coordinate
system for an isotropic material, as described in Equation 10 (assuming pseudoequilibrium).
Eq. 10
In Equation 10 E is Young’s modulus and υ is Poisson’s ratio for Ca(OH)2.
Substitution of the strain (Δr/r) from Equation 10 into Equation 9 and rearranging
yields Equation 11
.
Eq.11
Equation 11 can be used to estimate the stress in the outermost layer of Ca(OH) 2 as a
function of conversion, using densities of CaO and Ca(OH)2 from Perry and Green,
with Young’s Modulus from Monteiro and Chang.
The porosity of the Ca(OH)2 which was formed was assumed to be 0.19, which was
calculated using the average porosity of the CaO following dehydration of 0.73.
Figure 18: Particle breakage model.
3. Hydration in the presence of other gases similar to flue gases
3. Hydration in the presence of other
gases similar to flue gases
3.1 Introduction
Focusing on this topic and since in most cases the existence of CO 2 and SO2 is
predominant in the flue gases, the carbonation of calcium oxide by carbon dioxide, as
well the desulfurization process based on the use of CaO must be faced either
separately or in combination. Except carbonation and desulfurization under
investigation is the momentum of hydration as it will be explained in details.
3.2. The carbonation of calcium oxide by carbon dioxide
3.2.1 General
Current techniques for post-combustion carbon capture filter out CO2 from a power
plant's flue gases as they travel up a chimney. The filter is a solvent that absorbs the
CO2, before being heated, releasing water vapour and leaving behind the CO 2. In precombustion, the CO2 is filtered out by use of a catalytic converter before the fossil
fuel is burned and the CO2 is diluted by other flue gases. These methods can prevent
80% to 90% of a power plant's carbon emissions from entering the atmosphere
CaO based materials have a large range of applications including pre- and postcombustion carbon capture technologies and thermochemical fuel upgrading. They
are low cost, high abundance, have a large sorption capacity and fast reaction rates
during the chemical process. They capture CO2 in the temperature range 400-800oC
via the formation of calcium carbonate (CaCO3) which can be regenerated with
subsequent release of CO2, ready for compression and storage. However, after
multiple capture and regeneration cycles, the materials' capacity for capture decreases
due to the loss of surface area through sintering, a process that fuses powders together
to create a single solid object. Although the surface area can be restored through
hydration, the material suffers a reduction in mechanical strength. If these problems
can be overcome, CaO based materials could provide a low cost answer for carbon
capture on a very large scale.
3.2.2. Gas -solid reactions
A number of non-catalytic gas–solid reactions have been widely employed in the
industries for energy production and environmental protection. Particularly, the
carbonation reaction of CaO with CO2 has been studied for the applications such as
the CO2 separation from flue gas or from syngas, the storage of energy, chemical heat
pump, and the clean hydrogen production by reaction integrated coal gasification.
It is well known that the gas–solid CO2–CaO reaction proceeds through two rate
controlling regimes. At the very initial stage of reaction, the reaction occurs rapidly
by heterogeneous surface chemical reaction kinetics. Following this initial stage, as
compact layer of product CaCO3 is developed on the outer region of a CaO particle,
the rate of reaction decreases due to the diffusion limitation of reacting species
through the layer. It has been reported that the reaction does not proceed to the
complete conversion of CaO, with ultimate conversions in the range of 70–80% or up
to 90%. In order to describe such gas–solid reaction kinetics, various models have
been introduced.
Most classical are the continuous model and the un-reacted core model. Because the
continuous model assumes that the diffusion of gaseous reactant into a particle is
rapid enough compared to chemical reaction, it is not good for representing the CaO
carbonation reaction in diffusion control regime. Un-reacted core model known as
shrinking core model assumes that the reaction zone is restricted to a thin front
advancing from the outer surface into the particle, which is represented by Eq.1
t/τ = 1- (1-Χ)1/3
( 1α)
2/3
t/τ = 1- 3(1-Χ) + 2 (1-Χ)
( 1β)
where, t is the time; X, the conversion of CaO; and τ is the time required to completely
convert an unreacted particle into product. While Eq. (1a) is for chemical reaction
control regime, Eq. (1b) for diffusion control regime. This model could be applied for
the CaO-carbonation reaction kinetics. However, as the model predicts the complete
conversion, X = 1 at t = τ, it is not good for properly describing the actual kinetic
behavior in the diffusion control regime of CaO-carbonation. It is also inconvenient to
get the conversion using this model because the conversion X is implicitly given as a
function of time. Bhatia and Perlmutter developed the random pore model as given
below to correlate reaction behavior with the internal pore structure:
where ψ is a structural parameter depending on the surface area, porosity, and the
initial total length of pore system per unit volume, and k’, k are rate constants. Eq.
(2a) is for chemical reaction control regime, and Eq. (2b) for diffusion control regime.
They employed Eq. (2b) to obtain kinetic parameters. This model is informative for
understanding by what structural parameters the rate of (CaO-carbonation reaction) is
determined, however, very complex to employ.
From a practical point of view, in such processes for which the CaO-carbonation is
employed as listed above, a kinetic equation with the best fit to experimental
conversion data is very useful for the process design, or modeling.
Τhe capture of CO2 from air via two carbonation reactions:
(I) CaO + air (500 ppm CO2)→CaCO3
in the temperature range 300–450°C, and
(II) Ca(OH)2+air (500 ppm CO2)→CaCO3+H2O
in the temperature range 200–425°C.
shows that the rate of CaO-carbonation is initially chemically-controlled but
undergoes a transition to a diffusion-controlled regime, and can be well described by
the un-reacted core kinetic model.
The rate of Ca(OH)2-carbonation is predominantly chemically controlled and can be
well described by a kinetic model that considers the formation of an interface of water
molecules or OH-ions, and the intrinsic chemical reaction taking place only over the
surface that is not covered by CaCO3. Water catalyzes the CaO-carbonation to such an
extent that, in the first 20 min, the reaction proceeds to50% extent at a rate that is
about 22 times faster, and the reaction extent attains up to 80% at 400°C after 100
min. Within residence times of 0.11–0.17s, the uptake of CO2 from air containing 500
ppm is high during the first reaction minute (for example, it reaches up to 60% for
CaO with added H2O) and decreases with time following Avrami’s empirical rate law.
The kinetic models applied for the carbonation of CaO, Ca(OH) 2, and CaO with
added H2O are able to describe the reaction rates with reasonable accuracy.
3.3 Flue gas desulfurization with calcium oxide
The use of lime for treating flue gases is a proven technology. Flue gas is generated
from the thermal treatment process in Energy from Waste plants (EfW) and contains
acidic gases such as hydrogen chloride, sulphur dioxide and hydrogen fluoride. The
use of lime in the three main flue gas treatment processes of; dry, semi-dry and wet
processes shows its flexibility and adaptability in its worldwide application.
Calcium carbonate (chalk), calcium oxide (quicklime) or calcium hydroxide (hydrated
lime) can be used to neutralize acidic gases and remove sulphur dioxide from both
EfW plants and power stations. This ensures that plants comply with both local and
European environmental legislation for air emissions. Together with the new flue gas
treatment equipment technologies, lime is the most cost effective alkali that can be
used for this kind of treatment, with less dosage and less waste production compared
with other reagents.
The number of Energy from Waste plants in the UK is due to rise significantly in the
near future, as the cost of land-filling waste is set to increase dramatically. Lime
products can therefore provide a cost effective, efficient solution to the treatment of
flue gases generated from the energy recovery process, which reduces the waste
volumes sent to landfill.
Hydrated lime is fluidized in air and injected straight into the exhaust ducting.
Generally, over 99% of the HCl, over 95% of the HF and over 95% of SO 2 can be
removed. The neutralization reactions are as follows:
Ca(OH)2+2HCl→CaCl2+2H2O
Ca(OH)2+2HF→CaF2 + 2H2O
Ca(OH)2 + SO2 → CaSO3 + H2O
Ca (OH)2 + SO2 + 0.5O2 → CaSO4 +H2O
Transformed into calcium chloride, calcium sulphite, calcium sulphate and calcium
fluoride, the acidic gases are captured on bag filters as solids (similar to the semi-dry
scrubbing technique).
The excess hydrated lime can be re-circulated to improve utilization.
Apart from the content of available hydrated lime, the reactive surface area is also of
importance for removal efficiency. The high degree of fineness of industrial hydrated
limes also increases the efficiency in eliminating acid gas components.
3.3.1 The reactivation of CaO
Hydrated lime (Ca(OH)2) used in pulverized or fluidized bed combustors for SO 2
removal suffer from low reactivity and low utilization rate. In spite of being
economical and easily retrofit table in the existing utility units, dry sorbent processes
fail to be more competitive with other more expensive SO 2 control technologies due to
their poor SO2 removal efficiency and low sorbent utilization. Typically, less than
50% of the available calcium is converted to high molar volume calcium sulfate
product which causes pore blocking and pore mouth plugging and renders the sorbent
ineffective for any further SO2 capture. The spent sorbent from pulverized combustors
(PC's) exhibits less than 35% calcium utilization, while for circulating fluidized bed
combustors (CFB's), up to 45% calcium utilization is realized (Couturier et al., 1994).
The spent sorbent exhibits negligible reactivity towards SO 2 and in order to increase
the sorbent utilization the sorbent needs to be reactivated to expose the un-reacted
CaO. Reactivation of the underutilized sorbent would necessarily require, re-exposing
and/or redistribution of the CaO from the interior of the sorbent particle and
reactivation of the sintered CaO by converting it into a more reactive form. The
fundamental challenge and goal of the reactivation process is to redistribute the
CaSO4 predominantly from the surface of the particle to a more uniform distribution.
One of the methods for reactivating partially utilized sorbents is by the process of
hydration (Bobman et al., 1985). In this process, the un-sulfated CaO is reacted with
water to form Ca(OH)2. Due to higher molar volume of the hydroxide (33 cc/gmol),
compared to CaO (17 cc/gmol), the sorbent particles expands and the non-porous
CaSO4 shell cracks thereby exposing the hydrate (see also 2.4). Moreover, once this
reactivated sorbent is reintroduced into the combustor, calcination of Ca(OH) 2 further
increases the porosity and provides added exposure of CaO to SO2.
Hydration has been known to increase the utilization of spent sorbent from 35% to up
to 70% (Couturier et al., 1994). It is known that the effectiveness of the hydration
reactivation process is dictated by the duration of hydration, the hydration
temperature, and the solids concentration in the process. The temperature for drying
the hydration products has also been indicated to markedly affect the activity of the
reactivated product (Khan et al., 1995; Tsuchia et al., 1995).
3.4 Relation between carbonation and hydration of CaO
Both temperature and H2O concentrations played important roles in determining the
reaction rate and extent of carbonation. The mechanism of the carbonation of CaO
with and without H2O vapors present in the synthetic flue gas showed significant
differences.
Most noticeably, carbonation of CaO did not occur when T < 400°C without H2O, but
at 15% and 8% H2O present, significant CaO conversion to CaCO3 was achieved.
It appears that carbonation of CaO in real coal-fired flue gas proceeds via reaction:
CaO + CO2→CaCO3 and is catalyzed by H2O vapor,
presumably by the formation of a transient hydroxide species. Here we suggest that
when water vapor is present in the flue gas, the actual carbonation process of CaO
occurs via the following two-step reaction:
First, CaO reacts with H2O to form Ca(OH)2:
CaO + H2O →Ca(OH)2
Ca(OH)2 then reacts with CO2:
Ca(OH)2 + CO2→CaCO3 + H2O
Figure 19 shows that, at H2O partial pressure less than 20 kPa and temperature
>400°C, Ca(OH)2 cannot exist as a stable compound. However, it can be assumed that
Ca(OH)2 will still form when H2O encounters CaO as a transient intermediate.
The concentration of Ca(OH)2 intermediate could be very low, since Ca(OH) 2
calcines to CaO quickly. The higher the temperature is, the shorter the existence of
Ca(OH)2 transient species will be, and the less important its contribution to the
carbonation process will be.
However short the existence of Ca(OH)2 is, if a CO2 molecule meets
Ca(OH)2, CaCO3 should result according to reaction:
As noted elsewhere the carbonation of Ca(OH) 2 is much faster than carbonation of
CaO. Therefore, the presence of H2O vapor in the flue gas increases the carbonation
rate of CaO contained in the fly ash as observed with results in this study.
It should be noted that the contribution of reaction:
CaO + CO2→CaCO3
becomes less important when temperature decreases.
This can be seen in the relevant Figure which showed that lower temperatures led to
more pronounced differences between tests done with and without H2O. For example,
when the carbonation reaction tended to stabilize, there was <10% carbonation ratio
difference between 15% H2O and 15% N2 at 800°C, but at 500°C the difference was
47%.
Figure 19: Partial pressure of water vapor over Ca(OH)2.
At T< 400°C, there was no detectable carbonation reaction of CaO without H2O. But
when 8–15% of H2O vapor was added to the synthetic flue gas, carbonation of CaO
was obvious. This observation supports the proposed second mechanism that the
carbonation of CaO when H2O vapor was present also includes the two-step route:
CaO→Ca(OH)2→ CaCO3.
From experimental results, one might conclude that reaction:
was faster than:
since the difference in final CaO reaction products existed, i.e., there was no Ca(OH) 2
in the carbonated fly ash samples when T >300°C, but some Ca(OH)2 remained when
T<300°C, and the amount of Ca(OH)2 increased as the temperature decreased.
3.5 The effect of steam on Carbonation and Sulfurization of
Calcium oxide
According a recent paper from Poland (Halina Pawlak-Kruczek) the effect of steam
on different rates of carbonation reaction and sulfation can be explained by the change
of CO2 and SO2 migration to the internal surface of sorbent (CaO). The problem of
gas migration through the pores of solid bodies of various geometry and thickness in
the presence of sulfate (gypsum) phase in coal ashes was studied and concluded that
the influence of pore geometry on gas migration (and sorption) may be described in
the way presented in Figure 20. Pore size reduction resulting from the formation of
the thin, polar film of water molecules creates a barrier which blocks the access of
larger and polar molecules (SO2) to the part of the sorbent’s active surface.
This mechanism is less effective in the case of smaller and non-polar molecules of
CO2. As a result, sorbent pores act as a molecular sieve, thus influencing competitive
sorption of CO2 and SO2.
Figure 20: Model of water vapor film influence on SO2 and CO2 migration
Finally, the effect of steam on carbonation conversion when the competing
carbonation and sulfurization reactions occur is positive, i.e., carbonation conversion
is higher in comparison with the condition without steam. Sulfurization in the
subsequent cycles forms thermally stable CaSO4 which blocks the access to the
internal surface and leads to reduction in the CO2 capture capacity of CaO. The
negative effect of the competitive sulfurization reaction on carbonation increases in
the subsequent cycles for both gases, but to a lesser extent in the presence of steam.
The presence of steam definitely lowers the sulfurization conversion in comparison to
the condition without steam which can result from pore size reduction resulting by
creating of the thin, polar film of water molecules which blocks the access of larger
and polar molecules (SO2) to the part of the sorbent’s active surface. This mechanism
has been proven less effective in the case of smaller and non-polar molecules of CO2.
The ratio of carbonation to sulfurization in the subsequent cycle with 10% steam
concentration in the simulated flue gas is several times higher than for the similar
condition but without steam.
4. Hydration of CaO in the presence of other mineral phases.
4. Hydration of CaO f in Fly ashes
4.1. Introduction
After the individual examination of the hydration conditions of CaO either in higher
temperatures (Chapter 2) or in flue gases (Chapter 3), this chapter will deal with the
hydration of CaOf in the presence of other mineral phases, materials and by products
focusing and insisting on the hydration of CaO f present in fly ashes Clauses 4.2 and
4.3). In clause 4.4 is given a more precise approach to the problem raised through a
comparative examination of steam vs. water hydration of CaOf present in High
Calcium Fly Ashes in relation to their sulfurization behavior.
4.2.
Hydration of CaOf present in fly ashes
As it is referred in many studies (Kuusik et al., 2005), fly ashes formed in boilers
operating at different combustion technologies differ significantly in their chemical
and phase composition, as well as in their physical structure and surface properties.
Changes in the firing technology have altered the mineral of the ash produced – the
low temperature CFB ashes characterized by the lower content of silicates and
aluminates and higher content of free oxides show predominantly pozzolanic
hydration type.
The relationship between the composition of the solid fuel and its combustion
temperature and the hydration type of the produced fly ash has been studied by many
researchers.
The high content of free CaO indicates air binding properties. The high firing
temperature of boilers causes not only decomposition of limestone, formation of
calcium silicates and aluminates, but also dead burning of free CaO, which leads to
the formation of larger size crystals and decreased hydration reactivity. Hydration
reactivity could be characterized by slaking rate and temperature. To examine the
slaking properties of free CaO containing ashes, the calculation method had to be
modified using a 100% hydration of the tested ash.
Usually, particles of fly ashes formed at moderate temperatures (750–800oC) are
characterized by an irregular shape and porous uneven surface (Kuusik et al.,2005)
Figure 21(b), while in the case of ashes formed at 1250–1400 oC Figure 21(a) the
glassy phase significantly affects the formation of the particle shape and surface
properties: the particles tend to have a regular spherical shape and smooth surface.
(a)
(b)
Figure 21: Ashes characterized by an irregular shape and porous uneven surface
If ashes are sprayed by water then the phenomenon of aqueous carbonation of the ash,
lime – as the most reactive component – passes through several stages: hydration,
dissolution and diffusion of Ca2+ ions into the bulk solution, and carbonation
according to the following equations (Domingo et al., 2006; Garcia-Carmona et al.,
2003).
CaO + H2O Ca(OH)2  Ca2+ 2OH-(solid surface)
 Ca2+ 2OH- (bulk solution)
CO2 + H2O  H2CO3 HCO3- + H+ CO32- + 2H+
Ca2+ + CO32- CaCO3
The rate of lime slaking is influenced by the porosity, the amount of impurities,
particle size, time and temperature of the limestone calcination, the amount of water
added and agitation (Boynton, 1980; Ritchie and Xu, 1990). As even minute amounts
of extraneous substances present in slaking water affect the extent and rate of lime
solubility, water quality can be critically important in lime slaking (Boynton, 1980;
Giles et al., 1992; Potgieter et al.,2003; Ritchie and Xu, 1990; Xu et al., 1998). In the
presence of Ca2+- and OH- ions the slaking rate drops because the reaction is also
controlled by the diffusion of calcium hydroxide away from the surface (Ritchie and
Xu, 1990). Carbonate and sulphate ions in the slaking water are known to form
coherent layers of both CaCO3 and CaSO4.2H2O ,which partially or completely coat
the surface and thereby prevent the further dissolution of CaO and the formation of
further CaCO3 (Potgieter et al.,2003). Previous studies (Uibu and Kuusik, 2009; Uibu
et al., 2010) have shown that immersing ashes into de-ionized water results in lime
slaking and the dissociation of portlandite, such that the solution becomes saturated
with Ca2+ ions. In the case of continuous flow carbonation processes, constant
saturation of the re-circulating liquid phase with different species leached from ash
(mainly Ca2+, Mg2+, K+, Na+, OH-, SO42-) and absorbed from the flue gases (CO32-,
HSO3- , HCO3-) is expected. Process deceleration, which starts at the hydration step, is
mainly caused by two factors: the low porosity of ash and the composition of the recirculating liquid phase (Uibu and Kuusik, 2009).
Experiments with non-porous ashes showed that an excess amount M) of CO 32- and
HCO3- ions in the slaking water had the most severe effect on the availability of lime
and its utilization (<16%) in correlation with the solution concentration. SEM analysis
showed the formation of a coherent product layer on ashes particles, and XPS analysis
indicated that the surface layer consisted mainly of carbonates. The SEM observations
with element analysis confirmed that un-reacted free CaO remained inside the
particles. Excess amounts (0.05–0.1 M) of SO42-ions in the slaking water caused
moderate process deceleration.
The utilization of lime could be substantially enhanced by diluting the process water
or using ageing pre-treatment to increase the porosity of ash. Six-months of ageing
pre-treatment enhanced the utilization of lime in the aqueous carbonation process
from 16% to 37–68% even in the case of inhibiting aqueous conditions.
Comparative experiments showed that the porous structure of its particles supported
the fast and full hydration of lime as well as the diffusion of Ca 2+ ions into solution,
which resulted in complete carbonation regardless of the liquid phase composition.
According to previous research work, there exist carbonation process of Ca(OH) 2 and
CSH in cementitious materials. The reaction equations are expressed as follows:
Ca(OH)2 + CO2 CaCO3 + H2O
Calcium silicate hydrate:
(CSH) +CO2CaCO3 + SiO2 nH2O + H2O
The carbonation of CSH causes silica gel formation and its carbonation rate is rather
slower than that of Ca(OH)2. Colloidal hydration products of calcium silicates (CSH)
have a diminishing effect on the capillary pore diameter. Such a phenomenon
decreases capillary pore volume. Free SiO2 reacts with free CaO and water, forming
calcium silicate hydrate gel. Gel products fill large capillary pores and the volume of
over 10μm sized pores decreases
4.3. The hydration behavior of industrial fly ashes
The conclusions from the very interesting relative study (D. Gora, E.J. Anthonya,,
E.M. Bulewicz, L. Jia "Steam reactivation of 16 bed and fly ashes from industrialscale coal-fired fluidized bed combustors" Fuel 85 (2006) 94–106), concerning the
hydration behavior of sixteen ashes, obtained from different commercial-scale
combustors are presented below.
1. Hydration by saturated steam at 165°C is efficient in converting the CaO
present in FBC ash to Ca(OH)2, but due to pozzolanic reactions with the coalderived ash components, some of the CaO may be consumed in side reactions,
with the formation of hydrated aluminates, silicates and alumino-silicates,
such as katoite.
2. Steam hydration does not result in the formation of any gypsum.
3. The crystalline reaction products give rise to a number of weak and very weak
lines in the XRD spectrum, but only katoite, Ca2.93Al1.97(Si0.64O2.65)(OH)9.44,
could be identified with any confidence. Full identification was impossible,
because of the small amounts of such compounds, possibly containing
sulphate ions. In addition, some of the reaction products could be amorphous.
4. Different methods of assessing the amount of ‘free lime’ or reactive CaO in
hydrated FBC ashes yield results that agree only at the qualitative level. In the
sucrose method, the sugar solution might affect some Ca compounds other
than Ca(OH)2 and the TG curves may be distorted, making the determination
of Ca(OH)2 unreliable. However, the “free lime” from the sucrose method can
probably be used for comparative purposes.
5. Independent of the presence of any Ca(OH)2, the TGA curves indicate that the
hydrated ash samples can lose water practically continuously, with few, if any,
characteristic effects.
6. It is clear that the mass change of hydrated samples cannot be used as a simple
measure of the extent of hydration. An appreciable proportion of the water
taken up is combined in phases such as hydrated alumino-silicates.
7. For hydration to be complete, free CaO must no longer be present, but its
disappearance does not necessarily mean that it has been fully converted to
Ca(OH)2. It can be consumed in pozzolanic reactions and in some cases as
much as 10% of added CaO can react. Hence, in systems like FBC ashes, the
extent of CaO to Ca(OH)2 conversion can only be defined unambiguously if
both compounds are determined independently.
8. Hydration by pressurized steam reactivates FBC ashes for further SO 2 capture,
but the simple reaction involving CaO and SO2 may not be the only process
that occurs. It appears that some SO2 can be taken up even when neither CaO
nor Ca(OH)2 are detectable.
9. The difficulties encountered in this work indicate that the results of
experiments on real ashes may be difficult to interpret, since the behavior of
such complex material can be influenced by many factors. Whenever possible,
more than one experimental method should be used and the results carefully
cross-checked; modern analytical methods should be used critically and with
care.
4.4 Steam vs. water hydration of high calcium fly ashes in
relation to their sulfurization behavior
Table 2 shows the results of the free lime and Ca(OH)2 content in the FA fractions
after steam hydration. At any given hydration time, the free lime content was higher at
lower hydration temperatures For FA the free lime content was significantly reduced
after 30-min hydration at all temperatures used when compared with levels prior to
hydration. This apparently paradoxical type of result has been previously observed by
many researchers. This is regarded as clear evidence that, during the hydration
process, free lime can be released from the OCCs, as well as consumed by reactions
with the coal ash components, such as silica.
Table 2: Mean free lime and Ca(OH)2 contents following steam hydration (expressed as
CaO, wt%)
Hydration time
Steam temperature
150 °C
200 °C
250 °C
Free lime Ca(OH)2 Free lime Ca(OH)2 Free lime Ca(OH)2
FA
30 min
20.8±2.3 17.2±0.9 15.4±0.4 9.6±1.8 13.4±1.8 5.8±2.7
1h
22.6±2.3 17.8±2.7 14.4
7.9
13.3±1.2 5.7±1.2
2h
19.5
6.1
13.2
14.1
13.7
4.7
The Ca(OH)2 content always increased rapidly, even after a short hydration period of
30 min and then stayed at a relatively stable high level at any given temperature.
The fact that the Ca(OH)2 content can decrease is also strong evidence that during the
hydration process it can be consumed by reaction with other components present in
the system. In general, the most significant changes of free lime and Ca(OH) 2 content
typically occurred within 30-min hydration. After that there were only slight
fluctuations for longer periods of hydration, albeit that longer hydration times did not
always increase the extent of hydration from the view of Ca(OH)2 production
Using a comparison of values of Ca(OH)2 before and after hydration and the initial
CaO content in the ash prior to hydration, the apparent conversion of CaO to
Ca(OH)2 for each fraction was obtained, as shown in
Figure 22.
Figure 22: Apparent conversion of CaO to Ca(OH)2
FA shows very different sulfurization behavior compared with bottom ashes.
Hydration with either liquid water or steam caused no enhancement of the sulphur
capture capability Table 3 and, if anything, produced a deleterious impact despite the
fact that this ash had the highest ability for SO2 absorption when untreated. The
surface analysis on the sample of steam-hydrated FA, did not reveal any significant
increase in the BET area and pore diameter, which agrees with the sulfurization
results. The failure of FA (which has a mean size of ∼40 μm) to respond to
reactivation agrees with other observations ( Laursen et al. [9] and [14]), that the
existence of a well-defined core/annulus structure in the sulphated particle is optimal
for reactivation. FA particles are generally too small to have the core/annulus
structure and instead tend to show continuous sulphurization,
Table 3: SO2 absorption capacity of FA at 90 min sulphurization, mg SO2 g-1
sample
Unhydrated
Hydration
time
Temperature
Water
FA
510
Steam
5 °C
Ambient
60
°C
150
°C
200
°C
250
°C
30 min
–
510
467
–
487
–
1h
–
–
–
–
430
–
2h
–
–
–
–
415
–
4h
–
480
452
Even though the failure of FA to be reactivated by water agrees with some earlier
works. (Couturier et al.), it should be noted that FA reactivation has been reported
several times: Fluidized Bed Combustion Conference: (1) S. Julien, C.M.H. et al.,
(2) T. Khan, R. Kuivalainen, Y.Y. Lee 1995 and (3) Y. Tsuo, J. McClung, K.
Sellakumar 1999.
In the first two cases the ashes examined came from small pilot-scale rigs, and so this
may explain their results (i.e. particles are coarser than FA from a full-scale industrial
boiler); in the last case, the tests were done in a small industrial boiler, and a possible
explanation is that if the FA suffered some agglomeration during the hydration
process, then its residence time and utilization may have increased even if this ash
was not actually reactivated by hydration. An explanation is that the original FA
contained a significant amount of char carbon and the CaSO4 decomposed to a large
extent in the heating process during re-sulphurization. Moreover, there was no
significant evidence of reactivation due to any hydration treatment attempted.
5. The case of free CaOin HCFA
5. The case of free CaO in HCFA
5.1 General for Fly ashes. Calcareous and siliceous ashes
According to EN 450-1, fly ash is a fine powder of mainly spherical glassy particles
derived from burning of pulverized coal. It is obtained by electrostatic or mechanical
precipitation of dust like particles from the flue gases of furnaces fired with
pulverized coal, with or without co-combustion materials.
The types of mineral present in the coal and the accompanying rock are determining
factors of the chemical composition and the mineral phase constitution of the fly ash.
The minerals which are the most common and occur most widely in coal include
carbonates, clay minerals and iron sulfides. These types of minerals may account for
up to 95% of the entire mineral contained in the coal. When pulverized coal particles
are burnt in the flames of burner, the minerals accompanying the coal are heated to
temperatures >1300oC. (Figure 23). The majority of the mineral material melts and
then solidifies in a glassy amorphous form during the cooling process. The ash
particles are separated from the flue gas by electrostatic separation in multistage
electrostatic precipitators. The resulting glass content of the fly ash lies between 6085% of the mass. The main crystalline phases are quartz and iron oxide.
Figure 23: From coal particle to fly ash
So, depending on the origin of the coal, fly ashes have their main oxides (SiO 2, CaO)
in different percentages and therefore are divided as siliceous, consisting mainly of
SiO2 (LCFA, Low Calcium Fly Ashes) and calcareous, consisting mainly of CaO,
(HCFA, High. Calcium Fly Ashes). As consequence fly ashes express different
properties in cementitious mixtures and more specifically siliceous fly ashes have
pozzolanic properties while calcareous express additionally latent hydraulic
properties. Table 4 summarizes chemical composition range of siliceous and
calcareous ashes.
Table 4: Chemical composition of fly ashes
Component
Siliceous fly ashes
Calcareous fly ashes
SiO2
36-59
23-52
Al2O3
20-35
12-22
Fe2O3
3-19
4-10
CaO
1-12
5-41
MgO
0,7-4,8
1,5-4
K2O
0,5-6
1,2-3
Na2O
0,1-3,5
0,3-1
SO3
0,1-2
2-10
TiO2
0,5-1,8
LOI
0,5-5
<5
5.2 The contribution of fly ash as addition in cement and
concrete
It is generally adopted that fly ash is used extensively in concrete either as a
separately bathed material or as an ingredient in several types of blended cements. It
is used for economy and to improve the properties of concrete for certain applications.
As it is byproduct of another process like the combustion of pulverized coal, its reuse
as Supplementary Cementing Material leads to save energy by reducing the amount of
Portland cement, which is an energy intensive product. Its reuse also finally leads to
the Sustainable Development as, additionally, it contributes to the reduction of CO 2
emissions.
Except the economical and environmental impact, fly ash in concrete reduces the
temperature rise in fresh concrete. This property based on the slower reaction rate of
many fly ashes is a real help in limiting the amount of early heat generation and the
detrimental early temperature rise in massive structures. Fly ashes also generally
cause an increase in setting time, both initial and final set. It normally allows a
reduction in the quantity of mixing water in a concrete mixture necessary to produce a
target slump. Because of the fineness and rounded shape of its particles, generally
improves the cohesion and workability of the concrete at a given slump. Segregation
and bleeding are often reduced. Fly ash improves the pumpability of concrete
mixtures. As the concrete hardens the fly ash makes use of developed heat from
Portland cement hydration to accelerate pozzolanic reactions and, thereby promotes
the reaction of the fly ash with available calcium and alkali hydroxides. Using fly ash
in concrete generally allows a reduction in the required cement content and therefore
reduces the peak temperature developed in the concrete during curing.
Regarding the effect of fly ash in hardened concrete we must say that the early
strength development at three days is normally reduced by the use of class F fly ashes
and may be reduced with the use of class C fly ashes. Finally increased long-term
strength is achieved with most fly ashes in concrete.
Fly Ash has beneficial effect to two main characteristics of an adequate concrete as
are the good workability of fresh concrete and the dense microstructure of a hardened
concrete. More specifically the positive contribution of fly ashes in concrete is mainly
attributed to a combination of three different factors: i) physical effect, ii) chemical
effect and iii) the properties of the pore structure.
5.3 Comparison between HCFA and LCFA towards their use in
cement and concrete
There are two ways by which HCFA may replace cement clinker in the mixture:
Either as part of blended type cement or as partial substitution of the cement in the
concrete mixture. The consequences are the same and based to the dual character
(hydraulic and pozzolanic) of HCFA. The three factors also explained previously
(physical effect, chemical effect and properties of the pore structure) exist in the case
of HCFA and justify their behaviour.
Under a tight control of the entire design-production-application process; the use of
HCFA can be proved more beneficial than LCFA in terms of mechanical properties
because the contribution of HCFA to the hardening of the cementitious phases is
greater. In fact, numerous literature findings clearly demonstrate that high calcium fly
ashes provide better early age strengths as a result of the cementitious compounds
they possess. It is believed that calcium substitution in the glass phase is generally
increasing the reactivity of high-lime fly ashes providing for the formation of the
calcium-silicate and calcium-aluminate phases in the absence of an external source of
lime. Table 5 below gives a general comparison of basic properties between LCFA
and HCFA based concrete
Table 5: Comparison of basic properties of LCFA and HCFA-based concrete
Parameter
LCFA
Early-age Compressive Strength
Better
Later-age Compressive Strength
Susceptibility to DEF
Less
Reaction Rate
Slower
Ability to suppress expansion due to
Better
ASR
Ability to suppress expansion due to
Better
Sulphate attack
Creep
Higher
HCFA
Better
More
Faster
Less
Susceptibility to Inadequate curing
Demand for Air Entraining Admixtures
Chloride Resistance
Carbonation Resistance
Freeze Thaw Resistance
Influence on Drying Shrinkage
Workability
Expansion during wetting
Greater
Significant
Less
Less, almost none
Slightly better
Better
Much better
Almost none
Better
Less
Given that positive figure concerning HCFA the main reason for the skepticism in
their wide application lies to the variability in lime and sulfate contents which are
factors of great importance since these constituents are often considered for cement fly ash system failures. So HCFA need to be beneficiate for their incorporation in
cement and concrete.
5.4 Beneficiation tactics of HCFA
It is well discussed in the past that an analysis of the particularities of HCFA leads to
the fact that, without any dispute, their main problem is the elevated proportion of free
lime content since its hydration causes soundness problems and significant
temperature increase.
Additional problems of HCFA are:
 The variations in their chemical and mineralogical composition (inhomogeneity
problem), Major factors that contribute to the heterogeneity of fly ashes are: i) the
lignite quarry (i.e. the quality of lignite and the amount and composition of
organic matter, ii) the combustion conditions (i.e. the temperature and the
atmosphere inside the boiler) and iii) the physical condition of electrostatic
precipitators.
 The necessity for supplementary grinding in order to enhance their pozzolanic
and hydraulic properties (fineness problem). Coarse ashes are a major
subcategory of reject fly ashes, involving ashes of small specific surface,
containing high levels of residual carbon. Apart from being less reactive, such
ashes are also characterized by problematic setting behaviour when inserted into
cement paste environment.
 The periodically elevated proportions of SO3 content associated with undesired
effects similar to those occurring in cement (high sulphate problem)
 The periodically high percentages of LOI. This problem does not exists in
Hellenic Fly ashes as the totality of them have LOI less than the limit of 5% so it
will not be discussed in the continue. Moreover several technologies have been
developed facing successfully the problem.
Referring to the construction sector in general, the use of HCFA is foreseen according
to EN 197-1 as main constituent for different types of cements (CEM II or CEM IV,
A or B, W or M respectively). The particularities mentioned before, theoretically do
not cause problems to their partial incorporation in blended cements as their use is
controlled by the limits of the properties existing in EN 197-1. However the max
percentages of cement substitution by HCFA (CEM II: 20% for A, 35% for B and
more for CEM IV) in no case are achieved as other parameters (mainly sulfates) do
not permit it.
To that aspect the beneficiation of HCFA which is mainly addressed for the uses in
concrete will be also useful for the uses in blended cements leading to the increase of
their application rate.
5.5 Facing the increased values of CaOf in HCFA. Attempts
towards hydration of CaO
The beneficiation attempts discussed in the continue are referred to face the problem
of increased values of CaO f in parallel with the reduction of size and not to face other
problems as is mainly the high sulphates problem which is successfully confronted
with the pre-selection tool. The problem of the reduction of CaO f becomes more
difficult as exists the inhomogeneity problem meaning that the percentage of CaOf
varies. In the continuethis clause aims to describe the Hellenic attempts in order to
face the problems of elevated proportion of free lime content and that of necessity for
supplementary grinding.
A first approach for the solution of this problem is the introduction of a milling plant,
where the coarse fly ash is milled to the desired fineness. Such a plant, with parallel
hydrolization in order to face at the same time also the problem of elevated values of
free CaO, has been constructed in Greece and the treated fly ash has been used as the
main cementitious material for the construction of a dam with the RCC technique.
According to the Greek experience, the grinding unit consists of a closed circuit ball
mill with the relevant supplementary equipment (Figure 24).
The mill is consisted of three compartments and its dimensions are: 13mX2, 2m. Its
max through-put rate is 23t/h, its min 16,7t/h and its normal (for fineness R 45 20-30%)
is 20.83% which corresponds to the daily capacity of 500t. The residence time is
15min for 20,83t/h. It is protected with internal forged steel plates and has his load
consisting of forged steel grinding media of 30 and 40mm. Even though the wear of
grinding media was initially estimated to 223g/t of feed, the measured wear was much
lesser.
In the first compartment is suitably adjusted the spraying system, accepting water
form the water tank. Its function is to spray the suitable quantity of water (between
limits 5 to 12.5%) in relation to the current percentage of CaO f in order to reduce
(hydrolyze) the CaO f in values less than 3%. The system hydrolyzes at least 75% of
free lime content and the max quantity of spraying water is 48L/min. Water was
sprayed into the mill in controlled quantities depended on the initial value of
avail.CaO, so that the most of the lime was hydrated to Ca(OH)2 under hot moist
conditions. It is essential that the mill must operate at a relatively high temperature
(about 100oC) to speed the reaction and to boil surplus water.
The water system also serves to the cooling of special parts of mill (as e.g. the
reduction gear). The mill is connected with an air separator so that the coarse fractions
return to preheating cyclones and finally to mill for supplementary grinding. The fines
are separated using two separating cyclones.
Basic demands from such a milling system are: i) the reduction of size so that the
retained percentage on the 45 μm sieve will be 20-30%, which is an acceptable
fineness for HCFA to reveal their pozzolanic and hydraulic properties and ii) as
previously referred, the hydration of free lime content following the reaction:
CaO + H2O Ca(OH)2
to be at least 75% of feed content and so that free CaO should not exceed 3%. The
above described milling plant is now able to treat raw fly ash in accordance to the
relevant specifications per application, but also with regards to the desired fly ash
fineness (R45 : 50-15%) and free CaO content (between 12% and 2%).
Figure 24: The milling plant equipped with the partial hydrolization system
The milled fly ash collected in the cyclones after separator is distributed to three silos.
Each of themes has a capacity of 200m3. At the entrance of each silo, a continuous
sample collection system is installed (with its electronic control system) for the final
examination of the quality of ash. Quantities that do not fulfil the requirements are
rejected and collected in a fourth silo with capacity of 70m3 for further rejection with
silo vehicles. The three silos of accepted quality milled fly ash have a common
discharge point situated over a platform scale in order to weigh up the silo vehicles.
Operation mill control and therefore treated fly ash quality control were performed by
measurements of CaOf expressed as the subtraction from the total available CaO (CaO
+ Ca(OH)2) the CaO compound as Ca(OH)2. The measurement of available CaO is
carried out following the method described in EN 451.1 part 1, which is referred to
the determination of non combined aO (eg as CS, C2S, CaSO4 etc) in fly ashes. Along
with the chemical determination according EN 451.1, the weight loss, between 350
and 510 oC, of a representing sample of treated fly ash is measured using two same
model Perkin Elmer thermogravimetric devices (TGA-7). The difference in weight
refers to the weight loss caused by decomposition of Ca(OH) 2 following the reaction :
Ca(OH)2CaO + H2O
The existence and use of a second similar model instrument was extremely needed in
order to insure rapid and continuous analysis. Figure 25 shows the percentage
distribution of the CaOf in production of the mill.
Figure 25: The percentage distribution of the CaOf in production of the mill
6. Conclusions. Personal experience. Comments and suggestions
6. Application in Bełchatów ashes.
Comments and suggestions.
Conclusions.
6.1 Introduction
In the previous chapters (1-4) have been gradually reported literature data related to
all the factors affecting or potentially can affect the hydration of calcium oxide.
Chapter 5 showed the personal experience of facing similar problem in Greece. In this
chapter, and through the general data outline entitled "Flyash hydration in flue gas -an
experiment within a R / D Project", presented in Athens on November 19, will
attempt to align and implement the bibliographical findings to the project described in
your Patent. The comments referred in this chapter are mainly addressed to future
application in concrete where we are more experienced.
6.2 Bełchatów ash quality
Assuming that the chief engineers of the Bełchatów Power Plant whose main mission
is to produce energy (and not fly ash) operate the units of the station to the optimal
conditions towards this mission, the resulting fly ash as anthropogenic mineral has a
chemical and granulometric composition fluctuating in a given area. This assumption
is of major importance, as significant changes in the characteristics of the composition
maybe differentiate a little bit the analysis that follows. In any case the selection of
the final process must also be based on the fact that this process must be flexible (as
was Ptolemais milling plant) to successfully undertake fluctuations in chemical and
granulometric composition.
Concerning the crucial characteristics (CaOf and fineness) of the quality we have the
following figures taken from your presentation
Figure 26, Figure 27.
Figure 26 : CaOf variation (Bełchatów ashes)
Figure 27: Fineness variation (Bełchatów ashes)
Before commenting on these two specific parameters, we must note that, according to
relevant standards, Loss on Ignition (LOI) and reactive silica (SiO 2 react) as well,
should be considered for the use of fly ash in concrete. We observe also that the
(almost totality of sulphates) is less than 3,5% and according our experience the
relatively high percentage that is between 3,0 and 3,5 will not cause any problem
For future applications and for full confirmation that no problem concerning
sulphates will appear, we estimate that a periodic control of the soundness is
sufficient.
Commenting on the figures of free lime and fineness of Bełchatów ashes presented
before, we should note that, according our experience, free lime lies closer to the limit
of 2,5% than fineness (R45< 30% for concrete applications). That means that much
effort must be done concerning the reduction of size than the relevant for the
reduction of CaOf.
Staying at the proposed method in your patent simultaneous disintegration (reduction
of size) and hydration (reduction of CaO f) of fly ash by spraying water (or addition of
steam) in the flue gases after the boiler and in temperatures not greater than 400oC
(due to the undesired decomposition of the Ca(OH) 2 ), we must consider, according to
the literature review presented in chapters 2 and 3, the following potentially occurring
reactions:
Before the stage of water (or steam) spraying occurs:
i.
carbonization of the CaOf existing in fly ash due to direct contact in higher
temperatures with the CO2 presented in flue gases The reaction occurs before
the electrostatic filters and has as result the desirable reduction of CaO f in
parallel with the reduced CO2 emissions. The CaCO3 produced is a
constituent of fly ashes collected with themes and detected by XRD,
ii.
sulfurization of the CaOf existing in fly ash with the SO2 presented in flue
gases and deriving from the oxidation of sulfur presented in coals.
Under investigation is what exactly happens just after the injection of the water by
means that the instantly produced Ca(OH)2 reacts with the CO2 and SO2contained in
the flue gases to form additional quantities of calcium sulfates and CaCO3. Finally fly
ash shows decreased values of CaO f (as our intention is) but on the other hand its
chemical composition in sulfates is increased (to which extend?)fact that is undesired
due to the soundness problems, if the main application concerns concrete..
Even though the exact situation will be clearer after the experiments in lab (or better
in pilot plant) scale, from the literature review (several clauses of chapter 3) is
extracted that:
1. at T< 400oC, there was no detectable carbonation reaction of CaO without
H2O. But when 8–15% of H2O vapor was added to the synthetic flue gas,
carbonation of CaO was obvious. This observation supports the proposed
second mechanism that the carbonation of CaO when H2O vapor was present
also includes the two-step route:
CaO→Ca(OH)2→ CaCO3
From experimental results, one might conclude that reaction:
was faster than:
since the difference in final CaO reaction products existed, i.e., there was no
Ca(OH)2 in the carbonated fly ash samples when T > 300oC, but some
Ca(OH)2 remained when T<300oC, and the amount of Ca(OH)2 increased as
the temperature decreased.
2. the effect of steam on carbonation conversion when the competing
carbonation and sulfurization reactions occur is positive, i.e., carbonation
conversion is higher in comparison with the condition without steam
3. the presence of steam definitely lowers the sulfurization conversion in
comparison to the condition without steam which can result from pore size
reduction resulting by creating of the thin, polar film of water molecules
which blocks the access of larger and polar molecules (SO 2) to the part of the
sorbent’s active surface. This mechanism is less effective in the case of
smaller and nonpolar molecules of CO2. The ratio of carbonation to
sulfurization in the subsequent cycle with 10% steam concentration in the
simulated flue gas is several times higher than for the similar condition but
without steam.
Finally concerning reduction of size it was verified and explained by a two stages
mechanism in 2.4, that since the fact of the molar volume of calcium hydroxide is
almost double than those of calcium oxide, the formation of calcium hydroxide at the
core causes its volume expansion and destructs the calcium oxide shell.
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Behavior of Ca (OH)2/CaO pellet under dehydration and hydration.
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7. References
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The effect of surface carbonation on the hydration of CaO
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Year the Document was Publish: 1990
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8. Appendix
Appendix
Author’s Contact Details:
Prof. Stamatis Tsimas
Laboratory of Inorganic and Analytical Chemistry
School of Chemical Engineering
National Technical University of Athens
9, Heroon Polytechniou str., Zografou Campus
157 73, Athens, Greece
tel.: +30 210 772 3095
fax: +30 210 772 1727
http://www.chemeng.ntua.gr/the_people/s.tsimas

hydration of cao

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