Center for

By-Products

Utilization

PRELIMINARY DRAFT REPORT

CEMENT KILN DUST (CKD) - BASED SORBENT

FOR FLUE GAS DESULFURIZATION

By Tarun R. Naik and Fethullah Canpolat

Report No. CBU-2003-48

REP-541

December 2003

A CBU Report

Department of Civil Engineering and Mechanics

College of Engineering and Applied Science

THE UNIVERSITY OF WISCONSIN – MILWAUKEE

i

TABLE OF CONTENTS

Cement Kiln Dust (CKD) - Based Sorbent for Flue Gas Desulfurization .......................... 1

Introduction ..................................................................................................................... 1

Cement - Kiln Dust ......................................................................................................... 1

New Calcium-Based Sorbents for Flue Gas Desulfurization ............................................. 2

Properties of SO2............................................................................................................. 3

Properties of CO2 ; .......................................................................................................... 4

Why Use CKD ................................................................................................................ 5

Cementitious Minerals ................................................................................................ 5

Fineness....................................................................................................................... 6

Promoters .................................................................................................................... 6

Wet Scrubbing ................................................................................................................ 7

Cement Kiln Dust and Air Pollution. ............................................................................ 11

Dry Scrubbing ............................................................................................................... 11

Properties Required In Sorbents ................................................................................... 13

Previously Considered Sorbents ................................................................................... 13

Desulfurization Characteristic of Calcium-Based (CaO) Sorbent During Activation

Process .............................................................................................................................. 14

SiO2 - Modified Ca(OH)2 Sorbents ............................................................................... 15

Synthesis of Ca(OH) ..................................................................................................... 15

Structure and Morphology of Ca(OH)2......................................................................... 16

Modified Ca(OH)2 and Ca(OH)2-Based Sorbents ........................................................ 17

Characterization of Ca(OH)2 / fly ash sorbents for flue gas desulfurization ................ 17

Summary and Conclusions ............................................................................................... 19

REFERENCES ................................................................................................................. 20

1

Cement Kiln Dust (CKD) - Based Sorbent for Flue Gas Desulfurization

Introduction

Cement is an exceedingly important material for modern society. It is produced cheaply

and on a very large scale worldwide. World production of cement in 2002 was about 1.8 billion

tons (about 0.4 ton for each person in the world) [1].

“A typical portland cement is manufactured by feeding materials containing appropriate

proportions of lime, silica, alumina and iron into the upper end of a kiln. The mix passes through

the kiln at a rate controlled by the slope of the kiln and the speed at which the kiln rotates.

Burning fuel is forced into the lower end of the kiln where it produces temperatures of 1400-

1500 °C, changing the raw mix to a cement clinker. During this operation a small percentage of

the material in the form of dust (CKD) can vary from plat-to-plant depending on the raw

materials used and producing the same cement type will typically have relatively consistent

composition” [2].

A new concept in resource recovery is explored within tills report In that waste from one

industry was utilized to mitigate emissions from another. The waste utilized was bypass dust, a

type of Cement Kiln Dust (CKD). The industrial emission treated was sulfur dioxide from a

thermal electric power generating station.

Cement - Kiln Dust

The manufacture of portland cement includes the tumbling of fine ground raw materials

(75 to 80 % passing a 200 mesh) inside a rotary kiln. The tumbling action releases fine dust

particles, which are quickly swept out of the kiln by the hot combustion gases. This dust,

referred to as cement kiln dust (CKD), is captured by particulate emission control equipment. As

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a result of the cooling associated with the dust capture, the CKD provides nucleation sites for

minerals volatilized in the kiln system [3].

Cement kiln dust (CKD) is a by-product material of cement manufacturing industry. It is

a fine powdery material similar in appearance to portland cement. The principal constituents of

CKD are compounds of lime, silica, and alumina, and iron. The physical and chemical

characteristics of CKD depend on the raw materials used and the method of its collection

employed at a particular cement plant. Free lime is found in CKD. The concentration of free

lime is generally highest in the coarser particles of CKD captured closest to the kiln. Finer

particles of CKD contain higher concentrations of sulfates and alkalis. The primary value of

cement kiln dust is its cementitious property. Depending on the concentration of lime (CaO),

CKD can be highly cementitious. Therefore, CKD can be used as a replacement for other

cementitious materials such as portland cement, blast furnace slag cement, portland pozzolan

cement, blended cements, and the like.

CKD is composed primarily of finely ground particles of calcium carbonate, silicon

dioxide, calcium oxide, sodium, potassium chlorides and sulfates, metal oxides, portland cement

hydraulic minerals and other salts.

New Calcium-Based Sorbents for Flue Gas Desulfurization

Three sorbents for SO2 flue gas have been investigated. One is the mixture made by

agitating a high water-to-solids slurry of Ca(OH)2 and a reactive SiO2 such as diatomite or a

reactive SiO2 source such as perlite or pumice. This mixture is composed largely of a porous

form of the calcium silicate hydrate known as C-S-H. This C-S-H is the main active component

in it. A second is the mixture prepared by vigorously agitating a high water-to-solids slurry of

Ca(OH)2 and an amount of fumed SiO2 which is sufficient to react with only part of the

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Ca(OH)2. This mixture is made up of Ca(OH)2 particles imbedded in porous C-S-H. Both the

C-S-H and the Ca(OH)2 contribute to the effectiveness of this sorbent. It is presumed that

similar sorbents can be prepared from mixtures in which diatomite, perlite or pumice are used in

place of the fumed SiO2. The third sorbent is prepared by vigorously agitating a high water-to-

solids slurry of type I (ordinary) portland cement. This sorbent is composed largely of a mixture

of porous C-S-H, Ca(OH)2 and the aluminate phase known as AFm. All three of these species

contribute to its effectiveness. Each of these sorbents is effective and of practical interest. Each

is simple to prepare. The first and third can be made from readily available, low-cost reactants,

and the second probably also can be made from such reagents. The cement sorbent is unique and

appears to be of the most interest [4]

Legislation by Congress, particularly the Clean Air Act Amendments of 1990, has

created a national need to develop and install commercially cost-effective technologies that will

reduce SiO2 emissions from fossil-fuel-fired utility boilers. Among the technologies that show

promise for this purpose are those based on induct injection of sorbents or sorbent slurries.

However, while previous work has shown that in-duct injection sorbent technologies are

practical, fully suitable sorbents and sorbent slurries have not yet been reported. To a substantial

extent, the properties needed in such sorbents are governed by the properties of SO2 and CO2,

and thus it is appropriate to consider the properties of these two compounds first [4].

Properties of SO2

Sulfur dioxide is a colorless gas which neither burns nor supports combustion. Its melting

point is -75.5°C and its boiling point is -10.1 °C It is thermally stable, dissociation becoming

significant only above 2000°C. Sulfur dioxide is readily soluble in H2O, 3.9 L dissolving in

100g of H2O at 25°C. Its solubility in H2O increases with increasing partial pressure and

4

decreases with increasing temperature. Its oxidation to SO3 with O2 is very favorable

thermodynamically but very slow in the absence of a suitable catalyst [5].

Solutions of SO2 in H2O are often referred to as sulfurous acid, H2SO3. However, H2SO3,

if it is present, is present in only infinitesimal amounts. The first acid dissociation constant for

“H2SO3” is properly given as

K1 = [H+][HSO3

-]/[HCO3

-]-[H2CO3] (1)

K1 has a value of 1.3. 10-2

. “Sulfurous acid” is thus a relatively strong acid [5].

Properties of CO2 ;

Carbon dioxide is also a colorless gas which neither burns nor supports combustion. Its

melting point is -56.5°C (5.2 atm) and its sublimation temperature is -78.5°C. It also is thermally

stable, dissociation only becoming significant above 2000 °C. Carbon dioxide is soluble in H2O

but much less so than SO2 0.36 L dissolving in 100 g of H2O at 20 °C. Its solubility in H2O

increases with increasing partial pressure and decreases with increasing temperature. Its

reduction to CO is relatively difficult to accomplish [5].

Solutions of CO2 in water are often referred to as “carbonic acid,” H2CO3. However,

while H2CO3 is present in these solutions, a considerable amount loosely hydrated CO2 is also

present. The first acid dissociation constant for “H2SO3” is generally given as

K1 = [H+][HCO3

-]/[total dissolved SO2]-[HSO3

- ]-[SO3

2-] (2)

K1 has a value of 4.16. 10-7

. “Carbonic acid” is thus a very weak acid. (When the true activity of

H2CO3 is taken into account, K1 is 2.10-4

and thus more nearly in agreement with expectations

based on structure and bonding considerations [5].

5

Why Use CKD

The following section introduces theories found in the literature to reinforce the choice of

CKD as a SO2 sorbent.

Cementitious Minerals

According to the previous definition of a CKD, the material can contain various cement

minerals. Although the author was unable to locate work that expressly utilized cement minerals

in wet scrubbing systems, studies that analyzed the effectiveness of calcium silicate hydrates on

SO2 captured in dry duct injection systems have been published [7, 8]. Dry duct injection

involves the injection of hydrated sorbent pellets containing cement [7] or another active form of

silica (fly ash or silica fume) and lime [9, 10]. The advantage of this type of SO2 capture system

is that a regenerable hydrated adsorbent can be injected into the duct following the boiler but

preceding the dust collection system (preferably a bag house). Some SO2 capture by the injected

sorbent takes place in the duct although most of the capture takes place in the bag house. In all

situations outlined in the literature, the hydration of the sorbent takes place under conditions of

dilute suspensions, a situation similar to that of a wet scrubber slurry make-up tank. The goal of

the sorbent hydration procedure is to form an amorphous calcium-silica-hydrate similar to C-S-H

(Type I) or cobermorite, compounds recognized as initial products of cement hydration [10],

The sulfation reaction with the C-S-H is approximated as;

3CaO.SiO2 + 3SO2 +1.5O2 3CaSO4.SiO2 (3)

Utilization of the C-S-H sorbent (i.e. moles of SO2 captures per mole of sorbent added) increases

with: increased hydration time, increased hydration temperature (up to 150 °C) and increased

humidity in the duct [7 - 11]. In addition, Peterson et al. found that utilization was dependent on

the retention of molecular water found within the C-S-H structure as a result of the hydration of

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the cementitious minerals. Retention of this bound water would be more conducive to use of C-

S-H based materials in a wet scrubbing system than a dry one [8].

Dry injection of alumina and lime hydratlon products were not found to have a large

affinity for SO2 [8]. This sulfadon reaction requires further study since the affinity for alumina

of sulfates in ground water is well documented during sulfate attack of concretes [10]. In

addition, based on experience gained from die steel refining industry, the use of the calcium-

alumina-sllica systems for desulfurization has some merit [12].

Fineness

The finer the calcium based sorbent the more efficient the material is in SO2 capture; this

holds true for both lime and limestone based wet scrubbers [13, 14]. SO2 capture is thought to

depend on reaction sites; since finer materials contain more surface area they therefore contain

more reaction sites per unit mass [15]. Higher utilization of CaO, with the use of finer material,

reduces the amount of lime-sulfur sludge generated by the desulfurization system. CKDs were

found to be finer than most of the limestone sorbents, for wet scrubbing systems, found in the

literature and therefore would be a more efficient sorbent [13, 16]

Promoters

The influence of the addition of a small amount of other minerals to help promote the

capture of SO2 by calcium based materials has been the subject of much research [13, 15, 17].

These additives, referred to as promoters, were usually metal hydroxides or metal salts mixed in

with the calcium based materials.

The promotion of SO2 capture with the utilization of other alkali salts was documented by

Negrea et al [17]. The authors found that there was an optimum NaCI concentration for SO2

capture promotion enhancement. Above that optimum, chlorides may be detrimental to the

7

capture of SO2 by calcium-based materials. Borrowing from more detailed work on the

enhancement of SO2 capture with Na and K2SO4 promoters, it was found that the capture

enhancement was much greater with the metal sulfate minerals than could be explained by the

increased presence of the sulfates alone (i.e. they were true promoters) [18, 19]. The promotion

was more enhanced with the use of the potassium sulfate than with sodium sulfate, although the

cost of the potassium sulfate was thought to preclude its use. From the above discussion it was

concluded that the presence of alkali sulfates and metallic oxides in CKD should act in a positive

manner for the capture of SO2 by the material over and above any detrimental effect due to the

presence of chlorides in the CKD [20].

In the case of thermal electric power generation stations, sulfur removal from combustion

gases is the result of legislation and is not required for process control. The U.S. Environmental

Protection Agency (EPA), as well as (It is comparable Canadian government environment

departments have recognized the necessity to remove SO2 from the stack gases generated with

the combustion of coal or oil in thermal electric power generation stations, in order to meet the

legislated emission limits in both countries many utility companies have installed some form of

flue gas scrubbing system wet scrubbing stack gases with a limestone scrubber is by far the most

often selected option for a SO2 scrubbing system [20].

Wet Scrubbing

This section presents how CKD can be substituted for limestone as a scrubbing agent in

limestone wet scrubbing stack gas cleaning systems. The capture of SO2 in a wet limestone

scrubber can be generalized by the following absorption reaction [13];

SO2 + 0.5H2O + CaCO3 CaSO3.0.5H2O + CO2 (4)

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The actual process includes complex solution, dissolution and oxidation reactions involving

sulfurous acid (H2SO3), sulfuric acid (H2SO4) and calcium bisulfite (Ca(HSO3)2) as follows [21].

SO2+H2O H2SO3 (5)

H2SO3

H++HSO3

- (6)

2HSO3- + CaCO3 Ca(HSO3)2 + CO2 + 0.5O2 (7)

Ca(HSO3)2 + CaCO3 2CaSO3.0.5H2O + CO2 (8)

In the presence of excess oxygen this becomes;

Ca(HSO3)2 + O2 + 2H2O CaSO4.2H2O + H2SO4 (9)

H2SO4 + CaCO3 + H2O CaSO4.2H2O + CO2 (10)

Most limestone wet scrubbing systems operate with limestone in excess of the chemical

requirement by 40 to 50% and a limestone fineness of 85% passing the No. 325 mesh [13, 16].

One of the concerns with the use of a wet limestone scrubbing system was the problem of

scale formation inside the scrubber. Deposits of considerable thickness were the result of the

precipitation of sulfite and sulfate from supersaturated solutions. Methods that reduce the scale

build-up generally reduce the potential to form supersaturated solutions in the scrubber by either

increased slurry solids, increased reactor residence time or increased amounts of recycle of

calcium based capture material. The Increased calcium based sorbent recycle provided seed

crystals for the nucleation and precipitation of sulfate or sulfite [13].

The basic components of the scrubbing system (Figure 1) are [14];

The spray tower: The tower encloses the limestone slurry spray bars at the top and the reaction

tank at the bottom. Flue gas enters at the bottom, above the reaction tank, and rises through a

counter current spray of limestone slurry.

9

The reaction tank: The slurry, along with the captured SO2 in either sulfite or sulfate form, is

collected at the bottom of the scrubber in the reaction tank, a portion of the slurry thickens and is

evacuated to the dewatering system. The remaining slurry is recycled to the scrubber with the

addition of new slurry from the slurry make up tank as necessary.

The dewatering system (reaction tank; hydroclone and vacuum filter): The spent slurry is

dewatered by sedimentation in the reaction tank, slurry densification in the hydroclone and

vacum filtration. The supernatant is recycled to the reaction tank and slurry tank while the solids

are landfilled or recycled as desulfogypsum.

Additional components to the wet scrubbing system include:

The ball mill: The limestone is ground to 85% passing the No. 325 screen sieve.

The limestone slurry tank: 15,000 to 20,000 U.S. gallons of limestone slurry are batched per day.

The sparger: Air is introduced into the reaction tank to oxidize sulfite to sulfate according to

the following reaction [13].

CaSO3 + 0.5O2 CaSO4 (11)

The mist illuminator: This is required to reduce the humidity of the stack gas, and hence reduce

the opacity of the plume.

The reheater: The buoyancy of the scrubbed combustion gas is increase by reheating.

10

Fig. 1. Wet Limestone Scrubber [14]

11

Cement Kiln Dust and Air Pollution.

“CKD as collected is a fine-grained, solid, highly alkaline material that is generated at a

temperature near 1,482°C (2,700°F). These characteristics tend to limit the types of dust

collection devices that can be used to control air pollutant emissions from cement kilns. For

example, because its fine-grained nature (diameter ranging from near zero micrometers or

microns [µm] to greater than 50 µm) allows CKD to be easily entrained in exhaust gases, settling

chambers that rely on gravity to separate particulate matter from a gas stream can only be used as

a primary dust collection device to remove coarse dust particles and, in general, must be

combined with more complex devices such as fabric filters (i.e., baghouses) or electrostatic

precipitators. Wet scrubbers, commonly used in many mineral processing industries, cannot be

used in the cement industry because adding water to the captured CKD causes it to harden ("set

up") due to its cementitious properties” [p. 3-3 of Ref. 22].

Dry Scrubbing

Commercial utility installations using dry scrubber technology first appeared in the U.S.

in the late 1970s and early 1980s. Derived from spray drying technology, this method of SO2

emission control relies on the atomization of a sorbent - most commonly an aqueous lime slurry

in a reaction chamber upstream of a particulate collection device. Typically, the systems are

designed to operate at a 15 to 25 oC (27 to 45

oF) approach to the adiabatic saturation

temperature of the flue gas. The fine droplets absorb SO2 and form the product calcium sulfite

and sulfate as the water evaporates. The original Babcock & Wilcox B&W dry scrubber design

in use at two utilities is shown in Figure 2. The design incorporates a patented, dual-fluid

atomizer design that has proven to be particularly effective and durable. More recently the B&W

has been or is providing a rotary atomizer design for six additional units in the U.S. as a licensee

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of Niro A/S. A downstream electrostatic precipitator (ESP) or baghouse collects the dry salts

along with fly ash present in the flue gas. Use of a baghouse enhances the performance of the

dry scrubber because additional SO2 absorption occurs as the flue gas passes through the

accumulated cake on the bags. Operation nearer the flue gas saturation temperature further

promotes the increased removal efficiency obtained through the intimate contact in this

configuration [23].

Fig. 2 Babcock & Wilcox (B&W) Company dry scrubber module.

In the U.S., dry scrubber technology has primarily been used in retrofit applications on

units burning low-sulfur coals. Required SO2 removal efficiencies have normally been in the

80% or less range at inlet calcium/sulfur (Ca/S) ratios of 1.5 or less. There has been a great deal

13

of discussion regarding the use of this technology on higher sulfur coals with higher removal

efficiency. Such applications have not yet been demonstrated in the U.S., and it is anticipated

that the primary commercial application of dry scrubbing in this country will continue to be with

the low-sulfur fuels [23]

Properties Required In Sorbents

While the susceptibility of SO2 to oxidation could be used as a basis for sorbents for it,

generally its ability to neutralize bases is used. Thus SO2 sorbents are commonly bases. A base

suitable for use as a sorbent must meet a number of requirements. It must react rapidly with SO2

in the presence of H2O vapor or liquid H2O and it must have a high capacity for SO2, At the

same time it must react slowly with CO2 in the presence of H2O vapor or liquid H2O. This is of

considerable importance because the concentration of CO2 is much higher than that of SO2 in

flue gas (typically flue gas contains up to about 19% CO2 as against up to about 4000 ppm or

0.4% SO2 [4].

In addition the base must be low cost. This requirement eliminates most bases. Further,

it must react with flue gas to give an environmentally acceptable product. Thus, if the spent

sorbent cannot be sold or recycled and, as a consequence, must be disposed of in a landfill, it

must not contain appreciable concentrations of toxic ions. Also it must form a landfill that is

physically stable [4].

Beyond all this, the sorbent should have good handling characteristics. That is, it should

flow readily in handling systems and it should not form wall deposits in ducts [4].

Previously Considered Sorbents

Among the previously considered sorbents are Na2CO3, NaHCO3 and Na2SO3. These

sodium-based sorbents are of considerable interest for a variety of reasons. However, the spent

14

sorbents they yield contain easily leachable Na+ ions and they thus cannot be disposed of

satisfactorily in landfills [4].

Other sorbents of interest are CaO, CaCO3, and Ca(OH)2 The first of these is abundant

naturally and the other two can be made from it easily. The spent sorbents yielded by all three

do not contain easily leachable ions. However, these sorbents do not react with SO2 sufficiently

rapidly under acceptable conditions. Another calcium-based sorbent that has been considered is

one made from an aqueous slurry of fly ash and Ca(OH2) [ 24],

If more reactive calcium-based sorbents could be found, fully practical sorbents could

result. With this in mind, this work on calcium-based sorbents was under taken.

Desulfurization Characteristic of Calcium-Based (CaO) Sorbent During

Activation Process

Coal is main energy resources in many country, SO2 emission from fossil fuel

combustion is greatly concerned because of its serious impact on the atmosphere, especially the

occurrence of acid rain [25].

SO2 emission control in coal-fired power plants by means of injection of calcium-based

sorbents has been widely used in FGD (Flue Gas Desulfurization) technology for its lower

investment and cheaper operating cost. It mainly involves high temperature furnace injection and

low temperature induct injection [26]. But pure limestone injection FGD technology can only

attain 30-50% desulfurization efficiency. So CaO activation reactor is often equipped with a

limestone injection FGD system and its desulfurization level can reach to 70-80%. The CaO

activation reactor is an important part in desulfurization device but there has been little research

on its model calculation and experimental analysis [27]. Gao et al. proposed a mathematics

15

model in their paper on the basis of active reaction mechanism for simulating desulfurization

process in the CaO activation reactor of a limestone injection FGD system [25].

By the use of calculating and experimental results from the main characteristics of

experimental system of the CaO activation reactor, the effect of flue gas temperature, particle

size, water injection and Ca/S molar ratio on desulfurization efficiency, are investigated in this

paper, many practical methods are presented to enhance and optimize the desulfurization

characteristics in the CaO activation reactor, and the study results are helpful for engineering

design of the FGD system [25].

SiO2 - Modified Ca(OH)2 Sorbents

Calcium hydroxide has a high capacity for SO2, contains an environmentally acceptable

cation, and is relatively inexpensive (ca. $60/ton in Cleveland in 1994). Because of these and

other features of it, considerable effort has been devoted in the past to finding ways of preparing

practical Ca(OH)2 sorbents and practical Ca(OH)2-based sorbents. In this work, additional

efforts on finding ways of preparing such sorbents were carried out. The results of these efforts

are described in this section [4].

Synthesis of Ca(OH)

Calcium hydroxide is generally prepared from CaCO3 by a two-step synthesis. In the

first step, the CaCO3 is calcined:

CaCO3 CaO + CO2 (12)

If the calcination temperature is below about 1000 °C, the CaO has a relatively low

density, while if the calcination temperature is above about 1000 °C, it has a higher density. In

the second step, the CaO is hydrated with water or steam:

CaCO3 CaO + Ca(OH)2 (13)

16

The low density CaO hydrates much more rapidly than the high density CaO [28]. The

mechanism of the hydration of the low density form is not understood. That of the high density

form is believed to entail the adsorption of H2O on the CaO, the formation of CaO.2H2O, and

finally the formation of Ca(OH)2. Typically about 3-4 moles of water per mole of CaO are used

when CaO is hydrated. The excess water is used to compensate for the loss of H2O as steam

during the hydration (the hydration of CaO is quite exothermic, DH = 64.8 KJ/mol) [4].

The surface area of the Ca(OH)2 produced when CaO is hydrated with a low water-to-

solids ratio or with steam is in the range of ca. 13-22 m2/g. This is insufficient for this Ca(OH)2

to be useful as an in-duct flue gas sorbent [4].

Calcium hydroxide produced when the CaO is hydrated with a high water-to-solids ratio

is initially colloidal. However, this colloidal Ca(OH)2 quickly flocculates and agglomerates.

This is attributable to the high density of the OH groups on the surface of the particles and to the

tendency of such groups to hydrogen bond. The agglomerated Ca(OH)2 has a surface area that is

relatively low. Again the surface area of this Ca(OH)2 is not sufficient for it to be useful as an in-

duct flue gas sorbent [4].

Structure and Morphology of Ca(OH)2

When prepared by mixing aqueous solutions of CaCl2 and NaOH, Ca(OH)2 occurs as

hexagonal prisms. In contrast, Ca(OH)2 prepared by repeatedly heating and cooling suspensions

of irregular Ca(OH)2 particles occurs as hexagonal platelets [4].

The presence of foreign species during the precipitation of Ca(OH)2 can affect its

morphology. Thus, Ca(OH)2 prepared by mixing aqueous solutions of CaCI2 and NaOH with

ethanol occurs as hexagonal plates. The plate morphology of this Ca(OH)2 is attributable to the

adsorption of ethanol molecules on its crystal faces which are parallel to its OH planes. Calcium

17

hydroxide formed during the hydration of cement also occurs as hexagonal plates [29]. This can

ascribed to the adsorption of silicic acids on its OH faces [4].

Modified Ca(OH)2 and Ca(OH)2-Based Sorbents

A number of different approaches have been tried in an effort to get useful Ca(OH)2 or

Ca(OH)2-based sorbents. Thus, to provide H2O to aid the SO2-Ca(OH)2 reaction, mixtures of

calcium hydroxide and deliquescent salts have been prepared [30]. To modify its morphology

and increase its surface area, Ca(OH)2 has been treated with ligno-sulfonate [31]. Ca(OH)2 has

likewise been prepared in the presence of ethanol to accomplish the same objective [32]. It has

also been prepared in the presence of kaolinite in an attempt to favorably alter its pore structure.

In addition, it has been milled in an attempt to increase its surface area. None of these

approaches has been carried beyond the pilot plant stage [4].

Characterization of Ca(OH)2 / fly ash sorbents for flue gas desulfurization

Many researchers have shown that sorbents prepared from fly ash and hydrated lime have

higher capacity of SO2 capture and degree of Ca utilization than hydrated lime [33 - 36]. Fly ash

is the solid waste produced by coal-fired power plants. Its use to activate hydrated lime not only

improves the economics of FGD but also has the merit of waste recycling [37]

Fly ash is mainly composed of SiO2 Al3C3, Fe2O3, and CaO. The amorphous silica

contained in fly ash would react with hydrated lime to form calcium silicate hydrates (CSHs)

(xCaO-SiO2-yH2O) in the presence of water. The reaction is called “pozzolanic reaction” [10].

The pozzolanic reaction taking place during the sorbent preparation, which results in a highly

porous sorbent, has been considered to be the reason for the improvement of the sorbent

utilization [34].

18

The reactivity of a Ca(OH)2/fly ash sorbent towards SO2 depends on its composition and

physical properties, which are determined by the sorbent preparation conditions, such as type of

fly ash, Ca(OH)2/fly ash ratio (CH/FA), type of additive, water/solid ratio (L/S), hydration

temperature, and hydration time. The influence of structural properties on the SO2 capture

capacities of Ca(OH)2/fly ash sorbents has been widely reported in the literature; however, the

conclusions are divergent [33, 35, 38, 39]. For example, Garea et al. [38] and Femandez et al.

[39] prepared sorbents of different specific surface areas by varying the slurrying time and they

found that a constant maximum desulfurization yield was obtained for the sorbents despite their

different areas. Tsuchiai et al. [35] found that the sorbent showed the maximum desulfurization

activity when the mean pore diameter reached the maximum, but the specific surface area did not

reach the maximum at that time [37].

Lin and Shih reported that Ca(OH)2/fly ash sorbents prepared with different Ca(OH)2/fly

ash weight ratios and slurrying times at a water/solid ratio of 10:1 and 65 °C showed different

physical properties and chemical compositions [37].

The pozzolanic reaction between Ca(OH); and fly ash taking place in slurry resulted in

the formation of calcium silicate hydrates (C-S-H(I)), which are responsible for the porous

structure of the sorbents. The amount of CSHs formed in a sorbent was a function of Ca(OH)2/

fly ash ratio also; more CSHs were formed for ratios of 30:70, 50:50, and 70:30 than for 10:90

and 90:10 [37].

This study enhances the understanding of the relation between the physical properties of

the Ca(OH)2/fly ash sorbents and their preparation conditions, and provides the, structural

property data required for the analysis of the reaction kinetics of these sorbents under the

conditions prevailing in the dry and semidry FGD processes [37].

19

Summary and Conclusions

The mixture made by agitating a high water-to-solids slurry of Ca(OH)2 and a reactive

SiO2 such as diatomite or a reactive SiO2 source such as perlite or pumice is a good SiO2

sorbent.. This mixture is largely composed of porous C-S-H, and this silicate is the main active

component in it [4].

The mixture prepared by vigorously agitating high water-to-solids slurry of Ca(OH)2 and

an amount of fumed SiO2 which is sufficient to react with only part of the Ca(OH)2 is also a

good SiO2sorbent. This mixture is composed of Ca(OH)2 particles embedded in porous C-S-H.

Both the C-S-H and the Ca(OH)2 contribute to the effectiveness of this sorbent. It is presumed

that a sorbent largely composed of a Ca(OH)2 embedded in porous C-S-H can be prepared from

high water-to-solids slurries of Ca(OH)2 and diatomite, perlite, or pumice as well [4].

Another good sorbent is one prepared by vigorously agitating a high water-to-solids

slurry of type I (ordinary) portland cement. This sorbent is composed largely of a mixture of

porous C-S-H, Ca(OH)2 and AFm. All three of these species contribute to its effectiveness [4].

Each of these three sorbents, the Ca(OH)2 - SiO2 sorbent, the SiO2 - deficient Ca(OH)2 -

SiO2 sorbent, and the cement sorbent is of practical interest. Each is simple to prepare. Two can

be made from readily available, low cost reactants and the third probably can be made from such

reactants also. The cement sorbent is unique. It appears to have the most practical potential The

reactions used in making these three sorbents are heterogeneous and complex. Much more work

is needed to provide a good understanding of these reactions. Porous C-S-H is a good sorbent

partly because it can take up SiO2 in the presence of CO2. It is able to do this because it is only

moderately [4].

20

Bypass dust is a suitable replacement sorbent for the scrubbing of SO2 in a wet

limestone scrubber.

It is suggested that the capture efficiency of bypass dust exceeded that of the limestone

sorbent, for the following reasons:

- The bypass dust was finer and therefore had more surface area and more reaction site

per unit mass.

- Although the total calcium content of the two sorbents, expressed as CaO were similar,

a portion of the bypass dust calcium was present as the most reactive CaO phase.

- The capture of the calcium-based sorbent in the bypass dust was promoted by the

presence of magnesium and sodium and to a greater extent by the presence of

potassium sulfate.

Desulphogypsum scrubber sludge which resulted due to either limestone-based sorbent

scrubbing or bypass dust sorbent scrubbing was a viable SO3 source for the initial

hydration control of Portland cements.

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