Studies of the Major Factors Affecting Magnesium Limestone Dissolution

5 Studies of the Major Factors Affecting Magnesium Limestone Dissolution FRANK B. MESEROLE and ROBERT L . GLOVER Radian Corporation, Austin, TX 78759

Downloaded by CORNELL UNIV on August 3, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch005

DOROTHY A. STEWART Electric Power Research Institute, Palo Alto, CA 94394

This paper summarizes the results from a laboratory study funded by the Electric Power Research Institute (EPRI) to examine the factors which affect limestone reactivity and the availability of magnesium in limestone. The use of limestone in calcium-based SO wet scrubbers is well demonstrated technology. However, guidelines to assess the acceptability of a given limestone for a specific scrubbing system are incomplete. Size, calcium concentration, and inert content are generally the only criteria included in the limestone specifications. Experience as well as theoretical considerations indicate that these properties are insufficient to uniquely gauge the reactivity of the limestone. The chemical composition, the crystal lattice imperfections, and the composition of the specific scrubber solution may also affect the dissolution rate and hence the acceptability of the limestone. The availability of the magnesium in limestone to dissolve is also important since the presence of magnesium is beneficial to scrubber efficiency. However, not a l l the magnesium in limestones will dissolve under scrubber conditions. Therefore, this study was funded to examine key variables including temperature, particle size, pH, stir rate, and the magnesium level in the dissolving solution. 2

The use of limestone in calcium-based SO wet scrubbers is economically preferable at large facilities such as electrical utilities' coal-fired generating plants because of the increasing cost differential between limestone and lime. This differential is caused by the continually rising fuel costs incurred in the calcination of limestone to lime. However, the economic advantage of limestone is diminished if its utilization in a particular process is low. Low utilization results in higher reagent costs as well as increased disposal burden. This paper presents a laboratory apparatus and test procedure to measure the reactivity of different limestones at a variety of simulated scrubber operating conditions. The results from this test can then be used to choose the most appropriate limestone. 2

0097-6156/82/0188-0099$6.00/0 © 1982 American Chemical Society Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

100

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The limestone r e a c t i v i t y t e s t procedure c a l l s f o r the samples to be ground and screened to produce m a t e r i a l i n narrow p a r t i c l e s i z e ranges f o r t e s t i n g . S o l u t i o n composition and pH are maintained constant throughout the measurement p e r i o d (up to 20 hours) at p r e s e l e c t e d v a l u e s . The s e l e c t i o n of pH and s o l u t i o n compositions i s based upon s e v e r a l design c r i t e r i a i n cluding : •

d e s i r e d operating pH,

•

forced versus n a t u r a l o x i d a t i o n ,

•

q u a l i t y of makeup water,

•

degree of s o l i d s dewatering,

•

scrubber design.

and

Using an SO2 wet scrubber computer model, the above information can be used to p r e d i c t the s o l u t i o n composition i n a system at steady-state c o n d i t i o n s . Further d e s c r i p t i o n s of the t e s t equipment and procedure as w e l l as recent experimental data on a v a r i e t y of limestones are presented i n the f o l l o w i n g s e c t i o n s . Discussion The d i s s o l u t i o n of limestone i s known to be c o n t r o l l e d by both d i f f u s i o n of ions i n s o l u t i o n and surface r e a c t i o n r a t e s . The pH value i n f l u e n c e s which of these steps dominates i n the limestone d i s s o l u t i o n . For example, at pH values l e s s than 5 the d i f f u s i o n process dominates the d i s s o l u t i o n with l i t t l e dependence on surface r e a c t i o n . On the other hand, at pH values greater than 7 the r e a c t i o n at the limestone surface begins to dominate the d i s s o l u t i o n process. In the pH range between 5 and 7 both d i s s o l u t i o n steps can i n f l u e n c e the o v e r a l l r a t e . The bulk of the limestone d i s s o l u t i o n i n most SO2 scrubbers occurs i n the 5-6 pH range. Thus, both s o l u t i o n mass t r a n s f e r p r o p e r t i e s and the nature of the limestone must be considered at t y p i c a l operating c o n d i t i o n s . Therefore, the pH, s o l u t i o n composition, s o l u t i o n b u f f e r c a p a c i t y , and the nature of the limestone are important c o n s i d e r a t i o n s when designing f o r maximum limestone u t i l i z a t i o n . This paper deals p r i m a r i l y with the measurement of the i n f l u e n c e of limestone p r o p e r t i e s on the overall dissolution rate. The chemical r e a c t i o n s i n v o l v i n g calcium carbonate at the surface of the s o l i d s i n the scrubber pH operating range are: CaC0

3

+ H0 2

$ Ca

+

HCO3

+

OH"

Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.

(1)


5.

MESEROLE

ETAL.

101

Magnesium Limestone Dissolution

CaC0

3

+ H+ $ Ca++ + HCOI

(2)

CaC0

+ H2CO3 X Ca "* + 2HCO3

(3)

3

4

Reaction (2) i s thought to have a more r a p i d r a t e than r e a c t i o n s (1) and (3). This i s s u b s t a n t i a t e d by experimental data that show the l o g o f the d i s s o l u t i o n r a t e to be i n v e r s e l y p r o p o r t i o n a l to the pH i n the range of 2 to 5 (1). However, at pH values above 5, the hydrogen i o n c o n c e n t r a t i o n decreases s i g n i f i c a n t l y and thus t h i s step becomes l e s s s i g n i f i c a n t . This i s also the case f o r the r e a c t i o n shown i n equation (3). Therefore, equation (1) represents the predominate d i s s o l u t i o n mechanism at high pH. Since water i s the reactant i n equation (1), the d i f f u s i o n should be important only i n removing the products from the r e a c t i o n zone. The measurements presented i n the Experimental Results Section were designed to determine the r e l a t i v e e f f e c t s of the surface r e a c t i o n r a t e and the d i f f u s i o n o f products on the o v e r a l l d i s s o l u t i o n r a t e . The v a r i a b l e s i n these t e s t s were temperature, pH, s t i r r i n g r a t e , and type of limestone. The equation used f o r c o r r e l a t i n g the data i s : η D.R. = k-SA-(RS-l)

(4)

where, D.R. = d i s s o l u t i o n r a t e , k = r e a c t i o n r a t e constant, constant with and limestone type,

temperature

SA = limestone surface area, RS = degree o f s u b s a t u r a t i o n of calcium carbonate i n the boundary l a y e r , and η = exponent u s u a l l y equal to 1. The d i f f i c u l t y with applying equation (4) i s the u n c e r t a i n t y as to whether the RS value measured i n the bulk l i q u o r (which can be measured) i s the same as the RS i n the boundary l a y e r surrounding the limestone p a r t i c l e s (which cannot be measured). Test c o n d i t i o n s employing high s t i r r i n g rates and r e l a t i v e l a r g e p a r t i c l e s (-100 ym) have been chosen to minimize the d i f f e r e n c e s i n these two RS v a l u e s .


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Experimental Approach The experimental apparatus shown i n Figure 1 was used to evaluate limestone r e a c t i v i t y . Conditions i n the r e a c t o r approximate c o n d i t i o n s i n the r e a c t i o n tank of a limestone scrubbing system. As the limestone d i s s o l v e s , the pH i n the r e a c t o r w i l l begin to r i s e . Therefore, to insure a constant pH i n the experimental r e a c t o r , a low pH t e s t s o l u t i o n (see Table I) i s metered to the r e a c t o r . As the low pH feed i s added, an equal amount of r e a c t o r l i q u o r i s a l s o withdrawn to maintain the r e a c t o r volume a l s o remains constant (2.5L). In an a c t u a l scrubbing system, S0 absorbed from the f l u e gas w i l l provide enough a c i d i t y such that the pH remains approximately constant. Downloaded by CORNELL UNIV on August 3, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch005

2

TABLE I.

SCRUBBER FEED COMPOSITION (mmol/L) Normal Conditions

High Magnesium Scrubber

NaaSOit

24

24

HC1

25

25

0

87

MgCl pH

2

2.0

2.0

P r i o r to t e s t i n g , 3.0-3.5L of scrubber feed s o l u t i o n are saturated overnight with approximately 10 g of the limestone of interest. S a t u r a t i o n i s accomplished at the temperature at which the run i s to be performed. Ten grams of the limestone are ground and s i z e d to the d e s i r e d mesh. P i c t u r e s were taken using an o p t i c a l microscope to document the s i z e d i s t r i b u t i o n . The next day the f i l t e r e d s o l u t i o n i s then used as the i n i t i a l charge to the r e a c t o r . In t h i s way, the i n i t i a l r e a c t o r composition w i l l be approximately the same i n the beginning as during the run. The pH c o n t r o l l e r i s then set to the d e s i r e d value. The pH c o n t r o l l e r i s used to meter scrubber feed l i q u o r (pH 2) i n t o the r e a c t o r to maintain a constant r e a c t o r pH. After the s t i r r i n g r a t e has been set and the d e s i r e d temperature reached, 10.00 g of s i z e d limestone are added to the r e a c t o r . As the run proceeds, the accumulated volume l e a v i n g the r e a c t o r i s recorded. Samples of the r e a c t o r l i q u o r are a l s o taken p e r i o d i c a l l y and analyzed f o r calcium and magnesium by atomic a b s o r p t i o n . By maintaining a constant r e a c t o r pH, the composition remains f a i r l y constant throughout the run. Based upon the flow r a t e data and the s o l u t i o n composition, the f r a c t i o n of the stone d i s s o l v e d and d i s s o l u t i o n r a t e can then be c a l c u l a t e d .



5.

MESEROLE

ET AL.


103

Variable Speed Stirrer

o

Graduated Feed Tank

-

n

• β

Reactor

1 2"

Filter

Metering Pump Thermostat Figure 1.

Limestone reactivity apparatus.


Outlet Sample Container

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DESULFURIZATION


Experimental Results F i f t e e n runs using three limestones are reported f o r v a r i o u s reactor c o n d i t i o n s . A base case experiment was performed f o r each of three limestones: 1) Fredonia, 2) B r a s s f i e l d , and 3) P f i z e r (see Tables I I and I I I ) . Operating c o n d i t i o n s were then v a r i e d to show the e f f e c t of s t i r r a t e , temperature, and pH as w e l l as r e a c t o r feed composition. Each stone was ground and s i z e d to 120-200 mesh i n an attempt to minimize d i f f e r e n c e s i n surface area between the samples. Based upon o p t i c a l counts of the three s i z e d stones, each sample used i n the experiments had approximately the same average p a r t i c l e diameter (Fredonia - 134 ym, P f i z e r - 147 ym, B r a s s f i e l d - 133 ym). However, no attempt was made to c o r r e c t for any surface area d i f f e r e n c e s due to d i f f e r e n t limestone porosities. The experimental r e s u l t s are summarized i n Table IV. The run c o n d i t i o n describes the operating v a r i a b l e which was changed from the base l i n e c o n d i t i o n s . For Fredonia, the e f f e c t s of temperature (30°C, 50°C, 60°C), s t i r rate (500 rpm, 1000 rpm, 1500 rpm) and pH (5.0, 5.8) were examined. For B r a s s f i e l d , a high temperature (60°C), low pH (5.0), and high magnesium (87 mmol/L) cases were run i n a d d i t i o n to the base case. Base, low pH (5.0), and high magnesium (87 mmol/L) cases were conducted f o r P f i z e r . TABLE I I .

BASE CASE OPERATING CONDITIONS

S t i r Rate

500 rpm

Temperature

50°C

pH

5.8

P a r t i c l e Size

120-200 mesh

Reactor Feed

Composition

Na2S0i+

24 mmol/L

HC1

25 mmol/L

MgCl

2

0

mmol/L


5.

MESEROLE


TABLE I I I .

105


ET AL.

COMPOSITIONS OF LIMESTONE SAMPLES TESTED Composition MgC0

(wt%)

Limestone

CaC03

Inerts

Fredonia

96.8

1.5

1.7

Brassfield

88.3

9.0

2.7

Pfizer

94.6

4.8

0.6

3

The calcium and magnesium d i s s o l v e d from the limestones are c a l c u l a t e d by m u l t i p l y i n g t h e i r r e s p e c t i v e l i q u i d concentrations (mg/L) by the volume of l i q u o r l e a v i n g the r e a c t o r during some increment of time. By summing over the v a r i o u s time increments, the t o t a l amounts of calcium and magnesium which have d i s s o l v e d can be c a l c u l a t e d . To a i d i n a n a l y z i n g the data and to allow e x t r a p o l a t i o n to greater d i s s o l u t i o n f r a c t i o n s ( t y p i c a l l y only 40-50 percent of the stone d i s s o l v e s i n a 6 hour run), the data are f i t to a power f u n c t i o n of the form: 1 - W/W

ο

b

= at

(5)

where, W

= weight (g) of calcium or magnesium remaining i n l i m e stone at time t ; W

= i n i t i a l weight (g) of calcium or magnesium i n lime stone ;

a, b = constants; and t = time

(min.).

By d i f f e r e n t i a t i n g equation (5), the r a t e of d i s s o l u t i o n at any time ( t ) can be c a l c u l a t e d from: dW = -W

abt

b _ 1

(6)

Using equation ( 6 ) , the d i s s o l u t i o n rates of CaC03 and MgC03 were c a l c u l a t e d when 50 percent of the calcium and magne sium had d i s s o l v e d (see Table I V ) . The values reported i n Table IV represent the r a t e of d i s s o l u t i o n (dW/dt) d i v i d e d by the amount of CaC0 or MgC03 l e f t i n the limestone (50 percent of W ). The Fredonia base case d i s s o l u t i o n r a t e s from three r e p l i c a t e t e s t s were c a l c u l a t e d to be 1.77 χ 10~ , 1.75 χ 10" , and 2.05 χ 10" g/min/g CaC0 (s) at 50 percent CaC0 d i s s o l v e d . 3

0

3

3

3

3

3


Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982. 3

0.63 χ 10~

30°C 3

3.6 χ 101.5 χ Ι Ο "

pH 5.0

High Mg 3

3

3

3

3

3

3

3

3

— 0.996

3

3

0.48 χ Ι Ο "

—

1.4 χ 10-

3

3

0.26 χ l O ^

0.16 χ 10"

0.21 χ Ι Ο "

0.76 χ Ι Ο "

1.4 χ Ι Ο "

3

3

0.87 χ 10-

4.9 χ Ι Ο "

1.7 χ 10-

3

0.85 χ Η Γ

0.78 χ 10-

0.78 χ 10-

0.998

1.000

3

3

1

3

—

0.999

1.000

—

0.998

1.000

1.000

1.000

0.992

1.000

0.972

0.996

0.999

1.000

0.999

MgC0 D i s s o l u t i o n Dissolution Rate at t=t^g / g/min \ Correlation Coefficient U MgC0 (S) ;

3

3

0.997

1.3 χ Ι Ο " 1.6 χ 10-

0.998

4.0 χ Ι Ο "

pH 5.0

High Mg

Base

1.000

3

1.8 χ 10-

60°C

3

1.000

3

1.4 χ 10"

Base

0.999

0.989

3

3

1.000

0.964

3

2.2 χ 10~

0.996

3

3

0.999

3

2.5 χ 10-

8.9 χ 10-

pH 5.0

1.000

3

1500 rpm

3.8 χ Ι Ο "

60°C

0.999

3

1000 rpm

2.0 χ 10~

Base

1.8 χ Ι Ο " 1.8 χ 10"

3

Base

Base

Run Condition

1

th = time required to d i s s o l v e 50% o f the CaC0 th = time required to d i s s o l v e 50% of the MgC0 S t i r Rate = 500 rpm, Τ = 50°C, pH = 5.8, low Mg

2

l

Pfizer

Brassfield

Fredonia

Limestone

SUMMARY OF RESULTS

CaC0 3 D i s s o l u t i o n Dissolution Rate at t=t^g / g/min \ Correlation Coefficient \g CaC0 (S)/

TABLE IV.


5.

MESEROLE

ET

AL.


107

Comparing the CaC03 d i s s o l u t i o n rates f o r the three stones, Fredonia appears to d i s s o l v e only s l i g h t l y f a s t e r than e i t h e r P f i z e r or B r a s s f i e l d with B r a s s f i e l d the slowest of the three. The d i f f e r e n c e s i n the MgC03 d i s s o l u t i o n rates are more s i g n i f i c a n t ; again, Fredonia appears to be the f a s t e s t d i s s o l v i n g . Low pH increased the d i s s o l u t i o n rates of both CaC03 and MgC0 f o r a l l the stones as expected. Figure 2 g r a p h i c a l l y i l l u s t r a t e s the e f f e c t of pH on the CaC0 d i s s o l u t i o n r a t e . Fredonia shows the greatest i n c r e a s e i n d i s s o l u t i o n at the lower pH. However, both B r a s s f i e l d and P f i z e r a l s o show s u b s t a n t i a l increases i n d i s s o l u t i o n r a t e . Higher temperature seemed to i n c r e a s e the d i s s o l u t i o n rates of CaC03 and MgCÛ3 f o r both Fredonia and B r a s s f i e l d . Figure 3 shows the e f f e c t of temperature on the CaCÛ3 and MgC03 d i s s o l u t i o n rates f o r the Fredonia stone. The l i n e a r nature of the graphs suggests an Arrhenius form f o r the temperature dependence. This i n d i c a t e s that a temperature dependent r e a c t i o n ( s ) (such as the surface d i s s o l u t i o n ) may play a major r o l e i n l i m i t i n g the d i s s o l u t i o n of the Fredonia limestone at pH 5.8. Several runs were a l s o made f o r the Fredonia stone at v a r i ous s t i r r i n g r a t e s . As shown i n Table IV and Figure 4, the s t i r r i n g rate did not have a d r a s t i c e f f e c t on e i t h e r the calcium or magnesium d i s s o l u t i o n r a t e s . There was a s l i g h t increase seen i n the CaC03 d i s s o l u t i o n rate at 1000 rpm and 1500 rpm. However, the change i n the CaC03 d i s s o l u t i o n (approximately 30 percent from 500 rpm to 1500 rpm) i s not that s i g n i f i c a n t when compared to other f a c t o r s such as temperature. F i n a l l y , the magnesium l e v e l i n the d i s s o l v i n g s o l u t i o n did not seem to a f f e c t the CaC03 d i s s o l u t i o n rate of e i t h e r B r a s s f i e l d or P f i z e r . 3


3

Conclusions The r e s u l t s reported i n t h i s paper represent only three of the seven limestones which w i l l be tested during t h i s study. However, based upon the r e s u l t s to date the f o l l o w i n g conclusions can be made. 1.

CaC03 and MgC03 d i s s o l u t i o n data at various temperatures suggest an Arrhenius form of the temperature dependence. The c a l c u l a t e d a c t i v a t i o n energy of 11 Kcal/mole i n d i cate that the d i s s o l u t i o n at pH 5.8 i s r e a c t i o n r a t e , not d i f f u s i o n , l i m i t e d .

2.

There does appear to be some d i f f e r e n c e between limestones i n the CaCÛ3 d i s s o l u t i o n r a t e . However, the most s i g n i f i c a n t d i f f e r e n c e between stones i s the r a t e of d i s s o l u t i o n of MgC03.


Ο I

Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982. C a C 0 D i s s o l u t i o n Rate at 5 0 % D i s s o l u t i o n χ 1 0

3

3

g/min g C a C 0 (S) 3

lOCJik I

I

1

1

CJIOJ ι

-vl I

ι

00 I

C D Ο > -· N > I

I

I

L


N O i x v z r a n j i n s H a svo

801

ΗΠΉ

MESEROLE

E T

AL.

109



50-1

iJ

1

I

1

3.0

3.1

3.2

3.3

(60°C)

( ° )

(40°C)

(30°C)

50

C

1

- X 103(°K" ) Τ

Figure 3.

Dissolution rate at pH 5.8 vs. reciprocal temperature—Fredonia lime stone.


FLUE

GAS

DESULFURIZATION


110


5.

MESEROLE

ET AL.


3.

A low pH d i s s o l v i n g environment does increase both the CaCÛ3 and MgCÛ3 d i s s o l u t i o n rates as expected.

4.

High magnesium d i s s o l v i n g s o l u t i o n s d i d not s i g n i f i c a n t l y impede the CaC03 d i s s o l u t i o n rate f o r e i t h e r the B r a s s f i e l d or P f i z e r stones.

5.

S t i r r i n g rate appeared to have l i t t l e e f f e c t on the CaC03 and MgCÛ3 d i s s o l u t i o n rates f o r the Fredonia limestone at pH 5.8.


Literature Cited (1)

Plummer, L. N.; Parkhurst, D. L. "Critical Review of the Kinetics of Calcite Dissolution and Precipitation, Chemical Modeling in Aqueous Systems," Everett A. Jenne, Editor; ACS Symposium Series 93, 1979.

RECEIVED

November 20, 1981.


1

Studies of the Major Factors Affecting Magnesium Limestone Dissolution

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