Fossil Fuels Utilization - American Chemical Society

17 Effect of Deliquescent Salt Additives on the Reaction of SO with Ca(OH) 2

2

Rosa N. Ruiz-Alsop and Gary T. Rochelle

Downloaded by CORNELL UNIV on November 2, 2017 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch017

Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712

The effect of relative humidity, temperature, amount of lime and SO2 concentration on the reaction of SO2 with Ca(OH)2 solids dispersed in a sand reactor was studied. Of these variables, relative humidity showed the greatest impact on Ca(OH)2 reactivity. Small amounts of deliquescent salts (1 to 10 mole%) were added to the Ca(OH)2 by a slurrying and drying process, and their effect on the reaction rate was investigated. The reaction rate was studied at relative humidities of 10% to 74%, with other conditions similar to those encountered in bag filters during flue gas desulfurization by spray drying. The reaction solids were characterized by scanning electron microscopy, powder x-ray diffraction, coulter counter size distribution, BET (N2) surface area, energy dispersive x-ray spectrometry, and differential scanning calorimetry. Most of the deliquescent salts tested increased the reactivity of the Ca(OH)2 towards SO2. The most effective additives were: LiCl, KC1, NaCl, NaBr and NaNO3. Spray drying has become increasingly important in recent years as an a l t e r n a t i v e to wet scrubbing for sulfur dioxide control. In the spray dryer the sulfur-containing flue gas is contacted with a f i n e mist of an aqueous solution or a s l u r r y of an a l k a l i ( t y p i c a l l y Ca(0H)2 or soda ash). The sulfur dioxide is then absorbed in the water droplets and neutralized by the a l k a l i . Simultaneously, the thermal energy of the gas evaporates the water in the droplets to produce a dry powdered product. After leaving the spray dryer the dry products, including the f l y ash, are removed with c o l l e c t i o n equipment such as fabric f i l t e r s or e l e c t r o s t a t i c p r e c i p i t a t o r s . Fabric f i l t e r s are the preferred c o l l e c t i o n equipment as addit i o n a l sulfur dioxide removal takes place in the baghouse (1,2). T y p i c a l l y , under conditions such that 80% of t o t a l i n l e t SO2 is removed, 60 to 70% of the removal takes place in the spray dryer with 10 to 20% additional removal taking place in the bag f i l t e r s

0097-6156/ 86/0319-0208506.00/0 © 1986 American Chemical Society

Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

17.

RUIZ-ALSOP AND ROCHELLE

Effect of Deliquescent Salt Additives

209

(_3-6). Parametric studies in spray dryer p i l o t plants have demon strated that the main v a r i a b l e a f f e c t i n g S02 removal in the bag f i l t e r s , besides the stoichiometric r a t i o of Ca(0H>2 to SO2, is the approach to the adiabatic saturation temperature of the f l u e gases (_3, 5-9). The approach to the adiabatic saturation temperature in turn is correlated with the moisture content of the s o l i d s . Addi tives that w i l l modify the moisture content of the Ca(0H)2 s o l i d s in equilibrium with a gas phase of a given r e l a t i v e humidity would then be expected to change the r e a c t i v i t y of the Ca(0H)2 towards S0 . A few additives have been tested in spray drying systems. Calcium chloride has proven e f f e c t i v e in increasing the r e a c t i v i t y of limestone and Ca(0H)2 towards S02 (10, 11). Adipic acid was also tested (12) with mixed r e s u l t s . Sodium s u l f i t e , sodium hydro xide, Fe-H- compounds, Ethylene-diamine-tetra-acetic-acetic acid and disodium s a l t (EDTA) have been used as additives during simul taneous SO2 and ΝΟχ removal (Niro Process) (13). The emphasis in the Niro process was to improve the removal of N0 . The present work was undertaken to investigate, in a systematic manner, the kind of additives that could be used to improve Ca(0H)2 r e a c t i v i t y towards S02« A small fixed bed reactor was used to simu l a t e the conditions encountered in the bag f i l t e r s during spray drying flue gas d e s u l f u r i z a t i o n . Three d i f f e r e n t kind of additives were t r i e d : buffer acids, organic déliquescents and inorganic déliquescents. The inorganic s a l t s were selected according to t h e i r deliquescent properties, or the lowering of the vapor pressure of water over t h e i r saturated solutions. Of these three types of additives the deliquescent s a l t s were the only substances that increased the Ca(0H)2 r e a c t i v i t y .


2

X

Experimental

Apparatus

The general design of the experimental apparatus is given in Figure 1. A simulated f l u e gas was synthesized by combining nitrogen and sulfur dioxide from gas cylinders. The gas flow rates were measured using rotameters. Water was added by means of a syringe pump and evaporated at 120°C before mixing with the gas stream. The glass reactor (4 cm diameter, 12 cm height) was packed with a mixture of s i l i c a sand and powdered reagent Ca(0H)2 in a weight r a t i o of 40:1. The addition of sand is necessary to avoid channeling caused by Ca(0H)2 agglomeration (10). The 100 mesh s i l i c a sand was obtained from Martin Marietta Aggregates. The reactor was immersed in a water bath that maintained the system temperature within 0.1°C. Tubing upstream from the reactor was heated to prevent the condensation of moisture on the walls. Before going to analysis the gas was cooled and the water then condensed out by cooling water and an ice bath. The gas was analyzed f o r SO2 using a pulsed fluorescent SO2 analyzer (Thermoelectron Corporation model 40) and the concent r a t i o n continuously recorded. The reactor was equipped with a bypass, to allow the bed to be preconditioned and to allow the gas flow to be s t a b i l i z e d at the desired S02 concentration before beginning the experimental run. Before each experimental run the bed was humidified by flushing with pure nitrogen at a r e l a t i v e humidity of about 98% f o r 10 minutes then l a t e r with pure nitrogen at the r e l a t i v e humidity at which the experiment was to be performed for 8



FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

1.

Experimental

Apparatus


17.



211

minutes. This was done to simulate better the moisture conditions encountered in the bag f i l t e r s because the s o l i d s leaving the spray dryer were o r i g i n a l l y s l u r r y droplets. Preparation of the Samples with Additives


An aqueous solution containing the desired additive was prepared. Five ml of t h i s solution were then added to 1 g of Ca(0H>2 and s l u r r i e d . The sample was placed in a oven to dry at 75°C for about 1 4 hours then l a t e r sieved to separate the i n d i v i d u a l Ca(0H>2 p a r t i cles p r i o r to mixing with the s i l i c a sand and being placed in the reactor. Analysis The f r a c t i o n of Ca(0H>2 reacted at any given time can be calculated by integrating the S02 versus time curve obtained by the recorder on the S02 analyzer and doing a mass balance in the reactor. As a backup, the reacted s o l i d s are analyzed for s u l f i t e and hydroxide using acid/base and iodine t i t r a t i o n s . Characterization of the Reactant The experimental work was done using reagant grade Ca(0H)2 as a reactant. Two batches of Ca(0H)2 that w i l l be i d e n t i f i e d as lime 0 and lime A were used. These two batches of Ca(0H>2 d i f f e r s l i g h t l y in p a r t i c l e s i z e and BET surface area as shown in Table I. The use of these two d i f f e r e n t batches of Ca(0H>2 was not by choice, but rather forced by the depletion of the f i r s t batch of Ca(0H)2 (type 0). The p a r t i c l e s i z e d i s t r i b u t i o n of the Ca(0H>2 was determined by means of a Coulter Counter, model ΤΑΠ using as e l e c t r o l y t e a solution of 4 wt% CaCl2, saturated with Ca(0H>2. The surface area was determined using Brunauer, Emmett, and T e l l e r (BET) nitrogen absorption i s o therms. The BET surface area of a sample of 1 g of Ca(0H)2 A, after s l u r r y i n g with 5 ml of d i s t i l l e d water and drying at 75°C is also showed in Table I. The s l u r r y i n g and drying process caused a s l i g h t decrease in the surface area of Ca(0H)2. Figures 2 and 3 show Scanning Electron Micrographs of the two types of Ca(0H>2 used as the reactant. From these pictures it can be seen that the Ca(0H>2 p a r t i c l e s are in the micrometer s i z e range, are highly nonspherical, and have considerable surface roughness. The samples of Ca(0H>2 with additives that were prepared in the way discussed above were characterized using x-ray powder d i f f r a c t i o n , scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and BET surface area. According to x-ray d i f f r a c t i o n analysis, most of the additives p r e c i p i t a t e as a separate phase a f t e r s l u r r y i n g with the Ca(0H)2 then drying. The exception is CaCl2> where a Ca(OH)2«CaCL2«H20 phase is formed. This finding agrees with the result reported in the l i t e r a t u r e for the equilibrium of the Ca(0H>2CaCl'H20 system ( 1 4 ) . In the case of Ca(N03)2, the formation of the s o l i d phase Ca2N2C>7«2H20 has been reported ( 1 4 ) . This could not be confirmed using x-ray analysis as the d i f f r a c t i o n pattern of the Ca2N2C>7.2H20 was not a v a i l a b l e . X-ray d i f f r a c t i o n and scanning d i f f e r e n t i a l calorimetry showed the reaction product to be



212


Figure 2.

Figure 3.

SEM picture, lime 0, 4000 Magnification

SEM picture, lime A, 4000 Magnification


17.



213

CaSO^'^I^O. No gypsum or any other sulfate phase was detected either by x-ray d i f f r a c t i o n or scanning d i f f e r e n t i a l calorimetry, suggesting that any sulfate produced in the reactor w i l l be present in s o l i d solution with the CaS0 *^H 0. EDS was used to analyze i n d i v i d u a l p a r t i c l e s and it was found that the additives p r e c i p i t a t e together with the Ca(0H)2 and are, more or l e s s , uniformly d i s t r i b u t e d through the Ca(0H)2 p a r t i c l e s . SEM micrographs of the p a r t i c l e s of Ca(0H)2 with additives showed no s i g n i f i c a n t difference in s i z e or shape from the pure Ca(0H)2 p a r t i c l e s . The BET surface areas of the Ca(0H>2 with d i f f e r e n t s a l t additives were also measured. The surface areas ranged from 7.0 to 9.6m2/g depending on the additive used. No c o r r e l a t i o n was found between Ca(0H)2 r e a c t i v i t y and surface area. Downloaded by CORNELL UNIV on November 2, 2017 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch017

3

2

Effect of Process Variables Experiments were performed to determine the effect of r e l a t i v e humid i t y , temperature, i n l e t S02 concentration, and amount of Ca(0H)2, on the r e a c t i v i t y of pure Ca(0H)2 towards SO2. Relative humidity has the greatest impact on Ca(0H)2 r e a c t i v i t y . Figure 4 shows Ca(0H)2 conversion as a function of time at 19, 50 and 70% r e l a t i v e humidity. A l l three experiments were run at 2000 ppm i n l e t SO2 and 66°'C using 4 g of lime 0. The discontinuous l i n e in the figure corresponds to the Ca(0H)2 that would be converted i f a l l the SO2 were being removed. For a l l r e l a t i v e humidities, 100% of the SO2 is removed during the f i r s t one or two minutes of reaction, then the reaction rate decreases quickly at low r e l a t i v e humidities but slower at high r e l a t i v e humidities. The same kind of behavior was observed with the other batch of lime A which d i f f e r s s l i g h t l y in p a r t i c l e s i z e and BET surface area. Reactor temperature has a very moderate effect on Ca(0H>2 r e a c t i v i t y , as i l l u s t r a t e d by Figure 5 that shows the effect on Ca(0H)2 conversion versus time when the temperature is increased from 30.4°C to 64.4°C while keeping a l l other variables constant. The effect of the i n l e t SO2 concentration on the reaction rate was found to depend on the r e l a t i v e humidity. At 70% r e l a t i v e humid i t y the Ca(0H>2 conversion was p r a c t i c a l l y independent of the i n l e t SO2 concentration as i l l u s t r a t e d in Figure 6. At lower r e l a t i v e Table I:

P a r t i c l e Size and BET Surface Area of Reactant

Reactant

Coulter Counter Median P a r t i c l e Size (ym)

BET Surface Area (m2/e)

lime 0

7.4

8.2

lime A

5.6

9.4

-

8.8

lime A (slurried)

humidity the reaction rate is not affected by the S0 concentration •if the SO2 concentration is high. However, at lower l e v e l s of SO2 2




0

10

20

30

40

50

60

Time (Minutes)

Figure 4. Lime 0, Effect of Relative Humidity

25

Ca(0H)2 Conversion (X)

Time (Minutes)

Figure 5. Effect of Reactor Temperature, Lime A



17.


215


the reaction rate is affected by the S02 concentration as i l l u s t r a t e d by Figure 7 which shows the Ca(0H)2 conversion at 50% r e l a t i v e humid i t y . The fact that at low r e l a t i v e humidities and low SO2 l e v e l s the S02 concentration a f f e c t s the reaction rate could be explained by assuming that the reaction has zero order k i n e t i c s in SO2, but at low r e l a t i v e humidity and/or low SO2 concentration, mass transfer through the product layer becomes the c o n t r o l l i n g step instead of chemical reaction. The effect on the average Ca(0H>2 conversion of changing the amount of Ca(0H>2 in the reactor is i l l u s t r a t e d by Figure 8 which shows the Ca(0H>2 conversion when the amount of Ca(0H)2 in the reactor was reduced from 4 to 1 g, keeping the SO2 concentration at 1000 ppm and the r e l a t i v e humidity at 50%. From Figure 8 it can be seen that the amount of Ca(0H>2 present in the reactor makes a difference during the f i r s t minutes of reaction, but the effect is less marked at l a t e r times. These r e s u l t s are consistent with the e f f e c t of S02 concentration discussed above. When more Ca(0H)2 is present in the reactor, more S02 is removed at the entrance of the reactor, so the Ca(0H>2 present farther down in the reactor "sees" a lower concentration of S02» and the reaction rate is slower. At l a t e r times when the SO2 removal is lower the amount of Ca(0H>2 present w i l l not be as important. Effect of Organic Acids and Organic Déliquescents Two organic acids were selected as test additives f o r Ca(0H>2, adipic acid because of its extensive use in wet scrubbing, and g l y c o l i c acid, because of its deliquescent properties. Both of these acids proved to be detrimental to the reaction of SO2 with Ca(0H>2, as can be seen from Table I I . At 74% r e l a t i v e humidity the average Ca(0H)2 conversion in the reactor decreased from 22 to 11% when 5 wt% of g l y c o l i c acid was added to the Ca(0H>2, and from 22 to 20% when 1 wt% adipic acid was used as additive. Table I I :

Effect of Organic Acids and Organic Déliquescents on Ca(0H)2 Reactivity

500 ppm SO2, 1.0 g lime A, 74% RH, 64.4°C, 4.6 1/min N Additive

2

Average Ca(0H>2 conversion a f t e r 1 hour (%) 22.4

None ORGANIC ACIDS 5 wt% G l y c o l i c Acid 1 wt% Adipic Acid ORGANIC DELIQUESCENTS 5 wt% Monoethanolamine 5 wt% Ethylene Glycol 5 wt% TEG

11.3 20.3 19.6 20.3 20.5





17.



217

The organic déliquescents t r i e d were ethylene g l y c o l (EG), t r i ethylene g l y c o l (TEG), and mono-ethanolamine (MEA). The experimental runs made adding 5 wt% of EG, TEG, or MEA are presented in Table I I I . At 75% r e l a t i v e humidity, a l l three additives s l i g h t l y decreased the Ca(0H)2 converted after 1 hour of reaction, when compared with the conversion of pure Ca(0H)2 at the same experimental conditions.


Effect of Deliquescent Salts Additives Table III shows the experimental results of Ca(0H)2 with deliquescent s a l t s as additives, a t 74 and 54% r e l a t i v e humidity. From the results presented in this table we can see that the b e n e f i c i a l effect of s a l t s depends on the type of s a l t and the r e l a t i v e humidity. At high r e l a t i v e humidity (74%) a l l the deliquescent s a l t s t r i e d were successful in increasing the r e a c t i v i t y of the Ca(0H)2 towards SO2. At lower r e l a t i v e humidity (54%) some of the s a l t s do not perform as w e l l , and some, such as Ca(N(>3)2, do not have any b e n e f i c i a l effect at a l l . Also presented in Table I I I is the water a c t i v i t y over saturated solutions of the s a l t s at 25 and 100°C and 1 atm. The water a c t i v i t y is approximately equal to the r e l a t i v e humidity of the gaseous phase that would be in equilibrium with a saturated solution of the s a l t at that temperature and pressure. I f one of these deliquescent s a l t s is contacted with a gaseous phase of r e l a t i v e humidity greater than the water a c t i v i t y , the s a l t w i l l capture water from the gas phase and become a solution. This tendency to capture water has been extensivel y documented in the l i t e r a t u r e by studies of the growth of s a l t containing aerosols as a function of the atmospheric r e l a t i v e humid i t y (15-20). From the data presented in Table III it is clear that based in deliquescence alone most of the s a l t s tested, s p e c i f i c a l l y NaN03 and a l l the chlorides with exception of L i C l , should not have any b e n e f i c i a l effect at 54% r e l a t i v e humidity. Nevertheless, these s a l t s are among the ones that behave the best at that low r e l a t i v e humidity. A possible explanation for these findings would be an hysteresis phenomenon, and t h i s w i l l be discussed l a t e r in the paper. Influence of Salt Concentration. A series of experiments were performed to determine the influence of the s a l t concentration on the SO2 reaction rate. The s a l t s used were NaCl and NaN03 in concentrations ranging from 1 to 15 mole%. The experiments were carried out at a r e l a t i v e humidity of 54%, and a reactor temperature of 66°C. As can be seen from Figure 9, the conversion increases with increasing concentration of additive u n t i l about 10 mole%. After t h i s the curve l e v e l s o f f . The optimum concentration of additive is then about 10 mole% for 1:1 e l e c t r o l y t e s l i k e NaCl and NaN03. Effect of Prehumidification at 98% Relative Humidity. As was mentioned in the Experimental Apparatus Section, before each experimental run the fixed bed was prehumidified by flushing with pure nitrogen at a r e l a t i v e humidity of about 98% for 10 minutes before flushing with nitrogen at the r e l a t i v e humidity at which the experiment was to be performed. This prehumidification step could be the reason why some of the s a l t s are s t i l l e f f e c t i v e at a r e l a t i v e humidity lower than the one




30

25

Ca(OH)2 Conversion at 60 minutes (%)

20 15 ·

NaN03

•

NaCl

•

•

10

Y

0 J 0

Figure 9.

•

D

54X RH 66 C 1 g lime A 4.6 1/min N2

1

2

1—ι h — ι 1 4 6 8 10 12 Amount of Additive (mole X)

.

4

14

16

E f f e c t of Amount of A d d i t i v e on Ca(0H) R e a c t i v i t y 2


17.

Table I I I :

219


RU1Z-ALSOP AND ROCHELLE

Effect of Deliquescent Salts in Ca(0H>2 Reactivity and Deliquescent Properties of the Salts

500 ppm S02, 1.0 g Ca(0H>2 A, 4.6 1/min (0°C, 1 atm) N2


Additive (Mole%)

Ca(0H>2 Conversion at 60 Minutes (%) 74% RH 54% RH 66°C 64.4°C

None

22.4

5% Na2S04

28.3

5% Na S0

11.8

-

Water A c t i v i t y Over Saturated Solutions at 25°C at 100°C

-

.902

(1)

-

29.8

16.1

5% CaCL (**) 2

34.6

16.4

.850 (1)

.686( M )

10% NaCL

38.5

27.0

.753 (3)

.735

5% Ca(N0 ) (**)

39.4

12.3

-

13.2

2

3

3

2

5% Co(N0 ) 3

2

40.0

-

3

41.5

27.2 19.4

10% NaN0 10% NaN0 5% B a C l

2

5% N a S 0 2

2

2

3

10% KC1 10% NaBr 10% L i C l 100% S0 Removal 2

(*)

-

.553

(2)

.738 (3)

.549

(1)

.902 (3)

.71

(2)

.52

(2)

.842 (3)

.745

(1)

42.0

.577 (3)

.502

(1)

43.0

.112 (3)

.085

(1)

48.2

—

-

37.3

-

data at 75°C

(**) Solids phases were CaCL * Ca(OH) -H 0 and C a ^ O ^ •2H 0 respectively. (1) Source (15) (2) Extrapolated from (15) (3) Source (16) 2

(1)

.496

21.6

48.2

-

2

2

2




220

predicted from equilibrium considerations. Due to hysteresis it is possible that when the r e l a t i v e humidity was lowered to the experimental conditions a f t e r the prehumidification, some excess water remains in the s o l i d s . Strong hysteresis e f f e c t s have been reported in NaCl aerosols (21). To check i f hysteresis was responsible for the b e n e f i c i a l e f f e c t of some s a l t s at low r e l a t i v e humidities, experimental runs were made omitting t h i s step. Table IV shows the r e s u l t s obtained at 54 and 17.4% r e l a t i v e humidity with and without prehumidification of the bed at 98% r e l a t i v e humidity. The additives used were NaCl, NaN03, and KC1. At 54% r e l a t i v e humidity even when some decrease of the Ca(0H)2 conversion was found without the prehumidification, the r e s u l t s are s t i l l far superior to the pure Ca(0H)2 case. Hysteresis then, can not explain a l l the b e n e f i c i a l e f f e c t observed at 54% r e l a t i v e humidity. At 17.4% r e l a t i v e humidity a l l the b e n e f i c i a l effect in the case of the NaCl appears to be due to the prehumidification of the bed, i . e . , due to a hysteresis phenomenon. E f f e c t of Nitrogen Purity When experiments using NaCl as an additive were performed it was found that the Ca(0H)2 conversion given by chemical analysis of the reacted s o l i d s was consistently lower than the Ca(0H)2 conversion calculated by integration of the S02 versus time curve. This d i f f e r ence could be explained by the p a r t i a l oxidation of the s u l f i t e produced in the reaction by the oxygen present as an impurity in the nitrogen. To test t h i s p o s s i b i l i t y some experiments were run using oxygen free nitrogen (purity of 99.998%) instead of the n i t r o gen previously used that has a purity of 99.5%. Table IV:

E f f e c t of Prehumidification of the bed at 98% on Ca(0H)2 Reactivity

500 ppm

S02,

1.0

g Ca(0H)2 A, 4.6

1/min

(0°C, 1 atm)

Average Ca(0H)2 Conversion a f t e r 1 hr

Additive (Mole%) None 10% NaCl 10% NaN03 10% KC1

54% RH 66°C Prehumidified No Yes _ 11.2 27.0 27.2 37.3

23.2 23.7 19.3

RH

N2

(%) 17.4% RH 95°C Prehumidified Yes No 4.0 9.7 11.9 3.4

4.0

Table V shows the comparison of two experiments run at the same experimental conditions but using the two d i f f e r e n t types of n i t r o gen. A much better agreement was found between the Ca(0H)2 conversion obtained by chemical analysis and the one obtained by i n t e grating the SO2 curve when oxygen free nitrogen was used. The Ca(0H)2 conversion estimated by integration of the SO2 curve was the same in both experiments i n d i c a t i n g that the rate of SO2 removal is not affected by the nitrogen purity even when some oxidation of the


17.


Table V:


221

E f f e c t of Nitrogen Purity on Lime Reactivity

500 ppm S0 , 54% Relative Humidity, l.Og lime A, 4.6 1/min N, 2

Ca(0H) Conversion after 1 hour (%) By Integrating By Chemical S0 Curves Analysis 2

Type of Nitrogen

7

Common (99.5% purity) 0 Free (99.998% purity)


2

27.0 27.0

19.7 25.1

s u l f i t e appears to be taking place when the l e s s pure nitrogen was used. Discussion As shown in the r e s u l t s section, the r e l a t i v e humidity of the gaseous phase is the most important v a r i a b l e in the reaction of SO2 with dry Ca(0H)2 s o l i d s . These r e s u l t s are in agreement with r e s u l t s reported in the l i t e r a t u r e for S02 removal in the bag f i l t e r s of spray dryer p i l o t and commercial plants (3, 5-9). In the spray dryer plants, the moisture content of the gases is normally indicated as approach to the adiabatic saturation temperature (difference between the temperature of the gas and the adiabatic saturation temperature) rather than r e l a t i v e humidity. The other variables tested, i . e . temperature, amount of Ca(0H)2 and S02 concentration have a lower impact on the reaction rate. The d i f f e r e n t e f f e c t of S02 concentration at low and high r e l a t i v e humid i t y can be explained by assuming that the reaction has zero order k i n e t i c s in SO2 and that at low r e l a t i v e humidity the reaction rate is mass transfer controlled while at high r e l a t i v e humidities the reaction is controlled by reaction k i n e t i c s . Most of the deliquescent s a l t s t r i e d were e f f e c t i v e in increasing Ca(0H)2 r e a c t i v i t y towards S02. The extent of the b e n e f i c i a l e f f e c t being a function of the type of s a l t , the s a l t concentration, and the r e l a t i v e humidity. Some s a l t s are e f f e c t i v e at a lower r e l a t i v e humidity than would be predicted from their deliquescent properties. Hysteresis due to the prehumification of the bed appears to be p a r t i a l l y responsible for t h i s behavior. In other words some water remained absorbed in the s o l i d s a f t e r the r e l a t i v e humidity was reduced because of surface tension e f f e c t s and formation of oversaturated solutions. From the r e s u l t s presented in Table IV, it can be concluded that even when the hysteresis e f f e c t s can explain the improvement in r e a c t i v i t y at a very low r e l a t i v e humidity (17.4%) it can not explain a l l the improvement observed at 54% r e l a t i v e humidity. An alternate explanation proposed is that the chlorides and NaN03 modify the properties of the product CaS03 l/2H20 layer that is formed as the reaction takes place thereby f a c i l i t a t i n g the access of the S02 to the unreacted Ca(0H)2 which remains in the i n t e r i o r of the p a r t i c l e . NaCl and CaCl2 have been reported to enhance the s u l f u r dioxide r e a c t i v i t y of limestones in f l u i d i z e d bed combustion by a f f e c t i n g the pore structure of the lime during c a l c i n a t i o n which then increases the extent of s u l f a t i o n of the limestone (23). e


222


For an additive to be e f f e c t i v e it is necessary that the hydrox ide of the cation be very soluble, otherwise the cation w i l l p r e c i p i t a t e as the hydroxide and the anion as the Ca s a l t . For example, Co(N03>2 is very deliquescent but Co(OH)2 is insoluble so Co pre c i p i t a t e s as Co(OH)2 and adding cobalt n i t r a t e becomes equivalent to adding calcium n i t r a t e , which is not a very e f f e c t i v e additive.

Literature Cited


1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Kelly, M.E.; Dickerman, J . C . , Proc. Symp. on FGD, 1981, p. 761. Kelly, M.E.; Kilgroe, J.D.; Brna, T.G., Proc. Symp. on FGD, 1983, p. 550. Stevens, N.J., Proc. Symp. on FGD, 1981, p. 777. Parsons, E.L.; Hemenway, L.L.F.; Krag, O.T.; Brna, T.G.; Ostop, R.L. Proc. Symp. on FGD, 1981, p. 801. Bresowar, G.E.; Borsare, D.C.; Walki, K.W. "Dry Scrubber Design and Application. The C-E Approach", presented at Joint Power Generation Conference, 1981. Robards, R.F.; Aldred, R.W.; Burnett, T.A.; Humphries, L.R.; Widico, M.J. Proc. The EPA/EPRI 9th Symp. on FGD, 1985. Stevens, N.J.; Manavizadeh, G.B.; Taylor, G.W.; Widico, M.J. Proc. 3rd Symp. on Transfer and Utilization of Particulate Control Tech., 1982 Samuel, E.A.; Lugar, T.W.; Lapp, D.E.; Fortune, O.F.; Brna, T.G.; Ostop, R.L. Proc. Symp. on FGD, 1983, p. 574. Blythe, G.M.; Rhudy, R.G. Proc. 8th Symp. on FGD, 1984, p. 708734. Karlsson, H.T.; Klingspor, J . ; Linne, M.; Bjerle, I., APCA J . , 1983, 33, 23. Rhudy, R.G.; Blythe, G.M. Proc. 9th EPA/EPRI Symp. on FGD, 1985. Parson, E.L.; Boscak, V.; Brna, T.G.; Ostop, R.L. "SO Removal by Dry Injection and Spray Absorption Techniques", presented at the U.S. 3rd Symp. on the Transfer and Utilization of Parti culate Control Tech., 1981. Felsvang, K.; Morsing, P.; Veltman, P. Proc. 8th Symp. on FGD, 1984, p. 650-667. Seidell, Α.; Linke, W.F. "Solubilities of Inorganic and Metal Organic Compounds", Van Nostrand Co., Inc. Washington, D.C., 1958, Vol. 1. National Research Council, International Critical Tables, McGraw-Hill, New York, Vol. 3, 1930. Stokes, R.H.; Robinson, R.A., Ind. Eng. Chem., 1949, 41, 2013. Winkler, P. J . Aerosol Sci., 1973, 4, 373-387. Ferron, G.A. J. Aerosol Sci., 1977, 8, 251-267. Tang, I.N. J . Aerosol Sci., 1976, 7, 361-371. Tang, I.N.; Munkelwitz, H.R.; Davis, J.G. J . Aerosol Sci., 1977, 8, 149-159. Tang, I.N.; Munkelwitz, H.R. J. Aerosol Sci., 1977, 8, 321-330. Tang, I.N.; Munkelwitz, H.R.: Davis, J.G. J . Aerosol Sci., 1978, 9, 505-511. Chopra, O.K.; Smith. G.W.; Lenc, J . F . ; Shearer, K.M.; Myles, K.M.; Johnson, I. Proc. 6th Int'l Conf. Fluidized Bed Combus tion, 1980. 2

R E C E I V E D April 8, 1986


Fossil Fuels Utilization - American Chemical Society

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