15 Effects of Magnesium and Chloride Ions on Limestone Dual Alkali System Performance 1
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John C. S. Chang , Norman Kaplan , and Theodore G. Brna 1
Acurex Corporation, Research Triangle Park, NC 27709 Industrial Environmental Research Laboratory, U.S. Environmental Protection Agency, Research Triangle Park, NC 27711
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Pilot plant tests have been conducted to evaluate the effects of magnesium and chloride ions on system performance of limestone-regenerated dual alkali processes under closed-loop operating conditions. It was found that limestone reactivity and solids dewatering properties are very sensitive to magnesium ion concentrations. The total magnesium ion concentration should be maintained below 1000 ppm for satisfactory performance under normal operation. A model which assumes competitive surface adsorption of calcium and magnesium ions was used to interpret the data. Limestone reactivity decreased and solids dewatering properties worsened with the increase of chloride ion concentration. However, the effect of chloride ion accumulation was not significant until the concentration reached 80,000 ppm. Sodium-based dual a l k a l i (DA) f l u e gas d e s u l f u r i z a t i o n (FGD) processes have the features of c l e a r solution scrubbing, high SO2 removal e f f i c i e n c y , low liquid-to-gas (L/G) r a t i o , and high r e l i a b i l i t y ^ ) . Recent t e s t i n g has demonstrated the f e a s i b i l i t y of using limestone instead of lime for scrubbing l i q u o r regeneration which makes DA processes more competitive with slurry scrubbing processes^ ). Since the limestone DA process uses concentrated sodium s u l f i t e solution as the absorbent for SO2 removal, closed water loops are desired to minimize the sodium makeup requirements. However, the closed-loop operation ( i . e . , the only water leaving the FGD system is through evaporation and f i l t e r cake moisture) also promotes the buildup of soluble s a l t s in the r e c i r c u l a t i n g scrubbing l i q u o r . The primary sources of soluble s a l t s include makeup water, reagents, and f l u e gas. Previous findings i n d i c a t e that the accumulation of soluble s a l t s , e s p e c i a l l y c h l o r i d e ions, can have s i g n i f i c a n t e f f e c t s on system chemistry and scrubber performance of lime/limestone s l u r r y FGD processesv » ). The most s i g n i f i c a n t e f f e c t s observed include decreases of equilibrium pH, SO2 removal e f f i c i e n c y , and s o l i d s s e t t l i n g rate, and an increase in gypsum s c a l i n g p o t e n t i a l . 1
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0097-6156/86/0319-0176S06.00/0 © 1986 American Chemical Society
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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15.
CHANG ET
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Limestone Dual Alkali System Performance
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In order to broaden the data base of the limestone DA process and to evaluate the impacts of closed-loop operation, a series of p i l o t plant t e s t s was conducted under the sponsorship of U.S. Environmental Protection Agency^ (EPA) Industrial Environmental Research Laboratory in Research Triangle Park, NC (IERL-RTP). The t e s t i n g concentrated on evaluating the e f f e c t s of magnesium and chloride ions since appreciable accumulations of soluble s a l t s containing these two species are expected in a limestone DA system under closed-loop operating conditions. The current trend in power plant design is to use plant waste water (e.g., cooling tower blowdown) f o r FGD system makeup. Magnesium and chloride are the primary ingredients of soluble s a l t s which enter the FGD system with plant waste water. Additional chloride ions may enter the scrubbing l i q u o r through the absorption of HC1 produced during the coal combustion process. In addition to makeup water, limestone is also an important source of magnesium ions. If closed-loop operation is used, the dissolved s a l t s from e i t h e r of the above sources can be concentrated to l e v e l s which are considerably higher than those presently observed in most systems. This paper summarizes selected r e s u l t s from the p i l o t plant study of the limestone DA process with total magnesium ion concentration up to 2000 ppm and chloride ion concentration to 150,000 ppm. Test F a c i l i t i e s The IERL-RTP p i l o t f a c i l i t i e s include a three-stage tray tower with 7.5 m /min (approximately 0.1 MW) f l u e gas capacity (Figure 1). The f l u e gas is drawn from a gas-fired b o i l e r (no f l y a s h is present). Pure SO2 is injected to achieve the desired concentration in the f l u e gas. Regeneration of spent scrubbing l i q u o r is performed in the four-tank-in-series reactor t r a i n with a total residence time of 80 minutes. Limestone is fed to the f i r s t reactor as 45% s l u r r y . The feed rate is manually set as required f o r e i t h e r pH or reactant stoichiometry c o n t r o l . Soda ash is added to the fourth reactor as sodium makeup. Reactor e f f l u e n t s l u r r y flows by gravity to the thickener centerwell. C l a r i f i e d l i q u o r overflows from the thickener to the forward feed hold tank from which it is pumped to the tray tower. A horizontal belt f i l t e r is used f o r further dewatering of the thickener underflow s o l i d s . U l t r a v i o l e t spectrophotometry (DuPont 400 SO2 analyzer) was used to monitor the gas phase SO2 concentrations and SO2 removal e f f i c i e n c i e s . The pH of scrubbing l i q u o r in each reactor was measured hourly during p i l o t t e s t i n g . Solid dewatering properties were characterized by hold tank s l u r r y s e t t l i n g rate and f i l t e r cake insoluble s o l i d s concentration. Detailed descriptions of the t e s t f a c i l i t i e s and a n a l y t i c a l procedures were reported e a r l i e r ( 5 ) . 3
E f f e c t s of Magnesium Ions 2+
For the study of magnesium ion (Mg ) e f f e c t s on the system performance of the limestone DA process, Epsom s a l t (MgS04*5H20) was added to adjust the Mg2 concentration in the scrubbing l i q u o r . The base case system performance was established without the addition of epsom s a l t ; the steady-state concentration of M g was 355 Dpm. in subsequent t e s t s , a l l operating conditions -- except f o r Mg2* +
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Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
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FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS
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Figure 1.
Flow diagram of IERL-RTP dual a l k a l i p i l o t plant.
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
15.
CHANG ET AL.
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Limestone Dual Alkali System Performance
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concentration were maintained constant; the concentration of M g was gradually increased by Epsom s a l t addition to the f i r s t reactor. The maximum total M g concentration reached during t h i s t e s t series was 2000 ppm. The principal results of these tests are l i s t e d in Table I. Summaries of l i q u o r and s o l i d analyses are l i s t e d in Tables II and I I I , r e s p e c t i v e l y . 2+
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E f f e c t s of 1000 ppm Mg + A comparison of results obtained from run MG-1 with those from MG-2 indicated that the most s i g n i f i c a n t change observed with the increase of total Mg concentration up to 1000 ppm is the d e t e r i o r a t i o n of s o l i d s dewatering properties as reflected by the decrease of insoluble s o l i d s in the f i l t e r cake. The base case (run MG-1) f i l t e r cake contained 52% s o l i d s ; however, only 45% s o l i d s was obtained in the f i l t e r cake generated at 1000 ppm total M g concentration. No magnesium s u l f a t e was added t o the system f o r run MG-3. The objective of t h i s run was to evaluate the system performance with decreasing M g concentration. The mass balance indicated that the total Mg * concentration should d r i f t down to below 500 ppm. During the run, the total Mg concentration decreased from 1000 ppm t o about 625 ppm toward its end. A leak was discovered at the scrubber bleed/quench r e c i r c u l a t i o n pump i n l e t which introduced a i r into the process stream and therefore caused high oxidation. The high oxidation, as confirmed by s o l i d s analysis results in Table 3, was reflected by increases of the s u l f a t e - t o - s u l f i t e ratio to above 2.5. A f t e r the a i r leak problem was corrected, the s u l f a t e - t o - s u l f i t e r a t i o decreased, but the t e s t average was 2.4. Very stable operation was maintained f o r run MG-4 without magnesium s u l f a t e addition with the total Mg concentration s t a b i l i z i n g a t about 356 ppm. The s u l f a t e - t o - s u l f i t e ratio decreased to 2.0, the f i l t e r cake insoluble solids reached 53%, and the slurry s e t t l i n g rate was 2.2 cm/min. The results confirmed the base case s o l i d s quality and scrubber performance obtained from run MG-1.
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Effects of 2000 ppm Mg + 2+
Epsom s a l t was then added t o the f i r s t reactor to raise the M g concentration f o r run MG-5. No s i g n i f i c a n t changes of system performance were observed except the d e t e r i o r a t i o n of f i l t e r cake q u a l i t y when the total M g concentration reached 1000 ppm. The f i l t e r cake insoluble s o l i d s dropped from 53% t o 47% at 1000 ppm total Mg concentration and confirmed the results observed during runs MG-1 and MG-2. However, a f u r t h e r increase of the M g concentration caused s i g n i f i c a n t changes in system performance. When the total M g concentration exceeded 1500 ppm, pH decreases were observed throughout the system. The SO2 removal e f f i c i e n c y also decreased. F i l t e r cake q u a l i t y deteriorated f u r t h e r to below 40% insoluble s o l i d s . The s o l i d s s e t t l i n g rate also began to drop. At 2000 ppm total Mg concentration, a system upset with non-settling s o l i d s occurred. The s o l i d s s e t t l i n g rate dropped t o below 0.1 cm/min, while only 28% i n s o l u b l e s o l i d s were obtained from the f i l t e r cake. The pH of the regenerated l i q u o r was 6.2 (base case pH was 6.6) and SO? removal e f f i c i e n c y was 85% (base case removal e f f i c i e n c y was 92.7%). The solids content of s l u r r i e s in the 2+
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Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986.
Markuszewski and Blaustein; Fossil Fuels Utilization ACS Symposium Series; American Chemical Society: Washington, DC, 1986. a
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Ujwashed in the belt f i l t e r ( )Mass balance
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MG-1 1060 8.4 1.4 6.6 6.2 2990 6.7 91.8 137 23 45 7.0 18 45 3 55.6 0.67 52 0.41 18.6 78 1.08 1.9 2.1 1.08 2.15
MG-2 625 7.9 1.4 6.6 6.2 2990 6.6 91.0 132 22 45 6.1 16 48 3 57.4 0.70 74 0.38 16.9 85 1.06 2.2 2.6 1.08 2.4
MG-3 356 7.8 1.4 6.6 6.2 3050 6.7 92 138 22 45 6.9 20.1 53 3 54.0 0.74 62 0.42 12.6 92 1.03 2.2 2.1 1.16 2.0
MG-4 2000 8.2 1.4 6.2 5.6 3020 6.6 85 125 22 45 7.2 7.8 28 3 49.7 0.67 58 0.28 (d) 67 1.13
