Effects of Magnesium and Chloride Ions on Limestone Dualalkali System Performance

P i l o t p lan t t e s t s have been conducted t o evaluate the e f f e c t s o f magnesium and c h l o r i d e ions on system performance o f 1 imestone-regenerated dual a1 ka l i processes under closed-loop operat ing cond i t i ons . It was found t h a t l imestone r e a c t i v i t y and s o l i d s dewatering p roper t i es a r e ve ry s e n s i t i v e t o magnesium i o n concentrat ions. t o t a l magnesium i o n concentrat ion should be mainta ined below 1000 ppn f o r s a t i s f a c t o r y pe r fonance under normal operation. A model which assumes compe t i t i ve sur face adsorpt ion o f ca lc ium and magnesium ions was used t o i n t e r p r e t the data. Limestone r e a c t i v i t y and s o l i d s dewatering p roper t i es decreased w i t h the increase of c h l o r i d e i o n concentrat ion. s i g n i f i c a n t u n t i l t h e concen t ra t i on reached 80,000 ppm. INTRODUCTION The However, the e f f e c t o f c h l o r i d e i o n accumulation was no t 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 o f c l e a r s o l u t i o n scrub i g, h igh SO2 removal e f f i c i e n c y , low l i q u i d t o g a s (L/G) r a t i o and h i s h r e l i a b i l i t J 1 7 . Recent t e s t i n g has demonstrated the f e a s i b i l i t y o f using l imestone instead of l i m e f o r scrubbing l i q u o r r e e e r a t i o n which makes DA processes more compet i t ive w i t h s l u r r y scrubbing processesq2r. absorbent f o r SO2 removal, c losed water loops a re des i red t o min imize the sodium makeup requirements. l eav ing the FGD system i s through evaporat ion and f i l t e r cake mois ture) a l so promotes the b u i l d u p o f so lub le s a l t s i n the r e c i r c u l a t i n g scrubbing l i q u o r . The pr imary sources o f so lub le s a l t s i nc lude makeup water, reagents, and f l u e gas. Prev ious f i n d i n q s i n d i c a t e t h a t the accumulation o f so lub le 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 sys em hemist ry and scrubber pe r fonance of l ime/ l imestone s l u r r y FGD p r o c e s s e ~ f ~ * ~ f . The most s i g n i f i c a n t e f f e c t s observed inc lude decreases o f e q u i l i b r i u m 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 r a t e , and increase i n gypsum sca l i ng p o t e n t i a l . the impacts o f c losedloop operat ion, a se r ies o f p i l o t p l a n t t e s t s was conducted under the sponsorship o f U.S. Environmental P ro tec t i on Agency's (EPA) I n d u s t r i a l Environmental Research Laboratory i n Research T r iang le Park, NC (IERL-RTP). The t e s t i n g concentrated on evaluat ing the e f f e c t s o f magnesium and c h l o r i d e ions s ince apprec iab le accumulations o f so lub le s a l t s con ta in ing these two species a re expected i n a l imestone DA system under c losedloop opera t i ng condi t ions. Since the l imestone DA process uses concentrated sodium s u l f i t e s o l u t i o n as t h e However, the c losedloop opera t i on ( i .e., the o n l y water I n order t o broaden the data base o f the l imestone O A process and t o evaluate