6 Thermal Decomposition of Sulfite, Bisulfite, and Disulfite Solutions B. MEYER, M . RIGDON, T . BURNER, M . OSPINA, and K. WARD
Downloaded by UNIV OF GUELPH on August 13, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch006
University of Washington, Department of Chemistry, Seattle, WA 98195, and University of California, Lawrence Berkeley Laboratory, Materials and Molecular Research Division, Berkeley, CA 94720 K. KOSHLAP University of California, Lawrence Berkeley Laboratory, Materials and Molecular Research Division, Berkeley, CA 94720
This work shows that oxygen-free sulfite in lime/limestone slurries, exposed to sulfur dioxide, slowly decomposes under process conditions. In fact, auto-redox reactions of sulfur oxyacids can occur in a l l coal desulfurization systems, including coal-gasification systems and impurities present in commercial flue gas systems are capable of catalyzing the reaction under process conditions. Our experiments indicate that any large-scale coal u t i l i z a t i o n w i l l depend on appropriate control of the auto-redox reactions of sulfur species. It is well known that sulfite and bisulfite can be oxidized to sulfate by oxygen in a i r . It is less widely recognized that sulfite can be converted to sulfate by auto-redox decomposition reactions, even i f no oxygen is present. This latter reaction was f i r s t described by Priestley in 1790 (1) · Priestley conducted his experiments with sulfurous acid solution ("volatile acid of sulfur") sealed in a soft-glass tube. The latter was stored in a sand bath at 180°C for several months u n t i l brownish solid and liquid phases formed. Priestley recognized that the reaction products included elemental sulfur as well as sulfate. This auto-redox reaction was a l l but forgotten for almost a hundred and fifty years and has been widely ignored by chemists and chemical engineers during the last f i f t y years, probably because i t was assumed to be slow and insignificant. This assumption is incorrect. Already the earliest patent literature on desulfurization of coal gasification, "city gas", and coal combustion gas desulfurization (2) contains persistant and recurring reports of decomposition of sulfur scrubbing liquids, yielding unexpected and difficult-to-handle products, among them nascent, polymeric elemental sulfur which forms rubbery and sticky films causing breakdown of equipment. Our 0097-6156/82/0188-0113$6.00/0 © 1982 American Chemical Society Hudson and Rochelle; Flue Gas Desulfurization ACS Symposium Series; American Chemical Society: Washington, DC, 1982.
Downloaded by UNIV OF GUELPH on August 13, 2016 | http://pubs.acs.org Publication Date: July 1, 1982 | doi: 10.1021/bk-1982-0188.ch006
114
FLUE
GAS
DESULFURIZATION
research group has studied a v a r i e t y of systems, i n c l u d i n g lime/limestone s l u r r i e s , as w e l l as magnesium, sodium and potas sium systems, i n c l u d i n g i m p u r i t i e s containing t r a n s i t i o n metal ions and nitrogen compounds. The l a t t e r are almost absent i n o i l - d e s u l f u r i z a t i o n systems, but quite abundant i n heterocycles i n coal and therefore i n coal gases. We have found that ammonia s o l u t i o n saturated with s u l f u r dioxide gas sealed i n glass tubes p r e c i p i t a t e s u l f a t e c r y s t a l s , as w e l l as elemental s u l f u r w i t h i n l e s s than 72 hrs at 70°C. We also observed the same products at room temperature a f t e r l e s s than two months. Thus, the decomposition of s u l f i t e occurs at lower tempera ture and f a r quicker than g e n e r a l l y assumed, under c o n d i t i o n s w e l l w i t h i n the temperature range of many d e s u l f u r i z a t i o n pro cesses. The r e a c t i o n involves the auto-redox decomposition of sulfur (IV). Depending on the pH of the system, the o v e r a l l r e a c t i o n can be summarized as shown i n Equations I - I I I . 3S0
+
2
4H 0 2
3HS0 ~ 3
2
3S0 " + 3
H0 2
+·
2HS0 "
*
2S0 "~
->
2S0 ~
4
2
4
2
4
+
1/n
Sn
+
2H 0
+
1/n
Sn
+
H 0
+
1/n
Sn
+
20H~
+
(I)
3
+
(II)
3
(III)
Equation I a p p l i e s at pH
