Fossil Fuels Utilization - American Chemical Society

14 Technical Analyses of a Wet Process for Flue Gas Simultaneous Desulfurization and Denitrification S. G. Chang

Downloaded by UNIV LAVAL on September 28, 2015 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch014

Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 The technical and economic aspects of wet flue gas simultaneous desulfurization and denitrification systems are presented so that their practicality for utilization by utility industry can be assessed. The emphasis is on the kinetics of the systems based on the employment of ferrous chelates to promote the sol ubility of NO and the reactivity of NO with SO2 in scrubbing liquors. Analytical techniques are devel oped for characterizing reaction intermediates and products. Alternative approaches and novel ideas that could develop into a more efficient and cost-effective scrubber system employing metal chelate additives are discussed.

While the development of f l u e gas clean-up processes has been pro gressing for many years, a s a t i s f a c t o r y process is not yet available. Lime/limestone wet flue gas desulfurization (FGD) scrubberisthe most widely used process in the utility industry at present, owing to the fact that it is the most technically developed and generally the most economically a t t r a c t i v e . In spite of t h i s , it is expensive and accounts for about 25-35% of the c a p i t a l and operating costs of a power plant. Techniques for the post combustion control of nitrogen oxides emissions have not been developed as extensively as those for control of sulfur dioxide emissions. Several approaches have been proposed. Among these, ammonia-based s e l e c t i v e c a t a l y t i c reduction (SCR) has received the most attention. But, SCR may not be suitable for U.S. c o a l - f i r e d power plants because of r e l i a b i l i t y concerns and other unresolved technical issues (X). These include uncertain cat a l y s t l i f e , water disposal requirements, and the e f f e c t s of ammonia by-products on plant components downstream from the reactor. The s e n s i t i v i t y of SCR processes to the cost of ΝΗβ is also the subject of some concern. The development of a process that is simple and can allow an e f f i c i e n t removal of both SO2 and Ν 0 simultaneously in one system could provide economical advantage. In the 1970's, the Japanese pursued χ

0097-6156/86/0319-0159506.00/0 © 1986 American Chemical Society

In Fossil Fuels Utilization; Markuszewski, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

FOSSIL FUELS UTILIZATION: ENVIRONMENTAL CONCERNS

160

t h i s approach and developed several wet f l u e gas simultaneous desul f u r i z a t i o n and d e n i t r i f i c a t i o n processes (2) which can be c l a s s i f i e d into two types. The f i r s t type of process involves the i n j e c t i o n of gaseous ox idant, such as ozone or chlorine dioxide, into the f l u e gas to se l e c t i v e l y oxidize insoluble NO to the more soluble N0 . N i t r i t e and n i t r a t e ions are produced in the aqueous phase a f t e r N0 and/or N 0^ are absorbed into scrubbing liquors; and s u l f i t e / b i s u l f i t e ions are formed when S0 dissolves in the solutions. Interaction between n i t r i t e and b i s u l f i t e ions can take place (Figure 1) (3-7) to produce hydroxylamine disulfonate (HADS), hydroxylamine monosulfonate (HAMS), amine t r i s u l f o n a t e (ATS), amine disulfonate (ADS), sulfamate (SA), and hydroxylamine (HA). These nitrogen-sulfur compounds can be r e moved from the scrubbing liquors as potassium s a l t precipitates (2-8). The percentage of the absorbed N0 converted to n i t r a t e ion ranges from 10 to 50%. The two most common wastewater treatment methods for the soluble n i t r a t e ion are evaporation and b i o l o g i c a l d e n i t r i f i c a t i o n . Sulfate and s u l f i t e ions can p r e c i p i t a t e as calcium s a l t s by reaction with lime/limestone. Processes of t h i s type (2) , such as Chiyoda, Ishikawajima-Harima Industries, Mitsubishi, Moretana Calcium, and Moretana Sodium, are in most cases simple modifications of commercially available FGD technology. The second type of process is based on the addition of ferrous chelates to scrubbing liquors to enhance the absorption of NO by forming ferrous n i t r o s y l chelates in aqueous solutions (9-11). Ferfous n i t r o s y l chelate can then react with dissolved S0 to produce (12) N , N 0, dithionate, sulfate and various nitrogen-sulfur com pounds as mentioned previously, while some ferrous chelates are ox idized by reaction intermediates and residual oxygen in flue gas to f e r r i c chelates, which are inactive. Therefore, t h i s type of process requires regeneration of scrubbing liquors by removing dithionate, sulfate, and nitrogen-sulfur compounds from the solutions and reduc ing f e r r i c chelates back to ferrous chelates. Processes such as Asahi, Chisso, Kureha, and M i t s u i belong to t h i s category (2). Processes of both types were demonstrated to be highly e f f i c i e n t in S0 and N0 removal (more than 95% for S0 and 85% for Ν 0 ) . How ever, these wet processes have not reached the commercial stage yet because they are uncompetitive economically, according to cost eval uation (13). These cost evaluations, however, were made based on design and knowledge available in the 1970 s. Critiques (13) i n d i cated that these wet processes were in their early stages of devel opment, and with t h e i r maturation they could become competitive in cost. The major cost of the f i r s t type of process is associated with the requirement of costly gas phase oxidants. S i g n i f i c a n t amounts of corrosion-resistant material are required for t h i s type of process, regardless of which oxidant is u t i l i z e d . The development of coste f f e c t i v e methods for the oxidation of less soluble NO to more s o l uble N0 is essential for the future of t h i s type of process. An extensive review a r t i c l e on the k i n e t i c s and mechanisms of important reactions involved has been published by Chang et a l . (_3) · The major cost of the second type of process is associated with the low s o l u b i l i t y of NO, even in scrubbing liquor containing ferrous che l a t e s , the oxidation of ferrous to f e r r i c chelates by residual oxygen in flue gas, and the production of undesirable soluble products such 2

2

2


2

X

2

2

2

2

X

2

f

2


χ

14.

CHANG

Flue Gas Desulfurization and Denitrification

as dithionate ion and nitrogen-sulfur (N-S) compounds in the system. The need to refurbish chelates for the loss In processing also con tributes to the costs. A preliminary-level economic evaluation (13) performed by EPRI ( E l e c t r i c Power Research Institute) and TVA (Tennessee Valley Author ity) indicates that a combination of 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 (or bag house), ammonia-based SCR system, and wet lime/limestone FGD scrubber range between 20% to 185% cheaper than wet process f o r com plete control of particulates, Ν 0 and S0 . The lower percentage is for the second type and higher percentage f o r the f i r s t type of pro cess. Therefore, the second type of process appears to be more prom i s i n g and w i l l be the subject of further discussion in t h i s paper. Asahi process was chosen from among the second type processes for the cost evaluation (13). In the Asahi process, the flue gas enters a packed-bed absorber where it flows countercurrent to a 6.3 pH sodium-salt scrubbing solution containing F e (EDTA). The l i q u i d effluent from the absorber is_then pumped to a reducing tank where Fe + (EDTA) is reduced by HS0 to produce F e (EDTA) and dithionate ion. Most of the scrubbing liquors leaving the reducing tank are r e cycled to the absorber. Only about 10-20% of the l i q u o r is pumped to an evaporator system in the regeneration section. The concentrated solution from the evaporators is then pumped to a cooling c r y s t a l l i z e r where hydrated sodium dithionate and sulfate c r y s t a l s are produced under vacuum. These c r y s t a l s are separated from the mother liquor in a screw decanter and sent to a dryer operating at 120-150 C, in which the hydrated c r y s t a l s are converted to anhydrous sodium s a l t s . Most of the mother liquor from the decanter is recycled to the reduc ing tank and a smaller stream is passed through a N-S compounds t r e a t ment section. The potassium s a l t s of N-S compounds are separated in a screw decanter and sent to a thermal cracker f o r the decomposition at about 500 °C. The high c a p i t a l investment cost of the Asahi process is due to the necessity f o r large absorbers, evaporators, c r y s t a l l i z e r s , dryers, rotary k i l n crackers and screw decanter separators. The major oper ating and maintenance costs are e l e c t r i c i t y , f u e l o i l , steam and chemicals such as soda ash, EDTA and limestone. The requirement f o r consumption of large amounts of utilities is associated with the oper ation p r i n c i p l e and design of the Asahi process. According to the economic evaluation, equipment required f o r N0 and S0 absorption (such as packed-bed absorbers) accounts f o r 20% of t o t a l direct cap i t a l investment; f o r treatment of dithionate ion (such as evaporator, c r y s t a l l i z e r , dryer, and cracker) it accounts f o r about 40%; and f o r treatment of nitrogen-sulfur compounds (such as screw decanter and cracker) it accounts for only 2%. The development of more e f f i c i e n t ferrous chelates that can in crease the binding rate and equilibrium constant with NO, and also the reaction rate of ferrous n i t r o s y l chelates with s u l f i t e / b i s u l f i t e ion, would allow the employment of smaller absorbers, reducing tanks, and L/G (flow rate r a t i o of scrubbing liquors to f l u e gas) to achieve the same scrubbing e f f i c i e n c i e s . The determination of optimum scrub bing conditions and chemistry such that the formation of undesirable products can be depressed or eliminated would allow the reduction of cost in the area of scrubbing liquor regeneration. This paper ad dresses the k i n e t i c s and thermodynamics of important reactions inχ


161

2

2 +

3

2 +

3

e

X

2


162


volved, the a n a l y t i c a l techniques developed for the determination of reaction intermediates and products, and the areas for further r e search that could result in substantial improvement in e f f i c i e n c y and cost-effectiveness of t h i s type of process based on the addition of metal chelate additives. Kinetics and Thermodynamics Reversible Reaction of NO with Ferrous Chelates. The binding of NO to F e ( L ) can be expressed by the following equation: 2+


NO + Fe

(L) * Fe

(L) (NO)

(1)

where L represents chelates. With the temperature-jump technique, the formation and d i s s o c i a t i o n rate constants of several ferrous n i t r o s y l chelates have been measured as shown in Table I. The tem perature-jump apparatus employed is similar to that described by C z e r l i n s k i and Eigen (14). The result of Fe^+(H 0) 5 (NO) is in good agreement with that de termined by Kastin et a l . (15) using the same experimental technique. For both Fe (EDTA)(NO) and Fe (NTA)(NO), the relaxation times due to the temperature jump were too fast to be measured. However, an upper l i m i t of 10 Ms was established for the relaxation times for both complexes. By use of t h i s value with the equilibrium constants determined for Fe (EDTA)(NO) (16) and Fe +(NTA)(NO) (10), the lower l i m i t s of formation rate constants were calculated to be 7 χ 10? and 6 χ 10? £/mol-sec at 35 °C, which is in good agreement with that de termined by the temperature-jump technique. From the results l i s t e d in Table I, we can conclude that the formation rate of Fe +(EDTA) (NO) is at least 85 times faster than that of F e ( H 0 ) ( N O ) , whereas, the d i s s o c i a t i o n rate of Fe (EDTA)(NO) is about 250 times slower than that of Fe +(H 0) (N0) at 25 °C. 2

2+

2+

2+

2

2

2+

2

5

2+

2

2

5

Reaction of NO with S u l f i t e and B i s u l f i t e . We have recently studied the reactions of NO + S0$ ~ and NO + HSO3" using rapid-mixing con tinuous-flow and stopped-flow techniques in conjunction with UV spec trophotometry for detection of the reaction's product, N-nitrosohydroxylamine-N-sulfonate (NHAS). In continuous-flow experiments, the UV spectrum was recorded over a range of 200 to 400 nm, along with the pH of the reacting solution. In stopped-flow experiments, the spectrometer was set at a wavelength were NHAS absorbs. Wavelengths used for monitoring were in the range of 257 to 295 nm, where no other species in the solution were absorbed. A recording of the absorption was started and then the flow was stopped abruptly. The record of the absorption vs. time was used in rate constant determinations. A plot of the log of the rate constant vs. pH is shown in Figure 2 along with the log percent of s u l f i t e and b i s u l f i t e vs. pH. From the data we get the rate expression: z

ά

We obtain k

5

^ dt

]

1

a

(a)

2

= k [N0][HS0 -] + k , [ N 0 ] [ S 0 ~ ] a ô a ο

« 32 ± 10 M"

sec"

1

1

and k f = 620 ± 100 M" a

sec"


1

at


14. CHANG

163

HSO~ HSO' HSO, HNQ^ « * » HO^SNO « * » (HO^S) NOH. "(HO,S)-N Nitrous acid Nitrososulfonic Hydroxylamine Amine t r i s u l f o n i c acid d i s u l f o n i c acid acid ?

HSO,

NO + HSO"

HSO HNO„

H

N

2 2°2^ Hyponitrous

(HO-S)NHOH Hydroxylamine monosulfonic acid


| H

N

+ HN0

2

3

+

NH(HSO-) Amine d i s u l f o n i c ac 2

\ H S O -

|H

+

+H0 2

HSO,

HSO,

(NH-)(HSO-) Sulfamic acid

NH^OH

Hydroxylamine

N

2

+ HS0

Figure 1. Summary of reactions that can take place as a r e s u l t of interactions between n i t r i t e and b i s u l f i t e ions.

d ( N

d

H

t

A S )

= k (NO)(HS03) + k (NO)(S03) 0

b

• 298 Κ •284 Κ log percent of S as SOJ m

log k (M" sec* ) 1

1

\log percent of S^os HS0

7 PH

J

L

8

9

10

3

II

Figure 2 . A plot of the log of the rate constant vs. pH f o r the reaction between NO and H S 0 ~ / S 0 " . 3

3


4


2

5

2+

2

2+(acac) (N0) 24 ± 2

10

3

-1.0 χ

2+

36

10

> 6 χ

> 35 > 6

7

7

7.4

-11.0 -20.7

-15.8

5.1 -11.94

χ 10

-

7

χ 10

2.14 1.15

10

7.3

1.8 χ

-

6.3

12.0

6

-

-

4

3.7

8.6

9.1

5

-

-

-

10

5

2a

As°, eu

2.1 χ 105

> 3.3 χ

10 10

4

5

10

ΔΗ°, kcal/mol

where acac acetylacetone, oxal = oxalate, c i t = c i t r a t e , IDA = iminodiacetate, NTA « n i t r i l o t r i a c e t a t e , EDTA = ethylenediaminetetraacetate, and Chelex 100 = a crosslinked polystyrene divinylbenzene porous l a t t i c e with attached imino diacetate multidentate functional groups.

10

> 7 χ

Fe (EDTA)(NO)

2+

Fe (NTA)(NO)

2+

Chelex 100(Fe )(NO)

Fe +(IDA)(NO)

2

Fe +(cit)(IDA)(NO)

2

2

10

17 ± 14

(4.7 ± 2.0) χ

K.L/mol at 298 Κ

2.1 χ

2

(1.5 ± 0.6) χ

s-1

4.9 χ

10

5

k-1,

Fe +(cit)(OH") (NO)

(4.0 ± 3.0) χ

(7.1 ± 1.0) χ 10

mol/(L)

Fe +(cit)(NO)

2

Fe (oxal)(N0)

Fe

Fe (H 0) (N0)

2+

Ferrous Chelates

Table I. Kinetic and Thermodynamic Data for Reversible NO Coordination to Ferrous Chelates


70

m

ο ο ζ η

r

ζ

m

ζ < 55 ο ζ

m

H F Ν δ

c

oo

T1

c m r

CO C/5

τι Ο

14.


CHANG

165

e

25 C. The curve for t h i s rate expression is shown in Figure 2 also. The rate constant for the reaction with s u l f i t e ion is much larger than that obtained by Nunes and Powell (18), who studied the reaction using a s t i r r e d reactor by bubbling NO gas into a s u l f i t e solution and obtained an expression: 2

« .132[NO] + .45[NO][S0 "]

-

(b)

3

- 1


in M s e c at 25 °C and pH 13 to 14. They a t t r i b u t e the f i r s t term to the hydration of n i t r i c oxide and the second term to the reaction of dissolved n i t r i c oxide with s u l f i t e ion. NO + xH 0

+

2

ΝΟ(Η 0) 2

2

+ S0 ~

Χ

3

ΝΟ(Η 0) 2

(2)

Χ

-> " O N S 0 "

(3)

-v -ONSO3-

(4)

3

2

NO + S 0 ~ 3

where NO and NO(H 0) represent dissolved and hydrated n i t r i c oxide respectively. The reaction of hydrated n i t r i c oxide with s u l f i t e ion was assumed to be much faster than the reaction of dissolved n i t r i c oxide with s u l f i t e ion. The "ΟΝΒΟβ" intermediate was believed to react rapidly with a second molecule of n i t r i c oxide to form NHAS. They made no mention of whether the second n i t r i c oxide needed to be hydrated or not. Presumably, because of the r a p i d i t y by which the intermediate reacts with n i t r i c oxide, the n i t r i c oxide could be either in the dissolved or hydrated form. I f t h i s were not the case, the rate expression would be more complex. In our system, the hydrated form of n i t r i c oxide is formed p r i o r to mixing, so the sulfite-independent term would not be observed, and the rate constant we obtain is for the reaction of hydrated n i t r i c oxide with s u l f i t e ion. The rate of the reaction of dissolved n i t r i c oxide with s u l f i t e ion is s u f f i c i e n t l y slow that it would not contrib ute s i g n i f i c a n t l y to the formation of NHAS in the time scales in which we observed the reaction. In experiments where the o r i g i n a l s u l f i t e ion concentration was in excess of that of n i t r i c oxide, we observed concentrations of NHAS equal to half the o r i g i n a l n i t r i c oxide concen t r a t i o n shortly after mixing. This indicated that at least half of the n i t r i c oxide was in the hydrated form. Otherwise, the NHAS con centration would be lower because the dissolved n i t r i c oxide could not generate NHAS quickly. Other than measurements such as these, we have no indication of what the equilibrium constant is for reaction (2). In the analysis of experimental data and calculating rate constants, it was necessary to correct the r e s u l t s for the hydrolysis of NHAS whose k i n e t i c s follow. 2

X

Hydrolysis of N-nitrosohydroxylamine-N-sulfonate. produce nitrous oxide and sulfate: "ON(NO)S0 " 3

+

NHAS hydrolyzes to

2

N 0 + S0 " 2

4

(5)

The rate of t h i s hydrolysis was studied by Seel and Winkler (19) at pH 6.8-7.6 and Ackermann (20) at pH 5.5-11, who found that hydrol y s i s is catalyzed by acid as well as heavy metal ions. By adding EDTA


166


to the system it was possible to avoid the c a t a l y s i s due to heavy metal ions. Seel and Winkler have obtained a hydrolysis rate equation: - ^ M i =

k [H+][NHAS]

(c)

5

4

with k^ = 1.1 χ 10 M~l s e c ~ l . The rate obtained by Ackermann and Powell without EDTA has a hydrogen ion dependence that is l e s s than f i r s t order and can be expressed as: +

86


= k; [H ]°- [NHAS] 3

(d)

1

where k^ = 1.6 χ 1 0 sec"* . The primary effect of trace heavy metal ions appears to be an increase of the decomposition rate at neutral and a l k a l i n e pH conditions. Our k i n e t i c r e s u l t s of the hydrolysis of NHAS between pH 4 and 5.8 were obtained from the analysis of rapid mixing stopped-flow ex periments in the k i n e t i c studies of NO reaction with HSO3. Figure 3 shows the curve of absorption vs. time for a t y p i c a l stopped-flow ex periment at pH 4.6. The flow was stopped at 4 seconds on the scale shown in the figure. The absorbance i n i t i a l l y rose due to the con tinued reaction of NO with HSO3 and SÛ3 ~. As the reactants were depleted, the hydrolysis of NHAS caused the absorbance to decrease. The decaying absorbance curves obtained from these experiments were used to determine the hydrolysis rate constant for NHAS as a function of pH. Short i n t e r v a l s of the decay curve were used to calculate values of the rate constant. Intervals of the curve taken shortly after the flow was stopped yielded values which were low, due to the continuing formation of NHAS by NO + HSO3 and SO3. At longer times, the values became consistent when the NHAS formation reaction no longer occurred at a s i g n i f i c a n t rate. These values were averaged to obtain the hydrolysis rate constant. This is shown in Figure 4, along with r e s u l t s obtained at higher pH conditions by Ackermann (20) and Seel and Winkler (19). 2

2

Reaction of Ferrous N i t r o s y l Chelates with S0 ~/HS03. The reaction is known to produce a large number of products (12), including N 0, N , hydroxylamine disulfonates (HADS), SO^ ", S (>6> and F e . Reports of the reaction have indicated that it is complicated. There are contradictions in the l i t e r a t u r e as to what the reaction products are, as well as the k i n e t i c behavior. Sada and co-workers (21) studied t h i s reaction using a system where NO gas was continuously flowed into a Fe (EDTA) + Na S0 solution. They conclude that the reaction proceeds by S(IV) reacting with NO coordinated to the ferrous complex to produce N 0 and F e . The basis for t h i s conclusion is the appearance of higher concentrat i o n of N 0 in solutions of Na S0 and Fe (EDTA) than in solutions of Na S0 alone when n i t r i c oxide was bubbled through. They a t t r i bute NHAS to be the source of N 0. No rate was given for the reactions of Fe (EDTA) (NO) with either S0 "" or HSO3. In our study of the reaction degassed solutions of reagent grade sodium s u l f i t e and/or sodium metabisulfite were added to the spectro3

2

2

3

2

2

2+

2

3

3 +

2

2+

2

2

2

3

3

2

2+

2

3


14.

167


CHANG

2.0

r

Ο

ω w


φ

ο c σ JO

(11)

2

The l a s t step of the process is the same as that proposed (3,4) f o r the preparation of HADS from b i s u l f i t e ion and nitrous acid. The n i t r o s y l ion, NO", could dimerize and decompose to N 0. More work is required to c l a r i f y the HADS formation process. 2

Formation of Nitrogen-Sulfur Compounds. Once HADS is formed, it can further react to form other nitrogen-sulfur compounds (Figure 1). Chang et a l . (3) have published an extensive review a r t i c l e on the kinetics and mechanisms of important reactions involved. Oxidation of Ferrous Chelates to F e r r i c Chelates. Flue gas contains about 5% oxygen. When dissolved, oxygen can oxidize ferrous ions to f e r r i c ions which are i n a c t i v e for coordination with NO. The oxidat i o n rate of ferrous ions is accelerated in the presence of chelating agents, e.g., EDTA and NTA. This acceleration may be ascribed to the s t a b i l i z a t i o n of the oxidized form by the chelation. Kurimura et a l . (22) studied the oxidation of Fe (EDTA) and Fe (NTA) by dissolved oxygen in aqueous solutions and suggested that the oxidation proceeds by two p a r a l l e l reaction paths, with both protonated and unprotonated chelates reacting. The reaction mechanisms suggested (22,23) are as follows: 2+

2+

k£00 2 2

2+

Fe (HL) + 0

2

F e ^ i L ) + HO"

(12)

Fe (L) +

(13)

^23 3+

2+

Fe (L) + 0 0

H0

+ H+

2

Fe(II) + H0 H0

2

2

+ H

2

°i

2

Fe(III) + HO'

(15)

H

(16)

+

m

(14)

2°2


FOSSIL F U E L S UTILIZATION: E N V I R O N M E N T A L

170 Fe(II) + H 0 2

2

Fe(II) + OH

+

CONCERNS

Fe(III) + 0H~ + OH

(17)

Fe(III) + OH"

(18)

In these equations, Fe(II) and Fe(III) represent the ferrous and f e r r i c ion species respectively. The rate equation f o r the oxidation is: d[

^f

+1

- 4 k [ F e + ( H L ) ] [ 0 ] + 4 ^ [ F e ( L ) ] [0 ] 2

2+

22

(e)

2

2

3

2

1

1

For EDTA, k = 6.8 χ 1 0 and k • 2.7 χ 10 M" s e c " , while f o r NTA, k 3 = 80 M" s e c " at 25 °C. As can be seen from these r e s u l t s , protonated chelates react more rapidly than the unprotonated ones. In addition to oxidation by 0 , F e ( L ) can be converted to F e ( L ) in an aqueous system containing only F e ( L ) + NO + SO3 ". Preliminary experiments from our laboratory, performed by mixing Fe (NTA) or Fe (EDTA) with NHAS, have shown the formation of F e and the l i b e r a t i o n of N 0. The k i n e t i c study of t h i s reaction is in progress. 2 2

2 3

1

1


2

2+

2

3+

2+

2+

2

2+

3 +

2

Reduction of F e r r i c Chelates by HSO3 and Formation of Dithionate. Fe (EDTA) is reduced by HSO^, producing dithionate and a small amount of SO, " (24). The rate of reduction of Fe (EDTA) is f i r s t order in [HSO^] and [Fe (EDTA)], and inversely f i r s t order in [Fe (EDTA)]. J+

2

3+

3+

2+

-

d

[

F

e

( at

E D T A

3+

2+

> J = k[Fe (EDTA)][HS07]/[Fe (EDTA)] ο 6

(f)

1

The rate constant k is 8.8 χ 10" s e c " at 55 °C. The apparent a c t i vation energy is 21.3 kcal/mol. The following reaction mechanism was suggested: 3+

Fe (EDTA) + HS0" FeS0+ Fe

2 +

+ EDTA

SO" + H

+

FeS0+ + H 2 +

o

+

+ EDTA

(19)

=

Fe

+ SO"

(20)

•

Fe (EDTA)

(21)

2+

HS0

HS0 + HS0 3 J o

=

(22)

3

2

S 0 , " + 2H ^ο

+

o

(23)

A n a l y t i c a l Techniques One of the problems in studying the chemistry of wet processes f o r desulfurization and d e n i t r i f i c a t i o n of f l u e gas in order to determine an optimum design and operating condition of scrubbers is the d i f f i c u l t y in quantitatively detecting a l l important species produced. S p e c i f i c a l l y , there was no methodology available f o r the convenient determination of the large number of N-S compounds that can be pro duced. We have applied a laser Raman spectroscopic (25) and ion chrom atographic (26) techniques to the determination of N-S compounds in reaction mixtures. Both techniques require only a small amount of



14.

CHANG

171


the sample and allow simple, rapid and simultaneous determination of these compounds. Table I I shows Raman s h i f t s and r e l a t i v e molar in t e n s i t i e s of some species involved in flue gas scrubbers. The major l i m i t a t i o n of Raman spectrometry is the lack of s e n s i t i v i t y . Samples with low concentrations (0.001-0.01 M) of the compounds of interest require very long data a c q u i s i t i o n times. Using a Dionex 2010i ion chromatograph equipped with AG4 guard column, an AS4 anion separation column, and an anion f i b e r suppressor, we can make determinations of N-S compounds (Figure 5). For the determination of HAMS and s u l f a mate, a 1.5 mM bicarbonate eluent was used with a flow rate of 2.0 mL/min at a pressure of 700-800 p s i . For the determination of HADS, ADS, and NHAS, the AG4 column alone was used as the separation c o l umn with 12 or 18 mM carbonate eluents. The flow rate was 1.5 mL/min at a pressure of 200-300 p s i . Detection l i m i t s for the compounds, defined as the concentration that generated a peak with a height twice that of the background noise, are as follows: HAMS, 0.5 χ 10"° M; sulfamate, 0.6 χ ΙΟ" M; ADS 1.4 χ ΙΟ" M; HADS, 7.4 χ 10" Μ; and NHAS, 0.5 χ ΙΟ" Μ. As the retention time of a compound increases, the peak broadens and decreases in height. Thus it is advantageous to have as short a retention time as possible within the l i m i t a t i o n s of obtaining separation between the peaks of the chromatogram. 6

6

6

6

Future Research

Areas

The improvement over the existing Japanese processes can be made by developing a more e f f i c i e n t ferrous chelate such that it can provide better absorption e f f i c i e n c y for NO, faster reaction rates between NO and S02> and better s t a b i l i t y for the ferrous chelate toward oxida tion, compared to Fe (EDTA) or Fe (NTA) employed in Japanese pro cesses. The development of an e f f i c i e n t and c o s t - e f f e c t i v e method for the reduction of f e r r i c chelate to ferrous chelate without producing dithionate ions could make the process a t t r a c t i v e . In addition to these areas, the study of several a l t e r n a t i v e approaches and novel ideas could develop into a much more e f f i c i e n t and c o s t - e f f e c t i v e scrubber system employing metal chelate additives. (1) Immobilization of the ferrous chelate onto a s o l i d sub strate (27). I f an e f f i c i e n t immobilized ferrous-ion catalyst can be found, it could provide important im provements over the presently available Japanese pro cesses. These improvements include s i m p l i f i c a t i o n in processes design for species separation, reduction of water, energy, and catalyst consumption, and reduction of operating costs. (2) Investigation of non-ferrous metal chelates that can ef f i c i e n t l y absorb NO and provide s t a b i l i t y toward oxida t i o n . Many cobalt, copper, manganese, molybdenum, n i c k e l , osmium, rhenium, rhodium, ruthenium and vanadium chelates have been demonstrated to be able to coorddinate NO. Their application to f l u e gas scrubbing sys tems should be explored. (3) Development of metal chelates suitable f o r employment in a spray drying system (28,29). Spray drying systems have been demonstrated to be more c o s t - e f f e c t i v e than wet systems for control of SO2 emissions from power plants. 2+

2+


172


Table I I . Raman S h i f t s and Relative Molar Intensities of Some Species Involved in Flue Gas Scrubbers

Species

-1

Raman s h i f t (cm )

3

0.053 "0.025 0.125

818 1240 1331 Downloaded by UNIV LAVAL on September 28, 2015 | http://pubs.acs.org Publication Date: September 18, 1986 | doi: 10.1021/bk-1986-0319.ch014

Relative molar intensity

0.95

NO"

1050

N 0= 2 2

692 1115 1383

N 0-

1285

~0.18

967

0.12

1023

0.10

455

~0.07

981

1.00

b

0

b

o

2

S0= HSO~ S0=

weak** weak strong*

b

~0.9

HSO^

1055

S

1050

0.05

~700 1084

~0.20 1.43

~420 ~760 1058 1004 918 1097 1084 1063 1049

~0.13 "0.08 0.48 0.21 0.09 0.10 0.056 c 0.41

2°5

HADS

HAMS HA (pH < HA (pH > ATS ADS SA (pH < SA (pH >

a

b

7) 9)

1) 3)

SOT 981 c m 4

-1

5

l i n e = 1.000

Rauch, J . E.; Decius, J . C. Spectrochim. Acta 1966, 22, 1963.

c

No value obtained.


CHANG



14.


173

174


The addition of métal chelates to the a l k a l i spray s l u r r i e s f o r simultaneous control of SOo and NO could be very e f f e c t i v e . The Fe (EDTA) and Fe* (NTA) type metal chelates are not suitable for use in spray drying systems, however, because the metal chelates suitable for employment in spray drying systems must be able to absorb or react with NO i r r e v e r s i b l y under the scrubbing conditions. As the droplet passes through the flue gas, it w i l l heat up. This has the effect of reducing the l i q u i d volume through evaporat i o n and changing the k i n e t i c s due to the temperature dependence of the rate constants and equilibrium constants. The increase in temperature w i l l also reduce the s o l u b i l i t y of Ν 0 and S0 . I f the droplet loses a l l its water by evaporation, the NO attached to Fe (EDTA) or Fe +(NTA) type ferrous chelates w i l l dissociate and return to the gas phase. From the k i n e t i c information presented in t h i s paper, one can c a l culate (30) that only a small f r a c t i o n of absorbed NO w i l l react with dissolved S0 during about 10 sec res idence time of the droplet in spray drying chamber. A cost-effective method for the recovery of metal che lates must be available for the process to be a t t r a c tive.


2+

χ

2+

+

2

2

2

Acknowledgments We appreciate the support and encouragement of Michael Perlsweig, Joseph Strakey, and John Williams. This work was supported by the Assistant Secretary for F o s s i l Energy, U.S. Department of Energy under Contract No. DE-AC0376SF00098 through the Pittsburgh Energy Technology Center, Pittsburgh, Pennsylvania.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9.

Moore, T. EPRI Journal, November 1984, 26-33. Faucett, H. L.; Maxwell, J. D.; Burnett, T. A. "Technical Assess ment of ΝOx Removal Processes for Utility Application"; EPRI-AF568, March 1978. Chang, S. G.; Littlejohn, D.; Lin, Ν. H. ACS Symp. Ser. 1982, 188, No. 118, 127-152. Oblath, S. G.; Markowitz, S. S.; Novakov, T.; Chang, S. G. J. Phys. Chem. 1981, 85, 1017. Gomiscek, S.; Clem, R.; Novakov, T.; Chang, S. G. J. Phys. Chem. 1981, 85, 2567. Oblath, S. B.; Markowitz, T.; Novakov, T.; Chang, S. G. J. Phys. Chem. 1982, 86, 4853. Oblath, S. B.; Markowitz, S. S.; Novakov, T.; Chang, S. G. Inorg. Chem. 1983, 22, 579. Martin, A. E., Ed.; "Emission Control Technology for Industrial Boilers"; Noyes Data Corp.: Park Ridge, NJ, 1981. Littlejohn, D.; Chang, S. G. J. Phys. Chem. 1982, 86, 537.


14. 10. 11. 12. 13. 14. 15.


16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

CHANG


175

Lin, N.; Littlejohn, D.; Chang, S. G. I & EC Proc. Design & Deveop. 1982, 1, 725. Chang, S. G . , ; Littlejohn, D.; Lynn, S. Environ. Sci. & Tech. 1983, 17, 649. Littlejohn, D.; Chang, S. G. Lawrence Berkeley Laboratory report LBL-17962, submitted to Inorg.. Chem., 1985. Maxwell, J. D.; Tarkington, T. W.; Burnett, T. A. "Preliminary Economic Analysis of ΝOx Flue Gas Treatment Processes Using TVA and EPRI Economic Premises" EPRI CS-2075, December 1981. Czerlinski, G.; Eigen, M. Elektrochem. 1959, 63, 652. Kustin, K.; Taub, I. Α.; Weinstock, E. Inorg. Chem. 1966, 5, 1079. Hishinuma, Y . ; Kaji, R.; Akimoto, H . ; Nakajima, F . ; Mori, T . ; Kamo, T . ; Arikawa, Y . ; Nozawa, S. Chem. Soc. Jap. Bull. 1979, 52, 2863. Teramoto, M.; Hiramine, S.; Shimada, Y . ; Sugimoto, Y . ; Teranishi, H. J. Chem. Eng. of Japan 1978, 11, 450. Nunes, T. L . ; Powell, R. E. Inorg. Chem. 1970, 9, 1916. Seel, F.; Winkler, R. Z. Naturforsch. 1963, 18a, 155. Ackermann, M. N. "Alkaline Hydrolysis and Oxidation of Hydroxylamine-N-Sulfonate", Ph.D. Thesis, University of Cali fornia, Berkeley, 1966. Sada, E . ; Kumazawa, H . ; Takada, Y. IE & C Fundam. 1984, 23, 60. Kurimura, Y . ; Ochiai, R.; Matsuura, N. Chem. Soc. Jap. Bull. 1968, 41, 2234. Kurimura, Y . ; Kurizama, H. Chem. Soc. Jap. Bull. 1969, 42, 2238. Sato, T . ; Simizu, T . ; Okabe, T. Nippon Kagakukaishi 1978, 361. Littlejohn, D.; Chang, S. G. Environ. Sci. & Tech. 1984, 18, 305. Littlejohn, D.; Chang, S. G. Anal. Chem. 1986, 58, 158. Brodbeck, K.; Chang, S. G. "Study of Ferrous Ion on Chelex 100 for NO Absorption and Subsequent Reactivity with Oxygen", Law rence Berkeley Laboratory report LBL-20132, 1985. Chang, S. G.; Griffiths, E. "A Process for Combined Removal of SO2 and ΝOx from Flue Gas", LBL case no.: IB-612, U.S. patent pending.. Chang, S. G. "Spray Drying Process for Simultaneous Removal of SO2, ΝOx, and Particulates from Flue Gas", LBL case no.: IB-643, U.S. patent pending, 1986. Chang, S. G.; Littlejohn, D. Lawrence Berkeley Laboratory re port LBL-19398, 1985.

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