Encyclopedia of Environmental Science and Engineering, Volume I and II

1212 VAPOR AND GASEOUS POLLUTANT FUNDAMENTALS

the total (De Vitt et al., 1980). The following reactions take

place in the limestone systems:

CaCO (s) CaCO (aq)

2 CaCO (aq) 2H CaCHCO Ca

SO (g) SO (aq

3 3

3 3

2

2 2

�

�

�

   

))

SO (aq) H O H SO HSO H

HSO O (aq) SO H

Ca SO

2 2 2 3 3

3

1

2 2 4

2

2

 

 



 

  



→

→

�

44

2

CaSO (aq) CaSO (s) 4 4

 � → ↓

Or

Ca 2Cl CaCl (aq) CaCl (s)

2H SO H SO

H Cl HCl

Ca(H

2

2 2

4

2

2 4

 

 

 







�

�

�

→ ↓

CCO ) 2H Ca 2H CO

H CO CO HO

3 2

2

2 3

2 3 2 2

 



� 

�

The dissolution rate of limestone depends on the pH

values. The pH values encountered in practical operations

of limestone systems is in most cases between 5.5 and 6.5.

If the limestone systems operate at too low pH, SO 2 removal

efficiency will decrease. At too high pH, the scale formation

will be promoted. Other factors affecting the performance of

limestone systems include solids content, liquid-gas ratio,

and corrosion. A discussion can be found elsewhere (De Vitt

et al., 1980).

In selecting the FGD processes, the following should be

considered:

1) process type: wet or dry, regenerable or non-

regenerable.

2) chemical reagent used.

3) end-product produced: saleable product or dis-

posable waste.

The reagent, end product, principle of operation, and SO 2

removal efficiency of major FGD processes are shown in

Table 1 (Princiotta, 1978). For details of SO 2 removal refer

to the Stack gas cleaning sections.

Sulfur dioxide reacts slowly with a large excess of oxygen

in the presence of sunlight to form trioxide. Gerhard (1956)

showed that the process occurs with O 2 at a rate of 0.1–0.2%

per hour and Cadle (1956) with ozone, O 3 , at 0.1% per day.

Niepenburg (1966) illustrates the effects of oxygen in the

waste gas during combustion of oil.

The conversion of SO 2 to SO 3 is believed to be possible

at realistic rates because of the presence, on diverse surfaces,

of Fe 2 O 3 which acts as a catalyst. The SO 3 has a short lifetime

since it readily combines with water vapor in the atmosphere

to form sulfuric acid.

Oxides of Nitrogen, NOx

NOx is produced in all combustions which take place using air

as an oxygen supply and in those chemical industries employ-

ing nitric acid. More than 55% of the total NOx emissions of

20 million tons originate from stationary sources as shown in

Figure 1, and 93% of all stationary source NOx emissions are

from combustion of fossil fuels for utilities. Direct industry-

related emissions account for only 5% of the stationary source

total. Approximately 30% of all stationary source NOx is emit-

ted by coal-fired utility boilers. Uncontrolled NOx emissions

from coal-fired sources have been measured in the range 0.53

to 2.04 lb/10^6 BTU at full load (Ziegler and Meyer, 1979). The

NOx formed in combustion is from fixation of atmospheric

nitrogen and/or fuel nitrogen. Ermenc (1956) found that at

high temperature nitrogen and oxygen combine to form both

NO and NO 2. The yield of NO increases from 0.26% at 2800F

to 1.75% at 3800F. If the temperature is reduced slowly the

reverse reaction will take place, but if the products are quenched

by rapid heat exchange, the reverse reaction rate becomes small

and the oxides remain in the exhaust stream. The oxide NO can

usually be oxidized to form NO 2 according to:

2NO O 2NO .^2 →^2

Because this is a tri-molecular gas phase reaction, the con-

centration of NO and NO 2 tremendously affects the rate at

which the oxidation takes place. At low concentration, for

example 1–5 ppm in air, the reaction is so slow that it would

be negligible except for the photochemical reactions which

take place in the presence of sunlight. The dioxide also reacts

with oxygen to form ozone. The existence of nitrogen trioxide

at low concentration in polluted atmospheres is postulated

(Hanst, 1971) to form by the reaction with ozone.

NO O NO O^2 ^3 �^3 ^2

and remain in equilibrium with N 2 O 5

N O NO NO .2 5�^2 ^3

The NO 3 formation reaction only takes place after NO is sub-

stantially depleted in the atmosphere and O 3 begins to appear.

Without more stringent control of new sources, the NOx

emissions by 1995 are projected to be 66% higher than than

in 1985. Even with application of the best control method to

all new sources there is still a projected 24% increase over 10

year emissions (McCutchen, 1977). The typical NOx emis-

sion from nitric acid plants is 1000–3000 ppm. The federal

standard for new nitric acid plants is 3 lb NOx/ton 100%

HNO 3 —that is, about 200 ppm (Ricci, 1977). The most

widely used process for nitric acid plant tailgas cleanup is

catalytic decomposition of NOx to nitrogen and oxygen.

The current and projected values of the New Source

Performance Standards (NSPS) for NOx are discussed later

in this article. During recent years N 2 O formation rates

have been the subject of controversy, especially in fuel NOx

mechanisms.

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