Capsule Report: Adipic Acid Enhanced Lime/Limestone Test Results at the EPA Alkali Scrubbing Test Facility


     United States
     Environmental Protection
     Agency
 Industrial Environmental Research
 Laboratory
 Research Triangle Park NIC 27711
     Technology Transfer
     Capsule Report
     Adipic Acic
-Enhanced
     Lime/Limestone
     Test Results at the
     EPA Alkali Scrubbing
     Test Facility
                    I'
1'=- — •""- ^

-------

-------
Technology Transfer
EPA 625/2-82-029
Capsule Report
Adipic Acid-Enhanced
Lime/Limestone
Test Results at the
EPA Alkali Scrubbing
Test Facility
April1982
This report was developed by the
Industrial Environmental Researqh Laboratory
Research Triangle Park NC 27711

-------
Shawnee Test Facility

-------
1. Introduction
and Background
This is the fifth i t a series of capsule
reports describing the results of the
Shawnee Lime and Limestone Wet
Scrubbing Test P|rogram conducted by
EPA's Industrial Environmental
Research Laboratory, Research Tri-
angle Park, North Carolina (IERL-
RTP). In this program, flue gas desul-
furization (FGD) tests were conducted
at the EPA 10 WlW prototype Shawnee
Test Facility located at the Tennessee
Valley Authorit| (TVA) coal-fired
Shawnee Power Station near Paducah,
Kentucky. Bechtel National, Inc. of
San Francisco was the major contrac-
tor and test director, and TVA was the
constructor and acility operator.

This report describes the results of
adipic acid-enhanced lime and lime-
stone testing at the Shawnee Test Fa-
cility from July 1978 through March
1981. It also summarizes earlier adipic
acid additive test results from the
IERL-RTP 0.1 MW pilot plant, which
led to the testing at Shawnee. Also
reported are preliminary results from
the 100 MW full
being conducted
Power Plant of
scale demonstration
at the Southwest
Springfield City
Utilities, Springflield, Missouri and
from the 27 MW
boiler test at Ricl
Base.

As the emission
dioxide become
equivalent industrial

-------
2. Advantages
of Adipic Acid as
Scrubber Additive
A number of attractive features of
adipic acid as a scrubber additive are
presented below.
Handling
Adipic acid is non-toxic, non-hygro-
scopic, and usually comes in powder
form. It is easy to handle and no
hazards are encountered in the usual
applications other than possible dust
explosions, which are typical of any
organic dust. At Shawnee, it is rou-
tinely dry-fed directly to the effluent
hold tank, although it has been added
to the fresh limestone slurry makeup
tank in some instances.
Buffer Reaction Mechanism
The mechanism by which adipic acid
buffers the pH is simple. It reacts with
lime or limestone in the effluent hold
tank to form calcium adipate. In the
absorber, calcium adipate reacts with
absorbed S02(H2S03) to form CaS03
and simultaneously regenerates adipic
acid (the buffer reaction). The regen-
erated adipic acid is returned to the
effluent hold tank for further reaction
with lime or limestone. With a suffi-
ciently high concentration of calcium
adipate in solution, usually on the
order of 10 m-moles/liter to react with
the absorbed SO2, the overall reaction
rate is no longer controlled by the
dissolution rate of limestone or cal-
cium sulfite.
the tightness of the liquor loop, the
quantity of adipic acid required is
quite small in relation to the alkali
feed. At Shawnee, where a filter is
normally used as the final sludge
dewatering device, the adipic acid
consumption rate is usually less than
10 Ib/ton of limestone fed to the sys-
tem, and sometimes as low as 2 Ib/ton
of limestone. These values correspond
to only 0.6 to 3.0 tons of adipic acid
per day for a 500 MW plant.
Adipic acid has two pH buffer points.
These are pH 4.5 and 5.5 in the
absence of chloride in the liquor, and
about 4 and 5 with 5,000 to 7,000
ppm chloride. To fully utilize the buf-
fer capacity of adipic acid, therefore,
the slurry pH should be kept above
these values. At Shawnee, where the
chloride concentration is usually a few
thousand ppm, a slurry pH above
about 5.2 is sufficient to keep adipic
acid fully active (or ionized). The
optimum concentration range of
adipic acid at a pH above 5.2 is only
700 to 1,500 ppm for 90 percent
removal of approximately 2,500 ppm
inlet SC>2. Higher concentrations
would be required at a lower pH to
maintain equivalent buffer capacity in
the liquid. It should be noted that
most of the degradation products of
adipic acid, such as valeric and glutaric
acid, are also effective buffers.
Retrofit
Use of adipic acid in an existing lime
or limestone system does not require
modification of process flow configu-
ration or absorber design; therefore, it
is particularly suited for retrofit appli-
cations. The fact that it may be added
at any point in the slurry circuit pro-
vides a greater flexibility in the loca-
tion and installation of a simple solids
storage and feed system, a minimal
capital investment.
Quantity and Concentration
Depending on the operating param-
eters, the degree of degradation, and
Limestone Utilization
At a scrubber inlet pH of about 5.2,
the corresponding limestone utiliza-
tion is normally 80 percent or higher
for an adipic acid-enhanced system, as
compared to 65 to 70 percent in unen-
hanced limestone systems at an equiv-
alent SO2 removal. Thus the quantity
of waste solids generated is reduced in
an adipic acid-enhanced system.
Higher limestone utilization also con-
tributes to more reliable scrubber
operation by reducing the fouling
tendency. This increased reliability is a
very attractive feature of adipic acid-
enhanced systems, since reliability
problems have historically plagued
limestone FGD.
-------
Operating pH
With proper pH control and suffi-
ciently high adipic acid concentra-
tion (sufficient buffer capacity), the
scrubber performance is more stable,
and steady outlet SO2 concentrations
can be maintained, even with wide
fluctuations of inlet SO2 concentra-
tions.

With the lower operating pH (about
4.6 to 5.4) in an adipic acid-enhanced
limestone system, compared to the
higher pH (about 5.5 to 5.8) usually
needed for an unenhanced limestone
system, the system becomes more
amenable to other process concepts
and improvements. Potential advan-
tages of low pH operation are:
• Reduced adipic acid consumption.
Adipic acid degradation has been
found to decrease with decreasing
pH
• Easier forced oxidation in the
scrubber slurry loop or bleed
stream, and a smaller air (and
compressor energy) requirement
• Potential for essentially complete
limestone utilization with improved
scrubber operating reliability
• Reduced sensitivity of the system
to limestone type and grind. Fine
grinding of limestone is probably
not required
• Lower sulfite scaling potential
• Better prospects (sensitivity) for
automatic pH control

• Greater flexibility for SO2 emission
control. Higher sensitivity of SO2
removal at lower pH allows raising
pH to increase the adipic acid buf-
fer capacity and SC>2 removal when
needed
• Applicability to low-sulfur sub-
bituminous and lignite coals con-
taining alkaline ashes, which are
extractable only at low pH
• Lower costs due to all of the above
factors
Economics
Since limestone dissolution is not a
rate-controlling step in SO2 absorption
for an adipic acid-enhanced limestone
system, adipic acid should promote use
of less expensive and less energy-inten-
sive limestone rather than lime.

Adipic acid-enhanced limestone scrub-
bing has lower
operating costs
stone or MgO-
scrubbing. This
reduced limestc
projected capital and
than unenhanced lime-
enhanced limestone
is due primarily to the
ne consumption at the
lower operating! pH, the reduced grind-
ing cost, and the reduced quantity of
waste sludge generated.
Forced Oxidation
The mechanism1 by which adipic acid
promotes SO2 removal is not affected
by forced oxidapon. Therefore, it can
be used with both lime and limestone
in systems with) or without forced
oxidation.
Since forced ox
dation converts sulfite
to sulfate, it has an adverse effect on
SO2 removal in an unenhanced lime
system in which sulfite is the major
SOa scrubbing species. This is also true
in MgO-enhanced lime and limestone
systems in which the promotion of
SOa removal relies on an increased
sulfite-bisulfite buffer. When adipic
acid is used with lime, calcium adipate
becomes a major buffer species; there-
fore, both good SO2 removal and sul-
fite oxidation can be achieved using
within-scrubber-loop forced oxidation.

Chloride Effect
The effectiveness of adipic acid is not
adversely affected by chlorides, as is
the effectiveness of MgO in an MgO-
enhanced process. Tests at the 1ERL-
RTP pilot plant showed that SO2
removal efficiency obtained with
17,000 ppm chloride in the scrubbing
liquor was not significantly different
from that obtained without chloride
under similar levels of adipic acid con-
centration. Thus, use of adipic acid is
especially attractive for systems with a
very tightly closed liquor loop.
Solids Dewatering
Adipic acid does not significantly
affect the settling and filtration prop-
erties of oxidized or unoxidized
slurry solids, whereas magnesium does.

Total Dissolved Solids
Addition of adipic acid does not sig-
nificantly increase the total dissolved
solids in liquid as does magnesium.
High total dissolved solids in liquid
entrainment can increase particulate
emissions and fouling tendencies of •
equipment downstream of the
scrubber.
-------
3. Test Programs
The theoretical basis for the effect of
adipic acid on the performance of lime
and limestone scrubbers was first
developed in detail by G. Rochelle in
1977 ("The Effect of Additives on
Mass Transfer in CaCOs or CaO
Slurry Scrubbing of S02 from Waste
Gases," Industrial and Engineering
Chemistry Fundamentals, 16, pp.
67-75, 1977). In October 1977, EPA
began an investigation of adipic acid
with the 0.1 MW lERL-RTP pilot
plant to determine its effectiveness as
an additive to limestone scrubbers for
improving SO2 removal efficiency.
Initial results demonstrated, as pre-
dicted by Rochelle, that adipic acid
was indeed an.attractive and powerful
additive.
Based on the findings at the IERL-
RTP pilot plant, a program was set up
at the 10 MW Shawnee Test Facility to
develop commercially usable design
data for adipic acid as a chemical
additive. Actual testing at Shawnee
began in July 1978, and lasted through
March 1981. The test schedule for this
period is shown in Figure 1. Tests were
conducted over a period of 33 months,
using both lime and limestone with
and without forced oxidation. As can
be seen, major emphasis was placed
on limestone testing with forced
oxidation.
As part of EPA's continuing program
of FGD technology transfer, and to
further demonstrate the effectiveness
of adipic acid and to encourage its use,
EPA contracted with Radian Corpora-
tion in the spring of 1980 to conduct
a full-scale demonstration program
of adipic acid-enhanced limestone
scrubbing. The program, being con-
ducted with two 100 MW Turbulent
Contact Absorbers (TCA) located at
the Springfield City Utilities' South-
west Station near Springfield, Mis-
souri, continued through October
1981. Some preliminary test-results
are included in this report, as are data
from the industrial-sized (27 MW)
scrubber test conducted by PEDCo
Environmental, Inc. on the Bahco
system at Rickenbacker Air Force
Base.
During some factorial tests conducted
at the Shawnee Test Facility in 1979,
it was noticed that the rate of adipic
acid addition required to maintain a
desired concentration in the scrubber
liquor was substantially reduced when
the scrubber inlet pH was controlled at
5.0 or lower. In order to verify the
Shawnee findings, the EPA initiated
several programs to study the adipic
acid degradation phenomenon. Con-
tractors involved included the Univer-
sity of Texas at Austin, Radian Cor-
poration, Acurex Corporation, and
Research Triangle Institute. Programs
were set up to investigate the effects
of pH, oxidation, and catalysts such as
manganese and iron on adipic acid
degradation, to develop analytical pro-
cedures and to identify the degrada-
tion products. Although the adipic
acid degradation mechanism is com-
plex, the principal variables affecting
degradation and its major products
have been identified. The results of
these studies are beyond the scope of
this report and will be reported
separately.
Shawnee Test Facility
Tests with adipic acid at Shawnee have
been conducted on two parallel scrub-
ber systems: a venturi/spray tower sys-
tem (Train 100) and a TCA system
(Train 200). Each system has its own
slurry handling and dewatering facil-
ities, and each is designed to remove
both SO2 and particulate from
approximately 10 MW equivalent of
flue gas (up to 35,000 acfm at 300°F).
The flue gas, which normally contains
1,400 to 3,500 ppm by volume of
SO2, is obtained either upstream (con-
taining high fly ash loading of 2 to 7
grains/dry scf) or downstream (con-
taining low fly ash loading of 0.2 to
0.6 grain/dry scf) from Boiler No. 10
particulate removal equipment.

In June 1980, the venturi scrubber
was removed from Train 100, allowing
operation with the spray tower only.
Prior to the removal of the venturi
scrubber, operation with a true spray-
tower-only configuration was not pos-
sible without some interference from
the venturi, even with its adjustable
-------
plug wide open and minimum slurry
flow for flue gas cooling.

Shawnee Test Blocks
Tests conducted at the Shawnee Test
Facility can be classified into blocks
according to type of alkali, fly ash
loading in the flue gas, adipic acid
addition, and forced oxidation
scheme. Table 1 lists the combinations
of these variables which have been
tested at Shawnee, including factorial
tests.
Forced oxidation is achieved by air
sparging of the slurry in an oxidation
tank, either on the bleed stream to the
solids dewatering system or on the
recirculated sluriTy within the scrubber
slurry loop. For a one-scrubber-loop
forced oxidation system, the slurry
effluent from all scrubbers in the sys-
tem (e.g., the venturi scrubber and
spray tower at Shawnee constitute a
two-scrubber system, and the spray
tower alone or TCA, a one-scrubber
system) are sent to a single effluent
hold tank, whicl- is the oxidation tank.
For a two-loop forced oxidation sys-
tem, there are two scrubbers in series
(e.g., venturi and spray tower at
Shawnee) with effluent from each
scrubber going to a separate-tank;
the effluent hold tank for the up-
stream scrubber (with respect to gas
flow) is the oxidation tank. For either
one-loop or two-loop forced oxidation
systems, the oxidation tank may be
followed by a second tank, in series, to
provide further limestone dissolution
and gypsum desupersaturation time
prior to recycle to the scrubber.
Adipic Acid Test Block
Venturi/Spray Tower System (Train 100)
Lime with forced oxidation
Limestone with forced oxidation,
no adipic acid (Widows Creek simulation)
Lime and limestone (Venturi only)
Limestone factorial
Limestone without forced oxidation

Spray Tower System (Train 100)

Limestone factorial, no adipic acid
Limestone factorial
Limestone with forced oxidation

TCA System (Train 200)
Limestone without forced oxidation
Lime without forced oxidation
Lime without forced oxidation.
no adipic acid
Lime with forced oxidation
Limestone with forced oxidation
Limestone factorial
Limestone with forced oxidation,
no adipic acid
Limestone, low fly ash loading
(Springfield simulation)
Limestone without forced oxidation,
no adipic acid (Glitsch Grid packing)
Note: All tests are with adipic acid and high fly
1978
J

••

Bo
1979
J

•

nmm

iler
Nd. 10
Outage,

Train
200 -*-
on] Boiler
Nd. 9

•
••

Mi
•

MMB

mmmm

Ufm

ash loading unl

•
mm

•

mm
oaa

M
•

ess otherwise
J

•

-4-

^mM

•

mmm

mmam

—

_
i

•
•

) N

••M

•
•

-*•

1980
J

M J

_^_

Boiler
No

Du
, 10
tac
- Train"
on
No

S
~

Removed
, Venturi
from Train
100

mmaam

mmam

am
am

mmmm

m
m

100
Boiler
.9

DOWA
Process

• DerfiohstratiorJ

noted.

•

aaam

am i

1981
J F M A

_
mmaataaam

•

Figure 1. Shawnee Adipic Acid Test Schedule
-------
4. Test Results
It is beyond the scope of this capsule
report to present all of the Shawnee
test results from the test blocks listed
in Table 1. Therefore, only the typical
and important test results are pre-
sented below. Results of long-term
tests (longer than one month) are
included. Results from factorial or
partial factorial tests, which normally
lasted a minimum of 12 hours, includ-
ing 5 to 7 hours of steady state opera-
tion, are also included as figures to
illustrate the effects of pH and adipic
acid concentration on SO2 removal.
A summary of initial tests at the
IERL-RTP pilot plant and the prelimi-
nary results from the full-scale TCA
tests at Springfield are also given.

IERL-RTP Pilot Plant Test Results
The initial testing of adipic .acid as a
scrubber additive was carried out by
EPA beginning in October 1977 in the
0.1 MW in-house pilot plant located at
IERL-RTP. A single-loop limestone
scrubber was used for this purpose,
operated with forced oxidation in the
scrubbing loop. In addition to effects
Table 1.
Test Blocks Conducted at Shawnee
Test Fly Ash
Block Alkali Loading
Venturi/Spray Tower
1 Lime
2 Lime
s'a' Limestone
4 Limestone
5 Limestone
6 Limestone
7 Limestone
8 Limestone
9'k) Limestone
Spray Tower System:
10^a' Limestone
1 1 Limestone
12 Limestone
13 Limestone
TCA System:
14 Lime
15 Lime
16'a' Lime
17 Limestone
18 Limestone
ig'a) Limestone
20 Limestone
21 'c' Limestone
22'd' Limestone
23 'd) Limestone
System:
High
High
High
High
High
High
High
High
High

High
High
High
High

High
High
High
High
High
High
High
High
Low
Low
Adipic Acid Oxidation No. of Tanks
Addition Scheme in Oxid. Loop

Yes
Yes
Yes
Yes
Yes
Yes
Yes
Yes
No

Yes
Yes
No
No

Yes
Yes
No
Yes
Yes
Yes
No
No
Yes
Yes

2-Loop
No
2-Loop
2-Loop
1-Loop
1-Loop
Bleed Stream
No
2-Loop

1-Loop
No
1-Loop
No

1-Loop
No
No
1-Loop
1-Loop
No
1-Loop
No
1-Loop
No

2
—
2
1
2
1
—

2
—
2
—

1
—
_
2
1
—
1
: . .
1
—
(a) Includes long-term (greater than one month) tests.
(b) Widows Creek forced oxidation simulation tests.
(c) Glitsch Grid packing tests.
(d) Springfield adipic acid simulation tests.
-------
on SO2 removal and oxidation effi-
ciencies, these tests sought to deter-
mine whether adipic acid caused any
change in the properties of the oxi-
dized sludge. So that these properties
could be clearly seen, the system was
operated without fly ash. Chloride
was added as HCI and controlled at
the high levels expected for tightly
closed loop systems.

The results of the tests showed adipic
acid to be very effective in improving
SO2 removal efficiency, even when
operating at chloride levels as high as
17,000 ppm. A TCA scrubber, which
removed 82 percent of the inlet SO2
without the additive, yielded 89 per-
cent SO2 removal with 700 ppm adi-
pic acid, 91 percent removal with
1,000 ppm, and 93 percent removal
with 2,000 ppm adipic acid. The lime-
stone utilization was concurrently
increased from 77 percent without the
additive to 91 percent with 1,600 ppm
adipic acid. The observed effects thus
confirmed the theoretical expectations
in all respects.
showed no ser
adipic acid on
oxidizer, opera

The quality of
similar to that
In addition, the tests
ous interference by
the performance of the
:ing at pH 6.1.

tie oxidized sludge was
obtained when opera-
ting without adipic acid, although
small differences were detected. For
example, the fi
80 percent sol
tests) vs 84 pen
tered sludge averaged
ds (for 13 one-week
:ent solids for 11 tests
without the additive, when operating
at 97 to 99 pen
cases. The settl
(fly ash free at
cm/min during
:ent oxidation in both
ing rate of the slurry
50°C) averaged 2.3
the adipic acid tests
and 3.4 cm/min without adipic acid;
bulk settled densities averaged 1.0 and
1.2 g solids/cm3 slurry, respectively.
It was concluded from these results _
that the large improvements in sludge
quality that can be achieved by forced
oxidation are nst compromised by the
use of adipic pcid as a scrubber
additive.

Tests without forced oxidation also
demonstrated the efficacy of adipic
acid. Operating a TCA scrubber with
2,000 ppm adipic acid and 6 inches
H2O pressure drop, 92 percent SO2
removal was obtained at a limestone
utilization level of 88 percent. By
comparison, only 75 percent SO2
removal would be expected in the
pilot plant at these test conditions
without the additive. At this adipic
acid level, the unoxidized sludge fil-
tered to 49 percent solids; at lower
adipic acid levels (1,500 ppm or less),
the filterability of the slurry was the
same as that obtained without addi-
tives: 55 percent solids.

During the testing with adipic acid, the
scrubbing liquor had a noticeable odor,
even though the additive feed did not.
The odor has been identified as that of
valeric acid, CH3(CH2)3 COOH, an
intermediate product formed by side
reactions that degrade adipic acid at
scrubber operating conditions. At
Shawnee, this odor was rarely noticed
and was not a problem.
Flue Gas
Reheat.
Makeup Water
Clarified Liquor from
Solids Dewatering System
Bleed to Solids
Dewatering
System
Figure 2. Flow Diagram for Adipic Acid-Enhanced Scrut bing in the Venturi/Spray Tower System with Two Scrubber
Loops and Forced Oxidation
-------
Limestone Long-Term Tests with
Two Scrubber Loops and Forced
Oxidation
The venturi/spray tower system was
modified for two-scrubber-loop opera-
tion with forced oxidation as shown in
Figure 2 (see previous page). Two
tanks were used in the oxidation loop
(venturi loop); air was injected to the
first of these tanks through a simple
3-inch diameter pipe below the agita-
tor. Adipic acid was dry-fed into the
spray tower effluent hold tank. This
was accomplished by manually adding
one-pound increments hourly to main-
tain specified concentration, usually
totaling only a few pounds per hour. A
small screw feeder would serve the
purpose in a full-scale plant.

The main advantage of this two-loop
system, as far as forced oxidation is
concerned, is that it permits operation
of the first loop (venturi loop) at
lower pH for good oxidation effi-
ciency, while maintaining higher pH in
the second loop (spray tower loop) for
good SO2 removal. This configuration
also maximizes the limestone utiliza-
tion. With adipic acid enhancement,
however, some of these advantages can
also be obtained in a single-loop
scrubber because an adipic acid-
enhanced system can be operated at a
lower pH (4.6 to 5.4). Thus, good
oxidation and SOj removal both can
be achieved without an independent
first loop.

Table 2 summarizes the results of two
long-term tests (exceeding one
month), Runs 907-1A and 907-1B,
with adipic acid addition and with a
variable gas flow rate. The results
of Run 901-1A, a base case run with-
out additive and with a constant spray
tower gas velocity of 9.4 ft/sec, are
also included for comparison.

Run 907-1A was a month-long adipic
acid-enhanced limestone run with
forced oxidation, designed to demon-
strate operational reliability with
respect to scaling and plugging and to
demonstrate the removal enhancement
capability of the adipic acid additive.
This run was controlled at a nominal
limestone stoichiometry of 1.7 (corn-
Table 2.
Adipic Acid-Enhanced Limestone Tests on the Two-Loop Venturi/Spray
Tower System with Forced Oxidation
Run No.
901-1A 907-1A 907-1B
Onstream hours
Fly ash loading
Adipic acid cone, in venturi, ppm
Adipic acid cone, in spray tower, ppm
(controlled)
Spray tower gas velocity, ft/sec
Venturi liquid-to-gas ratio, gal/Mcf
Spray tower liquid-to-gas ratio, gal/Mcf
Venturi slurry solids cone., wt % (controlled)
Spray tower slurry solids cone., wt %
Venturi inlet pH (controlled)
Spray tower inlet pH
Venturi pressure drop, in. h^O
Oxidation tank level, ft
Oxidation tank residence time, min
Desupersaturation tank residence time, min
Spray tower effluent tank residence time, min
Spray tower limestone stoich. ratio (controlled)
Average percent SO2 removal
Average inlet SO2 concentration, ppm
Percent oxidation of sulfite to sulfate
Air stoichiometry, atoms oxygen/mole
SO2 absorbed
Overall percent limestone utilization
Venturi inlet liquor gypsum saturation, %
Spray tower inlet liquor gypsum saturation, %
Filter cake solids content, wt %
187
High
0

0
9.4
21
57
15
5.9
4.50
5.45
9.0
18
11.3
4.7
14.7
1.36
57
2,800
98

2.30
97
95
95
85
719
High
2,360

1,560
4.8-9.4
21-42
57-111
15
6.1
4.65
5.45
3.0-9.6
18
11.3
4.7
14.7
' 1 .77
97.5
2,350
98.5

2.0-3.85
88
110
105
87
1,666
High
2,180

1,510
5.4-9.4
21-37
57-100
15
5.9
4.65
5.35
3.5-9.2
18
11.3
4.7
14.7
1.70
97
2,500
98

1.9-3.3
92
105
110
85
pared to 1.4 for the base case run. Run
901-1 A) and 1,500 ppm adipic acid in
the spray tower. Venturi inlet pH was
controlled at a minimum of 4.5 by the
occasional addition of limestone to the
venturi loop.

Flue gas flow rate was varied from
18,000 acfm to a maximum of 35,000
acfm (spray tower gas velocity
between 4.8 and 9.4 ft/sec) to follow
the daily boiler load cycle, which nor-
mally fluctuated between 100 and
150 MW. The adjustable venturi plug
was fixed in a position such that the
pressure drop across the venturi was
9 inches H2O at 35,000 acfm maxi-
mum gas rate. Actual pressure drop
ranged from 3.0 to 9.6 inches H2O.
The slurry recirculation rates to the
venturi and spray tower were fixed at
600 gpm (L/G = 21 to 42 gal/Mcf) and
1,600 gpm (L/G = 57 to 111 gal/Mcf),
respectively.

The oxidation tank level was 18 ft and
the air flow rate was held constant at
260 scfm.
The run began on October 8, 1978 and
terminated November 13, 1978. It ran
for 719 onstream hours (30 days) with
no unscheduled outages. The scrubber
was down once for a scheduled 3-hour
inspection and again when the boiler
came down for 135 hours to install a
new station power transformer.
-------
Average SO2 removal for Run 907-1A
was 97.5 percent at 2,350 ppm average
inlet SO2 concentration. The SO2
removal stayed within a narrow range
of 96 to 99 percent throughout almost
the entire run. This was a significant
improvement over the 57 percent SO2
removal for the base case run, Run
901-1 A, at 9.4 ft/sec spray tower gas
velocity under similar conditions. On
October 19 and on October 27, SO2
removal dropped briefly to less than
90 percent when the pump which sup-
plied the slurry to the top two spray
headers was brought offstream for
repacking, and the spray tower slurry
flow rate was cut in half to 800 gpm.
At the reduced slurry recirculation
rate, S02 removal was 82 to 87
percent.

Venturi and spray tower inlet pH
averaged 4.65 and 5.45, respectively.
Overall limestone utilization was 88
percent and the spray tower limestone
utilization was 56 percent, demon-
strating the advantage of good lime-
stone utilization in a two-scrubber-
loop operation.

Average adipic acid concentrations
were 2,360 ppm in the venturi loop
and 1,560 ppm in the spray tower
loop.

Sulfite oxidation in the system bleed
slurry averaged 98.5 percent, with the
air stoichiometric ratio varied between
2.0 and 3.85 atoms oxygen/mole SO2
absorbed. The filter cake solids con-
tent was 87 percent.

The mist eliminator was clean during
the entire run. The system was free
of plugging and scaling and there was
no increase in solids or scale deposits
on the scrubber internals during Run
907-1 A.

Following Run 907-1A, a second
adipic acid-enhanced limestone long-
term run with forced oxidation was
made during which flue gas monitor-
ing procedures were evaluated by EPA.
This run. Run 907-1B, was made
under the same conditions as Run
907-1A except that the gas flow rate
was varied according to a "typical"
utility boiler load cycle rather than the
actual Unit No. 10 boiler load.
Run 907-1B beban on November 13,
1978 and terrr inated January 29,
1979. It ran for
(69 days) with
scrubber-relatec
was also out of
1,666 onstream hours
only 27 hours of
outages. The scrubber
iervice 146 hours when
Unit 10 came down for replacement of
a broken turbine thrust bearing.

Excluding boiler outages and sched-
uled inspection;, the combined Runs
907-1A and 90 7-1B operated for a
period of over
onstream factor
3 months with an
of 98.9 percent. No
deposits whatsoever were observed in
the mist eliminator for the entire 3-
month test period. On only one occa-
sion did solids accumulation cause an
outage; the cross-over line carrying
slurry effluent from the venturi to the
oxidation tank plugged with soft solids
and had to be cleaned out. Because of
problems associated with converting
the Shawnee venturi/spray tower sys-
tem to two-scrubber-loop operation,
this cross-over line followed a tortuous
path (see Figure 2). A properly
designed systerr
problem.
would not have this
Results of Run 907-1 B were as good in
every respect as those of Run 907-1 A.
Average SO2 removal remained within
a narrow band of 95 to 99 percent.
SO2 removal dropped briefly (typi-
cally 30 minutes) below 90 percent
five times when one of the two spray
tower recirculation pumps was taken
out of service for maintenance, effec-
tively cutting the slurry recirculation
rate in half.

Overall limestone utilization during
this run was 92 percent. Sulfite oxida-
tion averaged 98 percent and the waste
sludge filter cake quality was excel-
lent, having a solids content of 85
percent.

S02 emissions for Run 907-1A and
907-1B were calculated based on an
assumed coal heating value of 10,500
Btu/lb, on 100 percent sulfur overhead
(none in bottom ash), and on an
assumed excess air of 30 percent. This
excess air rate resulted in about 700
ppm inlet SO2 per 1.0 weight percent
sulfur in coal for the above conditions.
The average SO2 emission for the
entire 3-month operating period was
only 0.20 lb/106 Btu. The highest
24-hour average SO2 emission during
Run 907-1A was 0.37 lb/106 Btu, and
during Run 907-1B was 0.41 lb/106
Btu.

A material balance calculation for the
adipic acid consumption was made for
Run 907-1 B. Actual adipic acid feed
rate was 8.3 Ib/ton of limestone fed to
the system, of which 1.8 Ib/ton were
discharged with the filter cake (theo-
retical requirement) and 6.5 Ib/ton
were unaccounted for, giving an actual-
to-theoretical consumption ratio of
4.6.
Limestone Drying, Grinding and Classification System
-------
Limestone Long-Term Test with One
Scrubber Loop and Without Forced
Oxidation
Perhaps the most straightforward
illustration of the effectiveness of
adipic acid is demonstrated by a long-
term limestone test conducted on the
Shawnee TCA system, in which the
additive was introduced without any
system modifications.

Table 3 lists the results of the long-
term test, Run 932-2A, with adipic
acid enhancement. The results of a
base case run. Run 926-2A, without
the additive, are also included in the
table for comparison.

Figure 3 depicts the simple single-
loop, one-tank configuration of the
TCA system used for these runs. The
TCA contained three beds of 1-7/8
inch diameter, 11.5-gram nitrile foam
spheres retained between bar grids.
Each bed contained 5 inches static
height of spheres. Adipic acid was
manually fed by the operator to the
effluent hold tank.
Table 3.
Adipic Acid-Enhanced Limestone Test on the Single-Loop TCA System without
Forced Oxidation
Run No.
926-2A
932-2A
Onstream hours
Fly ash loading
Adipic acid concentration, ppm (controlled)
Scrubber gas velocity, ft/sec
Liguid-to-gas ratio, gal/Mcf
Slurry solids concentration, wt % (controlled)
Limestone stoichiometric ratio (controlled)
Total static bed height, inches of 1 1 .5 gram
nitrile spheres
Effluent hold tank residence time, min
Average percent SO2 removal
Average inlet SO2 concentration, ppm
SO2 make-per-pass, m-moles/liter
Percent oxidation of sulfite to sulfate
Scrubber inlet pH
Percent limestone utilization
Scrubber inlet liquor gypsum saturation, %
Centrifuge cake solids content, wt %
192
High
0
12.5
50
15
1.2

15
4.1
71
2,750
10.1
13
5.65
80
90
37*
833
High
1,620
8.4-12.5
50-75
15
1.2

15
4.1
96
2,450
4-18
21
5.30
82
110
61
*Clarifier underflow solids content.
Run 932-2A was made to demonstrate
both operational reliability with
respect to scaling and plugging of the
TCA and the SO2 removal enhance-
ment capability of the adipic acid
additive. The run began on Septem-
ber 26, 1978 and terminated on
November 2,1978, for a total of 833
onstream hours (35 days). During the
run, the scrubber was out of service
for 48 hours due to a boiler outage
caused by a tube leak, 5 hours for a
scheduled inspection, and 8 hours for
unscheduled outages to clean and
repair the scrubber induced-draft fan
damper. Excluding boiler outages and
scheduled inspections. Run 932-2A
operated with an onstream factor of
99.0 percent. As was typical of all
long-term runs, the scrubber was more
reliable than the boiler.

The run was controlled at a nominal
limestone stoichiometric ratio of
1.2 and 1,500 ppm adipic acid con-
centration in the slurry liquor. Slurry
solids concentration was controlled
at 15 percent. The flue gas flow rate
was varied between 20,000 and
Flue Gas
Reheat
Flue Gas
Alkali Slurry
Adipic Acid
Effluent Hold Tank
Makeup Water
Clarified Liquor
from Solids
Dewatering System
Bleed to Solids
Dewatering System
Figure 3. Flow Diagram for Adipic Acid-Enhanced Scrubbing in the TCA
System without Forced Oxidation
-------
30,000 acfm (8.4 to 12.5 ft/sec super-
ficial gas velocity) as the boiler load
fluctuated between 100 and 150 MW.
The slurry recirculation rate was fixed
at1,200gpm (L/G=50 to 75 gal/Mcf).
The effluent hold tank residence time
was only 4.1 minutes.

SO2 removal during Run 932-2A
averaged 96 percent at an average inlet
S02 concentration of 2,450 ppm.
Excluding the first few days of
unsteady state operation, S02 removal
stayed within a narrow range of 94 to
98 percent as the inlet SO2 concentra-
tion varied widely between 1,400 and
3,500 ppm. By contrast, SO2 removal
during the base case run without
adipic acid, Run 926-2A, averaged
only 71 percent at 2,750 ppm average
inlet SO2 concentration, and at con-
stant 12.5 ft/sec gas velocity and 50
gal/Mcf liquid-to-gas ratio.

SO2 emissions were calculated for Run
932-2A on the same basis as for the
venturi/spray tower runs, Runs 907-
1A and 907-1B. Excluding the first
few days of unsteady state operation,
the SO2 emissions for the 27-day
period from,October 6 through the
end of the run on November 2, 1978,
averaged only 0.26 lb/106 Btu. The
highest 24-hour average SO2 emission
during this period was only 0.44
lb/106 Btu.

The mist eliminator was completely
clean at the end of Run 932-2A and
the entire scrubber system was free of
scaling and plugging. Limestone util-
ization during the run averaged 82 per-
cent. Solids discharged from the cen-
trifuge averaged about 61 percent,
which is typical of unoxidized lime-
stone sludge.

An adipic acid material balance calcu-
lation was made for a 21-day period
during Run 932-2A. The actual adipic
acid feed rate was 9.2 Ib/ton limestone
feed, of which 4.2 Ib/ton were dis-
charged with the centrifuge cake (the-
oretical requirement) and 5.0 Ib/ton
were unaccounted loss, giving an
actual-to-theoretical consumption ratio
of 2.2. This ratio was less than the
value of 4.6 for venturi/spray tower
Table 4.
Adipic Acid-E
Forced Oxida

Onstream ho
Fly ash load!
Adipic acid c
Scrubber gas
Liguid-to-gas
Slurry solids
Scrubber inlt
Total static t
nhanced Lime Tests on the Single-Loop TCA System without
ion
Run No.
urs
ig
oncentration, ppm (controlled)
velocity, ft/sec
ratio, gal/Mcf
concentration, wt % (controlled)
t pH (controlled)
ed height, inches of 1 1 .5 gram
nitrile spheres
Effluent hold tank residence time, min
Average perc jnt SC>2 removal
Average inlet
SC>2 make-pe
Percent oxid;
Percent lime
SC>2 concentration, ppm
r-pass, m-moles/liter
ition of sulfite to sulfate
utilization
Scrubber inlet liquor gypsum saturation, %
Run 907-1 B v
forced, indicat
tion promotes

978-2A
116
High
0
12.5
50
8
7.0

15
4.1
83
2,350
10.3
21
92
125
fhen oxidation was All three runs

979-2A
177
High
615
12.5
50
8
7.2

15
4.1
93
2,900
14.3
14
92
90

980-2A
247
High
1,305
12.5
50
8
6.95

15
4.1
97.5
2,750
14.3
10
88
75
listed in Table 4 were
ng that forced oxida- operated under the same
idipic acid degradation. except
for the
adipic acic
conditions,
concentra-
However, the zctual feed rate of 9.2
Ib/ton limestore for Run 932-2A was
higher than the 8.3 Ib/ton limestone
for Run 907-1 B, because of the higher
moisture content in the discharge cake
for Run 932-2A without forced oxi-
dation. Thus, the net effect of forced
oxidation was to reduce the adipic
acid makeup requirements by approx-
imately 10 percent.

In summary, tie objectives of this
long-term test were met. High removal
was consistently achieved at a good
limestone utilisation, and no fouling,
scaling, or plugging occurred.
Lime Tests witi One Scrubber Loop
and Without Forced Oxidation
Tests with adipic acid in lime scrub-
bing also were impressive in enhancing
SO2 removal.
joth on the venturi/
spray tower and TCA systems. Table 4
shows some ty
acid-enhanced
Shawnee TCA
tion. The flow <
is shown in Fig
sical results of adipic
lime tests from the
without forced oxida-
Jiagram for these tests
jre 3.
tion. Run 978-2A was a base case test
without adipic acid. For Runs 979-2A
and 980-2A, adipic acid concentration
was controlled at a nominal 600 ppm
and 1,200 ppm, respectively (615 ppm
and 1,305 ppm actual). The scrubber
inlet pH was controlled at about 7.0
for all runs.

Average SO2 removal improved from
83 percent at 2,350 ppm average inlet
SO2 concentration for the base case
run, to 93 percent SO2 removal at the
higher inlet SO2 concentration of
2,900 ppm with 615 ppm adipic
acid, and to 97.5 percent removal at
2,750 ppm inlet SO2 with 1,305 ppm
adipic acid. Thus, with 600 to 1,300
ppm adipic acid, SO2 removal
improved by 10 to 15 percent over the
base case removal of 83 percent at
50 gal/Mcf liquid-to-gas ratio and
7.0 scrubber inlet pH.
Lime Test with One Scrubber Loop
and Forced Oxidation
Within-scrubber-loop forced oxidation
in a single-loop scrubbing system
-------
would not be expected to give good
SOj removal for a lime scrubber
because of the oxidation of the major
scrubbing species, sulfite ion, into non-
reactive sulfate ion. With adipic acid
addition, however, satisfactory S02
removal should be possible because
calcium adipate becomes the major
scrubbing species. In addition, the
lower pH at which a lime/adipic acid
system operates should facilitate sul-
fite oxidation.

Table 5 lists the test results of such a
lime run. Run 951-2E, using within-
scrubber-loop forced oxidation with
1,330 ppm adipic acid. The system
configuration used for this run was the
same as that shown in Figure 3, except
that the oxidizing air was injected into
the effluent hold tank (oxidation
tank). A single 3-inch diameter pipe
was used for this purpose, with the air
discharging downward at the center
of the oxidation tank 5 inches from
the tank bottom.

Run 951-2E was made with a scrubber
inlet pH of 5.0 (oxidation tank pH).
Higher pH increases the calcium sul-
fite scaling tendency and also
decreases oxidation efficiency. Lower
pH reduces the calcium adipate buffer
capacity and SO2 removal efficiency.

At 5.0 scrubber inlet pH and 50 gal/
Mcf liquid-to-gas ratio, sulfite oxida-
tion averaged 98 percent and the SO2
removal was satisfactory at 82 percent.

It should be noted that under the
operating conditions chosen for Run
951-2E, the major SC>2 scrubbing
species was calcium adipate because
there was little sulfite available. Since
sulfite is normally the major scrubbing
species in an unenhanced lime system
without forced oxidation, 862
removal was reduced. Higher 562
removal than the 82 percent in Run
951-2E should be achievable by simply
raising the adipic acid concentration
beyond the 1,330 ppm tested.

An SO2 removal of only 65 percent
would be predicted under the same
operating conditions as Run 951-2E,
Table 5.
Adipic Acid-Enhanced Lime Test on the Single-Loop TCA System with
Forced Oxidation
Run No.
951-2E
Onstream hours 103
Fly ash loading High
Adipic acid concentration, ppm (controlled) 1,330
Scrubber gas velocity, ft/sec 12.5
Liquid-to-gas ratio, gal/Mcf 50
Slurry solids concentration, wt % (controlled) 8
Scrubber inlet pH (controlled) 5.0
Total static bed height, inches of 11.5 gram
nitrile spheres 15
Oxidation tank level, ft 17
Oxidation tank residence time, min 4.1
Average percent S02 removal 82
Average inlet SO2 concentration, ppm 2,400
SO2 make-per-pass, m-moles/liter 10.4
Percent oxidation of sulfite to sulfate 98
Air stoichiometry, atoms oxygen/mole S02 absorbed 1.95
Percent lime utilization 97
Scrubber inlet liquor gypsum saturation, % 105
Scrubber inlet liquor SO3/HSO§ concentration, ppm 100
Centrifuge cake solids content, wt % 72
but without forced oxidation and
without adipic acid addition. With •
forced oxidation and without adipic
acid enhancement, the expected SO2
removal should be significantly lower
than 65 percent.
Limestone Long-Term Test with One
Scrubber Loop and Forced Oxidation
A one-scrubber-loop system has an in-
herent advantage over a two-scrubber-
loop system in its simple design and
lower capital and operating costs. If a
simple one-loop limestone (or lime)
system is operated with adipic acid,
which offers the advantage of lower
operating pH, then both good SO2
removal and sulfite oxidation can be
achieved with minimum cost.

This was illustrated in a long-term
adipic acid-enhanced limestone run.
Run 917-1 A, conducted on the
Shawnee spray tower system from
December 26, 1980, to March 13,
1981. Figure 4 shows the flow diagram
for this long-term run with forced oxi-
dation using two series tanks in the
slurry loop. Oxidation was forced in
the first tank while fresh limestone
was added to the second. Use of two
tanks in series in a within-scrubber-
loop forced oxidation system has
several advantages over a single tank:

• Lower pH in the first tank (oxida-
tion tank), which receives the
scrubber effluent slurry, gives bet-
ter oxidation efficiency

• Limestone blinding potential by
calcium sulfite is reduced because
liquor sulfite is oxidized in the first
tank before fresh limestone is
added to the second tank
• Limestone utilization is improved
with two tanks in series

• The second-tank offers extra time
for gypsum desupersaturation and
precipitation

• The second tank provides air-free
suction for slurry recirculation
pumps

In any within-scrubber-loop forced
oxidation system, irrespective of
whether it is additive promoted or not,
-------
Clarified Liquor from Solids
Dewatering System
Bleed to Solids
Dewatering System
Figure 4. Flow Diagram for Adipic Acid-Enhanced Limestone Scrubbing in the Spray Tower System with Forced
Oxidation and Two Tanks
the possibility exists for calcium sul-
fite blinding of limestone because the
recirculated slurry lacks the solid
CaSO3 crystal seeds. Under this envi-
ronment, and if the oxidation inten-
sity is not sufficiently high, liquor sul-
fite could build up to a level at which
CaSO3 begins to precipitate on alka-
line limestone particles, causing lime-
stone blinding, reduced dissolution,
and a pH drop. The problem is usually
avoided by increasing the air stoi-
chiometry to prevent the buildup of
sulfite in the liquor. The potential of
limestone blinding is further reduced
by the use of two tanks in series, as
described above, to permit sulfite
oxidation before limestone addition.
Table 6 summarizes the important test
results of Run 917-1A. As in the pre-
vious runs with forced oxidation, air
was injected into the oxidation tank
through a single 3-inch diameter pipe.
The system was onstream for 1,688
hours. During the run, the scrubber
Table 6.
Adipic Acid-Enhanced Limestone Test on the Single-Loop Spray Tower
System with Forced Oxidation and Two Tanks
Onstream ho jrs
Fly ash load! ig
Adipic acid CDncentration, ppm (controlled)
Scrubber gas
Liquid-to-gas
Slurry solids
velocity, ft/sec
ratio, gal/Mcf
:oncentration, wt % (controlled)
Scrubber inlet pH (controlled)
Oxidation tat
Oxidation tar
Effluent hole
Average inlet
SO2 make-pe
Percent oxide
Oxidation tar
Run No.
917-1A
k level, ft
k residence time, min
tank residence time, min
Average percent SO2 removal
SO2 concentration, ppm
•-pass, m-moles/liter
tion of sulfite to sulfate
Air stoichiorretry, atoms oxygen/mole SO2 absorbed
kpH
Percent limestone utilization
Scrubber inlet liquor gypsum saturation, %
Filter cake solids content, wt %
1,688
High
1,300-1,700
5.4-9.4
85-150
15
5.0-5.1
18
2.8
8.3
93.4
2,660
4.0-8.9
99.8
1.4-2.4
4.9
92.6
93
86
-------
was out of service for 78 hours due to
equipment problems and 84 hours due
to boiler outages. Excluding boiler
outages. Run 917-1A operated with an
onstream factor of 95.6 percent.

The run was controlled at a scrubber
inlet pH of 5.0 to 5.1 and an adipic
acid concentration of 1,300 to 1,700
ppm to obtain 90 percent or higher
SO2 removal. The flue gas flow rate
was varied between 20,000 and 35,000
acfm (5.4 to 9.4 ft/sec superficial gas
velocity) according to a "typical daily
boiler load cycle." The slurry flow rate
was fixed at 2,400 gpm (L/G = 85 to
150 gal/Mcf). Slurry solids concentra-
tion was controlled at 15 percent.

S02 removal during the run averaged
93.4 percent at 2,660 ppm average
inlet S02 concentration. At the low
L/G of 85 gal/Mcf, SO2 removal varied
from 87 to 92 percent with 1,300 ppm
adipic acid, and from 90 to 93 percent
with 1,700 ppm adipic acid. At the
high L/G of 150 gal/Mcf, the removal
was 97 to 99 percent. Daily average
SO2 removal was 92 to 95 percent.

Limestone utilization averaged 92.6
percent. Sulfite oxidation was excel-
lent at 99.8 percent and the filter cake
solids content was high, averaging
86 percent. Gypsum saturation in the
scrubber inlet liquor was only 93
percent.

The mist eliminator was completely
clean at the end of the run, and there
was no evidence of plugging or scaling
within the spray tower.

The actual adipic acid consumption
rate during Run 917-1A was only
5.4 Ib/ton of limestone feed, four
times the theoretical requirement.

Limestone Tests with Bleed Stream
Oxidation
A major advantage of the bleed stream
oxidation is its simple flow configura-
tion. In operation without forced oxi-
dation, the scrubber bleed stream
would be sent directly to the solids
dewatering system. To oxidize this
bleed stream, it is necessary only to
install an oxidation tank and the asso-
Table 7.
Adipic Acid-Enhanced Limestone Test on the Venturi/Spray Tower System
with Bleed Stream Oxidation
Run No.
915-1C
Onstream hours 127
Fly ash loading High
Adipic acid concentration, ppm (controlled) 4,140
Spray tower gas velocity, ft/sec 9.4
Venturi liquid-to-gas ratio, gal/Mcf 21
Spray tower liquid-to-gas ratio, gal/Mcf 57
Slurry solids concentration, wt % (controlled) 15
Scrubber inlet pH (controlled) 4.8
Venturi pressure drop, in. h^O 9
Oxidation tank level, ft 17
Effluent hold tank residence time, min 9.1
Average percent SO2 removal 96
Average inlet SO2 concentration, ppm 2,030
Percent sulfite oxidation in effluent hold tank 54
Percent sulfite oxidation in oxidation tank 98
Air stoichiometry, atoms oxygen/mole SO2 absorbed 1.8
Oxidation tank pH 4.8
Percent limestone utilization 88
Scrubber inlet liquor gypsum saturation, % 105
Oxidation tank liquor gypsum saturation, % 100
Centrifuge cake solids content, wt % 79
ciated agitator and compressed air sys-
tem anywhere between the effluent
hold tank and the solids dewatering
area. Thus, the bleed stream oxidation
scheme is particularly well suited for
retrofit when modifications of the
existing scrubber system for within-
scrubber-loop forced oxidation are not
possible due to physical constraints.

Bleed stream oxidation of unenhanced
lime or limestone slurry is usually not
feasible because the pH rise caused by
the residual alkali in the oxidation
tank makes it difficult to redissolve
the solid calcium sulfite. With adipic
acid-enhanced limestone scrubbing,
however, this constraint is removed
because of the low operating pH and
low residual alkali in the bleed slurry.
Thus, the oxidation tank can be main-
tained at a low pH for good sulfite oxi-
dation, while achieving high SO2
removal efficiency with a sufficiently
high concentration of adipic acid in
the scrubber liquor.
Table 7 gives the results of a typical
bleed stream oxidation test, Run 915-
1C, which was conducted with adipic
acid-enhanced limestone on the ven-
turi/spray tower system. The effluent
slurries from the venturi and the spray
tower were discharged into a common
effluent hold tank. The scrubber bleed
stream was pumped from the effluent
hold tank to an oxidation tank into
which air was injected through a
3-inch diameter pipe. The final system
bleed was withdrawn from the oxida-
tion tank and sent to the solids dewa-
tering system.

Good sulfite oxidation of 98 percent
was achieved in the oxidation tank at
4.8 pH and 1.8 air stoichiometry. SO2
removal was high at 96 percent with
4.8 scrubber inlet pH, 4,140 ppm
adipic acid, and 2,030 ppm inlet SO2
concentration.

Degradation of adipic acid was low, as
expected with the low pH operation.
-------
The actual-to-theoretical adipic acid
consumption ratio was only 1.26 for a
rate of 8.7 Ib/ton of limestone feed.
The centrifuge cake solids content was
79 percent.
Factorial Test Results
Full or partial factorial tests have been
conducted at Shawnee, primarily to
investigate the effects of adipic acid
concentration and pH on SO2
removal. These tests usually lasted
12 hours or longer, including at least
5 to 7 hours of steady-state operation.
Scrubber configurations used were:
venturi alone, spray tower alone, com-
bined venturi and spray tower, and
TCA. Limestone was used in all scrub-
ber configurations. Lime was used
only with the venturi alone. Only the
typical results from the TCA and spray
tower tests are presented below to
show the degree of effect of pH and
adipic acid concentration on SO2
removal.

Figures 5 through 7 show the results
of partial factorial limestone runs con-
ducted on the TCA system using two
tanks in series. Operating conditions
common to all runs were:
Fly ash loading: High (2 to 7
grains/dry scf)
Slurry solids concentration:
15 percent
Oxidation tank level (7 ft diameter):
18ft
Effluent hold tank level (20 ft dia-
meter): 6.2 ft
Air flow to oxidizer: 220 scfm (for
runs with forced oxidation)

Figure 5 shows the SO2 removal as a
function of adipic acid concentration
and slurry flow rate for the TCA with-
out spheres (grid tower). With a con-
trolled limestone stoichiometry of 1.2
(5.6 to 6.1 scrubber inlet pH) and a
slurry flow rate of 37 gpm/ft2, adipic
acid concentration greater than 2,000
ppm would be required to achieve
90 percent S02 removal.

Figure 6 is similar to Figure 5 except
that the data used for Figure 6 were
100
37 gpm/ft2
28 gpm/ft2
IN LET SO2 = 1800 - 2800 ppm
GAS VELOCITY = 8.4-1 2.5 ft/sec
SCRUBBER INLET pH = 5.6-6.1
(LIMESTONE STOICH. = 1.2)
HEIGHT OF SPHERES = 0 inch
WITH FORCED OXIDATION
40
100
90
80
O
5
LLI
val in the TCA with Four Grids and without Spheres
o
A 37 gpm/ft2
O 28 gpm/ft2
019 gpm/ft2
INLETS02= 1800-2800 ppm
GAS VELOCITY = 8.4 - 12.5 ft/sec
SCRUBBER IN LET pH = 5.6 - 6.1
(LIMESTONE STOICH. = 1.2)
HEIGHTOF SPHERES = 15 inches
WITH AND WITHOUT FORCED OXIDATION
J_
_L
400 800 1200 1600 2000
ADIPIC ACID CONCENTRATION, ppm
2400
Figure 6. Effect of Adipic Acid Concentration and Slurry Flow Rate on SO2
Removal in the TCA with Four Grids and 15 Inches of Spheres
-------
obtained using 15 inches (5 inches per
bed) of static height of spheres in the
TCA. With a controlled limestone
stoichiometry of 1.2,90 percent SO2
removal could be obtained with 2,000
ppm adipic acid and only 19 gpm/ft2
slurry flow rate, or with only 600 ppm
adipic acid at 28 gpm/ft2 slurry flow
rate. With 37 gpm/ft2, the required
adipic acid concentration is only
300 ppm to achieve 90 percent
removal. Both Figures 5 and 6 show
that, at 1.2 limestone stoichiometry
(5.6 to 6.1 scrubber inlet pH), S02
removal begins to "taper off" at
about 600 ppm adipic acid
concentration.
The effects of scrubber inlet pH and
adipic acid concentration on SO2
removal in the TCA are given in Figure
7. In comparing Figure 7 with Figures
5 and 6 (high pH data) at the same
slurry flow rate of 28 gpm/ft2, the low
pH curves of Figure 7 have a notice-
ably steeper slope for adipic acid con-
centration above 600 ppm than is the
case for the high pH data. At low pH,
the adipic acid is partially ineffective
because of a significant amount of un-
ionized adipic acid. For example.
Figure 6 shows that at a scrubber inlet
pH of 5.6 to 6.1 and 28 gpm/ft2,
75 percent S02 removal can be
achieved without adipic acid. To
achieve this same 75 percent removal,
Figure 7 indicates that 600 ppm adipic
acid is required at 5.3 scrubber inlet
pH, and 1,700 ppm at 4.6 inlet pH.
Therefore, operation at a very low pH
with adipic acid-enhanced limestone is
not as attractive as at the higher pH
from the process standpoint, since
adipic acid is only partially utilized for
SO2 scrubbing,' and the S02 removal
is far more sensitive to fluctuations in
both pH and adipic acid concentration.

Figures 8 and 9 show the results of
partial factorial limestone runs made
on the spray tower. Common operat-
ing conditions for these runs were:
Fly ash loading: High (2 to 7
grains/dry scf)
Slurry solids concentration:
15 percent
Gas velocity: 9.4 ft/sec
100
90
80
>
o
HI
DC
IN
O
O
cc
111
Q.
70
60
50
40
V PH = 5.6
<> pH = 5.3
O pH = 5.0
A pH = 4.6
A
INLET SO2 = 1800 - 2800 ppm
GAS VELOCITY = 10.4 ft/sec
SLURRY FLOW RATE = 28 gpm/ft2
HEIGHT OF SPHERES = 15 inches
WITH FORCED OXIDATION

J I I
400 800 1200 1600

ADIPIC ACID CONCENTRATION, ppm
2000
2400
Figure 7. Effect of Adipic Acid Concentration and Scrubber Inlet pH on SO2
Removal in the TCA with Four Grids and 15 Inches of Spheres
Liquid-to-gas ratio: 85 gal/Mcf
(2,400 gpm)
Air flow to oxidizer: 250 scfm (for
runs with forced oxidation)

For runs without forced oxidation, a
single effluent tank 20 ft in diameter
with 8.5-ft tank level was used. For
runs with forced oxidation, two tanks
in series were used with an oxidation
tank preceding the effluent hold tank.
The oxidation tank was 8 ft in dia-
meter with an 18-ft tank level.

Figure 8 gives the SO2 removal as a
function of adipic acid concentration
and spray tower inlet pH for runs
made without forced oxidation and at
a constant liquid-to-gas ratio of 85
gal/Mcf. SO2 removal is sensitive to
both pH and adipic acid concentration
within the ranges shown in the figure.
At a liquid-to-gas ratio of 85 gal/Mcf,
90 percent SO2 removal could be
achieved at 5.4 scrubber inlet pH and
1,200 ppm adipic acid, or 5.0 inlet pH
and 2,200 ppm adipic acid. At 4.6
inlet pH, the required adipic acid con-
centration is estimated to be in excess
of 3,000 ppm to yield 90 percent SO2
removal.

Figure 9 shows the effects of scrub-
ber inlet pH and adipic acid concen-
tration on SO2 removal for runs made
with forced oxidation. As in Figure 8,
the liquid-to-gas ratio was held con-
stant at 85 gal/Mcf for the runs shown
in Figure 9. By comparing the two
figures, it is seen that forced oxidation
dramatically improved the SO2
removal, especially at the scrubber
inlet pH below about 5.0. Forexample,
at 1,200 ppm adipic acid concentra-
tion and without forced oxidation,
SO2 removals were 59, 77, and 90 per-
cent at scrubber inlet pH of 4.6, 5.0,
and 5.4, respectively. The correspond-
ing SO2 removals with forced oxida-
tion were 87, 91, and 94 percent.

The reason for improved SO2 removal,
particularly at low pH, is that forced
oxidation eliminates bisulfite species,
-------
thereby reducing the SO2 vapor pres-
sure at the gas-liquid interface and
improving the S02 mass transfer effi-
ciency. This mechanism of improved
SO2 removal holds true when sulfite
is not a major scrubbing species and
the SO2 removal does not depend on
the sulfite-bisulfite buffer. In the case
of Figures 8 and 9, calcium adipate is
the major scrubbing reagent.

Therefore, it would be advantageous
to operate a low phi, adipic acid-
enhanced limestone or lime system
with within-scrubber-loop forced oxi-
dation which, in addition to improved
SO2 removal, requires low adipic acid
makeup, minimizes gypsum scaling
potential, and produces a sludge with
good disposal properties. Based on
Figure 9, 90 percent SO2 removal can
be achieved at 5.0 inlet pH and only
1,100 ppm adipic acid, or at 4.6 inlet
pH with 1,400 ppm adipic acid.
100
IN LET SO2 = 2380 - 3000 ppm
GAS VELOCITY = 9.4 ft/sec
L/G = 85 gal/Mcf
I 1
40
800 1200 1600
ADIPIC ACID CONCENTRATION, ppm
2400
Figure 8. Effect of Adipic Acid Concentration and Scrubber Inlet pH on SC>2
Rerr oval in the Spray Tower without Forced Oxidation
100
90
o
ai
rr
O
to
111
o
OL
80
70
60
50
O
IN LET SO2 = 2360 to 3090 ppm
GAS VELOCITY = 9.4 ft/sec
L/G = 85 gal/Mcf
_L
_L
_L
400 800 1200 1600
ADIPIC ACID CONCENTRATION, ppm
2000
2400
Figure 9. Effefct of Adipic Acid Concentration and Scrubber Inlet pH on SO2
Rempval in the Spray Tower with Forced Oxidation
-------
Springfield Full-Scale Demonstration
In August and September 1980, the
EPA, through its contractor. Radian
Corporation, conducted the first
demonstration of the commercial
feasibility of adipic acid addition to a
full-scale limestone scrubber. The host
facility was the Southwest Power Plant
of the City Utilities of Springfield,
Missouri. In that facility, 3.5 percent
sulfur eastern coal is burned in two
boilers with a total generating capac-
ity of 200 MW. The flue gas from the
electrostatic precipitators is scrubbed
by two parallel 100 MWTCA's.

During this initial two-month test per-
iod, seven different sets of test condi-
tions were examined. Table 8 presents
the major results of two baseline tests
without adipic acid conducted on
Modules S-1 and S-2, and seven tests
with adipic acid conducted on Module
S-1. All tests were without forced
oxidation.

SO2 removal improved from 68 to 72
percent for the baseline tests to 91,
95, and 96 percent with 840, 1,040,
and 1,650 ppm adipic acid, respec-
tively, at the same scrubber inlet pH of
5.5. At 5.0 inlet pH, SO2 removal
remained high at 84, 90, and 93 per-
cent with 1,250, 1,300, and 1,800
ppm adipic acid, respectively. At the
lower pH of 5.0, the limestone utili-
zation also increased from 76 to 84
percent for the baseline tests (pH 5.5)
to 84 to 97 percent.

These results are consistent with the
Shawnee findings. Furthermore, the
SO2 removal obtained at Springfield
lay within 1 to 3 percentage points of
model prediction based on the Shaw-
nee data under similar operating
conditions.

It should be noted that the odor asso-
ciated with the adipic acid testing also
was not a problem at Springfield.

Following these initial tests, the scrub-
ber system was shut down for sched-
uled maintenance. Subsequently, the
demonstration continued with adipic
acid testing, both with and without
forced oxidation. These results will be
reported separately by others.
Rickenbacker Industrial Boiler
Demonstration

In February, March, and April 1981,
the EPA, through its contractor,
PEDCo Environmental, Inc., con-
ducted adipic acid-enhanced limestone
scrubber tests on an industrial-sized
system. The testing was carried out at
the Rickenbacker Air Force Base on a
Research-Cottrell/Bahco system rated
at 55,000 scfm, or about 27 MW
equivalent. The tests, conducted with
certified instrumentation, indicated an
SO2 removal efficiency increase from
55 percent without adipic acid, to 90
to 95 percent with adipic acid. This
improvement was achieved at a scrub-
ber inlet pH of 5.0 and adipic acid
concentrations of between 2,000 and
2,500 ppm. More complete data will
be reported separately by others.
Table 8.
Results of Springfield Full-Scale Adipic Acid Demonstration

Test

Gas
Flow
lO^dscfm

Slurry
Flow*
gpm

Inlet
SO2
ppm dry

Scrubber
Inlet
pH
Adipic
Acid
Cone.
ppm

Av. SO2
Removal
%

Limestone
Util.
%

Test
Period
hrs.
Baseline
S-1
S-2
171
163
13,500
13,500
2,410
2,410
5.5
5.5
0
0
68
72
76
84
497
408
Adipic Acid
(S-1)
1
2
3
4
5
6
7

201
187
190
193
192
196
175

13,500
13,500
13,500
13,500
13,500
13,500
13,500

2,460
2,360
2,420
2,500
2,540
2,500
2,640

5.5
5.5
5.2
5.0
5.0
5.5
5.0

840
1,040
1,000
1,250
1,800
1,650
1,300

91
95
88
84
93
96
90

82
75
94
91
84
73
97

122
50
74
44
33
89
84
'Slurry contains approximately 10 wt % solids.
-------
5. Economics
The economics of limestone scrub-
bing, with or without additive, have
been projected (for forced oxidation
systems designejd to achieve an average
of 90 percent Sp2 removal from high-
sulfur flue gas. the capital investment
and revenue requirements are calcu-
lated using a Dejsign/Economics Com-
puter Program vj/hich was jointly devel-
oped by TVA and Bechtel under EPA
sponsorship.
For the purposes of this report, four
cases were studied, including a lime-
stone case with MgO additive. The
operating conditions for these cases
are presented in Table 9. The evalua-
tions were based on a 500 MW scrub-
bing facility incorporating forced
oxidation, and operating on flue gas
from a boiler burning eastern coal
containing 4 percent sulfur by weight.
The cases evaluated were:
Table 9.
Conditions for ^conomic Analysis of Limestone Scrubbing with Forced
Oxidation and vj/ith or without Additive
Capacity:
Coal:
Scrubber:
SO2 removal Efficiency:
Superficial gas velocity:
Number of trains:
Solids dewate'ring:
Onstream factor:
Effluent holdjtank residence
time: j
Oxidation tanik residence time:
Oxidation tank level:
Air sparger pressure drop:
Oxidation tank agitator Hp:
Solid sulfite oxidation:
Air stoichiomjetry:
Number of tahks:
Alkali: I
500 MW
4 wt % sulfur
TCA with 3 beds, 4 grids, and 5 inches
of static height of spheres per bed
90%
12.5 ft/sec
5, including one spare train
To 80% solids by thickener and rotary
drum vacuum filter
5,500 hr/yr

5 min
5 min
18ft
5 psi
0.002 brake Hp/gal
99%
1.7 Ib-atoms 0/lb mole SO2 absorbed
2 (effluent hold tank and oxidation tank)
Limestone
Case No.
1
Additive

Additive cone
Additive rate.

jntration, ppm
Ib/hr
L/G, gal/Mcf
Limestone stoiichiometry, moles
Ca/mole S02 absorbed
TCA inlet pH
Mode of oxidation

—
—
58

1.52
5.8
1 loop.
2 tanks
MgO

B^SOo'3)
104
50

1.20
5.4
bleed
stream
Adipic
Acid
800
83.3
50

1.20
5.6
1 loop.
2 tanks
Adipic
Acid
2,000
53.6
50

1.05
4.8
1 loop.
2 tanks
(a) Excess of mip\ar equivalent of chloride.
(b) Five times theoretical consumption.
(c) 1.4 times theoretical consumption.
-------
Case 1 — A limestone base case with-
out additive operated at rela-
tively high limestone stoichi-
ometry and liquid-to-gas ratio
to achieve 90 percent SO2
removal. It should be noted
that long-term reliability with
this mode of operation has
not been demonstrated at
Shawnee.

Case 2 —A limestone case with MgO
addition. Oxidation of the
scrubber bleed stream was
chosen because in-loop oxi-
dation is incompatible with
magnesium-enhanced scrub-
bing. As in Case 1, long-term
reliability has not been
demonstrated at Shawnee for
this mode of operation.

Case 3 —A limestone case with adipic
acid addition operated at high
pH. Although only 800 ppm
adipic acid is required to
obtain 90 percent SO2
removal, degradation of
adipic acid at high pH
requires about five times the
theoretical adipic acid addi-
tion rate.
Case 4 —A limestone case with adipic
acid addition operated at low
pH. For this case, 2,000 ppm
adipic acid is required. How-
ever, the low pH operation
requires only 1.4 times the
theoretical adipic acid addi-
tion rate and 1.05 limestone
stoichiometry.
The results of the economic evalua-
tions are presented in Tables 10 and
11. The capital investment and the
first-year revenue requirement in Table
10 include the dewatering equipment
(thickener and filter) but exclude the
waste sludge (filter cake) disposal area.
Table 11 lists separately the first-year
revenue requirement for the waste
sludge disposal area.

As shown in Table 10, both the total
capital investment and the first-year
revenue requirement are the lowest
for adipic acid-enhanced limestone
scrubbing at low pH (Case 4). The
total capital investment is reduced
by 4.8 percent, and the first year
revenue requirement reduced by
5.8 percent for the limestone/adipic
acid/low pH case (Case 4), compared
with the conventional limestone case
(Case 1). The revenue requirement
includes 14.7 percent annual capital
charge.

Total capital investment and operating
costs for adipic acid-enhanced lime-
stone at high pH (Case 3) are higher
than those for limestone/adipic acid
at low pH (Case 4), but are still lower
than those for the conventional lime-
stone (Case 1) or the limestone/MgO
case (Case 2). Total capital investment
is lower by 3.9 percent, and the first-
year revenue requirement is lower by
4.0 percent for Case 3, compared with
Case 1.

Table 11 illustrates the additional
savings that result from adipic acid
addition. Because of the lower pH
operation, and thus lower limestone
consumption, the amount of waste
solids produced is lower for limestone/
adipic acid cases (Cases 3 and 4) than
for a limestone case (Case 1). Assum-
ing a landfill disposal cost of $10/dry
ton, including 14.7 percent annual
capital charge, the first-year revenue
requirements for the sludge disposal
area are 0.97, 0.83, and 0.77 mills/
kWh for Cases 1, 3, and 4, respectively.
Table 10.
Results of Economic Analysis Excluding Waste Sludge Disposal Area
Total Capital Investment
Case No. $ MM (1982) $/kW Cost Factor
First Year Revenue Requirement

$ MM (1984) Mills/kWh Cost Factor
1
2
3
4
87.40
85.26
83.97
83.22
174.8
170.5
167.9
166.4
1.000
0.975
0.961
0.952
25.01
24.15
24.01
23.56
9.09
8.78
8.73
8.57
1.000
0.966
0.960
0.942
Revenue requirement includes 14.7% annual capital charge.
Raw material costs (1984): Limestone — $8.5/ton
MgO - $460/ton
Adipic Acid -$1200/ton
-------
Thus, the total first-year revenue
requirement, including the sludge dis-
posal area, is 9.34 mills/kWh for Case
4, compared with 10.06 milis/kWh for
Case 1. This is a reduction of 7.2 per-
cent, compared with 5.8 percent when
the sludge disposal cost is not
included.

These cost figures are cited as repre-
sentative of typical scenarios only,
and some variation from them would
be normally expected. Moreover, the
differences in total capital investments
and operating costs between these
cases are small. The principal conclu-
sion from these evaluations is that
adipic acid addition to a limestone
scrubbing system decreases cost con-
sistently when compared on the same
basis. I

It should be noted that adipic acid use
provides a level of flexibility in fuel
and reagent choice and control level
not available with other systems, and
in site-specific cases, may prove to be
much more economically advanta-
geous than indicated above.
Table 11.
Revenue Requirement in Waste Sludge Disposal Area
'
Case No.
1
2
3
4

Filter Cake,
dry tons/hr
48.7
41.6
41.6
38.3
First Y
sar Revenue Requirement, Mills/kWh (1984)
Total Excluding Sludge
Sludge Disposal Disposal
9.09
8.78
8.73
8.57
0.97
0.83
0.83
0.77
Total
10.06
9.61
9.56
9.34
Cost Factor
1.000
0.955
0.950
0.928
Revenue requirement includes 14.7% anijual
Sludge disposal cost assumes $10/dry tor
capital charges.
, including 14.7% annual capital charge.
CONVERTING UNITS OF MEASURE
Environmental Protection Agency policy is to express all jneasurements in Agency documents in metric units. In this
report, however, to avoid undue cost or lack of clarity, some English units are used. Conversion factors from English to
metric units are given below:
To Convert From
Btu
scfm (60°F)
cfm
°F
ft
ft/hr
ft/sec
ft2
ft2/tons per day
gal/mcf
gpm
To
J
nm3/hr (0°C)
m3/hr
°C
m
m/hr
m/sec
m2
m2/metric tons per day
l/m3
l/min
Multiply By To Convert From
1055
1.61
1.70
(°F-32)f1.8
0.305
0.305
0.306
0.0929
0.10^
0.134
gpm/ft2
gr/scf
in.
in. H2O
Ib
Ib-moles
Ib-moles/hr
Ib-moles/hr ft2
Ib-moles/min
psi
3.79 ton
To
l/min/m2
g/m3
cm
mm Hg
g
g-moles
g-moles/min
g-moles/min/m2
g-moles/sec
kPa
metric ton
Multiply By
40.8
2.29
2.54
1.87
454
454
7.56
81.4
7.56
6.895
0.907
-------
NOTICE

This report was prepared by D. A. Burbank and S. C. Wang of Bechtel National,
Inc., San Francisco, CA, as an account of work sponsored by the Environmental
Protection Agency (EPA) under Contract No. 68-02-3114. J. E. Williams, R. H.
Borgwardt, and J. D. Mobley were the EPA Project Officers.

Comments or questions regarding this report or requests for information regard-
ing adipic acid as an FGD system additive should be addressed to:

Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711

Neither Bechtel National, Inc., nor the EPA nor any person acting on their
behalf makes any warranty or representation, expressed or implied, with respect
to the accuracy, completeness, or usefulness of the information contained in this
report, or that the use of any information, apparatus, method, or process dis-
closed in this report may not infringe upon privately owned rights; or assumes
any liabilities with respect to the use of, or for damages resulting from the use
of, any information, apparatus, method, or process disclosed in this report.
-------