Seit der Einfilhrung der nassen 

g

schwefelung (REA) wurden die S02-Grenzwerte 

schrittweise verscharft. Demzufolge milssen alteren REA-Anlagen Leistungen oberhalb ihrer 

urspriinglichen Auslegung erbringen. Dariiber 

hinaus sind in nassen REAs niche nur Schwefelemissionen sondern auch Halogene, Staub und 

Quecksilber abzuscheiden. 

Seit mehr als 30 Jahren beschtiftigt sich URS 

mit der Forschung, Entwicklung, dem Probebetrieb sowie der Ertiichtigung nasser REASysteme. Bis jetzt wurden rund 52.280 MW 

Kraftwerkskapazitiit ertiichtig. Dabei wurde 

die Zuverltissigkeit durch die Verminderung 

von Ablagerungen, die Herstellung verwertbarer Nebenprodukte wie Flugasche und REAGips, sowie die sicherer Beherrschung der Emissionen (Staub, Halogene, 502 und Quecksilber) 

erhOht. URS hat gezeigt, dass unabhtingig von 

der urspriinglichen Auslegung, jede REA so 

modifiziert und ertiichtigt werden kann, dass 

auflerplanmiiflige Nichtverfiigbarkeiten ausgeschlossen werden konnen, und unabhangig von 

schwefelreicher oder schwefelarmer Kohle, 502-

Abscheidegrade von iiber 99% erreicht werden. 

Dabei werden auch Staub und Quecksilberemissionen zuverliissig vermindert. 

In dem vorliegenden Beitrag werden die Auslegungs- und Betriebsbedingungen dargestellt, 

die zu sub-optimaler Leistung und Ver.fiigbarkeit fiihren sowie mogliche Ertiichtigungsmaflnahmen, um bestehende REA-System an den 

Stand der Technik anzupassen. I 

Author 

Dr. Jonas 

S. Klingspor 

URS Corporation 

Ausfin/Texos/USA 

VGE!I IPOW E R TECH 

from coal-fired power plants. Some of the 

basic design principles used in the original 

FGD designs are still used today. However, 

early systems showed deficiencies in many 

areas as the basic chemistry, fluid dynamics and gas/liquid contact as well as mist 

eliminator technology were poorly understood and would take decades to refine. 

Initial FGD systems used predominately 

natural-oxidation chemistry often lead to 

scaling, build-up, and poor reliability. Later, inhibited-oxidation, based on addition 

of thiosulphate or elemental sulphur, was 

successfully used to control the chemistry 

to avoid scaling conditions. Eventually, 

forced-oxidation was introduced, which 

provided the opportunity to produce a salable by-product, wallboard-grade gypsum, 

while providing a robust chemistry much 

less prone to scaling. 

Fluid dynamics, in particulate gas distribution, was modelled in the early days 

through scale models, predominately focussing on the gas phase only while the 

interaction of flue gas and slurry was perceived as less important. The impact of 

spray nozzles on the flue gas distribution 

was poorly understood and frequently ignored. Therefore, the effect of nozzle spray 

angles; the spray coverage as a function of 

the nozzle size, type, density, and pressure 

of the spray nozzles; and the impact of wall 

sneakage were typically not included in the 

model. 

Since the introduction of wet FGD systems, 

emission standards have gradually been 

tightened. Wet FGD systems are now being 

used to control in addition to S02 halogens, 

particulate matter, and mercury. 

URS has been involved in research, development, testing, and upgrade of wet FGD 

systems for more than 30 years; and to date 

we have modified 52,280 MWe of wet FGD 

systems to improve reliability by eliminating scaling; produce a salable by-product 

namely fly ash and gypsum; and control 

emissions of particulate matter, halogens, 

S02, and mercury. URS has demonstrated 

that every wet FGD system, regardless of 

design and configuration, can be modified to achieve uninterrupted operation 

operating issues resulting in less than opti

mum performance and available upgrade 

options to improve system performance 

·to the performance level provided by new 

state-of-the-art FGD systems. 

Operating 

problems 

Background 

Wet FGD systems were first introduced in 

the late 1960s and through a multitude of 

design iterations have converged towards 

bottom·supported absorber towers, round 

or rectangular, with a standard set of ab· 

sorber internals to optimise performance 

and reliability. However, the road to a reliable design has involved multitude of 

design options, which in retrospect were 

implemented without full understanding of FGD chemistry and fluid dynamics. 

Wet FGD systems that have been provided 

commercially, on utility coal-fired boilers 

include: 

- Plump bob absorber, 

- Rectangular hung absorbers, 

- Cross-flow horizontal absorbers, 

- Co-current packed absorbers, 

- Disc and donut absorbers, 

- Double loop absorbers, 

- Venturi absorbers, 

- Rod absorbers, 

- Open spray tower absorbers, 

- Open spray tower and tray absorbers, 

and 

- Single and twin tower fountain absorbers 

Some of the more common absorber designs are illustrated in Figure 1. 

This paper details typical operating problems with wet FGD systems, as well as 

available upgrade solutions associated 

with different absorber designs. 

Typical operating problems 

The root cause of the absorber operating 

problems, listed above, can typically be divided into nine categories: 

1 -Unbalanced flow between modules, 

2 -Poor gas distribution in FGD system, 

101 -
p

g

liquid film thickness across the flue gas and 

slurry interface. The thickness of the gas 

film is a function of the relative velocity between the slurry and the flue gas. A higher 

gas velocity will yield a thinner gas film 

and potentially a higher mass transfer rate. 

For mass transfer into the slurry, it is often 

more useful to use the surface renewal concept instead of a liquid-film approach. The 

rate of surface renewal is a function of the 

internal mixing or turbulence in the slurry. 

A higher degree of turbulence results in a 

higher rate of surface renewal, therefore 

potentially a higher mass transfer rate. A 

typical spray pattern for two adjacent nozzles is shown in Figure 4. 

7-Reaction tank mixing and introduction 

of forced oxidation air, 

8-Scrubber chemistry, and 

9- Materials of construction. 

Some of the common flue gas distribution 

problems are illustrated in Figure 2. 

With older wet FGD systems, gas sneakage 

around nozzles inside the absorber as well 

as sneakage along the absorber walls was 

common due to narrow-angle nozzles and 

a low-nozzle density leading to poor spray 

coverage as shown in Figure 2. Poor spray 

coverage can also produce skewed distribution across the absorber. These common 

problems can occasionally be found in new 

absorber installations. Poor spray coverage 

and poor gas distribution impacts the performance of the absorbers and frequently 

limits the maximum achievable S02 removal efficiency to less than 95 %. 

Common flue gas maldistribution problems associated with the absorber inlet 

and outlet ducts are shown in Figure 3. 

Skewed distribution in the absorber inlet 

duct will often generate flue gas recirculation zones leading to build-up of solids 

in the vicinity of the wet/dry interface. 

Likewise, a skewed flue gas distribution at 

the absorber outlet duct will often lead to 

excess mist eliminator carryover, scaling 

of the absorber outlet duct, and stack and 

possibly stack rain. 

Available upgrade options 

Most wet FGD systems, regardless of initial 

physical design or chemistry, can be upgraded to achieve above 99% $02 removal 

efficiency and to operate uninterruptedly 

between scheduled outage cycles. Upgrades typically have to address fluid-dynamic related issues such as uneven distribution between absorber modules, uneven 

gas flow in the absorber inlet duct, poor 

gas/ liquid contact, skewed distribution in 

the absorber outlet duct, and poor mixing 

in the reaction tank. These issues typically 

are materialised through deposits and scaling in the absorber inlet and outlet ducts, 

and poor S02 removal efficiency. 

- 102 

to around 95 o/o, regardless of measures 

taken. Occasionally, inefficient gas/liquid 

contact devices are also found in modern 

wet FGD systems. 

A number of upgrades solutions are available to improve the performance of existing FGD systems: 

1 -Flow distribution improvements 

- turning vanes 

-perforated plates 

-bull nose 

-wall rings 

2 - Gas liquid contact devices 

- sieve trays 

-spray nozzle layout 

-double hollow cone nozzles 

3 -Chemistry 

- performance additives 

- forced oxidation 

Coverage 

0 

1 

2 

3 

4 

5 

6+ 

Hollow cone nozzles generate a hollow 

cone spray pattern by giving the slurry a 

centrifugal rotation before exiting the nozzle. Pigtail nozzle generates spiral hollow 

cone spray patterns by use of the pigtail 

but the spray pattern can more accuratly 

Fig. 2. Common absorber problems including spray coverage, 

flue gas distribution and flue gas sneakage. 

Fig. 3. 

Examples of common inlet and outlet duct flue gas rna/distribution problem. 
also create a high capacity for S02 removal. 

Fig. 4. Typical droplet generation and droplet-to-droplet collision by hollow cone nozzles. 

A high efficiency spray header in combination with double hollow cone nozzles provides substantial improvements in performance, as shown in Figure 6. On average, 

a high efficiency spray header and double 

hollow cone nozzles provides a 40 percent 

increase in performance, as determined 

by side-by-side testing of an upgraded absorber module and an unmodified absorber module. 

be characterised as a full cone nozzle. 

Also, pigtail nozzle generates an uneven 

slurry distribution in the cross section below the nozzle, which effects gas distribution inside the absorber tower. Both the 

pigtail and full cone nozzles are less often 

used in wet FGD systems because they generate a pressure centre below the nozzles, 

which forces the flue gas to travel around 

the spray cone generating a poor flue gas/ 

slurry mixing environment. A hollow cone 

nozzle, on the other hand, generates a lowpressure area below the tip of the nozzle, 

which attracts the flue gas and forces the 

flue gas to travel through the spray cone 

rather than around it and, therefore, providing an excellent flue gas/slurry mixing 

environment. 

Slurry nozzles used in wet FGD systems initially generate a liquid sheet as the slurry 

Slurry nozzles for wet FGD systems are often characterised by the Souter mean droplet size. A more useful characteristic is the 

exit velocity of the slurry. Use of the spray 

coverage at the 0.5 m distance from the 

nozzles to capture the most reactive region 

of the spray coverage has proven very sueleaves the nozzle. The liquid sheet breaks fig. s. High efficiency spray header and spray nozzles. 

up into smaller ligaments, which eventually produce droplets, as shown in Figure 4. 

$02 has successfully been measured inside 

wet FGD systems by multiple organisations 

and it has been concluded that the majority 

of the S02 removal occurs within a distance 

ofO.S m from the nozzle tip, in an area that 

only has thin slurry sheets, and before any 

droplets are formed. As the slurry breaks 

up into ligaments and droplets, the slurry 

becomes less and Jess turbulent and, therefore, less reactive. Although the relative velocity between the flue gas and the slurry 

droplets decrease some as the slurry travels away from the nozzle, the associated 

increase in gas film thickness does not explain the dramatic reduction in 502 removal with distance from the nozzle tip. As the 

nozzle injects the slurry, the slurry exhibits 

a forceful internal turbulence, which, over 

time, is consumed by generation of slurry 

ligaments and eventually droplets. By the 

time droplets are formed, they are close to 

stagnant; providing a substantial barrier 

j 

J 

§ 

!00 

IS 

10 

as 

eo 

11) 

.., 

40 100 

LJG. .,.s!Mac:f 

!.0 

for liquid-side mass transfer internally. fig. 6. Performance of high efficiency spray header and dovble hollow cone nozzles. 

If>.) 

VGB I P 0 W E R T E C H l 03 -
and four recycle pumps in operation. The 

upgraded module can achieve the same 

removal efficiency with three pumps in 

operation when compared to the unmodified module with four pumps in operation. 

Hence, substantial power savings can be 

realised by upgrading to a high-efficiency 

spray header and double hollow cone nozzle, or a substantially higher S02 removal 

efficiency. 

Sie'lc lroy 

A sieve tray provides a low-cost and highly 

efficient approach to improve gas/ liquid contact in wet FGD systems. An absorber 

tower can be equipped with one or two 

sieve trays. Typically designed to provide 

about 50 mm ofw.g. pressure drop, a sieve 

tray provides about the same removal efficiency as one spray header without requiring any of the complexity of a recycle 

pump and spray header system. In addition to providing improved so2 removal, 

a sieve tray also provides the following 

benefits: 

- Increases particulate removal, - Corrects gas distribution problems in the 

lower part of the absorber spray zone, 

- Protects the inlet duct from scaling, 

- Provides the lowest cost approach to a 

FGD upgrade, and 

- Provides a maintenance platform that 

avoids draining the reaction tank to gain access to the spray headers and mist 

eliminators. 

Typical tray layouts with round and 

hexagonal hole patterns are shown in 

Figure 7. 

The open area of a sieve tray is a function 

of gas velocity and liquid loading. Round 

holes are predominately used. However, 

at high gas velocities such as 6 m/s, the 

open area requirement is so large that the 

structural integrity of a sieve tray is compromised. The structural integrity can be 

maintained by switching to a hexagonal hole-pattern. The tray is typically partitioned to assure that the desired slurry/ 

Because the liquid spray density is inherently less in close proximity to the absorber 

walls, the pressure drop in this area is less 

than at any point in the spray zone. The 

flue gas takes advantage of the lower pressure drop along the absorber walls and 

thus wall sneakage is generated. Depending on the size of the absorber tower, wall 

sneakage can be significant or immaterial. 

Wall sneakage can often limit FGD system 

performance to less than 97 % so2 removal, in smaller absorber towers. Wall sneakage can be avoided by installing wall rings, 

a sloping plate extending from the absorber wall as shown in Figure 8. The spray 

nozzles located adjacent to the absorber 

wall must be arranged at a distance from 

the absorber wall such that the flue gas is 

directed in the middle of the nozzle spray 

cone. A wall ring positioned in this fashion 

also provides protection from wall erosion. 

The impact of the wall sneakage zone is 

much more prominent is small absorbers 

below 10 m in diameter than in larger absorber towers. Large absorber modules also 

experience wall sneakage, but the sneakage zone is relative small when compared 

to the total absorber cross section. 

y 

ash, was in many cases, used to provide 

the alkalinity for S02 removal avoiding the 

cost of a limestone grinding system and a 

limestone reagent. 

In a plump bob venturi system, the venturi 

portion of the absorber design is placed 

above the absorber and after the flue gas 

has passed through the venturi throat, the 

flue enters the absorber in a down comer 

located in the centre of the module, as 

shown in Fig u r e 9. At the bottom of the 

down comer, the flue gas makes a 180 degree turn, passes through the spray zone, 

and exits the absorber through an outlet 

duct located at the side of the absorber. 

The 180 degree turn after the down comer 

skews the flue gas heavily towards the absorber outer walls, and the side exit further 

skews the flue gas toward the side of the 

outlet duct. 

A sieve tray was installed at the elevation of 

the 180-degree flue gas turn at the bottom 

of the down comer. The pressure drop provided by the sieve tray (-so mm of w.g.) 

provides an even gas distribution profile 

downstream of the sieve tray. The single 

existing spray headers were replaced with 

flue gas mixing occurs. Fig. 8. Installation of wallrings to avoid wall sneakage. 

- 104 
two high efficiency spray headers equipped 

with double hollow cone nozzles. To make 

room for the second spray header, the twostage mist eliminator was replaced with 

an integrated diamond mist eliminator, 

which requires only one level of support 

beams. To overcome the lateral skewed 

gas distribution generated by the side exit, 

a perforated plate was located above the 

mist eliminator covering a 80 degree cross 

section on either side of the outlet duct 

centre line. To accomplish the best possible 

gas distribution, the perforated plate has a 

variable open area becoming increasingly 

more open in a direction moving away 

from the centreline of the outlet duct. 

Several of these plump bob absorbers 

have been upgraded using magnesium 

enhanced lime, limestone, and fly ash as 

reagent. With the upgrade approach described above, it is possible to obtain the 

same removal efficiency and reliability as 

a new state-of-the-art system. 

Rod scrubber 

Rod scrubbers were designed to provide 

gas/liquid contact through a number of rod 

decks consisting of l-inch pipes densely 

packed, and a flooding spray header above 

the upper rod deck, as shown in Figure 

10. The rod scrubber is similar to a way it 

achieves gas/liquid contacting. The aspect 

ratio of a rod scrubber is unusual with a 3 

m absorber side and a 10m absorber width 

to accommodate the rod deck without requiring an excess of structural supports. 

The rod scrubber exhibits a number of issues including poor gas/liquid mixing as 

gas and liquid mixing is inherently unstable unless confined by the type of partitions used with a sieve tray and wall effects 

due to unusual aspect ratio. Only two rod 

scrubbers were ever built. 

original design flue gas velocity was about 

3 m/s, thus, bringing the flue gas velocity 

to about 6 rn/s with the by-pass eliminated. A high velocity (hexagonal holes) tray was 

installed immediately above the inlet duct 

to distribute the flue gas evenly across the 

absorber cross section. Three levels of high efficiency spray headers, equipped with 

double hollow cone nozzles, were also 

installed. New larger recycle pumps were 

required to increase the liquid-to-gas ratio. 

Due to the unusual absorber aspect ratio, 

and the high S02 removal requirement, 

each spray level was equipped with wall 

rings. 

The horizontal flow mist eliminator is 

slanted backwards towards the gas flow to 

improve drainage and the droplet collection efficiency. Initially, the absorber had 

a 90-degree turn, without any flow correction devices, from the spray zone to the 

mist eliminator. That arrangement worked 

well at the original design flue gas velocity of 3 m/s. However, at 6 m/s, the sharp 

90-degree turn generates a noticeable uneven flue gas distribution and also an uneven droplet density distribution. To accommodate the higher flue gas velocity and 

droplet loading, a bull nose was installed 

in the upper spray zone to guide the flue 

gas around the 90-degree turn. Finally, a 

forced oxidation conversion was provided 

along with a new ball mill grinding system. 

.. .. .. 

system was converted from inhibited oxi

dation to forced oxidation, requiring upgrades of the dewatering system. A new 

limestone grinding system was also provided. The absorber tower was converted 

from a packed tower to a co-current open 

spray tower and new higher capacity and 

higher-pressure recycle pumps were installed. The high efficiency spray headers 

were equipped with double hollow cone nozzles. A higher-pressure nozzle was selected to improve the performance of the 

co-current header. With co-current spray 

headers, the pressure drop across the absorber module was reduced from 150 mm 

of w.g to 0 mm of w.g. 

Summary 

From the installation of the first FGD systems in the late 1960s to the end of the 

1980s, the industry had a poor understanding of fluid dynamics, the important role of 

gas/liquid contact and the slurry chemistry, in particular how to avoid scaling con· 

ditions. Many ofthe FGD systems installed 

during that period had serious deficiencies 

that affected performance and reliability. 

The wet FGD community has, over time, 

gravitated to integrated open spray and 

tray towers that can provide S02 removal 

efficiencies above 99 % and uninterrupted 

operation between scheduled outages. Unfortunately, there are still wet FGD systems 

installed today that do not take advantage 

This particular installation was originally 

designed with 40% flue gas by-pass around 

the FGD system. The station S02 removal 

was original about 50 % and after the upgrade, the flue gas bypass was completely 

eliminated and a station S02 removal efficiency above 98 % was achieved. The Fig. 1 0. Rod deck absorber before and after upgrade and associated CFD model. 

VGE3 I P 0 W E R T E C H 105 
Fig. 12. High eHiciency wet FGD system design. 

of high efficiency and high reliability de- The early FGD systems are frequently asked 

sign concepts. to provide performance levels not original

, 

- Even flue gas distribution through the 

spray zone and the mist eliminator, 

- A sieve tray, 

- High efficiency spray headers, 

- Double hollow cone nozzles, 

-Wall rings, 

- Stable chemistry (inhibited or forced 

oxidised), 

- Limestone quality excluding excess content of crystal modifiers, and 

- Mist eliminator wash systems using service water as source for mist eliminator 

wash water, ample wash header nozzle 

pressure and spray density. 

A high-efficiency, wet FGD system including a sieve tray, high-efficiency spray header, double hollow cone nozzles, and wall 

rings are shown in Figure 12. Absorber 

towers utilising this design approach can 

readily achieve S02 removal efficiencies 

above 99 % on low to high sulphur coals. 

This design approach also provides particulate removal efficiencies above 70 % percent and removal efficiencies of oxidised 

mercury above 90 o/o. I