Proceedings of POWER2009
ASME Power 2009
July 21-23, 2009, Albuquerque New Mexico, USAPOWER2009−81054
Spray Technology is Critical in Helping a Plant Achieve
Optimal Performance
Robert E. Van Durme, P.E.
Lechler Inc.
St. Charles, IL USA
ABSTRACT:
Spray nozzles are used in several locations inside a powerplantand have intimate contact with the processes in the plant.
Nozzles are used for inlet fogging, humidification, Wet andSemi-Dry FGD (Flue Gas Desulphurization) applications
using open spray towers, CFB (Circulating Fluidized Bed) andSDA (Spray Dry Absorber) scrubbers, NOx using SCR(Selective Catalytic Reduction) and SNCR (Selective Noncatalytic Reduction) technologies. As plants try to achieve
optimum efficiency, spray nozzles need to be designed,
installed, and maintained properly. In addition, operators anddesigners need to be aware when a spray nozzle needs to becleaned or replaced. A defective spray nozzle will have an
adverse affect on plant performance.
Several factors are to be considered in selecting the proper
nozzle. These include material of construction, flowrate,
spray angle, operating pressure, placement, droplet size,
maximum free passage, spray angle, and many more. New
developments in spray nozzle technology are allowing
powerplants to have better control and achieve/maintain thetighter regulations on key emissions in the plants. Examplesof nozzle type, their application, and recent
installations/retrofits will be discussed.

recover heat, fuel sources, and financial gains/losses from therequired cap and trade programs. Since the advent of thepowerplant, great strides have been made in making electrical
power clean, reliable, and abundant. Several improvements
have been made to continually improve the efficiency of theprocess. These include improved control technology,
engineered materials, streamlined processes, and years ofoperator experience.
Nozzles play an integral part in the process of power
generation, from when the coal is mined from the ground to
the final cleaning of the gas before it’s emitted into theatmosphere. Nozzle technology has contributed to theimproved performance of plants and the reduction of harmful
pollutants emitted into the atmosphere.

OVERVIEW:Powerplants (Graphic 1) are built for the sole purpose ofpower generation. When plants were originally built andoperated, the plant’s efficiency was the primary measure of
how well the plant was run and maintained. Since that time,
we have become aware of the environmental impact
powerplants can have on the local environment andpopulation. Environmental issues such as acid rain, ozone,
water pollution, and opacity have been indentified andcorrected to minimize its impact. Today, a plant’s efficiency
and effectiveness is determined by the political environment,
the required emission control devices it supports, the parasitic
loads to meet increased EPA requirements, its ability toGraphic 11
Basic Rankine Cycle

NOZZLE MATERIAL:
Nozzle material is very important as applications in thepowerplants have varying degrees of temperature, corrosion,and erosion aspects of performance. In addition, the nozzle
material can not contaminate the process or process fluid
being atomized. Depending on the application (Chart 1),
nozzles can be made from relatively low cost brass, stainlesssteel, or polymers (plastic) to exotic high cost materials suchas duplex stainless steels containing high levels of nickel andchromium, as well as ceramics.
Nozzle Material(s)PriceBrass, 303 SS, Plastics$304SS, 316 SS$$

Duplex Stainless
Ceramics$$$Application(s)Vehicle Rinsing
Floor Wash
General Cleaning
Cooling Towers
Mist Eliminators
SCR/SNCR
Inlet Fogging
Dust Suppression
Fuel Injection
Steam Condensation
Evaporative Cooling
Emergency Quench
Flue Gas Desulphurization
Coal Supply$ − Low Cost$$ − Medium Cost$$$ − High Cost

Chart 1
Typical Nozzle Materials Used at Powerplants
The nozzle material must be designed to withstand not only
the fluid being atomized but also the process it is placed intoservice. Using an inferior nozzle material increases youroverall operating costs due to shortened life, increased
maintenance, and unforeseen repairs. Nozzle erosion affects anozzle’s flowrate, spray pattern, and droplet size. (See Photo1 & 2)
Photo 1
Erosion of stainless nozzle after 5 weeks of service

Photo 2
No erosion of ceramic nozzle after
24 weeks of service

NOZZLE SPRAY PATTERNS:The four primary spray patterns that are used in the industry
are the solid stream, flat fan, full cone, and hollowcone:
The solid stream nozzle provides a solid column of water withno effective atomization of the fluid. This provides the largest
impact force of the different patterns. It is used for cleaning ofa small area. This pattern is seldomly used in the powerplants.
The flat fan nozzle distributes the water into a wide patternwith a constant minimal spray thickness across the spray.
These nozzles create large droplets at high velocity and impactthat are used for application of fluids on wide surfaces. Thisspray pattern also creates a push-broom effect on a surface
when oriented at an angle to the surface being sprayed.
Typical uses for these nozzles are for high pressure washing,and spraying of a moving surface such as a conveyor belt.
A full cone nozzle atomizes the droplets into a conical shapefrom the nozzle orifice. The droplets are dispersed throughout
the full cross section of the spray and tend to have smaller
droplet sizes and lower velocity in comparison to a similarly
sized flat fan nozzle. This distribution can be created fromtwin-fluid and hydraulic nozzles. Typical uses for hydraulicfullcones are mist eliminator panel washing, steam
condensing, inlet fogging, and packing rinsing. There areseveral more. Twin-fluid fullcones are used in aqueous
ammonia injection for SCR as well as SNCR, and evaporative
cooling.
Last, the hollow cone nozzle atomizes the droplets into a
conical shape from the nozzle orifice as well. However, thedroplets are dispersed along the circumference of the spraypattern with no droplets dispersed inside the cone of the spray.
The droplets have a higher velocity and smaller droplet size incomparison to fullcone nozzle due to the reduced energy
losses in the atomization chamber. Hollowcone nozzles are
only able to be atomized hydraulically. Typical uses for thesenozzles is evaporative cooling and gas scrubbing for flue gas
desulphurization.
NOZZLE TECHNOLOGY:
There are two nozzle types that are typically installed inpowerplants. They consist of the single-fluid (hydraulic)nozzles and twin-fluid (air-atomizing) nozzles (See Photos 3& 4). These nozzles come in various materials, sizes, spraypatterns, and quantities to perform the intended design
function. Twin-fluid nozzles create the smallest droplet sizes(Chart 2) but require the highest amount of energy input increating these droplets. In certain cases, hydraulic nozzles canbe operated at extremely high pressures to create droplets thatare comparable to twin-fluid droplet sizes.
Photo 3Large Single−fluid Hollowcone Nozzle used for
Wet Flue Gas Desulphurization

Typical Droplet Sizes
Nozzle Type

Droplet Range (µm)

Twin−fluid
Single−fluid (small)
Single−fluid (medium)
Single−fluid (large)

25 − 100
50 − 500
500 − 1,500
1,500 − 3,000

Chart 22
Droplet size range (Sauter Mean Diameter) produced
by different nozzles (Ref. 1 in. = 25,400 µm)
For most applications requiring relatively large droplets forsurface cleaning, hydraulic nozzle technology utilizing feed
pressure to atomize the fluid sprayed is adequate. Control
technology to monitor the process is relatively straightforward with a pressure gauge and flowmeter to monitor
pressure and flowrate respectively and valves to adjust flowand pressure. Nozzles used in these applications require
minimal maintenance. Typical issues with these nozzles arepluggage caused by internal debris or particles in the supply
fluid that have not been properly filtered (See Photo 5). Inorder to design a proper strainer system for your nozzle, themesh size of the strainer should be ⅓ to ½ of the free passageof the nozzle in use. The free passage area is defined as thelargest particle that may pass through the nozzle without
clogging.
Twin-fluid nozzles are also heavily used in powerplants.
Their use is for the gas conditioning/cleaning of NOx,
SO2 (~ 100 MW or smaller plant), SO3, and mercury in theflue gas. The nozzles and systems, their material, and spray
performance are critical in order to successfully remove thepollutants in the flue gas. Control technology is more
complex as two fluids (typically the chemical solution andcompressed air) need to be monitored and adjusted and kept inbalance to maintain spray performance. Twin-fluid nozzles,
due to their critical function in a plant’s APC (Air PollutionControl)

Photo 4Twin−fluid Nozzle used for Semi−Dry Flue Gas
Desulphurization OR Selective Catalytic Reduction

Photo 5
Hydraulic spiral nozzle used in Wet Flue Gas
Desuphurization plugged with internal debrissystem, require a high degree of reliability, maintenance, andcontrol. The process gas these nozzles get placed into serviceis laden with moisture, dust, and ash which promote buildup
on the nozzle surface and walls. Nozzle spray degradation(See Photo 6) increases droplet size, affects the spray pattern,
and can vary the flowrate, thereby affecting the entire APC
system of the plant. It can cause emissions from the plant toexceed limits. Poorly maintained nozzles cost the plantmoney in increased maintenance, increased operating
expenses, and reduced income potential from the cap and trade
programs.

to restrict air flow and affects nozzle performance and droplet
size. (See Photo 7).
Photo 7
Fouling of twin−fluid nozzle spraying slurry
after 1 week of service

Photo 6
Buildup around nozzle orifice of twin−fluid nozzle

The continuous buildup of slurry required maintenance ofremoving the lance, removing the nozzle, cleaning the nozzle
in a solution to dissolve the lime and re-assembling the nozzle
and lance to put back into service. Several of these lances are
typically required to achieve the required emission targets
which creates a high degree of maintenance time and cost.

SPRAY TECHNOLOGY DEVELOPMENTS:
Nozzle designs and their applications have continuously
evolved to help powerplants meet increasingly stringent
emission requirements. New nozzle designs have helped
improve spray performance through better atomization(smaller droplet size), reduced energy requirements, improved
spray patterns, and less maintenance costs. New corrosion anderosion resistant materials are now in the marketplace towithstand the extreme environments nozzles are placed into.
In most cases, the payback period for improved nozzle
technology is quite short. The savings they generate over their
24 hour, 365 day typical service time quickly pays for theirinitial cost outlay.
One application of nozzles is the atomization of lime slurry
into the flue gas for semi-dry flue gas desulphurization.
During this process, compressed air atomizes pump-fed lime
slurry into small droplets. The small droplets create a large
surface area for the flue gas to interact with and maximize
mass transfer of the SO2 and SO3 out of the flue gas. Inside
the nozzle chamber, the pressure differential between thecompressed air supply and slurry supply is in constant flux.
This allows for temporary penetration of lime slurry into theair supply. The water in the slurry quickly evaporates leaving
a deposit of slurry on the holes. Over time this buildup begins

Two steps were taken to decrease the maintenance frequency
on these nozzles. The first improvement was a redesign of the
air inlet holes. The air inlet holes were modified to create alarger pressure drop between the air supply and the mixing
chamber. This created a larger resistance to backflow ofslurry into the air holes. The second modification consisted ofa flushing sequence that injected an acid solution into the air
supply and through the air inlet holes to dissolve any limebuildup. This created a more dependable, low maintenancesolution for the powerplant (See Photo 8).
Photo 8
Modification of twin−fluid nozzle after 1 week ofservice and cleaning cycle.

SPRAY TECHNOLOGY DEVELOPMENTS (CON’T):
Another issue facing twin-fluid nozzle lances operating in thepowerplant is buildup on the exterior surfaces (See Photo 6).
The causes for this include flue gas flow around the nozzle
creating eddies, bad design of the nozzle orifice,erosion/corrosion of the nozzle material, and condensation on
the nozzle lance. The twin-fluid lance has two fluids that havea lower temperature relative to the flue gas and can be below
saturation temperature of the flue gas. The velocities in thelance are high, which create a high heat transfer coefficient of
the outer surface of the lance exposed to the flue gas. To help
alleviate this problem, the two fluids could be heated to above
saturation temperature. This creates additional energy input
into the system and can create more problems than it solves.
A solution to this problem is the addition of a second annulus
around the lance and pipe. (See Photo 9). The annulus
between the lance and the outer pipe is fed with low pressure(less than 1 psig) blower air that allows the outer skin surfaceof the outer pipe to remain closer to the flue gas temperature.
Photo 93
Cross section cutout of Twin−fluid lance with outerbarrier pipe to minimize material buildup on lance

angular momentum as it leaves the nozzle (See Photo 10).
This angular momentum is then transferred to the gas.
Photo 10
Droplets exiting from hollow cone nozzle orifice

To increase the residence time of the droplet in the flue gas
and to counteract this angular momentum, a new nozzle wasdesigned with separate spray chambers that are located on
opposite sides of the nozzle supply inlet. This nozzle istypically described as a dual hollowcone nozzle (See Photo11). It can be designed to have a fullcone or hollowcone spraypattern. The benefits of this technology when compared tosingle orifice nozzles include: smaller droplet sizes as the fluid
is being atomized into two sprays, increased mass transfer
with flue gas due to increased residence time and larger
surface area, and counter-acting angular momentum
transferred to the gas. This nozzle design is widely used inEurope and is installed in several coal fired powerplants in theUnited States.
Photo 11
Dual hollowcone nozzle spraying

Wet flue gas desulphurization technology was introduced into
the United States in 1970s. These are commonly known asWetFGD scrubbers. In their infancy, scrubbers were able toachieve 70-80% removal of the sulfur in the flue gas.
Currently, scrubbers are now able to achieve 99%+ removalwith the proper design and L/G (Liquid to Gas) ratio. Nozzles
are a crucial part in achieving these levels while minimizing
limestone requirements and pumping power. The primarynozzle that was utilized in the scrubbers is the single orifice
hollowcone nozzle (See Photo 3). These nozzles were placed
at several spray levels in the scrubber to help achieve a highL/G ratio and maximize the surface area of the droplets formass transfer removal of pollutants in the flue gas.
One design aspect of these nozzles is that the fluid entering thenozzle is tangentially fed. As the liquid leaves the orifice, therotational momentum of the spray is transferred into an

SPRAY TECHNOLOGY DEVELOPMENTS (CON’T):
Another nozzle development for WetFGD scrubbers is a dual
hollowcone nozzle that only sprays downward against the gasflow. This nozzle is typically described as the dual-down
hollowcone (See Photo 12) and is designed to utilize all of thedesign features of the dual orifice nozzle but could be placedon the top level spray header inside a scrubber. This nozzle
can also be designed with a fullcone or hollowcone spraypattern. Any slurry spraying upward into the mist eliminator
panels would cause fouling and plugging. These nozzles areused in Europe and are installed in several coal fired
powerplants in the United States. The use of the dual orifice
technology has helped powerplants reach near 100% removalof the sulfur in the flue gas.
Photo 12Dualdown hollowcones spraying inside scrubber

CONCLUSION:
Nozzles play an integral part in a plant’s performance andmust be designed for their intended service and performance.
It has been demonstrated that keeping nozzles clean andoperating as designed has a direct effect on a plant’sperformance. As plants are required to emit less pollutants,
new nozzle technologies need to be developed to meet theserequirements.

REFERENCES:
1.
2.
3.
4.

Moran, Michael J. and Shapiro, Howard N.,
Fundamentals of Engineering Thermodynamics © 1988
Environmental Engineering World – “Wet Scrubbers:
Match the Spray Nozzle to the Operation”, Lothar Bendig
Nasr, G.G., Yule, A.J., and Bendig, Lothar, Industrial
Sprays and Atomization © 2002
Photos courtesy of Lechler Inc.