ASHWIN PATNI, LECHLER INC., USA, OUTLINES THE BASIC DESIGNCONSIDERATIONS FOR A FLUE GAS COOLING SYSTEM AND THE POTENTIALADVANTAGES OF A LOWER TEMPERATURE OPERATION FOR CEMENT PLANTS.

Introduction
Gas cooling is an integral process component for mostcement plants around the world. Traditionally, it servesto protect baghouses from damage due to hot gases.
However, under current NESHAP regulations, theperformance of gas cooling using water sprays can be
critical to plant operations.
Cooling gases close to dew point approachtemperatures can have many advantages. A cooler
temperature in the presence of moisture can reduce
lime consumption for SO2 control. Mercury adsorption
is also more efficient at lower gas temperatures; using
powdered activated carbon and, in some cases, awell-designed gas cooling system can help in lowering
activated carbon usage while maintaining the levelof mercury capture. Lower gas temperatures also
have positive effects on ID fan energy consumption
by reducing overall gas volumetric flow rates, which
decrease with cooler temperatures.

As is well known, cement manufacturing is a hightemperature process. The temperature required toproduce clinker is in the range of 1500˚C. The flame
temperature required to heat the raw material istypically 2000˚C. These hot combustion gases passthrough the process stream counter current to material
flow and this transfers heat from gases to materialas they cross each other. There is additional processequipment, including cyclones or chains in long kilns,
which make heat transfer more efficient. Typically,
the exit temperature of the gases downstream ofcyclones for new plants is about 325˚C and for oldertechnology temperatures are slightly higher. Thesegases have a very high dust load, which consists mainlyof raw meal materials. To capture this dust a baghouseis utilised. Today’s filter media can handle continuous
gas temperatures of up to 260˚C. To protect thesebags, typically, a spray injection system is installed
to cool the gases to a lower temperature below the/ 55

normal operating temperature of filter media. In
newer kilns, the spray injection location is either in adowncomer duct or inside a gas conditioning tower. Agas temperature of around 120˚C can offer potential
advantages, which may reduce some costs associatedwith reagents required for emissions control and also
possibly reduce the power draw by ID fans. This article
discusses the basic design considerations for a gas
cooling system and the potential advantages of a lower
temperature operation.
Benefits of gas cooling in the cement
manufacturing process
Modern cement manufacturing is almost completely
a dry operation. Water sprays are one of the few
wet operations within the cement manufacturing

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2015
56 \ April
World Cementprocess. Water sprays pose multiple challenges toa cement plant operator, including maintaining astable and trouble free operation. In most cases, spraynozzle performance is the root cause of many issues.
The challenge for a cement plant is to assess spray
performance. Performance assessment is typicallyassociated with an understanding of the droplet size
being produced at various operating conditions. A
spray nozzle manufacturer should have detailed data
regarding droplet size information for any operating
condition. Estimated droplet sizes have been known todeviate considerably from the actual produced droplet
sizes, especially when the nozzles have worn over timeor have had scaling issues.
The droplet assessment can be a challenge when
it is impossible to see the sprays interacting with fluegas inside the tower or duct. Any
diagnosis has to be based on thespray nozzle manufacturers’ droplet
size data and measured by processparameters in the field. The droplet
size for most air atomising nozzles isa combination of air to liquid flow
ratios and other parameters suchas liquid viscosity and operatingpressure. Compressed air pressure is
not a good parameter for measuring
a nozzle’s performance. Droplet size
cannot be estimated on pressurealone and, therefore, it is importantthat air and water flow rates aremeasured to determine optimum
sprays. Cement plants should also beaware of their compressed air usage
as it can be very costly.
Most systems in the US operatewith dual fluid nozzles. These systems
require more energy to operate incomparison to a hydraulic system.
However, a dual fluid atomisation
design is much more resilient in termsof increasing flow capacity or altering
the produced droplet size. This can beeasily accomplished by adding morecompressed air and replacing pumps.
A hydraulic spillback system is notvery resilient in terms of a capacity
increment or changing the droplet
size while using the same equipment.
However, it has the lowest operating
cost, as a hydraulic system does not
require any compressed air.
The purpose of any spray nozzle
is to produce the most liquidsurface area for efficient gas–liquidinteraction for heat and/or masstransfer by utilising minimum
energy to generate such surface
area. Typically the energy input rises

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plane, which can cause
massive dust buildup thatcan fall under gravity
and damage downstream
components.
Water quality
Well or quarry water,which has very highdissolved solids, is typicallyused. Some nozzles havevery high propensity toscale internally with thisLeft: Typical Air Atomizing Nozzle with large droplets exiting the nozzle orificetype of water. This leads
from the edge. Right: RingGap™ nozzle with Annular Air shearing large droplets
the nozzle efficiency to gofrom the nozzle edge to create a very tight droplet spectrum.
down and cause hopperwetting. The droplet size
is usually based on newand clean nozzle performance and certain safety
exponentially as we try to generate smaller and smaller
margins are utilised to work with upset conditions,droplets. For a cement plant, this phenomenon issuch as scaling or nozzle wear. Once the droplet sizes
similar to the increase in energy input that is required
produced by the spray nozzle increase beyond theto grind smaller and smaller particles from a rock.
design envelope, wetting occurs.
Design criteria of a gas cooling system
Direct vs compound operation
Spray nozzles, like any other type of process
Available residence timeequipment, have a best efficiency operating point. For
Available residence time is defined as the straighta dual fluid spray nozzle with a constant air-to-liquid
available evaporation distance that can be utilised toratio, the droplet size produced is smaller at higher
achieve complete evaporation of spray droplets. It isoperating pressures. Typically, in direct mode, nozzles
the most important consideration when calculating
are efficient and require a lower air-to-liquid ratio
droplet size and selecting the type of spray nozzle. Itthan running the spray injection system in compoundis typically calculated by dividing the duct volume ofmode. Certain nozzle designs have worse outcomes
available straight evaporation distance by the actual
under compound mode. Nozzles are selected to copeflue volumetric flowrate at process temperature.
with a large variation in turndown. It is very importantthat the effect of droplet size is studied carefully for
Process condition
such large varying conditions.
This involves a study of detailed process conditions,
including variations in inlet temperature and changes
Benefits of lower flue gas outlet temperaturein volumetric gas flowrates. If all other variables areheld constant, the rate of water evaporation slows as
SO2 control
the gas temperature approaches closer to the boilingpoint of water. This parameter also has an effect on
Various cement plants in the US are required
droplet size and type of nozzle selection.
to control SO2 emissions. The preferred reagent
to scrub SO2 is lime and there are two common
Spray coverage/distribution
methods of injecting lime into the system. The first
It is very important to have optimum spray distribution
method is to introduce lime into the conditioning
based on nozzle design for an evaporative spray
tower or downcomer mixed with cooling water incooling application. Improper distribution can cause
a slurry form. The second method is to inject lime
issues such as sticky dust, which can stick to vessel
as powder just upstream of the ID fan. In bothwalls. Spray coverage also plays a vital role in efficient
cases the reaction between lime and SO2 is much
evaporation.
more efficient closer to dew point temperatures.
Lower temperature reduces the actual volume of
Flue gas flow distribution
the flue gas, increasing overall available residenceThis parameter is one of the most important andtime of the system and providing higher interaction
least studied from a spray injection system design
potential between lime and SO2 molecules. As
standpoint. The ideal gas distribution for efficient andthe outlet temperature of gas decreases, the lime
trouble-free evaporation is less than 20% RMS velocity
injection rate falls. This method alone can save
distribution. A mal-distribution can cause wall wetting
the plant thousands of dollars in lime costs. These2015
58 \ April
World Cement

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Mercury capture via dust shuttling or activatedcarbonElemental mercury is typically adsorbed on a highsurface area particle, such as powdered activatedcarbon. Cement dust can also adsorb mercury. In
physical adsorption, temperature is also a factor
that determines adsorption. At lower temperatures
the adsorption rate is higher and cooling the gases
increases the residence time of the system. Reducing thetemperature also improves the pollutant’s concentrationof inert flue gas, which provides a conducing condition
for removal while achieving higher efficiency.
Improved particulate control device performance
Cooling of gases also improves air-to-cloth ratios withina baghouse. At a lower temperature, the actual fluegas volume reduces the inlet velocities in a baghouseand improves its air-to-cloth ratio. In ESPs, humidityand lower temperatures reduce dust resistivity while
improving ESP performance. At lower temperatures, theresidence time inside an ESP increases, which could bebeneficial for overall dust removal performance.
Over the years, Lechler has been successful indesigning new and efficient nozzles. Once the system
has been designed, the spray nozzles are the easiest
component to replace without significant modification
to the existing system. In other words, by using anefficient spray design, more water can be injected
to cool the gases without upgrading compressors toachieve the desired outlet temperature and avoiding
wetting the hopper. Another potential benefit of highefficiency nozzles is that they can be installed in smaller
vessels, which reduces overall construction costs – if
installing new GCT towers.
It is commonly believed that in a cement planta hydraulic spray injection system can be difficult
to operate. Lechler has installed many hydraulicspillback systems, including a recent plant installation
located in Florida, which was designed to injectapproximately 150 gpm of water. This is madepossible by selecting the correct droplet size andmatching it to variation in process conditions.
Hydraulic systems have much lower operating costs
due to a lack of required compressed air.
If a gas cooling system is designed properly, it should
not experience wetting in any condition, unless there isan equipment malfunction.
Typical operational issues with gas coolingsystems
Hopper wetting
The most common issues that can cause hopper
wetting are caused by the nozzle or an interruption
in the compressed air supply. These issues can also be
caused by an increase in droplet size beyond the design2015
60 \ April
World Cement

envelope, which is required for complete evaporation.
Wetting can not only cause a plant shutdown but canalso damage equipment, such as screw conveyors or
ID fans. Cleaning of buildup and material accumulation
due to wetting requires significant plant resources,
which can be time consuming and costly.
Often the system is not designed properly, either
from lack of compressed air or improper selection ofdroplet size. This is a fairly common issue within cementplants. This problem should not occur if the droplet size
produced during system operation matches the droplets
selected according to system design.
Plants that experience hopper wetting will reactby raising the outlet temperature by reducing thewater injection rate as a first step. This step solves
the wetting issue, but only for the short-term. For thelong-term it increases the operating cost, and plants
do not generally benefit from cooler gases that can
result in operating cost savings. To achieve a stable
lower outlet temperature without wetting under allprocess scenarios, a gas cooling system must be carefullydesigned. Droplet estimates based on formulas andextrapolation of inadequate data more often result ininaccurate droplet size selection, which is the root causeof most issues.
Advances in spray nozzle technology
New regulations require some plants to reduce HCland SO2 and, as a result, lime injection has become
important to scrub these acid gases. In the US, plantsthat required acid gas emission controls quickly ran
into nozzle wear and wetting issues due to poor nozzle
design and material of construction. In certain nozzle
designs, very little wear can rapidly deteriorate nozzle
performance and cause severe hopper wetting. Theslurry velocity can approach 300 – 400 fps inside thenozzle, which is very high for common alloys, suchas stainless steel, from an erosion standpoint. Any
deviation or obstruction in the slurry path will cause
severe nozzle erosion. The nozzle design and correct
choice of construction material are critical for superior
nozzle performance and trouble-free operation using
slurry sprays. There are nozzle designs available withappropriate material of construction that can provide
guaranteed nozzle wear life of 2 years or more if lime
slurry is used. Lechler has had operating experience withslurry applications at three cement plants located in theUS. These lances have now been in operation without
any observable wear for over a year. Another cement
plant in California had severe nozzle wear issues. Thenozzle tips were wearing within three to four daysof operation and this prompted the plant to find analternate design. Lechler installed Laval nozzle lancesat this plant and no nozzle wear has been observed forover a year. It is expected that the same nozzles willcontinue to operate for more than three to four years
without replacement due to wear.
Another issue where new technology can improve
the operation is by preventing nozzles from scalingand plugging during operation times. Typically, incement plants, a high amount of dissolved solids arepresent in the process water. This causes scaling at thewet-dry interface inside the spray nozzle. Scales alter thenozzle performance, which leads to hopper wetting. Anew technology is available that can clean the nozzles
onsite without any manual input or labour. The Lechler
Online Cleaning System, or LOC®, has been proven tocompletely eliminate manual cleaning requirements,
which are present with both lime slurry injection andhard water usage. The LOC® system is a trouble-free
operation and is virtually guaranteed when using lime
slurry and/or hard water. This online cleaning system canalso guarantee zero wetting with such liquid media.
Another important area where new spray nozzle
technology has created an impact is the capacity toproduce superior droplet sizes. It is critical to produce
small droplets by using less compressed air. Lechler has
found that the droplets for most nozzles generated
at the edges produce the largest droplets. There is anew generation of nozzles that can reduce these large
droplet sizes to a very fine mist. The RingGapTM nozzle
is very efficient and produces a very narrow droplet size
spectrum.
Conclusion
Spray nozzles play an important role within cement
manufacturing operations. Under current regulations,a well-designed spray gas cooling system can reduce
maintenance issues and the cost of using expensive
chemical reagents required for emission control.
However, in order to design and achieve cooler
temperatures, a careful analysis of the design is required.
The droplet size at various design and operating points
must be measured carefully, as estimates have been
proven to be very unreliable. It is very important thatplants ask nozzle manufacturers to describe the methods
they use for nozzle droplet testing.
Spray nozzles should provide a high surface area forheat transfer between hot gases and water or slurry.
If design calculations are carried out correctly andencompass all design conditions, then a spray gas coolingsystem should not cause any operational issues.
Bibliography•Standard Practice for Determining Data Criteria and Processing forLiquid Drop Size Analysis ASTM E799-03.

•

LOCKE, J.E., WINSCHEL, R.A. and O’PALKO, B.A., Full Scale Field Trialof the Low Temperature Mercury Control (LTMC) Process.

•

FLORA, J.R.V. and VIDIC, R.D., Modeling Powdered Sorbent Injection inCombination with Fabric Filter for the Control Of Mercury Emissions.

•DI CELSO, G.M., Adsorption or Mercury Chloride onto ActivatedCarbon on a New Pilot Scale Plant.

•PATNI, A., ZIEGLER, K., FIELDS, R. and FISCHER, M., ‘Improvements
in SDA performance at Dynegy Baldwin and Havana Station’,Proceedings of Dry Scrubber Users Conference 2014.

•MOCK, R. ‘Balancing the Costs’, World Cement (April 2007).

•PATNI, A., ‘Optimizing Spray Nozzles’, International Cement Review(March 2008).

•‘Ready for the upturn’, ACC, International Cement Review(April 2013).

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