251
1 INTRODUCTION
Among different solutions for reducing nitrogen
oxides emission we can therefore mention the
following:introduction,throughdifferentmethods,of
waterorsteam,incombustionrooms,recirculationof
combustiongases,selectiveornon‐selective catalytic
reduction, use of monitoring systems and electronic
control of functional processes of energetic
installation providing fuel combustion in optimal
condit
ions for obtaining as low emissions’values as
possible.
TheInternationalMaritimeOrganization(I.M.O.),
inAnnexVIMARPOL73/78drawsattentionbothon
sulphuroxidesandespeciallyonnitrogenoxides,for
which methods of determination on board ships,
calculus methods, as well as reduction solutions are
emphasized.I.M.O. is at the same ti
me the main
regulating organism in the marine environment
pollutiondomain.(Uzunov,1997)
Regarding gas turbines installations, as well as
other naval energetic installations, in the matter of
emissionsreduction,themostusedonesarewateror
steam injection systems, combustion gases
Technologies for the Reduction of Nitrogen Oxides
Emissions
P.Arsenie,G.Martinas,C.Gheorghe&A.Arsenie
ConstantaMaritimeUniversity,Romania
ABSTRACT:Whenitcomestogasturbines,theirmainproblemconcerningpollutantemissionsisrepresented
bynitricoxides.Amongotheremissions,sulphuroxidesbeingmuchreducedduetotheuseofliquiddistilled
and gas fuels with a low content of sulphur. Using water or steam injection beca
me the favourite method
duringtheʹ80sandespeciallytheʹ90ssinceʺdryʺmethodsandcatalyticreductionwerebothatthebeginningof
thedevelopmentphase.Catalyticconvertorshavebeenusedsincetheʹ80sandtheyarestillusedalthoughthe
costsofrenewingthecatalystareveryhigh.Inthelasttwentyyearsagradualdecreasehasbeenregisteredon
thelimit
sofnitricoxidesfrom75ppmto25ppm,andnowthetargetisorientedtowardsthe9ppmlevel.The
evolutionofburningtechnologiesofcombustionmakesitpossibletocontrolthelevelofproductionofnitric
oxides even from the source without being necessary to useʺhumidʺ methods. This, of course, opened the
ma
rketforgasturbinesbecausetheycanfunctioneveninareaswithlimitedqualitywaterreserves,suchas
maritimeplatformsandinthedesert.Inthispaper,wearegoingtoshowthat,alt
houghwaterinjectionisstill
used,ʺdryʺcontroltechnologiesofburningbecamefavouritemethodsforthemajorityofusersontheindustrial
power generators market. The great dependency between the creation of nitric oxides and the temperature
revealstheeffectofdirectwaterorsteaminjectiononreducingnitricoxides.Recentresearchshowedtha
ta
reductionupto85%ofnitricoxidesmaybeobtainedbyusingthewaterorsteaminjectionalltogetherwiththe
improvementofaerodynamiccharacteroftheburningroom.
http://www.transnav.eu
the International Journal
on Marine Navigation
and Safety of Sea Transportation
Volume 9
Number 2
June 2015
DOI:10.12716/1001.09.02.13
252
recirculation or re‐combustion, as well as selective
catalyticreduction.(Woodyard,2006)
2 REDUCINGNOXEMISSION
Regardinggasturbines,themainproblemconcerning
pollutant emission is represented by NO
x. Among
otheremissions,SO
xbeingmuchreducedduetothe
usageofliquiddistillateorgasfuelswithareduced
contentofsulphur.
The next figure presents the method of reducing
NO
xemissioninthelast40years,inthecaseofusing
gasturbinesfirstbytheusageofsteaminjectioninthe
combustion rooms (the so called wet combustion
rooms) and then in the `90’s through the system of
dry combustion room with alow level of NO
x (Dry
LowNO
x Combustors). Thenewsystems already in
useconceiveareductionofemissionsofNO
xbelow9
ppm.
Figure1.ControlofNOxemissionsintime
Startingfrom1977itwasadmittedthatthereisa
rangeofmethodsofcontrolofnitrogenoxides:
using combustion rooms divided into several
burning areas in which production of nitrogen
oxidesvariesinordertodecreasethem;
usingaprimaryzoneofthecombustionroomvery
tightinordertodecreasetemperatureofthepeak
oftheflamebyattenuation;
using water or steam injected together with fuel
forcoolingthezoneinthevicinityoftheinjection
burner;
using inert gasses of evacuation which are re‐
circulatedinthereactionzone;
catalyticcleaningofevacuation.
Using water or steam injection became the
favouritemethodinthe`80’sandespeciallythe`90’s
fromthemomentwhen“dry”methodsandcatalytic
reduction were both at the beginning of the
development phase. Catalytic converters were used
from the `80’s and they are still used;
although
renewalcostsoftheacceleratorareveryhigh.
Inthelasttwentyyears,agradualdecreaseofthe
limitsofNO
xwasregisteredfrom75ppmto25ppm,
andnowthetargetisorientedtothelevelof9ppm.
Fuel combustion technologies evolution makes it
possibleforthecontrolof the level of productionof
NO
x right to the source without necessitating “wet”
methods.This,ofcourse, opened the market forgas
turbinesastheycanfunctioneveninthezoneswith
limitedqualitywaterreserves,forexamplemaritime
platformsandinthedesert.(U.S.Navy,2008)
Although water injection is still used, “dry”
technologies of
controlling combustion became the
favourite methods for the majority of users on the
marketofthepowerindustrialgenerators.
DLN(DryLowNO
x)wasthefirstacronymused,
still along with the apparition of requirements of
controlofNO
xwithoutimplicatinganincreaseinthe
level of carbon monoxide and hydrocarbons which
arenotburnt,ledtoDLE(DryLowEmissions).
The largest part of NO
x produced in the
combustion room is called „thermal NO
x”. This is
produced by a series of chemical reactions between
nitrogen (N
2) and oxygen (O2) in the air which is
foundathightemperatures andpressuresinside the
combustionroomofthegasturbine.
82%
18%
8%
10%
10%
28% 72%
Primary area
Dilution area
Production areas
2500 K
1200 K
Flame tube
Figure2.ProductionofNOxinaclassiccombustionroom
Therateofdevelopmentofreactionsisdependent
onthehightemperatures,andtherateofproduction
ofNO
xbecomesimportantatthetemperatureofthe
flame of approximately 2088 K (1815
o
C). Figure 2
presentsschematicthetemperaturesoftheflameand
areas of production of NO
x from the interior of a
combustionroom.Thedesignofthecombustionroom
determinescombustionofthewholequantityoffuel
in a seriesof areaspassing from the state ofrich in
fuel to that of poor in fuel, on the entire power
interval.
The great dependence between formation
of NOx
andtemperaturerevealsthedirecteffectofthewater
or steam injection on the reduction of NO
x. Recent
researches showedthat a reduction of up to 85% of
NO
x may be obtained through water or steam
injectiontogetherwithoptimizingtheaerodynamicof
thecombustionroom.
Insidearegularcombustionroom,suchastheone
infigure2,theairdebitwhichentersinthefirstarea
islimitedtoalmost10%.Therestofthedebitis
used
for the mixture of combustion and for cooling the
combustion room. The maximum temperature is
reachedinthefirstarea,anditisalmost2503K(2230
o
C) and after mixing the combustion fluid with the
coolingair,thetemperaturedecreasestothevalueof
1643K(1370
o
C).
Therearethreemaintechnologiesofreductionof
emissions of NO
x which are used for gas turbines.
Theseare:
Addingwater/steaminthecombustionroom(wet
combustionroom);
253
DrycombustionroomswithlowNOx(DLN);
Selectivecatalyticreduction.
There are also other technological solutions for
reducingNO
xemissionssuchasrecirculationofburnt
gases or humidifying the alimentation air, but these
arenotsomuchused.
Along with the apparition of international
regulations regarding environment pollution a vast
processofresearchandobservationbeganconcerning
themethodofformationofpollutantemissionandthe
influence of some
parameters over those as well as
lookingfortechnologicalsolutionsfortheirreduction.
Presently, researches lead especially in the
direction of conceiving reduction systems of
emissions,especiallythoseofNO
xandSOx. Another
research direction is that of determination or
predictionofformationprocessesofemissions.
Severalworkspresentasasolutionintheproblem
ofpollutionthe useofalternativefuels, suchasbio‐
diesel. The most important researches in this field
were performed at N.R.E.L. (National Renewable
Energy Laboratory), prevailing
in the field or auto
transport. Results show that decreased values of
sulphuroxidesandmechanicalparticlesareobtained,
andfornitrogenoxidesthereductionispossibleonly
by using some additives for improving the cetane
ratio.
3 SOLUTIONSFORREDUCINGNITROGEN
OXIDESEMISSIONS
Accordingtointernationalregulationsinthe
fieldof
marineenvironmentpollution,alongthetimeaseries
of control and reduction of pollutant emissions
measures were developed resulting from fuel
combustion.
The study of these measures made it easier to
understand the chemical processesof formation and
reductionofpollutantagents.Generally,thepurpose
which is targeted in
the study of pollution is to
respect standards of quality of air and water in the
marine environment. (Moldoveanu, 2005)
Internationalprogrammesofreductionofpollutionof
the marine environment may be divided into long
term and short term programmes of control of
pollution.
3.1 Recirculationofburntgases
Recirculation
of burnt gases, FGR (Flue Gas Re‐
circulation)isoneofthemostwell‐knowntechniques
in the field of the steam generators, used with the
purpose of reducing the content of the nitrogen
oxides(NO
x).Thistechniqueusesboththereduction
ofpressureofalimentationairoftheburningpointof
flame tube and the reduction of the temperature of
theflame.Thisisduetoanincreaseofthecontentof
inert gas in the burning area which leads to a
limitationofthe
thermalformationprocessofNOx.
Analysing the diverse applications of reduction
techniquesofemissionsfromsteamgenerators,itwas
notices that much better results are obtained in
reducing NO
x, if burnt gases are introduced along
with the fuel, unless air would be introduced along
withthefuel.
This technique (combined introduction of fuel
mixed with burnt gases) is called re‐circulated fuel
injection, FIR (Fuel Injection Re‐circulation). For
example, emissions of NO
x have been reduced from
thevalueof90 ppmtothevalue of 30 ppmusinga
systemtype5%FIR,whileitwasnecessarytheuseof
23 % of a classical FGR system „Windbox”, for
obtaining the same reduction of NO
x. Steam
generators used inthe naval field would necessitate
considerable quantities of auxiliary power for
conceivingrecirculation.
Figure3.Recirculationsystemofburntgases
In the next figure, an application of the
recirculation of burnt gases technology is presented
withaseparatedventilator.Quantitiesofre‐circulated
gasarecontrolledwithprecisionbythecontrolunitof
dosage(UCD).Researches performedonthissystem
showedthatareductionofover50%ofemissionsof
NO
xisobtainedifevacuationgassesarere‐circulated
inapercentof20%fromtheirtotalquantity.
Significationofnotationsinfigure3is:TF–flame
tube(combustionroom);A–burner;CE–evacuation
funnel; VRG – ventilator for recirculation of burnt
gases;VA–airventilator;
VCC–fuelcontrolvalve;
PC–fuelpump;UCD–dosagecontrolunit.
Thereductiondegreewhichmaybeobtainedisa
function of the fuel nature, concentration of NO
x
whichmaybeobtainedandtherecirculationdegree.
In the diagram in figure 8.2, dependence on these
parametersispresented.
Figure4.Theeffectofthemixtureair–fuelandburntgases
–fuelovertheproductionofNO
x
254
3.2 Waterorsteaminjection
WSI(WaterorSteamInjection)asanagentofdilution
in burnt gases re‐circulated lead to a significant
decreaseofthecontentofnitrogenoxides.Otherwise,
waterorsteaminjectionintheareaofcombustionhas
asimilareffectwiththatofrecirculationtechnology
of
burntgases.Insomecases,waterorsteamareinjected
directly in the flame, either by separate nozzles
disposedontheburnerorbyintegratedorificesinthe
fuel pulveriser. (Moran, 2006) For conventional
burners,usingthewaterinjectionsystempresentedin
the figure, a reduction of 25% of
NOx could be
obtainedwithareportwater‐fuel(WFR)equalto:
whichledtoanincreaseinthecontentofCO.
Figure5.Waterorsteaminjectionburner
Significanceofnotationsinfigure5is:D – flame
diffuser;CA‐burnerbody;TC–fueltube;PC–fuel
pulveriser;EA–combustionelectrodes;TIAA–water
or steam injection tubes; PA – water or steam
pulveriser.
WhensteamisusedforreducingNO
x,thismaybe
injected directly in the combustion room of in the
alimentation air, which by moving reaches also the
combustionroom.Forsomesystems,steamisinjected
inthe discharge air ofthe ventilator for introducing
freshairinthecombustionroom.
Thismethodismuchsimplerfroma
constructive
point of view but less efficient because only 40% of
thesteamreachespreciselythecombustionarea.
Figure6.Influenceoftheairexcesscoefficientandwateror
steam injection over the production of carbon monoxide
andnitrogenoxides
Thesystemisusedforgasturbinesalso.HereWSI
(WaterorSteaminjection)actionstillbyloweringthe
temperatureoftheflame,byensuringadilutionagent
having effect over the thermal mechanism of
formationofNO
x. Water may be injected directly in
the combustion room of the turbine, or it may be
turned into steam, using heat recuperated from
evacuationgasesandtheninjectedinthecombustion
room.
In order to realize the level of reduction of NO
x
obtained by using water, a larger quantity of steam
needstobeused.
3.3 Separationoftheairfromthefuel
The technology for separation of air from the fuel,
AFS (Air and Fuel Staging), is represented by the
division of the combustion areas in areas with sub‐
stoichiometric combustion
and areas with supra‐
stoichiometric combustion. The purpose of this
procedureistodeterminethatcombustionshouldbe
performedinconditionsofinsufficiencyofair,which
hasasaresulta lower rate of formation of nitrogen
oxides.
Further on, incomplete combustion products are
transferred and oxidised in the supra‐stoichiometric
combustionarea.Thistechnologyofseparationofair
from fuel is most often used in case of steam
generators which are using heavy marine fuel,
especially for aqua‐tubes generators having lower
valuesofthetransmissioncoefficientofheatresulting
therefore in a larger volume in which complete
combustion may
be performed. Over time, several
typesofburnerswerebuiltwithdifferentgeometries
withthepurposeofobtainingagoodstratificationof
thepoororrichinairareas,theseburnersbearingthe
name of low NO
x burners. A common trait for the
majorityofburnerswithlowNO
xisthattheprimary
combustion area is reach in fuel by reducing the
quantityofairintroduced.
Significance of the notation in Figure 7 is the
following: ACA – common air alimentation; JAP –
primaryairjet;JAS–secondaryairjet;JAT–tertiary
airjet.
Figure7.Airstratificationburner
255
Figure8. Air stratification system produced by Babcock &
Wilcox(XCL‐SBurner)
3.4 Re‐burningwithnaturalgases
Re‐burning with natural gases for the emissions’
controlofNO
xisamethodwhichproveditsefficiency
intheapplicationsfornavalsteamgenerators.Inthe
re‐burning technology (recycling of NO) a poor in
fuelcombustionisfirstperformed,followedbyarich
in fuel combustion. Chemical mechanism of re‐
burningimpliesrecyclingofnitrogenoxides,formed
inthe
firststageofpoorcombustionandeliminating
those by the reaction of hydro‐carbonated radicals
(composed by the cyanic type, CN or HCN) which
appearsintherichinfuelarea.Intheconditionofthe
richinfuelcombustion,thesecompoundsshallreact
inordertoformN
2leadingtoasubstantialreduction
ofNO.
Re‐burning with natural gases is a useful
technology for the reduction of nitrogen oxides
emissions. This process may reduce nitrogen oxides
emissions with up to 60 – 70%.Generally the
process and reactions produced necessitate the
separation of the steam generator in
three distinct
areas,presentedinfigure9.Theseareasare:
Figure9:Steamgeneratorwithre‐burning
Primary combustion zone (ZPA). In this zone
combustionmayberealisedwithanexcesscoefficient
of air reduced to a minimum value, in order to
decrease formation of NO
x and in order to provide
optimalconditionsforperformingre‐burning.Inthis
area,liquidfuelisburnt,stillwith10‐20%lessthanin
normal conditions of functioning (without re‐
burning);
Gasesre‐burningzone(ZRG).Inthisareathereis
a natural gas injected (10‐20%) over the
first
combustionarea. Thisleadstoarich infuelarea,in
which hydro carbonated radicals which reacts with
NO
xleadingtotheformationofN2;
Final combustion zone (ZFA). In this area an air
excess is added in order to oxide particles of liquid
fuelorgasleftun‐burnt.
Signification of notations in figure 9 is the
following: ZPA – primary combustion zone; ZRG –
gasre‐burningzone;ZFA–burningfinalzone;
AA–
air alimentation; ACP – primary fuel alimentation;
AGN – natural gas alimentation; GE – evacuation
gases.
4 CONCLUSION
Inrelatedpapers,especiallythoseofforeignauthors,
attentionisaimedfirsttonitrogenoxidesandthento
sulphur and carbon oxides. The great majority of
authors present solutions available
from the
productioncompaniesofnavalenergeticinstallations,
solutionswhichwerealreadypresentedinthispaper,
yet at the same time they emphasize the manner in
which pollutant emissions are produced, as well as
theireffectonthemarineenvironmentandthecosts
fordifferentconstructivesolutionsthattheyimpose.
Along
with the arise of international regulations
regardingenvironmentpollution,avastresearchand
observationprocessofthemannerofproductionof
pollutantemissionsbegan,andalsooftheinfluenceof
someparametersonthemaswellaslookingforsome
technologicalsolutionsineachcase.
Presently, researches are especially directed
towards conceiving some systems of reduction of
such emissions, especially those of NO
x and SOx.
Anotherresearchdirectionisthatofdeterminationor
predictionofemissions’formationprocesses.
According to international regulations regarding
the marine environment pollution, a series of
emissions of control and reduction of pollutant
emissions resulted from fuel combustion was
developedintime.
The study of these measures made it easier to
understand the chemical processesof formation and
reductionofpollutantagents.Generally,thepurpose
ofthestudyofpollutionistoobeyqualitystandards
of air and water in the marine environment.
Internationalprogrammesofreductionofpollutionin
the marine environment may be therefore divided
into long and short
term pollution control
programmes.
ACKNOWLEDGEMENTS
This article is a result of the project “Increasing
quality in marine higher education institutions by
256
improving the teaching syllabus according to
International Convention STCW (Standards of
Training, Certification and Watchkeeping for
Seafarers)withManilaamendments”.Thisprojectis
co funded by European Social Fund through The
Sectorial Operational Programme for Human
Resources Development 2007‐2013, coordinated by
ConstantaMaritimeUniversity.
Thecontentofthisscientific
articledoesnotreflect
the official opinion of the European Union.
Responsibility for the information and views
expressedinthearticleliesentirelywiththeauthors.
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