Combustion of solid alternative fuels in the cement kiln burner

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Combustion of solid alternative fuels in the cement kiln burner
Nørskov, Linda Kaare
Publication date:
2012
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Citation (APA):
Nørskov, L. K. (2012). Combustion of solid alternative fuels in the cement kiln burner. Technical University of
Denmark.

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and investigate your claim.
Combustion of solid alternative fuels
in the cement kiln burner
Linda Kaare Nørskov
Industrial PhD thesis
2012
Abstract
In the cement industry there is an increasing environmental and financial motivation for substituting conven-
tional fossil fuels with alternative fuels, being biomass or waste derived fuels. However, the introduction of
alternativefuelsmayinfluenceemissions,cementproductquality,processstability,andprocessefficiency. Al-
ternative fuel substitution in the calciner unit has reached close to 100% at many cement plants and to further
increase the use of alternative fuels rotary kiln substitution must be enhanced. At present, limited systematic
knowledge of the alternative fuel combustion properties and the influence on the flame formation is available.
Inthisprojectascientificapproachtoincreasethefundamentalunderstandingofalternativefuelconversionin
therotarykilnburnerisemployedthroughliteraturestudies,experimentalcombustioncharacterisationstudies,
combustionmodelling,datacollectionandobservationsatanindustrialcementplantfiringalternativefuels.
Alternativefuelsmaydifferfromconventionalfossilfuelsincombustionbehaviourthroughdifferencesinphys-
icalandchemicalpropertiesandreactionkinetics. Oftensolidalternativefuelsareavailableatsignificantlarger
particle sizes than solid fossil fuels due to the cost of downsizing. Through theoretical evaluation it is found
thatthedevolatilisationoflargefuelparticlesismainlylimitedbyinternalheattransferandthecharoxidation
isdominatedbyexternalO diffusionatconditionsrelevanttosuspensionfiredcombustion.
2
An experimental combustion reactor for simulating suspension fired combustion of large, single particles is
established and experiments are performed to investigate conversion pathways, ignition, devolatilisation, and
charoxidationtimesofpinewood,andthreetypesofdriedsewagesludgeasfunctionofparticlesizeandshape,
O concentration,andgastemperature. Resultsshowthatthemainfactorsaffectingthetimeofdevolatilisation
2
is the gas temperature and particle size and shape. Factors affecting char oxidation rates include gas tempera-
ture,O concentration,andparticlesizeandshape.
2
A one-dimensional mathematical model of the rotary kiln flame is developed to evaluate the influence of fuel
properties and combustion system parameters on the fuel burnout and flame temperature profile. Two alter-
native fuel cases are simulated; dried sewage sludge and refuse derived fuel firing. Firing sewage sludge or
refused derived fuel with large particles and high moisture contents at conditions similar to a coal fired flame
resultsinanelongatedflameandaburnouttimeexceedingtheavailabletimeinsuspension. Fuelpretreatment,
i.e. grinding and drying, is insufficient to ensure the dried sewage sludge to be converted within the available
time in suspension, however a partial particle downsizing without drying can be allowed for refuse derived
fuelfiring. Byincreasingthe entrainmentrate ofsecondaryair, theprimary airpercentage, theexcessairratio
andapplyingO enrichmentitisfoundthatfullconversionofthelargealternativefuelparticlesmaybereached.
2
The simplified mathematical model may serve as a tool for predicting the effect of introducing new fuels on
burnoutbehaviour,andflamepropertiessuchasflamelengthandgastemperatureprofileinarotarykilnflame.
i
Dansk resume´
I cementindustrien er der en øget miljømœssig og økonomisk motivation for at erstatte konventionelle fossile
brœndslermedalternativebrœndsler; biomasseogaffald. Indførelsenafalternativebrœndslerkandogpåvirke
emissioner, cementproduktkvalitet, processtabilitet og -effektivitet. I kalcinatoren er substitutionen med alter-
native brœndsler nået tœt på 100% på mange cementanlœg, og for at øge anvendelsen af alternative brœndsler
yderligeremåsubstitutioneniroterovnenøges. Dererbegrœnsetsystematiskvidentilgœngeligomdealterna-
tivebrœndslersforbrœndingsegenskaberogderesindflydelsepåflammedannelsenidag. Idetteprojektanven-
desenvidenskabeligtilgangforatøgedengrundlœggendeforståelseafomsœtningenafalternativebrœndsleri
roterovnsbrœnderen igennem litteraturstudier, eksperimentelle forbrœndingsstudier, forbrœndingsmodellering,
ogdataopsamlingogobservationerfraetindustrieltcementanlœg,derfyreralternativebrœndsler.
Forbrœndingsegenskaberneafalternativebrœndslerkanadskillesigfradekonventionellefossilebrœndslergen-
nemforskelleifysiskeogkemiskeegenskaberogreaktionskinetik. Oftevilfastealternativebrœndslerhaveen
markantstørrepartikelstørrelseendfastefossilebrœndslerpågrundafomkostningernevedneddeling. Enteo-
retiskevalueringviser,atpyrolyseafstorebrœndselspartiklerhovedsageligterbegrœnsetafinternvarmetrans-
port,ogkoksoxidationenerdomineretafeksternO diffusionvedsuspensionsfyretforbrœndingsbetingelser.
2
Eneksperimentelforbrœndingsreaktoreretablerettilsimuleringafsuspensionsfyretforbrœndingafstoreenkelt-
partikler og forsøg er udført for at undersøge omdannelsesvejen, antœnding, pyrolyse, og koksoxidationstider
af fyrretrœ og tre typer tørret spildevandsslam som funktion af partikelstørrelse og form, O koncentration og
2
gastemperatur. Resultaterne viser, at de vigtigste faktorer, som påvirker pyrolysen er gastemperaturen, par-
tikelstørrelsenogformen. Faktorer,sompåvirkerkoksoxidationen,inkluderergastemperatur,O koncentration,
2
partikelstørrelseogform.
Enendimensionelmatematiskmodelafroterovnsflammenerudvikletforatevaluereindflydelsenafbrœndsels-
egenskaber og forbrœndingssystemparametre på udbrœndingstiden og flammetemperaturprofilen. Indfyring af
to alternative brœndsler er simuleret; tørret spildevandsslam og affaldsfyring. Fyring med spildevandsslam og
affaldmedstorepartikleroghøjtvandindholdundersammebetingelsersomkulfyringresultererienforlœnget
flammeogenudbrœndingstid, somoverstigerdentilgœngeligetidisuspension. Brœndselsforbehandling, tør-
ring og neddeling, er ikke tilstrœkkelig til at sikre udbrœnding af spildevandsslam indenfor den tilgœngelige
tid,mensendelvisneddelingafutørretaffaldkantillades. Vedatøgeindblandingshastighedenafsekundœrluft,
primœrluftmœngden, overskudsluftforholdet og anvende iltberigelse kan fuld omdannelse af de alternative
brœndsleropnås.
Den simplificerede matematiske model kan bruges som et vœrktøj til at forudsige effekten af at introducere
nyebrœndslerpå udbrœndingogflammeegenskaber,somflammelœngdeoggastemperaturprofiliroterovnen.
ii
Preface
ThisIndustrialPhDprojectisperformedincorporationbetweentheResearchDepartmentofFLSmidthA/Sand
CHECResearchCentre, DepartmentofChemicalandBiochemicalEngineeringattheTechnicalUniversityof
Denmark,DTU.ThePhDprojectisco-fundedbyTheDanishAgencyforScience,TechnologyandInnovation.
The project is a part of the research platform ’New Cement Production Technology’, funded by the Danish
NationalAdvancedTechnologyFoundation,theTechnicalUniversityofDenmark,andFLSmidthA/S.
The supervisors of the project are Morten Boberg Larsen (FLSmidth), Kim Dam-Johansen (CHEC), Peter
Glarborg (CHEC), and Peter Arendt Jensen (CHEC) whom I would like to thank for valuable discussions and
supervision during the project. Furthermore, a special thank you to Michael Lykke Heiredal for performing
computational fluid dynamics (CFD) calculations in this project, to Paw Jensen who has contributed to a large
part of the thermogravimetric analysis experiments and combustion experiments in the single particle reactor
during his master project, to Troels Bruun Hansen for various matters and help regarding the single particle
combustion setup during his master thesis and later employment at CHEC Research Centre, to Weigang Lin
for conducting the swirl-flow burner experiments, and to Martin Hagsted Rasmussen for troubleshooting my
Matlab code. Thanks to all the other colleagues at FLSmidth and CHEC, not mentioned here, who have been
involvedintheproject.
31th July2012
LindaKaareNørskov
iii
Nomenclature
Roman
a ConstantinSherwoodcorrelation [-]
a Densityweightedvelocityratiobetweenajetandco-flowfluid [-]
a KineticparameterintheHR-SMORpyrolysismodel [sb−1/Kb]
A Area [m2]or[m2/s]
A Pre-exponentialfactor unitdependsonrateconstant
A Massratio [kg/kg]
b KineticparameterintheHR-SMORpyrolysismodel [-]
Bi Biotnumber [-]
c Particleconcentrationingas [m−3]
p
C Constantforwalltemperatureprofile [-]
C Bulkconcentrationofoxygen [molO /m3]
b 2
C Specificheatcapacity [J/(molK)]or[J/(kgK)]
p
d Particlediamter [m]
p
D Diameter [m]
D Diffusioncoefficient [m2/s]
E Activationenergy [J/molorJ/kg]
A
f Massfraction [-]
F Force [kgm/s2]
F Viewfactorforradiation [-]
g Gravitationalacceleration [9.80665m/s2]
h Heattransfercoefficient [W/(m2 K)]
H Enthalpy [J/mol]or[J/kg]
k Rateconstant [m/s] ( ) 
kc Chemicalreactionrateconstant  m3(gas n ) 
(molO )n-1 m2 surface ·s
 2 ( ) 
kc′ Chemicalreactionrateconstant (molO )nm-13(mga2ssunrface)·s
2
iv
 
k′′ Chemicalreactionrateconstant  ( gC ) 
c (atm)n· m2 surface ·s
k RateconstantforexternalO diffusion [m(s]
g 2
K Proportionalconstantforsecondaryairentrainment [-]
l Length [m]
L Length [m]
LHV Lowerheatingvalue [J/mol]or[J/kg]
m Reactionorderwithrespecttostructuralmodel [-]
m˙ Massflowrate [kg/s]
M Molarmass [kg/mol]
M Momentum [kgm/s]
n Numberofmoles [mol/kgfuel]
n ReactionorderwithrespecttoO [-]
2
n Constantorexponent [-]
n˙ Molarflow [mol/s]
N Absolutenumber [-]
Nu Nusselt’snumber [-]
P Pressure [Pa]
Pr Prandlt’snumber [-]
q Volumetricflow [m3/s]
q Heatfluxdensity [J/(s·m2)]
Q Heatflow [J/s2]
r Radius [m]
R Initialradius [m]
Rgas Idealgasconstant [[(m3 atm])/(molK)orJ/(molK)]
molC
R Reactionrate
m2 s
Ra Rayleigh’snumber [-]
Re Reynold’snumber [-]
S Surfacearea [m2]
S Swirlnumber [-]
Sc Schmidtnumber [-]
Sh Sherwoodnumber [-]
t Time [s]
T Temperature [K]
u Velocity [m/s]
V Volume [m3]
wt Massfraction [-]
X Massbaseddegreeofconversion [-]
y Molarfraction [-]
z Coordinatesofthetransientheattransferequation [-]
v
Greek
α Absorptivity [-]
α KineticparameterintheDAEMmodel [-]
α Stoichiometricnumberforcarbon [mol/kgfuel]
β KineticparameterintheDAEMmodel [-]
β Stoichiometricnumberforhydrogen [mol/kgfuel]
β Angleofrepose [o]
β Coefficientofvolumeexpansion [-]
γ Stoichiometricnumberforsulphur [mol/kgfuel]
Γ Kilnfillangle [rad]
δ Stoichiometricnumberfornitrogen [mol/kgfuel]
ε Emissivity [-]
ε Evaporationfactor [-]
κ Thermaldiffusivity [m2/s]
λ Excessairratio [-]
λ Thermalconductivity [W/(mK)]
λ Wavelength [m]
µ Dynamicviscosity [kg/(ms)]
µ StoichiometricnumberforH O [mol/kgfuel]
2
ξ Stoichiometricnumberforchloride [mol/kgfuel]
ρ Density [kg/m3]
σ Stefan-Boltzmann’sconstant [J/(sm2 K4]
τ Timeforconversion [s]
ϕ StoichiometricnumberforO [mol/kgfuel]
2
φ Stoichiometricfactorforcharconversion [molC/molO ]
2
ψ Volumefraction [-]
ω Rotationalspeed [rad/s]
Ω Viewfactorofradiation [-]
vi
subscripts
0 Initialorattimet = 0
air Air
ash Ash
b Bulk
b Bed
c Chemicalreaction
c Coating
cb Combustion
co Co-flow
CC Conductionthroughcoating
CGB Convectionfromgastobed
CGP Convectionfromgastoparticle
CGW Convectionfromgastowall
char Char
CR Conductionthroughrefractory
CS Conductionthroughsteelshell
CSA Convectionfromshelltoambientair
CV Controlvolume
CWB Conductionfromwalltobed
daf Dry,ashfree
e Equivalent
e(cid:11) Effective
end End
evap Evaporation
f Flame
f Film
fc Forcedconvection
fuel Fuel
g Gas
i Inner
inst Instantaneous
jet Jet
loss Loss
nc Naturalconvection
o Outer
org Organicmatter
p Particle
p Projected
pri Primaryair
vii
R Reaction
rc Interfacebetweenrefractoryandcoating
ref Reference
rgp radiationgastoparticle
RGP Radiationfromgastobed
RGP Radiationfromgastoparticle
RGW Radiationfromgastowall
RSA Radiationfromshelltoambientair
RWB Radiationfromwalltobed
s Surface
s Shell
sr Interfacebetweenshellandrefractory
sec Secondaryair
theo Theoretical
tra Transportair
tot Total
vol Volatile
w Wall
viii