changes in combustion properties of natural gas...
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Changes in combustion properties ofNatural gas when mixed with Hydrogen
PARISA SAYAD, ALESSANDRO SCHÖNBORN ANDJENS KLINGMANN
DEPARTMENT OF ENERGY SCIENCES, LUND UNIVERSITYPARISA.SAYAD@ENERGY.LTH.SE
Hydrogen-containing fules in gas turbinecombustors
oFuel mixtures comprising Hydrogen andMethane as main constituents are of increasinginterest to power generation using gas turbines:
• Hydrogen has been proposed as storagemedium for intermittent renewable energyproduced from wind or solar power
• Hydrogen introduction into existing naturalgas grids has been proposed as a means ofstorage and distribution
• Using synthesis gases as part of the fuel ingas turbines
Hydrogen-containing fules in gas turbinecombustors
oHydrogen is one of the main reactive componentsof synthesis gas
oSynthesis gas (syngas) can be obtained fromrenewable sources such as biomass, or fromtraditional fuels like coal or heavy oil.
oThe composition of syngas may vary according tothe particular gasification process and feed-stockused
Hydrogen-containing fules in gas turbinecombustors
Vol%
Indirect gasification Oxygen/steam blown
wet dry wet dry
CO2 13 21.7 21 32.8
H2O 40 0.0 36 0.0
N2 2 3.3 8 12.5
CO 14 23.3 11 17.2
H2 23 38.3 14 21.9
CH4 8 13.3 10 15.6
Fuel interchangability in gas turbine combustors
oFuel handling and injection systems
oReaching the required combustion temperature
oWobbe index
• Normalised heating value which is used tocompare the combustion energy output of differentcomposition fuel gases in an appliance
• If two fuels have identical Wobbe indicies, giventhe same pressure and valve settings, the energyoutput will also be identical
Premixed or diffusion flames
Premixed:
• Fuel and air is mixedprior to combustion
• Combustion may occurat non-stoichiometricconditions
• Combustion iscontrolled by diffusionand reaction rates(chemical kinetics).
Diffusion flames:
• Fuel and air is mixedand combusted at thesame time
• Combustion occurs atclose to stoichiometricconditions
• Combustion iscontrolled by mixing(diffusion)
Emissions at different equivalence ratio
Temperature or Equivalence ratio, f
Em
issio
ns
f =1Blow out
CO
NOx
Sw
eet
sp
ot
• Lean premixedcombustion canshow low CO andNOx emissions atthe same time
Fuel interchangability in gas turbine combustors
oFlame holding and combustionstability
• Flashback
• Blowout
• Autoignition
• Dynamic instability
oEmissions
• NOX
• CO
Flame holding and operability/Flashback
oFlashback occurs when the flamepropagates upstream from thecombustion chamber into thepremixing section.
• Boundary layer flame propagation(critical velocity gradient)
• Turbulent flame propagation incore flow
• Combustion instabilities
• Upstream flame propagationinduced by combustion inducedvortex breakdown
Flame holding and operability/Blowout
oBlowout is the processof flame extinction underlean conditions, bymeans of the flameleaving its stableanchored position andbeing drawn downstreaminto the combustorwhere it extinguishes.
oIt is likely to occur under part-loadaconditions and during transient operationasuch as during turbine start-up.
Flame holding and operability/Autoignition
oAutoignition occurs when the residence time of thefuel-air mixture in the premixing section exceeds thecritical time necessary to cause self ignition
Flame holding and operability/Influencingparameters
oFuel properties
• Reactivity and chemical kinetics
• Transport properties
oFlow-filed in the combustor
• Turbulence levels
The combustion behaviorof blended fuels, may becompletely different from
their separatecomponents, and cannot
be estimated by anarithmetic mean based
on proportions
Turbulent flamespeed
Fluid dynamics effects(turbulent intensity)
Laminarflame speed
Diffusion
Reaction rates
Flame holding and operability/Hydrogen properties
oCompared to Methane, Hydrogen has:
• Significantly higher reactivity
• Higher adiabatic flame temperature atstochiometric conditions in air
• Higher laminar flame speed. A hydrogenflame propagates five times faster than amethane flame at atmospheric conditions
• Different thermal and mass diffusion (Lewisnumber) which affects the flame behavior interms of resistance to stretch
Flame holding and operability/Hydrogen properties
oCompared to Methane, Hydrogen has:
• Significantly higher reactivity
• Higher adiabatic flame temperature atstochiometric conditions in air
• Higher laminar flame speed. A hydrogenflame propagates five times faster than amethane flame at atmospheric conditions
• Different thermal and mass diffusion (Lewisnumber) which affects the flame behavior interms of resistance to stretch
•More prone to flashback
•Higher risk of autoignition
•Stable combustion at lowerequivalence ratios
Why experimental approach?
oThe physics behind blowout and flashback is verycomplicated due to their unstable nature which isan interaction between chemical kinetics andturbulence
oFor this reason it is not possible to study thesephenomena using CFD modeling
oConducting blowout and flashback experiments ina real facility using Hydrogen containing fuels isnot a practical option
The atmospheric variable-swirl burner/Schematic
Combustion intensity: 20 MW/m3.atm
Thermal power: 17 KW
Volume: 0.0009 m3
Air mass flow rate: 3.5 gr/sec
The atmospheric variable-swirl burner/Layout
Air0.6 MPa
Air , 293 K0.6 MPa
Air, 293-1000 K0.1 MPa
Burner
CH4 CO H2
Pressureregulators0.4 MPa
Strainer
Mass-flow meters,pressure sensor,
Thermocouple
Mass-flow meter,pressure sensor,
Thermocouple
Hot exhaustGases
0.1-1 MPa
Solenoid-controlled shut-off
valves
Air293-1000 K
0.1 MPa
Mass-flowmeter, pressure
sensor,Thermocouple
Swirl mixerRadial flow
Axial flow
0.1 MPa
Spark
plug
Ignitioncircuit
Exhaust gasanalysis rack
Measurement techniques
oVelocity measurements using LDA
• In order to determine the swirl number for each flowcondition, the axial and tangential velocity profileswere measured 1 mm above the dump plane of thecombustor using LDA.
oEmission measurements
• The CO measurements in the exhaust gas werecarried out using a non-dispersive infrared (NDIR) gasanalyzer (Horiba).
Measurement techniques
oHigh-speed OH* chemiluminesence
• The occurrence of flashback and unsteadypropagation of the flame in the premixing tubewas recorded using high speed OH*
chemiluminescence imaging. This was done usinga high-speed camera (Vision Research PhantomV 611) equipped with an image intensifier(Hamamatsu C4598), a band-pass filter (ActonResearch 310.5±5.75nm) and a phosphateglasslens (UV-Nikkor 105 mm, f/4.5) to photograph OH*
chemiluminescence of the flame around 306 nmat high speed.
Experimental variables
oFlow Parameters
• Flow-field (Swirl Number)
• Inlet Temperature
• Total Mass Flow rate
oFuel Composition
• Syngas (H2 + CO + CH4 + N2 +CO2 )
• Hydrogen enriched methane
• Wet mixtures(H2 + CH4 + CO + H2O)
Experimental variables/ Swirl number
oBy adjusting the mass flow rate through therespective axial- and radial flow paths, and hencevarying the ratio between tangential and axialmomentum through the combustor, different flowpatterns can be achieved
o In order to characterize different flow cases, the swirlnumber is introduced
Where R is the radius of the swirler, Gt is the axial flux oftangential momentum and Ga is the axial flux of axialmomentum
Experimental results/ Velocitymeasurements
-10 -5 0 5 10
-1
-0.5
0
0.5
1
1.5
2
Y [mm]
Axi
alv
elo
city
/Ubu
lk
S=0.06
S=0.13
S=0.24
S=0.34
S=0.53
S=0.66
-10 -5 0 5 10-1.5
-1
-0.5
0
0.5
1
1.5
Y [mm]
Tan
gent
ialv
elo
city
/Ubu
lk
S=0.06
S=0.13
S=0.24
S=0.34
S=0.53
S=0.66
0 2 4 6 8 100
0.1
0.2
0.3
0.4
0.5
0.6
0.7
Axial momentum/ Tangential momentum
Sw
irl
num
ber
Experimental results/ Flashback limits
0 20 40 60 80 1000
0.2
0.4
0.6
0.8
1
H2
molar content [%]
Fla
shba
ckeq
uiva
lenc
era
tio
[CH4]=0.0, S=0.66
[CO]=0.0 , S=0.66
[CO]=[CH4], S=0.66
[CH4]=0.0, S=0.53
Experimental results/ Blowout limits
0 20 40 60 80 1000
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
H2
molar content [%]
Blo
wou
teq
uiva
lenc
era
tio
[CH4]=0.0, S=0.66
[CO]=0.0, S=0.66
[CO]=[CH4], S=0.66
[CH4]=0.0, S=0.53
Experimental results/ Stability range
0 20 40 60 80 1000
0.2
0.4
0.6
0.8
1
H2
molar content [%]
Sta
bili
tyra
nge
[CH4]=0.0, S=0.66
[CO]=0.0 , S=0.66
[CO]=[CH4], S=0.66
[CH4]=0.0, S=0.53
Experimental results/ Flashbackvisualisation
t = 0.0 msZ
[mm
]
-40
-20
0
20
40
60t = 0.444 ms t = 0.888 ms
t = 1.332 ms
Z[m
m]
-40
-20
0
20
40
60t = 1.776 ms t = 2.220 ms
t = 2.664 ms
Y [mm]
Z[m
m]
-20 0 20
-40
-20
0
20
40
60
t = 3.108 ms
Y [mm]-20 0 20
t = 3.552 ms
Y [mm]-20 0 20
t = 0.0 ms
Z[m
m]
-40
-20
0
20
40
60t = 2.072 ms t = 4.144 ms
t = 6.216 ms
Z[m
m]
-40
-20
0
20
40
60
t = 8.288 ms t = 10.360 ms
t = 12.432 ms
Y [mm]
Z[m
m]
-20 0 20
-40
-20
0
20
40
60
t = 14.504 ms
Y [mm]-20 0 20
t = 16.576 ms
Y [mm]-20 0 20
Fla
sh
back
du
eto
co
mb
usti
on
ind
uced
vo
rtex
bre
akd
ow
n
Fla
sh
back
inth
eb
ou
nd
ary
layer
Experimental results/ Autoignitiont = 0.0 ms
Z[m
m]
-40
-20
0
20
40
60t = 0.074 ms t = 0.148 ms
t = 0.222 ms
Z[m
m]
-40
-20
0
20
40
60t = 0.296 ms t = 0.370 ms
t = 0.444 ms
Y [mm]
Z[m
m]
-20 0 20
-40
-20
0
20
40
60t = 0.518 ms
Y [mm]-20 0 20
t = 0.592 ms
Y [mm]-20 0 20
t = 0.592 ms
Z[m
m]
-40
-20
0
20
40
60t = 0.888 ms t = 1.184 ms
t = 1.480 ms
Z[m
m]
-40
-20
0
20
40
60
t = 1.776 ms t = 2.072 ms
t = 2.368 ms
Y [mm]
Z[m
m]
-20 0 20
-40
-20
0
20
40
60t = 2.664 ms
Y [mm]-20 0 20
t = 2.960 ms
Y [mm]-20 0 20
Fla
sh
back
an
db
low
ou
tcau
sed
by
au
to-i
gn
itio
nin
the
pre
mix
ing
tub
e
Experimental results/ Autoignition
oEstimation of the bulk residence time in thepremixer in combination with gas-phasechemical kinetic modeling (Chemkin)suggested that the residence time of thereactants in the premixer (0.01 s) wassignificantly shorter than the autoignitiondelay time calculated from chemical kineticmodeling under these conditions (> 1.5 s).
Experimental results/ Autoignition
oRecirculation zones can significantly increasethe effective residence time of the reactants,and thereby assist autoignition.
o Surface reactions on the walls of premixingtubes can significantly reduce autoignitiondelays below what is predicted by gas phasekinetic modeling.
Characterization of fuels
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