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