Research on Gas Combustion

E. Van den Bulck

Departement Werktuigkunde

Katholieke Universiteit Leuven

Selected new research targets on gas combustion at the K.U. Leuven:

1. Enrichment of natural gas with hydrogen.

2. Control of burner outlet flows using small jets.

Hydrogen may become an important fuel in the near future.

Burners and gas appliances will need to be redesigned to cope with the challenging combustion properties of this fuel. An important property is:

Enrichment of natural gas with hydrogen

E. Van den Bulck & K. Vanoverberghe

The very high flame speed.

About the flame speed

1. To stabilise a flame, the air/gas velocity has to match the flame speed.

2. The flame speed of pure hydrogen is about 6,4 times the flame speed of natural gas, e.g., 3 meters/second for natural gas implies almost 20 meters/second for hydrogen!

3. The driving pressure is proportional to the velocity squared, e.g., a 1 mbar fan for a natural gas burner implies a 40 mbar fan for hydrogen!

Flame speed and flame stabilizationa 30 KW low-NOx natural gas flame

The gas/air velocity is matched by the upstream directed flame speed.

The flame speed of L-gas/H2 mixtures

0 0.2 0.4 0.6 0.8 10.4

0.5

0.6

0.7

0.8

0.9

1

molar hydrogen fraction, xH2

eq

uiv

ale

nc

e r

ati

o,

1700 C

1500 C

1300 C

Today’s applications

Tomorrow’s application?

Lean combustion lowers the flame speed AND combustion temperature.

Adding hydrogen to natural gas can be seen as the enrichment of natural gas.

1. Adding hydrogen increases the flame speed.

2. Leaner combustion, i.e. increasing the combustion air offsets this increase of flame speed.

3. With lean combustion, the flame temperature drops by about 300 C!

Can lean combustion of hydrogen enriched natural gas mixtures lead to near-zero NOx emissions whilst keeping the flame structure the same?

Experimental setup

L-gas

H2

30 kW swirl stabilized flame

NOx

First results of experiments

Volume fraction of H2 in L-gas (%)

0 20 40 60

Equ

ival

ence

rat

io (

%)

50

60

70

80

90

100 46

56

22

56

27

50

8

50 4

44 <1

40

NOx in mg/Nm3 (3% O2)

CO2 in g/MJ

Kyoto target (80 % of 56)

Selected new research targets on gas combustion at the K.U. Leuven:

Enrichment of natural gas with hydrogen.

Conclusions:

Kyoto goal (-20 % CO2) can be reached by adding 45 % of hydrogen.

Zero NOx is not just a bonus.

Higher air to gas ratio’s will need bigger heat exchangers

Control of burner outlet flows using small jets.

E. Van den Bulck

K. Vanoverberghe & M. Vanierschot

Experimental observations:

1. Jets are attracted by nearby walls

2. Jets can be deflected over large angles using secondary control jets

Experimental facilityQuartz glass combustion

chamberMass flow controllers

30 and 300 kW twin burners

Experimental gas burners

SWIRL

Control jets

1. Swirling air and gas

2. Secondary gas injection holes which are used for small control jets

Features:

Experimental observation of flow hysteresis induced b/t wall-effect.

S = 0..20 S = 70 S = 90 & S = 0

Computer simulations of flow hysteresis induced by the wall-effect

S = 0 S = 100

S = 45S = 0 S = 70

Wall-effect can be induced by small control jets in stead of swirl

A strong axial non-swirling jet can be deflected over 90 degrees by injecting 1% of the flow crosswise into the main jet.

Experimental validation

a 30 KW natural gas flame clinging to the refrac-tory bottom of the combustion chamber - no swirl needed.

Selected new research targets on gas combustion at the K.U. Leuven:

Control of burner outlet flows using small jets.

Conclusions:

Multiple flame structures from the same burner.

Self adapting burner with respect to, e.g., hum.

Many other applications of active control by secondary jets are emerging.