Experimental investigation of safety issues in ignition of methane-

hydrogen jets

G. Migliavacca1, C. Morreale

1, A. Maggioni

1, P. Lopinto

1, S. Marengo

1, G.

Solero2, M. Longa

2, G. Lombardi

2

1. Stazione Sperimentale per i Combustibili, San Donato Milanese ITALY

2. Energy Department - Politecnico di Milano, ITALY

1. Introduction

The usage of hydrogen as a general purpose fuel in the near future is one of the options

presently considered as a solution of many energetic and environmental problems. However

the production of amounts of hydrogen large enough to substitute the whole demand of

natural gas is not a realistic perspective for the next decade and, in the same time, the

complete switch of grids and appliances from natural gas to pure hydrogen would produce

many technical problems of difficult and expensive solution. On the contrary an increasing

tendency to enlarge the acceptable limits of natural gas parameters is growing in the national

and international standards and regulations, in order to allow new gas suppliers to enter the

gas market, including bio-gas producers. This principle could be extended to future hydrogen

producers from renewables or zero emission plants, distributing hydrogen in mixture with

methane through the existing grid and feeding the existing appliances. It could represent a

proper solution to reduce both pollutant and greenhouse emissions, but its compatibility with

the existing gas system, mainly in terms of safety, is still to be evaluated.

2. Experimental section

Different series of experiments have been carried out in order to characterise the possible

behaviour of a hydrogen-methane jet generated from an accidental, small scale leakage of gas

in a low pressure pipeline. The use of calibrated nozzles to simulate this situation has been

presented in previous works [1]; in the present paper this approach is applied to study

different aspects of the morphology of the jets and the corresponding flames. A test rig,

shown in Figure 1, has been used for the experiments, where different diagnostic techniques

have been adopted to analyse the different aspects of the studied phenomena. Particularly, the

visible, IR and UV spectra have been investigated and laser diagnostic techniques (OH-PLIF,

Rayleigh Thermometry) have been adopted to characterize the flame structure.

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Processes and Technologies for a Sustainable Energy

Fig. 1: Experimental setup for jets and flames studies.

3. Results and discussion

3.1. Jet morphology analysis

A first analysis has been carried out on the jet morphology in order to single out the possible

effects due to the presence of hydrogen in the mixture. IR emission from jets of gas mixtures

of different compositions, which had been passed through an electrically heated adduction

pipe, has been collected by the IR camera. A specific filter has been applied on the camera

objective in order to select the methane emission band only. This set-up allows to visualize

the distribution of methane in the jet propagation. This technique is insensitive to molecular

hydrogen, since this compound does not emit in the IR spectrum, hence it is possible to

evaluate the effect that hydrogen has on the diffusion of methane, but not to observe the

diffusion of hydrogen itself. Then the radial profiles of IR emission from methane has been

compared with the correlation by Schefer et al. [2, 3] here reported:

2

2/1/693.0exp LrY

Y

CL

where the ratio of the local concentration Y(r), in a free jet, on the corresponding central line

concentration YCL is expressed as a function of the radial coordinate (r) and L1/2 indicates the

position where the concentration is equal to ½ of YCL, thus measuring the jet spread. As

shown in Table 1 L1/2, estimated from the experimental profiles by means of numerical

regressions, is nearly constant for hydrogen concentrations lower than 30%, indicating that

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Ischia, June, 27-30 - 2010

small amounts of H2 do not influence the jet shape, even if a larger concentration of hydrogen

could be supposed to be present in the outer shell of the jet, as a consequence of it greater

diffusivity. The good agreement between experimental data and Schefer model radial profiles

is shown in Figure 2-a. The corresponding axial profiles show larger deviations from the

model predictions, mainly close to the nozzle, while a better agreement is observed at larger

distances, as visible in Figure 2-b. The presence of hydrogen may cause a more significant

variation on the axial profiles, since not only its higher diffusivity but also its lower density

acts on the shape of the jets.

Tab. 1: Mixing length as a function of mixture composition in non reacting free-jets.

0

0,2

0,4

0,6

0,8

1

1,2

0 5 10 15 20 25 30

Z/d

Y

Fr=270Fr=100

a) b)

Fig. 2: a) experimental (dots) radial profiles of methane relative concentration compared with Schefer model (solid lines);

b) experimental (dots) axial profiles of methane relative concentration compared with Schefer model (solid lines).

3.2. Flame morphology analysis In the second step of the work the attention has been focused on the morphological analysis of

the flames generated from the hydrogen-methane jets previously discussed. Here the radial

and axial OH profiles are reported for some selected flames. In Figure 3-a and b, the axial

profiles for flames of lower (780) and higher (1240) Reynolds numbers and at different

hydrogen concentration in the fuel are reported; they show regular profiles having a trend

proportionally increasing with the hydrogen content and asymptotically growing towards a

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Processes and Technologies for a Sustainable Energy

maximum which is outside the investigated area.

Re = 780

00,10,20,30,40,50,60,70,80,9

1

0 2 4 6 8

Axial distance [cm]

Re

lati

ve

in

ten

sit

y

CH4

90%CH4-10%H2

70%CH4-30%H2

50%CH4-50%H2

H2

a)

Re = 1240

00,10,20,30,40,50,60,70,80,9

1

0 2 4 6 8

Axial distance [cm]

Re

lati

ve I

nte

ns

ity

CH4

90%CH4-10%H2

70%CH4-30%H2

50%CH4-50%H2

H2

b)

Fig. 3: Integral profiles of OH relative concentration along the axial direction at different mixture compositions and Reynolds numbers.

In Figure 4-a-b-c the radial OH profiles are reported for the low Reynolds flames only, at

three different distances from the nozzle. It is possible to observe a slight increase of the

radial dimension of the flames increasing hydrogen concentration. Only in the case of a pure

hydrogen flame the maxima on the radial profiles are markedly shifted towards the outer

radius. This effect is also visible in the IR spectrum (Figure 4-d), in particular in the case of

the pure hydrogen flame, where an evident difference in the morphology of the flame is

observed. It is possible to observe that for pure hydrogen flames the OH maximum is reached

in a position closer to the nozzle, while for methane rich flames the maximum concentration is nearly constant throughout the investigated area.

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Ischia, June, 27-30 - 2010

a) b)

c)

d)

Fig. 4: a), b), c) radial profiles of OH relative concentration at different axial positions and

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Processes and Technologies for a Sustainable Energy

H2 concentrations; d) IR images of flames at different H2 concentrations.

4. Conclusions

The present work briefly summarizes some results from a wider study concerning the

accidental release of hydrogen-methane mixtures. The geometrical characteristics of jets and

flames, here evaluated and parameterized, can be used to define critical conditions and

properties, useful to predict possible safety issues in the handling of these mixtures in low

pressure pipelines and appliances. The length, width and morphology of the hydrogen-

methane jets are not significantly influenced by the presence of hydrogen, at least in terms of

methane distributions; also the corresponding flames do not show an appreciable increase of

the radial dimension, except when large amounts of hydrogen are present in the fuel mixture.

On the contrary the axial behavior of jets and flames appears more influenced by the presence

of hydrogen, mainly as a consequence of density effects on the jets and higher flame speed on

the flames.

5. References

1. Cavallini, M., Furci, A., Solero, G., Lopinto, P., Migliavacca, G. (2009): “Safety

issues of hydrogen-methane unintended releases and ignition”, Sixth Mediterranean

Combustion Symposium, 7-11 June 2009, Ajaccio

2. Schefer, R.W., Houf, W.G., Williams, T.C. (2008): “Investigation of small-scale

unintended releases of hydrogen: buoyancy effects”, International Journal of Hydrogen

Energy, vol. 33, p. 4702-4712

3. Schefer, R.W., Houf, W.G., Williams, T.C. (2008): “Investigation of small-scale

unintended releases of hydrogen: momentum dominated regime”, International Journal of

Hydrogen Energy, vol. 33, p. 6373-6384

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