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Preventing hydrogen detonations in road tunnelshydrogen trap concept

Mitja Ko�zuh a,b,*

a Faculty of Chemistry and Chemical Technology, University of Ljubljana, Ljubljana, Sloveniab Center of Excellence Low-Carbon Technologies (CO NOT), Ljubljana, Slovenia

a r t i c l e i n f o

Article history:

Received 3 June 2014

Received in revised form

3 August 2014

Accepted 7 August 2014

Available online 13 September 2014

Keywords:

Hydrogen

Road tunnels

Safety

Detonation prevention

Passive system

* Faculty of Chemistry and Chemical TechE-mail address: mitja.kozuh@fkkt.uni-lj.s

http://dx.doi.org/10.1016/j.ijhydene.2014.08.00360-3199/Copyright © 2014, Hydrogen Energ

a b s t r a c t

During research of new possible sources of energy, hydrogen was identified as a very

promising potential energy carrier. Because of its very good energy characteristics, it has

received a lot of research attention while its safety features are the ones that were its

drawback for potential use. Before it can be put in general use in transportation industry,

there were safety problems identified as hazard which has to be further analysed. The

main problem in the transport is the safe use of hydrogen in road tunnels where it should

be safe in case of possible accidents where its release could end up in fire, deflagration and

even detonation. In the article, concept of hydrogen trap on the ceiling is developed and

described based on the available data and research results from which passive safety

approach is suggested to be used in future designs of the road tunnels.

Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

Introduction

Depleting resources of carbon fuels poses a threat against the

mankind and calls for solutions which would enable

commuting with vehicles using alternative source of energy.

One possible solution is use of hydrogen which is a natural

selection based on the quantity and its good featureswhile the

other features regarding the safety are not so appealing and

just these features are the ones that prohibit the general use of

this source at the moment. Based on these problems, wide

number of experiments tries to determine all the areas where

we do not know all the problems related to hydrogen. These

experiments serve as the source of the mathematical and

physical models, which could enable engineers to design safe

solutions for the use of hydrogen. Using these models, we can

nology, University of Ljui.09y Publications, LLC. Publ

conduct experiments, which would drive extensive costs in

reality and for this reason we could make safe systems using

hydrogen as an energy carrier. Models, however, require

validation since we do not know if the models are describing

the reality, which is established through the tests performed

in different research facilities all over the world. Our aim was

to leverage the present knowledge and to propose a safe so-

lution for the mitigation of accidents involving hydrogen cars

passing through a road tunnel.

Efforts regarding the problem of hydrogen release during

the road accidents in tunnels. In the articles, we see that the

scientists wanted to ascertain how the concentration of

hydrogenmixedwith air would spread under the ceiling of the

tunnel [2,4,7,9,15,17,18,20,]. This was done by modelling with

different CFD codes, which more or less resulted in different

outcomes [13]. In addition, the problem with ignition of the

bljana, Ljubljana, Slovenia. Tel.: þ386 1 4798000.

ished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 4 3 4e1 7 4 3 9 17435

mixturewasmodelled in different scenarios, which again lead

to different consequences. Based on the results which are not

of the same qualitative area, the recommendation is that

there is a need for further investigation. Nevertheless, we

need to have profound understanding of the problem, but on

the other hand we need to stay pragmatic and direct the

research towards possible safe solutions instead of broad-

ening the research without the rationale behind it.

All the calculations with CFD models have certain amount

of assumptions behind them, so by using different set of as-

sumptions we can get different results in the end.

Safety concerns

Regarding safety, best results are achieved if the hydrogen is

immediately ignited after the accident, if released. In this

case, there is no potential for deflagration and detonation. If

there is no immediate ignition, then there is the problem of

mixing of the hydrogen with neighbouring air thus producing

cloud within the explosion limits posing hazard to the people

and infrastructure [8,10,14], In literature, we found that the

probability for such delayed ignition is low but the research

has been done to investigate the outcome of such delayed

ignition events. Dealing with consequences and their mitiga-

tion is one problem, while preventing this to happen and

reducing the probability of explosion is another problem. In

our opinion, the latter is even more important than the

former, so in our work we were dedicated developing

approach which would work in any situation by reducing the

probability of the explosion or reducing the size of it if the

amount of hydrogen is too large for the system to be optimally

effective.

Finding solution

When we search for a solution, we can use the knowledge

from the experiments, which were performed during the

different investigations and determine the proper solution for

the problem. The advantage of such approach is that we do

not need to additionally validate the results since they are

based on the tests. At the end, we have to test the solution

with the test which can provide us with a prompt answer to

the question whether we have solved the problem or whether

our reasoning was correct. This kind of approach could yield

relatively fast and usable results, which can then be trans-

ferred into practical use.

Calculations that accompany the process are simple

enough to present the problemwhile the test and possible CFD

calculations can be done afterwards when we establish if the

reasoning is sound and when we can expect possible solution

to the problem.

Cars using newnon-carbon technologies are now equipped

with the containers filled by hydrogen sized 20 gallons with

the pressure between the 350 and 700 bars. The mass of the

hydrogen is around 2 kg, and the volume of the hydrogen

released to the atmosphere pressure is approximately 50 m3.

In case of an accident, this container can release the

hydrogen to atmosphere although well protected within the

car to avoid the damage. In open country conditions, this

should not be problematic since the difference between the

densities of hydrogen to that of the air is large and the rising of

the hydrogenwill be very quick. It can be problematic in a road

tunnel where the released hydrogen can be trapped by the

ceiling of the tunnel. The shape of the ceiling can produce

concentration of the pressure in the case of delayed ignition of

the hydrogen mixture. Since hydrogen has a very broad range

of explosion concentrations, it is highly likely that, in some

instance, explosion would occur.

Immediate ignition of the hydrogen has probably the

lowest consequences for the accident but it cannot be ob-

tained in all the cases. In the case of release without ignition,

we should use passive systems to prevent possible detonation

and with those systems we should be able to reduce the

probability for detonation to the lowest possible limit.

Numerous experimentswere performed over the years and

a large amount of data collected related to gas explosions and

hydrogen explosions in particular.

Based on the data, we can deduct that there are some ge-

ometries and instances where hydrogen would not detonate

with high pressures. This is described in Gas explosion

handbook [3]. The geometry is based on the tubes that have

diameter less than l/3 or with channels where height of the

channel should be smaller than l where l is detonation cell

size.

All the experiments were conducted with horizontal tube/

channel alignment which is good for experiments but it is not

good from practical point of view so the vertical tubes/chan-

nels are preferred. Buoyant force of the hydrogen in air

mixture will tend to force hydrogen to collect in the upper part

of the tube thus there forming high concentration of the

hydrogen. The lower part of the tube/channel would have low

concentration of the hydrogen in the air mixture.

The ignition of the material would be in any case from

below and outside of the tube. The idea to compartmentalise

hydrogen has several advantages:

� To reduce the mass of hydrogen in possible reaction/

detonation.

� To design such geometry (diameter of the pipe or height of

the channel) which would give deflagration/detonation

with lowest possible overpressure or no overpressure at all.

� To prevent large fire from accident resulting in substantial

consequences.

� To reduce consequences of an accident to the lowest

possible extent.

During the last period, a lot of research has been carried out

to study hydrogen mixtures with air explosions within the

number of geometries and border conditions to learn how the

environment and obstacles interfere with probabilities for

explosion. It is interesting how much information can be ob-

tained from articles and literature available. The problem is

that all the research work is being done to improve and

develop models with which we can later on, without large-

scale experiments, foresee accidents and their conse-

quences. Based on experiment models, we can provide close

enough approximations, which can help design safe tech-

nology for use of hydrogen as a new energy carrier. To enable

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 4 3 4e1 7 4 3 917436

safe use of hydrogen and to develop new passive safety sys-

tems, we can either use models or we cause the results of

experiments which were a basis for the model development.

In our case, we decided to use the results of experiments

which can help us design passive safety systems which can

either prevent or mitigate the consequences of possible

hydrogen explosion within the road tunnel where hydrogen

vehicles will drive through.

Hydrogen trap within the ceiling

The main problem with hydrogen in the car during the acci-

dent is how it will be released and when it will be ignited. In

the previous research, it was established that there will be

release through the safety device on the cylinder. The time to

release 2 kg of hydrogen under the pressure of 700 bars is

approximately 84 s. It was calculated how the hydrogenwill be

released and how concentrations will be distributed under the

ceiling of the tunnel [13].

By covering the ceiling with tubes that have the diameter

lesser than l/3 or channels with height lesser than l, we can

make hydrogen traps for passive prevention of detonation

(see Fig. 1).

Different scenarios for hydrogen car accidentswithin the tunnel

When a hydrogen car causes an accident in the tunnel, there

are several possible scenarios how the accident will develop.

The first one is that no ignition takes place. The second one is

that the hydrogen is ignited immediately. In this case, we have

nothing to do since the flame is narrow and it is impossible to

extinguish the fire fast enough before all of it is released. Here

we expect that there will be no major consequences due to

Fig. 1 e The principle

just burning of the hydrogen. The third option is the late

ignition. This scenario is most difficult to prevent since we

have no mechanism with which we could prevent this late

ignition.

One of the factors affecting the scenario is velocity of the

air going through the tunnel. Mostly these velocities are

somewhere between 3 and 6 m/s which is normally natural

ventilation. During accident, ventilation is started and then

the velocities are higher.

When the hydrogen is released it goes relatively fast to the

ceiling of the tunnel. All the previous calculations were done

by CFD codes with assumptions that the tunnel ceiling is

plane or round. Different researchers have obtained contro-

versial results using different CFD codes (Fluent and Flacs)

[13]. It is probably reasonable to predict such results since all is

based on different tests and on different computer codes with

different calculation techniques and models. In Ref. [3] it is

stated that perforated top plates can reduce the explosion

pressure drastically. There was no experiment in a tunnel as a

follow-up for this conclusion. However, this can be done only

if the ceiling has an empty space above it. The problem with

false ceiling is that in this case a closed tube is formed, which

can trigger explosion like in ventilating duct.

Therefore, the perforated top plate should be done

differently.

Detonation cell size

Detonation cell size is a parameter which shows how reactive

and dangerous certain gas is. Cell size is affected by the con-

centration of the hydrogen and is the smallest around the

concentration of 30% by volume (see Fig. 2). The size of the cell

is in the range between the 12 mm and 1000 mm at concen-

tration of 13% [5,6,12]. According to Gas Explosion Handbook,

the relationship between the cell size and explosion is such

of hydrogen trap.

Fig. 2 e Detonation cell size vs. % of fuel in air.[19].

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 4 3 4e1 7 4 3 9 17437

that in the pipeswith diameter less than l/3 you cannot obtain

detonation as for channels the height should be less than l. Of

course, this information is also available in other source with

expert opinion, which suggests that the size of the pipe

diameter or channel height should be further reduced by

factor of two, to be on the safe side. The problem with this

statement is that we do not know the exact concentration

with which we will have to deal at the time of ignition.

To be able to predict concentration, it is necessary to first

forecast concentration which will be spread under the ceiling

of the tunnel. This concentration would normally be the

average concentration in the layer beneath the tunnel ceiling.

It can be assumed that the concentration in the layer is

stratified so the upper layer would have higher hydrogen

concentration.

Vertical tubes/channels on the ceiling could be preventing

hydrogen from contact with the main source of hydrogen.

When we have hydrogen in the close layer under the ceil-

ing, this hydrogen tends to stratify from ceiling downward. So

the high concentration of the hydrogen will tend to enter the

vertical tube and travel up to the closed top of the tube. There

will be mixing but not as much as to fill the pipe with the

average concentration.

Buoyancy speed of hydrogen is in the range of 1.2e9 m/s

[1]. The buoyancy speed also depends on the pressure in the

container, which in our case is between 350 and 700 bars. Once

hydrogen is released from the cylinderwith high pressure, it is

mixed with air and then the buoyancy forces began to lift the

hydrogen towards the ceiling where the concentration of

hydrogen under the ceiling began to rise. If we can put the

concentrated hydrogen under the ceiling in a separate vol-

ume, in which it is isolated from other free gas volume of the

tunnel we can prevent hydrogen explosion.

During release of hydrogen, its mixing with air is the

crucial moment. During this period, the concentration of the

mixture of hydrogen and air is in the region where explosions

can take place. If there is delayed ignition of hydrogen, it is

possible to use the passive feature of the tunnel with its ceil-

ing made of tubes/channels of proper diameter and proper

length.

This passive feature has several benefits as opposed to the

standard tunnel design. It separates small volumes of

hydrogen from the main tunnel volume. Normally one would

expect thatmixturewould be disturbed by different structures

on the ceiling which would contribute to the turbulence

mixing and consequently to strength of explosion. Since the

tubes are of diameter smaller than l/3 or for channels smaller

then as for channels the height should be less than l which

prevents gas in the tube/channel to detonate preventing the

amount of hydrogen to participate in explosion. The separa-

tion of hydrogen in the tube continues, so in the upper part of

the tube concentration of hydrogen is expected to be in the

higher concentration than in the tunnel. Therefore the prob-

ability for ignition is lower than in the region between the 4

and 75 vol%. The exact concentration depends also on the

velocity of air streaming through the tunnel, which is under

normal conditions somewhere around 5e6 m/s. The stream-

ing air mixes with the inner air, which can producemixture of

rather equal concentration. To prevent this, the length of the

tube should be rather long in comparisonwith the diameter. It

should be between 0.5 and 1.5 m. The length should be

determined from the thickness of the mixed hydrogen layer

under the ceiling of the tunnel determined on the basis of the

CFD simulation. This was conducted by two different groups

of researchers. The resultswere contradictorywhich indicates

that this area still needs further research as it is exhibited in

the article [13].

In the article, it is stated that the size of flammable cloud is

smaller if the ventilation level is low (1 m/s), therefore also its

hazard is lower. In our opinion, higher ventilation level like

6 m/s produces thinner layer of mixed hydrogen with air

which is prone to be trapped into the vertical tubes on the

ceiling preventing the hydrogen to participate in the explo-

sion. Problem arises if the ignition time is not so distant in

time and if there is a slight possibility for hydrogen to explode

sooner then the hydrogen is absorbed in the tubes/channels.

The hydrogen would be absorbed from the layer, which is

travelling beneath the ceiling as shown by CFD simulations.

We can assume that the buoyancy speed in the tubes can be

near 1.2 m/s at the entrance, which is at the end of the tube

reduced to near zero. Certain movement of hydrogen is still

there due to induced flow.

Scenarios regarding the release and the time ofignition

Because cannot accurately define the process of the release of

hydrogen and its mixing with air together with its conse-

quences after the ignition, we have to develop scenarios based

on available tests since the tests are the ones supplying the

data for calculation validation.

If the ignition of hydrogen is timed after the full release,

which is after, 84 s we are faced with the problem that at least

part of the hydrogen is in high concentration above the car

and the rest of hydrogen is spread under the ceiling being

absorbed by the vertical tubes beneath it. Since the time of

ignition is fairly quick, we cannot absorb all the hydrogen

from the source, so the explosion is possible but at least part of

the hydrogen is removed from the scene.

Alternate scenario is that the delayed ignition which hap-

pens after 2 min after release of hydrogen. With the speed of

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 4 3 4e1 7 4 3 917438

air in the tunnel around 5 m/s, we have hydrogen spread

under the ceiling in the distance of 600 m. If there were no

trapping of hydrogen the thickness of the hydrogen, layer

would be around 0.5 m.

From the start of release and to the time of the end of

release, the hydrogen cloud does spread for about 420 m. At

the same time, rising occurs due to buoyancy with the speed

of 5 m/s on average, which is, for the height of 7 m, no more

than 2 s. If we apply the lowest velocity of 1.2 m/s, it would be

6 s. In our case, the thickness of the ceiling layer would be

between 0.14 and 0.20mwhichmeans that the separationwill

be relatively fast.

Separation takes place even in a balloon if we fill in the

hydrogen and then the air to achieve desired concentration.

During tests, it was not possible to ignite hydrogen mixture

with the spark which means that separation already took

place. At the bottom, the concentration was too lean and at

the top concentration was too rich and in either case ignition

did not occur.

So from our reasoning, we can conclude that the most

dangerous period for explosion is during leakage of hydrogen

from the container andwhere themixture of hydrogen and air

is in the explosion limits and it is highly likely that there will

be some source of ignition. We can assume that when there is

a source of ignition there will be immediate fire and no

hydrogen explosion. According to some tests, there is evi-

dence that immediate fire is least harmful if there is no other

fuel present.

The results of the CFD modelling using FLACS commercial

code provided results that are mainly depending on buoyancy

and release mechanism while longitudinal ventilation is

claimed to have marginal effect on the process [15]. There are

differences in using FLUENT computational code, which raises

a certain degree of doubt regarding the distribution of con-

centrations along the tunnel and on the vertical axis.

Our assumptions are that we do not have any obstacles on

the ceiling since this would produce turbulence and conges-

tion, which would further result in higher explosion pres-

sures. Therefore, our aim is to incorporate all that we know

about gas explosions, to reduce the explosion pressures. Af-

terwards we should also incorporate possible passive mech-

anisms, which would reduce the possible active volume of

hydrogen.

Discussion and conclusions

Based on literature and available articles, we can deduct that

hydrogenwhen released in the tunnel poses risks for people in

the vehicles in the tunnel as well as risks for the vehicles and

infrastructure of the tunnel. Before using hydrogen as an en-

ergy carrier in the vehicles in the future, we have to be aware

of the amount of risk, which such use poses for traffic par-

ticipants and for the society as a whole. Research has been

conducted to understand mechanisms of hydrogen explo-

sions and on the risk of hydrogen use in tunnels. Tests were

conducted for tunnels as well as garages where hydrogen cars

will be stored. Based on tests and using validated CFD soft-

ware, a number of calculations were made to predict conse-

quences of possible hydrogen explosions in the tunnels

following the hydrogen release from hydrogen vehicle storage

container. Less was done in the area of prevention such ex-

plosions from happening. From our perspective, we should

dedicate at least certain amount of research to prevention

and, of course, mitigation of consequences of such accidents.

Passive hydrogen trap on the ceiling of the tunnel could

improve safety when using hydrogen in vehicles in the

tunnels.

There are two distinct safety features in this passive safety

concept which, in our opinion, cannot be modelled within the

CFD models but should be tested in the test facility to deter-

mine optimal parameters. The first feature is perforated upper

plate (ceiling) for which the gas explosion handbook says that

it reduces the explosion pressure. This is true for the tests

where stoichiometric mixture is tested within the rectangular

test tube. Another feature is the vertical tubes with the

diameter size less than l/3 or as for channels the height

should be less than l for which size no explosion within the

tubemight occur nomatter what concentration of hydrogen is

in the tube. Our thinking is that the concentrations are rising

towards the top of the tube, and if the tube is long enough it

cannot deliver hydrogen back during the explosion or fire

outside of the tube in the tunnel. Hydrogen is therefore trap-

ped in the pipes or some other porous structures still to be

researched on the ceiling. The size and the diameters of the

tubes should be determined on the experimental basis

depending on the concentrations observed under the ceiling

and later on in the tubes/channels.

The changing of the l depending on concentration is very

broad, therefore it is possible to use the detonation cell size for

concentration which is most probable and not the smallest

one which is connected with concentration around 30 vol%

being around 8 mm.

Moreover, the smallest one ensures more safety and, on

the other hand, expert opinion on this matter is to divide the

size by two [16] meaning that the size of the tube would be

1.4 mm which is quite a small size for practical use.

In the literature, we found that the ventilation can

dramatically reduce hazard from explosion but there was fear

that higher ventilation rate can produce near homogenous

mixture under close to stoichiometric conditions, which

would increase explosion hazard [13,11].

If we look at the velocity profile on vertical axis, we can see

that under the ceiling we will get a boundary layer which has

low velocities and, on the ceiling, velocity near zero. This

means that high hydrogen concentrations, which are under

the ceiling, would be the highest producing higher hazard for

explosion. By means of the hydrogen trap concept, we

remove higher hydrogen concentration volume of hydrogen

in the vertical tubes/channels where it cannot participate in

forming cloud that could deflagrate or even detonate. Since

the hydrogen in this fashion is compartmentalized, it cannot

be possible for it to participate in the explosion of any kind.

Even if it is ignited from outside it cannot detonate due to

diameter of tubes being less than l/3 or l for channel and

preventing explosion. In the worst case, we will get fire under

the ceiling.

There is a number of issues that are still open and subject

to possible future research. Some of the issues were already

mentioned in the paper:

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 9 ( 2 0 1 4 ) 1 7 4 3 4e1 7 4 3 9 17439

� /channels/channels Impact of tunnel air velocity on pro-

cess of separation of hydrogen entering tubes/channels.

� Concentration of hydrogen within the tube/channel.

� Behaviour of burning of hydrogen being ignited from

outside.

� Behaviour of hydrogen in the tubes/channels when there is

detonation in the road tunnel.

� Possible help to the process by impulse ventilation.

The next set of problems is connected with tubes should

they be round or rectangular andwhat kind ofmaterial should

be used to make them. In order to be able to get bigger di-

mensions rectangular tubes would be preferred because the

smaller dimension of the channel then equals detonation cell

size. Speaking of the material and the economic effect of the

proposal is in my opinion too early to predict because it is

technological aspect, which could be solved after confirma-

tion that the expected features are attained.

Regarding the length of the tubes, it should be determined

by experimental work as well as with cost benefit analysis and

the last but not the least by physical limitations of the tunnels.

The hydrogen left in the tubes/channels could be either left

in the tubes and then be removed slowly by the induced flow

of the air through the tunnel or be burned from the outside or

by sparklers inside the tubes like in nuclear plant

containment.

Acknowledgements

The work has been supported by the Slovenian Research

Agency and Center of Excellence Low-Carbon Technologies

(CO NOT).

The constructive comments of HE reviewers are gratefully

acknowledged.

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