H2Tech - Q1 2021 - 33

SAFETY AND SUSTAINABILITY

Hydrogen fuel risk assessment
and differing views of ignitability
M. MOOSEMILLER, Baker Engineering and Risk Consultants Inc., Chicago, Illinois
and J. K. THOMAS, Baker Engineering and Risk Consultants Inc., San Antonio, Texas

Hydrogen is being contemplated or
implemented as an alternative fuel in several parts of the world. Creating a largescale infrastructure for H2 fuels requires
an objective assessment of the associated
risks. Such an assessment requires consideration of the probability of ignition of
potential releases in a variety of storage,
loading and fueling situations.
In researching a book1 on estimating ignition probabilities for releases of flammables into the atmosphere, industry subject
matter experts (SMEs) were solicited for
their experience-based opinions on a range
of hypothetical release/ignition situations. Reasonable agreement was achieved
among the SMEs for most fuels. However, in the case of H2 releases, there was a
large divergence in opinion ranging from
near-zero expectation of ignition to 100%
ignition probability for the same event. A
corresponding divergence in ignition probability models was noted by Jallais.2
This broad span in H2 ignition probability opinions suggested that either the
SMEs were not as expert as was believed
(although they were), or that there were
underlying factors that led to such varying
experiences among the SMEs. In fact, the
literature published within the past 15 yr
suggests that physical phenomena may be
involved in H2 releases that might reasonably result in these differing experiences.
This article is intended to provide an
unbiased literature review on the subject,
as well as suggest how this literature and
ongoing H2 field testing might provide
consistent and more accurate expectations
for ignition probabilities that can be used
with confidence to evaluate H2 fuel risks.
H2 ignition properties and factors.
Mechanisms proposed for igniting H2
include those shown in FIG. 1. However,
even this list may not be exhaustive, as

indicated by Gummer and Hawksworth's
work through 2007,3 which indicated that
the proposed mechanisms do not explain
some reported H2 ignitions (or non-ignitions). In addition, these mechanisms are
not universally agreed upon, or in some
cases may not be significant enough to
cause ignition in H2 fuel applications.
As noted earlier, SMEs had widely different views of H2 ignitability based on H2
release events experienced during their
careers. As an example, the survey participants were asked to provide their estimates for the probability of ignition for an
H2 leak under the following conditions:
* Temperature = 100°F (38°C)
* Pressure = 100 psig (7 bar)
* Hole size = 1-in. (25-mm) diameter
* Release duration = 100 sec
* Weather = Typical daytime
conditions
* Release = Standard Class I
Division 2 process area.
The respondents were asked to estimate the likelihood of immediate igni-

tion of H2 (at or near the source) and
delayed ignition (ignition after a cloud
had developed). Their responses are provided in TABLE 1.
Considering that the responses for
the ignition probabilities of most hydrocarbon fuel release scenarios were in
much better agreement, it appears that
the SMEs were biased due to their prior
experiences with H2, which appear quite
diverse. Clearly, there must be some
variable(s) that were not accounted for
in the release conditions provided to the
survey participants, which the SMEs implicitly accounted for or assumed.
Anecdotal record of hydrogen ignition events. Gummer and Hawksworth3

noted that " Studies undertaken many
years ago on H2 vents [...] showed that
ignition was rare during fine weather, but
was more frequent during thunderstorms,
sleet, falling snow, and on cold frosty
nights. " This suggests that for some events,
weather conditions may be a factor.

Electrostatic ignition
* Ignition due to sparks, brush discharges and corona discharges.

Reverse Joule-Thomson effect
* Hydrogen is atypical in that its temperature can rise upon depressuring, potentially reaching its autoignition
temperature (AIT).

Hot surface ignition
* Hydrogen can be ignited by a hot surface, although this requires temperatures substantially higher that the
reported minimum AIT.

Diffusion ignition
* Ignition of a gas at a temperature well below its AIT has been reported experimentally in a shock tube at high speeds.

Adiabatic compression/turbulence
* In this case, the equipment geometry at or near the point of release drives compression that results
in a shock wave that leads to ignition.
FIG. 1. Hydrogen ignition mechanisms.
H2Tech | Q1 2021 33

H2Tech - Q1 2021

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