Most common odorants will contaminate fuel cells. Additionally, hydrogen's small molecule and high buoyancy make it challenging to find a compatible odorant. Research is being conducted on fuel cell compatible odorants, but there are none currently in use. Like liquefied natural gas, liquid hydrogen also can’t be odorized due to its cryogenic temperature.

Hydrogen flames can be nearly invisible in daylight, especially at low flowrates. The concentration of hydrogen does not have much effect on the color of the flame. Many hydrogen incidents or fires will have a bright orange hue, or even yellow flames. The color is primarily caused by contaminants that is either naturally in the air in certain environments, swept into the air during the release (such as duct), or surrounding materials which are also burning

The “Hydrogen Ready Appliances Assessment Report” published by the Northwest Energy Efficiency Alliance (NEEA) in February, 2023, is one of the most recent studies on this topic.  Several key items from the report pertaining to this question include the following:

1.    “There appears to be growing consensus that blends of up to 20% or perhaps even 30% are possible with little to no impact on current equipment. Higher fractions of hydrogen approaching 100% will likely require changes to equipment components and perhaps installation and
maintenance practices.”

2.    “…..there is currently no standardized testing, rating, or labeling program to enable differentiation of equipment manufactured for the North American market that is safe and efficient to operate with hydrogen blended fuels. From a market transformation perspective, this lack of a standardized program with these elements could represent a significant market barrier to the expanded use of hydrogen blended fuels.”

3.    There are “unanswered questions about the long-term use of hydrogen blends on typical materials used in new natural gas appliances. Hydrogen is known to cause some metals and plastics to become brittle over time, increasing the risk of failure of parts made from these materials. It will be important to understand the implications of hydrogen blends over the lifetime of materials used in modern, high-efficiency equipment such as gas-fired heat pumps and condensing heating equipment.”

4.    “Hydrogen integration initiatives are happening all over the world. In the United Kingdom, pilot programs are incorporating 20% hydrogen blends in natural gas in public networks, and in the Netherlands, pilots have replaced natural gas with 100% hydrogen. Utilities in the United States are currently testing up to 20% hydrogen blends within their training facilities, and the Gas Technology Institute conducted laboratory research that indicates that the water heaters and furnaces tested can maintain performance and safety with a 30% hydrogen blend in natural gas.”

However, there are still some safety issues that need to be addressed, such as odor, flammability, and potential skin burns.  A 2021 study* showed how two types of Sulphur-based odorants are compatible with 100% hydrogen gas allowing its identification at the 1% regulatory thresholds of gas in air by untrained participants.” This could help solve concerns in odor detection. High hydrogen concentration blends may also require hydrogen sensors and ventilation systems to maintain safe operations.  Being able to identify the light pale flame, almost invisible to the naked eye, and the lack of infrared heat to avoid skin burns is another consideration.**

See the link below to read more about hydrogen use in residential applications in Europe and the standards in use or under development in Europe and Australia. Current standards and test methods for appliances in the United States (Z21 standards) fail to incorporate hydrogen blends, even as a limit gas. https://neea.org/resources/hydrogen-ready-appliances-assessment-report

*Mouli-Castillo, J. 2021. “A comparative study of odorants for gas escape detection of natural gas and hydrogen,” in
International Journal of Hydrogen Energy, Vol 46, Issue 27, 14881-14893
https://doi.org/10.1016/j.ijhydene.2021.01.211

**PNNL (Pacific Northwest National Laboratory), “Hydrogen Compared with Other Fuels,” Hydrogen Tools,
https://h2tools.org/hydrogen-compared-other-fuels.

A design condition for vent stacks is to always assume that the hydrogen will ignite. The stack musts be designed such that it can withstand those conditions as well as minimize radiation to surrounding personnel and exposures. Although hydrogen fires have comparatively low radiation compared to hydrocarbon fuels, the radiation from large releases from vent stacks can be quite high. Documents such as NFPA 2 and API 521 provide direction on acceptable levels of radiation for different exposures. Also, the temperature of the flame is very high, so if the hydrogen flame contacts materials around the vent stack, they are likely to melt or ignite.

The purity required will be a function of the end use application. There are a variety of grades of hydrogen that can be purchased. The H2 purity will also vary based on source (GH2 or LH2) and production method. CGA G-5.3, Commodity Specification for Hydrogen, lists several typical purities of both liquid and gaseous hydrogen. Standard GH2 available from most suppliers is 99.95% hydrogen. Standard LH2 standard purity is 99.995 % by volume. Often the hydrogen is purer than stated, but the stated purity is based upon the level of analysis. For example, LH2 is usually at least 99.998% pure. Standard quality testing usually includes a “total purity” measurement as well as for typical impurities such as H2O, O2, CO, CO2, and hydrocarbons. Fuel cell applications usually are required to meet the SAE J2719 or ISO 14687 specifications for PEM fuel cell. These specifications list the minimum requirements for over a dozen impurities.

Yes, although not as common as high-pressure gas releases, high-velocity cold H2 gas has ignited during rupture disc and relief valve activation.

The water vapor cloud formed from venting cold hydrogen gas from a liquid hydrogen tank will vary in size depending upon atmospheric conditions including ambient temperature and humidity. There is not a direct relationship between the water vapor cloud and the flammable could of hydrogen, but it’s often used as a proxy. 

Initially upon release, it is possible that H2 vapor from an LH2 source will be slightly denser than air and will be roughly neutrally buoyant. Also, as the hydrogen cools the air and condenses water vapor, the resultant combined cloud can be denser than the ambient air and can drop toward the ground initially. However, the hydrogen will rapidly become buoyant as it warms and eventually separate from the water vapor cloud. In low wind conditions, the GH2 rises with the vapor cloud and for the most part is resident in the cloud. Higher wind conditions will result in faster mixing of the hydrogen with the air. The hydrogen and water vapor are blown by the wind. The hydrogen is resident within the cloud for a while, but then will rise above the visible water vapor cloud as they both move downwind.

Commercially available dispersion software packages can also be used for specific applications and release parameters.

In the case of fuel-rich mixtures like those in the question, the heat of combustion for the mixture should be calculated on the basis of the oxygen content of the mixture. The heat of combustion per mole oxygen is twice the heat of combustion per mole hydrogen, i.e., it is 286*2 kJ per mole O2. You should be able to do the calculations based on this reply.

H2-air flammability limits vary with temperature  . The H2-air lower flammability limit is virtually the same as the H2-O2 lower limit. However, the H2-O2 upper flammability limit increases substantially to about 95% at room temperature and gets even higher at elevated temperatures.

A pressure of 600 kPa (87 psi) is relatively moderate, so the combustion properties are similar to those at atmospheric pressure where the autoignition temperature of hydrogen is 585°C.