Hydrogen Codes and Standards Being Written
Codes and standards for the safe use of hydrogen fuel in commercial and residential applications are being written and existing natural gas codes and standards are used as a guide in writing hydrogen safety requirements. As a result, comparisons of hydrogen properties with natural gas properties are frequently made.

The U.S. Department of Energy and Natural Resources Canada are funding research to study hydrogen leaks into the air, which will provide information for standards development organizations to determine appropriate, safe storage requirements.


Hydrogen and Natural Gas Properties
Hydrogen and natural gas for vehicle fuel are both highly flammable gases that are usually stored at pressures ranging to 4000 psi for natural gas and 6000 psi for hydrogen. As such they must be treated with appropriate safety precautions. To aid officials in approving hydrogen installations, this paper explains the similarities and differences between the two gases concerning safety issues such as fire, explosion, and area electrical classification. The properties of methane are used to represent natural gas.

1. Leak Rate and Energy Content The hydrogen (H2) molecule is a smaller molecule than methane (CH4) and will leak through permeable materials where methane will not. However, the difference in leakage rates is very low. Hydrogen has approximately one-third the energy of methane by volume. Therefore, an equal pressure three times the volume of hydrogen will have the same total energy content as methane. For pinhole size leaks from high-pressure systems, this means about three times the volume of hydrogen will leak over methane. However, this represents an equivalent energy content release. By mass, hydrogen has approximately three times the energy of methane.

2. Lower Flammable (LFL) and Lower Explosive (LEL) Limits LFL and LEL for most flammable gases and vapours are similar and by convention the LEL is used. The LFL for hydrogen and methane are similar (4.0% for H2 and 5% for methane). However, hydrogen has a much wider range between LFL and LEL than methane (4.0% to 18% for H2 vs. only 5% to 5.7% for CH4). This means that a hydrogen gas concentration of over three times that of methane is required to produce an explosive mixture. LFL is used in place of LEL for hydrogen, which provides an additional safety factor. Twenty-five percent LFL for hydrogen equals 1% H2 in air but 25% LEL is 4.5% H2 in air. Therefore, gas detection using LFL gives an earlier warning for a hydrogen explosive mixture than a methane explosive mixture.

3. Upper Flammable and Explosive Limits A frequently referenced issue regarding hydrogen is its wide flammability range (4.0% to 75% H2 vs. 5% to 15% CH4 in air). When a gas plume approaches an ignition source, the lean, leading edge is ignited first, so in this regard, the ignition points of methane and hydrogen are similar. A significantly richer mixture of hydrogen will burn which means that a similarly concentrated cloud of hydrogen will be consumed quicker than a methane cloud.

4. Buoyancy Hydrogen is 14.5 times lighter than air and methane is 1.8 times lighter. Hydrogen will rise much more quickly causing greater turbulent diffusion, which reduces its concentration below the LFL more rapidly.

5. Diffusivity Hydrogen diffuses into air 4 times more readily than methane and therefore its concentration reduces faster.

6. Minimum Ignition Energy

7. At concentrations up to approximately 10% of hydrogen and methane in air, hydrogen has the same ignition energy as methane. As the hydrogen concentration increases toward a stoichiometric mixture of 29% H2 in air, the ignition energy drops to about one fifteenth of that for methane. Since we are generally concerned with the prevention of ignitable mixtures, the LFL is the important property. Energy levels required for methane or hydrogen ignition are so low that common ignition sources will ignite both gases. Temperature and Gas Groups Both gases have a T1 temperature rating. Methane is a group D gas with an auto-ignition temperature of 537C to 630C and hydrogen is a group B gas with an auto-ignition temperature of 520C to 585 C depending on the information source.

8. Flame Characteristics Unlike visible methane flames, hydrogen burns with a near invisible flame in daylight but contaminants in the air generally add some visibility. Hydrogen flames are visible at night. Due to the heat-absorbing water vapour created during hydrogen combustion and the absence of a carbon combustion reaction, the radiant heat from a hydrogen fire is significantly less than a hydrocarbon fire, which reduces risk of secondary fires. Combustible materials may actually be placed closer to a hydrogen flame than a methane flame. As radiant heat is low, there is less warning that one is approaching a hydrogen flame.

9. Burning Characteristics Hydrogen burning speed (time to peak pressure) is ten times greater than methane. This indicates that a hydrogen explosion will be of much greater severity but will be shorter lived. However, peak explosion pressures are equal to methane.

10. Odorant

11. Natural Gas is odorized so that leaks can be detected. Since natural gas distribution piping exists in so many places and is piped in to buildings and homes, odorization is prudent although not entirely effective safety measure. Leaks will only be detected if someone is present to smell them and respond. Hydrogen as an industrial gas or fuel cell vehicle fuel is not odorized. Sulphur-containing mercaptans will contaminate the catalysts of a fuel cell. Hydrogen Attack of Some Materials Hydrogen has been safely stored and transported under pressure for many years. The conditions that cause hydrogen attack and embrittlement and susceptible materials have been known for many years. Equipment for hydrogen service is selected to avoid these conditions and materials. References include ASTM standards, NASA TM-112540 Safety Standard for Hydrogen and Hydrogen Systems and ISO 15916 Basic
Considerations for Safety of Hydrogen Systems.

12. Carbon Monoxide Both hydrogen and methane are colorless, odorless and non-toxic, but can cause asphyxiation by displacing oxygen. When combusted, hydrogen produces only water vapour, natural gas produces mostly CO2 and some water. Incomplete combustion of natural gas will produce toxic carbon monoxide.

13. Government Authority approves Natural Gas Standards for H2 Use The Technical Standards and Safety Authority (TSSA), which regulates fuel safety for 13 million people in the province of Ontario, Canada, has stated in writing that the properties of hydrogen are similar enough to natural gas that natural gas standards and regulation can be used in Ontario with some additional requirements, until new hydrogen standards are produced.

Some H2 fueling stations in California have been approved using guidance from the California Fire Code and Uniform Fire Code Std. 52-1, which is based on NFPA 52 CNG Vehicular Fuel Systems Code. Hydrogen energy and fueling stations in Sweden and Hong Kong were installed and approved using CSA B108 Natural Gas Fuelling Stations Installation Code, CSA B51 Boilers, Pressure Vessels and Pressure Piping Code, and ISO 11439 Gas Cylinders for pressure vessels. In all cases, classified electrical components used were suitable for Class I Group B or IIc for hydrogen.

Summary
Like natural gas, hydrogen is a flammable gas that must be treated with respect, not fear. Hydrogen is generally stored at very high pressure at hydrogen fueling stations due to its low energy content by volume; however, the containers and piping comply with well-established ASME standards. Composite high-pressure cylinders complying with CSA B51 and European Union directives are allowed for hydrogen storage in Europe and Canada. (Photo above: Electrolyser, compressed gas storage and hydrogen dispenser at Toyota's Torrance, California facility)

The purpose of on-going development of hydrogen codes and standards is to provide consistent regulations for new applications of hydrogen and to achieve the goal of public acceptance of hydrogen as a pollution-free fuel that can be safely utilized.

Further information on hydrogen and natural gas can be obtained from:

http://www.nist.gov/srd/
http://www.hydrogensafety.info/
http://www.eere.energy.gov/hydrogenandfuelcells/
http://www.h2fc.com/news.html
http://www.hydrogensociety.net/
http://www.nrcan.gc.ca/es/etb/ctfca/index_e.html
http://www.hydrogen.org/index-e.html
http://www.fuelcellstandards.com/
http://www.aga.org/
http://www.gastechnology.org
http://www.nfpa.org
http://www.iccsafe.org

Acknowledgements
The author would like to thank Andrei V. Tchouvelev, Ph.D., V.P. Codes and Standards of Stuart Energy Systems for his extremely valuable consultation in development of this paper. Also to be thanked are those who reviewed the initial draft and provided helpful insight and comments.

Gary W. Howard, P. Eng.
ghoward@stuartenergy.com

Gary Howard graduated in 1981 with a B. Sc. in Metallurgical Engineering from Queen’s University. He has 23 years experience in safety as a forensic and safety engineer. He joined Stuart Energy in 2000 and currently serves as Director, Product Safety and Compliance. He is involved in electrolyser design requirements and hydrogen fuel station installations with a focus on hazardous area classification, ventilation, safety and code compliance. He is responsible for product compliance with international codes and standards and contributes to writing hydrogen related codes and standards as a member of the following organizations and committees:

*BNQ Canadian Hydrogen Installation Code
*California Fuel Cell Partnership
*Canadian Transportation Fuel Cell Alliance - Codes and Standards
*ICC International Mechanical Code
*ISO TC 197 WG 8, ISO 22734 Gaseous Hydrogen - hydrogen gas generators using the water electrolysis process
*ISO TC 197 WG 9 Gaseous hydrogen - Hydrogen generators using fuel processing technologies
*ISO TC 197 WG 11 Gaseous Hydrogen - Service Stations
*NFPA Industrial and Medical Gases (NFPA 50, 50A, 51, 51A, 50B, 55, 560)
*NFPA Vehicular Alternative Fuel Systems (NFPA 52 and 57)
*NFPA Hydrogen Coordinating Group
*NHA Codes and Standards Committee
*U.S. Fuel Cell Council - Codes and Standards