Hydrogen and CNG Stations – A New Risk Comparison
Based on the work of A. Tchouvelev, R. Hay and P. Benard in Comparative Risk Estimation of Compressed Hydrogen and CNG Refuelling Options.
Quantifying the risks from hydrogen fueling stations is a vitally important activity. As a benchmark, the comparison to risks from compressed natural gas (CNG) fueling option, will aid the development of codes and standards based on research and data as well as stakeholder education campaigns.
The Canadian Hydrogen Safety Program recently finished a significant new project for the Codes & Standards Working Group of the Canadian Transportation Fuel Cell Alliance. The project, Comparative Quantitative Risk Estimation of Hydrogen and CNG Refuelling Options, was a comprehensive risk analysis of various hydrogen sourcing and fueling site configurations. The clear conclusion of the project is that hydrogen fueling is as safe as, or safer than, CNG fueling.
The researchers conducted parallel analysis of hydrogen and CNG stations using quantitative risk assessment tools for a wide range of safety-critical issues. Using CNG as a basis for comparison gave the researchers a reference with established data about the public’s level of risk aversion. Hydrogen leak scenarios from sources including tube-trailer delivery vehicles, on-site reformers and electrolysers, and storage tanks were examined representing the concerns that provide key input to design and public acceptance decision making.
In 2005 TIAX released a Failure Mode and Effects Analysis (FMEA) study of hydrogen fueling options compared to a CNG fueling option. The TIAX study provided good guidance on the risks of various hydrogen technologies vs. CNG technology. The Canadian Hydrogen Safety Program study extended the comparison beyond the FMEA’s qualitative analysis with a detailed quantitative comparison of “stand-out” elements that are either technology or fuel related. The TIAX study came to several important conclusions that are worth repeating:
- None of the hydrogen scenarios considered presented high risk and generally all the hydrogen fueling options considered were at par with a CNG fueling option;
- In terms of medium risk, CNG fueling presents less risk due to the simplicity of the system and generally lower pressure;
- In terms of medium risk, reformer technology is marginally riskier due to higher complexity arising from the need to deal with two fuels, methane and hydrogen, a higher process temperature and a higher internal inventory of gases;
- Electrolyser-based and tube trailer options are approximately at par in terms of medium risk.
The Canadian Hydrogen Safety Program study reached a similar suite of conclusions. They found that producing hydrogen on-site by electrolysis presents a lower location-specific individual risk (LSIR) than producing hydrogen on-site by steam methane reforming (SMR). This is thought to be due to the complexity of the installation in the SMR case and because both hydrogen and natural gas are present in SMR.
Whether the hydrogen is sourced on-site or off-site, the LSIR is almost the same. However, on-site electrolysis appears to be less risky than tube trailer delivery, which in turn is safer than on-site steam methane reforming within the scope and assumptions of the considered scenarios.
A comparison of the relative risk associated with hydrogen and natural gas storage shows that a hydrogen storage facility presents a marginally lower (within 20%) risk compared to an identical CNG storage. This conclusion is based on the hazard from the thermal effects of an accidental horizontal-jet release from storage connecting piping. Regarding storage venting, a CNG storage facility may require either a larger clearance than an identical hydrogen storage facility or a higher vent stack to achieve the same level of thermal radiation from a vertical flare at human height.
A final conclusion is that an electrolysis fueling option that includes compressed hydrogen storage presents the lowest risk among the fueling options that were considered including a CNG station of equal fueling capacity to provide equivalent travel mileage.
A. Tchouvelev - A.V.Tchouvelev & Associates Inc., 6591 Spinnaker Circle, Mississauga, ON Canada L5W 1R2
R. Hay - Tisec Inc., 2755 Pitfield Boulevard, Montreal, QC Canada H4S 1T2
P. Benard - Hydrogen Research Institute, Université du Québec à Trois-Rivières, 3351 Boulevard des Forges, CP 500, Trois-Rivières, QC Canada G9A 5H7