Like gasoline or natural gas, hydrogen is a fuel that must be handled properly. It can be used as safely as other conventional fuels when simple guidelines are followed.
Bar charts presented in the following sections compare some key properties of hydrogen with those of several commonly used fuels – natural gas, propane, and gasoline vapor.
Legenda
- H2: Hydrogen
- LH2: Liquid Hydrogen
- NG: Natural Gas
- LNG: Liquid Natural Gas
Fuel Emission Comparison
- HFO
- MDO
- LNG
- H2
The Graphic above, shows the fuel emission comparison between Heavy Fuel Oil (HFO), Marine Diesel Oil (MDO), Liquid Natural Gas (LNG) and Hydrogen (H2).
Hydrogen has zero emissions in the environment compared to other fuels.
Natural Gas has lower emissions when compared to carbon fuels as HFO and MDO, but its greenhouse impact, if released in the atmosphere, is 24 times more harmful than carbon dioxide.
The Fuel Cell system is a great solution to reduce gas emissions from the transportation sector. Combusting fossil fuels to power conventional vehicles releases greenhouse gas (GHGs) emissions and other pollutants from the vehicle exhaust system (i.e., “tailpipe” emissions). In addition, there are also emissions associated with producing petroleum-based fuels (i.e., “upstream” emissions), notably emissions from oil refineries. Hydrogen Fuel Cells emit no tailpipe GHGs or other pollutants during operation, and depending on how hydrogen is produced, there can be substantially lower upstream GHG emissions associated with producing hydrogen fuel.
Energy Comparison Mega-Joules (MJ/Kg)
Hydrogen has a high energy content by weight, but not by volume, which is a particular challenge for storage.
In order to store sufficient quantities of hydrogen gas, it’s compressed and stored at high pressures.
For safety, hydrogen tanks are equipped with pressure relief devices that will prevent the pressure in the tanks from becoming too high.
Relative Vapor Density (Kg/m3)
Hydrogen is about 57 times lighter than gasoline vapor (as shown in the graphic above) and 14 times lighter than air.
This means that if it is released, it will rise and disperse rapidly. This upward tendency and diffusivity is a safety advantage.
Lower Flammability Limit (% Ratio)
Lower Flammability Limit is the minimum percentage of fuel vapor mixed in air that will ignite.
Gasoline’s minimum mixing ratio is 1.4 percent, whereas Hydrogen and Natural Gas, are at 4 percent and 5 percent, respectively, making the mix from these individual fuels four times less flammable.
Knowing that the technology used with hydrogen compare to petroleum fuel is much more restrictive, the overall hydrogen usage is safer than traditional fuel.
The energy required to initiate a combustion, at low concentrations of hydrogen in the air, is similar to other fuels. In optimal combustion conditions (a 29% hydrogen-to-air volume ratio), the ignition energy can be much lower.
Where Hydrogen is used, the design of the surrounding areas is studied to avoid any possibility of gas accumulation. Also, its natural diffusivity makes it difficult to reach the upper flammability limit which is 75%, or even the optimal combustion concentration which is 29%.
In addition, these safety measures are taken:
- Gas detector at a gas concentration 10% of the lower flammability limit (0.4%);
- Ventilated double wall pipes to avoid any risk of leakage;
- 30x per hour ventilation in hazardous areas.
Auto Ignition Temperature (ºC)
The auto-ignition temperature of a substance is the lowest temperature at which it will spontaneously ignite without the presence of a flame or spark.
The auto-ignition temperatures of hydrogen and natural gas are very similar. Both have auto-ignition temperatures over 500°C, much higher than the auto-ignition temperature of gasoline vapor, as shown above.
Gasoline has a relatively low auto-ignition temperature (247°C) which means that it can ignite in contact with hot metal parts such as a catalytic converter or exhaust manifold. This is not the case with hydrogen, which has an auto-ignition temperature of 585°C.
Fire Risk “Radar Chart”
L.F.L.: Lower Flammability Limit is the minimum percentage of fuel vapor mixed in air that will ignite.
Diffusivity: the speed that gases disperse in air (from high to low concentration).
Buoyancy: for gases, it’s the upward force with which gas rises in air (in general: the upward force exerted by a fluid that opposes the weight)
Fire Risk “Radar Chart”. As another measure of fire risk, the “Radar Chart” above compares three attributes of fuel vapors. All three of these attributes should be as high as possible to reduce the risk of a fire. In the chart above, the fuels closest to the center of the 3-D “radar” chart have the greatest risk.
By this 3-element risk metric, gasoline is the most dangerous, propane the next most dangerous, then methane (natural gas). Hydrogen is the safest fuel on all measures except lower flammability limit, where methane has slightly higher LFL than hydrogen.
Source: h2tools.org
LH2 vs LNG
H2
- It’s not poisonous nor a greenhouse gas (Natural Gas is)
- In its gas form, is more buoyant than Natural Gas
- In its liquid form, it requires less thermal energy to become buoyant than Liquid Natural Gas.
- If spilled, cools down structures less than LNG
- If ignited, will heat up the surroundings less than LNG
- If burned radiates less heat than LNG/NG (less than half)
- The safety distance where people can be without getting burns from a hydrogen fire is three times less than Natural Gas
LH2 (and H2) require the same as LNG
- Using proper stainless steels in all contacted components;
- Using the same safety measures, like monitoring, alarm and fire prevention to maintain the product handling safe for operation – as does LNG;
- Keeping areas with potential H2 leak risk sufficiently small and eliminating strong ignition sources in them to eliminate possibility of H2-air mixture ignition;
- Particular care in piping insulation
Uncombusted NG: The Methane Slip Issue
Methane slip is methane that escapes into the atmosphere, and is not used as a fuel in an engine.
Methane, as a gas can hide in engine combustion space crevices and corners, not fully combust and escape into the exhaust.
That is a very significant issue with methane (LNG) as a fuel, since methane is a very powerful greenhouse gas (25 times more harmful than CO2).
Both hydrogen fuel cell systems and LNG engines may have fuel leaks, but in the case of hydrogen it does not harm the environment.
| LNG | H2 | |
|---|---|---|
| Fuel Material (feedstocks) | Underground reserves and renewable biogas | Natural Gas, methanol and electrolysis of water |
| Gasoline Gallon Equivalent | 1 kg of LNG has 100% of the energy of 1.55 liters of gasol | 1 kg of Hydrogen has 100% of the energy of 3.785 liters of gasoline |
| Energy Content (Lower heating value) | 49.5 MJ/kg | 120.1 MJ/kg |
| Energy Content (Higher heating value) | 55.1 MJ/kg | 141.9 MJ/kg |
| Physical State | Cryogenic Liquid | Compressed Gas or Liquid |
| Autoignition Temperature | 540 °C | 585 °C |
| Maintenance Advantages | LNG engine requires less maintenance respect conventional fuels thanks to minor carbon content of the fuel and no Sulphur. This will reduce the components wear | Hydrogen is carbon free and the Fuel Cell system does not have moving part, does not create vibration and the emissions are just water and electricity. For those reasons it requires minimum maintenance |
| Energy Security Impacts | LNG is domestically produced from natural gas and renewable biogas | Hydrogen is produced domestically and can be produced from renewable sources |