H2 Safety, Reduced/Zero Emissions
Safety
Hydrogen is the lightest of all elements – it’s fourteen times lighter than air and rises quickly in the event of a leak, dispersing harmlessly into the atmosphere. Unlike gasoline, hydrogen will not pool on the ground, potentially contaminating water sources and soil, or creating a catastrophe if ignited.
Polluting fuels pose significant health and environmental safety threats when leaked, spilled, or combusted. Hydrogen leaks are non-toxic, this is in steep contrast to other fuels. If diesel gets into the waterways, it can cause serious pollution to rivers, streams and groundwater.
Hydrogen is less combustible. Gasoline in the air is flammable at a lower concentration limit of 1.4%, compared to hydrogen’s 4%. To put it another way, gasoline is two to three times more flammable in the air. The optimal mixture for hydrogen combustion is 29%—which in reality is quite unusual, since hydrogen rises and will generally diffuse. Gasoline vapor’s optimal mixture for combustion is only 2%—a ratio that is very easy to reach.
In the unlikely event of ignition, hydrogen burns faster than gasoline and with a lower radiant heat due to the absence of carbon. Therefore, the risk of secondary fires due to hydrogen is also lower. BTW: Modern pressurized hydrogen storage tanks are designed to make it impossible for sources of ignition (oxidizers) to come into contact with the fuel...
Robust Safety Measures
Hydrogen has been safely used in a variety of applications for over half a century—giving time for scientists and engineers to develop and implement rigorous safety protocols.
The Department of Energy and other global entities have created safety codes and standards based on decades of research and testing by private sector companies. These standards have helped build processes that mitigate potential harms.
Citing a 2003 white paper, “Twenty Hydrogen Myths,” by American physicist and Chairman/Chief Scientist of the Rocky Mountain Institute Amory B. Lovins, Hydrogen Fuel News notes hydrogen burns quickly with a nonluminous flame that “cannot readily scorch a person at a distance as it emits only one-tenth the radiant heat of a hydrocarbon fire and burns 7% cooler than gasoline.”
“In fact, to speak further to hydrogen safety compared to gasoline safety, typically, victims of hydrogen fires are not typically burned unless they are actually in contact with the flame,” the trade publication wrote. “Furthermore, they’re also not choked by smoke.”
Emissions
Fuel-Cells
As there is no carbon in H2 molecule, it's use in fuel-cells will only result in water as output. It is the cleanest fuel known to mankind.
Atmosphere
H2 is a light molecule if it in the atmosphere it will leak into space [2]. Some observed [1] that hydrogen in the atmosphere can destroy a molecule known as the hydroxyl radical that otherwise destroys methane—a gas that contributes to greenhouse warming, but the real problem here is methane itself, whose emission would be cut anyway due to a clean alternative being available now, the alternative being hydrogen.
Combustion Engines
For the combustion scenario additional care must be taken to avoid NOx emissions. The H2 car engine design described here [3] that enabled the exhaust gas to be re-introduced / recirculated that results in NOx emissions of 1 ppm (regular car NOx emissions is > 300 ppm).
There are other techniques; a stratification technique used in new diesel / hydrogen hybrid engines has varied concentration of hydrogen gas within different sections of the combustion chamber, these variations can affect the chemical reactions and control the amount of NOx produced by the engine, then the timing of the hydrogen injection is used to limit NOx production. 'We have shown in our system that if you make it stratified—that is, in some areas there is more hydrogen and in others there is less hydrogen—then we can reduce the NOx emissions below that of a pure diesel engine,' said the UNSW researcher [4].
Reference
[1] Cary Institute
[2] https://en.wikipedia.org/wiki/Atmospheric_escape
[3] Sciencedirect
[4] H2 Central