I heard a podcast recently about hydrogen fuel cell vehicles.
My question: is the hydrogen used in fuel cell vehicles odorized? Hydrogen is normally an odorless gas. How could someone know if hydrogen was leaking unless they could smell it?
J.
The technology is still in its infancy, so I serious, seriously doubt that there are any requirements to add odorizing agents to the hydrogen.
Unfortunately, the ways these things usually work, that sort of requirement will only be developed after a dozen or so garages have blown up due to hydrogen leaks.
Fuel Cell catalysts (and many other catalysts) are very sensitive to Sulfur compounds (most odorizing compounds are sulfur species - mercaptans, ets.).
On the positive side - hydrogen is probably the fastest diffusing gas and a leak will diffuse the fastest.
Hydrogen is also easily detected by instruments.
Or by the high-pitched, squeaky voice, but only if you talk to yourself while you’re working in your garage.
Hmmm, why does the word “Hindenburg” come to mind?
Yes, I realize dirigibles and fuel cells are very different situations.
J.
Unrelatedly, I’ve just thought of the worst name ever for the first mass produced hydrogen cell car.
We need hydrogen vehicles to save [sub] Oh, the [/sub] humanity.
Good one!
J.
That is not quite true. Although hydrogen has a high buoyancy at NTP (between 1.2 and 9 m/s), its diffusion velocity at NTP (the rate it spreads laterally) is <2 cm/s, so if a mass of hydrogen is caught under any kind of overhang or inverted catchment it may persist for a considerable duration. It also has a very high range of flammibility concentration (between 4% and 75% volume in NTP air) and high detonability range (between 18.3% and 59.0% in NTP air) with a detonation velociy of between 1,500 and 2,100 m/s (low order detonation, but well in excess of the speed of sound in air). The minimum energy for ignition in air is 0.017 mJ; the typical electric dischage from body static chage accumulation is 5-10 mJ. Although hydrogen has a flame temperature of 2,318 K (~3,700 °F) and a heat of combustion between 120 and 142 kg/J which is definitely hot enough to cause immediate 3rd degree burns to exposed flesh directly in the flame corridor, but because the flame burns clear and with a radiated energy of 17% to 25% of total energy it may be nearly impossible to observe the flame visually before contact. (All values taken from AIAA G-095-2004 Guide to Safety of Hydrogen and Hydrogen Systems.)
It is unwise to be sanguine about the hazards associated with handling hydrogen, as the aerospace industry has learned very well. The hazards can be mitigated somewhat by proper facility design (no overhangs or other inverted catchments or positive spark-free ventiliation), good fuel system design (fail-safe connections and check valves which minimize potential leakage), and training (always advance on a suspected hydrogen fire with a combustable or thermal probe, never transfer fuel in a confined location, et cetera) but it is nowhere nearly as benign as gasoline, kerosene, methanol, or even propane or dimethyl ether.
As previously noted it is impractical to add odorants if the hydrogen is to be used in conjunction with a fuel cell, and it is worth noting that current mass hydrogen production is performed by the energy intensive steam reforming of natural gas or coal gasification with little sequestration of the CO[SUB]2[/SUB] byproducts. (This could be done but at a considerable loss of net energy production.) The ‘clean’ production of hydrogen from dissociation of water via thermolysis or electrolysis is a very low efficienty process requiring very high temperatures (over 1000 K), and thus far no practical means of economically producing sufficient quantities of molecular hydrogen to substantially replace the use of hydrocarbon fuels for transportation use have yet been demonstrated, although there are some very interesting fermentative and photobiological possibilities that may allow the use of vast tracts of midocean are to be used and harvested for hydrogen production, albeit at questionable overall efficiencies for land transport.
Stranger
That is very true. I have worked in Industrial applications of hydrogen from steam reforming of natural gas / naphtha to coal gasification and have extensive experience designing, commissioning , chairing hearing committees on hydrogen safety.
Having said that, my analysis shows that hydrogen fueled cars do not make sense because the economics are not favorable.
Diffusion Velocity in NTP air (cm/s) : Hydrogen 2, Methane 0.51, Propane 0.34, Gasoline 0.34source
So Hydrogen diffusion velocity is 6 times that of gasoline. My statement - " hydrogen is probably the fastest diffusing gas and a leak will diffuse the fastest." still stands unless you show me contrary proof.
This is all speculation. What sort of overhang or inverted catchment are you talking about ? Even helium in a normal balloon is hard to keep for any considerable duration - hydrogen escapes even faster. And the statement is very misleading because you say mass of hydrogen and not volume of hydrogen.
1 ft3 of trapped hydrogen @100F / atm pressure, heating value = 324 BTU
1 ft3 of trapped propane @100F / atm pressure, heating value = 2517 BTU
So the propane is an order of magnitude in terms of energy content.
Again - what makes you say it is very high ? Very high compared to what ? Whats the significance of the range ? Hydrogen LEL/LFL (Lower Explosivity/Flammability Limit) is 4 % by volume in air. Propane is 2 %. So you can get explosions with propane at even lower concentrations.
Again - this is all in the realm of speculation since we have already established that hydrogen in air diffuses fast and it will be nearly impossible for a leak to build up to that level of concentration.
I would not dignify the rest of the hydrogen post with tabulated answers. Here is an actual test with pictures of a hydrogen car versus a gasoline car catching fire.
As to the Hindenberg - if it was filled with propane - the disaster would have been much worse.
Again - this is an opinion disguised as a fact. CO2 is routinely sequestrated in Ammonia plants as Urea. Ammonia is produced from Hydrogen (produced by steam reforming natural gas / naphtha or coal gasification followed by shift reaction).
Dakota Gasification exports about 152 million cubic feet per day of CO2 to Canada – about 50 percent of the CO2 produced when running at full rates.
This message board has already nailed that one down. It’s going to be the Graphene Zeppelin.
Nitpick, but the “diffusion velocity” was reported by two people above in units of cm/s. This is apparently a synonym for the “diffusivity” or “diffusion coefficient,” which is in units of cm^2/s. A diffusion front does not move at a constant speed, it expands like the square root of time.
Sorry, that is wrong. Diffusion velocity is not the same as diffusion coefficient. Without going into too much details, here is the definition fromNASA :
diffusion coefficient (NASA Thesaurus / NASA SP-7, 1965)
The absolute value of the ratio of the molecular flux per unit area to the concentration gradient of a gas diffusing through a gas or a porous medium where the molecular flux is evaluated across a surface perpendicular to the direction of the concentration gradient.
diffusion velocity (NASA SP-7, 1965)
1. The relative mean molecular velocity of a selected gas undergoing diffusion in a gaseous atmosphere, commonly taken as a nitrogen (N2) atmosphere.
Thanks for the definition, which certainly makes sense, since it describes an actual velocity. I was not familiar with the term “diffusion velocity” and was trying to make sense of the use above. Using the NASA definition, the term is the same as the thermal velocity, which is many orders of magnitude higher than the <2 cm/s that was cited. The thermal velocity for hydrogen is more than 20,000 cm/s. The diffusion coefficient in air is about 0.6 cm^2/s, consistent with the value cited, except in units of cm^2/s. Also, the “diffusion velocity” was described by one of the posters as the rate at which the gas spreads laterally. That certainly does not occur at a rate anywhere near the thermal velocity, which is close to the velocity of sound, nor can a “rate” be measured in cm^2/s.
As I mentioned, the velocity changes as the diffusion proceeds, so that the characteristic length over which the diffusion has occurred grows like the square root of time, rather than linearly in time. In fact diffusion will easily be swamped out by convection in any realistic situation over macroscopic distances.
You are welcome.
When it comes to binary or multicomponent gaseous diffusion, the Stefan-Maxwell(velocity) model for diffusion is more commonly employed than the Ficks (flux) model. (More so by chemical engineers). Hence you will see both diffusion velocity and Diffusivity constants in engineering databooks.
For Binary mixtures, the Maxwell-Stefan model readily reduces to the Ficks model. See slide 5 in this link.
The hydrogen used in fuel cell vehicles is not currently odorized, however sufficient technology has been researched to the extent that promise has been shown for the use of odorants with fuel cells. Penn State University and the US Department of Energy have investigated the topic. This technology involves non-sulfer odorant chemistries and the work shows promise to be benign to fuel cell catalysts. There is a Federal Regulation (40 CFR 192) that requires odorants be used for distributed fuels. Sensors cannot be distributed across a fueling infrastructure at the density required to detect all leaks that could cause harm to public health and property. Therefore, odorants should play a role in the widespread adoption of hydrogen fueling and the use of fuel cell vehicles by the general public.