Yes, the low density helps dissipation.
It is also very predictable while NatGas can vary a lot.
The low mol wt does make leaks an issue but common good practice works fine.
Not nearly as hard to seal as He.
Our garbage trucks are CNG, I suppose our school buses and city buses are as well. Never heard of one of them catching fire. Which of course only means I haven't heard of it
The problem with sloppy work is that the supply FAR EXCEEDS the demand
The autoignition temperature of natural gas is very high. Even gasoline has a high autoignition temperature. Lubricating oils are much more likely to ignite off a hot surface. In my experience of one engine fire, it was the lube oil that caused it after a double catastrophic turbocharger failure. The rotating assembly impacted the compressor housing hard enough to pull ten 10mm Timeserts out and rotate the cartridge shearing both the oil supply and return lines (1/2" and 1-1/4" tubing). The second turbo had similar damage but didn't shear the oil lines. Post failure investigation found no thrust bearings in the turbo.
Natural gas explosions blow a home to smithereens every few days. Granted there are a lot of terminals for natural gas - water heaters, stoves, ovens - and that many of these explosions are caused by construction crews or repair efforts that go badly. What is key is the surrounding building in which natural gas can accumulate to explosive mixtures.
Absent a constraining structure an explosion is less likely with a fuel gas unless there is a major rupture, such as a pipeline failure, that creates a very large mixed volume of explosive mixture.
More difficult for hydrogen is that it is stored at very high pressures, making any leak a large source of fuel. House pressure is on the order of 0.25 psi. Hydrogen is dispensed at 350 Bar or 700 Bar into vehicles with 1000 Bar site storage. CNG is 200-250 Bar for vehicles.
The lowest flash point temp of natural gas appears to be −188°C, explosive from 5-15% in air.
The flash point of hydrogen is anything above 20K. It will burn at 4%-75% mixture with air and 95% in oxygen.
Reported elsewhere - it was a fuel cell bus, so electrically operated.
Houses have pilot lights so the flash point is of concern. The flash point is largely irrelevant for vehicles because they don't typically have open arcs or flames present hence the importance of autoignition temperature.
The problem with hydrogen is that it has a very wide explosive range. If it can find an ignition source it will ignite.
It was at a refilling operation which may have had a spark source, like a bad electrical contact in the motor control system. It's not an internal combustion bus.
The difference in properties is even more significant because your average doofus with a BBQ is accustomed to fuel gas being somewhat forgiving of his doofusness. Reticulating hydrogen, as is dreamed about in some circles, will require higher pressures, (thus finding all the leaks in the infrastructure), different metallurgy (hence creating new leaks where this done wrongly), and lifting into orbit at least the first few doofi who haven't gotten the message.
![[ponder]](/data/assets/smilies/ponder.gif "[ponder] [ponder]")
