Chemistry - Can natural gas appliances run on Hydrogen?
Solution 1:
Pure hydrogen, as opposed to hydrocarbons, degrades metals. $\ce {H_2}$ molecules are tiny, so they wedge themselves into the relatively large metallic crystal structures and form hydrides. This is the entire reason nickel-metal hydride (NiMH) batteries work - the hydrogen is stored in an iron-hydride matrix. An appliance that might last 20 or 30 years burning natural gas might not make it past ten years burning $\ce{H_2}$, and a failure may take the form of a dangerous leak.
Solution 2:
I'm not entirely sure of this, feel free to correct me.
This probably depends on the type of appliance. I'll try two, the internal combustion engine and the stove.
Internal combustion engine
The nice thing about an internal combustion engine is that it takes in gas and oxygen, isolates it, and then ignites it. Besides, it uses a spark to ignite it. Hydrogen is more explosive than natural gas, so the isolation is quite useful here. And the reaction with air is more easily triggered by a spark rather than by heat.
On modification needed to make it work would be to recalibrate the apertures of the oxygen and hydrogen feeds such that the gas mixture is sucked in in a 1:2 ratio.
Secondly, there needs to be a cleaner way of disposing the water formed. Internal combustion engines assume that the exhaust is gas and particulate matter -- which can be compressed and expelled entirely, and isn't adhesive. Water can't be compressed. This makes it easier to expel, but some of it may adhere to the walls. As water, oxygen, and electricity are being supplied, this may quickly rust the walls of the cylinder. So there needs to be a way to protect the cylinder from rusting. Also, the engine will just stop working if the spark plugs get shorted by water. There needs to be a way to prevent that from happening, too.
Actually, a hydrogen internal combustion engine exists, but it looks like they got there by starting from scratch, not modifying a regular internal combustion engine.
The stove
The only issue I see here is that the aperture on a normal stove is too large. Oxygen may enter and end up exploding the whole setup.
In such a case, having one pipe (with a very small bore) for each individual flame may work better. This would probably have to be lit automatically -- I don't see how one can safely light a set of such pipes without there being an individual knob for each pipe.
Nowadays, the more efficient way of using hydrogen for fuel is in a fuel cell. This way, expensive hydrogen doesn't get wasted, and is instead reused (the energy input comes from other sources)
Solution 3:
A few addition comments.
The production of water would create a problem, as if being used as a heat source, how is the water going to escape? Steam and condensation can cause many problems, not to mention; there is the issue of steam and the safety risk to end user. All these factors also cause metal pipes, machinery and appliances to (potentially) rust. On top of the points mentioned above.
The Hindenberg is an interesting example of the flammability of Hydrogen and the effects of it's products when combusted.
As an point of interest, I have provided a link for a table of flammable gases:
Hydrogen's "Lower Explosive or Flammable Limit" (LEL/LFL) (%) is 4, in the order of methane and propane 5 and 2.1 respectively, bit it's "Upper Explosive or Flammable Limit" (UEL/UFL) (%) is much higher 75 as compared to 15 and 10.1 for methane and propane, respectively. Making it more highly combustible at concentrated levels, leading to the difficulties in containing such explosions, as mentioned in the other answer.
http://www.engineeringtoolbox.com/explosive-concentration-limits-d_423.html
I still remember clearly a video our Chemistry Lecturer showed us of the Hindenberg and how the sheet of water (produced from the combustion of Hydrogen) saved lives. That was over 30 years ago, so it made an impression and it's a subject I find interesting.
Solution 4:
The primary problem when switching to different fuels in appliances or engines is getting the fuel/air mix right
Previous answers contain a disturbing number of misleading hypothetical reasons why hydrogen can't work. Many are just plain wrong. For example, the idea that dealing with the water produced is ludicrous given that hydrocarbon fuels produce plenty of water without any major issues.
Hydrogen is a viable fuel if the devices are adjusted to use it (whether the transmission infrastructure needs to be modified is a different question).
The key issue is that most engines and appliances (eg gas cooker or boilers) are set up to use a specific gas/air mixture that optimises the combustion for the specific fuels that is being used. If you change the fuel, that mix changes (the mix being the specific amount of air that is mixed with the fuel to achieve optimal combustion).
So engines need to be adjusted to use hydrogen not hydrocarbon (which might involve hardware and software changes since hydrogen is a gas and petrol/gasoline needs to be vaporized but the air/fuel mix will be different).
Cookers and boilers have simple hardware to control the mixing of air and fuel. So changing to a different fuel needs that hardware to be changed. The UK has experience of doing this for gas-driven domestic appliances when the fuel changed from "town" gas (consisting of a mix of mainly CO and Hydrogen produced from coal) to natural gas (consisting mainly of methane) in the 1960s. The changeover required the hardware in cookers (which means the parts on burners that mix air and fuel) to be swapped out for parts optimised for methane rather than town gas.
So the answer to the question is: yes, but not without hardware changes.