Why can't you re-burn hydrogen (I KNOW this is a dumbass question, just am not seeing this, be nice!)

[ulrichburke]

Dear Anyone.

So you've got a car powered by hydrogen.  Totally environment friendly - so I've read - water comes out the other end of the process.  (Not seen one for real, just read about them!)

Now water is hydrogen plus oxygen, as you know, so some'ow (don't understand this bit!) the hydrogen you've 'used' is still there in the water.  Wouldn't it be possible to separate the two back out again so they're both gases and re-burn the separated-out hydrogen in the separated-out oxygen? And if you get MORE water coming out the other end of the process, couldn't you just set up a loop so the new water would go back to the separator, separated back out and re-reburned and so on?  Surely you'd end up with an infinite power-loop, constantly reusing the same hydrogen?

Now there's GOTTA be a problem with the above, or someone would've thought of it by now! But if by burning hydrogen in the first place you're getting water coming out the other end, I'm not seeing what the problem IS! (And if you've got a separator, the above aside, couldn't you just power everything with purified water?)

What am I missing here? (Apart from a brain!) 

Yours puzzledly

Chris.

[Endy0816]

You could seperate them again, via electrolysis for example, but will end up requiring more Energy than combining them yields.

[pzkpfw]

Firstly, a car that's hydrogen powered isn't "burning" the hydrogen the way you may be thinking; a chemical reaction between the hydrogen and oxygen is used to make electricity to power motors.

Yes, you could collect the water coming out of something hydrogen powered.

But, it takes energy to separate the hydrogen from the oxygen. (That's where the hydrogen is going to come from in the first place; factories or whatever using electricity to separate hydrogen and oxygen.)

(And, you'd have to carry that water around.)

You're not going to get a free ride.

[exchemist]

6 hours ago, ulrichburke said:

Dear Anyone.

So you've got a car powered by hydrogen.  Totally environment friendly - so I've read - water comes out the other end of the process.  (Not seen one for real, just read about them!)

Now water is hydrogen plus oxygen, as you know, so some'ow (don't understand this bit!) the hydrogen you've 'used' is still there in the water.  Wouldn't it be possible to separate the two back out again so they're both gases and re-burn the separated-out hydrogen in the separated-out oxygen? And if you get MORE water coming out the other end of the process, couldn't you just set up a loop so the new water would go back to the separator, separated back out and re-reburned and so on?  Surely you'd end up with an infinite power-loop, constantly reusing the same hydrogen?

Now there's GOTTA be a problem with the above, or someone would've thought of it by now! But if by burning hydrogen in the first place you're getting water coming out the other end, I'm not seeing what the problem IS! (And if you've got a separator, the above aside, couldn't you just power everything with purified water?)

What am I missing here? (Apart from a brain!) 

Yours puzzledly

Chris.

The reaction is 2H₂ + O₂ <-> 2H₂O, with a heat of combustion of 142MJ/kg of hydrogen. So for every kg you burn, you get 142megajoules (=39.4kWh) of heat out.

If you want to recover the hydrogen to use again, as you propose, you have to run this reaction backwards. That's what electrolysis of water does, for example. But that requires you to put in 142MJ of energy per kg of hydrogen produced.  So you get back to exactly where you started, with no net energy gain. (Conservation of energy means you cannot get something for nothing.) 

This is why, when people propose using hydrogen as fuel, what they envisage is using hydrogen as an energy storage medium. A cylinder of hydrogen is a lot lighter than a battery with the same energy content. But you need to make the hydrogen in the first place, which requires energy input, e.g. from renewably produced electricity, just as you do to charge up a battery.

Though you can also make hydrogen by thermal cracking of natural gas: CH₄  + heat -> 2H₂ + C, in which case you get hydrogen from fossil fuel and produce a lot of solid carbon as a byproduct. This is not widely commercialised yet. I'm not sure what one does with all the carbon. I suppose one could make it into bricks as a building material, or something. The one thing you must not do with it of course  is burn it!    

 

     

[studiot]

7 hours ago, ulrichburke said:

Dear Anyone.

So you've got a car powered by hydrogen.  Totally environment friendly - so I've read - water comes out the other end of the process.  (Not seen one for real, just read about them!)

Now water is hydrogen plus oxygen, as you know, so some'ow (don't understand this bit!) the hydrogen you've 'used' is still there in the water.  Wouldn't it be possible to separate the two back out again so they're both gases and re-burn the separated-out hydrogen in the separated-out oxygen? And if you get MORE water coming out the other end of the process, couldn't you just set up a loop so the new water would go back to the separator, separated back out and re-reburned and so on?  Surely you'd end up with an infinite power-loop, constantly reusing the same hydrogen?

Now there's GOTTA be a problem with the above, or someone would've thought of it by now! But if by burning hydrogen in the first place you're getting water coming out the other end, I'm not seeing what the problem IS! (And if you've got a separator, the above aside, couldn't you just power everything with purified water?)

What am I missing here? (Apart from a brain!) 

Yours puzzledly

Chris.

 

Hi Chris, the two most important Laws in Science are called the first and seconf Law of Thermodynaics.

They are the most important in my opinion, because they enabled the industrial revolution.

The first law states

"You can't get something for nothing" or "There is no such thing as a free lunch" or "You can't win, you can only break even".

The second law states

"You can't even break even you can only loose"

So think of an ordinary coal or log fire.

In theory you could collect all the heat from the combustion and use that as well as the portion you actually use.

But the first law tells you that you can only get out what you put in.
That is for a specific amount of fuel you get a specific amount of heat and no more.

However the second law bites you in the ass because some of the products of combustion appear as a gas and go up the chimney.
But to make a gas from a solid or liquid (fuel) requires an input of heat energy. This heat can only come from the heat of combustion as ther is nowhere else for it to come from.
If the combustion products did not go up the chimney they would collect in the fireplace.

Similarly if you took all the energy out of the water passing through a turbine at the bottom of a dam, the water would unot have enough energy left to flow away and you would be faced with a flood.

At each stage of any process there Mr 10% (or worse) always takes his cut.
So every time you recycle your hydrogen you would loose 10% (it would actually be more).

 

Does this help ?

 

[Intoscience]

Basically it's the old adage - There are only 2 things, unescapable in this life - Death and Taxes. 

Eventually the energy runs out - death, and you never get out everything you put in or started with - taxes.

 

[John Cuthber]

"Why can't you re-burn hydrogen"
Because you can't burn "ash".