# Is liquid hydrogen rocket propeller pollution free?

If one searches for the different fuel types used in rockets, one can find that over the years, NASA and other space agencies have used both solid fuels and liquid fuels. I can see that more and more rockets are propelled by using a combination of Liquid Hydrogen (LH) - Liquid Oxygen (LOX). Then, I assumed that the reaction would be: $$\ce{2H2(l) + O2(l) ->[heat] 2H_2O(v)}\tag{1}$$

By looking at this reaction, one can see that the product of this reaction is water (which most probably will be in a vapor state). Now, I have three questions:

1. Is this reaction correct?
2. If the product is water, does that mean that such a combustion reaction is pollution free?
3. I assume that I am wrong thinking that is pollution free. What would be the polluting agent in this situation and how bad is it, compared to let's say the burning of the same amount of Kerosene?

Thank you!

I'll address you questions in order:

1) Yes, that is basically the correct reaction, although not quite balanced; here is how I would write it:

$$\ce{2H2(l) + O2(l) -> 2H_2O(g)}$$

2) Ideally, if there were no side reactions, then yes, this particular reaction is "polution free", and the only product is water.

3) You are correct in your thinking that in the end this does not represent a net pollution-free energy source.

The reason for this doesn't lie in the reaction taking place at the rocket, but in how they produced the hydrogen in the first place. Unfortunately, there are no significant deposits of $\ce{H2}$ on our planet waiting to be pumped out and used as fuel. So we have to make it, which requires engergy.

This means that the $\ce{H2}$ is not a net energy source, rather it is essentially acting as a battery, storing the energy from some other source. So, you can think of the actual pollutants of the $\ce{H2}$ rocket fuel as being the products from whatever process went into generating it in the first place. This largely depends on what part of the world the hydrogen generation took place. They could have burned coal or other fossil fuel to produce the energy, or they could have built a dam to generate hydroelectric power, etc. Any of the negative aspects of producing a particular type of energy would be considered a negative product of the $\ce{H2}$ production.

• Same for $\ce{O2}$ as well. Plenty of it in the atmosphere, but energy has to be used to separate and liquefy it. – MaxW Feb 19 '17 at 21:01
• Absolutely; far from an energetically-downhill task. – airhuff Feb 19 '17 at 21:07
• Thank you very much for your answers. So, if you would get H2 from dams or wind mills, that would reduce the pollution, which is actually nice. But what about the answer from Oscar Lanzi. He states that the hot exhaust would form NOx. Do you also agree with this statement? – Physther Feb 19 '17 at 21:46
• I do. My answer is really in the perfect extreme of getting exactly the desired reaction. Oscar's basically correct statement is an example of my #2 about side reactions. The same thing happens in an automobile. Ideally, the chemical equation is hydrocarbon plus O2 gives CO2 and water. But, under the high temperatures of combustion, particularly the case of an auto where you have more N2 than O2, you get nitrogen oxides, partially burned hydrocarbons, etc. That said, you are not using atmospheric O2 in the case of rocket fuel, so that wouldn't be nearly the relative problem it is in an auto. – airhuff Feb 19 '17 at 22:20

Hydrogen is not quite pollution-free when it burns in air; some of the nitrogen will form NOx in the high-temperature environment. In a rocket with hydrogen and purified oxygen we get pure water in the combustion reaction but the hot exhaust can still form some NOx in air. Still, this is a lot less polluting (and more efficient for the rocket) than carbon-bearing fuels.

• Thank you Oscar. This is great and interesting information. So, would the NOx form because of the Nitrogen in the air, or? And is there any chance that this NOx would contribute to the formation of Ozone up in the stratosphere? As far as I know, NOx can react with VOC's, forming ozone, which would actually be beneficial for the stratosphere. – Physther Feb 19 '17 at 21:51
• Actually NOx is detrimental to ozone. Like other free radicals such as atomic chlorine, it sets off a free radical chain reaction that causes the ozone to break down to diatomic oxygen. See learner.org/courses/envsci/unit/text.php?unit=11&secNum=10. – Oscar Lanzi Feb 19 '17 at 23:15
• I definitely agree with what you said in your answer. My only further comment (not a disagreement) on the NOx-ozone cycle is that in the troposphere, during the daytime and in the presence of VOC's, NOx will promote photochemical ozone production down low where we don't want it. As you said, this case is not as bad as the case of carbon-bearing fuels, which also produce their own VOC's, among other things. – airhuff Feb 20 '17 at 19:04