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My friend who is a biology major proposed that we can produce water by burning any hydrocarbure or wood or ... to obtain water.

$\color{red}{f(x,y,z,\Lambda)}\ce{C}_x\ce{H}_y\Lambda_z+\color{red}{g(x,y,z,\Lambda)}\ce{O_2->}\color{red}{x}\ce{CO_2}+\color{red}{\frac{y}{2}}\ce{H_2O}+\color{red}{\text{other substances}+\text{heat}}$

I know this may be a stupid question but why isn't this method used to obtain pure water? Is it maybe because it releases a lot of heat and a lot of carbon dioxyde or is it because the fuel is more expensive than water?

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    $\begingroup$ In semiconductor fabs, the water for steam oxidation of silicon is made by burning hydrogen - it is 'easy' to get very pure $H_{2}$ and $O_{2}$ so this way no impurities (like Na) are introduced into the equipment. $\endgroup$
    – Jon Custer
    Jul 27, 2016 at 17:32
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    $\begingroup$ He proposed this solution for the water scarcity problem that is to produce large quantities of water. $\endgroup$
    – user5402
    Jul 27, 2016 at 17:36
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    $\begingroup$ You need to capture and separate your water from your $\color{red}{\text{other substances}}$ as well as any unburnt fuel. $\endgroup$
    – f''
    Jul 27, 2016 at 17:50
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    $\begingroup$ Like you say, fuel is more expensive than water. You need to burn one liter of fuel for two-three liters of water, and then you still have to condense it out of the hot exhaust gas. $\endgroup$
    – Karl
    Apr 14, 2019 at 19:01
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    $\begingroup$ @BPP Nothing that burns is much cheaper than oil or wood, or people would use it instead. $\endgroup$
    – Karl
    Apr 14, 2019 at 19:31

5 Answers 5

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The answer is in the formula. The 'other substances', which are either volatile matter or ash byproducts, are formed along with the water. The water is not pure because some of volatile substances can condense with the water which will require further treatment for removal. Removal of the volatile matter and the disposal of the rest of the 'other substances' will cost too much money for this solution to be economically feasible.

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    $\begingroup$ Since water and \color{red}{the other substances} have different degree of condensation we can easily separate them. Even better we can use only hydrocarbures or substances that don't produce nocive substances. $\endgroup$
    – user5402
    Jul 27, 2016 at 18:58
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    $\begingroup$ Distillation (which is a type of treatment process involving separation by differing condensation temperatures) is conceptually easy; however, relative to current technologies the energy requirement for distillation make implementation expensive. ... If alternative hydrocarbons are available then it is likely a source of water is available as well. Why would it not make more sense to distill impure water, which likely has much fewer contaminants, to obtain pure water. $\endgroup$
    – Agriculturist
    Jul 27, 2016 at 21:55
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    $\begingroup$ Maybe we could do some kind of isothermal combustion at a temperature where water is liquid and all other substances are gazes. $\endgroup$
    – user5402
    Aug 1, 2016 at 12:25
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The problem about drinkable water is that we need more "clean and unsalted" water, and less CO2; we don't need more water at the moment. With the climate unbalance, less snow (clean and unsalted) is accumulated in high mountains in winter so fewer water is released in rivers when the weather becomes warmer. It is a big problem between some middle east countries which are along the river because some built barrages on the river to get energy and other countries get less water because of that.

The question is much larger than just getting water.

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    $\begingroup$ This is a good point. Geo-politics surrounding clean water is an ancient problem. People have been squabbling over clean water and burning things for a long time. If there were an economical way to produce clean water from burning stuff we would have likely found out how to do it many years ago. $\endgroup$
    – Agriculturist
    Jul 27, 2016 at 22:21
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I know this may be a stupid question but why isn't this method used to obtain pure water? Is it maybe because it releases a lot of heat and a lot of carbon dioxyde or is it because the fuel is more expensive than water?

Cost. Fuel cost a lot more than water. And in most cases, it just does not make sense to burn fuel of any type to obtain pure water. It is cheaper to just use that energy from burning said fuel to power machinese to purify water from a local body of water (ie lake, river, ocean)

However, in space missions, hydrogen fuel is oxidized (electrochemically, never burnt as fires in space is an absolute nightmare) in a fuel cell to obtain both energy and drinking water.

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Why would we? Pure water is widely available and, where it isn't, there are cheaper and easier ways to get it.

Sure, you can make water from combustion of hydrocarbons. Better still by burning hydrogen (where it is the major product). But why do that if there are better ways to get potable water?

The majority of countries have widespread sources of clean (or easy to clean) water. There are lakes and rivers. extra water can be obtained by arranging to collect rain (often in artificial lakes called reservoirs).

Even in countries short of rain, rivers or lakes, there are usually vast quantities of salty water available. You can't drink it directly, but there are plenty of relatively cheap ways to make it potable (distillation or reverse osmosis use far less energy than generating water from the fuels that would provide power for those processes).

Maybe, were you a stranded astronaut on Mars with large supplies of oxygen and hydrogen, the best way to generate water would be to burn the hydrogen (this is part of the plot of a well-known movie). But why use this technique even in a desert country where you could generate far more useable water by running a desalination plant with the same fuel?

To summarise: if you need water the best way to get it will be the cheapest and easiest way to get it. Unless you are stuck on Mars, burning fuel will never be the cheapest or easiest way to do it.

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There is an attractive solution particularly well suited for the desert areas near the ocean, like California or Mexico. Use an old oil rig (or a retired aircraft carrier) for a floating desalination/hydrolysis plant (solar or nuclear powered).

H2 gas can be piped ashore at a few psi for many miles, over mountain ranges, where ever it is needed, far easier than pumping water$^1$.

At its destination, every pound of H2 burned will yield around 9 pounds (1 gallon) of water, plus energy from H2 combustion (in a turbine) can be used to for mechanical energy or to make electricity.

Burning wood or coal unfortunately yields a lot of CO2. Even natural gas, CH4, would yield 44 pounds of CO2 for every 36 pounds of water.

$^1$ the pure O2 by product of water hydrolysis would also be saleable.

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