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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 '16 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 '16 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 '16 at 17:50
  • $\begingroup$ 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 '19 at 19:01
  • $\begingroup$ @Karl You could burn any cheap substance but I understand this is impractical. $\endgroup$ – user5402 Apr 14 '19 at 19:23
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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 '16 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 '16 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 '16 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 '16 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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