When garbage is incinerated, it's not uncommon for that garbage to have so much moisture, that it takes just as much energy to drive off that moisture as is produced by burning the garbage.

I propose the following two part system:

The second part of the system consists of an incinerator which burns dried out garbage and uses it to drive a steam engine.

The first part of the system takes garbage, and feeds it, via a gear pump, into a long, cylindrical, vacuum dehydrator. The trash is conveyed through the cylinder by a tumbling device which ensures thourough air circulation. The cylinder is continuously heated by heat from the steam engine's condensor. The vacuum is maintained by a vacuum pump, which is directly driven by the steam engine. The dehydrated garbage is removed from the dryer via a second gear pump.

Because of the lowered boiling point of water inside the vacuum cylinder, any moisture in the trash will suck in heat, at a temperature below 100C... possibly even below atmospheric temperature. Thus, the dehydrator acts as a very effective heat sink for the steam condensor.

It's probably also possible to use some of the heat of the air/steam pumped out of the dehydrator to help heat the dehydrator.
-- goldbb, Mar 24 2009

do we want to burn trash? would not the goal be to reclaim as many elements as possible making heterogeneous homogeneous.

I was thinking along the lines that after all the reclaim milling/sorting/washing processes what is left over comes out as a thick paste ready for the microbes or your trash cooker .
-- wjt, Mar 24 2009


I think that the energy required to reduce air pressure would be comparable to the energy required to heat the trash and drive off the water.
-- bungston, Mar 24 2009


i second that. the energy price of boiling water with vacuum is exactly equal to the energy required to boil it with heat. No free lunch, conservation of energy applies.
-- WcW, Mar 24 2009


The movie adaption could look like a cross between Star Wars and Total Recall - Leia and Han, with bulging eyes, trying to shout for help, but their voices thinned with the atmosphere...
-- loonquawl, Mar 25 2009


I have pondered garbage drying techniques that involve exportation of coastal SoCal trash to the arid eastern california desert. I suspect that the energy to haul the trash would equal the energy to cook the water out of it on site. But if one could haul it all the way to Nevada there would likely be less stringent laws about burning it for energy.
-- bungston, Mar 25 2009


I'm pretty sure that would require more total energy than just burning, because machinery to maintain a vacuum is not very efficient.

However, this might still be a good idea, because moisture could prevent an incinerator from running at a high enough combustion temperature to really incinerate. If you burn some stuff at too low a temperature, you get very nasty exhaust.
-- colorclocks, Mar 26 2009


wjt, you're absolutely correct, but facilities to extract recyclables from mixed refuse take space, inve$tement, and, even the most advanced systems still rely on human personel to do much of the sorting. So sometimes it's just cheaper to burn it.

Anyway...

I realize that it takes just precisely the same amount of energy to boil water in a vacuum as to boil water at atmospheric pressure...

However, to boil water at atmospheric pressure requires that that heat be supplied at a higher temperature; we wouldn't be able to use waste heat from the steam condensor to do it.

(Of course, a slightly better way to deal with waste might be to cook it at high heat to turn as much of it as possibly into charcoal, then gasify the charcoal)
-- goldbb, Mar 26 2009


//do we want to burn trash? would not the goal be to reclaim as many elements as possible making heterogeneous homogeneous. //

No, it takes more energy to recycle almost everything than to make it new, except aluminum cans, and maybe one or two other things.

As for the idea, other have mentioned that it takes just as much energy to use a vacuum. But I'm sure there are other ways to dry out the garbage without actually boiling the water. Compression, laying it out in the sun, drilling holes at intervals that are likely to pierce most large liquid traps, etc.
-- Smurfsahoy, Mar 28 2009


I’m not sure I buy the thermodynamic argument for why this wouldn’t work. Regardless of whether it’s in the gas or liquid phase, it should take the same amount of energy to heat water at room temperature to 100°C. Heating water vapor from room temperature to 100°C should take the same amount of energy as lowering the pressure to where water boils at room temperature. So in a perfectly efficient world, we’re using the same amount of energy to heat our water as we’d be using to lower our pressure. Now, regardless of what temperature we do this at, turning liquid water into water vapor will take the same amount of energy, and this will be significantly more energy than we used to heat the water or create the vacuum. So we can burn fuel to keep our water at 100°C, or we can let the atmosphere cool down ever so slightly as it provides that energy for us.

Now, I have no idea as to how efficient vacuum pumps are in the real world (I’ve heard they’re not super-great), so that might make this unfeasible. But the thermodynamics shouldn’t get in the way, unless you’re trying to dry your garbage in space.
-- SirBobofBobton, Mar 29 2009


SirBobofBobton, That's not an accurate way of describing the problem.

Suppose that you've got water in a sealed container, at room temperature, with no air in the container.

To change the pressure in that container from atmospheric pressure to a sufficiently low pressure that it boils at atmospheric temperature, requires almost zero energy.

Once it's reached that low pressure, the amount of heat energy required to boil the water, at room temperature, is equal to the amount of heat energy required to boil the water at 100C/atmospheric pressure. Moving heat energy from the atmosphere into the container when both are the same temperature, is, however, a "free" operation.

Of course, moving the steam out of the container and into the atmosphere (to keep the pressure in the container low) requires energy -- *this* is the costly part.

The question is: does it require more energy to move the cold steam into the atmosphere, than it does to first heat the water from atmospheric temperature up to 100C, then boil the water, at atmospheric pressure?
-- goldbb, Mar 29 2009


//cold steam//
when you try this might I suggest Band-Aid brand bandages for that moment when you realize that your "cold steam" will condense into 100C water when it hits atmospheric pressure.
-- FlyingToaster, Mar 29 2009


Of course I know that the cold steam will become hot steam once it reaches atmospheric pressure.

After all, another word for vacuum pump is compressor, and everyone knows (or should know) that gasses become hotter when you compress them.
-- goldbb, Mar 30 2009


well... I imagine fiddling with 'vapour pressure' (whatever that is) equations would let you figure out how much energy you need to dewater something with a vacuum; it's not gonna be zero because you need to keep the chamber vacuumed up against 14psi. [21Q] was working on something like that when he was blatantly ripping off my 'Vacuum Reducer' post for his own nefarious uses ;)
-- FlyingToaster, Mar 30 2009



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