Make:

registrationsucks2 sez:
"But what if you had some kind of combustable material in the chamber, like waste organic matter? It would burn, obviously, and release extra energy. And with the chamber full of extra heat largely in the form of light, I'm thinking you'd get a very efficient combustion of the matter and some ammount of the smoke.

So, does anyone know if this would be worthwhile? Would the extra energy gained make it worth it? What would the emissions be, compared with just setting fire to it all? What happens to carbon at these kinds of temperatures?"

Here's a couple of things you could profitably google for: "pyrolysis" and "gasification".

Solar-heating of biomass can be a useful energy-harvester, but don't think "burn" think "gas". Heating biomass in the absence of oxygen results in a breakdown of organic material [pyrolysis] and release of flammable volatiles. The resulting gas can be used for fuel after cleaning [to remove tarry fractions] and cooling. You can clean & cool by bubbling the gas through a water column.

After pyrolysis you will be left with charcoal. You can either a) burn this for fuel, b) use it for biochar [carbon sequestering], or c) process into syngas.

To make syngas you heat your charcoal [again in the absence of oxygen] to red-heat, and pass steam over it. The resulting syngas is H2 + CO.

Obviously you need to take safety precautions with this - you'll be dealing with hot, flammable, poisonous gases, and to heat in the absence of air, you'll need sealed containers with the resulting issues of pressure build-up and explosion risk.

Note that syngas production has traditionally been carried out in a gasifier that burns part of the fuel to produce the heat required. This has the advantage of being low-tech and simple to build, but the major shortcoming is that the syngas is polluted not only with combustion byproducts, but also with [relatively inert] nitrogen from the air, which reduces its volumetric efficiency.

Using a separate heat source and processing the biomass anaerobically will result in a much better quality fuel gas than the traditional aerobic gasifiers you will read about.

If you want electricity, you can modify an internal combustion generator to run off the gas you produce - but keep in mind that you must cool the gas well for best efficiency.

Hmm, that's very interesting, and yet another use for solar energy that I had no idea existed. I'll look further into it.

If you wanted to keep it more basic tho, even if a lot less efficient, what about just burning the biomass?
I guess I'm mostly interested in what happens to the smoke and the carbon, it probabaly wouldn't be a good idea to just set fire to your compost heap and let it all into the atmosphere...

registrationsucks2 sez:
"If you wanted to keep it more basic tho, even if a lot less efficient, what about just burning the biomass?
I guess I'm mostly interested in what happens to the smoke and the carbon, it probabaly wouldn't be a good idea to just set fire to your compost heap and let it all into the atmosphere..."

Not sure that there's be any advantage in using solar power to assist with that. Concentrated solar gives you heat, as does burning biomass. Heat on it's own isn't easy to use, which is why using the energy to produce gas or electricity is generally considered more useful.

If you burn biomass, either directly, or after gasification, you are returning the carbon load to the atmosphere - but it came from there in the 1st place, so there's no net carbon impact. It's the loading of fossil carbon into the atmosphere that's the problem.

Of course if you did the pyrolysis step and then used the charcoal as bio-char you'd be carbon-negative.

But say you've got solar which on it's own gives you something like 600 deg C into a Stirling or whatever. If you were chucking a bunch of burnable things into your cylinder, would you get more heat? And how much?

It just seems like a good way of making use of lawn clippings or compost or old paper, etc. Burning it on it's own might not be viable (as in with damp kitchen scraps), so the solar would be a good way of incinerating it.

The charcoal / syn gas setup sounds a lot more productive, but also a lot more involved to set up. And potentially hazardous.

registrationsucks2 sez: "But say you've got solar which on it's own gives you something like 600 deg C into a Stirling or whatever. If you were chucking a bunch of burnable things into your cylinder, would you get more heat? And how much?

It just seems like a good way of making use of lawn clippings or compost or old paper, etc. Burning it on it's own might not be viable (as in with damp kitchen scraps), so the solar would be a good way of incinerating it."

You'll obviously get more heat, and equally obviously, I can't quantify how much that might be. However you'll lose efficiency if you're trying to combust damp stuff - a lot of energy will be wasted turning moisture into steam.

In my opinion you might be better off using solar to dry your biomass, then burning it to extract energy.

Trying to combine solar plus combustion in a single unit will likely compromise one mode or the other.

It looks like something that just needs to be played with and see what happens.
Could go either way.