Ian Krase

From time to time, people here talk about the potential for burning plastic waste in a rocket mass heater or other stove that reaches high temperatures. And almost always, the conventional response is that one should not do this because one risks forming dioxins by doing so.


I certainly do not wish to take the problem of dioxins lightly, and I understand that "backyard barrel" burning of plastic trash leads to a significant portion of all the dioxin pollution produced worldwide. However, I think that this question calls for serious analysis by an expert in chemistry and/or chemical engineering, and that it likely is safe to burn some types of plastic if the proper precautions are taken. Often the concern about dioxin ignores that different plastics are wildly different in chemistry.


Unfortunately, I am not an expert in chemistry, so I would be very interested if one can critique the following argument:

- Toxic dioxins all contain chlorine, fluorine, or bromine atoms. If there is no source of one of these halogens, no dioxins can be formed.
- Clean polyethylene and polypropylene are some of the most common waste / single use plastics, and have a very simple molecular structure, essentially the same as paraffin wax (or, for that matter, diesel fuel) only much longer and larger. They should burn cleanly if heated hot enough.
- There should not be any source of halogens in these fuels, so no dioxins should be able to be formed.
- Dioxins are mostly a concern when PVC or other plastics that contain halogens or halogen-containing additives are burned.


I can see the following potential issues:
- Do "clean" polyethylene and polypropylene actually contain significant quantities of additives or contaminants that do provide enough halogens to form dioxins?
- Or will the wood or other surface or atmospheric contaminants provide enough halogens? Does the plastic have to be immaculately free of salt for this to work?
- Are there non-halogen-requiring toxins of sufficient potency that the whole project should be dropped (and burning kerosene or wood shouldn't be)?
- Is identifying safe plastic waste to burn a futile task?



I DON'T KNOW the answer to any of these questions, and I think the topic deserves a serious, critical investigation as opposed to the cautionary approach that's been taken so far.

In at least some places, Orchard Supply Hardware stores sell a pack that contains a small piece of three different kinds of gasket material- some kind of rubber, corksheet, and something else.

To explain slightly, you can either put electricity (lots) *into* the peltier module aka TEG and have it be a heat pump / refrigerator, or you can let heat (lots) flow across the peltier module and it will output a small amount of electric power. Most peltier modules are designed for cooling electronics and micro-refrigerators, since that is what they are most practical for. Solid-state TEG/peltier units like we are describing are much less efficient (in either direction) than normal engines/heat pumps/refrigerators with compressors and moving parts, but they are extremely compact and reliable as long as they are not overheated or exposed to moisture (if unsealed).


In the 1950s, there were Russian TEGs used to power small radios from a (large) oil lamp. You had to hang the (hot) lamp outside in the (cold) Russian air. This was in an age when radios still used tubes (eat batteries), and kerosene was more common than batteries and easier to store.

Today, I doubt that in our kinds of applications it is ever going to be more practical to use TEGs rather than solar panels, which are available at less that 1USD a watt versus several USD a watt for the ones I saw at TEGmarket. For cases where the sun is not shining, there are better types of generators such as steam engines or internal combustion engines powered by gasified biomass. They have a kind of narrow temperature range, which is exactly what you don't want for efficiency with heat engines of any kind. The ideal application is something where your high temperature is no more than about 80 C (lest you cook the module) and the cold temperature is not realistically going to be colder than the ground or the weather. So realistically, "rotting compost/manure/whatever in the snow". This will provide miniscule amounts of power. Good heat sinking is absolutely essential.

(What you do want is something like a jet engine running white hot... with air going into the intake at 40 below. Or a gasoline/diesel/steam engine running at room temperature, for normal people.)

(Two realistic applications for this kind of technology that I've encountered: Small fans that automatically run when placed on a wood stove, and wristwatches powered by body heat, though the latter are viewed as a bit of a gimmick and I find my solar watch(s) work fine anyway)

I think that the manufacture and lifetime of solar panels is more ecological anyway, thermoelectrics are made of some weird materials and you need a lot more of them for the same amount of power.

OTOH, you can use peltier modules as heat pumps -- treat them like heating elements, but supply the cold side with something to hold them at room temperature and you will get twice as much heat out as you put electric power in. A specific application that I see for this is a regenerative heater for composting toilet ventilation. Warm air exhausting from the toilet will first pre-heat cold make-up air from outdoors, and then be heated by a small bank of peltier modules that transfer heat from either indoors ambient or from the warm exhaust air. As a result, the toilet can be continuously ventilated and stay warm for composting even in winter. Power would be provided by an independent small solar panel of about a hundred watts.



Another viable use of TEGs is to recover waste heat from anything that produces waste heat in the right temperature range. Some engineering is needed to keep this from just being silly and pointless, but in the right place they can be decisive. squeezing the last watt out of the flue of a rocket mass heater, maybe...?