[Stoves] torrefied biomass and plastic

[AJH]

On Tue, 11 Mar 2008 11:02:33 +0800, Robert Taylor wrote:

>Looking at my (developed-country) domestic waste stream for its potential 
>energy value, I would guess that the plastics outweigh all the other 
>components, but generating dioxins etc. within the home by burning them in a 
>stove doesn't seem like a clever thing to do. Thus it worries me when people 
>here mention practices such as using shredded plastic bags in briquettes, or 
>styrofoam pellets as gasifier fuel.

I'm no chemist but my understanding is that all biomass burning
contributes some (insignificant?) small amount of dioxins and the
conditions in small stoves will not degrade them.

The plastics that produce dioxins are the halogenated ones (polyvinyl
chloride, neoprene, polytetrafluoroethylene etc) and again to
incinerate these cleanly requires high temperature and rapid
quenching, the mineral acids so formed are problematical to say the
least. So these must be avoided.

I think the plastic films are simple linear compounds and should burn
cleanly given enough oxygen.

Polystyrene (styrofoam is this expanded) is a vinyl polymer, it is
difficult to burn cleanly as the it produces sooty particulates rather
readily and these are difficult to break up and burn out, as it has
phenyl groups linked together I suspect there will be nasty poly
cyclic aromatic compounds in the soot formed.

At the industrial scale the temperature and retention time will be
enough to guarantee these species are broken down to simple oxidation
product but I cannot see this going to completion in a simple cooking
stove. Tom Miles once mentioned a retention period of a small number
of seconds, a brief calculation of mass flow and flame length will
show this does not happen in a simple flame.

AJH