[Stoves] Burning tar

[John Davies]

Thank you Philip, for clarifying the make up of "tars".  I can now share my 
experiences on the burning of tars, with reasonable truth.

If I remember correctly the term for the nasty products of pyrolysis, and 
partial thermal decomposition of heavy oils and bitumen, are known as 
poly-cyclic-aromatics and usually have an offensive odour and are irritating 
to the respiratory tract.  These being common to a smoking wood or coal 
fire.

Can we burn these tars ? I believe that we do, or can do, this quite 
successfully in a TLUD stove, of which my gasifier coal stove is just a TLUD 
type, which is fine tuned to handle large volumes of these products. the 
secret is having a secondary burner which burns these products and other 
gasses in a "close to luminous flame". This is achieved in my coal stove by 
mixing preheated air in a turbulent fashion with the gasses as they leave 
the red hot coal bed.

The lack of coal fire smell ( or wood fire smell ) leads me to believe that 
the tars are completely combusted to CO2 and Water,  as long as there is 
sufficient excess oxygen. I am also of the opinion that as the excess air 
( oxygen ) is reduced in such a system, that the flame temperature tends to 
go higher, due to the reduction of nitrogen in the flame ( 80 % of air ) at 
the expense of producing CO in the flue gas.

This last opinion is based on the literature published on the " gas producer 
fire box" used on steam locomotives. These furnaces produced the greatest 
heat per unit of coal, when the excess air was reduced until the flue gas 
contained up to 15 % of the fuel uncombusted in the form of CO and H2.  The 
experiments that I did with a miniature locomotive using the system showed a 
complete absence of soot and tar in the flue system, as opposed to a 
continual sticky black coating of increasing thickness, with the normal coal 
combustion in the same locomotive.

It was the results of these experiments together with hands on workshops 
conducted with Paul Anderson that led to the current design of my coal 
stove.

I hope that this information is of some value.

John Davies

----- Original Message ----- 
From: "IPC" <ipcipc at mweb.co.za>


> Dragged kicking and screaming into the discussion!  Why me??
>
> "Tar" is a generic term for gunk derived ultimately from the pyrolysis of
> hydrocarbons.  It therefore comprises largely hydrocarbons, but unusual 
> ones
> with multiple ring structures and an excess of carbon - none of this nice 
> 1
> carbon : ~2 hydrogens of your common-or-garden hydrocarbon,