[Stoves] Gasifying coal?

[John Davies]

Greetings All,

Yours truly is now ready to continue with the "gassifier coal stove" 
research and development, after a self imposed exile of about 2 1/2 years. 
Starting today I will slowly get back into the mainstream and should be well 
into my work by the middle of next year.

Reading the latest mails on coal gasification prompts me to expand a little 
on the principles on witch my stove works.

Gasification is the heart of the process, but not exclusively the only 
process involved. The combustion principles follow that of any TLUD stove 
with a fuel cell combusting the fuel in such a way that a volume of 
combustible gas leaves the fuel bed to be burned in a close coupled 
combustion chamber.

During the first stage of the burn, there is pyrolisis generating a large 
volume of gas, but also gasification taking place in the hot coke above the 
pyrolisis zone as well as combustion of coke and some pyrolysis gasses in 
the fuel bed. I am convinced that a large portion of the CO2 produced is 
converted to CO before leaving the bed. I have no way of determining exactly 
what is happening in the various processes, but believe that all the 
processes are taking place in varying proportions, depending on the depth of 
hot coke above the pyrolisis zone.  At this stage a large volume of air is 
fed to the gas burner giving a smokeless exhaust from about a minute after 
lighting and for the duration of the burn. Naturally there are sulphur 
oxides in the exhaust, but judging from the lack of "burning coal smell" I 
come to the conclusion that all the tars and gasses are burned to 
completion, or very close to that.

The second stage of the burn consists of a combination of gassifying and 
burning the residual coke, with a CO flame present in the burner. The air to 
the burner at this stage is reduced to minimum which has the effect of 
drawing a much larger volume of air through the coke bed.

The draft for both stages is induced by a 2.7 m chimney, and the only 
control is that of the secondary air ( gas burner ). the primary air flow 
( fuel bed ) is controlled by the flow resistance of the coal bed and the 
draft remaining after addition of secondary air.

There is no chimney damper, which is essential, in order to have all parts 
of the stove running below atmospheric pressure. This should be a 
prerequisite of any indoor stove with a chimney exhausting outside. This 
allows for a construction that does not have to be perfectly sealed, as any 
leakage will always be air into the stove, with zero toxic emissions into 
the house.

I am satisfied with the performance of the existing prototype, but need to 
determine suitable materials which can withstand the combustion process, 
while being relatively cheap and easy to work with. This stove is strictly a 
batch burn stove, with the coal charge being predetermined for the burn time 
required,

I am hoping to adapt the process to a continuous burn stove, and have a 
partially built 1st. prototype, which awaits further time and effort.

The Gasification process creates a unique problem. CO at high temperatures, 
reacts with the steel wall of the fuel unit turning the sheet steel liner 
into a compound of iron oxide and carbon.  while the tube looks sound, it 
becomes quite brittle, and cracks and breaks like an egg shell.  My thoughts 
at this stage tend to opt for an enamel lining to protect the steel. 
Stainless steel might give a longer life, but also suffers the same fate, is 
expensive and difficult to work with.

I look forward to joining the discussion once again.

John Davies,
Secunda,
South Africa.


----- Original Message ----- 
From: "Crispin Pemberton-Pigott" <crispinpigott at gmail.com>
To: "'Discussion of biomass cooking stoves'" <stoves at listserv.repp.org>
Sent: Thursday, December 13, 2007 8:18 AM
Subject: Re: [Stoves] Gasifying coal?


> Dear Dr T
>
>>...and was
> delighted to find that it burned longer even than wood pellets, being so
> dense.
>
> Great stuff.  John Davies says that if you change the air flow (if I 
> recall
> correctly) it will gasify the coke too.
>
> I have a feeling that a fan stove is going to work better than variable
> chimney draft with small coal stoves
>
> Regards
> Crispin
>
>
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