[Stoves] Pellet stoves - risks

[Frank Shields]

Dear Andrew, Stovers, 

The problems I have had heating to a low temperature of, say, 300c is there is an outside or internal temperature that is much hotter. It could be a red hot coil of wire or glowing biomass combustion. This being the igniter for the flammable volatiles released and collected in a confined space along side the heat source. These gases need be dissipated to non flammable concentration as released and therefore the slow temperature rise I found with my set up to be important. 

So how best to heat biomass to 300c?

1) intermal combustion controlled by air. You will always have a very hot spot but no oxygen to ignite the gases produced. you will have some overly charred material above 300c but could be separated from the product perhaps. 

2) heat a container filled with biomass from the outside. Good control over the product. The gases need be released from the enclosed container and cannot be collected in an outside container where they have oxygen and can combust sending the temperature of the inside biomass to very high levels. This has been my problem.

3) heat from compression during creation of the pellets.  Not sure how hot this can become and if there is a means of letting the gases escape.  

Without a pellet machine and requiring a consistent product I have chosen the second means to make the controlled torrefied product in the lab. 

I have found it to not be as easy as it seems it should. : ) 


Regards

Frank

Frank Shields
franke at cruzio.com




> On Sep 6, 2015, at 11:04 AM, Crispin Pemberton-Pigott <crispinpigott at outlook.com> wrote:
> 
> Dear Andrew
> 
> Can we realistically run a pelleter at 280 C input and save energy? The output is already charring the material. Perhaps that is torrefaction on the fly, so to speak. If so, the MJ/kg should rise with processing. 
> 
> The pellets are have in Java are 8% moisture (Albasia). IF it gets really hot and was bagged, it should be zero.
> 
> Crispin
> 
> 
> Frank on the large scale I doubt it is a problem because as you heat wood up through 100C and drying and then on to the evolution of some VOCs to this 280 C torrefaction range you are always in an endothermic reaction, let it slide above 330C and it becomes mildly exothermic, so with a bit of feed back it should be possible to stay around280C with no thermal runaway. By thistemperature presumably lignin is quite plastic so pelleting power should be lower.I'm not sure what chemical changes are taking place but I'm assuming some hydroxyl groups  are being lost, making the material less able to attract water i.e changing from hydrophilic to hydrophobic, so unlike sawdust pellets torrefied pellets should store better without attracting moisture and disintegrating. What are they like when dropped in a glass of water for an hour or two?
> 
> With large scale it will also be easy to incinerate the low cv offgas given off bythe process.
> 
> AJH
> 
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