On Saturday 15 October 2011 06:56:03 Ron Larson wrote: > Trevor, Paul, and lists > > See Table 1 on p 12 of the draft IBI guidelines, which states that > the ash content shall be less than 50%. If you think this is the wrong > level, now is the time to speak up. > > See > http://www.biochar-international.org/sites/default/files/IBI_Guidelines >_for_Specifications_of_Biochars_for_October_2011_Public_Review.pdf >
Ronal, does this refer to total ash after the sample has been incinerated and include ash in the char matrix as well as "free" ash in a partly incinerated air starved fire? I can see the desire to not have loose ash in a product sold as biochar. > > If we gasified all volatile matter, > > the fixed carbon content of the char would rise to about 43%, > > and the ash content would rise to about 57%. > > But typically rice hull biochar has an ash content of about 40%. > > This means that there is still a lot of volatile matter that remains > > in the biochar. > > I've still not seen rice husks so take my comments with a pinch of salt. I'd be careful about using the term "gasified" in the first sentence, it could be taken to mean using sufficient oxygen to deliberately gasify the fixed carbon in the husks. If this is done completely then the incinerated sample would be 100% ash. A pyrolysed sample would keep all the ash locked in a char matrix. The amount of char would depend on the rate of temperature rise ( which we could expect to be high in the thin, low mass hulls) and the final temperature attained in the treatment A typical gasifier would still have some fixed carbon in the ash because the air supply is restricted to below that necessary to react out all the carbon, because things are not perfect and it is better to discard char than have any free oxygen downstream of the gasification process. This is the opposite of the internal combustion process where a slight excess air is necessary to prevent soot and CO being carried into the exhaust. > > In the case of rice hulls, > > the amount of volatile matter that remains in the biochar is > > determined by the rate of gasification. At times the yield in rice > > hull biochar by weight is as low as 30%. At times the yield is as > > high as 50%. Yes, this I would expect, biochar will consist of two parts, the fixed carbon ( which usually is usually dependant largely on lignin content of the feedstock and soots redeposited from secondary reactions) and the higher temperature tars. These latter are volatilised as temperature increases. > > If the rate of gasification is high, high temperatures within the > > reactor are reached. With high temperatures, more volatile matter is > > gasified. Yes, the heat of burning the carbon raises the temperature and this volatilises any surviving tars. > > > > We need operating temperatures well beyond 1000 C before fixed carbon > > gets gasified. Well the fixed carbon will remain static above 900C in an airless environment but if any oxygen is present it will continue to react with any carbon it meets but some carbon will have changed from an amorphous form to less reactive graphene like clumps. I'd like to learn more about how carbon graphitises with increased temperatures, we know diamond will do this but I see little about how biomass derived carbon might undergo this change. http://www.bris.ac.uk/Depts/Chemistry/MOTM/diamond/cphased.gif shows a phase diagram that suggests at low pressures and high temperatures graphite is favoured. > > In my opinion it makes little sense to gasify fixed > > carbon. Then keep air away and quench it fast. My question is: why do you want fixed carbon? It is after all a small portion of normal charcoal. > > > > http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioener > >gylists.org > > > > or, via email, send a message with subject or body 'help' to > > [email protected] > > > > You can reach the person managing the list at > > [email protected] > > > > When replying, please edit your Subject line so it is more specific > > than "Re: Contents of Stoves digest..." ... and please try not to repost whole digests to the list. AJH _______________________________________________ Stoves mailing list to Send a Message to the list, use the email address [email protected] to UNSUBSCRIBE or Change your List Settings use the web page http://lists.bioenergylists.org/mailman/listinfo/stoves_lists.bioenergylists.org for more Biomass Cooking Stoves, News and Information see our web site: http://www.bioenergylists.org/
