We have had the cristobalite discussion before but it must have been on the biochar list. At the small scale with relatively low temperatures we haven’t seen good documentation of cristobalite formation or assessment of its health hazard. Apparently sodium promotes formation of cristobalite at lower temperatures (~900C) but I do not know of a good reference that has analyzed cristobalite in local, low temperature combustion such as stove applications.
Cristobalite is a hazard with high temperature combustion (>1200C) as in suspension burning. Industrial rice husk burners often limit peak temperatures to about 750 C to minimize cristbalite formation. In industrial gasification and combustion of rice husk ash we have seen cristobalite formation in quantities of concern when the processes are exposed to higher temperatures. Australians measured cristobalite in biochar from an industrial gasifier that is highly oxidative, that is, it has high temperature zones, When you are burning charcoal in direct contact with air you get temperatures that greater than 2500F (1300C). In the US rice husk ash from a similar process is classified by OSHA as a hazardous material, which means that appropriate precautions must be taken for its handling and use. Tens of thousands of tons of rice husk ash are used for a variety of purposes. For an analysis of rice husk ash that is transformed by high temperature industrial processes, and a Material Safety and Data Sheet for handling it see http://www.scott-glenn.com/analysis.html Tom From: Stoves [mailto:[email protected]] On Behalf Of Kevin Sent: Friday, June 14, 2013 5:38 AM To: Discussion of biomass cooking stoves; Paul Olivier Cc: Jon Anderson Subject: Re: [Stoves] Fw: : Re: Insulation and stove life Dear Rebecca Firstly, I am concerned about the "Cristobalite Link" below, in that it seems to overlap "silica", "cristobalite", and other forms of quartz. See: http://www.quartzpage.de/gen_mod.html showing how various forms of quartz exist at various temperatures. Note that "cristobalite" is one specific crystalline phase of quartz. Secondly, what is important is the "respirable" dust that is actually respired. It is well known that respiring "silica dust" can cause "Silicosis". It is well known that "Small Particulate Matter emissions" from virtually any poor stove can cause serious health problems. Hence, the effort to design "stove systems" that minimize "Small Particulate Matter Emissions" into the living space, where they can potentially be respired. Rice Hull Ash, even at 90% silica in the ash pit, is not a problem, in that it is in the ash pit, where it is not respired. On the other hand, an "apparently superior fuel" that only had say 25% silica content in the ash pit would be vastly more hazardous, if it vented 10 or 100 times as much ash into the living space, in respirable form. This is where competent and meaningful stove design and testing comes into play. As has been said many times, "It is not so much the fuel, but the stove system design, that is good or bad." Best wishes, Kevin ----- Original Message ----- From: Rebecca A. Vermeer <mailto:[email protected]> To: Paul Olivier <mailto:[email protected]> ; [email protected] Cc: Jon Anderson <mailto:[email protected]> Sent: Friday, June 14, 2013 3:44 AM Subject: [Stoves] Fw: : Re: Insulation and stove life Hello Paul, Larry just told me that the silica content of rice hull ash is over 90%. At the ETHOS 2013 Conference, I saw a TURBO stove developed in the Philippines which used rice hull for fuel. Given your comment below regarding cristobalite “which is a nasty carcinogen” and severely hazardous to human health (see link below), would you recommend the use of rice hull as a household fuel for cookstoves? Rebecca Vermeer CRISTOBALITE LINK: http://nj.gov/health/eoh/rtkweb/documents/fs/1657.pdf From: Paul Olivier <mailto:[email protected]> Sent: Wednesday, June 12, 2013 12:01 AM To: Discussion of biomass cooking stoves <mailto:[email protected]> Subject: Re: [Stoves] : Re: Insulation and stove life Paal, One thing I look for on my burner is that all burner holes support a flame throughout the process. If channeling occurs during the process or if char is being burned as the process comes to a close, then one can see burner holes that do not support a flame. This means that CO2 is being discharged from the burner holes, and of course CO2 does not burn. When CO2 is formed, this represents a big inefficiency, since combustion takes place far below the pot. When this happens the sides of the reactor can easily turn red hot and melt. I do not know how it is possible to spot the presence of CO2 if the top of the reactor stays open and does not have a lid with burner holes. If one turns up the fan a bit too high resulting in channeling, it can happen that only a few holes (among a total of 80 in my case) do not support a flame. If I turn the fan down a bit and shake the reactor, this problem is immediately corrected. Also the effect of the presence of CO2 can be spotted by the cook in another way. The distribution of heat to the pan is not even. Also many of the positive characteristics of biochar are lost when biochar is combusted and is reduced to ash. The combustion of biomass and biochar takes place when channeling occurs, and the combustion of biochar takes place if the fan is not turned off at the end of the process. Rice hull ash and rice hill biochar are not at all the same thing when it comes to growing plants. Also rice hull ash can easily contain cristobalite, which is a nasty carcinogen. Under ordinary conditions, no farmer should be handling this stuff. Thanks. Paul On Wed, Jun 12, 2013 at 1:06 PM, Paal Wendelbo <[email protected]> wrote: Ron By end of flame the color of the char is red to yellow, that indicate a temperature of 700 to 800 ˚C and when there is no smoke, complete combustion has taken place. Is that not good for biochar? Regards Paal W _______________________________________________ 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://stoves.bioenergylists.org/ -- Paul A. Olivier PhD 26/5 Phu Dong Thien Vuong Dalat Vietnam Louisiana telephone: 1-337-447-4124 (rings Vietnam) Mobile: 090-694-1573 (in Vietnam) Skype address: Xpolivier http://www.esrla.com/ _____ _______________________________________________ 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://stoves.bioenergylists.org/ _____ _______________________________________________ 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://stoves.bioenergylists.org/
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