Tom,<br><br>When rice hulls are subjected to heat:<br><br><i>... there is virtually no change in size or shape of the particle until<br>~ 200°C. This is followed by a rapid shrinkage between<br>
200 and 400°C. Above 400°C, the particle shrinks at a slow<br>rate and stops shrinking after ~ 800°C...</i><br><br><i>Further heating to 1500°C did not show any change in<br>the particle shape or size. This suggests that the residue is<br>
structurally strong and stable. This phenomenon appears<br>to be unique to rice husk and some other straws. However,<br>most other biomass fuels do not exhibit this behaviour.<br>The particle finally softened and melted at temperatures<br>
above 1525°C...</i><br><br><i>It appears that silica forms molecular bonds with carbon,<br>which are not easily broken at the gasification temperatures.<br>Reactions leading to the formation of silicon carbide<br>from silica involve very high temperatures (above<br>
2500°C) and gasification temperatures are not high enough<br>for such reactions to take place.<br><br>In view of these results, we suggest that full carbon<br>conversion in rice husk may not be achievable. However,<br>this does not diminish the importance of rice husk as a<br>
fuel source because of its widespread and large availability<br>at affordable prices.</i><br><br>On the one hand, we see (previous email) that oxides such as K2O<br> fuse with silica on the surface of the rice husk char <br>
and forms glassy or amorphous phases, preventing the completion of the reaction. <br>This places an upper limit on the gasification process.<br>What happens in this fusion?<br>Is potassium silicate formed?<br><br>On the other hand, we see that silica forms molecular bonds with carbon,<br>
and that this also inhibits full carbon conversion.<br>Even at 1,525 C carbon is not gasified. At this temperature carbon and silica melt.<br><br>Thanks.<br>Paul<br><br><div class="gmail_quote">
On Mon, Oct 17, 2011 at 7:31 AM, Paul Olivier <span dir="ltr"><<a href="mailto:paul.olivier@esrla.com" target="_blank">paul.olivier@esrla.com</a>></span> wrote:<br><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Tom,<br><br>I found this which confirms what you are saying:<br><br><i>The melting point of SiO2 is 1410–1610°C, while that of<br>K2O and Na2O is 350 and 1275°C respectively. It has<br>been suggested that at higher temperatures, the low-melting<br>
oxides fuse with silica on the surface of the rice husk<br>char and form glassy or amorphous phases, preventing the<br>completion of reaction. This places an upper limit on local<br>temperature of the gasifier.<br></i><br>
Thanks.<br><font color="#888888">Paul</font><div><div></div><div><br><br><div class="gmail_quote">On Mon, Oct 17, 2011 at 5:36 AM, Paul Olivier <span dir="ltr"><<a href="mailto:paul.olivier@esrla.com" target="_blank">paul.olivier@esrla.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Tom,<br><br>The coffee husk is composed of several layers that are liberated from one another in the dry milling process.<br>There are some thin soft layers and some thicker hard layers. <br>It would be easy to take measurements with a caliper,<br>
and compare them to the thickness of the rice hull.<br><br>If silica reacts with potassium, do we end up with potassium silicate ( K<sub>2</sub>SiO<sub>3)</sub>?<br><h1>Potassium Silicate</h1>
<p>A liquid source of potassium and silica. Silica is often a forgotten element and
has many benefits in the plant including strengthening cell walls.</p>
<h2>Benefits</h2>
<ul><li>Biological Farmers of Australia (BFA) Registered Product 456AI.</li><li>Improves photosynthesis and raises brix in all plants.</li><li>Reduces the negative effects of excess aluminum, sodium and manganese.<br>
Improves plant growth, lifting yield and quality.</li><li>Enhances reproduction by improving pollination and increasing pollen fertility.</li><li>Produces an armour-like resilient layer in the outer cell wall resulting in:</li>
<li>Reduced lodging.</li><li>Decreased plant palatability.</li><li>Less fungal colonisation sites.</li></ul>Thanks.<br><font color="#888888">Paul</font><div><div></div><div><br><br><div class="gmail_quote">On Mon, Oct 17, 2011 at 3:36 AM, Tom Miles <span dir="ltr"><<a href="mailto:tmiles@trmiles.com" target="_blank">tmiles@trmiles.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex"><div link="blue" vlink="purple" lang="EN-US"><div><p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D">Paul,<u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D">The coffee husks I have seen are paper thin. Is your fuel like that or thicker? If it is think then it will “flash” pyrolyze. The rice husks would provide structure and could dampen the evolution or loss of nutrients. We have seen silica absorb potassium in the gas stream. So I don’t think that you can assume that you will lose it all even if your raw fuel reaches peak temperatures of 800-1000 C. The char probablu sees 800-900C. <u></u><u></u></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D"><u></u> <u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D">Tom<u></u><u></u></span></p><p class="MsoNormal"><span style="font-size:11.0pt;color:#1F497D"><u></u> <u></u></span></p>
<div style="border:none;border-top:solid #B5C4DF 1.0pt;padding:3.0pt 0in 0in 0in"><p class="MsoNormal"><b><span style="font-size:10.0pt">From:</span></b><span style="font-size:10.0pt"> <a href="mailto:stoves-bounces@lists.bioenergylists.org" target="_blank">stoves-bounces@lists.bioenergylists.org</a> [mailto:<a href="mailto:stoves-bounces@lists.bioenergylists.org" target="_blank">stoves-bounces@lists.bioenergylists.org</a>] <b>On Behalf Of </b>Paul Olivier<br>
<b>Sent:</b> Sunday, October 16, 2011 1:23 PM</span></p><div><br><b>To:</b> Discussion of biomass cooking stoves<br><b>Subject:</b> Re: [Stoves] why does coffee husk biochar smell like urine?<u></u><u></u></div>
<p></p></div><p class="MsoNormal"><u></u> <u></u></p><p class="MsoNormal" style="margin-bottom:12.0pt">Alex,</p><div><div></div><div><br><br>Thanks so much for this graph.<br><br>Alexis Belonio says that the temperature within his gasifiers on rice hulls reaches 1,000 C.<br>
If this is true in the case of rice hulls, <br>then the temperature within the reactor in the case of coffee husks is a good deal higher.<br>If this is true, a lot of the K is lost in the coffee husk gas.<br><br>Coffee husks biochar has a moment at which it swells or expands during gasification.<br>
The volume of char actually increases within the reactor.<br>But if I shake the reactor during the process, the volume of char decreases enormously.<br>If left unshaken, large cracks appear in the char, and the char forms clumps.<br>
<br>When I added rice hulls to the coffee husks on the one occasion,<br>cracks did not appear in the biochar within the reactor, and the burn was far more uniform.<br>The production of soot decreased a lot.<br><br>A few months ago, I put coffee husks in a large pan and attempted to torrefy them using gasifier heat.<br>
During this process, a lot of fumes were produced, and the coffee husks lost their oily aspect.<br>When I gasified these "torrefied" husks, the flame shifted from orange to a color in between orange and blue.<br>
<br>This opens up the possibility of torrefying the coffee husks to produce acetic acid and other compounds.<br>Just as we torrefy the coffee bean, we would torrefy the husk.<br>This would leave behind a solid that would no longer absorb water,<br>
as well as a solid that would thermally decompose at high temperatures in a superior manner.<br>I have no idea if this makes sense.<br><br>Thanks.<br>Paul<br><br><u></u><u></u></div></div><p></p><div><div></div><div>
<div><p class="MsoNormal">On Sun, Oct 16, 2011 at 7:58 PM, Alex English <<a href="mailto:english@kingston.net" target="_blank">english@kingston.net</a>> wrote:<u></u><u></u></p><p class="MsoNormal">Paul,<br> Attached is minimalist, two points, graph showing potassium volatility beginning at 400C with a ~37% loss at 800C.<br>
Courtesy of Preto and Hrbek;<br><a href="http://www.biochar.ca/files/Can%20Biochar%20Initiative%20Preto%20Dec08.pdf" target="_blank">http://www.biochar.ca/files/Can%20Biochar%20Initiative%20Preto%20Dec08.pdf</a><br><span style="color:#888888"><br>
Alex</span><u></u><u></u></p><div><div><p class="MsoNormal"><br><br><br>On 16/10/2011 2:25 AM, Paul Olivier wrote:<u></u><u></u></p><p class="MsoNormal" style="margin-bottom:12.0pt">Tom,<br><br>But the temperature in the reactor is much higher than 350 C.<br>
I have seen a sort of clinker in the biochar if I do not turn the fan off at the end of a batch.<br>At what temperature does the vaporization of K take place?<br><br>Paul<u></u><u></u></p><p class="MsoNormal"><u></u> <u></u></p>
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