<div>There's a common confusion here - a familiar one between chemical substances with similar-sounding names.......</div><div><br></div><div>Just as <b>alkanes</b> (eg methane, propane, hexane and all the paraffin family) are not remotely related to<b> alkalis</b> (eg caustic soda and similar metal hydroxides), so <b>silicon</b> (a non-metallic element) is only distantly related to the <b>silicone, siloxane</b> and other synthetic compounds that can cause minor problems in landfill gas and the digestion of municipal wastes. </div>
<div><br></div><div>Broadly speaking, <b>silicon</b> is commonly found in plants (esp grasses) in the form of silica (silicon oxide) and in many minerals as <b>silica </b>(eg sand) or <b>silicates</b>. These materials aren't digestible, so they cause no problems in biogas. They may accumulate in your digester or cause wear of moving parts - but many other minerals can do the same. Good design should deal with such issues easily enough.</div>
<div><br></div><div><b>Silicones, siloxanes</b> etc are fairly exotic synthetic compounds used in small quantities (usually) in many modern products - ranging from cosmetics via sealants to breast implants. They resemble organic (carbon-based) compounds but with the carbon replaced by silicon. They commonly include hydrogen and oxygen as well as silicon - and maybe other elements too.</div>
<div><br></div><div>When these compounds get into any kind of digestion system, traces can end up in the biogas - and when they burn, they form silica. Sand, in effect. In a heating application, this shouldn't be a huge problem. However, in any kind of gas engine used for electricity generation, the effect is similar to regularly slipping a few grains of sand into the fuel tank of your car. Not good for engine life! </div>
<div><br></div><div>It follows that silicon compounds would not be expected in the biogas produced from the digestion of crops, manures and other natural substrates, even if the feedstock is high in silica etc. </div><div>
<br></div><div>Moral 1: don't worry about silica!</div><div>Moral 2: read the names of chemicals with care - and don't guess about relationships!!</div><div>Moral 3: remember that tabloid reporters who write about aspiring starlets getting "silicon" implants probably ended up in that job because they failed Science 1.01!!!</div>
<div><br></div><div>Duncan Martin</div><div>Cloughjordan Ecovillage</div><div>Ireland</div><div><br></div><div><br></div><br><br><div class="gmail_quote">On 27 September 2011 23:26, David <span dir="ltr"><<a href="mailto:david@h4c.org">david@h4c.org</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div bgcolor="#FFFFFF" text="#000000">
<font face="Georgia"><br>
Les, all,<br>
</font><br>
On 9/23/2011 11:19 AM, Les Gornall wrote:
<blockquote type="cite">
<pre>Good question! The [arundo donax (giant reed)] is unique in it's ability to lock silicon into its stem tissues. That's why it is used for making oboe reeds. </pre>
</blockquote>
<br>
A more ubiquitous high silica plant, at least around here (Oregon)
where I live is equisteum, variously known as scouring rush and
horsetail.<br>
<br>
<blockquote type="cite">
<pre>When digested the silicone would most likely be expresses as siloxanes in the Biogas. But I have not seen any data. </pre>
</blockquote>
<br>
I tend to doubt that any resulting biogas would have siloxanes in
it. It's not my area of expertise, but my understanding is that the
siloxanes found in landfill gas are not "built up" from any natural
silica compounds, as would have to be the case if they were to be
produced simply because one is digesting natural organic matter that
has a lot of silica, but rather they result from the decomposition
of artificial silicates:<br>
<br>
<blockquote>
Volatile Methyl Siloxanes (VMS) are the result of hydrolysis of
Polydimethylsiloxane (PDMS), an organosilicon compound which is
used in a wide range of consumer applications. Due to its
widespread use, PDMS is deposited in landfill where it degrades in
lower molecular weight compounds (VMS). PDMS can also enter the
wastewater treatment plants were, because of its insolubility in
water, partitions to the sludge. Consequently, when this is fed to
anaerobic digester, PDMS can hydrolyse to VMS.<br>
<br>
The species detected by now in biogas are
hexamethylcyclotrisiloxane (D3), decamethyl-cyclopentasiloxane
(D5), octamethylcyclotetrasiloxane (D4), hexamethyldisiloxane
(L2), octamethyltrisiloxane (L3). The concentration values depend
on the origin of the biogases. [found <a href="http://www.tf.uniag.sk/Biohydrogen/state.htm" target="_blank">here</a>]<br>
</blockquote>
<br>
<br>
I am not familiar with giant reed grass, but I have seen reeds of
the sort used in instruments, and clearly one of the main
characteristics of those reeds is their stiffness; they have to
vibrate to fulfill their function. If that is characteristic of the
plant generally-- stiff, woody-- then I think it would make a very
poor biogas substrate unless it was harvested while still at a
tender stage, or perhaps exposed to fungi post-harvest. I would in
general assume that one could learn more about pretreatment options
by researching what is done with more widely used crops of the same
general sort-- miscanthus, switchgrass, et al.<br>
<br>
<br>
d.<br><font color="#888888">
<div>-- <br>
<div style="font:Georgia"><span style="font-size:110%">David
William House<br>
</span>
<div style="padding-left:3em;font-size:80%">"The Complete
Biogas Handbook" <code><a href="http://www.completebiogas.com" target="_blank">www.completebiogas.com</a></code><br>
<em>Vahid Biogas</em>, an alternative energy consultancy <code><a href="http://www.vahidbiogas.com" target="_blank">www.vahidbiogas.com</a><br>
<br>
</code></div>
<div style="padding-left:2em">"Make no search for water.
But find thirst,<br>
And water from the very ground will burst."
<div style="padding-left:2em;font-size:80%">(Rumi, a Persian
mystic poet, quoted in <em>Delight of Hearts</em>, p. 77) <br>
<br>
<a href="http://bahai.us/" target="_blank">http://bahai.us/</a></div>
</div>
</div>
</div>
</font></div>
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