[Digestion] Silicones, siloxanes and the like

[Duncan Martin]

There's a common confusion here - a familiar one between chemical substances
with similar-sounding names.......

Just as *alkanes* (eg methane, propane, hexane and all the paraffin family)
are not remotely related to* alkalis* (eg caustic soda and similar metal
hydroxides), so *silicon* (a non-metallic element) is only distantly related
to the *silicone, siloxane* and other synthetic compounds that can cause
minor problems in landfill gas and the digestion of municipal wastes.

Broadly speaking, *silicon* is commonly found in plants (esp grasses) in the
form of silica (silicon oxide) and in many minerals as *silica *(eg sand) or
*silicates*. 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.

*Silicones, siloxanes* 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.

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!

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.

Moral 1: don't worry about silica!
Moral 2: read the names of chemicals with care - and don't guess about
relationships!!
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!!!

Duncan Martin
Cloughjordan Ecovillage
Ireland




On 27 September 2011 23:26, David <david at h4c.org> wrote:

>
> Les, all,
>
> On 9/23/2011 11:19 AM, Les Gornall wrote:
>
> 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.
>
>
> A more ubiquitous high silica plant, at least around here (Oregon) where I
> live is equisteum, variously known as scouring rush and horsetail.
>
>  When digested the silicone would most likely be expresses as siloxanes in the Biogas.  But I have not seen any data.
>
>
> 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:
>
>  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.
>
> 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 here<http://www.tf.uniag.sk/Biohydrogen/state.htm>
> ]
>
>
>
> 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.
>
>
> d.
> --
> David William House
>  "The Complete Biogas Handbook" www.completebiogas.com
> *Vahid Biogas*, an alternative energy consultancy www.vahidbiogas.com
>
>  "Make no search for water.       But find thirst,
> And water from the very ground will burst."
> (Rumi, a Persian mystic poet, quoted in *Delight of Hearts*, p. 77)
>
> http://bahai.us/
>
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