[Digestion] Silicones, siloxanes and the like

Tue Oct 4 05:58:42 CDT 2011

Les & all

I think silica in plant tissues is mainly present as silica or silicate.
These are non-volatile inorganic compounds, so they don't evaporate or
digest. That means they end up in the digestate - just like soil particles
(which also contain silica or silicates) - not in the biogas. So there's no
problem, provided your digester is designed to prevent the build-up of grit,
mud etc - either by ensuring very good mixing to keep all solids in
suspension or by allowing for (at least intermittent) removal of sludge from
the lowest level of the digester.

Any digester fed with solid agri-products or by products should be so
designed - or its effective volume will gradually reduce, perhaps to near
zero - because grit, soil, mud etc are inevitable contaminants in the
feedstock. Even small amounts will build up over the years.

*(I think I once posted a report here on a digester treating pharmaceutical
wastes that was found to be ~90% full of a crystalline deposit, with 'rat
holes' linking the feed and discharge pipes to a small cavity around the -
inadequate - stirrer. With the working capacity reduced to the 10% of design
represented by that cavity, it was no surprise that the performance of the
digester had fallen off dramatically over the years it took for the deposit
in question to build up. Not quite the same problem - but a good
illustration of how bad things can get!)*

Burning is a very different process from AD, not only because of much higher
temperatures but also because of the physical disruption of the feedstock/
fuel. Silicon compounds are likely to be oxidized to silica (i.e. sand) and
there may also be carry-over of silica-based fines from soil etc in the
feedstock. That's what ended up on their fire tubes. It could occur with any
fuel - but the problem will be much greater with a silica-rich fuel (eg
anything grass-like, including straw or miscanthus) than with other fuels,
such as wood.

So there's no comparison between the behaviour of miscanthus in a furnace
and in a digester.

Duncan Martin
Cloughjordan Ecovillage
County Tipperary
Ireland

On 3 October 2011 13:48, Les Gornall <Les.Gornall at projen.co.uk> wrote:

>  Plenty of good information in these submissions and I am learning all the
> time!  However, my key point was that in the literature I can find no
> practical experiments on the digestion of plant materials containing high
> levels of the element silicon (however bound in the tissue) with  the
> digestate and gas analysis.  There is real scope for some good doctoral
> level research here and I would be the first to read it!    If you have
> sources I would value a reference or two.
>
>
>
> On a side issue a few months ago I had a verbal  report of some live
> experiments with miscanthus as a dry fuel in a biomass boiler and the
> experiment had to be stopped because of ‘hard deposits’ on the fire tubes.
> We still do not have a reason for this yet – any ideas?
>
>
>
> David and Duncan your endevours in the field of AD are much appreciated.
>
>
>
> *Best Regards*
>
> * *
>
> *Les Gornall*
>
> Process Consultant
>
> PROjEN BioEnergy
>
>
>
> *PROjEN PLC*
>
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>
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>
>
> *From:* digestion-bounces at lists.bioenergylists.org [mailto:
> digestion-bounces at lists.bioenergylists.org] *On Behalf Of *Duncan Martin
> *Sent:* 30 September 2011 17:19
> *To:* david at h4c.org; For Discussion of Anaerobic Digestion
> *Subject:* [Digestion] Silicones, siloxanes and the like
>
>
>
> 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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