[Digestion] Silicones, siloxanes and the like

Mon Oct 3 07:48:40 CDT 2011

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
Project Management and Engineering Solutions
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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<mailto: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<http://www.completebiogas.com>
Vahid Biogas, an alternative energy consultancy www.vahidbiogas.com<http://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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