[Digestion] Silicones, siloxanes and the like

Fri Sep 30 11:27:01 CDT 2011

For what its worth one thing we are seeing in certain areas is some level 
of siloxanes in biogas applications where industrial grade detergents are 
used for washing down of areas and flow into the biogas feedstock. 

Duncan Martin <duncanjmartin at gmail.com> 
Sent by: digestion-bounces at lists.bioenergylists.org
09/30/2011 12:23 PM
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[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]

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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and the Biogas Wiki http://biogas.wikispaces.com/

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