Siloxanes the Elephant in the Cupboard for MSW Biogas?

As more municipal waste biogas plants come on stream, which use mechanically separated "black bag" waste, there will be impurities persent, and there are concerns that one of those impurities "siloxane" will turn out to cost a lot of money to remove. Also, Anaerobic Digestion Plant operators will experience costly downtime for repairing badly damaged gas engines. We all know that unreliable power output costs in terms of the tarrif paid by the recieving electricity company, so this ends up as a double whammy. This could be an elephant lurking in the cupboard as the rapid uptake of municipal waste to energy fermentation just gets going.

That is why we were particularly interested to read the article excerpted below. We have included only part of this excellent and well-informed article below. So, we expect that many of you will follow the link below and visit the original web site for the full article. All comments giving your views on whether this danger is significant will be apperciated:

Siloxanes are a subgroup of silicones containing Si-O bonds with organic radicals. They are widely used for a variety of industrial processes. They are also commonly added to consumer products, including detergents, medical products and devices, shampoos, cosmetics, paper coatings, and textiles. Although most siloxanes disperse into the atmosphere where they are decomposed, some end up in wastewater (approximately 17,000 tons per year in the US).  Siloxanes do not decompose in the activated sludge process, but generally end up as a significant component in the sludge.

As sludge undergoes anaerobic digestion, it may be subjected to temperatures up to 60 C. At this point the siloxanes contained in the sludge will volatize and become an unwanted constituent of the resulting biogas.

List of Siloxane Removal Options

The author has provided the folloing list of methods for siloxane removal:

Currently, there are six primary technologies for removing siloxanes from biogas. These include the following:

–Activated carbon is widely used to remove organic substances from gases and liquids due to its superior adsorbent properties. An example of a product based on this technology is SAGPack from Applied Filter Technology.

–Activated alumina absorbs siloxanes from biogas. When the alumina becomes saturated, its absorption capability can be recovered by passing a regeneration gas through it.

–Refrigeration with condensation in combination can be used to selectively remove specific compounds by lowering the temperature or pressure of the gas, and then allowing the compound to precipitate out to a liquid, and then settle out.

–Synthetic resins remove VMS’s through adsorption. They can be specially formulated to remove specific classes of compounds.

–Liquid absorbents are used by a small number of landfill operators to treat biogas prior to use in combustion devices such as gas turbines. An example of a product based on this technology is Selexol from Dow Chemical.

–Membrane technology is a relatively recent development in siloxanes removal. At present, however, membranes are subject to acid deterioration from the acidic content usually found in raw biogas.

Of these technologies, activated carbon appears to be among the most dominant in the industry. This material can be purchased relatively cheaply, for less than $1 per pound when purchased in bulk form.

Although there are a number of options available for cleaning contaminants such as siloxanes from biogas, none has yet proven to be “ideal.” Therefore there is active interest in the development of new and better technologies for cleaning biogas.

It appears that siloxanes comprise a very significant problem for the production and use of biogas. End users are demanding efficacy, and they also require ease of use in terms of being able to drop the technology into existing biogas treatment processes without significant re-engineering. End users also demand adaptability to accommodate the varying nature of biogas streams and the various types and amounts of contaminants that need to be treated. And of course, as always users are looking for the most cost-effective options.

A technology that can satisfy these criteria appears well-positioned to be readily introduced and adopted into this market, and in turn could help biogas achieve its full potential as an important component of the worldwide energy mix.

View the original article here