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Siloxanes and Biogas

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. This problem can be exacerbated by the fact that silicones-based anti-foaming agents are frequently added to the digesters; these silicones sometimes biodegrade into siloxanes. Unfortunately, when siloxanes gasses are burned, they are usually converted into silicon dioxide particles, which are chemically and physically similar to sand. Needless to say, this can cause significant internal damage to turbines and other motors (just imagine sand grinding away in your automobile’s engine). Siloxanes can also be a problem with biogas used for fuel cells.

According to one report, removal of siloxanes can save a 5 million-gallon-per-day wastewater treatment facility $60,000 to $130,000 per year in operating costs.

Siloxane Removal Options

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.

A Nanomaterials Approach

One rather novel approach to removing siloxanes from biogas is being developed by NEI Corporation of New Jersey, a company that specializes in nanotechnology. NEI is developing a nanomaterials-based drop-in technology designed to significantly enhance the capacity of activated carbon to remove siloxanes and other contaminants from digester and landfill biogas. According to the company, the higher selectivity and ability to regenerate the media will render energy production from biogas more cost effective. The material developed will be environmentally benign and economical.

The materials can be combined to absorb multiple biogas contaminants, and thus could be useful for multiple steps within the biogas purification process. NEI plans to manufacture their materials and sell them to biogas producers, either directly or through intermediaries such as consultants, producers of activated carbons, and others.

The Biogas Market

A biogas cleaning technology is likely to be subject to the overall dynamics of the global biogas market itself. Biogas is a rapidly growing market; one industry report estimates that the total global market size for all biogas equipment is around $3 billion. This market appears likely to continue to expand, shaped by several important drivers. For example, the Kyoto Protocol “Clean Development Mechanisms” initiative calls for the development of power generation with biogas from landfills, wastewater treatment plants, and anaerobic digester plants. This has prompted interest in biogas in regions such as Europe, where the regulatory environment is favorable.

Estimates state that in US there are around 500 landfill gas plants either currently in operation or soon to go online. These plants, along with other opportunities in wastewater treatment and agricultural sites, may present a viable domestic opportunity for biogas related technologies.

Within this market, the primary criterion for a biogas cleaning technology is efficacy, for instance the ability to remove siloxanes to a level where the biogas is no longer a danger to use in delicate motors. Ideally, this would be below the 100 parts-per-billion range. Other desired characteristics include ease-of-use (the ability for a technology to be dropped into an existing biogas production process without extensive engineering), ease-of-handing, and cost-effectiveness.

That being said, this market does appear to present some challenges for the newcomer. Although there does not appear to be any one single dominant product in this market niche, some of the companies active in this area represent major global corporations, including Dow Chemical  and Applied Filter Technology. These companies may comprise formidable competition with which a smaller company may have difficulty competing.

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.

Dick McCarrick is an analyst with Foresight Science & Technology.

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