Methods of Siloxane Removal from Biogas
There are at least 5 methods of siloxane removal from biogas which have been used. This remarkable range of methods is described below:
Activated Carbon Technique
Activated carbon is widely used to remove organic substances from gases and liquids because it has excellent adsorbent properties and a large surface area.
A proprietary gas filtration process has been devised based upon the catalytic oxidizers. This method has been used on 50 MW gas turbines which were degrading rapidly from silica fouling.
This catalytic gas filtration process was designed by Filtration and Media Group (FMG) of Union Gap, WA, now thought to be part of Applied Filter Technology, AFT.
The FMG process consists of a series of adsorber vessels containing polymorphous porous graphite (PPG), which is a specialized form of activated carbon.
Each bed is used in rotation before it requires regeneration, which takes place off-site.
We understand that the process uses SAGTM technology (segmented activity gradient) developed by Applied Sorbent Technologies (AST) of Bellevue, WA and now also now part of Applied Filter Technology, AFT for the removal of poorly adsorbed species in the presence of high concentrations of competing organic contaminants in air and gas streams.
Activated Alumina Method
Siloxane contaminants are in theory removed by passing biogas through a bed containing activated alumina, which absorbs the siloxanes. The alumina eventually reaches saturation.
When the activated alumina becomes saturated with siloxanes, the absorption capability of the activated alumina can be recovered by passing a regeneration gas through the bed of activated alumina, however, it would be necessary to obtain specialist advice before doing this.
An internet site reference reports that a system containing two or more beds of activated alumina can use one bed to remove siloxanes from biogas while one or more of the other beds are being regenerated.
Refrigeration with Condensation in Combination
Condensation can be used to preferentially remove selected compounds by lowering the temperature or pressure of the gas and letting the compound condense to a liquid form so it can settle out. Condensing can be achieved by either refrigeration or through de-pressurization of a pressurized system.
The City of Dayton WWTP in the US, is reported installed a three stage condenser to remove siloxanes and various other deleterious substances. The condenser has been described as successful.
Synthetic Resins
Synthetic resins, like activated carbon, can remove VMSs through adsorption onto their surface. They have surface area densities (i.e., surface area to mass ratio) equivalent to activated carbon, and can be specially formulated to remove specific classes of compounds.
A pilot scale project,has been reported as having been designed by American Purification, Inc., of Newport Beach, CA. It was a regenerative vapor recovery system using a polymeric adsorbent. The pilot unit processed 50 cfm of gas, and reported removal of siloxane compounds to non-detectable levels.
According to American Purification, regeneration of a full scale unit is accomplished using microwave technology. The regeneration is completed in a vacuum under a nitrogen blanket and into a condensation system to simplify disposal.
Liquid Absorbents
 Liquid absorbants are reported to have been used in at least seven installations in Europe to remove organics and siloxane from landfill gas. In the US, liquid absorption is also being it is reported, being used by a handful of landfill operators to treat biogas prior to use in combustion devices such as gas turbines.
One is called SELEXOL as manufactured by Union Carbide. SELEXOL is designed to provide not only siloxane removal but also CO2, H2S, COS, mercaptan, and BTEX removal from biogas.
Ecogas, Inc. a division of Getty Synthetic Fuels, is thought to have developed a sytsme using the Union Crabide product.
Their system is reported to use SELEXOL in conjunction with a condenser and activated carbon. Reported removal efficiencies are high, and the method may be suiatble for use for higher purification of biogas for upstream utilization.
Membranes
Membrane technology is a technology which has moved ahead considerably in recent years advancing in recent years, and a number of pilot scale installations have been operated to upgrade biogas to pipeline quality using membrane technology.
A project has reportedly been completed in New York state to remove CO2 and organics from digester gas.
There is no doubt potential for membrane use, but first the question of acid deterioration from the acidic content that is usually seen in biogas, will need to be resolved.
Also see:
Yahoo Answers here.
Further sources of information on siloxane removal from landfill gas are the specialist landfill gas engine and turbine equipment manufacturers.
We have further information on trace components in landfill gas on the following page:
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