The Development of Technology to Remove Siloxanes from Biogas (P)
Albert Pegus*, Paul D Lickiss*
John Hayward ‡, David Hayward ‡
* Department of Chemistry, Imperial College London, Exhibition Rd,
South Kensington, London, SW7 2AZ
‡ PpTek Ltd, Orchard Business Park, North End Rd, Yapton,
West Sussex, BN18 0GA.
Landfill and sewage sites generate ‘green’ electricity by using the methane gas produced by decomposition of waste to power engines that drive generators. The gas produced from these sites is contaminated with volatile siloxanes such as (Me2SiO)n (n=4, D4; n = 5, D5; n = 6, D6) that form silicon dioxide (silica) during the combustion process. These siloxanes are derived from the many personal care products and other household items that use the desirable surface and structural properties of silicone polymers. The silica deposits on the combustion surfaces as a solid layer that builds up to a level where it is necessary to de-rate the engine to 80% (or less) of its capacity. The deposits also damage the engine particularly the combustion parts, for example, the build up produces collision between the piston and valves causing valves to break and pieces of silica to stick under the valve seats. Frequently an engine with a projected run time before overhaul of 35,000 hours will require strip down and repair at 4,000 – 6,000 hour intervals. The government has a target that 10% green energy be generated by 2010 but siloxane contaminated gas from landfill and sewage sites is preventing efficient green energy being produced from these sources and thus from fulfilling their full potential in this market.
This project aims to remove the siloxanes from methane before the gas enters the engine by passing them through a filter of absorbent material to trap and hold them prior to regenerating the material for further use. Experiments have been developed to characterise the absorption characteristics of various filter media used by PpTek Ltd. to reduce siloxane levels present in biogas. Standard flows of nitrogen containing D4, D5 and D6 at various concentrations were passed through 1.5 g of filter medium at temperatures ranging from 23 – 80 º C. Experiments performed at 23 º C showed a linear relationship for the total mass of adsorbed siloxanes for siloxane concentrations between 1.93 – 6.45 g m3. Temperature variation illustrates that fresh media operates most efficiently at lower temperatures but is capable of removing > 90% of total siloxane content in the gas at 80 º C. Desorbtion experiments of used filter media were performed at 135 º C under atmospheric pressure and vacuum. The results showed that desorbtion under vacuum results in a 5% increase in the removal of volatile organic compounds (VOC’s) compared with the desorbtion under atmospheric pressure. Subsequent adsorption experiments also give evidence that the vacuum desorbed filter media are more efficient (>17%) in removing VOC’s than the atmospheric desorbed media.
We acknowledge funding from the Faculty of Physical Sciences, Imperial College London, via the Fleming Fund for partial support of this project.