Activated carbon filters work by passing the biogas derived from the AD plant through a metal tower filled with activated carbon. Normally the gas will have been dried by running through a chilling plant to remove excess water which would be absorbed by the Carbon.
The siloxanes and other contaminants are trapped by the carbon allowing the filtered gas to pass through and into the engine. The size and design of Carbon filters will depend on the site specifics, flow and anticipated Siloxane loading, often sites will implement a set of towers in series which are rotated to get best optimisation of the Carbon. The issue here is that optimisation is based on a very small subset of data taken from bag samples submitted to the lab for analysis which have been shown to be subject to influence from time and temperature causing condensing of the siloxanes onto the bag walls and a resultant variable result. In house tests by STS managed to achieve figures of +/- 100% by manipulating these factors.
Bag analysis is also relatively expensive so users tend to do as few tests as they can – and then rely on the results to produce a schedule for changing the filters. In a perfect world that would be fine, but that assumes no change in the loading of siloxanes received into the plant and a consistent performance of the Carbon and consistent water removal from the gas stream – three things which simply don’t happen. The solution is to monitor the gas flow to the engine post filter on a regular basis, in the case of the STS Siloxane Monitor that is hourly. This provides the user with a constantly updated dataset on which to make informed decisions regarding the correct time to replace the filters. This is a win-win situation, changing the filters too early when Siloxane breakthrough has not occurred will cost unnecessary money and changing the filter too late when breakthrough has already occurred will costs even more money in damage to engine components and degradation of engine oil quality and life time.
STS installed a monitor in 2018 on a major utilities waste water treatment works. When first installed the customer was running the gas through an old and un-serviced regenerative filter system with the gas being supplied to multiple CHP engines. Oil life was at approx 300 hours and breakdowns from siloxane build up on heads and plugs a constant issue. After about 8 weeks the utility brought on line a pair of 6 m3 activated carbon towers and routed the gas to one engine through these.
The results were fairly dramatic, total siloxanes dropped from a peak of 340 mg/m3 to less than 1.0 mg/m3 and have remained below 3 mg/m3 for over a month. Since the introduction of the filters the oil life has extended to over 800 hours and the engine has only required the replacement of 2 spark plugs with no major overhaul or downtime. The monitor is now being used to assess the life span of the filter and to identify a rising trend to enable optimal replacement to maximise the carbon usage and to protect the engine.
Following this initial installation the utility is now working to put carbon filtering in front of all their engines and the STS Siloxane monitor will be re-sited to monitor the filter series.
