Biogas Derived from Food Waste AD Plant - Case Study
STS installed the first monitor on a food waste AD plant in 2017.
The plant in the UK is run by a water company and is co-located with a waste water sewage works although this is not used as feed for the Combined Heat & Power generation units.
The Plant takes in food waste from both urban collections and also a considerable amount from commercial operations including supermarkets, wholesalers and the food manufacturing industry. There has been much dispute about the presence of siloxanes in such plants but the evidence here suggests that there is a significant contamination of the gas supply by siloxanes. The origin of these is a deeper question and is explored in a paper written by David Ward in more detail here: Siloxanes in Food Waste.
To protect their 2 Jenbacher engines the company have installed a pair of 2M3 activated carbon filters and a chilling plant. The chiller is designed to take as much of the moisture out of the gas as possible (as this will rapidly saturate the carbon filters) and also may have the effect of condensing out some of the siloxanes. (consideration should be given to where the condensate is returned to as this is often back into the works therefore essentially recycling the siloxanes) The gas is passed through both filters and then sent to the engine in its now "clean" state.
The Siloxane Monitor is housed in a GRP kiosk outside of any zoned area. This has resulted in long pipe runs for the sample lines to feed the instrument and so in turn STS have supplied and fitted heated lines which prevent the siloxanes from condensing out in the pipework due to diurnal temperature shifts. The heated line comes with a temperature controller housed in the kiosk which maintains the line at approx 60C well above any dew point. This instrument has 2 sample lines, a pre and a post filter which enables the operator to both trend the filter performance and to identify any anomalies in the incoming load which could suggest an issue with the feedstock being used.
Certain feedstocks seem to have much higher siloxane levels than others - perhaps due actually to packaging and coatings than the food itself. On these large plants taking commercial food waste from tankers there is always the chance that the product may be contaminated with cleaning products, oils and lubricants which may all be silicone based. Evidence has shown that silicones can break down in the anaerobic digestion process to form siloxanes.
On this particular site the Siloxane levels normally run at about 10-15mg/m3 on the unfiltered gas and then to less than 0.5mg/m3 after the filter when newly changed. As the filter begins to saturate so the amount of Siloxanes which pass through the filter begin to increase and are reflected in the readings as below.
In this instance the operators have decided to change their filters when the post filter levels hit between 6-8 mg/m3. This prevents the siloxane level breaching the engine manufacturers recommendation and also prevents contamination to the oil which can lead to shorter change intervals. Having the trended data also means that the operator can understand the performance of the filter and predict when the next change should be due -preventing panic ordering of replacement filters and potential engine damage.