The pre-purification of biogas is essential to optimally convert biogas into green gas. With this in view, a large Italian processor of household waste, along with a pioneering company in upgrading bio and process gases decided to rely on DESOTEC’s expertise.

The company based in northern Italy provides the collection and processing of household waste. Plastic waste is separated from biodegradable material by means of sorting systems. Fermentation of the biodegradable fraction generates biogas, with which the company produces green electricity and heat. The company recently invested in an upgrade of the fermentation infrastructure in order to be able to produce bio methane to the same quality as natural gas. For this it now has five biogas lines, each with a capacity of 1,250 m³/h.

For the pre-purification of untreated biogas – in which volatile organic compounds (VOC’s) and H₂S must be removed from the biogas – this company initially planned to invest in fixed activated carbon filters, which would have had to be emptied and filled using big bags and requiring human intervention. This approach turned out not to be practically feasible for a variety of reasons - unhealthy and too expensive. It not only demands considerable investment in permanent activated carbon filters and a lot of man hours, but also a great deal of dust and emissions from contamination would be released on each occasion during replacement.

DESOTEC, which became involved in the project only at a later stage, therefore suggested working with mobile activated carbon filters. Our customer was immediately convinced by this approach and made a number of adjustments to the site that facilitated this. A number of concrete tips allowed the company to make considerable savings in costs by further reducing the consumption of activated carbon.

To this end, DESOTEC installed two mobile activated carbon filters in series on each biogas line. These are filters of type AIRCON BG, each of which is filled with 11 tons of activated carbon. By using the ‘lead-lag’ principle, the second filter is able to capture contamination penetrating through once the first filter starts to fill up. We are constantly able to make maximum use of the take-up capacity of the activated carbon by means of this rotation system. In the interim the purification process continues to run optimally.

This purification allows the original quantity (3 to 5 g/m³) of volatile organic compounds to be radically cut to below the detection limit. We also supply impregnated activated carbon, which is essential for removing H₂S (hydrogen sulphide) from the untreated biogas. Our concrete optimisation tips also effectively resulted in lower consumption of activated carbon. So the system completely complies with the expectations, to the great satisfaction of this customer.

At DESOTEC’s facilities, all used carbon is analysed so the right measures can be taken for handling and removing the saturated carbon out of the mobile filters. All molecules that were adsorbed on the activated carbon at the customers’ site, are desorbed inside DESOTEC’s reactivation furnaces. These contaminants are then fully destroyed, in accordance with National and European legislation, by an incineration and neutralisation setup. The entire installation and it's emissions are under continuous on-line monitoring, which guarantees that only harmless water vapour is seen exiting the chimney.