At one of the DMT’s plants recently a safety incident occurred. An Activated Carbon Filter overheated due to an incorrect mode of operation, causing damage to the installation. This led to a significant financial loss including business interruption impacts.
To raise awareness for correct operation of Activated Carbon Filters, DMT publishes this S.H.E. bulletin to prevent this from happening again. Below you can find the outline of the cause, the technical background and recommendations.
Biogas upgrading facilities require proper pre-treatment of the Biogas in order to remove contaminants such as Hydrogen Sulphide (H2S). At the affected site, the installation has been successfully operated for many months.
In August 2016, the system was operated for a period of time with no oxygen present in the biogas stream. The oxygen injection unit of the digester was not in use. This resulted in the biogas stream containing high amounts of H2S and no oxygen present.
The absence of oxygen combined with the high H2S concentration in the biogas, quickly saturated the activated carbon. Usually this would not be a problem when the biogas stream was transferred by a timer to the redundant fresh Active Carbon Filter and the saturated carbon filter was replaced.
However it appeared that both filters were saturated which resulted in several automatic shut downs due to the critical H2S level alarm after the filters.
The operators incorrectly made several attempts to regenerate the saturated carbon by feeding air directly into the inlet of the filter, by connecting an external air compressor pump.
The compressed air was injected into the filters, while the biogas stream was not present. The overload of oxygen and the absence of any cooling effect of the biogas stream resulted in overheating by an exothermal chemical reaction (hot-spots) with a meltdown of the filter as a result.