The emissions released during the burning of biogas are subject to legal limits. In order to comply with these limits, exhaust aftertreatment is required by oxidation catalyst. Over time, decreasing pollutant conversion occur due to aging of the catalyst. Therefore, after only 1.5 years working time, compliance with the formaldehyde limit is no longer possible with certainty. The main reason is the deposition of inorganic compounds called oil ashes. These flue gas components from biogas engines block the catalytic active species and plugs pores.