Wood ash biomethane upgrading system: a case study

In this work, a biomethane fuel was obtained from biogas, using a solid matrix of ashes obtained from a central heating plant fed by wood. Biomethane composition was studied in an experimental pilot plant that treats organic waste through dry anaerobic digestion. European limits were considered and respected for the biomethane injection into the gas grid. The process was able to completely remove carbon dioxide (CO2) during its initial phase. After 50 h of test CO2 concentration started to appear in the outlet gas. This behaviour is related to the ash content, reactive phases (e.g., Ca hydroxides) and fine particle size (<0.2 mm). Coupled with the CO2 removal, European limits for the biomethane injection into the gas grid must consider also trace compounds (Limits for Sulphur 150 mg/Nm3 and CO2 3%vol.). After 95 h of the experimental test, the gas injection limits are still maintained both for total sulphur trace compounds and both for CO2 limits. Uptake of 0.7 g/kg for H2S and 115 gCO2/kgash were registered from the pilot plant