Potential of wood ash in purification of biogas

Abstract

Carbon dioxide and hydrogen sulfide are the main contaminants in biogas, which reduce its calorific value and cause wear on metallic appliances through corrosion. There are no comparative studies on the potential of dry adsorption and wet wood ash (carbonation) process on biogas upgrade. The current study for the first time investigates the effects of different process parameters on the use of wood ash for purification of biogas and compares the performance of the two processes. The chemical composition of wood ash was characterized using X-ray fluorescence (XRF) while biogas was analysed using gas chromatography and a digital biogas analyser. The effects of process parameters on biogas carbonation method were investigated. The highest CO2 uptake and methane increment achieved by use of wood ash slurry was 2.30 mmol/g-wood ash and 88%, respectively. This high CO2 uptake was possibly due to the high content of oxides (46% TS) in natural wood ash. The optimum conditions of adsorbent/water ratio and biogas flow rate for methane enhancement were 1:4, and 100 ml/min, respectively. In dry adsorption processes, an increase in mass of activated wood ash from 2.5 to 35 g increased CO2 removal from 8.9 to 67.9%. Furthermore, the evaluation of carbonation adsorption indicated that the process followed pseudo-first order kinetics. Activation of wood ash did not significantly improve its CO2 uptake by carbonation while the uptake by activated wood through dry adsorption was comparable to that of raw wood ash through carbonation process. Conclusively, raw wood ash by carbonation process is a good candidate recommendable for purifying biogas in household digesters.