Upgrading biogas to biomethane using untreated groundwater-NaOH absorbent: Pilot-scale experiment and scale-up estimation for a palm oil mill

Abstract

A caustic absorbent like NaOH used in biogas upgrading can be recycled by regeneration at a moderate temperature. Using untreated groundwater to prepare the absorbent is cost saving, but the of CO2 and H2S removal efficiencies from biogas need to be confirmed. 60 experiments were performed in pilot scale scrubber for data on operating conditions vs removal efficiency. It was found that the removal efficiencies were not significantly different between using groundwater and softened water to prepare the NaOH, even though 0.3–0.4% of precipitate was found in NaOH solution when using groundwater. 99% removal efficiencies of CO2 and H2S were achieved at the minimum NaOH concentration of 4 g/L and L/G ratio of 0.3. The operating conditions for specific biomethane concentration requirements are tabulated, and the relation of operating conditions to CO2 removal efficiency is provided as an empirical correlation. Moreover, there was a good agreement between the experimental data and the simulation by Aspen HYSYS using the Acid Gas-Caustic Wash method. The caustic scrubber was then simulated for the 500 m3/h biogas production rate, and the chosen column diameter and height were 1 m and 6 m, respectively, which is 30 fold smaller in volume compared to the bio scrubber for which no CO2 removal was observed. For bio methane production, this study reveals the potential to use a cost saving caustic scrubber to replace the bio scrubber.