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Upgrading biogas to biomethane using thermally regenerable absorbent:

pilot-scale experiment and scale-up estimation for a palm oil mill

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Thiwa Rattanaya , Prawit Kongjan , Charun Bunyakan , Alissara Reungsang ,
Rattana Jariyaboon 2,*

1 Energy Technology program, Faculty of Engineering, Prince of Songkla University, Hatyai 90110, Thailand
2 Department of Science, Faculty of Science and Technology, Prince of Songkla University, Pattani 94000, Thailand
3 School of Engineering and Resource, Walailak University, Nakhon Si Thammarat 80161, Thailand
4 Department of Biotechnology, Faculty of Technology, Khon Kaen University, Khon Kaen 40002, Thailand
5 Research Group for Development of Microbial Hydrogen Production Process from Biomass, Khon Kaen University,
Khon Kaen 40002, Thailand
*Corresponding author: rattana.sa@psu.ac.th
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Abstract
A process to upgrade a biogas product to biomethane has been investigated.

Recycling of an absorbent is important for low-cost upgrading of biogas by chemical
absorption. This study investigated the alkaline recovery of absorbent solution by thermal

decomposition at various temperatures (75, 85 and 95 °C) and various vacuum levels (0.2, 0.4
and 0.6 bar). During heating, CO and H S were released from the absorbent solution, which
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increased the pH. The highest pH 10.58 was reached for NaHCO solution from initial pH 8.86
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while NaHS solution gave the highest pH 10.38 when treated at 95 °C and 0.6 bar vacuum

level. The biogas upgrading experiments were performed in pilot scale scrubber. It was found
that 99% removal efficiencies of CO and H S were achieved at the minimum NaOH
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concentration of 4 g/L and L/G ratio 0.3. The performance of the recovered alkaline solution
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was also tested The results show that CO and H S removal efficiencies of alkaline solution
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before and after recovery were similar, at 80 and 99% respectively Moreover, there was a
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good agreement between the experimental data and the simulation by Aspen HYSYS. The
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caustic scrubber was simulated for the 0 0 m /h biogas production rate, and the chosen
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column diameter and height were m and m, respectively. The economic analysis of the
process shows payback period of 3 and 4 years for using waste heat and biomass boiler as
heating source, respectively. This study demonstrated the potential approach to applying the
using of thermally regenerable absorbent for upgrading biogas to biomethane.

Keywords: biomethane, biogas upgrading, absorption, scrubbing, thermally regenerable
absorbent