A compact, economic scrubber to improve farm biogas upgrading systems

Abstract

This paper presents a theoretical and experimental study of a new biogas upgrading technology and its application at farm scale. The process is designed for flows of raw biogas of up to 50 Nm3/h (55% CH4 and 45% CO2). The upgrading system includes a physical absorption of CO2 at 7–10 bars into water and desorption at atmospheric pressure. First, the traditional bottom column design is improved to avoid the usual formation of biogas bubbles and leaks. As a second technological breakthrough, the process water is successfully recycled using a static mixer to enhance CO2 desorption from the water. Finally, the scrubbing system is entirely characterized: carbon dioxide absorption into the column is modeled using the transfer unit height (HTUOG) and the number of gas transfer units (NTUOG); the CO2 desorption step is performed in the static mixer at atmospheric pressure and modeled. Experiments with this new scrubber were conducted on a farm with raw biogas inlet flow rates ranging between 15.6 and 42 Nm3/h. It proved possible to upgrade biogas by up to 77% (QG = 40.7 Nm3/h, QL = 8.243 m3/h, P = 7.924 bar and T = 285 K) with good absorption efficiency (57.5%), a high methane recovery ratio (94%) and low power consumption (0.26 kWh/Nm3). The comparative analysis of the experimental results with the modeling proposed provides good references to evaluate this simple, cheap, compact technology and its potential for the production of bio-methane (no solvent recirculation) or fuel (solvent recirculation).