Upgrading Biomethane from Biogas using Microbubble Technique in Absorption System

Abstract

Biomethane is an interesting alternative energy produced from biogas upgrading. Concentration of methane (CH4) in the biogas is increased more than 90% and can be used as a renewable energy. Biogas improvement can be done by eliminating carbon dioxide (CO2) and hydrogen sulfide (H2S) from biogas until meeting biomethane specifications. There are many ways to clean biogas for producing biomethane, such as adsorption, chemical absorption, pressure swing adsorption (PSA), cryogenic separation, biological methane enrichment, and membrane separation. Absorption using water as an absorbent is one of interesting processes which is able to combine with other techniques. The aim of this research project was to study the production of biomethane by using the absorption process with water based absorbent. The absorption was combined with a technique of microbubbles generated by using a venturi ejector, A size 0.25 and 0.5 inches, as a tiny bubble generator can create the bubble size range of 20-30 μm which can be called microbubble gas. The bubbles were mixed with the water before sending through the absorption unit. The treated gas is separated and released out from the water absorbent while CO2 still remains in the water. The simulated biogas was a mixure of CO2 and N2 gas at 20-40% CO2 concentration and biogas from the anaerobic fermentation were finally used as a feed gas in this study. The production of biomethane in this research project was carried out by designing and developing equipment set, as follows; 1) The first step was to design and a gas bubble generation set and equipped with construct a CO2 absorption column with a diameter of 0.18 meters and height 1.0 meters. The simulated biogas was applied to confirm that the CO2 can be removed from the system. Observing the N2 concentration in outlet treated gas stream was monitored and proved the upgrading of biogas to biomethane. 2) Step 2, the experimental prototype was developed to reduce pressure drop. A tube absorber of 0.016 meters in dia. 10 meters long was designed for CO2 absorption in water instead of the absorption column. Treated gas was separated from water absorbent in a series of 2 columns. A spiral nozzle was fixed at top of column to spray the tiny absorbent dropets. 3) The CO2-rich water stream from the gas separator was sent to a column of regeneration unit. The water was sprayed through spray nozzle to make small droplet of water. The counter current flow between the water droplets and air in regeneration column allows the CO2 desorbing from the water which is readity to reuse. The process variables ware studied to produce bio-methane consist of venturi ejector size, gas flow rate, water flow rate, L/G ratio, CO2 concentration in the initial simulated biogas feed. The air flow rate feeding to the regeneration unit was monitored for the effect on the system efficiency. Based on the study, it was found that size of bubble is decreased with decreasing the gas flow rate to the venturi ejector. The 0.5-inch venturi nozzle can produce microbubble size of 20-30 um that helps to increase CO2 absorption in water. The operating condition for the absorption unit with microbubble of 4 L/min gas flow rate, 15 L/min water flow rate, and L/G ratio of 3.75 can effectively capture CO2 for more than 80% from the simulated biogas at 35-50% CO2. The CO2 absorption from real biogas by the prototype experimental was effectively performed with high concentration of methane to 96% by volume, which corresponds to the purity requirements for biomethane. In addition, it was found that the amount of methane loss in the system was only about 0.013%. The economic analysis for the biomethane production system with the absorption column system together with microbubble was found the operating cost of 10.63 baht /1 m3 biogas. The system design approach in this research can be possibly be applied in the production of biomethane from industrial biogas.