| | Methods | Process description and performance |
| | 1 | Membrane separation (MS) | Membrane separation uses membranes in form of hollow fiber bundles made of polymeric materials like polysulfone, polydimethylsiloxane, and polyimide or incorporated in a stainless-steel tube. The materials used are permeable to carbon dioxide (CO2), moisture (H2O), and ammonia (NH3) but are less permeable to oxygen (O2) and hydrogen sulfide (H2S) but less permeable to methane (CH4)and nitrogen (N2). A flow called “permeate” mainly composed of CO2, H2O, NH3, and other residues penetrates the micropore, but CH4-rich gas (called “retentate”) passes through the membranes without removal. A set of multiple modules are used to provide sufficient surface area [131]. | | 2 | Water scrubbing (WS) | Water scrubbing is based on different solubility of carbon dioxide in water with respect to that of methane. Based on Henry’s law, carbon dioxide has got 26 times higher solubility than methane in water at 25°C. Raw biogas is first passed through a scrubber to remove hydrogen sulfide, utilizing the same type and amount of desulfurization solvent utilized for the membrane unit. The desulfurized biogas is compressed to 4–6.5 bar and passed through a washing column from the bottom, where it meets water injected from the top leading to absorption of carbon dioxide by water as methane from the top of the washing column. The methane is dried before refining by means of an activated carbon filter which removes traces of VOCs. The water living in the scrubber is rich in CO2 and has 5–6% methane content in the compressed biogas. The water with methane and CO2 is taken to a flash column where the pressure drops to 2–4 bar to facilitate methane separation. Upon methane separation, the water is taken to a stripping column for removal of CO2, before it can be reused. Water scrubbing technology has CO2 removal efficiency greater than 98% with methane slip from 1 to 2%. | | 3 | Chemical absorption (CA) with amine solvent | This is a variant the scrubber techniques, since it uses organic amines as solvent, such as monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA), and diglycolamine (DGA). Its operating principle is like water scrubbing using amine. Using amine which is more selective in absorbing CO2 with respect to water, hence more CO2 per unit volume is removed, hence requiring smaller upgrading units in size [131]. However, the process requires some amount of thermal energy for amine regeneration inside the stripper, and amine solvent make-up to avoid loss of process efficiency [131]. In this process, raw biogas is desulfurized and fed to the absorber where it meets the amine solvent (MEA), which absorbs CO2 from biogas in an exothermic reaction that increases the temperature of the amine solvent. Biomethane then exits from the top of the and supplied to the refining phase followed by final compression. The “rich” amine solvent now has CO2 contents heated in a heat exchanger, using heat from the “lean” amine solvent living in the stripping column, and is directed to the absorber before being fed to the stripper. The gas is heated in the re-boiler before before the stripping reaction. After the makeup phase, CO2-free amine solvent is sent to the heat exchanger. Traces of steam and amine solvent are removed from the off-gas and recirculated to the stripping column with the remaining portion which is mainly CO2 being released to the atmosphere. The CA technology allows the highest methane recovery because of its low methane slip of (up to 0.04%[131]. | | 4 | Pressure swing adsorption (PSA) technology | This method makes use of the ability of a porous adsorbent medium to adsorb molecules out of a gas mixture and release them based on pressure applied released by applying different values of pressure. The technique takes advantage molecular dimensional differences of CO2 (0.34 nm) and CH4 (0.38 nm). An adsorbent material having cavities of 0.37 nm retains CO2 in the pores and allows CH4 to flow. The most used adsorbent materials are zeolites and activated carbons as they are more efficient. The process starts with a pretreatment phase using activated carbon for removal of H2S and drying to remove water, before compressing about 4 bar and fed to PSA unit which has four columns in series (Skarstrom cycle), packed with zeolites as adsorbent material. Compressed biogas is fed in a column where CO2 is adsorbed by the adsorbent material with methane passing through. In the second column, pressure drop enables CO2 desorption; in the third phase, the column is cleaned from residual CO2 injecting a part of biomethane, and in the fourth phase, the gas is repressurized. The residual VOCs are removed in another activated carbon filter followed by compression of biomethane to about 24 bars. The zeolite must be replaced after some time. The pressure swing adsorption technology has the lowest efficiency with methane slip varying from 1.8% to 2%. |
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