Table 4. Overview of upgrading technologies for biogas purification (115, 118, 120, 122, 125, 128, 130, 136, 142, 146).
| Upgrading technology | φ(CH4)/% | φ(CO2)/% | γ(H2S)/(mg/L) | φ(CH4)loss/% | E/(kWh/Nm3) | Advantages | Disadvantages | |||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Physical | ||||||||||||
| Water scrubbing absorption | 95-98 | <2 | <2 | <2 | 0.20-0.50 | high efficiency, simultaneous removal of H2S, low CH4 losses, tolerance to impurities, possible regeneration, simple operation | expensive investment and operation, clogging due to bacterial growth, requires huge amount of fresh water | |||||
| Organic solvent scrubbing | 93-98 | <2 | <1 | <4 | 0.10-0.33 | economical, simultaneous removal of organic components, H2S, NH3, HCN and H2O, energetically more favourable than washing with water, regeneration with low-temperature waste heat | expensive investment and operation, difficult operation, insufficient operation when stripping/vacuum applied, reduced operation by glycol dilution with water | |||||
| Pressure swing adsorption (PSA) | >96-98 | 1-2 | 2 | <4 | 0.16-0.43 | low energy used, no chemicals required, no water demand, high pressure but regenerative, no microbial contamination and impurities | H2S pretreatment required, expensive investment and operation, complex setup | |||||
| Membrane separation | 90-99 | 1–3 | 2 | <5 | 0.18-0.35 | H2S and H2O are removed together with CO2, simple construction and operation, no chemicals required | unstable over the long term, pretreatment required, multiple steps required (modular system) to reach high purity | |||||
| Chemical | ||||||||||||
| Chemical scrubbing | >98 | <1 | <4 | <0.1 | 0.05-0.18 | high efficiency, cheap operation, regenerative, more CO2 dissolved than with water, very low CH4 losses | use of chemicals, corrosion, expensive investment, heat required for regeneration, decomposition and toxicity of the amines or other chemicals | |||||
| Biological | ||||||||||||
| Hydrogenotrophic removal | 98 | 7.8 | 38 | <1 | mild process conditions, enhancement of CH4, no unwanted end products, low operating costs | still on an experimental basis, tested only on a small scale, further developments to increase the H2 gas-liquid transfer | ||||||
| Photosynthetic removal | 97-99 | 10 | 0-0.5 | <1 | 0.05-0.10 | mild process conditions, tolerance to high CO2 concentrations and pH values, extraction of high value-added products | poor gas-liquid mass transfer of CO2 and H2, pilot scale, limitation on investment and operating cost data | |||||
| Novel | ||||||||||||
| Cryogenic separation | 99 | <2 | <1 | <0.1 | 0.42-1 | high purity of CO2 and CH4, no chemicals required, upgraded biogas at high pressure, no further compression is required, low extra energy cost to reach liquid biomethane | high capital and operating costs, high energy required for equipment such as compressors and heat exchangers, pretreatment required, removing H2S and H2O prior to cryogenic separation | |||||
| In situ upgradation technology | 95 | <2 | cost-effective, easy to operate | high CH4 loss, appropriate only for a small scale, limited by gas-liquid mass transfer | ||||||||
| Hybrid technologies | 95-98 | <1 | low operating costs, high CO2 and H2S-capture efficiency, higher yields of pure CH4, competitiveness and less energy consumption | small scale production, limited by enzyme lifetime, high enzyme production costs | ||||||||
E=energy consumption (kWh/Nm3), Nm3=normal cubic metre of biogas at standard conditions (T=273.15 K and p=101 325 Pa)