Biogas upgrading
Raw biogas from digestion processes comprises mainly energy-rich methane (CH4) and carbon dioxide (CO2), besides substances such as hydrogen sulphide (H2S) and water (H2O). To enable the use of biogas as a natural gas substitute or vehicle fuel, it is first necessary to enhance its gas quality and technical combustion characteristics. This includes not only raising of the calorific value through CO2 separation (methane enrichment), but also the removal of contaminants such as water and hydrogen sulphide.Depending on the individual project and local conditions, different processes can be chosen for CO2 separation and methane enrichment. The following technologies are usually applied in our plants:
- Pressure less amine scrubbing
- High-pressure water scrubbing
- Membrane technology
Pressure less amine scrubbing
With this method the CO2 contained in the raw biogas is removed almost completely by means of a selective amine scrubbing solution. This process takes place in a scrubbing column, in which the biogas and the amine scrubbing solution are contacting in a counter flow reaction. The amine solution absorbes CO2, as a result of which the methane concentration in the bio methane can be increased up to 99%. The pressure less operation of the scrubbing process allows minimum electrical energy consumption, as no compression of the raw biogas > 50 mbar is necessary.
The CO2 loaded scrubbing solution is regenerated in a downstream regeneration stage and can afterwards re-used in the scrubbing process. Energy-efficient regeneration of the scrubbing solution is achieved by way of a combination of thermal heating and pressure relief.
Optimum interconnection of the heat exchangers enables the thermal energy in the system to be utilised in the best possible way and thus minimises the amount of additional heat energy input required. For further optimisation of the energy consumption, a proportion of the heat input can be recovered (heat extraction) and used as process heat for other purposes (e.g. for heating of the digester). In addition to CO2 separation, pressure less amine scrubbing allows the simultaneous removal of H2S from the biogas. As an option, an upstream activated carbon filter can be incorporated into the plant. The drying of the upgraded biomethane, i.e. H2O removal, can be realised in a downstream process stage.
Process characteristics and benefits
- Very low electrical energy consumption -> less than 0.1 kWh per Nm³ of raw biogas
- High flexibility in terms of throughput rate -> wide control range of 1:10
- Low methane loss -> high yield of marketable bio methane
- Low methane emission -> no additional process stage required for emission control
- Significantly lower heat input requirement for regeneration compared to other amine scrubbing processes thanks to 2-stage regeneration with intelligent heat exchanger interconnection
|
|