Power requirements of biogas upgrading by water scrubbing and biomethane compression: Comparative analysis of various plant configurations

摘要

Biogas upgrading by water scrubbing followed by biomethane compression is an environmentally benign process. It may be achieved using various plant configurations characterised by various power requirements with associated effects on biomethane sustainability. Therefore, the current study has been undertaken to systematically investigate the power requirements of a range of water scrubbing options. Two groups of water scrubbing are analysed: (1) high pressure water scrubbing (HPWS) and (2) near-atmospheric pressure water scrubbing (NAPWS). A water scrubbing plant model is constructed, experimentally validated and simulated for seven upgrading plant configurations. Simulation results show that the power requirement of biogas upgrading in HPWS plants is mainly associated with biogas compression. In contrast, in NAPWS plants the main power is required for water pumping. In both plants the compression of the biomethane from atmosphereic pressure to 20 MPa also contributes remarkably. It is observed that the lowest specific power requirement can be obtained for a NAPWS plant without water regeneration (0.24 kW h/Nm3 raw biogas) but this plant requires cheap water supply, e.g. outlet water from a sewage treatment plant or river. The second is HPWS without flash (0.29 kW h/Nm3 raw biogas). All other HPWS with flash and NAPWS with water regeneration plants have specific power requirements between 0.30 and 0.33 kW h/Nm3 raw biogas. Biogas compression without upgrading requires about 0.29 kW h/Nm3 raw biogas. The thermodynamic efficiency of biogas upgrading is between 2.2% and 9.8% depending on the plant configuration while biomethane compression efficiency is higher, about 55%. This result implies that the upgrading process has a remarkable potential for improvement whereas compression is very close to its thermodynamic limit. The potential for minimising energy dissipation in the state-of-the-art HPWS upgrading plant with flash by applying a rotary hydraulic pumping device is evaluated at about 0.036 kW h/Nm3 raw biogas meaning the specific power requirement reduction of 10%.