Pressure swing adsorption
Pressure Swing Adsorption (PSA) is a technique which consists of four adsorber vessels, a compressor and an H2S removal step. The four adsorber vessels contain adsorbent material (carbon molecular sieve) which adsorbs N2, O2, H2O, CO2 and H2S. The H2S is adsorbed irreversible by the adsorbent material and would thus poison the material. Therefore, an additional H2S removal step is included. This is a disadvantage, because removing H2S is a complex step.
The four adsorber vessels each operate in an alternating cycle of adsorption, regeneration and pressure build-up. During the adsorption phase, the biogas enters at the bottom of one of the adsorption columns. CO2 is then adsorbed by the adsorption material. The gas leaving at the top of the vessel contains more than 97% CH4. Just before the adsorption material is saturated with CO2 and other impurities, the adsorption column is stopped to regenerate the adsorption material. Another adsorption column is then switched to the adsorption mode in order to obtain a continuous operation. Regeneration of the adsorption material is achieved by a stepwise depressurization of the column to atmospheric pressure and finally to near vacuum conditions. During regeneration also significant amounts of CH4 can be recovered which were trapped in the voids of the adsorption material. The waste stream consists of N2, O2, H2O, CO2 and the amounts of CH4. The waste stream can be recycled to recover more methane, but another option is to lead the stream to a gas engine linked to a generator. The fact that the adsorbent material also removes N2 and O2 is a huge advantage. Before the column can be set to the adsorption mode, pressurization takes place.
The advantages of a PSA-plant are the high enrichment of CH4 (more than 97%) and the low level of emissions and waste. Next to that, the carbon molecular sieves also adsorb N2 and O2. The disadvantage is the fact that H2S needs to be removed which is a complex process.
For the PSA-plant the investment costs are calculated as well as the annual costs. Combining these two costs leads to a cost price of €0.25 per Nm3 upgraded biogas. The investment costs equal €680,000 and the running costs account to €187,250.