Selective desorption of carbon dioxide from sewage sludge for in-situ methane enrichment: Enrichment experiments in pilot scale

The application of in-situ methane enrichment for upgrading the biogas from anaerobic digestion to vehicle fuel or natural gas quality, has been studied in pilot scale and by computer simulation of the desorption step. Pilot plant experiments have been performed using a 19m ³ and 15m ³ continuously stirred tank reactor operating with municipal sewage sludge at mesophilic conditions connected respectively to a 90dm ³ and 140dm ³ external bubble column for selective desorption of CO ₂. The results show that the CH ₄ yield is unchanged during the experiments, and accordingly there is no evidence that the oxygen in the air used in the desorption of CO ₂ has a negative impact on the CH ₄ producing activity. The sludge recirculation system must be designed to avoid leakage of air into the digester, in order to maintain a low N ₂ concentration in the biogas. At best, a biogas with a volume fraction of 87% CH ₄ and φN2=2% was obtained. The CH ₄ loss however amounted to 8%, which is unsatisfactory. The experimental results are compared with previous data in the literature and explanations are deducted for the difference in the performance. Computer simulations reveal that the sludge flow rate recirculated through the desorption column should be as low as possible to minimize the loss of CH ₄. An increased air flow rate through the desorption column and an increased desorption column volume will promote the desorption of CO ₂ and improve the ratio of CO ₂ to CH ₄ desorption.