Pilot-scale optimization of a physical–chemical biogas upgrading system based on a high alkalinity absorbent at ambient pressure and temperature

Abstract

The optimization of an innovative process consisting of chemical absorption–desorption at ambient pressure and temperature with EDTA-Fe/carbonate solutions devoted to biogas upgrading was conducted. The influence of parameters such as the initial pH (9–10), inorganic carbon concentration (IC) (4000–8000 mg/L), biogas flowrate (BF) (30–90 L/d), air flowrate (AF) (300–1500 L/d), L/G ratio (0.7–3) and EDTA-Fe concentration (Fe) (0–30 mM) on biomethane composition was evaluated. In addition, the effect of carbon-coated iron nanoparticles on CO2 absorption performance was investigated. The L/G ratio governed the O2 concentration in the biomethane. Interestingly, the addition of EDTA-Fe was not necessary for the complete removal of H2S from the biogas. BF, AF and IC exerted a significant influence on the biomethane CO2 concentration (BF > AF > IC), while the initial pH induced no effect. On the other hand, the supplementation of iron nanoparticles did not significantly influence on the CO2 absorption performance. The optimal conditions in a 7 L absorption-7 L desorption system were: BF = 90 L/d, AF = 1500 L/d, L/G = 0.7, IC = 8000 mg C/L, initial pH = 9.5 and Fe = 0 mM. Under these operational conditions, the biomethane obtained was free of H2S and average concentrations of CO2, O2, N2 and CH4 of 1.7 ± 0.1 %, 0.7 ± 0.1 %, 2.7 ± 0.5 % and 94.9 ± 0.6 %, respectively, were recorded for 3 weeks of continuous operation. This biomethane complied with the European standard EN 16273 on the biomethane use for injection into natural gas networks.