Producing “green” methanol from syngas, derived from anaerobic digestion biogas

Abstract

An anaerobic digester of sewage sludge or agro-industrial waste produces biogas and ammonia-rich digestate. Three H₂-producing processes exist: dry reforming of methane (from biogas), catalytic decomposition of methane (from biogas after CO₂ capture), and catalytic decomposition of ammonia (from digestate). Dry reforming of methane offers the best syngas yield at 700 °C and for a 50–50 vol % CH₄/CO₂ biogas. Catalytic decomposition of methane achieved a H₂ yield of 95%. Finally, the digestate was stripped and NH₃ was further completely decomposed into H₂ and N₂, for a complete NH₃ conversion at 650 °C. A methanol valorization case study of a wastewater treatment plant of 300000 person equivalents with an anaerobic digester is examined. The methanol production from syngas (H₂/CO) and H₂ product streams is simulated using Aspen Plus®. This anaerobic digester process will daily generate 4485 m³ CH₄, 2415 m³ CO, and 320 kg NH₃. The methanol production will be 183 kg·h⁻¹ (1600 t·y⁻¹). The additional H₂ from ammonia’s catalytic decomposition (631 m³·d⁻¹) can be valorized with excess biogas in the anaerobic digester-associated combined heat and power unit. Due to a significantly higher ammonia concentration in manure, catalytic decomposition of ammonia will produce more H₂ if manure would be co-digested.