A comprehensive comparison of three renewable natural gas production technologies: Energy, exergy, economic, and environmental assessments

This study aims to conduct a comprehensive comparison of three technologies for producing renewable natural gas (RNG) from biomass, evaluating their technical, economic, and environmental perspectives: (i) Catalytic hydrothermal gasification (CHG) technology; (ii) Gasification and methanation (G&M) technology; (iii) Anaerobic digestion (AD) technology. Energy analysis reveals that the CHG system achieves the highest energy efficiency (81.30 %), attributed to its superior energy recovery and utilization capabilities. The AD system exhibits 50.17 % lower energy efficiency compared to the CHG system, primarily due to incomplete biomass conversion into biogas. Exergy analysis indicates that the CHG system demonstrates the highest exergy efficiency (63.38 %). The reaction unit constitutes the primary source of exergy losses across the three RNG production systems. Energy utilization diagram (EUD) analysis of the RNG production reaction in the CHG and G&M systems reveals that the CHG system experiences lower exergy losses, owing to its single-step conversion and milder reaction conditions. Economic evaluation highlights that the CHG system offers the most favorable economic performance, driven by its moderate investment cost (24.50 M€), high RNG and steam production, and a competitive RNG break-even cost of 0.41 €/Nm³. Raw material costs and by-product steam prices are critical factors influencing the economic viability of the process. Life cycle assessment reveals that the CHG and G&M systems exhibit superior environmental performance, whereas the AD system performs poorly due to the significant volume of digestate requiring treatment.