Biomass to bio-CNG from sorghum varieties for biogas production, energy recovery, and emission mitigation

Sorghum (Sorghum bicolor) has emerged as a promising lignocellulosic substrate for anaerobic digestion; however, its biogas production potential and energy recovery efficiency are strongly influenced by varietal characteristics and operating temperature. In this study, eleven sorghum varieties were evaluated for their biogas production potential, biodigestibility, and theoretical energy recovery efficiency under mesophilic (37 ℃) and thermophilic (55 ℃) conditions. The experiments were conducted in 250 mL batch-scale anaerobic digestors operated for 60 days. A clear difference in biogas yield was observed, with consistently higher yields in thermophilic reactors. Among the evaluated varieties, Combined Sorghum Variety-48 (CSV-48), Combined Sorghum Variety-49 Sweet sorghum (CSV-49SS), yellow sorghum, and white sorghum showed superior biogas productivity and higher total solids, volatile solids, and chemical oxygen demand removal efficiencies. Assuming 50% methane content, Bio-Compressed Natural Gas (Bio-CNG) potential ranged from 36.06 to 83.27 Nm³ t^{− 1} FM under thermophilic conditions. Corresponding electricity and heat generation ranged from 146.3 to 337.9 kWh t^{− 1} FM. Sorghum-derived Bio-CNG could replace approximately 33.9–78.3 L of diesel or 37.9–87.4 L of petrol per tonne of fresh biomass, reducing CO₂ emissions by 90.9–209.8 kg CO₂-eq. t^{− 1} FM on diesel basis and 87.5–201.1 kg CO₂-eq. t^{− 1} FM on a petrol basis. These findings highlight that identifying high-performing sorghum varieties with favourable lignocellulosic composition and a balanced C/N ratio can improve process stability, substrate biodegradability, and energy recovery efficiency. Using the identified high-yielding varieties for industrial purposes instead of random varieties can reduce operational variability in sorghum-based biogas systems.