From biomass residues to syngas: Intensified biogas upgrading through nickel hydroxyapatite-catalysed dry reforming

Industrial lignocellulosic biomass (LB) residue obtained after steam explosion (SE) pretreatment was subjected to anaerobic digestion (AD) under mesophilic conditions (37 ± 1 °C) at a substrate-to-inoculum ratio of 1:2 using a pressure-transducer-based gas measurement system. Over a 36-day batch digestion period, specific biogas and methane yields of 97.69- and 67.27-mL g −1 VS fed at STP, respectively, were obtained under the constraints of pressure-based gas quantification. The produced biogas was converted into syngas rich in hydrogen (H 2 ) and carbon monoxide (CO), via dry reforming (DRM) using eggshell-derived nickel-based hydroxyapatite (Ni-HAP E ) and nickel-based catalyst supported on a commercial Hydroxyapatite (Ni-HAP C ). Characterisation of LB residue revealed an unexpectedly high apparent lignin content which was attributed to pseudolignin. Pressure fluctuations associated with invasive gas sampling were also recorded on the AD system, suggesting that measurement artefacts primarily contributed to low recorded gas production, with pseudolignin acting as a contributing factor. The importance of non-invasive gas sampling when using pressure-based AD systems is highlighted. DRM tests at 800 °C and atmospheric pressure with Ni-HAP E and Ni-HAP C showed consistently high CH 4 conversion (>90 %), while CO 2 conversion varied strongly with feed composition, ranging from 60 % at CO 2 /CH 4 = 3.47 to > 90 % at ratios of 1.0 and 0.82. H 2 and CO selectivities peaked near the stoichiometric ratio (CO 2 /CH 4 = 1.0), both exceeding 90 %, but declined sharply under CO 2 -rich (H 2 = 70 %, CO = 50 %) or CH 4 -rich (H 2 = 75 %, CO = 65 %) conditions. These findings provide practical guidance for improving AD, DRM and advancing bioenergy pathways. • Integrated AD of industrial lignocellulosic residue with DRM of AD-derived biogas. • Methane potential assessed using a pressure-transducer-based BMP system. • Residue characterization revealed features consistent with pseudolignin formation. • Valorization of eggshell waste produced stable hydroxyapatite catalyst support. • Ni/HAP catalyst achieved >90 % CH 4 conversion and high H 2 /CO selectivity in DRM.