The paper illustrates the redefinition of the concept of biorefinery and its application onto the biogas field. The proposed integrated framework merges in a synergistic way the second and third generation biorefineries with the aim of reducing the CO₂ emissions and maximizing the utilization of biomass potential. The specific focus of the paper is on the mathematical modelling of the main conversion steps of the lignocellulosic residuals into biosyngas by means of thermal treatment (pyrolysis, combustion, gasification) and, next, into biomethanol (heterogeneously catalyzed synthesis). The models are implemented in user-friendly programmes and adopted to estimate biomass conversion and process yield. A final study of sensitivity analysis is provided to open the way for the next process optimization based on detail models.