Heat integration in digestate-to-methanol systems based on pyrolysis and alkaline water electrolysis: A comparative assessment of digestate drying and heat supply strategies

The expansion of anaerobic digestion has led to increasing production of biogas digestate, highlighting the need for efficient and integrated valorization pathways. This study develops and thermodynamically evaluates an integrated digestate-to-methanol system comprising anaerobic digestion, biogas upgrading, digestate processing, pyrolysis, alkaline water electrolysis, together with downstream methanol synthesis and distillation. Six scenarios are assessed by coupling two digestate drying technologies (steam drying and air drying) with three heat-supply strategies (direct electric heating, heat pumps, and integrated straw pyrolysis). Process models are developed in Aspen Plus, and pinch analysis is applied to quantify minimum heat demand and the theoretical potential for internal heat recovery at system level. Results indicate that drying and heat supply mainly affect heat integration, whereas energy and carbon efficiencies are dominated by the definition of product slate. Heat pumps consistently outperform direct electric heating and deliver the highest energy efficiencies when only fuels (methanol and bio-methane) are considered as products. Integration with straw pyrolysis typically improves carbon efficiency and methanol yield, with the benefit becoming more obvious when biochar and captured CO2 are also included in the product slate. Overall, when only fuels are targeted, the air drying system with heat pumps achieves the highest energy efficiency (56.3%), while the air drying system integrated with straw pyrolysis attains the highest carbon efficiency (41.9%). When carbon sequestration and multi-product valorization are prioritized, the air drying system integrated with straw pyrolysis emerges as the most favorable configuration, achieving the highest overall energy efficiency (73.8%) and carbon efficiency (85.3%).