Pathways for the thermally optimal design and practice of anaerobic digestion in large-scale biogas plants: Heat transfer modeling and energy analysis

Abstract

A flexible and configuration-dependent heat model framework is developed, validated, and used to perform in-depth energy analyses and to identify the optimal design and operating conditions in wet anaerobic digesters in stirred reactors with multi-membrane gasholders in modern biogas plants. This framework enables a detailed geometrical representation of the gasholder and digester at different scales, such that different gasholder designs from single- to multi-membrane systems with varying configurations can be studied and compared to increase thermal efficiency. The model operates in the dynamic regime and includes longwave infrared surface-to-surface heat exchanges. This includes the impact of dynamic environmental conditions (e.g., ambient air temperature, wind, rain, and solar radiation) on the thermal behavior of the unit. Throughout the parametric analyses, the influences of key operating and design parameters, as well as synergistic features such as the feasibility and efficiency of various waste heat recovery strategies, are investigated on the basis of the thermal performance of the digester. The analysis shows that, based on a 3,000-m3 double-membrane biogas plant with a raw biogas self-consumption of 7.4%, thermal autarky can be achieved using (i) a three-membrane gasholder (−2.1%), (ii) outgoing digestate heat recovery (−3.7%), and (iii) recovery of heat from biogas purification (−1.6%).