Eco-efficiency and entropy generation evaluation based on emergy analysis: Application to two small biogas plants

The research presented in this paper demonstrates how emergy analysis can be used to measure the eco-efficiency of unit operation processes (UOP). To illustrate this, two small biogas plants in France were analyzed, both with a nominal biogas production of almost 300 Nm3/d. One plant is an Upflow Anaerobic Sludge Blanket (UASB) digester treating dairy effluents while the other is a Continuous Stirred Tank Reactor (CSTR) farm biogas plant with a cogeneration of a nominal power of 45 kWe. The objectives of the study were to show: (a) the long-term sustainability of both UASB and CSTR biogas plants by assessing the respective eco-efficiencies using emergy analysis; (b) whether emergy analysis can holistically account for the true benefits of biogas plants; (c) how energy change or transformation is generated in any UOP systems. Energy analysis revealed some dysfunctions for both systems. The results show that it is possible to improve anaerobic digestion and efficiency of the process working on heat supply and insulation for the UASB-AD (anaerobic digester) plant and on insulation for the CSTR-AD plant. Emergy analysis of the UASB-AD plant through emergy ratios and indices confirm that eco-efficiency was not optimal due to a low biogas production and high sodium hydroxide consumption. For the CSTR, emergy indices show a better eco-efficiency than for the UASB with a biogas transformity close to 4 × 105 sej/Jbiogas in the same range as literature values. However, digester operation and maintenance should be upgraded as well as the use of digestate in order to improve the eco-efficiency of this kind of energy conversion system. The discussion chapter explores the applicability of emergy analysis to improve sustainability of UOP taking into account relationships between emergy, entropy generation and information theory. Entropy generation decreases after improving efficiency and sustainability, in accordance with the maximum empower principle.