Thermodynamic and exergoeconomic analysis of energy recovery system of biogas from a wastewater treatment plant and use in a Stirling engine


Biogas biomethanisation from SS represents an alternative renewable energy source.

Thermodynamic, thermoeconomic and exergetic analysis of energy recovery process.

The main source of irreversibility and exergy destruction is the boiler.

Energy and exergy economic value of exhaust gases is significant.

Application of biogas obtained to produce steam to power a Stirling engine.


The aim of this research it is to show how the biogas biomethanisation from primary and secondary treatment of activated sludge from a wastewater treatment plant (WWTP), can be an alternative renewable energy option from fossil fuels, which offers competitive advantages and points out new horizons for the use of this fuel. This will allow to achieve some important priorities of energy plans in EU countries: to reduce the organic matter deposited in landfills and CO2 emissions and to find viable solutions to minimize the environmental impact of sewage sludge (SS).

This study analyses the biogas combustion and energy recovery processes from a thermodynamic, thermoeconomic and exergetic point of view.

The results show that the boiler of the process is the main source of irreversibility and exergy destruction. Moreover, the energy and exergy economic value of exhaust gases from the combustion chamber, are significant and worthwhile to be exploited. For this reason, the present study explores the applicability and suitability of integrating a Stirling engine in such process. The study reveals that it is possible to create a small micro-cogeneration system which leads to sustainable waste management and energy savings in the treatment plant itself.


  • Biogas;
  • Sludge;
  • Thermoeconomic;
  • Exergetic;
  • Anaerobic digestion;
  • Stirling engine


  • B, boiler;
  • BOE, official state bulletin of Spain;
  • CIEMAT, Center for Energy, Environmental and Technological Research;
  • CORINAIR, core inventory of air emissions;
  • d. m., dry matter;
  • EU, European Union;
  • EUROSTAT, statistical office of the European Communities;
  • EWL, the European waste list;
  • E2020, European strategy for smart, sustainable and inclusive growth;
  • GW, gigawatt;
  • Ha, hectare;
  • HRB, heat recovery boiler;
  • ktoe, kilo tonnes oil equivalent (=4.18 × 1013 J);
  • LHV, lower heating value of fuel [kcal/Nm3];
  • MAGRAMA, Spanish Ministry of Agriculture, Food and Environment;
  • Mtoe, million tonnes oil equivalent;
  • NMVOC, non-methane volatile organic compounds;
  • NTU, number of transfer units;
  • PER, renewable energy Spanish plan;
  • PNIR, Spanish integrated plan of waste;
  • PNLD, Spanish plan of sewage sludge;
  • ppm, parts per million;
  • RNL, Spanish register of sludge;
  • SS, sewage sludge;
  • VOCs, volatile organic compounds;
  • WWTP, wastewater treatment plant

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