The environmental impact of electricity from biogas: a mitigation strategy involving heat recovery

Lead University: The University of Manchester

Professor Adisa Azapagic, Dr Alessandra Fusi

Collaboration: Università degli Studi di Milano

Associate Professor Marco Fiala, Dr Jacopo Bacenetti

Summary

Given the growing scarcity of primary energy resources, increasing the efficiency of energy conversion processes is one of the key challenges for optimising primary energy use.

In most biogas plants in Europe, the use of waste heat is non-optimal and in many cases is limited only to the heating of the digester, while the remaining heat is either not used at all, or only partially. Waste heat utilisation is made problematic by several issues: (i) biogas plants, generally located in farms, are usually far from big and continuous thermal demands; (ii) the transport of heat is difficult and the construction of heating system plants requires considerable investments; (iii) there is an opposite seasonality between waste heat availability and thermal energy requirement in agricultural activities such as stable and greenhouse heating and drying, (iii) low economic value of heat. In this context, the Organic Rankine Cycle (ORC) technology could represent an effective solution to the above waste heat issues.

The aim of this study is, therefore, to assess the potential benefits arising from the valorisation of waste heat when ORC systems are implemented. For this purpose, ten biogas plants were assessed by using life cycle assessment. The findings of this study indicate that the adoption of the ORC micro turbine could represent a viable solution for improving energy efficiency and reducing environmental impacts of electricity from biogas. Nonetheless, due to technical reasons, the ORC system cannot be applied for low-capacity plants (<150 kW).

Highlights

  • AD plants using animal slurries and waste are normally small and therefore not suitable for ORC.
  • Small AD plants fed with slurries have lower environmental impacts than bigger ORC plants for most impacts, including global warming potential

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