Electrical energy production from biosolids: a comparative study between anaerobic digestion and ultra-high-temperature gasification

Abstract

Biosolids management is one of the most expensive and complicated processes in sanitation engineering. Anaerobic digestion (AD) is often employed for the stabilization of biomass and for energy production, as approximately 50% of the carbon entering the anaerobic digester is recovered as methane (CH₄). Gasification has been used recently for the thermal reformation of biosolids to synthesis gas (syngas), which primarily consists of carbon monoxide (CO) and hydrogen (H₂). In the present work, the net electrical energy production from biosolids has been calculated, for a typical activated sludge wastewater treatment plant, with an inlet flow rate of 75,708 m³/d (equal to 20 Mgd). The calculations suggest that the ultra-high-temperature gasification (UHTG) system can achieve a net electrical energy output of about 15.40 MJ/kg(dry biosolids), whereas the AD system can achieve values between 8.45 MJ/kg(dry biosolids). The latter values correspond to approximate net electrical energy power of 18.8 kW for UHTG, versus 9.9 kW for AD, for a wastewater treatment plant with capacity of 1000 m³/d; thus, the UHTG process yields approximately 190% of the energy that may be produced by the AD process.

Keywords:

• anaerobic digestion
• ultra-high-temperature gasification
• biosolids
• municipal sludge
• waste to energy
• energy
• net electric energy yield

Acknowledgements

The author is grateful to Dr. S. Noll for his contribution in selecting the appropriate engines and calculating the engine yields for biogas and syngas combustion.