Abstract
This paper evaluates the technical and economic feasibility of using a steam accumulator in the thermal hydrolysis process for the treatment of sludge. The increase in the efficiency of anaerobic digestion and biogas valorisation through the combined use of heat and power engines was studied based on scenarios from the wastewater treatment plant of the city of Burgos (Spain). These scenarios were evaluated based on the installation of steam accumulation to average the thermal needs of the process. Biogas production was estimated based on the plant’s operating conditions between 2011 and 2016. Results indicated a process enhancement from 33 to 100% due to a better use of exhaust gases. Consequently, increases of 20.0% in the net biogas and 13.1% in electrical energy production were obtained, along with decreases of 66.8% in the thermal power needs and 61.9% in the biogas consumed by the recovery boiler. The economic savings were 98,213 €/year, due to a decrease in the need to purchase electrical energy from the network. The return on investment period was 2 years after introducing a steam accumulator to the process and replacing the boiler with a new, smaller one.
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Abbreviations
- BP:
-
Biogas production of the digesters (m3/day) reported at STP (0 °C and 100 kPa)
- CHP:
-
Combined heat and power
- C F :
-
Annual cash flow (€)
- Cp W :
-
Specific heat capacity of water (4.186 kJ/kg °C)
- D l :
-
Degree of loading of the engines (%)
- Dsr:
-
Degree of steam release (kg/h m2)
- Ds max :
-
Maximum degree of steam release (kg/h m2)
- E g :
-
Thermal power recovered from the exhaust gases in the recovery boiler (kW)
- E p :
-
Electrical power generated by the engines (kW)
- fc :
-
Filling coefficient
- F Eg :
-
Mass flow of exhaust gases of the Guascor engines (kg/s)
- Fs :
-
Steam flow of the thermal hydrolysis process (kg/h)
- FSA:
-
Free surface area of water (m2)
- Fs acum :
-
Mass flow of steam provided by the steam accumulator (kg/h)
- Fs boiler :
-
Mass flow of steam provided by the boiler (kg/h)
- F W_feed :
-
Flow of feed water (kg/s)
- I 0 :
-
Initial capital invested (€)
- LHV:
-
Low heating value of the biogas (kJ/m3)
- L s :
-
Thermal power of live steam obtained in the recovery boiler at 190.2 °C (kW)
- Ms:
-
Mass of steam needed for the thermal hydrolysis process (kg)
- m sat_water :
-
Saturated water stored in the accumulator (kg)
- N :
-
Number of engines
- P acc :
-
Pressure of the accumulator (kPa)
- P VS :
-
Daily mass flow of volatile solids (VS) of primary sludge (kg/day)
- SMPp, SMPw :
-
Specific methane production of primary sludge and WAS (m3 CH4/kg VS)
- T 0 :
-
Temperature of the feeding water (15 °C)
- T fw :
-
Preheating temperature of feed water (°C)
- THSA:
-
Thermal hydrolysis and steam accumulator scenario
- THP:
-
Thermal hydrolysis plant
- t inj :
-
Injection time of live steam per hydrolysis cycle (min)
- t cycle :
-
Total time of the reactor cycle (min)
- T in, T out :
-
Inlet temperature and outlet temperature of the recovery boiler of exhaust gases (°C)
- TpB cons :
-
Consumption of thermal power derived from biogas needed for the engines and the recovery boiler (kW)
- TpB dig :
-
Thermal power of the biogas produced (kW)
- TpB eng :
-
Thermal power associated with biogas available for the engines (kW)
- TpB rb-av :
-
Average thermal power of the biogas consumed in the recovery boiler (kW)
- Tp W :
-
Thermal power of feed water (kW)
- V acum :
-
Volume of the accumulator (m3)
- WAS:
-
Waste activated sludge
- W VS :
-
Daily mass flow of volatile solids (VS) of WAS sludge (kg/day)
- ∆h :
-
Change in enthalpy of water associated with difference in pressure
- η rb :
-
Efficiency performance of the recovery boiler (%)
- η b :
-
Efficiency performance of the burner in the recovery boiler (%)
- λ water]1260 :
-
Enthalpy of water evaporation at 1260 kPa
- ρ :
-
Density of water (kg/m3)
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Acknowledgements
The author wishes to thank the plant manager of the Burgos WWTP for the great support during the implementation of this research. This research was possible thanks to financial support from Ministerio de Economía y Competitividad and ERDF through Project UNLE15-EE-3070.
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García-Cascallana, J., Borge-Díez, D. & Gómez, X. Enhancing the efficiency of thermal hydrolysis process in wastewater treatment plants by the use of steam accumulation. Int. J. Environ. Sci. Technol. 16, 3403–3418 (2019). https://doi.org/10.1007/s13762-018-1982-6
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DOI: https://doi.org/10.1007/s13762-018-1982-6