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Enhancing the efficiency of thermal hydrolysis process in wastewater treatment plants by the use of steam accumulation

  • J. García-Cascallana
  • D. Borge-Díez
  • X. GómezEmail author
Original Paper

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.

Keywords

Biogas valorisation Energy balance Anaerobic digestion Thermal storage 

List of symbols

BP

Biogas production of the digesters (m3/day) reported at STP (0 °C and 100 kPa)

CHP

Combined heat and power

CF

Annual cash flow (€)

CpW

Specific heat capacity of water (4.186 kJ/kg °C)

Dl

Degree of loading of the engines (%)

Dsr

Degree of steam release (kg/h m2)

Dsmax

Maximum degree of steam release (kg/h m2)

Eg

Thermal power recovered from the exhaust gases in the recovery boiler (kW)

Ep

Electrical power generated by the engines (kW)

fc

Filling coefficient

FEg

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)

Fsacum

Mass flow of steam provided by the steam accumulator (kg/h)

Fsboiler

Mass flow of steam provided by the boiler (kg/h)

FW_feed

Flow of feed water (kg/s)

I0

Initial capital invested (€)

LHV

Low heating value of the biogas (kJ/m3)

Ls

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)

msat_water

Saturated water stored in the accumulator (kg)

N

Number of engines

Pacc

Pressure of the accumulator (kPa)

PVS

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)

T0

Temperature of the feeding water (15 °C)

Tfw

Preheating temperature of feed water (°C)

THSA

Thermal hydrolysis and steam accumulator scenario

THP

Thermal hydrolysis plant

tinj

Injection time of live steam per hydrolysis cycle (min)

tcycle

Total time of the reactor cycle (min)

Tin, Tout

Inlet temperature and outlet temperature of the recovery boiler of exhaust gases (°C)

TpBcons

Consumption of thermal power derived from biogas needed for the engines and the recovery boiler (kW)

TpBdig

Thermal power of the biogas produced (kW)

TpBeng

Thermal power associated with biogas available for the engines (kW)

TpBrb-av

Average thermal power of the biogas consumed in the recovery boiler (kW)

TpW

Thermal power of feed water (kW)

Vacum

Volume of the accumulator (m3)

WAS

Waste activated sludge

WVS

Daily mass flow of volatile solids (VS) of WAS sludge (kg/day)

∆h

Change in enthalpy of water associated with difference in pressure

ηrb

Efficiency performance of the recovery boiler (%)

ηb

Efficiency performance of the burner in the recovery boiler (%)

λwater]1260

Enthalpy of water evaporation at 1260 kPa

ρ

Density of water (kg/m3)

Notes

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.

Supplementary material

13762_2018_1982_MOESM1_ESM.docx (28 kb)
Supplementary material 1 (DOCX 27 kb)

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Copyright information

© Islamic Azad University (IAU) 2018

Authors and Affiliations

  1. 1.Department of Electrical, Systems and Automatic Engineering, School of Electrical, Industrial and InformaticsUniversity of LeónLeónSpain
  2. 2.Chemical and Environmental Bioprocess Engineering Department, Natural Resources Institute (IRENA)University of LeónLeónSpain

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