Towards Energy Self-Sufficient Wastewater Treatment for Ireland Karla Dussan and RFD Monaghan National University of Ireland Galway Engineers Ireland, Dublin, 7 June 17
Outline 1. Introduction 2. Wastewater treatment and waste management in Europe 3. The Irish context 4. Objective and methodologies in EPA project 5. Thermal conversion: Gasification and Combustion 6. Results 6.1 Technical implications of the technology 6.2 Techno-economic performance of the system 7. Other waste management applications 8. Concluding remarks 2
Thermal Energy Research Made Efficient Research Group Mechanical Engineering Energy conversion and end-use Renewable and conventional fuels Combustion, gasification, pyrolysis Combined heat and power (CHP) Thermal comfort in the built environment 3
Wastewater treatment in Europe Population: 5 million people -45 kwh/p.e./y α Level of treatment, technologies, process operation, sludge treatment and disposal. Energy savings at WWT plants: - Benchmarking/audit process. - Energy savings: Aeration systems, pumping stations, process control. - Energy recovery: Cogeneration from biogas. - Switzerland: 38% reductions in energy costs leading to 8 M per year in cost savings. - Austria: 9 sites achieving ~% reductions in energy costs. Urban Waste Water Treatment maps http://www.eea.europa.eu/themes/water/waterpollution/uwwtd/interactive-maps/urban-wastewater-treatment-maps-1 4
Distribution, % Wastewater treatment in Europe Population: 5 million people Sludge production: ~14 Mt/year Sludge α Population, stringent directives 0 80 60 0 Agriculture/composting Incineration Others Urban Waste Water Treatment maps http://www.eea.europa.eu/themes/water/waterpollution/uwwtd/interactive-maps/urban-wastewater-treatment-maps-1 5
Wastewater treatment in Ireland >5 WWT plants in Ireland: Still much to implement for energy management and optimisation. Energy consumption reported from 35 to 60 kwh/ p.e. every year Distribution: -75% aeration systems 15-% sludge treatment -25% inlet works Ringsend WWT plant, Dublin, Ireland G. McNamara, et al., South East European Conference on Sustainable Development of Energy, Water and Environment Systems, Ohrid, Macedonia, 14. 6
Wastewater treatment in Ireland >5 WWT plants in Ireland: 53,0 dry t per year of (treated) sludge. Only 18 sludge treatment sites with anaerobic digestion for waste management/energy recovery: 3.92 Mp.e. up to 235 t per day WWT with sludge treatment No. of active sites No. of inactive sites No. of sites with CHP >0,000 p.e. 1 0 1 0,000-0,000 p.e. 7 1 5,000-0,000 p.e. 4 3 5 <,000 p.e. 1 1 2 7
Wastewater treatment in Ireland >5 WWT plants in Ireland: 53,0 dry t per year of (treated) sludge. Only 18 sludge treatment sites with anaerobic digestion for waste management/ energy recovery: 3.92 Mp.e. up to 235 t per day -70% of VS conversion 0-160 t per day of digestates Where is all sludge going? Concerns? 96% is used for landspreading on agricultural land No treatment 1% Anaerobic digestion % Lime treatment 28% Drying 9% Composti ng 12% Sewage sludge treatment Irish Water 14 8
Wastewater treatment in Ireland >5 WWT plants in Ireland: 53,0 dry t per year of (treated) sludge. Only 18 sludge treatment sites with anaerobic digestion for waste management/ energy recovery: 3.92 Mp.e. up to 235 t per day -70% of VS conversion 0-160 t per day of digestates Where is all sludge going? Concerns? Treatment and transportation costs Organic pollutants/biological risks Heavy metals run-off and leaching to water bodies? Plant and animal uptake of heavy metals? Land availability? Sewage Sludge Directive 86/278/EEC 9
Thermal conversion: Waste-to-Energy Combustion & Gasification: The ultimate waste volume reduction methods. Disposing > % of sewage sludge through incineration: Denmark, Austria, Germany, Belgium, Switzerland and the Netherlands. Objectives: Evaluate the feasibility of thermal conversion for energy recovery from sludge and digestates in the Irish context. Create a model for the thermal conversion of sludge. http://www.economist.com/node/15172621 Evaluate the integration of anaerobic digestion (current capabilities) with thermal conversion technologies.
Biomass and wastes gasification Gas reforming & char conversion Oxidation Drying & volatiles Gomez-Barea et al. Prog Energy Combust Sci, 36 (4), 444 Flexible operation with various types of solid fuels. High conversion: >80% + Stable ash Fuel gas product: Syngas CHP, chemical synthesis, synthetic natural gas Heat integration. 11
Modelling of biomass and waste gasification Advanced kinetics-transport models Three components: chemical reactions, Complex dc transport, and heat dt transfer Conversion: empirical and approximated kinetic models. Data not readily available Transport: empirical fluidisation models, computational fluid dynamics. i k C Computationally expensive Accurate i i Gómez-Barea et al. Prog Energy Combust Sci, 36 (4), 444 Thermodynamic equilibrium models Simplified Full conversion: reactions nigi Data available i approach chemical equilibrium. o fˆ nig i RT ni ln i o f Computationally inexpensive Not as accurate Coupled with global heat balance. i i 12
Pseudo-equilibrium modelling of gasification Stoichiometric air/o2 flow ER Actual air/o flow 2 Experimental data: Campoy et al., Fuel Process Technol 14, 121, 63-69 Campoy et al., Fuel Process Technol 09, 90, 677-685 Jand et al., Ind Eng Chem Res 06, 45, 834-843 Kersten et al., Ind Eng Chem Res 03, 42, 6755-6764 Li et al., Biomass Bioenerg 04, 26, 171-193 Petersen et al., Chem Eng Process 05, 44, 717-736 Xue et al., Energy Fuels 14, 28, 1121-1131 13
Pseudo-equilibrium modelling of gasification Syngas composition: Satisfactory carbon distribution in CO and CO 2. Inaccurate H 2 production: water-gas shift reaction. Chemical yield/heating value of syngas: Satisfactory prediction due to excess of H 2 and CH 4 deficit benefiting energy balance. Dry gas concentrations / vol% LHV dry syngas / MJ Nm -3 (Pseudo-equilibrium model) 0 Air gasification Dry gas concentrations / vol% LHV dry syngas / MJ Nm -3 (Pseudo-equilibrium model) 60 Steam gasification H 2 CO CO 2 H 2 CO CO 2 CH CH 4 4 LHV 0 LHV 0 Dry gas concentrations / vol% LHV dry syngas / MJ Nm -3 (Experimental) 0 60 Dry gas concentrations / vol% LHV dry syngas / MJ Nm -3 (Experimental) 14
Energy recovery in WWT: Methodologies Various scenarios: Integration of thermal conversion to anaerobic digestion. Combustion or air gasification as thermal conversion technologies. Three combined heat and power systems. Techno-economic analysis: Model built in Matlab R15: Heuristic factors and thermodynamic analysis. Performance indicators: How much energy demands can be offset by sludge conversion? How is the carbon footprint of the plant affected? How much would these system cost? 15
Energy recovery in WWT: Methodologies 1 tpd Energy efficiency of the concept Coverage Carbon footprint Energy generated Energy required 0 3 g CO2 per m treated WW kg CO2 per t dry sludge Economic indicators Cost of treatment Cost of electricity Sp. investment COT COE SCI 16
Composition, % mass fraction Technical implications of air gasification Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. Moisture content in sludge y M,2, wt% 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER 0 C H N S Ash Raw sludge MAD digestate SS digestate TAD digestate 17
Technical implications of air gasification Moisture content in sludge y M,2, wt% Composition, % mass fraction Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER T GS, K 10 12 1600 1800 1894 0 C H N S Ash Raw sludge MAD digestate SS digestate TAD digestate 18
Technical implications of air gasification Moisture content in sludge y M,2, wt% Composition, % mass fraction Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. 1600 1800 12 10 T GS,min = 73 K 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER T GS, K 10 12 1600 1800 1894 0 C H N S Ash Raw sludge MAD digestate SS digestate TAD digestate 19
Technical implications of air gasification Moisture content in sludge y M,2, wt% Moisture content in sludge y M,2, wt% Composition, % mass fraction Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. 0 C H N S Ash Raw sludge MAD digestate SS digestate TAD digestate 1600 1800 2.5 3.0 12 3.5 10 1.5 2.0 2.5 3.0 3.5 4.0 4.0 4.5 5.0 T GS,min = 73 K Equivalence ratio ER 5.5 6.0 6.5 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER T GS, K 10 LHV dry syngas, MJ Nm -3 2.0 12 1600 1800 1894 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Technical implications of air gasification Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. Cold Gas Efficiency= Energy in gas fuel Energy in solid fuel 0 Moisture content in sludge y M,2, wt% Moisture content in sludge y M,2, wt% 1600 1800 2.5 3.0 12 3.5 10 4.0 4.5 5.0 T GS,min = 73 K 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER 5.5 6.0 6.5 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER T GS, K 10 LHV dry syngas, MJ Nm -3 2.0 21 12 1600 1800 1894 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Technical implications of air gasification Properties of sewage sludge as a fuel: moisture content and energy content. Moisture content: Higher gasification temperature, greater energy demand. Equivalence ratio: Higher ER, lower oxygen consumption, better fuel properties, greater energy demand. Moisture content in sludge y M,2, wt% 55 60 65 70 75 80 85 90 95 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER Cold gas efficiency, % 55 60 65 70 75 80 85 90 95 0 Moisture content in sludge y M,2, wt% Moisture content in sludge y M,2, wt% 1600 1800 2.5 3.0 12 3.5 10 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER 4.0 4.5 5.0 T GS,min = 73 K 5.5 6.0 6.5 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER T GS, K 10 LHV dry syngas, MJ Nm -3 2.0 22 12 1600 1800 1894 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0
Technical implications of air gasification Heat integration α Low drying demands, higher gasification temperatures. Full heat coverage: At least % moisture or ER in gasification of 2. Power generation α High drying demands, high ER in air gasification. Full power coverage: ER > 1.8 and moisture <%. Assuming that an combustion engine is used for CHP: Coverage Energy generated Energy required 0 Moisture content in sludge y M,2, wt% 160 1 1 1 1 1 0 90 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER C hr, % 80 90 0 1 1 1 1 1 160 170 Moisture content in sludge y M,2, wt% C el, % 70 80 90 0 1 1 1 1.5 2.0 2.5 3.0 3.5 4.0 Equivalence ratio ER 60 70 80 90 0 1 1 1 1 23
Energy recovery in WWT: Different CHP technologies Properties Scales Low BTU fuels Installation costs Combustion + Steam cycle kw 0 MW Suitable for sludge Gasification + Steam cycle kw 0 MW Suitable Gasification + IC engine kw 1.5 MW 1-3.5 MW Suitable after simple machine modification Gasification + Gas turbine 0 kw >0 MW Only available for >0 MW TC Low High High High CHP Low Low High High Power efficiency 4-% 4-% 25-% -% (-60%) Heat recovery >60% >60% 35-55% -45% Emissions 25-0 ppmv NO X -0 ppmv CO 25-0 ppmv NO X -0 ppmv CO 0.5-1.5 ppmv NO X <2 ppmv CO < ppmv NO X < ppmv CO 24
Techno-economic performance of energy recovery systems 1. Combustion + Steam cycle 2. Gasification + Syngas combustor + Steam cycle 3. Gasification + IC engine 4. Gasification + Gas turbine TC1 ADTC1 TC2 ADTC2 TC3 ADTC3 TC4 ADTC4 Electricity coverage W gen /W dem, % 0 1 0 0 Energy surplus Energy deficit Case TC1 Case TC2 Case TC3 Case TC4 Case AD Energy surplus ADTC1 ADTC2 ADTC3 ADTC4 0 0 1 0 2 Heat coverage Q rec /Q dem, % *ADTCn = Anaerobic digestion + Thermal conversion 25
Techno-economic performance of energy recovery systems Higher energy recovery: slightly more cost-efficient sludge treatment, inexpensive electricity. Costly technologies due to flue gas treatment. AD does not affect costs of electricity generation: higher process efficiency. Cost of treatment COT, 15 t -1 sludge 0 0 0 TC1 ADTC1 TC2 ADTC2 TC3 ADTC3 TC4 ADTC4 AD-CHP plants kw el - 2 MW el Biomass & wastes 0 60 80 0 1 1 160 Total energy coverage, % 26
Techno-economic performance of energy recovery systems Higher energy recovery: slightly more cost-efficient sludge treatment, inexpensive electricity. Costly technologies due to flue gas treatment. AD does not affect costs of electricity generation: higher process efficiency. Cost of electricity COE, c 15 kwh -1 1,000 0 TC1 ADTC1 TC2 ADTC2 TC3 ADTC3 TC4 ADTC4 AD-CHP plants, kw el - 2 MW el Biomass & wastes 1 0 0 1 0 Electricity coverage, % 27
Techno-economic performance of energy recovery systems High dependence on heat recovery efficiency. AD lowers carbon footprint by improving energy recovery. Boilers using biosolids or biogas: same order of carbon footprint. Gas turbines: higher emissions than combustion engines due to gas treatment. Net carbon emissions, g CO 2 e m -3 0 0 0 0 TC1 ADTC1 TC2 ADTC2 TC3 ADTC3 TC4 ADTC4 0 0 1 0 2 Heat coverage, % WWT + AD + Drying No energy recovery WWT + AD + Drying Energy recovery 600 0 0 kg CO 2 t -1 dry sludge 28
Techno-economic performance of energy recovery systems Economy of scale is of the essence in profitable and sustainable thermal technologies. Waste management α Low treatment costs. Power generation α Low levelised energy costs α Scales above 1 dry t per day. Strategies: Centralised treatment, co-processing of wastes. Levelised cost of electricity COE, c kwh -1 0 80 60 0 [1 tpd, 52c kwh -1 ] [160 tpd, 26c kwh -1 ] 0 80 1 160 0 Costs of treatment COT, t -1 225 0 175 1 [ tpd, 195 t -1 ] [ tpd, 185 t -1 ] 0 80 1 160 0 Sludge feed rate, tpd Sludge feed rate, tpd Combustion + Steam turbine (TC1) Gasification + Combustion engines (TC3) 29
Other waste management applications: Brown bin waste Use of brown bin waste (organic fraction of municipal solid waste) for the production of biomethane (renewable natural gas). R. O Shea, I. Kilgallon, D. Wall, J.D. Murphy, Applied Energy, 175 (16), 229-239. What other sustainable technologies (other than landspreading) can be implemented for biosolids management in future biomethane production sites?
Other waste management applications: Brown bin waste Base case: No use of biogas/ biomethane for energy demands 31
Comparisons Other waste management applications: Brown bin waste AD + biogas upgrade + digestate to agricultural land AD + biogas upgrade + digestate to gasification + CHP No. of sites 5 5 (adjacent to AD sites) Capacity Transportation of final product to disposal/use Energy footprint of treatment system Carbon footprint of treatment system Heat integration 0,000-1,000 t per annum wet waste 9-12 MW th biomethane 16-77 kw OR 133-6 GWh per annum 4,000-5,0 t per annum of dry digestate 460-6 kw el 0.2-1.5 kw OR 1.7-14 GWh per annum 8-12 MWh per t dry digestate 6-9 MWh per t dry digestate 12-14 kt CO 2-eq per annum OR 2.2-3.0 t CO 2-eq per t digestate Over % of biomethane would be required for heat demands 3.6-6.3 kt CO 2-eq per annum OR 0.7-1.5 t CO 2-eq per t digestate Gasification provides 15% of heat requirements and power surplus >% 32
Concluding Remarks 1) Energy recovery: feasible covering potentially all energy demands on-site. 2) Integration of anaerobic digestion and thermal conversion offers higher process efficiencies. 3) Costs of operation are competitive Although a heavy capital expenditure is required (large scales are profitable). 4) Thermal conversion can reduce further carbon footprint of wastewater treatment processes. 5) Scale and capital investment are key issues. 6) It is vital evaluating optimal sludge collection schemes for centralised TC and energy recovery. 7) Decentralised thermal treatment and waste co-processing. 33
Thank you Karla Dussan Mechanical Engineering Department Therme Research Group National University of Ireland Galway karla.dussan@nuigalway.ie 34