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NAWTEC 2011 - Lancaster (PA), 16-18 May 2011

A Model to Estimate the Mass and Energy Balances of Bio-drying

Dept. of Energy - Politecnico di Milano Marco Ragazzi, Elena Rada Dept. of Civil and Environmental Engineering - University of Trento

Federico Viganò, Stefano Consonni

Summary

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· Motivations and technologies for bio-drying

· Mass / energy balances of bio-drying · Experimental observations · Modelization · Results for four case studies

· Conclusions

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Energy recovery paths for residual waste

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Residual waste

Direct combustion

Mass burn in WtE plants

Mass reduction to the atmosphere Solid residues to landfill

MBT - Mechanical Biological Treatment

Bio-drying Mechanical treatment RDF

Electricity & Heat Cement factories

Dedicated RDF to Energy plants Power plants

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Bio-drying

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Its basic purpose is reducing the moisture in the waste, thereby decreasing its mass and increasing its heating value Energy for moisture evaporation is provided by the exothermal reaction carried out by aerobic bacteria.

It requires large amounts of air to evacuate evaporated moisture and to provide oxygen for bacteria.

A small amount of volatile solids contained in the organic fraction of waste (Organic Volatiles = OV) is consumed Reduction of mass follows from evaporation of moisture and oxidation of part of the Organic Volatiles.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Bio-drying

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Waste is shredded and laid in piles inside an appropriate space called bioreactor. Air is forced to pass through the piles to provide oxygen for the biological activity and to evacuate evaporated moisture, as well as oxidation products The process typically takes 7 to 14 days, depending on: ­ plant arrangement ­ waste moisture ­ waste biological activity. Air flow rate varies with these same variables, with typical values between 3 to 12 m3 kg-1MSW. Air distribution can be either up-flow or down-flow Height of the piles should be limited to approx 6 m to have good air distribution and acceptable pressure drops

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Bio-drying with ventilation in open circuit

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S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Basic motivations of bio-drying

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Since authorizations of WtE plants are typically given on a mass basis (tons per year), the decrease of mass (increase of HV) allows increasing the power output of the WtE plant downstream. Bio-dried material can be stored much more easily than "fresh" Residual Waste Bio-dried material can be used to generate RDF for a number of applications (dedicated WtE plants, cement kilns, co-combustion in power stations Easier acceptance by the Public

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Industrial plants: Corte Olona (Pavia)

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S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Industrial plants: Bergamo (pile)

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S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Industrial plants: Bergamo (biofilter)

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S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Benefits / Drawbacks of bio-drying

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Significant mass reductions and LHV increases with very small energy penalties Nearly eliminates odors --> easier storage and transport Due to low moisture, subsequent mechanical treatment for RDF production (if applicable) is easier Increase of LHV gives a (slight) increase in the efficiency of WtE plant downstream

Added capital cost

Added emissions Added auxiliary consumption and (small) energy losses

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Basic physical mechanism of bio-drying12

CO2 H2O

(partial) oxidation of Organic Volatiles is the driving force of bio-drying

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Our simulation model

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Model carries out mass and energy balances of bio-drying

It is based on a simulation software designed for power plant applications --> components DO NOT represent actual physical processes; they simply reproduce the same mass and energy balances. Resulting model is MUCH SIMPLER than a biological model.

Simulation is steady-state ---> all variables are represented by their time-averaged values Model cannot predict time-dependent variations nor the duration of the process. It requires a number of assumptions to be derived from experimental observations

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Experimentation at University of Trento14

Two experimental bio-reactor of 1 m3 volume each, see:

Ragazzi M, Rada E C, 2009, "MSW Bio-Drying Eco-Balance and Kyoto Protocol", Third International Symposium MBT&MRF, Mechanical Biological Waste Treatment and Material Recovery Facilities, Hanover, Gottingen, Cullivier Verlag, pp. 372-379, Hanover, 12-15 May 2009.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Experimental observations

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Mass reduction is mainly due to moisture evaporation and, to a small extent, to the consumption of Organic Volatiles (OV) Mass reduction is proportional to the amount of organic material in the waste

Heat release comes not only from the OV_consumed, but by the entire mass of OV_initial in the waste.

Energy balances show that the heat released is usually larger than [MOV_consumed * HHVOV_initial] Leachate production is very small, and basically due only to condensation of air humidity on cold surfaces.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Mass / Energy break-down

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Initial Organic Volatiles (OV)

Mass break-down

fraction consumed

Mconsumed

final fraction

M final M initial M consumed

HHVinitial HHVconsumed

Mass Minitial Energy Minitial HHVinitial

Energy break-down

final fraction

Mfinal HHVfinal

Mconsumed HHVconsumed

fraction consumed

Minitial HHVinitial Mconsumed HHVconsumed

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Interpretation of experimental data

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Mass reduction averages 63% of total organic content of the waste.

Leachate production is about 2% of mass reduction.

Air flow is 10­11 Nm3 kg-1MSW for very high organic contents (above 50%), and decreases with organic contents. For an organic content of about 30%, it's around 6 Nm3 kg-1MSW In most case, the air discharged by the process is nearly saturated.

Air temperature varies significantly with time. Mean value for 15 days-long runs is 33-37°C, while piles are much warmer, around 50°C.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Observed values of

100 95 90 85 80 75 70 65 60 20 30 40 50

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Parameter , %

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Total organic content, % by mass on wet basis

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Functional model

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S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Assumptions and model calibration

Inputs for the model Reference conditions: Air pressure*, mbar Air temperature, °C Relative humidity, % Process targets: Total mass reduction, % of total organic content Percolate production, % of mass reduction Mean temperature of oxygen depleted air, °C Mean relative humidity of ox. depl. air, % Process parameters: Thermal losses, kJ/kg MSW Mean temperature of piles, °C

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950 15 60 63 2 35 90 30 50

* Lower than atmospheric due to the pressure drop through the piles.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Four case studies

Case study Source Separation Level (SSL) Composition (% mass) Organic matter (with moisture + ash) Organic Volatiles (OV, no moisture, no ash) C Cl H N O S Ash Moisture Total LHV, MJ/kg HHV, MJ/kg

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

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A 35% 44,67 15,57 26,00 0,28 4,04 14,55 0,70 0,03 15,91 38,49 100,00 9,75 11,56

B 50% 54,45 18,98 24,53 0,27 3,86 15,31 0,73 0,03 12,79 42,48 100,00 9,03 10,90

C 50% 33,90 11,81 25,17 0,23 3,86 16,58 0,62 0,04 18,25 35,24 100,00 9,27 10,95

D 65% 34,46 12,01 27,50 0,27 4,23 15,44 0,64 0,03 17,00 34,89 100,00 10,38 12,14

Results - 1

Case study: Source Separation Level (SSL) Total mass reduction, kg/100 kgMSW of which: OV consumed Moisture evaporated Leachate produced Parameter LHV of bio-dried waste, MJ/kg HHV of bio-dried waste, MJ/kg Energy efficiency % on LHV basis % on HHV basis Other performance indexes Moisture reduction, Dkg/kgINITIAL Increase of LHV, % Increase of HHV, % O2 dep. air produced, Nm3/kgMSW

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

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A 35% 28,14 3,65 23,93 0,56 0,823 13,19 14,77 97,17 91,82 0,636 35,22 27,77 8,84

B 50% 34,30 4,36 29,25 0,69 0,882 13,38 14,98 97,28 90,31 0,705 48,07 37,46 10,12

C 50% 21,36 2,68 18,25 0,43 0,793 11,51 13,01 97,71 93,40 0,530 24,25 18,77 6,77

D 65% 21,71 2,73 18,55 0,43 0,797 12,98 14,56 97,95 93,97 0,544 25,11 20,03 6,87

Results - 2

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Case study: Source Separation Level (SSL) Bio-dried waste, % mass, wet basis C Cl H N O S Ash Moisture Total

A 35% 33,29 0,34 5,13 0,88 18,71 0,04 22,14 19,47 100,00

B 50% 33,80 0,36 5,28 0,95 21,02 0,04 19,46 19,09 100,00

C 50% 29,98 0,26 4,56 0,73 20,14 0,04 23,21 21,06 100,00

D 65% 33,07 0,31 5,05 0,75 18,74 0,04 21,71 20,32 100,00

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Conclusions and future developments

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Mass / Energy balances of bio-drying can be evaluated by a physical, simple model that couples the mass/energy equations (rather than a biological model). Interpretation of available experimental data has allowed simulating some interesting case studies, generating realistic outcomes Mass reduction ranges 20-35%. LHV of bio-dried waste is 25-50% higher than LHV of Residual Waste

LHV content (product M*LHV) of bio-dried waste is nearly the same of LHV content of initial Residual Waste

Targeted experimental activity is needed to properly calibrate and validate the model.

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

Conclusions and future developments

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Thank you for your attention !

www.mater.polimi.it

S. Consonni - Mass and Energy Balances of Biodrying - NAWTEC 2011

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