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ANAEROBIC DIGESTION OF CLEMSON’S CAFETERIA
FOOD WASTEBiting Li and Jessica Ketchum
Senior Design Project
Biosystems Engineering
Clemson University
INTRODUCTION
CLEMSON’S DINING HALL FOOD WASTE
• 313 tons annually produced by the main two dining halls Harcombe and Schilletter can be used for AD• Summer: 360 lb/day of food waste
• Fall & Spring: 2,600 lb/day of waste food
WASTING OUR WASTE!!
• In the United States, food waste is the 2nd
largest component of municipal waste.
• The EPA estimates 14.5% of the 251 Million Tons of MSW in 2012 was food.(36.4 M.tons)
• More than 97% was dumped in landfills when it could have been composted or anaerobically digested.
• Anaerobic digestion can produce biological methane, allowing the US to rely less on non-renewable energy, and also nutrient rich digestate that can be utilized as plant fertilizer or soil amendment.
http://www.epa.gov/epawaste/nonhaz/municipal/
WHERE DO WASTES GO?
• Problems associated with landfill• Running out of space
• Causing health & environmental problems
• Example: Cairo in Egypt• Manshiyat Naser, a ward of Cairo
• “Garbage City”, slum settlement
• Zabbaleen: the garbage collectors
• Increasing & extending landfill is not the best option…
(Curry, 2010)
Figure: “Garbage City” located outside Cairo, Egypt (L), Zabbaleen family (R)
WHAT ARE OTHER WAYS TO TREAT WASTES?
• Incineration (burn)• Advantages
• Reduce volume
• Produce heat and steam generate power
• Disadvantages• Toxicity of the gases and ash
• Containing heavy metals
(Curry, 2010)
• Pyrolysis / Gasification• Temp. > 430°C• Chemical decomposition
• 2 main byproducts
• Syngas: CO + H2
• Used as fuel, b/c
~1/2 energy content
of natural gases
• Biochar ash
• Rich in C; used as
fertilizers
• Carbon negative
BIOLOGICAL PROCESSING WAYS
• Composting
• Aerobic reaction
• Organic constituents CO2 + heat + stable fertilizer
• “Carbon-neutral”
• No energy produced
• Anaerobic Digestion
• Anaerobic reaction
• Organic materials biogas + neutral digestate sludge
• CH4 can be used as fuel
• Digestate can be used as soil conditioners • Provide moisture content
• Supply nutrients
• Protect from soil erosion
(Curry, 2010)
BRIEF COMPARISON
Table: Efficiency Comparison of Renewable Technologies
PROJECT GOALS FOR THE DESIGN
• Processing of Clemon’s Dining Halls’(Harcombe & Schilletter) Food Waste
• 60% of the biogas produced from Clemson’s 313 tons of food waste annually are CH4 (Harcombe and Schilletter)
• 38074.74 m3 of biogas (fall & spring) • 22844.844 m3 CH4 (60%)
• 3305.24 m3 of biogas (summer & winter)• 1983.146 m3 CH4 (60%)
• Biogas total production: 41380 m3/year
• CH4 total production: 24828 m3/ year
POSSIBLE CONSTRAINTS OF DESIGN
• Time……• Amount and sources of food waste depends on time
• High variability b/w spring, summer, fall, Football food waste both in types and amount
• Safety……
• “Potential Errors in the Quantitative Evaluation of Biogas Production in Anaerobic Digestion Processes” (Walker et. al.)
Note: Up to 10% difference in Volume corrections at STP (IUPAC vs. NBS)
in many Anaerobic Digestion recent publications
CONSIDERATIONS DURING DESIGN
• Safety & Health issue • potential pathogens
• Ecological & Ethical concern• emission of product
• storage for CH4
• global warming problem
• Life Cycle
• supplement ions for bacteria (Fe, Mg)
• Ultimate Use• Energy & Soil conditioner
http://hajahubacademy.tumblr.com/post/27818028851/2012-07-23-workshop-permaculture-with-uni
QUESTIONS OF USER, CLIENT AND DESIGNER
• User’s questions
• How big the digester will be?
• Can I have one in my backyard without smelling bad odor?
• How do I check or evaluate functions of a digester?
• Client’s questions
• How long will it take to produce good amount of methane (and hydrogen)?
• How much will it cost to build a digester?
• Is it easy to move and assemble?
• Designer’s questions
• How to minimize the emission of methane into atmosphere?
• How big the reactor should be according to the food waste input?
• How much it costs?
LITERATURE REVIEW Governing Equations, Literature Data, etc.
ANAEROBIC DIGESTION PROCESS
http://www.intechopen.com/books/biomass-now-sustainable-growth-and-
use/microbial-biomass-in-batch-and-continuous-system
GOVERNING EQUATIONS
1. Volumetric Organic Loading Rate V’=(Ci)*(Q/V)
2. Hydraulic Retention Time, HRT Ĩ=V/Q
3. Buswell and Mueller (1952) (Formula/VS to obtain CH4 yield)
(Curry & Pillay, 2012)
4. Alkalinity and pH (Bicarbonate/Carbonate/NH4+↔NH3)
pH= - log[H+]
5. C6H12O6→3CO2 + 3CH4
6. Henry’s Law
p=kHc
GOVERNING EQUATIONS…
• The mass of the substrate can be converted to biogas in multiple ways.
• Buswell and Mueller (1952)-Takes mass of waste, using VS and chemical composition, to covert to volume of biogas produced.
• Using a conversion factor for kg COD/kg VS for specific substrate, biogas volume can be obtained with a VS value.
• Other Factors:
• Microbes: pH, Alkalinity, Temperature and Toxicity
• Mixed Culture to perform different steps of AD
Theoretical Yield of Biogas will be greater than actual due to Assumptions:
1. All VS=Organic Matter
2. Inhibition Factors not considered
3. Retention Time long enough for full AD
INCREASING C/N RATIO
• Microbes desired C/N ratio ranges from 20:1 to 30:1
• Problem of the mono-substrate anaerobic digester
• Low C/N ratio of 13.8 – 18.2
• Protein
• Co-digestion method
• Why?
• Balance C & N sources
• Improve digestion rate & biomethane yield
• Our Design?
• Adding paper towels & shredded paper
HARD DATA & EQUATIONS FROM LITERATURE
• Analysis of Food Waste
• Ultimate Analysis: Elemental basis
• Dryness of Input (b = dryness)• D = 1 for b </= 15%
• D = 1- exp(-0.3/(b-0.1))for b > 15%
• Digester Sizing Consideration• Volume [m3]= Flow Rate [m3/day] * VS Concentration [kg/m3] / OLR
[kg/(m3*day)]
• Cvs = (VS/TS) [%] * Density of dry substances [kg/m3]
C H O N S
% 48 6.4 37.6 2.6 0.4
Kg/ mol 5.45 0.46 7.26 6.35 14.55
(Curry & Pillay, 2012)
PRELIMINARY DATA
PH Density (ρ)
[g/cm3]
Water Content (Ɵ) [%]
TS/Wet [%] VS/Wet [%] VS/TS[%]
6.313 0.9812 76.14 23.86 22.92 95.935
• Summary Table of Measurements
• Experimental Procedures for Determining TS and VS
Weight [g] Empty
bowl
Filled bowl
with wet
food
Wet food
waste
(t=0)
FW after
24hrs @ 105°C
FW after
48hrs @ 105°C
FW after
7.5hrs @ 550°C
No. 1 46.6302 63.3120 16.6818 3.9681 3.9558 0.1638
No. 2 44.7587 59.5740 14.8153 3.5433 3.5293 NA
No. 3 44.9524 61.7362 16.7838 4.0516 4.0381 0.1609
ADM1-ANAEROBIC DIGESTION MODELING 1 SOFTWARE
• The IWA Anaerobic Digestion Modeling Task Group
• Established in 1997 by Congress
• Goal- To develop a generalized AD model
• Includes disintegration & 4 Major steps of AD in all their complexity
• Implemented as a differential and algebraic equation (DAE) set
• 32 dynamic concentration state variables using MATLAB AND SIMULINK software
http://spectrum.library.concordia.ca/7485/1/Curry_MASc_S2011.pdf
WHY IS THIS A GOOD APPROACH?
• Numerous Papers state Buswell Percent Error 275-
300% (Both Lab and Industrial Scale)
• ADM1 <35 % error
• ADM1 with Transformer <10% error
DESIGN METHODOLOGY & MATERIALS
BOUNDARY SYSTEM
Anaerobic
Digester
Biogas
CVEnergy Input Energy Output
(Temp. maintenance, mixing…)
(Energy of CH4 generatedfrom a gas turbine)
Heat loss(Radiation…)
Digestate
Paper towel
Food waste
Water
THEORETICAL YIELD
• Elemental basis: C, H, O, N & S
• C22H34O13N (C:H:O:N = 22:34:13:1)
• Using Buswell’s equation: a=22, b=34, c=13, d=1
• C22H34O13N +7.75 H20 11.625 CH4 + 10.375 CO2 + NH3
• V (biogas) = 1.0186 m3/kg VS
• Experimental biogas yield (300% overestimated)
• Corrected biogas yield = 0.3395 m3/kg VS
• Our goal is to yield 60% of CH4 out of biogas
• CH4 yield = 0.2037 m3/kg VS
(Curry & Pillay, 2012)
SYSTEM DESIGN STEPS
(Curry & Pillay, 2012)
Calculation of density needs 15%
• Digester type: CSTR - continuous flow & mixing
• Dryness of input
• Density = 1- exp(-0.3/(0.2286-0.1)) = 0.903 dry tons/m3 = 903 dry kg/m3
• Mass flow rate =
• Our dryness is 22.86 %
• Add water to the food waste
SYSTEM DESIGN STEPS…
FLOW RATE, HRT AND VOLUME OF REACTOR
# lbs. Daily * Weeks /Density of FW
PT mass flow rate was calculated by C:N
Divide by 0.75 to
get a Working
Volume with a
Headspace of
25%
Final Reactor Volume=348 m3Substrate Fall/Spring Winter/Summer
Food Waste 1.50 0.21
Paper Towels 0.86 0.12
Added Water 8.09 1.12
Total Q (QFW+QPT+QH2o) 10.45 1.45
Flow Rate (Q) [m3/day]
Ƭ (HRT) [days] Fall/Spring Winter/Summer
15 156.8 21.8
20 209.0 29.0
25 261.3 36.3
30 313.5 43.5
35 365.8 50.8
Volume V=Ƭ*Q [m3]Ƭ (HRT) [days] Fall/Spring Winter/Summer
15 209.0 29.0
20 278.7 38.7
25 348.3 48.4
30 418.0 58.1
35 487.7 67.7
Working Volume WV=V/0.75
REACTOR TANK
Height [ft] Height [m] Radius [ft] Radius [m]
40 12.2 10.4 3.2
25 7.6 13.15 4
15 4.6 17 5.2
Volume of a Cylinder: V=r2h∏
h=40’
D=20.8’
h=25’
D=26.3’
h=15’
D=34’
Best Optionhttp://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106
• Pre-fabricated Concrete Panels
• Required Volume is 348 m3
• Cladded, rigid insulation on exterior
http://www.fairtex.com.ng/insulation.php
REACTOR COVER• Double Membrane Gas Storage –
Tank Mounted | DMGS TM
• Company: SATTLER
• ¼ to ¾ Spherical, bolted to concrete tank
• Hermetically sealed
• Structural Analysis for Snow Load and Wind Pressure
http://www.sattler-global.com/environmental-engineering/gas-
storage-dmgs-tm-1083.jsphttp://www.sattler-global.com/environmental-engineering/gas-storage-dmgs-tm-1083.jsp
MIXING AND FLOW
Contents of unmixed digester become stratified into following layers:
Gas
Scum
Supernatant
Active Digester Sludge
Digested Sludge
Grit
• CSTR- Homogeneous 2-Layer remains after mixing
• Mixing options:
-Impeller
-CO2 Injection
• Energy Required?
http://en.wikipedia.org/wiki/Chemical_reactor
REACTOR MIXING
http://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106
• Specifically designed for Biogas Tanks
• Up to 39.4 ft. Depth
• Adjustable Height with Cable Winch
(Cleaning Made Easy/No Disruption of AD)
• Positioned 47” horizontally into the tank
• Height Indication Controls
• 135º +/- Angle Rotation
• Running 50% time
• Product Name: MGD
(SUMA America, Inc.)
http://www.gosuma.com/Ruehrwerke_Biogas_E.php?we_objectID=106
REACTOR HEATING• Constant Mesophillic Range Needed (35-40º C)
• Specifically designed for thermal processing of manure, sludge and liquid biomass
• Cold substrate can be circulated against heated substrate for heat recovery purposes
• Economical heat recovery for digester
• Extremely low energy loss
• Heat exchange surface: 12.3 m²
DOUBLE HELIX HEAT EXCHANGERS
STAINLESS STEEL PIPING
http://www.farmatic.com/en/biogas-components/heat-exchangers.html
REACTOR DESIGN SAFETY MEASURES
• Pressure Relief Valve
• Flame Arrester
• Vacuum Breaker
• Burst/Rupture Disk
http://www.turbosquid.com/3d-models/water-pressure-relief-valve-3d-model/470491http://www.atechsis.com/en_EN/arrete_flammes/arrete_flammes_en_li
gne.html
http://www.lowes.com/pd_21507-34146-MVB+3/4_0__?productId=3353894
http://singapore.corrom.com/Rupture_Disc.htm
ENERGY OUTPUT & YIELD
• Energy value of methane
• 1m3 CH4 36MJ = 10 kWh
• Theoretical Energy Output from Methane
• Theoretical Energy Generated from the system(η = 35%)
SAVING BILLS
• The least electricity bills we could save per day is in summer:
• 226.39 kWh/day * 11 cents/kWh = $24.9/day
• The most electricity bills we could save per day during fall or spring semester:
• 1667.27 kWh/day *11 cents/kWh = $183.4/day
http://www.npr.org/blogs/money/2011/10/27/141766341/the-price-
of-electricity-in-your-state
ALTERNATE DESIGN
• Currently focusing on single CSTR
• Interested in 2-stage CSTR
• 1st Stage containing acid forming bacteria
• May increase stability since methanogens have a high pH sensitivity (Bonomo, 2011)
Acetogenesis &
Methanogenesis
(2)Acidogenesis(1)
HRT 1 < HRT 2
SUSTAINABILITY MEASURES
SUSTAINABILITY MEASURES
• Contributions
• Economic: produce energy & save bills
• Ecological: reduce environmental issues
• Social: bring alternative energy
• Ethical: green & concern
• Efficiency
• Societal issues
• Less FW, less rodent/insect issues
• Odor emission of H2S
• Active Carbon or Iron Oxide Coated wood chips
• C & H2O footprint
• Lower Carbon Footprint; but be aware
• Burning H2 small amount H20
http://www.ptj.com.pk/Web-2011/04-2011/Dyeing-Benninger.htm
LCA-LIFE CYCLE ASSESSMENT• LCA Cradle to Grave
• Consider Impacts on Human Health, Ecosystem, Climate Change, Resources
• Important Consideration when comparing AD to Landfill life cycle—TIMELINE
• (1 yr? 5 yrs? 10 yrs?
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.
Inputs:
-Water
-Energy
-Raw
Materials
Outputs:
CO2
Methane
H2S
Digestate
Michael Carbajales-Dale, Asst. Professor, Clemson University, Intro to LCA, 2014.
BUDGET
ANAEROBIC DIGESTION: 3 SOURCES OF VALUE
1. Electricity Generation: Converting biogas through electric generator with FIT contact
-Sold to Grid at price range (0.132$/kWh) to (0.269$/kWh)
($30-$60/day in Summer) ($220-$450/day in Spring and Fall)
-2009--CU purchased 133,410,000 kWh for $7.16 million
-2011--Decrease in use/rising energy cost (122,127,434 kWh at $10.2 million)
2. Heat Generation: Burning the biogas or capturing heat given off when run through electrical
generator
3. Tipping Fees- Fee paid for AD of organic waste
(Waste from restaurants, farms and meat processing plants)
http://www.investopedia.com/terms/f/feed-in-tariff.asp(Banks, 2006)
Total Savings $$ $60-125,000/year
CAPITAL COSTS:
CSTRThe first method calculates the base capital cost
by multiplying the base generator size by the
estimated average capital cost per kilowatt (kW).
• Minimum capital cost set to $300,000
The second method is the one that is currently
being used by the workbook. This method has a
minimum capital cost of $250,000 with an addition
$5,000 added per kW of capacity
(Anderson, 2012)
TIME LINE
REFERENCE
1. Banks, C.J. et. al. (2011). Anaerobic digestion of source-segregated domestic food waste: Performance assessment by mass and energy balance. BioResourceTechnology, 102(2), 612-620.
2. Dr. Sandra Esteves and Desmond Devlin-Technical report food waste chemical analysis, PDF of Final Report produced March 2010, Company: Wales Center of Excellence for Anaerobic Digestion.
3. Curry N. & Pillay P. (2012). Biogas prediction and design of a food waste to energy system for the urban environment. Renewable Energy, 41 (2012) 200-209.
4. http://www.ptj.com.pk/Web-2011/04-2011/Dyeing-Benninger.htm
5. http://hajahubacademy.tumblr.com/post/27818028851/2012-07-23-workshop-permaculture-with-uni
6. http://www.alternative-energy-action-now.com/hydrogen-power.html
APPENDICES• Theoretical yield
• Assume 1 mol of N; Percentage of C, H, O, N, S and their kg/mol values are given
• N= (150 tonnes) * (1000kg/tonnes) * (2.6%) /(6.35kg/mol) = 614.173 mol
• C = (150 tonnes) * (1000kg/tonnes) * (48%) /(5.45kg/mol) = 13211.009 mol
• H = (150 tonnes) * (1000kg/tonnes) * (6.4%) /(0.46kg/mol) = 20869.565 mol
• O = (150 tonnes) * (1000kg/tonnes) * (37.6%) /(7.26kg/mol) = 7768.595 mol
• C:H:O:N = 13211.009 : 20869.565 : 7768.595 : 614.173~~ 22 : 34 : 13 : 1
• Buswell’s equation: a=22, b=34, c=13, d=1
• (4a-b-2c+3d)/4 = 7.75; (4a+b-2c-3d)/8 = 11.625; (4a-b+2c+3d)/8 = 10.375
• C22H34O13N +7.75 H20 11.625 CH4 + 10.375 CO2 + NH3
• 1 mol C22H34O13N 11.625 mol CH4
• (150 tonnes) * (1 mol C22H34O13N/ 520 g) * (1/1 mol C22H34O13N) * 11.625 mol CH4 * (16g/1mol CH4) = 53.654 tonnes CH4
• Density (CH4) = 0.66kg/m3 V (CH4) = 81294 m3
• 1 mol C22H34O13N 10.375 mol CO2 density (CO2)=1.842kg/m3 71489 m3
• Total biogas generated for 150 tonnes of food waste = 152783 m3
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