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ABE 482Environmental

Engineering in Biosystems

September 22

Lecture 8

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Today

Anaerobic digester parameters

Biogas production

Digester heat balance

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Anaerobic Digestion Parameters

Consider the Continuous Stirred TankReactor (CSTR)

Hydraulic retention time

Specific loading rate

Specific biogas production

Specific methane production

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Hydraulic Retention Time (HRT)

Not same as length of time biomass is in the digester

unless there is perfect plug flow Optimum retention time = 10-25 days for material

with DM% = 4-12 For higher DM%, retention time is not the rate

limiting parameter!

/day)(mratefeedDaily)(mdigesterofVolume(days)HRT

3

3

=

(kg/day)rateremovalSolids

(kg)TankinSolidsofMassSRT =

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Specific Loading Rate (SLR)

Well-designed anaerobic digester willmaximize SLR

Varies according to feedstock and

digester type Optimum about 3-6 kg ODM/m3-day

)(mDigesterofVolumeODM/day)(kgfedOMofMass)

day-mODMkg(SLR 33 =

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Biogas Production

Biogas: CH4, CO2, H2O vapour, H2S, H2 andN2

Density of CH4 = 0.71 kg/m3

@ stp Density of CO2 = 1.96 kg/m

3 @ stp

Density of biogas?

Volume of biogas?

Will actual biogas production be higher orlower than theoretical value?

Theoretically: Biogas (kg/day) = ODMR (kg/day)

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Specific Biogas Production (SBP)

Indicator of digester efficiency Should be higher than 1.5 (target

2.5)

)(mVolumeDigester/day)(mProductionBiogas)(daySBP

3

3

1-=

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Biogas production

15 - 3055 600.31 0.35MSW

20640.37 0.61Sheep manure

10 1558 610.49 0.76Pig manure

9 3058 600.31 0.56Poultrymanure

n/an/a0.20 0.33Cattle manure

RetentionTime(days)

MethaneContent(%)

GasProduction(m3/kg dry solids)

Material

Table 1: Biogas production rates for a variety of feedstock materials.

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Specific Methane Production (SMP)

Relates energy production to the potentialenergy in feedstock

Between 0.3 0.4 for some energy crops

Based on table data, 0.8 for some manure

ODM/day)(kgRateLoadingOM

/day)m(CHofVolumeODM)/kgCH(mSMP

3

44

3=

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Example 2aCalculation of biogas production

Feedstock:

Mass = 50 t/day ODM = 13.5 t/day OMR = 9 t/day

Biogas is 60% CH4 Volume of digester is 3,000 m3

Calculate: Volume of biogas produced Specific loading rate

Hydraulic retention time Specific biogas production Specific methane production

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Example 2bCalculation of biogas production

A bench-scale digestion plant of mixed

wastes produced the following results Reactor volume = 2 L

Feedstock CODinf= 97,000 mg/L

CODeff= 4,000 mg/L Biogas = 1.13 m3/m3-day

CH4 = 59%

Daily feed rate = 55 mL

Determine the volume of CH4 produced perkg of waste digested (SMP)

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Energy Calculations

Energy balance of an anaerobicdigester involves

Fuel value of biogas

Conversion of biogas to useful energy

Digester heat input

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Fuel Value of Biogas

Lower Calorific Value of CH4

= 35.7MJ/m3

How much energy is produced when 1

m3

of biogas is combusted (60%CH4)?

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Example 3Fuel value of biogas

Calculate the fuel value (kW) of thebiogas produced from Example 2a

7500 m3/day @ 60% CH4

Calorific value of CH4

= 35.7 MJ/m3

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Conversion of Biogas to UsefulEnergy

Convert fuel energy to useful energy

Hot water in a boiler Electricity and hot water

Methane converted to hot water withefficiency = 85%

Methane converted to electricity withefficiency = 32%

Heat also recovered Engine jacket water (80C) efficiency = 35%

Engine exhaust (500C) efficiency = 18%

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Conversion of Biogas to UsefulEnergy

Convert to hot water only (boiler)

7,500 m3 biogas per day produces 136,400 MJper day of hot water or 1580 kW of continuoushot water

Convert to electricity and hot water (CHP

unit) 7,500 m3 biogas produces 595 kW of electricity 7,500 m3 biogas produces 650 kW of hot water

(engine jacket) 7,500 m3 biogas produces 335 kW of hot water

(exhaust) Total: 1580 kW of electricity and hot water

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Example 4Calculating heat loss and input

The 3,000 m3 digester was designed with a

0.33 surface area to volume ratio. Theinsulation thickness is 0.1 m and has aconductivity of 0.035 W/m-C. The digesteris running at a temperature of 37C.

Calculate the heat loss through the surfaceif the ambient air temperature is 5C.

The feedstock enters the digester at a

temperature of 10C and a rate of 50t/day. Calculate the heat input required toraise the temperature to 37C.

Specific heat capacity of water = 4.19 kJ/kg-C

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Digester Heat Balance

Balance between energy produced

from biogas and energy required tomaintain mesophilic temperatures

Other energy requirements of

anaerobic digester? Stirring, pumping, etc.

Energy produced depends on OMR, so

for feedstocks with lower ODM,energy produced will be lower

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Example 5Digester heat balance

If the 7,500 m3 per day of biogas is used to

produce hot water only, what percentage ofthis hot water energy is required fordigester heating?

What percentage of the hot water energyfrom a CHP unit is required for digesterheating?

What percentage of energy produced (hotwater only) is required for digester heatingduring a Saskatchewan winter?

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12 weeks3 weeks digestion, plus

5 weeks composting

Process time required to produce

mature compost

Nil100 150 m3

/MgBiogas production

Energy demandEnergy surplusEnergy balance

100%20%Odours

100%50%Space requirement (footprint)

CompostingAnaerobic Digestion

Comparison Table From: Introduction to Anaerobic Digestion,Wolfgang Muller and Axel Huttner, ORA -Organic Resource Agency Ltd., Malvern Hills Science Park, Geraldine Road, Malvern, Worcestershire WR143SZ, and IGW Ingenieurgemeinschaft Witzenhausen Fricke & Turk, and GmbH, Bischhuser Aue 12, D-37213 Witzenhausen, Germany. Presented at the Biowaste: Digesting the Alternatives Seminar, April 2005,UK.

A Comparison of Anaerobic Digestion and Composting

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References

Wikipedia, 2007. Anaerobic Digestion. Online:

http://en.wikipedia.org/wiki/Anaerobic_digestionAccessed: October 25, 2007.

Class notes for ChEng 882. 2006. University ofSaskatchewan.

Kiely, G. 1997. Environmental Engineering. McGrawHill:USA.

Chesshire, M. 2005. Process Calculations for Full-Scale Digesters. Biogas from Energy Crops andAgrowastes. 15th Jyvskyl Summer School:Cropgen.

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Question for next class

Calculate the $ value of the electricity produced if all theMSW waste and manure waste produced in Canada each

year was anaerobically digested. 25 million wet tonnes of MSW/year 132 million wet tonnes of manure/year

30% dairy cow 30% beef cow

20% pig 20% other

Assume digester runs 365 days a year, 24 hours a day Assume you can sell electrical energy for 14 cents/kW-hr Assume you can convert methane to electricity with an

efficiency of 32%

How many Canadians will this serve?

Use: Table 1 from lecture, ASABE standards for manurecomposition, internet for electricity consumption of Canadians.