Converting Dairy Manure into Energy.An introduction to anaerobic digestion of farm waste

Stefan GrimbergAssociate Professor

Dept. Civil Environ. EngClarkson University

Thank you to Floyd Schanbacher

Anaerobic Digester Potential Benefits

Odor Control Reduced liability

Manure stabilization Help with nutrient management Soil stability

Energy generation Offset farm electricity requirements from grid Sale of extra energy Heat generation to reduce fuel required

Reuse of bedding solids Offset bedding cost

Plug Flow Digesters

Little mixing Few mechanical parts Simple

Mixed Digesters

Mechanical mixing in tankMore gas productionSlightly higher capital cost

Behringen co-fermentation plant in Germany

Community Digester

Digester Feed

Large-scale Digester Performance

Capital Cost of Manure Management Technology

Source: AgStar Publication:

Digester economics

Dr. Fick’s estimated fixed of $660 per cow is a little low for the farms we are looking at.

Tinesdale farm (in Minnesota), with1800 cows, invested about $1,000 per head.

Good estimate $1,000 per cow? At this capital assumption AD system become

economical based on sold energy alone at power sale price of $0.09/kWh.

(Mehta, 2002)

Case Study

•Gordondale Farms, Nelsonville, WI•750 head dairy•Sell biogas to utility that generates electricity on site, utility owns and operates generator•Sell at 0.015 $/kWh generated

Martin, J.H., 2005. An evaluation of a mesophilic, modified plug flow anaerobic digester for dairy cattle manure. Final report for EPA contract GS10F-0036K

Summary

Martin, 2005

Martin, 2005

Martin, 2005

Martin, 2005

Martin, 2005

Martin, 2005

Martin, 2005

Martin, 2005

Economic Evaluation

Capital costs with co/gen: $748,000 Annual sale of power: $18,396 Avoided annual bedding cost: $60,000 Sale of extra solids: $8,600 Total benefits: $86,996 Payback period: Approx 6 years.

Digester Systems Model

Digester Solids Sep.

Cogen

ManureOther Input

Biogas

Electricity offsetSurplus electricitySurplus heat

Spread liquid fraction

Solids for beddingSale of solidsSolids field application

Heat

Example

500 cow dairy Free stall barn Scraped manure Include flush from milking parlor Northern NY

Model Estimate: 500 cow dairyItem Cost/Benefit

Capital Cost Digester/Cogen system $656,180

Total Annual Cap. Cost over 10 yrs $68,774

Annual Operating Costs Maintenance, Repair $23,553

Potential Annual Benefits Fiber sales $9,133

Electric Sales (surplus) $17,707

Thermal Sales (surplus) $12,368

Methane Credit $7,646

Renewable Energy Tax Credit $4,203

Farm savings-Electric (internal) $25,171

Farm savings-Thermal (internal) $8,495

Tax Offset $16,739

Onsite Fiber value $10,000

Total Annual Benefits $111,463

Annual Net Benefit per Cow $38.27

Annual Net Benefit per Cow if excess heat cannot be sold $13.54

Annual Net Benefit per Cow without excess heat and methane credits -$1.8

Run Digester Model

Demonstrate: Effect of size Power cost Bedding off set

Any scenarios of interest?

Conclusions

Anaerobic digestion of manure has significant potential to decrease fossil fuel use in rural areas.

Community digesters may be very cost effective due to larger scale and mixed feed composition