Cooperative Extension Lewis County

Final Report June 2010 (updated)

Feasibility Study of Anaerobic Digestion and Biogas Utilization Options for the Proposed Lewis County Community Digester

www.manuremanagement.cornell.edu

Feasibility Study of Anaerobic Digestion and Biogas Utilization Options for the Proposed Lewis County Community Digester

By:

Curt Gooch, P.E.1, Senior Extension Associate

Jennifer Pronto1, Research Support Specialist

Brent Gloy, Ph.D2, Professor

Norm Scott, Ph.D1, Professor

Steve McGlynn1, Research Support Specialist

Christopher Bentley1, Undergraduate Student

1Biological and Environmental Engineering Department

2Department of Applied Economics and Management

334 Riley-Robb Hall

Cornell University

Ithaca, New York 14853

June 11, 2010

Updated June 30, 2010

Foreword

The Feasibility Study of Anaerobic Digestion and Biogas Utilization Options for the Proposed Lewis

County Community Digester project is not a feasibility study in its strictest definition, but rather an

assessment of the farm and non-farm biomass resources available in and around the village of Lowville,

an investigation into the available options for co-digesting them (various combinations of materials and

site locations), an estimation of the biogas that could be produced by the various scenarios, the resulting

energy produced, and net energy available for use, and an economic profitability assessment for each of

the options investigated. The scope of work for this project was somewhat dynamic as adjustments

were continually made based on progress of evaluating the information at hand. This report was

written to provide the findings and recommendations of the feasibility study to the client, the Lowville

Digester Workgroup, and also to serve as an educational tool for the stakeholders of this and future

proposed centralized anaerobic digester projects.

The proposed Lewis County Community Digester project exemplifies the full potential of a centralized

anaerobic digester. Manure and, waste biomass materials (processing byproducts from multiple

sources), are mixed together and heated to produce biogas; a locally generated, clean burning,

renewable energy. Waste biomass is generated daily by food processing plants and restaurants, public

facilities and institutions such as schools and hospitals, and at private residences. Co-digesting manure

and these materials reduces the burden on landfills and reduces greenhouse gas (GHG) emissions. The

U.S. dairy industry has formally committed to reducing its GHG emissions by 25% by 2020 and this

project is an example of how this can be effectively accomplished, from a technical/applied perspective.

In fact, the Lewis County Community Digester project demonstrates the vision behind “Dairyville 2020”

– the Innovation Center for U.S. Dairy’s Dairy Power Initiative flagship project. The major shortcomings

at this point are high capital costs and less than required energy purchase prices needed to make such

systems economically feasible.

Acknowledgements This document is the culmination of a team effort by the authors and many others who provided their

assistance and support. The authors wish to acknowledge and thank the following individuals/groups

for their contributions:

Senator Joseph Griffo, 47th District in New York State, for funding this project and for his continued

interest.

The dairy farmers of Lewis County who completed the farm surveys.

Representatives for the non-farm biomass suppliers who completed the non-farm surveys.

Drs. Dave C. Ludington and Michael B. Timmons, Professor Emeritus and Professor, respectively, of

Biological and Environmental Engineering at Cornell University for their efforts in reviewing drafts of the

feasibility study and for their constructive inputs and suggestions.

Members of the Lowville Digester Workgroup for their confidence in the Cornell team to provide a

feasibility study that would contain unbiased information and for their teamwork and collaboration

while the feasibility study was being conducted.

Ms. Christine Ashdown (Cornell Office of Sponsored Programs) for her timely efforts in developing the

contract for this project and for her continued support to funded project opportunities pursued by

members of the Cornell PRO-DAIRY program.

Ms. Michele Ledoux (Cornell Cooperative Extension – Lewis County) for her trust in the Cornell team

and for her work in securing the funding and performing contract administration tasks that resulted in a

workable means to performing this work.

Ms. Norma McDonald (North American Sales Manager, Organic Waste Systems, Inc.) for providing key

information on energy crop digesters suitable for U.S. applications needed to perform the annual

economic profitability analysis for the energy crop digester scenarios investigated.

Mr. Todd Vernon (Senior Sales Manager, GE Energy – Jenbacher) for providing key information on the

Jenbacher engine-generator sets needed to perform the annual economic profitability analysis.

Mr. Frans Vokey (Cornell Cooperative Extension – Lewis County) for his overall leadership of the

Lowville Digester Workgroup and Cornell collaboration, and for all of his efforts in planning and running

project meetings.

Mary Beth Anderson (community resident) for her assistance in collecting samples from non-farm

biomass suppliers and for work on distributing and collecting non-farm biomass surveys.

Mike Durant (Soil and Water Conservation District) for designing the project map.

Table of Contents

Foreword

Acknowledgements

Table of Contents

Table of Figures

Table of Tables

Abbreviations and Acronyms

Executive Summary p. 1

Introduction p. 15

Chapter 1. Basics of Centralized Dairy Manure-based Anaerobic Digestion, Biogas Utilization, and Nutrient Recovery Systems

p. 23

Chapter 2. Literature Review of Centralized AD Projects p. 39

Chapter 3. Farm and Community Biomass Survey p. 49

Chapter 4. Biomass Sample Collection and Analysis p. 61

Chapter 5. Biomass Transportation p. 71

Chapter 6. Preliminary Investigation of Five AD Scenarios p. 77

Chapter 7. Final AD Scenario Selection and Details p. 93

Chapter 8. Next Steps and Recommendations p. 115

References p. 117

Appendix

A. Glossary of terms p. 121

B. Farm-based Survey p. 127

C. Non Farm-Based Survey p. 131

D. Substrate Sampling Report p. 133

E. Biochemical Methane Potential; Laboratory Procedure p. 137

F. Projected Farm Survey Responses p. 139

Table of Figures Page Figure 1. New York State map showing location of project-site ................................................................. 16 Figure 2. Typical CAD system process flow diagram ................................................................................... 23 Figure 3. A CAD in Jutland, Denmark .......................................................................................................... 24 Figure 4. Danish above-grade complete mix vertical digesters in background .......................................... 29 Figure 5. Thermal to electric conversion efficiency of six NYS on-farm engine-generator sets. (Source: Gooch, Pronto, Ludington, Unpublished, 2010) ......................................................................................... 32 Figure 6. Basic process flow diagram for advanced biogas clean-up for biomethane production. ........... 34 Figure 7. Advanced digestate treatment to segregate and concentrate nitrogen, phosphorus, and potassium. ................................................................................................................................................... 38 Figure 8. Landfill tipping fees ($/ton) by region of the U.S. (Repa, 2005) .................................................. 47 Figure 9. Landfill tipping fees ($/ton), developed from Figure 8 (Repa, 2005). ......................................... 47 Figure 10. Lowville regional map with collaborating dairy farms superimposed along concentric circles of various radii centered on downtown Lowville............................................................................................ 54 Figure 11. Quantity (millions lbs/yr.) of substrates (wet weight). ............................................................. 57 Figure 12. Biochemical Methane Potential (BMP) data (cumulative biogas yield) for substrate 4. ........... 62 Figure 13. Graphical representation of biochemical methane potentials for all substrates tested. .......... 63 Figure 14. Estimated annual minimum, maximum, and average methane production by substrate. ....... 66 Figure 15. Estimated aggregated annual minimum, maximum, and average methane production of non-farm biomass substrates and manure. ....................................................................................................... 66 Figure 16. Nutrient concentrations for pre- and post-digestion conditions for N, P, K.............................. 70 Figure 17. Diagram of estimating a break-even tipping fee for non-farm biomass substrate suppliers. ... 75 Figure 18. CAD Site 1 for Scenario Nos. 1 and 2. ........................................................................................ 78 Figure 19. Remote AD Site 2 for Scenario Nos. 3, 3a, and 3b. .................................................................... 79 Figure 20. Remote AD Site 3 for Scenario Nos. 3, 3a, and 3b. .................................................................... 80 Figure 21. Process flow diagram for Scenario No. 1 using the average annual total volume of the seven non-farm biomass substrates. .................................................................................................................... 81 Figure 22. Process flow diagram for Scenario No. 2 using the average annual total volume of the three non-farm biomass substrates. .................................................................................................................... 83 Figure 23. Process flow diagrams for Scenario No. 3 using the average annual total volume of the three non-farm biomass substrates for Site 2 and Site 3. All manure and digestate are trucked. ..................... 85 Figure 24. Process flow diagram for Scenario No. 3a using the average annual total volume of three non-farm biomass substrates for Site 2 and Site 3. Manure and digestate are pumped and trucked. ............ 87 Figure 25. Process flow diagram for Scenario No. 3b using the average annual total volume of three non-farm biomass substrates for Site 2 and Site 3. ........................................................................................... 89 Figure 26. Final Scenario No. 2 process flow diagram. ............................................................................... 94 Figure 27. Energy crop anaerobic digester process flow diagram. ............................................................. 94 Figure 28. Comparison of the volume of manure sent to the CAD and volume of CAD effluent received, by farm. ....................................................................................................................................................... 96 Figure 29. Comparison of the volume of manure sent to the CAD and volume of CAD effluent received, by farm, taking into account each farm's nutrient balance situation....................................................... 112 Figure 30. Image of residential food waste sample collected. ................................................................. 134 Figure 31. Meat and butcher waste from substrate number 4. ............................................................... 135

Final%20report.doc#_Toc266177221Final%20report.doc#_Toc266177226

Table of Tables Page Table 1. Typical fuel-to-power efficiency values (adapted and updated from Wright, 2001). .................. 33 Table 2. St.Albans/Swanton, VT project statistics ...................................................................................... 39 Table 3. LREC project statistics ................................................................................................................... 41 Table 4. Dane County, WI (Waunakee cluster) project statistics ............................................................... 42 Table 5. Cornell project statistics ................................................................................................................ 43 Table 6. York, NY project statistics .............................................................................................................. 43 Table 7. Salem, NY project statistics ........................................................................................................... 44 Table 8. Perry, NY project statistics ............................................................................................................ 45 Table 9. Port of Tillamook project statistics................................................................................................ 46 Table 10. Summary of current (2009) farm survey data ............................................................................. 51 Table 11. Summary of nutrient balance information as provided in farm surveys ................................... 53 Table 12. Summary of non-farm biomass survey results............................................................................ 56 Table 13. Select Lewis County crop farm data ............................................................................................ 58 Table 14. BMP analysis results for all substrates tested ............................................................................. 63 Table 15. Biogas production potential of non-farm biomass substrates and manure ............................... 65 Table 16. Potential biogas production of available energy crop acreage ................................................... 65 Table 17. CES lab results for each non-farm biomass substrate: nutrients ................................................ 67 Table 18. CES lab results for each non-farm biomass substrate: solids...................................................... 67 Table 19. Estimated annual mass of nitrogen series for raw AD feedstock .............................................. 68 Table 20. Estimated annual mass of phosphorus and potassium series for raw AD feedstock ................ 68 Table 21. Predicted annual mass of nitrogen for post-digested AD feedstock ......................................... 69 Table 22. Predicted annual mass of phosphorus and potassium for post-digested AD feedstock ........... 70 Table 23. Capital and annual cost estimates for a project-owned trucking fleet ....................................... 72 Table 24. Contracted trucking fleet example schedule .............................................................................. 73 Table 25. Scenario No. 3a means of manure and digestate transport ....................................................... 87 Table 26. Comparison of the five AD scenarios .......................................................................................... 91 Table 27. Scenario No. 2 participating farms and associated manure generation ..................................... 94 Table 28. Scenario No. 2 feedstock volumes .............................................................................................. 97 Table 29. Potential methane and biogas production volumes for each feedstock in Scenario No. 2 CAD 98 Table 30. Capital costs ($) for Scenario No. 2 CAD system, engine-generator set, and biogas clean-up

system, and totals for two different energy sale options ............................................................... 101 Table 31. Annualized capital costs ($) for the Scenario No. 2 CAD system based on minimum, maximum,

and average biogas production quantities...................................................................................... 102 Table 32. Scenario No. 2 CAD, annual operating and maintenance expenses ($) .................................... 103 Table 33. Scenario No. 2 CAD, total annual costs ($) ................................................................................ 103 Table 34. Scenario No. 2 CAD net annual economic profitability ($) 2,3 for various biomethane sale prices

and biogas production volumes (no tipping fees received) ............................................................ 104 Table 35. Scenario No. 2 CAD net annual economic profitability ($) 2,3 for various electrical energy sale

prices and biogas production volumes (no tipping fees received) ................................................. 104 Table 36. Scenario No. 2 CAD net annual economic profitability ($) 2,3 for various biomethane sale prices

and biogas production volumes, including current tipping fee paid by substrate supplier #8 ...... 105 Table 37. Scenario No. 2 CAD net annual economic profitability ($) 2,3 for various electrical energy sale

prices and biogas production volumes, including current tipping fee paid by substrate supplier #8 ......................................................................................................................................................... 105

Table 38. Scenario No. 2 CAD net annual economic profitability ($)2 for various biomethane sale prices and tipping fee revenues ................................................................................................................ 106

Table 39. Scenario No. 2 CAD, net annual economic profitability ($)2 for various electrical energy sale prices and tipping fee revenues ...................................................................................................... 106

Table 40. Annualized capital costs ($) for energy crop digester system .................................................. 108 Table 41. Net annual economic profitability ($) for various electricity prices and feedstock costs ......... 109 Table 42. Capital cost estimate for construction of on-farm short-term manure storage per farm ........ 111 Table 43. Scenario No. 2 CAD nitrogen series annual masses by feedstock source and totals ................ 111 Table 44. Scenario No. 2 CAD phosphorus and potassium series masses by feedstock source and totals

......................................................................................................................................................... 111 Table 45. Farm survey responses based on projections for two years ..................................................... 139 Table 46. Farm survey responses based on five year projections ............................................................ 140

Abbreviations and Acronyms AD Anaerobic digestion BMP (1) Best Management Practice BMP (2) Biochemical Methane Potential Btu British thermal unit (mmBtu = 1 x 106 Btu), (TBtu = 1 x 1012 Btu) CAD Centralized anaerobic digester CAFO Concentrated Animal Feeding Operation cfm Cubic feet per minute CCE-LC Cornell Cooperative Extension of Lewis Count CIP Clean-in place wastewater CMMP Cornell Manure Management Program CNMP Comprehensive Nutrient Management Plan CBM Compressed biomethane CH4 Methane CHP Combined heat and power CNG Compressed natural gas CO2 Carbon dioxide COD Chemical oxygen demand Decatherm = 1 million Btu ESP Electrical service provider FOG Fats, oils, and greases ft3 Cubic foot gal US gallon (3.8 liters) GE General Electric Company GHG Greenhouse gas GWh Giga-Watt hours GWP Global Warming Potential gpm Gallons per minute H2 Hydrogen H2S Hydrogen sulfide HRT Hydraulic retention time kg Kilogram kW Kilowatt kWh Kilowatt-hour L Liter LCE Lactating cow equivalent LWWTP Lowville Wastewater Treatment Plant Lb(s) US pound LNG Liquefied natural gas m3 Cubic meter mmscf Million standard cubic feet MW Megawatt MWh Mega-Watt hours N2 Nitrogen N2O Nitrous oxide NH3 Ammonia

NPK Nitrogen, phosphorus and potassium content of fertilizer/organic matter NRCS Natural Resources Conservation Service NYS New York State OLR Organic loading rate O&M Operations and maintenance PPA Power Purchase Agreement REC Renewable energy credit RNG Renewable natural gas STP Standard Temperature and Pressure TSS Total suspended solids SCFM Standard cubic feet per minute (adjusted for temperature and pressure) SLDM Sand-Laden Dairy Manure SLS Solid-liquid separator SPDES State Pollutant Discharge Elimination System VFA Volatile fatty acids VS Volatile solids VSS Volatile suspended solids yd3 Cubic yard

1

Executive Summary The region surrounding Lowville, New York has multiple existing large scale renewable energy systems,

including wind and hydro-power. In the spirit of broadening the area’s renewable energy systems,

members of the Lowville Digester Work Group (comprised of representatives from Cornell Cooperative

Extension of Lewis County (CCE-LC), Kraft® Foods, Lewis County Economic Development Office,

residents, dairy farmer representatives, Lewis County Farm Bureau, and the Soil and Water Conservation

District) desire to develop a locally-owned and operated biomass-based renewable energy system. The

energy produced would stay local and the system would provide direct benefits to Lewis County

farmers, businesses, and residents. This desire prompted an investigation of anaerobic digestion

technology and its application in a centralized anaerobic digester (CAD) system that would use both

farm and non-farm biomass feedstock sources as input materials.

The Lowville Digester Work Group, in June of 2009, commissioned Cornell University (Ithaca, New York)

to conduct this feasibility study through funding provided by Senator Joseph Griffo of the 47th District in

New York State. Cornell worked closely with the Lowville Digester Work Group to develop the feasibility

study scope of work and key parts of its implementation.

The scope of the feasibility study consisted of multiple biomass related components including: resource

assessments, sampling and laboratory analyses (biochemical methane potential and nutrient

concentration investigation), methane production estimations and trucking analyses. The scope of work

also included biogas to energy conversion quantifications, digester site option investigations, and

economic profitability analyses. The major findings pertinent to each of these areas investigated are

provided below; the report contains additional information and details.

Biomass Resource Assessment

Many potential sources of farm and non-farm biomass in and around Lowville were initially identified by

members of CCE-LC. Project specific surveys, one for use in assessing the dairy farms and one for

assessing the non-farm biomass sources, were developed by Cornell University and CCE-LC. Identified

farms were surveyed by members of CCE-LC while non-farm biomass sources were surveyed by the

Lewis County Economic Development office.

2

The farm survey results revealed that there are 25 dairy farms (herd size ranges from 62 to 787 cows)

within an 18-mile radius of downtown Lowville with a total of 5,327 lactating cow equivalents (LCEs). All

of these farms have long-term manure storages (6-month or longer), and use organic bedding material

to bed their cow stalls. Five of the farms reported they have excess organic nutrients (nitrogen,

phosphorus, and potassium), while nine farms indicated that they are nutrient deficient, and 11 are in

balance. An opportunity exists for this project to help farms better manage their nutrients and lessen

the need to purchase commercial fertilizers. The survey responses also showed that the number of LCEs

would increase by approximately 675 cows over two years, and then by 150 more cows after five years.

It should be noted that the actual change in cow numbers in the future (increase or decrease) will be

driven primarily by dairy farm profitability.

The non-farm survey results revealed there are 11 potential sources of biomass (local food processors,

food vendors and residents were surveyed) in the local area that could be aggregated and co-digested

with manure. The minimum estimated useable quantity of substrates from the six non-farm biomass

sources with the highest volumes, was 110 million lbs/year, and the maximum quantity of useable

substrate was 160 million lbs/year. Two of the potential sources (whey mixed with CIP water and post-

digested sludge) provide the bulk (largest volume) of the non-farm biomass available for digestion.

Initial survey results prompted investigation into additional sources of biomass for co-digestion to

further increase potential biogas production. This included manure from sand-bedded dairy farms,

which was ruled not to be an option at this time due to the small farm sizes and comparatively large

capital equipment cost to effectively separate bedding sand from manure. Potential biomass sources

from Fort Drum, a nearby United States Army base, Reed Canary grass from fallow ground along the

Black and Beaver Rivers, and sludge from the Lowville Wastewater Treatment plant (LWWTP) were also

considered and investigated but due to availability, harvesting, and handling issues, all were deemed not

feasible for inclusion at this time, and therefore were not included in further analysis.

Energy crops (corn silage and haylage) fed directly to an energy crop digester were also considered. Two

farms, one north of Lowville and the other south of Lowville, that are currently solely cash crop farms

were included, but kept separate, in the overall analysis.

3

Biomass Sampling and Laboratory Analysis

Based on the information available from the 10 completed non-farm biomass surveys1, the decision was

made to obtain samples from five of the 10 potential feedstock sources, with one source having two

different materials analyzed, for a total of six potential feedstock materials analyzed. These included

waste grease, meat processing by-products, mixed food scraps, post-digested sludge, and diluted whey.

Sub-samples of the collected materials were analyzed in triplicate at the Cornell Agricultural Waste

Management Laboratory to quantify the biogas and methane (CH4) produced by these materials, on a

unit basis. As expected, the laboratory results showed that the waste grease material produced the

highest unit yield (363 L CH4/kg raw substrate2) and the diluted whey the least (2 L CH4/kg raw

substrate2). Sub-samples were also analyzed at an EPA certified laboratory, to quantify their nutrient

composition.

Biogas and methane production estimates for dairy manure were obtained from previous work

conducted at the Cornell Agricultural Waste Management Laboratory where several manure samples

had previously been obtained from commercial New York State dairy farms and analyzed using the same

procedure (Labatut and Scott, 2008).

Methane Production Estimation

The methane (CH4) production for dairy manure and each identified non-farm biomass substrate was

estimated by multiplying the methane production (on a unit mass basis) by the annual estimated

biomass quantity provided in each of the completed surveys. Using this approach, the estimated

minimum annual methane production was 10 thousand ft3 CH4/yr for waste grease (due to its

comparative low quantity available) and the maximum was 157 million ft3 CH4/year for manure (due to

the comparatively high quantity available).

Energy crop methane production estimates were developed using typical yields (wet tons/acre) for corn,

grass, and alfalfa silage for Lewis County, applied to the cropland currently farmed by the two

potentially collaborating cash crop farmers (2,000 and 400 acres). Total biomass yields were multiplied

by unit methane yields for each crop; overall, the estimated annual methane yield from the energy crop

digester was 97 million ft3 CH4/year.

1 Substrate supplier #11 was not initially surveyed; it was discovered subsequent to the conclusion of the survey period. 2 Expressed on a wet weight basis

4

Energy Potential Quantification

Assuming that manure from 15 selected farms3 and the three non-farm biomass substrates with the

highest volumes are co-digested, and using the average estimated gross and parasitic electrical energy

values, the resulting potential net electrical energy available from the CAD facility would be

approximately 8,880 MWh/year. Assuming a typical residence uses 7,250-kWh/year, approximately

1,225 homes could be powered by the CAD facility. If all net energy available were used for biomethane

sale, the CAD facility would be capable of producing 80,800 million Btu’s, which would have a residential

value (at a price of $13.81/1,000 ft3 natural gas) of $1,115,900.

Trucking Analysis

The proposed project would encompass facilitating the transport of raw manure to the centralized

anaerobic digester (CAD) facility (30 million gallons per year), and CAD effluent (42-48 million gallons per

year), back to the collaborating farms at no cost to the collaborating farms. The CAD effluent is a higher

volume than the manure proportion of the influent due to the inclusion of non-farm biomass substrates

at the CAD facility, which would be transported to the CAD by each substrate supplier at their cost. Two

options for the transport of manure and CAD effluent were analyzed; initiating a project-owned trucking

fleet, or contracting with an existing trucking company. A 6,000-gallon manure tanker truck was

assumed for all trucking-related analyses.

The analysis of a project-owned trucking fleet, with an estimated initial capital cost of $1.5 million and

estimated annual expenses of more than $420,000, was deemed not economically feasible at this time.

Contracting with an existing trucking company is the recommended option to pursue in order to simplify

the overall CAD facility start-up by lessening the capital cost and reducing the risks. Although this option

entails higher annual costs, (estimated to be $1.3 million dollars in total annual expense), the project-

run fleet is a possibility to pursue at any time following project start-up.

3 These 15 farms referred to are the selected farms under Scenario No. 2

5

Digester Site/Configuration Scenarios Investigated

Five different digester site/configuration scenarios were initially analyzed and presented, along with

other interim project findings, to the Lowville Digester Work Group at a December 2009 meeting. The

five scenarios explored were:

Scenario No. 1: Co-digest manure from all (25 farms) dairy farms surveyed, and seven (out

of 11 total) non-farm biomass substrates at a central location adjacent to the LWWTP.

This option makes use of all manure and most non-farm biomass substrates discovered by

the completed surveys.

Scenario No. 2: Co-digest manure from 14 dairy farms, and three non-farm biomass

substrates at a central location adjacent to the LWWTP. This option was explored to

reduce trucking costs by reducing the number of collaborating farms.

Scenario No. 3: Co-digest manure from only 12 dairy farms, and one non-farm biomass

substrate at one of two remote sites, and co-digest manure from four dairy farms and two

non-farm biomass substrates at a second remote site. This option was explored to

determine the impacts of having multiple, smaller, regional digesters to further reduce

trucking costs.

Scenario No. 3a: Identical to Scenario No. 3, except that 33% of the manure would be

piped to each remote digester site, and the remainder would be trucked. This option was

also pursued to determine impacts on trucking costs.

Scenario No. 3b: Identical to Scenario No. 3, but includes 400 acres of energy crops

digested at one remote site and 2,000 acres of energy crops digested at the second

remote site. This option was explored to investigate the impacts of including an energy

crop digester on overall biogas production and profitability.

The Lowville Digester Work Group chose Scenario No. 2 CAD, as described above, for complete

investigation at the December, 2009 meeting, and it was decided that one additional farm would be

included in the scenario before performing an economic profitability analysis.

6

The remainder of the Executive Summary provides details and the results of a complete analysis

performed for the Scenario No. 2 CAD, and since the Lowville Work Group also requested a detailed

analysis of an energy crop digester co-located with the Scenario No. 2 CAD manure and non-farm

biomass digester, this information is also provided below.

Scenario No. 2 CAD System Overview

The estimated annual average volume of non-farm biomass substrates available for co-digestion by

three local suppliers was 16 million gallons per year (range 13 to 19 million gallons per year) and the

manure volume available from the 15 targeted collaborating farms was 30 million gallons per year.

Therefore, the CAD should be sized to handle at least on average 122,400 gallons of influent per day.

Using a digester hydraulic retention time of 22.54 days, the digester treatment volume needed was

calculated to be 2.8 million gallons. A digester configuration of one or multiple tanks can be used to

accomplish this overall size requirement. The average estimated capital cost for a complete mix digester

system of this size was $5.89 million (range $4.73 to $7.14 million).

The annual cost to transport manure to the CAD site (adjacent to the existing LWWTP) and digester

effluent back to the collaborating farms was estimated to be on average $1.12 million annually (range

$1.07 million to $ 1.17 million). It was assumed that the trucking cost for the non-farm biomass material

to the CAD site would be paid by the substrate suppliers, as is currently the case.

The average estimated gross volume of biogas produced was 188 million ft3/year (range 140 million to

237 million ft3/year). Using a biogas methane concentration of 60%, the annual estimated volume of

methane produced was 113 million ft3/year (range 84 million to 142 million ft3/year).

4 22.5 days is the average of 20 and 25 days, which are common retention times for similarly sized systems

7

Two of the most commonly implemented biogas utilization options were investigated:

1) Use biogas to fuel a reciprocating engine-generator set5

2) Sell cleaned biogas as renewable natural gas, biomethane, by first removing

impurities (carbon dioxide, hydrogen sulfide, and moisture) using pressure-swing

adsorption gas clean-up technology6.

For option 1, it was assumed that thermal energy harvested from the engine-generator set would be

used to meet all of the digester heating requirements (warming the CAD influent to target operating

temperature and then maintaining it); field experience has shown that this is an appropriate assumption

to make. For option 2, it was assumed that 20 percent of the biogas produced by the digester would be

needed to meet this demand; this assumption needs to be confirmed, based on information about the

design of each digester system considered, specifically, how well the vessel is insulated and the

exposure it has to winter wind and temperature. The overall estimated annual parasitic heating

requirement was 20,200 million Btu’s per year (range 15,000 to 25,500 million Btu’s per year). Using the

average estimated parasitic heating requirement, the annual cost to provide this heat ranged from

$81,000 to $282,000 per year for a natural gas purchase price range of $4 to $14 per decatherm,

respectively.

For parasitic electrical requirements, for both biogas utilization options, the average estimated parasitic

electrical energy requirement of the CAD system was determined the same way. Calculations were

performed using data from vendor information obtained for other similar sized systems to determine

the electrical energy requirement per gallon of influent material; the results were that the average

electrical energy requirement was found to be 0.0313 kWh per gallon of influent7 (range 0.0121 to

0.0505 kWh per gallon of influent). Applying these energy values to the CAD system, the estimated

average annual parasitic electrical energy requirement was 1,400,000 kWh per year (range 540,000 to

2,257,000 kWh per year). Using the average estimated parasitic energy requirement, the estimated

annual cost to provide this energy ranged from $112,000 to $252,000 for an electrical energy purchase

price range of $0.08 to $0.18 per kWh, respectively.

5 Other, less commonly used methods exist for converting biogas to electrical energy (e.g. microturbines) 6 Other methods are available for scrubbing biogas to make biomethane (e.g. membrane separation, regenerative amine wash)

7 Influent is defined as the biomass on the in-flow side of a treatment, storage, or transfer device

8

Nutrient Management Implications

Assuming the non-farm biomass imported for co-digestion supplies excess nutrients to the post-

digestion product that would be available for sale to area crop farms, the project could potentially

receive $226,000/year in total revenue. Of the $226,000/year, $86,000/year would be derived from the

sale of nitrogen, $121,000/year would be derived from the sale of phosphorus, and $19,000/year would

be derived from the sale of potassium.

Economic Profitability Analysis- Scenario No. 2 CAD

A net annual economic profitability analysis was performed for the Scenario No. 2 CAD to determine if

this scenario was economically viable considering the options of: 1) selling electrical energy at a price

range of $0.08 to $0.18 per kWh, or 2) selling biomethane (cleaned biogas) at a price range of $4 to $14

per decatherm. For both of these options, separate net annual economic profitability analyses were

performed, which included a tipping fee equal to the tipping fee being paid by one of the three non-

farm biomass suppliers whose substrate was selected for co-digestion (the other two tipping fees were

not provided by the completed surveys).

For all of the analyses, the cost of capital (discount rate) used was 5%, the economic life of the digester

was 20 years, and the replacement cycle of the engine-generator set was 10 years. Trucking costs to

haul manure to the CAD site and effluent to collaborating farms was included as an annual cost. Other

annual costs included operation and maintenance of (1) the CAD system (based on data obtained from

vendor quotes for other similar systems), (2) the engine-generator set ($0.018 per kWh) and (3) the

biogas clean up system.

The results of the net annual economic analysis showed that for all energy sale options investigated it

was more costly to own and operate the system each year, than the system would receive in revenue

annually. In other words, no option was found to be economically profitable.

Based on these results, a final net annual economic profitability analysis was conducted to determine

the tipping fees needed for the two energy sale options investigated to result in a Scenario No. 2 CAD

financially break-even situation. For the option of selling electrical energy at a price ranging from $0.08

to $0.18 per kWh, the break-even tipping fee range was determined to be $21 to $9 per ton,

9

respectively. For the option of selling biomethane at a price range of $4 to $14 per decatherm, the

break-even tipping fee range was determined to be $29 to $16 per ton, respectively.

The calculated break-even tipping fee ranges were substantially below the average tipping fee of over

$70 per ton currently charged by landfills for the northeastern U.S., but somewhat higher than the

calculated tipping fee currently being paid by the non-farm biomass supplier considered for this

project with the most biomass available annually.

Energy Crop AD System Overview

The proposed Lowville energy crop digester is an anaerobic digester designed to process high solids

energy crop materials (corn silage and/or haylage). Such digesters are widely used in Germany and

other European countries and produce about eight times the biogas as digesters fed manure only

(Effenberger, 2006).

Silage corn and grass hay would be harvested and ensiled as if they were going to be fed to dairy cattle.

Sufficient quantities would be stored to enable the energy crop digester feed hopper (usually a walking

floor bin) to be filled once-a-day, year round, normally with a pay loader. Several times per day, the

control system would automatically transfer a portion of the feedstock into the digester; screw

conveyors (augers) are normally used due to the high solids content of corn silage and haylage.

The energy crop digester economic analysis performed for this feasibility study used “in-the-bunk” silage

prices ranging from $30 to $55/ton, meaning that the costs to grow and harvest the crops and ensile

and store them are covered by the purchase price.

In addition to the energy crop feedstock, a small portion of manure is also normally added to the energy

crop digester, about 10 percent by weight, to help stabilize digester pH and to provide some dilution

water to lessen the power required to provide in-vessel mixing.

Energy crop digester effluent, laden with organic nutrients, is the consistency of digested manure. For

this feasibility study, it is assumed the effluent would be stored on-site for a short period of time and

periodically trucked to the energy crop source farms for longer-term storage and for subsequent use as

fertilizer to grow the next rotation of energy crops. Some of the surplus nutrients from the Lowville CAD

10

system could also be trucked to the collaborating farms to meet the overall fertilizer requirements for

the crops grown on those farms.

Economic Profitability Analysis - Energy Crop AD System

The same net annual economic profitability analysis was performed for the energy crop AD system. For

this analysis, the only energy sale option investigated was the sale of electrical energy8, using a sale price

range of $0.08 to $0.18 per kWh with varying feedstock (fermented corn silage and haylage) prices

between $30 and $55 per wet ton.

Again, the cost of capital (discount rate) used was 5 percent, the economic life of the digester was 20

years, and the replacement cycle of the engine-generator set was 10 years. Trucking costs to haul

digester effluent to collaborating farms was included as an annual cost, as it would be paid by the

project. Other annual costs included operation and maintenance of: (1) the CAD system (based on data

obtained from industry vendors), (2) the engine-generator set ($0.018 per kWh) and (3) biogas clean up

system.

The results of the net annual economic analysis showed that for all digester feedstock and energy sale

price options investigated it was more costly to own and operate the system each year than the

system would receive in revenues annually. This is the same result that was found for the Scenario No.

2 CAD options investigated.

Recommendations and Future Work

The recommendation for a CAD system is based on conducting thorough and complete technical and

economic feasibility analyses, as well as the vision of the Lowville Digester Work Group. Based on this,

the recommendation is to further investigate one centrally-located complete mix AD, sited adjacent to

the LWWTP that would co-digest manure from 15 targeted collaborating dairy farms and targeted non-

farm biomass substrates (currently the following three substrates: whey, post-digested sludge, and

glycerin) that are by-products generated nearby.

8 Biogas clean-up to biomethane was not investigated, since economic profitability analysis results for the Scenario No. 2 CAD showed little difference in the bottom line when compared to electrical energy sales.

11

The future net annual economic profitability behind this recommendation is encouraging, given that, (1)

the calculated tipping fee needed for the system to break-even is well below the average tipping fee

charged in the northeastern U.S. and many predict regulations will be instituted in the near future

restricting the land-filling of organic matter, (2) future regulations aimed at reducing the impact of fossil-

fuel derived energy (specifically GHG emissions and climate change) would likely positively impact

renewable energy projects, (3) energy produced from such projects would have less price volatility than

fossil fuel-based energy products, and (4) the annual economic profitability will improve with reductions

in capital cost by receiving grants and/or premium payments for renewable energy.

If future efforts are put forth to further investigate one CAD, it is recommended that the two major

areas provided below be addressed in the order presented below and that the bullet items under each

be included.

A. Address Economic Barriers to Project Implementation

Identify other potential sources of non-farm biomass that are currently being land-

filled or otherwise disposed of that could be received by the CAD with a tipping fee

paid by the supplier.

Continue the education and outreach efforts concerning this project and the goals and

objectives of local community members, targeted at collaborating and non-

collaborating dairy farmers and non-farm biomass substrate suppliers to develop

project support targeted towards securing public funding.

Secure grant funding or subsidies that could help offset the capital cost of the CAD

and/or supplement the revenue(s) received for system outputs (raw biogas,

electricity, biomethane, and/or organic nutrients).

Validate the trucking analysis and farm biomass pick-up options determined under

this effort.

Investigate the willingness of non-farm biomass suppliers to enter into reasonable

long-term contracts , with a negotiated tipping fee.

Investigate the willingness of the end user(s) of the net energy produced by the CAD

facility to enter into reasonable long-term contracts.

Explore the potential for selling raw biogas to a local end user.

12

Investigate the possibility of the sale of CAD surplus heat combined with woody

biomass heat to local industry or the community (district heating).

B. Advanced Project Due Diligence

Perform more complete laboratory testing of the targeted substrates mixed

proportionally with manure to better solidify the quantity of biogas that would be

produced by the system.

Perform a value engineering/economic analysis that includes looking at the digester

treatment volume vs. biogas production potential.

Conduct an in-depth site and environmental impact assessment for the targeted

construction site.

Investigate the legal issues for various digester ownership options.

Determine the permit(s) that will be required by the New York State Department of

Environmental Conversation (NYSDEC)9.

Conduct an in-depth investigation into the site improvements that will be required at

each farm in order to participate in the project, and develop an associated budget.

Investigate contracting with an existing trucking company to provide transportation of

farm biomass.

Assess renewable energy credits (RECs) and carbon credits as applied to centralized

digesters.

Conduct a net energy analysis for the proposed system.

Develop a request for proposals (RFP) package to be distributed to AD system

designers.

Validate the economic profitability analysis using the results of the proposed RFP.

Continue investigation into future opportunities, such as manure nutrient extraction

equipment and resulting product marketing opportunities for organic nitrogen,

phosphorus, and potassium.

Continue assessment of alternative biogas market opportunities such as the sale of

biomethane as a vehicle fuel.

9 There are currently no operating dairy manure-based CAD systems in NYS, and an initial inquiry made by Cornell to NYSDEC on

behalf of this project revealed that NYSDEC is not readily prepared to state what permit(s) is/are needed.

13

Nomenclature

Effort was made to make terminology throughout this report consistent to allow for a more clear

understanding of the information presented. Please refer to this list as necessary.

Centralized Anaerobic Digester (CAD) facility The term used to describe the proposed manure and substrate co-digestion AD system, and all of the integrated components.

Energy crops Field crops grown specifically as a feedstock source for an energy crop AD system

Feedstock Describes the entire influent to the CAD

Lewis County Community AD project The name of the proposed project

Lowville Digester Work Group The local volunteer group of decision-making stakeholders on behalf of the project

Manure

Effluent from a dairy housing barn made up of cow urine and feces, bedding, and other minor components such as gravel, undigested feed, and/or milking system gray water.

Methane production potential Quantification of a biomass substrate to produce methane

Non-farm biomass substrates Organic by-product material from local processors of farm products; otherwise referred to as food waste

Non-farm biomass substrate suppliers

Local food processors and vendors who have, upon initial survey, shown interest in supplying organic material for co-digestion; otherwise known as food waste sources

14

15

Introduction

The proposed Lewis County community anaerobic digester (AD) project (see Figure 1) was initiated in

early 2008. Cornell University was contracted to perform a feasibility study of the proposed project in

May 2009, with a targeted completion of December 2009. Three interim project meetings were held by

the Cornell team to present interim project findings and assess progress in October and December, 2009

and March, 2010. After some changes in scope of the project, the final feasibility report was completed

in May 2010.

Lowville goals and objectives

Interest in a community AD from several Lewis County, NY constituents grew from the initial set of goals

developed from multiple community viewpoints. The Lowville Digester Work Group was formed from a

group of local stakeholders interested in determining the application of anaerobic digestion technology

to meet the goals set forth, and to oversee development of the proposed project. The following are the

initial project goals developed by the Lowville Digester Work Group (committee document, 2008):

Goals for the community:

Encourage continued economic growth

Lessen the negative impact of farms on county residents (e.g., farm-based odor)

Reduce the environmental footprint

Goals for the region’s dairy industry:

Provide greater flexibility in manure handling and nutrient management that results in an

economic advantage versus today

Reduce odor associated with manure storage and land application

Allow a greater number of animals per unit of land area with less environmental risk

Goals for local industry:

Gain access to sustainable energy at lower (versus today) cost.

16

Figure 1. New York State map showing location of project-site

Scope of Work

The following questions were posed in the scope of work document developed prior to the beginning of

the feasibility study, and used throughout the study by Cornell University and the Lowville Digester

Work Group to guide the project.

Biomass

1) What is the annual on-farm (manure) and Village of Lowville non-farm biomass potentially

available for anaerobic digestion, by source?

2) How much biomass can be secured, by source?

3) How many farms are currently prepared (on an infrastructure basis) to store raw manure

short- term and digester effluent long-term?

4) What infrastructure upgrades are needed for those farms not currently prepared to store

raw manure short-term and/or digester effluent long-term?

Biomass/biogas transportation

5) What options exist for transporting manure from the farms to the digester location(s) and

digestate back to the farms and what is the estimated cost associated with this?

17

6) What options exist for transporting non-farm biomass to the digester location(s) and what

is the estimated cost associated with this?

7) What are the results of an economic sensitivity analysis on biomass transportation cost?

8) What is the feasibility of transporting biogas or biomethane (processed biogas) to a

utilization site?

Anaerobic digestion

9) What are the AD technology options available?

10) Which option is best suited for the application?

11) What are the estimated capital and operating and maintenance costs associated with the

AD and associated equipment?

12) Is it best to truck all biomass destined for digestion to one site or to have an array of

digesters strategically located within the county?

Biogas/energy conversion

13) How much biogas can potentially be produced with the secured biomass?

14) Is biogas clean-up required and if so what option is best?

15) How much energy can be extracted from the biogas?

16) What are the results of a sensitivity analysis performed on the sale price for the energy?

Nutrients

17) What is the expected nutrient value of the manure once digested (tons total-N, ammonia-

N, total-P, ortho-P, and potassium)?

18) What is the anticipated increase in digester effluent volume and nutrient composition

with the importation of securable non-farm biomass sources?

Impacts on the community

19) How many truck loads of manure will be transported to the digester site(s) per day?

20) What labor force is anticipated to operate the overall facility?

18

Economics

21) What is the estimated total annual cost for various digester/biogas utilization scenarios?

Designated responsibilities

In addition to the questions set forth in the scope of work, the same document designated which tasks

each group involved in the project would be responsible for. It was decided that the Cornell Manure

Management Program Team (CMMPT) would provide leadership and overall project coordination to

facilitate the completion of the feasibility study. CMMPT developed and maintained a project schedule

identifying specific tasks, responsible parties and targeted completion dates. Specific responsibilities are

outlined below.

CMMPT

o Deliverables: CMMPT will complete and provide the following items to Cornell

Cooperative Extension of Lewis County:

Initial Findings (written report and oral presentation)

Interim Report (written report and oral presentation)

Final Report (written report and oral presentation)

o The work tasks and components of the feasibility study include:

Gather information from existing centrally located community ADs or

completed feasibility studies that are relevant.

Develop a survey for completion by select dairy farms within Lewis County

Develop a survey to all potential substrate suppliers within Lewis County

Aggregate and analyze results of the above surveys

Perform all calculations required to answer the questions outlined above

Prepare all reports and make oral presentations

Lewis County Cornell Cooperative Extension (CCE)

o Identify farms within a specified radius of possible digester site(s)

o Implement the farm survey and provide reports/summaries to CMMPT

19

o Organize project meetings

Lewis County Soil and Water Conservation District (SWCD)

o Using data provided by the Cornell Cooperative Extension, create a map identifying all

potential participating dairy farms within the selected radius of the Village of Lowville

Wastewater Treatment Plant (LWWTP) and other potential digester sites. Incorporate

information on road infrastructure into map so that feasible transportation routes can

be considered.

o Using data provided by the Cornell Cooperative Extension, create a map identifying all

potential substrate suppliers within the selected radii of the LWWTP and other

potential digester sites.

Village of Lowville

o Implement a survey to quantify all potential non-farm biomass substrates within Lewis

County

o Provide completed surveys and results to CMMPT for analysis and use

Lowville Digester Work Group

o Assist with the identification of potential sites for the proposed central AD

o Assist in identifying potential buyers of final products

o Inform community about the project and generate support

Project approach

Cornell University, in agreeing to perform the feasibility study for the Lewis County community AD,

responded to the Lowville Digester Work Group’s request with the following plan of action:

Develop a plan of necessary work to be accomplished on the local level

Aggregate and analyze results of local work

Calculate total quantity and characteristics of digester inputs

o Farm

20

o Kraft

o Other

Assist local creation of a map of cooperating farms and other biomass sources

Calculate costs and feasibility of farm-based biomass transportation

Measure biogas producing potential of assumed substrate inputs and calculate projected

biogas production

Review biogas clean up options

o Cost

o Scale

o Availability

Determine the best use of biogas produced

o Generation of electricity

Cost of interconnection

Sale to grid or private

o Sale of cleaned biogas

Sale to Kraft

Sale to community

Sale of energy back to farms

Used to power vehicles/farm trucks

o Market price of each option

o Cost of implementing each option

Analyze all final products from digester and determine marketability

o Solids

Bio-security issues

21

o Heat

o Electricity

o Nutrient-laden liquid effluent

o Compost

o Other

o Determine revenue from each potential sale

Devise a strategy to return organic material/nutrients to farms

o Solids and/or liquids

o Transportation

o Delivery infrastructure feasibility on a farm level

Overall cost benefit analysis for project

Formulate questions for Lewis working group before proceeding, based on initial findings

Incorporate new visions to final recommendation

Develop a mid-study interim report

Develop a final feasibility study report

22

23

Chapter 1. Basics of Centralized Dairy Manure-Based Anaerobic Digestion, Biogas Utilization, and Nutrient Recovery Systems A centralized dairy manure-based anaerobic digestion and biogas utilization system is one where dairy

manure, the system’s stable feedstock, is aggregated from multiple farms, blended together, and co-

digested in a heated vessel for 15 to sometimes more than 30 days. In many cases, non-farm biomass

substrates such as food processing and bio-fuel processing by-products, organic industrial wastes, and

culled and leftover human foods are co-digested with dairy manure. Digestate (digester effluent) is

generally stored short-term on-site at the centralized facility, and then transported back to source farms

for storage until it is used to replenish cropland with nutrients (nitrogen (N), phosphorus (P), and

potassium (K)) and organic matter. Digestate can be further treated, as described later in this chapter,

to achieve various undigested fiber recovery and nutrient conservation and management goals and

objectives. A typical process flow diagram for a centralized digestion system is shown in Figure 2.

Figure 2. Typical CAD system process flow diagram

Centralized digesters are best located where they are strategically placed to minimize transportation of

manure and non-farm biomass substrates and to maximize output energy and digestate utilization. CAD

24

can effectively improve fertilization of cropland by returning CAD effluent to a strategically located site

at the farm, for ease of use in spreading on cropland.

Centralized digestion systems are common-place in Denmark and other European countries; a

centralized digester in Jutland, Denmark is shown in Figure 3.

Figure 3. A CAD in Jutland, Denmark

Overall, centralized digestion of manure provides the opportunity for economies of scale to come into

play that generally cannot happen on individual farms. The capital and operating costs per unit of

influent treated (i.e., cents per gallon) is generally less in larger systems than smaller systems. Another

reason centralized digestion is given due consideration is that it is likely to have the size needed to

justify and pay for a full-time crew to operate the facility. Further, centralized digestion provides the

opportunity for more efficient use of organic nutrients by the collaborating farmers. Digestate can be

sampled more frequently than on-farm, thus better quantifying the nutrients sent back to each

collaborating farm. Also, anaerobic digestion provides a steady and consistent material that is well

suited for secondary or tertiary treatments that can include enhanced nutrient management by

farmers.

25

A common concern with centralized anaerobic systems is biosecurity (disease control). Commingling

of source farm manure and non-farm biomasses is part of the centralized digestion model that

cannot be avoided. Farmers can be especially concerned about biosecurity since manure that may

contain infectious disease causing organisms can be brought onto their farms. However, the risk of

this is lessened when manure is digested; further risk reductions occur when influent or digestate is

pasteurized before being returned to the farm.

Additional information about dairy manure-based centralized digestion systems is provided in this

chapter with the goal of preparing the reader for the following chapters where the work and feasibility

study findings are presented. More in-depth information about on-farm and centralized anaerobic

digestion can be obtained by reviewing the references cited herein.

Centralized digester feedstock materials

Centralized digesters are generally fed two or more of the three different types of biomass materials.

The three types are categorized based on availability, specifically those that are:

Continuously available such as manure, certain food processing wastes like whey, etc.

or at least almost continuously (e.g. some slaughterhouse waste sources)

Seasonally available such as grape puree, onion tops, carrot skins, etc.

Available year-round but not consistently such as processed foods that have exceeded

their shelf life

Manure

For most centralized digestion systems, manure is the stable feedstock material. Not only is it

continuously produced by dairy cattle, it also provides a key role in co-digestion with other, more

biologically convertible materials as it moderates pH due to its buffering capacity.

The average U.S. dairy cow produces 150 lbs. of raw manure per day that contains 20 lbs. of total

solids (TS), 17 lbs. of volatile solids (VS), 1 lb. of (N), 0.17 lbs. of (P), and 0.23 lbs. of (K) per day while

a dry cow and a replacement (heifer) produces measurably less (ASABE, 2005). A portion of the

manure VS are biologically converted to biogas. Digestion of raw manure from a dairy cow produces

on average, 80 ft3 biogas per cow-day (Ludington, 2008).

26

Non-farm biomass sources

Any biomass can be digested. Digestion of various biomass materials is largely a function of materials

handling (conveying material from storage into a digester), biodegradability, maintaining a balanced

state within the digester vessel, and economics. Many of the suppliers of non-farm biomass substrates

available for anaerobic digestion currently pay significant tipping fees to the local landfill authority in

order to dispose of their unwanted processing by-products.

In New York State, many farmers are interested in mixing non-farm biomass substrates with manure

due to:

1. The increased biogas production potential the mixture produces

2. The associated tipping fees for allowing substrate suppliers to unload their by-product

on the farm.

Non-farm biomass can have lower solids content than raw manure, so when combined with manure

the resulting mixture needs to be mixed within the digester to keep the solids in suspension.

Some materials, like fats, oils, and greases readily break down in an AD while others like corn silage take

much longer to fully do so. Many non-farm biomass substrates have the potential to produce several

orders of magnitude of biogas per unit of influent mass compared to manure. An example of biogas

production from co-digesting manure with food wastes is between 368 and 560 ft3 biogas per cow-

day, as found on one New York State dairy farm (Gooch et al., 2007).

Like manure, non-farm biomass generally contains measurable levels of nutrients (N, P, and K) that

must be considered when assessing the impact centralized digestion will have on a collaborating

farm’s ability to comply with their Comprehensive Nutrient Management Plan (CNMP).

A centralized anaerobic digester (CAD) that looks to co-digest measurable volumes of non-farm

biomass substrates needs to have reasonable assurance that these are available and securable by

long-term contract or are able to be replaced with alternate biomass sources. This is important

because the capital cost of the centralized digester will be directly affected by the volume of non-

farm biomass sources digested and the associated biogas production potential.

27

Anaerobic Digestion

Direct environmental benefits of an anaerobic digestion system include conservation and phase

transformation of manure nutrients (N), (P), and (K) during digestion resulting in an effluent rich in

organic, crop available-nutrients needed to grow feed for livestock and people alike. Since the digestion

process significantly reduces odors associated with untreated biomass stored long-term, digestate can

more effectively be used to fertilize crops. This reduces the need to purchase synthetic fertilizers that

require large amounts of fossil fuels to produce, thus reducing the greenhouse gas (GHG) emissions

associated with crop production. Improvements in water quality are also associated with less use of

synthetic fertilizers.

Anaerobic digesters can be thought of as an extension of a cow’s stomach. Both rely on operative

microbes that flourish in the absence of oxygen to transform foodstuff into useable energy. Operative

microbes are most successful at doing this when they are consistently fed a diet that meets their

nutritional needs and the digester temperature and pH are maintained at target values.

The anaerobic digestion process overall involves three groups of anaerobic microbes. First, hydrolytic

bacteria initiate a process called hydrolysis. These bacteria use extra cellular enzymes to convert

organic insoluble fibrous material into soluble material; however, inorganic solids and hard-to-digest

organic material are not able to be converted.

Next, acid forming bacteria convert the soluble carbohydrates, fats, and proteins to short-chained

organic acids. The acids produced in step two become the food source for the methanogens, which

produce methane gas in the third step.

Various methanogenic species grow in different temperature regimes.

1. Psychrophilic methanogens grow in the lowest of the temperature ranges, less than 68°F.

Methanogens in this range grow slowest and produce the least biogas per unit of time.

Covered lagoon systems, especially those in northern climates, will be in this range much

of the year (Wright, 2001).

28

2. Mesophilic methanogens grow in an optimum temperature of about 100°F which is the

most common operational temperature for digesters in the U.S.

3. Thermophilic methanogens grow in an optimum temperature of about 130°F. The higher

operating temperature increases the rate of biomass degradation, increases pathogen

reduction, and allows for shorter retention times thus reducing the capital cost of the

digester vessel.

Digester Types

In the U.S. there are basically three different types of anaerobic digestion systems used today to process

dairy manure. They are: plug-flow, complete mix, and covered lagoon. Of these three, a complete mix

system is the system of choice for use in a centralized digester because the likelihood of co-digestion of

dairy manure with non-farm biomasses is very high. (Digester influent concentrations less than 10

percent total solids are common when co-digesting manure with most food processing by-products

and require mixing to minimize solids settling.)

Complete mix digesters can be either horizontal flow or vertical flow systems. Each is briefly discussed

below.

Complete Mix Digester, Horizontal Flow System

Horizontal-mix digesters incorporate agitation systems in digester vessels. The mixing system is

mainly utilized in scenarios that have influent total solid concentrations greater than 12 percent (not

common with dairy manure-based systems) or less than 10 percent.

Complete Mix Digester, Vertical System

Vertical mixed digester tanks can be either below-grade (atypical) or above-grade (typical) as shown

in Figure 4. Cast-in-place concrete, welded steel, bolted stainless steel, and bolted glass-lined steel

panels are all used to construct vertical tanks.

The mixing process is achieved by various methods, depending on the preference of the system

designer and the overall goals of the system. In one method, an external electrical motor (about 10-

20-Hp) turns a vertical shaft, concentric with the digester tank, which has several large paddles

29

attached. The shaft speed is about 20 RPMs. This system is common for solid top tanks.

Another method uses submersed impeller agitators each driven by either an electrical motor or a

centrally located hydraulic motor. These systems have a much higher blade speed, perhaps 1,750

RPMs, and can be used with both flexible top and solid top applications. One clear advantage of the

first method is the electrical motor is easy to service and replace.

Vertical tanks are insulated during the construction process to reduce the maintenance heating

requirement (heat to maintain digester operating temperature). Significant heat can be lost from

vertical tank digesters if they are not properly insulated. Applicable insulation options are to spray

the tank with foam insulation or to use rigid board insulation attached to the tank and then covered

with metal cladding.

Figure 4. Danish above-grade complete mix vertical digesters in background

Biogas

Anaerobic digestion produces a continuous supply of biogas in quantities sufficient to not only power

the digestion plant but also to utilize the excess in various ways. Producing electricity and/or thermal

30

heat from biogas results in a net reduction of greenhouse gases (GHG). Anaerobic digestion of dairy

manure also mitigates methane emissions otherwise caused by traditional manure handling and storage

practices.

Production of biogas is dependent mainly on the digester hydraulic retention time (HRT), digester

operating temperature, and the biochemical energy potential of the influent. Higher biomass

conversion efficiencies by thermophilic (~135°F) methanogens allow for shorter hydraulic retention

times and consequently reduced capital costs as compared to mesophilic (~100°F) systems. Biochemical

energy of an influent material is most accurately evaluated by conducting long-term (6-month) bench-

top reactor tests (Angenent, 2009) but is generally estimated by measuring the VS content in the

influent. Biochemical methane trials can also be conducted in the laboratory to estimate the biogas

production potential of a biomass sample. Jewell (2007) reported that an appropriate estimation of the

methane (CH4) production is to use a value of 0.5 L CH4/gram of VS degraded. If the dry biogas is 60

percent CH4 this is equivalent to 13.4 ft3 biogas/lb. of VS degraded.

Composition and energy value

On-farm digester monitoring has shown that biogas is comprised mainly of ~60% methane and ~40%

carbon dioxide (CO2), with trace levels of 0.2 to 0.4 percent hydrogen sulfide (H2S). Even though H2S

concentrations are low, biogas is highly corrosive and prudence is needed to avoid pre-mature biogas

transport and utilization equipment failures.

Pure (dry) methane has a low heating value of 896 Btu/ft3 (at standard temperature and pressure: 68°F

and 1 atm) (Marks, 1978). Since biogas is only ~60% methane, its heating value is ~40% lower or about

540 Btu/ft3. Raw biogas is considered to be saturated with water vapor.

Utilization: fuel source for engine-generator sets

Using biogas as an energy source to fuel on-site engine-generator set(s) is the most common use of

biogas today. Large engines that had been adopted for landfill biogas years ago are now widely

available for use at centralized digestion sites. Most are spark-ignited systems with a few compression

ignited systems that also use about 10 percent diesel fuel concurrently as a fuel source.

31

Overall, these “low Btu or dirty gas” engines work well with the exception of difficulties arising from

hydrogen sulfide (H2S). Hydrogen sulfide is very corrosive at low temperatures since it converts to

sulfuric acid. To date, most on-farm biogas-fired engines combat the corrosiveness by running the

engine nearly continuously (keeps the temperature high) and changing oil more frequently than for

cleaner fuel source scenarios.

Recently, some U.S. farmers have implemented methods to reduce H2S concentrations from biogas prior

to utilization. Methods include chemical reaction and biological reduction systems. Scrubbers are

mainstream equipment on European digester systems.

Overall, there are two basic types of generators:

1. Induction generators run off the signal from the utility and are used to allow parallel hook

up with the utility. Induction generators cannot be used as a source of on-farm backup

power since the system needs the signal from the utility line to operate properly.

2. Synchronous generators could be run independently of the utility but matching the

utilities power signal would be very difficult so these types of generators would be used if

the system were not connected to the utility grid.

Most generator systems manufactured today have controls that will allow the engine-generator set to

synchronize with the utility’s electrical frequency and still operate in island mode when there is a

disruption of the grid power. These systems can be set up to “black start” if desired.

Thermal-to-electrical conversion efficiencies for biogas-fired internal combustion engine-generator sets

are less than desirable, but are about the same as other fuels. On-farm digester monitoring has shown

that the conversion efficiency ranged from 22 to 28 percent, as shown in Figure 5.

The electricity production depends on the amount and quality of gas as well as the efficiency of the

engine-generator. Typically, 33-38 kWh/day will be produced per 1,000 ft3/day of biogas produced

(Koelsch et al., undated and EPA, 1997). Some engine-generator set manufacturers show biogas-to-

electrical energy conversion efficiencies as high as 42% in their advertisement literature. As with all

32

large capital purchases, careful evaluation of those systems is needed to ensure they are economically

feasible.

As already mentioned, engine water jacket heat, and sometimes exhaust heat as well, is harvested and

used as the primary means to heat the digester. In the winter, most if not all of this harvested heat is

needed, while in the summer a good portion of it is dumped to the ambient via forced-air/water heat

exchanger.

Figure 5. Thermal to electric conversion efficiency of six NYS on-farm engine-generator sets.

(Source: Gooch, Pronto, Ludington, Unpublished, 2010)

Utilization: fuel source for microturbines

Two New York State dairy farms have microturbines in operation to power generators to produce

electricity. The main interest in microturbines is the premise that they require less maintenance on a

daily basis and also on a long-term basis, and most recently that they potentially produce less exhaust

emissions. Biogas pressure needs to be increased from typical digester pressure values to about 60 psi

before being injected into a microturbine. Corrosion-resistant small-scale compressors are available to

compress raw biogas to this pressure thus lessening the need for an H2S scrubber.

The typical fuel-to-power efficiencies of various biogas utilization options are shown in Table 1. These

efficiency figures do not account for increases due to the use of co-generated heat.

33

Table 1. Typical fuel-to-power efficiency values (adapted and updated from Wright, 2001).

Prime Mover Type Efficiency

Spark ignition engine 18-42%

Compression ignition

engine (Diesel)

30-35% above 1 MW

25-30% below 1 MW

Gas turbine 18-40% above 10 MW

Microturbine 25-35% below 1 MW

One source states the operation and maintenance cost of $0.015 per kWh are estimated for engine-

generators (EPA, 1997). On-going engine-generator set service contracts are offered by one company

that sells them for $0.015 to $0.02 per kWh produced depending on the pre-existing maintenance

performed on the set and presence of an H2S scrubber.

Utilization: fuel source for boilers

On-farm biogas utilization by a boiler is the second most popular use of the energy. Natural gas boilers

can be modified to use biogas as a fuel source. The main modification involves increasing the pipe

delivery size and orifices in the burners to accommodate the lower density fuel. Decreasing the

concentration of H2S in the biogas can extend the life of the boiler equipment. Boilers are mainly used

to provide primary or secondary heating of the digester and in some cases also to provide domestic

heating of farm offices and lounge areas. One farm used boiler heat to heat a calf barn, but this use is

limited.

Utilization: fuel source for other uses

Raw biogas can also be used as a fuel source for drying equipment such as grain dryers, separated

manure solids dryers, evaporators, etc. Other possible uses fall under the category of those needing

fully cleaned (scrubbed) biogas, commonly known as “biomethane”. These possible uses include any

that currently use natural gas (almost pure methane) and as a vehicle fuel. There are two primary

methods to process biogas into biomethane. They are: 1) chemical and, 2) physical removal of

impurities (CO2, H2S, and water vapor). Details of these processes are beyond the scope of this report

but the general flow process diagram is shown in Figure 6.

34

Figure 6. Basic process flow diagram for advanced biogas clean-up for biomethane production.

Advanced Centralized Digester Information

Specific information on a CAD system is presented below.

System electrical demand

Modern CADs require electrical energy to operate with the highest electrical demand normally

associated with the pumps and agitation equipment. The electrical energy used to operate a system

is known as parasitic electrical energy. With all centralized digester systems it is important to

implement a design that is energy efficient. Electrical energy efficiency can be expressed in various

ways including as a function of the: 1) influent volume (annual kWh/annual influent), 2) vessel

treatment volume (annual kWh/tank size), and 3) energy production (kWh consumed/kWh

produced). All systems that are not electrically efficient result in reduced sale of electrical power

and/or increased purchase of electrical energy from the utility.

System thermal (heat) demand

Anaerobic digesters require a controlled heating system for operation. There are two different heat

demands in most systems; they are: 1) differential heat, and 2) maintenance heat. Differential heat is

the heat needed to raise the influent temperature to digester target operating temperature and

35

represents by far the largest heating requirement of the system. Maintenance heat is needed in

most, but not all systems, to maintain digester contents at target operating temperature.

When an engine-generator set is used to convert biogas to electricity, the heat of combustion is

harvested from the engine and used to heat the digester. In this scenario, the heating efficiency of

the digester heating system is less important than if heat is provided by a biogas-fired boiler. Under

the later scenario, a primary goal of the digester system is normally to sell raw or processed biogas

and thus the need exists to minimize the parasitic heating requirement. Installations where heat

sales are important can utilize digester effluent/influent heat exchangers can be used to minimize the

parasitic heating requirement by preheating digester influent.

Biosecurity/disease control

Dairy manure is known to contain various pathogens that survive outside the cow. Not all cows on all

farms have the same contagious pathogens. The centralized digestion model involves commingled

digested manure and non-farm feedstock(s) being returned to the source farms resulting in justified

biosecurity concerns.

The hydraulic retention time (HRT) of complete mix digesters varies at the microscopic level from

manure particle to manure particle. Some manure particles will remain in the digester for greater

than the theoretical HRT while some will short-circuit due to the agitation process and exit sooner.

Data collected from one New York State dairy farm that co-digested dairy manure with several non-

farm biomass sources using a complete mix digester showed that the average reduction of the

commonly measured fecal coliform (an indicator organism) and Mycobacterium paratuberculosis

(Johne’s disease) was 98.4 and 94.8 percent, respectively (Wright et al., 2003).

In Denmark, mixing of non-farm biomass materials with manure is common practice and when this is

done, the Danish government requires the food waste/manure mixture to be pasteurized (70°C for

one hour) prior to being land applied in order for the farm to be in compliance with standard manure

application laws. Observation has shown that pasteurization normally occurs at the centralized

digester site, prior to digestion.

36

Operational considerations

Experience has shown that well-designed centralized digesters can be operated successfully for long

uninterrupted periods of time continuously (24 hours per day, seven days per week, and 365 days per

year) when adequate management and maintenance is provided. Centralized digesters are complex and

involve:

Physical systems including containment vessels and influent /effluent pits

Mechanical systems including pumps, agitators, and sensors

Biological systems including methanogens

The daily success of such a system is deeply rooted in personnel who take “ownership” in the system

and are provided the resources needed to make it successful.

General operational challe