PB83-190298
Resource Conservation and Utilization in Animal Waste Management -
Volume III. Utilization of Animal Manures as Feedstocks for Energy
Production
John H. Martin, et al
Cornell University
Ithaca, New York
March 1983
U.S. DEPARTMENT OF COMMERCE
National Technical Information Service
NITS
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EPA-600/2-83-024c
March 1983
RESOURCE CONSERVATION AND UTILIZATION IN
ANIMAL WASTE MANAGEMENT - VOLUME III
Utilization of Animal Manures as
Feedstocks for Energy Production
by
John H. Martin, Jr.
Raymond C. Loehr
Cornell University
Department of Agricultural Engineering
Ithaca, New York 14853
Grant Number R806140010
Project Officer
Lynn R. Shuyler
U.S. Environmental Protection Agency
Robert S. Kerr Environmental Research Laboratory
Ada, Oklahoma 74820
ROBERT S. KERR ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
ADA, OKLAHOMA 74820
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TECHNICAL REPORT DATA
(Please read Inaructions on the reverse before completing)
1. REPORT NO.
EPA-600/2-83-024C
2.
4. TITLE AND SUBTITLE . ,,..,. . . . , ...
Resource Conservation and Utilization in Animal Waste
Management - Volume III
Utilization of Animal Manures as Feedstocks for Energy
7. AuVfiSHWr1011
John H. Martin, Jr., and Raymond C. Loehr
9. PERFORMING ORGANIZATION NAME Af>
Department of Agricultural
Cornell University
Ithaca, NY 14853
JD ADDRESS
Engineering
12. SPONSORING AGENCY NAME AND ADDRESS
U.S. Environmental Protection Agency
Robert S. Kerr Environmental Research Laboratory
P.O. Box 1198
Ada, OK 74820
3. RECIPIENT'S ACCESSION NO.
PBS ? 190298
5. REPORT DATE
March 1983
6. PERFORMING ORGANIZATION CODE
8. PERFORMING ORGANIZATION REPORT NO.
10. PROGRAM ELEMENT NO.
APBC
11. CONTRACT/GRANT NO.
R-806140
13. TYPE OF REPORT AND PERIOD COVERED
Final, Vol. Ill
14. SPONSORING AGENCY CODE
EPA/ 600/15
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This study critically examined the feasibility of using thermochemical processes
such as combustion, pyrolysis, and partial oxidation and anaerobic digestion as
methods for utilizing livestock and poultry manures as renewable sources of energy.
Technical, economic, and environmental quality aspects were considered.
Results of this study indicate that livestock and poultry manures can, at best,
supply only a small fraction of U.S. energy requirements and cannot signficantly
reduce the dependence of U.S. agriculture on petroleum fuels. It was found that live-
stock and poultry manures generally are not suitable for use directly as fuels or
feedstocks for pyrolysis and partial oxidation due to high moisture content.
It also was found that the technical feasibility of manurial biogas production
has been adequately demonstrated and a rational basis for system design and operation
has been established. Although manurial biogas production is technically feasible,
economic feasibility was found to be site specific depending on available biogas
utilization options. Odor control appears to be the principal environmental quality
benefit associated with manurial biogas production. Potential adverse water quality
impacts associated with manures may be reduced, but are not eliminated.
17.
a. DESCRIPTORS
Agricultural Wastes
Waste Disposal
Anaerobic Digestion
Pyrolysis
Partial Oxidation
Combustion
13. DISTRIBUTION STATEMENT
Release Unlimited
KEY WORDS AND DOCUMENT ANALYSIS
b.lDENTIFIERS/OPEN ENDED TERMS
Water Conversion
Processes
Feedstock for Energy
Production
19. SECU-RIT.Y CLASS (This Report)
Unclassified
20. SECURITY CLASS (This page)
Unclassified
c. COSATl Field/Group
02/A, C, E
21. NO. OF PAGES
8«
22. PRICE
EPA Form 2220-1 (9-73)
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DISCLAIMER
Although the research described in this article has. been funded wholly or
in part by the United States Environmental Protection Agency through contract
or grant R-806140 to Cornell University, it has not been subjected to the
Agency's required peer and policy review- and therefore does not necessarily
reflect the views of the Agency, and no official endorsement should be inferred.
Mention of trade names or commercial products does not constitute endorsement
or recommendation for use.
ii
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FOREWORD
EPA is charged by Congress to protect the Nation's land, air and water
systems. Under a mandate of national environmental laws focused on air and
water quality, solid waste management and the control of toxic substances,
pesticides, noise, and radiation, the Agency strives to formulate and imple-
ment actions which lead to a compatible balance between human activities and
the ability of natural systems to support and nurture life. In partial
response to these mandates, the Robert S. Kerr Environmental Research Lab-
oratory, Ada, Oklahoma, is charged with the mission to manage research
programs to investigate the nature, transport, fate, and management of
pollutants in ground water and to develop and demonstrate technologies for
treating wastewaters with soils and other natural systems; for controlling
pollution from irrigated crop and animal production agricultural activities;
for controlling pollution from petroleum refining and petrochemical indus-
tries; and for managing pollution resulting from combinations of industrial/
industrial and industrial/municipal wastewaters.
This phase of the project was initiated to critically examine the
feasibility of using thermochemical processes such as combustion, pyrolysis,
partial oxidation, and anaerobic digestion as methods for utilizing livestock
and poultry manures as renewable sources of -energy. Results indicate that
livestock and poultry manures can only supply a small fraction of U.S. energy
requirements. The information presented is useful in determining which
systems will be useful in the development of Best Management Practices for
animal waste management systems.
Clinton W. Hall, Director
Robert S. Kerr Environmental
Research Laboratory
111
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ABSTRACT
Temporary but serious shortfalls in supplies of natural gas,
liquified petroleum gas, gasoline, and distillate fuels have led to the
examination of various opportunities for the agricultural sector to be-
come a producer as well as a consumer of energy. This study critically
examined the feasibility of using thermochemical processes such as com-
bustion, pyrolysis, and partial oxidation and anaerobic digestion as
methods for utilizing livestock and poultry manures as renewable sources
of energy. Technical, economic, and environmental quality aspects were
considered.
Results of this study indicate that livestock and poultry manures
can, at best, supply onlya small fraction of U.S. energy requirements
and cannot significantly reduce the dependence of U.S. agriculture on
petroleum fuels. It was found that livestock and poultry manures
generally are not suitable for use directly as fuels or feedstocks
for pyrolysis and partial oxidation due to high moisture content.
It also was found that the technical feasibility of manurial biogas
production has been adequately demonstrated and a rational basis for
system design and operation has been established. Although manurial
biogas production .is technically feasible, economic feasibility was
found to be site specific depending on available biogas utilization
options. Odor control appears to be the principal environmental quality
benefit associated with manurial biogas production. Potential adverse
water quality impacts associated with manures may be reduced, but are
not eliminated.
This report was submitted in partial fulfillment of Grant No.
R806140010 by Cornell University under the sponsorship of the U.S.
Environmental Protection Agency. This report covers the time period
of 1 October 1978 to 31 December 1980.
iv
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CONTENTS
Foreword i i i
Abstract iv
Figures vi
Tables vi i
Acknowledgements x
1. Introduction 1
Background 1
Objectives and Scope 3
2. Conclusions and Recommendations 4
3. Thermochemical Processes 6
Processes 6
Suitability of Manures as Feedstocks 9
Plant Nutrient Losses 13
Environmental Quality Impacts 16
Summary and Conclusions 20
4. Anaerobic Digestion 22
Historical Background 22
Process Fundamentals 24
Summary of Research Results 27
Economic Feasibility 37
Environmental Quality Impacts 61
Summary 69
5. References 71
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FIGURES
Number Page
1 Basic Components of Thermochemical Processes 10
2 Relationship Between Moisture Content and
Weight of Moisture H
3 Relationship Between Moisture Content and Heat
Required for Evaporation of Water in Comparison
with the Heating Value of Cattle Manure 12
4 Prices Paid by Farmers for Fertilizer Materials,
1968-1979 14
5 General Schematic for the Two-Step Conceptual-
ization of the Anaerobic Digestion Process 25
6 General Schematic for the Three-Step Conceptual-
ization of the Anaerobic Digestion Process 26
7 Relationship Between Biogas Production and Loading
Rate for Dairy Manure, 35°C .32
8 Effect of Detention Time on Dairy Manure Biogas
Production, 35°C 33
9 Relationship Between Biogas Production and Loading
Rate for Laying Hen Manure, 35°C 34
10 Effect of Detention Time on Laying Hen Manuare
Biogas Production, 35°C 34
11 Relationship Between Biogas Production and Loading
Rate for Swine Manure, 35°C 35
12 Effect of Detention Time on Swine Manure Biogas
Production, 35°C 35
13 Relationship Between On-Farm Utilization of Biogas
Generated Electricity and Resultant Income for
a 100-Cow Dairy Farm 48
14 Waste Stabilization as Indicated by Volatile Solids
Destruction as a Function of Digester Retention
Time 52
VI
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TABLES
Number
1 Estimates of the Quantities of Manure Produced
Annually by U.S. Livestock and Poultry and
Quantities of Manure that are Economically
Recoverable for Use 2
2 On-Farm Energy Consumption in U.S. Agriculture 2
3 Energy Balances for Batch and Continuous Retort
Pyrolysis of Beef Cattle Manure 8
4 Elemental Analysis of Potential Feedstocks for
Thermochemical Processes 9
5 Comparison of the Monetary Values of Livestock
and Poultry Manures Based on Nitrogen,
Phosphorus, and Potassium Content, $/fonne of
Dry Matter 13
6 Concentrations of Minerals Present in Ash and Char
Residues Produced by Combustion, Pyrolysis, and
Partial Oxidation of Poultry and Beef Cattle
Manures 15
7 Illustration of the Impact of Plant Nutrient
Losses on the Monetary Value of Manures Used
to Replace Conventional Fuels 16
8 Comparison of the Sulfur Content of Beef Cattle
Feedlot Manure and Conventional Fuels 17
9 Comparison of the Nitrogen Content of Manures and
Conventional Fuels 18
10 Composition of Fuel Gases and Chars Produced from
Manure by Pyrolysis and Partial Oxidation 19
11 Biogas Production from Dairy Manure - Research Results 28
12 Biogas Production from Laying Hen Manure - Research
Results 29
13 Biogas Production from Swine Manure - Research Results 30
14 Biogas Production from Beef Cattle Manure - Research
Results 31
vn
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Number Page
15 Estimates of Biogas Production Potential for Typical
Livestock and Poultry Production Units 36
16 Comparison of Estimates of the Cost of Producing
Biogas from Dairy Cattle Manure 38
17 Comparison of Estimates of the Cost of Producing
Biogas from Beef Cattle Manure 38
18 Summary of Costs for Conventional Sources of Energy 39
19 Comparison of Assumed Values Used in Estimating the
Cost of Producing Biogas from Dairy Cattle Manure 40
20 Comparison of Assumed Values Used in Estimating the
Cost of Producing Biogas from Beef Cattle Manure 41
21 Comparison of the Energy Densities of Biogas and
Methane to Natural Gas and Liquid Fuels 42
22 Effect of Compression on the Energy Density of Refined
Biogas with Comparison to Liquid Fuels 43
23 Quantities of Biogas Equivalent to One Liter of
Gasoline or Diesel Fuel 43
24 Critical Properties of Methane, Propane, Butane, and
Carbon Dioxide 44
25 Opportunities for On-Farm Use of Biogas as a Boiler
Fuel 45
26 Effect of Thermal Efficiency on Fuel Cost for Biogas
Generated Electricity 49
27 A Typical Heat Balance for a Spark Ignited L-Head
Engine 50
28 Comparison of Demand for Thermal Energy and Availability
of Waste Heat Recovered from a Biogas Fueled Engine-
Generator Set on a Typical New York State 100-Cow
Dairy Farm 51
29 Comparison of Demand for Thermal Energy and Availability
of Waste Heat Recovered from a Biogas Fueled Engine-
Generator Set on a Typical Central Iowa Farm 52
30 Estimated Costs and Revenues for Producing Biogas
from Beef Feedlot Manure 53
viii
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Number Page
31 Comparison of the Nutrient Composition of Beef
Cattle Manure Before and After Anaerobic ,-A
Digestion ^
32 Comparison of the Nutrient Composition of Dairy
Cattle Manure Before and After Anaerobic
Digestion 55
33 Comparison of the Nutrient Composition of Swine
Manure Before and After Anaerobic Digestion 55
34 Percentages of Anaerobically Digested Beef Cattle
Manure Constituents Recovered in Centrifuge Cake 56
35 Comparison of the Nitrogen and Amino Acid Content
of Anerobically Digested Beef Cattle Manure
Effluent and Centrifuge Cake 56
36 Influence of Dried Centrifuge Cake as a Component
of Rations for Sheep and Steers on Dry Matter
Intake and Apparent Digestibility 58
37 Influence of Wet Centrifuge Cake and Effluent on Dry
Matter Intake and Digestibility of Rations Fed to
Sheep and Steers 59
38 Performance of Beef Cattle Fed Anaerobically Digested
Manure as a Finishing Ration Component SO
39 Performance of Beef Feeder Cattle Fed Anaerobically
Digested Manure as a Finishing Ration Component 63
40 Comparison of the Physical and Chemical Characteristics
of Livestock and Poultry Manures Before and After
Anaerobic Digestion °^
41 Comparison of the Sulfur Content of Manurial Biogases
and Conventional Fuels °7
42 Comparison of SOX and NOX Emissions when Biogas,
Natural Gas, and Fuel Oil are Used as Boiler Fuels 68
43 Comparison of Air Pollutants Emitted when Methane is
Used as a Boiler Fuel Versus As a Fuel for Internal
Combustion Engines 68
44 Comparison of SOX and NOX Emissions Associated
with Manurial Biogas Production and Purification
with SN6 Production from Coal 69
ix
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ACKNOWLEDGEMENT
Financial support for this investigation was provided by the U. S.
Environmental Protection Agency under Grant No. R806140010 and the College
of Agriculture and Life Sciences, Cornell University. Mr. Lynn R. Shuyler;
U. S. Environmental Protection Agency, Ada, Oklahoma, served as the Project
Officer.
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SECTION 1
INTRODUCTION
During the past decade, the United States has experienced temporary but
serious shortfalls in supplies of natural gas, liquified petroleum gas,
gasoline, and distillate fuels. Among the many consequences of these events
has been an increased awareness of the finite nature of fossil fuel reserves
and the need to develop renewable supplies of energy. Another consequence
has been a better understanding of the dependence of production agriculture
on fossil fuels. The production of agricultural commodities has been esti-
mated to require approximately 2.6% of the energy consumed annually in the
U.S. (Economic Research Service, 1974).
These shortfalls and concerns have led to the examination of various
opportunities for the agricultural sector to become a producer as well as
a consumer of energy. Consideration has been given to the use of a variety
of agricultural residues including livestock and poultry manures as fuels
and as feedstocks for physical-chemical and biological energy conversion
processes such as pyrolysis and anaerobic digestion.
BACKGROUND
The dry matter content of manures excreted by U.S. livestock and poul-
try exceeds 100 million tonnes per year (Van Dyne and Gilbertson, 1978).
Assuming the average heating value of manurial dry matter to be 1.7 X 1010
joules (J)/tonne (Schlesinger et al., 1978), the potential energy value of
manures produced in the U.S. is more than 1.7 X 1018 J/yr. This is roughly
equal on the basis of energy content to 300 million barrels of crude oil or
16 million therms of natural gas.
Much of the manure produced by U.S. livestock and poultry is not
collectible since it is being produced on pasture or range. Van Dyne and
Gilbertson estimated that less than one-half of all livestock and poultry
manures can be economically recovered for use elsewhere with almost all
collectible manures produced in confinement facilities. Table 1 compares
estimates of the quantities of manures produced annually by U.S. livestock
and poultry and the quantities that are economically recoverable for use
elsewhere. It is noteworthy that dairy cattle are the single largest source
of collectible animal manures.
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TABLE 1. ESTIMATES OF THE QUANTITIES OF MANURE PRODUCED ANNUALLY BY U.S.
LIVESTOCK AND POULTRY AND QUANTITIES OF MANURE THAT ARE ECONOMI-
CALLY RECOVERABLE FOR USE (Van Dyne and Gilbertson, 1978).
Total Production
Beef Cattle (range)
Beef Cattle (feed lots
Dairy Cattle
Swine
Sheep
Laying Hens
Broilers
Turkeys
Total
Dry Weight,
106 tonnes/yr
47.3
) 9.5
22.9
12.1
3.5
3.1
1.9
1.1
101.3
% of
total
46.7
9.3
22.6
12.0
3.4
3.0
1.9
1.1
100.0
Economically Recoverable
Dry Weight,
106 tonnes/yr
1.7
9.5
18.5
5.0
1.5
3.0
1.9
0.9
42.0
% of
total
4.1
22.6
44.0
12.0
3.7
7.0
4.5
2.1
100.0
TABLE 2. ON-FARM ENERGY CONSUMPTION IN U.S. AGRICULTURE
(Economic Research Service, 19.74).
Direct inputs
Crops
Petroleum
Electricity
Livestock
Petroleum
Electricity
Subtotal
Indirect inputs
Fertilizer
Petroleum
Feeds & Additives
Animal & Marine Oils
Farm Machinery
Pesticides
1017J/yr
7.91
0.12
2.64
0.38
11.05
6.20
2.05
1.05
0.38
0.33
0.12
% of Total U.S.
Energy Consumption*
1.0
0.02
0.3
0.05
1.4
0.7
0.5
21.18
2.6
19
7.91 X 10lsJ/yr.
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Assuming that possibly 50% of the energy content of collectible animal
manures is recoverable in a usable form (Huffman, 1978), roughly 4 X 1017 J
of energy is potentially available annually from livestock and poultry
manures in the U.S. This represents approximately 15% of total (direct and
indirect) on-farm energy consumption in the U.S. and exceeds estimated
direct energy consumption for livestock production (Table 2).
OBJECTIVES AND SCOPE
Although the use of livestock and poultry manures as feedstocks for
energy conversion processes is attractive in principle, the merits of this
approach for utilizing animal manures remain debatable. The objectives of
this report are, in the context of U.S. agriculture, to:
1. Examine a) thermochemical processes such as combustion, pyroly-
sis, and partial oxidation, and b) anaerobic digestion as alterna-
tives for utilizing livestock and poultry manures as renewable
sources of energy. These alternatives are characterized in terms
of the forms of energy, possible by-products, and residuals
produced.
2. Assess the economic incentives for the utilization of animal
manures as renewable sources of energy based on: a) the monetary
value of the fuel or energy produced, b) opportunities for
utilization, and c) opportunities for by-product recovery. These
incentives are compared to estimates of production costs.
3. Identify possible environmental quality benefits and negative
impacts associated with the above energy conversion processes.
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SECTION 2
CONCLUSIONS AND RECOMMENDATIONS
CONCLUSIONS
1. Livestock and poultry manures can, at best, supply only a small frac-
tion of U.S. energy requirements but represent a renewable source of
energy that should not be overlooked.
2. Energy produced from animal manures cannot significantly reduce the
dependence of U.S. agriculture on gasoline and diesel fuel, and other
petroleum products.
Thermochemical Processes
1. Livestock and poultry manures generally are not suitable for use
directly as fuels or feedstocks for pyrolysis or partial oxidation due
to high moisture content. Broiler litter and manures produced in
semi-arid and arid climates are exceptions.
2. Of the thermochemical processes considered (combustion, pyrolysis, and
partial oxidation) combustion is the most attractive.
Anaerobic Digestion
1. The technical feasibility of manurial biogas production has been ade-
quately demonstrated and a rational basis for system design and opera-
tion has been established.
2. Of the available biogas utilization options, on-site generation of
electricity appears to be the most practical and generally applicable.
3. The economic attractiveness of producing biogas for on-site generation
of electricity depends on several site specific variables including
on-site demand for and cost of electricity, the price to be paid for
excess electricity, and opportunities for waste heat utilization.
4. Anaerobic digestion does not enhance the value of manures as feed-
stuffs. Thus, by-product credits based upon the monetary value of
anaerobically digested manures as feedstuffs are not justified.
5. Odor control appears to be the principal environmental quality benefit
associated with manurial biogas production. Potential adverse water
quality impacts associated with manures may be reduced but are not
eliminated.
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RECOMMENDATIONS
In future research, emphasis should be placed not only on production
but also utilization of energy from livestock and poultry manures.
Thermochemical Processes
.1. Emphasis in any future research should be placed on the direct use of
manures as boiler fuels.
2. In all future investigations, characterization of manures used as feed-
stocks and detailed energy and material balances should be basic
requirements.
3. Possible environmental quality benefits as well as negative impacts
also should be components of any future studies of thermochemical
conversion of animal manures into usable energy.
Anaerobic Digestion
1. Biogas production costs and the relationships between these costs and
system design and operation should be more rigorously examined.
2. The efficiency and cost as related to engine type (spark ignited,
diesel, or gas turbine) of using biogas for on-site generation of
electricity also should be more rigorously examined.
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SECTION 3
THERMOCHEMICAL PROCESSES
The possibility of using thermochemical processes such as combustion,
pyrolysis, and partial oxidation to recover usable forms of energy from
livestock and poultry manures has been the subject of recurring interest and
limited research for more than a decade. This section reviews the charac-
teristics of these processes and compares the forms of energy and residuals
produced. It also examines the suitability of livestock and poultry manures
as feedstocks for these processes, trade-offs between energy production and
plant nutrient losses, and environmental quality implications based upon the
limited data available. These topics address what appear to be the more
critical questions concerning the use of manures as renewable sources of
energy using thermochemical processes.
PROCESSES
Combustion, pyrolysis, and partial oxidation are the principal types of
thermochemical energy conversion processes applicable to organic wastes such
as animal manures. Although these processes differ substantially in many
respects, they have the common characteristic of employing high temperatures
to thermally decompose or crack complex organic molecules into simpler
constituents. Depending on the process used, either thermal energy or
solid, liquid, and gaseous fuels can be produced. Production of chemical
process feedstocks such as ammonia synthesis gas also is possible but will
not be considered in this report.
Combustion
Combustion is the simplest and most familiar thermochemical energy
conversion process. It produces thermal energy as a final product and ash
and other products of combustion as residuals. During combustion, complex
organic molecules are thermally cracked, ultimately producing elemental
carbon, hydrogen, and possibly sulfur, which are then rapidly oxidized
producing heat and commonly light. Residuals include: carbon dioxide
(C02), carbon monoxide (CO) if combustion is not complete, water, oxides of
sulfur (SOX) if sulfur is present, oxides of nitrogen (NOX), and ash.
Pyrolysis
Pyrolysis which is also known as destructive distillation also employs
high temperatures to crack complex organic molecules into simpler constitu-
ents. It differs from combustion in that it is a reducing rather than an
oxidizing process and is endothermic. However, the products produced which
6
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include solids, liquids, and gases can have value as fuels. Commonly, one
or more of these products are burned to provide the required process heat.
The solid component which is commonly referred to as char contains car-
bon and ash as principal constituents. The heating value of char produced
by pyrolysis of manures is variable depending on ash and carbon content.
Ash content is directly related to the composition of the manure used as a
feedstock while char carbon content is a function of process conditions.
Nelson (1973) reported that increasing the temperature for pyrolysis from
538° C to 815° C decreased the carbon content of poultry manure char from
44% to 36%. Reported heating values for chars produced from manures range
from 7.0 X 109 J/tonne (White and Taiganidies, 1971) to 1.8 X 1010 J/tonne
(Garner et al., 1972). Garner et al., noted that char produced by pyrolysis
of beef cattle manure at 400-500° C was difficult to burn.
Liquids produced during pyrolysis are typically a complex mixture of
water, alcohols, light oils, and tars. Garner et al., (1972) reported that
the chemical complexity of the oils and tars produced during pyrolysis of
beef cattle manures was such that stearic acid was the only compound that
could be positively identified. Estimated heating values for light oils and
tars were 2.3 X 1010 J/tonne and 3.0 X 1010 J/tonne respectively. Benzene
and toluene have been reported to be the principal constituents of light
oils produced during pyrolysis of municipal and industrial solid wastes
(Sanner et al., 1970).
Both combustible and noncombustible gases are produced during
pyrolysis. The major constituents of pyrolysis off-gases are hydrogen (H2),
nitrogen (N2), carbon dioxide (C02), carbon monoxide (CO),, methane (CH,J,
ethylene (C2Htt), and ethane ^Hg). Longer chain hydrocarbons (CnHn)
also may be present. The energy content of pyrolysis off-gases, which also
is called fuel gas, can vary widely but generally does not exceed 12 X 106
J/m3 of dry gas (Beck 1981). This is much less than other gaseous fuels
such as natural gas which has an energy density of 38 X 106 J/m3.
Product mix, i.e., the relative quantities of solids, liquids, and
gases produced during pyrolysis, depends on the characteristics of the
material pyrolyzed, the process temperature, and the possible presence of
catalysts. Generally, increasing process temperature increases production
of gases and reduces the quantities of oils and tars. Heavy oil can be
produced as a principal end product, however, by using steam and carbon
monoxide as reactants under high temperature and pressure (Appell et. al.,
1971). As previously noted, char carbon content and thus heating value also
decreases as process temperature increases.
Partial Oxidation
Partial oxidation is a modification of the pyrolysis process in which a
portion of the feedstock is burned in the reactor under controlled condi-
tions to energize the pyrolysis reaction. The principal advantage of
partial oxidation as compared to conventional pyrolysis is the reduction of
heat transfer problems. The products produced during pyrolysis and partial
oxidation are similar and are influenced by the same variables.
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Process Comparison
Overall thermal efficiency (the percentage of feedstock energy that
ultimately can be converted to heat or power) is the most logical basis for
comparing combustion, pyrolysis, and partial oxidation as processes for
converting livestock and poultry manures into usable forms of energy. Such
comparison, however, is not possible except in very general terms, due to
the lack of detailed energy balances for these processes from either full or
pilot plant scale studies.
Although no supporting data are available, it appears reasonable to
assume that the thermal efficiency of using manure as a boiler fuel should
be comparable to that for coal, 60 to 70%. Available data for pyrolysis
(Table 3) suggest that 76 to 85% of feedstock energy can be recovered as
gas, oil, and char. This is without consideration of process energy
requirements or thermal inefficiencies associated with utilization of the
fuels produced. Thus, it appears reasonable to conclude that the overall
thermal efficiencies of pyrolysis and partial oxidation are at best
comparable to and probably less than that for combustion.
TABLE 3. ENERGY BALANCES FOR BATCH AND CONTINUOUS RETORT PYROLYSIS OF
BEEF CATTLE MANURE.
Batch Pyrolysis* Continuous Retortt
Moisture content of manure, % 3.6 2.91
Yields, per dry tonne of manure
Gas, m3 410 663
Oil, 4 51 215
Water, i 150 478
Char, kg 342 337
Heating Values
Manure, J/kg 1.65 X 107 1.77 X 107
Gas, J/m3 1.68 X 107 4.6 X 106
Char, J/kg 1.69 X 107 1.48 X 107
Oil, J/i. tt 2.5 X 107 2.5 X 107
Energy Balance, J/tonne
of manurial dry matter
Input 1.65 X 1010 1.77 X 1010
Gas
Oil
Char
Total
Recovery, %
6.89 X 109
1.28 X 109
5.78 X 109
1.40 X 1010
85
3.05 X 109
5.38 X 109
4.99 X 109
1.34 X 1010
76
* Schlesinger et. al., 1972
t Massie and Parker, 1973
tt Estimated by authors
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Perhaps the most significant difference between pyrolysis and partial
oxidation and combustion is the form of energy produced. Pyrolysis and
partial oxidation are fuel producing processes requiring a subsequent
utilization step to produce thermal energy. If the gases, oils, and chars
produced by pyrolysis or partial oxidation of animal manures are to be used
as boiler fuels, there appears to be no advantage to using these processes
instead of combustion. Due to the low energy densities of the gases and
chars and the chemical complexity of the oils and tars produced by
pyrolysis, use of these products as boiler fuels is most probable.
SUITABILITY OF MANURES AS FEEDSTOCKS
The value of any organic material including animal manures as a fuel or
a feedstock for thermochemical processes such as pyrolysis or partial
oxidation depend on elemental composition and moisture content. As noted by
Whetstone et al. (1974), materials with high carbon and hydrogen contents
are most desirable with hydrogen being of greater value. Oxygen is
detrimental since it reduces heating value. Sulfur and nitrogen can result
in emissions of sulfur and nitrogen oxides, which are classified as air
pollutants.
As shown in Table 4, beef cattle feedlot manure compares favorably, on
the basis of elemental analysis, with other materials that can be used as
fuels or feedstocks for thermochemical processes. The values listed for
carbon, hydrogen, etc. are for beef cattle manure as it is produced. As
manures accumulate in barns, feedlots, and storage facilities, losses of
carbon, hydrogen, nitrogen, and possibly sulfur will occur and ash content
will increase due to microbial activity and weathering. Thus, the value of
manures as sources of energy also will be reduced.
TABLE 4. ELEMENTAL ANALYSES OF POTENTIAL FEEDSTOCKS FOR THERMOCHEMICAL
PROCESSES.
Subbituminous*
Material
Source
Carbon, %
Hydrogen
Oxygen
Nitrogen
Sulfur
Ash
Total
Moisture,
% wb.
Lignite*
ND-Beulah
Steam
42.4
6.7
43.3
1.7
0.7
6.2
101.0
34.8
Oil Shale*
CO-Green
River deposit
23.8 tt
2.6
12.3
0.5
1.0
59.8
100.0
0.2
coal
WY-monarch
seam
54.6
6.4
33.8
1.0
0.4
3.8
100.0
23.2
Solid Waste*
Average Mun-
icipal Refuse
35.4
4.4
28.2
0.4
0.2
31. 4§
100.0
20.7
Beef
Cattlet
Feedlot
Waste
42.6
5.5
23.7
2.8
0.5
24.9
100.0
29.1
* Feldmann (1971)
t Herzog et al. (1973)
tt Includes 4.6% carbon as carbonate
§ Includes metals, glass, ceramics
9
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Thermochemical energy conversion processes consist of three basic
components: drying, thermal decomposition, and recovery and utilization of
the energy produced (Figure 1). The first two components are endothermic
and require thermal energy inputs. The magnitude of these energy inputs is
an important factor in determining net energy production. Before the
necessary temperatures for thermal decomposition can be achieved, removal of
moisture is necessary. This makes feedstock moisture content a critical
factor.
FEEDSTOCK
DRYING
THERMAL
DECOMPOSITION
ENERGY RECOVERY
AND UTILIZATION
Energy
ENERGY FOR USE
Figure 1. Basic components of thermochemical'processes.
The relationship between moisture content, % wet basis (wb.), and the
height of water present on a total solids basis is illustrated in Figure 2.
Figure 3 shows the relationship between: a) the heat required for evapora-
tion of moisture in the feedstock, and b) the feedstock moisture content, %
wb., for an initial feedstock temperature of 20°C. Assuming a gross energy
(heat) value for manurial dry matter of 12,755 kJ/kg (Garner et al., 1972)
and a thermal efficiency of 65%, feedstock moisture content must be less
than 71% wb. for combustion to be theoretically self sustaining. This
assumes complete combustion of carbon, hydrogen, and sulfur and does not
consider evaporation of reaction water.
As illustrated in Figure 3, feedstock moisture content must be below
60-65% wb for significant net energy production via combustion. For
pyrolysis, it has been estimated that an additional 930 kJ/kg of total
solids is required (Garner et. al., 1972). This results in an additional
reduction of net energy production potential.
10
-------
ui
0 a. 2
I- •*
X \
o o
o>
I
0
0
10
I
I
I
I
70
20 30 40 50 60
MOISTURE CONTENT, % WET BASIS
Figure 2. Relationship Between Moisture Content and Height of Moisture.
80
-------
CE
LU
I
U.
O
P
go
CL
-------
As produced, the moisture content of livestock and poultry manures
exceeds 75% wb. Thus, although manures are attractive as thermochemical
process feedstocks on the basis of elemental analysis, they are unsuitable
without drying or addition of low moisture content organic matter. Only
manures produced in semi-arid and arid areas or manures which contain
substantial quantities of bedding or litter materials such as broiler
litter can be realistically considered as feedstocks for direct combustion,
pyrolysis, or partial oxidation.
PLANT NUTRIENT LOSSES
A majority of the investigations that have evaluated the use of live-
stock and poultry manures as boiler fuels and feedstocks for other thermo-
chemical processes were conducted during the late 1960's and early 1970's.
During this time period, abundant supplies of low cost inorganic fertilizers
were readily available, and the monetary value of manures as fertilizer
materials often did not justify expenditures required for such utilization.
Thus, losses of manurial plant nutrients associated with the thermochemical
processes were of little concern and received little attention.
Prices for inorganic fertilizer materials (Figure 4) and therefore the
monetary value of manures as fertilizer materials (Table 5) have increased
substantially over the past decade. Thus, loss of plant nutrients when
manures are utilized as fuels or thermochemical process feedstocks can
translate into a significant opportunity cost if using manures as fertilizer
materials is an available option. This section has two objectives. The
first is to summarize the available information concerning losses of
nitrogen, phosphorus, and potassium associated with combustion, pyrolysis,
and partial oxidation. The second is to compare the opportunity cost of
manurial plant nutrient losses to the value of energy produced.
TABLE 5. COMPARISON OF THE MONETARY VALUES* OF LIVESTOCK AND POULTRY
MANURES BASED ON NITROGEN, PHOSPHORUS, AND POTASSIUM CONTENT t,
$/TONNE OF DRY MATTER.
Year Dairy Cow Beef Feeder Laying Hen Broiler
1968
1974
1979
$13.18
$28.35
$31.25
$18.66
$42.23
$46.99
$19.57
$46.35
$50.88
$21.03
$47.38
$51.13
* Based on values presented in Figure 4.
t Agricultural Engineers Yearbook (1980)
Under the high temperatures common to combustion and other thermo-
chemical processes, organic nitrogen is converted to ammonia. Subsequently,
13
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240
200
160
O)
c
£ 120
80
O AMMONIUM NITRATE, 33.5 %N
A SUPERPHOSPHATE, 44-46% P205
Q MURIATE OF POTASH
60 % K20
40 -
1968
1970
1972
1974
YEAR
1976
1978
1980
Figure 4. Average Prices Paid by U.S. harmers for
Fertilizer Materials, 1968-1979 (Agricultural
Prices, 1S6S-1S80).
14
-------
ammonia molecules can be thermally cracked producing elemental nitrogen and
hydrogen. When manures are used as boiler fuels, loss of nitrogen is
essentially complete with both ammonia and elemental nitrogen becoming
components of stack gases. For pyrolysis and partial oxidation, elemental
nitrogen is a significant constituent of the gas produced; values from 15%
to 40% of dry gas volume have been reported (White and Taiganides, 1971;
Garner et al., 1972 and Huffman et al., 1978). Although this nitrogen can
be recovered via ammonia synthesis, it will be lost during combustion.
Nelson (1973) reported significant concentrations of ammonia nitrogen
in condensate and char produced when poultry manures were pyrolyzed.
Reported values for condensate ammonia nitrogen concentration ranged from
42,000 mg/Jl to 59,000 mg/l and ranged from 2.6% to 3.8% by weight for
chars. Dunn et al. (1976) reported that the aqueous fraction of condensate
produced during pyrolysis of beef cattle manure contained less than 1%
nitrogen. The nitrogen content of oil and tar and char fractions is a moot
point, however, if energy production is the process objective since these
residuals probably have greater value as fuels and the nitrogen present will
ultimately be lost during combustion.
Although it is generally assumed that neither phosphorus nor potassium
losses occur where manures are utilized in thermochemical processes, mater-
ial balances to support this assumption are lacking. Mineral analyses of
chars and ashes do suggest, however, that these materials are fairly concen-
trated sources of phosphorus and potassium as well as calcium and magnesium
(Table 6). This has led to suggestions that residual chars and ashes have
value as fertilizer materials (Davis et. al., 1972 and Khara et. al.,
1975). It has been noted, however, that soluble salts such as sodium
chloride which are harmful to plants are also concentrated in these chars
and ashes (Nelson, 1973; Whetstone et. al., 1974; and Kreis, 1979). Thus,
the value of these residuals as sources of phosphorus and potassium needs
critical examination. In addition, the availability of the plant nutrients
contained in these residuals remains undetermined.
TABLE 6. CONCENTRATIONS OF MINERALS PRESENT IN ASH AND CHAR RESIDUALS
PRODUCED BY COMBUSTION, PYROLYSIS, AND PARTIAL OXIDATION OF
POULTRY AND BEEF CATTLE MANURES.
Poultry Manure*
Minerals
Phosphorus
Potassium
Calcium
Magnesium
Combustion
ash, 600°C
11.4
24.6
10.8
2.8
Low tempera-
ture (Brown)
char, 482°Ctt
10.9
27.1
11.8
3.0
High tempera-
ture (Black)
char, 815°Ctt
8.4
27.5
9.4
2.4
Beef Cattle Manuret
Partial oxidation
ash
12.5 - 20.0
4.3 - 4.9
* Nelson, 1973
t Khara et al. 1975
tt Samples combusted at 600°C prior to mineral analysis
15
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Even though knowledge of the magnitude of plant nutrient losses that
occur when manures are used as fuels and feedstocks for other thermochemical
processes is imperfect, it is possible to illustrate the monetary signifi-
cance of such losses. Table 7 presents such an illustration comparing the
monetary value of manures used as replacements for conventional fuels with
and without consideration of the opportunity costs associated with plant
nutrient losses. As shown, using manures as sources of energy via thermo-
chemical processes can involve a significant economic trade-off if oppor-
tunities for plant nutrient utilization exist. This is especially true if
the use of coal is an available alternative for energy production.
TABLE 7. ILLUSTRATION OF THE IMPACT OF PLANT NUTRIENT LOSSES ON THE
MONETARY VALUE OF MANURES USED TO REPLACE CONVENTIONAL FUELS.*
Replacement of
Fuel Oil, No. 2t Natural Gastt Anthracite Coal§
Gross value, $/tonne
of dry matter* 103.32 51.66 45.92
Net value, $/tonne
of dry matter
Dairy
Beef
Laying Hen
Broiler
72.07
56.33
52.44
52.19
20.41
4.67
0.78
0.33
14.67
-1.07
-4.96
-5.21
* Based on 1979 values presented in Table 5.
t $8.10/GJ
tt $4.05/GJ
§ $3.60/GJ
# $12.755 GJ/tonne
ENVIRONMENTAL QUALITY IMPACTS
Although the use of livestock and poultry manures as fuels and as
feedstocks for other thermochemical processes has been considered in a
number of studies, the possible environmental quality impacts have received
little attention. Thus, a detailed analysis and comparison of possible
impacts and benefits is not possible. However, the environmental quality
impacts associated with the processes can be outlined based on knowledge of
process fundamentals and the limited data base that is available.
The use of manures as boiler fuels will result in the production of
potential air pollutants that are commonly associated with combustion such
16
-------
as oxides of sulfur (SOX) and nitrogen (NOX), carbon monoxide, and
particulates. Both sulfur dioxide (S02) and sulfur trioxide (S03) are of
particular concern. S02 can damage plants and S03, which is hygroscopic,
combines readily with water creating sulphuric acid.
The quantity of SOX produced during combustion is determined by fuel
sulfur content. When compared to conventional fuels (Table 8), beef feedlot
manure contains more sulfur on a mg/kJ basis than most conventional fuels
including low sulfur (anthracite) coal. Although data are not available,
it is probable that other manures are comparable in sulfur content. How-
ever, the need for control of SOX emissions to meet ambient air quality
standards when animal manures are used as boiler fuels as suggested by
Sweeten (1980) is unclear. The need for control of SOX emissions appears
unlikely, however, as the use of manures as boiler fuels will most probably
be limited to rural areas.
TABLE 8. COMPARISON OF THE SULFUR CONTENT OF BEEF
CATTLE FEEDLOT MANURE AND CONVENTIONAL FUELS.
Fuel
Sulfur, %
as S by wt
Sulfur,
mg/kJ
Anthracite coal* 0.8 0.27
High volatile B
bitumenous coal* 2.8 1.03
Fuel oil, mid-continent* 0.35 0.08
Fuel oil, California* 1.16 0.26
Pipeline natural gas* Trt Trt
Municipal sewage
sludge solids tt
Beef cattle feedlot
manure §
Beef cattle feedlot
manure #
0.70
0.77
0.80
0.40
0.55
0.59
* Schlesinger et al., 1978
t Tr - trace
tt Ballou et al., 1980
§ Sweeten and Higgins, 1980
# Engler et al., 1975
17
-------
The formation of nitrogen oxides during combustion results from the
oxidation of atmospheric nitrogen and varies with equipment design and
operating conditions. It appears, however, that nitrogen present in fuels,
particularly as ammonia, can contribute to the formation of nitrogen
oxides. For conventional fuels such as coal and fuel oil, nitrogen content
is minimal (Table 9) and contribution to nitrogen oxide formation during
combustion is of little concern. The nitrogen contents of animal manures
and sewage sludge solids are substantially higher than those of conventional
fuels. Thus, there is the possibility of increased nitrogen oxide emissions
when manures are used in place of conventional boiler fuels.
TABLE 9. COMPARISON OF THE NITROGEN CONTENT OF MANURES
AND CONVENTIONAL FUELS.
Nitrogen, % Nitrogen,
Fuel as N by wt mg N/kJ
Anthracite coal* 0.8 0.27
High volatile B
bitumenous coal* 2.8 0.48
Fuel oil, mid-continent* 0.5 0.11
Municipal sewage
sludge solidst
Beef feeder manurett
Dairy cow manurett
Laying hen manurett
Broiler manurett
4.5
4.9
3.9
5.4
6.8
2.68
2.88
2.29
3.18
4.00
* Schlesinger et al., (1978)
t Ballou et al., (1980)
tt Agricultural Engineers Yearbook (1980)
As discussed earlier, solids, liquids, and gases are produced when
organic materials such as manures undergo pyrolysis or partial oxidation.
Since closed reactors are used for these processes, any possible environ-
mental quality impacts will be associated with the utilization or disposal
of the products produced. Available data (Table 10) suggests that both the
fuel gas and char produced by pyrolysis or partial oxidation of manures are
attractive as fuels due to their low sulfur content. However, the relation-
ship between feedstock sulfur content and the quantities of sulfur present
in the gases and chars remains undefined as does the sulfur content of the
oils and tars produced.
18
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TABLE 10. COMPOSITION OF FUEL GASES AND CHARS PRODUCED
FROM MANURE BY PYROLYSIS AND PARTIAL OXIDATION.
Batch Pyrolysis
A B
Partial Oxidation
C D
Gas composition,
% volume basis
Oxygen
Nitrogen
Carbon Dioxide
Carbon Monoxide
Hydrogen
Methane
Ethyl ene
Ethane
Hydrogen Sulfide
0.0
0.0
24.5
18.0
27.5
22.7
0.0
0.0
0.0
4
19
18
18
30
--
13t
0.0
0.3-2.5
28.4-38.3
13.1-18.7
11.5-17.3
18.1-26.2
6.2-7.9
2.7-4.1
0.3
0.0
0.0
7.1
14.2
12.8
27.4
32.3
5.9
0.0
0.0
Char composition,
% weight basis
Carbon
Hydrogen
Oxygen
Nitrogen
Sulfur
49.4
0.4
0.4
1.1
0.3
63
*
*
*
*
32.0-41.2
0.3-1.6
*
2.0-2.5
*
*
*
*
*
*
A. Schlesinger et al., 1972
B. White and Taiganides, 1971
C. Halligan et al., 1974
D. Huffman et al., 1978
* Not reported
t Combustibles and illuminants
As shown in Table 10, the nitrogen content of the chars and fuel gases
produced during pyrolysis and partial oxidation of manures is variable.
Consequently, there will be the possibility of nitrogen oxide emissions
when these materials are used as fuels. Fuel gases produced via partial
oxidation should contain larger quantities of nitrogen oxides as compared to
gases produced by pyrolysis since limited quantities of air are limited to
support controlled combustion with partial oxidation.
As noted in Table 3 and also by Nelson (1973), significant quantities
of water, as much as 478 £ per tonne of manurial dry matter, are produced
during pyrolysis even when low moisture content feedstock is used. The same
is true for partial oxidation. These wastewaters appear similar to those
produced during the conversion of coal to coke containing high concentra-
19
-------
tions of ammonia nitrogen (Cousins and Mindler, 1972). As noted earlier,
Nelson (1973) reported condensates produced during pyrolysis of laying hen
manure contained ammonia nitrogen concentrations ranging from 42,000 mg/£ to
59,000 mg/a. This waste stream could be a serious liability if land
application to utilize the nitrogen present is not an available option.
Possible environmental quality problems related to disposal of ash
produced during combustion of manure or char also have received little
attention beyond recognition of possible problems associated with concentra-
tion of salts. If land disposal is practiced, it has been suggested that
land requirements would be comparable to those for disposal of the original
quantity of manure (Kreis, 1979). Thus, one of the apparent advantages of
these processes, reduction in land requirements for manure disposal, is
negated. The alternative of landfilling presents possible groundwater
contamination problems.
SUMMARY AND CONCLUSIONS
Analysis of the limited information that is available suggests that use
of either combustion, pyrolysis or partial oxidation for the recovery of
usable forms of energy from livestock and poultry manures is not overly
attractive. All three processes require low moisture content feedstocks to
achieve reasonable levels of thermal efficiency (Figure 3). Although a
detailed comparison of thermal efficiencies, i.e., feedstock energy produced
to energy available of the ultimate point of use, was not possible, it
appears that direct combustion is the most desirable of the three processes
in situations where low moisture content manures are available. The
comparatively low overall thermal efficiencies of pyrolysis and partial
oxidation (Table 3) are compounded when the thermal efficiences associated
with the ultimate use of the fuels produced also are considered.
The use of manures directly as fuels or as feedstocks for pyrolysis and
partial oxidation can involve significant opportunity costs if options for
utilizing manurial nitrogen, phosphorus, and potassium are available. The
value of phosphorus and potassium contained in char and ash fractions is
unclear due to the concentration of salts that can be harmful to plants.
Possible adverse environmental quality impacts associated with using
animal manures as fuels and feedstocks for pyrolysis and partial oxidation
have received little attention. It appears, however, that the potential for
adverse impacts exists but sufficient data to assess the possible signifi-
cance is lacking. Of particular concern are the possible impacts of: a) the
wastewater generated during pyrolysis and partial oxidation, b) char and ash
disposal, and c) sulfur and nitrogen oxides in the gases that are produced.
One aspect of using combustion, pyrolysis, and partial oxidation to
produce usable forms of energy from animal manures not discussed in this
section is economic feasibility. This omission was not accidental.
Although there have been several estimates of capital and operating costs,
there has been little agreement between estimates due to differences in
assumptions. This results from the lack of any full scale or even signifi-
cant pilot plant scale experience with these processes and manures.
20
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This study initially attempted to develop a rational basis for ranking
combustion, pyrolysis, and partial oxidation as processes for recovering
usable forms of energy from animal manures. The criteria to be used
included material and energy balances as well as costs. However, this
information was not available. It appears, however, that combustion, using
manures directly as fuels, is the most desirable of the three processes.
This conclusion is based on the following: (1) process simplicity,
(2) apparently greater thermal efficiency, and (3) absence of a liquid
waste stream. The above conclusion is in agreement with that reached by
Whetsone et al., (1974) in an earlier analysis of this subject area.
21
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SECTION 4
ANAEROBIC DIGESTION
In recent years, there has been growing interest in the U.S. in the
possibility of using anaerobic digestion to produce usable energy from
livestock and poultry manures. This topic has been the subject of several
extensive studies. Although these studies have demonstrated the technical
feasibility of manurial biogas production, economic feasibility in the U.S.
remains unclear. This section:
1. Reviews the history of anaerobic digestion as a unit process,
discusses process fundamentals, and summarizes the results of
manurial biogas production research.
2. Examines the economic feasibility of manurial biogas production
based on estimates of production costs and biogas utilization
options.
3. Considers possible environmental quality implications of manurial
biogas production.
HISTORICAL BACKGROUND
It has been known for more than a century that a flammable gas, now
commonly called biogas, is produced when organic materials decompose in the
absence of oxygen. Although the concept of using anaerobic digestion to
produce energy is not new, this process has been utilized only to a very
limited extent solely as an energy producing process. It is, however, used
extensively for the stabilization of wastes, particularly wastewater
sludges. The following briefly reviews the history of anaerobic digestion
as a process for waste stabilization and energy production particularly from
animal manures.
Credit for first recognizing the potential of anaerobic digestion as a
waste stabilization process is generally given to Louis Mouras (McCabe and
Eckenfelder, 1958; Griffiths, 1960). Around 1860, Mouras, a Frenchman,
received a patent for a sealed cesspool known as the Mouras automatic
scavenger. This tank was used to liquify wastes from several households
prior to discharge to a sewer.
It appears that the potential for using the combustible gas produced
during the liquification of wastewater solids was first recognized by an
Englishman, Donald Cameron (Metcalf and Eddy, 1935). In 1895, Cameron
22
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designed and constructed the first septic tank for the city of Exeter,
England and used the gas produced for street lighting.
In the early 1900's, several different types of dual purpose tanks that
combined sedimentation and anaerobic digestion of wastewater solids were
developed. Examples include the Travis hydrolytic tank, the Biolytic tank
and the Imhoff tank. Imhoff tanks were widely used, particularly in Germany
and the U.S. (Grifiths, 1960). Use of separate tanks for the digestion of
wastewater solids also began during this period. In 1911, the first
separate tanks for anaerobic digestion in the U.S. were constructed at the
wastewater treatment plant in Baltimore, Maryland (Wagenhals et al., 1925).
Since the 1920's, the anaerobic digestion process has been studied
extensively. Among the more significant results of these studies have been
major improvements in digestion tank design and the use of heat to
accelerate the digestion process.
Recognition of the potential of using anaerobic digestion to produce
usable energy from animal manures also dates back to the 1800's. Results of
a student's experiments produced the observation by Pasteur that anaerobic
digestion might be used to produce biogas from animal manures for heating
and illumination (Tietjen, 1975). In the 1930's, Buswell and others
conducted a number of comprehensive studies on this topic (Buswell and
Boruff, 1933; Jacobs, 1934; Buswell and Hatfield, 1936). Studies of
manurial biogas production also began in India in the late 1930's (National
Academy of Sciences, 1977).
A number of manurial biogas plants were built on farms in France,
Germany, and Algeria in response to war-related disruptions in supplies of
conventional fuels during and immediately after World War II (Tietjen,
1975). Many of these plants were quickly abandoned, however, as supplies of
conventional fuels were reestablished. In 1976, only one of the more than
1000 digesters built in France and Germany for the production of manurial
biogas was known to be in operation (Jewell et al., 1976).
Although manurial biogas production received little attention in the
U.S. and Europe in the years following World War II, the research program
iniated in India in the late 1930's was continued and expanded (National
Academy of Sciences, 1977). This research program has led to the
construction of several thousand small-scale manurial biogas plants to
provide fuel for cooking and illumination in rural areas of India.
Construction of numerous manurial biogas plants also has occurred in the
People's Republic of China, Korea, and Taiwan.
A renewal of interest in the anaerobic digestion of animal manures
occurred in the United States beginning in the early 1960's (Hart, 1963;
Cassell and Anthonisen, 1966; and Gramms et al., 1971). Emphasis was on
waste stabilization, however, with energy production a very secondary
concern. This reflected the environmental concerns and low energy costs of
the period. Although it was demonstrated that anaerobic digestion could be
an effective process for stabilizing animal manures, there was little
interest due to high costs which were not adequately offset by the value of
the biogas produced.
23
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With the oil embargo of 1973 and subsequent increases in crude oil
prices acting as catalysts, the concept of manurial biogas production became
the subject of renewed interest in the U.S. in the mid-1970's. Since 1975,
several full scale, experimental systems for manurial biogas production have
been constructed and operated for extended periods of time. Examples
include digesters located at Cornell University, The Pennsylvania State
University, University of Missouri, Washington State Reformatory Dairy Farm,
and U.S. Meat Animal Research Center. Results from these studies are
discussed elsewhere in this report.
PROCESS FUNDAMENTALS
Anaerobic digestion is a microbial process in which complex organic
compounds are degraded in the absence of oxygen producing biogas and other
reduced compounds as end products. Methane (CH^) and carbon dioxide (C02)
are the principal constituents of biogas with methane generally constituting
between 50% and 70% of the gas produced. The other 30% to 50% is primarily
carbon dioxide. Hydrogen (H2), hydrogen sulfide (H2S), ammonia (NH3),
nitrogen (N2), and water vapor also can be constituents of biogas but are
normally present only in trace amounts.
The microbial conversion of organic carbon to biogas traditionally has
been viewed as a two-step process involving two distinct groups of bacteria
(Metcalf and Eddy, Inc., 1979). The first step of the two-step process
scheme (Figure 5) usually is described as the acid phase. In this phase,
complex organic compounds; carbohydrates such as sugars, starches,
cellulose, and hemicellulose; proteins; and lipids (fats) are hydrolyzed and
fermented. The resultant compounds that can be utilized subsequently for
methane production include:
A) Fatty acids containing a maximum of six carbon atoms (formic,
acetic, propionic, butyric, valeric, and caproic acids).
B) Normal and isoalcohols containing from one to five carbon atoms
(methanol, ethanol, propanol, butanol, and pentanol).
C) Inorganic gases (hydrogen, carbon monoxide, and carbon dioxide).
24
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ACID _
PHASE
METHANE -
PHASE
COMPLEX ORGANIC COMPOUNDS
(CARBOHYDRATES, PROTEINS, LIPIDS)
HYDROLYSIS AND FERMENTATION
\
i
ORGANIC ACIDS
AND ALCOHOLS
\
r
CO, C02, H2
METHANOGENESIS
C02
i
CH,, & C02
Figure 5. General schematic for the two-step conceptualization of the
anaerobic digestion process.
Collectively, the microorganisms associated with the first step of the
anaerobic digestion process are known as "acid formers." Included in this
group are facultative and obligate anaerobic bacteria capable of producing
proteolytic, lipolytic, ureolytic, or cellulytic enzymes.
In the second step of the two-step process scheme, the products of
hydrolysis and fermentation are converted to methane and carbon dioxide.
The formation of methane from substrates such as ethanol, butyrate, and
hydrogen involves substrate oxidation and reduction of atmospheric carbon
dioxide as illustrated by the following equations:
2 C2H5OH + C02 —> 2 CH3COOH +
4 H2 + C02 —> DV + 2 H20
(1)
(2)
For substrates such as acetic and propionic acids, methane is formed by
reduction of carbon dioxide that was formed during substrate oxidation as
illustrated by the following equations:
4 C2H5COOH + 8 H20 —> 4 CH3COOH + 4 C02 + 24 H
3 C02 + 24 H —> 4 CH,, + 6 H20
4 C2H5COOH + 2 H20 —> 4 CH3COOH + C02 + 3 CH,,
(3)
(4)
(5)
25
-------
CH3 COOH —
C0
(6)
The principal genera of bacteria responsible for methane production are
Methanobacterium, Methanobacillus, Methanococcus, and Methanoscarcinia
(Higgins and Burns, 1975).Although methane can be produced from a number
of compounds, most of the methane produced during the anaerobic digestion of
complex organic materials such as manures is derived from the methyl group
of acetate (Mclnerney and Bryant, 1980).
Recently, it has been suggested that anaerobic digestion may be more
accurately described as a three-step process involving three and possibly
four distinct groups of bacteria (Mclnerney and Bryant, 1980). This scheme
is shown in Figure 6. The principal difference between this and the
two-step conceptualization of the anaerobic digestion process is the
identification of the fermentation of organic acids and alcohols to acetate,
carbon dioxide, and hydrogen as a separate step. The production of acetate
and possibly other acids from hydrogen and carbon dioxide also may be
considered a separate step as indicated in Figure 6.
ORGANIC MATTER
CARBOHYDRATES, PROTEINS & LIPIDS
HYDROLYSIS AND FERMENTATION
FATTY ACIDS
1
ACETOGENIC
DEHYDROGENATION
ACETATE
Ho + CO,
ACETOGENIC
HYDROGENATION
ACETATE
DECARBOXYLATION
T
REDUCTIVE METHANE
FORMATION
T
C0
C0
Figure 6. General schematic for the three-step conceptualization of the
anaerobic digestion process (Mclnerney and Byrant, 1980).
26
-------
Although the previous discussion has focused on carbon transformations,
other transformations also occur during anaerobic digestion. As shown in
Figures 5 and 6, proteins are utilized as substrates by the acid forming or
fermentative group of bacteria. Proteins are first hydrolyzed producing
constituent amino acids which are subsequently deaminated yielding ammonium
ions and an a-Keto acid. The a-Keto acid is then transformed into acetic
acid and subsequently to methane. The ammonium ions not utilized to satisfy
microbial nutrient requirements generally remain in solution. For highly
nitrogenous wastes such as animal manures, residual ammonia concentrations
can be substantial and desorption of ammonia can occur if pH levels result
in a significant concentration of unionized or free ammonia. Thus, ammonia
can be a trace consituent of biogas.
Microbial transformations of phosphorus also occur during anaerobic
digestion. During hydrolysis, organically bound phosphorus is transformed
into inorganic forms such as orthophosphates. Inorganic phosphates are
soluble compounds that can be utilized to satisfy microbial nutrient
requirements. Excess inorganic phosphates remain in solution and are
discharged with the stabilized waste.
The microbial formation of sulfides, particularly hydrogen sulfide
(H2S), during anaerobic digestion also is of importance. Sulfur is a
constituent of certain amino acids and is released as sulfate (SOi^2) when
these sulfur amino acids are hydrolyzed. The sulfate ion is then reduced to
sulfide (S~2) by Desulfovibrio and subsequently to hydrogen sulfide (H2S).
Since hydrogen sulfide is only slightly soluble in water, it becomes a
constitutent of biogas.
SUMMARY OF RESEARCH RESULTS
The production of biogas from livestock and poultry manures has been
the subject of a number of laboratory, pilot-plant, and full scale studies.
Tables 11 through 14 summarize the results from several of the more
comprehensive studies that have been conducted. These studies not only have
demonstrated that manurial biogas production is technically feasible but
also have provided a rational basis for system design and operation by
delineating relationships between parameters such as loading rate and
detention time and process performance. Examples of these relationships
are presented in Figures 7 through 12.
Table 15 presents biogas production estimates for typical livestock and
poultry enterprises. Based on maximum reported values for biogas production
on a nr/kg VS added basis (Tables 11 through 14). As shown, the gross
amount of energy that could be produced is appreciable. It should be
recognized that biogas available for utilization will be less, perhaps
significantly less due to digester heating requirements, than the estimates
presented and will vary with system design, method of operation, and
climatic conditions. It also should be recognized that it may not be
economically desirable to maximize biogas production due to the reactor
volume necessary.
27
-------
TABLE 11. BIOGAS PRODUCTION FROM DAIRY MANURE-RESEARCH RESULTS.
ro
oo
Loading Rate,
Source KG VS*/m3-day
Morris, 1976t
Pigg, 1977
Converse et al.,
1977
Jewell et al .,
1980
15.21
7.60
3.80
2.55
5.1
3.9
3.0
2.2
6.22
10.51
11.05
7.50
3.19
11.05
7.50
3.19
Detention
Time, Days
5
10
20
30
10
15
20
30
10.4
6.2
10
15
30
10
15
30
VS*
Reduction,
%
18.8
29.0
35.1
37.6
__
__
—
29.4
27.9
26.2
27.8
31.7
30.2
34.1
40.6
Gas Production,
m3/KG VS*
added
0.12
0.25
0.32
0.36
0.22
0.29
0.34
0.39
0.27
0.25
0.20
0.28
0.31
0.23
0.33
0.36
Gas
m3/KG VS* Composition, Temp.,
removed % C>V °C
0.63
0.87
0.92
0.95
—
__
—
0.87
0.86
0.78
1.00
0.98
0.77
0.99
0.89
61
64
63
63
54
54
54
54
57
54
60
55
58
60
55
57
32.5
32.5
32.5
32.5
35
35
35
35
35
60
35
35
35
35
35
35
*Volatile solids
-------
TABLE 12. BIOGAS PRODUCTION FROM LAYING HEN MANURE-RESEARCH RESULTS.
ro
10
Source
Gramms et al .
1971
Converse et al . ,
1981
Loading Rate,
KG VS*/m3-day
1.9
1.9
3.8
3.8
1.80
1.65
1.85
2.14
2.29
VS*
Detention Reduction,
Time, Days
10
15
10
15
36
46
45
38
36
%
67
67
64
57
62
56
56
51
65
Gas Production,
m3/KG VS*
added
0.30
0.36
0.25
0.28
0.37
0.44
0.44
0.46
0.51
Gas
m3/KG VS* Composition, Temp.,
removed
0.45
0.53
0.48
0.49
0.60
0.79
0.79
0.90
0.78
% CHi,
58
58
53
52
58
61
63
63
64
°C
32.5
32.5
32.5
32.5
35
35
35
35
35
*Vo1atile solids
-------
TABLE 13. BIOGAS PRODUCTION FROM SWINE MANURE-RESEARCH RESULTS.
CO
o
Loading Rate,
Source KG VS*/m3-day
Gramms et al.
1971
Kroeker et al.,
1975
Fischer et al.,
1979
1.9
1.9
3.8
3.8
1.0
2.1
4.0
Detention
Time, Days
10
15
10
15
30
15
15
VS*
Reduction,
%
52
61
44
59
44
36
63 '
Gas Production,
mVKG VS*
added
0.25
0.42
0.36
0.43
0.76
0.65
0.58
Gas
m3/KG VS* Composition, Temp.,
removed % CH4 °C
0.49
0.69
0.83
0.73
1.74
1.90
1.10
61
60
58
59
67
68
59
32.5
32.5
32.5
32.5
35
35
35
*Volatile solids
-------
TABLE 14. BIOGAS PRODUCTION FROM BEEF CATTLE MANURE-RESEARCH RESULTS.
Loading Rate,
Source KG
Loehr and Agnew,
1967
Hashimoto, et al.,
1978
VS*/nr-day
1.35
2.71
3.51
4.80
14.88
11.45
5.15
3.42t
VS*
Detention Reduction,
Time, Days
10
10
10
10
4
6
12
20
%
58.6
45.3
34.4
44.6
39.8
46.1
52.8
44.2
Gas Production,
mVKG VS*
added
0.56
0.55
0.74
0.70
0.42
0.43
0.56
0.38
Gas
m^/KG VS* Composition, Temp.,
removed
0.96
1.21
2.15
1.57
1.06
0.94
1.06
0.86
% CH,,
58
57
52
53
50
53
55
58
°C
35
35
35
35
55
55
55
55
*Volatile solids
tNo mechanical mixing
-------
0.40
o
I- uj
O O
3 0
O <
rt™
0.30
OJ
ro
> 0.20
o>
5 0.10
A
V-O.OI6X + 0.39
R * -0.87
O MORRIS (1976)
A PIGG0977)
0 CONVERSE ET. AL.0977)
• JEWELL ET. AL. (1980)
6 8 10 12
LOADING RATE, kg VS/m3-doy
14
rigure 7. Relationship Between Biogas Production and Loading j^ate for
Dairy i-ianure, 350C.
16
-------
O MORRIS (1976)
A PIGG (1977)
D CONVERSE ET. AL.(I974)
• JEWELL ET. AL. (1980)
0.40
0 0
0.30
0 <
O CO
£>
O
g
GO
°-20
0.10
o
A
e
o
X
1
1
10 20
DETENTION TIME , days
30
Figure 8. Effect of Detention Time on Dairy Manure Biogas
Production, 35°C.
33
-------
5 o0.60
g<
§ ^0.40
a.
o>
en jc
< ^
O 10
Q E0.20
CD
A
O GRAMMS ET. AL. (1971)
A CONVERSE ET. AL. (1981)
z
Q _ 0.60
O ^
a: en 0.40
a. >
Ow
O E 0.20
CD
012345
LOADING RATE , kg VS / m3- day
Figure S.. Relationship Between Biogas Production
and Loading Rate for Laying Hen Manure, 35°C.
O GRAMMS ET. AL.U97I)
A CONVERSE ET. AL. (1981)
0
A
1
1
1
10 20 30 40
DETENTION TIME, days
50
Figure 10. Effect of Detention TiiTie on Laying Hen
Manure Biogas Production, 35 C.
34
-------
0.80
z"
; PRODUCTIOI
g VS ADDED
0 0
4* b>
0 0
g*E 0.20
CD
n
A
A
D
0 °
o
0
0 GRAMMS
A KROEKER
D FISCHER
1 1 1 1
ET. AL. (1971)
ET. AL. (1975
ET. AL (1979)
1 I
01 23456
LOADING RATE, kg VS/m3-day
Figure 11. Relationship Between Biogas Production and
Loading Rata for Swine Manure, 35°C.
0.80
Z
2 o 0.60
H~ *' *
O Q
^ o
Q <
Q
oc g 0.40
Q. <*
O»
0) -*•
< ^
O *>
2 e 0.20
m
o
A
Q
@'
0
0 GRAMMS ET. AL (1971)
0 A KROEKER ET. AL.(I975)
~ Q FISCHER ET. AL.(I979)
1 1 1 1 1 1
> 10 20
DETENTION TIME, days
Figure 12. Effect of Detention Tir.ie on Swine Manure
Biogas Production, 35°C.
35
30
-------
TABLE 15. ILLUSTRATION OF BIOGAS PRODUCTION POTENTIAL FOR TYPICAL LIVESTOCK AND
POULTRY PRODUCTION UNITS.
CO
CTl
Volatile Biogas Production,
Number Solids Loading
of Produced, Rate, kg m3/KG VS
Animals kg/day* VS/m-day added m3/Day
Dairy cow, 100 468 2.2t 0.39t 182
-------
ECONOMIC FEASIBILITY
Although it has been established that substantial quantities of energy
in the form of biogas can be obtained from livestock and poultry manures,
the economic feasibility of manurial biogas production is questionable.
Assuming that alternative sources of energy are available, adequate income
must be produced to provide an attractive rate of return for invested
capital and management. Otherwise, other investment opportunities will be
more attractive. The following sumarizes estimates of biogas production
costs and examines biogas utilization and by-production recovery options as
sources of income to offset costs and provide the returns necessary for the
economic feasibility of this technology.
Biogas Production Costs
Even though the subject of manurial biogas production has been studied
extensively, utilization of this technology under commercial conditions is
just beginning to occur. Thus, opportunities for detailed analyses to
determine production costs have been lacking. However, there have been
several attempts to estimate the cost of producing biogas from dairy and
beef cattle manures using information obtained from research studies. The
results of these estimates are summarized and compared in Tables 16 and 17.
Estimated production costs for electricity include costs associated with
engine-generator set operation and are based on the assumption that waste
engine heat is used to satisfy digester heating requirements. Thus, biogas
production cost estimates which are components of the electricity production
cost estimates are based on total rather than net biogas production. When
compared to current (1981) prices for conventional sources of energy (Table
18), it appears that manurial biogas can be produced at a cost that is
competitive with conventional fuels particularly fuel oil and liquified
petroleum gas and also electricity.
37
-------
TABLE 16. COMPARISON OF ESTIMATES OF THE COST OF PRODUCING BIOGAS
FROM DAIRY CATTLE MANURE.
Source
Abeles et al.
(1978)*
Coppinger et al.
(1979)t
Persson et al .
(1979)t
Jewell et al.
(1980)*
Jewell et al.
(1980)t
No. of
Animals
100
200
400
100
50
100
300
500
150
100
300
Production Cost
Biogas, $/GJ net Electricity, $/kWh
7.22 0.320
4.26 0.045
3.23 0.036
4.96tt
4.38
3.07
2.13
1.89
9.00
6.00
4.00
*Earth supported, plug flow digester
•(•Conventional type, rigid wall digester
ttDoes not include labor costs
TABLE 17. COMPARISON OF ESTIMATES OF THE COST OF PRODUCING BIOGAS
FROM BEEF CATTLE MANURE.
Source
No. of
Animals
Production Cost
Biogas, $/GJ net Electricity, $/kWh
Hashimoto et al
(1978)
Lizdas et al.
(1980
10,000
80,000
6.33
5.97
*Does not include electrical generation costs.
tlncludes C02 removal to improve generator set efficiency.
0.022*
Hashimoto and
Chen (1981)
400
4,000
40,000
13.71
5.46
2.77
0.3191-
0.0751-
0.0251-
38
-------
TABLE 18. COMPARISON OF ESTIMATED PRODUCTION COSTS FOR MANORIAL BI06AS AND
BIOGAS GENERATED ELECTRICITY WITH COSTS FOR CONVENTIONAL
ENERGY SOURCES.
Energy Source Unit Cost* Unit Cost, $/GJ
Fuel Oil, No. 2 $0.325/£ $ 8.10
Liquified Petroleum Gas 0.198/£ 7.76
Natural Gas 0.155/m3 4.05
Coal, Anthracite 0.109/KG 3.60
Electricity 0.05/KwH 13.89
Biogas
Dairy Manure — $1.89 to 9.00t
Beef Cattle Manure — 2.77 to 13.71tt
Biogas Generated Electricity
Dairy Manure $0.036 to 0.32/kWht
Beef Cattle Manure 0.022 to 0.319/kWhtt
*Consumer prices excluding taxes, Ithaca, New York, January 1981.
tRange of values, Table 16.
ttRange of values, Table 17.
Although the biogas production cost estimates summarized in Tables 16
and 17 appear to be reasonable, the differences among these estimates should
be viewed critically. As shown in Tables 19 and 20, the assumptions
underlying these estimates vary substantially. For example, the value for
net energy production, GJ/cow-year, assumed by Jewell et al. (1980) is
substantially higher than the values used by other investigators for biogas
produced form dairy manure (Table 19) reflecting a lower estimate of biogas
requirements for digester heating. The differences in interest rates used
for amortizing capital costs also should be noted. Current (1981) interest
rates are about 16 percent.
Biogas Utilization
It is generally agreed that effective utilization of essentially all
available energy is critical to the economic feasibility of producing biogas
from livestock and poultry manures. Manurial biogas can be used as a fuel
for trucks, tractors, and other vehicles, as a boiler fuel, and as a fuel
for on-site generation of electricity. The following discusses and analyzes
these utilization options.
39
-------
TABLE 19. COMPARISON OF ASSUMED VALUES USED IN ESTIMATING THE COST OF PRODUCING BIOGAS
FROM DAIRY CATTLE MANURE.
-Fa
O
Source
Abeles et al.
(1978)*
Copplnger et al.
(1979)t
Persson et al.
(1979)t
Jewell et al.
(1980)*
No. of
Animals
100
200
400
100
50
100
500
Net Energy Output,
GJ/YR
590
1108
2216
760
456
956
4964
GJ/COW-YR
5.9
5.5
5.5
7.6
9.1
9.6
9.9
Capital Cost, Interest Labor Required,
$
$22,500
55,100
84,700
20,000
15,400
20,800
53,500
$/GJ net Rate, %
38.14 9.5
49.74 12.0
38.22
38.14 6.0
33.75 10.0
21.73
10.77
HR/YR
144
219
328
548tt
102
165
665
*Earth supported, plug flow digester.
tConventional type, rigid wall digester.
ttNot included in biogas production cost estimate.
-------
TABLE 20. COMPARISON OF ASSUMED VALUES USED IN ESTIMATING THE COST OF PRODUCING BIOGAS
FROM BEEF CATTLE MANURE.
Source
Hashimoto et al.
(1978)*
Lizdas et al.
.P. (1980)tt
Hashimoto et al.
(1981)*
No. of
An i ma 1 s
10,000
80,000
400
4,000
40,000
Net Energy Output,
GJ/YR
46,100
121,276
2,039
20,929
211,359
GJ/COW-YR
4.6
1.5
5.1
5.2
5.3
Capital
$
901,000
3,085,000
65,000
327,000
1,640,000
Cost,
$/GJ net
19.54
25.44
31.88
15.62
7.76
Interest
Rate, %
9
9
14
14
14
Labor Required,
HR/YR
t
12,000
t
t
t
*Thermophilic digestion, 55°C.
tLabor included in cost estimates but requirements, HR/YR, not stated.
ttMesophilic digestion, 35°C.
-------
The well documented dependence of production agriculture on gasoline
and diesel fuel for the operation of trucks, tractors, and other vehicles
has made using biogas conceptually attractive as a substitute. However,
typical manurial biogas containing 60% methane has an energy density of 22.2
MJ/m3 which is substantially less than that of liquid fuels including
liquified petroleum gas (Table 21). Even with removal of carbon dioxide,
the energy density of refined biogas (100% methane) is still slightly less
than even that of natural gas due to the absence of heavier hydrocarbons
such as ethane and propane.
TABLE 21. COMPARISON OF THE ENERGY DENSITIES OF BIOGAS AND
METHANE TO NATURAL GAS AND LIQUID FUELS.
Fuel Energy Density*
Biogas (60% Methane)
Refined Biogas (100% methane)
Natural Gas (79% CHM 14% C2H6)
Gasoline
Diesel Fuel, No. 2-Dtt
Fuel Oil, No. 2tt
22.2 MJ/m3t
37.0 MJ/m3t
38.3 MJ/m3t
35,600 MJ/m3
40,100 MJ/m3
(35.6 MJ/A)
(40.1 MJ/*)
Liquified Petroleum Gas 25,500 MJ/m3 (25.5 MJ/£)
*Schlesinger et al., 1978
t20°C, 760 mm
ttASTM grades
Although the energy density of refined biogas can be substantially
increased by compression (Table 22), it still is not comparable to liquid
fuels. Thus, sizable fuel tanks would permit only limited periods, one to
two hours, of vehicle operation (Table 23). In addition the energy required
for compression is not insignificant. Smith et al. (1977) estimates that
11.2% of the energy available in uncompressed methane would be required for
compression to 3.4 MPa.
In order to achieve an energy density that is comparable to liquid
fuels, liquification of refined biogas is necessary (Table 22). However,
unlike propane and butane which are the principal constituents of
liquified petroleum gas, methane is difficult to liquify due to its low
critical temperature, -82.4°C (Katz et al, 1959). Above this temperature,
methane cannot be liquified solely by pressure; refrigeration also is
necessary. In contrast, both propane and butane can be liquified at ambient
42
-------
TABLE 22. EFFECT OF COMPRESSION ON THE ENERGY DENSITY OF REFINED
BI06AS WITH COMPARISON TO LIQUID FUELS.
Fuel
Refined Biogas (100% methane)
Liquified Methane
Gasoline
Diesel Fuel, No. 2-D
Liquified Petroleum Gas
Pressure,
MPa* Gauge
0
0.7
1.4
2.1
2.8
3.4
4.6
—
—
2.0
Energy Density,
MJ/m3
37
293
550
818
1,099
' 1,380
23,300
35,600
40,100
25,500
*Pascals x 106; LBf/in2 x 6.89 x 106 = MPa
TABLE 23. QUANTITIES OF BIOGAS EQUIVALENT TO ONE LITER
OF GASOLINE OR DIESEL FUEL.
Biogas
(60% methane)
Refined Biogas
(100% methane)
Pressure,
MPa Gauge
0
0.7
2.0
7.0
0
3.4
7.0
Volumetric
Gasoline, a
1603.6
199.2
52.3
76.3
962.2
25.8
13.0
Equivalent to -
Diesel Fuel, I
1806.3
224.4
58.9
86.0
1083.8
29.0
14.7
43
-------
temperatures (Table 24). In addition to the need for refrigeration, almost
all impurities must be removed before the methane component of biogas can be
liquified. Typical requirements are no more than 0.002% carbon dioxide,
0.0001% water, and 2.3 x 10~5 mg of hydrogen sulfide per m3 of gas (Kohl and
Risenfield, 1974). Molecular sieves would probably be necessary to achieve
this level of purification. Using the Linde process, approximately 30% of
the energy available in uncompressed methane would be required for
liquification (Smith and Van Ness, 1959).
TABLE 24. CRITICAL PROPERTIES OF METHANE, PROPANE, BUTANE AND
CARBON DIOXIDE (KATZ ET AL., 1959).
Critical Temperature, Critical Pressure,
Compound °C* MPa Gauget
Methane (CHJ -82.4 4.6
Propane (C3H8) 93.5 4.3
Butane (C4H10) 152.0 3.8
Carbon Dioxide (C02) 31.0 7.4
*That temperature above which a gas can not be liquified by pressure alone.
tThat pressure under which a substance may exist as gas in equilibrium with
a liquid.
Although use of biogas as a fuel for trucks, tractors, and other
vehicles is technically feasible, this method of utilization is not
generally viewed as practical particularly in situations where liquid fuels
are available. Even with carbon dioxide removal and compression, the low
energy density of methane would permit only limited periods, one to two
hours, of vehicle operation even with sizable fuel tanks. In order to be
competitive with liquid fuels, liquification of refined biogas which is
technically complex and significantly reduces the quantity of available
energy is necessary.
Of the available alternatives, use of biogas as a boiler fuel to
produce heat, hot water, or steam is the most attractive from standpoints of
simplicity and thermal efficiency. It also can be attractive economically
if costly energy sources such as fuel oil, liquified petroleum gas, and
electricity can be replaced. However, on-farm use of boiler fuels and
therefore opportunities for using biogas as a substitute are limited as
illustrated by Table 25. For large beef cattle feedlots, opportunities for
using biogas as a boiler fuel also are limited. Sweeten (1979) estimated
44
-------
TABLE 25. OPPORTUNITIES FOR ON-FARM USE OF BIOGAS AS A BOILER FUEL.
(Jewell et al., 1976).
Fuel
Gasoline
Diesel Fuel
Electricity*
Boiler Fuels
- Fuel Oil
- Electricityt
Total
Biogas Productiontt
Boiler Fuels, %
of Biogas Production
40 Cow
Amount
5,530 £
2,760 £
25,160 KwH
6,435 £
11,800 KwH
Da i ry
GJ x 102/yr
1.97
1.08
0.90
2.58
0.42
6.95
4.52
66.4%
100 Cow
Amount
13,700 £
8,270 9.
37,900 KwH
6,435 £
24,350 KwH
Dairy
GJ x 102/yr
4.88
3.32
1.36
2.58
0.88
13.02
10.30
33.6%
1000 Head Beef
Amount GJ
26,685 £
26,190 £
38,520 KwH
13,170 £
18,000 KwH
Feed lot
x 102/yr
9.50
10.50
1.39
5.28
0.65
27.32
58.40
10.2%
*Excluding water and space heating
tFor water and space heating
ttNet energy production, plug flow digester
-------
that only 30% of the biogas produced on a 30,000 head beef feedlot could be
used on-site as a boiler fuel, primarily for steam flaking of corn or
sorghum.
Sale of biogas as substitute natural gas (SN6) also is possible.
However, biogas purification which involves removal of carbon dioxide,
hydrogen sulfide and water vapor to levels specified for pipeline quality
natural gas and compression are required prerequisites. Even for large
feedlot size systems, biogas purification to meet pipeline quality standards
is costly. Hashimoto et al. (1979) estimated that biogas purification
equipment necessary for the sale of manurial biogas as SNG would account for
30% of the total cost of a biogas production system for a 10,000 head beef
cattle feedlot. Thus, sale of biogas as SNG can not be realistically viewed
as a possible option for smaller farm size systems.
Since the low energy density of biogas makes it generally unsuitable
for use as a vehicular fuel and demand for boiler fuels on farms and
feedlots is limited, on-site generation of electricity appears to be the
most practical and generally applicable biogas utilization alternative.
This utilization option is particularly attractive since the Public Utility
Regulatory Policies Act of 1978 (PURPA) requires utilities to offer to
purchase electrical energy and capacity from co-generation and small (less
than 80 MW rated capacity) power producing facilities that use renewable
resources such as biogas as a primary fuel (Berger et al., 1978). Thus,
biogas utilization is not necessarily limited to on-site demand for
electricity. The following examines the biogas utilization approach
focusing on the factors that affect income realized and therefore economic
feasibility.
The revenue that can be derived from biogas generated electricity is
dependent on several inter-related variables. One is the cost of purchased
electrical energy which determines the monetary value of biogas generated
electricity used on-site. Unlike other forms of energy, the cost of
purchased electrical energy and thus the value of biogas generated
electricity used as a replacement varies substantially. In late 1981, the
cost of electricity in the U.S. ranged from less than $0.05/KwH to over
$0.10 KwH reflecting differences in methods of generation, fuels used, etc.,
among utilities. The cost of purchased electricity from an individual
utility also can vary depending on quantity used, peak demand, and time of
use.
Another variable affecting the revenue that can be realized is the
price that is paid by utilities for purchased electrical energy. Although
PURPA requires electric utilities to purchase biogas generated electricity,
the price paid may be substantially less than the value of electricity
utilized on-site. Under rules issued by the Federal Energy and Regulatory
Commission, state public utility commissions are required to establish rates
for electrical energy purchased from co-generation and small power producing
facilities using "avoided cost" as the basis for rate making (Schiefen,
1981).
46
-------
Avoided cost is the cost that a utility would incur if it were to
generate or purchase from another source an equivalent amount of power. If
the utility has adequate generating capacity, avoided cost is the cost of
fuel not used. This may be less than $0.02/KwH for coal or exceed $0.06/KwH
for oil generated electricity (New York State Electric and Gas Corporation,
1981). Avoided cost also may include costs that would be associated with
construction of additional generating capacity or long term contractual
purchases from other utilities. If the price paid for biogas generated
electricity is based solely on the cost of fuel not used, there will be a
substantial difference between the value of electricity used on-site and
sold. Including avoided costs related to generating capacity would
significantly reduce this differential, reflecting only distribution and
administrative costs.
It is generally agreed that on-farm generation of electricity using
biogas should be done in parallel with an electric utility to eliminate the
need for generating capacity to meet peak demands and to facilitate sale of
excess electrical energy. Maximizing on-site use of biogas generated
electricity is most desirable, however, due to the previously discussed rate
differential. The impact of this rate differential on income produced by
biogas generated electricity for a 100 cow dairy farm is illustrated in
Figure 13. In this example, reducing on-site utilization from 80% to 40%
reduces income by $840/year.
The revenue derived from biogas generated electricity also is a
function of the efficiency of this conversion process. As illustrated by
Table 26, reported values for the thermal efficiency of converting biogas to
electricity and therefore the fuel cost per KwH of electrical energy
produced vary significantly. Thermal efficiency affects not only the
quantity of electricity produced and the fuel cost/KwH but also generated
cost. Koelsch and Walker (1981) have estimated a cost of $0.018/KwH for
converting biogas to electricity at a thermal efficiency of 20%. At a
thermal efficiency of 17.6% (Table 26), generation cost will increase to
$0.020/KwH.
Waste heat recovery and utilization is commonly cited as a method of
enhancing the thermal efficiency of the biogas to electricity conversion
process. As shown in Table 27, approximately three-quarters of the energy
input for an internal combustion engine is converted to heat. A reasonably
well designed waste heat recovery system should capture about two-thirds of
this thermal energy for digester heating and other uses. As shown in Tables
28 and 29, a substantial fraction of recovered waste heat can be utilized in
northern climates. Demand, however, exceeds availability in winter months
and there is a substantial surplus during the summer. Thus, the signifi-
cance of waste heat recovery and utilization on the thermal efficiency of
the biogas to electricity conversion process is dependent on climate and by
extension geographical location.
47
-------
6200 -
5800 -
5400 -
5000 -
UJ
O
o
- 4600 -
4200 -
3800 -
ASSUMPTIONS:
I. 7.75XI04 KW-h/yr
2. $0.08/kW-h FOR PURCHASED
ELECTRICITY DISPLACED
3. $0.05/kW-h FOR ELECTRICITY
SOLD
0 20 40 60 80 100
ON-SITE UTILIZATION, % OF TOTAL
Figure 13. Relationship Between On-Far,,i Utilization
of Biogas Generated Electricity and
Resultant Income for a 100 Cow Dairy Farr.i.
48
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TABLE 26. EFFECT OF THERMAL EFFICIENCY ON FUEL COST FOR BIOGAS GENERATED ELECTRICITY.
UD
Engine Type
Spark Ignitedtt
Spark Ignited#
DieseU
Diesel§
Fuel Consumption, MJ/HR
Biogas Diesel Fuel
511
351
329 60
289 48
Fuel Cost, Thermal
$/HR* ' Efficiency, %t
2.56
1.76
2.13
1.83
17.6
20.6
18.6
30.0
Electrical
Output, KW/HR
25
20 •
20
28
Fuel Cost/
kWh,$/kWh
0.102
0.088
0.106
0.065
*Biogas - $5.00/GJ, Diesel Fuel - $8.10/GJ.
tFrom biogas to electricity
ttCoppinger et al., 1979
#Persson and Bartlett, 1981
§Ortiz-Canavate et al., 1981
-------
TABLE 27. A TYPICAL HEAT BALANCE FOR A SPARK IGNITED L-HEAD ENGINE.
(Obert, 1968).
Heat of Combustion, % Coolant Load, %
Engine Coolant
Exhaust Port 7 18
Head 19 49
Cylinder _13^ 33
Subtotal 39 100
Exhaust Gases 37
Friction 4
Brake Power 18
Unaccounted 2
Total 100
By-Product Recovery
Cost analyses by several investigators; Burford and Varani (1978),
Hashimoto et al. (1978), and Lizdas et al. (1980); have indicated that for
large scale facilities such as those using beef feedlot manure as a
feedstock the monetary value of biogas produced will only partially offset
total production costs (Table 30). These investigators have concluded,
however, that producing biogas from manure can be economically feasible due
to the value of digested manure or constituents thereof as feedstuffs (Table
31).
50
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TABLE 28. COMPARISON OF DEMAND FOR THERMAL ENERGY AND AVAILABILITY OF WASTE
HEAT RECOVERED* FROM A BIOGAS FUELED ENGINE-GENERATOR SET ON A
TYPICAL NEW YORK STATE 100 COW DAIRY (Koelsch and Walker, 1981).
Utilized
Month
January
February
March
April
May
June
July
August
September
October
November
December
Demand,
24.9
22.5
28.2
28.7
21.1
15.0
13.4
13.7
17.4
24.9
32.3
31.3
GJt Available, GJtt
11.9
9.2
14.0
18.1
29.8
32.9
33.0
34.0
29.8
25.0
19.8
17.1
274.6
Utilized/Available =
Quantity, GJ
11.9
9.2
14.0
18.1
21.1
15.0
13.4
13.7
17.4
24.9
19.8
17.1
195.6
71.2%
Percentage
100
100
100
100
71
46
41
40
58
100
100
100
*50% recovery of biogas energy
tExcluding digester heating
ttAfter digester heating
51
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TABLE 29. COMPARISON OF DEMAND FOR THERMAL ENERGY AND AVAILABILITY OF
WASTE HEAT RECOVERED* FROM A BI06AS FUELED ENGINE-GENERATOR
SET ON A TYPICAL CENTRAL IOWA FARM. (Smith et al., 1977).
Utilized
Month
January
February
March
Apri 1
May
June
July
August
September
October
November
December
Demand,
47.2
36.5
37.2
16.2
10.1
6.0
2.7
2.7
7.3
61.9
87.9
43.9
GJt Available, GJtt
26.8
24.3
23.6
17.0
38.9
52.2
63.5
67.3
39.1
(47.7)§ 38.8
(51.6)§ 26.6
20.0
438.1
Utilized/Available =
Quantity, GJ
26.8
24.3
23.6
16.2
10.1
6.0
2.7
2.7
7.3
38.8
26.6
20.0
205.1
48.6%
Percentage
100
100
100
95
26
11
4
4
19
100
100
100
*55% recovery of biogas energy
tExcluding digester heating
ttAfter digester heating
§Thermal energy required for drying corn
52
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TABLE 30. ESTIMATED COSTS AND REVENUES FOR PRODUCING BIOGAS FROM
BEEF FEEDLOT MANURE.
Burford and
Varani
(1978)
Hashimoto,
et al.
(1978)
Lizdas,
et al.
(1980)
System capacity, head
of feeder cattle 50,000 10,000 80,000
Total production costs,
$/year 2,229,000* 390,400* 724,350
Methane value, $/year 475,000t 76,500t 258,700§
By-product credit,
$/year (value of
residue as a feedstuff) 2,378,000# 414,000** 3,138,800 tt
(413,900)§§
* includes centrifugation of effluent for feedstuff recovery.
t $1.66 per GJ.
§ $3.00 per GJ.
# $83.00 per tonne of dry matter.
** $100.00 per tonne of dry matter.
tt $86.18 per tonne of crude protein (cottonseed meal equivalent).
§§ $11.34 per tonne of crude protein (urea equivalent).
This conclusion is based on the assumption that anaerobically digested
beef cattle manure can be used to replace soybean or cottonseed meal in
finishing rations for feeder cattle. In these cost analyses, it was implied
that beef cattle manure prior to digestion has no nutritive or monetary
value as a feedstuff since no opportunity cost for using this manure as a
feedstock for biogas production was included. The logic underlying this
assumption is unclear particularly when published values for the nutrient
composition (Bhattacharya and Taylor, 1975 and Ensminger and Olentine,
1978) and the estimated monetary value (Smith and Wheeler, 1979) of beef
cattle manure as a feedstuff are considered. In the following discussion,
information concerning the value of anaerobically digested manures as
feedstuffs is analyzed to determine if the previously noted assumption and
conclusion are valid.
Nutrient Transformations--
In the cost analysis cited, it was suggested that anaerobic digestion
increases or "enhances" the value of beef cattle manure as a feedstuff.
This hypothesis also has been suggested by Jewell et al. (1978) and Meckert
(1978). When nutrient composition data for beef cattle manure before and
after anaerobic digestion are compared on a dry matter basis (Table 31), it
appears that enhancement has occurred. On this basis, the concentrations of
53
-------
crude protein and amino acids have increased and those of digestible fiber,
cellulose and hemicellulose, are essentially unchanged. Comparison on a
weight volume basis shows, however, that the digestion process has not
increased but rather only concentrated crude protein and amino acids and
then only on a dry matter basis.
TABLE 31. COMPARISON OF THE NUTRIENT COMPOSITION OF BEEF CATTLE MANURE
BEFORE AND AFTER ANAEROBIC DIGESTION* (Prior and Hasimoto, 1981).
Grams per Liter
Component
Dry Matter
Cellulose
Hemicellulose
Lignin
Crude Protein
Amino Acids
Gross Energy
Ash
Influent
7.22%
-X of Dry
10.5
26.2
3.1
34.8
14.3
4661t
9.8
Effluent
3.98%
Matter-
10.6
20.1
6.4
61.6
23.5
4655t
17.1
Influent
72.2
7.6
18.9
2.2
25.1
10.3
336. 5§
7.1
Effluent
39.8
4.2
8.0
2.6
24.5
9.4
185. 3§
6.8
Percent change,
gm/liter Basis
-44.9
-44.7
-55.7
+18.2
- 2.4
- 8.7
-44.9
- 4.2
* Average of 5 and 13 day HRT, 55°C.
t Kilocalories per gram of dry matter.
§ Megacalories per liter.
Loss of dry matter during anaerobic digestion results from the
microbial transformation of carbonaceous materials to methane and carbon
dioxide which is a universal characteristic of the process. Thus, any
constituent will appear to increase on a dry matter basis if the loss of
that constitutent is less than the dry matter loss.
Although it may be suggested that the concentration of crude protein
and amino acids enhances the value of beef cattle manure as a feedstuff, the
validity of this suggestion is questionable. First, the loss of digestible
fiber is a loss of available energy. Significant losses of digestible fiber
occur when dairy cattle manure is anaerobically digested (Table 32). Simple
carbohydrates (sugars and starches) and lipids also are reduced during
anaerobic digestion (Table 33). These losses are not unexpected, however,
since simple carbohydrates and liptds are readily available microbial
substrates that are utilized for biogas synthesis.
54
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TABLE 32. COMPARISON OF THE NUTRIENT COMPOSITION OF DAIRY CATTLE MANURE
BEFORE AND AFTER ANAEROBIC DIGESTION* (Jewell et al., 1978).
Grams per Liter Percent change,
Component Influent Effluent InfluentEffluent gm/liter Basis
Dry Matter 8.00% 5.34% 80.0 53.4 -33.2
-% of Dry Matter-
Cellulose
Hemicellulose
Lignin
Ash
24.2
18.3
8.2
11.4
21.0
13.1
11.9
16.2
19.3
14.6
6.5
9.1
11.2
7.0
6.0
8.8
-42.0
-52.0
- 8.3
- 3.3
* 12 day HRT, 35°C.
TABLE 33. COMPARISON OF THE NUTRIENT COMPOSITION OF SWINE MANURE BEFORE
AND AFTER ANAEROBIC DIGESTION (Fischer et al., 1978).
Component
Hemicellulose
Cellulose
Lipids
Protein
Volatile Acids
Starch
Glucose (free)
Lignin
Grams
Influent
11.79
7.07
8.45
11.62
4.44
0.67
0.22
2.54
per Liter
Effluent
4.35
2.70
2.78
6.35
0.52
0.03
0.02
2.57
Percent
Loss
63
62
67
49
88
96
91
--
Second, the concentration of nutrients on a dry matter basis is of
value only if these nutrients can be effectively recovered. Results of
studies involving centrifugation as a liquid-solids separation process sug-
gest that this is not possible (Hashimoto et al., 1978). Estimated maximum
recoveries for total solids and organic nitrogen using a polyelectroyte to
enhance dewatering were 50% and 60%, respectively. Actual recoveries have
been significantly lower (Tables 34 and 35).
55
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TABLE 34. PERCENTAGES OF ANAEROBICALLY DIGESTED BEEF CATTLE MANURE CON-
STITUENTS RECOVERED IN CENTRIFUGE CAKE (Hashimoto et al., 1978)
Digester Rentention Time, Days
Parameter
Total Solids
Volatile Solids
Fixed Solids
Suspended Solids
Total Nitrogen
Organic Nitrogen
Number of Observations
20
31 ± 2*
33 ± 2
24 ± 5
45 ± 2
12 ± 1
18 ± 4
4
12
34 ± 4
32 ± 4
39 ± 7
49 ± 4
13 ± 4
20 ± 4
16
6
35 ± 3
34 ± 3
39 ± 8
51 ± 3
20 ± 4
23 ± 3
9
* Mean ± standard deviation.
TABLE 35. COMPARISON OF THE NITROGEN AND AMINO ACID CONTENT OF ANAERO-
BICALLY DIGESTED BEEF CATTLE MANURE EFFLUENT AND CENTRIFUGE CAKE
(Hashimoto, et al., 1978).
Percent of Dry Matter
Parameter
Total Nitrogen
Organic Nitrogen
Total Ami no Acids
Essential Amino Acids*
Effluent
5.05
4.27
25.25
10.84
Centrifuge Cake
2.96
2.36
13.44
5.75
* Phenylalanine, threonine, methionine, valine, leucine, isoleucine, lysine,
histidine, arginine.
These comparisons of the nutrient compositions of beef cattle, dairy
cattle, and swine manures before and after anaerobic digestion indicate that
the possible value of these materials as feedstuffs is decreased
substantially.
Digestibility Studies--
Interest in using anaerobically digested beef cattle manure (effluent)
and effluent solids concentrated by centrifugation (wet cake and dry cake)
as feedstuffs has resulted in several digestibility studies to determine
results are summarized in Tables 36 and 37. The data illustrate that using
digested manure or constituents thereof as components of rations fed to
56
-------
sheep and beef cattle typically decreased digestibility of dry matter,
organic matter, energy, nitrogen, and other constituents. These decreases
indicate that the manures fed were not comparable nutritionally to the
conventional feedstuffs replaced.
The decreases in nitrogen digestibility (Tables 36 and 37) are
particularly interesting in light of the previously noted assumption
regarding the use of digested beef cattle manure as a substitute for soybean
or cottonseed meal. The studies cited in Tables 36 and 37 also have shown
that using anaerobically digested beef cattle manure as a feedstuff can
create palatability problems which significantly reduce dry matter intake.
Feeding Trials--
The potential value of anaerobically digested beef cattle manure as a
feedstuff also has been examined in a number of feeding trials. Results
from studies with heifers and steers are summarized in Table 38. In the
first study (Burford and Varani, 1978), wet centrifuge cake, 30% dry matter,
replaced cottonseed meal, straw, and limestone in a steam flaked milo-
alfalfa hay based finishing ration. Prior et al. (1981) used digester
effluent, approximately 4% dry matter, in place of soybean meal in a ground
corn-brome grass hay based finishing ration. In both of these feeding
trials, negative as well as positive control rations were used. In the
negative control rations, soybean meal was replaced by ground corn. In the
trial involving heifers, undigested beef cattle manure (influent) also was
evaluated as a replacement for soybean meal.
In the Burford and Varani study (Table 38), the use of wet centrifuge
cake as a substitute for cottonseed meal significantly reduced feed
consumption and weight gains. Feed conversion efficiency, kg of feed
consumed/kg of weight gain, also was adversely affected but to a much lesser
degree. The reduced level of feed consumption for the experimental ration
indicates palatability could have been a problem. The authors noted that
the wet centrifuge cake had a "strong" chemical odor resulting from the use
of a commercially available feed preservative, "Grazon", which was added to
the centrifuge cake.
The use of digester effluent (Prior et al., 1981) in place of soybean
meal in a finishing ration for steers also reduced weight gain and feed
conversion efficiency (Table 38) even though feed consumption increased.
The steers fed the negative control ration also outperformed the positive
control group in this feeding trial. In the subsequent feeding trial
involving heifers, average daily gain for animals fed digester effluent was
comparable to the positive control group (Table 38). However, average daily
gain for the heifers fed the influent also was comparable to the positive
control. Feed consumption and feed conversion efficiency data for this
feeding trial have not been published to date (1982).
Results of feeding trials involving the use of digested beef cattle
manure (effluent) and centrifuge cake (wet cake) also have been presented by
Lizdas et al. (1980). In this study, a protein supplement in a steam flaked
corn, corn silage, and beet pulp finishing ration was partially replaced.
57
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TABLE 36. INFLUENCE OF DRIED CENTRIFUGE CAKE AS A COMPONENT OF RATIONS FOR
SHEEP AND STEERS ON DRY MATTER INTAKE AND APPARENT DIGESTIBILITY
(Prior et al., 1981).
Percent of Ration Dry Matter
Sheep*
Dry Matter Intake, kg/day
Apparent Digestibility, %
Dry Matter
Organic Matter
Energy
Nitrogen
Ash
Steerst
5
1.027
72.5
73.5
70.2
58.5
59.5
10
0.970
71.6
73.5
70.0
55.1
46.3
15
0.886
72.2
75.2
71.4
56.8
40.0
20
0.947
68.0
71.5
68.1
51.4
28.4
Dry Matter Intake, kg/day
Apparent Digestibility, %
4.85
5.37
4.91
* Replacing alfalfa hay.
t Replacing brome grass hay, corn, soybean meal and limestone.
§ Acid detergent fiber.
# Neutral detergent fiber.
5.36
Dry Matter
Organic Matter
Energy
Nitrogen
Ash
ADF§
NDF# (cell walls)
Cellulose
Cell Contents
77.2
78.5
75.4
63.4
51.2
54.9
62.5
59.8
83.7
71.1
72.7
70.1
60.3
31.5
27.2
59.5
38.8
75.8
72.1
73.7
70.8
61.8
50.6
48.6
61.0
59.4
79.0
62.9
65.4
63.8
54.2
28.0
43.1
66.9
56.5
59.2
58
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TABLE 37. INFLUENCE OF WET CENTRIFUGE CAKE AND EFFLUENT ON DRY MATTER INTAKE AND DIGESTIBILITY
OF RATIONS FED TO SHEEP AND STEERS.
in
10
Feedstuff
Wet Cake*
(to steers)
Effluentt
(to sheep)
Effluentt
(to steers)
% of
Ration
Dry Matter
0
30
0
6.5
0
6.5
Neg. 0§
Dry
Matter
69.9
67.1
81.4
75.4
76.1
73.9
77.6
Apparent
Organic
Matter
69.9
67.1
82.8
77.1
—
Digestibi
lity, 1
Ash Energy
« _ «
66.2
39.8
—
_ _ —
81.3
74.4
—
/
0
Nitrogen
52.5
52.6
72.6
58.8
61.5
61.5
70.5
Dry Matter
Intake,
kg/day
11.7
9.2
0.78
0.84
5.29
5.50
5.04
Feedstuff
Reduced or
Replaced
Cottonseed
Meal
Soybean
Meal
Soybean
Meal
* Richter (1979).
t Prior et al. (1981).
§ Negative control.
-------
TABLE 38. PERFORMANCE OF BEEF CATTLE FED ANAEROBICALLY DIGESTED MANURE A FINISHING RATION
COMPONENT.
Burford and Varani (1978)
Steers - Control
Wet Cake, 18%*
Prior et al . (1981)
Steers - Control
Effluent, 6.45%t
Negative Control
Heifers - Control
Influent, 6.45%t
Effluent, 6.45%t
Negative Control
Total
Weight
Gain, kg.
128.6
103.9
159.0
136.9
162.2
108.0
107.8
106.4
103.1
Average Feed
Daily Feed Con- Conversion
Gain, sumption, Efficiency,
kg/Day kg/Day kg/kg
1.67 14.22 8.50
1.35 11.92 8.85
0.93 8.21 8.83
0.80 8.55 10.69
0.94 7.59 8.07
1.28 § §
1.28 § §
1.27 § §
1.20 § §
Number
of
An i ma 1 s
36
36
10
10
10
20
20
20
20
Duration
of Study,
Days
77
77
168
168
168
126
126
126
126
* % of ration, "as fed" basis.
t % of ration, dry matter basis.
§ Not reported.
-------
Although reported values for average daily gain and feed conversion effi-
ciency for both trials (Table 39) appear reasonable, neither positive nor
negative controls were included in the experimental design for these feeding
trials.
Results of future feeding trials may show that using anaerobically
digested manures as feedstuffs significantly reduces unit feed costs, $ per
kg of weight gain. It should be recognized, however, that unit fixed costs
will increase if either average daily gain or feed conversion efficiency is
adversely affected. This point has been commonly overlooked in studies
evaluating the use of animal manures as feedstuffs.
ENVIRONMENTAL QUALITY IMPACTS
The production of biogas from livestock and poultry manures as well as
the utilization of biogas as a fuel are generally viewed as attractive from
an environmental quality standpoint. Both odor control and reduction of the
water pollution potential of animal manures are commonly cited as important
benefits. It has been suggested that anaerobic digestion could be
designated as a best management practice (BMP) for addressing manure related
nonpoint source water quality problems under provisions of the Clean Water
Act (PL 95-217) addressing such problems (Office of Technology Assessment,
1980). Although such environmental quality benefits may result from
anaerobic digestion, analysis of the significance of these possible benefits
as well as impacts has been lacking. The objective of this section is to
examine and discuss this subject.
Pollution Characteristics of Manures
Waste stabilization is frequently cited as an important benefit of
using anaerobic digestion to produce biogas from animal manures. When
concentrations of total and volatile solids and chemical oxygen demand in
manure slurries before and after anaerobic digestion are compared (Table
41), it can be seen that substantial reductions do occur. These reductions,
which are the result of the microbial conversion of carbonaceous compounds
into methane and carbon dioxide, translate directly into reduced
carbonaceous oxygen demand if these wastes enter surface waters.
It should be recognized, however, that the carbonaceous fractions of
livestock and poultry manures are not completely stabilized during anaerobic
digestion particularly at the short retention times, 10 to 15 days,
necessary to economically optimize biogas production (Figure 14). In order
to produce a highly stabilized effluent, a retention time of at least 30
days is necessary.
Although anaerobic digestion is a waste stabilization process,
stabilization is limited to carbonaceous materials. As shown in Table 41,
concentrations of manurial nitrogen and phosphorus are essentially unchanged
by the digestion process. However, the fraction of manurial nitrogen
present as ammonia and therefore the potential nitrogenous oxygen demand of
manures is substantially increased. This oxygen demanding process, known as
nitrification, is common in both terrestrial and aquatic environments.
61
-------
80
^
E
cy^
*
o
o
40
ro
o
H
z
UJ
UJ
20
BIODEGRADED VOLATILE
SOLIDS
BIODEGRADABLE
VOLATILE SOLIDS
Figure 14.
5 10 15 20 25
RESIDENCE TIME, days
Waste Stabilization as Indicated by Volatile Solids Destricucion
as a Function of Digester Retention Time (Morris, 1975).
30
-------
OJ
TABLE 39. PERFORMANCE OF BEEF FEEDER CATTLE FED ANAEROBICALLY DIGESTED MANURE AS A FINISHING
COMPONENT (Lizdas et al., 1980).
Total Average Feed
Weight Dally Feed Con- Conversion Number Duration
Gain, Gain, sumption, Efficiency, of of Study,
kg kg/day kg/day kg/kg Animals Days
Effluent*
Wet Cake*
115.5 1.7 10.5 6.3 20 69
90.6 1.5 10.4 6.9 19. 5t 79
* Average crude protein content of 77 gm/head/day.
t Weighted average, 2 animals removed during the trial.
-------
TABLE 40. COMPARISON OF THE PHYSICAL AND CHEMICAL CHARACTERISTICS OF LIVESTOCK AND POULTRY
MANURES BEFORE AND AFTER ANAEROBIC DIGESTION.
en
Laying Hen Manure* Beef Cattle
Concentration, Change in Concentration,
gm/liter Concentration, gm/liter
Manuret
Change in
Concentration,
Parameter Influent Effluent % Influent Effluent %
Total Solids 113.7 56.0 -51 70.1 36.6
Volatile Solids 76.5 32.1 -58 61.8 29.2
Chemical Oxygen
Demand (COD) 119.0 70.4 -41 74.9 40.2
Total Nitrogen 8.8
Ammonia Nitrogen
as N 5.2
Phosphorus as P 2.9
Potassium as K 3.4
9.2 + 4 4.3 3.9
7.1 +36 1.1 1.9
2.1 -28
3.6 + 6
-48
-53
-46
- 9
+73
—
—
* Converse et al., 1981, 35°C, retention time 36-46 days.
t Hashimoto et al., 1978, 55°C, retention time 12 days.
Continued ..
-------
TABLE 40. continued
Dairy Cattle Manurett
Swine Manure§
Concentration, Change in Concentration, Change in
gm/liter Concentration, gm/liter Concentration,
Parameter
Total Solids
Volatile Solids
Chemical Oxygen
Demand (COD)
Total Nitrogen
Ammonia Nitrogen
as N
Phosphorus as P
Potassium as K
Influent
77.5
64.9
77.2
2.6
0.9
0.6
1.9
Effluent
,59.0
45.8
51.0
2.9
1.3
0.5
2.0
%
-24
-29
-34
+12
+44
-17
+ 5
Influent
73.5
60.3
80.6
3.9#
1.7
1.3
1.2
Effluent
35.7
23.9
35.9
4.1#
2.9
1.2
1.2
%
-51
-60
-55
+ 5
+70
- 8
0
ttConverse et al., 1977, 35°C, retention time 10.4 days.
§ Fischer et al., 1978, 35°C.
# Total Kjeldahl nitrogen.
-------
Stoichiometrically, 4.57 gm of oxygen are required for the oxidation of one
gm of ammonia nitrogen to nitrate nitrogen. Thus, the nitrogenous oxygen
demand exerted if anaerobically digested manures are permitted to enter
surface waters will be substantial and probably differ little from
undigested manures. Other water quality problems such as eutrophication
resulting from soluble forms of phosphorus and ammonia toxicity to fish and
other aquatic organisms also will be undiminshed.
Although the water quality benefits associated with manurial biogas
production are questionable, it is clear that this process can provide a
measure of odor control. Under controlled anaerobic conditions, the produc-
tion of reduced compounds (organic acids, amines, mercaptans, and sulfides)
which occur under uncontrolled anaerobic conditions and resultant odors are
minimized. It is important, however, to recognize that although manure
odors may be considerably reduced in both presence and offensiveness as
noted by Welsh et al. (1977), they are not eliminated.
The actual environmental quality benefits of waste stabilization as a
by-product of manurial biogas production depends on digester performance and
total system management. Possible environmental quality problems will
relate to odors and water quality impacts associated with storage and
pasture and cropland disposal of digester effluent. The problems that do
occur and the benefits realized will be the result, however, of management
and not biogas production.
Biogas Utilization
The possible impacts of biogas utilization on air quality have received
little attention. Data that are available indicate, however, that impacts
should be minimal particularly with respect to emissions of oxides of sulfur
(SOX) and nitrogen (NOX). The sulfur content of fuels determines the
quantities of sulfur dioxide and sulfur trioxide produced during combus-
tion. Formation of sulfur trioxide is negligible, however, with small scale
combustion equipment where firebox temperatures do not exceed 420°C
(Danielson, 1973). Nitric oxide and nitrogen dioxide emissions also are
characteristic of combustion processes. These emissions appear to be
primarily a function of temperature and equipment design (Danielson, 1973),
but the presence of ammonia in fuels may increase emissions of nitrogen
oxides (Kuo and Jones, 1978).
Although manurial biogas contains hydrogen sulfide, it compares
favorably on the basis of sulfur content per unit energy with other fuels
particularly coal and fuel oil derived from high sulfur crudes (Table 41).
The difference in sulfur content between biogases produced from dairy and
poultry manure should be noted. On the basis of combustion products
produced, manurial biogas appears to compare favorably with conventional
alternatives when used as a boiler fuel with conventional alternatives
(Table 42).
Published information concerning emissions of air pollutants that occur
when biogas is used to fuel internal combustion engines appears to be
lacking. Reported values for methane indicate, however, that using biogas
66
-------
to fuel Internal combustion engines will increase emissions of oxides of
nitrogen, carbon monoxide, and hydrocarbons when using biogas as a boiler
fuel is the basis of comparison (Table 43). These higher emission levels
are characteristic of internal combustion engines. Emission of sulfur
oxides should be comparable.
TABLE 41. COMPARISON OF THE SULFUR CONTENT OF MANURIAL BI06ASES
AND CONVENTIONAL FUELS.
Fuel
Anthracite Coal
High Volatile B
Bitumenous Coal
Fuel Oil, Mid-continent
Fuel Oil, California
Diesel Fuel, No. 2-D
Pipeline Natural Gas
Biogas 60% CH^/40% C02
Dairy Manuret
Poultry Manure§
Sulfur, %, as S by wt.*
0.8
2.8
0.35
1.16
0.5 max.
trace
0.06-0.19
0.16-0.71
Sulfur, mg S/KJ
0.27
1.03
0.08
0.26
0.11
trace
0.04-0.10
0.09-0.39
* Schlesinger et al., 1978.
t Coverse et al., 1977.
§ Converse et al., 1981.
67
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TABLE 42. COMPARISON OF SOX AND NOX EMISSIONS WHEN BIOGAS, NATURAL GAS,
AND FUEL OIL ARE USED AS BOILER FUELS.
gm/GJ
Sulfur Oxides (SOX) Nitrogen Oxides fNUx)
Manurial Biogas
Beef Manure*
Poultry Manure*
Natural Gast
Fuel Oil, No. 6t§
310 (S02)
387 (S02)
Insignificant
720
462
805
27 - 163
236
* Kuo and Jones, 1978.
t Daniel son, 1973.
§ No. 6 Fuel Oil, 1.6% sulfur by weight.
TABLE 43. COMPARISON OF AIR POLLUTANTS EMITTED WHEN METHANE IS USED AS A
BOILER FUEL VERSUS AS A FUEL FOR INTERNAL COMBUSTION ENGINES
(Brooklyn Union Gas Company, 1978).
gm/m3 gm/GJ
Products of Boiler:InternalBoilerInternal
Combustion Fuel Combustion Engine Fuel Combustion Engine
Nitrogen oxides
(NOX) 8 72 227 2040
Carbon monoxide
(C02)
Hydrocarbons
3
2
8
8
91
45
230
230
Estimates of the quantities of sulfur dioxide and nitrogen oxides
generated in association with biogas production and purification are
summarized in Table 44. Data for coal gasification are provided as a basis
for comparison. These data indicate that air quality impacts of biogas
production, with the possible exception of sulfur dioxide emissions related
to biogas purification, should be of little concern. However, the
68
-------
monoethanolamine gas purification process for removal of carbon dioxide and
hydrogen sulfide is practical only for large scale systems. Thus, it
appears that iron sponges will be more extensively utilized for removal of
hydrogen sulfide. This is more desirable from the standpoint of air quality
since elemental sulfur is the end-product of iron sponge regeneration.
TABLE 44. COMPARISON OF S02 AND NOX EMISSIONS ASSOCIATED WITH MANURIAL
BIOGAS PRODUCTION AND PURIFICATION WITH SNG PRODUCTION FROM COAL.
Gm/GJ Net Energy
Sulfur DioxideNitrogen Uxides
Biogas Production*
Beef Manure, Fresh 46 69
Beef Manure, Feedlott 97 80
Poultry Manure 387 805
Biogas Production and Purification§
Beef Manure, Feedlot
SNG From Coal
2200#
43
78
Nil
* Kuo and Jones (1978).
t Manure accumulated over periods of 5-6 months.
§ Ballay et al. (1980) monoethanolamine process for purification.
# Would be nil if iron sponge is used for H2S removal.
SUMMARY
This section evaluated the feasibility of producing biogas from live-
stock and poultry manures using anaylses of technical, economic, and envi-
ronmental quality considerations as a basis. As discussed, the technical
feasibility of manurial biogas production has been demonstrated in a number
of studies and a rational basis for system design and operation has been
established.
The economic feasibility of manurial biogas production is less clear
due to uncertainties concerning biogas production costs and biogas utiliza-
tion. Of the available alternatives for biogas utilization, on-site genera-
tion of electricity appears to be the most practical and generally applica-
ble. On-site generation of electricity in parallel with an electric utility
permits utilization of all available biogas. However, revenues produced to
offset production costs and provide the attractive rate of return to
invested capital necessary to stimulate investments in this technology
depend on several variables. These include the on-site demand for and the
cost of electricity, the price to be paid by utilities for excess electrical
energy, and opportunities for engine-generator set waste heat utilization.
69
-------
An additional variable is the thermal efficiency of converting biogas to
electricity which appears to be equipment dependent. Due to the site
specific nature of these variables, the economic feasibility of this
technology also should be viewed as site specific.
None of the available experimental evidence supports the assumption
that anaerobic digestion "enhances" the value of animal manures as feed-
stuffs. Thus, the assumption of by-product credits made in several
evaluations (Table 31) do not appear justified. Although anaerobically
digested manures may have some value as feedstuffs, this value is, at best,
only comparable to these manures "as produced". Thus, any by-product value
realized would be offset by the opportunity cost associated with first using
the manure as a feedstock for biogas production instead of directly as a
feedstuff.
It does not appear that there are any readily identifiable negative
environmental quality imputs of importance related to producing or utilizing
manurial biogas. However, it also does not appear that there are any sub-
stantial environmental quality benefits associated with biogas production
and utilization. One exception is odor control which appears to be the most
attractive environmental quality aspect of manurial biogas production.
70
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