Methane Fuel Gas from Livestock Wastes
A Summary
Prepared by:
James C. Barker, Professor and Extension Specialist
Biological and Agricultural Engineering
North Carolina State University, Raleigh, NC
Published by: North Carolina Cooperative Extension Service
Publication Number: EBAE 071-80
Last Electronic Revision: March 14, 2001
In 1979 the total energy (liquid, gas, electricity) used in North Carolina in all of production
agriculture (crops and livestock) amounted to approximately 56 trillion BTUs. (One gallon of
gasoline equals 120,000 BTUs.) If North Carolina can be viewed as representative of other states,
then production agriculture used 3% of the nation's energy. If all livestock and poultry wastes
currently recoverable in North Carolina were utilized to produce optimum rates of methane, then
approximately 6% of the total energy used in production agriculture could be replaced by methane.
Extrapolated to a national level, optimum methane generation from all recoverable livestock and
poultry wastes could replace 0.2% of the nation's energy consumption. In North Carolina this
would amount to approximately 3 trillion BTUs. This optimized scenario represents a significant
amount of energy, but it is not, however, the panacea that would allow production agriculture or
even animal agriculture to become energy independent.
METHANE DIGESTION - UTILIZATION SYSTEM COMPONENTS
Animal Production Facilities
Manure / Effluent Handling System
Digestor Tank
Heating & Mixing System
Process Monitoring
Biogas Cleaning & Handling System
Biogas Storage
Digestor By-Product Handling / Storage System
Biogas Utilization System
Equipment Building
The AMOUNT OF GAS and the RELIABILITY desired have a great INFLUENCE on SYSTEM
COST & COMPLEXITY.
ANAEROBIC DIGESTION PROCESS
Yields methane from controlled breakdown of organic wastes
by bacteria in the absence of oxygen.
ORGANIC
MATTER
Acid-Forming
Carbohydrates
Proteins
Fats
Bacteria
SIMPLE
ORGANIC
ACIDS
Methane-Forming
BIOGAS
Bacteria
Acetic
Propionic
Butyric
Formic
Methane
Carbon
Dioxide
Water
Vapor
Ammonia
Hydrogen
Sulfide
ANAEROBIC DIGESTION OPTIMIZING FACTORS
TEMPERATURE
Mesophilic = 95oF (35oC) ± 5oF (2.8oC)
Thermophilic = 140oF (60oC)
Each 20oF decrease cuts gas production in half.
LOADING RATE
Semi-Continuous Batch Loaded
Digestor Retention Time = 15 - 30 days
Manure Solids Content = 6 - 10 %
Digester Design Range = 0.1 - 0.3 lbs volatile solids / ft3 / day
PROCESS STABILITY
Recommended pH Range = 7.0 - 7.5
Anaerobic digestion process tends to become acidic (lowered pH).
Digester failure may occur if pH gets below 6.5.
Hydrated lime may be added to stabilize pH.
DIGESTER MIXING
Prevent settling and maintain contact between bacteria and manure
Maintain uniform temperature
Prevent surface scum formation
Facilitate release of the biogas
NUTRIENTS
Good digestion requires carbon : nitrogen ratio between 15:1 to 30:1.
Most fresh animal manures fall within this range and need not adjustment.
Excessive bedding or exposed feedlot manure may present a nutrient imbalance.
TOXIC SUBSTANCES
Oxygen
Ammonia
Abnormal dosages of antibiotics, disinfectants, or cleansing agents
DIGESTER START-UP
Fill tank with water and warm to desired operating temperature.
Add seed sludge from municipal sewage treatment plant to 15% of digester volume to reduce
start-up time.
Add fresh manure beginning gradually and increasing over a 3-week period to desired loading
rate.
Good gas production should occur in about 4 weeks of start-up.
Unassisted, bacteria may require 2 - 3 months to multiply to an efficient population.
DIGESTER DESIGN CRITERIA
ANIMAL WEIGHT, lbs
TOTAL FRESH MANURE &
URINE, gal/day
SOLIDS CONTENT, %
Before Dilution
After Dilution
TOTAL WASTE VOLUME AFTER
DILUTION, gal/day
VOLATILE SOLIDS
PRODUCTION, lb VS/day
DAIRY
Per
Head
1400
BEEF
Per
Head
800
POULTRY
SWINE LAYERS
Per Head Per Bird
135
4
12.5
6.1
1.35
0.032
15
8
15
8
10
6.7
25
8
23.5
11.5
2.0
0.10
12.0
4.7
0.65
0.038
DIGESTER LOADING RATE, lb
VS/ft3 digester/day
DIGESTER VOLUME, ft3/head
DETENTION TIME, days
PROBABLE VOLATILE SOLIDS
DESTRUCTION, %
BIOGAS YIELD PER LB
VOLATILE SOLIDS DESTROYED,
ft3/lb VS
BIOGAS YIELD PER FT3 OF
DIGESTER VOLUME, ft3/ft3/day
0.25
47.0
15
0.25
19.2
12.5
0.125
5.3
20
0.125
0.3
22.5
35
45
50
60
11
13
12
13
1.0
1.4
0.75
1.0
DIGESTER TANK SIZE
EXAMPLE 1 : 100 - COW DAIRY
· Fresh Manure @ 15% Solids
=
· Milk Center Washwater
=
· Dilution Water Req'd for 8% Solids
=
· Total Waste Volume Generated
=
· Digester Retention Time
=
· Digester Tank Capacity = 15 x 2350
=
Round tank: 18 ft diameter x 18.5 ft tall
1250 gpd
500 gpd
600 gpd
2350 gpd
15 days
35,250 gallons
EXAMPLE 2 : 200 - SOW FARROW-TO-FINISH SWINE OPERATION
· Fresh Manure @ 10% Solids
· Additional Water from Leaking Waterers, Foggers,
etc.
· Total Waste Volume Generated
· Digester Detention Time
· Digester Tank Capacity = 20 x 4245
Round tank: 24 ft diameter x 25 ft tall
=
2830 gpd
=
1415 gpd
=
=
=
4245 gpd
20 days
84,900 gallons
EXAMPLE 3 : 50,000 - BIRD LAYER OPERATION
· Fresh Manure @ 25% Solids
· Dilution Water Req'd for 8% Solids
· Total Waste Volume Generated
· Digester Detention Time
· Digester Tank Capacity = 22.5 x 5060
Round tank: 27 ft diameter x 26.5 ft tall
=
=
=
=
=
1620 gpd
3440 gpd
5060 gpd
22.5 days
113,850 gallons
GAS PROPERTIES
COMPOSITION, by volume
MOLECULAR WEIGHT, lbm/lb-mole
SPECIFIC GRAVITY, air = 1
DENSITY @ STP, lbm/ft3
HEAT VALUE, BTU/lbm
ENERGY CONTENT @ STP, BTU/ft3
GAS CONSTANT, ft-lbf/lbm-oR
CRITICAL TEMPERATURE, oR
CRITICAL PRESSURE, lbf/in2
CRITICAL SPECIFIC VOLUME,
ft3/lbm
OCTANE RATING
IGNITION TEMPERATURE, oF
AIR-TO-METHANE RATIO FOR
COMPLETE COMBUSTION, by
volume
EXPLOSION LIMITS IN AIR, by
volume
METHANE
Methane 100%
BIOGAS
Methane - 60%
16.04
0.554
0.0447
23,850.
1,066.
96.3
343.6
673.
Carbon Dioxide - 35%
Hydrogen Sulfide 1.67%
Ammonia - 1.67%
Water Vapor - 1.67%
26.18
0.904
0.073
8,937.
652.
59.0
-
0.099
-
130.
1250.
-
10:1
-
5% - 15%
-
ENERGY CONTENT OF VARIOUS FUELS
COAL
PEAT (air-dried to 30%
moisture)
WOOD (air-dried to 20%
moisture)
· White Oak
· American Elm
· White Pine
CRUDE OIL
NO. 2 FUEL OIL
DIESEL FUEL
GASOLINE (regular)
GASOHOL
BUTANE (L.P.)
PROPANE (L.P.)
ETHYL ALCOHOL (190
25,000,000
BTU/ton
12,400,000
BTU/ton
6,400
24,400,000
18,700,000
13,900,000
6,000,000
138,000
138,000
120,000
116,500
104,000
92,000
80,000
BTU/lb
BTU/cord
BTU/cord
BTU/cord
BTU/barrel
BTU/gallon
BTU/gallon
BTU/gallon
BTU/gallon
BTU/gallon
BTU/gallon
BTU/gallon
proof)
ELECTRICITY
NATURAL GAS
BIOGAS (60% methane)
3,414
1,000
600
BTU/kwhr
BTU/ft3
BTU/ft3
STORAGE PRESSURE - DENSITY - ENERGY CONTENT
TEMPERATURE = 60oF
COMPRESSIBILITY
DENSITY
ENERGY CONTENT
PRESSURE FACTOR METHANE BIOGAS METHANE BIOGAS
------------BTU/ft3------3
psi
--------------lb/ft ------------14.7 (atm)
1.0
0.0423
0.0690
1,008
616
15
1.0
0.0432
0.0704
1,029
629
20
1.0
0.0575
0.0939
1,372
839
25
1.0
0.0719
0.117
1,715
1,049
30
1.0
0.0863
0.141
2,058
1,258
35
1.0
0.101
0.164
2,401
1,468
40
1.0
0.115
0.188
2,744
1,678
45
1.0
0.129
0.211
3,088
1,888
50
1.0
0.144
0.235
3,431
2,098
60
1.0
0.173
0.282
4,117
2,517
70
1.0
0.201
0.329
4,803
2,937
80
1.0
0.230
0.376
5,489
3,357
90
1.0
0.259
0.423
6,176
3,776
100
1.0
0.288
0.470
6,862
4,196
125
1.0
0.360
0.587
8,577
5,245
150
1.0
0.432
0.704
10,293
6,294
175
1.0
0.504
0.822
12,008
7,343
200
0.99
0.581
0.949
13,863
8,477
250
0.98
0.732
1.19
17,452
10,672
300
0.98
0.885
1.44
21,114
12,911
350
0.97
1.04
1.69
24,760
15,141
400
0.97
1.19
1.94
28,395
17,364
450
0.96
1.36
2.21
32,335
19,773
500
0.94
1.52
2.48
36,269
22,179
600
0.93
1.85
3.02
44,114
26,976
700
0.92
2.19
3.57
52,212
31,978
800
0.91
2.53
4.13
60,327
36,891
900
0.89
2.91
4.75
69,393
42,435
1000
0.88
3.27
5.34
77,979
47,686
1500
0.82
5.26
8.59
125,528
76,762
2000
0.78
7.38
12.04
175,954 107,599
2500
0.79
9.11
14.86
217,158 132,796
3000
0.81
10.66
17.39
254,156 155,421
3500
0.84
11.99
19.56
285,925 174,848
4000
4500
5000
0.88
0.92
0.96
13.08
14.07
14.99
21.34
22.97
24.46
311,918
335,651
357,407
190,744
205,257
218,561
BIOGAS CLEANING
CO2
H2S
REDUCES ENERGY
CONTENT
HIGHLY CORROSIVE
H2O
CONDENSATION
Removed by water scrubbing, caustic scrubbing, solid
or liquid absorption, and pressure separation
Removed by passing through iron sponge (wood
shavings mixed with iron oxide)
Removed by passing through frost-proof condensers
(water traps)
BIOGAS STORAGE
Must be stored as a gas; unlike L.P. gas which liquifies at 160 psi, methane requires 5000 psi
for liquefication.
Container size renders low pressure storage of gas impractical for prolonged periods.
Example: One day's net accumulation of uncompressed gas requires storage volume about
equal to one-half of digestor tank volume.
Propane tanks can be used up to 200 psi pressure; higher pressures on-farm are complex,
expensive, and dangerous.
Low-pressure containers include butyl rubber, hypalon, or polystyrene bags; or a cover
floating over a liquid reservoir.
SAFETY PRECAUTIONS
Methane is explosive when mixed with air 5 to 15% by volume.
Methane is odorless, colorless, lighter than air, and difficult to detect.
o Buildings should be well ventilated.
o Explosion-proof motors, wiring and lights should be used.
o Flame arrestors should be placed on gas lines leading to burners or engine carburetors.
o Perform periodic system checks for gas leaks.
o Utilize gas detection and alarm devices.
BIOGAS PRODUCTION RATES
INFLUENCED BY :
Type of livestock or poultry
Biodegradable organic matter content of manure
Length and type of manure collection and storage
Flush systems
ENERGY POTENTIAL
Animal
Weight
lbs
Biogas
Production*
Energy
Content
Gross
Net**
---------BTU/head/day----3
ft /head/day
---46.4
27,800
18,000
27.6
16,600
10,700
3.9
2,300
1,500
0.29
180
110
Dairy Cow
1400
Beef Feeder
800
Market Hog
135
Poultry Layer
4
* 60% methane
** Assumes 35% of gross energy is used to operate the digester.
BIOGAS UTILIZATION ON LIVESTOCK FARMS
Use directly for cooking, lighting, space heating, water heating, grain drying, gas-fired
refrigeration or air conditioning.
Transform into electricity through internal combustion engine-driven generator.
EXAMPLE 1 :
Assume a well-insulated three-bedroom home takes about 900,000 BTU/day for heating
during cold weather.
Assume that 35% of the biogas produced is needed to maintain the digester temperature.
It would take the total manure from 50 dairy cows, 600 hogs, or 7870 layers to produce
enough biogas each day for home heating.
EXAMPLE 2 :
The national average electrical usage for dairies is 550 kwhrs/cow/year.
Assume that 35% of the biogas produced is needed to maintain the digester.
Assume that the combined efficiency of an internal combustion engine-driven generator is
20%.
A dairy could generate 70% of its electrical needs with biogas.
EXAMPLE 3 :
The Rocky Mount Swine Development Center uses about 55 kwhrs of electricity and 5.75
gallons LP Gas per hog per year including feed mill and incinerator.
Assume that 35% of the biogas produced is needed to maintain the digester.
Assume that the generating efficiency is 20%.
This farrow-to-finish operation could supply 40% of its energy needs with biogas.
EXAMPLE 4 :
An average tractor fuel tank occupies about 8 ft3 (60 gallons).
8 ft3 of biogas compressed to 3,000 psi would run a 100-hp tractor about 1 hour.
8 ft3 of biogas compressed to a more realistic working pressure of 300 psi would run this
tractor about 5 minutes.
BIOGAS NET RETURNS
ELECTRICITY EQUIVALENT
kwhrs*
value**
--------------per head per year-------------Dairy Cow
385
$15.45
Beef Feeder
230
9.20
Market Hog
32
1.30
Poultry Layer
2.5
0.10
* 20% combined generating efficiency
** 4¢ per kwhr
NATURAL GAS EQUIVALENT
Dairy Cow
Beef Feeder
Market Hog
Poultry Layer
* $3.60 per mcf
mcf
value*
---------------per head per year---------------6.6
$23.75
3.9
14.15
0.55
2.00
0.04
0.15
BIOGAS NET RETURNS
PROPANE (L.P. GAS) EQUIVALENT
Dairy Cow
Beef Feeder
Market Hog
PoultryLayer
* 58¢ per gallon
gallons
value*
--------------per head per year----------72
$41.60
43
24.80
6
3.49
0.45
0.26
NO. 2 FUEL OIL EQUIVALENT
Dairy Cow
gallons
value*
------------per head per year-------------48
$42.10
Beef Feeder
Market Hog
Poultry Layer
* 88¢ per gallon
28
4
0.30
25.05
3.53
0.26
RETAIL COST COMPARISONS OF VARIOUS FUELS
FEBRUARY 1, 1980
FUEL
RALEIGH AREA PRICE
QUOTES OR ESTIMATIONS
Natural Gas
$3.60 / mcf
Crude Oil
25,00 / barrel
Coal
110.00 / ton
Wood (Air-Dried to 20% Moisture)
80.00 / cord
White Oak
American Elm
White Pine
Butane
0.60 / gallon
Propane (L.P.)
0.58 / gallon
No. 2 Fuel Oil
0.88 / gallon
Diesel Fuel
1.05 / gallon
Gasoline (regular)
1.08 / gallon
Gasohol
1.15 / gallon
Electricity
0.04 / kwhr
Biogas (60% methane)
Ethyl Alcohol (190 proof)
1.50 / gallon
DOLLARS/MILLION
BTU
$3.60
4.17
4.40
3.28
4.28
5.75
5.77
6.30
6.38
7.61
9.00
9.87
11.72
$7 - $32
18.75
Distributed in furtherance ot the Acts of Congress of May 8 and June 30, 1914. Employment and
program opportunities are offered to all people regardless of race, color, national origin, sex, age, or
disability. North Carolina State University, North Carolina A&T State University, U.S. Department of
Agriculture, and local governments cooperating.
EBAE 071-80
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