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 Bacteria	SIMPLE ORGANIC ACIDS	Methane-Forming Bacteria	BIOGAS
Carbohydrates Proteins Fats		Acetic Propionic Butyric Formic		Methane Carbon Dioxide Water Vapor Ammonia Hydrogen Sulfide

ANAEROBIC DIGESTION OPTIMIZING FACTORS

TEMPERATURE

Mesophilic = 95^oF (35^oC) ą 5^oF (2.8^oC)
Thermophilic = 140^oF (60^oC)
Each 20^oF 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

	DAIRY Per Head	BEEF Per Head	SWINE Per Head	POULTRY LAYERS Per Bird
ANIMAL WEIGHT, lbs	1400	800	135	4
TOTAL FRESH MANURE & URINE, gal/day	12.5	6.1	1.35	0.032
SOLIDS CONTENT, %
Before Dilution	15	15	10	25
After Dilution	8	8	6.7	8
TOTAL WASTE VOLUME AFTER DILUTION, gal/day	23.5	11.5	2.0	0.10
VOLATILE SOLIDS PRODUCTION, lb VS/day	12.0	4.7	0.65	0.038
DIGESTER LOADING RATE, lb VS/ft³digester/day	0.25	0.25	0.125	0.125
DIGESTER VOLUME, ft³/head	47.0	19.2	5.3	0.3
DETENTION TIME, days	15	12.5	20	22.5
PROBABLE VOLATILE SOLIDS DESTRUCTION, %	35	45	50	60
BIOGAS YIELD PER LB VOLATILE SOLIDS DESTROYED, ft³/lb VS	11	13	12	13
BIOGAS YIELD PER FT³ OF DIGESTER VOLUME, ft³/ft³/day	1.0	1.4	0.75	1.0

DIGESTER TANK SIZE

EXAMPLE 1 : 100 - COW DAIRY

ˇ Fresh Manure @ 15% Solids	=	1250 gpd
ˇ Milk Center Washwater	=	500 gpd
ˇ Dilution Water Req'd for 8% Solids	=	600 gpd
ˇ Total Waste Volume Generated	=	2350 gpd
ˇ Digester Retention Time	=	15 days
ˇ Digester Tank Capacity = 15 x 2350	=	35,250 gallons
Round tank: 18 ft diameter x 18.5 ft tall

EXAMPLE 2 : 200 - SOW FARROW-TO-FINISH SWINE OPERATION

ˇ Fresh Manure @ 10% Solids	=	2830 gpd
ˇ Additional Water from Leaking Waterers, Foggers, etc.	=	1415 gpd
ˇ Total Waste Volume Generated	=	4245 gpd
ˇ Digester Detention Time	=	20 days
ˇ Digester Tank Capacity = 20 x 4245	=	84,900 gallons
Round tank: 24 ft diameter x 25 ft tall

EXAMPLE 3 : 50,000 - BIRD LAYER OPERATION

ˇ Fresh Manure @ 25% Solids	=	1620 gpd
ˇ Dilution Water Req'd for 8% Solids	=	3440 gpd
ˇ Total Waste Volume Generated	=	5060 gpd
ˇ Digester Detention Time	=	22.5 days
ˇ Digester Tank Capacity = 22.5 x 5060	=	113,850 gallons
Round tank: 27 ft diameter x 26.5 ft tall

GAS PROPERTIES

	METHANE	BIOGAS
COMPOSITION, by volume	Methane - 100%	Methane - 60%
		Carbon Dioxide - 35%
		Hydrogen Sulfide - 1.67%
		Ammonia - 1.67%
		Water Vapor - 1.67%
MOLECULAR WEIGHT, lbm/lb-mole	16.04	26.18
SPECIFIC GRAVITY, air = 1	0.554	0.904
DENSITY @ STP, lbm/ft³	0.0447	0.073
HEAT VALUE, BTU/lbm	23,850.	8,937.
ENERGY CONTENT @ STP, BTU/ft³	1,066.	652.
GAS CONSTANT, ft-lbf/lbm-^oR	96.3	59.0
CRITICAL TEMPERATURE, ^oR	343.6	-
CRITICAL PRESSURE, lbf/in²	673.	-
CRITICAL SPECIFIC VOLUME, ft³/lbm	0.099	-
OCTANE RATING	130.	-
IGNITION TEMPERATURE, ^oF	1250.	-
AIR-TO-METHANE RATIO FOR COMPLETE COMBUSTION, by volume	10:1	-
EXPLOSION LIMITS IN AIR, by volume	5% - 15%	-

ENERGY CONTENT OF VARIOUS FUELS

COAL	25,000,000	BTU/ton
PEAT (air-dried to 30% moisture)	12,400,000	BTU/ton
WOOD (air-dried to 20% moisture)	6,400	BTU/lb
ˇ White Oak	24,400,000	BTU/cord
ˇ American Elm	18,700,000	BTU/cord
ˇ White Pine	13,900,000	BTU/cord
CRUDE OIL	6,000,000	BTU/barrel
NO. 2 FUEL OIL	138,000	BTU/gallon
DIESEL FUEL	138,000	BTU/gallon
GASOLINE (regular)	120,000	BTU/gallon
GASOHOL	116,500	BTU/gallon
BUTANE (L.P.)	104,000	BTU/gallon
PROPANE (L.P.)	92,000	BTU/gallon
ETHYL ALCOHOL (190 proof)	80,000	BTU/gallon
ELECTRICITY	3,414	BTU/kwhr
NATURAL GAS	1,000	BTU/ft³
BIOGAS (60% methane)	600	BTU/ft³

STORAGE PRESSURE - DENSITY - ENERGY CONTENT

TEMPERATURE = 60^oF

COMPRESSIBILITY		DENSITY		ENERGY CONTENT
PRESSURE	FACTOR	METHANE	BIOGAS	METHANE	BIOGAS
psi		--------------lb/ft³-----------		------------BTU/ft³----------
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	0.88	13.08	21.34	311,918	190,744
4500	0.92	14.07	22.97	335,651	205,257
5000	0.96	14.99	24.46	357,407	218,561

BIOGAS CLEANING

CO₂	REDUCES ENERGY CONTENT	Removed by water scrubbing, caustic scrubbing, solid or liquid absorption, and pressure separation
H₂S	HIGHLY CORROSIVE	Removed by passing through iron sponge (wood shavings mixed with iron oxide)
H₂O	CONDENSATION	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.
- Buildings should be well ventilated.
- Explosion-proof motors, wiring and lights should be used.
- Flame arrestors should be placed on gas lines leading to burners or engine carburetors.
- Perform periodic system checks for gas leaks.
- 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	Biogas	Energy	Content
	Weight	Production*	Gross	Net**
	lbs	ft³/head/day	---------BTU/head/day---------
Dairy Cow	1400	46.4	27,800	18,000
Beef Feeder	800	27.6	16,600	10,700
Market Hog	135	3.9	2,300	1,500
Poultry Layer	4	0.29	180	110
* 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 ft³of 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

	mcf	value*
	---------------per head per year-----------------
Dairy Cow	6.6	$23.75
Beef Feeder	3.9	14.15
Market Hog	0.55	2.00
Poultry Layer	0.04	0.15
* $3.60 per mcf

BIOGAS NET RETURNS

PROPANE (L.P. GAS) EQUIVALENT

	gallons	value*
	--------------per head per year-----------
Dairy Cow	72	$41.60
Beef Feeder	43	24.80
Market Hog	6	3.49
PoultryLayer	0.45	0.26
* 58˘ per gallon

NO. 2 FUEL OIL EQUIVALENT

	gallons	value*
	------------per head per year--------------
Dairy Cow	48	$42.10
Beef Feeder	28	25.05
Market Hog	4	3.53
Poultry Layer	0.30	0.26
* 88˘ per gallon

RETAIL COST COMPARISONS OF VARIOUS FUELS

FEBRUARY 1, 1980

FUEL	RALEIGH AREA PRICE	DOLLARS/MILLION
	QUOTES OR ESTIMATIONS	BTU
Natural Gas	$3.60 / mcf	$3.60
Crude Oil	25,00 / barrel	4.17
Coal	110.00 / ton	4.40
Wood (Air-Dried to 20% Moisture)	80.00 / cord
White Oak		3.28
American Elm		4.28
White Pine		5.75
Butane	0.60 / gallon	5.77
Propane (L.P.)	0.58 / gallon	6.30
No. 2 Fuel Oil	0.88 / gallon	6.38
Diesel Fuel	1.05 / gallon	7.61
Gasoline (regular)	1.08 / gallon	9.00
Gasohol	1.15 / gallon	9.87
Electricity	0.04 / kwhr	11.72
Biogas (60% methane)		$7 - $32
Ethyl Alcohol (190 proof)	1.50 / gallon	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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Methane Fuel Gas from Livestock Wastes A Summary

METHANE DIGESTION - UTILIZATION SYSTEM COMPONENTS

ANAEROBIC DIGESTION PROCESS

Methane Fuel Gas from Livestock Wastes
A Summary