CO2 from farm animals

 

Anaerobic Digestion:
Biogas and CO2 Production from Manure

 

Cow emissions are more damaging the planet than CO2 from cars 

The world’s 1.5 billion cattle are most to blame. Livestock are responsible for 18 per cent of the greenhouse gases that cause global warming, more than cars, planes and all other forms of transport put together. It generates 65 per cent of human-related nitrous oxide, which has 296 times the Global Warming Potential (GWP) of CO2. Most of this comes from manure.

A cow weighing 1,329 lbs. produces 115 lbs. of manure a day

 

 

Algae can be successful grown at a large dairy, poultry, pig, sheep and goat farms. The animal’s manure can be used to feed the algae and produce production power for the farm and the grid…Algae installations can also be adapted to the CO2 rich exhaust of new and existing ethanol refineries. 

 

 

Below, a scheme of an agricultural biogas plant including slurry, energy crops and 
organic residues as feedstock and including different pathways of biogas utilization

 

 

 

With the use of liquid-based systems, the primary method for reducing emissions is to recover the methane before it is emitted into the air. Methane recovery involves capturing and collecting the methane produced in the manure management system. This recovered methane (a medium Btu gas with about 500-600 Btu/ft3) can be flared or used to produce heat or electricity. Three methane recovery technologies are available:

 

  Covered anaerobic digesters are the simplest form of recovery system, and can be used at dairy or swine farms in temperate or warm climates. In this system, manure is mixed with water and pumped into outdoor lagoons. The covered lagoons are air-tight and provide the anaerobic conditions under which methane is produced and recovered.
 
  Complete mix digesters present a methane recovery option for all climates. They are heated, constant-volume, mechanically-mixed tanks that decompose medium solids swine or dairy manure (3-8% total solids) to produce biogas and a biologically stabilized effluent. The manure is collected daily in a mixing pit where the percent total solids can be adjusted and the manure can be pre-heated. A gas-tight cover placed over the digester vessel maintains anaerobic conditions and traps the methane that is produced. The produced methane, representing about 8 to 11 percent of the total manure, is removed from the digester, processed, and transported to the end use site.
 
  Plug flow digesters only work with dairy scraped manure and cannot be used with other manures. These are constant volume, flow-through units that decompose high solids dairy manure (>11% solids) to produce biogas and a biologically stabilized effluent. The basic plug flow digester design is a long tank, often built below ground level, with a gas-tight, expandable cover. A gas-tight cover collects the biogas and maintains anaerobic conditions inside the tank. The amount of methane produced is about 40 cubic feet per cow per day.


As noted, the amount of methane produced from aerobic decomposition (dry management) is small in comparison to the emissions from liquid management. Currently, no feasible options exist for reducing methane emissions from dry manure management.

The recovery of methane from manure management systems can significantly reduce the overall emission of greenhouse gases.  Utilities can work with large livestock producers to reduce overall emissions of methane from animal waste lagoons by encouraging producers to cover their lagoons and collect the methane for electricity generation or on-farm fuel.

 

 

 

To get an idea of the size of an anaerobic digester, consider one designed for 200 milking cows with a 20 day retention time:

Assuming each high-producing milking cow produces 2.2 ft3  manure per day, the daily volume of manure from these milking cows would be: 200 cows x 2.2 ft3 manure/day/cow = 440 ft3 manure/day

If dilution water is needed for manure flow ability or added from the milking center at a rate of 3 gallons per cow per day, the additional volume added daily would be: 200 cows x 3 gallons water/cow/day ÷ 7.5 gallons water/ft3 water = 80 ft3 water/day

The total material added daily to the digester, therefore, would equal: 440 ft3 manure/day + 80 ft3 water/day = 520 ft3 material/day

To hold 20 days worth of manure and water, the digester volume would need to be: 520 ft3/day x 20 days = 10, 400 ft3 A digester with a rigid cover, a 3 ft head space for gas collection, and a material volume (no bedding included) of 10,400 ft3, would be approximately 15 ft deep and 33 ft in diameter.

 

Different types of manures

Comparisons of different types of manures
Manure % Moister % Nitrogen % Phosphorus % Potassium
Human 66-80 5-7 3-5.4 1.0-2.5
Cattle 80 1.67 1.11 0.56
Horse 75 2.29 1.25 1.38
Sheep 68 3.75 1.87 1.25
Pig 82 3.75 1.87 1.25
Hen 56 6.27 5.92 3.27
Pigeon 52 5.68 5.74 3.23
Sewage --- 5-10 2.5-4.5 3.0-4.5

 

The Gas

Composition

General Composition of Bio-Gas Produced From Farm Wastes
CH4 methane 54 - 70%
CO2 carbon dioxide 27 - 45%
N2 nitrogen 0.5 - 3%
H2 hydrogen 1 - 10%
CO carbon monoxide 0.1%
O2 oxygen 0.1%
H2S hydrogen sulfide trace
 		 The gas produced by digestion, known as marsh gas, sewage gas, dungas, or bio-gas, is about 70% methane (CH4) and 29% carbon dioxide (CO2) with insignificant traces of oxygen and sulfurated hydrogen (H2S) which gives the gas a distinct odor. (Although it smells like rotten eggs, this odor has the advantage of being able to trace leaks easily.)

The basic gas producing reaction in the digester is: carbon plus water = methane plus carbon dioxide (2C + 2H2O = CH4 + CO2). The methane has a specific gravity of 0.55 in relation to air. In other words, it is about half the weight of air and so rises when released to the atmosphere. Carbon dioxide is more than twice the weight of air, so the resultant combination of gases, or simply bio-gas, when released to atmosphere, will rise slowly and dissipate.
As a general rule, pure methane gas has a heat value of about 1,000 British Thermal Units (BTU) per cubic foot (ft3). One BTU is the amount of heat required to raise one pound (one pint) of water by 1°F (0.56°C). Five ft3, or 5000 BTU of gas, is enough to bring 1/2-gallon of water to the boil and keep it there for 20 minutes. If you have a volume of bio-gas which is 60% methane, it will have a fuel value of about 600 BTU/ft3, etc.

 

Fuel Value
x		 Amount of Gas From Different Wastes
x
Fuel Value of Bio-Gas and Other Major Fuel Gases
Fuel gas Fuel value (BTU/ft3)
Coal (town) gas 450-500
Bio-gas
540-700
Methane 896-1069
Natural gas (methane or propane-based) 1050-2200
Propane 2200-2600
Butane 2900-3400
Cubic Feet of Gas Produced by Volatile Solids of Combined Wastes
Material Proportion Ft3 Gas Per lb VS Added CH4 Content of Gas (%)
Chicken Manure 100% 5.0 59.8
Chicken Manure & Paper Pulp 31% 69% 7.8 60.0
Chicken Manure & Newspaper 50% 50% 4.1 66.1
Chicken Manure & Grass Clippings 50% 50% 5.9 68.1
Steer Manure 100% 1.4 65.2
Steer Manure & Grass Clippings 50% 50% 4.3 51.1
The fuel value of bio-gas is directly proportional to the amount of methane it contains (the more methane, the more combustible the bio-gas).
This is because the gases, other than methane, are either non-combustible, or occur in quantities so small that they are insignificant. Since tables of "Fuel Values of Bio-Gas" may not show how much combustible methane is in the gas, different tables show a wide variety of fuel values for the same kind of gas, depending on the amount of methane in the gas of each individual table.
The actual amount of gas produced from different raw materials is extremely variable depending upon the properties of the raw material, the temperature, the amount of material added regularly, etc. Again, for general rule-of-thumb purposes, the following combinations of wastes from a laboratory experiment can be considered as minimum values