1. College of Agriculture Fisheries and Forestry.
Fiji National University
College of Agriculture Fisheries and Forestry.
IFS 401 Lecture 6, 2016
Biogas .

2. What is Biogas?
Most organic matter begins the process of decomposition when it is exposed to oxygen and sunlight.
However, organic matter can also decompose without any oxygen, by the process of anaerobic fermentation.
This happens due to the bacteria present in the matter which acts during the absence of oxygen.

3. Landfills see a lot of such decay, especially when the waste material becomes wet and receives little sunlight.
As a result, a lot of methane and nitrous oxide is produced and released into the atmosphere.
Biogas is the result of this decay, and it is an energy source like no other.

4. In other words
Biogas typically refers to a mixture of different gases produced by the breakdown of organic matter in the absence of oxygen.
Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste.
It is a renewable energy source and in many cases exerts a very small carbon footprint.

5. Biogas can be produced by anaerobic digestion with anaerobic bacteria, which digest material inside a closed system, or fermentation of biodegradable materials

6. Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of hydrogen sulfide (H2S), moisture and siloxanes.
The gases methane, hydrogen, and carbon monoxide (CO) can be combusted or oxidized with oxygen.
This energy release allows biogas to be used as a fuel; it can be used for any heating purpose, such as cooking.
It can also be used in a gas engine to convert the energy in the gas into electricity and heat.

7. Biogas is primarily methane (CH4) and carbon dioxide (CO2) and may have small amounts of hydrogen sulfide (H2S), moisture and siloxanes.

8. Production Equipment
The Main parts of a typical biogas plant consist of the following components:-
Inlet
Digester
Gas holder
Outlet
There are different types of biogas production plants . The main two types are as follows :-
Fixed-dome Plant
Floating-drum Plants

9. Fixed dome type
A fixed-dome plant consists of a digester with a fixed, non-movable gas holder, which sits on top of the digester
Advantage - The costs of a fixed-dome biogas plant are relatively low. It is simple as no moving parts exist. There are also no rusting steel parts and hence a long life of the plant (20 years or more) can be expected. 

10. Floating dome type
Floating-drum plants consist of an underground digester and a moving gas-holder.
The gas-holder floats either directly on the fermentation slurry or in a water jacket of its own.
The gas is collected in the gas drum, which rises or moves down, according to the amount of gas stored
Advantage- Floating-drum plants are easy to understand and operate. They provide gas at a constant pressure, and the stored gas-volume is immediately recognizable by the position of the drum.

11. Floating dome type

12. 3. Sausage Bag Gas Plant Material is polyethylene plastic
Gas to kitchen
Sludge used as a fertiliser
The digester is constructed of thick reinforced plastic.
The cost of this gas plant may be less of that of the Gobar and it is very quick to construct. Yet it may require importing. A fence may also be required around it to protect it from damage.
A hole in the ground accommodates the bag, which is filled two-thirds full with waste water. Gas production fully inflates the bag, which is weighted down and fitted with a compressor to increase gas pressure
Quick to construct
May require importing
May require protection from sharp objects & vermin

13. Biogas can be compressed, the same way natural gas is compressed to CNG, and used to power motor vehicles.
In the UK, for example, biogas is estimated to have the potential to replace around 17% of vehicle fuel.
It qualifies for renewable energy subsidies in some parts of the world.
Biogas can be cleaned and upgraded to natural gas standards, when it becomes bio methane

14. What is biogas plant?
A biogas plant is an anaerobic digester that produces biogas from animal wastes or energy crops.
Energy crops are cheap crops grown for the purpose of biofuels, rather than food.
Biofuels are liquid, gaseous, or solid fuel made from live or recently dead organic material known as biomass, as opposed to fossil fuels, which are composed of ancient biological materials.

15. Biogas is a type of biofuel created via anaerobic, or oxygen- free, digestion of organic matter by bacteria.
A biogas plant is composed of a digester and a gas holder.

16. What is a digester?
The digester is an airtight container in which the waste is dumped and decomposed, and the gas holder is a tank that harnesses the gases emitted by the slurry.
Bacteria within the digester tank breaks down the waste and, as it decomposes, gases such as carbon monoxide, methane, hydrogen, and nitrogen, are released.

17. Through a pressurized system, the gas holder conducts the flow of these gases upward into a hole in its drum.
The hole is specially designed to allow gases to pass freely into the holder while prohibiting any gases from escaping back into the digester.
In a controlled environment, the gases are later combusted, or reacted, with oxygen to create an energy source for such processes as heating and vehicle propulsion

18. Construction of a biogas plant may vary depending on the amount of gas needed, the amount of waste at hand, and whether the digester is designed for batch feeding or continuous feeding.
Batch feeding systems use mostly solid wastes that are added to the tank in installments, and continuous feeding models feed mostly liquids to the digester.

19. A biogas plant may be constructed either above or below ground, with advantages and disadvantages to both models.
An above ground biogas plant is easier to maintain and benefits from solar heating, but takes more care in construction because it must be built to handle the internal pressure of the digester.
A below ground biogas plant is cheaper to construct and easier to feed, but is more difficult to maintain.

20. To facilitate fast decomposition with optimal gas production, digesters are often kept between the temperatures of 29°C and 41°C (84.2°F-105.8°F).
In an attempt to neutralize the slurry, more acidic carbon dioxide, which is a desired gas, will be created.

21. The slurry within the tank must also be frequently stirred to prevent a hard crust from forming on top of the waste.
A crust can trap the gases within the slurry and impede the machinery’s ability to harness the gases.

22. Biogas is increasingly preferred to fossil fuels, or fuels made from ancient organic matter like coal or oil.
Carbon, in small amounts, is a vital component of a healthy atmosphere, but becomes problematic when too much is added into circulation.
The carbon contained in fossil fuels has been buried for such a long time that it is no longer part of the carbon cycle.

23. When it is released through burning of fossil fuels, it raises the carbon concentration.
Biogas, however, comes from live or recently dead organisms whose carbon content is still within the cycle, so burning these fuels does less to upset the carbon concentration in the atmosphere.

24. In addition to carbon output, biogas fuel is often preferred to fossil fuels because it is a low cost, renewable source of energy and it uses otherwise wasted materials.
Biogas is also a valuable energy source for developing nations, as it can be produced in small-scale sites.

25. Biogas fuel, however, also has its critics.
Some argue that energy crops detract from food agriculture and will create a global food shortage.
Biofuels may also cause deforestation, water pollution, soil erosion, and a negative economic impact on oil producing nations.

26. One may find the structure of biogas to be a little confusing
One may find the structure of biogas to be a little confusing. Comprising mainly of methane and carbon dioxide, one has to wonder how these greenhouse gases are helping anybody.
However, the magic happens when the gas is burnt.
The use of biogas as a fuel happens because it reacts with oxygen and releases energy, which is clean in nature.

27. The resulting reaction uses up the gases and prevents them from rising into the atmosphere.
Biogas plants are slowly becoming popular due to the many benefits associated with them.
They are already being used for public transport, industrial heating and many more applications.

28. Advantages of Biogas
1. Renewable Source of Energy: To begin with, biogas is considered to be a renewable source of energy. Since it often produced from materials that form sewage and waste products, the only time it will be depleted is when we stop producing any waste.
2. Non-Polluting: It is also considered to be non-polluting in nature. The production of biogas does not require oxygen, which means that resources are conserved by not using any further fuel.

29. 3. Reduces Landfills: It also uses up waste material found in landfills, dump sites and even farms across the country, allowing for decreased soil and water pollution.
4. Cheaper Technology: Applications for biogas are increasing as the technology to utilize the biogas it gets better. It can be used to produce electricity and for the purpose of heating as well. Compressed Natural Gas (CNG) is biogas that has been compressed and can be used as a fuel for vehicles. Production can be carried out through many small plants or one large plant.

30. 5. Large number of Jobs: Either way, work opportunities are created for thousands of people in these plants. These jobs are a blessing in rural areas, which are the targeted grounds for the use of biogas. In fact, biogas can easily be decentralized, making it easier to access by those living in remote areas or facing frequent power outages.
6. Little Capital Investment: Biogas are easy to set up and require little capital investment on a small scale basis. In fact, many farms can become self sufficient by utilizing biogas plants and the waste material produced by their livestock each day. A single cow can provide enough waste material within a day to power a light bulb the entire day.

31. 7. Reduces Greenhouse Effect:
 It also reduces the greenhouse effect by utilizing the gases being produced in landfills as forms of energy.
This is a major reason why the use of biogas has started catching on.
It recycles most forms of biodegradable waste and works on simple forms of technology.
8. Manure for Agricultural crops

32. Disadvantages of Biogas
1. Little Technology Advancements: First of all, the current systems in place used to create biogas are not as efficient as they get. Little new technology has been introduced for streamlining the process and making it more cost effective. As a result, large scale industrial production of biogas is still not on the energy map. Although it could solve the energy issues being faced by countries all over the world, very few investors are willing to put in the startup capital. It is also not the best idea to construct one biogas plant per house, which means that a central system will have to be put into place.

33. 2. Contain Impurities: Biogas contains a number of impurities even after refining processes have been put into place. When compressed for use as fuel, these can become corrosive to the metal parts of engines.eg Hydrogen sulphide when it combines with a water vapor it becomes corrosive in nature. 3. Not Attractive on Large Scale: The process of using biogas on a large scale is not economically viable and it is very difficult to enhance the efficiency of biogas systems.

34. 4. Unstable:
 It is also somewhat unstable, making it prone to explosions if the methane comes in contact with oxygen and become flammable in nature.
Even with all of the disadvantages present, countries have started to apply the uses of biogas in everyday life.
Public transportation has been renewed and made efficient with the help of CNG.
Remote locations that are off the electric grid receive a steady supply of power from these plants.
The future use of biogas is bright, even with the problems it faces.