Biogas through Anaerobic Digestion Systems

 

What is Biogas?

 

Biogas is a mixture of Methane (CH4) and Carbon Dioxide (CO2) that is produced through a natural occurring decomposing and fermentation process of any organic matter, by bacteria that functions in the absence of oxygen.

 

Benefits of having a biogas plant?

The most common problem on any intensive farming operation is the accumulation of organic waste matter. The most problematic is the manure pond that causes bad smells and attracts flies. By installing a Biogas plant you can address your manure-handling problems, reduce flies, and reap the benefits of self-generated electricity and heat at a fixed price for the life of the plant.

 

How does the plant work?

 

In order to create a suitable living environment for the bacteria that is producing the gasses by their natural living processes, we have to build an airtight tank that will hold the water and organic matter mixture (the bacteria’s food). The tank must constantly be mixed to ensure that the organic particles are made available for the bacteria to eat and the tank should be heated to allow optimal living conditions for the bacteria. This tank will be called the Digester Tank.

The bacterial digestion process that takes place on the inside of the Digester tank produces the Methane and Carbon Dioxide gasses that are released as evaporating gas from the liquid. This gas is collected from the top of the tank and treated in order to allow it to be used as a fuel source in various ways. First the Sulphurous (H2S) component in the gas is removed and then any excess moisture is taken out of the gas stream. These components must be removed to protect the generator and other gas burning equipment from corrosion.

 

What can you put into a Digester to make Biogas?

 

Organic matter that can be fed into the tank as feedstock can be chicken litter, pig manure, cow manure, all components of slaughterhouse waste from all animals, any plants (except too much wood), food waste, fat, oil and human sewage. Each type of feedstock need to be processed in a suitable way to allow maximum efficiency of the bacterial digestion process inside the tank.

 

What comes out of the Digester?

 

The discharge from the digester tank is in the form of a liquid mixture of dissolved and spent food particles and fibres. The liquid almost 95% moisture can be sprayed onto farmland through an irrigation system as a faint fertiliser. The same quantity of material that is fed into the digester should also be taken out of the tank at the same frequency as the loading rate. The discharge from the digester can immediately be pumped to the irrigation system or it can be stored in the pond for later distribution.

 

What can you do with Biogas?

 

Once the gas is cleaned it can be used as fuel for a standard internal combustion engine driven generator to produce electricity. Biogas can also be used for gas cooking stoves, gas heater equipment, hot water boilers and geysers. When a generator is used, the excess heat from the generator exhaust gasses can be harvested by a heat exchanger to produce steam or hot water for general heating purposes.

By operating a biogas plant you will be able to supply your own energy source for electricity and heat at a fixed cost over the life of the plant. Also in terms of environmental awareness you can run closer to a carbon-neutral farm or factory by destroying waste in a responsible manner and harvesting the energy that would generally have been lost into the environment as harmful greenhouse gasses. It can give you the necessary environmental credits required by some exclusive retailers in order to improve the saleability of your product.

 

How Big is the Layout of the Plant?

 

The plant will occupy a minimum of 15mX20m land area with additional space required for each additional storage tank (8m diameter each) based on the required size of your operation.

 

What Daily Operation is Required?

 

Depending on the routine of your farm you can feed the digester continuously, once a day, or once a week. With operations such as dairy farming where animals aren’t necessarily kept in a contained area with automated manure flushing systems it is necessary to manually collect as much manure as is possible. Any digester’s feed can also be supplemented by additional organic material available on the farm such as grass, wasted food crops and waste from dairy processing. In systems with automated manure flashing the sewage stream will accumulate in a sump where water and solids are separated in a continuous manner.  The high solids sludge will be periodically pumped into the digester via a manual or semi-automatic control system.  Once a day the equivalent amount of material that was pumped into the digester will have to be extracted from the tank into the discharge pond from where it can be distributed to irrigation systems. For the extraction task the operator only needs to open the waste valves and run the pump until the required operating level on the thanks level indicator has been reached. The digester generates gas at a constant rate equivalent to the amount of average daily feed. The gas flows from the digester into an accumulation-holding bag from where a small pressurisation blower will distribute it to equipment that consumes the gas.

 

Maintenance required on the plant.

 

The main maintenance item on the plant will be the generator engine.  The engine installed inside the generator is a typical sparc ignition engine comparable to a petrol engine. It will require the similar type of services as any other engine at regular intervals.

On the gas handling system all the condensate traps should be checked and drained on a weekly basis.

After a period of time when one can notice the sulphurous smell in the gas it will indicate that the H2S filter medium (consisting of iron-oxide pellets) has to be replaced

 

 

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