Biogas is a form of energy produced by anaerobic digestion ( the decomposition of constituents of biodegradable matter in an oxygen-free environment). It is a mixture of gases mainly carbon dioxide and methane. Approximate biogas compositon:
– Methane (CH4) 55 to 65 %
– Carbon dioxide (CO2) 30 to 35 %
– Water vapour 1 to 5 %
– Hydrogen sulphide (H2S) 0 to 3 %
– Hydrogen (H2) 0 to 1 %
The decomposition of waste material during anaerobic digestion is caused by bacterial action rather than high temperatures. It takes place in almost any biological environment, but is favoured by warm, wet and low oxygen conditions.
Anaerobic digestion also occurs in two major situations created by human activities:
– Sewage (human waste) or animal manure.
– Landfill gas produced by domestic refuse buried at landfill sites.
In nature, there also many raw materials (organic materials) from which biogas can be extracted: human and animal manure, leaves, twigs, grasses, garbage, agricultural and industrial wastes with organic content greater than 2 %.
The biogas produced can be used for cooking, heating, lighting (using gas lamps), electricity generation, operation of farm and other energy needs. The effluent produced from the process can be used as a fertilizer for crops.
Landfill Biogas Blower systems
Landfill biogas is transported from the landfill wellfield to the blower facility ba way of the main gas collection header. LFG then typically passes through a liquid knockout vessel for the removal of gas particles and liquid, before being routed to the blower. Block valve and bypass valves may be actively used at some landfills if there is a desire to route gas around the liquid knockout vessel. This is generally not recommended because condensate removal helps to protect processing instrumentation and equipment.
The blower facility and associated control equipment can either be housed inside a building or be exposed to the elements otside. It should be centrally located with room for expansion and supplied power. It should also have the capacity to handle 100 percent of the LFG peak production estimate, plus additional size for LFG migration control. Butterfly valves are often installed on the inlet and outlet piping for each blower being used to allow for continuous blower operation during scheduled maintenance and shutdowns.
The purpose of the LFG blower (also known as a compressor) is to create a vacuum for the extraction of gas from collection wells and trenches under pressure, the pulling of the LFG to the blower, and the pushing of the LFG to the flare or other treatment equipment. This process is known as actively controlling LFG, which contrasts with passive LFG control. Passive systems, where LFG is typically allowd to vent into the atmosphere with little or no treatment, most often are not advocated for modern landfill operations. The primary mechanical component of the blower system is the gas compressor or blower itself. Other associated equipment may include:
- Valves (automatic block, check)
- Flow metering and recording
- Gages to measure pressure, temperature, etc
- Condensate treatment and handling equipment
- Electrical equipment
Selection of the appropriate blower is determined by such factors as the quantity and end use of the LFG, the vacuum required to extract the gas, the pressure required for processing, etc. The main types of blowers used for LFG applications include single and multi-stage centrifugal blowers: I.E. constant vacuum/pressure, variable gas volume machines, incorpating a butterfly valve at the unit inlet.
Biogas and the environment.
There is a significant quantity of unused organics (animal waste) in rural areas. The organics can produce large quantities of methane gas and present technology may be utilized and adapted to local conditions.
Bio-digester design and output expectations must be tailored to the rosources, climatic conditions and builiding materials available. To minimize capital cost for equipment, it is important to ensure that the digester is appropriately designed.
Anaerobic digestion also occurs in ponds, marshes and manure pits where there is an abundance of rotting organic materials. Bubbles are often seen coming up to the surface and upon combustion of the bubbles (filled with methane), a violet blue, smokeless and odourless flame is observed. Methane is about 23 times more potent as a greenhouse gas than carbon dioxide ( CO2).
Potential feedstock for bio-digesters
The raw materials that can be considered as suitable substances for the production of biogas through the bioconversion process in a digester are:
1. Crop Residue – Sugar cane trash, weed, corn and crop stubble, straw, spoiled fodder, etc. Studies have shown that the content of water soluble substances such as sugars, animo acids, proteins and mineral constituents decreases with age of the plant and it is low enough to limit the rate of the digestion process. Thus, the decomposition of crop residues takes a longer time than manures due to their fibrous content and larger particle sizes.
2. Manure – Cattle-shed waste (dung, uringe, litter), sheep and goat droppings, slauther house waste (blood, meat), fishery waste, leather, etc. from the agricultural community represent a significant source of feedstock for the biogas production. Manure is a source of carbon and nitrogen required for the successful operation of the fermentation process. The quantity and composition of animal waste is dependent on the type of animal. Pultry, for example, produces more volatile solids, nitrogen and phosphorus, per unit weight. In addition, the composition of manure depends to a large extent on the feed ration of the animal. animals feeding o grass alone show much lower nitrogen content in their manure and urine.
3. Human waste – Human faces and urine are raw materials that can be used for biogas production.
4. By products and waste from agriculture based industries and aquatic growth – Oil cakes, bagasse, rice bran, seeds, water from fruit and vegetable processing, filter press mud from sugar factories, marine algae, seaweed, etc, can also be sued as feedstrock for bio digesters.
Requirements for a successful system
- Keep the digester away from naked flames and electrical equipment than may spark.
- Buildings should be well ventilated.
- Explosion proof motors, wiring and lights should be used.
- Perform periodic system checks for gas leaks.
- Follow Bunsen burner safety rules when testing the gas with a flame.
- Utilize gas detection and alarm devices in enclosures.
- Do not divet the effluent from the unit directly into lakes or streams.
Safety concerns related to biogas generation include health hazards and risks of fire or explosion. Biogas is flammable and can be explosive when mixed with air.
- Acceptance by potencial users.
- Ability to use the gas when produced.
- Sufficient demand for gas.
- Availability of sufficient raw materials to meet the production requirements.
- Adequate maintenance and operational control.