Biogas Power Plant Working, Site Selection and Application:
Introduction – Biogas is a combustible gas mixture of methane, carbon dioxide, water and hydrogen sulphide produced during the anaerobic decomposition of organic matter. During anaerobic digestion, wastes are treated and degraded and biogas is produced. Anaerobic treatment also has the advantage over aerobic treatment of a smaller emission of greenhouse gases. Biogas Power Plant can be recovered and used either directly for cooking, lighting or it can be transformed into any kind of thermal, electrical or mechanical energy. It can also be compressed, much like natural gas, and used to power motor vehicles. The calorific value of biogas is about 6 kWh/m3, what corresponds to about half a liter of diesel and can be utilised directly as a heat source or to produce electricity. In all cases, the biogas must be dehumidified and purified before combustion, since it may damage the gas engine.
Therefore, biogas is a renewable green energy source.
Biogas Power Plant consists mostly of methane (CH4, about 65-70%) carbon dioxide (CO2, about 25-30%) and varying quantities of water (H2O) and hydrogen sulphide (H2S) and some trace amounts of other compounds, which can be found, especially in waste dump biogas (e.g. ammonia, NH3, hydrogen H2, nitrogen N2, and carbon monoxide, CO).
The amount of each gas in the mixture depends on many factors such as the type of digester used and the kind of organic matter.
The main applications of biogas are:
- Domestic lighting and heating
- Fuel for I.C Engines
- Used to make fuel cell (the electrolyte used in Potassium Hydroxide KOH)
The gas needs to be upgraded by removing CO2 with the help of water scrubbing.
Various technologies to generate electricity from biogas on a household level are available. In principle, the chemical energy of the combustible gases is converted into mechanical energy in a controlled combustion system by a heat engine. This mechanical energy then activates a generator to produce electrical power. The most common heat engines used for biogas energy conversion are gas turbines and combustion engines. Combustion engines can be either internal or external combustion engine.
For small-size heat engines, combustion engines are popular as they are more efficient and less expensive than small gas turbines. Biogas is burnt for running a generator (e.g. micro turbine). The installation is usually less than 5 kWe (Kilowatts-electrical). Instead of burning fuel to merely heat the space or water, some of the energy is converted into electricity in addition to heat. This electricity can be used within the home or business or, if permitted by the grid management, sold back to the electric power grid. Plants producing electricity more than 5 kWe and less than 500 kWe generally sells the excess energy into the electricity grid.
Biogas Power Plant systems show the environmental friendly way of energy production and have a positive impact on climate change.
The technology is easily adaptable and can be applied at household or community level. To minimise distribution losses, it is normally useful for a single house or small business because of the low power output.
Biogas Power Plant converts wet biomass into biogas by the process of anaerobic fermentation. Biogas Power Plants are very popular in rural areas where biomass (cow dung, agricultural waste, etc.) are abundant. A typical biogas plant is shown in Fig.4.56.
The biomass (cow dung) is mixed with water to form slurry in mixing tank. The slurry is stored in the inlet chamber. From the inlet chamber the biomass is fed into the digester where the biomass undergoes anaerobic fermentation and then the biogas (methane) is released as a result of biomass decomposition.
The slurry fed in the plant comprises mainly of cow dung mixed with other biodegrable waste. The slurry when confined in a place without air, gives rise to mainly two types of bacteria – Acid forming bacteria and Gasifying bacteria. Volatile acids are formed from carbohydrates, fats and proteins by the acid forming bacteria and carbondioxides are formed by gasifying bacteria. This phase is called as liquification phase. After this phase, the methane bacteria work upon the volatile acid, producing methane and carbon dioxide with the help of introcelluar enzyme. This phase is called gasification phase.
The whole process is governed by factors like temperature of slurry, loading rate, detention period, pH value, nutrients concentration, solid concentration, toxic substance, etc.
The Biogas produced consists of 55-60% methane and 40-45% carbonmonoxide with little amount of hydrogen and hydrogen sulphide.
The gas being lighter in weight rises to the top of the digester and stored there. A dome structure is made at the top to prevent the biogas from escaping out into the atmosphere. The plant has a strong foundation made up of concrete.
The slurry move to the outlet chamber after being fermentated. The moving of slurry is due to difference in the gas pressure. The slurry at the outlet can be used as manure.
A gas outlet pipe is provided on the dome structure which is governed by a valve. When the biogas is required, the valve is opened and when not required, it is closed.
The biogas from the outlet pipe is supplied for various uses like cooking, domestic heating and lighting, I.C engine, etc.
The I.C engine is coupled to a generator which produce the electricity. The biogas need to be upgraded before using it in an I.C engine by removing CO2 and heating the gas in order to make the gas free from moisture.
The dangers of biogas are mostly similar to those of natural gas, but with an additional risk from the toxicity of its hydrogen sulphide fraction. Biogas can be explosive when mixed one part of biogas to 8-20 parts air. When the tank is open for cleaning or repair work, open flames, sparks, and smoking should be avoided. If light is needed, a flashlight or sunlight reflected off of a mirror should be used. Biogas leaks smell like rotten eggs (hydrogen sulphide). If someone enters a biogas digester they should always have someone with them in case they stop breathing due to low oxygen intake.
It is important that a biogas system should never have negative pressure as this could cause an explosion or kill the digesting bacteria. Negative gas pressure can occur if too much gas is removed or leaked.
Frequent smell checks must be performed on a biogas system. If biogas is smelled anywhere, windows and doors should be opened immediately. If there is a fire, the gas should be shut off at the gate valve of the biogas system.
Site Selection of Biogas Power Plant:
While selecting the site for a biogas plant, various points should be considered.
- Land should have high bearing stress.
- Easy water availability.
- Adequate space for making composite pit.
- Sun light should be available whole day through out the year.
- It should be atleast 3 meters away from foundation of building.
- Biomass should be easily available (near the cattle shed).
- Should be away from drinking water source.
- Foundation surface should be plane and should be at a higher elevation so that in rainy season water logging problems do not occur.
Advantages of Biogas Power Plant:
- Generation of renewable, green electricity.
- Low operating costs.
- Underground construction minimizes land use.
- Long life span.
- Reduces greenhouse gases.
- Increases family income by selling back electric energy to the electric power grid.
- No maintenance problems due to the absence of moving parts.
- No corrosion problems.
- Requires expert design, skilled construction and expert maintenance required.
- Biogas production below 15°C, is no longer economically feasible.
- High capital costs.
- Variable gas pressure.
- Scum formation.