INTRODUCTION
Biogas is the mixture of gases produced by the breakdown of organic matter in the absence of oxygen (anaerobically), primarily consisting of methane and carbon dioxide. Biogas can be produced from raw materials such as agricultural waste, manure, municipal waste, plant material, sewage, green waste or food waste. Biogas is a renewable energy source. In India, it is also known as "Gobar Gas".
Biogas is produced by anaerobic digestion with methanogen or anaerobic organisms, which digest material inside a closed system, or fermentation of biodegradable materials. This closed system is called an anaerobic digester, bio- digester or a bioreactor. 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.
Biogas can be compressed after removal of Carbon dioxide, the same way as natural gas is compressed to CNG, and used to power motor vehicles. In the United Kingdom, 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. Biogas is considered to be a renewable resource because its production-and-use cycle is continuous, and it generates no net carbon dioxide. As the organic material grows, it is converted and used. It then regrows in a continually repeating cycle. From a carbon perspective, as much carbon dioxide is absorbed from the atmosphere in the growth of the primary bio-resource as is released, when the material is ultimately converted to energy.
Bio-CNG is produced from biogas, through a simple and convenient process-involving desulphurisation, up-gradation and compression. First, biogas is de-sulphureted if the hydrogen sulphide content is over 2,500 ppm. Then, the de- sulphureted biogas is upgraded to make its composition similar to CNG, followed by the compression and bottling of the resulting bio-CNG.
BENEFITS OF BIOGAS PLANTS
- A non-polluting and renewable source of energy is created in biogas plants. Under the process organic waste is converted to useful fuel. It is an excellent way of energy conversion. Electrical power can be feed to the grid.
- Many types of raw material (other than dung) can be used in the plant: Poultry droppings, Agro & Farm Waste, Kitchen Waste, Vegetable & Fruit Market Waste, Food Processing Waste and other Bio Degradable Waste.
- It destroys Methane, which is a potent greenhouse gas with a heat trapping capacity of approximately 21 times that of carbon-di-oxide. It thus leads to reduction of global warming.
- Biogas as a gas provides improvement in the environment, sanitation and hygiene by proper management of waste. It improves ground water quality as anaerobic digestion provides several water quality benefits.
- Biogas digesters can destroy more than 90% of disease causing bacteria that can otherwise enter surface water. Thus it reduces risk to human and animal health.
Purification Methods
Removing biogas impurities other than CO₂ is referred to as biogas cleaning, while further separating CO₂ from biogas is known as biogas upgrading. Commonly used methods are together with the number of industrial plants worldwide using them and their achieved CH₄ purities.
Water scrubbing
Removes both CO₂ and H₂S simultaneously by taking advantage of their higher water solubility compared to CH₄. To enhance absorption, biogas is usually compressed. After absorption in the scrubber, the purified biogas, which contains more than 90% CH₄, is collected from the top of the scrubber, while liquid effluent containing high concentrations of CO₂ and a trivial amount of CH₄ is treated in a flash tank to recover CH₄. The water is then regenerated in the stripper with air blown (air sparing) into the reactor.
Pressure swing adsorption
Uses the adsorbent's pressure dependent gas adsorption rate to capture preferred gases at a high pressure, and then release the adsorbents at a low pressure to regenerate the adsorbent. In the pressurized vessel (step 1), impurities having high gas adsorption rates are adsorbed, while enriched CH₄ is collected from the top of vessel. The saturated vessel is then depressurized to around atmospheric condition (step 2) for desorption of the impurities. As the gas released in this step contains both impurities and a small amount of CH₄, it is recycled. The pressure is further decreased to a near vacuum in step 3, which de-adsorbs captured gases and regenerates the adsorbents. The gas that leaves the vessel in this step mainly consists of CO₂, N₂ and O₂. Pressure is built up in step 4 for the next cycle.
Membrane Separation
The design principle of membrane Separation is that under a certain pressure, gases with high permeability (e.g., small molecular size and low affinity) can be transported through the membrane while gases with low permeability are retained. As shown in Fig. high permeable impurities such as CO₂, O₂ and H₂O pass through the membrane as permeate, while low permeable CH₄ is retained and collected at the end of the hollow column.
