CBG or Compressed Bio Gas consists of mainly methane (more than 90%) and other gases like carbon dioxide (less than 4%), etc. CBG is produced by anaerobic digestion of biomass and waste sources like agricultural residue, cattle dung, sugarcane press mud, municipal solid waste, sewage treatment plant waste, etc. This Biogas can be purified to remove hydrogen sulphide (H2S), carbon dioxide (CO2), water vapor and when this purified biogas (methane content more than 90%) is compressed to maximum 250 bar and filled up in cascades (group of high pressure cylindrical vessels), it is called Compressed Bio Gas or CBG.
Hydrogen sulfide is a corrosive gas. Presence of carbon-dioxide in the bio-gas reduces its calorific value. Hence the bio-gas needs to be purified. Various technologies are used for removal of hydrogen sulfide, such as Biological Fixation, Iron chloride dosing, Water scrubbing, Activated carbon, Iron Hydroxide or Oxide and Sodium Hydroxide etc.
An Anaerobic Digester is a device for optimizing the anaerobic digestion of biomass and to produce biogas for energy production. Anaerobic digesters are made out of concrete, steel, brick, or plastic. They are shaped like silos, troughs, basins or ponds, and may be placed underground or on the surface. All designs incorporate the same basic components: a pre-mixing area or tank, a digester vessel(s), a system for using the biogas, and a system for distributing or spreading the effluent (the remaining digested material).
There are two basic types of digesters viz., batch type digester and continuous digester besides other process like covered lagoon.
In a CSTR digester, organic material is constantly or regularly fed into the digester. The material moves through the digester either mechanically or by the force of the new feed pushing out digested material. Continuous digesters produce biogas without the interruption of loading material and unloading effluent. They are better suited for large-scale operations. Proper design, operation, and maintenance of continuous digesters produce a steady and predictable supply of usable biogas.
Horizontal plug-flow reactor design incorporates progressive steps of narrow vertical mixing using gas injections throughout the length of the long rectangular channel.
Batch-type digesters are the simplest to build. Their operation consists of loading the digester with organic materials and allowing it to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated.
Anaerobic digester reactor design that is made up of a lined pit and a flexible plastic cover. This system captures biogas under an impermeable cover, while taking advantage of the low maintenance requirement of a lagoon.
In order to improve the calorific value and energy content, methane concentration shall be increased and in turn CO2 & hydrogen sulphide (H2S) shall be removed. Some of the available technologies for removal of H2S are Iron chloride dosing, Water scrubbing, Activated Carbon and Amine Process. For removal of carbon dioxide, the following technologies are prevalent:
This technology is prevalent for large bio-gas systems in India. With this technique, carbon dioxide is separated from the biogas by adsorption on a surface under elevated pressure. The adsorbing material, usually activated carbon or zeolites, is regenerated by a sequential decrease in pressure before the column is reloaded again, hence the name of the technique. Hydrogen sulphide and water needs to be removed before the PSA-column.
It is a non-cryogenic gas separation technology. It utilizes the different selectivity of adsorbents for gas molecules to separate gas components. Using special solids, or adsorbents, VSA segregates certain gases from a gaseous mixture under minimal pressure. These adsorbents (e.g., zeolites) form a molecular sieve and preferentially adsorb the target gas species at near ambient pressure. The process then swings to a vacuum to regenerate the adsorbent material.
Carbon dioxide has a higher solubility in water than methane. Carbon dioxide will therefore be dissolved to a higher extent than methane, particularly at lower temperatures. In the scrubber column carbon dioxide is dissolved in the water, while the methane concentration in the gas phase increases. The gas leaving the scrubber has therefore an increased concentration of methane.
Dry membranes for biogas upgrading are made of materials that are permeable to carbon dioxide, water and ammonia. Hydrogen sulphide, and oxygen permeate through the membrane to some extent while nitrogen and methane only pass to a very low extent. Usually membranes are in the form of hollow fibers bundled together.
In this system, Carbon dioxide is not only absorbed in the liquid, but also reacts chemically with the amine in the liquid and the chemical reaction is strongly selective. Apart from the above, there are alsoother technologies like Cryogenic upgrading etc.
CBG bottling unit will consist of High Pressure compressor and Cascade of storage cylinders. Dried and purified form of biogas goes into the suction of High Pressure Compressor, where it compresses the gas to desired working pressure (~250 Bar).
The compressed biogas will be filled and stored in cascades which are a group of high pressure cylindrical vessel. For delivery of CBG, 3000 litre metal cascades or cascades of higher capacity shall be used. Steel cylinder cascades and composite cascades may be used for transportation of CBG. For steel cylinder cascades, IS 7285 shall be applicable. If type-3 or type-4 composite cascades are used, respective BIS specifications like IS 15935 or relevant specifications shall be applicable.
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