07-08-2012, 03:50 PM
Enrichment technique
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Biogas enrichment process
A variety of processes are available for enrichment i.e. removing CO2, H2S and water
vapour. Commonly CO2 removal processes also remove H2S. One of the easiest and
cheapest methods involved is the use of pressurized water as an absorbent liquid. In this
method, biogas is pressurized and fed to the bottom of a scrubber column where water is
sprayed from the top. In counter-currently operated absorption process, the carbon dioxide
and hydrogen sulfide present in the biogas is absorbed in down going water and methane
goes up and is collected in vessel. However, water requirement in this process is high but it
is the simplest method of removing impurities from biogas.
Concept of alternative Bio-CNG
Biogas contains a large proportion (about 40 % by volume) of carbon dioxide, a heavier and
non combustible gas and some fraction of hydrogen sulphide. Hence it is needed to enrich
biogas by removing these undesirable gases to save compression energy and space in bottle and
corroding effect, which can be done by scrubbing. The scrubbing system is found to enrich methane
about 95 % or more depending upon biogas inlet and water injection pressure. Biogas can be used
for all applications designed for natural gas, assuming sufficient purification.
Scope of the technology
Enriched biogas is made moisture free by passing it through filters, after which it is compressed up to 200 bar pressure using a three stage gas compressor. Compressed gas is stored in high pressure steel cylinders as used for CNG. There is large potential of this technology in buses, tractors, cars, auto rickshaws, irrigation pump sets and in rural industries. This will help to meet our energy demand for rural masses thus reducing Biogas Scrubber burden of petroleum demand, moving towards energy security and will improve economic status by creating employment generation in rural areas.
Cylinders filled from one 120 m3
biogas bottling plant = 8 cylinders /day (Capacity 6 kg/ cylinder)
As 6 kg CNG cylinder = 6 litre Petrol
So, gas filled in these cylinders will be equivalent to = 6 x 8
= 48 litres of Petrol/day
or Diesel/Petrol savings = 17520 litres/annum.
= 876000 Rs./annum (Approx.)
= 9 lakh Rs./annum (from one bottling plant)
Therefore, from only one biogas bottling plant, enriched biogas filled cylinders will be able
to replace fuel worth Rs. 9 lakh annually in the country. Increasing the number of bottling plants
will subsequently increase the diesel/petrol savings. The whole cost can be recovered within
two-three years from the installation of the plant.
1.1 Need for Hydrogen Sulphide Removal:
Hydrogen Sulphide is a highly toxic and corrosive gas and is a major pollutant in fossil fuel based industries, sulphur based chemical Industries and biogas based units Sweetening of biogas is essential because of its toxicity which affects the environment and the highly corrosive nature which corrodes the metallic parts like boiler tubes, engines coming in contact with it in few hundred hours of operation. The toxicity of H2S is more than hydrogen cyanide (1).
IC engine application requires H2S concentration to be less than 1000 ppm, limit which is dependent on individual engine manufacturer, but lower the level better is for the life of the engine.
2. PROCESSES FOR REMOVAL OF HYDROGEN SULFIDE: Different processes like absorption and desorption by amine followed by Claus process for sulfur recovery, Liquid red-ox process, absorption by caustic solution and the subsequent biological regeneration of the same are being used for scrubbing hydrogen sulfide from the gas stream. Amongst the various methods for removal of hydrogen sulfide, modified versions of Claus process and Liquid red-ox process are the most common and widely used. Claus process which is a vapor phase oxidation process in multiple stages scrubs hydrogen sulfide at higher concentrations (more than 15% v/v) and converts to sulfur, has an efficiency of around 97 % for a three stage process (1). The Liquid red-ox process can treat gas economically and efficiently, with fluctuating gas flow and with low to medium level (0.5 % – 10 %) but fluctuating hydrogen sulfide concentrations.
3. ISET PROCESS:
ISET process developed by CGPL, IISc, Bangalore, is a modified liquid red-ox process using chelated polyvalent metal ion with a stabilizing agent. The process utilizes the oxidation reduction potential of chelated iron in aqueous medium, for scrubbing hydrogen sulfide from the biogas.
The need for developing a modified liquid redox process mainly arose due to lack of a suitable and reliable indigenous technology in the country and the imported liquid redox systems being expensive. The present technology is about a third of the cost of the one of the technologies available outside India (1). The systems based on Claus process are likely to be far more expensive at the flow rates of relevance to applications considered here (600 m3 per hr at 7.5 % hydrogen sulfide in the gas that corresponds of 1.5 tonnes per day of sulfur)