Elimination Of Hydrogen Sulfide From Biogas

Biogas is a type of gas that has several other gasses present in it. Biogas is produced by the breakdown of various natural and organic matters. Biogas can be made with organic matter without the presence of oxygen and mainly carbon dioxide and methane like plant waste, food remains sewage and agricultural waste. Other gases present in biogas are methane, carbon dioxide, hydrogen sulfide, nitrogen, hydrogen, and ammonia. The percentage of methane and carbon dioxide is maximum in biogas.


Hydrogen Sulfide


Hydrogen sulfide is a gas or a chemical compound that is colorless in nature. It has a foul smell that is similar to the smell of rotten eggs. It is a highly corrosive, flammable, and poisonous gas. Hydrogen sulfide is present in biogas. The level of hydrogen sulfide increases in biogas with the presence of waste that is high in protein.


The high protein waste generally contains sulfur-based acids known as amino acids. These amino acids trigger the biogas and end up boosting the level of hydrogen sulfide in biogas. As we all know hydrogen sulfide is a highly corrosive and poisonous gas. It can cause major damage to pipelines, instruments, and also equipment.


Dangerous To Mankind


Hydrogen sulfide is a harmful and toxic gas. It is generally shipped as a compressed and liquefied gas. It is very harmful to humans in several ways. Here are few ways by which hydrogen sulfide is harmful to humans.


  • Contact with Eye or Skin: If hydrogen sulfide is exposed to the skin or eyes it can cause irritation and result in painful rashes and dermatitis. It can result in a burning and irritation sensation in the eyes. Direct contact with hydrogen sulfide can also give a person frostbite and other skin problems.


  • Inhalation: Hydrogen sulfide is absorbed by the lungs very fast. It can make the body weak. It can cause trouble in breathing and can result in chest congestion.


The Desulfurization Process


As we all know that hydrogen sulfide removal from biogas is necessary as it is a highly flammable, toxic, and corrosive gas that can be dangerous to humans and can also ruin piping and other instruments. Here are the methods by which we can remove hydrogen sulfide from biogas.


  • Caustic Scrubbing: In this method hydrogen sulfide and oxygen both are removed by the use of several other chemicals like trisodium phosphate, ammoniacal solutions, and sodium phenolate. In this potassium hydroxide, calcium and sodium are used to purify hydrogen sulfide.


  • Water Scrubbing: In this method, gas is passed through the path of water. Water then absorbs some amount of hydrogen sulfide. This is only effective for the removal of hydrogen sulfide and not carbon dioxide with it. It is also the easiest and most cost-effective way of hydrogen sulfide removal from biogas.


  • Absorption with Solid Chemical: This is a great way of hydrogen sulfide removal and can be done only if there are no other impurities to be removed. This method uses ferric oxide and shaving of wood is placed in an absolutely dry scrubber. It is then kept to react with the hydrogen sulfide gas to be purified.



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The Process of Hydrogen Sulfide Removal from Biogas

Hydrogen Sulfide is the major pollutant in biogas. It is very important to remove hydrogen sulfide from the biogas because of it’s corrosive and poisonous nature. If not removed then it may cause substantial harm to the device, piping, and instrumentation.

The corrosiveness and poisonousness of hydrogen sulfide vary based on its presence in ppm. There is not only one process of hydrogen sulfide removal from biogas, there are many ways from which it can be removed. Many new methods like the removal of hydrogen sulfide by activated carbon, red rock, etc. have been also tested these days.

Here is the process of desulphurization by the biological method.

Desulphurization by biological method

In this desulphurization of biogas by biological methods, micro-organisms perform all the activities. Many of these microorganisms belong to the Thiobacillus family. Oxygen in stoichiometric amounts must be added to biogas to perform microbiological oxidation. This approximates 2-6% air present in biogas relying on the presence of the amount of hydrogen sulfide.

The desulphurization can be simply done by adding air or oxygen directly to the storage tank or the digester performing at the exact time as that of the gasholder. Systems will not need inoculation because the Thiobacilli are omnipresent.  They get the essential nutrients and the important microaerophilic surface as they develop on the dig estate surface.  They shape clusters of sulfur which are yellow-colored. Relying on the conditions, the time of reaction, the place and proportion of added air in the hydrogen sulfide, it’s engagement can be lessened up to 95% to smaller than 50 ppm.

Overdose of the air can cause the pump to fail thus safety measures be taken primarily to avoid such a situation.  If the concentration of air is up to 6-12 % in the biogas then it becomes an explosive substance, being sure of the methane quantity.  At the interface of gas and liquid, there’s a little danger of corrosion in the digester which are made of steel and especially those who are without any rust protection.so it is better to keep the causative measure strictly while working in these areas.

Dosing of Iron chloride (FeCl)

Dosing of Iron chloride can stand directly to feed substrate or to digester slurry, then with the obtained hydrogen sulfide, the iron chloride will react to form iron sulfide particles(salt). This process is very beneficial in decreasing massive hydrogen sulfide concentration but not in obtaining a stable and low degree of hydrogen sulfide.    So, in this aspect, this process of iron chloride dosing can just be considered as a biased removal method. This process is good for avoiding corrosion which may take place in the remaining upgrading cycle.

The cost involved in all such processes is very limited because all you need to invest is in just a storage tank for the dosing pump and to make and keep an iron chloride solution. But it doesn’t end up here because, on the contrary, the functional expense will be a little high because of the main cost for iron chloride.

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