Hydrogen Sulfide or H2S present in biogas is an impurity that environmentally hinders the utilization of biogas as a source of energy. Also, the concentration of hydrogen sulfide varies according to the feedstock. The concentration in the biogas is a function of the inorganic sulfate content and digester feed substrate. High-protein wastes contain amino acids, which influences the H2S levels.
What is the need to remove the gas?
The inorganic sulfate in the feedstock during the digestion process gets reduced by the sulfate-reducing bacteria present in the digester. This way, it ends up contributing more to the sulfide level present in the biogas. The H2S contained in the gas causes corrosiveness, bad smell along with sulfur emissions after the gas gets burned. If the biogas is to be used in fuel cells, turbines and internal combustion engines, it is important to remove hydrogen sulfide from the gas to prevent damage to the equipment.
Techniques involved in the removal process of H2S
Here are several techniques for the removal of H2S gas.
- Iron oxide pellets
- Iron sponge
- Water scrubbing
- Sodium Hydroxide scrubbing
- Activated carbon
- Biological removal on filter beds
Commonly used methods to remove H2S
The concentration level of the different components of biogas impacts its usage. While internal combustion engines function well when H2S is managed below 100 ppm, boilers are capable of withstanding concentrations to up to 1000 ppm. The most common methods used for H2S removal from biogas are iron chloride dosing and oxygen/air dosing.
Iron chloride dosing – Iron chloride is fed to the digester slurry directly or given to the feed substrate in the pre-storage tank. The produced H2S reacts with iron sulfide to produce iron sulfide salt. The method is quite effective in lowering the H2S levels.
The method of iron chloride dosing can be treated as a partial removal technique process to avoid corrosion in the remaining part of the upgrading process equipment. It further needs to be contemplated with a removal down to 10 ppm.
Such a removal process costs less because you need a storage tank for holding the iron chloride solution along with a dosing pump. Yet, the operational costs would be expensive because of the price of iron chloride.
Oxygen/air dosing – Biological desulphurization or oxygen/air dosing of biogas is performed with the help of microorganisms. Almost every microorganism oxidizing sulfide belongs to the Thiobacillus family. For the oxidation of sulfide, it is important to add amounts of oxygen in specific ratios to the biogas.
This simplest method involves adding oxygen directly to the storage tank or digester. The micro-organisms strive on the surface of the digester, which provides necessary space and nutrients. Depending on the reaction time, temperature, place and amount of air added, the H2S concentration gets reduced to about 50 ppm.
Safety measures should be taken to prevent overdosing of oxygen. Keep in mind that biogas in oxygen causes explosion depending on the percentage of methane.
When choosing from the different techniques, you have to consider factors such as cost of the chemicals used, adsorption capacity, vacuum trucks to empty filters, disposal of wastes, and so on. Or else, there could be a risk of ignition of the used or spent adsorbent.
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