How to Remove Hydrogen Sulphide from Biogas? (3 Methods)

Hydrogen sulphide which is usually present in trace quantities in biogas is poisonous and harmful for health, and also imparts corrosive property to it. Process of removing hydrogen sulfide from gas is called desulfurisation. Like carbon dioxide, it can also be removed by water and caustic scrubbing.

A brief description of some of the commonly used desulfurisation methods are as follows:

Method # 1. Water Scrubbing:

According to this method gas is passed through water which absorbs part of hydrogen sulfide. However, water-scrubbing for desul­furisation is not that effective as it is for CO₂ removal. Low partial pressure of H₂S leads to low mole fraction absorption of hydrogen sulfide in water which makes it relatively less effective for desulfurisation.

Method # 2. Caustic Scrubbing:

This method works or the principle that when caustic solution are made to react with CO₂ bearing gas streams, an irreversible carbonate-forming reaction followed by another irreversible potassium hydro-sulfide forming reaction takes place which appears as a yellow precipitate.

Following chemical equations represent the two chemical reactions:

It is possible to remove hydrogen sulfide and carbon dioxide by using other chemicals like ammonical solutions, trisodium phosphate, sodium phenolate and other alkacid processes but these suffer from limitation of high costs of purification. In caustic scrubbing hydroxides of potassium, sodium and calcium [KOH, NaOH and Ca (OH)₂] are used for purification.

Method # 3. Solid Chemical Absorption:

This is one of the simple and practical methods for desulfurisation which is especially appropriate for situations where other impurities are not necessary to be removed. In this method ‘iron sponge’ consisting of ferric oxide and wood shavings and kept in dry scrubber is made to react with gas to be purified.

According to studies carried by H.M. Lapp et al., one bushel (0.0532 m³) of iron sponge can remove 8.2 lbs (3.7 kg) of sulfur. For removing upto 0.2 per cent of hydrogen sulfide which is the normal range, 1.1 ft³ (0.03 m³) of iron sponge is needed for a plant running on wastes from 1000 hogs with each hog contributing 7 ft³ (0.2 m³) of biogas per day. It is possible to convert (regenerate) used sponge back to free sulfur and ferric oxide which can be re-used for de-sulfurisation.

Following chemical reactions take place in this process:

The above method is quite economic and easy to operate and maintain. In place of ferric oxide, zinc oxide can also be used for desulfurisation which has added advantage of removing organic sulfur compounds such as carbonyl sulfide and mercaptans. Use of zinc oxide is however less economic as compared to ferric oxide which is easily available from iron filings and scrap.

Some other commonly used absorbents are activated alumina, activated carbon, silica gel and molecular sieves. Molecular sieves are preferred as they help to remove both carbon dioxide and hydrogen sulfide. Molecular sieves are also used to remove contaminants from industrial gases.

One commonly used class of absorbents are alumino-silicates which are found to be quite effective in absorbing hydrogen sulfide and water vapour. These absorbents produce clean dry gas without use of high pressure components.

As an added advantage molecular sieves do not absorb methane unless bed temperature falls below -20°F (-28.8°C) which barring high altitude hilly-regions is never the case in this country. It is to be kept in mind that molecular sieves are not inexpensive and quickly get saturated with contaminants. As reported by H.M. Lapp, et al., based on results from industry, 2.7 lb of molecular sieve is needed to remove 1 ft³ of CO₂ from biogas, or equivalently 42.3 kg of molecular sieve is needed to remove 1 m³ of CO₂.

Microbiological Methods of Purification:

P.G Anglo, et al., carried out studies at the National Institute of Science and Technology in Manila (Philippines) on production, and desulfurisation of biogas from swine wastes using microbiological methods. A purple photosynthetic bacterium, Chromatium sp. was isolated from the methane fermenter and was found as an active agent for removing hydrogen sulfide from biogas.

Separate 7-days old cultures of the two purple bacteria, Chromatium sp. and Rhodopseudomonasspheroid.es briefly called Rs-1 and Rs-2 respectively, were inoculated in meat boullion agar in 300 ml effluent medium (1:3) and incubated at 35°C in lighted anaerobic conditions. Sodium sulfide solution (24 per cent, 0.5 ml) was introduced into the liquid cultures and air was replaced with nitrogen gas.

For comparison, the two organisms were also inoculated separate­ly in 300 ml meat boullion containing the same amount. The residual hydrogen sulfide was noted daily until all the gas was consumed. Further experiments were conducted to study the desulfurisation activity of Rs-1. Duplicate flasks of meat boullion with sodium sulfide as above were used throughout.

In the temperature experiment, incubation at 18°C, 25°C and 35°C was tested under lighted conditions. In the light and dark experiments, one set of flasks was covered with tinfoil and another set was exposed to light bulbs at 35°C. To test the desulfurisation efficiency of isolates Rs-1 and Rs-2, two-weak old separate flask cultures of the bacteria in meat boullion were connected to the 6-litre biogas jar fermenters.

Percentage removal of hydrogen sulfide was determined every day. The study established capability of certain photosynthetic bacteria like Chromatium sp. for use in desulfurisation of biogas. According to P.G. Angle et al., desulfurisation was most effective at 35°C under lighting conditions of 1000 to 1200 lux.

T.S. Ghou, et al., studied the effectiveness of hydrogen sulfide removal with silica gel. Hydrogen sulfide can be effectively removed till silica gel is saturated as indicated by the change in colour from blue to black. It is possible to reuse silica gel is following regeneration, Flow rate of biogas, regeneration temperature, degree of saturation of silica gel and its water content help to evaluate scrubbing efficiency of silica gel. A mathematical model for evaluat­ing adsorbed efficiency was also developed.

D.R. Pande made a comparative study of some of the common and efficient scrubbing methods for biogas purification. He discussed adsorption method using molecular sieves made of natural zeolites which is claimed to be economic, efficient and simple.