Hydrogen Sulphide

Hydrogen sulphide (H2S) is a colourless, strongly smelling gas that, depending on the accumulated amount, can be both toxic to human health and corrosive to unprotected machinery. In sufficient quantities, it can also be flammable. It forms under anoxic conditions and frequently accompanies the formation of methane during the degradation or organic material.

Because it is produced by biological processes, mechanisms to adequately treat hydrogen sulphide are of critical importance during any process that uses anaerobic digestion as a basis for creating and accumulating biogas used in the production of energy.

Apart from the main components of methane and carbon dioxide, biological degradation of biogas under anaerobic conditions can also lead to the production of trace gases of which hydrogen sulphide can be one of the most significant. As the gas is highly corrosive , its presence in biogas must therefore be reduced to safe levels prior to its use in any commercial application.

Hydrogen sulphide can be formed when organic matter degrades in the absence of oxygen. It is a common occurrence in natural processes such as in marshes where high organic loadings result in an exhaustion of available oxygen and a concomitant bloom of sulphate reducing bacteria.

In municipal waste landfill sites, the burial of organic material rapidly leads to the production of anaerobic conditions within the waste mass and, with the humid atmosphere and relatively high temperature that accompanies the organic degradation process, biogas is produced as soon as the air within the waste mass has been reduced. If there is a source of sulphate bearing material, such as plasterboard or natural gypsum (calcium sulphate dihydrate), under the right conditions sulphate reducing bacteria converts this to hydrogen sulphide. The bacteria cannot survive in air but the moist, warm, anaerobic conditions of buried landfilled waste that contains a high source of carbon – even in inert landfills, the paper and glue used in the fabrication provides a rich source of carbon – is an excellent environment for the formation of hydrogen sulphide.

In industrial anaerobic digestion processes hydrogen sulphide can be formed from the reduction of sulphate and the degradation of amino acids and proteins within organic compounds. Sulphates are relatively non-inhibitory to methane forming bacteria but can be reduced to H2S by sulphate reducing bacteria, of which there are several genera. Other sources of H2S can be the presence of elemental sulphur or sulphate salts contained within waste water. Concentrations of H2S can vary from a few ppm if the substrate contains iron salts, up to a maximum of about 4% by volume. With certain types of wastes the concentration can be even higher.

The Organics Solution

1. Define Objectives
Data collection at the outset should be directed towards understanding the performance objectives of the finished facility. As well as the straightforward requirement to reduce H2S, there are often other equally important factors which must be addressed, such as the rate of start-up and the ability to respond to rapid changes in loading.
2. Define Constraints

There are often many constraints to a specific installation. These include aspects which may be obvious, such as access limitations or power requirements, as well as less obvious issues, such as the maximum gas temperature on the inlet and fluctuating pressures and the alteration of composition of the inlet gas.

3. Define the Solution
Once a clear understanding has been obtained of the situation in which equipment performance is to be expected, it will be possible to commence the process of conceptually designing and building a solution. Where this is a standard application, such as with POME or anaerobic digestion biogas, this will be a matter of choices. Should an untested application be involved, care must be taken to address all requirements and constraints.
4. Deliver and Install the Solution
The final stage of the process is delivery. From detailed design, through procurement, manufacture, delivery, installation and commissioning, there are many details which must be coordinated. Through regular internal and external reporting, as well as maintenance of on-line systems, Organics keeps close coverage of all aspects of delivery, installation and successful commissioning of all equipment.

Frequently Asked Questions About H2S

What is hydrogen sulphide (H2S)?

Hydrogen sulphide (H2S) is a type of gas that occurs naturally and is found during drilling for natural gas and crude oil, but it can be produced also by human activity. The presence of low levels of H2S can be detected by the smell which has been likened to rotten eggs. H2S gas is colourless and flammable and can be both poisonous and corrosive. In certain situations, H2S can be detected by smell in concentrations as low as 0.6 parts per billion (ppb). 

Why is H2S produced from landfilled waste?

Landfill sites can produce H2S in the presence of wastes plasterboard or other products that contain sulphur. It can also be present if there is the presence of gypsum (CaSo4.2H20) such is frequently found along the coast of the Mediterranean.

H2S levels on or close to landfills can also change significantly throughout the day with concentrations higher at night or in the early morning when wind is less active.

What is the environmental impact of H2S?

Impact on human health

Living close to industries which could potentially produce uncontrolled emissions of H2S can increase the risk of exposure. Prolonged exposure to high enough levels of H2S can cause unconsciousness and those affected may continue to experience headaches, reduced attention span and altered motor functions. Effects of H2S gas exposure may not become apparent for up to 72 hours following removal from the affected environment.

In most cases, people are outside when H2S is present and, under normal conditions, it mixes with ambient air where the risk of exposure is significantly reduced. People who work in atmospheres where there may be a risk of H2S production are strongly advised to carry a wearable gas detector .

What is the environmental impact of H2S?

Impact on industrial facilities

As H2S is heavier than air it usually accumulates in low lying areas of poorly ventilated spaces. During the production and collection of biogas in landfill sites or anaerobic digestion plant, H2S gas in the presence of air and moisture can form sulphuric acid which is capable of corroding metals within machinery that is often not seen.

Unless specific steps are taken against the accumulation of H2S and its subsequent reaction to a corrosive agent, catastrophic and premature damage to equipment can ensue.

What treatments are effective against H2S?

In the presence of elevated levels of H2S, cleaning or scrubbing of biogas is of critical importance in the production of energy from biogas produced from organic material. Many different methods are available, including:

  • Different types of wet biogas scrubbers mostly using NaOH
  • Dry type biogas scrubbers using iron compounds to absorb the H2S
  • The addition of iron salts to the inflow of a digester for precipitation of H2S
  • Chelated iron complex solution wet scrubbing
  • Dry-bed adsorption methods
  • Bio-catalytic H2S removal by bacteria

Most of these methods have a high investment and/or operational costs which include for chemicals and waste disposal.
Another method is by using bio-scrubber technology, the main advantage of which is that, in most circumstances, no additional costs for chemical-additions are incurred. The bacteria involved in the process (genera thiobacillus) are ubiquitous and as long as correct environmental conditions are maintained, the bacteria will function reliably and predictably in the removal of hydrogen sulphide from gas streams.
Unless specific steps are taken against the accumulation of H2S and its subsequent reaction to a corrosive agent, catastrophic and premature damage to equipment can ensue.

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