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Hydrogen Sulphide (H2S)

Hydrogen Sulphide (H2S) is a highly toxic colourless transparent gas, heavier than air, that has a molecular weight of 38 vapour density 1.189 (air = 1). Hydrogen Sulphide (H2S) is a poison that can paralyse your breathing system and can kill you in minutes. Even small amounts are dangerous to your health. Small amounts of H2S will have an offensive odour similar to rotten eggs. At slightly higher concentration H2S may have a sickly sweet odour. At high concentrations no smell can be detected as H2S paralysing the sense of smell therefore H2S cannot be detected by sense of smell at higher concentrations.

H2S is formed by decomposition of organic materials in landfill sites. H2S means that each molecule of gas is made up of two hydrogen atoms and one sulphur atom.

H2S can be found in large amounts in some landfill sites. Workers involved are at risk from exposure to H2S which, is accelerated by heat or hot weather. On still, foggy days, H2S tends to accumulate in low places in dangerous concentrations if it is warmer than the surrounding air will tend to rise. Readily dispersed by wind movements and air currents.

H²S is a by-product of landfill and is best disposed of by burning in a flare. H2S burns with a blue flame producing SO2 that is also a toxic gas.

Possible sources of exposure to H²S in landfill sites

  •  Migration of gas from landfill sites
  • Uncapped landfill sites
  • Leachate collection chambers
  • Drains
  • Open wells
  • Vents
  • Open flares
  • Low lying area (i.e. ditches and trenches)
  • Confined spaces
  • Stagnant water

H2S is more deadly than carbon monoxide (CO) and almost as toxic as hydrogen cyanide (HCN) gas, that was used in some States of America for criminal executions.

H2S is flammable and forms explosive mixtures with oxygen or air, explosive limits 44-46% by volume in air.


    0.13 PPM Minimal perceptible odour

    4.60 PPM Moderate odour easily detectable

    10 PPM Eye irritation

    28 PPM Strong unpleasant odour but tolerable

    100 PPM Loss of sense of smell after few minutes coughing, eye irritation

    200-300 PPM Respiratory tract irritation after one hour of exposure

    500-700 PPM Loss of consciousness and possible death in 20-30 minutes

    700-1000 PPM Rapid unconsciousness and death

    1000-2000 PPM Unconsciousness at once, death in minutes

Death may occur even if individual is removed to fresh air at once.

There are many ways to be alerted to the presence of H2S gas. Your nose is usually the first but unfortunately can be the last. You can smell as little as one part (ppm) of H2S in a million parts of air. If the concentration of gas is say, 100-200 ppm the sense of smell is lost very quickly, giving a false sense of security. The only way is to use gas detectors that will detect H2S gas.

Note: Some detectors do not read above 50 PPM.

Enclosed Vs. Open Flares

This Paper Considers The Benefits Offered Using The Two Types of Enclosed Flare System’s Versus an Open Flare SystemThe Open Flare System There is information out there that is being used as an efficiency of combustion of landfill gas on an open flare system of 98%. This is not correct for landfill gas. The test was with a flammable gas Propylene, ( C3H6 ) which is derived from petroleum hydrocarbon i.e. Propane Gas referencing (EPA 430-R-99-012 August1999) and in our opinion should not be used for landfill gas. It is almost impossible to read an open flare burning landfill gas. All open flares should be outlawed, as in the rest of the world and only used as a temporary flare for emergency use. It should be rated at maybe 5% not 98% efficient and should not be used for carbon credit trading, as it requires verification of emissions. This has all the hallmarks of pumping out methane gas that has not been burnt in great amounts with no control over the combustion process whatsoever. This type of system should never be used. It’s a hazard and a contributor to global warming and acid rain.

The Fixed Tip Enclosed Flare System Most of them are an open flare with a shroud around them. These are more efficient than an open flare and can be controlled to a point. The drawback is they have a fixed tip (exit orifice) so can only be set to one given flow rate. With this design type, the flow rate, once the parameters change, can be as bad as open flares. Any engineer will tell you that a fixed tip has to be sized for a given flow on this type of flare. They normally have a turn down 5-1 which means you have to manually turn a fixed tip off and on. The combustion efficiency of this type of flare will always be questionable because whenever the site condition changes, which it does all the time, it needs constant manual adjustments in order to comply with regulations and in order to be acceptable for carbon credit trading. In no way can this type of fixed tip flare system be 99% efficient. With no site parameters ever changing, it is impossible to achieve site condition without changes taking place all the time.

The Hilo Enclosed Sonic Flare System This flare does not use a fixed tip, it uses a variable tip which is controlled by PLC which adjusts to give optimum combustion at any flow rate and will self adjust to field conditions. It is proven to be the best flare system on the market today and is based on the ideal combustion process for landfill gas and does not rely on the design sizing of a fixed tip as in the above so called enclosed flare. This system is ideal for carbon credits because it can verify exhaust emissions greater than 99.9% at any flow rate and has a proven track record of doing so in more than just the landfill industry. Customers should ask any flare manufacture to prove that they can reach the percentages that they are saying on landfill gas and not reference information that does not have anything to do with landfill gas (as above when test are done using a different type of gas, in this case Propane, which has no relevance to landfill gas whatsoever).

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