Hydrogen Sulphide: How Nature Uses it and Dredges Get Rid of it

Bison grazing near Mud Volcano, Yellowstone National Park, Wyoming, USA

Yellowstone National Park is not only renowned for its active geology. There is also an abundant wildlife, roaming free over the area. One would guess, the local wildlife would be accustomed to these interesting geological features. Or would be disturbed by all the tourist gathering at the literal hotspots in the park. Well, you might be surprised. At times, e.g. bison tend to flock around the same hydrothermal features that attract the tourists. You don’t notice it from the pictures, but when you are there, you’ll smell it instantly: rotten eggs. As a dredger, you know what this means: danger!

The odour of rotten eggs is caused by the toxic gas hydrogen sulphide1. At Yellowstone National Park, this gas is released by the fumaroles, mudpots  and geysers2. The bison might not be aware about the danger of inhaling this vapour. But they do know, that all the bugs and parasites, that live on their skin have a lower resistance for the toxin and fall of their hosts. Well, sometimes the bison fall for their own trap and get intoxicated themselves3. That is one of the dangers of hydrogen sulphide, above a certain a threshold, your senses get numbed and you don’t recognise the danger anymore.

Gas bubbles expanding in the vacuum of the dredge pump

In dredging operations, hydrogen sulphide usually has a biological origin, rather than a geological. In seasonally warm water, algae bloom near the end of the summer and die some time later. The decomposing biomass can release hydrogen sulphide amongst methane and carbon dioxide. Quietly trapped in bubbles between the grains in the sediment, they get disturbed by the dredge and enter the suction line. It is only in the dredge pump, that these bubbles get expanded and wreak havoc to the dredging process.

Gas removal concept before the dredge pump

The trick is to remove the gas bubbles before they enter the dredge pump. Several systems do exist but usually the inspection piece is modified and separates mixture and gas. It is up to the rest of the system what happens with the foam that gets extracted. It might be blown overboard or properly re-handled responsibly to protect the environment. Either way, the dredge pump will be able to operate at a more continuous load, the mixture density increase and total production will be higher.

Example of production increase in relation to gas removal rate

A good write-up about the dangers of hydrogen sulphide in dredging4 can be found at ‘The Art of Dredging’. There is also an article about the application of degassing systems to lower the dangers of hydrogen sulphide5. Even as the vapour is released at a single location on the ship, you still have to be aware of what you are doing and operations have to be adapted accordingly.

My personal experience with this nasty gas is only limited to commissioning degassing systems6, not actually working with them over longer periods. Even so, I got my impressions of life aboard under these circumstances. There was one occasion, where just in the week before our commissioning of our delivered degassing system, there was a severe accident. During our commissioning trials we had several warning alarms and whenever we went outside we had to wear personal gas detectors. If you did not report within an agreed period, alarms would ring and a search party dispatched. So, I am happy for every degassing installation delivered. It saves lives and fuel and cares about the environment.

Example of a standard degassing installation from Damen Dredging Equipment

References

  1. Hydrogen sulfide; Wikipedia
  2. Mudpots at Yellowstone National Park; US NPS
  3. Poison gas kills five bison in Yellowstone; Bozeman Daily Chronicle
  4. H2S (hydrogen sulphide); The Art of Dredging
  5. Degassing systems for dummies; The Art of Dredging
  6. Retrofit degassing lifts dredger efficiency; Maritime Journal

See also