Waste of the wastes, Part 2

Another waste of anaerobic digestion is hydrogen sulfide gas.  Hydrogen sulfide is also produced as a waste product in all types of gasification, as long as sulfur is present in the feedstocks used in the the process.  Hydrogen sulfide is very toxic and can be deadly in the parts per million range.  In order to produce a safe product from digestion or gasification the hydrogen sulfide must be removed.  Luckily numerous technologies exists to get rid of this harmful gas.  

One simple technique is to pass the sour gas (gas that has hydrogen sulfide in it) over iron shavings.  Iron reacts very quickly with hydrogen sulfide and removes a majority of the hydrogen sulfide from the gas.  The exiting clean gas is then considered sweet gas.  This process is known as a non regenerative process because the iron that reacts with the hydrogen sulfide becomes spent or useless after a certain amount of hydrogen sulfide has come into contact with it.  Other technologies follow this same type of process but use a different scavenger.  The scavenger, iron shavings in this case literally grab the hydrogen sulfide out of the passing gas and convert it into a non toxic form.  Other scavengers include iron oxides, and zinc oxides.  

A regenerative process that is typically used in the natural gas industry is an amine gas treating unit.  This process uses a mixture of water and amines (to be discussed later) to absorb the hydrogen sulfide from the passing sour gas stream.  The sweet gas is allowed to continue upward and on with processing while the water/amine/hydrogen sulfide solution falls and is regenerated.  Regeneration occurs when the temperature is increased and the pressure is dropped.  This can be described with a simple bottle of soda analogy.  When a bottle of soda is allowed to warm up and the cap is suddenly taken off (decreasing the pressure inside the bottle) a bunch of fizz is produced.  This fizz is simple carbon dioxide escaping from the liquid phase to the gas phase.  So back to amines.  Amines are simple compound that resembles ammonia.  Ammonia is a compound that contains one nitrogen atom bonded to three hydrogen atoms.  An amine is produced when at least one of these hydrogens bonded to the nitrogen is replaced with a larger hydrocarbon chain.  Amines have the great property of  being slightly basic.  When the hydrogen sulfide comes into contact with the water a slight acid is produced.  And when a acid and base come into contact they react.  A simple acid base reaction that may ring a bell is heartburn.  Acid from your stomach travels up your esophagus causing irritation.  Once you feel this pain you take some Tums.  Tums make this feeling go away (hopefully), because Tums are made up of basic compounds that react with the acid and neutralize it, or turn it into water.