Biogas hydrogen sulfide scrubbing project
For a farm-sized system, probably the best thing to use is a pair of old 200litre/44gallon drums. They will corrode fairly quickly and have to be replaced, but that is cheaper than the hassle of keeping plastic drums from melting in the heat.
Fill the drums with steel turnings or bashed up rusty tin cans or anything that is iron or steel. The easiest way to do that is to chop out the 3/4" bung in the top and weld in a 4" or larger pipe fitting. The 2" bung on the other side should have a length of the largest size of steel pipe that you can fit inside a 2" pipe nipple, and that goes nearly to the bottom of the drum and that is the gas inlet from the gas bag. The new and larger bung should be the gas outlet from the top of the drum with a 2" reduction to plastic pipe, with which everything is connected. The wet biogas will flow slowly through the drum and the metalic iron and the rust will react with the Hydrogen sulphide to make ferrous sulphide. You should have two of these drums made up and rotate them at suitable intervals to make sure that no H2S gas is getting through the system, or your intermediate gas compressor for Biogas CO2 Scrubbing will not last for very long.
To regenerate the sulphide stripper all you have to do is to unscrew the 4" bung on the used drum and let the air into the drum, as slowly as possible. The best way is to let the air just diffuse into the drum through the open bung over several days. The air will react with the iron sulphide to produce flowers of sulphur, metallic iron again, and lots of heat!!! Enough heat to melt a plastic drum and to also create a rising current of air through the drum to speed up the reaction should it get away on you. When you swap the drums, the process will cycle again until the drum is half full of elemental sulphur and is so corroded from the inside out, that it's time to replace the drum.
For home-size systems, a easy solution is to put a section of 4" PVC about a foot long in the line. Fill this with steel wool, the type used for scrubbing pots. This will rust away similar to the metal above. It is very efficient because of the high amount of surface area presented by the steel wool. Threaded connections make it easy to replace the wool. The easiest way to determine when to replace the wool is when you can smell the hydrogen sulfide at the burners.
[edit] References and resources
- Taylor, John Poe , Removal of Hydrogen Sulfide from Biogas, August 2003. This thesis offers good general information about the chemistry of scrubbing H2S.
- Subbukrishna, DN, et al, Hydrogen Sulfide Removal from Biogas by Iset Process, 2006. One could deduce a small-scale process from this article, but it is not written for that purpose. Nevertheless it has good information.
