Environmental Engineering Reference
In-Depth Information
mechanical structure is installed in the top of the digester for these bacteria
to grow on. A drawback with this technology is that if too much oxygen is
injected it will affect the digestion process negatively. Care also has to be
taken to ensure that explosive mixtures between methane and oxygen are
not formed. Depending on how the biogas is going to be utilised, traces of
oxygen and nitrogen (which will remain after the oxygen has been consumed
if air is injected) may be a problem if the gas is going to be upgraded since it
is hard to separate nitrogen from methane. The same principle that is used
for removal in the digester can also be used after the digester, using a
biological filter with bacteria that can oxidise hydrogen sulphide. In this
case, a column filled with large surface area carriers onto which bacteria can
grow are used and the biogas is cleaned by letting it pass through the
column. The sulphur that is formed remains in the liquid phase in the filter.
Physical removal
Hydrogen sulphide can be removed from raw biogas using absorption
technologies with water or organic solvents. One of the oldest methods for
hydrogen sulphide removal is sodium hydroxide scrubbing. Chemical
absorption can also be used and, in that case, the liquid medium is a
water solution containing NaOH, FeCl
2
or Fe(OH)
3
. Hydrogen sulphide
reacts and forms sulphide or sodium hydrogen sulphide. A caustic solution
has increased solubility compared with water. The liquid can be regenerated
with microorganisms that are able to biologically oxidise sodium sulphates.
Organic solvents that have greater solubility of hydrogen sulphide than
water can also be used. The liquid is regenerated by air stripping, which can
give elementary sulphur. Another alternative is to use a process in which the
biogas is cooled down so that liquid carbon dioxide is formed in which
hydrogen sulphide (but also other impurities) is dissolved.
Hydrogen sulphide can also be removed by adsorption using activated
carbon. To make the removal more efficient, the carbon can be dosed with
KI or H
2
SO
4
, which will catalyse between added air and the hydrogen
sulphide. If the raw biogas is going to be upgraded, hydrogen sulphide will
be removed together with carbon dioxide to some extent, but the level of
removal depends on the upgrading technology used. However, it is often
advantageous to remove it before the upgrading process. In a PSA
upgrading plant, hydrogen sulphide will be removed, but since it is
irreversibly bound to the activated carbon it is advantageous to remove it
before upgrading.
Search WWH ::
Custom Search