Environmental Engineering Reference
In-Depth Information
of hydrogen sulfide. Hydrogen sulfide removal from biogas can be accomplished
using different methods:
Absorption of H
2
S in an alkaline solution and subsequent microbial oxidation of
sulfide with molecular oxygen to (solid) elemental sulfur (S
0
) that can be recov-
ered. This process is called the
Thiopaques
process and is commercialized by
Paques, the Netherlands.
Reactive absorption of H
2
S in a chelated or acidic ferric iron (Fe
+3
) solution
under formation of elemental sulfur (S
0
) and ferrous iron (Fe
+2
). Sulfur can read-
ily be separated from the solution, and ferric iron can be regenerated by microbial
or chemical oxidation with oxygen.
Ferrous iron dosage to the digester in order to precipitate sulfide formed as iron
sulfide (FeS).
For effective use of biogas as transportation fuel or for introduction into the natural gas
network, biogas has to be enriched in methane. This is primarily achieved by carbon
dioxide removal, which increases the heating value of the gas to enable longer driving
distances with a fixed gas storage volume. Four different techniques for CO
2
removal
from biogas are commercially applied:
Absorption with water
Pressure swing adsorption (PSA)
Absorption with Selexol
TM
Chemical absorption with amines
Absorption of CO
2
with water or water scrubbing is the most commonly used
technique. The technique is based on the higher solubility of CO
2
in water
compared to CH
4
.
Biogas can be utilized directly or after upgrading to natural gas quality for electric-
ity and heat production in a CHP plant. Guidelines for conducting calculations on the
conversion of chemical redox energy available in methane combustion to electrical
energy production are shown in Figure 14.12.
Integration of material and energy balances in anaerobic digestion facilities is
another challenge for engineers involved in the implementation of anaerobic
digestion. Figure 14.13 shows an example scheme for an energy self-sufficient,
integrated waste processing plant based on anaerobic digestion, which uses the heat
produced in the CHP for upgrading (drying) of the solid and liquid fractions of
the digestate.
14.5 OUTLOOK AND DISCUSSION
One of the fundamental strong points of the anaerobic digestion process for sustain-
able energy production is its firm foundation on thermodynamic grounds. Methane is
the organic compound with the lowest energy content per electron, and therefore, all
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