H 2 S aq
ð Þ Ð H
þ
þ HS
À
ð9:4Þ
HS
À
Ð H
þ
þ S
À2
ð9:5Þ
9.2.2.1 Physical Absorption
Physical absorption process is the most common and older process used for removal
of H 2 S from biogas stream. In this process, no chemical solution is used.
Physical absorption is mainly done by water and organic solvents such as cold
formaldehyde propylene carbonate and methylpyrrolidone. The process is very
environment friendly and could meet the demand of H 2 S removal from high
concentration H 2 S containing biogas. For this reason, we have to move forward to
chemical absorption process (Lin et al. 2013; Awe et al. 2017).
9.2.2.2 Chemical Absorption
Chemical absorption is used effectively for the removal of high-concentration H 2 S at
larger scale biogas plants. In chemical-based absorption system, H 2 S is absorbed by
various aqueous solution such as amines solution, chelated iron solutions, nitrite
solutions, alkaline salt solutions and caustic solutions (Zicari 2003; Horikawa et al.
2004).
9.2.2.2.1 Amine Solution
Different amine solutions such as monoethanolamine (MEA), diethanolamine
(DEA) and methyldiethanolamine (MDEA) are used for chemical absorption of
H 2 S. The amines are water soluble which lead to the efficient absorption of acid
gas as it has one hydroxyl group and one amino group and has the capability to
remove H 2 S by absorption. After absorption, amine dissolves H2S in an aqueous
amine (Siefers et al. 2010; Belmankhout et al. 2009).
RNH 2 þ H 2 S Ð RNH
þ
3 þ SH
À
ð9:6Þ
9.2.2.2.2 Chelated Iron Solutions
In this process, chemical absorption of H 2 S into iron-chelated solutions takes place
with the formation of elemental sulphur as a by-product. This method is advantageous over others due to high H 2 S removal efficiency and low use of chemicals for
the reason that the iron-chelated solutions function as a pseudo-catalyst that can be
regenerated (Siefers et al. 2010; Awe et al. 2017). The major advantage of using
iron-chelated solution is that it can remove H 2 S from the biogas stream with the
formation of H 2 S into a more stable or valuable product, as do processes that
transform H 2 S into S. Iron-chelating-based process comprises of physical absorption
of H 2 S onto the water undergoing dissociation which is explained in Eq. (9.5).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
243
ð Þ Ð H
þ
þ HS
À
ð9:4Þ
HS
À
Ð H
þ
þ S
À2
ð9:5Þ
9.2.2.1 Physical Absorption
Physical absorption process is the most common and older process used for removal
of H 2 S from biogas stream. In this process, no chemical solution is used.
Physical absorption is mainly done by water and organic solvents such as cold
formaldehyde propylene carbonate and methylpyrrolidone. The process is very
environment friendly and could meet the demand of H 2 S removal from high
concentration H 2 S containing biogas. For this reason, we have to move forward to
chemical absorption process (Lin et al. 2013; Awe et al. 2017).
9.2.2.2 Chemical Absorption
Chemical absorption is used effectively for the removal of high-concentration H 2 S at
larger scale biogas plants. In chemical-based absorption system, H 2 S is absorbed by
various aqueous solution such as amines solution, chelated iron solutions, nitrite
solutions, alkaline salt solutions and caustic solutions (Zicari 2003; Horikawa et al.
2004).
9.2.2.2.1 Amine Solution
Different amine solutions such as monoethanolamine (MEA), diethanolamine
(DEA) and methyldiethanolamine (MDEA) are used for chemical absorption of
H 2 S. The amines are water soluble which lead to the efficient absorption of acid
gas as it has one hydroxyl group and one amino group and has the capability to
remove H 2 S by absorption. After absorption, amine dissolves H2S in an aqueous
amine (Siefers et al. 2010; Belmankhout et al. 2009).
RNH 2 þ H 2 S Ð RNH
þ
3 þ SH
À
ð9:6Þ
9.2.2.2.2 Chelated Iron Solutions
In this process, chemical absorption of H 2 S into iron-chelated solutions takes place
with the formation of elemental sulphur as a by-product. This method is advantageous over others due to high H 2 S removal efficiency and low use of chemicals for
the reason that the iron-chelated solutions function as a pseudo-catalyst that can be
regenerated (Siefers et al. 2010; Awe et al. 2017). The major advantage of using
iron-chelated solution is that it can remove H 2 S from the biogas stream with the
formation of H 2 S into a more stable or valuable product, as do processes that
transform H 2 S into S. Iron-chelating-based process comprises of physical absorption
of H 2 S onto the water undergoing dissociation which is explained in Eq. (9.5).
9 Recent Trends in Biogas Upgrading Technologies for Biomethane Production
243