che_july-2024 - 24

Cover Story
Part 2
Regenerative H2S-Removal
Technologies for
Biogas Upgrading
Advances in H2S-removal technologies for biogas upgrading are essential for mitigating
environmental issues like air pollution and corrosion
Christopher Ristevski and
Rosanna Kronfli
Macrotek Inc.
B
iogas, a renewable energy
source derived from
organic waste and typically
comprised mainly
of methane, holds great promise
for sustainable energy production.
However, the presence of hydrogen
sulfide (H2S) in biogas poses
challenges to its utilization due to
its corrosive and toxic nature. Effective
removal of H2S is crucial for
upgrading biogas to meet quality
standards for various applications,
including power generation and injection
into natural gas pipelines.
This article explores the different
types of H2S removal technologies
for biogas upgrading, with a focus on
the advantages of regenerative technologies
in addressing the unique
challenges of H2S removal.
Biogas produced from anaerobic
digestion processes often contains
impurities, with H2S being a predominant
contaminant. While H2S is naturally
occurring, its presence in biogas
streams can lead to equipment corrosion,
air pollution and health hazards.
Therefore, upgrading biogas to
remove H2S is essential for ensuring
safety, protecting infrastructure
integrity and complying with regulatory
requirements. Moreover, purified
biogas can be utilized efficiently in
various applications, contributing to
more sustainable energy practices
and reducing reliance on fossil fuels.
H2S removal technologies
Several technologies are available for
removing H2S from biogas streams,
each with its own unique mechanisms
and advantages.
Chemical
scrubbing.
Chemical
FIGURE 1. In a liquid redox system, H2S is converted to elemental sulfur
using a chelated iron catalyst, which is continually regenerated and reused
in the system
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scrubbing involves removing H2S by
absorption and neutralization.
A base,
such as sodium hydroxide
or potassium
hydroxide, is
metered into the
scrubbing liquid and
reacts with the H2S
to form salts. Often,
an oxidizing agent,
such as sodium hypochlorite,
is added
to convert unstable
sulfite salts to stable
sulfate salts. Chemical
scrubbing is not
selective to H2S,
and CO2 interference
can result in increased
chemical consumption.
While effective, chemical scrubbing
requires significant chemical consumption,
especially if CO2 is present.
It also generates large volumes
of wastewater, which increases
operating costs.
Biological desulfurization. Biological
desulfurization utilizes sulfur-oxidizing
microorganisms to
convert H2S into elemental sulfur
or sulfuric acid. This regenerative
process occurs in bioreactors or
biofilters, offering a sustainable and
environmentally friendly approach
to H2S removal. However, biological
desulfurization may be limited by
process kinetics and sensitivity to
operating conditions.
Adsorption. Adsorption
technologies
involve the physical adsorption
of H2S onto porous media, such
as activated carbon or zeolites. Adsorption
offers versatility and can
achieve high removal efficiencies,
but the spent adsorbents require
regeneration or disposal, adding to
operational costs.
Scavengers. Scavengers are chemicals,
such as metal oxides or triazines,
that irreversibly react with H2S. For
both adsorption and scavengers, a
standby unit is often installed in order
to replace the spent media without
interrupting the process. While effective,
scavengers generate waste
products that require disposal, contributing
to environmental concerns
and operational costs.
Liquid redox. Liquid redox systems
(Figure 1) utilize a catalyst,
such as chelated iron, to convert
H2S into solid sulfur. The chelated
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