primary objective of biogas purification is to remove biogas components like H2S,
water vapour, NH3, particles, etc. These components not only cause
environmental hazard and processing problems but also dilute the energy density
of biogas. The major contaminants of biogas and their related removal
technologies are discussed as follows:
Major Contaminants and related Biogas Purification Technologies:
Biogas generated from anaerobic digestion is usually
saturated with water vapor. Water vapour may condense into water or ice and
thus result in corrosion and clogging issues. Most biogas utilization processes
require relatively dry gas, so removal of water vapor is required.
- Passive cooling:
Biogas pipe line is run though underground for a short period of time. Water
condenses from the biogas as it cool down. The condensate either discharges to
sewer or recycle back.
- Refrigeration and Pressurization:
Heat exchangers can be used to cool down the biogas so that the water vapour
gets condensed. Biogas can be further pressurized to dry it more.
- Absorption: Biogas can be passed
through drying medium like glycol, hygroscopic salts, silica gel, aluminum oxide
etc. to absorb water. These drying medium can be regenerated by drying them at
high temperature and sometime at high pressure as well. Eventually the drying
media has to be replaced.
H2S is a toxic and
corrosive gas and its concentration in raw biogas may vary based on the feedstock.
H2S in biogas has to be reduced to harmless level to protect the
downstream processes and equipments as well as toxicity to human health.
- Water scrubbing: Biogas is feed from
the opposite direction of water flow to create a solution of H2S in water.
The water can be regenerated and scrubbing water discharged can be
- Activated Carbon:
Biogas is fed through an activated carbon filter which removed sulphides
by adsorption. Activated carbon media can be regenerated.
- Iron Hydroxide or
is led through a media composed of woodchips and iron oxide or hydroxide.
H2S is removed as iron oxides react with sulfides (H2S) to produce iron
sulfide. Bed can be regenerated several times before requiring
- Biofiltration: Biofiltration
uses microbes living on a packed medium to remove sulfides. Sulfides in
the biogas get absorbed into a liquid film and are then metabolized by the
microbial cells. It is available as above grade packed towers or below
grade systems, filled with natural media like wood chips or peat moss.
A simplified Biogas Purification Diagram
can cause corrosion to the downstream process equipments. It can also form
nitrogen oxides (NOx) from the combustion of biogas.
Ammonia is soluble in
water, so it can be removed using the following two methods:
water vapour removal methods
dust and oil particles from the compressors may be present in the biogas.
Filter: The particles in the biogas
can be removed by using filter of 2 to 5μm size. These filters need to be
replaced in regular interval as part of maintenance.
presence of siloxanes in biogas cause abrasive siloxane deposits on equipments
and reduce their life significantly.
Silica gel: Silica gel has good
potential of removing of certain siloxane compounds from biogas. It has High
- Cooling: Siloxanes can be removed
from the cooled gas with condensation water.
- Activated carbon: Activated carbon removes siloxane from biogas
can cause corrosion to mechanical parts of the plant. They can also form
dioxins and furans during combustion of biogas.
Activated carbon can be used to remove halogenated hydrocarbons.
Digestion Guideline by the Ministry of Environment (MOE) of British
Columbia (BC), Canada.
Petersson, A. and
Wellinger, A., (2009); Biogas Upgrading Technologies –Developments and Innovations,
by IEA Bioenergy.
Processing for Utilities, American Biogas Council
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