Raw or unprocessed biogas contains
pollutants that are released both before and after combustion. The common components
of biogas are methane, CO2, O2, N2, H2, and H2S with smaller amounts of other
contaminants such as sulfur compounds, ammonia, and siloxanes. Burning unprocessed
biogas creates significant air pollution. The contaminants are explained below by
Wikipedia in terms of pre- and post-combustion dangers.
Sulfur compounds
“Toxic, corrosive and foul smelling hydrogen sulfide (H2S)
is the most common contaminant in biogas. If not separated, combustion will
produce sulfur dioxide (SO2) and sulfuric acid (H2SO4), which are corrosive and
environmentally hazardous, Other sulfur-containing compounds, such as thiols
may be present.
Ammonia
“Ammonia (NH3) is produced from organic compounds
containing nitrogen, such as the amino acids in proteins. If not separated from
the biogas, combustion results in NOx emissions.
Siloxanes
“In some cases, biogas contains siloxanes. They are
formed from the anaerobic decomposition of materials commonly found in soaps
and detergents. During combustion of biogas containing siloxanes, silicon is
released and can combine with free oxygen or other elements in the combustion
gas. Deposits are formed containing mostly silica (SiO2) or silicates (SixOy)
and can contain calcium, sulfur, zinc, phosphorus. Such white mineral deposits
accumulate to a surface thickness of several millimeters and must be removed by
chemical or mechanical means.
“Practical and cost-effective technologies to remove
siloxanes and other biogas contaminants are available.
Other important contaminants in biogas include volatile
organic compounds (VOCs) and terpenes. These five: sulfur compounds like H2S, siloxane,
ammonia, VOCs, and terpenes are the main biogas contaminants.
Anaerobic Digesters
Anaerobic digestion
is a good way to capture methane and fertilizer from waste. Agricultural waste,
food waste, and yard waste can be used as feedstock. Manure is often a
component of the feedstock and may be added to hasten the process. Anaerobic
digestors still produce air pollutants including carbon monoxide (CO), nitrogen
oxides (NOx), and sulfur dioxide (SO2). The resultant gas also produces more
formaldehyde than other fuel sources when burned. Other potential environmental
issues with anaerobic digesters (ADs) include the risk of spills, the need for significant
transport of wastes, and nuisance odors.
Anaerobic Digester. Source: Power Knot
Operators of AD
facilities must acquire air quality permits before construction. These must
consider the current quality of local air to determine if additions will
approach or exceed pollutant limits. Dispersal of pollution must also be
considered which takes into account weather data such as prevailing winds.
Regulators acquire comparable data on AD facilities with stack tests that
measure emissions and pollutant level predictions for comparable facilities.
Biogas Processing and Improvements
A 2023 paper in Biogas
Production and Process Control Improvements addresses process control improvements
which refers to the whole of the biogas collection and generating process. The
authors state some basic data and facts about biogas and some interesting conclusions
about the state of biogas generation as a science. The abstract is below.
Abstract
“Biogas is a sustainable energy produced from
biodegradable organic matter through anaerobic digestion. Biogas mainly
contains methane (CH4) and carbon dioxide (CO2) and various contaminants, such
as water vapor (H2O), ammonia (NH3), hydrogen sulfide (H2S), methyl siloxanes
(MSs), nitrogen (N2), oxygen (O2), halogenated volatile organic compounds
(VOCs), carbon monoxide (CO) and hydrocarbons whose presence largely depends on
the source of biogas. The value of biogas is enhanced further by the production
of organic manure as substitutes for chemical fertilizers in crop production.
The vast range of applications makes biogas an attractive sustainable energy
choice. Bioenergy generation capacity, mainly from biogas globally, has shown
promising worth over non-renewable energy options; hence, its production has
expanded rapidly during the previous two decades, from 42.38 Exajoules/year in
2000 to about 57.74 Exajoules/year in 2023, an increase of about 36.24%. This
study presents the biogas production and potential improvement measures that
include factors influencing the process and quality of biogas produces such as
C/N (carbon nitrogen ratio, the pH, dilution of feedstock, composition and
nutritive value of feedstock, residence period, mixing and stirring, temperature,
presence of toxicants, loading time and redox conditions). The main barriers to
biogas production and use include lack of technical know-how, lack of subsidies
and incentives for biogas production and use. Access to appropriate and
affordable biogas technologies, the development of a comprehensive policy on
the construction and operation of bio-digesters as well as utilization of
biogas and sales of biogas energy products and services are important for an
increasing adoption of biogas.”
The company DESOTEC
offers mobile filtration as a way to process raw biogas so that it can be used
in some machinery. Their units “remove contaminants, such as siloxanes,
which are abrasive, and hydrogen sulfide (H₂S), which causes corrosion and the
acidification of engine oil.”
Upgrading raw
biogas to biomethane requires removal of VOCs, siloxanes, terpenes, H₂S, and Ammonia
(NH₃). Biomethane is processed to the point where it is 99% methane and can be
accepted by gas pipeline grid systems. According to DESOTEC:
“Various technologies are available to separate CH₄ from
CO₂ and other molecules in the gas stream. Membranes are one of these technologies,
but require protection from siloxanes and other components. DESOTEC’s mobile
activated carbon filtration is an effective pretreatment.”
“Scrubbers and bioscrubbers can also be used to remove
CO₂, but are less effective at treating H₂S and volatile organic compounds
(VOCs). When placed after these technologies as a polishing step, DESOTEC’s
sustainable mobile filtration solution ensures that the final product is pure.”
Siloxane removal
is one of the biggest challenges for biogas, particularly landfill biogas. Siloxanes
are “chemical structures containing silicon and oxygen bonds and are found
in various products like cosmetics and cleaning agents.” They can damage
equipment even in small amounts. New research published in the journal Environmental
Progress & Sustainable Energy shows that low-cost adsorbents composed of clinoptilolite
(a naturally occurring mineral) and biochar (a type of charcoal) can cost-effectively
remove siloxane gases from the biogas stream. Some tables, graphs, and the paper’s
conclusions and recommendations are shown below.
References:
Biogas.
Wikipedia. Biogas - Wikipedia
Six
Reasons Anaerobic Digesters Aren’t as Environmentally Friendly as You Think.
Power Knot. March 1, 2021. Six
Reasons Anaerobic Digesters Aren't as Environmentally Friendly as You Think -
Power Knot
Scientists
identify low-cost adsorbents for removing impurities from landfill gas. Science
X staff. January 6, 2025. Scientists
identify low-cost adsorbents for removing impurities from landfill gas
Experimental,
economic, and life cycle carbon footprint assessment of low‐cost adsorbents for siloxane removal
from landfill gas. Rarosue J. Amaraibi et al. Environmental Progress &
Sustainable Energy (2024). Experimental,
economic, and life cycle carbon footprint assessment of low‐cost
adsorbents for siloxane removal from landfill gas
Air
Quality Permitting For Anaerobic Digesters, Part 1. John Hinckley. BioCycle.
September 2017. Air
Quality Permitting For Anaerobic Digesters | BioCycle
Biogas
Production and Process Control Improvements. Moses Jeremiah Barasa Kabeyi,
Oludolapo Akanni Olanrewaju and Joseph Akpan. From Biomass to Biobased Products.
Edited by Eduardo Jacob-Lopes, Leila Queiroz Zepka and Rosangela Rodrigues Dias.
January 12, 2024. Biogas
Production and Process Control Improvements | IntechOpen
Biogas
and Renewable Energy. Desotec. Biogas
and Renewable Energy | Desotec
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