Biomethane
Biomethane is defined by ANP Resolution No. 8/2015 as a gaseous biofuel constituted essentially by methane, derived from the purification of biogas. Biogas, in its turn, is defined as the raw gas obtained from the biological decomposition of organic products or wastes. The main components of biogas are methane and carbon dioxide (CO2).
In the BioValue project the main interest is on biomethane from biogas obtained from the anaerobic biodigestion of organic waste in biorefineries. However, biogas can also be produced in landfills and sewage treatment plants, as well as in digesters operating with animal wastes.
Specifications and equivalence with natural gas (NG)
Biomethane specifications in Brazil are regulated by the National Agency of Petroleum, Natural Gas and Biofuels (ANP). A Resolution ANP No. 8/2015 establishes the specifications for biomethane from products and organic waste from agroforestry and commercial activities. For biomethane from landfills and sewage treatment plants, the specifications are established by ANP Resolution No. 685/2017. However, ANP Resolution No. 685/2017 changes the specifications table for biomethane from agroforestry and commercial organic products and waste, and therefore its specifications can also be found in ANP Resolution No. 685/2017.
The biomethane regulated by ANP resolutions is intended for vehicular use (CNG) and in residential, commercial, industrial and power generation facilities, being also allowed its mixture with natural gas (NG). The equivalence of biomethane with natural gas is corroborated by ANP Resolution No. 8/2015 that treats biomethane analogously to natural gas, being interchangeable with natural gas delivered to distribution in the Northeast, Midwest, South and Southeast Regions.
The specifications for biomethane (ANP Resolution No. 685/2017) and natural gas (ANP Resolution No. 16/2008) are identical for many characteristics, such as: higher calorific value, Wobbe index, maximum contents of CO2, sulfur and H2S, and water and hydrocarbon dew points. The main differences are in the contents of methane, ethane, propane, butanes, and other heavier hydrocarbons. For example, in biomethane the minimum methane content is a mole fraction of 90% for any region in the country, while for natural gas this content is a minimum of 68% in the North Region and a minimum of 85% in the Northeast, Midwest, South and Southeast Regions. In the case of ethane, propane, butanes and heavier hydrocarbons, there are specifications for natural gas; in the case of biomethane, these levels need only to be determined if there is an addition of NG, propane or LPG to the biomethane.
Transforming biogas to biomethane
The upgrading of biogas to biomethane is carried out with a sequence of separation processes for the removal of H2S, CO2 and water, among other contaminants.
In general, the removal of H2S, since it is corrosive, is the first step of the upgrading process. Some technologies that can be employed for H2S removal are: precipitation, adsorption, absorption, microaeration, membrane separation, and oxidation. Microaeration employs bacterias capable of oxidizing H2S in the presence of O2. However, the injection of O2 can lead to the need for its subsequent removal, because the O2 content is specified at a maximum mol fraction of 0.8%, according to ANP Resolution No. 685/2017. Technologies to avoid this limitation combine biological desulfurization with a pre-washing with NaOH to absorb the H2S before the microbiological process. In the final polishing, sulfur precipitation can be employed.
Water removal is generally the second stage of biogas purification and is also important for the methane and CO2 separation process by the adsorption process (PSA). Moisture removal is generally done by adsorption, absorption, compression/refrigeration or condensation.
Finally, for the separation of methane and CO2, absorption processes with ethanolamines, membrane separation or adsorption (PSA) are employed. The PSA adsorption process has good energy competitiveness and can also remove contaminants other than CO2.
Commercialization forms
As biomethane is interchangeable with the natural gas delivered to distribution in the Northeast, Midwest, South and Southeast Regions, the processes of compression, distribution, pressure reduction and the forms of commercialization are the same adopted for NG. Biomethane can be transported through pipelines and in the form of compressed gas by means of transport vehicles suitable for this purpose. Biomethane can also be transported through a dedicated pipeline where there is only delivery of biomethane coming from landfills and sewage treatment plants to industrial consumers.
Marketed biomethane must also meet specifications for potentially corrosive components such as H2S, CO2 and water (water dew point) to preserve the integrity and safety of equipment and facilities in distribution, halogenated components to meet occupational health requirements, and siloxanes due to the formation of silicon oxides deposits on vehicle catalysts and silica films on burners. Biomethane must be odorized when it is injected into the distribution network. Biomethane transported as compressed gas must be odorized by the producer.
Fundamental Bibliographical References
Advances in biogas valorization and utilization systems: A comprehensive review