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What is biomethane (BM)?

Reactores biometano Esquema biometano

Biomethane —also known as renewable natural gas (RNG) or Sustainable Natural Gas (SNG)— is the methane produced from biomass. It has similar properties to natural gas. Biomethane is renewable, because it is obtained from renewable organic matter (biomass), following treatment of the biogas obtained in the anaerobic digestion process.

During the anaerobic digestion processes, the organic matter is broken down by bacteria in the absence of oxygen to produce biogas and a sludge containing most of the inorganic substances (nitrogen, phosphorus, etc.), together with other non-biodegradable compounds.

Biogas is generated at municipal solid waste (MSW) landfills, as a result of breakdown of the accumulated organic matter. It can also be produced in reactors or digesters that process biomasses with a high organic matter content: sludge from wastewater treatment plants (WWTPs), waste from arable and livestock waste, etc.

Biogas is a mixture of gases, containing 40–70 % methane (CH4), 30–40 % CO2, 2–8 % of H2O and traces of other compounds such as SH2, O2, N2, NH3, siloxane and particulates. It has to be treated using a cleaning process and upgraded to obtain BM.

Cleaning and upgrading

The cleaning process involves separating off the H2O, SH2, siloxane, etc. The “upgrading” or enrichment process separates off the CO2 to increase the methane proportion to levels similar to that of natural gas of fossil origin.

Esquema biometano

Several different technologies can be used to upgrade biogas: water washing, chemical absorption, pressure swing adsorption (PSA), membrane separation, cryogenic processes, etc. Each has its own different capacities and associated costs. Upgrading processes are still expensive and work on their technological development continues.

Uses of biomethane

Vehiculo GLP Vehiculo GLP

Traditionally, biogas has been used for energy purposes by transforming it into electricity and heat in reciprocating engines. However, a number of factors, including a fall in the profitability of biogas valorisation plants, the development of upgrading technologies and the opportunities of new transport fuels, have increased interest in new ways of harnessing biogas, such as by transforming it into biomethane, where it can be used as Vehicular Natural Gas (VNG) in suitably fitted-out vehicles or injected into the natural gas network, which requires additional processes such as compression, quality control, etc.

Biomethane, used either as VNG or for injection into the gas network, must meet certain requirements. The standard for these uses is UNE-EN 16723 (Natural gas and biomethane for use in transport and biomethane for injection in the natural gas network, which consists of two parts:

Part 1: UNE-EN 16723-1

Specifications for biomethane for injection in the natural gas network.

Part 2: UNE-EN 16723-2

Automotive fuels specification.

Some statistics

The following figures show that there a significant margin of development¡ for biomethane in Europe.

Number of biogas plants in Europe in 2015

Número de plantas de biogás en Europa en 2015

Source: EBA (European Biogas Association)

Evolution of the number of biogas plants in Europe

Evolución del número de plantas de biogás en Europa

Source: EBA (European Biogas Association)

Evolution of the number of biomethane plants in Europe

Evolución del número de plantas de biometano en Europa

Source: EBA (European Biogas Association)

Bio-SNG (Bio-Synthetic Natural Gas)

Bio-SNG has the same uses as biomethane, since both are very similar in composition to natural gas.

Essentially, the difference lies in the form of production; unlike biomethane, which us produced by upgrading the biogas from the biochemical anaerobic digestion process, Bio-SNG is obtained through methanation of the syngas (mostly CO and H2) produced through gasification of biomass.

Esquema bio-sng

Gasification is a thermochemical process whereby biomass is partially oxidised, to convert it into what is called syngas. This syngas is mainly composed of CO and H2, although it can also contain CO2, CH4, and also N2 if the gasification has been performed with air instead of oxygen.

Methanation is normally a reaction whereby the syngas is catalytic converted into methane and water: CO+3H2 → CH4+H2O,

The gasification and methanation technologies for this application are still under development and demonstration.

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