What is it?

The biomethane is a gas with the same chemical-physical properties of the Natural Gas (NG). Both are CH4 rich gases (up to 99.0%). Differently from the NG which is recovered from non renewable fossil sources, the biomethane is recovered from the biogas which is produced from several renewable sources like agricultural products, zootechnical and agricultural wastes, FORSU and algae.

 

The Biogas has a content of CH4 in the range of 40-80% with the balance of CO2. By the GM upgrading process the CO2 is selectively removed from the Biogas producing the biomethane which can be utilized as a substitute of the NG.


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Why choose GM

GM technology allows a total selective recovery of the methane and CO2 contained in the biogas feed. The GM upgrading units are very flexible to treat biogas produced by difference sources. The ultimate goal is achieved by working closely with the client.
GM has a strong organization and structure to follow the client from the feasibility study to the plant management.
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How does it work?

The Biogas, after a pre-treatment to remove incidental contaminants like H2S, NH3 and Siloxane, is compressed to the absorption pressure of about 15 bar (g) for the selective removal of the CO2.The process is based on the chemical absorption of the CO2 by a countercurrent washing of the biogas with a water solution of Potassium Carbonate in an absorption column filled with metal packing.The biogas depurated from the CO2 is recovered from the top of the absorption column at 15 bar (g) as biomethane with a minimum CO2 content of 1% by vol (typically 2.0%) to be further processed (cooling, drying, compression

 

and odorization) for either grid injection or use as vehicle fuel (NG substitute).The non-condensing gases if contained in the biogas like O2 and N2 are collected in the biomethane which after the final cooling/drying can achieve a typical CH4 content of 98.0-99.0%.The pressure of 15 bar (g) of the biomethane at the battery limits is a benefit to reduce the power for a further compression required by the use of the biomethane as vehicle biofuel.The CO2 enriched Potassium Carbonate solution, leaving the bottom of the absorption tower, feeds a tank where, by reducing the pressure, the small amount of CH4 dissolved into

 

the solution for solubility is released by flash and recycled to the compressor suction.The absorbing solution is finally fed to a regeneration column where the absorbed CO2 is released by stripping with steam produced by reboiling the solution with an external source of heat. The CO2 is recovered from the top of the regeneration column at high purity (>99.90% dry basis) and, after cooling, can be vented without further treatment or liquefied and re-utilized as raw material for multiple uses (like dry ice, soft drinks and other).



ENERGY CONSUMPTION
0,15 - 0,2 kWh/Nm3 of biogas
NO THERMA
ENERGY COST
CO2 in the biomethane
0,5 - 1%
RESIDUAL METHANE IN THE OFF-GAS.
0,06% (no post-treatement required)
METHANE CAPTURE
>99,95%

Process scheme

  1. Biogas
    • Pressure: any
    • CO2: 20 - 60 %
    • CH4: 80 - 40 %
    • Other gases: as per grid injection
      regulations/fuel
      standards.
  2. Dried biomethane
    • Pressure: 4 - 15 bar(g)
    • CO2: 0,5 - 1 %
    • CH4: balance
    • Other gases: as per grid injection
      regulations/fuel
      standards.
    • Dew point: -5°C@ 70bar(g)
  3. Off-Gas
    • Pressure: 0 bar(g)
    • CO2: 99,94%
    • CH4: 0,06%
    • H2S: as per grid injection
      regulations/fuel
      standards.
1

Technology

GM has selected a simple and safe CO2 removal process wide referenced in the world in several different applications. Simple because is utilizing well know and reliable equipments successfully proven in the industry since many years. Safe because free from utilisation of hazardous products and componentsThe up to date technologies and know-how are applied during the activities of designing, construction and operation of the unit.
2

Costs

The maintenance cost is reduced at the minimum thanks to the selection of top quality equipments and to the adoption of first class engineering standard. The operating cost is at the lowest range compared to the competitors technologies available on the market.
3

Efficiency

The efficiency, in term of amount of CH4 recovered from the Biogas, is extremely high overcoming the 99.90%. The final purity of the biomethane from the upgrading unit is very high and, depending on the characteristics of the biogas, a CH4 content of 98.0-99.0 is easily achieved.
4

Recovered CO2

By implementing the optional CO2 recovery unit supplied by GM, the totally of the CO2 removed from the biogas is available at high purity for food industry like dry ice production, soft drinks and breweries or other industrial application like cryogenics uses and greenhouse or algae growing.
Comparing technologies
“Some say that the optimal biogas upgrade technology has yet to be discovered....”
We disagree.
CHEMICAL
ABSORPTION

The solutions of Amines or K2CO3 utilised in Chemical Absorption are the most efficient and generate the purest Biomethane with high heating values.

Due to the low solubility of methane in the absorption solutions, the processes do not require any post- treatment of the Off-gas, as the loss of Biomethane in the process is negligible.

K2CO3 solution additionally is more stable (no degradation of the solution during the process) than an Amines solution and not dangerous to manipulate. K2CO3 also requires less regeneration heat and allows for a simpler more compact plant layout.
PHYSICAL
ABSORPTION

Physical absorption uses either Pressurized Water Scrubbing (PWS) or organic solvents.

Water scrubbing when compared to chemical scrubbing absorbs less CO2 and requires larger columns with greater liquid circulation.

The Methane recovery is also lower, as Methane partially dissolves in water, therefore the Off-gas requires post- treatment by thermal or catalytic oxidation before it can be released into the atmosphere.
MEMBRANE
SEPARATION

Membrane separation exploits the different molecular size of each component of Biogas to separate each other by a semi- permeable membrane.

As the separation of the CO2 from the raw Biogas is only partial through each pass, the scrubbing process requires several stages of membranes to achieve an acceptable separation, increasing investment and operating costs.

Membranes are very sensitive to impurities contained in the raw Biogas and periodical or emergency replacements are required during the normal operation.
PRESSURE SWING
ADSORPTION

In Pressure Swing Adsorption (PSA) CO2 is adsorbed in vessels filled with a molecular sieve made-up of activated carbon or zeolites.

The adsorption takes place at an elevated pressure while the saturated vessels are regenerated by cyclical depressurization.

The Off-gas however requires a post-treatment to oxidize the significant residual methane content prior to it being released to the atmosphere.
CONCLUSIONS:
The high CO2 absorption rate and minimal Methane loss of an Amines chemical scrub solution.


The environmentally friendly features of a PWS.


The limited unit footprint of a Membrane Separation process.


No Off-gas post-treatment.

The base configuration of the GM upgrading unit can be expanded by adding optional units according
to the clients or to the market requirements.

Removal of contaminants from the biogas

The biogas from the anaerobic digestion can be contamined by differents impurities like hydrogen sulfhide (H2S), ammonia (NH3) and siloxanes (R2SiO)n, depending on the quality of the feed to the digesters and on the applied digestion technology.GM can offer suitable technologies for the removal of the contaminants in order to meet the following targets:ul>
  • Reliable operation of the GM upgrading unit;
  • iBiomethane specification according to the required standards;
  • Food grade for the recovered CO2 (if required);
  • Gases emission according to the local regulations;
  • Drying of the
    biomethane

    The biomethane from the GM upgrading unit is saturated by water at about 10°C.GM can offer a suitable drying unit to achieve the biomethane dew point required by the final users standards.

    Heat recovery 

    As the hot off-gas (CO2 + H2O) from the CO2 stripping column shall be to some extent cooled, the available heat can be advantageously recovered for other services like an integration to the heating for the anaerobic digestion or to the building heating system.GM can offer a package heat recovery unit customized according to the clients requirements.

    Recovery of the CO2
     

    The high purity CO2 from the stripping column of the GM upgrading unit, instead to be vented as normally is done with the upgrading competitors processes, can be liquified and sold in the market at food grade quality.
    GM can offer a package CO2 recovery unit customized according to the clients requirements.
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