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P.T.C. System



Example for purification of a biogas and obtaining pure biomethane

This innovative "One-pot" process consists in capturing gaseous pollutants in a physicochemical treatment whose liquid effluents are subsequently digested by the aerobic bio-purification process in the treatment plant.

(A)    Simultaneous process of absorption (capture) and organic chemical modification of Volatile Organic Compounds (organic and inorganic).
This operation is carried out in a single operation on a collection installation by physicochemical washing.

(B)  The final destruction of the capture products after the simultaneous process of absorption and chemical modification
This ultimate operation (B) is carried out in a biological treatment plant. The organic compounds present and formed During the condensation reaction are digested by the process of natural aerobic bio-purification of the purification plant.

The originality of the process resides firstly in the choice of the reagent which combines with the pollutants to be treated and secondly in the final natural destruction in the wastewater treatment plant that does not generate any new gaseous pollution.

The example below describes the treatment of 100 m3 of biogas from the methanisation of sludge from an urban sewage treatment plant.

The 100 m3 of biogas consists of:

  • 70 m3 of pure methane (biomethane) (3.123 mol)
  • 30 m3 (about 60 kg) of carbon dioxide (CO2) to be removed and recovered (1,338 moles)
  • 0.4 kg of hydrogen sulphide (H2S) to be removed (11.76 moles)
  • Possibly traces of siloxanes

The amounts of base and reagent are calculated on the basis of Spreadsheet and load sheets .

Equipment used :

  • Absorption column 800 liters (column + tank), Height: 3 m,
    base area: 0.28 m2 (diameter 60 cm), plate lining,
    mist eliminator at the air outlet.
  • Fluid circulation pump adjustable from 0 to 25 m3/H.
  • Adjustable polluting effluent feed valve from 0 to 1000 m3/H.

Operating load for a batch operation:

  • Water : 300 L.
  • 30% soda: 390 kg
  • Pure reagent : 7.3 kg or 18,4 kg of solution à 40%

Matériel type

The operation is carried out over a period of 4 hours with a gas flow rate of 25 m 3 / h.
The end of the reaction is controlled by pH-metry (about 8.5), and the absence of H2S verified with lead acetate paper.
The reaction medium is odorless and the suspension of CO 3 Na 2 (carbonate of soda) is separated on a centrifuge.

The capture and transformation of the 30 m3 of CO2 present in the biogas yielded in principle 142 kg of sodium carbonate (Na2CO3). On this theoretical weight, 100 kg are obtained after drying (70%), which can be used, for example, in cement plants in the cement manufacturing cycle:

Ca(OH)2 + Na2CO3 = CaCO3 + 2 Na+ + 2 OH- (See note *)

The remaining 30% soluble in the colorless and odorless reaction medium rejoin the wastewater treatment system of the
STEP before being discharged into the natural environment. This release helps compensate for the pH of surface waters
in the oceans, which has been declining for years, from 8.25 to 8.14, and due to the increase in anthropogenic
CO2 emissions in the atmosphere.

* Note: At the end of the reaction, sodium carbonate may also be advantageously displaced by calcium chloride
in order to obtain practically insoluble calcium carbonate according to the reaction:

Na2CO3 + CaCl2 --> CaCO3 + 2 NaCl

If it is not desired to recover the carbonate, the soda will be replaced by the potash. The carbonate of potash is very
soluble in the reaction medium, and will not precipitate, as carbonate of soda has done.

The clear, colorless and odorless reaction medium is evacuated to the all-water pit to be subjected to the aerobic
bio-purification process of the purification plant.

The flux calculation worksheet to determine the quantities of reagents:

The calculation of the operating costs for evaluating the economic potential of the process:

In this test, it is clear that for the destruction of H2S, the treatment with bleach is three times more expensive than treatment with GASWASH system.
Treatments with ClO2 or H2O2 are even more expensive: 10 times more expensive.

This example of biogas treatment includes:

  • Complete elimination of H2S.
  • Decarbonation (complete elimination of CO2).
  • The lowering of 4% water methanization at 40 ° C. (1 to 1.5%)
  • The total removal of siloxanes and organochlorines or fluorinated if present in trace amounts.

In this example, GASWASH system was used to purify 100 m3 of biogas which supplied 70 m3 of pure biomethane but
also to capture and remove 30 m3 of carbon dioxide and to eliminate 0.4 kg of H2S.

See also the examples of the DAVID Process- Odors

Report on the 100 m3 of biogas treated with GASWASH process

The 100 m3 of biogas produced from sludge from STEP avoided the production and release into the atmosphere of
230 kg of CO2 responsible for global warming during methanisation.

Out of these 100 m3 of biogas, the capture of the 30 m3 of CO2 present after sludge methanisation provides between 142
and 100 kg (70%) of sodium carbonate (Na2CO3) with the complete elimination of H2S (Hydrogen sulphide) and other
traces of volatile compounds.

The 30% soluble in the reaction medium rejoin the sewage system before being released into the natural environment.

This release helps compensate for the pH of surface waters in the oceans, which has been declining for years,
from 8.25 to 8.14, and due to the increase in anthropogenic CO2 emissions in the atmosphere.

This capture of 30 m3 of CO2 by the GASWASH system avoided the release into the atmosphere of 60 kg of additional CO2 responsible for global warming.

The 70 m3 of methane of purity greater than 98.5% (biomethane) obtained can be used as an energy source or
as a precursor for the manufacture of biohydrogen, for example with the VABHYOGAZ process.

Cost of pure reagents for GASWASH treatment:

  • Treatment of 100 m3 of biogas: 0,224 € / m3 or for 70 m3 of pure biomethane: 0,32 € / m3
  • For the 30 m3 of CO2 present in the biogas and disposed of: 0.75 € / m3

Energy equivalence



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