Biogas purification and methane-enrichment

Lise Appels1, Raf Dewil1, Jan Baeyens2,3

1 Department of Chemical Engineering, Katholieke Universiteit Leuven2 Department of Chemical Engineering, University of Birmingham

3 School of Engineering, University of Warwick

Introduction

• Biogas from anaerobic digestion• Composition:

– CH4 : 65-70%

– CO2 : 30-35%

– Water vapour

– Traces of H2S, oxidised sulphur compounds, H2, halogenated compounds (landfills), siloxanes,…

• Not useable as such for introduction in gas grid or as CNG

• Need to increase energy content and remove trace gases

Removal of impurities

• Required gas quality = function of application

• Purification methods– Traditional: scrubbing, pressure swing adsorption,

cryogenics– Gas membranes

Application H2S CO2 H2O TracesGas heater (boiler) < 1000 ppm no no yes (e.g. siloxanes)

CHP < 1000 ppm noavoid condensation yes (e.g. siloxanes)

Vehicle fuel yes yes yes yesGas Grid yes yes yes yes

Carbon dioxide removal

• Removing CO2 leads to:– Increased heating value– Consistent gas quality, similar to natural gas

• Removal options:– Absorption (scrubbing)– Pressure swing adsorption– Cryogenic separation– Membrane technology

Absorption

• Simultaneous removal of H2S and CO2

(polar comounds)

• Most common solvent = water• Efficiency = function of solubility

– Dependent on P, T, pH

• Pressure scrubbing

Absorption (2)

• Other adsorbants– Ca(OH)2 solution (formation of CaCO3 and CaS)

– Organic solvents:• Polyethyleneglycol (Selexol®, Genosorb ®)• Alkanol amines

• Low presure generation is possible here• Regeneration of organic solvent with steam

Pressure swing adsorption

• Adsorbents such as activated carbon and molecular sieves

• Selectivity: different mesh sizes• Adsorption: high pressure

Desorption: depressurisation

• Simple design & operation• Costly with high pressure drops & heat

requirements• Dry biogas is needed

Cryogenic separation

• Boiling point:– CH4 : -160°C

– CO2 : -79°C

• Removal of CO2 in liquid form by cooling biogas mixture at elevated pressure

• Expensive, only tested in pilot plants

• Recovery of CO2 is easily feasable

Membranes

• Transport of components through membrane driven by partial pressure and is dependent on the permeability of the component through the membrane

• Selectivity of silicone membraneComponent Selectivity

N2 ~O2 1

CO2 1.4

CH4 2.2

C2H6 6.0

C3H8 10.1

Membranes (2)

• High pressure is required (flux)• Some permeabilities:

• Some CH4 losses occur

Silicone 0.20

PEI/dehesive 0.18

PEI/PDMS 2.99

PVDF/dehesive 0.33

Economic evaluation

• Relative costs for 55m3/h digester biogas

Removal of water

• Mostly achieved by– Condensation

– Drying over silicagel or Al2O3 if low dew points need to be achieved

• Alternative– Absorption in glycol or hygroscopic salts

Removal of H2S

• Concentration in gas can be limited– Fe3+ addition– Activated carbon catalyst– Micro-organisms

• Desulphurisation of biogas– Addition of oxygen (safety!)– Biological removal by biofilm– NaOH scrubbing

Removal of siloxanes

• Adsorption on activated carbon(difficult to desorb)

• Other adsorbents (molecular sieves, silica gel, polymer pellets)

• Cryogenic condensation• Chemical abatement

– Caustic or acidic catalysed hydrolysis of Si-O bond

Biogas compression and storage

Pressure Storage device MaterialLow (0.14 - 0.41 bar)

Water sealed gas holder Steel

Low Gas bag Rubber, plastic, vinylMedium (1.05 - 1.97 bar)

Propane or butane tank Steel

High (200 bar)Commercial gas cylinders Alloy

Conclusions

• Biogas applicable as such in a limited number of beneficiation methods

• Purification is required for transportation/storage• Various purification methods are applicable• Mostly a combination is necessary because of

the myriad of pollutants present