Biogas Purification and Methane-Enrichment for Industrial / Vehicular use

SANJAY SHARMAPSA NITROGEN LIMITED

D-9/6 OKHLA INDUSTRIAL AREA PHASE-INEW DELHI 110020 INDIA

Email : psanitrogen@airtelmail.in

Introduction

• Biogas from anaerobic digesters

• Approximate Composition:– CH4 : 55-75%– CO2 : 25-45%– Water vapour : Fully Saturated– Traces of H2S : Upto 1%

• Not suitable without enrichment for use as Automobile Fuel or as CNG

• It is required to increase it’s Calorific Value and to remove trace of harmful gases

Removal of impurities• Required gas quality in terms of application

• Purification ( Enrichment)methods– Absorption(scrubbing)– Pressure Swing Adsorption ( PSA)– Cryogenics– Gas membranes

Application H2S CO2 H2O Traces

Gas heater (boiler) < 1000 ppm No No yes (e.g. siloxanes)

Vehicle fuel yes yes yes yes

Gas Grid yes yes yes yes

Carbon dioxide removal

• Removal of CO2 leads to:– Increased Calorific Value of the gas.

– Consistent gas quality, similar to natural gas.

• Removal options:– Absorption (scrubbing)

– Pressure swing adsorption ( PSA)

– Cryogenic separation

– Membrane technology

Absorption ( Scrubbing)

• Simultaneous removal of H2S and CO2

• Most common solvent used is water

• Efficiency  of scrubbing depends on the solubility which depends on– Pressure, 

– Temperature 

– pH of water.

Pressure swing adsorption

• Adsorbents such as activated carbon and molecular sieves are used.

• Selectivity of adsorbtion is based on different molecular weight and molecular size

• Adsorption is carried out at high pressureDesorption is carried out at lower pressure.

• Advantages of PSA process• Simple design & operation• Consistent purity of gas is achieved.• Clean process requiring very less floor area• Dry product gas is produced

Cryogenic separation• Boiling point:

– CH4 : (‐)160°C

– CO2 : (‐)79°C

• Removal of CO2 in liquid form by cooling biogas to Critical Temperature at Critical Pressure

• Expensive, only tested in pilot plants

• Requires Cryogenic vessels to store and transport

• Recovery of CO2 is possible

Economic evaluation

• Relative costs for Medium size Purification plants

• Larger capacities favour PSA systems in terms of economics.

Removal of water

• Mostly achieved by– Condensation by cooling to low temperatures

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

• Alternative– Absorption in glycol or hygroscopic salts

Removal of H2S

• Most common processes for removal of H2Sare– By catalytic Removal. ( Chemical Process)

– Absorption on Activated carbon ( Physical Process)

– Micro‐organisms ( Biological Process)

Principle of PSA Biogas Purification

PSA BIOGAS PURIFICATION SYSTEM IS THE GAS SEPARATION SYSTEM WITH SPECIALLY DESIGNED ADSORBENT.

This adsorbent is called Molecular Sieves (MS) having a micro poreon its surface and adsorb CO2, N2, H2S and H2O moleculesselectively under a certain pressure. After adsorption process,adsorbent is regenerated by depressurizing. These PSA systemsproduce the Purified Methane enriched gas continuously byrepeating above adsorption and regeneration process

Molecular Sieves （MS）

Molecular Sieves

CO2Magnified View of

5Å

CO2

CO2

3.23Å

CH4

CH4

CH4CH4

CH4

H2S

CO2

CO2

Flow Diagram of PSA System

Biogas

Methane

Exhaust Gas

AdsorptionColumns

Coarse H2S Removal

Silencer

Biogas Compression &Storage

Storage Pressure Storage device Material of Construction

Low (Up to 0.2 Barg)

Water sealed gas holder Steel

Low Gas bag Rubber, plastic, vinyl

Medium (Up to 20 Barg)

Similar to Propane / LPG

tank Steel

High (Up to 250 barg)

Commercial gas cylinders Alloy Steel

Conclusions

• Biogas, if purified, can be used for various application replacing fossil fuels, Including Vehicular Fuel.

• Purification is definitely required for transportation/storage & Vehicular use.

• Various purification methods are available. Each method has relative advantages.

• PSA system has the unique advantage of simultaneous removal of various impurities.