Process for converting biogas to a gas rich in methane

1. Process for converting biogas to a gas rich in methane comprising the steps of:

mixing a carbon dioxide-comprising biogas with steam to form a mixture comprising carbon dioxide, methane and steam;

electrolysing the mixture comprising carbon dioxide, methane and steam in a high temperature solid oxide electrolyser cell unit, to obtain a gas comprising mainly hydrogen and carbon monoxide; and

catalytically converting hydrogen and carbon monoxide in the gas comprising hydrogen and carbon monoxide to methane in one or more methanation steps to obtain a gas rich in methane.

2. Process according to claim 1 , wherein the one or more methanation steps include adiabatic methanation followed by non-adiabatic methanation.

3. Process according to claim 1 , wherein the mixture comprising carbon dioxide, biogas and steam also comprises approximately 0.1-200 ppm of sulphide compound.

4. Process according to claim 1 , wherein the gas comprising hydrogen and carbon monoxide is desulphurised after electrolysis and prior to methanation.

5. Process according to claim 1 , wherein the mixture comprising biogas, carbon dioxide and steam is co-electrolysed according to the following reactions:

CO 2 =CO+0.5O 2   (1)

H 2 O=H 2 +0.5O 2   (2).

6. Process according to claim 1 , wherein the high temperature solid oxide electrolyser cell unit comprises fuel electrode material with limited or no steam reforming activity.

7. Process according to claim 6 , wherein the fuel electrode material does not comprise nickel or the fuel electrode material is all ceramic.

8. Process according to claim 6 , wherein the fuel electrode material comprises compounds or elements selected from the group consisting of LSCM, Cu, CeO 2 , titanates and combinations thereof.

9. Process according to claim 6 , wherein the fuel electrode material comprises Ni-YSZ, SYSZ or Ni-SSZ electrodes having a thickness of less than or equal to 10 microns.

10. Process according to claim 1 , wherein the high temperature solid oxide electrolyser cell unit operates thermoneutrally.

11. Process according to claim 2 , wherein the sulphide is removed from the hydrogen-rich gas by absorption on a metal oxide absorbent.

12. Process according to claim 11 , wherein the metal oxide absorbent is zinc oxide and/or is based on copper.

13. Process according to claim 1 , wherein carbon monoxide and hydrogen are converted to methane in a methanation step according to the following reactions:

CO+3H 2 =CH 4 +H 2 O  (3)

CO 2 +4H 2 =CH 4 +2H 2 O  (4).

14. Process according to claim 1 , wherein the one or more methanation steps are catalysed by a catalyst including metals selected from the group consisting of Group 6B, Group 8 of the Periodic Table and combinations thereof.