TECHNOLOGY FOCUS - BIOLOGY - Preferred Components: The methanogenic microorganisms are methanogenic archaea which is Methanosarcina barkeri, Methanobacterium thermoautotrophicus, Methanothermobacter thermoautotrophicus, Methanococcus maripaludis, Methanothermobacter marburgensis and/or Methanocaldococcus jannaschii.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Method: In the method: the gas feed rate of hydrogen and carbon dioxide into the reaction vessel is at least 0.5 vvm, preferably at least 1 vvm in total; the hydrogen is at least partially provided by electrolysis of water and/or brine; the electrolysis of water and/or brine uses at least partially electric energy generated using a renewable and/or non-renewable energy source; the electric energy is generated using a renewable energy source which is solar energy, wind power, wave power, tidal power, water or hydro power, geothermal energy, biomass and/or biofuel combustion; and the carbon dioxide is at least partially provided by oxygen enriched combustion or gasification using oxygen being at least partially provided by electrolysis of water and/or brine. In the method: inside the reaction vessel an absolute pressure above normal atmospheric pressure is applied, an absolute pressure of at least 1.5 bar, preferably 1.5-1000 bar is applied; and the gas feed rate of hydrogen and carbon dioxide into the reaction vessel is at least 0.5 vvm in total, where inside the reaction vessel an absolute pressure of at least 1.2 bar is applied. Preferred Components: The nitrogen concentration in the liquid phase inside the reaction vessel is 0.0001-20 mmol/l. The nitrogen is ammonia, amino acids, aqueous solution of ammonia, molecular nitrogen, ammonium compounds and/or inorganic or organic nitrogen compounds. The nitrogen is an ammonium compound comprising ammonium chloride, ammonium carbonate, ammonium sulfide, ammonium hydrogen sulfide, ammonium phosphate, ammonium hydrogen phosphate, ammonium sulfate, ammonium, ammonium hydrogen carbonate, ammonium carbamate, ammonium nitrate, ammonium nitrite and/or ammonium hydroxide. The volumetric ratio of hydrogen to carbon dioxide in the gas feed introduced into the reaction vessel is 3:1-4.5:1, preferably 4:1.    TECHNOLOGY FOCUS - INSTRUMENTATION AND TESTING - Preferred Components: The bioreactor further comprises: at least one device for measuring the head pressure and/or the off-gas concentration, and optionally at least one device for adjusting or maintaining the partial pressure ratio of hydrogen to carbon dioxide inside the reaction vessel; and a device for providing, controlling and/or measuring an absolute pressure inside the reaction vessel above normal atmospheric pressure. The system further comprises: at least one device for oxygen enriched combustion or gasification for producing carbon dioxide; and at least one device for delivery, recovery, purification, enriching, storing, recycling and/or further processing of a member selected from the group consisting of off-gas, hydrogen, water, nitrogen, oxygen, chlorine, carbon dioxide, hydrogen sulfide, sodium sulfide, methanogenic microorganisms, medium and medium components, methane and other substances of the process from the reaction vessel and bioreactor. The renewable energy source comprises solar energy, wind power, wave power, tidal power, water/hydro power, geothermal energy, biomass and/or biofuel combustion. EXAMPLE - No suitable example given.