(677b) Hydrogen Storage and Carbon Dioxide Valorization through Reductive Calcination of Mineral Carbonates

Georg Baldauf-Sommerbauer, Susanne Lux, Matthaeus Siebenhofer

Institute of Chemical Engineering and Environmental Technology, Graz University of Technology

The calcination of mineral carbonates is a widely used preparation reaction for metal oxides in industry (see Eq. 1). Carbon dioxide that has been fixed in mineral form is released, with the well-known detrimental effect on the global carbon balance and climate. This drawback of calcination can be overcome in reductive calcination. Carbon dioxide can be used as a means of storage for renewably produced hydrogen as methane and/or synthesis gas CO/H₂.

MCO₃ = MO + CO₂ (1)

The conversion of carbon monoxide and dioxide into methane is feasible for temperatures below 550 °C from a thermodynamic point of view. Due to kinetic hindrance, however, heterogeneous catalysis is needed to reach high yields. The Gibbs enthalpy for the reverse water gas shift reaction (RWGS: CO₂ + H₂ à CO + H₂O) decreases roughly linear from approximately +30 kJ mol^-1 at 0 °C to approximately -6 kJ mol^-1 at 1000 °C. The RWGS is also kinetically hindered and needs to be catalyzed for conversions near the thermodynamic equilibrium. If the carbon dioxide emitting step, namely calcination, is already performed in reductive hydrogen atmosphere, Eq. 2 can be formally derived for the direct production of carbon monoxide and methane from mineral carbonates. Calculations of the Gibbs enthalpy revealed that methane and carbon monoxide production by reductive calcination of the industrially relevant carbonates of iron, magnesium, and other metal carbonates is possible.

(a+b+c)MCO₃ + (b+4c)H₂ = (a+b+c)MO + aCO₂ + bCO + cCH₄ + (b+2c)H₂O (2)

Numerous experiments were conducted to elucidate the relationship between product gas composition, mineral carbonate, and process temperature and pressure. Experimental investigations were conducted using a tubular reactor setup with mineral iron (= siderite, 5-10 mm size fraction), magnesium (= magnesite, 5-8 mm size fraction), and magnesium/calcium (= magnesite/dolomite, 5-8 mm size fraction) carbonates as a fixed bed. The inlet gas composition was set to 90-100 vol.% hydrogen and 0-10 vol.% nitrogen at a total flow of 30 L h^-1 at standard temperature and pressure.

Thermogravimetric experiments showed that reductive calcination proceeds at lower temperatures than calcination in inert nitrogen atmosphere. The minimum temperature needed for reductive calcination at isothermal conditions using the tubular reactor setup increases from siderite (~330 °C), through magnesite (~430 °C) to magnesite/dolomite (~475 °C).

Reductive calcination experiments with magnesite in hydrogen atmosphere (430-530 °C, 1-13 bar total pressure) revealed a maximum methane yield of ~37 % (475 °C, 13 bar). Methane formation is facilitated by increase of pressure at moderate temperatures below 475 °C. The yield of carbon monoxide ranges from 27 to 47 %. It increases from 430 to 505 °C and decreases at higher temperatures. Pressures above 1 bar slightly decrease carbon monoxide yield. When the magnesium carbonate fraction of a mixed magnesite (M) - dolomite (D) (M:D = 1.0:2.3) is calcined reductively (475-535 °C, 1-13 bar total pressure), only traces of methane could be detected in the product gas. Compared to pure magnesite, the carbon monoxide yield is enhanced. Up to 68 % of the total carbon can be released as carbon monoxide. Siderite (FeCO₃) differs from magnesite when the methane yield is compared. Experiments in the temperature region 330-380 °C at 1-11 bar pressure did show that the formation of methane is facilitated by low temperatures and elevated pressures. Optimization of the process parameters temperature, pressure, feed flow, size fraction, and bed length revealed that methane yields above 60 % without any additional catalyst are obtainable.