Chemical energy storage in gaseous hydrocarbons via iron Fischer–Tropsch synthesis from H2/CO2—Kinetics, selectivity and process considerations

Abstract

The potential of a new practical application of Fischer–Tropsch synthesis is investigated, the production of C2–4 components to increase the heating value of substitute natural gas (SNG), starting from CO2 and H2, produced from renewable electricity. This process route offers the possibility to convert electrical energy into chemical energy. The resulting chemical energy carrier can be stored in the natural gas grid, easy to distribute.An iron-based catalyst promoted with potassium (100g Fe/2g K) is studied over a wide range of operation conditions to investigate its suitability to produce C2–C4 components from H2/CO2 mixtures. The achieved hydrocarbon distribution (α=0.2–0.3) allows for the production of Substitute Natural Gas components (68 C% C1, 30 C% C2–C4, C5+ approx. 2 C%). The catalyst stability is good, at least for 50 days. The hydrocarbon selectivity remains almost constant during the experiment, methane becoming slightly more predominant over time. Catalyst activity seems to be strongly influenced by the (H2O/H2)out ratio (possibly due to oxidation), which correlates with CO2 conversion. At high values of (H2O/H2)out, the activity of the catalyst seems to change and cannot be described using the same reaction rate kinetics determined for lower (H2O/H2)out values. The maximal CO2 conversion achieved is 44% (p=2Mpa, (H2/CO2)in=8). Experimental results show that higher conversions could not be achieved neither with an increase in temperature nor in modified residence time. The H2/CO2 inlet ratio is the most promising parameter to reach high CO2 conversions without a high oxidation potential in the product gas. Interesting catalytic effects have been identified, however experimental results will be supported by additional work in order to get a better understanding of the CO2 hydrogenation under Fischer–Tropsch conditions with iron catalysts.