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

doi:10.1016/j.cattod.2014.05.020

Catalysis Today

Volume 242, Part A, 15 March 2015, Pages 184-192

https://doi.org/10.1016/j.cattod.2014.05.020 Get rights and content

Highlights

•
Short chain gaseous hydrocarbons can be produced from H₂/CO₂ using 100 Fe/2 K (g/g) catalyst.
•
Catalyst activity strongly influenced by high (H₂O/H₂)_out (oxidation potential).
•
(H₂/CO₂)_in is the most promising parameter to reduce the oxidation potential.
•
Kinetic parameters calculated for CO₂-shift and FT reactions using 100 Fe/2 K (g/g).

Abstract

The potential of a new practical application of Fischer–Tropsch synthesis is investigated, the production of C_2–4 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 (100 g Fe/2 g 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 = 2 Mpa, (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.

Graphical abstract

Introduction

The increasing contribution of renewable electrical energy to primary energy supply involves, due to its fluctuating production, the development of new storage technologies to stabilize the electrical grid. Chemical energy carriers, e.g. gaseous hydrocarbons, with a high energy density and a developed infrastructure for storage, distribution and utilization are seen as an important option for medium- and long-term storage [1], [2]. The production of chemical fuels from electrical energy may also become relevant in a completely renewable energy system. Wind and solar energy offer the highest potential to produce energy from renewable sources without constraints and probably hydrocarbon fuels will remain relevant for process heat (gas) or transport (liquid, e.g. for airplanes, ships) (Fig. 1.1).

Synthesis of gaseous hydrocarbons using CO₂ as C-source has recently gained special interest since it may be attractive to produce H₂ using excess electrical renewable energy via water electrolysis, a proven electrochemical technology at small scale. The availability of CO₂ is high and CO₂ may be directly separated after its formation in corresponding industrial processes. Potential sources of concentrated CO₂ are biogas facilities, biomass combustion or gasification (leading to CO/CO₂ mixtures) and blast furnace offgases. The use of CO₂ as carbon source offers the possibility to replace natural gas or compounds from petroleum refining (liquefied petroleum gas, LPG), thus avoiding fossil CO₂ emissions. Methane is the most common gaseous hydrocarbon and the main component of Substitute Natural Gas (SNG). Hydrogenation of CO₂ to methane has been widely studied in literature (e.g. Wang and Gong [3]; Kopyscinski et al. [4]). It is usually carried out with a nickel-based catalyst and high conversions and selectivity to methane can be reached.

Short chain hydrocarbons, (C₂H₆, C₃H₈, C₄H₁₀) are used to increase the calorific value of methane (raw SNG) (HHV (high heating value) = 39.8 MJ/m³) before being fed into a high calorific value gas grid (H-gas) (HHV = 40–47.2 MJ/m³, Table 1). Little information has been found in literature for this route using H₂/CO₂ mixtures [5], [6], most of the research has been focused on the production of short chain alkenes [7], [8] for the chemical industry or long chain hydrocarbons [9], [10], [11] for transportation fuels from H₂/CO₂. Previous studies describe the hydrogenation of CO₂ as a two-step catalytic reaction, with CO as intermediate product. In the first step CO₂ is converted to CO in the CO₂-shift reaction (Eq. (1)). Afterwards, CO reacts in the Fischer–Tropsch reaction to form organic products (Eq. (2)). The alkenes can be hydrogenated to a variable extend in-situ to alkanes (Eq. (3)) ${CO}_{2} + H_{2} \leftrightarrow CO + H_{2} O$ $n CO + 2 n H_{2} \to C_{n} H_{2 n} + n H_{2} O$ $C_{n} H_{2 n} + H_{2} \to C_{n} H_{2 n + 2}$

The aim of the present study is to investigate the potential and limits of a new practical application of Fischer–Tropsch synthesis, the production of short chain hydrocarbons from H₂/CO₂ gas mixtures. Fundamental catalytic effects of CO₂ hydrogenation under FT conditions with iron catalysts should be identified and different process considerations evaluated.

Section snippets

Iron catalyst

Hydrogenation of CO₂ can be conducted mainly with catalytic systems based on group VIII metals (iron (Fe), cobalt (Co), Nickel (Ni), Ruthenium (Ru) [3], [12], [13]). Ni-based catalysts are the standard methanation catalysts. The possibility to produce C₂, C₃ and C₄ hydrocarbons when using H₂/CO₂ as gas mixture with nickel catalysts has not been reported in literature. This might be due to the high H₂/CO ratio prevailing in the reaction. Usually almost no CO is observed in the final product.

Iron catalyst formation

The formation of the iron catalyst takes place with H₂ and CO₂ in a molar ratio 5:1, reaction conditions are T = 297 °C and pressure 1 MPa. During this formation the metallic iron, obtained via the reductive pre-treatment, converts, at least partially, into various iron carbide and oxide phases [23]. Fig. 3.1 presents CO₂ conversion, yield of CO, yield of organic products and yield of carbon (determined by carbon balance, Eq. (8)) as a function of time. According to Eqs. (1), (2), CO appears as an

Process considerations

The use of iron-based catalyst for the production of gaseous hydrocarbons offers the possibility to carry out the CO₂-shift reaction and FT synthesis with the same catalyst. This eliminates the use of two different reactors with different temperature levels (i.e. approx. T = 900 °C for CO₂-shift and e.g. T = 250 °C FT reaction) and intermediate cooling of the product gas. However, the use of an iron catalyst accounts for some challenges mainly concerning the presence of water vapor as byproduct. The

Acknowledgements

This work was financially supported by the German Bundesministerium für Bildung und Wissenschaft (BMBF, FKZ: 033RC1010A) (M. Iglesias G., G. Schaub). Help in the experimental work from Friedemann Mörs is gratefully acknowledged.

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

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Iron catalyst

Iron catalyst formation

Process considerations

Acknowledgements

Fuel

Catal. Today

Chin. J. Catal.

Appl. Catal., A: Gen.

Appl. Catal., A: Gen.

Chem. Eng. Sci.

Appl. Catal., A: Gen.

Appl. Catal., A: Gen.

J. Catal.

J. Mol. Catal.

Microporous Mesoporous Mater.

Chem. Ing. Tech.

Chemical storage of renewable electricity via hydrogen-principles and hydrocarbon fuels as an example

Front. Chem. Sci. Eng.

Chem. Lett.

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