Sustainable thermochemical ammonia production using
redox oxides

D. Guban,^a J. Vieten^a,b,
M. Roeb^a, C. Sattler^a,b

^a Institute of Solar Research,
German Aerospace Centre (DLR), Cologne, Germany

E-mail address: Dorottya.Guban@dlr.de

^b Faculty of Mechanical Science and
Engineering, Institute of Power Engineering,

Professorship of Solar Fuel production, TU
Dresden, Germany

E-mail address: Josua.Vieten@tu-dresden.de

There
is an ever growing demand for ammonia production that already reached globally
200 million tons per year by 2018 and is forecasted to increase to over 350
million tons per year by 2050 [1]. The
application segment is dominated by the fertilizer industry, since the most
important fertilizer and the world's most widely produced chemical is urea. Ammonia
is synthesized via the Haber-Bosch process, for which the required hydrogen and
nitrogen are currently provided by using fossil fuels. This work proposes a
novel approach to produce ammonia from the raw materials water and air only by
utilizing solar energy directly.

The
solar ammonia production route consists of two coupled solar-heated
thermochemical cycle processes, which aim to specifically remove oxygen from
gases. This applies both to gases (oxides containing molecular bound oxygen like
water vapour), as well as for oxygen molecules-containing gas mixtures (air). The
applied redox material is used in a two-stage process, first oxidised by the
corresponding gas, and then thermally reduced. The energy required for this is
provided by concentrated solar radiation that has the potential to play a major
role in the future global energy mix.

In
the first cycle, H₂O is used for oxidation and hydrogen is produced.
This process has already been successfully tested by DLR on a pilot scale. [2] The second
innovative cycle uses air as the oxidant. The air is deprived of oxygen and
thereby pure nitrogen is produced. Both gases together, N₂ and H₂,
are subsequently converted to ammonia in the well-established Haber-Bosch
process.

The
focus of the current work is the unexplored solar thermal air separation for
the production of nitrogen for the Haber-Bosch process, which requires N₂
with oxygen contamination below 10 ppm. During thermochemical
reduction-oxidation (redox) cycles the redox material (typically metal oxide)
is thermally reduced at high temperature while heat is converted to chemical
energy in an amount equal to the enthalpy of the reduction reaction, and with
the release of gaseous oxygen. During the reduction step this stored energy can
then be converted back to heat by the reverse reaction to drive the air
separation step for N₂ production. The absorbed O₂ during
the oxidation step could later be released and utilized for fertilizer
production, e.g. in the Ostwald process for ammonium nitrate synthesis.

Perovskites
are promising redox materials that are described with the general formula AMO_3-δ,
where the A site typically features an alkali, alkaline earth or rare earth
metal cation, whereas the M site is occupied in most cases by transition metal
cations. The occurrence of an oxygen non-stoichiometry δ (δ = 0–0.5)
in AMO_3-δ perovskites, as well as the close structural relationship
between perovskites and their defect-ordered reduced form A₂M₂O₅
(brownmillerite) allow for fast redox kinetics. Furthermore, the partial
reduction that is described by Eq. 1 ensures the stability of these materials
since during the redox cycle oxygen moves through the lattice without a
decomposition of the crystal structure.

                                                                                                                                            (1)

Another
advantage of perovskites is their tuneable composition, since the A and M sites
can be occupied by a number of ions. The possible formation of a given
composition is limited by the Goldschmidt tolerance factor[3]:

                                                                                                                                                                 (2)

In
most cases, the perovskite forms the desired ideal cubic lattice if 1 < t
< 1.02 is fulfilled. The tolerance factor can be controlled by selecting the
required alkali metal on the A site, while the B site can be occupied by a
number of transition metals with the possibility of adding other metals in
small quantity. With this method the reduction enthalpy can be tuned in the
range of ΔH⁰ = 100-500 kJ/mol^-1.

The
selection of the metal ions was based on the rule that the oxygen affinity of
the material should be low enough to allow thermal reduction in air at moderate
temperatures, but high enough to enable full re-oxidation of the reduced form
to its initial state in air. A measure of the reducibility of an oxide is its
reduction enthalpy (ΔH_red), e.g. a lower ΔH_red
results in a lower reduction onset temperature.

In
this work the feasibility of using perovskites as redox materials for solar
thermochemical air separation is demonstrated. The choice of the applied redox
material SrFeO_3-δ is based on our previous results. [4]Furthermore,
the raw materials to make SrFeO_3-δ, strontium carbonate and
iron oxide are very inexpensive, due to abundant natural resources at world
market prices below $1 per kg, allowing for economical use as a redox material.
The concept is first validated in laboratory-scale experiments performed in an
IR heated furnace, followed by scaling up the process to a 20 kW solar reactor.

SrFeO_3-δ
was synthesized by a solid state reaction between the raw materials, strontium
carbonate and iron oxide. In order to ensure the smooth transfer of the
material as well as to avoid fine dust formation the reactor 2-3 mm spherical
particles were prepared by mixing-spheronization method. In the laboratory
scale tests 50g of the synthesized beads
were packed in a fixed bed reactor heated via infrared radiation in gas flows
of synthetic air and nitrogen while the oxygen content of the outlet gas was monitored
continuously. The process was demonstrated in a 20 kW scale solar rotary kiln
with 2000 g of redox material. The designed reactor was first tested by heating
it up with a solar simulator by using xenon lamps, then after filling the
reactor with the redox material the particle bed was heated by the solar
furnace of DLR in Cologne.

The
laboratory scale experiments showed that after the high temperature reduction
step, when oxidation begins, the outlet oxygen concentration drops to a value of
approximately 3 × 10⁻⁶ bar for some time, giving an outflow of
purified nitrogen with low oxygen impurities, from the gas entering the system
with 20% oxygen. As the oxidation progresses, the oxygen vacancies in the
material become filled and it slowly loses capability to remove oxygen at low
concentrations, so that the outlet concentration increases over time.

To
investigate cycle stability of the material, five cycles were performed at
identical thermal conditions of 800 ºC reduction and 350 ºC
oxidation for air purification. An additional five cycles were performed at
identical conditions but with an inlet gas of 1% oxygen in a 500 sccm nitrogen
flow. This is of interest as the SrFeO₃ cycle alone would be a
rather energy intensive route to producing nitrogen, but coupling it with an
already established technology for producing low purity nitrogen, such as a
pressure swing adsorption unit, could offer a very efficient route to removing
the remaining trace oxygen and producing high purity nitrogen. The production
versus purity relationships for each cycle is given in Fig. 1. The narrow
grouping between curves of different cycles shows the excellent repeatability
of the process and lack of degradation of the material, at least over the
limited number of cycles performed.

Fig.
1 Production curves for five
identical purification trials from both synthetic air and from a mixture with
1% O₂. Reproduced with permission from [5].

A
solar rotary driven kiln was designed and built to demonstrate the air
separation process in larger scale with ca. 2000 g redox material. The proposed
reactor scheme is shown in Fig 2. The reduction temperature (800 ºC) is reached
by concentrated sunlight.

Fig.
2 Scheme of a solar rotary kiln for
air separation.

Summary

In
the DüSol research project, the technology of sustainable fertilizer production
is developed and demonstrated on the basis of solar thermal redox cycle
processes. The focus is on the unexplored step of solar thermal air separation
for the production of nitrogen. For this reaction, corresponding materials are
identified by thermodynamic calculations and qualified and optimized on a
laboratory scale. In combination with material development, a prototype reactor
is designed based on computer-aided calculation tools. In a test campaign this
reactor is being tested and the solar thermal nitrogen production demonstrated.
These experimental works go hand in hand with the overall process simulation
and optimization, which lead to a comprehensive economic analysis.

References

[1]      

1.             U.S. Geological Survey, Mineral commodity
summaries2014.

2.             Säck, J.P., et al., High temperature hydrogen
production: Design of a 750KW demonstration plant for a two step thermochemical
cycle. Solar Energy, 2016. 135: p. 232-241.

3.             Goldschmidt, V.M., Die Gesetze der Krystallochemie.
Naturwissenschaften. 14(21): p. 477-485.

4.             Vieten, J., et al., Perovskite oxides for
application in thermochemical air separation and oxygen storage. Journal of
Materials Chemistry A, 2016. 4(35): p. 13652-13659.

5.             Bulfin, B., et al., Air separation and selective
oxygen pumping via temperature and pressure swing oxygen adsorption using a
redox cycle of SrFeO3 perovskite. Chemical Engineering Science, 2019. 203:
p. 68-75.