Minnesota Corn Research & Promotion Council
Minnesota Corn Growers Association
Final Project Report
Title: Development and Evaluation of a Novel, Small Pilot-Scale Non-Thermal Plasma
Process for the Production of Nitrogen Fertilizer from Wind and Other Renewable
Resources
1
Principle Investigators: Michael Reese1 and Dr. Roger Ruan2,
Renewable Energy Director, Univ. of Minnesota -West Central Research & Outreach Center
2
Professor, Univ. of Minnesota – Department of Bioproducts and Biosystems Engineering
Introduction
Nitrogen is the key limiting nutrient for corn production. The United States imports over 50% of
the nitrogen fertilizer required for corn production. The current nitrogen supply chain for
Minnesota corn producers primarily relies on moving anhydrous ammonia (or derivative forms
including urea) from Canada and the Gulf Coast States into the Midwest Corn Belt. There are no
commercial nitrogen fertilizer production plants in Minnesota. As a result, the state imports $400
million to $800 million retail value per year of nitrogen fertilizer. The nitrogen fertilizer market,
driven by corn production, represents an opportunity for Minnesota farmers to use their land for
both corn and nitrogen fertilizer production utilizing wind and other forms of electrical energy.
There are multiple reasons to develop Minnesota-based nitrogen fertilizer production base
utilizing wind and other electrical energy resources including:
1) Stranded wind resource due to low transmission capacity
2) Declining domestic ammonia production resulting in more imports
3) Nitrogen fertilizer price closely tethered to fossil fuels
4) High nitrogen fertilizer demand and robust infrastructure
5) Provides energy sink in “Smart Grid” applications
6) Enhances security for domestic food, feed, and bio-fuel production
7) Supports rural economic development
8) Helps achieve greenhouse gas emission reduction targets
9) Provides hydrogen economy transition bridge
The conventional processes of producing nitrogen fertilizer, either through natural gas steam
methane reforming or the electrolysis of water, are both well understood. The natural gas
process breaks down and scrubs methane (CH4) to produce hydrogen gas. Electrolysis uses
electricity to breakdown water (H2O). Once hydrogen gas is produced, both processes are very
similar in that nitrogen gas is separated from air and then combined with hydrogen under high
pressures and temperatures- known as the Haber-Bosch Process. Both steam methane reforming
and electrolysis production routes have been used extensively in large scale production facilities
(1,000 tons plus per day) around the world. Since the energy costs of natural gas steam methane
reforming has historically been lower than electrolysis, it has become the dominant production
route. However, dynamics of energy costs are ever changing and local production of energy is
becoming cost competitive with imported sources.
Electrolysis of water produces a purer form of hydrogen and therefore may be more applicable
for small-scale local production plants (10 - 100 tons per day) than steam methane reforming.
However, the high level of pressure required for the traditional Haber-Bosch Process is a limiting
factor in reducing the economies of scale of localized production facilities due to the high energy
(and cost) requirements of compression. Therefore, a novel process that can operate at
atmospheric pressures and low temperatures will be more cost effective and competitive. The
University of Minnesota has completed a pilot-scale hydrogen and ammonia production facility
at the West Central Research and Outreach Center (WCROC) near Morris. The baseline
information on the production efficiency and cost of producing ammonia using the traditional
Haber Bosch Process are now available. The pilot plant is designed to test and compare
alternative ammonia production processes. Dr. Roger Ruan has developed a lab-scale nonthermal plasma ammonia production process within his lab at the Department of Bioproducts and
Biosystems Engineering (BBE). The lab-scale non-thermal process combines hydrogen and
nitrogen gases at atmospheric pressure and low temperatures to form anhydrous ammonia. The
project team proposed to scale-up the non-thermal production process to a small pilot-scale
system and deploy the system at the West Central Research and Outreach Center. A small-scale
system was refined in the lab but was not deployed in the pilot plant.
Objectives and Hypothesis
The concept of synthesizing ammonia at atmospheric pressure and low temperatures through the
non-thermal plasma (NTP) assisted catalytic reactions was proven by Dr. Ruan and his coworkers. We hypothesize that this NTP based process could be developed into technologies
which could be deployed to locations near wind farms where hydrogen could be produced from
electrolysis of water using wind generated electricity. Therefore, the goal of the proposed
project was to develop, evaluate, and demonstrate a small pilot scale NTP based ammonia
production technology which utilizes locally produced hydrogen, e.g., hydrogen produced from
electrolysis of water with wind electricity. The specific supporting objectives of the project
were:
(1) develop and optimize the hydrogen-nitrogen to ammonia process by selecting high
activity and selectivity catalysts and NTP parameters
(2) develop scale-up parameters and pilot system design
(3) fabricate the pilot system
(4) test, evaluate, and demonstrate the pilot system
Methods and Procedures
Ammonia is produced mostly from hydrogen derived from natural gas through the well-known
Haber-Bosch process [ref]. The basic reaction is as follows:
N2 + 3H2 = 2NH3
The Haber-Bosch process is typically carried out over iron catalysts at temperatures around 400600°C and pressures ranging from 200 to 400 atmospheres. These severe thermal and pressure
conditions make implementation of the Haber-Bosch process in small scale and low cost very
difficult.
Using NTP for synthesis of ammonia was previously reported in the literature. However, the
concentration produced within the reported processes was very low. Dr. Ruan and his coworkers began their efforts to develop an NTP based process to produce ammonia at high
concentration. Their work, funded by a grant from the University of Minnesota IREE, resulted
in a process able to produce ammonia at concentrations around 12% at atmospheric pressure and
temperatures (115-240°C) much lower than those used in the Haber-Bosch process (Figure 1).
Figure 1. Traditional Haber-Bosch Process which is utilized in the WCROC ammonia pilot
plant.
Figure 2 shows the general concept of our novel wind to ammonia approach. NTP reactor is the
heart of our approach. NTP is electrically energized matter in a gaseous state which is not in
thermodynamic equilibrium, and can be generated through electric discharge in a gaseous
volume. These species have an energy level in the range of 2 to 10 eV at temperature close to
ambient condition. It is known that these kinds of species can alter organic or inorganic
compounds through at least three mechanisms: (1) decomposition, (2) structural rearrangement,
and (3) fragment elimination of polymers. One or all of these mechanisms may be responsible
for or promote the disassociation of hydrogen and nitrogen which is necessary for the synthesis
of ammonia from hydrogen and nitrogen through catalytic reactions. Our study shows that NTP
and catalysts form a synergetic catalysis system that can work efficiently at low temperature and
atmospheric pressure, thanks to the energetic characteristics of NTP. NTP on one hand directly
causes N2 and H2 to dissociate and form NH3 with or without catalyst, and on the other hand,
provides ionization energy necessary to produce electrons for the catalysis system to function.
Figure 2. New approach to ammonia production from renewable hydrogen and energy.
Therefore, one of the key/critical research tasks of the project was to improve and optimize the
NTP assisted catalytic process. Key process parameters we investigated were catalysts, applied
power voltage and frequency, N2/H2 volume ratio, N2/H2 flow velocity, catalyst loading and
temperature. After the process was optimized, we developed scale-up parameters and system
design, and built and tested the system.
Ammonia Synthesis by NTP-assisted Catalysis on Cs-promoted Ru Catalysts
In the Haber-Bosch process, a series of reactions take place at temperatures around 400-600°C
and pressures ranging from 200 to 400 atmospheric pressure. The main difficulty in this reaction
is the dissociation of dinitrogen (N2), a rate-limiting step in the process.
N2 → 2N(ad)
H2 → 2H(ad)
N(ad) + H(ad )→ NH(ad)
NH(ad) + H(ad) → NH2(ad)
NH2(ad) + H(ad) → NH3(ad)
A transitional catalyst is used to activate N2 and facilitate its dissociation. Metal catalysts prepared
from Ru and supported on MgO show considerable activity toward ammonia synthesis at the
temperature as low as 300-400 °C and under the close to ambient pressure for a N2/H2 mixture.
A number of studies reported ammonia synthesis reaction at atmospheric pressure achieved by
using solid state proton (H+)-conducting cell-reactor, radio frequency and microwave plasma or
in dielectric barrier discharge plasma with or without metal oxides-base catalysts. A number of
studies have also demonstrated the synthesis of ammonia from hydrogen and nitrogen using a
NTP process. These researchers found that NH radicals were formed in the nitrogen-hydrogen
plasma and the NH radicals would be quenched by hydrogen molecules to form ammonia.
However the yields with all these methods were still very low.
NTP is electrically energized matter in a gaseous state which is not in thermodynamic
equilibrium, and can be generated through electric discharge in a gaseous volume. A simple NTP
reactor may consist of two electrodes with a space (the discharge volume) and sometimes one or
two insulating or dielectric layers in between and connected to a high voltage power supply.
When a high voltage is applied to the electrodes, an electric field is generated across the space
between the electrodes, which, if sufficiently high, causes electric discharge. The energy in NTP
is thus directed preferentially to the electron-impact dissociation and ionization of the
background gas to produce NTP species including electrically neutral gas molecules, charged
particles in the form of positive ions, negative ions, free radicals, energetic electrons, and quanta
of electromagnetic radiation (photons). These species have an energy level in the range of 2 to 10
eV at temperature close to ambient condition. It is known that these kinds of species can alter
organic or inorganic compounds through at least three mechanisms: (1) decomposition, (2)
structural rearrangement, and (3) fragment elimination of polymers.
Our lab scale experiment device is illustrated in Figure 3. Basically, it is a dielectric barrier
discharge plasma reactor where Ru catalyst supported on different oxide particles were packed
between the two dielectric barriers. When a certain voltage is applied to the electrodes, electrical
discharge takes place between the dielectric barriers and on the surface of the catalyst particles,
and therefore the device creates both dielectric barrier discharge and surface discharge.
Figure 3. A schematic diagram showing the experimental device.
We know that Cs-promoted Ru catalyst on MgO support showed no activity at atmospheric
pressure without NTP. Figure 4 shows the ammonia yields under different conditions as
indicated with the assistance of NTP. NTP alone was able to promote ammonia synthesis,
suggesting that NTP species provided energy to dissociate N2 and H2 allowing addition-reactions
to form NH3. When the catalyst support MgO particles were packed into the gap between the
dielectric barriers, the ammonia yield was greatly improved. Although MgO is not an ammonia
synthesis reaction catalyst, the presence of MgO particles promotes intensive surface discharges
which are expected to favor dissociation of N2 and H2 and promote ammonia formation.
Addition of Ru catalyst to the system resulted in a marginal increase in ammonia yield, which
was a surprise. However, when the catalyst system included both MgO support and Cs promoter,
the yield increased dramatically. Therefore, promoter Cs seems to be a critical component in this
reaction system.
4
Ammonia yield (%)
3.5
3
2.5
2
1.5
1
0.5
0
NTP
NTP + MgO
NTP + Ru /MgO
NTP + Cs-Ru
/MgO
Figure 4. Ammonia synthesis under different conditions.
(VN2:VH2=1:3, N2 and H2 total flow rate 60ml/min, voltage 5000V, frequency 8000Hz)
To understand these results, we continued to examine the functions of individual components of
the reaction system. Figure 5 shows that an electron must be passed onto the antibonding orbital
of N2 through the d orbital of Ru in order to weaken the triple bond of dinitrogen. The weakened
triple bond can then be broken with additional energy, in this case, energy provided by NTP.
However, the energy provided by NTP is only about 6 eV, which is insufficient to ionize Ru
which has an ionization energy of 7.36eV (Figure 6). When promoter Cs is attached to Ru, the
situation changes dramatically. Cs, with an ionization energy of only 3.89eV, can easily be
ionized, producing electrons which are passed onto Ru, and then to di-nitrogen. Putting all these
together we have a synergetic catalysis system that can work efficiently at low temperature and
atmospheric pressure, thanks to the energetic characteristics of NTP.
Figure 5. The pathway of ammonia synthesis.
8.00
7.50
Ionization energy (eV)
7.00
Mg, 7.65
Ru, 7.36
Ti, 6.83
6.50
6.00
NTP, 6.00
5.50
5.00
4.50
4.00
Cs, 3.89
3.50
3.00
Figure 6. Ionization energy for different elements and average NTP energy.
Obviously NTP on one hand directly causes N2 and H2 to dissociate and form NH3 with or
without catalyst, and on the other hand provides ionization energy necessary to produce electrons
for the catalysis system to function.
NTP Pilot System Process Description
Apparatus
Based on the results we obtained in the above section, the concept of synthesizing ammonia at
atmospheric pressure and low temperatures through the non-thermal plasma (NTP) assisted
catalytic reactions was proven. Therefore, we improved and developed a non-thermal plasma
(NTP) synthesis pilot system coupled with catalysis (illustrated in Figure 7) and continued to
study the ammonia synthesis process and the key parameters affecting ammonia conversion
efficiency. The NTP reactor consisted of a quartz tube and a stainless steel tube placed coaxially
with 1.5 mm space (the discharge volume) as the two electrodes and connected to a high voltage
power supply. The inner diameter of the outside quartz tube was 35 mm and the outer diameter
of the inside stainless steel tube was 32 mm. The width of the electrode (made of stainless steel)
attached around the outside quartz tube has the range from wire length up to 2 cm. Catalysts
were packed to a length matched with the length of the electrode between the two dielectric
barriers. When a certain voltage is applied to the electrode, electrical discharge takes place
between the dielectric barriers and on the surface of the catalyst particles. The voltage and
frequency of the power supply could be adjusted during synthesis process. So far, the usual
voltage range applied in this NTP system was from 5,000 volts to 10,000 volts and the usual
frequency range applied in this NTP system was from 8,000 Hz to 16,000 Hz. An oscilloscope
was also connected to the electrode in order to monitor the changes of voltage and frequency and
also to record the related waveform information for further energy consumption calculations.
The feeding gases (reactants) were nitrogen and hydrogen, which were provided by nitrogen and
hydrogen cylinders for small scale tests. Before synthesis, nitrogen gas was used for keeping air
out of the whole system. Two mass flow meters (Smart mass Flow, BROOKS) were used to
control the exact flow rates for nitrogen and hydrogen individually. Feeding gas flow rates can
range from 2 ml/min to 30 L/min. Synthesis reaction takes place under atmospheric pressure.
After reaction in the NTP reactor, all the gas products (which include synthesized ammonia,
unreacted nitrogen and hydrogen gas, dissociated nitrogen and hydrogen continuously to
synthesized on absorbent surface, and some NH or NH2 groups) will flow into the adsorption
bed and be absorbed by the absorbents. So far, the best absorbent we tested was Molecular Sieve
13X (60/80 mesh). Then the remaining unreacted gases will be cycled back into the NTP reactor
by a gas pump. When the whole system reaches equilibrium, G1 will remain constant. G2 is the
circulation gas flow rate which can be measured by a rotor flow meter. In our best synthesis
condition, we use five parallel reactors instead of one reactor to achieve best conversion yield.
Power
G1
N2
H2
NTP Reactor
Adsorption
with catalyst
bed
G1
G2
Gas pump
Figure 7. NTP system with circulation and adsorbent; G1 is the feeding gas inflow rate and G2 is
the circulation gas flow rate.
Catalysts
The catalysts used for synthesis included Ru as the main catalyst, Al2O3 as the main support and
Cs, K, and Ba as promoters. So far, tests results showed that catalyst Ru loading of 10%
performed the best. After impregnation, the catalysts were dried under infrared light for about 4
h and then calcined at 500 °C in a muffle furnace for 4 h. Then the catalysts were reduced at 500
°C using hydrogen gas for 4 h prior to application.
Results and Discussion
Pilot scale system
We developed a lab scale pilot system as shown in Figure 8. The pilot system has cooling,
absorbent, and circulation as well as five reactors in parallel with band or wire electrodes. The
experiment data have shown greatly improved synthesis efficiency. Details are shown in the
following sections.
Figure 8. Lab scale pilot NTP system
Feeding gas ratio effect
N2/H2 ratios of 1:2.8, 1:2.9, 1:3.0, 1:3.1 and 1:3.2 were tested on the one-reactor NTP system
and results showed that the difference is not significant but ammonia production rate was slightly
higher at the ratio of N2/H2 = 1:2.8 compared with the other ratios. This indicates that reaction
under the condition of higher N2/H2 ratio was more desirable for ammonia concentration. The
possible reason is that nitrogen has a higher chemical bond energy which needs higher energy to
break the bond, and the high density of nitrogen gas in the plasma system could lead to higher
density of active nitrogen molecular. The possibility of active nitrogen molecule reacting with
hydrogen would be increased.
N2/H2 ratio of 5:1, 4:1, 3:1, 2:1, 1:1, 1:2, 1:3 were further tested on the pilot system. Results
show that N2/H2 ratio=3:1 produced the highest amount of ammonia. This confirms that
ammonia synthesis has higher efficiency under a N2 rich environment (Figure 9).
0.9
0.8
0.7
0.6
Energy 0.5
(gNH3 0.4
/kWh)
0.3
0.2
0.1
0
1:3
1:2
1:1
2:1
N2/H2 ratio
3:1
4:1
5:1
Figure 9. The N2/H2 ratio effect on energy efficiency, Voltage=7000v, Frequency=10,000Hz,
GF=4L, one reactor.
Flow rate effect
Previous results on one-reactor NTP system clearly showed that higher flow rate can produce
ammonia much more efficiently in the current NTP plasma catalytic synthesis system. Therefore,
we are using relatively larger sized catalyst to reduce residence time in order to reduce ammonia
decomposition in the synthesis reactor and improve the energy efficiency. Tests on pilot system
show that higher gas flow rate does improve energy efficiency, but there is a limit of the total
flow rate. When the total gas flow rates reach 5 to 6 liters/min, the energy efficiency does not
change much (Figure 10).
Energy
(gNH3/
kWh)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0.1
0.5
1
2
3
4
5
Gas Flow (Liters/min)
6
7
8
Figure 10. The Gas Flow effect on energy efficiency, Voltage=7000v, Frequency=10,000Hz,
N2/H2=3/1, one reactor.
Optimization of frequency and voltage setting in plasma system
With our recently upgraded new power supply, higher frequency (up to 25,000Hz) and higher
voltage (up to 30,000v) can be achieved at the same time without affecting each other. Therefore,
the feasible range of both frequency and voltage is expanded. Tests show that higher voltage
resulted in a higher ammonia concentration but the energy consumption was also much higher.
However, frequency effect did not show the same trend as voltage effect. Results also show that
10,000Hz and 5000v is the best combination for achieving highest energy efficiency (Figure
11&12).
0.9
0.8
0.7
0.6
Energy 0.5
(gNH3/ 0.4
kWh)
0.3
0.2
0.1
0
8,000
10,000
12,000
Frequeny (Hz)
14,000
16,000
Figure 11. The Frequency effect on energy efficiency, Voltage=7000v, N2/H2=3/1, GF=4L, one
reactor.
1.2
1
0.8
Energy
(gNH3/0.6
kWh)
0.4
0.2
0
5000
6000Voltage 7000
(Volts)
8000
9000
Figure 12. The Voltage effect on energy efficiency, Frequency=10,000Hz, N2/H2=3/1, GF=4L,
one reactor.
Number of reactors effect
In previous research, we tested s parallel reactor system with two reactors and results showed
that the ammonia yield was increased by about 30% with two parallel reactors under the same
flow rate conditions. After we optimized the other system parameters including N2/H2 ratio,
flow rates, voltage and frequency, we continued to optimize the number of reactors in our pilot
system under the optimized conditions of all other parameters. Current results showed that 4
reactors achieve the best energy efficiency (Figure 13). This result indicates that for a given
power supply, the number of reactors should be optimized to achieve best energy efficiency.
Naturally, the configuration and size of individual reactors must be considered.
0.9
0.8
0.7
0.6
Energy0.5
(gNH3/0.4
kWh)
0.3
0.2
0.1
0
1
2
3
Number of reactors
4
5
Figure 13. The number of reactors effect on energy efficiency, Voltage=5000v,
Frequency=10,000Hz, N2/H2=3/1.
Electrode width effect
The width of the electrodes also affected the energy efficiency due to its electric field generation.
We made a group of electrodes with different width. And the results show that the energy
efficiency was the highest when the electrode had a width of 2cm (Figure 14).
1.00
0.80
0.60
Energy
(gNH3/0.40
kWh)
0.20
0.00
2.6
2
1
Electrode width (cm)
0.5
wire
Figure 14. The electrode width effect on energy efficiency, Voltage=5000v,
Frequency=10,000Hz, N2/H2=3/1, four reactors.
Incorporating cooling and circulation with absorption during catalysis
To improve the conversion efficiency, a cooling device and a circulation loop were designed as
shown in Figure 15. Specifically, several improvements have been made further increasing the
synthesis efficiency. A vertical reactor was designed with gas flowing from top to bottom in
order to prevent ununiformed distribution of catalysts and high temperature gradient in the
system, and to increase gases resident time. The temperature of the feeding gas was lowered
using a cooling device before the gas flowed through the plasma assisted catalysis ammonia
synthesis system. This addressed the difficulty in controlling temperature rise during reaction. It
was found that low temperature shifts the reaction equilibrium towards ammonia production as
expected. We developed a parallel reactor system to further improve ammonia yield.
The experiment results showed that the cooling system using liquid nitrogen was very effective
on maintaining low temperature and system stability. The feeding hydrogen and nitrogen were
treated through the cooling system (liquid nitrogen) before they flowed through the plasma
assisted catalysis ammonia synthesis system. This was done to solve the temperature controlling
problem because the lower temperature is in favor of the ammonia production equilibrium. It
turned out that ammonia production rate is very stable during hours of continuous running.
Adsorbent Molecular Sieve 13X has the highest capability and therefore was selected as
ammonia adsorbent in NTP system as shown in Table 1. After reaction was terminated, ammonia
was desorbed from the adsorbent. Nitrogen and hydrogen gas both return to the cooling device
before they enter the reactor. When circulation is on, nitrogen and hydrogen keep entering the
system. We studied the feasibility of the circulation and adsorbent system to improve ammonia
concentration. The data related to reaction and adsorption is shown in Table 2.
Table 1. Ammonia adsorption capability of different adsorbents
Molecular Sieve
13X
MgCl2
Amberlyst 15
0.14
0.06
0.116
Adsorption
Capability
(gNH3/gadsorbent)
Table 2. Reaction and adsorption data
Resident time
on Catalyst (s)
Resident time
on adsorbent
(s)
Reaction rate
on Catalyst
(kgNH3/(kgcat.*h))
Adsorption Capacity
on Adsorbent
(kgNH3/Kg Adsorbent)
0.5
2.5
0.031
0.14
Power system effect
We improved our power system with a new transformer. Higher frequency (up to 25,000Hz) and
higher voltage (up to 30,000V) can now be achieved at the same time. The ammonia yield was
doubled with this new transformer (Table 3).
Table 3. Power system effect
Transformer
type
/maximum
voltage kV
15
30
N2
ml/min
H2
ml/min
800
800
400
400
Total Gas flow rate
ml/min
1200
1200
Improving ratio:
Voltage
kV
Frequency
kHz
Ammonia produced g
NH3/h
9
10
8
13
0.133
0.306
2.3
Catalyst development
Our previous study showed that Ba and Cs could be used as promoters for the Ru catalyst. Cs
can prevent Ru metal from congregating and thus improve catalyst efficiency. Recent tests
showed that higher ammonia yield was achieved when both K and Ba were used as promoters.
In addition, we used pellet pressure equipment to make catalysts with larger particle sizes.
Results show that although the ammonia yield was not improved apparently, catalyst with larger
particle size allowed higher flow rates into the reactor. Different catalyst formulas were also
tested. Results show that the ammonia yield improved about 40% when the content of Ru was
doubled and Al2O3 was used as support (Table 4).
Table 4. Catalyst effect
Ru
%
N2
ml/min
5
800
10
800
Improving ratio:
H2
ml/min
Total Gas flow rate
ml/min
Voltage
kV
Frequency
kHz
Ammonia produced
g NH3/h
400
400
1200
1200
9
9
8
8
0.085
0.133
1.56
Aspen model simulation
We have established an Aspen model to simulate the NTP synthesis process. The simulation results
are shown in Figure 15. Material balance and heat balance has been determined. The system design
is based on ~1 ton ammonia/hour production capacity.
Figure 15. Aspen simulation results
Aspen Model illustration:
1) NTP-CAT Model represents the catalytic non-thermal plasma synthesis reactor (non-thermal
plasma coupled with catalysis)
2) COOLER Model represents first heat exchanger for the gas stream cooling;
3) AIRCOOL Model represents second heat exchanger for the gas stream cooling;
4) ABS Model and ABSHEAT Model together represent the ammonia absorption and
desorption system; in fact, we use several ammonia absorbers in turn (not shown in the
flowsheet).
Aspen Model Material stream illustration:
1) N2 and H2 stream together are reactant gas;
2) 1NH3N2H2 stream is the outlet stream of non-thermal plasma coupled with catalysis reactor;
3) 2NH3N2H2 stream is the outlet stream of the first heat exchanger;
4) 3NH3N2H2 stream is the outlet stream of the second heat exchanger;
5) N2+H2 stream is the cycling stream;
6) NH3OUT stream is the stream after desorption.
Aspen Model Heat stream illustration:
1) The heat obtained from the first heat exchanger is used for the ammonia desorption in the
absorber; the heat transfer medium is high temperature heating oil.
Aspen Model Assumptions:
1) The system design is based on ~1 ton ammonia/hour production capacity; 20t/day.
2) The electrical power provided to the non-thermal plasma coupled with catalysis reactor is 500
kW.
3) The simulation uses a 15% single pass ammonia conversion rate, and a 100% complete
conversion with the circulation system.
4) The temperature for desorption is 200 C.
5) The remaining heat will be removed by the cooling air.
6) The absorption system can completely absorb the ammonia produced.
Conclusions
Through this research, we selected and evaluated different catalysts and developed a unique
ruthenium based catalyst system. We also explored the feasibility of absorbents and evaluated
the effect of various processing parameters and conditions. Temperature and applied electric
field are among the key process variables in addition to catalysts and absorption. Therefore, we
developed a new process that involves cooling of incoming feeding gas, product absorption, and
direct recycling of unreacted gas after absorption. The cooling was proven to be a significant
improvement in reaction rate and catalyst life and stability. The new absorbent is able to capture
100% of the ammonia in the exiting gas. The direct recycling through circulation is expected to
increase the conversion efficiency to 100%, which is extremely important as it removes the need
to separate the product ammonia from the unreacted gas in order to recycle the unreacted gas.
We have also developed and fabricated a small pilot multi-tube reactor system, which has
cooling, absorbent, circulation and five reactors in parallel with band or wire electrodes. All
system parameters have been optimized for the pilot system. Higher N2/H2 ratio was generally
more desirable for improving ammonia yield and energy efficiency since nitrogen has a higher
chemical bond energy and needs higher energy to break the bond. And experiments also showed
that too much nitrogen did not guarantee a higher yield. Higher feeding gas flow rate was very
helpful in improving energy efficiency. However, when the GF reached a certain high level, such
improving effect was not that obvious. Higher voltage would decrease the energy efficiency very
effectively but there was an optimum value for the frequency. There was also an optimal number
of reactors to achieve the highest energy efficiency with the single power supply used. We have
also established an Aspen model to simulate the NTP synthesis process. Material balance and
heat balance has been determined. This can become the basis for future commercial NTP system
design. This pilot system has been kept running continuously for 10 hours and have shown
greatly improved synthesis efficiency and stability.
Future work recommendations
The project team was able to leverage the research conducted within this sponsored project, to
receive additional funding from the University of Minnesota MnDRIVE program as well as the
Legislative Citizens Commission on Minnesota Resources through the Environmental Trust
Fund. Further efforts are needed to improve the reaction rate and conversion and energy
efficiency. Once the technical objectives are accomplished, the NTP based ammonia synthesis
technology does not need to be operated under extreme temperature and pressure conditions like
the Haber Bosch process. NTP based ammonia synthesis technology offers high scalability and
portability, therefore has high potential for distributed production of ammonia on locations
where ammonia fertilizers are needed and wind- generated electricity or biomass-based hydrogen
is readily available. Future work should emphasize the discovery and development of new
synthetic pathways using safe and environmentally friendly substances or renewable feedstock,
selective chemistry with the help of catalytic agents, and energy efficient processes, maximizing
raw material efficiency, and minimizing or eliminating waste. Detailed future work could include
the following:
1. Development of catalyst system for low temperature ammonia synthesis
1) Design catalyst systems
2) Evaluate catalyst performance under different process conditions
3) Optimize catalysis for best conversion and energy efficiency
Catalysts have a key impact on energy efficiency. We will still use Ru based catalysts but focus
on different promoters and supports. We have studied potassium, barium and caesium promoters.
Other alkali or alkaline earth metal oxides will be investigated further. Supports to be studied
include carbon black, activated carbon, high surface area graphite, carbon nanotubes, magnesia
and alumina. Different preparation methods including nano-catalysts (Saadatjou et al. 2014) will
be explored to increase the specific surface areas to boost the activity. We expect that carbon
support materials would improve the stability of the structure of the ruthenium particles
especially when ruthenium loading is relatively high and could help promoters work more
effectively.
2. NTP reactor development and process optimization
We will combine our catalyst preparation process with the design of our NTP reactor type. If we
want to install catalysts in our current reactor configuration which consists of a quartz tube and a
stainless steel tube placed coaxially, we will need to find a better method for catalyst loading.
Two possible reactor configurations are shown in Figure 16&17.
Catalyst coating layer
Porous material
N2
N
H2
H
Power
Figure 16. New Reactor Type A
N2
Catalyst coating layer
H2
Power
N2
H2
Catalyst coating layer
Figure 17. New Reactor Type B
References
Aika, K., Shimazaki, K., Hattori, Y., Ohya, A., Ohshima, S., Shirota, K. and Ozaki, A. (1985)
Support and promoter effect of ruthenium catalyst: I. Characterization of alkali-promoted
ruthenium/alumina catalysts for ammonia synthesis, Journal of Catalysis 92: 296-304.
Amorim, J., Baravian, A.G. and Sultan, G. (1996) Absolute density measurements of ammonia
synthesized in N2H2 mixture discharges, Applied Physics Letters 68: 1915-1917.
Bai, M., Bai, X., Zhang, Z. and Bai, M. (2000) Synthesis of ammonia in a strong electric field
discharge at ambient pressure, Plasma Chemistry and Plasma Processing 20: 511-520.
Bai, M., Zhang, Z., Bai, X., Bai, M. and Ning, W. (2003) Plasma synthesis of ammonia with a
microgap dielectric barrier discharge at ambient pressure, IEEE Transactions on Plasma
Science 31: 1285-1291.
Bielawa, H., Hinrichsen, O., Birkner, A. and Muhler, M. (2001) The Ammonia‐Synthesis
Catalyst of the Next Generation: Barium‐Promoted Oxide‐Supported Ruthenium,
Angewandte Chemie International Edition 40: 1061-1063.
Deng, S., Le, Z., Ruan, R., Yu, F., Reese, M., Cuomo, G. and Chen, P. (2007). Nonthermal
plasma synthesis of ammonia using renewable hydrogen. The 234th ACS National
Meeting. Boston.
Elmoe TD, Sorensen RZ, Quaade U, Christensen CH, Norskov JK, Johannessen T. A highdensity ammonia storage/delivery system based on Mg(NH3)6Cl2 for SCR-DeNOx in
vehicles. Chem Eng Sci. 2006; 61: 2618–2625.
Heath Himstedt, Mark Huberty, Alon McCormick, Lanny Schmidt, and Ed Cussler, Ammonia
Conversion Enhanced by Absorption, in preparation, April 2014.
Liu, C.Y. and Aika, K.-i. (2004) Ammonia absorption into alkaline earth metal halide mixtures
as an ammonia storage material, Industrial & Engineering Chemistry Research 43: 74847491.
Mark S. Huberty, Andrew L. Wagner, Alon McCormick and Edward Cussler*. Ammonia
absorption at Haber process conditions. DOI: 10.1002/aic.13744, AIChE Journal,
Volume 58, Issue 11, pages 3526–3532, November 2012
Marnellos, G. and Stoukides, M. (1998) Ammonia synthesis at atmospheric pressure, Science
282: 98-100.
Mizushima, T., Matsumoto, K., Sugoh, J.-I., Ohkita, H. and Kakuta, N. (2004) Tubular
membrane-like catalyst for reactor with dielectric-barrier- discharge plasma and its
performance in ammonia synthesis, Applied Catalysis A: General 265: 53-59.
Murata, S. and Aika, K.-I. (1992) Preparation and characterization of chlorine-free ruthenium
catalysts and the promoter effect in ammonia synthesis.: 1. An alumina-supported
ruthenium catalyst, Journal of Catalysis 136: 110-117.
Saadatjou, N., Jafari, A. and Sahebdelfar, S. (2014) Ruthenium Nanocatalysts for Ammonia
Synthesis: A Review, Chemical Engineering Communications.
Shur, V. and Yunusov, S. (1998) On the path to catalysts for the low-temperature ammonia
synthesis, Russian chemical bulletin 47: 765-776.
Tekin A, Hummelshoj JS, Jacobsen HS, Sveinbjornsson D, Blanchard D, Norskov JK, Vegge T.
Ammonia dynamics in magnesium ammine from DFT and neutron scattering. Energy
Environ Sci. 2010; 3: 448–456.
Ruan, R., Deng, S., Le, Z., Cheng, Y., Lin, X. and Chen, L. (2014). Non-thermal plasma
synthesis with carbon component. U. S. Patent. US, Regents of the University of
Minnesota.
Uyama, H. and Matsumoto, O. (1989) Synthesis of ammonia in high-frequency discharges. II.
Synthesis of ammonia in a microwave discharge under various conditions, Plasma
Chemistry and Plasma Processing 9: 421-432.