DOI: 10.3303/CET2188081
Paper Received: 29 April 2021; Revised: 17 July 2021; Accepted: 1 October 2021
Please cite this article as: Šulc R., Ditl P., 2021, The Potential of Liquefied Oxygen Storage for Flexible Oxygen-Pressure Swing Adsorption
Unit, Chemical Engineering Transactions, 88, 487-492 DOI:10.3303/CET2188081
CHEMICAL ENGINEERING TRANSACTIONS
VOL. 88, 2021
A publication of
The Italian Association
of Chemical Engineering
Online at www.aidic.it/cet
Guest Editors: Petar S. Varbanov, Yee Van Fan, Jiří J. Klemeš
Copyright © 2021, AIDIC Servizi S.r.l.
ISBN 978-88-95608-86-0; ISSN 2283-9216
The Potential of Liquefied Oxygen Storage for Flexible
Oxygen-Pressure Swing Adsorption Unit
Radek Šulc*, Pavel Ditl
Czech Technical University in Prague, Faculty of Mechanical Engineering, Department of Process Engineering, Technická
4, Prague, Czech Republic
Radek.Sulc@fs.cvut.cz
The pressure swing adsorption (PSA) units are widely used as an oxygen source. Start-up time taking minutes
is an undeniable advantage of PSA technology compared to cryogenic air separation start-up time taking hours
or days. The increasing share of renewable electricity causes intraday electricity price fluctuations. These
fluctuations can be an opportunity to improve the economy of a plant and/or to accumulate electricity in the form
of liquefied products. This paper aims to demonstrate the possibility of a flexible PSA unit connected to a small
oxy-fuel combustion unit. Two options were analyzed: i) LOX supply at electricity price peak, and ii) liquid oxygen
energy storage (LOES). The cold energy needed for oxygen liquefaction will be obtained utilizing liquefied
nitrogen (LIN) delivered from a large air separation unit (ASU). The analysis was carried out for the Czech
Republic, the Federal Republic of Germany, and the Kingdom of Denmark. These countries differ significantly
in the energy mix.
1. Introduction
The Pressure Swing Adsorption (PSA) units are widely used as oxygen sources where oxygen is produced in
gaseous form. Start-up time taking minutes is an undeniable advantage of PSA technology compared to
cryogenic air separation start-up time taking hours or days. The capacity of a pressure vessel limits the storage
of pressurized gaseous oxygen.
The increasing share of renewable electricity causes intraday electricity price fluctuations. These fluctuations
can be an opportunity to improve the economy of a plant (Miller et al., 2008) depending on market conditions
(Cao et al., 2017), and to accumulate electricity in the form of liquefied products (Casperi et al., 2019a, 2019b).
Miller et al. (2008) analyzed the energy and capital costs of a cryogenic air separation unit (ASU). They proposed
a simplified economic model enabling to take hourly variations in electricity prices into account. Cao et al. (2017)
presented a dynamic model for pre-emptive ASU performance prediction depending on market conditions.
Casperi et al. (2019a) designed a flexible ASU with an integrated liquefaction cycle and liquid assist operation.
This ASU allows changing the power demand from 3.5 to 28 MW without violating operational constraints by
changing the nitrogen and oxygen production. The flexibility of the designed process was tested over a time
horizon of two days with historical electricity prices, and an improvement of 14 % was found in comparison with
quasi-stationary scheduling (Casperi et al., 2019b). This way, the energy can be stored in the form of a cold
energy storage system. Liu et al. (2020) studied the applicability of single-and multi-component fluid cycles for
liquid air energy storage (LAES) to increase the performance of cold cycles for air liquefaction.
The increasing number of sizeable cryogenic air separation units connected to oxy-combustion power plants or
integrated gasification combined cycle plants can generate large amounts of nitrogen whose utilization is limited
but can be utilized for energy storage in the liquid form.
This paper aims to demonstrate the possibility of a flexible PSA unit connected to a small oxy-fuel combustion
unit. The following two options were analyzed: i) LOX supply in the electricity peak, and ii) the liquid oxygen
energy storage (LOES) where the cold energy needed for oxygen liquefaction will be obtained utilizing liquefied
nitrogen (LIN) delivered from a large ASU unit.
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2. Oxygen production by Pressure Swing Adsorption
Šulc and Ditl (2021b) investigated the potential of consumed energy savings in the PSA unit used as an oxygen
source for the oxy-fuel combustion unit to reach the ecologically friendlier processing. They analyzed the
following four options: i) single or dual compression, ii) utilization of waste compression heat for coal or biomass
dewatering, iii) utilization of dry waste gas from the PSA unit for coal or biomass dewatering, and iv) energy
recovery by an expansion of pressurized oxygen (GOX) before combustion. The analysis was carried out for
on-site oxygen production using two-bed Pressure Swing Adsorption for 95 % purity and oxygen recovery
characterized by the air ratio of 10 Nm3 Nm-3 producing 101 Nm3 h-1 of gaseous oxygen (GOX). They identified
the highest potential of energy saving for dual compression and utilization of low-grade waste compression heat
for fuel drying. 10 % of the electrical energy may be saved, and the specific energy consumption decreases
from 0.805 kWh kgO2-1 for a single compression to 0.728 kWh kgO2-1 using dual compression. It represents a
saving of 85.68 MWh y-1 for the annual operating time of 8,160 h. Utilization of low-grade waste compression
heat for fuel drying was enabled to reduce fuel consumption depending on fuel moisture, e.g., by 5.3 % or 10.4
% of lignite or wood, respectively, for reference fuel conditions. The data mentioned above were obtained for
ambient air and compressed air at an outlet temperature of 30 °C and outlet pressure of 750 kPa (a) at the PSA
unit inlet. The pressure losses of inter-and after-coolers were taken into account.
3. Intra-day electricity prices
The electricity cost is a major cost item of oxygen production by pressure swing adsorption. Electricity prices
vary from country to country, from supplier to supplier, and depend considerably on annual electricity take-off
(Šulc and Ditl, 2021a). For the analysis, the day-ahead prices reported by ENTSO-E Transparency Platform
were used. No taxes (VAT and recoverable taxes) and levies were not taken into account. The prices vary during
the year, months, and days. Therefore, the data for the randomly selected 2nd Wednesday in January, April,
July, and October of the year 2020 respecting the winter, spring, summer, and autumn seasons, respectively,
were overtaken. The analysis was carried out for the Czech Republic, the Federal Republic of Germany, and
the Kingdom of Denmark. These countries differ significantly in the energy mix. The Czech Republic generated
an average approx. 36 % in nuclear power plants, 49 % in thermal power plants, and 14 % by renewable energy
sources in selected days (Figure 1 in detail). Unlike this, Denmark generated an average of approx. 25 % of
energy in thermal power plants, 75 % renewable energy sources, and no energy is produced in nuclear power
plants. The energy mix of Germany was between both countries. Germany generated an average approx. 12 %
of energy in nuclear power plants, 40 % in thermal power plants, and 47 % renewable energy sources.
a) Thermal power plants b) Renewable energy sources (RES)
Figure 1: Share of sources for electricity production: a) thermal power plants (coal, gas, waste combustion), b)
renewable energy sources (solar, wind, biomass, water)
The day-ahead prices for January 8, April 8, July 8, and October 7 of the year 2020 and their comparison for
the Czech Republic, Germany, and Denmark are presented in Figure 2. The significant morning and evening
peaks are visible from 7 to 12 a.m. and from 7 to 9 p.m. In some cases, a significant price falls due to an excess
of renewable energy sources.
488
a) January 8, 2020 b) April 8, 2020
c) July 8, 2020 d) October 7, 2020
Figure 2: Day-ahead prices for the Czech Republic, Germany, and Denmark: a) January 8, 2020; b) April 8,
2020; c) July 8, 2020; d) October 7, 2020
4. LOX supply at electricity price peak
The benchmark specific electricity consumption of liquefied oxygen (LOX) is 638 kWh tLOX-1 (EIGA, 2019). The
difference between the specific electricity consumption of LOX and GOX produced by the PSA unit may be
utilized for cost-saving. In this case, the oxygen needed for the process is supplied by the PSA unit during a
period in which the electricity price is low. During the electricity price peak, the oxygen needed for the process
is produced from liquefied oxygen (LOX) supplied from large ASU facilities continuously operated through a
day. Liquefied oxygen is evaporated by ambient air.
The following assumptions were adopted for the following model for calculating the cost-saving:
1) the electricity cost is taken only into account; the investment cost and other operational costs such as
depreciation, maintenance, etc. are not included,
2) the LOX price is estimated based on the benchmark specific electricity consumption of LOX production and
the daily averaged day-ahead electricity price,
3) the LOX transportation cost is not included.
The daily average electricity price cel-daily (EUR MWh-1) was obtained numerically by the trapezoidal integration
method. The off-peak average electricity price cel-offpeak (EUR MWh-1) was calculated analogically by the same
procedure but for the off-peak period. The 24-hour operation and peak periods from 7 a.m. to 12 a.m. and from
6 p.m. to 9 p.m. were assumed. The electricity cost Cel-PSA-daily (EUR d-1) for PSA unit operated for an operation
time toper (h) was calculated:
Cel-PSA-daily = cel-daily ePSA toper , (1)
where ePSA (MWh tO2-1) is the specific electricity consumption of the PSA unit. The electricity cost Cel-PSA-off-peak
(EUR d-1) for PSA unit operated during off-peak period toff-peak (h) was calculated:
Cel-PSA-off-peak = cel-off-peak ePSA toff-peak. (2)
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The LOX cost CLOX-peak (EUR d-1) for LOX consumed during peak period tpeak (h) was calculated:
Cel-LOX-peak = cel-daily eLOX tpeak = cel-daily eLOX (toper - toff-peak ), (3)
where eLOX (MWh tLOX-1) is the specific electricity consumption of LOX production. Then, the cost saving is
estimated as it follows:
cost-saving (%) = 100( Cel-PSA-off-peak + Cel-LOX-peak)/ Cel-PSA-daily . (4)
The cost-saving calculated using the proposed model is presented in Tables 1, 2, and 3 for the Czech Republic,
Germany, and Denmark.
Table 1: LOX supply at electricity price peak – the Czech Republic
Description Unit January 8 April 8 July 8 October 7
Input data
Daily average day-ahead price *1 EUR MWh-1 44.84 24.54 42.59 38.00
Off-peak average electricity price *1 EUR MWh-1 42.40 23.30 41.15 34.34
LOX price based on daily average el. price*1,2 EUR tO2-1 28.61 15.66 27.17 24.24
Single compression *3,4
PSA unit: cost for daily average el. price EUR d-1 866 474 823 734
PSA unit: cost for off-peak production EUR d-1 546 300 530 442
LOX consumed during electricity price peaks EUR d-1 229 125 227 194
Oxygen cost by combined PSA+LOX EUR d-1 775 425 747 636
Cost saving *5 % 10.5 10.3 9.2 13.3
Dual compression *3,4
PSA unit: cost for daily average el. price EUR d-1 784 429 744 664
PSA unit: cost for off-peak production EUR d-1 494 271 479 400
LOX consumed during electricity price peaks EUR d-1 229 125 217 194
Oxygen cost by combined PSA+LOX EUR d-1 723 397 697 594
Cost saving *5 % 7.8 7.5 6.4 10.5
Note:*1 Operation time = 24 h d-1, peak period: from 7 a.m. to 12 a.m. and from 6 p.m. to 9 p.m.
Note:*2 Specific electricity demand for LOX production: 0.638 MWh tO2-1.
Note:*3 PSA unit: ambient air: temperature of 20 °C, pressure of 100 kPa (a), relative humidity of 70 %;
compressed air; outlet temperature of 30 °C, outlet pressure of 750 kPa (a); specific electricity demand: single
compression 0.805 MWh tO2-1, dual compression 0.728 MWh tO2-1.
Note: *4: Calculation was performed for the specific oxygen production capacity of 1 t h-1.
Note: *5 Cost savings: related to the cost of PSA production for daily average electricity price.
Table 2: LOX supply at electricity price peak – Germany
Description Unit January 8 April 8 July 8 October 7
Input data
Daily average day-ahead price *1 EUR MWh-1 33.31 26.01 40.99 36.82
Off-peak average electricity price *1 EUR MWh-1 29.67 24.44 39.28 33.37
LOX price based on daily average el. price*1,2 EUR tO2-1 21.25 16.59 26.15 23.49
Single compression *3,4
PSA unit: cost for daily average el. price EUR d-1 644 502 792 711
PSA unit: cost for off-peak production EUR d-1 382 315 506 430
LOX consumed during electricity price peaks EUR d-1 170 133 209 188
Oxygen cost by combined PSA+LOX EUR d-1 552 448 715 618
Cost saving *5 % 14.2 10.9 9.7 13.1
Dual compression *3,4
PSA unit: cost for daily average el. price EUR d-1 582 454 716 643
PSA unit: cost for off-peak production EUR d-1 346 285 457 389
LOX consumed during electricity price peaks EUR d-1 170 133 201 188
Oxygen cost by combined PSA+LOX EUR d-1 516 417 667 577
Cost saving *5 % 11.4 8.1 6.9 10.3
Note:*1-5 see Table 1 in detail.
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Table 3: LOX supply at electricity price peak – Denmark
Description Unit January 8 April 8 July 8 October 7
Input data
Daily average day-ahead price *1 EUR MWh-1 24.44 25.72 38.17 32.50
Off-peak average electricity price *1 EUR MWh-1 20.91 24.20 35.05 26.87
LOX price based on daily average el. price*1,2 EUR tO2-1 15.59 16.41 24.35 20.73
Single compression *3,4
PSA unit: cost for daily average el. price EUR d-1 472 497 737 628
PSA unit: cost for off-peak production EUR d-1 269 312 451 346
LOX consumed during electricity price peaks EUR d-1 125 131 195 166
Oxygen cost by combined PSA+LOX EUR 394 443 646 512
Cost saving *5 % 16.6 10.8 12.4 18.5
Dual compression *3,4
PSA unit: cost for daily average el. price EUR d-1 427 449 667 568
PSA unit: cost for off-peak production EUR d-1 244 282 408 313
LOX consumed during electricity price peaks EUR d-1 125 131 195 166
Oxygen cost by combined PSA+LOX EUR d-1 368 413 603 479
Cost saving *5 % 13.8 8.0 9.6 15.7
Note:*1-5 see Table 1 in detail.
Table 4: On-site oxygen liquefaction and liquid oxygen energy storage (LOES) – Denmark
Description Unit January 8 April 8 July 8 October 7
Input data
Daily average day-ahead price *1 EUR MWh-1 24.44 25.72 38.17 32.50
Average electricity price for storage period *1 EUR MWh-1 5.35 20.07 19.52 10.66
LIN price based on daily average el. price*1,2 EUR tN2-1 13.42 14.12 20.95 17.84
Single compression *3,4
PSA unit: cost for daily average el. price EUR d-1 472 497 737 628
PSA unit: cost for off-peak production EUR d-1 429 439 675 555
PSA unit: cost for stored production (LOES) EUR d-1 22 81 79 43
LIN consumed for liquefaction (LOES) *5 EUR d-1 63 66 98 83
Oxygen cost by combined PSA+LOES EUR d-1 514 586 851 682
Gain (+)/loss (-)*6 % -8.8 -17.9 -15.5 -8.6
Dual compression *3,4
PSA unit: cost for daily average el. price EUR d-1 427 449 667 568
PSA unit: cost for off-peak production EUR d-1 313 320 491 404
PSA unit: cost for stored production (LOES) EUR d-1 19 73 71 39
LIN consumed for liquefaction (LOES) *5 EUR d-1 63 66 98 83
Oxygen cost by combined PSA+LOES EUR d-1 395 459 660 526
Gain (+)/loss (-)*6 % 7.6 -2.1 1.0 7.3
Note:*1 Operation time = 24 h d-1, storage period from 1 a.m. to 6 a.m., peak period: from 8 a.m. to 10 a.m. and
from 7 p.m. to 10 p.m.
Note:*2 Specific electricity demand for LIN production: 0.549 MWh tN2-1.
Note:*3 see Table 1 in detail.
Note: *4: Calculation was performed for the specific oxygen production capacity of 1 t h-1.
Note: *5 Specific LIN consumption for oxygen liquefaction: 0.935 kgLIN kgO2-1.
Note: *6 Gain/loss: related to the cost of PSA production for daily average electricity price.
5. On-site oxygen liquefaction and liquid oxygen energy storage (LOES)
When the electricity price is lowest, the second PSA unit is started, and the on-site produced gaseous oxygen
is liquefied and stored in the storage tank. The cold energy needed for oxygen liquefaction will be obtained
utilizing liquefied nitrogen (LIN) delivered from large ASU facilities continuously operated throughout the day.
During the electricity price peak, the stored liquefied oxygen produced during the storage period is regasified
and supplied to the consumer technology, e.g., in the oxyfuel combustion unit. Outside the electricity price peak,
the oxygen needed is provided by the master PSA unit. The required second PSA unit, which is not fully
exploited during the day, may seem to be a disadvantage of this solution. However, the critical pieces of
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equipment in the industry are usually doubled to ensure the reliability of the plant operation. The assumptions
analogical to the previous option were adopted for calculating the effectiveness of the process proposed. The
24 h operation storage period from 1 a.m. to 6 a.m., and peak periods from 8 a.m. to 10 a.m. and from 7 p.m.
to 10 p.m. were assumed. Assuming the liquefaction of gaseous oxygen (95 % O2, 5 % Ar) at 400 kPa by LIN
evaporated at the pressure of 101 kPa, the specific LIN consumption of 0.935 kgLIN kgO2-1 was calculated using
ASPEN+ software. The LIN cost CLIN-storage (EUR d-1) for LIN consumed for oxygen liquefaction during the
storage period tstorage (h) was calculated:
Cel-LIN-storage = cel-daily eLIN tstorage, (5)
where eLIN (MWh tLIN-1) is the specific electricity consumption of LIN production. The benchmark specific
electricity consumption of liquefied nitrogen (LIN) is 549 kWh tLIN-1 (EIGA, 2019).
Then, the gain(+)/loss(-) rate is estimated as it follows:
gain/loss (%) = 100( Cel-PSA-off-peak + Cel-PSA-storage + Cel-LIN-storage)/ Cel-PSA-daily . (6)
The effectiveness of on-site oxygen liquefaction and storage calculated using the proposed model is presented
in Table 4 for Denmark only. The positive benefit was found only for the PSA unit with double compression when
the electricity price in the storage period was approx. three-four times lower than the daily average electricity
price. In the Czech Republic and Germany, the price volatility was insufficient to reach positive effectiveness.
7. Conclusions
This paper aims to demonstrate the possibility of a flexible PSA unit connected to a small oxy-fuel combustion
unit. The following two options were analyzed: i) LOX supply in electricity price peak, and ii) liquid oxygen energy
storage (LOES). The cold energy needed for oxygen liquefaction will be obtained utilizing liquefied nitrogen
(LIN) delivered from a large air separation unit. The theoretical potential of LOX supply in electricity price peak
was found to be around 10-14 % of cost-saving compared with the daily operation of PSA unit when the off-
peak average electricity price was from 90 to 86 % of the daily average electricity price respectively. Widening
the price gap, the potential is growing. Unlike this, the on-site oxygen liquefaction and storage was found to be
effective only for PSA units with double compression when the electricity price in the storage period was approx.
three-four times lower than the daily average electricity price. Combining the PSA unit and the electricity
accumulator seems to be a more prospective technology for more effective on-site oxygen production.
Acknowledgements
This work was supported by the Ministry of Education, Youth and Sports of the Czech Republic under OP RDE
grant number CZ.02.1.01/0.0/0.0/16_019/0000753 "Research center for low-carbon energy technologies".
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