Air Separation Unit Research Articles

A novel hybrid approach to pink and turquoise hydrogen production via oxy-fuel combustion

Hydrogen has been considered a clean energy source essential for net zero by 2050. However, the current green hydrogen production process is not economically feasible compared to the existing hydrogen production pathways. Therefore, this study proposes a hybrid operation of pink and turquoise hydrogen as a stepping-stone until the maturity of green hydrogen. Pink and turquoise hydrogen have their disadvantages: pink hydrogen requires an electric heater (EH) with high power load, and turquoise hydrogen faces economic drawbacks due to the high cost of air separation units (ASU). The proposed hybrid operation can mitigate these weaknesses: (1) by using low-grade waste heat from turquoise hydrogen to reduce the EH power load, and (2) eliminating the ASU process by utilizing high-purity oxygen produced from pink hydrogen for oxy-fuel combustion. We conducted energy, environment, techno-economic, sensitivity, and scenario-based cost analysis for the independent and hybrid operations. With the hybrid operation, total energy and specific energy consumption decreased by 11.5 % and 11.7 %, respectively, compared to independent operation. The environmental assessment showed the CO2 eq. flow of the hybrid operation decreased by 200 % compared to the independent operation, and the total expense and levelized cost of hydrogen (LCOH) decreased by 10.7 % and 10.7 %, respectively. Finally, the analysis revealed the hybrid operation can alleviate uncertainties about cost fluctuations of feedstocks and utilities, as well as the volatility of by-product costs. Therefore, the hybrid operation integrating pink and turquoise hydrogen is expected to serve as a stepping-stone to the transition to green hydrogen.

Efficient enhancement of cryogenic processes: Extracting valuable insights with minimal effort

Cryogenic systems are widely used in industries but are known for their high energy consumption. Designing these systems involves numerous variables, objectives, and constraints. This study introduces an optimization approach applied to three cryogenic processes: Natural Gas Liquids (NGLs) recovery, Air Separation Unit (ASU), and propane mixed refrigerant (C3MR) liquefaction, focusing on minimizing energy input to enhance efficiency and LNG output in a mega LNG plant. Exergy analyses identified energy losses in compressors (47 %), columns (37 %), heat exchangers (10 %), and valves/mixers (5 %), highlighting the need for improved compressor design. Optimization showed that inefficiencies could be reduced by adding inter-cooled compression stages and isentropic/isothermal compression routes. The ASU optimization involved five independent variables with optimized conditions demanding 30 MW of compression power while achieving 51 % separation efficiency. Heat exchangers are the main source of exergy loss in the standalone C3MR cycle, accounting for 61.82 % of 69.0 MW followed by compressors ∼29.27 %. When simulated with utilities, total exergy loss increases to 249.0 MW, with compressors accounting for 78.34 %, exergy loss from heat exchangers drops to 17.94 %. Results affirm the practicality of the proposed method for optimizing complex cryogenic systems, providing valuable engineering insights and potential for significant energy savings.

Process optimization, exergy analysis, and GHG emissions of ammonia production systems by gasification of high-sulfur petroleum coke with CO2 capture

High-sulfur petroleum coke is commonly used as a fuel because of its high carbon content and calorific value. Direct combustion of high-sulfur petroleum coke has the problem of emitting a lot of CO2 and sulfide. Gasification is an environmentally friendly way to utilize high-sulfur petroleum coke. Therefore, this study first established hydrogen production systems with CO2 capture through high-sulfur petroleum coke gasification. The hydrogen is then reacted with nitrogen from air separation unit for producing ammonia, which is easy to transport and store. After optimizing and analyzing the key parameters of the established models, exergy and life cycle GHG emissions analysis are carried out to evaluate the technical performance and environmental impact. In the system with an 85 % gasification conversion rate of petroleum coke, the ammonia production capacity, exergy efficiency, and GHG emissions are 2300 kmol/h, 45.39 %, and 1468 kg CO2-eq/t NH3. When the conversion rate is elevated by ten percentage points, the ammonia production and exergy efficiency are increased to 2664 kmol/h and 51.95 %, while the GHG emissions are only reduced by 85 kg CO2-eq/t NH3 since most of the CO2 produced by the system is captured. Improving the petroleum coke conversion rate can significantly increase the exergy efficiency of ammonia production system, but has little impact on reducing GHG emissions during the process.

Pressurized oxy-fuel combustion with sCO2 cycle and ORC for power production and carbon capture

The pressurization of oxy-fuel combustion presents the opportunity to recuperate more latent heat from the flue gas within the boiler. This study proposed a coal-fired pressurized oxy-fuel combustion plant coupled with the sCO2 Brayton cycle for simultaneous power production and carbon capture. Two configurations, i.e., sCO2 recuperator split and sCO2 bottom cycle, are conceived to fully exploit the flue gas heat from the boiler. Waste heat from various sources, including the air separation unit, the CO2 compression and purification unit and the sCO2 cycle, are recuperated by the organic Rankine cycle (ORC) to augment the net power efficiency. Sensitive analyses, including the turbine parameters, the recuperator split ratio, and the ORC parameters, are conducted to optimize the design and operation conditions of the plant. The plant net power efficiency ranges between 39.4 % and 42.9 %, inclusive of CO2 compression. The levelized cost of electricity (LCOE) is calculated, and the LCOE results are in the range of 101.60 $/MWh and 106.07 $/MWh for different configurations. These findings suggest the system's viability for future coal-fired plant with carbon capture.

New procedure for designing an innovative biomass-solar-wind polygeneration system for sustainable production of power, freshwater, and ammonia using 6E analyses, carbon footprint, water footprint, and multi-objective optimization

The correct management of renewable polygeneration systems is a highly effective approach that can not only help expand renewable energy systems but also ensure sustainable production. In the present work, a new procedure for designing renewable polygeneration systems is introduced. In this procedure, the management of polygeneration systems is discussed from various aspects, including climatic change, techno-economic and environmental conditions, layout design, ecological sustainability, and the sociological approach to polygeneration systems in a practical form. As a case study, an innovative biomass-solar-wind polygeneration system to produce power, ammonia, and freshwater for Ahvaz in Iran was designed and evaluated according to the developed procedure. The integration of multi-effect distillation and humidification-dehumidification desalination has been employed for freshwater production. For ammonia production, syngas produced in the gasification unit and nitrogen obtained from the air separation unit were utilized. Power production has been achieved using a combination of the hydrogen-ammonia Brayton cycle, the supercritical carbon dioxide cycle, and the organic Rankine cycle. The designed system was evaluated using energy, exergy, exergoeconomic, exergoenvironmental, emergoeconomic, emergoenvironmental, carbon footprint, and water footprint analyses. The layout design of the proposed system and the sociological study of the system from a systemic approach have enhanced the visibility of its application. Additionally, the investigation of biomass fuel variations and combinations according to climate, along with optimization and dynamic analyses, are key measures that allow for a more comprehensive evaluation of the system from various management perspectives. The results show that the proposed system is capable of producing 523.34 m3/day freshwater, 10.1 MW of power, and storing 17.75 ton/day of ammonia in optimal state. The energy and exergy efficiency of the polygeneration system in the optimal state is 29.97% and 13.12%. The total cost rate and total environmental impact rate of the entire system in this case are equal to 0.48 $/s and 19.79 mPts/s, respectively. Meanwhile, the defined renewable system has an ecological sustainability index of 1.76.

A process flow of an air separation unit with an energy storage function: Utilizing distillation potential to absorb energy storage air and its performance

The integration of liquid air energy storage (LAES) and air separation units (ASUs) can improve the operation economy of ASUs due to their matching at refrigeration temperature. A process flow of an ASU with energy storage utilizing the distillation potential of the ASU to absorb the released air due to storing energy (i.e., the energy storage air) is proposed. Its novelty is thus: the ASU can be used to absorb the energy storage air to maximize the air utilization and improve the energy efficiency of the integrated system. It can be simulated using Aspen Plus software. Based on the simulated data, an optimal process flow is determined by analyzing the effect of a reduction in discharge of energy storage air on the distillation conditions of the ASU, and its energy efficiency analysis and economic evaluation are conducted. When the discharge of energy storage air is reduced by 50 % during energy storage and the stored liquid air is directly recovered into the ASU during energy release, a proposed process flow with largest absorption for energy storage air could be obtained. Its product exergy efficiencies for energy storage and release are 37.80 % and 37.57 %, respectively. The overall exergy efficiency of the LAES and the electrical round-trip efficiency of the proposed system are both 67.48 %, the electricity cost saving ratio is 6.43 %, and the payback period of the LAES is 2.4 years. The investment in an air booster accounts for the largest proportion of the overall cost (being some 49.3 % of the total investment required for such a LAES system). After the proposed system is applied throughout the industry, carbon dioxide emissions can be decreased by 15.12–26.76 Mt/year on the grid side.

Integration of Chemical Looping Combustion in the Graz Power Cycle

: Effective decarbonization of the power generation sector requires a multi-pronged approach, including the implementation of CO2 capture and storage (CCS) technologies. The Graz cycle features oxy-combustion CO2 capture in a power production scheme which can result in higher thermal efficiencies than that of a combined cycle. However, the auxiliary consumption required by the air separation unit to provide pure O2 results in a significant energy penalty relative to an unabated plant. In order to mitigate this penalty, the present study explores the possibility of chemical looping combustion (CLC) as an alternative means to supply oxygen for conversion of the fuel. For a midscale power plant, despite reducing the levelized cost of electricity (LCOE) by approximately 12.6% at a CO2 tax of EUR 100/ton and a natural gas price of EUR 6.5/GJ and eliminating the energy penalty of CCS relative to an unabated combined cycle, the cost reductions of CLC in the Graz cycle were not compelling relative to commercially available post-combustion CO2 capture with amines. Although the central assumptions yielded a 3% lower cost for the Graz-CLC cycle, an uncertainty quantification study revealed an 85.3% overlap in the interquartile LCOE range with that of the amine benchmark, indicating that the potential economic benefit is small compared to the uncertainty of the assessment. Thus, this study indicates that the potential of CLC in gas-fired power production is limited, even when considering highly efficient advanced configurations like the Graz cycle.

Direct oxidative coupling of methane into ethylene in a catalyst-enhanced planar solid oxide fuel cell stack reactor

This study presents an efficient method to convert methane into valuable olefins using a Solid Oxide Fuel Cell (SOFC) membrane reactor. Methane, abundant in natural and shale gas, serves as a sustainable feedstock due to its favorable carbon-hydrogen ratio. The SOFC membrane reactor eliminates costly air separation units and safety risks associated with explosive methane-oxygen mixtures. A planar SOFC stack achieves over 30 % methane conversion, 40 % ethylene selectivity, and power generation capabilities. Optimization with mixed-conducting ceramics enhances electrical performance without compromising catalytic activity. The stack's scalability is demonstrated, with the 5-cell configuration surpassing the 2-cell setup. This innovative approach offers a sustainable solution to methane waste, producing high-value chemicals while reducing environmental impact.

Techno-economic analysis of blue ammonia synthesis using cryogenic CO2 capture Process-A Danish case investigation

Ammonia is a vital chemical with numerous applications. Currently, the primary methods for generating the necessary reactants for ammonia production involve steam methane reforming (SMR) and cryogenic air separation unit (CASU), while the Haber-Bosch process converts these reactants into ammonia. However, the SMR process releases substantial amounts of CO2, making it imperative to employ an efficient and cost-effective CO2 capture technology to mitigate emissions. This investigation focuses on evaluating the cryogenic CO2 capture (CCC) process for blue ammonia production and provides a thorough economic analysis, estimating both the initial investment costs and operational expenses involved in producing blue ammonia. The results indicated that the CCC process can capture 90% of the CO2 content in the flue gas emitted by the SMR, incurring an energy penalty of 0.724 MJe/kgCO2 while capturing CO2 in the liquid phase with purities exceeding 99.9%. In this case, the estimated CO2 capture costs would be 18.05, 45.1, and 16.65 USD/ton in 2021, 2022, and 2023, respectively. This represents a 40% reduction compared to the CO2 capture costs associated with conventional amine-based technology. The results of this study indicate that the annual electricity costs for ammonia production increase by 38.5% and 64.2% when employing the CCC and amine-based processes, respectively. This investigation employed an isothermal reactor for ammonia synthesis, using the heat from the exothermic reaction in a water ammonia absorption refrigeration cycle (ARC) to condense and purify ammonia. The results show that the ARC system can effectively condense ammonia at −6 °C, producing a liquid ammonia stream with 99.3% purity. This leads to a 95% reduction in power consumption compared to a vapor compression refrigeration cycle (VCRC). Consequently, this method has the potential to decrease the annual operational costs for ammonia production by 2.92%, 2.69%, and 3.13% in 2021, 2022, and 2023, respectively. This study indicated that the hydrogen production unit incurs the highest initial investment costs, as well as operating costs, in the blue ammonia production process, followed by CASU and the Haber-Bosch process.

Thermodynamic and exergy assessment of a biomass oxy-fuel combustion with supercritical carbon dioxide cycle under various recycle flue gas conditions

The ongoing development of second-generation oxy-fuel combustion system with higher oxygen concentration have presented a potential pathway for efficient biomass utilization. It also signified the meticulous role of recycled flue gas in altering the combustion behaviour in the combustor. Accordingly, investigation into different recirculation modes focusing on recycle ratio, wet/dry conditions, and excess oxygen ratio are conducted with an indirect supercritical power cycle integrated biomass oxy-fuel combustion system. Energy and exergy analyses, as well as composition of flue gas produced are presented. Energy and exergy efficiencies increased with reducing recycle ratio, with respective highest efficiency of 31.33% and 44.37% obtained at excess oxygen ratio of 1.01 and recycle ratio of 0.5 under wet condition. Investigation of CO2 and H2O concentration in produced flue gas depicted a separated stage profile under dry recycle condition, different from the slight diverging profile observed under wet condition when the recycle ratio varied. Concentration of NOx and SOx increased with decreasing recycle ratio and are generally higher under dry condition. Selective exergy analyses of the proposed power cycle presented highest exergy loss in the combustor, followed by the air separation unit, heat exchanger, CO2 compression and purification unit, and turbine generator. Based on the obtained result, wet recycle flue gas enhanced the combustion performance in the combustor but slightly impaired the convective heat transfer in the heat exchanger. The performance of proposed biomass power cycle is optimized at low recycle ratio of 0.5 under wet recycle condition.

Novel Separation Processes and Their Applications

Separation processes are an integral part of any process flow sheet. Various techniques can beused to separatethe mixture depending on the raw mix. Sometimes, two or more methods must be used to get the desiredproduct. Differences in chemical and physical properties also help decide the separation technique that mustbe used. An external agent, any form of energyor matter, can act as the driving force for the separation. Someof these techniques are conventional processes like distillation, filtration, adsorption, and absorption, whichhave already been well-studied and extensively used. These days newer separation processes called novelseparation processes, such as membrane separation, gas separation, supercritical fluid extraction, use ofultrasonics, chromatographic separation, magnetic projection, and liquid- liquid extraction, among manyothers, are gaining importance in the area of research and implementation. Novel processes were firstimplemented as analytical tools in laboratories. However, they developed rapidly to become significant commercially and technically. The development in gas separation techniques has led to using Liquified Natural Gas in Air Separation Units to produce high-purity nitrogen and oxygen4. Novel separation methods also include ultrasound to enhance separation processes like extraction, demulsification, and crystallisation. Pressure-driven processes are a subset of membrane separation techniques where pressure is utilised as a driving force for separation, with asemipermeable membrane acting as a barrier. This method has various applications ranging from wastewater treatment to dairy processing. Chiral chromatography is used for enantiomeric separations by the use ofHPLC. A new magnetic separation process proposes the separation of plastics by submerging them in a paramagnetic medium and attaching a magnet. This results in moving the particles inside the medium with different trajectories, thereby separating them. This article will scrutinise and briefly describe the essential aspects and developments of novelseparation processes and their applications.

Systems design and techno-economic analysis of a novel cryogenic carbon capture process integrated with an air separation unit for autothermal reforming blue hydrogen production system

In the transition of the global energy paradigm toward decarbonization, hydrogen is expected to play a crucial role as an energy vector. To enhance the sustainability of hydrogen as an energy source, it is important to reduce costs and carbon emissions in the hydrogen production. Among various hydrogen production technologies, the autothermal reforming (ATR)-based blue hydrogen production method can stand out as a promising option in terms of both economic and environmental considerations. However, there are several bottlenecks: (i) ATR requires an air separation unit that increases the cost burden, and (ii) conventional amine-based carbon capture method consumes a significant amount of energy. To address these bottlenecks, this study proposes a novel cryogenic carbon capture process for ATR-based blue hydrogen production. In the proposed process, air separation unit are utilized not only for oxygen production but also for carbon capture. The compressed air from the air separation unit provides cooling energy to the mixture gas, leading to solidification and separation of carbon dioxide. Despite imposing an additional burden on the air separation unit, it significantly reduces the energy and cost associated with carbon capture. Consequently, there exists a trade-off relationship between the increased load on the air separation unit and the decreased load in carbon capture. This trade-off was evaluated by comparing it with the conventional absorption-based carbon capture process in terms of energy and economic aspects. The proposed process exhibits a 36.4% reduction in total energy consumption, with a particularly approximately 49.5% decrease in the energy consumed for carbon capture (0.278 kW h/kg-CO2). The economic performance indicates an 11% decrease in the levelized cost of hydrogen (1.62$/kg-H2), and a 45.3% decrease in CO2 avoidance cost (40.1$/kg-CO2). The captured solid CO2 can offer advantages during storage and transportation. Furthermore, the potential utilization of cooling energy of solid CO2 from an end-user perspective is expected to contribute to the establishment of a new industrial value chain.

Techno-economic analysis of small-scale ammonia production via sorption-enhanced gasification of biomass

Biomass gasification is a green and efficient pathway for lowering the carbon footprint of ammonia production. Decentralized, small-scale biomass gasification plants (<150 t NH3/day output) are more practical than large plants due to sustainable large-scale biomass supply constraints. Small-scale biomass gasification for ammonia production faces the challenges of high energy consumption and production costs. Process intensification through sorption-enhanced gasification (SEG) integrates steam gasification and in-situ CO2 separation using regenerable CaO sorbents, directly producing hydrogen-rich syngas (∼70 vol%), which streamlines syngas conditioning and increases cost-effectiveness. Two SEG process configurations for small-scale biomass-to-ammonia (∼45000 t of agricultural residues/year) were proposed: SEG process A, which produced low-carbon ammonia without air separation, and SEG process B, which employed an air separation unit to deliver carbon-negative ammonia by capturing CO2. The two processes were compared against a dual fluidized bed gasification (DFBG) process, which generated green ammonia without an air separation, like SEG process A. Of the three cases, SEG process B showed the lowest biomass (2.14 t biomass/t NH3) and water consumption (0.47 t H2O/t NH3) and avoided 2.8 t CO2/t NH3, making it carbon-negative. While the DFBG process and SEG process A had higher energy efficiencies, SEG process B posted the lowest net energy consumption (36.6 GJ/t NH3). Furthermore, SEG process B had the lowest ammonia production cost ($692/t NH3) and payback time (9.4 years), making it the most economically viable. Sensitivity analysis showed that favorable carbon credit policies could boost the economics of SEG process B to match large-scale plants.

Performance and cost potential for direct-fired supercritical CO2 natural gas power plants

Direct-fired supercritical CO2 (sCO2) power cycles are being explored as an attractive alternative to natural gas combined cycle (NGCC) plants with carbon capture and storage (CCS). Therefore, understanding their performance and cost potential is important for the commercialization of the technology. This study presents detailed techno-economic optimization results of natural gas-fired, utility-scale power plants based on the direct sCO2 power cycle, which is lacking in public literature. The study considered multiple plant configurations with varying levels of thermal integration with the plant air separation unit (ASU) to shows the systematic impact of thermal integration on plant performance and economics. Several design variables for each power cycle configuration were identified and optimized to minimize the levelized cost of electricity (LCOE). The optimized direct sCO2 power plants achieved ∼48 % plant efficiency (HHV basis) which is similar or slightly higher plant efficiencies than state-of-the-art NGCC plants based on the F-class gas turbine with carbon capture and storage (CCS). The LCOE of the optimized direct sCO2 plants is 13–17 % higher than the reference NGCC plants with CCS due to high capital costs associated with the ASU and sCO2 power block, though there is significant room for improvement due to high uncertainty in component capital costs for these new plants. Based on the results, direct sCO2 power cycles have the potential to achieve >50 % efficiency (HHV basis) with >98 % CO2 capture rate which can lead to significant efficiency improvement and CO2 emissions reductions compared to NGCC plants with CCS.

Oxygen Storage Incorporated Into Net Power and the Allam–Fetvedt Oxy-Fuel sCO2 Power Cycle—Techno-Economic Analysis

Abstract With the planned future reliance on variable renewable energy, the ability to store energy for prolonged time periods will be required to reduce the disruption of market fluctuations. This paper presents a method to analyze a hybrid liquid-oxygen (LOx) storage/direct-fired supercritical carbon dioxide (sCO2) power cycle and optimize the economic performance over a diverse range of scenarios. The system utilizes a modified version of the NET Power process to produce energy when energy demand exceeds the supply while displacing much of the cost of the air separation unit (ASU) energy requirements through cryogenic storage of oxygen. The model uses marginal cost of energy data to determine the optimal times to charge and discharge the system over a given scenario. The model then applies ramp rates and other time-dependent factors to generate an economic model for the system without storage considerations. The size of the storage system is then applied to create a realistic model of the plant operation. From the real plant operation model, the amount of energy charged and discharged, the capital expenditures (CAPEX) of each system, energy costs and revenue and other parameters can be calculated. The economic parameters are then combined to calculate the net present value (NPV) of the system for the given scenario. The model was then run through the SMPSO genetic algorithm in Python for a variety of geographic regions and large-scale scenarios (high solar penetration) to maximize the NPV based on multiple parameters for each subsystem. The LOx storage requirements will also be discussed.

Novel Study on Cryogenic Distillation Process and Application by Using CHEMCAD Simulation.

The purpose of this study was to examine the cryogenic separation process used in air separation plants with the CHEMCAD simulation program. A preliminary analysis was carried out with the literature about the operation of the process. In light of the technical information, the simulation of the cryogenic separation process was utilized with the program. While no changes were analyzed in the basic parts of the process, the filtration of the air and separation from moisture and carbon dioxide were not included in the simulation. The simulation showed the three basic components of air, nitrogen, oxygen, and argon obtained as a product of the desired purity rather than a one-to-one demonstration of the applications in the industry. In addition, the low-temperature separation process of an air separation unit was studied to obtain high-purity products (99.49-99.99%) achieving the expected separation efficiency. As a result, the production of all three substances in desired purity was achieved successfully.

Analysis and optimization of a sustainable hybrid solar-biomass polygeneration system for production of power, heating, drying, oxygen and ammonia

In this study, we introduce an innovative polygeneration system that combines solar and biomass energies to generate power, heating, hot air for the drying process, oxygen, and ammonia. The proposed polygeneration system comprises a solar tower subsystem (STS) integrated with two energy storage tanks (cold and hot), a gas turbine cycle (GTC), an organic Rankine cycle (ORC), a proton exchange membrane (PEM) electrolyzer, a thermoelectric generator (TEG), a cryogenic air separation unit (CASU), and an ammonia synthesis reactor (ASR). The GTC and the ORC equipped with the TEG are utilized for power production. The steam for the gasification process, hot air for the drying process and heating are produced by three regenerators. The PEM electrolyzer is used for the decomposition of water into hydrogen and oxygen, while the air separator (cryogenic method) is employed for the separation of air into nitrogen and oxygen. The separated oxygen is stored in an oxygen tank, while nitrogen and hydrogen are utilized for the ammonia generation. The system is evaluated from energy, exergy, economic and, environmental standpoints. Based on the findings, the proposed system is capable of generating approximately 163 MW of net power, 150 MW of heating, 100 MW of drying, 21.7 kmol/hr of oxygen and 23.24 kg/hr of ammonia. The total energy and exergy efficiencies, as well as the life cycle cost (LCC) and CO2 emissions are determined to be 70.68 %, 48.71 %, 1.175 × 109 $ and 381.3 kg/MWh, respectively. Additionally, a multi-objective Grey Wolf optimization algorithm is utilized to optimize the exergy efficiency, LCC, and CO2 emissions. The optimized results yield an exergy efficiency of 51.02 %, an LCC of 1.16 × 109 $, and a CO2 emissions of 359.73 kg/MWh. The proposed system demonstrates encouraging thermodynamic and economic performances, providing an appealing solution for efficient utilization of abundant solar and biomass resources.

Utmost substance recovery and utilization for integrated technology of air separation unit and liquid air energy storage and its saving benefits

The integration of air separation units (ASUs) and liquid air energy storage (LAES) (ASU-LAES) can bring very good economic benefits based on their resource complementarity at the same low-temperature energy level. Two types of novel process flows are proposed in this paper for ASU-LAES. These flows can use the ASU to recover the maximum amount of refrigerated air from energy storage process to the front or back of the air compressor (i.e., pre- or post-machine recovery), so as to improve the energy efficiency and economic benefits. The pre-machine recovery process flow can achieve an electrical roundtrip efficiency of up to 76.38%. Although a lower refrigeration expansion ratio is used in the post-machine recovery process flow, it can form an air refrigeration cycle without passing through air compressor. Its exergy loss from expansion and compression is smaller, which maximizes liquid air storage. The electrical round-trip efficiency can reach up to 72.9%, and the larger storage scale increases its economic benefit compared to the pre-machine recovery. Based on the peak-to-valley price ratio of 3.4:1, the post-machine recovery process flow can save up to 13.23% of electricity cost compared to the conventional ASU. The dynamic payback period of the LAES is 2.27 years. Compared to ASU-LAES without recovery of refrigeration air, the electricity cost-saving ratio is improved by 134.9%, and the dynamic payback period of the LAES is shorten by 41.8%.

Waste oxy-fuel combustion integrated with supercritical CO2 cycle and high-temperature electrolysis technologies for e-methanol production: A feasibility analysis

The municipal solid waste (MSW) oxy-fuel incineration power plant based on the supercritical carbon dioxide (sCO2) cycle integrated with high-temperature electrolysis technology has been developed with the aim of compensating the energy penalties of air separation unit (ASU), reducing the carbon emission and producing renewable methanol by CO2 hydrogenation process. Results shows that the suitable excess oxygen ratio, flue gas circulation ratio and methanol synthesis temperature are 1.1, 0.74 and 240 °C, respectively. The thermal efficiency of proposed system reaches 74.83%, above the that of traditional MSW incineration power plant. In addition, environmental analysis shows that the carbon retention index (CRI) is 97.76%, and the goal of near zero carbon emission can be achieved. Economic analysis demonstrates that the levelized cost of methanol (LCOM) of proposed system is 728.40 $/t, which is more than double the current market price of methanol. However, with the technological breakthrough and expansion of market scale in the further, e-methanol shows economic competitiveness when the cost of water electrolysis unit is reduced to 40 M$/250 MW and carbon emission credit is higher than 70 $/t. This work is innovative, and provides a high-efficiency and sustainable MSW disposal and energy recovering scheme for e-methanol production.

Performance analysis of a modified Allam cycle combined with an improved LNG cold energy utilization method

The Allam cycle is a promising power cycle that could achieve 100% carbon capture as well as high efficiency. In order to further enhance system operating performance, here we propose a modified Allam cycle with an improved liquified natural gas (LNG) cold energy utilization method. The flow rate fluctuation of LNG is suppressed by variable speed adjustment of the air compressor, and the cold energy of LNG is transferred to liquid oxygen, which could implement a stable cold energy supply. The whole process is modeled including air separation unit and LNG supply path. Furthermore, the system thermodynamic and economic performance is evaluated through parametric analysis, and the proposed system superiority is highlighted by comparing with conventional Allam-LNG cycle. The results indicate that the system could achieve 70.93% of net thermal efficiency, 65.17% of electrical efficiency, and $403.63 million of net present value, which performs 5.76% and 6.48% enhancement of efficiency and 11% improvement of economic revenue. Moreover, the system off-design operation is assessed; 87% to 100% of compressor speed adjustment range is determined that could cope with −13% to 9% of LNG flow rate fluctuation.