Two-stage CO2 Research Articles

Application of a semi-empirical modelling approach to a Two-Stage Rotary CO2 compressor

2-stage Rotary CO2 compressors are currently in use in refrigeration and heat pump applications, including occasionally including vapour injection. The need of correctly assessing the compressor’s efficiency by means of a computationally fast model arises for cycle performance prediction and optimization. In response, an innovation on an existing semi-empirical modelling approach has been introduced in this paper to describe 2-stage Rotary compressors. The model has been validated with experimental data of a CO2 compressor available in open literature, giving maximum errors of around ±3% for mass flow and ±1.82% for power prediction. Calibration of a simplified 1-stage model is also performed to analyse to which extent it is necessary to complexify the model, and to identify key or neglectable modelling elements. It is found that doing this increases the error of the compressor performance prediction. Model analysis has been done to identify the inefficiencies introduced by each physical effect considered.

Coke oven gases processing by vacuum swing adsorption: Carbon capture and methane recovery

Coke oven gas (COG), a by-product gas from the steel industry with great potential for chemical utilization, contains multiple high-value gases including H2, CO2, and CH4. Our previous work has successfully achieved hydrogen production from COG with a purity greater than 99.99 %. For further separation of COG-to-hydrogen tail gas, a two-phase vacuum swing adsorption (VSA) strategy for CO2 capture and CH4 upgrading was carried out in this study. The accuracy of the 4-bed 8-step and 4-bed 7-step cycles established in the in-house mathematical model was verified through a series of multi-component breakthrough and VSA experiments for the two-stage CO2 capture and CH4 upgrading processes employing zeolite NaY and activated carbon RB3, respectively. To achieve more excellent separation performance, several novel VSA cycles were designed and developed in terms of each stage, and the Pareto fronts of purity and recovery of CO2 or CH4 for each cycle were obtained through a multi-objective optimization framework. Based on this, the separation performance and energy consumption of the optimal cycle for each stage were parametrically analyzed under various feed gas concentrations, vacuum pressures, and feed velocities. Moreover, the CO2 product with a purity of 95.58 % and a recovery of 90.11 % can be obtained ultimately by using a 4-bed 8-step cycle for the first stage and a 4-bed 7-step cycle for the second stage of the CO2 capture process under 0.1 bar vacuum pressure. The CH4 upgrading process using a 4-bed 7-step cycle at 0.1 bar vacuum pressure produced a product with 92.07 % purity and 98.81 % recovery. The specific energy consumption was 32.17 kJ/mol and 1.80 kJ/mol for CO2 capture and CH4 upgrading, respectively. Recovery of high-purity CO2 and CH4 at such medium vacuum pressure and low energy consumption through a two-phase VSA strategy from COG has great application potential.

Performance simulation analysis of a novel dual-piston two-stage carbon dioxide linear compressor

In this study, an innovative two-stage linear CO2 compressor with dual piston is proposed and its simulation model is developed. The dynamic characteristics of the two-stage dual-piston compressor are comprehensively analyzed. The simulation results indicate that the dual-piston configuration leads to gas forces oscillating around the zero axis, resulting in a reduction in the mean gas force, allowing for the replacement of the resonance spring with gas springs and magnetic springs. The dynamic performance of the two-stage linear compressor is compared with that of a single-stage linear compressor. The moving center offset of the dual-piston two-stage compressor is reduced by approximately 90 %. The gas equivalent stiffness and the equivalent damping at the TDC (Top Dead Center) get lower than the single-stage one by 25.7 % and 11.6 % as the pressure ratio is set at 2.5. The leakage during the compression process gets lower by more than 20%. The dual-piston compressor demonstrates excellent capacity and modulation capabilities, with a theoretical increase in cooling capacity of over 300 W when the stroke is increased from 6 mm to 8 mm.

Two-stage conversion of CO2 to methanol and dimethyl ether using CuO–ZnO–Al2O3/protonated Y-type zeolite catalysts

The CuO-ZnO-Al2O3/protonated Y-type zeolite (CZA/HYZ) catalysts were prepared via co-precipitation method and employed in a two-stage CO2 conversion process using a dual fixed–bed column at different temperatures and 50 bar. Catalytic components of CZA/HYZ were copper species that have been identified with XPS. Remarkably, the fine structures of CZA/HYZ catalysts metal atoms were confirmed with XANES and EXAFS spectra. Molecular configurations of catalytic species in CZA/HYZ catalyst simulated from the XANES/EXAFS spectra-analyzed fine structural parameters were schematically displayed. The optimal catalytic performances of CZA/HYZ (CH3OH conversion=78.0%, CH3OCH3/HCOOH selectivity=91.7%/8.3%, CH3OCH3/HCOOH yield=71.5%/6.5%) were achieved at 250 ºC. The Arrhenius equation and a pseudo-first-order/second-order model were used to evaluate the activation energies and rate constants of CH3OH and CH3OCH3 formations at various catalytic temperatures. The low activation energies (2.720 and 1.160 kJ mol−1) and Gibbs energies (3.26 and −40.00 kJ mol−1) of CH3OH and CH3OCH3 formations at 250 ºC, demonstrated that their spontaneities were remarkably improved via CZA/HYZ, respectively. The total income from a 10–ton per day (10–TPD) for a petrochemical refinery plant waste gas utility process was USD $43,557d−1, as well a payback of 3.23 years, based on cost evaluation. The proposed process provides a continuous two-step process for manufacturing high–value-added chemicals using industrial CO2 emitted and syngas.

Flowsheet modelling and techno-economic analysis of CO2 capture coupled pyro-hydrolysis of CaCl2 waste for HCl acid regeneration

Pyro-hydrolysis of CaCl2 waste for HCl regeneration, which entails high temperatures and substantial energy consumption, remains challenging due to concerns regarding cost and CO2 emissions. This study presents a low-emission process integrating CO2 capture with SiO2-assisted CaCl2 pyro-hydrolysis technology for HCl regeneration, with a comparative techno-economic feasibility evaluation using Aspen Plus. When employing the same process components as for FeCl2 pyro-hydrolysis, it is demonstrated that 18 wt% HCl acid, a similar concentration to that generated from FeCl2 pyro-hydrolysis, can be produced from SiO2-assisted CaCl2 pyro-hydrolysis with a higher energy consumption. The CaCl2 pyro-hydrolysis were further optimized with the integration of water recycling and exhaust heat recovery, reducing the water and energy consumption down to −1.1 kg/kg-HCl gas and 21,601 kJ/kg-HCl gas, respectively. The water consumption is lower than that of the FeCl2 process (+ 5.4 kg/kg-HCl gas) and the energy consumption is at a comparable level with the FeCl2 process (20,989 kJ/kg-HCl gas). Furthermore, with the integration of a two-stage CO2 capture unit, the CO2 emission was reduced down to 0.07 kg/kg-HCl gas with a CO2 capture efficiency of 91%. Economic analysis results reveal that the capital and annual operating cost for the CO2 capture coupled CaCl2 pyro-hydrolysis is 12.5 million A$, and 4.1 million A$, respectively. The NPV, IRR, and payback duration of 13 million A$, 12.04%, and 8.6 years, respectively, could be achieved for the optimized CaCl2 pyro-hydrolysis scenario. The sensitive analysis results suggest that the HCl market price and annual operating cost are the most influential parameters and should be especially considered in a future deployment.

Thermodynamic analysis of hybrid two-stage CO2 mechanical compression–ejector cooling cycle

In the present study, the main results of thermodynamic analysis of the hybrid two-stage carbon dioxide (CO2) transcritical mechanical compression–ejector cooling cycle using R245ca are provided. In the proposed cycle, an increase in the energy efficiency of a mechanical compression refrigeration machine (MCRM) is provided by additional supercooling of CO2 after the gas cooler due to the use of cold obtained in an ejector cooling machine (ECM). The ECM uses part of the superheated vapor heat after the high-pressure compressor and the intermediate pressure heat after the low-pressure compressor. This solution provides intermediate cooling of CO2 vapor without using an external cooling medium. The proposed method of computation makes it possible to determine the optimal parameters of the hybrid cooling cycle for the design conditions, ensuring the maximum possible MCRM efficiency. At the same time, the method considers the need to determine the optimal gas cooler pressure – a parameter that has a significant impact on efficiency growth. The effect of the intermediate pressure is extremely insignificant and is defined as the geometric mean value of the product of the gas cooler and evaporator pressures. The results show an increase in efficiency of the two-stage CO2 MCRM by up to 31.6% at high temperatures of the environment. In addition, as a result of the exergy analysis, components have been identified, the improvement of which can lead to an additional increase in the efficiency of the entire system. As follows from the data obtained, the greatest attention should be paid to improving the ejector and reducing throttle losses in the CO2 cycle.

Thermodynamic performance assessment and application feasibility analysis of small heat pump water heater using CO2/R41 azeotropy mixture as refrigerant for sustainable development

In order to improve the application possibility of CO2/R41 azeotropy mixture in the refrigeration industry, the optimum mass ratio of composition is defined and determined according to the thermodynamic properties and safety. On the basis of the first and second laws of thermodynamics, a model is proposed to evaluate the performance of CO2/R41 single-stage transcritical cycle (CO2/R41 cycle) in which a throttling valve is integrated and CO2/R41 azeotropy refrigerant is adopted. The performance of CO2/R41 cycle at the optimum mass ratio of CO2/R41 azeotropy mixture is then compared with that of a CO2 two-stage transcritical cycle with throttling valve (CO2 two-stage cycle) and that of a CO2 single-stage transcritical cycle with expander (CO2 expander cycle). The results show that the optimum mass ratio of CO2/R41 azeotropy mixture is 0.583/0.417. Compared with CO2 two-stage cycle and CO2 expander cycle, CO2/R41 cycle has the advantages of a simpler structure, lower optimum high pressure, appropriate discharge temperature of the compressor, and higher COP. Therefore, the CO2/R41 azeotropy mixture with the mass ratio of 0.583/0.417 is eco-friendly and can be considered as a good alternative refrigerant for application in the refrigeration industry. The simple CO2/R41 cycle shows great feasibility to replace the complex CO2 two-stage cycle and CO2 expander cycle.

Performance analysis of two-stage CO2 transcritical refrigeration cycles with absorption subcooling and mechanical recooling

Although the two-stage CO2 transcritical refrigeration cycle with dedicated absorption subcooling is favorable for energy saving through employing the heat to reduce the loss of throttling processes, some important issues are essential to address exactly for the performance improvement, e.g., extending the working range of heat sources and enhancing the efficiency for the mid-temperature heat utilization. Thus, the two-stage CO2 transcritical refrigeration cycle with dedicated absorption subcooling and mechanical recooling is presented and studied thermodynamically. Two kinds of the proposed system are considered, e.g., sole-subcooler for the low-temperature heat source (the scheme A) and dual-subcooler for the mid temperature one (the scheme B). The performance of the proposed system and traditional one is compared, and the effect of critical parameters on the novel cycle is analyzed in detail. For the scheme A, the lower limit of heat source temperature is extended from 80 °C to 77 °C, and the compressor power goes down by 3.3%. For the scheme B, the lower limit of heat source temperature is extended from 139 °C to 128 °C, and the power consumption is reduced by 6%. Moreover, the payback period of the scheme A and scheme B is 7.9 years and 6.4 years, respectively. The paper is helpful to increase the performance of two-stage CO2 refrigeration cycle through the heat recovery and utilization, and promote the energy saving of cold storages.

A 6-kW thermally self-sustained two-stage CO2 methanation reactor: design and experimental evaluation of steady-state performance under full-load conditions

This study designed and evaluated a 6-kW, two-stage CO2 methanation reactor, with the aim of effectively recovering the heat generated by the reaction and condensation of the by-product H2O, securing a high CO2 conversion (≥99%) under low pressure without any external thermal input for catalyst temperature control, i.e., thermally self-sustained. The H2 feed was split, with the majority being fed into the first reactor and the remainder into a second reactor. The heat-carrier flow was also split between the stages and, thereby, the second reactor was cooled by supplying only a fraction of the heat-carrier that had been heated in the first reactor. Reactor inner diameters of 16 and 25 mm, as well as parallel and counter flow heat exchange configurations were compared in the second reactor, using a 1-kW test rig. The results showed that the use of a 16-mm reactor diameter and a parallel flow configuration in the second reactor realize a higher total heat recovery with the 99% CO2 conversion. The steady-state performance of the designed 6-kW two-stage reactor was evaluated at 200 kPa (G) under full load. The reactor consequently recovered more than 70% of the total generated heat at 99% CO2 conversion, increasing the heat-carrier temperature from room temperature (inlet heat-carrier temperature) to 137 °C (outlet heat-carrier temperature).

Thermodynamic and economic analysis of two-stage CO2 Heat Pump with reverse Brayton cycle

At present, the output water temperature of most CO2 heat pumps is below 95°C. Herein, the reverse-Brayton cycle is innovatively applied to the CO2 heat pump to increase its output water temperature. A numerical model of a two-stage reverse-Brayton cycle with refrigerant injection and internal heat exchanger is proposed. The effects of discharge pressure, suction temperature, injection mass ratio, and pressure of refrigerant injection on COP of this system at different heat source temperatures are analyzed. Simulation results show that the highest COP can reach 3.05 with a discharge temperature of 129.1°C. The highest discharge temperature is 157.1°C and the temperature lift is 87.1°C with a COP of 2.85 correspondingly. High temperature hot water or steam is supplied by the proposed cycle with a respectable COP, which will broaden the application scope of CO2 heat pumps in industries. Moreover, relationship between COP and the payback period is revealed under typical conditions to evaluate the benefit of the proposed system.

An interval-parameter two-stage optimization model for CO2 collection, distribution, transportation, utilization, and storage planning

Geological carbon dioxide (CO2) utilization and storage have been widely recognized as one of the important options to deliver greenhouse gas emissions reduction. Reasonable planning is critical to promote CO2 utilization and storage. However, CO2 emissions gas collection exhibits a stochastic probability distribution, and CO2 utilization and storage features fluctuation demands, which have gone beyond current determine planning techniques. To fulfill the current research gap, this study develops an interval-parameter two-stage programming-based CO2 collection, distribution, transportation, utilization, and storage optimization model, integrating interval parameter planning and two-stage planning into a general framework. Therefore, the model can address uncertainties expressed as random probabilistic distributions and discrete intervals, tackle dynamic facilities capacity expansion issues, develop optimal predefined CO2 distribution policy, and generate recourse schemes to address gas shortage or gas surplus issues. The model is examined by a typical hypnotical case study in China. The results revealed that the model could generate a set of first-stage reasonable CO2 distribution and facilities capacity expansion schemes to maximum system benefits and the highest feasibility. Besides, a set of two-stage CO2 outsourcing purchases and facilities capacity expansion in reserve storage regions solutions were also generated to address the gas oversupplies and shortage issues. The modeling approach enriches the current CO2 utilization and storage distribution research content under multiple uncertainties.

Performance assessment of a two-stage CO2 small scale MT refrigerator

F-GAS regulation has introduced important limitations in the use of refrigerants in the small size commercial refrigeration sector, with a maximum GWP=150 since 2022, forbidding the use of R404A. Among alternatives, there are propane, low-GWP HFC/HFO mixtures or pure HFOs: each of them can be considered as an option depending on the refrigerant charge limit for the application or by the total cost accepted by customer. In case of remotely connected units with potential refrigerant leakages, such as condensing units, the use of CO2 is considered as an option, due to the its safety indexes (no toxic and no flammable) which enables the use of larger amount of refrigerant. Being the CO2 refrigerant cycle trans-critical in warm climates, it may have low COPs and not fulfil the limits imposed by Ecodesign rules. In this paper, a condensing unit working with R744 is experimentally tested to assess the performance according to EN13215:2016. To limit high temperature at the compressor outlet, a two-stage system working with an inverter-driven hermetic rolling piston compressor is considered with intercooler and flash gas removal at intermediate pressure. It has been verified that seasonal energy performance ratio (SEPR) can be over the threshold value of 2.3 applied since 2018.

CO2 high temperature heat pump – a promising solution

The study focuses on obtaining hot water above 80°C for industrial purposes. Compared to the electrical heating the use of a heat pump appears much more efficient. A comparative analysis using different refrigerants is carried out. The cold source is waste heat from an industrial process. The exergetic analysis reveals the advantages and weaknesses of using CO2 as refrigerant. Based on the exergy destruction concept several schematics of two-stage CO2 Heat Pumps are considered. For high temperature water heating, a CO2 Heat Pump system represents a promising solution.

Thermodynamic analysis of the optimal operating conditions for a two-stage CO2 refrigeration unit in warm climates with and without ejector

The natural refrigerant R744 seems to be the long term solution to be imposed in the food retail industry, where low and medium refrigeration temperatures are usually required, despite the technical difficulties related to high the pressure, especially in warm climates, leading to trans-critical operation with the consequent COP reduction. To overcome such difficulties, different approaches have been proposed in literature, being very popular the use of parallel compressor or its combination with ejector expansion devices. The efficient operation of trans-critical R744 systems requires the control of the gas cooler pressure, as well as the pressure of the flash-tank. In the case of ejector expansion devices, the ratio of entrained mass flow rate must be also included in the control variables and, at the same time, its use implies additional restrictions. The current paper presents the optimization methodology for the operating conditions of a two-stage CO2 refrigeration unit for both cases, with and without ejector. The curves of optimal combinations of gas cooler and flash-tank pressures, understood as operational control laws, are provided together with the COP achieved. The operating conditions and performance of the ejector case are compared showing COP improvements of up to 13%. The results also show that the operating region of the control variables is limited due to the use of the ejector.

Numerical study on an original oxy-fuel combustion power plant with efficient utilization of flue gas waste heat

In this work, an original oxy-fuel combustion natural gas power plant with two-stage power generation subsystems is proposed to realize the high-efficiency power generation with low-cost carbon capture. H2O and CO2 derived from flue gas is used as diluent to moderate the high temperature in oxy-fuel system. N2 Brayton cycle as the primary subsystem is utilized to generate electricity from GT exhaust heat. Organic Rankine cycle and transcritical CO2 cycle is used as the secondary subsystem for the low-grade power generation. Accordingly, organic Rankine cycle system and transcritical CO2 cycle system are established and simulated. The cryogenic LNG provides cold energy for the two-stage power generation subsystems and CO2 capture to reduce the power consumption of compression. The numerical results show that when 94.8% of CO2 with 97.2% purity is captured, the power consumption of carbon capture is approximately 0.57 kWh/kg-CO2 and 0.07 kWh/kg-CO2 in O2/CO2 and O2/H2O combustion atmosphere. Transcritical CO2 cycle is revealed to be more efficient for the utilization of flue gas waste heat (including the latent heat of flue gas) than organic Rankine cycle. The net power generation efficiency of transcritical CO2 cycle system is 58.78% in O2/CO2 atmosphere and 54.87% in O2/H2O atmosphere.

Performance Analysis of Two-stage Compression CO2 Transcritical Cycle with R290 Mechanical Subcooling Unit

The performances of two-stage compression CO2 transcritical cycle with R290 mechanical subcooling unit are analyzed. The overall COP variation with the compressor discharge pressure and the subcooling degree are discussed in detail. With the increase in subcooling degree and compressor discharge pressure, the COPs of the SMS, OTLMS, OTHMS, TTLMS, and TTHMS system increase first and then decrease. Under the working condition of optimal discharge pressure and optimal subcooling degree, TTHMS shows obvious advantage in COP over the system with no subcooling under the same working condition. When the evaporation temperature is -30 °C, COP of TTHMS is improved by 22.35%. When the ambient temperature is 40 °C, COP of TTHMS is 1.51 higher than that without mechanical subcooling. The COP of SMS system reaches up to 4.26, which is 22.19% higher than that of OTHMS at the evaporation temperature of 10 °C. At an ambient temperature of 35 °C, the COP of TTHMS is 2.15, which is 64.82% higher than that of OTHMS. The power consumption ratio of SMS is significantly higher than those of other systems and the highest power consumption ratio is 29.98%.

CO2 absorption and diffusion in ionic liquid [P66614][Triz] modified molecular sieves SBA-15 with various pore lengths

Mesoporous molecular sieves SBA-15 with four pore structures were loaded with an ionic liquid [P66614][Triz] to adsorb CO2 from raw biohythane. The hybrid sorbents of SBA-15 loaded with [P66614][Triz] were characterized by using N2 adsorption analyzer, thermogravimetric analyzer, scanning electron microscopy, and high-resolution transmission electron microscopy. The molecular sieve SBA-15 (with the most probable pore size of 4.3nm) loaded with 50% ionic liquid (IL), which was called SBA-15 (4.3) −50% IL, exhibited the fastest CO2 absorption rate and the shortest equilibrium time, which was approximately one-third of the equilibrium times of the other three hybrid sorbents. An intraparticle diffusion model was used to clarify that SBA-15 (4.3) −50% IL (with the shortest pore length of 120nm) executed a two-stage CO2 absorption process, which reduced the CO2 absorption time. The rate constant kI in the first stage of the CO2 absorption of SBA-15 (4.3) −50% IL was approximately four times as much as those of the other three hybrid sorbents because the cross-section area of its total pores was approximately three times higher than those of the other three hybrid sorbents. Results showed that pore length had significant effect on CO2 absorption performance.

A novel flue gas pre-treatment system of post-combustion CO2 capture in coal-fired power plant

A novel flue gas pre-treatment system for post-combustion CO2 capture (PCC) in coal-fired power plant was proposed to simultaneously reduce energy and water consumption. A flash evaporation device (FE) and mechanical vapour compressor (MVC) were utilised to provide low-temperature water for washing and cooling flue gas, collect recovered water, and absorb recovered latent heat from flue gas. The system successfully decreased flue gas temperature and moisture content, and provided heat for solvent regeneration to reduce regeneration energy consumption. The Aspen Plus’ rate-based model was used to simulate reference and novel systems based on a 300MW coal-fired power plant. The results indicate that energy consumption in the one- and two-stage CO2 capture systems decreased by 16.3% and 20.6%, respectively. To achieve lower energy consumption, key parameters of the two-stage CO2 capture system were optimised through simulation. The energy consumption of the system with the optimum parameters was 22.5% less than the reference system. Furthermore, water consumption was 27.9% lower than that of the reference system. This novel system has the potential to reduce energy and water consumption in coal-fired power plants with PCC.

Pyrolysis and oxidation of methyl acetate in a shock tube: A multi-species time-history study

High-temperature methyl acetate (MA) pyrolysis and oxidation were studied behind reflected shock waves using laser-absorption species time-history measurements of CO, CO2, OH and H2O. The shock tube experiments with very dilute fuel mixtures covered a temperature range of 1401–1605K for MA pyrolysis (0.2% MA/Ar) and 1423–1674K for MA oxidation (0.4% O2, ϕ=1), and pressures around 1.5atm. The dominant sensitivity of CO2 concentration to MA unimolecular decomposition reactions enabled accurate determination of the rate coefficient and dissociation branching ratio by monitoring the CO2 time-history during MA pyrolysis. A recent kinetic mechanism developed by Yang et al. [13] originally for interpreting flow reactor and low-pressure flame data was adopted to simulate and compare with the current shock tube data. The measured CO and CO2 time-histories during MA pyrolysis were both well-predicted by the modified Yang et al. [13] mechanism. A relatively complete description of MA oxidation behavior was given by measuring CO, CO2, OH and H2O time-histories at the same temperature and pressure (1480K, 1.5atm). A unique two-stage CO2 formation during MA oxidation was clearly observed in the measured CO2 time-histories over the entire temperature range, with the rapid pre-ignition CO2 formation analyzed to be associated with the initial MA dissociation. Despite the overprediction of MA ignition delay times by 18–40% between 1623 and 1423K, the kinetic model successfully captures the plateau levels and the peak values of all the measured species profiles, as well as simulates the characteristic two-stage formation of CO2 observed experimentally.

Energy and exergy performance comparison of different configurations of an absorption-two-stage compression cascade refrigeration system with carbon dioxide refrigerant

This paper presents a comparative study of six configurations of an absorption–compression cascade refrigeration system. The cascade system is composed of a single-effect or a double-effect LiBr/H2O absorption chiller in cascade with a two-stage CO2 compression chiller. The cooling capacity of the absorption chiller is used to enhance the efficiency of the compression chiller. The system runs by a micro-turbine which provides the supplied energy to both chillers. System components are modeled and analyzed through the energy and exergy approaches. The performance parameters of the systems and the second law efficiency are calculated in different operating conditions. The results show that a proper choice of discharge and inter-cooling pressure along with a well calculated generator temperature can upgrade the energy utilization factor of the system considerably. It is also concluded that at a specific discharge pressure, a system with inter-cooler, after-cooler and double-effect absorption chiller would result in the highest improvement of the system.