Development of CO2 liquefaction cycles for CO2 sequestration
Highlights
►We model several CO2 pressurization methods for their evaluation. ►We develop new CO2 liquefaction cycles and model it using HYSYS software. ►Liquefaction cycle using NH3 consumes 5.1% less power than multi-stage compression.
Introduction
In order to mitigate the global warming, CO2 is captured from stationary sources and sequestered in underground geological formations. Since about one third of all CO2 emissions from fossil fuel energy sources comes from fossil fuel burning power plants, which have the highest density of CO2 emissions in terms of mass per power output [1], they provide an appropriate target in the attempt to mitigate the global warming.
CO2 capture and sequestration (CCS) consists of three processes: capture, which is typically done by amine absorption; transport, which is done by either pipeline or ships; and storage, which can be done in underground geological formations such as oil wells, which can be utilized for enhanced oil recovery (EOR). Among the three processes, CO2 capture has attracted the most attention in the literature since it is more technically challenging. Since CO2 captured in post-combustion using amine absorption is at atmospheric pressure, CO2 needs to be pressurized to a supercritical pressure, e.g. 150 bar, before injection into an oil well for EOR. As for storage or shipping in tankers, the captured CO2 is liquefied at a pressure of about 6 bar so that its volume is reduced. Conventionally, CO2 pressurization is done using multi-stage compression with intercooling. CO2 compression consumes about 100 kWh/ton CO2 [2] while the energy efficiency of a power plant can be decreased by 3–4% points [3]. The other pressurization approach is to liquefy the CO2 and pump it to the target injection pressure, and then the pressurized CO2 is evaporated so that phase change does not occur inside the well. The two approaches are shown in Fig. 1.
Many studies in the open literature developed models for the CO2 compression process. Amrollahi et al. [4] used GT PRO software [5] to model a CO2 compression cycle. Pfaff et al. [6] and Cifre et al. [3] used EBSILON software [7] to model a CO2 compression cycle. Sanpasertparnich et al. [8] used an in-house code to model a CO2 compression cycle. Moller et al. [2] used IPSEpro software [9] to model a CO2 compression cycle. Romeo et al. [10] modeled and optimized CO2 compression processes using EES software [11].
Few authors investigated the CO2 liquefaction process. Aspelund et al. [12], [13] studied CO2 liquefaction for ship transport. They considered three liquefaction pressures (20, 55 or 95 bars) where CO2 is liquefied after compression by either seawater or by using an open cooling cycle that they patented. Moore et al. [14] carried out a similar study. According to Moore et al.’s preliminary analysis, a 35% reduction in power is possible when liquefying the CO2 using absorption chillers and pumping the CO2 instead of compressing it. Botero et al. [15] compared different compression strategies for CO2 compression using HYSYS software [16]. They investigated using absorption chillers and cold seawater for CO2 liquefaction at three liquefaction pressures.
Proprietary CO2 liquefaction processes are available in some applications such as in the food and beverage industries. However, such applications of CO2 liquefaction differ from power plant in terms of quantity and quality of feed gas and product gas. Several liquefaction cycles exist for natural gas liquefaction but they differ significantly from CO2 liquefaction in terms of liquefaction temperature and cooling curves. While absorption chillers are a mature technology and many manufacturers exist, they have low efficiency and are considered complex and expensive [17].
There is a lack of literature in detailed comparison between CO2 compression and CO2 liquefaction and pumping processes. No author has carried out a comprehensive study on whether liquefying and pumping CO2 for injection consumes less power than multi-stage CO2 compression, and investigated the design of several CO2 liquefaction cycles (cascade and single-refrigerant). Further, there has been no research conducted on optimizing the CO2 liquefaction pressure, and none of the authors verified or validated their models.
The objectives of this paper are (1) to carry out a comprehensive comparison between CO2 compression and CO2 liquefaction and pumping processes for EOR application by developing several CO2 liquefaction cycles and validating or verifying the developed models, (2) to optimize the CO2 liquefaction pressure, and (3) to evaluate the performance of the open CO2 liquefaction cycle.
Section snippets
CO2 compression cycle
In order to inject the captured CO2 into an oil well for EOR, it needs to be pressurized to a supercritical pressure. The injected CO2 is in supercritical vapor state. The injection pressure used in this paper is 150 bar, which is typical for an EOR project. Pressurizing CO2 is done using eight multi-stage centrifugal compressors with intercooling, so that it approaches isothermal compression. HYSYS process simulation software was used to model the CO2 compression plant as shown in Fig. 2. The
Absorption chillers for CO2 liquefaction
NH3 absorption chillers can be used to liquefy CO2 if there is enough waste heat. If there is 13.35 MW waste heat at 130 °C, NH3 absorption chillers can be used to liquefy the CO2 at −12 °C or 25 bar liquefaction pressure. The liquefaction load is 5.84 MW. The total power consumption (CO2 compressors and pump) for this option is 4.37 MW, which is the lowest power consumption for preparing the captured CO2 for injection (1.88 MW or 30.08% power savings as compared to the multi-stage compression).
Conventional open CO2 liquefaction cycle
The second way to pressurize CO2 is to liquefy it and then pump it to the desired pressure. The open CO2 liquefaction cycle for liquefying the CO2 from a stationary source is a patented cycle by Aspelund et al. [12]. According to Aspelund et al., the liquefaction of CO2 is best achieved using their open CO2 liquefaction cycle. The purpose here is to model the open CO2 liquefaction cycle at ambient conditions equivalent as the previous models.
The working principle of the cycle is that it uses
Comparison
Table 12 lists the least power consumption for all explored options. It shows that the developed VCC that uses NH3 as a refrigerant and recovers the cold energy in the liquefied CO2 at the optimum liquefaction pressure consumes less energy than the conventional multi-stage compression with intercooling by 5.12%. Further, the NH3–CO2 cascade cycle consumes less power than the single-refrigerant cycle if the cold CO2 energy were not recovered. Nonetheless, cascade cycles are more complex and
Conclusion
Several options were investigated for pressurizing the captured CO2 gas for EOR applications. The investigation was carried out through development of HYSYS models with models validated in good agreements. The liquefaction of CO2 was optimized using a generic HYSYS model coupled with the Matlab optimization tool that considers all power consumption and all recoverable cooling. The optimization results were graphed for different efficiency values which can be used as a tool to compare power
Acknowledgements
The authors would like to thank the sponsors of the Center for Environmental Energy Engineering (CEEE), University of Maryland, College Park, MD, USA and the Petroleum Institute of Abu-Dhabi, UAE, for their financial support of this research project.
References (24)
- et al.
On the off-design of a natural gas-fired combined cycle with CO2 capture
Energy
(2007) - et al.
Integration of a chemical process model in a power plant modelling tool for the simulation of an amine based CO2 scrubber
Fuel
(2009) - et al.
Optimised integration of post-combustion CO2 capture process in greenfield power plants
Energy
(2010) - et al.
Integration of post-combustion capture and storage into a pulverized coal-fired power plant
International Journal of Greenhouse Gas Control
(2010) - et al.
Optimization of intercooling compression in CO2 capture systems
Applied Thermal Engineering
(2009) - et al.
Ship transport of CO2 technical solutions and analysis of costs, energy utilization, exergy efficiency and CO2 emissions
Chemical Engineering Research and Design
(2006 September) - et al.
Gas conditioning – The interface between CO2 capture and transport
International Journal of Greenhouse Gas Control
(2007) Thermodynamic evaluation of refrigerants in the vapour compression cycle using reduced properties
International Journal of Refrigeration
(1988)- et al.
A generalized analysis for cascading single fluid vapor compression refrigeration cycles using an entropy generation minimization method
International Journal of Refrigeration
(2000) - et al.
Thermodynamic analysis of optimal condensing temperature of cascade-condenser in CO2/NH3 cascade refrigeration systems
International Journal of Refrigeration
(2006)
Optimization of propane pre-cooled mixed refrigerant LNG plant
Applied Thermal Engineering
Carbon dioxide recovery and disposal from large energy systems
Annual Review of Energy and the Environment
Cited by (82)
Research progress of carbon capture technology based on alcohol amine solution
2024, Separation and Purification TechnologyOptimisation of ship-based CO<inf>2</inf> transport chains from Southern Europe to the North Sea
2024, Carbon Capture Science and TechnologyNanomaterials for carbon capture and their conversion to useful products for sustainable energy production
2024, Nanomaterials in Biomass Conversion: Advances and Applications for Bioenergy, Biofuels, and Bio-based ProductsEnergy and exergy analysis of a novel ejector powered CO<inf>2</inf> liquefaction system (EPLS) and comparative evaluation with four other systems
2023, Energy Conversion and ManagementResearch on re-liquefaction of cargo BOG using liquid ammonia cold energy on CO<inf>2</inf> transport ship
2023, International Journal of Greenhouse Gas Control