Energy Saving Potential of CO2 Transportation Processes in Cold

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Energy Saving Potential of CO2 Transportation Processes in Cold Climate Locations Eivind Brodal, Steve Jackson, and Oddmar Eiksund Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03037 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 30, 2016

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Energy Saving Potential of CO2 Transportation Processes in Cold Climate Locations Eivind Brodal*, Steven Jackson and Oddmar Eiksund

UiT-The Arctic University of Norway

Abstract Carbon Capture and Sequestration (CCS) is a technology that can help reduce CO2 emissions and thereby mitigate global warming. The temperature of the available cooling medium effects many of the processes relevant to CCS and in particular, low ambient temperature helps reduce the power consumed by compression and liquefaction processes needed to prepare CO2 for transportation, e.g. in pipelines or tanker ships. The aim of this paper is to look at energy usage in different geographic locations and identify the benefits of preparing CO2 for transportation in a cold climate. The main finding is that the pipeline alternative consumes 10–15% more energy in the Middle East than in Northern Norway, and 25–30% more if the CO2 is liquefied. Lower temperature also offers an opportunity to simplify CO2 compressor design. The most efficient refrigerant for CO2 liquefaction is NH3 (R717), but CO2 (R744) is a practical alternative for cold climates.

1. Introduction Worldwide CO2 emissions from power plants using fossil fuel represent approximately 26% of total CO2 emissions1. Carbon Capture and Sequestration (CCS) can therefore be a central technique to


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stabilize the CO2 concentrations in the atmosphere. CCS projects require three very different process steps: CO2 capture, transportation and sequestration. In the sequestration step there are two further possibilities: geological storage and Enhanced Oil Recovery (EOR). When injected into geological storage, CO2 must be supplied at a high pressure, usually 70–100 bar;2 for EOR the pressure is typically 150 bar.1 Since most capture processes release CO2 at low pressure, the pressure of the CO2 must be increased significantly within the transportation process. Sources of CO2 are often remote from the sequestration location and therefore CO2 must also be transported significant distances. Transportation is possible in both pipelines and tankers (ships or trucks). If transportation is via a pipeline, the CO2 is compressed to a high pressure (above the critical pressure) and transported in supercritical form to avoid multi-phase flow in the pipelines. CO2 transported this way (at 150 bar and 20 °C) will have a density of 0.9 ton/m3. If CO2 is transported by tanker, the CO2 will be liquefied and will have a higher density (1.2 ton/m3 at 6.5 bar and -51 °C). To avoid freezing, CO2 must be condensed at a temperature above -56.6 °C because CO2 has a triple point of -56.6 °C and 5.17 bar. At the storage location, the liquid CO2 must be pumped up to injection pressure. Lucquiaud et al.3 have investigated a state of the art CCS process and found electric power consumption to be 250–300 kWh(e)/ton CO2, where ‘(e)’ indicates that the energy is from electricity. Since the compression of CO2 to pipeline pressure is expected to consume 90–120 kWh(e)/ton CO2,4 the contribution of the transport link to overall CCS energy consumption could be more than 30% of the total. This article looks at how ambient temperature will affect the power needed to compress and/or liquefy the CO2 being transported. For pipeline transport, this article assumes that the CO2 is pressurized to 150 bar with conventional 7stage multi-stage compression cycle. Multi-stage integrally geared type compressors have been used in


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high-pressure CO2 applications for decades.5 This type of compressor typically uses intercooling between each stage and has a pressure ratio per stage of around 2.6 In the last couple of stages of compression it is sometimes possible to liquefy CO2 with ambient temperatures and replace some compressor stages with pumps. At lower pressures, refrigeration cycles with ammonia (R717), CO2 (R744), R134a or propane (R290) can be used to liquefy CO2 which can then be pumped to pipeline pressure. While some researchers have found that CO2 liquefaction by refrigeration can provide significant energy savings for compression7 the energy required for refrigeration is also significant and other have found that overall savings in the power of only 5% can be expected.8 Given the relatively modest savings, CO2 compression to pipeline pressure will only utilize cooling to ambient temperature in-between compression stages in this report. When large tanker ships are used to transport liquefied CO2 the pressures will be in the range 5–7 bar9 and condensation must be achieved with the help of a refrigeration cycle. Refrigerants like R717, R744, R134a and R290 can be used in the liquefaction process. Refrigeration cycles are more energy efficient in cold climates because the compressors do not need to lift the pressure as high in order to condense the refrigerant, as well as reduced flash gas after the expansion valve. Researchers have earlier looked at the link between ambient temperature and refrigeration process performance, such as Seo et al.10, and In particular the significance of low ambient temperatures to the Snøhvit liquefied natural gas (LNG) processing plant, located in Melkøya, Northern Norway.11 Otherwise, little research has been found on the geographical aspect in CO2 transportation. Some research shows that R717 based refrigeration processes offer advantages when liquefying CO2 due to low costs12 or high COP.8 However, ammonia has several inherent disadvantages:13 It is toxic and moderately flammable; and it must be used at pressures below atmospheric if the temperature is below -34 °C in the evaporator. Cascade refrigeration process, with R744 and R717, have also been studied1314

, using evaporators operating in the range -45 °C to -55 °C and condensers operating in the range 30 °C


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to 40 °C and in recent years, R744 is becoming a more mainstream refrigerant.15 R744 refrigeration systems in particular have different operating characteristics in warm and cold climates due to a low critical temperature (31 °C). Most research concerning refrigerant R744 has been done in countries with cold climates,16 where it under certain conditions it may outperform the conventional alternatives.17 In this paper all of the refrigerant options outlined above will be studied and the condensing pressure of CO2 in the liquefaction process will be set to 6.5 bar, in line with the findings of other researchers.9, 18 The premise for the present work is that the amount of energy needed for CO2 transportation will vary significantly based on the ambient temperature at the physical location of the carbon capture unit and associated transportation process. The impact of ambient temperature on CCS power consumption has been explored previously, for example in 2012 by Alabdulkarem et al.8 who provided a discussion of CO2 liquefaction cycles and the benefit of using seawater extracted below surface level with a temperature of 27 °C, instead of seawater from the surface with a temperature of 35 °C. In 2015, Lee et al. also published a study considering temperature variation in the seawater used for cooling in CO2 liquefaction processes.19 The main goal for this paper is to analyze potential energy saving in cold climates, for both pipeline and tanker based transport by combining global seawater temperature data and numerical process models. Hence, this article has a different focus than earlier papers such as Leeds et al.19 Melkøya is used as a base case in this article because it has a low ambient temperature and is a potential candidate for CCS due to the natural gas industry in this location. The gas power plant in Melkøya contributes nearly 2% of Norway’s total CO2 emissions.11 In addition, the gas processing plant operates with CO2 reinjection, which means that much of the CCS infrastructure, e.g. pipeline to geological storage, already exists.


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2. Methods: Modelling Schemes to Prepare CO2 for Efficient Transportation In this article, the main focus is to investigate how the ambient temperature at different locations will change the energy use for preparing CO2 for transportation. Two different processes using seawater as a heat sink are studied here: 1. Compression to Pipeline Pressure 2. Liquefaction for Transport by Tanker Creating accurate models of these processes is important, since they will later form the basis of the performance comparison. Numerical models were developed in Matlab® with enthalpy data calculated using CoolProp®. CoolProp® is a software package that computes pure component thermophysical data with high accuracy.20 Coolprop® is written in C++, but wrappers generate high-level interfaces for numerous languages such as MATLAB, Microsoft Excel, Labview, Python and C#.20 Seawater data, provided by the Japan Meteorological Agency21, is combined with the process models developed in Matlab® to generate location specific performance data using Matlab’s built in map function ‘contourfm’. More detail on the modelling basis and the use of ambient temperature data is provided below.

2.1

General Modelling Basis

Carbon capture processes will be designed to handle a wide range of application specific operating parameters, e.g. flue gas pressure and composition. In order to create a general performance model that is simple, transparent and can be easily recreated, we assume in this study the following set of simplifying parameters: 

The carbon capture process releases pure CO2 gas at 1.0 bar.



All compressors and pumps have isentropic efficiency, 𝜂is=0.85.



All stages in compression processes have the same pressure ratio.


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

All heat exchangers, evaporators, condensers operate with a 5.0 K temperature difference. For example, the CO2 stream is cooled down to five degrees over ambient temperature (Ta +5°C) before each compressor stage.



The pressure drop in all heat exchangers, evaporators and condensers is set at zero.



In the liquefaction of CO2, it is assumed that the refrigerant in the external refrigeration cycle is saturated liquid when entering the expansion valves and saturated gas when leaving the evaporator.

2.2

Modelling Compression to Pipeline Pressure

In this study, pure CO2 is pressurized from 1.0 bar to 150 bar in a 7-stage compression process, with intercooling. Each stage has an equal pressure ratio (i.e. 𝑝r,7−stage = 1501⁄7 ≈ 2.0). If the ambient temperature is low, the CO2 is condensed before reaching 150 bar and a pump is used to reach the final pressure. Hence, the CO2-stream can bypass some high-pressure compressor stages, which saves energy. A flow diagram of the system is shown in Figure 1. Figure 2 illustrates these processes at different ambient temperatures in a ph diagram.

Figure 1. CO2 compression process with seven stages, intercooling to Ta+5°C before each compressor (with streams having ambient temperature Ta). If Ta is low, CO2 is liquefied and pumped to 150 bar.


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Figure 2. ph diagram of a CO2 compression processes with pressure ratio of 2.0. CO2 is cooled down to Ta+5°C before each compression. Processes with warm Ta= +25°C (left) and cold Ta= -15°C (right) ambient temperature.

2.3

Modelling Liquefaction for Transport by Tanker

In this process, pure CO2 is pressurized from 1.0 bar to 6.5 bar in three stages with intercooling. Each compressor stage has an equal pressure ratio (i.e. 𝑝r,3−stage = 6.51⁄3 ≈ 1.9). At 6.5 bar the CO2 is cooled down to -51 ᵒC and condensed using a refrigeration cycle. A flow diagram of this liquefaction process is shown in Figure 3. The process is shown in Figure 4 for ambient temperature of 5 ᵒC.

Figure 3. CO2 liquefaction process where CO2 is condensing at 6.5 bar (-51 °C) using a refrigeration cycle with evaporator temperature TR.


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Figure 4. ph diagram of a CO2 liquefaction process (Ta= +5 °C) where CO2 is condensing at 6.5 bar (-51 °C) using refrigeration. Refrigeration Schemes used in the CO2 Liquefaction Process The literature suggests that refrigerant R717 is very efficient for condensing CO2, and is here modeled in both a refrigeration cycle with one-stage and a two-stage refrigeration cycle with open intercooler. However, R717 evaporates below atmospheric pressure at -56 °C, which is avoided in two alternatives discussed using R744 in either two stages with open intercooler, or in a cascade with R717. In total, this paper will look at the four different refrigeration processes that are summarized in Table 1. Table 1. Different refrigeration processes used in CO2 liquefaction.

Refrigeration Configuration Process A

R717 with two stages and open intercooler. Both compressors have the same pressure ratio. Figure 6 shows the process in a ph diagram for Ta= 30 ᵒC.

B

Single stage R717.

C

R744 with two-stage open intercooler. Both compressors have the same pressure ratio. The refrigeration cycle is transcritical at high ambient temperature (Ta>23 ᵒC). A backpressure valve lifts the high pressure to 100 bar if Ta>23 ᵒC.

D

Cascade using R717 and R744. R744 is condensed at -20 ᵒC and R717 is vaporized at -25 ᵒC in the heat exchanger (see Figure 5).


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Flow diagrams of the three different refrigeration cycles investigated in this report are illustrated in Figure 5, including associated operating temperatures. The log pressure-enthalpy diagram in Figure 6 shows the two-stage R717 process with open intercooler.

Figure 5. Flow diagrams of the refrigeration cycles studied: process B (left), process A and C (middle), and process D (right).

Figure 6. Log ph diagram of “Refrigeration process A” (two-stage open intercooler with R717).

2.4

Ambient Sea Temperature Data

The Japan Meteorological Agency (JMA) provides average monthly global Sea Surface Temperature (SST) data with a resolution of 1° latitude and 1° longitude.21-22 This data can be downloaded in WMO


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binary code format from the JMA website.21 This data has formed the basis for modelling the variation in power consumption with geographic location. In this study, three geographic locations with very different climates have been selected to illustrate performance variation: Melkøya (Norway), Oristano (Italy) and Ras Laffan (Qatar). These locations were selected because they illustrate a wide range of ambient temperature data and because they have local industries where CO2 capture can be relevant. At Melkøya, in particular, CO2 is already removed from natural gas as part of LNG production and re-injected in an offshore field. Figure 7 shows the variation in SST in these locations in 2015, and the yearly average SST. The SST data and the process models described above were combined in Matlab to generate performance data around the world for each of the processes considered here. The following section presents the results of this work.

Figure 7. Monthly average sea surface temperatures (ambient temperature Ta) in Melkøya (Norway), Oristano (Italy) and Ras Laffan (Qatar), downloaded from the JMA website.21

3. Results: Performance Variations in Different Climates The results generated from the combination of ambient SST data and process models described above are presented below.


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3.1

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Energy Consumption and Ambient Temperature

Figure 8 shows the specific energy consumption for each of the selected process schemes at different ambient temperatures. Table 2 summarizes the specific energy usage for three different ambient temperatures (5 ᵒC, 15 ᵒC and 25 ᵒC).

Figure 8. Specific energy usage for compression and four liquefaction alternatives with different refrigerant schemes. Average sea surface temperatures from 2015 shown for Melkøya (Norway), Oristano (Italy) and Ras Laffan (Qatar). Table 2. Specific energy usage for different processes and ambient temperatures. Percentage increase in energy consumption from the respectively 5 ᵒC cases are shown in parenthesis.

Ambient temperature

Process energy consumption [kWh(e)/(ton CO2)] CO2 liquefaction

CO2 compression

A

B

C

D

5ᵒC

77.6 (0%)

81.8 (0%)

88.8 (0%)

88.8 (0%)

88.3 (0%)

15ᵒC

81.4 (4.9%) 85.4 (10.1%)

92.9 (13.6%) 104.9 (28.2%)

102.5 (15.4%) 117.8 (32.7%)

106.3 (19.7%) 140.4 (58.1%)

100.0 (13.3%) 112.9 (27.9%)

(Ta)

25ᵒC


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3.2

Sensitivity test of important assumptions in the refrigeration alternatives

The low R717 evaporator pressure results in a large compressor pressure ratio pr. In general the isentropic efficiency of a compressor is reduced at larger pr and hence the assumption that all refrigerant-compressors have isentropic efficiency 𝜂is =0.85 will be particularly optimistic for the R717 refrigerant processes. Another factor affecting the relative efficiency of the refrigeration processes is the assumption of 5 K temperature difference in the heat exchanger connecting the two refrigeration loops. The efficiency of the cascade process is very sensitive to variations in this parameter. Figure 9 illustrates the sensitivity of the performance with respect to compressor efficiency and temperature assumptions in cascade heat exchanger. The basis for the data presented in Figure 9 is that the compressor stages in each refrigerant cycle are divided into a sequence of compressors with 𝜂is =0.85 and pr ≈2. In addition Process D’’ in Figure 9 includes a 10 K temperature difference out of the cascade heat exchanger.

Figure 9. Process A, B, C and D (solid lines) are the same as before. Process A’, B’, C’ and D’ are modified having sequence of compressors in the refrigerant cycle with pressure ratio pr ≈2 and 𝜂𝑖𝑠 =0.85. Process D’’ has also a 10 K temperature difference out of the cascade heat exchanger (instead of 5 K).


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3.3

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Energy Consumption and Seasonal Variation

Figure 7 shows how the sea surface temperature in the coastal waters around Melkøya varies throughout the year. Using this as the process cooling temperature, Figure 10 presents how the energy consumption of different processes is effected by yearly temperature variation.

Figure 10. Specific energy consumption for different processes in Melkøya (2015).

3.4

Energy Consumption and Geographic Location

By combining global average SST and process performance variation with temperature one can determine yearly average energy consumption by geographic location. Energy consumption for both the CO2 compression alternative and the most efficient CO2 liquefaction process (alternative A) is shown in Figure 11. Figure 12 compares the energy efficiency relative to processes located in Melkøya.


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Figure 11. Specific energy consumption. Top: CO2 liquefaction using “refrigeration process A.” Bottom: CO2 compression.


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Figure 12. Specific energy consumption relative to Melkøya. Melkøya is marked as “*”.

3.5

R744 as an Alternative Refrigerant in Cold Climate

Energy consumption for the liquefaction process using refrigeration alternative C is illustrated in Figure 13. Alternatives A and C both use refrigeration cycles with two-stages and an open intercooler, but alternative C uses R744 instead of R717. Alternative D uses a cascade with R717/R744. Figure 13 includes a comparison of the difference in specific energy usage between C and the two other refrigeration processes A and D.


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Figure 13. Top: Specific energy consumption for liquefaction using “refrigeration process C” (R744 with two-stage open intercooler). Middle: Comparison of alternative C and A (A is a R717 cycle with twostage and open intercooler). Bottom: Comparison of alternative C and D (D is a cascade with R717/R744).

4. Discussion Globally, the yearly average sea temperatures range from 0 ᵒC to 30 ᵒC. However, the temperature data from the Japan Meteorological Agency21 does not take into account conditions close to land, i.e. the effect of shallow water. Since carbon capture facilities are likely to be located on-shore, the most relevant seawater temperature is that of the sea ‘close to land’. Therefore, the specific ‘close to land’ sea temperatures for the locations used here may show small variations from the temperature data used to generate Figure 11 results of this study. However, the impact of increased sea temperature


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close to land will have a negative impact on the power consumption for all options and in cold locations where the sea is sometimes warmer than the ambient air, the effect could potentially be positive. Seawater temperature also depends on the depth. For example, Alabdulkarem et al. discussed applying seawater extracted below the surface level with a temperature of 27 °C, instead of seawater from the surface with a temperature 35 °C.8 No study of the impact of the variation of sea temperature with depth or variations due in ‘close to land’ locations has been studied in detail here and could therefore be identified for an area of future investigation.

4.1

Compression to Pipeline Pressure (150 bar)

The variation in energy consumption for the CO2 compression process with geographic location is shown in Figure 11, which illustrates that a facility located in a Middle Eastern country will use approximately 10 – 15% more energy than in Northern Norway. This performance gain is essentially ‘free’ for facilities located in cold climates since no fundamental re-design is needed to take exploit lower cooling water temperatures. The design of any operating facility must, however, be optimized across the range of temperatures found in location where it is placed. Further discussion on this is presented at the end of this section. Figure 8 also shows the ambient temperature levels where stages in the CO2 compressor can be bypassed (around 26 ᵒC and -4 ᵒC). That is, above 26 ᵒC all seven compressor stages are used, and below -4 °C only five stages are operational. These break points are potentially interesting in terms of equipment design since the elimination of compressor stages would have a positive impact on the compressor equipment costs.

4.2

Liquefaction for Transport by Tanker (6.5 bar and -51 ᵒC)

Figure 8 shows that process A (using R717 with two-stage open intercooler) is the most efficient refrigeration process modeled in this study. Table 1 also illustrates that if the ambient temperature increases from 5 ᵒC to 25 ᵒC with refrigeration process A, the energy usage at the warmer location is


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28% higher. Figure 11 shows that a facility in the Middle East needs approximately 25 – 30% more energy than in Northern Norway. This supports the findings of earlier studies such as the life cycle costs study presented in 2015 by Seo et al.10 However, there are a number of operational drawbacks associated with such R717 refrigeration cycles. Due to the low pressure in the evaporator (R717 boils at 0.28 bar and -56 ᵒC), humid air can leak into the refrigerant cycle causing freezing problems. The density in the evaporator is also low, resulting in compressor(s) with large inlet volume flow rate. Low density in the evaporator also means that the R717 schemes may have trouble achieving the same temperature difference as other schemes (i.e. they will need a larger evaporator). Ammonia is also toxic and moderately flammable, which are important safety issues. Refrigeration process D with R744/ R717 in a cascade does not require operating pressures below atmospheric, but does consume more power and still uses ammonia. Refrigeration process C does not use ammonia and is as efficient as the other alternatives to process A when the ambient temperature is low.

4.3

R744 as an Alternative Refrigerant in Cold Climate

The variation in specific energy consumption between refrigeration processes A, C and D is illustrated in Table 1 and Figure 13. Table 1 shows that when the ambient temperature is 5 ᵒC, a system with a twostage open intercooler with R744 (process C) will only require 8% more energy than one with R717 (process A). Figure 13 shows that the variation between locations in the Barents Sea, the Norwegian Sea and the North Sea is less than 12 kWh(e)/ton CO2, i.e., less than 11%. Figure 8 shows that refrigeration alternative D is much more efficient than alternative C for ambient temperatures above 20 ᵒC. This trend reflects the fact that process C goes transcritical at high ambient temperatures (Ta>23 ᵒC). However, the difference in energy usage between refrigeration processes C


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and D becomes small at low ambient temperatures and alternative C avoids the operational drawbacks associated with using ammonia discussed above. Figure 9 shows that the compressor efficency and the temperature difference in the cascade heat exchanger are important when comparing process D and C. Modified and more efficient cascade refrigeration processes with flash tank intercoolers are discussed by Mosaffa et al.14 Alternative C can also be further optimized because of the high pressure e.g. through the use of an ejector-expansion23. Determining accurately which real refrigeration process is the most efficient at low ambient temperatures is probably a question of equipment and optimization.

4.4

Seasonal Ambient Temperature Variation

Figure 7 shows the variation in sea surface temperature outside Melkøya, and illustrates that the SST in Melkøya exhibits a relatively low seasonal variation in temperature. This reduced variability is a potentially important feature of this location because in all of the processes studied the equipment items needed in a working facility (i.e. compressors, exchangers, etc.) would be designed to operate across the full range of expected ambient temperature. Therefore, each of the processes studied here could reasonably be expected to operate with higher average efficiency in a location with low seasonal variation in SST. This aspect of the yearly average performance of each scheme represents a potential area for future research.

5. Conclusion The results from this study show that there is potentially a significant energy reduction when preparing CO2 for transportation in a cold climate. Figure 11 shows that a facility located in a Middle Eastern country could use approximately 10 – 15% more energy than one in Northern Norway when transporting compressed CO2 in pipes, and 25 – 30% more when transporting liquefied CO2 with tanker ships and trucks. The results also show that there could also be equipment cost savings when these processes are designed for operation in cold climate locations. For example, the number of compressor


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stages needed depends on the ambient temperature. The refrigeration cycles will also require less inlet volume flow rates due to reduced flash gas after the expansion valve. Four alternative refrigeration processes have been reviewed as part of this study. Of these processes, R717 with two-stage open intercooler (Process A) is the most efficient. However, there are operational drawbacks associated with R717 (ammonia) and in cold climate locations a two-stage R744 process (Process C) could be a practical alternative with little impact on power consumption. Therefore, in cold locations, alternatives C and D must be evaluated to find the overall optimum process scheme. Geographical locations with a small seasonal variation in temperature, like Melkøya, could also have additional advantages since processes here always will operate close to an optimized design condition. This aspect of the yearly performance represents a potential area for future research of each scheme. Carbon capture and sequestration is an important technology to reduce global warming. In a global perspective, it is perhaps a good idea to develop such facilities in cold climates, since the energy needed for compressing CO2 in deep wells at such locations will be less. Liquefying CO2 for transportation with tank boats or trucks to the wells, where it can be long term stored, requires significantly less energy in cold climates.

AUTHOR INFORMATION Corresponding Author * Tel.: +47 77660364. E-mail: [email protected]


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Energy Saving Potential of CO2 Transportation Processes in Cold

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