Carbon Dioxide Capture From Internal Combustion Engine Exhaust Using Temperature Swing Adsorption

In order to reduce the CO₂ emissions in the transportation sector, one can electrify the vehicle, switch to biofuel, or capture and store CO₂ on board. In this study, integration of an on board CO₂ capture and storage unit with an internal combustion engine has been proposed. The technology can be applied for various internal combustion or Stirling engines with targeted applications in the transportation sector. Truck transport for goods delivery is used as an example for on board CO₂ capture and storage system design. The investigated system integrates a temperature swing adsorption system for CO₂ capture with a turbo-compressor system to compress and liquefy the captured CO₂ using the waste heat of the exhaust gases of the engine. Energy and exergy analyses of the proposed CO₂ captured system are studied in details. The CO₂ capture system for engine exhaust stream (car, truck, bus, ship, or train) can capture 90% of the emitted CO₂, without any energy penalty. This system can be integrated into overall mobility system (fuel-engine-CO₂-fuel), where captured CO₂ can be recycled as conventional liquid or gaseous fuels produced from renewable energy sources.

Introduction

Among the challenges of the energy transition, reducing CO₂ emissions of the transportation sector is one of the most difficult. Electrification of the vehicles is of course a good solution to replace fossil fuel for mobility. This path is however penalized by the density of the electricity storage in the batteries. Fossil fuels are a finite resource, hence they should be replaced by bio-based fuel equivalent. This however requires lots of land that may compete with the food production. CO₂ capture is also an alternative, if one can capture with limited energy penalty, and converts captured CO₂ into fuels using renewable electricity.

In the year 2014, CO₂ emissions due to human activities accounted for about 65% of greenhouse gas emissions globally (IPCC, 2014). In 2016, transportation sector was accountable for about 28 percent of CO₂ emissions in USA (United States Environmental Protection Agency, 2016). In 2016, according to European Automobile Manufacturers Association, 2.7 million commercial vehicles were produced in the European Union. This number shows a huge potential for on board CO₂ capture technology for vehicles. However, it could be very difficult to capture CO₂ from vehicles due to large number of vehicles. There has been limited research on CO₂ capture from vehicles due to mobile nature of source, relatively smaller production rate, discontinuous emissions, space limitation, and on board CO₂ storage. Ligterink et al. (2016) has reported 2.65 kg of CO₂ emission per liter diesel consumption by heavy duty vehicles. In 2015, according to the European Environment Agency (2017), road transportation sector contributed about 0.746 giga tons CO₂ emissions. Hence, CO₂ capture from vehicles could be an attractive way to reduce the CO₂ emissions significantly.

Figure 1a shows typical composition of exhaust gas from diesel engine. CO₂ and pollutant emissions are about 12 and 1% (CO, HC, NO_x, SO₂, PM), respectively (Majewski and Khair, 2006). Figure 1b presents emission standard of European Union for heavy duty vehicles (Delphi, 2012), NO accounts for 90% of total NOx emissions, with the remainder being NO₂ (Hebbar, 2014). The diesel engine has an efficiency of about 35%, and about 25 and 40% energy is lost in cooling system and exhaust heat, respectively (Hossain and Bari, 2014).

FIGURE 1

Figure 1. (a) Composition of exhaust gas from diesel engine; (b) Euro emissions standard for heavy duty vehicles.

For CO₂ capture, amine absorption, membrane separation, cryogenic separation and adsorption are main technologies for post combustion CO₂ capture from power plant and process industry. The amine absorption for capturing CO₂ is commonly used in power plant and process industry including natural gas sweetening (Sharma et al., 2017). The amine absorption process is energy intensive: 3.6–4.0 MJ/kg using mono-ethanolamine and 2.0–3.8 MJ/kg using advanced amines, for 90% CO₂ capture rate (Bui et al., 2018). For 90% CO₂ capture, performance of amine absorption process and membrane process are similar with about 10% loss in the plant efficiency (Wang et al., 2017). For natural gas power plant with 85% CO₂ capture using amine, efficiency of the integrated plant decreases by over 8% due to the energy penalty of CO₂ capture (Tock and Maréchal, 2014). Amine absorption process is difficult to use in mobile applications, although it has been proposed in marine application (Luo and Wang, 2017). Pressure swing adsorption (PSA) is well established gas separation technology, which has found applications in air separation, hydrogen purification, and natural gas industry. Further, temperature swing adsorption (TSA) is an attractive technology for CO₂ capture that requires low grade waste heat which may be available close to the CO₂ emission source (See Figure 2). In a TSA cycle, cooled exhaust gases are passed through the adsorbent bed, where CO₂ is adsorbed in the material and the remaining gases are released into the environment. Once adsorbent bed is saturated with CO₂, it is heated to recover the CO₂ from the material. After CO₂ recovery from the adsorbent bed, it is cooled down from the desorption temperature to the adsorption temperature. Note that heat is removed during adsorption step, whereas heat is supplied during desorption step of a TSA cycle.

FIGURE 2

Figure 2. Simple schematic of TSA cycle with four steps. Q, Heat Supplied or Removed; T_ads, Absorption Temperature; T_des, Desorption temperature.

The temperature of engine exhaust normally range from 350 to 700°C (Dimitrova and Maréchal, 2017; Kanchibhotla and Bari, 2018). Further, the heat of cooling system can also be recovered as around 95°C (Abdelghaffar et al., 2002). The waste heat from engine exhaust and cooling system has been used in Rankine cycle to generate mechanical power for heavy duty trucks (Grelet et al., 2016) and cruise ships (Luo and Wang, 2017). Sprouse and Depcik (2013) have reviewed many studies on the use of organic Rankine cycle for the waste heat recovery from the exhaust of internal combustion engine, and claimed 10% improvement in the fuel economy. In order to overcome the dynamic nature of the engine waste heat availability, a heat storage material can be used to maintain the steady supply of heat to the Rankine cycle (Dinker et al., 2017). Lu et al. (2017) used a thermal fluid for the heat transfer between engine exhaust stream and working fluid of organic Rankine cycle, this mechanism is useful in maintaining steady operation of organic Rankine cycle.

The idea of the study is to investigate the possibility of integrating a Rankine cycle and a temperature swing adsorption system for vehicles. Proll et al. (2016) evaluated a fluidized bed TSA system for CO₂ capture from flue gas stream, and in terms of heat transfer, fluidized bed reactor was found better than fixed, and moving bed reactors. Gibson et al. (2016) have evaluated several adsorption materials and process designs for CO₂ capture from gas fired power plant. They have developed an adsorption model to predict the separation efficiency and to obtain the optimum separation conditions. Ntiamoah et al. (2016) performed cyclic experiments on single adsorption column, and product (hot) CO₂ was used to supply the heat of desorption in the desorption step. In order to increase the purity of product CO₂, they used product CO₂ purge before the desorption step. Marx et al. (2016) studied cyclic behavior and separation performance of TSA for post-combustion CO₂ capture. They performed break-through, heating and cooling, and steady-state cyclic experiments to evaluate separation performance of the process.

This work combines organic Rankine cycle with TSA to capture the CO₂ from exhaust stream of an internal combustion engine. Amine doped adsorbents are selected for CO₂ capture, as they show good performance in the presence of water (Huck et al., 2014). Part of the mechanical power produced by organic Rankine cycle is used to generate cold utility using CO₂-based heat pump (turbo-compressor 1). This cold utility is used to remove heat of adsorption, and condense the water from engine exhaust stream. The remaining mechanical power generated by organic Rankine cycle is used to compress and liquefy the produced CO₂ (turbo-compressor 2). The CO₂ capture system has energy self-sufficiency, and does not require any external power. In other words, TSA with turbo-compressors is an attractive choice for CO₂ capture from vehicles without any energy penalty. The CO₂ capture system for engine exhaust stream aims at capturing 90% of the emitted CO₂ (i.e., 2.11 kg CO₂ per liter of diesel consumption). The captured CO₂ can be utilized as a carbon source for producing green fuel (methane or liquid fuels) by integrating hydrogen produced from renewable energy resources.

Energy and Exergy Balance of Engine, TSA and CO₂ Compression

Figure 3 describes the integrated CO₂ capture system, based on 1 l diesel consumption in an internal combustion engine. First of all, diesel engine exhaust is analyzed for energy content and its composition. Figure 3 shows simple heat and mass flows for CO₂ capture system. One liter diesel contains 37.27 MJ energy (lower heating value), which is divided into three parts by the internal combustion engine: 13.05 MJ as mechanical power to drive the vehicle, 14.21 MJ as waste heat in exhaust gas, and 10.02 MJ as heat removed using coolant (Hossain and Bari, 2014).

FIGURE 3

Figure 3. Simple heat and mass flows for CO₂ capture from diesel engine exhaust: exhaust cooling, TSA cycle, product CO₂ compression, and liquefaction.

The exhaust gas stream is cooled down to 25°C, and water is condensed and removed. The cooled exhaust gas stream (saturated with water at 25°C) goes to the adsorption bed, where CO₂ is attached to the adsorbent. Finally, CO₂ is desorbed from the adsorbent at high temperature, and then compressed and liquefied (multi-stage compression with inter-stage cooling). After desorption step, adsorbent bed is cooled down so that it can be used in the next TSA cycle. Belsim Vali models were developed for exhaust cooling and CO₂ compression with inter-stage coolers. These models compute heat available from the exhaust stream at different temperature levels, and total compression power and inter-stage cooling requirements for CO₂ compression. A TSA model was developed in gPROMS process simulator, based on the mathematical model shown in Table B of Supplementary Material (Casas et al., 2013). The TSA model has adsorption, preheating, desorption, and precooling steps. A mixture of N₂ (84%) and CO₂ (16%) was used to calculate heat of desorption, preheating of bed, heat of desorption and precooling of bed, at different adsorption, and desorption temperatures. The developed TSA model uses PPN-6-CH₂TETA (Table C in Supplementary Material; Huck et al., 2014) as an adsorbent material. Finally, adsorption temperature of 30°C and desorption temperature of 150°C were chosen (see Figure A in Supplementary Material).

Figure 4 shows enthalpy-temperature profiles for exhaust cooling, heating and desorption of adsorbent, cooling and adsorption of adsorbent, and product CO₂ compression and liquefaction. The exhaust stream contains 14.21 MJ/l-diesel waste heat, heating and desorption step requires 7.96 MJ/l-diesel heat, 8.17 MJ/l-diesel heat has to be removed during cooling and adsorption step, and 1.63 MJ/l-diesel heat has to be removed for CO₂ compression and liquefaction.

FIGURE 4

Figure 4. Enthalpy-temperature profiles for exhaust cooling, adsorption cooling, desorption heating and CO₂ compression, and liquefaction for 1 l diesel.

Table 1 presents energetic and exergetic analyses of an internal combustion engine, TSA and CO₂ compression and liquefaction (Al-Najem and Diab, 1992; Kul and Kahraman, 2016). The CO₂ capture system design looks feasible from the exergetic point of view (Table 1 and Figure 5). In total, 3.04 MJ of net exergy is available. See the Supplementary Material for Exergy Calculations given in Table 1. In reality, this system is dynamic in nature due to exhaust variations and TSA cycle. The reported exergy analysis is for steady-state operation of the system which gives indication on the heat availability and heat requirements by the system. Assuming a 50% exergy efficiency, therefore there is a potential to produce the equivalent of 1.52 MJ of mechanical power for the CO₂ capture and storage system. Assuming an isentropic efficiency of 75% for the compressors, this value can be compared with the compression power needed to produce CO₂ in the liquid form (compression at 75 bar): 0.88 MJ or compressed CO₂ at 150 bar: 1.05 MJ. For 1 l diesel consumption in an internal combustion engine, 2.11 kg CO₂ is captured by the system, which has a volume of 2.96 l as liquid CO₂ product (at 75 bar) or 4.53 l as compressed CO₂ product (at 150 bar).

TABLE 1

Table 1. Energy and exergy analyses of the internal combustion engine, the CO₂ capture system, and compression power required for CO₂ liquefaction (1 l diesel).

FIGURE 5

Figure 5. Q vs. 1-T0/T plot for CO₂ capture system (1 l diesel); T0 (reference temperature) = 298 K.

Design of CO₂ Capture Systems for a Delivery Truck

The preliminary exergy analysis shows that there is the opportunity by system integration to generate cold, heat, and work that is needed to capture the exhaust CO₂ using the energy available in the exhaust gases. Referring to the usage in a mobile application, there is a need to generate cooling for adsorption step, and therefore production of cooling capacity is considered at a temperature lower than the 40°C. The overall system design concept combines exhaust cooling, TSA cycle, product CO₂ compression and cooling, Rankine cycle and heat pumping, as shown in Figure 6.

FIGURE 6

Figure 6. Overview of CO₂ capture system design: integration of Rankine cycle (RC) and heat pump (HP).

The CO₂ capture system is designed for 1 day operation of delivery truck in a city, which travels 250 km in 8 h (~20 l diesel/100 km, Delgado et al., 2017). The diesel engine emits 117.2 kg of CO₂ by consuming 50 l diesel, and 105.5 kg of CO₂ (90% capture) should be captured and stored by the CO₂ capture system. The working capacity (or CO₂ loading) of the adsorbent material is 0.1 kg-CO₂/kg-adsorbent (Verdegaal et al., 2016). Finally, 1 h adsorption-desorption cycle time has been assumed (Gibson et al., 2016). Initially, three cases for CO₂ capture system are analyzed, as summarized in Table 2.

TABLE 2

Table 2. Details of CO₂ capture system specification (1 day operation, 250 km travel, 50 l diesel consumptions).

Adsorption on Truck and Desorption in the Parking Lot

In total, 105.5 kg of CO₂ should be captured by the adsorbent. It requires 1055 kg of adsorbent, with CO₂ loading of 0.1 kg-CO₂/kg-adsorbent. Hence, the total mass of the system is 1160.5 kg. The density of adsorbent is 0.805 kg/liter, and so the volume of adsorbent or capture system is 1310.6 l. In the parking lot, CO₂ can be converted into methane, and exothermic heat of methanation reaction can be used to recover CO₂ from the adsorbent bed.

Adsorption-Desorption on Truck With CO₂ Liquefaction (Liquid CO₂)

In this case, CO₂ is captured, compressed, liquefied, and stored in a storage tank. The critical point of CO₂ is 31.1°C and 73.8 bar. In order to store liquid CO₂ around 25°C in summer, an efficient cooling system would be required for operation of CO₂ capture and storage system. The diesel engine consumes 6.25 l diesel per hour that means 13.19 kg CO₂ should be captured per hour (1 l diesel = 2.34 kg CO₂ emission, 90% CO₂ capture, see Figure 3). Hence, the CO₂ capture system requires 131.88 kg/h (163.8 l/h) of adsorbent. The duration of a TSA cycle is 1 h, and the regenerated adsorbent material can be used in the next TSA cycle. The estimated mass of storage tank is 273.6 kg to store 105.5 kg (or 147.96 l) liquid CO₂ at 75 bar and 25°C (Turton et al., 2012). Further, typical weight of storage tank (modified CrMo steel) is 0.72 kg/liter for compressed natural gas at 200 bar (Ashok Leyland Report, 2012), and this light weight tank can also be used to store liquid or compressed CO₂.

gPROMS model was developed for TSA cycle, whereas Belsim Vali models were developed for exhaust cooling, CO₂ compression with inter-stage cooling, Rankine cycle and heat pump. CO₂ based Rankine cycle (160 and 75 bar) is used to extract heat from the exhaust gas stream, and to produce the mechanical power in a turbine. This mechanical power is used in CO₂ based heat pump (75 and 50 bar) to generate cold utility for removing heat of adsorption from bed and precooling of bed from desorption temperature to adsorption temperature. The lower pressure level for Rankine cycle and upper pressure level for heat pump are same (75 bar), so that streams from Rankine cycle and heat pump cycle can be combined for discharging waste heat into the environment. Figure 7 shows integrated operations of CO₂ based Rankine cycle and heat pump. Further, heat rejected from CO₂ based Rankine cycle is used for supplying heat of desorption and preheating of bed from adsorption temperature to desorption temperature. Finally, compressors are used to compress the product CO₂ after the desorption step. The mechanical power generated using turbine is sufficient to run compressor for CO₂ based heat pump and compressors for product CO₂.

FIGURE 7

Figure 7. Integrated operation of CO₂-based Rankine cycle (RC) and heat pump (HP).

In order to perform heat integration of the system, heat cascade model (Maréchal and Kalitventzeff, 1998) was used for heat integration among TSA cycle, exhaust cooling, inter-stage heat exchangers/coolers for CO₂ compression, Rankine cycle, and heat pump. Figure 8 presents composite and grand composite curves for cooling of exhaust gas stream, heat of adsorption and precooling, heat of desorption and preheating, CO₂ based Rankine cycle and heat pump, and product CO₂ compression. In Figure 8, no external hot utility is required to close the heat balance which shows the feasibility of the CO₂ capture system. The composite curves provides minimum energy targets (i.e., hot and cold utilities) that can be used in the heat exchanger network synthesis. Figure 9 presents a simple schematic of CO₂ capture system design, without showing heat integration between hot and cold streams. In Figure 9, streams in red color show Rankine cycle operation, whereas streams in blue color show heat pump cycle operation. It can be seen that Rankine cycle and heat pump cycle streams are merged to discharge waste heat (RH1) into the environment. The product CO₂ stream from desorption step is shown in green color, and it is compressed from 1 bar to 50 bar, before merging with heat pump cycle. The product CO₂ is removed from the heat pump cycle at 75 bar and 30°C. Alternatively, a separate product CO₂ compression and liquefaction system is also possible. The purity of product CO₂ has been calculated using adsorbed phase CO₂ and bulk phase gas mixture (i.e., cooled exhaust gas) inside the adsorption bed. The purity of product CO₂ is about 99.5%, assuming 40% inter-particle empty space.

FIGURE 8

Figure 8. Grand composite (A) and composite (B) curves for the CO₂ capture system.

FIGURE 9

Figure 9. Heat and mass flow details of design of CO₂ capture system (1 h operation or 6.25 l diesel consumptions).

In order to achieve the target temperatures of hot and cold streams with minimum utilities, hot and cold streams should exchange heat whenever possible. In this work, the heat exchanger network synthesis (stage-wise superstructure model) has been studied by mathematical programming method. For this, a mixed integer linear programming (MILP) optimization problem, written in AMPL, has been solved using CPLEX solver (Yee and Grossmann, 1990; Mian et al., 2016). In the optimization problem, minimization of number of heat exchangers is considered as objective function. There are 11 hot streams (AD1, AD2, EE1, EE2, RC4, RH1, HP3, HP4, CC1, CC2, and CC3) and 7 cold streams (DE1, DE2, RC1, RC2, RC3, HP1, and HP2) in the CO₂ capture system. Table 3 presents heat exchanger network synthesis results, which has 20 heat exchangers. See Supplementary Material for grid representation of HEN. In Table 3, overall heat transfer coefficient (U) for a heat exchanger, with low heat transfer coefficients streams (exhaust cooling, precooling, adsorption, preheating and desorption), is assumed to be 25 W/(m².K) (Clausse et al., 2011; Ribeiro et al., 2013; Marx et al., 2016). Further, the U-value for a heat exchanger, with high heat transfer coefficients streams (Rankine cycle, heat pump and cooling utility), is assumed to be 2000 W/(m².K) (Yang, 2016). Finally, U-value of 500 W/(m².K) is assumed for a heat exchanger with one low heat transfer coefficient stream and other high heat transfer coefficient stream. Table 3 presents estimated heat transfer areas for different heat exchanges, and AD1-DE1 heat exchanger (precooling and preheating of adsorption bed) has largest heat transfer area (11.23 m²). Total heat transfer area of all heat exchangers is 22.47 m². More accurate heat exchanger area may be calculated by estimating overall heat transfer coefficients for different streams/heat exchangers. This heat transfer area can be provided in compact volume by using micro-channel heat exchangers, wherever it is possible (Hesselgreaves, 2001). TSA is a cyclic process with four steps, and so the heat exchangers inside the adsorbent beds should be heated and cooled in a cyclic way. Hence, heating and cooling of heat exchangers material will increase the thermal duty of the system.

TABLE 3

Table 3. Heat exchanger network synthesis for CO₂ capture system.

Adsorption-Desorption on Truck With CO₂ Compression (Compressed CO₂)

In this case, product CO₂ is compressed and stored at high pressure. The mass of compressed CO₂ storage tank is much higher compared to the mass of storage tank for liquid CO₂ (in Case 2). This case is suitable when compressed methane or natural gas is used as a fuel in the vehicle. Fuel tank for compressed fuel can also be used to store the compressed product CO₂. Three pressures for storing compressed CO₂ are considered: 75, 100, and 150 bar. It can be noted in Table 2 that compressed CO₂ volume at 75 bar (811.11 l) is significantly larger when compared at 100 and 150 bar (226.41 l). The mass of storage tanks, for storing compressed CO₂ at different pressures, are calculated as function of CO₂ volume (0.72 kg/liter; Ashok Leyland Report, 2012). Compressed CO₂ storage at 100 or 150 bar found to be better (lowest weight and volume) compared to 75 bar.

The CO₂ capture system design in Case 2 (i.e., adsorption-desorption on truck with liquid CO₂ product) is an attractive choice due to its lower weight and lower volume. For daily operation of a delivery truck, the total mass and volume of the adsorbent beds and captured CO₂ with storage tank are about 343.9 kg and 311.8 l. Additionally, some weight and space will be required for piping, turbo-compressors and heat exchanger network. In general, about 2 m³ space is available over the truck cabin (www.uhaul.com). Hence, temperature swing adsorption based CO₂ capture system can easily be placed over the truck cabin. Figure C in Supplementary Material presents estimated volume of CO₂ capture and storage system for delivery truck. The CO₂ storage tank can be placed below the truck cabin, so that it can easily be removed and replaced.

Green Fuel Production and Use of Alternate Fuels

The captured CO₂ by the system can be sequestrated in underground or used as feedstock to produce gas or liquid green fuels and chemicals. Conversion of CO₂ into chemicals is a good option from economic perspective due to their high prices (Chen et al., 2018). Table D (in Supplementary Material) presents the conversion of 1 kg of CO₂ into green fuel by co-electrolysis (solid oxide electrolysis cell) using renewable electricity (Wang et al., 2018). The renewable electricity for CO₂ conversion into green fuels can be provided by the PV panels. For calculating total area of PV panels in Switzerland, 400 W/m² average annual solar irradiation (17.28 MJ/day/m²; www.meteoswiss.admin.ch) has been considered in Table D (Supplementary Material). Further, solar irradiation to electricity conversion efficiency of 20% has been assumed for the PV panels.

The delivery truck consumes 50 l (41.6 kg) diesel for 250 km travel, or 1,885 MJ energy based on the lower heating value of diesel fuel. Assuming same efficiency of the engine for different fuels, Table 4 presents amount of alternate fuels used, CO₂ produced, CO₂ captured, fuel produced using captured CO₂, renewable energy consumed, and PV panel area. The fuel produced / fuel used ratio is different for different carbon based green fuels. In Table 4, H₂ fuel is also reported for comparison purpose. The efficiency of electrolysis (electricity to H₂) is assumed to be 80% (Schaaf et al., 2014), H₂ efficiency in the internal combustion engine is considered same as for diesel engine (Verhelst and Wallner, 2009). In order to use H₂ in existing internal combustion engine, fuel supply system has to be modified to avoid pre-ignition in the premixed H₂ and air (Salvi and Subramanian, 2015). Further, high NO_x emission has been reported for H₂ in spark ignition engine due to increase in combustion temperature (Shivaprasad et al., 2018). Figure 10 presents weight of fuel with storage tank and CO₂ capture system with liquid CO₂ storage, for different fuels. For compressed CH₄ fuel, it is assumed that captured CO₂ can be stored in the same storage tank. For different fuels, the total weight (fuel with storage tank, adsorbent material, and CO₂ with storage tank) are compared at initial point (0 km), mid-point (125 km), and end point (250 km). For carbon based fuels, added weights of fuel and CO₂ capture and storage systems are comparable.

TABLE 4

Table 4. Use of alternate fuels in the delivery truck for 250 km travel.

FIGURE 10

Figure 10. Weight comparison between on-board storage of H₂ and different carbon based fuels with CCS-LS for conventional internal combustion engine.

CO₂ Capture System and Liquid Storage for Different Vehicles

The CO₂ capture system and liquid storage (CCS-LS) can be integrated on different types on vehicles such as cars, trucks, buses, ships, and trains. Average weights, payloads and fuel (diesel) consumptions for different types of vehicles are reported in the Supplementary Material (Table E). The added weight of CCS-LS includes weights of CO₂ with storage tank and adsorbent material. For comparison basis, each vehicle is assumed to be traveled 250 km (diesel fuel), and travel time for each vehicle is reported in the Supplementary Material (Table F). Figure 11 presents percentage added weight of CCS-LS with respect to total weight of vehicle (i.e., empty weight of vehicle + payload on vehicle) and payload on vehicle. For ferry and cruiseship, payloads are estimated based on maximum number of on-board persons. As expected, percentage added weight of CCS-LS is larger for small vehicles (car, SUV) compared to large vehicles (ferry, cruiseship, train). CCS-LS can be used on vehicle to reduce the CO₂ emissions by allowing a higher share of renewables used in the transport and reducing the fossil CO₂ emissions to environment and the same time to generate cooling by using waste heat available in the engine exhaust stream and cooling system.

FIGURE 11

Figure 11. Percentage added weight of CO₂ capture system and liquid storage (CCS-LS) with respect to total weight (A) and payload (B) of different vehicles.

Conclusions and Future Works

This study presents a system for CO₂ capture from the exhaust stream of an internal combustion engine. Energy and exergy analyses of the CO₂ capture system has been performed in details. Three CO₂ capture system designs have been compared, and adsorption-desorption on vehicle with liquid CO₂ product found to be better than the other two designs. The system design includes integration of temperature swing adsorption, Rankine cycle, heat pump (i.e., cold generation), and CO₂ compression and liquefaction on vehicle. The proposed system design has energy self-sufficiency, as it converts waste heat available in the exhaust stream into mechanical power (via Rankine cycle) to drive the heat pump compressor and product compressors. The captured CO₂ can store renewable energy by converting product CO₂ into green fuels using co-electrolysis. About 90% of the carbon present in the fuel can be recycled using CO₂ capture system, and the remaining 10% carbon can be supplied from the biomass. This study compares conversion of captured CO₂ into methane, methanol, DME and gasoline. Further, percentage added weights of CO₂ capture system and liquid storage for several vehicles are calculated, that shows high potential for the integration of CO₂ capture in the transportation sector. Finally, weights of on-board hydrogen storage for fuel cell electric vehicle and CO₂ capture system with liquid storage for conventional diesel vehicle are compared.

This study is an initial effort for capturing CO₂ from vehicles, and it may require several years to realize such system in practice. In this study, fixed composition and flow rate of the exhaust gas have been assumed, to calculate the heating/cooling requirements of the TSA. This choice is suitable for train and ship transports, but exhaust variations are expected for cars, trucks, and buses. Further, steady-state operation of Rankine cycle is very critical, and so an intermediate thermal fluid will be required to store the exhaust heat and to maintain the Rankine cycle operation. In future, dynamic study on the system will be performed. This system analysis and design have several assumptions and uncertainties, which should be improved by experimental data. Hence, we are planning to develop a prototype of the CO₂ capture system.

Data Availability Statement

The raw data supporting the conclusions of this manuscript will be made available by the authors, without undue reservation, to any qualified researcher.

Author Contributions

All authors listed have made a substantial, direct and intellectual contribution to the work, and approved it for publication.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

This research project is financially supported by the Swiss Innovation Agency lnnosuisse and is part of the Swiss Competence Center for Energy Research SCCER EIP.

Supplementary Material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenrg.2019.00143/full#supplementary-material

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