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nitrogen carrier gases
Journal of CO₂ Utilization 34 (2019) 725–732
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Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Carbon dioxide conversion by solar-enhanced microwave plasma: Effect of specific power and argon/nitrogen carrier gases
T
Sina Mohseniana, Dassou Nagassoua, Rasool Elahia, Peng Yub, Melisa Nallarb, Hsi-Wu Wongb, ⁎ Juan P. Trellesa, a b
Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA 01854, United States Department of Chemical Engineering, University of Massachusetts Lowell, Lowell, MA 01854, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Atmospheric pressure microwave plasma Solar thermochemistry Solar-plasma interaction CO2 decomposition
Plasma-chemical processes are ideally suited for the conversion of low-value gaseous feedstock such as carbon dioxide (CO2) into fuels and chemicals. However, the reliance on low-pressure operation of traditional plasma methods to achieve high efficiency can limit their economic viability and net sustainability benefits. SolarEnhanced Microwave Plasma (SEMP) CO2 conversion exploits the interaction between solar radiation and a CO2Ar or CO2-N2 microwave discharge to achieve increased energy efficiency while operating at atmospheric pressure conditions. Moreover, by incorporating solar energy, SEMP processes exhibit the same specific energy input (SEI) as conventional plasma methods. The effects of electric power, solar power, flow rate, and gas composition on solar absorption, conversion, and energy efficiencies in SEMP CO2 conversion are investigated. Experimental results show up to ∼21% absorption of solar power obtained under CO2-Ar operation. In all the studied conditions, the absorbed solar radiation leads to increased CO2 conversion efficiency; e.g. an increase from ∼8% to ∼9% is obtained at 900 W electrical power and 75 W absorbed solar power. The use of nitrogen as auxiliary gas leads to higher conversion and energy efficiencies than those using argon. The highest conversion efficiency obtained with CO2-N2 is ∼15.5%, whereas the highest conversion efficiency using CO2-Ar is ∼9.5% – both observed at the same SEI of 1.8 eV/molecule. The results suggest the potential of the SEMP processing of CO2-N2 mixtures, as found in flue gas from fossil-fuel power plants, as a viable approach for CO2 conversion and utilization.
1. Introduction Through the last decades, the continuous consumption of fossil fuels in power plants has significantly contributed to the dramatic increase in the amount of carbon dioxide (CO2) in the earth’s atmosphere with respect to pre-industrial levels [1,2]. To control the rising atmospheric CO2 concentration, several CO2 utilization methods are being investigated. The conversion of gaseous CO2 into synthesized fuels and chemicals using renewable energy is an increasingly appealing alternative given its potential for shifting and/or reducing CO2 emissions while helping fulfill global demands for fuels and chemicals [3,4]. The conversion of CO2 requires high energy processes due to the high thermodynamic stability of CO2 (i.e. large negative formation enthalpy of −394 kJ/mol [5]). The required energy can be directly provided by renewable energy sources, such as concentrated solar radiation in solar thermochemical CO2 decomposition approaches [6,7]. Alternatively, electricity-based methods, such as electrochemical and plasma
⁎
processes, can readily exploit renewable electricity to provide the energy required for CO2 decomposition reactions [8,9]. For example, solar photovoltaic and/or wind energy can provide the electrical energy to establish and sustain an electrical discharge and drive CO2 conversion reactions [10]. Plasma sources are traditionally categorized as thermal or nonthermal. Thermal plasmas are often generated at atmospheric pressure conditions and are characterized by highly-efficient electrical-tothermal energy conversion that makes them attain very high temperatures (typically of over 10 kK) for both, free electrons and gas species [11,12]. Non-thermal (cold) plasmas are commonly attained at lowpressures, and present electron temperatures of 1–2 eV (i.e. 11.6–23.2 kK), which are significantly higher than those of the gas species (from near atmospheric to a few hundred Kelvin). Chemical processes based on non-thermal plasmas tend to present significantly higher energy efficiency and selectivity than those based on thermal plasmas. Non-thermal plasma sources have shown high potential for
Corresponding author at: 1 University Avenue, Lowell, MA, 01854, United States. E-mail address: [email protected] (J.P. Trelles).
https://doi.org/10.1016/j.jcou.2019.09.002 Received 14 August 2019; Received in revised form 30 August 2019; Accepted 3 September 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Solar-Enhanced Microwave Plasma (SEMP) reactor system: the interaction between concentrated solar radiation and microwave plasma leads to enhanced CO2 decomposition.
higher energy efficiency). The relative low energy efficiency of atmospheric pressure microwave plasma CO2 decomposition is one of the most important challenges in plasma-chemical processing. Experimental observations have largely shown that the use of higher SEI results in higher conversion efficiency and lower energy efficiency [20,22]. Such trade-off is a key factor limiting the further advancement towards the viability of plasmabased CO2 decomposition methods. Therefore, strategies that increase the conversion efficiency without increasing the electric power input to the plasma (i.e. maintaining the same SEI) [14] are highly desirable. In that regard, our previous work [25,26] on the development of the approach known as Solar Enhanced Microwave Plasma (SEMP) processing found that the absorption of concentrated solar energy by atmospheric pressure microwave plasma increases the amount of deposited power into the plasma, increasing conversion efficiency and energy efficiency without altering the SEI. In the present work, the effect of process parameters such as electric and solar power, gas composition, and total gas flow rate on solar radiation absorption and CO2 conversion are investigated.
CO2 decomposition in low temperature and catalyst-free environments. Particularly, the high energy electrons in non-thermal electrical discharges, such as dielectric barrier discharge (DBD) [13,14], gliding arc [15,16], and microwave discharge, can induce vibrational excitation of CO2 molecules, which is a highly efficient channel for CO2 decomposition [17–19]. Among non-thermal plasmas, microwave plasmas are deemed the most effective plasma sources for CO2 decomposition due to their capability to induce molecular dissociation processes via concurrent thermal decomposition and vibrational excitation pathways [20,21]. Microwave plasmas are typically generated from low pressures, of a few mTorr, up to atmospheric pressure. CO2 decomposition via lowpressure microwave plasma has demonstrated very high energy efficiency. Particularly, efficiencies of ∼47% were obtained by Bongers et al. [20] using a microwave discharge operating at 200 mbar and at a specific energy input (SEI) of ∼0.8 eV/molecule, where SEI is a process parameter that quantifies the relative electrical energy intensity of the process. The experimental results of Bongers et al.’s work indicate that the plasma gas temperature was close to the vibrational temperature, and that the plasma operated in the thermal decomposition limit. Therefore, the thermal dissociation mechanism was predominant over the electron-induced vibrational mechanism. Moreover, Rooij et al. [22] reported a maximum energy efficiency of ∼45% operating in the range of 100–200 mbar, also for an SEI of ∼0.8 eV/molecule. The temperature measurements in their experiments indicated that the maximum conversion was obtained at the maximum gas temperature conditions. However, the obtained conversion efficiency did not approach the thermodynamic maximum of ∼55%. Even though the high energy efficiencies of low-pressure microwave plasma CO2 decomposition processes, low-pressure (vacuum) processing would incur additional operation costs (compared to atmospheric pressure operation) that limit its economic viability. Increasing the operating pressure from a few mbar to ∼1 atm can significantly decrease the process energy efficiency due to higher vibrational to translational energy relaxation [14,29]. At atmospheric pressure, the maximum energy efficiency for microwave plasma CO2 decomposition reported in the literature was ∼27%, obtained by Spencer et al.[23] at a SEI of ∼1 eV/molecule. Their results indicated that with increasing the SEI, energy efficiency decreases but conversion efficiency increases. The investigators concluded that increasing the SEI causes the thermal equilibration of vibrational and rotational temperatures, leading to the lower values of energy efficiency. Furthermore, Belov et al.[24] investigated the effect of gas pressure and gas flow rate on the conversion and energy efficiency of microwave plasma CO2 decomposition. The highest energy efficiency at atmospheric pressure operation reported by them was ∼7%. Their results indicated that the higher the gas flow rate at constant power, the higher the energy efficiency - a finding consistent with the results reported by Spencer et al. (i.e. the lower SEI, the
2. Methods The SEMP process seeks to exploit the interaction between an atmospheric pressure microwave plasma and an influx of concentrated solar radiation in order to enhance CO2 decomposition. Fig. 1 schematically illustrates the SEMP system. The system includes a parabolic concentrator to gather and direct a flux of concentrated solar radiation into the SEMP reactor. Inside the reactor, a microwave plasma is sustained by the interaction of input electric power carried by microwaves and a flow of feedstock gas. The interaction between concentrated solar radiation and microwave plasma, starting at the concentrator’s focal point inside the reactor and proceeding downstream, causes the absorption of photons by plasma species (e.g. excited molecules, ionized atoms, etc.), leading to step-wise vibrational excitation and thermal dissociation of the CO2 molecules. To evaluate SEMP CO2 processing, the following metrics for the absorption of solar radiation and for the effectiveness of CO2 decomposition were used. The absorption of solar radiation by microwave plasma was evaluated by measuring the intensity of solar radiation after its transit through the reactor with and without the plasma. The absorption efficiency (ηa) is calculated by:
ηa =
I0 − I , I0
(1) −2
−1
where I0 is the net radiation intensity (W-m -sr ) at the reactor’s exit when the plasma is off, which is assumed equal to the incident radiation, and I is the net radiation intensity at the reactor’s exit when the 726
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with a quartz window that acts as the solar aperture placed at the based of a stainless-steel conical chamber. The conical chamber directs the incident solar radiation into the reaction chamber, and receives the working gas stream. The working gas is injected via two tangential holes on the side of the conical chamber, resulting in a swirl flow pattern inside the discharge tube. The reaction chamber consists of a dielectric tube that pierces perpendicularly through the tapered waveguide to form a transverse electric mode (TE101) inside the chamber. Since the discharge tube in the SEMP reactor experience intense heating due to its exposure to concentrated solar radiation and the confinement of the atmospheric pressure plasma, the tube is made of boron nitride, given that boron nitride at high temperatures remains transparent to microwave energy (in contrast to quartz, often used in the discharge tube of microwave plasma systems, which presents high absorption of microwave energy when heated). Inside the chamber, plasma ignition is obtained by briefly inserting a thin tungsten wire, which causes the local accumulation of the incident microwave energy. The swirl flow pattern stabilizes the microwave plasma and cools the discharge tube by forced convection. A stainless-steel cut-off tube, used to confine the microwave power and promote its absorption by the plasma, surrounds the reaction chamber. Fig. 2(b) presents the implemented SEMP reactor under operation. The figure depicts the exposure to concentrated solar radiation from a high-flux solar simulator placed in front of the optical aperture. The conical chamber channels the flux of concentrated solar radiation into the discharge tube, where plasma efficiently interacts with solar radiation, starting at the focal point of the concentrator and continuing downstream along the reactor chamber. The processed gas stream exits the reactor through the exhaust port at the end of the reaction chamber.
plasma is present. Both quantities, I0 and I, are obtained by integrating their respective spectral intensities, i.e. I = ∫ Iλ dλ , where Iλ is the spectral intentity (W-m−2-sr−1-nm-1) and λ is wavelength (nm). The absorbed solar power by the plasma (Wabs) is given by:
Wabs = ηa Wsi,
(2)
where Wsi is the input solar power injected into the reactor (W). The conversion efficiency (ηc), which quantifies the relative amount of CO2 gas decomposed during the SEMP process, is given by:
ηc =
co2 co2 N˙ in − N˙ out , co2 N˙ in
co N˙ in 2
(3)
co N˙ out2
and are the input and output molar flow rates of CO2 where (mol-s−1) respectively. The specific energy input (SEI) indicates the consumption of electrical power per molecule of input gases and is defined as:
SEI =
∑ j = CO2, N2, Ar
Wpi j N˙ in
, (4)
where is the input molar flow rate (mol-s−1) of the working gas stream (i.e. mixtures of carbon dioxide with argon or with nitrogen), and Wpi is the input electric power into the plasma (W). SEI is typically expressed in units of eV/molecule. The electrical energy efficiency (ηe) provides a metric of the relative amount of electrical energy effectively that is consumed for conversion of CO2 gas and is defined as: j N˙ in
co ΔH ηe = N˙ out , Wpi
(5)
where ΔH is the decomposition enthalpy of a CO2 molecule (i.e. 2.9 eV/ molecule). Our previous work 26 focused on the SEMP reactor and process design and on the investigation of the effect of solar input power on solar absorption, energy efficiency, and CO2 conversion efficiency for a set of fixed process parameters (specifically: 700 W electric power, 8 slpm flow rate and 7 Ar/CO2 flow ratio). In this study, the effects of electric power and solar power, gas composition, and gas flow rate on solar energy absorption and CO2 conversion are investigated. Specifically, CO2-Ar and CO2-N2 mixtures (where argon and nitrogen are auxiliary gases) are used as working gases. The effective electric power deposited into the plasma (i.e. direct minus reflected microwave power – see section 3.2) varied between 600 and 900 W. The solar input power into the reactor varied in three levels, namely: 0, 320, and 525 W.
3.2. Experimental set-up Fig. 3 shows a schematic representation of the SEMP CO2 conversion system. The microwave system is operated at the industrial frequency of 2.45 GHz via a 1.2 kW magnetron (Panasonic NN-T945SF) fed by a 4 kV AC power supply. Electromagnetic energy is directed via WR340 rectangular waveguides towards the reaction chamber, which displays a tapered waveguide section to increase electromagnetic energy density and facilitates the formation and maintenance of the plasma. Wave tuning and control components include a launcher, an isolator, a directional coupler, stub tuners, and a sliding short circuit to tune wave reflection. To monitor the forward and reflected power, and to calculate the electric power deposited into the plasma, a power monitor connected to the directional coupler was used. In all the experiments, the stub tuners and short circuit were tuned such that the plasma absorbed electric power stayed in the range of 600–900 W. A 6.5 kW high-flux solar simulator from Kinoton GmbH is used to provide the SEMP reactor with concentrated solar radiation. The simulator is fitted with a Xe short-arc lamp as the radiation source, whose emission matches closely the daylight spectrum, especially throughout the 400–700 nm wavelength range. A coated quartz glass inside the simulator filters the UV emission to avoid the ozone production. The solar simulator can deliver 350 to 525 W of solar power to the reactor by varying the simulator’s electrical current between 80 and 120 A using a potentiometer in the simulator’s rectifier. Mass flow controllers (ALICAT MC) were used to control the flow rate of CO2, Ar and N2 in the range of 0–10 slpm for attaining the desired CO2-Ar and CO2-N2 working gas mixtures. A water circulation system consisting of two water pumps immersed in a water tank was used for cooling the isolator’s waveguide and the tapered waveguide flanges. This cooling limits the heating of the discharge tube, particularly at the focal point (e.g. plasma generation point), which is the hottest part of the reaction chamber. An optical spectrometer (Avantes AvaSpec-ULS2048) fitted was used to measure the solar radiation intensity in the range from 200 to 1100 nm. The spectrometer was connected to an optical fiber placed at the reactor’s exhaust port.
3. Experimental components and devices 3.1. SEMP reactor The reactor design seeks to promote and exploit the interaction between the flux of concentrated solar radiation and the microwave plasma [25,27]. The interaction of solar photons with plasma species can promote the absorption of photons by vibrationally-excited CO2 molecules or by electronically-excited carrier gas molecules (Ar or N2). The latter presents the potential for sustaining the plasma with lower consumption of electric power (a potential verified by the results in section 4.2). In addition, a distinct feature of the SEMP process is that it results in the absorption of solar radiation by gas in absence of any catalytic medium. Such characteristic is in marked contrast to diverse solar thermochemical processes, which rely on a thermo- or photocatalytic medium [28,29]. Therefore, adding a catalytic medium downstream of the plasma-radiation interaction region in order to utilize un-absorbed radiation and further drive thermo-catalytic reactions could further enhance the SEMP process. Fig. 2(a) shows the design of the SEMP reactor. The reactor counts 727
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Fig. 2. SEMP reactor: (a) cross-sectional side view of the reactor design and (b) implemented SEMP reactor in operation in front of a high-flux solar simulator.
larger power-delivery system, limiting the viability of the CO2 conversion process. Using an easy to ionize gas besides CO2 promotes the ionization process, facilitating plasma ignition and keeping the plasma more stable [23,25,30]. In the experiments, plasma is initiated using pure argon gas (Airgas, 99.997% purity) and introducing a tungsten wire into the reactor chamber to concentrate microwave energy and to promote ionization (i.e. plasma ignition). Then, at the desired flow rate of Ar, CO2 is added gradually to reach the desired total flow rate and CO2-Ar ratio. Given the need for an auxiliary, easily ionizable gas such as argon, together with additional complexity (and expense) of using such a gas, studying the effect of the Ar/CO2 flow ratio is of primary importance for characterizing SEMP CO2 processing. Fig. 4 shows the experimental results of the solar absorption efficiency (ηa) and absorbed solar power (Wabs) as function of the Ar/CO2 flow ratio. The input solar power, electric power, and total gas flow rate were fixed at 320 W, 900 W, and 8 slpm, respectively. The Ar/CO2 gas flow ratio varied in the range of 1–7. The maximum solar absorption efficiency obtained is ∼21% (i.e. the microwave plasma absorbs ∼75 W of solar radiation power) at the Ar/CO2 flow ratio of 7. Absorption efficiency and absorbed power decrease to ∼7% and 30 W, respectively, with decreasing the flow ratio to 1. This result indicates the significant effect on solar energy absorption caused by the fraction of argon in the working gas stream. The presence of argon in the plasma affects the ionization processes, resulting in changing the optical characteristics of the plasma (emissivity and absorptivity), as manifested by the change in solar absorption with the Ar/CO2 ratio. In addition, higher Ar/CO2 ratios increase the stability of the plasma, leading to higher solar absorption efficiency. Fig. 5 presents the measured CO2 conversion efficiency (ηc) and electrical energy efficiency (ηe) in the SEMP reactor as a function of Ar/
Additional details of the SEMP reactor design and experimental setup are described in [25]. 3.3. Product analysis The processed gas exited the SEMP reactor was collected after passing through an ice bath for quenching (see Fig. 3). The processed gas samples were collected in Tedlar® bags and analyzed by a Shimadzu GC-2014 gas chromatography (GC) system equipped with a thermal conductivity detector (TCD). For the gas analysis, 0.5 ml of the gas were collected from each bag and injected into a Restek packed column (80,486 − 810, 2 m) connected to the TCD. Peak identification and calibration were achieved from results obtained using standard gas mixtures containing CO, CO2, and C1 to C4 hydrocarbons (Scotty Specialty Gases). The GC was programmed with an injection temperature of 250 °C and a split ratio of 10.0, respectively. The programmed temperature regime for the GC oven was as follows: start at 35 °C, ramp up to 50 °C at 7.5 °C min−1, hold for 1 min, ramp up to 100 °C at 5 °C min−1, hold for 3 min, ramp up to 200 °C at 10 °C min−1, hold for 7 min, ramp up to 250 °C at 25 °C min−1, and hold at 250 °C for 10 min. 4. Results and discussion 4.1. Effect of CO2-Ar flow ratio The use of argon gas facilitates the ignition and stabilization of the atmospheric pressure microwave plasma. In a pure CO2 plasma, most of the deposited electric power is transferred to the vibrational excitation modes of CO2 molecules rather than being consumed in ionization of the gas. Therefore, sustaining a pure CO2 plasma requires a significantly
Fig. 3. Schematic diagram of the solar enhanced microwave plasma system setup (GC: Gas Chromatography, OES: Optical Emission Spectroscopy). 728
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Fig. 6. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as functions of electric power and solar power. (Conditions: 8 slpm total flow rate, 7 Ar/CO2 flow ratio).
Fig. 4. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as a function of the Ar/CO2 flow ratio. (Conditions: 900 W electric power, 8 slpm total flow rate, 320 W solar input power).
results in Fig. 5 show that at the Ar/CO2 flow ratio of 7, the plasma energy efficiency increases from around 15.5% to 17%. 4.2. Effect of electrical and solar power on CO2-Ar Results of absorbed solar power and solar absorption efficiency as a function of electric power deposited into the plasma are shown in Fig. 6, for two different levels of solar input power: 320 and 525 W. The gas flow rates of CO2 and Ar were kept constant at 1 slpm and 7 slpm, respectively (i.e. Ar/CO2 flow ratio equal to 7), to isolate the effect of electrical and solar power on solar absorption. Operation at the Ar/CO2 flow ratio of 7 demonstrated a stable atmospheric pressure microwave plasma for a wide range of electric power input. In this experiment, the electrical energy into the plasma varied in the range of 600–900 W. Absorption efficiency and absorbed solar power increase with increasing the electric power deposited into the plasma. This behavior appears to be the consequence of the enhancement of electronic excitation processes due to the increase of electric power absorbed by electrons. Electronic excitation of plasma is potentially detrimental in terms of changing the net radiative absorption properties of the plasma (e.g. higher power density plasma emits more radiation, which could neglect the absorption of solar radiation). From the obtained results, it is observed that solar radiation absorption increases with increasing the amount of electric power absorbed by plasma. Nevertheless, the solar power absorption efficiency is greater for lower solar input power. This finding indicates that at higher solar input power levels, more solar power gets unutilized. However, the higher consumption of electrical and solar power results in higher solar power deposited into the plasma, which may improve the CO2 conversion process. The variation of CO2 conversion and energy efficiency, for three different levels of solar input power: 0, 320, and 525 W, as a function of total deposited power (electrical plus solar power) are shown in Fig. 7. The CO2 conversion efficiency for plasma-only operation (i.e. 0 W solar power) increases from around 6% to 8.5% with increasing the electric power input. The enhancement of conversion efficiency with increasing power at a fixed gas flow rate is directly related to the enhancement of specific electrical energy delivered to the vibrational excitation and thermalization of CO2 molecules [33]. However, the effect of electric power is opposite for the energy efficiency, e.g. increasing the electric power leads to decreasing the energy efficiency from 16.5% to 15.5%. This phenomenon can be understood by considering that a significant portion of power is expected to be deposited over the surface of the plasma volume rather than into the plasma bulk because of the socalled skin depth effect [34,35]. In addition, increasing the electrical deposited power for a fixed amount of working gas causes the thermal
Fig. 5. Conversion efficiency (black) and energy efficiency of CO2 decomposition (blue) in the SEMP reactor as a function of flow ratio. (Conditions: 900 W electric power, 8 slpm total flow rate).
CO2 flow ratio for zero solar power (i.e. plasma-only operation - solar simulator turned off) and 320 W solar input power. The CO2 conversion and energy efficiencies for plasma-only at the flow Ar/CO2 ratio of 1 are around 4.5% and 8.5%, respectively. By increasing the Ar/CO2 flow ratio to 7, both conversion and energy efficiencies increase to around 8% and 15.5%, respectively. Increasing the fraction of argon increases the overall rate of ionization, which consequently increases the impact of electrons with CO2 molecules. Therefore, the observed enhancement of efficiencies by increasing the Ar/CO2 flow ratio appears to be due to the enhancement of electron-induced vibrational excitation of CO2 molecules [31,32]. Fig. 5 also depicts the effect of introducing high-flux solar radiation on both, conversion and energy efficiencies for various gas flow ratios. The enhancement of conversion and energy efficiencies due to introducing solar power is greater for higher flow ratios than for lower ones; e.g., the conversion efficiencies for flow ratios of 7 and 1 are enhanced by 11% and 4.5%, respectively. This result is consistent with the results presented in Fig. 4, i.e. the higher the Ar/CO2 flow ratio, the more solar energy is absorbed by the plasma. Furthermore, since for a fixed flow rate and electrical input power, the specific energy of the plasma remains constant, the increase in plasma energy efficiency was solely due to the increased absorbed solar power. For example, the
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Fig. 8. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as a function of SEI for Ar/CO2: 7/1.
Fig. 7. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of total power for Ar to CO2 of 7 and 8 slpm total flow rate.
equilibrium of vibrational and rotational temperatures, reducing the selectivity of molecular decomposition, leading to the lower energy efficiencies - a behavior previously reported in [23,24,33]. The results in Fig. 7 also indicate that the effect of solar power is more dominant than the effect of electric power in the conversion process, i.e. at a specific amount of total (electric plus solar) power, increasing the solar input power leads to higher conversion efficiency. As mentioned earlier, electromagnetic power absorption into the plasma is limited by the skin depth effect. However, in SEMP, the microwave plasma can absorb the solar radiation, independently of the skin depth effect, leading to an increase in deposited energy into the plasma and enhancing both, conversion and energy efficiencies. As shown in the results in Fig. 6, the amount of absorbed solar power increases with increasing both solar and electrical power input. This behavior is consistent with the results presented in Fig. 7. For example, at 900 W electric power, around 7.5% enhancement of conversion efficiency is obtained by increasing the solar power input from 320 to 525 W. However, the enhancement of conversion efficiency for 600 W of electric power is only around 3%.
Fig. 9. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of SEI for Ar/CO2: 7/1.
Fig. 9 illustrates the variation of CO2 conversion efficiency and energy efficiency for the two different levels of solar input power: 0 W (plasma-only) and 320 W as a function of SEI. By increasing the SEI, the conversion efficiency at both solar power levels increases. However, the conversion efficiencies for the solar-plasma configuration are greater than those for plasma-only. The maximum conversion efficiencies observed with and without adding solar radiation are 9% and 10%, respectively, which correspond to the highest SEI of 2.75 eV/molecule. The enhancement of conversion efficiency by adding solar radiation to the system reaches the maximum of 11%, obtained at 1.6 eV/molecule, at the highest electric power level (900 W) and for 8 slpm total flow rate. This result is consistent with the one obtained from the solar absorption analysis (Fig. 8) in which the maximum amount of absorbed solar power obtained is 1.6 eV/molecule. The results in Fig. 9 also indicate an inverse relationship between energy efficiency and SEI for both solar-plasma and plasma-only systems, i.e. increasing the SEI leads to the decrease in energy efficiency from 17% to 10% for the plasma-only system. However, as it is expected from the conversion efficiency results, the energy efficiency of the solar-plasma system, varying from 19% to 11%, is higher than that for the plasma-only system. Therefore, by adding solar energy, the amount of energy deposited into the plasma increases without increasing the SEI, leading to the observed higher energy efficiencies. Finally, the obtained results show that, although increasing the SEI leads to more efficient CO2 conversion, increasing SEI cannot lead to
4.3. Effect of specific energy input (SEI) Specific Energy Input (SEI), defined in Eq. 4, is a process parameter that quantifies the electric energy intensity of the process. Therefore, SEI correlates with the electrical energy cost of the process. Fig. 8 shows the variation of solar absorption efficiency and absorbed solar power as a function of SEI at a solar input power of 320 W and a fixed Ar/CO2 gas flow ratio of 7. Experiments were initially performed in a plasma-only configuration (i.e. 0 W solar power) in order to attain the widest range for SEI possible (while maintaining a stable plasma for flow rates of 4–9 slpm and electric powers of 600–900 W), varying from 0.9 to 2.7 eV/ molecule. The maximum solar absorption efficiency obtained is ∼21%, i.e. the microwave plasma absorbs ∼75 W of solar radiation, which occurs at a SEI of ∼ 1.6 eV/molecule, corresponding to 900 W electric power and 8 slpm total gas flow rate. The last two points of the plots in Fig. 8 correspond to the gas flow rate of 5 slpm. As it was found from Fig. 6, an increase in electric power at a fixed gas flow rate increases solar energy absorption. However, lower swirl gas flow rate inside the reactor makes the plasma bulb less concentrated and less stable which may influence the solar absorption. Therefore, it can be concluded that increasing SEI cannot enhance the absorption efficiency, independently; i.e. at lower flow rates, the more solar power is unutilized because of the less plasma stability, resulting in lower absorption efficiency and absorbed solar power. 730
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concurrent increases in solar radiation absorption and conversion efficiency.
4.4. Effect of nitrogen gas In previous sections, argon gas was used as the auxiliary gas to initiate and stabilize the CO2 plasma by facilitating ionization kinetics. However, CO2 is emitted from power plants mixed with nitrogen (as well as smaller amounts of oxygen, water vapor, and other residual gases) as what is commonly known as flue gas. Therefore, using nitrogen instead of argon as an auxiliary gas provides a more practical impact on the sustainability and viability of SEMP CO2 decomposition processes. The volume fraction of nitrogen in flue gas of oil burner power plants is in the range of 78–80%; however, the fraction of CO2 gas is in the range of 8–11%. Therefore, the ratio of N2 to CO2 in flue gas is in the range of ∼7–10 [36]. In the SEMP operations investigated in this work, the microwave plasma was initiated using pure nitrogen gas. Then, CO2 gas was added gradually to the reactor to reach a N2/CO2 gas flow ratio of 8.75, which is in the range of flue gas emission from power plants and is a ratio that leads to a stable plasma. Fig. 10 presents the variation of solar absorption efficiency and absorbed solar power as functions of SEI at 320 W solar input power. SEI varies from 0.9 to 1.9 eV/molecule with gas flow rates from 4 to 9 slpm and deposited electric power of 600–900 W. The solar absorption efficiency obtained is in the range of ∼6.5–8%, i.e. the microwave plasma absorbs ∼23-29 W of solar radiation. However, according to the results for solar absorption in CO2-Ar plasma presented in Fig. 8, for this range of SEI, solar absorption efficiency and absorbed solar power are in the range of 13–21% and 48–75 W, respectively. These results indicate that the absorption of solar radiation in CO2-Ar microwave plasma is more efficient than that in CO2-N2 plasma. As mentioned in section 4.1, using an easily-ionizable gas such as argon besides CO2 promotes ionization kinetics, making the plasma more stable. In addition, the higher ionization reactions can affect the optical characteristics of the plasma and lead to the enhancement of the solar radiation absorption. From the results presented in Figs. 8 and 10, it could be expected that the use of argon as the auxiliary gas would result in a higher amount of solar power deposited in the plasma. However, the use of CO2-N2 gas mixtures is of greater relevance for direct CO2 utilization technologies. The maximum solar absorption efficiency obtained with CO2-N2 is ∼8%, i.e. the microwave plasma absorbs ∼29 W of solar radiation power which occurred at SEI of ∼1.6 eV/molecule, corresponding to 900 W electric power and 7.8 slpm total gas flow rate. (The data point for SEI ∼ 1.9 in Fig. 10 corresponds to the gas flow rate
Fig. 11. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of SEI for N2/CO2: 8.75/1.
of 6.68 slpm at 900 W electric power.) This result is consistent with the one obtained in Fig. 8 (CO2-Ar plasma), i.e. less solar power is deposited in the plasma for high SEI (i.e. fixed electric power and low flow rate), resulting in lower absorption efficiency and absorbed solar power. The variation of CO2 conversion efficiency and energy efficiency for the two different levels of solar input power, 0 and 320 W, as a function of SEI is presented in Fig. 11. The conversion efficiency at both solar power levels increases with SEI; however, the energy efficiency decreases. In addition, both conversion and energy efficiencies for various SEIs are greater with the solar power input. These results are consistent with those obtained for CO2-Ar operation. However, the values of conversion and energy efficiencies in CO2-N2 are greater than those under CO2-Ar operation. This behavior could be caused by the fact that N2 molecules can vibrationally excite CO2 molecules; whereas under CO2-Ar operation, CO2 molecules can be vibrationally excited only by electron impacts. Heijkers et al. [37] theoretically found that at around 85% of N2 fraction in a CO2-N2 plasma, vibrationally excitation of CO2 molecules induced by N2 molecules contributes to around 50% of CO2 dissociation; however, the contribution of electron induced vibrational excitation of CO2 molecules is only around 8%. Therefore, the higher vibrational excitation induced by N2 in CO2 could explain the observed higher CO2 conversion efficiency in the CO2-N2 process with respect to that in the CO2-Ar plasma process.
5. Summary and conclusions CO2 conversion via Solar-Enhanced Microwave Plasma (SEMP), an approach that exploits the interaction between concentrated solar radiation and microwave plasma, has the potential to lead to greater conversion and energy efficiencies than traditional plasma-based methods while operating at atmospheric pressure conditions. The effects of specific power input and CO2-Ar and CO2-N2 mixtures on solar power absorption, CO2 decomposition, and energy efficiency during SEMP CO2 conversion have been investigated. The experimental results under CO2-Ar operation, for Ar/CO2 ratios from 1 to 7, indicate that the greater the Ar/CO2 ratio, the greater the amount of absorbed solar power, and the higher the conversion and energy efficiencies. Also, the higher the electrical energy deposited in the plasma, the higher the solar radiation absorption efficiency. Although increasing the electric power input to the plasma leads to increasing the conversion efficiency, it also leads to decreasing the energy efficiency. Importantly, the obtained results also indicate that solar power is more effective on both conversion and energy efficiencies than electric power because of circumventing skin-depth power absorption limitations inherent in microwave plasmas. Furthermore, the
Fig. 10. Solar power absorption efficiency (black) and absorbed solar power (blue) as a function of SEI for N2/CO2: 8.75/1. 731
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experimental measurements showed that increasing the specific energy input (SEI) increases the conversion efficiency, but cannot increase the solar absorption efficiency independently. The use of a CO2-N2 mixture at a N2 to CO2 of 8.75 leads to stable operation of the plasma and better process performance than that obtained using CO2-Ar, while providing closer resemblance to the expected performance of the processing of flue gas from fossil-fuel power plants. Similar trends for solar power absorption and CO2 conversion obtained for CO2-Ar were observed during CO2-N2 operation. However, the results indicate that although the solar absorption efficiency under CO2-N2 operation is lower than that for CO2-Ar, the overall conversation efficiency using CO2-N2 is greater. Such result seems to be caused by the higher contribution of N2 molecules to the vibrational excitation of CO2, compared to that attained by electron collision. The results suggest the potential of the SEMP processing of CO2-N2 mixtures, as found in flue gas from fossil-fuel power plants, as a viable approach for CO2 conversion and utilization.
[15] [16]
[17] [18]
[19]
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
[23]
[24]
Acknowledgements
[25]
The authors acknowledge the financial support provided by the U.S. National Science Foundation through award CBET-1552037.
[26]
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Contents lists available at ScienceDirect
Journal of CO2 Utilization journal homepage: www.elsevier.com/locate/jcou
Carbon dioxide conversion by solar-enhanced microwave plasma: Effect of specific power and argon/nitrogen carrier gases
T
Sina Mohseniana, Dassou Nagassoua, Rasool Elahia, Peng Yub, Melisa Nallarb, Hsi-Wu Wongb, ⁎ Juan P. Trellesa, a b
Department of Mechanical Engineering, University of Massachusetts Lowell, Lowell, MA 01854, United States Department of Chemical Engineering, University of Massachusetts Lowell, Lowell, MA 01854, United States
A R T I C LE I N FO
A B S T R A C T
Keywords: Atmospheric pressure microwave plasma Solar thermochemistry Solar-plasma interaction CO2 decomposition
Plasma-chemical processes are ideally suited for the conversion of low-value gaseous feedstock such as carbon dioxide (CO2) into fuels and chemicals. However, the reliance on low-pressure operation of traditional plasma methods to achieve high efficiency can limit their economic viability and net sustainability benefits. SolarEnhanced Microwave Plasma (SEMP) CO2 conversion exploits the interaction between solar radiation and a CO2Ar or CO2-N2 microwave discharge to achieve increased energy efficiency while operating at atmospheric pressure conditions. Moreover, by incorporating solar energy, SEMP processes exhibit the same specific energy input (SEI) as conventional plasma methods. The effects of electric power, solar power, flow rate, and gas composition on solar absorption, conversion, and energy efficiencies in SEMP CO2 conversion are investigated. Experimental results show up to ∼21% absorption of solar power obtained under CO2-Ar operation. In all the studied conditions, the absorbed solar radiation leads to increased CO2 conversion efficiency; e.g. an increase from ∼8% to ∼9% is obtained at 900 W electrical power and 75 W absorbed solar power. The use of nitrogen as auxiliary gas leads to higher conversion and energy efficiencies than those using argon. The highest conversion efficiency obtained with CO2-N2 is ∼15.5%, whereas the highest conversion efficiency using CO2-Ar is ∼9.5% – both observed at the same SEI of 1.8 eV/molecule. The results suggest the potential of the SEMP processing of CO2-N2 mixtures, as found in flue gas from fossil-fuel power plants, as a viable approach for CO2 conversion and utilization.
1. Introduction Through the last decades, the continuous consumption of fossil fuels in power plants has significantly contributed to the dramatic increase in the amount of carbon dioxide (CO2) in the earth’s atmosphere with respect to pre-industrial levels [1,2]. To control the rising atmospheric CO2 concentration, several CO2 utilization methods are being investigated. The conversion of gaseous CO2 into synthesized fuels and chemicals using renewable energy is an increasingly appealing alternative given its potential for shifting and/or reducing CO2 emissions while helping fulfill global demands for fuels and chemicals [3,4]. The conversion of CO2 requires high energy processes due to the high thermodynamic stability of CO2 (i.e. large negative formation enthalpy of −394 kJ/mol [5]). The required energy can be directly provided by renewable energy sources, such as concentrated solar radiation in solar thermochemical CO2 decomposition approaches [6,7]. Alternatively, electricity-based methods, such as electrochemical and plasma
⁎
processes, can readily exploit renewable electricity to provide the energy required for CO2 decomposition reactions [8,9]. For example, solar photovoltaic and/or wind energy can provide the electrical energy to establish and sustain an electrical discharge and drive CO2 conversion reactions [10]. Plasma sources are traditionally categorized as thermal or nonthermal. Thermal plasmas are often generated at atmospheric pressure conditions and are characterized by highly-efficient electrical-tothermal energy conversion that makes them attain very high temperatures (typically of over 10 kK) for both, free electrons and gas species [11,12]. Non-thermal (cold) plasmas are commonly attained at lowpressures, and present electron temperatures of 1–2 eV (i.e. 11.6–23.2 kK), which are significantly higher than those of the gas species (from near atmospheric to a few hundred Kelvin). Chemical processes based on non-thermal plasmas tend to present significantly higher energy efficiency and selectivity than those based on thermal plasmas. Non-thermal plasma sources have shown high potential for
Corresponding author at: 1 University Avenue, Lowell, MA, 01854, United States. E-mail address: [email protected] (J.P. Trelles).
https://doi.org/10.1016/j.jcou.2019.09.002 Received 14 August 2019; Received in revised form 30 August 2019; Accepted 3 September 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.
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Fig. 1. Solar-Enhanced Microwave Plasma (SEMP) reactor system: the interaction between concentrated solar radiation and microwave plasma leads to enhanced CO2 decomposition.
higher energy efficiency). The relative low energy efficiency of atmospheric pressure microwave plasma CO2 decomposition is one of the most important challenges in plasma-chemical processing. Experimental observations have largely shown that the use of higher SEI results in higher conversion efficiency and lower energy efficiency [20,22]. Such trade-off is a key factor limiting the further advancement towards the viability of plasmabased CO2 decomposition methods. Therefore, strategies that increase the conversion efficiency without increasing the electric power input to the plasma (i.e. maintaining the same SEI) [14] are highly desirable. In that regard, our previous work [25,26] on the development of the approach known as Solar Enhanced Microwave Plasma (SEMP) processing found that the absorption of concentrated solar energy by atmospheric pressure microwave plasma increases the amount of deposited power into the plasma, increasing conversion efficiency and energy efficiency without altering the SEI. In the present work, the effect of process parameters such as electric and solar power, gas composition, and total gas flow rate on solar radiation absorption and CO2 conversion are investigated.
CO2 decomposition in low temperature and catalyst-free environments. Particularly, the high energy electrons in non-thermal electrical discharges, such as dielectric barrier discharge (DBD) [13,14], gliding arc [15,16], and microwave discharge, can induce vibrational excitation of CO2 molecules, which is a highly efficient channel for CO2 decomposition [17–19]. Among non-thermal plasmas, microwave plasmas are deemed the most effective plasma sources for CO2 decomposition due to their capability to induce molecular dissociation processes via concurrent thermal decomposition and vibrational excitation pathways [20,21]. Microwave plasmas are typically generated from low pressures, of a few mTorr, up to atmospheric pressure. CO2 decomposition via lowpressure microwave plasma has demonstrated very high energy efficiency. Particularly, efficiencies of ∼47% were obtained by Bongers et al. [20] using a microwave discharge operating at 200 mbar and at a specific energy input (SEI) of ∼0.8 eV/molecule, where SEI is a process parameter that quantifies the relative electrical energy intensity of the process. The experimental results of Bongers et al.’s work indicate that the plasma gas temperature was close to the vibrational temperature, and that the plasma operated in the thermal decomposition limit. Therefore, the thermal dissociation mechanism was predominant over the electron-induced vibrational mechanism. Moreover, Rooij et al. [22] reported a maximum energy efficiency of ∼45% operating in the range of 100–200 mbar, also for an SEI of ∼0.8 eV/molecule. The temperature measurements in their experiments indicated that the maximum conversion was obtained at the maximum gas temperature conditions. However, the obtained conversion efficiency did not approach the thermodynamic maximum of ∼55%. Even though the high energy efficiencies of low-pressure microwave plasma CO2 decomposition processes, low-pressure (vacuum) processing would incur additional operation costs (compared to atmospheric pressure operation) that limit its economic viability. Increasing the operating pressure from a few mbar to ∼1 atm can significantly decrease the process energy efficiency due to higher vibrational to translational energy relaxation [14,29]. At atmospheric pressure, the maximum energy efficiency for microwave plasma CO2 decomposition reported in the literature was ∼27%, obtained by Spencer et al.[23] at a SEI of ∼1 eV/molecule. Their results indicated that with increasing the SEI, energy efficiency decreases but conversion efficiency increases. The investigators concluded that increasing the SEI causes the thermal equilibration of vibrational and rotational temperatures, leading to the lower values of energy efficiency. Furthermore, Belov et al.[24] investigated the effect of gas pressure and gas flow rate on the conversion and energy efficiency of microwave plasma CO2 decomposition. The highest energy efficiency at atmospheric pressure operation reported by them was ∼7%. Their results indicated that the higher the gas flow rate at constant power, the higher the energy efficiency - a finding consistent with the results reported by Spencer et al. (i.e. the lower SEI, the
2. Methods The SEMP process seeks to exploit the interaction between an atmospheric pressure microwave plasma and an influx of concentrated solar radiation in order to enhance CO2 decomposition. Fig. 1 schematically illustrates the SEMP system. The system includes a parabolic concentrator to gather and direct a flux of concentrated solar radiation into the SEMP reactor. Inside the reactor, a microwave plasma is sustained by the interaction of input electric power carried by microwaves and a flow of feedstock gas. The interaction between concentrated solar radiation and microwave plasma, starting at the concentrator’s focal point inside the reactor and proceeding downstream, causes the absorption of photons by plasma species (e.g. excited molecules, ionized atoms, etc.), leading to step-wise vibrational excitation and thermal dissociation of the CO2 molecules. To evaluate SEMP CO2 processing, the following metrics for the absorption of solar radiation and for the effectiveness of CO2 decomposition were used. The absorption of solar radiation by microwave plasma was evaluated by measuring the intensity of solar radiation after its transit through the reactor with and without the plasma. The absorption efficiency (ηa) is calculated by:
ηa =
I0 − I , I0
(1) −2
−1
where I0 is the net radiation intensity (W-m -sr ) at the reactor’s exit when the plasma is off, which is assumed equal to the incident radiation, and I is the net radiation intensity at the reactor’s exit when the 726
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with a quartz window that acts as the solar aperture placed at the based of a stainless-steel conical chamber. The conical chamber directs the incident solar radiation into the reaction chamber, and receives the working gas stream. The working gas is injected via two tangential holes on the side of the conical chamber, resulting in a swirl flow pattern inside the discharge tube. The reaction chamber consists of a dielectric tube that pierces perpendicularly through the tapered waveguide to form a transverse electric mode (TE101) inside the chamber. Since the discharge tube in the SEMP reactor experience intense heating due to its exposure to concentrated solar radiation and the confinement of the atmospheric pressure plasma, the tube is made of boron nitride, given that boron nitride at high temperatures remains transparent to microwave energy (in contrast to quartz, often used in the discharge tube of microwave plasma systems, which presents high absorption of microwave energy when heated). Inside the chamber, plasma ignition is obtained by briefly inserting a thin tungsten wire, which causes the local accumulation of the incident microwave energy. The swirl flow pattern stabilizes the microwave plasma and cools the discharge tube by forced convection. A stainless-steel cut-off tube, used to confine the microwave power and promote its absorption by the plasma, surrounds the reaction chamber. Fig. 2(b) presents the implemented SEMP reactor under operation. The figure depicts the exposure to concentrated solar radiation from a high-flux solar simulator placed in front of the optical aperture. The conical chamber channels the flux of concentrated solar radiation into the discharge tube, where plasma efficiently interacts with solar radiation, starting at the focal point of the concentrator and continuing downstream along the reactor chamber. The processed gas stream exits the reactor through the exhaust port at the end of the reaction chamber.
plasma is present. Both quantities, I0 and I, are obtained by integrating their respective spectral intensities, i.e. I = ∫ Iλ dλ , where Iλ is the spectral intentity (W-m−2-sr−1-nm-1) and λ is wavelength (nm). The absorbed solar power by the plasma (Wabs) is given by:
Wabs = ηa Wsi,
(2)
where Wsi is the input solar power injected into the reactor (W). The conversion efficiency (ηc), which quantifies the relative amount of CO2 gas decomposed during the SEMP process, is given by:
ηc =
co2 co2 N˙ in − N˙ out , co2 N˙ in
co N˙ in 2
(3)
co N˙ out2
and are the input and output molar flow rates of CO2 where (mol-s−1) respectively. The specific energy input (SEI) indicates the consumption of electrical power per molecule of input gases and is defined as:
SEI =
∑ j = CO2, N2, Ar
Wpi j N˙ in
, (4)
where is the input molar flow rate (mol-s−1) of the working gas stream (i.e. mixtures of carbon dioxide with argon or with nitrogen), and Wpi is the input electric power into the plasma (W). SEI is typically expressed in units of eV/molecule. The electrical energy efficiency (ηe) provides a metric of the relative amount of electrical energy effectively that is consumed for conversion of CO2 gas and is defined as: j N˙ in
co ΔH ηe = N˙ out , Wpi
(5)
where ΔH is the decomposition enthalpy of a CO2 molecule (i.e. 2.9 eV/ molecule). Our previous work 26 focused on the SEMP reactor and process design and on the investigation of the effect of solar input power on solar absorption, energy efficiency, and CO2 conversion efficiency for a set of fixed process parameters (specifically: 700 W electric power, 8 slpm flow rate and 7 Ar/CO2 flow ratio). In this study, the effects of electric power and solar power, gas composition, and gas flow rate on solar energy absorption and CO2 conversion are investigated. Specifically, CO2-Ar and CO2-N2 mixtures (where argon and nitrogen are auxiliary gases) are used as working gases. The effective electric power deposited into the plasma (i.e. direct minus reflected microwave power – see section 3.2) varied between 600 and 900 W. The solar input power into the reactor varied in three levels, namely: 0, 320, and 525 W.
3.2. Experimental set-up Fig. 3 shows a schematic representation of the SEMP CO2 conversion system. The microwave system is operated at the industrial frequency of 2.45 GHz via a 1.2 kW magnetron (Panasonic NN-T945SF) fed by a 4 kV AC power supply. Electromagnetic energy is directed via WR340 rectangular waveguides towards the reaction chamber, which displays a tapered waveguide section to increase electromagnetic energy density and facilitates the formation and maintenance of the plasma. Wave tuning and control components include a launcher, an isolator, a directional coupler, stub tuners, and a sliding short circuit to tune wave reflection. To monitor the forward and reflected power, and to calculate the electric power deposited into the plasma, a power monitor connected to the directional coupler was used. In all the experiments, the stub tuners and short circuit were tuned such that the plasma absorbed electric power stayed in the range of 600–900 W. A 6.5 kW high-flux solar simulator from Kinoton GmbH is used to provide the SEMP reactor with concentrated solar radiation. The simulator is fitted with a Xe short-arc lamp as the radiation source, whose emission matches closely the daylight spectrum, especially throughout the 400–700 nm wavelength range. A coated quartz glass inside the simulator filters the UV emission to avoid the ozone production. The solar simulator can deliver 350 to 525 W of solar power to the reactor by varying the simulator’s electrical current between 80 and 120 A using a potentiometer in the simulator’s rectifier. Mass flow controllers (ALICAT MC) were used to control the flow rate of CO2, Ar and N2 in the range of 0–10 slpm for attaining the desired CO2-Ar and CO2-N2 working gas mixtures. A water circulation system consisting of two water pumps immersed in a water tank was used for cooling the isolator’s waveguide and the tapered waveguide flanges. This cooling limits the heating of the discharge tube, particularly at the focal point (e.g. plasma generation point), which is the hottest part of the reaction chamber. An optical spectrometer (Avantes AvaSpec-ULS2048) fitted was used to measure the solar radiation intensity in the range from 200 to 1100 nm. The spectrometer was connected to an optical fiber placed at the reactor’s exhaust port.
3. Experimental components and devices 3.1. SEMP reactor The reactor design seeks to promote and exploit the interaction between the flux of concentrated solar radiation and the microwave plasma [25,27]. The interaction of solar photons with plasma species can promote the absorption of photons by vibrationally-excited CO2 molecules or by electronically-excited carrier gas molecules (Ar or N2). The latter presents the potential for sustaining the plasma with lower consumption of electric power (a potential verified by the results in section 4.2). In addition, a distinct feature of the SEMP process is that it results in the absorption of solar radiation by gas in absence of any catalytic medium. Such characteristic is in marked contrast to diverse solar thermochemical processes, which rely on a thermo- or photocatalytic medium [28,29]. Therefore, adding a catalytic medium downstream of the plasma-radiation interaction region in order to utilize un-absorbed radiation and further drive thermo-catalytic reactions could further enhance the SEMP process. Fig. 2(a) shows the design of the SEMP reactor. The reactor counts 727
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Fig. 2. SEMP reactor: (a) cross-sectional side view of the reactor design and (b) implemented SEMP reactor in operation in front of a high-flux solar simulator.
larger power-delivery system, limiting the viability of the CO2 conversion process. Using an easy to ionize gas besides CO2 promotes the ionization process, facilitating plasma ignition and keeping the plasma more stable [23,25,30]. In the experiments, plasma is initiated using pure argon gas (Airgas, 99.997% purity) and introducing a tungsten wire into the reactor chamber to concentrate microwave energy and to promote ionization (i.e. plasma ignition). Then, at the desired flow rate of Ar, CO2 is added gradually to reach the desired total flow rate and CO2-Ar ratio. Given the need for an auxiliary, easily ionizable gas such as argon, together with additional complexity (and expense) of using such a gas, studying the effect of the Ar/CO2 flow ratio is of primary importance for characterizing SEMP CO2 processing. Fig. 4 shows the experimental results of the solar absorption efficiency (ηa) and absorbed solar power (Wabs) as function of the Ar/CO2 flow ratio. The input solar power, electric power, and total gas flow rate were fixed at 320 W, 900 W, and 8 slpm, respectively. The Ar/CO2 gas flow ratio varied in the range of 1–7. The maximum solar absorption efficiency obtained is ∼21% (i.e. the microwave plasma absorbs ∼75 W of solar radiation power) at the Ar/CO2 flow ratio of 7. Absorption efficiency and absorbed power decrease to ∼7% and 30 W, respectively, with decreasing the flow ratio to 1. This result indicates the significant effect on solar energy absorption caused by the fraction of argon in the working gas stream. The presence of argon in the plasma affects the ionization processes, resulting in changing the optical characteristics of the plasma (emissivity and absorptivity), as manifested by the change in solar absorption with the Ar/CO2 ratio. In addition, higher Ar/CO2 ratios increase the stability of the plasma, leading to higher solar absorption efficiency. Fig. 5 presents the measured CO2 conversion efficiency (ηc) and electrical energy efficiency (ηe) in the SEMP reactor as a function of Ar/
Additional details of the SEMP reactor design and experimental setup are described in [25]. 3.3. Product analysis The processed gas exited the SEMP reactor was collected after passing through an ice bath for quenching (see Fig. 3). The processed gas samples were collected in Tedlar® bags and analyzed by a Shimadzu GC-2014 gas chromatography (GC) system equipped with a thermal conductivity detector (TCD). For the gas analysis, 0.5 ml of the gas were collected from each bag and injected into a Restek packed column (80,486 − 810, 2 m) connected to the TCD. Peak identification and calibration were achieved from results obtained using standard gas mixtures containing CO, CO2, and C1 to C4 hydrocarbons (Scotty Specialty Gases). The GC was programmed with an injection temperature of 250 °C and a split ratio of 10.0, respectively. The programmed temperature regime for the GC oven was as follows: start at 35 °C, ramp up to 50 °C at 7.5 °C min−1, hold for 1 min, ramp up to 100 °C at 5 °C min−1, hold for 3 min, ramp up to 200 °C at 10 °C min−1, hold for 7 min, ramp up to 250 °C at 25 °C min−1, and hold at 250 °C for 10 min. 4. Results and discussion 4.1. Effect of CO2-Ar flow ratio The use of argon gas facilitates the ignition and stabilization of the atmospheric pressure microwave plasma. In a pure CO2 plasma, most of the deposited electric power is transferred to the vibrational excitation modes of CO2 molecules rather than being consumed in ionization of the gas. Therefore, sustaining a pure CO2 plasma requires a significantly
Fig. 3. Schematic diagram of the solar enhanced microwave plasma system setup (GC: Gas Chromatography, OES: Optical Emission Spectroscopy). 728
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Fig. 6. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as functions of electric power and solar power. (Conditions: 8 slpm total flow rate, 7 Ar/CO2 flow ratio).
Fig. 4. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as a function of the Ar/CO2 flow ratio. (Conditions: 900 W electric power, 8 slpm total flow rate, 320 W solar input power).
results in Fig. 5 show that at the Ar/CO2 flow ratio of 7, the plasma energy efficiency increases from around 15.5% to 17%. 4.2. Effect of electrical and solar power on CO2-Ar Results of absorbed solar power and solar absorption efficiency as a function of electric power deposited into the plasma are shown in Fig. 6, for two different levels of solar input power: 320 and 525 W. The gas flow rates of CO2 and Ar were kept constant at 1 slpm and 7 slpm, respectively (i.e. Ar/CO2 flow ratio equal to 7), to isolate the effect of electrical and solar power on solar absorption. Operation at the Ar/CO2 flow ratio of 7 demonstrated a stable atmospheric pressure microwave plasma for a wide range of electric power input. In this experiment, the electrical energy into the plasma varied in the range of 600–900 W. Absorption efficiency and absorbed solar power increase with increasing the electric power deposited into the plasma. This behavior appears to be the consequence of the enhancement of electronic excitation processes due to the increase of electric power absorbed by electrons. Electronic excitation of plasma is potentially detrimental in terms of changing the net radiative absorption properties of the plasma (e.g. higher power density plasma emits more radiation, which could neglect the absorption of solar radiation). From the obtained results, it is observed that solar radiation absorption increases with increasing the amount of electric power absorbed by plasma. Nevertheless, the solar power absorption efficiency is greater for lower solar input power. This finding indicates that at higher solar input power levels, more solar power gets unutilized. However, the higher consumption of electrical and solar power results in higher solar power deposited into the plasma, which may improve the CO2 conversion process. The variation of CO2 conversion and energy efficiency, for three different levels of solar input power: 0, 320, and 525 W, as a function of total deposited power (electrical plus solar power) are shown in Fig. 7. The CO2 conversion efficiency for plasma-only operation (i.e. 0 W solar power) increases from around 6% to 8.5% with increasing the electric power input. The enhancement of conversion efficiency with increasing power at a fixed gas flow rate is directly related to the enhancement of specific electrical energy delivered to the vibrational excitation and thermalization of CO2 molecules [33]. However, the effect of electric power is opposite for the energy efficiency, e.g. increasing the electric power leads to decreasing the energy efficiency from 16.5% to 15.5%. This phenomenon can be understood by considering that a significant portion of power is expected to be deposited over the surface of the plasma volume rather than into the plasma bulk because of the socalled skin depth effect [34,35]. In addition, increasing the electrical deposited power for a fixed amount of working gas causes the thermal
Fig. 5. Conversion efficiency (black) and energy efficiency of CO2 decomposition (blue) in the SEMP reactor as a function of flow ratio. (Conditions: 900 W electric power, 8 slpm total flow rate).
CO2 flow ratio for zero solar power (i.e. plasma-only operation - solar simulator turned off) and 320 W solar input power. The CO2 conversion and energy efficiencies for plasma-only at the flow Ar/CO2 ratio of 1 are around 4.5% and 8.5%, respectively. By increasing the Ar/CO2 flow ratio to 7, both conversion and energy efficiencies increase to around 8% and 15.5%, respectively. Increasing the fraction of argon increases the overall rate of ionization, which consequently increases the impact of electrons with CO2 molecules. Therefore, the observed enhancement of efficiencies by increasing the Ar/CO2 flow ratio appears to be due to the enhancement of electron-induced vibrational excitation of CO2 molecules [31,32]. Fig. 5 also depicts the effect of introducing high-flux solar radiation on both, conversion and energy efficiencies for various gas flow ratios. The enhancement of conversion and energy efficiencies due to introducing solar power is greater for higher flow ratios than for lower ones; e.g., the conversion efficiencies for flow ratios of 7 and 1 are enhanced by 11% and 4.5%, respectively. This result is consistent with the results presented in Fig. 4, i.e. the higher the Ar/CO2 flow ratio, the more solar energy is absorbed by the plasma. Furthermore, since for a fixed flow rate and electrical input power, the specific energy of the plasma remains constant, the increase in plasma energy efficiency was solely due to the increased absorbed solar power. For example, the
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Fig. 8. Solar power absorption efficiency (black) and absorbed solar power (blue) by the microwave plasma as a function of SEI for Ar/CO2: 7/1.
Fig. 7. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of total power for Ar to CO2 of 7 and 8 slpm total flow rate.
equilibrium of vibrational and rotational temperatures, reducing the selectivity of molecular decomposition, leading to the lower energy efficiencies - a behavior previously reported in [23,24,33]. The results in Fig. 7 also indicate that the effect of solar power is more dominant than the effect of electric power in the conversion process, i.e. at a specific amount of total (electric plus solar) power, increasing the solar input power leads to higher conversion efficiency. As mentioned earlier, electromagnetic power absorption into the plasma is limited by the skin depth effect. However, in SEMP, the microwave plasma can absorb the solar radiation, independently of the skin depth effect, leading to an increase in deposited energy into the plasma and enhancing both, conversion and energy efficiencies. As shown in the results in Fig. 6, the amount of absorbed solar power increases with increasing both solar and electrical power input. This behavior is consistent with the results presented in Fig. 7. For example, at 900 W electric power, around 7.5% enhancement of conversion efficiency is obtained by increasing the solar power input from 320 to 525 W. However, the enhancement of conversion efficiency for 600 W of electric power is only around 3%.
Fig. 9. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of SEI for Ar/CO2: 7/1.
Fig. 9 illustrates the variation of CO2 conversion efficiency and energy efficiency for the two different levels of solar input power: 0 W (plasma-only) and 320 W as a function of SEI. By increasing the SEI, the conversion efficiency at both solar power levels increases. However, the conversion efficiencies for the solar-plasma configuration are greater than those for plasma-only. The maximum conversion efficiencies observed with and without adding solar radiation are 9% and 10%, respectively, which correspond to the highest SEI of 2.75 eV/molecule. The enhancement of conversion efficiency by adding solar radiation to the system reaches the maximum of 11%, obtained at 1.6 eV/molecule, at the highest electric power level (900 W) and for 8 slpm total flow rate. This result is consistent with the one obtained from the solar absorption analysis (Fig. 8) in which the maximum amount of absorbed solar power obtained is 1.6 eV/molecule. The results in Fig. 9 also indicate an inverse relationship between energy efficiency and SEI for both solar-plasma and plasma-only systems, i.e. increasing the SEI leads to the decrease in energy efficiency from 17% to 10% for the plasma-only system. However, as it is expected from the conversion efficiency results, the energy efficiency of the solar-plasma system, varying from 19% to 11%, is higher than that for the plasma-only system. Therefore, by adding solar energy, the amount of energy deposited into the plasma increases without increasing the SEI, leading to the observed higher energy efficiencies. Finally, the obtained results show that, although increasing the SEI leads to more efficient CO2 conversion, increasing SEI cannot lead to
4.3. Effect of specific energy input (SEI) Specific Energy Input (SEI), defined in Eq. 4, is a process parameter that quantifies the electric energy intensity of the process. Therefore, SEI correlates with the electrical energy cost of the process. Fig. 8 shows the variation of solar absorption efficiency and absorbed solar power as a function of SEI at a solar input power of 320 W and a fixed Ar/CO2 gas flow ratio of 7. Experiments were initially performed in a plasma-only configuration (i.e. 0 W solar power) in order to attain the widest range for SEI possible (while maintaining a stable plasma for flow rates of 4–9 slpm and electric powers of 600–900 W), varying from 0.9 to 2.7 eV/ molecule. The maximum solar absorption efficiency obtained is ∼21%, i.e. the microwave plasma absorbs ∼75 W of solar radiation, which occurs at a SEI of ∼ 1.6 eV/molecule, corresponding to 900 W electric power and 8 slpm total gas flow rate. The last two points of the plots in Fig. 8 correspond to the gas flow rate of 5 slpm. As it was found from Fig. 6, an increase in electric power at a fixed gas flow rate increases solar energy absorption. However, lower swirl gas flow rate inside the reactor makes the plasma bulb less concentrated and less stable which may influence the solar absorption. Therefore, it can be concluded that increasing SEI cannot enhance the absorption efficiency, independently; i.e. at lower flow rates, the more solar power is unutilized because of the less plasma stability, resulting in lower absorption efficiency and absorbed solar power. 730
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concurrent increases in solar radiation absorption and conversion efficiency.
4.4. Effect of nitrogen gas In previous sections, argon gas was used as the auxiliary gas to initiate and stabilize the CO2 plasma by facilitating ionization kinetics. However, CO2 is emitted from power plants mixed with nitrogen (as well as smaller amounts of oxygen, water vapor, and other residual gases) as what is commonly known as flue gas. Therefore, using nitrogen instead of argon as an auxiliary gas provides a more practical impact on the sustainability and viability of SEMP CO2 decomposition processes. The volume fraction of nitrogen in flue gas of oil burner power plants is in the range of 78–80%; however, the fraction of CO2 gas is in the range of 8–11%. Therefore, the ratio of N2 to CO2 in flue gas is in the range of ∼7–10 [36]. In the SEMP operations investigated in this work, the microwave plasma was initiated using pure nitrogen gas. Then, CO2 gas was added gradually to the reactor to reach a N2/CO2 gas flow ratio of 8.75, which is in the range of flue gas emission from power plants and is a ratio that leads to a stable plasma. Fig. 10 presents the variation of solar absorption efficiency and absorbed solar power as functions of SEI at 320 W solar input power. SEI varies from 0.9 to 1.9 eV/molecule with gas flow rates from 4 to 9 slpm and deposited electric power of 600–900 W. The solar absorption efficiency obtained is in the range of ∼6.5–8%, i.e. the microwave plasma absorbs ∼23-29 W of solar radiation. However, according to the results for solar absorption in CO2-Ar plasma presented in Fig. 8, for this range of SEI, solar absorption efficiency and absorbed solar power are in the range of 13–21% and 48–75 W, respectively. These results indicate that the absorption of solar radiation in CO2-Ar microwave plasma is more efficient than that in CO2-N2 plasma. As mentioned in section 4.1, using an easily-ionizable gas such as argon besides CO2 promotes ionization kinetics, making the plasma more stable. In addition, the higher ionization reactions can affect the optical characteristics of the plasma and lead to the enhancement of the solar radiation absorption. From the results presented in Figs. 8 and 10, it could be expected that the use of argon as the auxiliary gas would result in a higher amount of solar power deposited in the plasma. However, the use of CO2-N2 gas mixtures is of greater relevance for direct CO2 utilization technologies. The maximum solar absorption efficiency obtained with CO2-N2 is ∼8%, i.e. the microwave plasma absorbs ∼29 W of solar radiation power which occurred at SEI of ∼1.6 eV/molecule, corresponding to 900 W electric power and 7.8 slpm total gas flow rate. (The data point for SEI ∼ 1.9 in Fig. 10 corresponds to the gas flow rate
Fig. 11. Conversion efficiency (black) and energy efficiency (blue) of SEMP CO2 decomposition as a function of SEI for N2/CO2: 8.75/1.
of 6.68 slpm at 900 W electric power.) This result is consistent with the one obtained in Fig. 8 (CO2-Ar plasma), i.e. less solar power is deposited in the plasma for high SEI (i.e. fixed electric power and low flow rate), resulting in lower absorption efficiency and absorbed solar power. The variation of CO2 conversion efficiency and energy efficiency for the two different levels of solar input power, 0 and 320 W, as a function of SEI is presented in Fig. 11. The conversion efficiency at both solar power levels increases with SEI; however, the energy efficiency decreases. In addition, both conversion and energy efficiencies for various SEIs are greater with the solar power input. These results are consistent with those obtained for CO2-Ar operation. However, the values of conversion and energy efficiencies in CO2-N2 are greater than those under CO2-Ar operation. This behavior could be caused by the fact that N2 molecules can vibrationally excite CO2 molecules; whereas under CO2-Ar operation, CO2 molecules can be vibrationally excited only by electron impacts. Heijkers et al. [37] theoretically found that at around 85% of N2 fraction in a CO2-N2 plasma, vibrationally excitation of CO2 molecules induced by N2 molecules contributes to around 50% of CO2 dissociation; however, the contribution of electron induced vibrational excitation of CO2 molecules is only around 8%. Therefore, the higher vibrational excitation induced by N2 in CO2 could explain the observed higher CO2 conversion efficiency in the CO2-N2 process with respect to that in the CO2-Ar plasma process.
5. Summary and conclusions CO2 conversion via Solar-Enhanced Microwave Plasma (SEMP), an approach that exploits the interaction between concentrated solar radiation and microwave plasma, has the potential to lead to greater conversion and energy efficiencies than traditional plasma-based methods while operating at atmospheric pressure conditions. The effects of specific power input and CO2-Ar and CO2-N2 mixtures on solar power absorption, CO2 decomposition, and energy efficiency during SEMP CO2 conversion have been investigated. The experimental results under CO2-Ar operation, for Ar/CO2 ratios from 1 to 7, indicate that the greater the Ar/CO2 ratio, the greater the amount of absorbed solar power, and the higher the conversion and energy efficiencies. Also, the higher the electrical energy deposited in the plasma, the higher the solar radiation absorption efficiency. Although increasing the electric power input to the plasma leads to increasing the conversion efficiency, it also leads to decreasing the energy efficiency. Importantly, the obtained results also indicate that solar power is more effective on both conversion and energy efficiencies than electric power because of circumventing skin-depth power absorption limitations inherent in microwave plasmas. Furthermore, the
Fig. 10. Solar power absorption efficiency (black) and absorbed solar power (blue) as a function of SEI for N2/CO2: 8.75/1. 731
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experimental measurements showed that increasing the specific energy input (SEI) increases the conversion efficiency, but cannot increase the solar absorption efficiency independently. The use of a CO2-N2 mixture at a N2 to CO2 of 8.75 leads to stable operation of the plasma and better process performance than that obtained using CO2-Ar, while providing closer resemblance to the expected performance of the processing of flue gas from fossil-fuel power plants. Similar trends for solar power absorption and CO2 conversion obtained for CO2-Ar were observed during CO2-N2 operation. However, the results indicate that although the solar absorption efficiency under CO2-N2 operation is lower than that for CO2-Ar, the overall conversation efficiency using CO2-N2 is greater. Such result seems to be caused by the higher contribution of N2 molecules to the vibrational excitation of CO2, compared to that attained by electron collision. The results suggest the potential of the SEMP processing of CO2-N2 mixtures, as found in flue gas from fossil-fuel power plants, as a viable approach for CO2 conversion and utilization.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
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Acknowledgements
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The authors acknowledge the financial support provided by the U.S. National Science Foundation through award CBET-1552037.
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