Steam Plasma Methane Reforming for Hydrogen Production

Plasma Chem Plasma Process https://doi.org/10.1007/s11090-018-9891-5 ORIGINAL PAPER Steam Plasma Methane Reforming for Hydrogen Production M. Hrabovsky1 • M. Hlina1 • V. Kopecky1 • A. Maslani1 P. Krenek1 • A. Serov1 • O. Hurba1 • Received: 13 December 2017 / Accepted: 3 April 2018 Ó Springer Science+Business Media, LLC, part of Springer Nature 2018 Abstract Reactions of methane with water and CO2 in thermal plasma generated in a special plasma torch with a water-stabilized arc were investigated. Steam plasma with very high enthalpy and low mass flow rate was produced in a dc arc discharge which was in direct contact with water vortex surrounding the arc column. Composition of produced gas, energy balance of the process and its efficiency were determined from measured data. The output H2/CO ratio could be adjusted by a choice of feed rates of input reactants in the range 1.1–3.4. Depending on experimental conditions the conversion of methane was up to 99.5%, the selectivity of H2 was up to 99.9%, and minimum energy needed for production of 1 mol of hydrogen was 158 kJ/mol. Effect of conditions on process characteristics was studied. Comparison of measured data with results of theoretical computations confirmed that the reforming process produces gas with composition which is close to the one obtained from the thermodynamic equilibrium calculations. Relations between process enthalpy, composition of produced syngas and process characteristics were determined both theoretically and experimentally. Keywords Thermal plasma
Steam plasma
Methane reforming
Hydrogen
Syngas Introduction Hydrogen is one of the most promising alternative fuels for the future. Hydrogen has gained an importance as a clean fuel in combustion engines, fuel cells and gas turbines, as well as a raw material for the production of gasoline, methanol, ethanol, and other high value chemicals. Numerous technologies for hydrogen production have been explored in recent decades. Hydrogen can be produced by several non-plasma technologies like natural gas reforming, coal and biomass gasification, and water electrolysis. Among many & M. Hrabovsky [email protected] 1 Institute of Plasma Physics, ASCR, Za Slovankou 3, 18200 Prague 8, Czech Republic 123 Plasma Chem Plasma Process processes used for hydrogen production [1, 2], reforming of hydrocarbons play an important role. Today, most of hydrogen production is based on natural gas reforming. Natural gas is a naturally occurring hydrocarbon gas mixture consisting primarily of methane. Methane is the most commonly adopted hydrocarbon for H2 generation. Hydrogen is mainly produced from natural gas by three different chemical processes: steam reforming, dry reforming, and partial oxidation [3–5]. In the steam-methane reforming, methane reacts with steam under pressure 0.3–2.5 MPa in the presence of a catalyst to produce syngas, which is a mixture of hydrogen, carbon monoxide, and a relatively small amount of other components. The reaction (1) of methane with steam is endothermic, energy needed for realization of the reaction must be added by an external source. At present the methane steam reforming is the major industrial hydrogen source. CH4 þ H2 O ! CO þ 3H2 DH ¼ þ 206 kJ=mol ð1Þ In the dry reforming, methane reacts with carbon dioxide to produce syngas with higher content of carbon monoxide. The dry reforming of methane has recently attracted significant interest due to simultaneous utilization and a reduction of two abundant and undesirable greenhouse gases, CH4 and CO2. Syngas is produced in the reaction (2) of methane with CO2. This process produces syngas with a H2/CO molar ratio close to 1, which is suitable for the synthesis of fuels by the Fischer–Tropsch synthesis. CH4 þ CO2 ! 2 H2 þ 2CO DH ¼ þ 247 kJ=mol ð2Þ Since the steam as well as the dry methane reforming are highly endothermic reactions, an energy supply is required. The partial oxidation of methane is also an attractive alternative to converting methane to syngas. This reaction (3) is an exothermic reaction, therefore, it can reduce the energy demand for the reforming process. CH4 þ 1=2 O2 ! CO þ 2H2 DH ¼ 36 kJ=mol ð3Þ Syngas can be also produced by the reforming of other hydrocarbons, such as ethanol, propane, or even gasoline. Subsequently, in what is called the ‘‘water–gas shift reaction,’’ the carbon monoxide and steam are reacted using a catalyst to produce carbon dioxide and more hydrogen. In a final process step called ‘‘pressure-swing adsorption,’’ carbon dioxide and other impurities are removed from the gas stream, leaving essentially pure hydrogen. The above discussed non-plasma technologies still possess some problems and limitations [3–5]. Important problems are related to the necessity of a catalyst usage. High conversion levels are achieved mainly due to the usage of catalysts. During the process the catalyst is gradually deactivated due to an agglomeration under high temperatures and due to the carbon deposition. Besides conventional technologies, plasma reforming is a new efficient process, which bring some beneficial characteristics. Plasma process offers a unique way to induce gas phase reactions. Comparing to non-plasma technologies, plasma methods offer several important advantages, especially high quality of produced gas and easy control of its composition, compactness, fast response time, the compatibility for a broad range of hydrocarbons, and removal of problems with catalysts. Moreover, plasma technologies are environmentally clean processes since they are not greenhouse gas emitters. Pollutants emission can be avoided because the plasma decomposes efficiently all complex molecules. 123 Plasma Chem Plasma Process Both non-thermal and thermal plasmas have been used in hydrocarbons conversion investigations. Most of published investigations on hydrocarbons plasma reforming have been based on non-equilibrium, non-thermal plasmas [6–17]. Several kinds of non-equilibrium plasma sources have been used, such as dielectric barrier discharge (DBD) [7–10], corona discharge [11, 12], atmospheric pressure glow discharge [13], gliding arc discharge [14–16], and high voltage pulsed plasma [17]. Chemical reactions leading to hydrocarbon reforming are stimulated by high energy electrons that are present in the non-thermal plasmas. Between main benefits of the non-thermal plasmas belong high reactivity and selectivity, low energy consumption and an operation at relatively low temperatures. For increasing reaction conversion rate, a catalytic process was introduced in some publications. Generally, the non-equilibrium plasmas are inhomogeneous in a discharge space which results in limited reaction region. So the conversions and the treatment capacity are restricted. The operating power in most of non-equilibrium plasma experiments was in the range of tenth to hundreds W, the feed-gas flow rates were from tenth of ml/min to 1 l/min. In recent years several papers have been published describing experiments with hydrocarbon reforming using the microwave discharge [18–21]. The highest power reported in the microwave gasification systems was below 100 kW. While for low microwave power the generated plasma is not in the thermodynamic equilibrium, the thermal plasma can be produced by a microwave heating at a power higher than several kW. Comparing with techniques based on the non-thermal plasmas, the thermal plasma processing exhibits significant advantages like large treatment capacity, little by-products, and high energy conversion efficiency. The potential of the thermal plasma follow from high temperatures in larger volumes, high energy densities and presence of chemically active species. Thermal plasma was used for hydrocarbons reforming in [22–26]. Conversion of methane by reaction with CO2 and H2O in a plasma reactor with a three-phase high-voltage plasma torch with the power 80–120 kW was studied in [22]. High methane conversion levels (91–98.3%) and low energy consumption 31.8–35.9 MJ per 1 kg of converted methane were achieved. DC arc steam plasma generated in a water torch with the power 1.05 kW was used for methane conversion by a reaction with CO2 [23]. The conversion levels of 49–74% with an energy consumption 18–28.4 kWh/kg were obtained. Lower power was used in experiments with 2 kW DC arc argon plasma where acetylene production by methane pyrolysis was studied [24]. The mixture of Ar/CH4/CO2 was used as a plasma-forming gas in the system with two anodes and the arc power 18–32.4 kW for CO2 reforming of methane [25]. Tao et al. [26] studied CH4/CO2 reforming in nitrogen thermal plasma jet with and without catalysts. The plasma power was 9.6 kW, the conversions of CH4 and CO2 were high up to 92.32 and 82.19% respectively, and an energy needed for the gas reforming was 290 kJ/mol. In this work we studied the dry and the steam methane reforming in a thermal plasma produced in the special plasma torch, where steam plasma is produced in a direct contact of an electrical discharge with a water surface [27]. Plasma jet generated in this torch is characterized by extremely high temperature and enthalpy. The mean enthalpy of produced plasma is one order higher than plasma enthalpies in commonly used plasma torches with a gas-stabilized arc discharges. Consequently, only small amount of plasma generated in this torch carries enough energy needed for a realization of endothermic reforming reaction. Thus a control of composition of produced gas by a combination of input gases is possible in a wide range, as the contribution of plasma gas to a final produced gas composition is limited. Moreover, because reaction gases are injected into high temperature plasma flow close to the torch exit, the reaction rate is high and gas molecules are decomposed to their 123 Plasma Chem Plasma Process basic constituents. Relation between an energy available for reforming process and a resulting gas composition is studied by changing the plasma torch power and the feeding rates of input reactants. Equilibrium Composition of Reaction Products and Energy Balances of Steam and Dry Methane Reforming Theoretical calculations of composition and thermodynamic properties of mixtures corresponding to Eqs. (1) and (2) were calculated in the temperature range 400–6000 K and the pressure 0.1 MPa assuming an existence of thermodynamic equilibrium and omitting electrically charged components. Along with the basic atoms C, H and O, the following substances were taken into account: C2, C3, CO, CO2, C3O2, CH, CH2, CH3, CH4, HCO, H2CO, C2H, C2H2, C2H4, O2, O3, H2, OH, HO2, H2O and H2O2. The equilibrium composition was calculated by solving the equation system comprising the equations for their equilibrium constants completed by the usual balance equations. The values of equilibrium constants were taken from the NIST-JANAF Thermochemical Tables [27]. The equilibrium composition of gases for components corresponding to a steam reforming (Eq. 1) is shown in Fig. 1, for a dry reforming (Eq. 2) in Fig. 2. Only components with the molar ratios higher than 10-3 are shown in Figs. 1 and 2. The values of molar ratios for the other substances included in the computations were below 10-3 in the whole temperature range. It can be seen that the composition of gases corresponding to the Eqs. (1) and (2), i.e. the mixtures of H2 and CO with very small amount of other components, is achieved at temperatures in the range 1200–2300 K. For temperatures below 1200 K the methane conversion rapidly decreases with the temperature reduction. Based on the resulting composition of both systems the equilibrium enthalpies HM of resulting mixtures were calculated. The values of thermodynamic data of individual species were taken from JANAF [27]. The energy Q needed for obtaining syngas with the composition given in Figs. 1 and 2 can be calculated as the difference of enthalpy of the mixture HM at corresponding temperature and the sum of heats of formation of methane and water, or methane and CO2 respectively: Fig. 1 Equilibrium composition of syngas for steam methane reforming 123 Plasma Chem Plasma Process Fig. 2 Equilibrium composition of syngas for dry methane reforming QðT Þ ¼ HM ðT Þ QðT Þ ¼ HM ðT Þ 0 ðCH4 Þ Df Hgas 0 ðCH4 Þ Df Hgas 0 ðH2 OÞ Df Hliquid ð4Þ 0 ðCO2 Þ Df Hgas ð5Þ Calculated energies Q(T) for mixtures corresponding to the steam and the dry reforming are shown in Fig. 3. The relation between characteristics of the dry and the steam methane reforming and an energy spent for realization of the process can be determined from calculated gas composition shown in Figs. 1 and 2 and energies presented in Fig. 3. Figures 4 and 5 present dependence of basic reforming characteristics, evaluated from calculated gas composition, on an energy spent for the reforming process. This energy includes chemical reactions enthalpies and an energy necessary for heating of mixtures to corresponding temperature, in case of the steam reforming also an energy needed for heating of water from the standard temperature and for water evaporation. The same characteristics were evaluated from experimental data and are presented in the following sections. The characteristics of reforming process presented in Figs. 4 and 5 are defined as follows: CH4 and CO2 conversion: Fig. 3 Energies Q for production of syngas at temperature T in steam and dry methane reforming 123 Plasma Chem Plasma Process Fig. 4 CH4 conversion, CO2 conversion, H2 yield and CO yield in dependence on process enthalpy for dry methane reforming Fig. 5 CH4 conversion, H2 yield and CO yield in dependence on process enthalpyfor steam methane reforming CH4 conversion ¼ mol CH4in mol CH4out  100 mol CH4in ð6Þ mol C02in mol C02out  100 mol C02in ð7Þ CO2 conversion ¼ H2 and CO yields: H2 yield ¼ mol H2out  100 mol H2 Oin þ 2mol CH4in ð8Þ molCOout  100 molCO2in þ molCH4in ð9Þ CO yield ¼ The minimum energy needed for a production of gas composed of H2 and CO with minimum content of other components can be determined from dependences shown in Figs. 4 and 5. For both processes these minimum energies are almost the same, 337 kJ/mol for steam reforming and 333 kJ/mol for dry reforming to achieve 98% of methane conversion. 123 Plasma Chem Plasma Process Fig. 6 Experimental reactor PLASGAS Experimental System The experiments were performed in plasma reactor PLASGAS (Fig. 6) which was used also for gasification of biomass [28], solid and liquid organic waste [29] and RDF [30]. Plasma torch with hybrid water/gas stabilization of arc is attached at the top of the reactor. Steam plasma is produced in this torch in direct contact of an arc discharge with inner surface of water vortex [31–33]. The principle of arc stabilization is characterized by extremely high plasma enthalpy and temperature, and very low plasma flow rate. The characteristics of plasma jet injected into the reactor volume (Table 1) are more than one order different from the ones used in the other published thermal plasma reforming experiments. The internal volume of the reactor was 200 l. The reagents, i.e. methane, carbon dioxide and water were injected into the volume of the reactor, where they were mixed with plasma. Gaseous reaction products were fed into the quenching chamber where Table 1 Operation parameters of plasma torch and experimental conditions Dry reforming Steam reforming Arc current (A) 325–450 325–450 Arc power (kW) 88–136 88–136 Torch efficiency (%) 0.57–0.60 0.57–0.60 Plasma mass flow rate (g/s) 0.37–0.44 0.37–0.44 Mean plasma enthalpy (MJ/kg) 136–186 136–186 Bulk plasma temperature (K) 15,400–16,300 15,400–16,300 H2O/Ar volume ratio in plasma 2.2–3.0 2.2–3.0 Input methane flow rate (slm) 75–150 75–150 Input CO2 flow rate (slm) 51–134 – Input water flow rate (g/min) – 38–94 123 Plasma Chem Plasma Process their temperature was reduced to 300 °C in a water spray with an automatically controlled flow rate. The gas was then exhausted by the water jet pump into the filter and the scrubber where solid particles were separated. The water ejector installed between the filter and the cyclone maintained the reactor at the slight under-pressure of several hundreds of Pa. Gas was then fed into the afterburning chamber where it was combusted. The measuring system included monitoring of the plasma torch operation parameters, measurement of inner wall temperatures in several positions inside the reactor, calorimetric measurements on cooling water loops, and monitoring of flow rates of added reaction gases and water. The flow rate of produced syngas was measured using a Pitot tube flow meter. As this measurement was complicated by a high level of signal fluctuations, the additional method of syngas flow rate evaluation was used which was based on a measurement of concentration of argon in produced gas for a case with known amount of argon supplied into the reactor. The gas temperature was measured at the input and the output of the quenching chamber by thermocouples. A sampling probe for composition measurements was located at the reactor output before the quenching chamber, the second sampling probe was positioned after the filter. A quadrupole mass spectrometer Pfeiffer Vacuum Omnistar GSD 301 with direct inlet was used as a main gas on-line analyzer. A freezing unit is placed between the mass spectrometer and the sampling probe to avoid water condensation in the mass spectrometer. Contrary to non-plasma processes, in the plasma systems the energy carrying medium enters the reaction volume at substantially higher temperature than the temperature needed for the reaction. Input reactants, i.e. methane, carbon dioxide, and steam or water are injected into plasma flow and after mixing with plasma they are heated and decomposed. The temperature of flowing reactants then decreases to the value determined by the plasma power and the flow rates of input reactants. Thus the temperature in reactor could be easily controlled by the plasma torch power and feeding rates of reactants. Basic parameters of the plasma torch and specification of reactants injected into the reactor are presented in Table 1. Experimental Procedure and Results Methane and additional reagents were introduced into the reactor with various combinations of input flow rates in the ranges specified in Table 1. The range of variation of the torch power is given also in Table 1. Certain amounts of steam and argon, specified in Table 1, are introduced into the reactor by plasma flow. For realization of the steam reforming an additional water was supplied to reach ratio of methane to steam corresponding to the Eq. (1). In experiments with the dry reforming carbon dioxide was added to balance molar concentrations of C and O, some hydrogen and oxygen came from steam plasma. Thus, the conditions in the dry reforming experiments do not correspond exactly to the Eq. (2), but an amount of steam in plasma flow was substantially lower than an amount of supplied carbon dioxide, as plasma density was extremely low. The flow rate and the composition of gas at the output of the reactor were measured for various combinations of the plasma torch power and the input flow rates of methane, carbon dioxide and water. The range of variation of experimental parameters is given in Table 1. The temperature in the reactor volume was controlled by the plasma power and flow rates of reactants and it was kept between 1200 and 1400 K. The composition of gas reaction products was analysed for various experimental conditions. Besides gas products also small amount of solid carbon 123 Plasma Chem Plasma Process particles was produced. The solid particles were captured in the filter tower. No liquid products were identified. An example of measured syngas composition for several flow rates of reactants and flow rate of sample argon 50 slm in case of steam reforming is presented in Fig. 7. For the test run presented in Fig. 7 the arc current was kept constant at I = 400 A, feed rates of methane and water were varied. The feed rates of methane and water are given below the graph. Figure 8 shows syngas composition for the reaction of methane with CO2. Small amount of steam was supplied in plasma. Measurements were performed for a number of various combinations of arc power and reactant flow rates. From measured data the following process characteristics were evaluated: CH4 and CO2 conversions defined by Eqs. (6) and (7), H2 and CO yields defined by (8) and (9) and H2 and CO selectivities defined as H2 selectivity ¼ 2ðmol CH4in mol H2out  100 mol CH4out Þ þ mol H2 Oin ð10Þ Fig. 7 Syngas composition for several combinations of flow rates of methane and water. Arc power 120 kW 123 Plasma Chem Plasma Process Fig. 8 Syngas composition for various torch powers and constant feed rates of reactants CO selectivity ¼ ðmol CH4in mol COout mol CH4out Þ þ ðmol CO2in mol CO2out Þ  100 ð11Þ For all combinations of arc power and reactants flow rates the values of process characteristics (3)–(8) were determined by the value of ratio of energy available for the process to the amount of input methane which we call process enthalpy. The process enthalpy is defined as process enthalpy ¼ torch power torch loss reactor loss ðmol CH4 Þin ð12Þ The dependences of evaluated characteristics on process enthalpy are in Figs. 9, 10, 11, 12, 13, 14, 15 and 16. It can be seen in Figs. 9 and 10 that for high values of the process enthalpy almost complete methane conversion was achieved, CO2 conversion in the dry reforming was maximum 93%. Both the CH4 conversion and the CO2 conversion decrease with reduction of the process enthalpy below 700–800 kJ/mol. 123 Plasma Chem Plasma Process Fig. 9 Dependence of CH4 conversion on process enthalpy in steam reforming Fig. 10 Dependence of CH4 and CO2 conversion on process enthalpy for dry reforming Fig. 11 Dependence of H2 yield on process enthalpy Figures 11 and 12 show H2 and CO yields, again in dependence on the process enthalpy. It can be seen that for higher values of process enthalpy both the H2 and the CO yields are close to 100%. Values of H2 and CO selectivities are shown in Figs. 13 and 14. The values of H2 and CO selectivity were between 80 and 100%, similarly like for all other characteristics the highest values were obtained for high process enthalpies. Values of H2 and CO yields as well as H2 and CO selectivities slightly decreased with process enthalpy reduction, more steep decrease was found for the dry reforming at enthalpies below 500 kJ/mol. 123 Plasma Chem Plasma Process Fig. 12 Dependence of CO yield on process enthalpy Fig. 13 Dependence of H2 selectivity on process enthalpy Fig. 14 Dependence of CO selectivity on process enthalpy Figure 15 presents H2/CO ratio determined from the measured gas composition. For both the steam and the dry reforming this ratio was close to the values corresponding to Eqs. (1) and (2) with hydrogen content a little bit higher than theoretical values, which are 3:1 for steam reforming and 1:1 for dry reforming. Higher hydrogen content corresponded to higher hydrogen amount supplied in reagents and in plasma. The last characteristic evaluated from the measured results was energy cost of hydrogen production, which is shown in Fig. 16 for both the steam and the dry reforming. The energy cost, defined as an energy spent for production of 1 kg of hydrogen, is lower for 123 Plasma Chem Plasma Process Fig. 15 Ratio H2/CO in produced gas Fig. 16 Dependence of energy cost of hydrogen on process enthalpy steam reforming. Minimum values of this energy in our experiments were 86 MJ/kg for steam reforming and 116 MJ/kg for dry reforming at an enthalpy of 500 kJ/mol. Discussion and Conclusions High enthalpy steam plasma generated in arcs with water stabilization provides special beneficial properties for methane reforming process. High plasma temperature ensures high rate of chemical processes, ultraviolet radiation from plasma in the region of interaction of plasma with injected reagents prevents formation of molecules of higher hydrocarbons. Due to high plasma enthalpy relatively small mass of plasma carries enough energy needed for endothermic reforming reactions. Thus the composition of produced gases is only slightly influenced by steam plasma. In addition to the steam reforming with high hydrogen content also a process generating syngas with composition close to the product of dry reforming can be realized. By an addition of various combinations of water or steam and CO2 the ratio H2/CO can be easily controlled. In all measured conditions, for the steam as well as the dry reforming, the produced gas was composed of hydrogen and carbon dioxide with small amount of other gases (CO2, CH4, and O2). Argon detected in measurements presented in Figs. 7 and 8 was used as a tracer and it was added into the reactor for determination of the output gas flow rate. Measured composition of gas was close to the calculated composition in the temperature 123 Plasma Chem Plasma Process range 1200–2500 K. For stable conditions the composition was stable. The composition changed after any change of experimental parameters (flow rate of input reactants or arc power) occurred within 30 s, as can be seen in Figs. 7 and 8. The delay is probably caused by a transport time in gas sampling circuits, the time of change of composition in the reactor may be even shorter. This short reaction time and easy possibility of control of composition of produced gas are important advantages of plasma treatment. The process can be started on or shut down, as well as gas composition can be changed, with a short delay time. Values of all main characteristics of the conversion process, which were evaluated from measured data, were determined by a value of process enthalpy, defined as the ratio of energy spent for endothermic process to the input number of methane moles. The measured hydrogen yield, methane conversion, and H2 and CO selectivity are high, for high values of the process enthalpy they are above 90%. The ratio of H2/CO was 3–3.4 for steam reforming and 1.1–1.2 for dry reforming, which are values close to the calculated theoretical values. Higher content of hydrogen in case of dry reforming was given by additional hydrogen supplied in steam plasma. For lower values of reaction enthalpies, the values of all characteristics given in Figs. 9, 10, 11, 12, 13, 14 and 15 were lower. In the same time the energy cost of hydrogen given in Fig. 16 is also lower for low values of the reaction enthalpy. Thus optimum conditions for hydrogen production and optimum value of process enthalpy should be determined from a compromise between energy cost, hydrogen yield and selectivity. We analyzed, both theoretically and experimentally, the relation between energy consumed in the reaction and main characteristics of steam and dry reforming process. Measured dependences of process characteristics on energy had the same character as the theoretical ones which are presented in Figs. 4 and 5. However, theoretical values of energy spent in the reactions with maximum conversion characteristics were smaller than the values determined in experiments. Higher experimental values of process enthalpy are probably related to non-complete mixing of plasma with input reactants and non-homogeneous distribution of temperature in the reactor volume. Consequently not all plasma energy was transferred to the reacting substances. The residence time of gases in the reactor volume, calculated from measured gas flow rates, was in the range of 18–35 s. This time seems to be sufficient in case of proper mixing of plasma with gases and if no dead zones in the reactor exist. Configuration of plasma reactor, its volume and properties of plasma jet should be optimized in the following research to achieve maximum energy efficiency. Depending on experimental conditions the conversions of methane was up to 99.5%, the selectivity of H2 was up to 99.9%, and minimum energy needed for production of 1 kg of hydrogen was 79 MJ/kg for the steam reforming and 116 MJ/kg for the dry reforming (Fig. 16). These values correspond to the energy spent for the process in the reactor volume, not total energy consumed in the experiments, which is determined also by the torch efficiency and power loss in the reactor. The minimum value of total energy calculated from plasma torch power was 192 MJ/kg for the steam reforming. This value can be compared with the energy cost 166 MJ/kg for hydrogen produced by water electrolysis [33]. Further development of the plasma process is possible, it should be directed to optimization of a plasma torch for an increase of its energy efficiency and reduction of energy loss in the reactor. 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