Article pubs.acs.org/IECR
Experimental Investigation of the Carbothermal Reduction of ZnO Using a Beam-Down, Gravity-Fed Solar Reactor Erik E. Koepf,† Suresh G. Advani,† Ajay K. Prasad,*,† and Aldo Steinfeld*,‡ †
Center for Fuel Cell Research, Dept. of Mechanical Engineering, University of Delaware, Newark, Delaware 19716, United States Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zürich, Switzerland
‡
ABSTRACT: Carbothermal reduction of ZnO with beech charcoal has been performed in a directly irradiated, beam-down, continuously gravity-fed, 10 kW solar thermochemical receiver−reactor using a high-flux solar simulator. Upon reaching mean cavity temperatures between 1113 and 1446 K, a mixture of ZnO powder and charcoal particles was fed continuously from hoppers into the axisymmetric reactor to create a moving reactant bed along the inclined ceramic reaction tiles. Product gases including zinc vapor exited through a centrally located outlet before passing through a condenser and then into a filter battery for collection. Solar thermochemical energy conversion efficiencies of up to 12.4% were recorded, with reactant conversion as high as 14%. Products collected from the reactor indicated acceptably high Zn content, with product yield exceeding 75% on average. Peak Zn production rates were as high as 0.135 mol/min for a total reactant feed rate of 1.5 g/s and a reaction temperature of 1435 K.
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INTRODUCTION Solar-produced Zn from ZnO is an attractive fuel applicable to power generation in a zinc−air battery or for further processing to generate syngas via H2O and CO2 splitting redox cycles.1,2 As precursors to liquid hydrocarbon fuels, the thermochemical production of H2 and CO using concentrated solar energy represents a sustainable alternative to fossil energy reserves. The first step of the Zn/ZnO cycle proceeds at 2000 K according to eq 1.3 Two primary challenges with direct thermal dissociation of ZnO are (i) the extremely high temperature requirement4 and (ii) the need to prevent recombination of the gaseous products O2 and Zn.5 By introducing a carbon-based reducing agent into the reaction, both of these issues can be surmounted simultaneously. The production of Zn can still be five times less carbon intensive via a solar-driven carbothermal route than via the conventional route supplying process heat by fossil fuel combustion.6−9 Further, if the carbonaceous reducing agent is harvested sustainably from process waste streams or biomass sources (such as beech charcoal), the process becomes CO2 neutral. The solar-driven carbothermal reduction is described in eqs 2−4. First step of solar-driven thermal dissociation: T > 2000 K
ZnO(s) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn(g) +
1 O2 , 2
CO2 (g) + C(s) → 2CO(g)
which are endothermic above 1230 K. As the oxygen from ZnO is transferred to either CO or CO2 in the carbothermal reduction reaction, processing of the product stream is simplified to traditional condensation techniques10 to harvest the solid Zn particles. The direct thermal dissociation of ZnO, eq 1, is negligible at this temperature and thus is not considered a significant reaction pathway.11 For a two-step water-splitting redox cycle, the solar Zn can be further processed via exothermic reactions with H2O and CO2 as follows: Second step exothermic oxidation: T < 1300 K
Zn + H 2O ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO(s) + H 2 , T < 1300 K
Zn + CO2 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ ZnO(s) + CO,
ΔH = −67 kJ/mol (6)
ZnO which is a reactant in step 1 is recovered as a product in step 2. The net inputs to the two-step process are sunlight, water, and carbon, and the net useful outputs are H2 and CO. Both of these product gases can be employed for direct power production via a fuel cell or combustion engine, or as precursors to Fischer−Tropsch synthesis for liquid hydrocarbon fuel production.3 From eq 2, it can be seen that the molar production rates of Zn during a ZnO carbothermal reduction experiment can be calculated from an oxygen balance as ṅZn = ṅO = ṅCO + 2ṅCO2. The production of Zn via solarcarbothermal reduction reactions has been studied extensively,12 and many solar receiver−reactor concepts have
(1)
First step of solar-driven carbothermal reduction: T > 1230 K
ZnO(s) + νC ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Zn(g) + (2ν − 1)CO + (1 − ν)CO2 (2)
where ΔHcarbothermal = 350.4 kJ/mol evaluated at 1473 K and ν represents the stoichiometric molar ratio of carbon to zinc oxide. The carbothermal reduction reaction proceeds via the following two intermediate reactions:
© XXXX American Chemical Society
ΔH = −104 kJ/mol (5)
ΔHnon‐carbon = 456 kJ/mol
ZnO(s) + CO(g) → Zn(g) + CO2 (g)
(4)
Received: April 2, 2015 Revised: August 8, 2015 Accepted: August 10, 2015
(3) A
DOI: 10.1021/acs.iecr.5b01249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
reaction surfaces. A stabilized rotating flow is generated at the receiver window by injecting inert gas from an array of radial and tangential jets, which transitions into an attached single-cell vortex inside the reaction cavity. The vortex attaches to the centrally located gas outlet at the bottom of the cavity and effectively removes product vapor from the reaction cavity while simultaneously preventing fouling of the quartz window by particle deposition which could attenuate solar power delivery to the reaction cavity.27 The design objective of the solar receiver−reactor concept was to simultaneously achieve three goals without the use of complex receiver−reactor rotation mechanisms: (i) create a cavity reaction environment to achieve high and uniform temperature, (ii) uniformly disperse reactants along the cavity walls such that they are directly exposed to incoming solar radiation, and (iii) provide sufficient residence time for adequate reactant conversion. Although the use of a rotating reaction cavity15,16 can also achieve these goals, scale-up and maintenance of a rotating reactor assembly can represent substantial engineering challenges. Figure 1 depicts the receiver−reactor as well as a reaction-tile section view. Pertinent design details of the receiver−reactor
been investigated for both the carbothermal8,13,14 and thermal dissociation of ZnO.15−19 Typical solar reactors that utilize solid particles as reactants can be categorized based on their reactant feeding mechanism (batch, continuous, etc.) and whether concentrated solar power is delivered directly or indirectly to the reactants inside the reaction cavity. The simplest case utilizes a batch reaction of the ZnO/carbon mixture, irradiated indirectly by an intermediate absorber surface that has been heated by concentrated sunlight and reradiates onto the reaction site. This approach has been successfully demonstrated on both small and large scale with various carbonaceous feedstock, yielding thermal efficiencies as high as 34%.6,10,13,20 However, the incorporation of an intermediate solar absorption surface incurs an energy penalty,21 and batch processing via packed beds imposes reaction limitations as well as capacity limitations. Alternatively, reactors that bring concentrated solar radiation directly into the reaction cavity allow the reactant to simultaneously serve the purpose of absorber, insulator, and chemical reactant.22 Further, by feeding small particle reactants continuously, heat and mass transfer limitations are reduced17 and better process control is possible as feed rates can be varied to suit the desired reaction conditions. A major drawback of direct high-flux solar irradiation is the need for optical access through a transparent window, a troublesome and critical component when operating at high radiative fluxes, high temperatures, high pressures, or pressure fluctuations, and in the presence of condensable metal vapors. Various aerosol-type reactors have been proposed and demonstrated for ZnO decomposition,17,23 as well as for other materials such as ceria.24 A rotating reactor utilizing quasicontinuous particle feeding (through a screw feeder inserted into the reaction cavity) was developed at the Paul Scherrer Institute (PSI) for ZnO decomposition15 and led to a scale-up demonstration project of closely related technology.16 A beam-down reactor with rotating cavity walls and a single particle inlet was successfully designed and tested to recycle hazardous waste material using solar process heat.25 Recently, the University of Delaware developed a novel, beamdown, gravity-fed, solar thermochemical receiver−reactor designed for continuous solid particle decomposition.18 In this work, a mixture of ZnO and beech charcoal has been fed continuously to the beam-down solar reactor to achieve carbothermal reduction of ZnO at temperatures between 1113 and 1446 K. This paper will summarize pertinent reactor design details, address the beam-down experimental setup, and describe in detail the results of the solar-driven reduction experiments carried out with a solid-particle mixture of ZnO and beech charcoal in a 1:1 molar ratio (ZnO:C).
Figure 1. Beam-down solar thermochemical reactor: (1) steel shell [diameter, 864 mm; height, 451 mm]; (2) annular solids exit [gap width, 25 mm]; (3) central product vapor and gas exit [diameter, 16 mm]; (4) array of 15 hopper assemblies, one above each reaction tile; (5) supporting insulation material; (6) alumina tile reaction surface [length, 300 mm]; (7) water-cooled aperture [diameter, 80 mm]; (8) water-cooled quartz window [diameter, 240 mm]; (9) particlemetering spline assembly; (10) particle hopper equipped with vibration motors; (11) incoming beam-down cone of concentrated sunlight [average rim-angle, 49°].
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SOLAR REACTOR DESIGN The solar reactor is cylindrical with a diameter of 864 mm and a height of 451 mm. Its reaction chamber consists of an axisymmetric cavity comprising 15 trapezoidal alumina reaction tiles arranged to form an inverted cone-shaped reaction surface. Solid particle reactants are fed individually to each tile from above by a hopper equipped with a metering spline/feeder system. Further details on the reactor concept and design have been reported previously by Koepf et al.,18 and a detailed description of the solid particle delivery system has also been reported.26 The receiver−reactor is closed to the atmosphere, and concentrated solar radiation enters from above, passing through a central quartz window and then through a watercooled aperture before it is directly incident on the inclined
will be summarized here. As depicted in Figure 1, the reaction cavity comprises 15 high-purity alumina tiles (300 mm long with an average width of 50 mm, and 12 mm thick) which are supported by layers of porous ceramic insulation to create an interlocking inverted cone-shaped reaction cavity at a 40° inclination. A solid particle hopper and delivery system sits above each tile reaction surface and can be controlled individually. Concentrated solar energy enters from above at an average rim-angle of 49°, passing through a water-cooled quartz window (240 mm diameter) before converging at the focal plane where an 80 mm water-cooled aperture is mounted. B
DOI: 10.1021/acs.iecr.5b01249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 2. Schematic setup for solar reactor experiments at the high-flux solar simulator at the Paul Scherrer Institute.
amount of argon ( 95%) adhered to the top surface of a 25.4 mm thick cast-aluminum plate via two-sided high thermal conductivity adhesive tape (3M, Thermally Conductive Interface Tape, 152.4 mm wide roll). The mirror is water-cooled via 13 through-bored water-lines along the width of the plate. The mirror was hung from an independently mounted frame, allowing for the in situ adjustment of the relative mirror− reactor angle during experiments. The mirror was hung from three points, the front edge (closest to arc-lamps) was centered and fixed to a ball-joint, and the two rear mountings were affixed through a threaded rod attached to a controllable wheeled motor assembly mounted on two tracks on the top of the frame. By adjusting the length of the two rods via their attached motors, the mirror angle relative to the incoming solar-beam and reactor could be adjusted during experiments to ensure optimum optical alignment of the system. In addition, the entire mirror could be raised to a horizontal position to allow access to the reactor between experiments. A critical aspect of the experimental setup was aligning the reactor and mirror to the solar simulator and light cone. As shown in Figure 3, the measured focal height of the lamp system was 2.54 m. The mirror angle could be fine-tuned in situ during experiments by adjusting the height of the lower two corners of the mirror independently while monitoring the flux image around the reactor aperture through the camera mounted inside the arc array. Power incident on the reactor aperture plane was determined from measured radiative flux calibration data,28 along with a ray-tracing analysis between the arc-lamps and the reactor aperture to account for self-shading of the reactor, as can be seen in Figure 3. After self-shading is taken into account, up to 7.5 kW of radiative power was E
DOI: 10.1021/acs.iecr.5b01249 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research components, as well as for heat-transfer analysis of the reactor system. Thermocouples were embedded behind every third reaction surface tile, denoted B1 in Figure 5. Thermocouples were also added to the back of the mirror to maintain adequate cooling. Reaction cavity temperature was monitored by a thermocouple (K1) embedded in the roof insulation, just inside the cavity volume. The schematic shown in Figure 5 also labels select important dimensions along with thermocouple type (type-K or type-B) and their relative locations. Thermocouples mounted behind the reaction tiles allowed for confirmation that uniform heating and reactant feeding was taking place inside the reactor. Additionally, key components of the reactor design are also indicated in Figure 5, for instance, the position of alumina pins which suspend the cavity roof insulation and the position of data acquisition access ports. The beam-down reactor was designed such that a region of 45° conical revolution starting at the center of the aperture and about the vertical center-line does not interfere with the top section of the reactor, window mount, or powder-feeding assembly (also indicated in Figure 5). To mitigate the effects of radiation spillage and generally manage temperature on the top surface of the metallic shell of the reactor, a radiation shield was fabricated and installed. An aluminum cooling plate assembly was constructed and mounted on top of the reactor to provide active cooling and to properly seal the alumina pins that hold the ceramic roof inside the reactor. Figure 6B shows the reactor top (5) with the cooling plate (6) installed. Alumina pins (8) can be seen protruding through the plates, and the cavity-access holes through which reactants are fed can also be seen (7). The cooling plate comprises two thinner plates with milled grooves that allow copper cooling lines to be wound inside. After the two plates are compressed together, the entire assembly is mounted to the top of the reactor. With the added cooling, compression fittings could be used to hold and seal the alumina pins via high-temperature silicone cord stock pieces. Figure 6A shows a photograph of the water-cooled outlet tube used for product removal and condensation. The outlet tube was designed to be inserted into the reaction cavity such that it could draw out product gases and withstand the hightemperature environment. Wall cooling along the outlet length of the reactor provided for sufficient Zn vapor condensation in the presence of carbon monoxide. The device is equipped with a high-purity alumina tip (not shown) that shields direct radiation, and the outlet length is wrapped tightly with copper tubing that carries chilled water (2). The device is mounted and sealed to the reactor via a metallic vacuum gasket and flange. An exhaust hose connects the bottom of the outlet tube to the filter battery, after which a portion of the stream is diverted for gas analysis and the remainder is vented to the atmosphere. Particle Reactant Feeding. Experiments were designed to carbothermally reduce the 1:1 molar ratio (ZnO:C-fixed) mixture of ZnO and crushed beech tree charcoal. Using 15 hoppers and feeding-spline assemblies described previously,26 the reactant mixture could be fed to the individual reaction tiles at flow rates between 0.2 and 0.6 g/s with feed-spline angular velocities in the range of 10−30 rpm. APS 1−5 μm ZnO powder from the Merelex Corporation (ZN-OX-03-P.05UM, lot no. 1871513147-754; CAS Registry 1314-13-2) was used and was certified 99.9% pure. Commercially available, pulverized, and unpyrolyzed beech charcoal (0.5−1.0 mm, Buchenholzkohlengriess 18 from Chemviron Carbon, product code Q018) was ground into a fine powder by hand and hand-
Figure 6. Water-cooled outlet tube (A) and roof cooling plate (B). The product outlet tube (1) is wrapped on the exterior by copper cooling lines (2). Inert gas can be delivered to the tip of the device via a concentric thin-walled tube design (3), and water-cooling (4) is also delivered such that the entire device can be sealed properly to the bottom of the reactor. During operation, a ceramic tip protects the device from direct radiation (not shown). The cooling plate (6) is mounted to the top of the reactor shell (5), through which alumina pins holding the cavity ceiling protrude (8). Slots (7) through which reactants are fed by the feeding mechanism are also shown.
mixed by weight into ZnO samples at a 1:1 molar ratio. Properties of the beech charcoal and ZnO are shown in Table 1. Table 1. ZnO and Beech Charcoal Properties beech charcoal
ZnO
property
value
property
value
molar weight density fixed-carbon volatiles ash moisture
12 g/mol 1505 kg/m3 85%
