Research Papers

Continuous Solar Carbothermal Reduction of Aerosolized ZnO Particles Under Vacuum in a Directly Irradiated Vertical-Tube Reactor

[+] Author and Article Information
Majk Brkic

Solar Technology Laboratory,
Paul Scherrer Institute (PSI),
Villigen 5232, Switzerland;
Department of Mechanical and
Process Engineering,
ETH Zurich,
Zurich 8092, Switzerland

Erik Koepf

Solar Technology Laboratory,
Paul Scherrer Institute (PSI),
Villigen 5232, Switzerland

Anton Meier

Solar Technology Laboratory,
Paul Scherrer Institute (PSI),
Villigen 5232, Switzerland
e-mail: anton.meier@psi.ch

1Corresponding author.

Contributed by the Solar Energy Division of ASME for publication in the JOURNAL OF SOLAR ENERGY ENGINEERING: INCLUDING WIND ENERGY AND BUILDING ENERGY CONSERVATION. Manuscript received December 15, 2015; final manuscript received January 31, 2016; published online February 23, 2016. Assoc. Editor: Wojciech Lipinski.

J. Sol. Energy Eng 138(2), 021010 (Feb 23, 2016) (14 pages) Paper No: SOL-15-1428; doi: 10.1115/1.4032685 History: Received December 15, 2015; Revised January 31, 2016

A solar-driven aerosolized particle reactor under vacuum was tested for carbothermal reduction of zinc oxide using concentrated solar power. The reactor concept is based on the downward flow of zinc oxide and carbon particles, which are indirectly heated by an opaque intermediate solar absorption tube. The particles are rapidly heated to reaction temperature and reduced within residence times of less than 1 s. In the continuous feeding experiments, maximum sustained temperatures close to 2000 K and heating rates as fast as 1400 K min−1 could be achieved for pressures between 1 and 1000 mbar. Reactant conversions of up to 44% were obtained at 1000 mbar. It was found that a reduction in system pressure leads to a decreased particle residence time (as low as 0.09 s), and therefore low conversion (as low as 1%), thus partially diminishing the positive thermodynamic effects of vacuum operation. Experimental results validate the robust and versatile reactor concept, and simultaneously highlight the necessity of balancing the system design in order to optimize the conflicting influence of vacuum operation and reacting particle residence time.

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Fig. 1

Schematic of the vacuum aerosol reactor as a cross-sectional view, and an enlarged view of the graphite absorber tube showing the hot reaction zone and particle flow. The graphite tube radial and vertical axes are denoted by r and ζ, respectively.

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Fig. 2

Temperature distribution at 1 mbar measured along the vertical axis of the graphite absorber tube as a function of different incident flux densities. The origin of the y-axis denotes the default temperature measurement position in experiments (at the focal point of HFSS and half-reflector height as shown in Fig. 1). The reaction zone is also defined with a length of 140 mm, and average temperature of the reaction zone is calculated along the height of the reflector (−70 mm < ζ < 70 mm) as such. Dashed lines indicate ± 1% error region of the thermocouple accuracy.

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Fig. 3

Schematic layout of the experimental setup, showing the solar vacuum aerosol reactor at PSI's HFSS. Reactants are introduced by a rotary particle feeder (1) into the graphite absorber tube (2). Reaction products are precipitated on the condenser (3). Particles exiting the reactor are collected in a collection vessel (4) and entrained fine particles are withheld by means of filter paper (5). Inert Ar gas flows are controlled by mass flow controllers (FC), and the system pressure is recorded at the top of the reactor (P). Online gas analysis includes infrared (IR: CO, CO2, and CH4) and TC detectors (TC: H2) as well as a micro-gas chromatograph (GC). Additional instrumentation consists of needle valves to adjust system pressure (6) and Ar gas flow at p < 10 mbar (7). An overpressure relief valve (8) provides safety when the setup is pressurized at completion of experiment.

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Fig. 4

Volume-based density (left) and cumulative (right) particle size distributions of activated charcoal (C), ZnO powder, and the blend of the two after stoichiometric mixing in a swing mill for 10 min

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Fig. 5

Temperature at the focal height of the HFSS and molar flow rates of evolved gases for a typical pulsed feeding experiment at p = 100 mbar. Arrows denote gas species at select peak heights, where solid lines are CO, dashed lines are H2, and filled peaks are CO2. Segment (A) corresponds to three feed pulseswith m˙  = 0.68 g·min−1. Segments (B) and (C) correspond to two feed pulses with m˙ = 1.36 g·min−1 and m˙  = 2.04 g·min−1, respectively.

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Fig. 6

Oxygen conversion for pulsed feeding experiments, investigating effect of system pressure, feed rate, and mean temperature TR of the reaction zone. Feed rates are indicated by marker type: solid markers denote low incident a flux density of 600 kW·m−2, with TR ∼1400 K; open markers denote a high incident flux density of 900 kW·m−2, with TR ∼ 1600 K. The error bars denote the standard deviation within feed pulses.

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Fig. 7

Temperature at the focal height of the HFSS, incident flux density, and evolved gases CO, CO2, and H2 for a typical continuous feeding experiment at p = 10 mbar (experiment #8 in Table 2). Feeding intervals with feed rate m˙ = 0.68 g·min−1 for duration Δtf = 10 min are indicated by vertical dashed lines.

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Fig. 8

Oxygen conversion (left) and particle Knudsen number (right) for continuous feeding experiments investigating the effect of system pressure and mean temperature of reaction zone TR. Flux density levels are indicated by marker type: squares refer to experiments at 600 kW·m−2 corresponding to reactor mean temperature TR ∼ 1400 K, circles to 900 kW·m−2 with TR ∼ 1600 K, and triangles to 1200 kW·m−2 with TR ∼ 1700 K, respectively. The error bars denote the standard deviation between two replicate experiments.

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Fig. 9

Particle residence times in the reaction zone as a function of system pressure for spherical particles of different size. Equivalent particle diameters considered were taken from characteristic values of the reactant particle size distribution listed in Table 1.

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Fig. 10

Mean particle velocity vp in the reaction zone and its ratio to the fluid gas velocity vg as function of system pressure for spherical particles of different size. Equivalent particle diameters considered were taken from characteristic values of reactant particle size distribution listed in Table 1.

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Fig. 11

Oxygen conversion at 1 mbar nominal pressure and various incident flux densities for four flow conditions, two of which contain obstruction to particle flow within the reaction zone and one no argon sweep gas. Conversion increases with higher degrees of flow obstruction Φ.

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Fig. 12

Temperature at the focal height, incident flux density, and evolved gases for a long feeding experiment at p = 100 mbar. Feeding interval with m˙   = 0.68 g·min−1 for Δtf = 30 min is indicated by vertical dashed lines. The HFSS was shutdown simultaneously with the feeding system.

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Fig. 13

XRD patterns of ZnO reactant and product samples taken from the long-term feeding experiment, collected at different locations as shown in Fig. 3. Samples were taken from powder remaining in the particle feeder (1), graphite tube (2), condenser (3), particle collection vessel (4), and particle filter paper (5).

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Fig. 14

SEM images of collected samples from the long-term feeding experiment at different reactor locations described in Fig. 3. (a) Overview and (b) detail of precipitated reaction products on the condenser (location 3). (c) Overview and (d) detail of agglomerated particles collected in collection vessel (location 4) below the reactor. (e) Overview and (f) detail of fine particles collected from filter paper on the gas outlet (location 5).




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