Solar Energy Capture Transform Store Utilize
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This document discusses different methods of collecting and utilizing solar energy. It begins by explaining that solar energy comes from nuclear fusion reactions in the sun and reaches Earth as photons. There are two main types of solar collectors: flat plate collectors and concentrating collectors. Flat plate collectors use absorber plates and carrier fluids like water to collect solar energy for thermal processes. Concentrating collectors use reflectors to focus sunlight onto small areas, achieving higher temperatures suitable for applications like solar cooling or furnaces. The document also discusses passive solar energy systems that design structures to naturally collect, store, and distribute solar heat.
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analysis showed very high purity of zinc and crystalline. The second step is known in the
literature. Finely divided carbonaceous materials, e.g. coal are injected into the FeO melt
(which floats above the iron as it forms). The coal is dissolved in the FeO and reduces it,
thus creating CO gas. The gasification of coal takes place very rapidly owing to the high
temperature, the carbon content of the melt and the mixing created by the evolving gas.
A conceptual scheme of the solar reactor is shown.
Article Outline
1. Introduction
2. Thermodynamic analysis
3. Reduction of ZnO with Fe
3.1. Experimental results of solar reduction of ZnO with Fe
4. Reduction of FeO with carbon
5. Concept of a solar reactor
6. Conclusions
References
1. Introduction
In recent years, the thermal production of zinc from its oxide with the aid of concentrated
solar radiation has been attracting increasing interest. Results of the work on direct
decomposition of ZnO at high temperatures [1], reduction of ZnO with CH4 [2 and 3] and
with carbon [4] have been published in the last few years. Zinc can be reoxidized with
steam to generate hydrogen [5] or be used in zinc/air batteries. The direct thermal
decomposition of ZnO requires very high temperatures. Only at above 2000 °C, a
significant yield can be obtained (see Fig. 1 for equilibrium yield of zinc as a function of
the temperature at a pressure of 1 bar absolute) [6]. However, the vapor also contains
oxygen, which needs to be separated before the reverse reaction takes place. Quenching
of the gases at this temperature is difficult, unless rapidly diluted with cold inert gas.](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-17-320.jpg)
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Fig. 1. Thermodynamic equilibrium compositions of thermal decomposition of ZnO
Reduction of ZnO can be also achieved with zinc sulfide [7]. Zinc sulfide is the main ore
found in nature (sphalerite). It is relatively an inert material, which is unaffected when
heated with carbon. The following reaction between sulfide and oxide
ZnS(s)+2ZnO(s)→3Zn(g)+SO2(g), ΔH1820 K
0
=918 kJ/mol, (1)
does not take place spontaneously until a temperature of 1547 °C (ΔG=0) is reached (see
Fig. 2 for thermodynamic equilibrium) [6].
Fig. 2. Thermodynamic equilibrium compositions for Eq. (1) [6].
In addition, the separation of SO2 has to be performed at high temperatures or through a
rapid quenching process. For these reasons, the first stage in the recovery of zinc by all
commercial processes is the conversion of the sulfide into a more reactive oxide,
according to the following equation:](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-18-320.jpg)
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ZnS(s)+1.5O2(g)→ZnO(s)+SO2(g), ΔH1200 K
0
=431 kJ/mol. (2)
The reduction of ZnO with solid carbon is especially important because the gasification
of carbon is part of the process. The source of carbon can be different carbonaceous
materials, such as coal and biomass. The reaction can be described as follows:
ZnO(s)+C(s) Zn(g)+CO(g), ΔG0
=352.8−0.29T kJ/mol. (3)
This reaction proceeds in two successive and reversible gas–solid reactions [8], as
follows:
ZnO(s)+CO(g) Zn(g)+CO2(g), (4)
C(s)+CO2(g) 2CO(g). (5)
Eq. (5) is the gasification step. The amount of the CO2 in the products is mainly dictated
by this reaction. High temperature of the product vapors leaving the reactor and excess of
carbon will maintain a low content of carbon dioxide. The product gas consists of
roughly equal proportions of zinc vapor and carbon monoxide, with a small amount of
carbon dioxide. As the temperature of these gases drops and the dew point of this mixture
(830 °C) is reached, zinc starts to condense. This cooling process is complicated by the
fact that Eq. (5) proceeds rapidly in the reverse direction. When this occurs, a large
proportion of the condensing zinc will be oxidized and the excess ‘blue powder’, a dust
consisting of particles of metallic zinc coated with ZnO, will be formed. These particles
will not coalesce to form liquid metal. Therefore, the condenser is a critical equipment to
the process—it has to be capable of cooling the vapors so rapidly that reoxidation is
eliminated. Two industrial solutions are utilized. In the most common way, the condenser
volume contains a pool of molten lead. By means of rotating blades, a dense spray of lead
droplets fills the condenser. The product hot gas from the reactor is forced to flow
through this spray. It is thoroughly scrubbed and its temperature drops from above 1000
°C to less than 500 °C at the exit. The zinc vapors are absorbed and rapidly dissolved in
the lead droplets. Hot lead at a temperature of 550 °C contains about 2.5% zinc. The lead
is circulated out of the condenser and chilled to a temperature of 450 °C. At this
temperature, the solubility of zinc in lead is 2.25%. The excess zinc is released and it
forms a layer of molten zinc floating above the lead. Because the lead is able to dissolve
only small amounts of zinc, the amount of lead to be circulated must be 400 times as
much as the produced amount of zinc. In the second type of condenser, the vapors
leaving the reactor pass through a chamber holding a bath of molten zinc, which is held at
a controled temperature of about 500 °C by cooling coils. A motor-driven graphite
impeller is immersed in the zinc bath. The impeller rotates and fills the chamber with a
spray of zinc droplets. The zinc vapors condense as a metal on the droplets and only a
small chance of reoxidation is possible [7]. The present paper describes a two-step cyclic
solar process that eliminates the complication of rapid cooling of the product gases, since](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-19-320.jpg)
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The second step involves the feeding of carbonaceous material such as charcoal into the
molten FeO, reducing it back to iron, according to the following reaction:
C(s)+FeO(l)→CO(g)+Fe(l), ΔH0
=153 kJ/mol. (7)
The CO gas is obtained separated from the zinc and can be used directly as a fuel or
through a reaction with steam to generate hydrogen and carbon dioxide, as follows:
CO+H2O→H2+CO2, ΔH0
=−41.16 kJ/mol. (8)
Eq. (7) is a gasification process that takes place very quickly due to the high temperature,
the high heat and mass transfer between the FeO medium and the carbon, and the mixing
created by the evolving gas. An additional advantage of this two-step process is the
omission of the requirement for feed preparation. In a direct carbothermal reduction, this
requires mixing of ZnO with carbon in a measured proportion and preparation of
briquettes from the powder. When ZnO from zinc/air batteries is recycled, size reduction
is usually required. In the present process, relatively large-size fractions can be reduced,
due to the contact with the iron, and the high heat and mass transfer mechanisms.
2. Thermodynamic analysis
The equilibrium compositions [6] as a function of the temperature for (6) and (7) are
illustrated in Fig. 4 and Fig. 5, respectively [6]. These calculations demonstrate that
above 1535 °C, the melting point of iron (the melting point of FeO is 1430 °C), the
conversion of both reactions is complete. The energy balance of the entire process at
1900 K is summarized in Table 1.](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-21-320.jpg)
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Fig. 4. Thermodynamic equilibrium compositions of the reduction of ZnO(s) by Fe(l)
(Eq. (6)) [6].
Fig. 5. Thermodynamic equilibrium compositions of the recovery of Fe(l) from FeO(l)
using solid carbon (Eq. (7)) [6].
Table 1. Energy balance for the entire cycle](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-22-320.jpg)
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The solar energy could be used in steps 1–4 of this process. The total solar input in this
example is:
ΔHsolar=ΔH1+ΔH2+ΔH3+ΔH4=447 kJ/mol of ZnO.
The theoretical total heat output in this process can be summarized as follows:
ΔHheat out=ΔH5+ΔH6+ΔH7+ΔH8+ΔH9+ΔH10=826 kJ/mol of ZnO.
Thus, the solar contribution is
The enthalpy gain (Fgain) is defined as the theoretical total heat output divided by the
enthalpy of Eq. (9) between the amount of carbon used in the process and oxygen
(possible heat output without the process), as follows:
C(s)+O2(g)→CO2(g), ΔH1900 K
0
=−396 kJ/mol of carbon. (9)
In this case, Fgain will be 826/396=2.09.
3. Reduction of ZnO with Fe
An alternative to the use of carbon as a reducing agent is the metallothermic reduction of
ZnO with either solid or liquid iron (e.g. at 1538 °C—the melting point of Fe), according
to the following reaction:
ZnO(s)+Fe(s or l)→Zn(g)+FeO(s or l), ΔH1811 K
0
=208 kJ/mol. (10)
This process has been receiving increasing attention recently, since electric arc furnace
(EAF) dust contains significant amounts of zinc, mostly in the form of ZnO, which can
be recovered. ZnO in the dust can react with either solid or liquid iron. The kinetics of the
reduction of iron ZnO powder by solid iron powder, formed into cylindrical briquettes at
the temperature range of 1073–1423 K, shows that the reaction is chemically controled
[8] with activation energy of 230 kJ/mol. Once a product layer of zinc is formed, the
reaction is limited by the diffusion of zinc gas away from the reaction interface. It was
found [9] that at around 1400 °C, ZnO is reduced rapidly in the presence of an iron bath](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-23-320.jpg)
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The theoretical total heat output in this process can be summarized as follows:
ΔHheat out=ΔH5+ΔH6+ΔH7+ΔH8+ΔH9+ΔH10=826 kJ/mol of ZnO.
Thus, the solar contribution is
The enthalpy gain (Fgain) is defined as the theoretical total heat output divided by the
enthalpy of Eq. (9) between the amount of carbon used in the process and oxygen
(possible heat output without the process), as follows:
C(s)+O2(g)→CO2(g), ΔH1900 K
0
=−396 kJ/mol of carbon. (9)
In this case, Fgain will be 826/396=2.09.
3. Reduction of ZnO with Fe
An alternative to the use of carbon as a reducing agent is the metallothermic reduction of
ZnO with either solid or liquid iron (e.g. at 1538 °C—the melting point of Fe), according
to the following reaction:
ZnO(s)+Fe(s or l)→Zn(g)+FeO(s or l), ΔH1811 K
0
=208 kJ/mol. (10)
This process has been receiving increasing attention recently, since electric arc furnace
(EAF) dust contains significant amounts of zinc, mostly in the form of ZnO, which can
be recovered. ZnO in the dust can react with either solid or liquid iron. The kinetics of the
reduction of iron ZnO powder by solid iron powder, formed into cylindrical briquettes at
the temperature range of 1073–1423 K, shows that the reaction is chemically controled
[8] with activation energy of 230 kJ/mol. Once a product layer of zinc is formed, the
reaction is limited by the diffusion of zinc gas away from the reaction interface. It was
found [9] that at around 1400 °C, ZnO is reduced rapidly in the presence of an iron bath
until the zinc concentration reaches about 3%, and subsequently reduction is slow.
Stirring the iron bath increases the rate of reduction of zinc very significantly.
3.1. Experimental results of solar reduction of ZnO with Fe
Experiments have been conducted at the WIS solar furnace. Two grams (0.03 mol) of
ZnO powder and 4 g (0.07 mol) of Fe were heated under a flow of argon in an alumina
tube (99.7% Al2O3, 22 cm length, 2.5 cm diameter), partially placed inside a cavity box
with a side aperture diameter of 6 cm, through which the concentrated solar radiation](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-25-320.jpg)
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Fig. 7. Powder X-ray diffraction of the product material of the ZnO reduction obtained on
the ‘cold finger’ and the residue in the alumina tube reactor.
4. Reduction of FeO with carbon
The second step of the process is the reduction of FeO with carbon and recovery of the
iron with CO as a product gas (Eq. (7)). The equilibrium compositions at different
temperatures are shown in Fig. 5. Thermodynamically, at 1000 °C, 100% conversion of
FeO to iron can be obtained. This process is widely used in the industry [10 and 11].
Industrial scale reduction of iron ore containing solid FeO and carbon monoxide takes
place, according to the following equation:
FeOn+nCO→Fe+nCO2. (11)
Carbon monoxide is produced in a blast furnace from the reaction of oxygen in the blast
air with hot coke and other reducing agents such as injected coal or oil, according to the
following reaction:
C+0.5O2→CO, ΔH1000 °C
0
=−113.7 kJ/mol. (12)
Carbon monoxide is also produced from CO2, the product of Eq. (11), with carbon by the
Boudouard reaction ( Eq. (5)). If FeO is present as a liquid, the main reaction that takes
place is that given in Eq. (7). If coal is used as the reducing agent, the gases actually
participating in the reduction process originate from the coal pyrolysis (i.e. hydrogen and
hydrocarbons) and coal gasification. Coal also contains ash, moisture (10% typically,
which reacts endothermally in the water–gas reaction) and sulfur. The kinetics and
reaction mechanism of the reduction process of FeO, especially in slag, either in solid](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-27-320.jpg)
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phase ore in a smelting reduction, are reported in the literature [12, 13 and 14]. The
reduction of FeO can also be achieved with hydrogen, as follows:
FeO+H2→Fe+H2O, (13)
and by a mixture of hydrogen and carbon monoxide resulting, for example, from methane
reforming or coal gasification processes.
In the presence of CO, in addition to H2, (PCO2/PCO) and (PH2O/PH2), partial vapor pressure
ratios are coupled at any given temperature by the gas phase equilibrium [10] as follows:
(14)
5. Concept of a solar reactor
The beam-down optics of a solar tower [15 and 16] can be used to provide concentrated
solar radiation in a ground reactor containing a liquid matter from its ceiling through a
terminal concentrator (known as compound parabolic concentrator—CPC). The concept
of a solar reactor is conceived in Fig. 8. A central compartment is used for the reduction
of ZnO and production of zinc. ZnO powder is injected into the iron melt creating a
liquid layer of FeO, which floats on the iron layer. The zinc vapors, the only product in
this compartment, exit at the top of this compartment directly to a condenser where the
vapors first cooled from the reaction temperature (1500 °C typically) to about 907 °C,
condensed to zinc liquid, and the liquid is further cooled to about 500 °C. If the final goal
of the zinc is to produce hydrogen via reaction with steam [5], the most suitable condition
of the zinc is a liquid phase at around 500 °C. Part of the zinc produced during the day
can be stored as a liquid for the night, when production of hydrogen can take place. This
results in a chemical storage of solar energy.
)](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-28-320.jpg)
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Fig. 8. Schematic concept of a solar reactor for the two-step reduction of ZnO(s) with
Fe(l).
The central compartment is surrounded by peripheral compartments (i.e. six hexagonal
peripheral compartments surrounding a central hexagon) (Fig. 8), where the reduction of
FeO(l) to Fe(l) takes place. There are openings at a certain height in the compartments
through which FeO(l) flows gravitationally to the peripheral compartments and at the
bottom, Fe(l) flows back to the center. The reduction of FeO(l) can be executed by the
injection of carbon, charcoal or coal into the FeO(l) layer. If carbon is used, the main
product is CO(g). CO is cooled and can be converted in a conventional process to CO2
and H2, according to Eq. (8). CO2 can be separated, recycled and used to inject the carbon
into the reactor. At the temperature of the reactor, CO2 reacts with the solid carbon to
produce CO, according to the Boudouard reaction [17]. According to Eq. (5), the
enthalpy of this endothermic reaction, which equals ΔH0
=172.5 kJ/mol, can be obtained
from the sun, resulting in CO2 gasification of the carbonaceous material. Both C(s) and
CO(g) can reduce FeO(l) and the product gas will be a mixture of CO and CO2. Instead or
in addition to the recycled CO2, steam can be injected as well and used to feed the
carbon. In this case, the carbon is gasified according to Eq. (15) [17], and both CO and
H2 can be used to reduce FeO as follows:
C(s)+H2O(g) CO(g)+H2(g), ΔH0
=131.4 kJ/mol. (15)
Finally, the peripheral compartments can be equipped with means to inject oxygen in
case solar energy is not available to avoid freezing of the Fe and FeO melts. The oxygen
reacts with the carbon in an exothermic reaction to provide heat for maintaining the
temperature of the bath and for reduction of FeO(l), if necessary, according to Eq. (12).](https://image.slidesharecdn.com/solarenergycapturetransformstoreutilize-210718091554/85/Solar-Energy-Capture-Transform-Store-Utilize-29-320.jpg)
Solar Energy Capture Transform Store Utilize
- 1. 1 SOLAR ENERGY Solar Energy, radiant energy produced in the sun as a result of nuclear fusion reactions It is transmitted to the earth through space in quanta of energy called photons, which interact with the earth's atmosphere and surface. The strength of solar radiation at the outer edge of the earth's atmosphere when the earth is taken to be at its average distance from the sun is called the solar constant, the mean value of which is 1.37 × 10 6 ergs per sec per cm 2 , or about 2 calories per min per cm 2 . The intensity is not constant, however; it appears to vary by about 0.2 percent in 30 years. The intensity of energy actually available at the earth's surface is less than the solar constant because of absorption and scattering of radiant energy as photons interact with the atmosphere. The strength of the solar energy available at any point on the earth depends, in a complicated but predictable way, on the day of the year, the time of day, and the latitude of the collection point. Furthermore, the amount of solar energy that can be collected depends on the orientation of the collecting object
- 2. 2 DIRECT COLLECTION OF SOLAR ENERGY The direct collection of solar energy involves artificial devices, called solar collectors, that are designed to collect the energy, sometimes through prior focusing of the sun's rays. The energy, once collected, is used in a thermal process or a photoelectric, or photovoltaic, process. In thermal processes, solar energy is used to heat a gas or liquid, which is then stored or distributed. In the photovoltaic process, solar energy is converted directly to electrical energy without intermediate mechanical devices . Solar collectors are of two fundamental types: flat plate collectors and concentrating collectors. Passive Solar Energy Active solar heating systems involve installing special equipment that uses energy from the sun to heat or cool existing structures. Passive solar energy systems involve designing the structures themselves in ways that use solar energy for heating and cooling. For example, in this home, a "sun space" serves as a collector in winter when the solar shades are open and as a cooler in summer
- 3. 3 when the solar shades are closed. Thick concrete walls modulate wide swings in temperature by absorbing heat in winter and insulating in summer. Water compartments provide a thermal mass for storing heat during the day and releasing heat at night. Flat Plate Collectors In thermal processes, flat plate collectors intercept solar radiation on an absorber plate in which passages for so-called carrier fluid are integral or to which they are attached. The carrier fluid (liquid or air) passing through these flow channels has its temperature increased by heat transfer from the absorber plate. The energy transferred to the carrier fluid, when divided by the solar energy incident on the collector and expressed as a percentage, is called the instantaneous collector efficiency. Flat plate collectors generally have one or more optically transparent cover plates intended to minimize heat losses from the absorber plate, in an effort to achieve maximum efficiency. Typically, they are capable of heating carrier fluids up to 82° C (180° F) with efficiencies between 40 and 80 percent. Solar Heating Flate plate collectors utilize the sun's energy to warm a carrier fluid, which in turn provides usable heat to a household. The carrier fluid, which in this case is water, flows through copper tubing in the solar collector, and in the process absorbs some of the sun's energy. Next, the carrier fluid moves to the heat exchange, where the carrier fluid warms water that is used by the household. Finally, a pump moves the carrier fluid back to the solar collector to repeat the cycle. Flat plate collectors have been used efficiently for water and comfort heating. Typical residential applications employ roof-mounted, fixed collectors. In the
- 4. 4 northern hemisphere, they are oriented in a southerly direction; in the southern hemisphere, they are oriented to face north. The optimum angle at which to mount collectors relative to the horizontal plane depends on the latitude of the installation. Generally, for year-round applications such as providing hot water, collectors are tilted (relative to the horizontal plane) at an angle equal to the latitude angle ± 15°, and are oriented to face true south (or north) within ± 20°. In addition to the flat plate collectors, typical hot-water and comfort heating systems include circulating pumps, temperature sensors, automatic controllers to activate the circulating pump, and a storage device. Either air or a liquid (water or a water-antifreeze mixture) can be used as the fluid in the solar heating system, and a rock bed or a well-insulated water storage tank typically serves as an energy storage medium. Concentrating Collectors For applications such as air conditioning, central power generation, and numerous industrial heat requirements, flat plate collectors generally cannot provide carrier fluids at temperatures sufficiently elevated to be effective. They may be used as first-stage heat input devices; the temperature of the carrier fluid is then boosted by other conventional heating means. Alternatively, more complex and expensive concentrating collectors can be used. These are devices that optically reflect and focus incident solar energy onto a small receiving area. As a result of this concentration, the intensity of the solar energy is magnified, and the temperatures that can be achieved at the receiver (called the "target") can approach several hundred or even several thousand degrees Celsius. The concentrators must move to track the sun if they are to perform effectively, and the devices used to achieve this are called heliostats. Solar Furnaces One important high-temperature application of concentrators is for solar furnaces. The largest of these, located at Odeillo in the Pyrenees Mountains of France, uses 9600 reflectors with a total area of approximately 1860 sq m (about 20,000 sq ft) to produce temperatures as high as 4000° C (7200° F). Such furnaces are ideal for research requiring high temperatures and contaminant-free environments-for example, materials research. Central Receivers Central electric power generation from solar energy is under development. In the central receiver, or "power tower," concept, an array of reflectors mounted on computer-controlled heliostats reflect and focus the sun's rays onto a water boiler mounted on a tower. The steam thus generated can be used in a conventional
- 5. 5 power-plant cycle to produce electricity. Solar Cooling Solar cooling can be achieved through the use of solar energy as a heat source in an absorption cooling cycle. One component of standard absorption cooling systems, called the generator, requires a heat source. Because temperatures in excess of 150° C (300° F) are generally required for the absorption device to perform effectively, concentrating collectors are more suitable than flat plate collectors for cooling applications. IMPORTANCE OF ENERGY CONVERSION AND STORAGE The conversion of raw materials into usable energy (electricity or heat) and storage of the energy produced, are two very important aspects of everyday life. Whilst most of the electricity generated is converted from primary energy sources (fossil, nuclear, hydro), there are many other techniques increasing in popularity. The most important is the conversion of sunlight into electricity using solar cells. Solar power stations are now feeding power into local electricity distribution networks. Solar collectors are used to harness the heat of the Sun to heat water or buildings. Fuel cells are becoming a commercial reality for generating electricity in a variety of applications. Substantial effort has been devoted to exploring the generation of electric power from the effect of gaseous plasma or liquid metal moving through a magnetic field, also known as magneto hydrodynamics. Thermoelectric and thermionic conversion processes are being investigated for possible use in space vehicles. Batteries are used for both energy conversion and storage. Improved technology is leading to longer life and better performances. Battery storage plants are now used for load levelling applications in power systems. Hydrogen is a useful energy resource but its role in the future is more likely to be in the area of energy storage and transportation. Energy can be stored by other than chemical means. These include: mechanical energy storage, primarily flywheels; capacitor banks, which are used for reactive power compensation or for supplying a large amount of energy in a very short time for pulsed power applications; inductive energy storage; compressed air energy storage in natural underground caverns and aquifers; superconducting magnet energy storage which is often used for power system control; and thermal energy storage using phase change materials, solar ponds, hot water tanks or ice. The INSPEC database provides comprehensive coverage of developments in these areas. Papers can be found in Section A
- 6. 6 (Physics Abstracts: Chapters A8630 and A8640) and Section B (Electrical and Electronics Abstracts: Chapter B8400). The main energy conversion and storage processes and their relevant INSPEC classification codes are: Energy conversion Electrochemical conversion, general A8630D, B8410 Primary cells A8630E, B8410C Secondary cells (inc. lead-acid batteries) A8630F, B8410E Fuel cells (inc. molten carbonate, solid oxide and phosphoric acid fuel cells) A8630G, B8410G Photoelectric conversion; solar cells A8630J, B8420 Phototelectrochemical conversion A8630K, B8410 Magnetohydrodynamic energy conversion A8630L, B8430 Thermoelectric conversion (inc. alkali metal thermoelectric converters) A8630M, B8460 Thermionic conversion A8630N, B8460 Photosynthesis (inc. bioenergy conversion) A8630P Chemical energy conversion (inc. coal gasification) A8630Q Thermal energy conversion (inc. heat engines, heat pumps and ocean thermal energy conversion) A8630R Photothermal conversion (inc. solar collectors, solar ponds, space heating and refrigerators) A8630S, B8460 Energy storage Mechanical (inc. flywheels and compressed air energy storage) A8640C, B8470 Thermal (inc. solar ponds and tanks, phase-change materials) A8640F Chemical A8640H Hydrogen energy A8640K Capacitor storage (inc. capacitor banks) B8470 Inductive energy storage B8470
- 7. 7 inductive energy storage Relevant INSPEC Thesaurus terms include: Energy conversion terms: bioenergy conversion chemical energy conversion direct energy conversion fuel cells heat engines heat pumps magnetohydrodynamic conversion ocean thermal energy conversion photoelectrochemical cells photothermal conversion primary cells radioisotope thermoelectric generators secondary cells solar absorber-converters solar cell arrays solar cells solar energy conversion solar heating solar ponds thermionic conversion thermoelectric conversion thermoelectric devices Energy storage terms: battery storage plants capacitor storage compressed air energy storage energy storage energy storage devices flywheels hydrogen economy Superconducting magnet energy storage B847
- 8. 8 mechanised energy storage superconducting magnet energy storage thermal energy storage
- 9. 9 PHASE CHANGE MATERIALS FOR STORAGE solar space heating systems commonly use dense materials, such as brick, concrete, and adobe, as thermal storage materials. Water, stored in plastic, fiberglass, or glass-lined steel containers, is not only the lowest cost, widely available thermal storage material/system, but it also has the highest thermal energy storage capability. On a weight basis, these high-density storage materials cannot compete with water.
- 10. 10 Heat storage materials absorb heat through standard heat transfer mechanisms— radiation, conduction, and convection. As the materials cool off at night or on cloudy days, they subsequently release the stored heat in the same fashion. Active space heating systems commonly use tanks of water or bins of rock (rock bins) as a thermal storage material. Water is the typical thermal storage medium in solar water heating systems. These heat storage materials deal with "sensible" heat. This means as they absorb heat, their temperature increases and they become warm to the touch. Phase change materials (PCMs) are "latent" thermal storage materials. They use chemical bonds to store and release heat. The thermal energy transfer occurs when a material changes from a solid to a liquid, or from a liquid to a solid. This is called a change in state, or "phase." Initially, these solid-liquid PCMs perform like conventional storage materials; their temperature rises as they absorb solar heat. Unlike conventional (sensible) storage materials, when PCMs reach the temperature at which they change phase (their melting point) they absorb large amounts of heat without getting hotter. When the ambient temperature in the space around the PCM material drops, the PCM solidifies, releasing its stored latent heat. PCMs absorb and emit heat while maintaining a nearly constant temperature. Within the human comfort range of 68° to 86°F (20° to 30°C), latent thermal storage materials are very effective. They store 5 to 14 times more heat per unit volume than sensible storage materials such as water, masonry, or rock. Solid-solid PCMs absorb and release heat in the same manner as solid-liquid PCMs. These materials do not change into a liquid state under normal conditions. They merely soften or harden. Relatively few of the solid-solid PCMs that have been identified are suitable for thermal storage applications. Liquid-gas PCMs are not yet practical for use as thermal storage. Although they have a high heat of transformation, the increase in volume during the phase change from liquid to gas makes their use impractical. The PCM applications described below are with liquid- solid materials.
- 11. 11 The main applications for PCMs are when space restrictions limit larger thermal storage units in direct gain or sunspace passive solar systems. Phase change materials may be used in solar domestic hot water heating or passive solar space heating systems. They are usually more expensive than conventional heat storage materials. Glauber's salt (sodium sulfate decahydrate), calcium chloride hexahydrate, and paraffin wax are the most commonly used PCMs in solar heating systems. Although these compounds are fairly inexpensive, the packaging and processing necessary to get consistent and reliable performance from them is complicated and costly. Because the chemicals in some PCMs separate and stratify when in their liquid state, PCMs have not always resolidified properly. When temperatures dropped, they did not completely solidify, reducing their capacity to store latent heat. These problems have been addressed by packaging phase change materials in thin or shallow containers, and by adding thickening and clumping agents. These types of PCMs, however, compare unfavorably with the newer generation of low-cost, highly efficient, linear crystalline alkyl hydrocarbons. Researchers now label these older compounds as "limited utility" PCMs. Phase change materials perform best in containers that—when combined with the PCM—total one inch (25.4 millimeters). The packaging material should conduct heat well; and it should be durable enough to withstand frequent changes in the storage material's volume as phase changes occur. It should also restrict the passage of water through the walls, so the materials will not dry out. Packaging must also resist leakage and corrosion. Steel and polyethylene are common packaging materials. Situating the PCM containers so warm air flows over both sides increases their performance. Research on solid-liquid phase change materials has concentrated on the following materials: linear crystalline alkyl hydrocarbons, fatty acids and esters, polyethylene glycols, long alkyl side chain polymers, the solid state series of pentaerythritol,
- 12. 12 pentaglycerine, and neopentyl glycol, low melting metals and alloys, quaternary ammonium clathrates and semi-clathrates, and salt hydrides. Additional research has led the development of PCM materials that may be designed for applications in the temperature range of just above 32° to 257°F (0°C to 125°C). By blending adjacent alkyl hydrocarbon chains, a mixture having a desired single melting temperature may be produced without significant decrease in thermal energy storage. Phase change drywall, a new development in PCMs, is currently under research. Phase change drywall incorporates phase change materials inside common wallboard to increase its heat storage capacity. PCM wallboard replaces the heavier, more expensive, conventional thermal mass used in passive solar space heating applications. A successfully tested PCM for drywall, K-18, is a low-cost alkyl hydrocarbon blend that melts and freezes congruently at 25°C (77°F). This is the recommended temperature for maximizing residential heating and cooling benefits. Research is being conducted on methods of incorporating PCMs into other lightweight building materials such as plywood, as well as ceiling and floor tiles. Possible commercial applications include use in paving materials to minimize nighttime icing on bridges and overpasses, while also reducing surface damage from firefighters)pumps with thermal storage.
- 13. 13 STORING SOLAR ENERGY BY USING CHEMICALS The concept of a closed loop thermochemical energy storage system using ammonia
- 14. 14 If solar energy is to become a major contributor to our energy supply, means to store it have to be found. One promising method applicable is "closed loop thermochemical energy storage using ammonia". In this system, ammonia (NH3) is dissociated in an energy storing (endothermic) chemical reactor as it absorbs solar thermal energy. At a later time and place, the reaction products hydrogen (H2) and nitrogen (N2) react in an energy releasing (exothermic) reactor to resynthesise ammonia. 2 NH3 + Heat N2 + 3 H2 Feeding the reactors with pure reactants is possible through the natural separation of reactants and products in the storage system: at the pressures applied, ammonia condenses. By ensuring that the stuff leaving each reactor transfers its own thermal energy (sensible heat) to the stuff going in - using heat exchangers - most of the solar energy is stored in the change in composition of the chemicals which are kept at ambient temperature. Main advantages of the closed loop thermochemical storage system using ammonia Apart from the ability of the ammonia system to allow for continuous energy supply on a 24-hour basis, other advantages, that are not
- 15. 15 necessarily shared by other solar thermochemical or photochemical systems, make this process unique: A high energy storage density, by volume and mass. The reactions are easy to control and to reverse and there are no unwanted side reactions. All constituents involved are environmentally benign. There exists a history of industrial application with the associated available expertise and hardware. A readily achievable turning temperature of 400o C to 500o C (depending on the pressure). This helps to reduce thermal losses from dish receivers, avoids some high temperature materials limitations, and allows lower quality (and hence cheaper) dish optics to be used. All reactants for transport and handling are in the fluid phase, which provides a convenient means of energy transport without thermal loss. This is an important point, particularly if large arrays of paraboloidal dishes are being considered as the method for solar energy collection. At ambient temperature the ammonia component of reactant mixtures condenses to form a liquid, whilst the nitrogen and hydrogen remains as a gas. This means that only one storage vessel is required for reactants and products. The success with the prototype solar closed loop using ammonia confirms that the process works. It is not only simple but also very much predictable and controllable. Solar energy can thus be effectively captured and converted without fears of transients. Ferro-reduction of ZnO using concentrated solar energy Michael Epstein , , a , Koebi Ehrensberger b and Amnon Yogev a a Solar Research Facilities Unit, Weizmann Institute of Science, Rehovot 76100, Israel b AAC Infotray AG, Winterthur, Switzerland
- 16. 16 Available online 2 October 2003. Abstract In recent years, the production of zinc from its oxide using solar energy has been attracting increasing interest. This is a promising process for the conversion of solar radiation to its chemical form and storage. When carbon is used as a reducing agent, the main product gases are zinc, CO, and CO2. A major problem is the need for rapid cooling and separation of the zinc to avoid back reaction and reoxidation. In industry, lead splash condenser is used to remove the zinc vapor rapidly. The lead is circulated out of the condenser and chilled, so that the solubility of the dissolved zinc is reduced and part of the molten zinc is separated and floats on the lead. Because lead is capable of dissolving only a small amount of zinc, the amount of lead to be circulated is about 400 times as much as the amount of produced zinc. This technology is complicated and cumbersome. This paper describes a new approach for two-step process for the reduction of ZnO that can potentially solve this problem. The two-step process can be characterized by the following equations: ZnO(s)+Fe(l)→Zn(g)+FeO(l), ΔH0 =177 kJ/mol, FeO(l)+C(s)→Fe(l)+CO(g), ΔH0 =153 kJ/mol. Both steps are endothermic, require temperatures in the range of 1300–1600 °C, and can be carried out using concentrated solar energy. In the first step, iron reduces ZnO and Zn vapors are distilled out (zinc is miscible in iron, but at the relevant temperatures, and at atmospheric pressure it volatilizes). In the second step, carbon is injected into the FeO melt and reduces it back to iron. The CO obtained in the second step is separated from the zinc vapors. Basically, this is a gasification process. The carbon is converted to CO. When using coal, the ashes form slag on the surface of the melt and can be removed. The advantages of this process compared to the direct carboreduction are: (i) high rates of heat and mass transfer mechanisms between the iron melt and the ZnO powder; (ii) avoiding the necessary preparation of the feed as required in the direct process, mixing ZnO and carbon in a measured proportion, preparation of briquettes; if ZnO is recycled from zinc/air batteries, there is no need for size reduction and, in principle, large-size fractions can be processed; (iii) avoiding the major difficulty of separation of the CO and providing the possibility for simpler zinc condenser compared to the lead splash condenser. The process has higher thermodynamic gain and higher contribution of solar radiation. This paper analyzes the thermodynamics of the two- step process and compares it to the direct carboreduction of ZnO. The kinetics and mechanism of the reactions are discussed. Experimental results with solar energy for the first step are described. A mixture of ZnO and iron was heated at the Weizmann Institute of Science (WIS) solar furnace. At 1600 °C, 90% of the theoretical yield was obtained after 5 min of testing. The zinc vapors were condensed and X-ray diffraction
- 17. 17 analysis showed very high purity of zinc and crystalline. The second step is known in the literature. Finely divided carbonaceous materials, e.g. coal are injected into the FeO melt (which floats above the iron as it forms). The coal is dissolved in the FeO and reduces it, thus creating CO gas. The gasification of coal takes place very rapidly owing to the high temperature, the carbon content of the melt and the mixing created by the evolving gas. A conceptual scheme of the solar reactor is shown. Article Outline 1. Introduction 2. Thermodynamic analysis 3. Reduction of ZnO with Fe 3.1. Experimental results of solar reduction of ZnO with Fe 4. Reduction of FeO with carbon 5. Concept of a solar reactor 6. Conclusions References 1. Introduction In recent years, the thermal production of zinc from its oxide with the aid of concentrated solar radiation has been attracting increasing interest. Results of the work on direct decomposition of ZnO at high temperatures [1], reduction of ZnO with CH4 [2 and 3] and with carbon [4] have been published in the last few years. Zinc can be reoxidized with steam to generate hydrogen [5] or be used in zinc/air batteries. The direct thermal decomposition of ZnO requires very high temperatures. Only at above 2000 °C, a significant yield can be obtained (see Fig. 1 for equilibrium yield of zinc as a function of the temperature at a pressure of 1 bar absolute) [6]. However, the vapor also contains oxygen, which needs to be separated before the reverse reaction takes place. Quenching of the gases at this temperature is difficult, unless rapidly diluted with cold inert gas.
- 18. 18 Fig. 1. Thermodynamic equilibrium compositions of thermal decomposition of ZnO Reduction of ZnO can be also achieved with zinc sulfide [7]. Zinc sulfide is the main ore found in nature (sphalerite). It is relatively an inert material, which is unaffected when heated with carbon. The following reaction between sulfide and oxide ZnS(s)+2ZnO(s)→3Zn(g)+SO2(g), ΔH1820 K 0 =918 kJ/mol, (1) does not take place spontaneously until a temperature of 1547 °C (ΔG=0) is reached (see Fig. 2 for thermodynamic equilibrium) [6]. Fig. 2. Thermodynamic equilibrium compositions for Eq. (1) [6]. In addition, the separation of SO2 has to be performed at high temperatures or through a rapid quenching process. For these reasons, the first stage in the recovery of zinc by all commercial processes is the conversion of the sulfide into a more reactive oxide, according to the following equation:
- 19. 19 ZnS(s)+1.5O2(g)→ZnO(s)+SO2(g), ΔH1200 K 0 =431 kJ/mol. (2) The reduction of ZnO with solid carbon is especially important because the gasification of carbon is part of the process. The source of carbon can be different carbonaceous materials, such as coal and biomass. The reaction can be described as follows: ZnO(s)+C(s) Zn(g)+CO(g), ΔG0 =352.8−0.29T kJ/mol. (3) This reaction proceeds in two successive and reversible gas–solid reactions [8], as follows: ZnO(s)+CO(g) Zn(g)+CO2(g), (4) C(s)+CO2(g) 2CO(g). (5) Eq. (5) is the gasification step. The amount of the CO2 in the products is mainly dictated by this reaction. High temperature of the product vapors leaving the reactor and excess of carbon will maintain a low content of carbon dioxide. The product gas consists of roughly equal proportions of zinc vapor and carbon monoxide, with a small amount of carbon dioxide. As the temperature of these gases drops and the dew point of this mixture (830 °C) is reached, zinc starts to condense. This cooling process is complicated by the fact that Eq. (5) proceeds rapidly in the reverse direction. When this occurs, a large proportion of the condensing zinc will be oxidized and the excess ‘blue powder’, a dust consisting of particles of metallic zinc coated with ZnO, will be formed. These particles will not coalesce to form liquid metal. Therefore, the condenser is a critical equipment to the process—it has to be capable of cooling the vapors so rapidly that reoxidation is eliminated. Two industrial solutions are utilized. In the most common way, the condenser volume contains a pool of molten lead. By means of rotating blades, a dense spray of lead droplets fills the condenser. The product hot gas from the reactor is forced to flow through this spray. It is thoroughly scrubbed and its temperature drops from above 1000 °C to less than 500 °C at the exit. The zinc vapors are absorbed and rapidly dissolved in the lead droplets. Hot lead at a temperature of 550 °C contains about 2.5% zinc. The lead is circulated out of the condenser and chilled to a temperature of 450 °C. At this temperature, the solubility of zinc in lead is 2.25%. The excess zinc is released and it forms a layer of molten zinc floating above the lead. Because the lead is able to dissolve only small amounts of zinc, the amount of lead to be circulated must be 400 times as much as the produced amount of zinc. In the second type of condenser, the vapors leaving the reactor pass through a chamber holding a bath of molten zinc, which is held at a controled temperature of about 500 °C by cooling coils. A motor-driven graphite impeller is immersed in the zinc bath. The impeller rotates and fills the chamber with a spray of zinc droplets. The zinc vapors condense as a metal on the droplets and only a small chance of reoxidation is possible [7]. The present paper describes a two-step cyclic solar process that eliminates the complication of rapid cooling of the product gases, since
- 20. 20 the zinc vapor and the carbon monoxide are obtained separately. The process is shown schematically in Fig. 3. At the first step, particles of ZnO are injected into a bath of molten iron. The iron reduces the solid ZnO and the zinc is volatilized as metallic vapors, according to the following reaction: ZnO(s)+Fe(l)→Zn(g)+FeO(l), ΔH0 =177 kJ/mol. (6) Fig. 3. Scheme of a two-step process for the production of pure zinc.
- 21. 21 The second step involves the feeding of carbonaceous material such as charcoal into the molten FeO, reducing it back to iron, according to the following reaction: C(s)+FeO(l)→CO(g)+Fe(l), ΔH0 =153 kJ/mol. (7) The CO gas is obtained separated from the zinc and can be used directly as a fuel or through a reaction with steam to generate hydrogen and carbon dioxide, as follows: CO+H2O→H2+CO2, ΔH0 =−41.16 kJ/mol. (8) Eq. (7) is a gasification process that takes place very quickly due to the high temperature, the high heat and mass transfer between the FeO medium and the carbon, and the mixing created by the evolving gas. An additional advantage of this two-step process is the omission of the requirement for feed preparation. In a direct carbothermal reduction, this requires mixing of ZnO with carbon in a measured proportion and preparation of briquettes from the powder. When ZnO from zinc/air batteries is recycled, size reduction is usually required. In the present process, relatively large-size fractions can be reduced, due to the contact with the iron, and the high heat and mass transfer mechanisms. 2. Thermodynamic analysis The equilibrium compositions [6] as a function of the temperature for (6) and (7) are illustrated in Fig. 4 and Fig. 5, respectively [6]. These calculations demonstrate that above 1535 °C, the melting point of iron (the melting point of FeO is 1430 °C), the conversion of both reactions is complete. The energy balance of the entire process at 1900 K is summarized in Table 1.
- 22. 22 Fig. 4. Thermodynamic equilibrium compositions of the reduction of ZnO(s) by Fe(l) (Eq. (6)) [6]. Fig. 5. Thermodynamic equilibrium compositions of the recovery of Fe(l) from FeO(l) using solid carbon (Eq. (7)) [6]. Table 1. Energy balance for the entire cycle
- 23. 23 The solar energy could be used in steps 1–4 of this process. The total solar input in this example is: ΔHsolar=ΔH1+ΔH2+ΔH3+ΔH4=447 kJ/mol of ZnO. The theoretical total heat output in this process can be summarized as follows: ΔHheat out=ΔH5+ΔH6+ΔH7+ΔH8+ΔH9+ΔH10=826 kJ/mol of ZnO. Thus, the solar contribution is The enthalpy gain (Fgain) is defined as the theoretical total heat output divided by the enthalpy of Eq. (9) between the amount of carbon used in the process and oxygen (possible heat output without the process), as follows: C(s)+O2(g)→CO2(g), ΔH1900 K 0 =−396 kJ/mol of carbon. (9) In this case, Fgain will be 826/396=2.09. 3. Reduction of ZnO with Fe An alternative to the use of carbon as a reducing agent is the metallothermic reduction of ZnO with either solid or liquid iron (e.g. at 1538 °C—the melting point of Fe), according to the following reaction: ZnO(s)+Fe(s or l)→Zn(g)+FeO(s or l), ΔH1811 K 0 =208 kJ/mol. (10) This process has been receiving increasing attention recently, since electric arc furnace (EAF) dust contains significant amounts of zinc, mostly in the form of ZnO, which can be recovered. ZnO in the dust can react with either solid or liquid iron. The kinetics of the reduction of iron ZnO powder by solid iron powder, formed into cylindrical briquettes at the temperature range of 1073–1423 K, shows that the reaction is chemically controled [8] with activation energy of 230 kJ/mol. Once a product layer of zinc is formed, the reaction is limited by the diffusion of zinc gas away from the reaction interface. It was found [9] that at around 1400 °C, ZnO is reduced rapidly in the presence of an iron bath
- 24. 24 until the zinc concentration reaches about 3%, and subsequently reduction is slow. Stirring the iron bath increases the rate of reduction of zinc very significantly. 3.1. Experimental results of solar reduction of ZnO with Fe Experiments have been conducted at the WIS solar furnace. Two grams (0.03 mol) of ZnO powder and 4 g (0.07 mol) of Fe were heated under a flow of argon in an alumina tube (99.7% Al2O3, 22 cm length, 2.5 cm diameter), partially placed inside a cavity box with a side aperture diameter of 6 cm, through which the concentrated solar radiation enters, impinging directly on the walls of the lower part of the tube (Fig. 6). The average solar concentration at the aperture of the cavity was about 3000 and the input power to the cavity was 6 kW of heat. The top part of the tube was protruding from the cavity box. This part is equipped with a ‘cold finger’ condenser and a proper water-cooled sealing. The experiments have been conducted at 1473 K, corresponding to the solid–solid process, and at 1863 K, where the iron was in a liquid phase. In the first case, 0.5 g of zinc was condensed after about 5 min of heating, corresponding to about 30% of the theoretically possible yield. In the case of the high temperature, 1.5 g of zinc were condensed at the ‘cold finger’ after 5 min of heating (corresponding to about 90% yield). Powder X-ray diffraction analyses (Rigaku, Rotaflex RU200B with graphite monochromator and rotating anode, CuK -radiation) of the material settled on the ‘cold finger’ show that the zinc is of high purity and crystallinity (Fig. 7). The residues inside the alumina reactor tube were also analyzed and FeO, Fe and ZnO were found in most of the cases (Fig. 7). Table 1. Energy balance for the entire cycle The solar energy could be used in steps 1–4 of this process. The total solar input in this example is: ΔHsolar=ΔH1+ΔH2+ΔH3+ΔH4=447 kJ/mol of ZnO.
- 25. 25 The theoretical total heat output in this process can be summarized as follows: ΔHheat out=ΔH5+ΔH6+ΔH7+ΔH8+ΔH9+ΔH10=826 kJ/mol of ZnO. Thus, the solar contribution is The enthalpy gain (Fgain) is defined as the theoretical total heat output divided by the enthalpy of Eq. (9) between the amount of carbon used in the process and oxygen (possible heat output without the process), as follows: C(s)+O2(g)→CO2(g), ΔH1900 K 0 =−396 kJ/mol of carbon. (9) In this case, Fgain will be 826/396=2.09. 3. Reduction of ZnO with Fe An alternative to the use of carbon as a reducing agent is the metallothermic reduction of ZnO with either solid or liquid iron (e.g. at 1538 °C—the melting point of Fe), according to the following reaction: ZnO(s)+Fe(s or l)→Zn(g)+FeO(s or l), ΔH1811 K 0 =208 kJ/mol. (10) This process has been receiving increasing attention recently, since electric arc furnace (EAF) dust contains significant amounts of zinc, mostly in the form of ZnO, which can be recovered. ZnO in the dust can react with either solid or liquid iron. The kinetics of the reduction of iron ZnO powder by solid iron powder, formed into cylindrical briquettes at the temperature range of 1073–1423 K, shows that the reaction is chemically controled [8] with activation energy of 230 kJ/mol. Once a product layer of zinc is formed, the reaction is limited by the diffusion of zinc gas away from the reaction interface. It was found [9] that at around 1400 °C, ZnO is reduced rapidly in the presence of an iron bath until the zinc concentration reaches about 3%, and subsequently reduction is slow. Stirring the iron bath increases the rate of reduction of zinc very significantly. 3.1. Experimental results of solar reduction of ZnO with Fe Experiments have been conducted at the WIS solar furnace. Two grams (0.03 mol) of ZnO powder and 4 g (0.07 mol) of Fe were heated under a flow of argon in an alumina tube (99.7% Al2O3, 22 cm length, 2.5 cm diameter), partially placed inside a cavity box with a side aperture diameter of 6 cm, through which the concentrated solar radiation
- 26. 26 enters, impinging directly on the walls of the lower part of the tube (Fig. 6). The average solar concentration at the aperture of the cavity was about 3000 and the input power to the cavity was 6 kW of heat. The top part of the tube was protruding from the cavity box. This part is equipped with a ‘cold finger’ condenser and a proper water-cooled sealing. The experiments have been conducted at 1473 K, corresponding to the solid–solid process, and at 1863 K, where the iron was in a liquid phase. In the first case, 0.5 g of zinc was condensed after about 5 min of heating, corresponding to about 30% of the theoretically possible yield. In the case of the high temperature, 1.5 g of zinc were condensed at the ‘cold finger’ after 5 min of heating (corresponding to about 90% yield). Powder X-ray diffraction analyses (Rigaku, Rotaflex RU200B with graphite monochromator and rotating anode, CuK -radiation) of the material settled on the ‘cold finger’ show that the zinc is of high purity and crystallinity (Fig. 7). The residues inside the alumina reactor tube were also analyzed and FeO, Fe and ZnO were found in most of the cases (Fig. 7). Fig. 6. A scheme of the experimental solar cavity reactor used for the reduction of ZnO by iron.
- 27. 27 Fig. 7. Powder X-ray diffraction of the product material of the ZnO reduction obtained on the ‘cold finger’ and the residue in the alumina tube reactor. 4. Reduction of FeO with carbon The second step of the process is the reduction of FeO with carbon and recovery of the iron with CO as a product gas (Eq. (7)). The equilibrium compositions at different temperatures are shown in Fig. 5. Thermodynamically, at 1000 °C, 100% conversion of FeO to iron can be obtained. This process is widely used in the industry [10 and 11]. Industrial scale reduction of iron ore containing solid FeO and carbon monoxide takes place, according to the following equation: FeOn+nCO→Fe+nCO2. (11) Carbon monoxide is produced in a blast furnace from the reaction of oxygen in the blast air with hot coke and other reducing agents such as injected coal or oil, according to the following reaction: C+0.5O2→CO, ΔH1000 °C 0 =−113.7 kJ/mol. (12) Carbon monoxide is also produced from CO2, the product of Eq. (11), with carbon by the Boudouard reaction ( Eq. (5)). If FeO is present as a liquid, the main reaction that takes place is that given in Eq. (7). If coal is used as the reducing agent, the gases actually participating in the reduction process originate from the coal pyrolysis (i.e. hydrogen and hydrocarbons) and coal gasification. Coal also contains ash, moisture (10% typically, which reacts endothermally in the water–gas reaction) and sulfur. The kinetics and reaction mechanism of the reduction process of FeO, especially in slag, either in solid
- 28. 28 phase ore in a smelting reduction, are reported in the literature [12, 13 and 14]. The reduction of FeO can also be achieved with hydrogen, as follows: FeO+H2→Fe+H2O, (13) and by a mixture of hydrogen and carbon monoxide resulting, for example, from methane reforming or coal gasification processes. In the presence of CO, in addition to H2, (PCO2/PCO) and (PH2O/PH2), partial vapor pressure ratios are coupled at any given temperature by the gas phase equilibrium [10] as follows: (14) 5. Concept of a solar reactor The beam-down optics of a solar tower [15 and 16] can be used to provide concentrated solar radiation in a ground reactor containing a liquid matter from its ceiling through a terminal concentrator (known as compound parabolic concentrator—CPC). The concept of a solar reactor is conceived in Fig. 8. A central compartment is used for the reduction of ZnO and production of zinc. ZnO powder is injected into the iron melt creating a liquid layer of FeO, which floats on the iron layer. The zinc vapors, the only product in this compartment, exit at the top of this compartment directly to a condenser where the vapors first cooled from the reaction temperature (1500 °C typically) to about 907 °C, condensed to zinc liquid, and the liquid is further cooled to about 500 °C. If the final goal of the zinc is to produce hydrogen via reaction with steam [5], the most suitable condition of the zinc is a liquid phase at around 500 °C. Part of the zinc produced during the day can be stored as a liquid for the night, when production of hydrogen can take place. This results in a chemical storage of solar energy. )
- 29. 29 Fig. 8. Schematic concept of a solar reactor for the two-step reduction of ZnO(s) with Fe(l). The central compartment is surrounded by peripheral compartments (i.e. six hexagonal peripheral compartments surrounding a central hexagon) (Fig. 8), where the reduction of FeO(l) to Fe(l) takes place. There are openings at a certain height in the compartments through which FeO(l) flows gravitationally to the peripheral compartments and at the bottom, Fe(l) flows back to the center. The reduction of FeO(l) can be executed by the injection of carbon, charcoal or coal into the FeO(l) layer. If carbon is used, the main product is CO(g). CO is cooled and can be converted in a conventional process to CO2 and H2, according to Eq. (8). CO2 can be separated, recycled and used to inject the carbon into the reactor. At the temperature of the reactor, CO2 reacts with the solid carbon to produce CO, according to the Boudouard reaction [17]. According to Eq. (5), the enthalpy of this endothermic reaction, which equals ΔH0 =172.5 kJ/mol, can be obtained from the sun, resulting in CO2 gasification of the carbonaceous material. Both C(s) and CO(g) can reduce FeO(l) and the product gas will be a mixture of CO and CO2. Instead or in addition to the recycled CO2, steam can be injected as well and used to feed the carbon. In this case, the carbon is gasified according to Eq. (15) [17], and both CO and H2 can be used to reduce FeO as follows: C(s)+H2O(g) CO(g)+H2(g), ΔH0 =131.4 kJ/mol. (15) Finally, the peripheral compartments can be equipped with means to inject oxygen in case solar energy is not available to avoid freezing of the Fe and FeO melts. The oxygen reacts with the carbon in an exothermic reaction to provide heat for maintaining the temperature of the bath and for reduction of FeO(l), if necessary, according to Eq. (12).
- 30. 30 6. Conclusions The use of iron as a reducing agent for ZnO may have an interesting alternative to the direct reduction by carbon or hydrocarbon. The main advantages are the pure zinc that is obtained and the possibility to refrain from a complicated and expensive quencher and separator required in the direct carboreduction process, such as the lead splash condenser. The reaction rates of the reduction of solid ZnO by liquid iron are very high and this phenomenon was demonstrated by the use of concentrated solar energy. In addition, even solid iron can be used as a reducing agent at temperature ranges of 1100–1300 °C, with lower conversion rates. Containment materials of construction for a large-scale solar reactor can be adopted from the vast experience of the iron production industry. The newly developed beam-down optics for a solar tower plant can be specifically suitable for this application. The concept for the solar reactor revealed in this paper enables a good practice for the maintenance of the liquid iron/iron oxide bath at a constant temperature during periods of solar radiation fluctuations, clouds and night hours, by injection of oxygen when needed.