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The use of concentrated solar energy as the source of process heat avoids emissions of greenhouse gases and other pollutants derived from the combusti...
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Environ. Sci. Technol. 2000, 34, 4177-4184

Recycling of Hazardous Solid Waste Material Using High-Temperature Solar Process Heat. 1. Thermodynamic Analysis BEATRICE SCHAFFNER Solar Process Technology, Paul Scherrer Institute, CH-5232 Villigen, Switzerland WOLFGANG HOFFELNER,HAIYAN SUN MGC-Plasma AG, CH-4132 Muttenz, Switzerland ALDO STEINFELD* Institute of Energy Technology, Department of Mechanical and Process Engineering, ETH - Swiss Federal Institute of Technology, ETH-Zentrum, CH-8092 Zurich, Switzerland and Solar Process Technology, Paul Scherrer Institute, CH-5232 Villigen, Switzerland

The thermochemical conversion and recycling of hazardous solid waste materials is investigated using high-temperature solar process heat. Two important sources of wastes contaminated with heavy metal oxides are considered: (1) electric arc furnace dust (EAFD) and (2) automobile shredder residue (ASR). The chemical equilibrium composition of these complex materials and the energy required to process them, using carbon, methane, or pyro-coke as reducing agents, are computed for temperatures in the range 300-2000 K. Metals can be extracted from their oxides in reducing atmospheres at above 1300 K for both EAFD and ASR: Zn is obtained in the gas phase, while Fe, Pb, and Cu are obtained in the condensed phase. The thermal energy requirements for converting EAFD at 1500 K are 3008 kJ/ kg and 4143 kJ/kg using C(gr) and CH4 as reducing agents, respectively. For converting ASR at 1500 K, 2455 kJ/kg are required. The solar exergy conversion efficiency, i.e., the efficiency of converting solar energy into the chemical energy of the reaction products (given by the Gibbs free energy change of product oxidation), can be as high as 69% for the EAFD conversion and 87% for the ASR conversion. Major sources of irreversibilities are those associated with the reradiation losses of the solar reactor and the heat rejected during the quenching. The use of concentrated solar energy as the source of process heat avoids emissions of greenhouse gases and other pollutants derived from the combustion of fossil fuels and further offers the possibility of converting waste materials into valuable commodities for processes in closed and sustainable materials cycles.

1. Introduction Solid waste materials, derived from a wide variety of sources, e.g. municipal waste incineration residuals, discharged batteries, dirty scraps, contaminated soil, sludge, and dusts, * Corresponding author fax: [email protected]. 10.1021/es0000495 CCC: $19.00 Published on Web 08/31/2000

+41-56-3103160;

 2000 American Chemical Society

e-mail:

and other by-products from the metallurgical industry, contain hazardous compounds that cannot be discharged to the environment. They are usually vitrified in a non-leaching slag and finally disposed of in hazardous waste storage and landfill sites, where they need to be continuously monitored (1). However, limited storage space, increasing storage costs, and environmental regulations have urged the need for technologies that recycle these toxic materials into useful commodities rather than deposit them in dump sites for an undetermined period of time. The chemical transformation of these materials into elemental components offers the possibility of converting waste materials into valuable feedstock for processes in closed materials cycles (2). High-temperature thermal processes are well suited for the treatment and conversion of complex solid waste materials. However, combustion processes are not suitable for recycling of metal-containing wastes because of the need for well-controlled reducing atmospheres. Waste materials containing carbonaceous compounds can be converted by thermal pyrolysis and gasification into synthesis gas and hydrocarbons that can be further processed into hydrogen, ammonia, methanol, Fischer-Tropsch’s chemicals, and other valuable synthetic chemicals. Waste materials containing metal oxides can be converted by carbothermal reduction into metals, nitrides, carbides, and other metallic compounds. Thus, the chemical products from such transformations can be used as feedstock for a variety of manufacturing processes or can be used as fuels. Materials recycling requires high-temperature energyintensive processes. The current commercial recycling techniques by blast, induction, arc, and plasma furnaces are major consumers of electricity and high-temperature process heat and, consequently, major contributors of greenhouse gas emissions and other pollutants (3, 4). The magnitude of the energy consumption and pollution released depends on the feedstock composition, which varies widely in the recycling business. For example, electric arc furnace dust collected during the treatment of galvanized steel alloys (representing over 50% of all zinc applications) can contain up to 75 wt % ZnO, while municipal solid waste fly ash contains about 4 wt % ZnO. A recent life cycle assessment on the electrolytic extraction of zinc from ZnO indicates that, assuming an European electricity mix with a 15% share of renewables and a 32% share of nuclear, total greenhouse gas emissions are 2 kg of CO2-equivalent per kg zinc, derived mainly from the electricity consumption (5). For coal-firedbased electricity, emissions can be as high as 10 kg CO2equivalent per kg zinc (6). These emissions can be significantly reduced or even completely eliminated by using a clean source of hightemperature process heat. Concentrated solar radiation can supply clean thermal energy at temperatures exceeding 1500 K for driving these endothermic processes. The advantages are 2-fold: (1) greenhouse gases and other pollutants derived from the combustion of fossil fuels are avoided; and (2) the chemical products are valuable feedstock for processes in closed materials cycles. The chemical products can also be used as fuels with a solar-upgraded energetic value because the solar input increases their energy content. The use of solar process heat for the recycling of waste materials is further justified by thermodynamic arguments. Conventional fossil-fuel-based recycling furnaces are unable to add only the thermodynamically required work due to process inefficiencies or the need to add heat at high VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 1: Approximate Temperatures at Which ∆G°rxn Equals Zero for Reactions 1-3 and the Corresponding ∆H°rxn When the Reactants Are Fed at 298 K and the Products Are Obtained at the Temperature Indicated redox paira

T|∆Grxn(1))0 [K]

∆H°rxn(1) [kJ/mol]

T|∆Grxn(2))0 [K]

∆H°rxn(2) [kJ/mol]

T|∆Grxn(3))0 [K]

∆H°rxn(3) [kJ/mol]

ZnO/Zn Fe2O3/Fe PbO/Pb Cr2O3/Cr MgO/Mg CdO/Cd Cu2O/Cu

2350 3060 2720 >3500 >3500 1830 2900

559 1232 338

1230 930 580 1540 2120 780 370

419 593 124 1008 736 183 66

1120 890 660 1260 1760 800 540

537 914 229 1121 880 288 168

a

429 405

Some metal oxides are reduced to lower valence oxides before complete reduction to the metal.

temperatures. Since electric power must usually be obtained from a high-temperature thermal reservoir created by the combustion of fossil fuels, it is thermodynamically wasteful to use electrical energy, or the equivalent Gibbs free energy of a reducing agent, in excess of what is required by ∆G of the reaction in order to compensate for the needed additional process heat. Frequently that is the case in many commercial furnaces with the result that the energy cost is a substantial portion of the value of the final product. Higher electricity consumption means also higher CO2 emissions from the process. This excess use of electricity and fuels is particularly observed in conventional processes that involve the reduction of metal oxides. The Gibbs free energies of formation of many stable metallic oxides, such as ZnO, Fe3O4, MnO2, MgO, CdO, SiO2, and TiO2, are large negative numbers that decrease in magnitude with temperature, whereas their enthalpy of formation remains relatively independent of temperature (7-9). Therefore, the ratio of electric to thermal energy required for their reduction decreases as the temperature is increased. Any endothermic process with these characteristics is an attractive candidate for the use of concentrated solar energy. Examples of metal oxides reduction processes that have been studied experimentally in solar furnaces include the production of Fe, Al, Mg, Zn, TiC, SiC, CaC2, TiN, Si3N4, AlN, and other metallic nitrides and carbides by carbothermal and CH4-thermal reduction of their oxides in Ar or N2 atmospheres (10-14). Preliminary feasibility tests with aluminium melts and complex inorganic mixtures were also conducted in a solar furnace with a rotary-kiln solar reactor (15). When the raw waste materials contain organic and other carbonaceous compounds, such as residuals from shredded cars or construction wood, their thermochemical conversion into fluid fuels using solar process heat also offers significant advantages over the conventional combustion routes. Incineration of these kinds of hazardous wastes creates heavily contaminated residual ashes and off-gas dusts that hinder operation, lower efficiency, and increase the cost of disposal (16-18). In contrast, for the solar process, the C-containing materials are not burned and consequently the reaction products are not contaminated by the products of combustion (CO2, NOx, etc.). Furthermore, the fluid fuels resulting from the thermochemical conversion have a solar-upgraded calorific value and therefore are cleaner fuels. An alternative approach could be to generate solar electricity and to operate a conventional plasma, induction, or electrolytic furnace. However, in view of the low energy conversion efficiency from solar to electricity and finally to process heat, and because of the intrinsic thermodynamic advantages of using high-temperature solar heat for this application, the direct solar thermochemical approach was selected. In the present study two important sources of hazardous waste are considered: EAFD (electric arc furnace dust) and ASR (automobile shredder residue). About 1.6 million tons of EAFD are estimated to be produced annually from scrap melting in North America, Western Europe, and Japan (2). 4178

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This amount represents typically 1.5-2 wt % of the hot metal production. About 5 million tons of ASR, a fluffy residue derived from shredded automobiles, are estimated to be produced annually in Europe (19). Both EAFD and ASR contain complex mixtures of metal oxides; the latter also contains a significant portion of carbonaceous compounds. This paper describes a sustainable approach to the recycling of waste materials using solar energy. We present a thermodynamic study of the recycling process of EAFD and ASR to assist the reactor engineering design. Chemical equilibrium compositions are computed over a wide range of temperatures for the carbothermal and CH4-thermal reduction processes. A second law analysis is conducted to determine the maximum solar exergy conversion efficiency [efficiency of converting solar energy into the chemical energy of the reaction products (given by the Gibbs free energy change of product oxidation), definition follows in eq 6] and to identify the major sources of irreversibility. This information determines the constrains to be imposed on the design and operation of the solar chemical reactor.

2. Chemical Thermodynamics The thermal dissociation of metal oxides (in the absence of a reducing agent), the carbothermal reduction of metal oxides (using C(gr) as reducing agent), and the CH4-thermal reduction of metal oxides (using CH4 as reducing agent) are complex processes, but the overall reactions can be represented by

MxOy ) xM + 0.5yO2

(1)

MxOy + yC(gr) ) xM + yCO

(2)

MxOy + yCH4 ) xM + y(2H2 + CO)

(3)

respectively, where M denotes the metal and MxOy denotes the corresponding metal oxide. Thermodynamics predict that, at temperatures where the Gibbs free energy change of the reaction (∆G) is negative, reactions 1-3 proceed spontaneously to the right by supplying heat in an amount equal to the enthalpy change of the reaction (∆H). Except at the phase transitions, the enthalpy change for most metal oxides remains almost independent of temperature, whereas the Gibbs free energy change decreases with temperature. The temperatures at which ∆G°rxn of reactions 1-3 equals zero, T|∆Grxn)0, are listed in Table 1, for the relevant metal oxide red-ox pairs. Also included is the corresponding ∆H°rxn when reactants are fed at 298 K and the products are obtained at T|∆Grxn)0, i.e., ∆H°rxn includes both the heat of the reaction and the sensible and latent heat required for bringing the reactants to T|∆Grxn)0. Temperatures exceeding 2000 K are needed for the thermal dissociation. In the presence of carbon, the uptake of oxygen by the formation of CO (and CO2) brings about the reduction of the oxides at much lower temperatures. For example, the carbothermal reduction of

TABLE 2: Main Metal Elemental Composition of EAFD and ASR element

EAFD [mass %]

ASR [mass %]

Zn Fe Pb Cu Cd Cr Cl S Si alkaline earth elements C

37.8 13.5 10.1 0.23 0.09 0.12 4.8 0.6 1.7 0-5 not determined

2.0 14.0 0.8 2.5 0.02 0.18 1.6 0.4 5 0-10 38

ZnO proceeds at above about 1230 K and is endothermic by 419 kJ/mol; the CH4-thermal reductions of ZnO proceeds at above about 1120 K and is endothermic by 537 kJ/mol. Typical elemental compositions of EAFD and ASR are shown in Table 2. Main metals in EAFD are Zn, Fe, and Pb; present in lower quantities are Si, Ca, Mg, and Cl. Main components in ASR are carbonaceous compounds (represented as carbon), alkaline earth elements, Si and Fe, and in lower quantities are Zn, Pb, Cu, and Cd (19). The zinc and lead content in the EAFD is due principally to the galvanized scrap, while the Cu content in ASR is due principally to the electronic scrap. In both EAFD and ASR, metals are present primarily as oxides. The chemical equilibrium composition of the systems EAFD and ASR was computed over the temperature range 300-2000 K using the code HSC Outokumpu (20). In the calculation model, the existence of liquid and solid solutions was excluded from consideration. These are of course important, but we believe their exclusion does not substantially affect the main conclusions that concern energy balances and energy efficiencies. Phases that were thermodynamically stable but are known not to be formed in the conventional furnaces because of kinetic limitations were also excluded. Representative species compositions were used; for 1 kg EAFD: 3.46 mol ZnO, 0.91 mol ZnFe2O4, and 0.12 mol Pb3O4; for 1 kg ASR: 29 mol C, 3 mol H2O, 1.66 mol SiO2, 1.18 mol Fe2O3, 0.39 mol CuO, 0.37 mol Al2O3, 0.31 mol ZnO, 0.05 mol Cl2, 0.02 mol Pb3O4. Oxides of alkaline earth elements, such as MgO and CaO, are inert up to 1800 K and have been excluded for simplicity. Results of the chemical equilibrium calculations are given in Figures 1(a)-(d) and 2(a)-(c). Species with less than 10-5 mol have been omitted from the graphs. Figure 1(a) shows the equilibrium composition as a function of temperature for EAFD when C(gr) is used as the reducing agent and SiO2 as the main vitrification agent. The amount of carbon added is slightly in excess of the stoichiometric amount given by eq 2. Already at quite moderate temperatures, above about 700 K, lead oxide is carbothermally reduced to lead. In the range 600-1200 K, hematite is reduced to the lower-valence iron oxides, magnetite and wustite, before complete reduction to iron. Zinc carbonate is thermodynamically stable below 500 K, but it will decompose to ZnO at higher temperatures and release CO2. Over the temperature range between 1000 and 1300 K, ZnO is reduced to Zn, both liquid and gas. These reductions produce mixtures of CO and CO2 in the gas phase which react with carbon according to C(gr) + CO2 ) 2CO. The equilibrium composition of the carbon-oxygen system, also known as the Boudouard equilibrium, depends strongly on the temperature as well as on the carbon/oxygen ratio. With excess carbon present, as is the case in this system, CO2 is the stabler of the two oxides of carbon at low temperatures. At the higher temperatures at which FeO and ZnO are reduced, CO is the stabler species. This shift in the equilibrium as the temper-

ature increases is clearly seen in Figure 2(a), which shows the partial pressure of gases in chemical equilibrium. At temperatures below 900 K, CO2 is the main gas component; at above 900 K, the equilibrium is shifted to CO. At 1500 K, the equilibrium composition consists principally of Zn(g) and CO(g) in the gas phase and Fe, Pb, and SiO2 in the condensed phase. Figures 1(b) and 2(b) show the equilibrium composition and partial pressures as a function of temperature for EAFD using methane as the reducing agent and SiO2 as the main vitrification agent. The amount of methane added is slightly in excess of the stoichiometric amount given by eq 3. Similar to the case shown in Figure 1(a), iron, lead, and zinc oxides are completely reduced to the metals above 1300 K. However, the system is complicated by the fact that the dissociation of CH4 to C(gr) and H2 is thermodynamically favorable at above about 800 K. The kinetics of solid carbon deposition is heterogeneous in nature and requires the nucleation of carbon on some catalytic site. However, the presence of freshly formed iron might catalyze the cracking reaction. At 1300 K, the equilibrium composition consists principally of CO, H2, and Zn(g) in the gas phase. As seen from Figure 2(b), the molar ratio of H2 to CO is approximately equal to 2, which corresponds to the syngas quality required for methanol synthesis. The condensed phase is similar to the one obtained when using C(gr) as the reducing agent. The equilibrium composition of ASR, using the carbon contained in the carbonaceous materials as the reducing agent in the form of pyro-coke is shown in Figure 1 parts (c) and (d), where in 1 (d) the species present in low concentrations are plotted. Pyro-coke refers to the carbon-rich residuals derived from the pyrolysis of carbonaceous materials. The amount of this carbon is in excess of the stoichiometric required carbon for conducting reaction 2. Similar to the reduction of EAFD, hematite is reduced to magnetite, wustite, and iron over the temperature range 600-1200 K. CuO and PbO are already reduced at moderate temperatures, while ZnO starts to be reduced above 1000 K. Complications arise because of the presence of water and its reaction with carbon to produce H2 and CO, which in turn can further act as reducing agents of metal oxides or react to yield CO2 and CH4. The methanation reaction is known to proceed catalytically below 800 K, and the presence of Cu and other metals may act as catalysts. For clarity, the gas phase is plotted in Figure 2(c). CO2, CH4, and H2O are the main gas components at below about 800 K. Over the temperature range 800-1200 K, the main chemical transformations with metal oxides occur. At 1300 K, the equilibrium composition consists principally of CO, H2, and Zn(g) in the gas phase and Fe, Pb, Cu, Zn(l), SiO2, and Al2SiO5 in the condensed phase. Silicon carbide (SiC) and other carbides can be formed at temperatures above 1700 K, and when N2 is used as carrier gas to sweep out product gases, Si3N4 and other metallic nitrides can also be formed. In general, it can be observed that metals can be extracted from their oxides in reducing atmospheres for both EAFD and ASR. Zn is obtained in the gas phase and can be separated from the gas mixture by distillation, using for example leadspray or zinc-splash condensers that are conventionally employed in the Imperial Smelting and other furnaces (21, 22). Fe, Pb, and Cu are obtained in the condensed phase. Pb may be distilled as a gas above its boiling point 2023 K; the separation of Fe and Cu may be hindered by the formation of alloys. The separation of products, both in the gas and condensed phases, require post-processing and refining processes that may have implications on the reactor design and its operating temperature. At this same level of thinking about the chemical thermodynamics, one recognizes that the final composition of the system will depend on the chemical kinetics. The VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Chemical equilibrium composition as a function of temperature for (a) the carbothermal reduction of EAFD, (b) the CH4-thermal reduction of EAFD, and (c) and (d) the carbothermal reduction ASR. Species with mole fraction less than 10-5 moles are omitted. (Note: the ordinate in Figure 2(d) is shown in a different scale.)

FIGURE 2. Partial pressures of gases in chemical equilibrium as a function of temperature for (a) the carbothermal reduction of EAFD, (b) the CH4-thermal reduction of EAFD, and (c) the casbothermal reduction of ASR. Species with mole fraction less than 10-5 mol are omitted. chemical kinetics are complex because of the complexity of these heterogeneous chemical systems. They will depend on the intrinsic chemical reaction rates, on whether condensed phase solutions are formed, and on the rates of heat and

mass transfer, which in turn depend on the reactor design. In addition, reverse reactions such as re-oxidation of metals may affect the process efficiency but are unlikely to occur when metals are produced in reducing atmospheres of CO VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 3: Standard Enthalpy Changes for the Reaction ZnO + C(gr) @ 298 K f Zn + CO(g) @ 298 K step 1 2 3 4

heat ZnO (∫TT12CpdT) (∫TT12CpdT)

heat C(gr) enthalpy change of reaction: ZnO + C(gr) ) Zn(g) + CO(g) cool CO(g) cool Zn phase transformation: Zn(g) f Zn(l) phase transformation: Zn(l) f Zn(s)

temp [K]

∆ H° [kJ/mol]

298-1500

61

298-1500 1500

23 350

1500-298 1500-298 1180 692

-38 -43 -115 -7

TABLE 4: Required Thermal Energy for the Conversion of EAFD and ASRa waste material (and reducing agent) EAFD (carbon) EAFD (methane) ASR (carbon)

energy requirement [kJ/kg] 3008 4143 2455

a Carbon and methane are used as reducing agents of EAFD; carbon contained in the carbonaceous materials of ASR is used as the reducing agent of ASR. Reactant species compositions are the same as for Figure 1. Product compositions are the equilibrium compositions at 1500 K given in Figure 1. For all three cases, the reactants are fed at 298 K, and the products are obtained at 1500 K.

and H2 (with little CO2), and products are rapidly quenched. Both the thermodynamics and kinetics of the chemical system determine the boundary conditions for the reactor design and define the operating parameters under which the reactor must function.

3. Energy Balance The input and output of thermal energy for effecting the transformations can be divided into four steps: (1) sensible heat required to heat the reactants from 298 K up to the reaction temperature; (2) enthalpy change of the reaction at the reaction temperature; (3) sensible heat removed to cool the products from reaction temperature to 298 K; and (4) enthalpy change of phase transformations. As an example, Table 3 shows the energy balance for the carbothermal reduction of ZnO at 1500 K. The total energy required is 434 kJ/mol. If the sensible and latent heat of the hot products is recovered, the total energy requirement is reduced to 231 kJ/mol. In practice, only a portion of the energy may be recovered. Table 4 gives the required process heat for the conversion of 1 kg of EAFD using carbon and methane as reducing agents and of 1 kg of ASR using the carbon contained in its carbonaceous compounds as the reducing agent. The species compositions of the reactants are the same as for Figure 1. For all three cases, the reactants are fed at 298 K, and the products are obtained at 1500 K. The compositions of the products are the equilibrium compositions at 1500 K given in Figure 1.

4. Second Law Analysis: Solar Exergy Conversion Efficiency Solar chemical reactors for highly concentrated solar systems usually feature the use of a cavity-receiver type configuration, i.e., a well-insulated enclosure designed to effectively capture incident solar radiation entering through a small opening, the aperture. At temperatures above 1000 K, the net power absorbed is diminished mostly by radiative losses through the aperture. The solar energy absorption efficiency of a solar 4182

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reactor, ηabsorption, is defined as the net rate at which energy is being absorbed divided by the solar power coming from the concentrator. For a perfectly insulated blackbody cavityreceiver (no convection or conduction heat losses), it is given by (23, 24)

ηabsorption ) 1 -

( ) σT4 IC

(4)

where C is the flux concentration ratio of the solar concentrating system defined as the ratio of the solar flux intensity achieved after concentration to the incident normal beam insolation, T is the nominal cavity-receiver temperature, and σ is the Stefan-Boltzmann constant. ηabsorption expresses the capability of a solar reactor to absorb incoming concentrated solar energy but does not include the losses incurred in collecting and concentrating solar energy. Losses in power and concentration depend on the power level and on whether parabolic troughs, power towers, or distributed dishes are used (29) and are due to geometrical imperfections (such as misalignment and segmented approximation to the exact reflector profile), optical imperfections (such as reflectivity and specularity of the mirrors, glass absorption, and shading effects), and tracking imperfections. Since the net energy absorbed should match the enthalpy change of the reaction, the total solar energy required is

Qsolar )

∆Hrxn ηabsorption

(5)

The measure of how well the solar energy can be converted into chemical energy for the given processes is the solar exergy efficiency, ηexergy, defined as

ηexergy )

-∆Gfuel cell Qreactants + Qsolar

(6)

where Qreactants is the heating value of the reactants (i.e., the enthalpy change for their complete oxidation), Qsolar is the total solar energy input given by eq 5, and ∆Gfuel-cell is the maximum possible amount of work that may be extracted from the products at ambient temperature (i.e., the Gibbs free energy change for their complete oxidation), also known as the exergy of the products. To help apply this term, one can think of solar processes that lead to fuels as ideal cyclic processes such as that shown in Figure 3, depicted for the solar carbothermal recycling of EAFD. It uses a solar reactor, a quenching device, and a fuel cell. The complete process is carried out at constant pressure. In practice, pressure drops will occur throughout the system. If one assumes, however, frictionless operating conditions, no pumping work is required. The reactants may be pre-heated in an adiabatic heat exchanger where some portion of the sensible and latent heat of the products is transferred to the reactants; however, for simplicity, a heat exchanger has been omitted. The reactor is assumed to be a perfect blackbody cavity-receiver, so that eq 4 applies for calculating its solar energy absorption efficiency. The reactant species compositions are the same as for Figure 1(a), which uses a representative composition of EAFD, carbon as the reducing agent, and SiO2 as the main vitrification agent. The reactants enter the solar reactor at 298 K and are further heated to the reactor temperature at 1500 K. Chemical equilibrium is assumed to be achieved inside the reactor. The irreversibility in the solar reactor arises from the non-reversible chemical transformation, the heat

FIGURE 3. Schematic of an ideal cyclic process for the solar carbothermal reduction of EAFD. Concentrated solar energy is the source of high-temperature process heat. Reactants (composition similar as in Figure 1(a)) are fed at 298 K. The solar reaction is assumed a perfect blackbody cavity-receiver operating at 1500 K and under a solar flux concentration ratio of 2000. Products exit the reactor having the chemical equilibrium composition at 1500 K given in Figure 1(a) and further undergo quenching to 298 K. Fuels contained in the products are sent to an ideal fuel cell for the purpose of calculating the exergy of the products. Oxidized solid fuels and waste materials derived from consumer goods are recycled to the solar reactor.

TABLE 5: Exergy Efficiency for the Three Solar Recycling Processes under Consideration: the Carbothermal Recycling of EAFD Using the Chemical Compositions of Figure 1(a), the CH4-Thermal Recycling of EAFD Using the Chemical Compositions of Figure 1(b), and the Carbothermal Recycling of ASR Using the Chemical Compositions of Figure 1(c),(d)a EAFD

reducing agents Qreactants (heating value of reactants) fuels in products ∆Gfuel cell (exergy of products) ∆Hrxn ηabsorption

Qsolar ηexergy a

C ) 2000 C ) 5000 C ) 2000 C ) 5000 C ) 2000 C ) 5000

ASR

reaction 2

reaction 3

reaction 2

C 2975 kJ C, CO, Zn, Fe, Pb -4107 kJ 3008 kJ

CH4 6731 kJ H2, CO, CH4, C, Zn, Fe, Pb -7697 kJ 4143 kJ 0.857 0.943 4834 kJ 4393 kJ 0.67 0.69

C (pyro-coke) 11412 kJ C, CO, H2, Fe, CH4, Cu, Zn, Pb -12225 kJ 2455 kJ

3509 kJ 3189 kJ 0.63 0.67

2865 kJ 2603 kJ 0.86 0.87

The parameter is the solar flux concentration ratio, C, assumed to be 2000 and 5000. All values are normalized for 1 kg reactants.

transfer from the sun at 5800 K to the reactor, and reradiation losses to the surroundings

solar reactor’s irreversibility [kJ‚K-1] )

(

)

(

)

Qsolar + 5800 K

Qreradiation + (∆S|Reactants @298K f Products @1500K) 300 K

where Qreradiation is the radiation heat loss by the reactor at 1500 K to the surroundings at 298 K, and Qreradiation ) (1ηabsorption)‚Qsolar. The products exit the reactor having the chemical equilibrium composition at 1500 K given in Figure 1(a) and further undergo rapid cooling to 298 K. It is assumed that the chemical composition of the products remains unchanged upon cooling in the quencher, except for the phase changes. After quenching, the fuels contained in the products are sent to an ideal fuel cell for the purpose of

calculating the exergy of the products, ∆Gfuel-cell. A similar scheme to the one shown in Figure 3 can be used to calculate the exergy efficiency of the solar CH4-thermal recycling of EAFD and the solar carbothermal recycling of ASR. Table 5 is a numerical description of the components shown in Figure 3, for the three recycling processes under consideration: (1) the carbothermal recycling of EAFD, using the chemical compositions of Figure 1(a); (2) the CH4-thermal recycling of EAFD, using the chemical compositions of Figure 1(b); and (3) the carbothermal recycling of ASR, using the chemical compositions of Figure 1(c),(d). The calculations are performed for two cases of solar flux concentration ratio: 2000, which can be easily achieved in present large-scale solar concentrating systems, and 5000, which may be achieved in advanced large-scale solar concentrating systems with the help of secondary CPC (compound parabolic concentrators) (25). These advanced systems are also being developed using VOL. 34, NO. 19, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Cassegrain optics to redirect sunlight onto a solar receiver located on the ground level (26). For a solar concentration varying in the range 2000-5000, maximum exergy efficiencies vary between 63 and 67% for the carbothermal reduction of EAFD, between 67 and 69% for the CH4-thermal reduction of EAFD, and between 86 and 87% for the carbothermal reduction of ASR. Major sources of irreversibilities are those associated with the reradiation losses of the solar reactor and the heat rejected during the quenching. Evidently, the exergy efficiency increases with the solar flux concentration ratio because of the lower reradiation losses. However, the increase is not significant and does not necessarily justify the use of a more expensive solar concentrating system. A solar flux concentration of 2000 corresponds to a maximum steady-state temperature of 2437 K and it seems to be appropriate for processes operating at 1300 K, as long as the conduction and convection losses are kept to low levels. The maximum steady-state temperature is the highest temperature an ideal blackbody solar cavityreceiver is capable of achieving when solar energy is being reradiated as fast as it is absorbed. It is given by (I‚C/σ)0.25(27). For higher operating temperatures, higher solar flux concentration ratios are necessary. The second law analysis indicates that a favorable aspect of using solar energy at high temperatures is the potential of achieving high exergy conversion efficiencies. High efficiencies directly translate to lower solar collection area and associated costs of the heliostat field, which amount to 4050% of the capital cost for the entire solar recycling plant. Under these assumptions, a recent economic assessment for a large-scale recycling plant indicates that the solar thermal extraction of zinc from ZnO is competitive with other renewable technologies, such as electrolytic processes using solar electricity (28). Besides zinc, the recovery of other metals from waste materials might also become cost competitive when the negative value of the raw materials and the avoidance of their storage costs are considered. Especially for highly contaminated waste materials containing high concentration of heavy metals, the solar thermal process might become cost competitive with conventional fossilfuel-based processes at current fossil fuel prices even before the application of governmental subsidies and/or credit for pollution avoidance. In general, it can be stated that the solar thermal technology for recycling waste materials is a favorable medium to long-term prospect provided the cost of energy will account for the environmental externalities from fossil fuel burning such as the costs for CO2 mitigation and other pollution abatement measures.

Acknowledgments Financial support by the BfE-Swiss Federal Office of Energy is gratefully acknowledged.

Literature Cited (1) Schweizerischer Bundesrat. Technische Verordnung u ¨ ber Abfa¨lle vom 10. Dezember 1990; Bundeskanzlei: Bern, Switzerland, 1991.

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Received for review March 7, 2000. Revised manuscript received June 26, 2000. Accepted July 3, 2000. ES0000495