Environ. Sci. Technol. 2003, 37, 165-170
Recycling of Hazardous Solid Waste Material Using High-Temperature Solar Process Heat. 2. Reactor Design and Experimentation BEATRICE SCHAFFNER, ANTON MEIER, AND DANIEL WUILLEMIN Solar Process Technology, Paul Scherrer Institute, CH-5232 Villigen, Switzerland WOLFGANG HOFFELNER RWH Consult GmbH, CH-5452 Oberrohrdorf, Switzerland
3000 kJ kg-1, discharging at least 0.7 kg of CO2/kg of EAFD when coke serves as both the reducing agent and the primary energy source. In practice, emissions are much higher because of inefficiencies associated with heat transfer. Alternatively, the use of concentrated solar energy as the source of process heat avoids greenhouse gas emissions and other pollutants derived from the combustion of fossil fuels and further offers the possibility of converting hazardous solid waste material into valuable commodities for processes in closed and sustainable material cycles. This paper describes the design, fabrication, and testing of a novel solar chemical reactor for recycling EAFD. The main objective of the proposed process is the extraction of metals, especially Zn and Pb, by carbothermic reduction of their metal oxides using solar process heat.
ALDO STEINFELD*
Chemical Aspects
Institute of Energy Technology, Department of Mechanical and Process Engineering, ETHsSwiss Federal Institute of Technology, CH-8092 Zurich, Switzerland
The typical elemental composition of EAFD is shown in Table 1. Main metals are Zn, Fe, and Pb that are present primarily in their oxidized state. Thermodynamic equilibrium computations for the carbothermic reduction of EAFD, when C(gr) is used as the reducing agent and SiO2 is employed as the main vitrification agent, indicate that the chemical equilibrium composition of the system at 1500 K consists principally of Zn(g) and CO in the gas phase and Fe, Pb, and SiO2 in the condensed phase. Silicon carbide (SiC) and other carbides can be formed at temperatures above 1700 K, and when N2 is used as a carrier gas to sweep out product gases, Si3N4 and other metallic nitrides can be formed (ref 1 and literature cited therein). The information on the chemical equilibrium composition, chemical kinetics, and energy requirements determines the constraints to be imposed on the design and the operation of the solar chemical reactor. The reactor should be able to operate at temperatures above about 1300 K under a reducing atmosphere. It should also be able to handle gaseous, liquid, and solid products. The pressure inside the reactor should be kept below atmospheric pressure for avoiding leaks of toxic gases. The residence time of the reactants should be controllable and adjusted to match the rate of the reaction, which in turned is coupled to the rate of heat transport. Since Zn and Pb are formed in the gas phase, a technique for condensing and separating the metals and for preventing their reoxidation should be implemented downstream. Finally, the reactor should be able to absorb efficiently the incoming solar radiation.
A novel high-temperature solar chemical reactor is proposed for the thermal recycling of hazardous solid waste material using concentrated solar power. It features two cavities in series, with the inner one functioning as the solar absorber and the outer one functioning as the reaction chamber. The solar reactor can handle thermochemical processes at temperatures above 1300 K involving multiphases and controlled atmospheres. It further allows for batch or continuous mode of operation and for easy adjustment of the residence time of the reactants to match the kinetics of the reaction. A 10-kW solar reactor prototype was designed and tested for the carbothermic reduction of electric arc furnace dusts (EAFD). The reactor was subjected to mean solar flux intensities of 2000 kW m-2 and operated in both batch and continuous mode within the temperature range of 11201400 K. Extraction of over 90% of the toxic compounds originally contained in the EAFD was achieved while the condensable products of the off-gas contained mainly Zn, Pb, and Cl. The use of concentrated solar energy as the source of process heat offers the possibility of converting hazardous solid waste material into valuable commodities for processes in closed and sustainable material cycles.
Introduction In a previous paper (1), we proposed a novel thermochemical process for recycling hazardous solid waste material using concentrated solar power. The motivation for the development of such a solar-driven process emerged from the fact that current recycling techniques by blast, induction, arc, and plasma furnaces are characterized by their high energy consumption and their concomitant environmental pollution, derived mainly from the combustion of fossil fuels for heat and electricity generation (2-4). For example, the theoretical thermal energy requirement for the carbothermic reduction of electric arc furnace dust (EAFD) at 1500 K is * Corresponding author fax: +41-56-3613160; e-mail: aldo.
[email protected]. 10.1021/es020056o CCC: $25.00 Published on Web 11/26/2002
2003 American Chemical Society
Solar Reactor Design and Fabrication The patented solar reactor concept (5) proposed for the solar thermal recycling process is depicted in Figure 1. It features two cavities in series. The inner cavity is a graphite enclosure with a small windowed opening, the aperture, to let in concentrated solar power. It serves as the solar receiver, radiant absorber, and radiant emitter. The outer cavity is a well-insulated enclosure containing the inner cavity. It serves as the reaction chamber that is subjected to thermal radiation from the inner cavity. Because of multiple reflections among its inner walls, the inner cavity approaches a blackbody absorber in its ability to absorb incoming solar irradiation. It further emits infrared radiation through its exterior walls that is directly absorbed by the reactants contained in the outer cavity. Such a two-cavity reactor concept has the following advantages: (i) The reactants are exposed to uniform irradiation emitted by the inner cavity, even for nonuniform incident solar fluxes. (ii) The inner cavity serves VOL. 37, NO. 1, 2003 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Main Elemental Composition of EAFD (1)a element
EAFD (wt %)
element
Zn Fe Pb Cu Cd Cr
37.8 13.5 10.1 0.23 0.09 0.12
Cl S Si alkaline earth elements C
a
EAFD (wt %) 4.8 0.6 1.7 0-5
