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Journal of Environmental Quality 30:1392-1401 (2001)
© 2001 American Society of Agronomy, Crop Science Society of America, and Soil Science Society of America

TECHNICAL REPORT
Waste Management

Oxidation Kinetics of the Combustible Fraction of Construction and Demolition Wastes

Ni-Bin Chang*,a, Kuen-Song Linb, Y.-P. Suna and H.Paul Wanga

a Dep. of Environmental Engineering, National Cheng Kung Univ., Tainan, Taiwan, Republic of China
b Dep. of Chemical Engineering, Wu-feng Institute of Technology, Chia-yi, Taiwan, Republic of China

* Corresponding author (a1211{at}mail.ncku.edu.tw)

Received for publication April 14, 2000.

   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Proper disposal of construction and demolition wastes (CDW) has received wide attention recently due to significantly large quantities of waste streams collected from razed or retrofitted buildings in many metropolitan regions. Burning the combustible fractions of CDW (CCDW) and possibly recovering part of the heat content for economic uses could be valuable for energy conservation. This paper explores the oxidation kinetics of CCDW associated with its ash characterization. Kinetic parameters for the oxidation of CCDW were numerically calculated using thermal gravimetric analysis (TGA) and the resultant rate equations were therefore developed for illustrating the oxidation processes of CCDW simultaneously. Based on three designated heating rates, each of the oxidation processes can be featured distinctively with five different stages according to the rate of weight change at the temperature between 300 K and 923 K. In addition, Fourier transform infrared (FTIR) spectroscopy was employed, associated with a lab-scale fixed-bed incinerator for monitoring the composition of flue gas. Carbon dioxide (CO2) was found as a major component in the flue gas. The fuel analysis also included an ash composition analysis via the use of X-ray powder diffraction (XRD), atomic absorption (AA) spectroscopy, inductively coupled plasma–atomic emission spectroscopy (ICP–AES), and scanning electron microscopy–energy dispersive spectroscopy (SEM–EDX). The ash streams were identified as nonhazardous materials based on the toxicity characteristic leaching procedure (TCLP). Overall, the scientific findings gained in this study will be helpful for supporting a sound engineering design of real-world CCDW incineration systems.

Abbreviations: AA, atomic absorption • ABS, acrylonitrile–butadiene–styrene copolymer • CCDW • combustible fractions of construction and demolition waste • CDW, construction and demolition waste • FTIR, Fourier transform infrared • ICP–AES, inductively coupled plasma–atomic emission spectroscopy • PE, polyethylene • PP, polypropylene • PS, polystyrene • PU, polyurethane • PVC, polyvinyl chloride • SEM–EDX, scanning electron microscopy–energy dispersive spectroscopy • TCLP, toxicity characteristic leaching procedure • TGA, thermal gravimetric analysis • XRD, X-ray powder diffraction


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
DEMOLITION waste is defined as a waste produced from razed buildings and other civil engineering structures. Wastes from the construction, remodeling, and repairing of individual residences, commercial buildings, and other structures are classified as construction wastes (Spivey, 1974; Oglesby et al., 1989; Spencer, 1989, 1990; Apotheker, 1990; Kalin, 1991; Wood, 1992; Gavilan and Bernold, 1994; Chang et al., 1997). Construction and demolition waste, however, may be produced from environmental disasters such as earthquakes, hurricanes, tornadoes, and floodwater that cause major structural damage to buildings (Tansel et al., 1994). These waste streams are often regarded as rubbish. Construction and demolition waste sometimes constitutes between 20 and 25% of municipal solid wastes (MSW) in Taiwan (Environmental Protection Administration, 1999). Table 1 presents the typical composition and potential recoverable heat content of CDW in a local study. Only the wood and plastic items can be burned in the incineration systems when considering the needs of energy recovery.


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  		Table 1. Composition and recoverable heat of Taiwan construction and demolition waste (CDW).

 
The composition of CDW is variable, including dirt, stones, bricks, block, concrete, steel, glass, plaster, lumber, shingles, plumbing, heating, and electrical parts (Stein, 1987; Gavilan and Bernold, 1994; Seo and Hwang, 1999). Proper management of CDW turns out to be critical in many developed and developing countries as economic development proceeds. Both the industry and the public sectors have faced increasing concerns with respect to recycling potential energy as part of the renewable resources and items as secondary materials from the CDW streams as well as their associated effects on the environment (Ravindrarajah, 1987; Lauritzen, 1994; Perez, 1994; Traenkler and Walker, 1994). Extensive study has been conducted to reevaluate alternative waste management technologies for reducing the volume of CDW destined for landfilling (Lauritzen, 1994; Gavilan and Bernold, 1994; Brooks et al., 1995; Seo and Hwang, 1999). The rising cost of landfilling the CDW, however, diminishes its potential from a long-term perspective (Ferguson, 1994; Freeman, 1994; Gavilan and Bernold, 1994; Hendriks, 1994).

Construction and demolition waste streams generally contain 10 to 15% combustible materials, such as wood, wallpaper, plastics, and rubber (Gavilan and Bernold, 1994; Seo and Hwang, 1999). Plastic or wood wastes that are sometimes abundant in the CDW streams also represent a significant quantity of energy (Buekens and Schoeters, 1986; Spencer, 1990; Perez, 1994; Lauritzen, 1994; Brooks et al., 1995). The depletion of natural fuel resources has renewed interest to convert CCDW into useful heat energy. Therefore, recovery of CCDW in a form with the highest possible value (i.e., heat energy) would be economically and environmentally attractive. Thermochemical processes, such as the oxidation process and gasification process, appear to be promising approaches to convert such wastes into useful energy. Thus, the purpose of this paper is to explore an in-depth investigation of oxidation kinetics of CCDW by TGA–differential thermal analysis (DTA) associated with Fourier transform infrared (FTIR) spectroscopy in order to provide the essential information for a sound engineering design and operation of the real-world incineration systems. In addition, the physicochemical properties of incineration ashes, explored via the use of the toxicity characteristic leaching procedure (TCLP), X-ray powder diffraction (XRD), and microscopy–energy dispersive spectroscopy (SEM–EDX), may present potential recovery and reuse of incineration ash.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Physicochemical Properties of Combustible Construction and Demolition Wastes
A sampling program was performed in 1999 to gain a deeper understanding of the physical composition, chemical content, and calorific values of CCDW in which both proximate and ultimate analyses were emphasized. The operation of proximate analyses was based on American Society for Testing and Materials (ASTM) Standard D3172. The ultimate analyses concerning CCDW samples were established with a Heraeus (Hanau, Germany) elementary analyzer (EA). Heating values were assessed by using the standard calorimeter. The samples prepared for this experimental practice mainly contained about 20% plastics and 80% wood. Plastics are generally mixed with some materials, such as polypropylene (PP), polyurethane (PU), polystyrene (PS), polyethylene (PE), polyvinyl chloride (PVC), and acrylonitrile–butadiene–styrene copolymer (ABS). Due to the limited size of the TGA sample holder, the CCDW samples therefore were milled into pieces with a size around 60 to 100 meshes, implying that the CCDW samples had uniform size and were suitable for oxidation of TGA tests as anticipated in the real-world incineration system. An elementary analyzer (Heraeus) performed the elemental analysis in this study.

Oxidation Kinetics
Kinetic parameters, such as the order of reaction, activation energy, and preexponential factor, can be derived from the TGA outputs. Preliminary data of oxidation kinetics in relation to wood, plastics, or CCDW can be measured in a typical practice of TGA (Model SDT 2960 and Thermal Analyst 2000; TA Instruments, New Castle, DE), determining both the weight loss and the temperature difference simultaneously. The variations of sample weight recorded based on a homogeneous 10-s time interval may exhibit a measuring sensitivity of 0.1 ± 0.01 µg. In the experimental settings, about 10 mg of wood, plastics, and CCDW samples were burned from 300 to 923 K at a heating rate of 2, 6, or 10 K min-1, respectively, in the flowing air environment with a flow rate of 100 mL min-1. Routine calibrations, such as TGA weight calibration, differential thermal analysis (DTA) baseline calibration, and temperature calibration, are generally performed once a month to maintain the integrity of these instruments. Aluminum oxide was used as a reference in all the tests. Detailed descriptions of the experimental methods and additional discussions, which cover the important techniques of apparent kinetic parameter evaluation from the TGA traces, can be found in the literature (Friedman, 1965; Petrovic and Zavago, 1986; Nikoo, 1987; Chen et al., 1997; Aggarwal and Dollimore, 1997; Galwey and Brown, 1997; Reggers et al., 1997). The overall rate equations of conversion factors expressed with respect to the Arrhenius relation form for oxidation of CCDW are as follows:

[1]

[2]

[3]

[4]

[5]
where t is the reaction time (min); Ap is the preexponential factor (min-1); Ea is the activation energy (kJ mol-1); k is the Arrhenius rate constant; T is the reaction temperature (K); R is universal gas constant (8.314 kJ kg mol-1 K-1); W is the mass of the sample at time t (kg); X is the residual mass fraction (conversion); g[O2] is the m-th order for the oxygen composition (mol); f(X) is the n-th order for the inertial material; and Wo and Wf are the initial and final (or ash residues) mass of the samples (kg), respectively.

Analysis Based on Lab-Scale Fixed-Bed Incinerator
The experiments using a batch type lab-scale fixed-bed reactor for burning the CCDW samples were also carried out to collect the essential information of this feasibility study. Such a finding may further determine if future engineering applications could be warranted. A tube reactor, with a volume of 100 mL and made of 316 stainless steel, enclosed three thermowells (i.e., top, middle, and bottom sections). Electrically programmable heated jackets installed within the thermowells were employed for heating the reactor. Oxidizing CCDW samples between 300 and 923 K in the flowing air environment (i.e., 120 mL min-1, gas hourly space velocity [GHSV] = 1000 h), with O to C ratios ranging from 1.0 to 1.3 on a molar basis, was regarded as an acceptable experimental condition. Gas products such as CO2 or CO formed in the combustion reactor had to be guided through a water cooler before entering the FTIR for further identification. During such a process, the condensates from the flue gas, such as HCl, were separated and collected in the two knockout drums (i.e., 500 and 300 mL, in series) and then scrubbed with a 1 M NaOH solution before discharging from the device. Composition of noncondensable gases generated in the oxidation process was then analyzed by on-line FTIR (10-cm gas cell) spectroscopy. The resultant data were recorded on a Digilab (Cambridge, MA) FTIR spectrometer (FTS-40) with a full capacity for computerized data storage and data handling via the use of a 64-scan data accumulation device with a resolution of 4 cm-1.

Physicochemical Properties of Incineration Ash of Combustible Construction and Demolition Wastes
The residuals of ash collected from the lab-scale fixed-bed incinerator were analyzed in accordance with the TCLP requirement first. The leachates collected from the TCLP test underwent an acid-digestion process before the investigation of heavy metal concentrations by both atomic absorption (AA) spectroscopy (Hitachi [Tokyo, Japan] Model Z-8100) and inductively coupled plasma–atomic emission spectroscopy (ICP–AES) (Jobin Yvon [Kyoto, Japan] Model JY32/38). Morphological observations of ash samples were examined by SEM–EDX (Jeol [Tokyo, Japan] Model JSMr840). The identification of solid phases and crystallinities of metal oxides in ash was achieved using XRD (Rigaku [Tokyo, Japan] Model D/MAX III-V). Samples retrieved from the ash were scanned from 10 to 50°(2{theta}) based on a scan rate of 3°(2{theta}) min-1, while a computer library system recorded the specific peak intensities and 2{theta} values in accordance with various metal oxides.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Generally, wood or wood-like materials, such as cellulose, lignin, hemicelluloses, refuse-derived fuel (RDF), and plastic wastes involve a complex set of chemical reactions, often influenced in the incineration processes (Costa and Camino, 1985; Dobry et al., 1986; Oren et al., 1987; Zayed et al., 1987; Ragland and Aerts, 1991; Wang et al., 1998b; Lin et al., 1998, 1999; Chang et al., 1999). When wood or plastics wastes are heated, the solid part decomposes by thermal scission of chemical bonds. The species formed by this initial step may be volatile and may undergo additional bond-breaking reactions to form CO or CO2 ultimately (Leminux and Prudhomme, 1985; Harris et al., 1986; Spurrell et al., 1987; Tonn and White, 1990; Hueglin et al., 1997; Ouedraogo et al., 1998). While pyrolysis of plastics, wood, or paper wastes has been studied in a variety of ways (Thurnner and Mann, 1981; Lede et al., 1985; Aho, 1987; Oren et al., 1987; Cordero et al., 1990; Wang et al., 1998a; Lin et al., 1998, 1999; Chang et al., 1999; Park et al., 1999; Di Blasi et al., 1999), the validity of some of the kinetic methods employed to characterize various plastics and wood materials using TGA and FTIR techniques can be found in many applications (Friedman, 1965; Petrovic and Zavago, 1986; Nikoo, 1987; Aggarwal and Dollimore, 1997; Galwey and Brown, 1997). The following sections summarize the main findings in this study.

Physicochemical Properties
Table 2 presents the outcome of proximate and ultimate analyses as well as heating values of the CCDW samples. The selected sample of CCDW contained more than 85% combustible materials, of which 5% may turn out to be ash residues after incineration. It is worthwhile to note that the carbon content constitutes more than 45% of the sample by weight. The heating values of three selected samples are of interest in energy recovery and the order of magnitude of them is as high as 18540, 18550, and 18790 kJ kg-1, respectively.


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  		Table 2. Proximate and ultimate analyses of the combustible fractions of construction and demolition waste (CCDW).

 
Oxidation Kinetics of Combustible Construction and Demolition Wastes
Thermal gravimetric analysis has been extensively used as a means for determining devolatilization characteristics and kinetic parameters (Friedman, 1965; Petrovic and Zavago, 1986; Nikoo, 1987; Aggarwal and Dollimore, 1997; Galwey and Brown, 1997; Wang et al., 1998b; Lin et al., 1998, 1999; Chang et al., 1999). In this study, TGA routinely recorded the loss of the weight of CCDW samples as the temperature increased at a predetermined rate. It also provides the net weight loss and kinetic parameters based on simplified assumptions that do not necessarily correspond with the complex chemical reactions in the thermal degradation process. Such practices could exhibit some comparative advantages with regard to the minimization of uncertainties in engineering applications via the assessment of oxidation kinetics over an entire temperature range.

Kinetic parameters for oxidation of CCDW were therefore numerically calculated in terms of the rate of weight loss obtained through the TGA practices based on different heating rates. A representative kinetic model should allow one to account for the variability of CCDW in the thermal degradation process as long as it is capable of describing the weight loss over time. Since the experiments were conducted based on lower heating rates, limitations of mass and heat transfer within the samples were thus neglected. Instead of focusing on the direct assessment of blend CCDW material, an estimation of the global oxidation rate in terms of the weight fractions of those key components appearing in the CCDW streams may become achievable. Such a practice of simulation analysis has been proven successful via an appropriate calibration in this study. They include wood (75%), wallpaper (3%), PP (5%), PU (5%), PE (3%), ABS (5%), and PVC (4%). Table 3 further summarizes the derived kinetic parameters of wood, wallpaper, plastics, and the outcome associated with CCDW gained in this study for comparative purposes. Typical TGA curves in Fig. 1 obtained for the oxidation of wood, PU, and CCDW show that a single-stage process could describe the thermal degradation of wood, while PU or CCDW have to be characterized by a two-stage process. Obviously, oxidation rate of CCDW is mainly affected by the oxidation rates of wood and PU simultaneously.


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  		Table 3. Average calculated kinetic parameters{dagger} for oxidation of main components in the combustible fractions of construction and demolition waste (CCDW).

 


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  		Fig. 1. Thermal gravimetric analysis (TGA) curves of (a) weight change (%) and (b) derivative weight (% K-1) for the oxidation of the combustible fraction of construction and demolition wastes (CCDW) ({circ}), wood ({square}), and polyurethane (PU) ({Delta}) based on a heating rate of 10 K min-1.

 
The residence time and temperature required for a full degree of oxidation of CCDW are very important for determining the extent to which the wood or polymeric materials with high molecular weight can be thermally cracked. In the subsequent experiment of lab-scale fixed-bed oxidation, CCDW samples were fully oxidized within a time period of 10 to 30 min and at temperature range between 300 K and 923 K. At temperatures higher than 823 K, the rate of yield of incineration ash was about 4% regardless of the temperature in the reactor. The other achievement shown in Table 3 includes the findings of the kinetic parameters, such as activation energy and preexponential factor, during the oxidation of CCDW samples. Such predictions were numerically calculated from the weight loss of CCDW samples at different heating rates based on the modified Friedman algorithm (Friedman, 1965; Petrovic and Zavago, 1986; Nikoo, 1987; Cordero et al., 1990; Day et al., 1993; Aggarwal and Dollimore, 1997; Galwey and Brown, 1997). The last column in Table 3 also summarizes the information related to the oxidation of wood, paper, plastics, and CCDW from different sources in comparison with the previous efforts. However, most kinetic studies for oxidation of wood, paper, and plastics found in the literature seldom have such a thorough and in-depth collection and integration based on kinetics, ash identification, and engineering application. Overall, the following rate equations derived from the weight loss curves for the blended CCDW material are:

For T = 300–600 K:

[6]

For T = 601–923 K:

[7]

The rate equations above, for addressing the behavior of CCDW oxidation, generally have three kinetic parameters, consisting of the conversion factors (X) (i.e., subscript 1 and 2 denote the first and the second step, respectively), residence time (t), and the reaction temperature (T). The residence time and conversion factor are two parameters mainly prepared for engineering design that are normally used for sizing the reactor as well as optimizing the operational conditions. The oxidation temperature, on the other hand, is a key factor for determining the desired feature of system environment and ash residue. The first-stage activation energy (Ea) for the oxidation of CCDW can then be estimated as approximately 123 kJ mol-1.

Figure 2 further describes the situations in which CCDW samples were oxidized at different heating rates. They are 2, 6, or 10 K min-1 in the TGA practice. The heating rates used in this study appear to have a little bit more effect on the generation of ash residues and also slightly influences the reaction rates of the samples. In particular, the inorganic ashes or slag must encapsulate unconverted carbons in the early stages during oxidation at higher heating rates and may generate a higher amount of ash residues. But heat transfer would have a greater effect in shifting the curves toward higher temperature as the heating rate increases. Theoretically, Eq. [2] implies that a higher temperature is required to achieve the same degree of conversion when choosing a higher heating rate. Therefore, as shown in Fig. 2, the curves illustrating smaller residual weight based on the lower heating rate (i.e., 2 K min-1) appears on the left-hand side as compared with those based on relatively higher heating rates (i.e., 6 or 10 K min-1). Furthermore, there are three principal reactions for CCDW oxidation processes. They can be identified easily (Fig. 2) associated with five distinctive stages within the temperature range between 300 and 923 K. It appears that a little bit of moisture (H2O) or volatile organic carbons (VOCs), which are generally used as the painting materials for wood and wallpaper, start to vaporize right above 300 K near the first and second stages. These two stages are easily handled at relatively lower temperatures and smaller reaction rates. The first- or second-order reaction kinetics existing in the third or fourth stage usually result in a larger reaction rate and goes on until the temperature reaches 600 K (X = 0.35) or 700 K (X = 0.04). The third stage represents pyrolysis of the sample, while the fourth stage is primarily focused on the reaction of char burning in which the destruction of wood, wallpaper, and polymer materials dominate the entire oxidation process. Finally, the reaction in the last stage may end up a special condition when the value of X is smaller than 0.04. The time required for CCDW oxidation to reach a given temperature is generally longer at the lower heating rate.



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  		Fig. 2. Comparison of residual weight (%) with respect to different heating rates for the oxidation of the combustible fraction of construction and demolition wastes (CCDW) (2 K min-1 [{circ}], 6 K min-1 [{square}], 10 K min-1 [{Delta}]).

 
Figure 3 shows the comparative results of instantaneous reaction rates (i.e., Eq. [2]) over the designated temperature window. Again, they are designed based on different heating rates in the experiment. This figure unambiguously pinpoints a set of salient peaks and transition zones associated with different heating rates. The occurrence of those peaks represents systematic changes in the oxidation processes. One of the typical examples is the ignition of cellulose wastes (wood or wallpaper) and plastics. Obviously, as the reactor encounters higher heating rates, the peak time appears earlier and the peak zone comes up to a smaller bandwidth in the oxidation process.



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  		Fig. 3. Comparison of derivative weight (% K-1) with respect to different heating rates for the oxidation of the combustible fraction of construction and demolition wastes (CCDW) (2 K min-1 [{circ}], 6 K min-1 [{square}], 10 K min-1 [{Delta}]).

 
The availability of oxygen in the incineration systems plays an important role when the wastes face bonding dissociation via the interaction with free radicals. Obviously, the maximum reaction temperature decreases with increasing oxygen concentration. Such an argument can be evidenced by the curves in Fig. 4, which show that as the peak zone appears at a smaller bandwitdth and the peak reaches a higher value, the system may require a larger molar ratio of O to C in the oxidation process. This implies that an increase of O to C ratios may result in an increase of oxidation rate.



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  		Fig. 4. Thermal gravimetric analysis (TGA) curves of the combustible fraction of construction and demolition wastes (CCDW) for illustrating the effect of different oxygen concentrations of 21% ({circ}), 15% ({square}), and 10% ({Delta}) based on a heating rate of 10 K min-1.

 
Fourier Transform Infrared Analysis
Figure 5 illustrates the outputs of FTIR analysis that are presented by a four-stage analysis. Of the noncondensable gases produced in the oxidation process, carbon dioxide was a major component identified by the on-line FTIR spectroscopy. It is known that solvent applied for wood coating may contain pentachlorophenol along with some chlorine species that are generally used as a wood preservative. Thus, CCDW streams might also have such trace chlorines. Burning the CCDW samples within the temperature range between 373 and 1023 K may result in the emergence of CO2 and CO that can be characterized by wave numbers around 2300 to 2380 cm-1 and 2050 to 2240 cm-1, respectively, in the FTIR analysis. It implies that the oxidation reaction can be initialized while CO2 and CO appear in the flue gas. In the early or final stage of the CCDW oxidation process (T < 600 K or T > 800 K), the flue gas might emit trace amounts of vinylchloride due to the existence of PVC, which contains some vinylchloride species. Such a phenomenon can be characterized by the wave number around 900 to 975 cm-1 in the FTIR analysis. In addition, CCDW might be humid because of its porous properties. At around 600 K, a higher concentration of CO2 is measured by the on-line FTIR, which is mainly due to the full combustion of PVC, PU, wood, and wallpaper; indicated as the third stage in Fig. 5. At between 600 and 700 K, defined as the fourth stage of CCDW oxidation process, the full combustion of PE, ABS, wood, and wallpaper is apparent. Overall, close agreement exists from both quantitative and qualitative aspects in relation to the oxidation of individual plastics and wood. The experiment, conducted by gas chromatography (Hewlett–Packard [Palo Alto, CA] 5890A) with a mass selective detector (HP 5972), did not find any trace organic compounds like polychlorinated dibenzo-p-dioxins (PCDDs)–polychlorinated dibenzo-p-furans (PCDFs) or polycyclic aromatic hydrocarbons (PAHs). Those trace organic compounds, extracted with a dichloromethane solvent (Merck [Darmstadt, Germany], purity >99%) as condensate and identified quantitatively by gas chromatography–mass spectrometry (GC–MS) along with an automatic sampler (HP-7673A), were not found in the oxidation process. However, the high temperature environment in the reactor could prevent the possible formation of trace organic compounds, such as PAHs, polychlorinated biphenyls (PCBs), and PCDDs–PCDFs.



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  		Fig. 5. Products in the flue gas identified by infrared spectra at (a) 900 K, (b) 700 K, (c) 500 K, and (d) 400 K (VC, vinylchloride; CO, carbon monoxide; CO2, carbon dioxide; H2O, water; resolution = 4 cm-1).

 
Characterization and Application of Combustible Construction and Demolition Waste Ashes
The TCLP test is usually used for the identification of the environmental effect of the incineration ash (Someshwar, 1996; Chang et al., 1997; Manninen et al., 1997; Chang et al., 1998; Thy et al., 1999). The leachability of heavy metals in the incineration is mainly affected by the composition of CCDW, its combustion history, and the method for ash handling. Table 4 indicates that the leaching test in relation to the incineration ash of CCDW exhibits positive results referring to the official standards in Taiwan. Therefore, such observation supports the fact that incineration ash of CCDW cannot be classified as hazardous wastes. Applying XRD and SEM techniques can make additional observations. Figure 6 presents the microstructure and chemical composition of such ash. It mainly contains the chemical compounds CaO, K2O, and SiO2, as well as some trace stable metal oxide aggregates, such as ZnO or CuO, with irregular-shaped crystals. They are typically 2 to 7 µm in diameter. The TCLP test clearly exhibits several application potentials regarding using the ash as structural fill, additives of concrete, soil stabilizer, landscape work, conduit bedding, basement for roads, landfill embankment, and garden soil.


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  		Table 4. Toxicity characteristic leaching procedure (TCLP) test of the ash produced from the conbustible fractions of construction and demolition waste (CCDW) incinerated at 900 K.

 


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  		Fig. 6. (a) X-ray powder diffraction (XRD) pattern and (b) scanning electron microscopy (SEM) micrograph of the incineration ash produced at 900 K (C, CaO; K, K2O; Q, quartz; U, CuO; Z, ZnO).

 
Engineering Application of Scientific Findings in This Study
The disastrous earthquake that occurred on 21 Sept. 1999 in central Taiwan caused overwhelming damage to many buildings, resulting in large quantities of CDW, which require further disposal immediately. Such an environmental disaster, resulting in a tragedy with more than 2400 deaths and the destruction of more than 100000 buildings, has generated many CDW streams in the vicinity of the earthquake epicenter. It has been estimated by the rescue teams of the Army Corps in Taiwan that overall CDW streams eventually may end up approximately 30 million tonnes by weight (Army Corps, personal communication, 1999).

Recovering these damaged houses in the earthquake areas also requires significant rebuilding efforts that may generate more quantities of CDW in the later stage. Proper disposal by landfilling might cause a remarkable increase of environmental costs because of limited landfill space left in that area. This issue has received wide attention recently due to the emerging effects by the accumulated CDW streams disposed elsewhere in the impact zone. Reusing part of the CDW as secondary materials might result in significant cost savings in civil engineering projects. Based on the scientific findings in this study, Fig. 7 illustrates the mass and energy balance of an incineration system on the basis of one tonne per hour (TPH), which might provide the essential information of what will be the design and operational conditions in the future. The combustion temperature designed for burning the CCDW streams is 1173 K. The information on oxidation of CCDW gained in this study and previous research may fully support the feasibility for the possible recovery of both heat energy and ash simultaneously (Tonn and White, 1990; Ragland and Aerts, 1991; Morris, 1996; Lea, 1996; Manninen et al., 1997). Municipal incinerators or several brick kilns in the vicinity of the earthquake areas could be options for such an application. However, the oxidation mechanism in the real-world incineration system is derived by flame combustion rather than electrically heating the air. This could become an uncertainty in applications. But kinetic parameters for oxidation of CCDW provide a fundamental understanding of the incineration phenomenon. Designing a more uniform temperature field in the combustor as the flame combustion proceeds with homogenous feeding of the waste inflows can further minimize the gap or discrepancy between them.



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  		Fig. 7. Mass and energy balance of the conceptual combustible fractions of construction and demolition waste (CCDW) incineration system.

 

   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
The combustible fractions of construction and demolition wastes represent a major component of construction and demolition wastes, typically ranging from 10 to 15% in the waste streams. Environmental disasters such as earthquakes and hurricanes may cause major structural damage to buildings, which require significant rebuilding efforts and produce a remarkable quantity of CCDW. To effectively handle those waste streams in incineration systems, kinetic parameters for oxidation of CCDW were numerically calculated in this study from TGA weight loss data at different heating rates. The oxidation of CCDW can be satisfactorily described by the following rate equations:

For T = 300–600 K:

For T = 601–923 K:

The global oxidation reaction rate calculated from key component fractions, such as wood, wallpaper, and plastics, using the weighed summation method, is found to be well suited to the findings of the TGA kinetics in relation to the blend CCDW material. Based on three different heating rates, three principal reactions in the CCDW oxidation process were identified with five distinctive stages in terms of various weight changes at the temperature between 300 and 923 K. Qualitative delineation for the degradation step turns out to be consistent with regard to the obtained TGA data in terms of individual plastics and wood. Fourier transform infrared spectroscopy was employed as a means for monitoring the flue gas in the CCDW oxidation process. Upon an increase in the reaction temperature, the concentration of carbon dioxide is apparent at the wave numbers around 2300 to 2380 cm-1, indicating an occurrence of oxidation reactions. The physicochemical and leaching properties of the incineration ashes were also investigated by several contemporary instrument analysis techniques, including TCLP, XRD, SEM–EDX, AA, or ICP–mass spectroscopy (MS). Overall, CCDW streams represent a significant quantity of potential energy that remained as part of the energy sources consumed in processing petroleum. Recovery of valuable heat energy could be beneficial for solving other pending environmental problems. Generally speaking, incineration of excess amounts of CCDW streams to produce heat energy is an appealing alternative resource recovery for the future.


   ACKNOWLEDGMENTS
 
The authors gratefully acknowledge the financial support of the National Science Council, Taiwan, R.O.C. (NSC-85-2621-P-006-033 and NSC-87-2211-E-006-011) and Chen-Pin Co. (88-001).


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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