Environ. Sci. Technol. 2000, 34, 2931-2938
Metal Behavior during the Low-Temperature Pyrolysis of Chromated Copper Arsenate-Treated Wood Waste LIEVE HELSEN* AND ERIC VAN DEN BULCK Department of Mechanical Engineering, Katholieke Universiteit Leuven, Celestijnenlaan 300A, 3001 Heverlee, Belgium
In the frame of a study that aims at optimizing the pyrolysis of chromated copper arsenate- (CCA-) treated wood with respect to a minimal metal release upon a maximal mass reduction, a set of experimental studies has been carried out in order to gain more insight in the metal (Cr, Cu, and As) behavior during the pyrolysis process. These experiments are described in detail in previous publications. In this paper, the experimental observations are translated in a mechanism that describes the metal (in particular As) behavior during pyrolysis. Two hypotheses concerning the As release are tested for their validity: (a) there exists a unique relation between the As release and the mass reduction of wood or wood components; (b) the As is released according to its own kinetic scheme consisting of a single first-order reaction. The last one results in a preexponential factor A ) 0.39 min-1 ) 6.5 × 10-3 s-1 and an activation energy E ) 20.4 kJ/mol, which are chemically meaningful values for a reduction reaction. The following model is derived: “The release of As is controlled by two consecutive reactions: the reduction of As(V) to As(III), followed by the volatilization of As(III), in the form of the volatile compound As4O6”. Because of the good predictions made with the simple onereaction kinetic scheme, it is expected that the volatilization of As(III) occurs faster than the reduction to As(III). Additional experimental work is needed to verify this approximation, but the simple one-reaction kinetic scheme already offers a good initial model.
Introduction Waterborne salts have been used to preserve wood from insects, fungi, and water damage for many years. One of the more common formulations contains copper, chromium, and arsenic salts and is known as chromated copper arsenate or CCA. For example, in 1990 the North American woodtreating industry produced 437.7 million ft3 of CCA-preserved wood (1). After impregnation of the wood with a CCA solution, the metal compounds are fixed to the cell walls of the wood matrix. Substantial amounts of CCA remain in the wood for many years, and the disposal of scrap wood is a growing problem in Europe and the United States. In Germany, for example, the total amount of wood waste is around 6-8 million ton/yr (2). In France, about 26 million poles treated * Corresponding author telephone: 00-32-16-32.25.05; fax: 0032-16-32.29.85; e-mail:
[email protected]. 10.1021/es991102w CCC: $19.00 Published on Web 06/07/2000
2000 American Chemical Society
with CCA (railway, electricity, and telephone) are in service. Every year 500 000 poles (50 000 ton) are taken out of service, which means that the waste disposal problem will last for at least 50 yr without putting new poles in service (3). Telephone poles, railway sleepers, timber from landscape and cooling towers, wooden silos, hop-poles, cable drums, and wooden playground equipment generate wood waste for which environmentally benign disposal technologies need to be developed. The number of waste disposal sites is decreasing, and redundant poles, piling, and lumber, which constitute a large volume of material, may not be accepted at the limited number of sites in the future. Extraction experiments have been carried out on CCAtreated wood and evaluated as a method to recover the metal compounds into either fresh wood preservatives or other useful industrial materials (4, 5). Among the disadvantages of this recycling method are the huge amount of chemical solvents used, the high cost of size reduction, and the long duration of the process. The effect of the extraction process on the combustion characteristics of the wood residue was not reported. Recycling of the resulting solutions was described neither. Pasek and McIntyre (1) stated that the resulting solution is not easily recycled, and multistage extraction processes are needed for high CCA concentrations. Numerous studies and experiments have been carried out on burning contaminated wood by various organizations, in particular in the United States and Canada but also in Europe, Germany, The Netherlands, Denmark, Switzerland and the U.K. (3). Their conclusions reflect three common points. Burning this wood waste emits highly toxic smoke and fumes in the environment. The municipal waste incinerators and most of the industrial waste incinerators are not equipped to retain this type of toxic elements, especially at the concentrations involved. Each attempt to mix the polluted wood with other waste streams has caused the destabilization of the combustion conditions in the incinerators, resulting in the appearance of highly dangerous and uncontrollable chemical compounds. Conventional pyrolysis systems (fixed bed, batch or grate; fluidized bed; rotary kiln, ...) operate at too high a temperature to prevent the release of metal vapors and often require that the wood is chopped before processing (2, 6-8). Percentages of arsenic (As(III)) volatilized have been reported (1, 9-14) to range between 8 and 95%. Amounts of copper and chromium volatilized are not well documented but are found to be much lower than for arsenic. Public concern has been raised over the possible formation of toxic smoke when CCA wood is burned in wood stoves, fireplaces, or boilers. Pyrolyzing the CCA-treated wood at low temperature is a promising solution to the growing disposal problem since low temperatures and no oxidizing agents are used. In turn, this leads to a smaller loss of metals than in combustion or even a total recuperation of the metals. The recent consciousness of the need for abatement of air pollution leads to further interest and investigation of pyrolysis as a major process for the disposal of enormous quantities of cellulosic wastes and residual materials. However, the current literature (1, 4, 10, 15, 16) clearly shows that the mechanism of metal volatilization during the thermal decomposition of CCAtreated wood is not yet completely understood. A study has been set up to design a low-temperature pyrolysis facility for the CCA-treated wood waste such that at least 90% of the metals are contained in a concentrated solid product stream, and the pyrolysis gases are used to their maximum potential with respect to energy recuperation (17). During this study, several experiments have been carried VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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out with the aim to gain more insight in the behavior of the metals (Cu, Cr, and A:s) during the pyrolysis process. Among these experiments are lab-scale pyrolysis study using different temperatures and residence times (18), determination of mass balances over the total system using inductively coupled plasma mass spectrometry (ICP-MS) (17, 19), sequential extraction of the pyrolysis residue (19), speciation study of As in the pyrolysis residue using hydride generation coupled to ICP-MS (19), study of metal microdistribution in wood and pyrolysis residue by energy-dispersive X-ray analysis coupled to scanning electron microscopy (SEM-EDX) (20), thermogravimetry (TG) and evolved gas analysis (EGA) (21), and TG analysis of the major arsenic compound in CCAtreated wood (CrAsO4) (22). Detailed descriptions and discussions of these experimental studies can be found in the given references and are thus not repeated here. In this paper, the most important conclusions with respect to the metal behavior and influence of metals on the pyrolysis behavior, drawn from the experimental observations, are translated into a hypothesis concerning the behavior of metals during pyrolysis of CCA-treated wood. Furthermore, a physical and a chemical model are tested for their validity to describe the As release during low-temperature pyrolysis. The development of these models is based on As releases, measured during the lab-scale and TG experiments. Therefore, a brief description of these experiments is given in the Experimental Section of the present paper. The paper concludes with a simple kinetic scheme describing the release of As during the pyrolysis of CCA-treated wood.
Previous Research Work Previous experimental studies, carried out by the authors (17-21, 23), have contributed to the understanding of the influence of CCA on the pyrolysis of wood and the metal behavior during the pyrolysis process. A kinetic scheme for the pyrolysis of CCA-treated wood is derived (22) through kinetic evaluation of the non-isothermal TG analysis results (least-squares evaluation of the DTG curves). The kinetic scheme consists of two independent parallel reactions, followed by one subsequent reaction. The pyrolysis of untreated wood, on the other hand, is described by a scheme of three independent parallel reactions. Thus, the presence of CCA influences the pyrolytic behavior of wood, resulting in a different reaction scheme. Comparing both schemes, each having its own kinetic constants, reveals the following observations. The first peak for CCA-treated wood is characterized by lower activation energy, preexponential factor and peak temperature, and higher contribution to volatiles production, resulting in a slightly higher peak with approximately the same shape but shifted toward lower temperatures. The second peak for CCAtreated wood, on the other hand, is characterized by a higher activation energy and preexponential factor, resulting in a more narrow peak located at lower temperatures and a lower contribution to volatiles production. This peak is thus shifted to lower temperatures, although the activation energy is increased. The third peak is better represented by a subsequent reaction rather than a third independent reaction. The kinetic parameters obtained for this last peak have very low values, resulting in an ill-defined peak. Based on all previous experiments and the kinetic scheme derived for CCA-treated wood, the following scenario, with respect to the influence of the metals on the pyrolysis behavior, is suggested: (i) Originally, the metals Cu, Cr, and As are preferentially bound to the cellulose and lignin compounds in the CCAtreated wood, which means that no metals are bound to the hemicellulose compound (24). (ii) The thermal decomposition of hemicellulose (first peak) is shifted to lower temperatures in the presence of 2932
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CCA. The metals may have a catalytic effect on the thermal decomposition of hemicellulose, resulting in a lower activation energy and a higher release of volatile compounds. (iii) The cellulose compound is originally loaded with CrAsO4 precipitates and small amounts of other Cu and Cr precipitates or complexes (24), which may hinder the thermal decomposition at first, resulting in a high activation energy for the second peak. However, once the decomposition of cellulose starts, it continues very fast (very high value for preexponential factor), resulting in a very narrow peak. As the temperature increases, the CrAsO4 precipitate decomposes into Cr2O3 and As2O5, which further dissociates in As2O3 and O2 at 327.2 °C (22). The other metal (copper and chromium) compounds, precipitated on or complexed with cellulose, are not volatile. The DTG peak temperature for the second independent reaction (which may be attributed to cellulose decomposition) is 327.9 °C, which is remarkably close to the dissociation temperature of As2O5. There may be a correlation between the decomposition of both compounds. An explanation could be that once the major part of the CrAsO4 precipitate on cellulose has decomposed and the resulting As2O5 has dissociated, the decomposition of cellulose continues very fast. This decomposition must thus be catalyzed by the metals that remained in the solid matrix after the decomposition of CrAsO4 or accelerated by the presence of O2 that results from the dissociation of As2O5, giving rise to the lower DTG peak temperature as compared to untreated wood (371.4 °C). (iv) The intermediate products formed by the thermal decomposition of hemicellulose and cellulose may decompose further at higher temperatures together with lignin. The small values of preexponential factor and activation energy give rise to a very wide DTG peak and thus a very slow charring process. Since lignin is originally complexed with CuCrO4 and CrAsO4 and small amounts of other Cu and Cr compounds (24), these metal compounds may delay the decomposition process. (v) At the end of the pyrolysis process, and maybe even earlier, the metals have been combined to agglomerates (17) and are no longer accessible for catalysis, which may be another reason for the slow charring process at higher temperatures.
Experimental Section Sample Preparation. Samples for Lab-Scale Pyrolysis Study. The type of wood used in the lab-scale pyrolysis study is Pinus sylvestris peeling that is impregnated with a 3.3% solution of type CCA 1-C salt: 32.5% CuSO4‚5H2O, 41.1% Na2Cr2O7‚2H2O, and 26.4% As2O5‚2H2O. The peeling wood chips are between 2 and 135 mm long, between 2 and 17 mm wide, and between 0.5 and 2 mm thick. Double impregnation of the peeling is carried out in order to simulate a CCA level that is higher than the levels found in CCA-treated wood waste (limiting case for industrial scale processes or problems). The peeling is treated twice for class 4 impregnation (wood may be in contact with groundwater and sweet water, requiring a salt retention of 9 kg of CCA/m3 of wood). A full-cell treatment (according to the procedure “Bre´antBethell”) is applied to the wood chips, which have a moisture content of less than 22%, in an industrial autoclave in five stages. (1) After the wood is placed in the cylinder (D ) 2 m and L ) 22 m) and its door is hermetically sealed, a vacuum of -88 kPa is applied for at least 30 min (step 1). (2) Subsequently and without introducing air, the cylinder is filled with preservative by opening the valve, which is connected to the tank containing the diluted CCA solution (step 2). (3) More solution is pumped until the pressure is raised to 1200 kPa and the valve is automatically closed (step 3).
(4) Finally, the pressure is released, while at the same time preservative is removed from the cylinder (step 4), (5) A low vacuum is applied to prevent bleeding of preservative from the surface of the treated wood (step 5). The duration of this total cycle is approximately 3 h. Fixation of the metals in the wood starts and is allowed to continue for about 1 month (for fixation, a minimum of 3 weeks is required, not including the days with temperatures below freezing point), while the samples are left to air-dry under shade and sheltered from rain to prevent leaching. After this fixation period, the peeling samples are treated once again according to the same procedure and using a fresh CCA treating solution. The treatment procedure as well as the post-preservation conditioning are carried out according to the French norm 7925-NF C67-100-1500-3/82 (25), which is developed for the treatment of electricity and telephone poles. The samples have been prepared by the company Beaumartin (France). The exact retention of this double-treated sample is not known, but the metal (Cr, Cu, and As) content is determined using ICP-MS, resulting in mean concentrations of 11.9 mg/g for Cr, 4.29 mg/g for Cu, and 10.5 mg/g for As in the dried CCA-treated wood (23). Samples for TG Study. The type of wood used in the TG study applied to CCA-treated wood is Pinus sylvestris (sapwood) that is impregnated with type CCA 3 oxide: 19.9% As2O5, 30.0% CrO3, 11.1% CuO, and 39.0% H2 O. The crushed wood chips are between 10 and 50 mm long, between 2 and 10 mm wide, and between 0.5 and 3 mm thick. These chips are impregnated with a 3.3% solution of type CCA 3 oxide. A full-cell treatment (according to the procedure “Bre´antBethell”) is applied to the wood chips, which have a moisture content of less than 22%, in a lab-scale autoclave in five stages. (1) After the wood is placed in the cylinder (volume of 10 L) and its door is hermetically sealed, a vacuum of 1 kPa is applied for 15 min (step 1). (2) Subsequently and without introducing air, the cylinder is filled with preservative by opening the valve, which is connected to the tank containing the diluted CCA solution (step 2). (3) More solution is pumped until the reaction vessel is filled completely, after which the valve is closed and the content of the cylinder is pressurized by compressed air: 800 kPa for 40 min (step 3). (4) Finally, the pressure is released, while at the same time preservative is removed from the cylinder (step 4), (5) A low vacuum (2.5 kPa for 5 min) is applied to prevent bleeding of preservative from the surface of the treated wood (step 5). The impregnation process was carried out at the lab of the Technisch Centrum van de Houtnijverheid (TCHN, Brussels). The duration of the total cycle is approximately 70 min, wherafter fixation of the metals in the wood starts. The treated wood samples are placed in a conventional oven with air circulation at a preset temperature of 105 °C for 60 min, after which the fixation is allowed to continue for about 3 months, while the samples are left to air-dry. The exact retention of the CCA-treated sample is not known, but the metal content of the dried sample is determined using ICP-MS, resulting in mean concentrations of 14.2 mg/g for Cr, 5.93 mg/g for Cu, and 11.5 mg/g for As in the CCA-treated wood (21). The CCA-treated wood samples are cut into smaller pieces to be used in the TG analysis. In this study, the samples used throughout are more or less cylindrically shaped and have a diameter of less than 2 mm. Consequently, heat transfer effects are minimized. Lab-Scale Pyrolysis Experiments. The influence of pyrolysis temperature and residence time of the wood chips on the metal content of the resulting pyrolysis residue and the
FIGURE 1. Experimental facility for the pyrolysis of CCA-treated wood chips: updraft fixed-bed pyrolysis reactor system. mass reduction is determined in a lab-scale updraft fixedbed pyrolysis reactor system (18). Therefore, the reactor temperature is varied between 300 and 450 °C, and the duration of the pyrolysis process is varied between 20 and 60 min. The experimental setup is schematically shown in Figure 1. The reactor that holds the wood is a stainless steel circular tube with a diameter of 5 cm and a length of 20 cm. In a typical experiment, 10 g of CCA-treated wood is loaded into the reactor. A flow of heated nitrogen (5 N m3/h) is forced through the reactor. The reactor tube is thermally guarded with a heating tape with a controlled heating power such that the reactor outlet temperature as well as the mean reactor temperature equal the inlet temperature. At the outlet of the reactor, the nitrogen flow mixed with noncondensable pyrolysis gases and microscopic tar droplets is cooled in a highly compact coiled, water-cooled heat exchanger. Upon leaving the gas cooler, the gas stream is passed through four Whatman filters, treated with the strong base tetra-nbutylammoniumhydroxide in series, and drawn to the outside by a fan. The reactor temperature is monitored at three positions (top, middle, and bottom) in the reactor, and an integral mean temperature (averaged in time and space) is calculated as
T h)
1 t - t0
∫
tTtop(t)
t0
+ Tmiddle(t) + Tbottom(t) dt 3
(1)
The duration of the pyrolysis process (t - t0) is defined as the time between the moment (t0) that the reactor temperature is 20 °C lower than the pyrolysis temperature and the end of the pyrolysis experiment (t). In the period [t0, t] the pyrolysis process can be classified as nearly isothermal, and the value of the reactor temperature is set to the integral mean temperature. In Figure 2, for example, the measured reactor temperature profiles at the three positions are plotted for a pyrolysis experiment characterized by a pyrolysis temperature of 400 °C and a duration of 20 min. The corresponding integral mean temperature, calculated acVOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 2. Measured reactor temperature profiles for pyrolysis at 400 °C during 20 min: Tbottom (dashed line), Tmiddle (dash-dotted line). and Ttop (solid line). cording to eq 1, is 394 °C. Before pyrolysis starts, the wood chips are dried by forcing a heated ((120 °C) nitrogen stream through the wood. At the end of the pyrolysis experiment (at time t), a cold nitrogen stream is forced through the reactor to cool the solid pyrolysis residue. The influence of reactor temperature and duration of the pyrolysis process on the weight percentage of As in the pyrolysis residue and on the mass reduction of the wood sample is described elsewhere (26). TG Study with Corresponding Metal Release. The release of metals (Cr, Cu and As) during the pyrolysis of CCA-treated wood is also studied by analyzing the pyrolysis residues, resulting from TG experiments characterized by different combinations of temperature and residence time, for metals. This TG study is performed in a DuPont Instruments 951 thermogravimetric analyzer. The CCA-treated wood sample, approximately 20 mg in weight, is introduced into a quartz sample pan and heated according to a preset temperature profile, using nitrogen (50 mL/min) as purge gas. The treated wood sample is heated to 100 °C at a rate of 20 °C/min, maintained at 100 °C for 10 min, after which the temperature is slowly (10 °C/min) raised to 400 °C and further maintained at 400 °C. This experiment is repeated five times, but each experiment is terminated after different residence times, represented in Figure 3 by points 1-5. The experimentally determined metal concentrations of the five pyrolysis residues and the corresponding residual masses are presented elsewhere (21, 26).
Discussion Arsenic is identified (17, 18, 21, 26) as the problematic compound with respect to metal release during the pyrolysis of CCA-treated wood. Until now, the mechanism controlling the As release is not known. Since chromium(III) arsenate is the major As compound in CCA-treated wood and the decomposition of pure CrAsO4 probably results in the formation of solid Cr2O3 and gaseous As4O6 and O2 according to the scheme (22):
4CrAsO4(s) f 2Cr2O3(s) + 2As2O5(s)
(2)
2As2O5(s) f 2As2O3(l) + 2O2(g)
(3)
2As2O3(l) f As4O6(g)
(4)
the dissociation of arsenic pentoxide (As2O5) according to eq 3 and/or the volatilization of arsenic trioxide (As2O3) according to eq 4 may be responsible for the release of As 2934
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FIGURE 3. Temperature profile used in the TG experiments applied to CCA-treated wood samples with the five termination points indicated.
FIGURE 4. Arsenic release (wt %), as measured in the lab-scale (O) and TG (×) experiments, as a function of mass reduction (wt %) of the solid residue relative to the dried wood. during the pyrolysis process. However, the presence of other compounds (such as hemicellulose, cellulose, lignin, etc.) may influence the behavior of the CrAsO4 precipitate. There may be a correlation between the release of As and the release of other volatile compounds resulting from wood decomposition. Here, two hypotheses are considered: (1) As release is assumed to be determined by the mass reduction of wood(components); (2) As is assumed to be released according to its own kinetic scheme, independent of the release of other volatile compounds; in order to attain a plausible theory describing the As release, which is in agreement with all the experimental observations. Relation to Mass Reduction. Both the release of As and the mass reduction of the solid residue are strongly dependent on temperature and residence time of the wood sample at a given temperature (18, 21, 26). Moreover, As compounds are complexed with or precipitated on wood components. A possible explanation for the behavior of As during pyrolysis could be that the As release is related to the mass reduction of the wood(components). Arsenic release, as measured in the lab-scale and TG experiments, is plotted against the mass reduction of the solid residue in Figure 4. This figure shows that an increasing trend exists between As release and mass reduction. Both
variables may be correlated, but mass reduction may not be the only parameter that influences the As release. The relation between both variables seems to be not unambiguous. Since As is originally “bound” to the cellulose and lignin compounds in wood, the release of As may be determined by the mass reduction of these wood compounds. A plot of the As release as a function of the mass reduction of cellulose + lignin shows again an increasing trend between both variables but no unique relationship. The same conclusions are drawn from the plot of As release as a function of mass reduction of cellulose and the plot of As release as a function of mass reduction of lignin. Since a unique relation between As release and the mass reduction of one or more wood compounds is not found, it is concluded that the As release is not adequately described by this physical model. The correlation between the decomposition of CrAsO4 and cellulose, suggested earlier based on the equal DTG peak temperatures of both compounds, can thus not be translated in the hypothesis that the As release is “caused” by the cellulose decomposition. Rather, it is stated that the presence of CrAsO4 at first hinders the decomposition of cellulose (see scenario formulated in Previous Research Work), which is reflected in a high activation energy for the corresponsding DTG peak, but once the CrAsO4 precipitate decomposes, which not yet implies that the As is released to the gas phase, the decomposition of cellulose continues very fast, which is reflected in a high preexponential factor for the corresponding DTG peak. Reaction Kinetics Scheme. The next hypothesis tested assumes that As is released according to its own kinetic scheme, consisting of a single reaction of order n, independent of the release of other (volatile) compounds. Development of the Kinetic Scheme. The model of pseudohomogeneous kinetics is used to describe the heterogeneous reaction corresponding to As volatilization. The rate equation, assuming an Arrhenius temperature dependency of the rate constant, is given by
dw ) k(T)wn dt
-
(5) FIGURE 6. Coefficient of linear regression as a function of reaction order for isothermal conditions: (*) 350, (O) 400, and (+) 450 °C.
with
(-E RT )
k(T) ) A exp
FIGURE 5. Linear regression applied to experimental points assuming a first-order reaction at three temperatures: (*) 350, (O) 400, and (+) 450 °C.
(6)
where w represents the fraction of the decomposable amount. In the present context (study of As release), decomposable should be interpreted as releasable. The kinetic constants are the reaction order n, the rate constant k, the preexponential factor A, and the activation energy E. R represents the universal gas constant (8.314 J/mol K), and T is the absolute temperature. At isothermal conditions, integration of this equation between t0 and t is easily carried out resulting in (w0 ) wt)t0 ) 1):
for n ) 1 -ln(w) ) k(T)(t - t0)
(7)
w1-n - 1 ) k(T)(t - t0) n-1
(8)
for n * 1
If As release can be described by a single reaction of order n, a plot of the left-hand side of one of these integrated equations as a function of (t - t0), for a constant temperature T, should deliver a straight line with the rate constant k as
the slope. These plots are made for three pyrolysis temperatures (350, 400, and 450 °C) using the results obtained from lab-scale and TG experiments, assuming different reaction orders. The plot is presented in Figure 5 for a reaction order of 1. The resultant coefficients of linear regression (r 2) and the levels of significance for the correlations obtained from linear regression are given as a function of the reaction order (n) in Figures 6 and 7, respectively. The levels of significance are calculated according to the F-test. The values obtained for the coefficients of linear regression are high enough to confirm the hypothesis that As release can be described by a single reaction of order n. Furthermore, the levels of significance for the correlations obtained from the linear regression are all higher than 95% for reaction orders between 1 and 3. Figure 6 shows that the reaction order has no remarkable influence on the coefficient of linear regression. Therefore, the reaction order must be determined based on chemical considerations. For reaction orders higher than 1, the reaction has to be initiated by collisions between molecules. Since the reaction considered here is heterogeneous in nature, collisions between molecules are not very likely to happen. Consequently, the reaction order is set to 1, and the integrated isothermal rate equation is given by eq 7. Here, (t - t0) stands for the duration of the isothermal pyrolysis process (heating up is occurring at t < t0). As a first estimate, the mass of As in the pyrolysis residue at time t0 VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Level of significance (%) for the correlations obtained from linear regression as a function of reaction order for isothermal conditions: (*) 350, (O) 400, and (+) 450 °C. is set to the initial As content, which corresponds to the assumption that no As is released during heating up (Figures 6 and 7 are made based on this assumption). Better estimates of these values are attained through an iterative procedure: linear interpolation (for 350 °C) or extrapolation (for 450 °C) of the values measured in the TG experiments (at 300 and 400 °C) delivers the initial values. These values are then recalculated using the kinetic scheme derived here and the temperature profiles measured, which requires two iteration steps to obtain converged values. This linear regression analysis, with the reaction order set to 1, yields the best fit straight lines for the three temperatures (with (t - t0) in minutes):
-ln(w) ) 0.0078(t - t0) + 0.042
(9)
for a pyrolysis temperature of 350 °C with r 2 ) 0.95 and level of confidence 97.5%;
-ln(w) ) 0.0085(t - t0) - 0.0044
(10)
for a pyrolysis temperature of 400 °C with r 2 ) 0.80 and level of confidence 99.4%; and
-ln(w) ) 0.013(t - t0) + 0.049
(11)
for a pyrolysis temperature of 450 °C with r 2 ) 0.94 and level of confidence 96.7%. The slopes of these lines (0.0078, 0.0085, and 0.013 min-1) are the kinetic rate constants at these particular temperatures. Since the rate constant is assumed to obey an Arrhenius temperature dependency (given by eq 6), the intercept and slope of a plot of ln(k) as a function of 1/T deliver ln(A) and -E/R, respectively. The integral mean temperatures calculated from the measured temperature profiles over the time interval [t0, t] for different values of (t - t0) are not exactly 350, 400, or 450 °C. The different values of T h , corresponding to the different values of (t - t0), obtained for one setpoint of the pyrolysis temperature, are averaged, resulting in T h) 346 °C ( 5 °C for a setpoint of 350 °C, T h ) 390 °C ( 9 °C for a setpoint of 400 °C, and T h ) 427 °C ( 7 °C for a setpoint of 450 °C. Using these mean temperatures in the plot ln(k) versus 1/T (see Figure 8) and applying linear regression to the three points renders the following equation:
ln(k) ) 2936
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(12)
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FIGURE 8. Temperature dependence of the rate constant according to the Arrhenius equation: ln(k) versus 1/T delivers the rate frequency factor and activation energy from intercept and slope. with a coefficient of linear regression of 0.82. The rate frequency factor and activation energy calculated here are as follows: A ) 0.39 min-1 ) 6.5 × 10-3 s-1 and E ) 20.4 kJ/mol. The errorbars in Figure 8 originate from incorporating the experimental uncertainties associated with the determination of the As content of the solid residue. As stated above, the temperatures used in this derivation are integral mean temperatures (in time) and averaged in space based on the measured temperature profiles. They represent the temperature of a mean particle; some particles may be at a temperature that is slightly higher or lower than the calculated mean temperature. Evaluation of the Kinetic Scheme. To reversely check the kinetic model, the integral rate equation describing the As release during pyrolysis of CCA-treated wood is used in combination with the measured temperature profiles T(t) in order to calculate the resultant As content of the pyrolysis residues. For nonisothermal conditions, numerical integration of the temperature-dependent rate constant over time is required:
-ln(w) )
∫ k(T) dt t
t0
(13)
The calculated As content of the pyrolysis residues is compared with the experimental values (lab-scale and TG experiments) in the parity plot, presented in Figure 9. Figure 9 shows that the agreement between calculated and experimental values is fairly good, which means that this simple first-order single-reaction kinetic scheme succeeds in describing the release of As during the pyrolysis of CCA-treated wood. It should be noted that the experimental As contents are higher than the calculated values for all TG results, while for most of the lab-scale experiments the opposite is true. This may be explained by the fact that the lab-scale experiments do not consider one particle at one particular temperature but consider a reactor filled with particles at a mean pyrolysis temperature. Consequently, the collection of these lab-scale results combined with the TG results, which are carried out in the kinetically controlled regime, results in the description of an “averaged” As behavior. Furthermore, the TG experiments are carried out with treated wood samples having a higher CCA content as compared to the lab-scale experiments. TG analysis performed on CCA-treated wood samples with different CCA content (21) showed that the relative concentration of metals (Cr, Cu, and As) in the pyrolysis residue increases with the CCA concentration of the original sample. Consequently, the relative amount of
(
[As(III)]g ) [CrAsO4]0 1 -
1 (k e-tkred kvol - kred vol krede-tkvol)
)
(15)
and for the individual reactions:
d[As(III)]s ) kred[As(V)]s dt
(16)
d[As(III)]s ) kvol[As(III)]s dt
(17)
-
FIGURE 9. Parity plot: experimental As content of the pyrolysis residues (O, lab-scale; *, TG experiments) is compared with the calculated values using the first-order single reaction kinetic scheme. metals found in the pyrolysis residues during the TG experiments should be higher, which is in agreement with the experimental observations. Hence, the kinetic scheme derived here does not account for the influence of the original CCA content. According to eq 13, w is determined by the temperature profile T(t) only, which explains the positive deviation for the TG experiments and the negative deviation for the lab-scale experiments. To check whether the resultant values of the kinetic constants (A ) 0.39 min-1 ) 6.5 × 10-3 s-1 and E ) 20.4 kJ/mol) fall within the range of chemically meaningful values, an exhaustive search in already published work is performed. This leads to the finding that rate constants of the same order of magnitude are found for the reduction reaction of Cr(VI) to Cr(III) by Pizzi (24, 27), who studied the fixation of CCA in wood. In the system wood + CrO3, the Cr(VI) reduction was described by a first-order reaction with A ) 1.1 × 10-3 s-1 and E ) 1.98 kJ/mol, while for the system wood + CCA a first-order reduction reaction with k ) 0.0023 min-1 at a temperature of 80 °C was reported (A and E values were not given). Because of this similarity, the reaction responsible for the As release may be identified as a reduction reaction. The presence of As(III) in the pyrolysis residue (see results of speciation study (19)), while As is originally present as As(V) in CCA-treated wood (as CrAsO4), confirms this hypothesis. Furthermore, the scenario formulated in Previous Research Work also considers the reduction reaction of As2O5 to As2O3 and O2. As(V) is thus reduced to As(III), which is more mobile and more toxic, during the pyrolysis process, and the resultant As(III) is probably volatilized as As4O6, which is a volatile compound. This volatilization reaction is not instanteneous, since As(III) is found in the pyrolysis residue. Consequently, the release of As during the pyrolysis of CCAtreated wood may be better described by two consecutive reactions (reduction followed by volatilization) instead of one single reaction. The suggested scheme for As behavior is then
CCA + wood f CrAsO4 f As(III) f As4O6
(14)
which is a sequence of fixation before pyrolysis and reduction and volatilization during pyrolysis. The As release according to the two consecutive pyrolysis reactions scheme (reduction followed by volatilization) is then described by the following rate equation (28):
To determine the kinetic constants for these reactions (kred and kvol), a speciation study (separation As(III) and As(V)) has to be performed on pyrolysis residues resulting from processes with different temperatures and residence times. However, the rate equation for the two consecutive reactions, given by eq 15, simplifies to eq 7 for kred , kvol, which means that the volatilization is assumed to occur much faster than the reduction reaction. Although the volatilization reaction is not instanteneous, it may occur faster than the reduction reaction. In this case, the kinetic scheme consisting of one single reaction serves as a good approximation. The determination of the actual value of kvol relative to kred will decide in favor of this simple or the more complex scheme however requires additional experimental work. The good agreement between experimental and calculated values (see Figure 9), using the simple scheme derived here, already shows that the single-reaction scheme with the corresponding rate constants is of high value as a first approximate to describe the As release during pyrolysis. To conclude this paper, the scheme presented here is compared with the observations, concerning the As release during the pyrolysis or burning of CCA-treated wood, postulated by other researchers. McMahon et al. (15) reported that negligible amounts of arsine (AsH3) are formed during CCA wood combustion. Moreover, they expected thermal decomposition of the pentavalent arsenic in the wood to yield arsenic trioxide (As2O3 or As4O6) as well as various arsenites (AsO33-) and arsenates (AsO43-). Essentially all of the volatilized As recovered was found in the condensed (particulate) form and consisted of both arsenites and arsenates. The volatile arsenic trioxide, however, could not be trapped efficiently. Hirata et al. (16) stated that arsenic compounds are first reduced to As2O3 with heating, after which it is gassified according to the equilibrium:
2As2O3 h As4O6
(18)
and generally accepted to be As4O6 for temperatures up to 1073 °C. Cornfield et al. (4) did not detect arsine or other metal compounds in volatile nonparticulate form. They suggested that the metals released are all present in particulate form. All these observations confirm the hypothesis that As is released as the volatile arsenic trioxide (As2O3 or As4O6), which is very difficult to capture and therefore rarely detected. Arsine (AsH3) is not found in the gas exhaust since it decomposes at 300 °C. The only way to release arsenic as As(V) compounds is in the particulate or aerosol phase, as condensed arsenates. These can be captured by scrubbing and filtering. The decomposition of wood components may give rise to methyl groups that can methylate arsine (at temperatures below 300 °C), resulting in volatile organic As(III) compounds that are less toxic than arsenic trioxide. Since earlier studies do not report kinetic schemes or rate constants to model the As release during the pyrolysis of CCA-treated wood, the simple one reaction scheme derived in this paper may be of value. A more advanced model should account for the influence of VOL. 34, NO. 14, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the initial CCA content and incorporate the volatilization reaction following the reduction reaction. Both extensions need more experimental work: pyrolysis experiments using CCA-treated samples with different CCA content and speciation study of the pyrolysis residues.
Acknowledgments The authors thank the Department of Chemical Engineering of the Katholieke Universiteit Leuven for their valuable help in conducting the quantitative analysis of the samples. We are especially grateful to Prof. C. Vandecasteele from the Chemical Department for his comments through the course of the study. Also the assistance of Steven Mullens and Prof. Jules Mullens in the TG analysis at the Laboratory of Inorganic and Physical Chemistry of the Limburgs Universitair Centrum are acknowledged. Furthermore, we are grateful to Beaumartin S.A. and Mr. J. S. Hery in particular for the financial support, the helpful discussions, and the wood samples.
Literature Cited (1) Pasek, E. A.; McIntyre, C. R. Presented at the 24th Annual IRG Meeting, Orlando, FL, 1993; IRG/WP 93-50007. (2) Whelte, S.; Meier, D.; Moltran, J.; Faix, O. In Developments in Thermochemical Biomass Conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Chapman & Hall: London, 1997; pp 206219. (3) Chartherm treated wood recycling, http://www.beaumartin. tm.fr/. (4) Cornfield, J. A.; Vollam, S.; Fardell, P. Presented at the 24th Annual IRG Meeting, Orlando, FL, 1993; IRG/WP 93-50008. (5) Honda, A.; Kanjo, Y; Kimoto, A.; Koshii, K.; Kashiwazaki, S. Presented at the 22nd Annual IRG Meeting, Kyoto, Japan, 1991; IRG/WP/3651. (6) Bridgwater, A. V.; Bridge, S. A. In A review of biomass pyrolysis and pyrolysis technologies; Bridgwater, A. V., Grassi, G., Eds.; Elsevier Applied Science: London, 1991; pp 11-92. (7) Bridgwater, A. V.; Peacocke, G. V. C. Presented at the 2nd Biomass Conference of the Americas, Portland, OR, 1995. (8) Bridgwater, A. V. Presented at the Biomass and Renewable Energy Seminar, Loughborough University, U.K., March 1995. (9) Wilkins, E.; Murray, F. Wood Sci. Technol. 1980, 14 (4), 281288.
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(10) Dobbs, A. J.; Phil, D.; Grant, C. Holzforschung 1978, 32 (1), 3235. (11) Marutzky, R. Holz Roh Werkst. 1990, 48, 19-24. (12) Kramer, J. E.; Lustenhouwer, J. W. A.; Van Weenen, J. C.; Brinkkemper, M. A. C. Gebruik van afvalhout; Rapport 17; Ministerie van Volksverhuizing, Ruimtelijke Ordening en Milieubeheer (VROM): Leidschendam, The Netherlands, 1985. (13) McMahon, C. K.; Bush, P. B.; Woolson, E. A. Presented at the 78th Annual Meeting of the Air Pollution Control Association, Detroit, MI, 1985; Paper 85-56.3. (14) Verbranding van afvalhout, Deel 1; Nationaal Onderzoeksprogramma Hergebruik van Afvalstoffen (NOH): The Netherlands, 1987; Project 51766.01. (15) McMahon, C. K.; Bush, P. B.; Woolson, E. A. For. Prod. J. 1986, 36, 45-50. (16) Hirata, T.; Inoue, M.; Fukui, Y. Wood Sci. Technol. 1993, 27, 35-47. (17) Helsen, L.; Van den Bulck, E.; Hery, J. S. Waste Manage. 1998, 18, 571-578. (18) Helsen, L.; Van den Bulck, E. In Developments in thermochemical biomass conversion; Bridgwater, A. V., Boocock, D. G. B., Eds.; Blackie Academic & Professional: London, 1997; pp 220-228. (19) Van den Broeck, K.; Helsen, L.; Vandecasteele, C.; Van den Bulck, E. Analyst 1997, 122, 695-700. (20) Helsen, L.; Van den Bulck, E. Holzforschung 1998, 52, 607-614. (21) Helsen, L.; Van den Bulck, E. J. Anal. Appl. Pyrolysis 1999, 52, 65-86. (22) Helsen, L.; Van den Bulck, E. J. Anal. Appl. Pyrolysis 2000, 53, 51-79. (23) Helsen, L.; Van den Bulck, E.; Van den Broeck, K.; Vandecasteele, C. Waste Manage. 1997, 17 (1), 79-86. (24) Pizzi, A. J. Polym. Sci. 1982, 20, 739-764. (25) Norme Franc¸ aise enregistre´e; l’Union technique de l’Electricite´, 12 place des Etats-Unis, 75783 Paris Cedex 16, France, 1982; NF C67-100. (26) Helsen, L. Ph.D. Dissertation, Katholieke Universiteit Leuven, Belgium, 2000. (27) Pizzi, A. J. Polym. Sci. 1981, 19, 3093-3121. (28) Pizzi, A. J. Polym. Sci. 1982, 20, 707-724.
Received for review September 27, 1999. Revised manuscript received April 17, 2000. Accepted April 20, 2000. ES991102W
