Reactivity in Oxygen and Carbon Dioxide of Char Formed in the

The reactivity in oxygen and carbon dioxide of chars obtained from the pyrolysis of a refuse-derived fuel (RDF) was investigated. RDF chars were obtai...
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Ind. Eng. Chem. Res. 2000, 39, 864-872

Reactivity in Oxygen and Carbon Dioxide of Char Formed in the Pyrolysis of Refuse-Derived Fuel Valerio Cozzani† Dipartimento di Ingegneria Chimica, Chimica Industriale e Scienza dei Materiali, Universita` degli Studi di Pisa, via Diotisalvi n.2, I-56126 Pisa, Italy

The reactivity in oxygen and carbon dioxide of chars obtained from the pyrolysis of a refusederived fuel (RDF) was investigated. RDF chars were obtained in a fixed-bed pyrolysis reactor at low heating rates (60 °C/min) and temperatures between 500 and 800 °C. The heterogeneous gasification kinetics of the chars were studied using thermogravimetric methods and were compared to data available in the literature. The RDF chars were found to have a reactivity quite similar to that of chars obtained from municipal solid wastes and wood, but higher than that of graphite of about 5 orders of magnitude in oxygen. Raising the final temperature of the pyrolysis process from 500 to 800 °C resulted in the decrease of the H/C ratio and in a significantly lower reactivity in oxygen of the char. Introduction Environmentally compatible disposal and energy recovery from municipal solid wastes (MSW) is a problem that has received increasing attention in the past years. The high quantities of MSW that are produced call for the development of low environmental impact processes, allowing energy production from wastes by thermochemical processes.1,2 In this field, besides conventional incineration processes, a growing interest is present in Europe for the development of industrial-scale gasification units.3-5 In gasification processes, two important steps are always present: (i) a pyrolysis stage in which the solid feed undergoes devolatilization reactions to yield volatiles (gases and tars) and a solid fraction (char); (ii) a gasification stage in which the char undergoes heterogeneous reactions to yield gaseous products and an inert residue (ash). These two steps may also be present in combustion or incineration processes, in particular if the solid feed has a high volatile content, as in the case of biomass. Pyrolysis and gasification stages may be sequential or contemporary, depending on the features of the process. However, the gasification stage is the rate-controlling step because the gasification reactions, involving the heterogeneous oxidation of the solid product fraction, are slower than the pyrolysis process.4 Usually, gasification and combustion processes should provide the complete gasification of char, to avoid the emission of unburned carbon in ashes. Thus, information on char reactivity is necessary to afford the optimization of gasification reactors and to allow these processes to comply with environmental regulations. A relevant amount of data is present in the literature on the mechanisms and the material balances involving char formation in the pyrolysis of waste and lignocellulosic materials.6,7 However, less information is present on the reactivity of char obtained from the pyrolysis of wastes. Lignocellulosic wastes and plastic wastes are the two main components of MSW and have quite different † Tel.: (+39)-050-511212. Fax: (+39)-050-511266. Email: [email protected].

pyrolysis and gasification behaviors. Data on the reactivity of chars obtained from biomass are reported by several authors.8,9 On the other hand, less attention was devoted to the influence of the plastic fraction on the overall gasification reactivity of char. Only recently Henrich et al.5 reported kinetic data for the oxygen and carbon dioxide reactivity of char formed from MSW at slow heating rates in a rotary kiln pyrolysis reactor. The comparison carried out with chars obtained from the pyrolysis of thermohardening plastics pointed out that important differences are present between the reactivity of chars obtained from plastic and lignocellulosic wastes. Previous work also evidenced that plastic wastes may cause relevant char formation also by secondary gasphase mechanisms.10 This study focused on the characterization of the gasification reactivities in oxygen and carbon dioxide of chars obtained from the pyrolysis of a refuse-derived fuel (RDF). RDF is commercially produced from municipal solid wastes, reducing the content of the noncombustible fraction by shredding, sorting, and metalrecovery processes. RDF chars were obtained in a laboratory-scale fixed-bed pyrolysis reactor at temperatures between 500 and 800 °C. The heterogeneous gasification kinetics of RDF chars was studied using thermogravimetric methods. The data obtained were compared to those present in the literature and allowed a relative ranking of RDF char reactivity. The mechanism of char gasification was also discussed with respect to the experimental results obtained. Experimental Section Techniques. A laboratory-scale fixed-bed reactor (FBR) was used for RDF char production. Sample heating rates ranged between 0.5 and 1.5 °C/s and final temperatures between 500 and 800 °C were used. A residence time of 60 min at the final temperature was accomplished. Further details on reactor characteristics and mode of operation are given in previous papers.11,12 The low heating rates and low temperatures used for char production may well represent the actual conditions of rotary kilns and of other large-scale commercial devices used in waste pyrolysis and gasification processes.5

10.1021/ie990534c CCC: $19.00 © 2000 American Chemical Society Published on Web 03/16/2000

Ind. Eng. Chem. Res., Vol. 39, No. 4, 2000 865 Table 1. Ultimate Analysis of the RDF Used for Char Production and of the Chars Obtained at Different Pyrolysis Temperatures (*) By Difference % C N H O* ashes H/C ratio

RDF 48.3 0.6 7.6 31.9 11.6 0.157

500 °C char

600 °C char

700 °C char

800 °C char

37.0 1.1 2.1 24.4 35.4

42.2 0.8 1.3 18.6 37.1

43.6 0.6 1.1 17.1 37.6

49.2 0.5 0.1 12.0 38.2

0.057

0.031

0.025

0.008

A Mettler TG-25 thermobalance and a Netzsch STA 409C thermoanalyzer were used for thermogravimetric (TG) runs. Simultaneous TG and differential scanning calorimetry (DSC) data were obtained from the Netzsch thermoanalyzer. A constant heating rate of 10 °C/min (0.167 °C/s) and typical sample weights of 1.5-4 mg were used in experimental runs. TG-FTIR simultaneous measurements were carried out using a Bruker Equinox 55 spectrometer. The spectrometer was coupled to the Netzsch analyzer using a 4-mm internal diameter Teflon tube. The 800-mmlong transfer line and the head of the TG balance were maintained at 200 °C. IR measurements were carried out in a low-volume gas cell (8.7 mL) specifically developed by Bruker and maintained at 220 °C. The gas flow from the TG outlet to the IR gas cell was 60 mL/ min and a residence time of 35 s could be evaluated for the evolved gases. The effects of different oxidizing conditions and gasifing agents on the weight loss behavior were tested by performing TG runs in various gaseous mixtures. Oxygen-nitrogen mixtures with 6.1%, 11.7%, and 21.0% oxygen by volume and carbon dioxide-nitrogen mixtures with 19.6%, 46.3%, and 100.0% carbon dioxide by volume were used. Gas flows of 60-200 mL/min were used during experimental runs. Materials. The pyrolysis chars were produced from a commercial RDF supplied by Daneco (Udine, Italy). The RDF was obtained from municipal solid wastes, reducing the content of the noncombustible fraction by mechanical sorting and metal-recovery processes. A rough composition of the RDF used for char production is the following: lignocellulosic wastes, 67.3 wt %, plastic wastes, 21.1 wt %, and inerts, 11.6 wt %. The analysis of the mineral fraction of the RDF by atomic emission spectrometry showed the presence of several metal species (values are in mg/kg of RDF): iron (2432), lead (620), copper (332), nickel (101), manganese (98), and minor amounts of chrome (5), vanadium (6), and cadmium (2). The solid product fractions obtained from the RDF pyrolysis in the FBR at temperatures of 500, 600, 700, and 800 °C were collected. The furnace temperature used for char production will be used in the following to identify the char samples. The results of the ultimate analysis of the char samples and of the RDF used for char production are reported in Table 1. As expected, the chars were found to have a high content of inert materials (35-40 wt %) and to be mainly composed of carbon. As shown in previous studies, char yields from lowheating-rate pyrolysis processes of RDF are only slightly dependent on pyrolysis temperature.11 The char yields13 obtained from the FBR ranged between 28 wt % at 500 °C and 26 wt % at 800 °C.

Results and Discussion Experimental Results. Figure 1 shows the weight loss curves obtained from TG runs at 10 °C/min in 100% nitrogen, 21% oxygen, and 100% carbon dioxide for the 500 °C and the 800 °C RDF char. The data reported in this and in the following figures were calculated as the mean of at least three experimental runs. The differences in weight loss with respect to temperature in different runs were less than 2%. The figure shows that for both char samples oxidation in air takes place between 300 and 500 °C at a heating rate of 10 °C/min and oxidation in carbon dioxide takes place at temperatures between 700 and 1000 °C. Heating the 800 °C char in nitrogen results in a limited weight loss. This is possibly due to desorption of surfaceadsorbed oxygen and to thermal rearrangements of the char structure. Besides this phenomenon, the 500 °C shows a 10% weight loss step between 550 and 700 °C. This weight loss step was also found for the 600 °C, but is not present in the case of the 700 and 800 °C chars. Figure 1a shows that the 10% weight loss step between 550 and 700 °C is also present during the heating of the char in carbon dioxide. The weight loss curve of the 500 °C char in the 21% oxygen-nitrogen mixture also shows a 10% weight loss in the same temperature range (the second weight loss step in Figure 1a). To check if this second weight loss step is due to char oxidation, a run was carried out in the Mettler TGA by changing the purge gas from 21% oxygen to 100% nitrogen at 500 °C. When a gas flow of 200 mL/min is used, the TG furnace has a residence time of 6 s at 500 °C. Thus, at temperatures higher than 550 °C (5 min after the change in the gas supply) the oxygen concentration in the surroundings of the sample was reasonably negligible. Nevertheless, as shown by the dash-dotted line in Figure 1a, the sample experienced the same weight loss as that shown in 21% oxygen. Thus, it is quite clear from the figure that a common phenomenon is likely to cause the 10% weight loss step observed either in 100% N2 or in 21% O2 for the 500 and 600 °C chars at temperatures between 550 and 700 °C. TG-FTIR runs at 10 °C/min were carried out to obtain data on the products evolved during the weight loss of the 500 °C char in pure nitrogen. Carbon monoxide, hydrocarbon (C1-C3), and carbonylic compounds were not detected in the evolved gases from IR spectra analysis. On the other hand, carbon dioxide was formed during the weight loss, as shown by the integral of the absorbance between 2400 and 2240 cm-1, reported in Figure 2. Similar results were obtained for a char sample oxidized at 500 °C for 30 min in air and then heated at 10 °C/min to 800 °C in pure nitrogen. The weight loss experienced by the 500 and 600 °C chars is well in the range of calcium carbonate decomposition to carbon dioxide and calcium oxide. The lignocellulosic wastes from which the char was obtained were shown to contain relevant quantities of carbonates, used as fillers in paper production.11 The almost complete absence of CO and the formation of CO2 confirms that the weight loss experienced by the 500 and 600 °C chars at temperatures between 550 and 700 °C is not due to oxidation phenomena and is attributable to carbonates decomposition. Figure 3 shows the sample conversion obtained from the constant heating rate TG runs in the temperature range of 300-600 °C for different O2 concentrations.

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Figure 1. TG weight loss curves of RDF chars using different purge gases (constant heating rate of 10 °C/min, 100 mL/min). (a) 500 °C char. (b) 800 °C char.

Figure 2. Integral of absorbance between 2400 and 2240 cm-1 (CO2) with respect to temperature during the heat-up in 100% N2 of RDF chars (10 °C/min, 60 mL/min N2 flow). Continuous line, 500 °C char; dashed line, 500 °C char previously oxidized in air at 500 °C for 30 min.

Sample conversions between 700 and 1000 °C for different CO2 concentrations are shown in Figure 4. Conversion was defined as

ξ)

W0 - W W0 - Wf

(1)

where W is the sample weight, W0 the initial weight, and Wf the final weight in the temperature range considered (300-600 °C for O2; 700-1000 °C for CO2). For the 500 and 600 °C chars, to avoid the interference of the second weight loss step due to carbonates decomposition, conversion in O2 was calculated using the weight at 550 °C as Wf. As expected, both the figures show that reducing the oxidizing species concentration in the TG furnace resulted in the decrease of the reaction rate. Figure 5a reports the differential thermogravimetric (dTG) curves obtained for the four char samples in 21% oxygen. The maximum in the weight loss rate, commonly used in the literature to characterize char reactivity,14 occurs at a temperature higher than about 100 °C for the 800 °C char. This difference is also

evidenced in Figure 5b, where the DSC curves obtained for the 500 and 800 °C char in the simultaneous TGDSC measurements are reported. A 100 °C difference is present between the temperatures of the maximum heat flow, although for both chars the integration of the heat flow curve yielded a value of about 7000 kJ/kg for the apparent heat of combustion. The temperature values of the maximum heat generation during char oxidation are in good accordance with those obtained for the weight loss rate, thus confirming the different reactivities of the chars. These results point out that, as generally found in the literature, the reactivity in oxygen of the char decreases, increasing the pyrolysis temperature. The results obtained in 100% carbon dioxide are reported in Figure 6a. The comparison of the DSC curves recorded simultaneously by the instrument for the 500 and 800 °C char is shown in Figure 6b. A mean value of about 950 kJ/kg was found by subtracting the baseline and integrating the heat flow curve for the endothermic heat of reaction of both the char samples. These results make it clear that limited differences are present in the carbon dioxide reaction of the different RDF chars. This is possibly caused by the higher temperatures at which the reaction process with carbon dioxide takes place, which may reduce the influence of the factors as the difference in the active surface area of the samples. Kinetic Data Analysis. The experimental data reported in Figures 3 and 4 were used to obtain quantitative information on the reactivity of the different chars. A simple kinetic analysis was performed using a single-step lumped reaction model:

CHAR + OS f GASEOUS PRODUCTS where OS is the oxidizing species (O2 or CO2). The main reaction represented by this scheme is carbon oxidation to carbon monoxide. For each char, the TG experimental data were correlated using an nth order Arrhenius rate equation:

dξ ) APgne(-Ea/RT)(1 - ξ)m ) K(T,Pg)(1 - ξ)m (2) dt where A is the frequency factor, Ea the activation energy

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Figure 3. RDF chars conversion during TG runs up to 600 °C using different oxygen concentrations (constant heating rate of 10 °C/min, 100 mL/min). Dots, experimental data; lines, kinetic model predictions. (a) 500 °C char. (b) 600 °C char. (c) 700 °C char. (d) 800 °C char.

of the reaction, R the gas constant, T the temperature, Pg the oxidizing species partial pressure, K an apparent kinetic constant, n the order of reaction with respect to gas partial pressure, m the reaction order with respect to char conversion, and ξ the sample conversion. If the partial pressure of the oxidizing species is not modified, an apparent pre-exponential factor may be defined: A′ ) APng . Equation 2 allows the direct correlation of TG data to the apparent kinetic constant:

K(T) )

(dξdt ) (1 - ξ)m

(3)

The values of conversion may be obtained from the experimental data using eq 1. The conversion rate may also be calculated from dTG data using the time derivative of eq 1:

dW dt dξ )dt (W0 - Wf)

( )

(4)

Thus, the terms dξ/dt and (1 - ξ) in eq 3 may be easily obtained from experimental TG data. The value of the

reaction order m is unknown, but a best-fit estimate may be easily obtained. It is clear from eq 2 that, using an Arrhenius nth order kinetic model, if the partial pressure of the gas is constant, the logarithm of K must show a linear dependence on the reciprocal of the temperature:

1 ln(K(T)) = a + b T

(5)

where a ) -Ea/R and b ) ln(A′). To estimate the bestfit value of the order of reaction m with respect to sample conversion, tentative values of 0, 0.5, 1, 1.5, and 2 where assumed and the parameters a and b in eq 5 were calculated for each value of m using a linear leastsquares method. Thus, it was possible to estimate the value of m that minimized the model error with respect to the values of ln(K) calculated from experimental data. The best-fit value of m obtained with this procedure resulted in m ) 1 for all the chars, either in oxygen or in carbon dioxide. Therefore, from the best-fit values of the parameters a and b in eq 5 calculated assuming m ) 1, it was possible to evaluate the apparent kinetic constant A′ and the activation energy Ea for each char and for each oxidizing gas partial pressure. Limited differences ((5%)

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Figure 4. RDF chars conversion during TG runs up to 1000 °C using different carbon dioxide concentrations (constant heating rate of 10 °C/min, 100 mL/min). Dots, experimental data; lines, kinetic model predictions. (a) 500 °C char. (b) 600 °C char. (c) 700 °C char. (d) 800 °C char.

were found between the values of the activation energy obtained for the different chars and different oxygen concentrations. Even smaller differences were found for the runs in carbon dioxide ((2%). These results suggested one to assume for the present analysis, at least as a working hypothesis, a mean activation energy value, Ea, obtained from a fitting procedure extended to all the chars and all the oxidation conditions. The use of a common value of the activation energy required the recalculation of the corresponding pre-exponential factors, A′. The results were used to evaluate the reaction order with respect to the oxidizing species n. The following expression was derived for n:

n)

(ln Ai′ - ln Aj′) (ln Pg,i - ln Pg,j)

(6)

where Ai′ and Aj′ are the A′ values obtained at oxygen partial pressures Pg,i and Pg,j, respectively. The values of n estimated for the four chars in oxygen and carbon dioxide are reported in Table 2. The limited differences in the results obtained for the different chars suggested that a mean value could be used in the kinetic modeling. The “true” pre-exponential factors A could thus be calculated for each char from eq 2 and are reported in Table 2.

The pre-exponential factors reported in the table for the different RDF chars with respect to O2 oxidation evidence a decrease in char reactivity with respect to the temperature used for char production. The reactivity decreases by a factor of 5 if the pyrolysis temperature is increased from 500 to 800 °C, as shown in Table 2 by the values of the ratio of the pre-exponential factor of each char to that of the 800 °C char. The decrease in reactivity is more pronounced between 500 and 700 °C, while smaller differences are present between the 700 and 800 °C chars. Previous measurements11 carried out on similar RDF chars produced in the FBR furnace used for the present study showed a moderate increase of the internal surface area due to microporosity, changing from 20 to 60 m2/g as the pyrolysis temperatures were increased from 500 to 900 °C. The marked decrease in reactivity shown by the values of the pre-exponential factors in Table 2, despite the increase in internal surface area, points out that at higher pyrolysis temperatures a strong reduction of the active surface area to the total surface area ratio takes place. An additional element is given by the decrease in the H/C ratio with respect to char pyrolysis temperature, shown in Table 1. All these factors strongly suggest that, as found for other carbonaceous materials,9,15 thermal annealing

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Figure 5. Experimental dTG (a) and DSC (b) curves obtained in 21% oxygen for the different RDF chars (constant heating rate of 10 °C/min, 150 mL/min).

Figure 6. Experimental dTG (a) and DSC (b) curves obtained in 100% carbon dioxide for the different RDF chars (constant heating rate of 10 °C/min, 150 mL/min).

plays a role in the decrease of RDF char reactivity with pyrolysis temperature. Contrary to the results found for O2, the preexponential factors estimated for the CO2 reactivity of the chars reported in Table 2 do not show a clear trend with pyrolysis temperature. However, the limited differences in reactivity result in small differences of the values of A (less than a factor of 1.3) and justify the use of a single value of the pre-exponential factor A in the kinetic model. As discussed previously, the reason for this difference between O2 and CO2 reactivity may be the higher CO2 reaction temperatures. Table 3 summarizes the values of the kinetic parameters obtained. On the basis of the present analysis, the gasification reaction mechanism seems to be similar in oxygen and in carbon dioxide. Although higher temperatures are required, the results obtained for heterogeneous gasification of RDF char in oxygen and in carbon dioxide are compatible with an adsorption-desorption mechanism similar to that proposed for heterogeneous gasification of coal chars in oxygen by Laurendau17 and Essenhigh.18 With this kinetic model, an activation

energy of 120-150 kJ/mol for the overall reaction rate of coal chars in O2 was estimated.17 The resulting global conversion rate dependence on the oxidizing species partial pressure should be comprised between 0 and 1, the actual value depending on the relative influence of the adsorption and desorption steps on the overall reaction rate. Thus, despite the relevant morphological differences of the waste-derived lignocellulosic chars with respect to the coal chars,11 the values obtained for the activation energy and the reaction order with respect to oxygen concentration are well in accordance with the three-step reaction mechanism proposed for the heterogeneous oxidation of coal char. A value of 1 for the reaction order with respect to sample conversion should be expected on the basis of a Langmuir-Hinshelwood kinetic mechanism in the absence of pore diffusional limitations. Complicating phenomena, such as internal diffusion in the different porosity classes and the changes in morphology due to the mass, should be accounted for by using more complex models.20,21 Although eq 2 oversimplifies the actual dependence of the reaction rate on conversion,

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Table 2. Values of the Parameters Obtained from the Kinetic Analysis of the Different RDF Char Samples in Oxygen and Carbon Dioxide reactivity in oxygen

reactivity in carbon dioxide

char sample

A (s-1 atm-n)

A/A800°C char

n

A (s-1 atmn)

A/A800°C char

n

500 °C 600 °C 700 °C 800 °C

1.08 × 1010 4.58 × 109 2.84 × 109 1.89 × 109

5.68 2.42 1.50 1.00

0.54 0.66 0.61 0.72

3.89 × 107 3.79 × 107 4.85 × 107 4.15 × 107

0.93 0.91 1.17 1.00

0.66 0.70 0.73 0.75

Table 3. Parameters Used in the Kinetic Model A (s-1 atm-n) Ea (kJ/mol) m n

oxygen

carbon dioxide

see Table 2 162 1 0.64

4.2 × 107 221 1 0.72

the mechanistic model used herein is sufficient to represent the influence of sample conversion on the reaction rate, at least because rate limitations due to internal diffusion may be neglected. Figures 3 and 4 show a comparison between the sample conversion predictions obtained from the kinetic model and the experimental TG data in oxygen and carbon dioxide. Both figures point out that despite the several assumptions used to correlate the experimental data, the kinetic model reproduces with sufficient precision the TG main weight loss step during RDF char oxidation. Nevertheless, the kinetic parameters for RDF char oxidation obtained from this approach are obviously only apparent values. Furthermore, the kinetic model validity is limited to the low heating rates and temperature ranges used in the present work. However, a single-step reaction model has been widely used to characterize the intrinsic reactivity of carbonaceous materials, thus allowing the comparison of the results obtained to those reported in the literature. Comparison to Literature Data of RDF Char Reactivity. Char reactivity is influenced by a number of factors, such as operating conditions, particle sizes, characteristics of the devices used for char production, and reaction regime used for experimental data production. These problems are known to have caused a large scatter of the data reported in the literature for coal char reactivity.16 Nevertheless, a collection of experimental kinetic data obtained from reasonably similar oxidation conditions is reported in Table 4 for chars obtained from biomass and waste-derived materials. Data concerning soot and graphite were also included for the sake of comparison. The table also shows the available information on the materials and conditions used for char production. The Arrhenius plots for oxidation in 21% oxygen and in 100% carbon dioxide are reported in Figure 7 for some of the materials included in the table. Table 4 reports the estimated values for the reaction order with respect to oxygen partial pressure. This is generally found to be comprised between 0 and 1 for all the chars and carbonaceous materials, either in O2 or in CO2. Values between 0.5 and 1 for the reaction order with respect to sample conversion, m, are generally used for kinetic data correlation in oxygen. With respect to carbon dioxide, a unitary value of m was used for all the chars considered in Table 4.. However, it must be remarked that in most cases the value of m was assumed to be equal to 1 in kinetic data interpretation, while only in a few cases it was actually estimated from the kinetic analysis. Nevertheless, the values found for the RDF chars are well in the range of those present in the literature.

With respect to oxygen reactivity, it can be observed from Table 4 that the chars obtained from substrates mainly composed of lignocellulosic materials (cellulose, wood, MSW) have activation energies comprised between 110 and 160 kJ/mol. Thus, the results obtained for the RDF chars (162 kJ/mol) fall well in the range of previous results. An even better agreement of the kinetic data can be observed if the global reactivity is considered. Figure 7a evidences that the reactivity in 21% oxygen of MSW, pine wood, and cellulose chars are very close to those found for the RDF chars. The slightly lower activation energies shown by the MSW and pine wood chars may have originated with the lower temperatures used in the experimental investigation.18 The minor differences in O2 gasification reactivity of chars derived from MSW, RDF, and wood suggest that the influence of the plastic fraction on the overall char reactivity is limited. Previous results12 obtained for the pyrolysis of thermoplastic polymers in the FBR showed that these materials mainly yield a secondary char, produced by a gas-phase mechanism. On the contrary, biomass, MSW, and RDF chars are mainly the result of the primary pyrolysis process. Thus, the overall reactivity of char obtained from MSW and RDF seems to be mainly influenced by that of the lignocellulosic materials (paper, cardboard, biomass) present in the wastes. With respect to CO2 reactivity, almost all the chars in Table 4 have activation energies between 210 and 240 kJ/mol. Figure 7b shows that the global reactivity of the RDF char in 100% carbon dioxide is very similar to that of MSW. As in the case of oxygen, chars obtained from the pyrolysis of polyethylene and of electronic scrap have a lower reactivity, while graphite and diesel soot are the more refractory to undergo the gasification process. As a matter of fact, these two materials show a reactivity that is more than 2 orders of magnitude lower than that of the RDF char. The data in Figure 7b confirm that as found in the case of reactivity in oxygen, the chars formed in the pyrolysis of RDF are quite reactive materials. These results suggest that the ashes may have a catalytic effect on the oxidation reactivity of MSW and RDF chars. As shown by the ash ultimate analysis reported in the Experimental Section, RDF and RDF chars have a high content of inorganic compounds that may act as catalysts. The reactivity differences between the various chars follow similar trends in oxygen and in carbon dioxide. The materials showing higher reactivity with respect to O2 also have higher reactivity with respect to CO2. However, the differences in the reactivities of the various chars in carbon dioxide (about 3 orders of magnitude) are lower than those in oxygen (about 5 orders of magnitude). Smaller influences on reactivity are detected also with respect to the different operating conditions used for char production. This is possibly due to the higher temperatures required by the reaction process.

Ind. Eng. Chem. Res., Vol. 39, No. 4, 2000 871 Table 4. Kinetic Parameters Reported in the Literature for the Reactivity in Oxygen and Carbon Dioxide of Chars Obtained from the Pyrolysis of Lignocellulosic Materials and Wastes conditions for char production Figure 7: curve n 1 2

3 4 5 6 7 8 9

material cellulose char pine wood char pine wood char pine wood char hardwood char graphite soot diesel soot polyethylene char electronic scrap char MSW char RDF char RDF char

heating rate final residence (°C/min) temp. (°C) time (min) 5 slow 20 18000 3

550 n.a. 700 600 600

n.a. n.a. n.a. 1 n.a.

60 slow slow 60 60

800 700 500 500 800

60 60 60 60 60

kinetic parameters in O2 kinetic parameters in CO2 Ea (kJ/mol) 160 132 152 125 109 167 130 207 234 119 110 162 162

m

n

1 0 0 0.49 0 1 1 1 1 1 1 1 1

0.78 0.87 n.a. 0.53 0.92 0.39 0.81 1.00 0.00 0.50 0.60 0.64 0.64

Ea (kJ/mol) n.a. n.a. n.a. n.a. n.a. 211 241 n.a. n.a. 163 231 221 221

m

n

n.a. n.a. n.a. n.a. n.a. 1 1 n.a. n.a. 1 1 1 1

n.a. n.a. n.a. n.a. n.a. 0.36 0.60 n.a. n.a. 0.59 0.27 0.72 0.72

ref n (22) (9) (23) (9) (24) (5) (5) (25) (10) (5) (5) this work this work

magnitude in oxygen and 3 orders of magnitude in carbon dioxide. Increasing the severity of the pyrolysis process by raising the final temperature from 500 to 800 °C resulted in a significantly lower reactivity of RDF chars in oxygen, while it had a minor effect on the reactivity in carbon dioxide. The increase of the char internal surface area and the decrease of the H/C ratio suggest that also in the case of RDF thermal annealing plays a role in the lower reactivity of char obtained at higher temperatures. The results obtained either in oxygen or in carbon dioxide could be easily interpreted, assuming an adsorption-desorption mechanism for RDF char gasification similar to that proposed in the literature for the oxidation of coal char. Nomenclature A ) frequency factor (s-1 atm-n) A′ ) apparent frequency factor (s-1) Ea ) activation energy (J/mol) K ) kinetic constant (s-1) m ) reaction order with respect to sample conversion W ) sample mass n ) reaction order with respect to gas partial pressure Pg ) gasifing agent partial pressure (atm) R ) universal gas constant (8.31 J/(mol K)) t ) time (s) T ) temperature (K) W ) sample weight (mg) ξ ) sample conversion Figure 7. Arrhenius plots obtained in 21% oxygen (a) and 100% carbon dioxide (b) for biomass and waste chars of Table 4.

Conclusions Chars obtained from the pyrolysis of RDF at low heating rates (60 °C/min) and low-medium temperatures (500-800 °C) were found to be quite reactive materials toward gasification processes in oxygen and carbon dioxide. Even if the kinetic analysis was based on a simple single-step reaction model, the data obtained allowed at least a comparative analysis of RDF char reactivity. The kinetic parameters obtained were found to be well in the range of those reported for char obtained from other lignocellulosic materials, such as wood char and char obtained from MSW pyrolysis. RDF char reactivity was higher than that of graphite of about 5 orders of

Subscripts 0 ) initial value f ) final value

Literature Cited (1) Buekens, A. G.; Schoeters, J. G. European Experience in the Pyrolysis and Gasification of Solid Wastes. Conserv. Recycl. 1986, 9, 253. (2) Kaminsky, W. Possibilities and Limits of Pyrolysis. Makromol. Chem., Macromol. Symp. 1992, 57, 145. (3) Fritz, C. J. Noell Konversionsverfahren zur Verwertung und Entsorgung von Abfallen; Emil-Freitag-Verlag: Berlin (D), 1994. (4) Bridgwater, A. V. The Technical and Economic Feasibility of Biomass Gasification for Power Generation. Fuel 1995, 74, 631. (5) Henrich, E.; Burkle, S.; Meza-Renken, Z. I.; Rumpel, S. Combustion and Gasification Kinetics of Pyrolysis Chars from Waste and Biomass. J. Anal. Appl. Pyrol. 1999, 49, 221. (6) Antal, M. J., Jr.; Varhegyi, G. Cellulose Pyrolysis: The Current State of Knowledge. Ind. Eng. Chem. Res. 1995, 34, 703.

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Received for review July 21, 1999 Revised manuscript received January 4, 2000 Accepted January 10, 2000 IE990534C