Gas Production by Pyrolysis of Municipal Solid Waste at High

Energy & Fuels 1995,9, 648-658

648

Gas Production by Pyrolysis of Municipal Solid Waste at High Temperature in a Fluidized Bed Reactor Angela N. Garcia,* Rafael Font, and Antonio Marcilla Departamento de Ingenieria Quimica, Universidad de Alicante, Apartado 99, Alicante, Spain Received November 15, 1994@

A fluidized bed reactor was used to study the production of gases from municipal solid waste (MSW) pyrolysis at high nominal temperatures (700-850 "C). Biomass decomposition followed by tar cracking reactions took place inside the reactor. Yields of 10 pyrolysis products (methane, ethane, ethylene, propane, propylene, acetylene, butylenes, hydrogen, carbon monoxide, and carbon dioxide) as a function of operating conditions were analyzed. The results were compared with the data obtained by the pyrolysis of MSW in a Pyroprobe 1000 where tar cracking is small. Correlations between the yields of the products analyzed and those of methane are presented and their evolution is discussed.

Introduction The yields of the products obtained from a pyrolysis process are due to the solid decomposition (primary reactions) plus the reactions undergone by primary volatiles (secondary reactions). The extent of the secondary reactions depends on the experimental equipment and the operating conditions. For example, in recent papers, Griffin et a1.l studied the effects of both the heating rate and particle size on the yields of volatiles produced from pyrolysis of bituminous coal in an electrical screen heater and Hajaligol et a1.2 studied the pressure effects on the distribution of products generated from rapid pyrolysis of cellulose in a screen heater reactor. It is well-known that high residence times at high temperatures favor secondary reactions of the volatiles. Various experimental devices can be used to carry out primary plus secondary processes. When high heating rates are required, a fluidized bed reactor is widely used, Igarashi et al.3 used a dual-fluidized bed reactor to pyrolyze municipal solid waste (MSW); Kuester4 used a fluidized sand bed reactor to obtain olefins from pyrolysis of different biomass; Scott and Piskorz5t6used a similar reactor to pyrolyze wheat straw, aspen, poplar wood, etc.; Font et al.7,s,g studied the pyrolysis of almond shells in a fluidized sand bed reactor at low and high temperatures; Beaumont and SchwoblO used a reactor to pyrolyze beech wood, etc. On the other hand, Antalll used a semibatch laminar flow reactor t o pyrolyze ~

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cellulose and lignin, Sanner et a1.12and Chaparro et al.13 used an electric furnace to study MSW pyrolysis, and Williams and Besler14pyrolyzed MSW in a static batch reactor. In the present work, primary secondary reactions of MSW pyrolysis in a fluidized sand bed reactor are studied, and the results obtained are compared with the data from MSW pyrolysis in a Pyroprobe 1000 (Garcia et al.15), where secondary reactions rarely occur (since the residence time of the volatiles in the hot zone of the probe is very short) and primary reactions take place almost exclusively (the yields of C3, C4 hydrocarbons do not diminish when increasing the pyrolysis temperature, which proves that the extent of the secondary tar reactions in a Pyroprobe is small compared with the secondary reactions in a fluidized bed reactor). The following aspects are considered in this paper: 1. Influence of pyrolysis temperature on yields of compounds obtained from pyrolysis of MSW. 2. Influence of residence time of volatiles in the hot zone reactor on yields of compounds obtained from pyrolysis of MSW. 3. Analysis and discussion of the correlation of the different product yields and their possible mechanisms of formation. 4. Comparison of the results presented in this paper with those obtained using a Pyroprobe 1000 Analytical Apparatus.

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Experimental Section

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@Abstractpublished in Advance ACS Abstracts, May 15, 1995. (1)Griffin, T. P.; Howard, J . B.; Peters, W. A. Energy Fuels 1993, 7, 297-305. (2) Hajaligol, M. R.; Howard, J . B.; Peters, W. A. Combust. Flame 1993,95,47-60. (3) Igarashi, M.; Hayafune, Y.; Sugamiya, R.; Nakagawa, Y.; Makishima, K. J . Energy Res. Technol. 1984, 106, 377-382. (4) Kuester, J . L. Specialist' Workshop on Fast Pyrolysis of Biomass; Diebold, J., Ed.; Solar Energy Research Institute: Golden, CO, 1980; SERYCP-622-1096,pp 253-270. (5) Scott, D. S.; Piskorz, J . Can. J. Chem. Eng. 1982, 60, 666-674. (6) Scott, D. S.; Piskorz, J. Can. J . Chem. Eng. 1984, 62, 404-412. (7) Font, R.; Marcilla, A,; Verdu, E.; Devesa, J. Ind. Eng. Chem. Prod. Res. Deu. 1986, 25, 491-496. ( 8 ) Font, R.; Marcilla, A.; Devesa, J.;Verdu, E. Ind. Eng. Chem. Res., 1988,27, 1143-1149. (9)Font, R.; Marcilla, A.; Verdu, E.; Devesa, J . Ind. Eng. Chem. Res. 1990,29, 1846-1855.

The pyrolysis equipment used was a fluidized bed reactor, as shown in Figure 1. It is similar to the reactor described by Font et al.7 to pyrolyze almond shells, although some (10)Beaumont, 0.;Schwob, Y. Ind. Eng. Chem. Process Des. Deu. 1984,23, 637-641. (11) Antal, M. J. Jr. Znd. Eng. Chem. Prod. Res. Deu. 1983,22,366-

2 7 -. 5 -.

(12) Sanner, W. S.; Ortuglio, C.; Walters, J. G.; Wolfson, D. E. U.S. Department of the Interior, Bureau of Mines, 1970, RI 7428. (13) Chaparro, M.; Jimenez, M.; Vazquez, J. Ing. Quim. 1989, 21 (2431, 215-218. (14)Williams, P. T.; Besler, S. J.Inst. Energy 1992, 65, 192-200. (15)Garcia, A. N.; Font, R.; Marcilla, A. J . Anal. Appl. Pyrol. 1992, 23, 99-119.

0887-0624/95/2509-0648$09.00/00 1995 American Chemical Society

Pyrolysis of Municipal Solid Waste

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Figure 1. Experimental reactor: (A)heating zone; (B) helium entry; (C) reaction zone; (D) gas diffuser; (E) thermocouples; (F)gas outlet; (G) gas purge inlet; (H) feed hopper; (I) feed valve. aspects in the reaction zone as well as in the gas collection systems were modified. m o l y s i s was carried out in a 18/8 stainless steel reactor, heated by a cylindrical refractory oven. The external body and internal body of the reactor are cylinders whose dimensions are 100 (diameter) x 476 (height) mm and 69 (diameter) x 432 (height) mm, respectively. The carrier gas enters the reactor, flows down the jacket, between both bodies, passes across the gas diffuser, and flows up through the sand bed and then continues through the reaction zone, pushing the pyrolytic gases to the gas outlet. The inert fluidized bed was sand of 105-210 pm particle size, calcinated at 850 "C and washed with HC1. The bed surface temperature was controlled automatically a t four different temperatures (according to the experiment): 700, 750, 800, and 850 "C. The reactor was not isothermal, showing a temperature gradient from the sand bed to the top of the reactor. Two chromel-alumel thermocouples were used to control and measure the temperature profile, which depends on the nominal temperature selected. The surface of the sand bed was a t the selected temperature but it decreased from the bed to the reactor head (around 300 "C). For example, when the

Energy & Fuels, Vol. 9,No. 4, 1995 649 sand bed temperature was 850 "C, the temperature was 750 "C a t 20 cm above the bed and 656 "C if the bed temperature was 750 "C. The inert gas used was helium. The advantage of using helium instead of nitrogen is to guarantee an inert atmosphere in the reaction system, since if Nz were detected, it would indicate the entrance of air. The differences between heat transfer abilities of He and Nz only affect the heating rate of the solid when it is dropped onto the bed (that is, the primary reactions of the solid); consequently, the same results may be expected when the value of this heating rate is kept constant (under different conditions of gas flux and pyrolysis temperature) regardless of the nature of the gas. In this case, a correlation model was applied t o simulate the primary and secondary reactions as well as the heat transfer process.16 According to this correlation model, the heating rate of the sample is around 670 "C/s when the nominal temperature is 850 "C and around 425 "C/s if the nominal temperature is 700 "C. Therefore, if these conditions are maintained, the results should be similar, whatever the inert gas selected is. The reactor was operated at atmospheric pressure. The experimental procedure was as follows: a 0.8-5 g sample of dry pellets of MSW was placed in the feeder (see Figure 1);the flow of the inert gas was fixed and the oven was switched on. When the reactor reached the selected temperature, the exit flow was shifted, from a bypass in the tubes of the collection system, to the feeder in order to eliminate the air. It was not hot enough t o modify the sample before being dropped. After a few minutes, the feed valve was opened and the sample dropped onto the hot fluidized sand bed (between 300 and 1743 g of sand, according t o the experiment). From some tests carried out at room temperature, it was observed that the MSW pellets remained in the upper part of the sand bed when discharged. Consequently, it can be considered that the primary decomposition took place on the bed surface and the secondary tar cracking occurred in the zone of the reactor between the surface sand bed and the exit section. After the sample was discharged, gases evolved from the reactor were cooled and collected at room temperature, for approximately 20 min in a PVC bag, provided with a septum for sampling and analysis. The PVC bag was previously calibrated by displacement of water. The compounds obtained by MSW pyrolysis were identified and quantified by a Shimadzu GC-14A gas chromatograph using both FID and TCD detectors and two different columns: (a) alumina column with FID for analyzing methane, ethane, ethylene, propane, propylene, acetylene and butylenes; (b) Carbosieve SI1 column with TCD for hydrogen, nitrogen, carbon monoxide, and carbon dioxide. After each experiment, the temperature profile in the upper part of the reactor was measured in order to carry out a kinetic study of the process (Garcia et a1.9. The solid residue was also collected and calcinated and the ash content and char produced were determined. The refuse used was MSW from a treatment plant in Alicante (Spain). Table 1shows the composition of MSW used in this research. The refuse collected was very heterogeneous. Nevertheless, samples of around 1 kg of refuse were dried, crushed, and pelletized into the final pellets used, which can consequently be considered representative of average refuse. A wide study of the MSW thermogravimetric analysis (Garcia et al.17)led t o the conclusion that the decomposition of this refuse can be considered as the decomposition of two independent fractions which decompose at this different temperature range: (a) around 310-380 "C (depending on the heating rate) of the cellulosic fraction, and (b) in the temperature range 200-500 "C of a heterogeneous fraction. Furthermore, the thermogravimetric analysis of the MSW used (16)Garcia, A. N.; Font, R.; Marcilla, A. J . Anal. Appl. Pyrol., in press. (17) Garcia, A. N.; Marcilla, A,; Font, R. Thermochim.Acta., in press.

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660 Energy & Fuels, Vol. 9, No. 4, 1995 Weight loss (%) I 100

1

1

\

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Figure 2. Weight loss vs temperature in a dynamic TG (heating rate equals 2 0 "C/min). Table 1. Composition of MSW Used in This Research moisture of MSW in the landfill 44% EDX (wt % on a moisture-free basis) first sample second sample MSW ash MSW ash Na 1.75 4.07 1.17 2.77 0.80 2.92 1.31 5.56 Mg

Al Si P S

c1 K Ca Ti Fe

2.06 3.28 0.49 2.46 4.01 1.99 12.7 0.10 0.72

5.76 15.4 1.16 4.70 2.21 2.11 41.8 2.36

1.61 2.81 0.22 1.85 3.72 3.02 8.61

8.31 13.5 0.99 4.93 1.43 2.38 42.9 0.49 1.89

0.10 0.42

Elemental Analysis MSW (wt % on a moisture-free basis) first sample second sample C N H S (organic)

ash 0 and C1 (by dim

38.53 0.078 5.46 0.38 23.30 32.25

40.93 1.38 5.32

0 26.70 25.67

in this study shows a minor although significant weight loss (around 6.5-9.8%) in a temperature range of 710-790 "C, depending on the heating rate (Figure 2). As the decomposition of any organic matter must be completed at these high temperatures, it was considered that this weight loss could be due t o the decomposition of CaC03 present in the MSW ash. In order to prove this hypothesis, the residue obtained by thermal decomposition of MSW (T,mtial= 5 0 "C, Tfinal= 475 "C, heating rate = 50 "C/min, tfinal = 3 min) was pyrolyzed up to 900 "C in a TG with a mass spectrometer (the sample was previously heated until 475 "C, to avoid the system being obstructed by the tars generated in the pyrolysis of the raw sample). This experiment revealed the COz formation (around 750 "C) and the CO formation (around 772 "C). The quantification of these peaks was 5.2% for COz and 2.0% for CO. If it is considered that around 80% of CO obtained at this 2CO temperature was formed by the reaction COz + C (Guti6rrez-Riosl8), the initial COZgenerated would be 6.5%, which agrees with the results obtained by TG, thus supporting the hypothesis that the reaction CaC03 CaO + COz took place. No other inorganic decomposition reactions were taken into account. The carbonates of alkaline metals melt before decomposing. Some carbonates decompose at higher temperatures (SrC03, BaC03, etc.) and other carbonates (MgC03, FeC03, PbC03, etc.) decompose at lower temperatures (Guti-

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(18)Gutierrez Rios. Quim.Inorg. (Revert6 Ed., Barcelona) 1985.

Brrez-Riosl*). Since Ca2+was the main cation detected by EDX (energy dispersion X-rays) in the refuse studied (Table 11, the COz generated by CaC03 decomposition was taken into account in the data analysis. The mean value of the weight loss percentage due to CaC03 decomposition was 8.0%, when the mean percentage of ash in the sample was 26.7%.

Results and Discussion 1. Fluid-DynamicStudy. Several fluidization runs were performed at room temperature in a glass tube, in order to select the appropriate sample size and the inert gas flow to be used in the fluidized bed reactor at high temperatures, taking into account the following aspects: (a) Sample Size. Small sizes favor the heat transfer within the particle, while on the other hand, the use of a fluidized bed reactor, where the solid drops against the flow of inert gas, limits to a minimum the particle size, if entrainment is to be avoided. Pellets of 4 mm average diameter were selected, since a larger size would hinder the heat transfer while smaller diameters are difficult to obtain and pellets would be less representative of the refuse (due to its heterogeneity). (b)Inert Gas Flow. High flow rates improve the sand-solid mixing and the heat transfer between the hot bed and cold solid, while on the other hand, an excessive flow rate may cause a considerable entrainment of fines. Furthermore, the gas flow must be limited in order to provide a minimum volatile residence time to enable the study of tar cracking. In order to determine the optimum helium flow, several runs were carried out in a glass tube. It was observed that MSW pellets remained on the surface without fluidizing (the value of the theoretical minimum fluidization velocity of MSW pellets is 49 c d s at 298 K). Neverthqess, good mixing between sand and pellets is achieved in the upper part of the reactor when the sand is properly fluidized and sand particles, carried upwards by the wake of the bubbles, intermittently cover the pellets. The minimum fluidization velocity of sand (Umf), at the different operating temperatures (700,750,800, and 850 "C), was calculated theoretically using the Ergun equation (Kunii and Leven~piel'~). This means that Umf slightly depends on temperature in the selected range: 1.32 c d s at 700 "C, 1.28 c d s a t 750 "C, 1.24 c d s at 800 "C, and 1.20 c d s at 850 "C. Consequently, if the helium flow is kept constant in the range 700-850 "C, the fluidization state of the sand bed as well as the heat transfer between the hot bed and the cold solids will be similar. The selected value for the gas velocity was 3.6 c d s measured a t each nominal temperature, which is 3 t i m e s the Umf at 850 "C (the highest operating temperature) and 2.7 times the Umfat 700 "C (the lowest one). 2. Effects of Temperature and Residence Time on the Yields of Products in MSW Pyrolysis. In order t o study the maximum yields of gases that can be obtained from the pyrolysis of MSW, 49 experiments were carried out at 4 sand bed temperatures: 700,750, 800, and 850 "C. Methane, ethane, ethylene, propane, propylene, acetylene, butylene, CO, C02, and H2 were detected and quantified in each experiment. (19)Kunii, D.; Levenspiel, 0. Ruidization Engineering; John Wiley and Sons: New York, 1969.

Energy & Fuels, Vol. 9, No. 4, 1995 651

Pyrolysis of Municipal Solid Waste Both the height of the bed (or the mass of sand) and the weight of MSW discharged onto the hot bed were modified in each experiment. In this way, the mean residence time of volatiles was different in each run, depending on the mass of volatiles produced and the volume of the upper part of the reactor, from the top of the fluidized bed to the exit section in the reactor head. As this upper part of the reactor is hot, the volatiles produced by the thermal decomposition of the solid when in contact with the sand bed also undergoes a thermal decomposition, from their formation (at the temperature of the sample) until they are cooled (at the exit section). Therefore, the gases collected are the result of both primary and secondary reactions. Since the reactor is not isothermal (temperature ranging from that of the sand bed to approximately 300 "C at the reactor head), it is difficult to find a parameter representative of the extent of the tar cracking because the different fractions of volatiles pass through the upper part of the nonisothermal reactor with different velocities. A magnitude that could be approximately proportional to the residence time is the ratio between the volume (V) of the upper part of the reactor, without sand, and the mass ( m )of the MSW discharged onto the bed. When V is constant, the greater the value of m, the lesser the residence time. When m is constant, the greater the value of V , the greater the residence time. The experiments carried out show that both parameters are important; although the free volume is kept constant (the same mass of sand), different amounts of sample dropped leads to different percentages of gas obtained and it decreases as the mass of sample increases. Due to this fact, both factors were taken into account. The ratio Vlm is only a first approximation to the real residence time. If a kinetic study needs to be carried out,16 different aspects must be taken into account to simulate the decomposition process: volume of primary gases and tars at the sand bed temperature (estimated from the experimental results in the Pyroprobe), the ratio volume of primary volatiles (gases tars)/volume of primary gases, weight percentage of water, acetic acid and methanol in the primary volatiles, final yields of gases analyzed obtained in the fluidized bed reactor, temperature profile in the reactor, average molecular weight of the gases obtained at the exit of the reactor, etc. Due to the complexity of calculating a real residence time, plots of the percentage of products vs Vlm are presented as a guide to show the general trends t o the experimental data obtained. Figure 3 shows the variations of the yields vs the ratio Vlm for the compounds analyzed at the four nominal temperatures. In these graphs, the primary reactions are represented by Vlm = 0 (reactor free volumelsample mass equals zero) obtained in a Pyroprobe 1000. As stated previously, these yields can be considered representative of the primary reactions, since according to Garcia et al.,15the residence time of the volatiles in the hot zone of the probe in the Pyroprobe 1000 is very short (therefore, secondary reactions in the Pyroprobe should be very low; furthermore, the fact that the yields of hydrocarbons such as propane, butane, or butylene did not diminish when the pyrolysis temperature was increased proves that the extent of the secondary tar reactions is small). The extrapolations of the results in the fluidized bed reactor t o Vlm = 0 show a good

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agreement with the results in the Pyroprobe, thus confirming the hypothesis that tar cracking takes place mainly in the upper part of the reactor, from the top of the sand bed to the exit of the reactor. If some important extent of the tar cracking reactions took place inside the fluidized bed, the extrapolations of the curves drawn in Figure 3, when Vlm equals t o zero, would intercept on the yield axis at values far above the primary yields obtained with the Pyroprobe 1000. The fluidized bed acts virtually like a hot plate to heat the substrate. Figure 3 is presented only t o show the trends and different behaviors followed by the pyrolytic products. It allows us to classify the pyrolytic products in different groups: compounds whose yield increases or decreases with the residence time and compounds whose yield increases or decreases with the temperature. The lines plotted suggest these different tendencies. A correlation model taking into account all the possible reactions could be developed in order t o obtain more information about the formation and decomposition of these products, but this has not been done due to the heterogeneity of the tars evolved and the presence of a great number of reactions. The dispersion of the results, especially for minor compounds, is important. The percentage of ash in the samples used varied between 17 and 28% which shows the heterogeneity of the refuse. When the same experiment was repeated twice and the variation coeffcient was calculated, values between 2 and 40% were obtained (the highest values of variation coeffcient were obtained for the minor compounds). In spite of this dispersion, the results obtained can be useful: for example, consider that if the yields obtained in two duplicated runs for a given compound were 0.01 and 0.02%) the variation coefficient would be around 50%, but the order of magnitude obtained would be clear and the information deduced could be useful. Other possible statistical evaluation of the experimental data would be to consider a correlation model which generated theoretical data and compare these theoretical data with the experimental ones. A correlation model was developed to simulate the decomposition process16 which allowed us t o calculate the theoretical values corresponding to the overall gases produced. In spite of the data scattering, the variation coeficient obtained between theoretical and experimental data was around 10% (no model which correlated the different pyrolytic products individually was developed due to its complexity). From Figure 3, the following behaviors for gases as a function of the "residence time" could be deduced: (a) Compounds whose yields increase when the residence time of volatiles in the reactor also increases (high values of Vlm): methane, ethylene, CO, COa, and Ha. This is also the tendency shown by total gas yield. These graphs show the shift of the curves to lower residence times when increasing pyrolysis temperature. (b) Compounds which clearly undergo cracking: acetylene, ethane, butylene, propylene, and propane. When Vlm increases, either maximums or a continuous decrease can be observed. The acetylene yield increases when the residence time increases in the range 700800 "C, and a t 850 "C and high residence times a maximum appears. This indicates that the acetylene formation is favored by tar and hydrocarbon cracking,

Garcia et al.

652 Energy & Fuels, Vol. 9, No. 4, 1995

:u 5110'c % memane

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Garcia et al.


but can also be cracked at high temperatures and residence times. The ethane behavior is similar to that of acetylene, although cracking takes place at lower residence times. According to Sundaram et a1.,20ethylene and methane are the major products obtained in ethane cracking. The net tendency of propane yields is a continuous decrease when temperature and residence time increase, thus, indicating that, if formed by secondary reactions, cracking is also very fast. Both combined effects result in a net decrease in propane production. According to Van Damme et the major products of propane cracking are methane, ethylene, hydrogen, and propylene. This agrees with the results obtained in the present paper, since methane, ethylene, and hydrogen increase their yields with the residence time of volatiles in the reactor and propylene formation is favored by the secondary reaction before being cracked. It can be seen that, at the four operating temperatures, the propylene yields show maximum values which shift to lower Vlm values when temperature is increased. This indicates that the tar cracking that leads to the propylene formation takes place at the initial stages of the reaction and then the propylene is also cracked. The evolution of butylene is similar to that of propylene. Devesa22obtained similar results in his study on the pyrolysis of almond shells in a similar fluidized bed reactor in the temperature range 700-900 "C. Antalll studied the pyrolysis of cellulose and lignin, analyzing the evolution of the pyrolytic products. According to his results, the yields of methane, hydrogen, ethylene, CO, and C 0 2 increase with the residence time (the rise in yields of carbon oxides being less marked) while ethane and propylene show maximum yields when pyrolyzing cellulose; these results agree with those shown in this paper. However, in lignin pyrolysis, the yields of methane and carbon dioxide seem t o be less dependent on residence time than in the case of cellulose. Yields of hydrogen and ethylene increase with the extension of secondary reactions, while yields of propylene and ethane acetylene (analyzed together) decrease. From the previous discussion, it could be deduced that the yields of pyrolytic hydrocarbons are the consequence of formation and cracking reactions, some of which predominate over others, depending on the biomass pyrolyzed, hydrocarbons produced, and operating conditions. Table 2 shows the mean values of the products analyzed, as a function of temperature, when the overall yield is maximum (i.e., at high values of V/m),as well as the mean weight of the char obtained, the mean weight of the sample ash, and the values of (tar + H20) calculated by difference. The amount of char was calculated by weighing the residue (char ash) obtained in each experiment and calcinating it to determine its percentage of ash. Subtracting the ash from the global residue gives the value of the char. As the solids remained in the hot zone of the reactor a long time (the

+

+

(20) Sundaram, K. M.; Van Damme, P. S.; Froment, G. F. AIChE J . 1981, 27 (61, 946-951. (21) Van Damme, P. S.: Naravanan. S.: Froment. G. F. AIChE J . 1975,21 (61, 1065-1073. (22) Devesa, J. Produccidn de Gases por Pirolisis de Cascara de Almendra a Elevadas Temperaturas. Ph.D. Thesis, University of Alicante, 1990.

Table 2. Mean Values of Experimental Yields Obtained at High Residence Time as a Function of Temperature (wt % on a Moisture-Free Basis) methane ethane ethylene propane propylene acetylene butylene

co

COZ

Hz total gas char

ash (tars a

+ HzO)"

700°C

750°C

800°C

850°C

2.9 0.74 2.7 0.088 1.4 0.085 0.38 13.8 18.0 1.0 41.1 13.7 23.0 22.2

3.4 0.70 3.4 0.077 1.2

4.2 0.50 4.5 0.025

4.2 0.21 3.4 0.001 0.1 0.14 0.005 18.2 19.3 1.5 47.1 8.0 21.3 23.6

0.10 0.20 16.0 19.0 1.0 45.0 12.4 22.8 19.8

0.80 0.17 0.06 18.7 18.1 1.0 48.0 10.5 19.7 21.8

By difference.

Table 3. Mean Values of Maximum Experimental Yields as a Function of Temperature (wt % on a Moisture-Free Basis) ethane ethylene propane propylene acetylene butylene

700°C

750°C

800°C

850°C

0.74 2.7 0.12 1.7 0.090 0.60

0.70 3.4 0.10 1.6 0.10 0.55

0.66 4.5 0.10 1.6 0.17 0.55

0.60 3.6 0.092 1.4 0.18 0.50

gases evolved from the reactor were collected for 20 min but solids were not taken out of the reactor until this was at room temperature) it was considered that the carbonate minerals had been decomposed to their oxides during the pyrolysis run in the temperature range studied (700-850 "C). The yields of ethane, ethylene, propane, propylene, acetylene, and butylene could be higher than yields shown in Table 2, at low residence times, before' extensive cracking takes place. The highest yields of these hydrocarbons, at lower residence times, are shown in Table 3. According t o the results obtained, the maximum yields of total gases (at high residence times) are obtained at 800 "C and remain almost constant at 850 "C. CO and CH4 also reach their corresponding maximums at 800 "C. C02 yield does not depend on temperature in the range studied. The only compound which increases its yield is H2, probably as a consequence of further hydrocarbon cracking, producing Ha and carbon black. The yields of total gases obtained in the range of temperature studied (700-850 "C) do not suffer great changes. This may be due to the fact that the four temperatures selected are high thus favoring the generation of gases vs the generation of tars and char. Therefore, in this range, changes in the individual species (which are formed or cracked in the secondary reactions) are more important. Obviously, high residence time can influence the results but the influence is less when the temperature increases. Table 4 shows the mean values of the product yields obtained by MSW pyrolysis in a Pyroprobe 1000 where the residence time of the volatiles in the hot zone of the probe is very short. No significant changes are detected in the percentage of gases generated (between 19 and 28%). Differences between primary and secondary reactions can be deduced by observing the values shown in Tables 2 , 3 , and 4.



Table 4. Mean Values of Experimental Yields Obtained by Pyrolysis of MSW in a Pyroprobe 1000 (Pyrolysis Time = 20 s) (wt % on a Moisture-Free Basis) methane ethane ethylene acetylene propane propylene butane butylene

+

co con

total gas

700°C

750°C

800°C

850°C

0.67 0.40 0.36 0.16 0.37 0.13 0.33 3.2 13.2 18.8

0.85 0.43 0.57 0.16 0.47 0.14 0.44 4.2

1.0 0.45 0.77 0.14 0.58 0.15 0.54 5.3 14.9 23.8

1.3 0.50 1.1 0.15 0.65 0.15 0.60 7.5 16.0 28.0

14.1 21.4

In order to estimate the residence time of the volatiles in the reactor (in units of s instead of cm3/g), the expression residence time = (free volume)/(helium flux) was considered as a first approximation. According to this expression, a higher limit for the residence time in the experiments presented in this paper is 10 s. In accordance with the model proposed to simulate the process,16where aspects such as the volatiles generated in each experiment and the temperature profile in the reactor have been taken into account, the highest value of the residence time is around 5 s. The results obtained by Antalll show a decrease in the ethane and propylene yields when the temperature is increased in the range 700-750 "C, when cellulose is pyrolyzed at residence times of 1-6 s. Tyler23 and Doolan et al.24found that the maximum yields of these products are obtained at high temperatures (830 "C for propylene and 1000 "C for ethylene) when the range of residence times is 0.1-0.2 s. These facts are in good agreement with the results presented in this paper. Ethylene yield vs temperature shows a maximum and the propylene yield decreases in the temperature range 700-850 "C at high residence times (as can be deduced from Table 2). However, both yields increase in the same temperature range at V/m = 200 cm3/g, showing that the maximum yields are obtained at low residence times when temperature is increased. 3. Correlation of Results. In order to correlate the results obtained by MSW pyrolysis in a fluidized bed reactor, logarithms of yields corresponding to each product obtained vs logarithm of methane yields have been plotted for each run (Figure 4). This type of plots (logarithmic plots of a product yields vs the logarithm of yields of another product) allows us to understand the relative evolution of some compounds vs others. According t o Font et al.,25methane yield is considered as an indicator of the extent of pyrolytic reactions, since it is a compound whose yield increases from low temperatures and residence times until high temperatures and residence times. Other pyrolytic products, for example, CO, increase more than CH4 with secondary reactions but the CO formation is also faster and maximum yields of CO are obtained a t lower temperatures and residence times (see Table 5 and Figure 3a,h). Thus, with these characteristics, the points of the graph logarithm of a product yields vs logarithm of CO yields would be located almost in the same vertical (23) Tyler, R. J. Fuel 1980, 59, 218-226. (24) Doolan, K. R.; Mackie, J. C.; Tyler, R. J. Fuel 1987, 66, 572578. (25) Font, R.; Marcilla, A.; Devesa, J.;Verdu, E. J.Anal. Appl. Pyrol. 1994,28, 13-27.

line, independently of the residence time and the temperature of the runs. This is due to the fact that the CO formation is more independent of this parameter than the CH4 formation. Actually, Figure 4 shows similar information such as figures and tables about yields of products vs temperature and residence time but from a different point of view. From Figure 4, different behaviors can be deduced: (a)Yields of ethane, propane, propylene, and butylene clearly show maximum at high yields of methane, as a consequence of the secondary cracking undergone by these hydrocarbons. (b)Yields of acetylene continuously increase with the methane yield. As previously discussed, the acetylene yield increases with the secondary reaction extension and only undergoes cracking a t the highest temperature and residence times tested. (c) The ethylene vs methane graph shows a good linear correlation between both products; with the slope close to 1. Thus, it may be assumed that both of them follow a similar mechanism. (d) CO yield increases with the methane yield, with a linear correlation having a slope lower than 1. It can be concluded that methane formation is favored over CO formation. (e) COZonly shows a slight increase with the extension of secondary reactions. These results are quite different from those obtained when considering primary yield products (Garcia et al.15). In the present case, with primary and secondary reactions, only ethylene shows a good correlation with methane. On the other hand, in the case of the results obtained with the Pyroprobe 1000 (i,e., mostly primary reactions), all the hydrocarbons correlate linearly with methane, with different slopes (more or less close t o unity) depending on the hydrocarbon considered. In this paper, the yields of products have been presented as weight percentage on a moisture-free basis, without considering the percentage of ash in the sample. In the case of MSW, this aspect is particularly important, since the calcium carbonate, which is part of the ash, decomposes above 650 "C generating CO2. Pyrolytic COS is, therefore, formed in two ways: pyrolysis of the organic fraction of MSW and decomposition of the calcium carbonate present in MSW. As commented in the Experimental Section, the mean percentage of C02 that could be formed from the inorganic fraction of MSW is around 8.0%for an average ash content of 26.7%. The residue obtained in each experiment was weighed and calcinated; therefore, the percentage of ash in the sample was determined by difference. With these data, it is possible t o estimate the amount of CO2 generated from the decomposition of the ash and, consequently, the values of the COZyield produced from the organic fraction. In the fluidized bed reactor, the solids remain a long time in the hot zone and gases evolved from the reactor are collected for 20 min (sufficient time to decompose the CaC03 present in the sample). On the other hand, in the Pyroprobe 1000, the sample is only heated for 20 s from the initial temperature to the programmed temperature at a high heating rate (20 "C/ms nominal heating rate, although the estimated rate was around 300 "C/s) (Garcia et al.15). Under these conditions, it is considered that the total


Garcia et al. l.OE+W

~

A 7OOoC

7750°C 0 850°C

1,OE-01

.A

0

A

0

A

W

A

A

a

e

1, E 4 1

1,OE4l

l,OE+W

l,OE+Ol

A

1 ,E52

% methane

% methane

% ethylene

1-1

l,M+W

l,oE+ol

% butylene

08°C

1,OE-Ol

A

.A

1,OEcW

T%

I

A

0 0

1.oE.02

0

A

b

0

f

1,OE4l 1 .OEQl

l,E+W

l.M+Ol

1,OE.03 1, E 4 1

l,OE+W

% methane

% propane

l,E+W

1 ,o bo1

% methane

%total C Q

j p q

0

1,OE.02

0

I l,E+W

1 ,OE43

0

1.OE-04 1,OE-Ol

I

l.OE+W

i,OE+Ol

[s

1,OE41 1,OE-Ol

l,OE+W

% methane

1,OEcOl

% methane

% propylene 1,OE+02

l,OE+O1

% co

.

750'C

..

l,OE+W

TO

1,OE.Ol

0 0

A

8

1

I

l.OE+W ~

t

I d 1,OE.02 1,OE-01

I

I

I h 1 .OE+W

% methane

l.OE+Ol

1.OE.01 I 1,OE-01

l,OE+W

J

1,OE+01

% methane

Figure 4. Correlations between the yields of several products and the methane yield obtained in each run in the fluidized bed reactor (weight percentage on a moisture-free basis).


Energy & Fuels, Vol. 9,No. 4, 1995 667

Table 5. MSW Organic Fraction Pyrolysis. Mean Values of Experimental Yields Obtained at High Residence Time as a Function of Temperature (wt % on a Moisture- and Ash-Free Basis) methane ethane ethylene propane propylene acetylene butylene

co

c02 H2 total gas char tars H20a tarsQ tars (Pyroprobe)

+

700°C

750°C

800°C

850°C

4.2 1.1 3.8 0.14 2.2 0.12 0.55 19.6 14.3 1.6 47.6 20.9 31.5 15.7 48.7

5.2 0.95 4.6 0.092 1.6 0.14 0.28 22.7 16.0 1.2 52.8 17.4 29.8 14.0 47.9

6.3 0.7 5.9 0.034 1.1 0.23 0.19 25.9 13.9 1.6 55.9 15.0 29.1 13.3 47.0

6.1 0.32 4.5 0.001 0.17 0.19 0.007 26.1 15.7 2.4 55.5 11.8 32.7 16.9 39.9

By difference.

decomposition of CaC03 in the Pyroprobe 1000 can only take place at 900 "C, whereas there is only partial decomposition at lower temperatures. This fact is useful when analyzing the results obtained and the correlations proposed, and must be considered when comparing these results with those found in literature about other organic refuse. The tendencies followed by total C02 and organic COS are similar. Both of them show a slight increase of their yields with the residence time. The sample and ash heterogeneity is also reflected in a high dispersion of results. Table 5 shows the mean values of the yields of the individual gases as well as those of the total gases on an ash- and moisture-free basis (Y,,,), i.e., the yields of gases generated from the organic fraction of MSW: yield of gas - CO, inorg - MSW mass - moisture - CO, inorg - ash

seen, around 60-70% of primary tars are cracked into gas in the secondary reactions. 4. Comparison with Other Results from the Literature. Some papers about MSW decomposition may be found. Igarashi et al.3used a dual fluidized bed reactor to pyrolyze MSW, Sanner et a1.12and Chaparro et al.13used an electric furnace. Under these conditions, both primary and secondary reactions take place to a great extent. The results shown by these researchers can be compared with those presented in this paper: Igarashi et al.3 obtained 6.7% of CH4, 1.7% of ethane, 6.7% of ethylene, 3.2% of propylene, and 21.5% of CO at 700 "C; Sanner et a1.12 working a t 750 and 900 "C, obtained 4.7-4.9% of CH4, 0.80-0.1% of ethane, 2.73.2% of ethylene, 0.84% of propylene, and 5.7-12.3% of CO; and Chaparro et al.13 obtained CH4 in the range 3.1-14.4%, 1.1-2.2% for ethane, 0.34-5.7% for ethylene, and 1.9-1.6% for propane propylene at 500,700, and 900 "C(all these yields are expressed on a moistureand ash-free MSW basis). Yields obtained by Igarashi et al. are somewhat higher than those presented in this paper, whereas the data obtained by Sanner et al. and Chaparro et al. are very similar to those obtained by us (except the methane yield of 14.4% a t 900 "C). In this paper, the study of the MSW pyrolysis on the basis of primary and secondary reactions is presented. Typical components of MSW-plastics, fats, proteins, etc.-have been pyrolyzed in a fluidized bed reactor by different researchers. From data shown by Stammbach et al.27about pyrolysis of polyethylene and by Kaminsky28about pyrolysis of pure polyethylene and plastics contaminated by food refuses, it could be deduced that the pyrolysis of plastics generates higher yields of hydrocarbons and aromatics (mainly methane, ethylene and benzene) than those obtained by MSW pyrolysis. In effect, yields of methane can reach around 20% at 800 "C, ethylene around 13-18%, and benzene around 15-20%.

+

-

Conclusions

Water yields were determined by gas chromatography obtaining lower values than those expected, probably as a consequence of condensation along the gas collection system. Due to this, the water yield was estimated together the values of tars and other condensable volatile5 by difference (Table 5). However, the percentage of water considered in secondary reactions could be the same as that obtained in primary reactions according to Boroson et a1.,26 who concluded that water generated in primary reactions hardly suffers a reaction in secondary crackings. This value was determined by the pyrolysis of MSW in a Pyroprobe 1000,15where there are no condensation problems since the gases generated flow directly into the column of the chromatograph (Pyroprobe GC equipment) and they do not pass through a collection system at room temperature. This value, expressed on a moisture- and ash-free basis, is around 15.8%. Considering this value, the percentage of tars can be calculated by difference (Table 5). Table 5 also includes the percentage of tars obtained by pyrolysis of MSW in the Pyroprobe 1000. As can be

From the study about MSW pyrolysis in a fluidized bed reactor, the following conclusions can be deduced: 1. The yield of total gas obtained increases in the range 700-800 "C from 41.1 to 48.0% (weight percentage on a moisture-free MSW basis) or from 47.5 to 56% (weight percentage on a moisture- and ash-free basis). Between 800 and 850 "C, this yield remains almost constant. 2. Pyrolytic gases shown different behaviors as a function of the residence time. While the formation of methane, CO, C02, and hydrogen is clearly favored by high residence times, ethylene shows some evidence of cracking at 850 "C at long residence times and the other hydrocarbons-acetylene, ethane, butylene, propane, and propylene-undergo cracking to different extensions at increasing residence times: ethane cracks a t higher residence times a t 800 and 850 "C, and possibly 750 "C, but shows a net increase with residence time at 750 "C; net depletion of acetylene is indicated at 850 "C but is ambiguous a t the other three temperatures; propylene and butylene yields show maximum values which shift to lower residence times when temperature is increased

(26) Boroson, M. L.; Howard, J. B.; Longwell, J. P.; Peters, W. A. M C h E J. 1989,35 (11,120-128.

(27) Stammbach, M. R.; Hagenbucher, R.; Kraaz, B.; Richarz, W. chi mi^ 1988, 42, (7/8), 252-256. (28)Kaminsky, W. Resour. Recovery Conserv. 1980,5(3), 205-216.

+

Garcia et al.


and a net decrease in propane production is observed when residence time is increased in the temperature range studied. At high residence time and at 800-850 "C, the yields obtained of the different compounds are the following (weight percentage on a moisture-free basis): methane 4.2% ethane 0.50-0.21%; ethylene 4.5-3.4%; propane 0.025-0.001%; propylene 0.80-0.1%; acetylene 0.170.14%;butylene 0.06-0.005%; carbon monoxide 18.5%; carbon dioxide 18.1-19.3%; hydrogen 1.0-1.5%. These yields expressed as weight percentage on a moisture- and ash-free basis are as follows: methane 6.2%;ethane 0.7-0.32%; ethylene 5.9-4.5%; propane 0.034-0.001%; propylene 1.1-0.17%; acetylene 0.23-

0.19%;butylene 0.19-0.007%; carbon monoxide 26.0%; carbon dioxide 13.9-15.7%; hydrogen 1.6-2.4%. 3. From the correlations between the different gases analyzed and methane, it can be concluded that only ethylene shows a good correlation with the methane, in a wide range of temperatures and residence times, thus indicating that they are formed in similar ways. 4. Primary tars evolved are cracked around 70% at 800 "C, increasing the gas yield (from 23.8%(primary yields) to 48.0 (secondary yields)).

Acknowledgment. Support for this work was provided by CICYT-Spain,Research project AMB93-1209. EF940192H

Gas Production by Pyrolysis of Municipal Solid Waste at High

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