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Tar cracking from fast pyrolysis of large beech wood particles
Journal of Analytical and Applied Pyrolysis 62 (2002) 83 – 92
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Tar cracking from fast pyrolysis of large beech wood particles J. Rath *, G. Steiner, M.G. Wolfinger, G. Staudinger Institut fu¨r Apparatebau, Mechanische Verfahrenstechnik und Feuerungstechnik, Technische Uni6ersita¨t Graz, Inffeldgasse 25, A-8010 Graz, Austria Received 5 July 2000; accepted 24 November 2000
Abstract Pyrolysis experiments in a thermogravimetric analyser and in a muffle furnace were carried out with spruce and beech wood. The particle size of the samples of spruce wood was varied in the range 0.5–20 mm and the heating rate was varied in the range 5 – 60 K/min. Additionally, drop-in experiments in the muffle furnace were carried out with both spruce and beech wood. These experimental data were compared with the results from calculation of pyrolysis of ‘large’ particles with the simulation program PARSIM. Agreement could only be obtained when the tar decomposition outside of the particle was taken into account. The pre-exponential factor and the ultimate yield for cracking of tar from beech wood are given. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Tar cracking; Pyrolysis; Beech wood; Secondary reaction
Nomenclature A pre-exponential factor (s−1) activation energy (kJ·mol−1) E gas constant (kJ·mol−1·K−1) R t time (s) T temperature (K) V amount of tar unconverted (wt.%) V* amount of non-reactive tar (wt.%) * Corresponding author. Fax: + 43-3168737492. E-mail address: [email protected] (J. Rath). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 2 1 5 - 1
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J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
1. Introduction For the simulation of the processes inside large solid fuel particles undergoing pyrolysis a computer program (PARSIM [1–3]) was developed. Results from validation experiments at low heating rates in a muffle furnace were in excellent agreement with simulation results for the mass loss, the temperatures in the centre and on the surface of the particle and also for the evolution of volatiles. In practice (e.g. fluidised bed technology) the fuel particles are dropped into the hot furnace atmosphere. Results from drop-in experiments with large solid fuel particles into a muffle oven are also in good agreement with simulation results for the mass loss and the temperatures in the centre and on the surface of the particle. However for the evolution of the volatiles, only poor agreement between simulation and experiment was found. The differences were assumed to be due to thermal degradation of the volatiles at the drop-in experiments. Detailed experiments were carried out to test this assumption.
2. Experimental setup Fig. 1 shows a diagram of the thermogravimetric analyser (TGA, manufactured by DMT) used. Detailed information can be found in Refs. [4,5]. This thermogravimetric analyser was used for experiments with small particles (0.5– 1.0 mm) in a packed bed and for experiments with single particles of a size of 6 mm [6]. The sample basket was made of Incoloy 800 wire mesh and is shown in Fig. 2. The samples were heated from 105 to 850°C at heating rates of 5, 20 and 60 K/min. The results from experiments with small particles at heating rates of 5 K/min were taken to derive kinetic data for the pyrolysis reactions. These kinetic data were then used in the simulation program PARSIM. During the experiments the reactor of the TGA was flushed with 2.4 l·min − 1 (s.t.p.) of nitrogen. In order to protect the microbalance of the TGA from the product gases, it was purged separately with 1.2 l·min − 1 (s.t.p.) of nitrogen during the experiments. The pyrolysis gases leaving the TGA were first cleaned in a tar trap. The tar trap was cooled by water to a temperature of about 15°C. The composition of the non-condensable product gases was analysed by a BOMEM MB 100 FTIR-spectrometer (Hartmann & Braun) and a CALDOS-hydrogen analyser (Hartmann & Braun). From the FTIR, quantitative information for carbon monoxide, carbon dioxide, methane and water was obtained. The data from the CALDOS-hydrogen analyser were corrected for their sensitivity with respect to other gas components (see [7]) using the data from the FTIR. Validation experiments with large particles were carried out in a muffle furnace (this being in fact a ‘TGA’ for large particles, see Fig. 3). This furnace is provided with a special sluice device which enables a quick drop-in of particles on a sample basket inside the preheated furnace. The sample basket is suspended by a balance above the furnace and enables the registration of the sample weight during the pyrolysis. With the muffle furnace, drop-in experiments [8] and exper-
J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
Fig. 1. Thermogravimetric analyser (TGA).
Fig. 2. Sample baskets for experiments with small particles in the TGA and in the muffle furnace.
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Fig. 3. Muffle furnace for experiments with large particles.
iments with constant heating rates were carried out. For the drop-in experiments large particles with a particle size of 10, 15 and 20 mm were used. For the experiments with constant heating rates small particles (0.5–1.0 mm) and large particles (20 mm) were used. Because of the wider measurement range and the lower accuracy of the muffle furnace balance compared to the TGA balance, higher sample masses in the muffle furnace were necessary. In order to get comparable conditions inside the basket of the TGA and the muffle furnace, a six basket arrangement for small particles in the muffle furnace was used. Fig. 2 shows the sample baskets used during the experiments in the TGA and in the muffle furnace. The small particles inside each basket of the six basket arrangement face conditions similar to the small particles inside the sample basket of the TGA. For experiments with large particles in the muffle furnace a flat basket, also made of Incoloy 800 wire mesh, was used. The experiments with constant heating rates in the muffle furnace were carried out in the temperature range from 105 to 850°C at heating rates of 5 and 20 K/min. For the drop-in experiments the muffle furnace was pre-heated to a constant temperature of 850°C. During all experiments the furnace was purged with 800 l·h − 1 (s.t.p.) of nitrogen. The pyrolysis gases leaving the muffle furnace were also cleaned in a tar trap at a temperature of –15°C because of the expected higher amounts of tar. In the experiments with constant heating rates the BOMEM MB 100 FTIR-spectrometer and the CALDOS-hydrogen analyser were used. With this setup, carbon monoxide, carbon dioxide, methane and hydrogen were detected. In the drop-in experiments, the analysers URAS (Fisher & Rosemount, carbon monoxide and carbon dioxide) and HYDROS (Hartmann & Braun, hydrogen)
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Table 1 Experimental conditions: TGA and muffle furnace
Maximum temperature (°C) Pressure (atm) Heating rate (K·min−1) Maximum sample mass (g) Reactor volume (cm3)
TGA
Muffle furnace
1050 1 5, 20, 60 0.5 40
850 1 5, 20 4 900
were used because of the high concentrations expected. In the drop-in experiments a flame ionisation detector (FID, Testa) was used in addition, in order to get the propane-equivalent of the pyrolysis gases. The gas was sampled upstream of the tar trap for FID analysis. Table 1 gives an overview of the experimental conditions during the experiments in the thermogravimetric analyser and the muffle furnace.
3. Sample preparation Experiments were carried out with small particles of spruce wood and beech wood of the size fraction 0.5– 1.0 mm, with large particles (cubes) of spruce wood with a particle size of 6, 10, 15 and 20 mm and with large particles (cubes) of beech wood with a particle size of 20 mm. The preparation of small particles was done by means of a hammer-mill and sieving. The large particles were sawn by hand. All samples were dried for 2 h at a temperature of 105°C in a drying oven before the experiment. Table 2 shows the proximate and ultimate analysis of the spruce wood and the beech wood used.
Table 2 Proximate and ultimate analysis of spruce and beech wood
Volatiles Fixed C Ash C H O N S
Spruce wood (wt.% dry)
Beech wood (wt.% dry)
86.97 12.78 0.25 50.19 6.10 43.54 0.16 0.01
88.15 11.46 0.39 49.59 6.06 44.08 0.26 0.01
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Fig. 4. Influence of particle size and heating rate on the CO yield, the CO2 yield, the H2 yield, the yield of other volatiles (tar, methane, water,…) and the char yield, spruce wood, pyrolysis 105 – 850°C, nitrogen atmosphere.
4. Results and discussion
4.1. Experiments with spruce wood Fig. 4 shows the respective results, for experiments with constant heating rates (validation of experimental setup) and for drop-in experiments with spruce wood. It gives an overview of the experimental results for the yield of carbon monoxide, carbon dioxide, hydrogen, other volatiles and char from experiments with different heating rates and different particle sizes. The main component of ‘other volatiles’ is tar. The yield of total volatiles is the sum of the yields of the individual gases.
4.1.1. Validation of experimental setup In order to exclude the possibility of different conditions in the TGA and the muffle furnace, a number of tests under identical conditions were made with spruce wood in both installations. During these experiments the heating rates were low and constant. The results of experiments with small particles (0.5–1.0 mm) in the TGA on the one hand and in the muffle furnace on the other, are in good agreement. At heating rates of 5 and of 20 K/min the same respective yields for the total volatiles, for the different gas components and for char were achieved. The average residence time of the volatiles in the muffle furnace is about four times longer than in the TGA. In experiments with low heating rates the volatiles leaving the particle inside the reactor are only exposed to temperatures almost equal to the temperature of their formation. Most of the volatiles were formed at temperatures in the range 250–450°C. As can be seen in Fig. 4, the difference in the average residence time of the volatiles in the reaction zones of the TGA and of the
J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
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muffle furnace does not produce different gases at low heating rates. The conclusion from this test series is, that the conditions for heat-up and reaction are the same in both devices. The different results from experiments with different but low heating rates and different particle sizes which can be seen in Fig. 4, are therefore not due to decomposition reactions of the volatiles inside the reactor, but are due to changes of the pyrolysis reaction process inside the particles. A detailed investigation of the influence of the particle size and the heating rate on the yield of different pyrolysis products can be found in [6]. The simulation program PARSIM takes account of these effects.
4.1.2. Drop-in experiments In the drop-in experiments the yield of total volatiles achieved was always slightly higher than in the experiments with constant heating rates. But the yields of carbon monoxide, carbon dioxide, hydrogen and other volatiles were completely different. The only meaningful explanation for these differences is the occurrence of decomposition reactions of the large molecules during their residence time inside the muffle furnace. In the drop-in experiments the volatiles are exposed to a temperature of 850°C for 1 s. Boroson [9,10] carried out experiments with sweet gum hardwood and found that at temperatures of 650°C and residence times of 1 s of the volatiles considerable decomposition of tar occurs in the reactor. He also found that this cracking of tar leads to an increase of the yield of carbon monoxide, carbon dioxide, hydrogen and small fragments of hydrocarbons. 4.2. Comparison between simulation and experimental results for beech wood A simple first order kinetic model (Boroson [9,10], see Eq. (1)) for the cracking of tar was used. This kinetic model was fed with a volatile composition as calculated by the simulation program PARSIM for beech wood. Table 3 shows a comparison of the kinetic data proposed by Boroson for sweet gum hardwood and our own data evaluated for the cracking of tar from beech wood. The value of the activation energy is the same as for sweet gum hardwood. From results of drop-in experiments using different residence times of the volatiles in the hot muffle furnace, the ultimate yield and the pre-exponential factor from our Table 3 Kinetic data for tar cracking
Biomass
Boroson [6,7] Sweet gum hardwood
This work Beech wood
A [s-1] E [kJ/mol] V* [wt.%, of primary tar]
104.98 93.37 11
105.14 93.37 22
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Fig. 5. Calculated and experimental temperatures and relative sample mass, pyrolysis of beech wood, cube 20 mm, muffle furnace, 850°C, 800 l·h − 1 (s.t.p.) nitrogen, drop-in experiment.
own measurements were determined. Boroson found a primary tar yield of 52.8 wt.% (of wood) and an ultimate tar yield at long residence times at high temperatures of 5.79 wt.% (of wood). So from the primary tar 11 wt.% was left. This ultimate yield was considered to be due to a non-reactive tar fraction. In this paper the fraction of non-reactive tar from primary tar from beech wood was determined to be 22 wt.% of primary tar. The rate of tar cracking is modeled as a first order reaction on the difference between the ultimate yield of tar V* and the amount of tar left at that time V(t). −
dV E =A·exp − ·(V(t) −V*). dt RT
(1)
Figs. 5 and 6 show a comparison between measurements from a drop-in experiment of a 20 mm particle (cube) of beech wood into the muffle furnace (preheated to 850°C) [11] and results from the simulation with PARSIM of the same drop-in experiment. Fig. 5 shows a reasonable agreement between measured and calculated data for the mass loss, the temperatures in the centre and on the surface of the particle during pyrolysis. Fig. 6 shows the measured FID-propane equivalent, the propane equivalent as calculated with PARSIM alone and the propane equivalent calculated with PARSIM combined with the above tar cracking model. Fig. 6 also demonstrates the ability of the tar cracking model to link the PARSIM results to the measured FID propane equivalent. For the calculation of the tar cracking reaction inside the muffle furnace, the residence time distribution obtained from step-response experiments was taken into account.
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Fig. 6. Calculated and experimental formation of tar (indicated as propane equivalent), pyrolysis of beech wood, cube 20 mm, muffle furnace, 850°C, 800 l·h − 1 (s.t.p.) nitrogen, drop-in experiment.
5. Conclusion From the results of experiments in a thermogravimetric analyser and a muffle furnace, an explanation was found for the differences between the simulation program PARSIM and validation experiments in our muffle furnace. The differences were caused by decomposition reactions of the volatiles in the hot oven atmosphere during drop-in experiments. In order to describe these decomposition reactions, a first order kinetic model was taken from the respective literature. The activation energy value proposed in the literature for sweet gum hardwood was found to be appropriate for describing the cracking of tar from pyrolysis of beech wood. The pre-exponential factor and the ultimate yield are slightly different for beech wood. Acknowledgements This research was supported by the Austrian Science Foundation FWF (Project No. S06803) and the European Commission in the Framework of the Non Nuclear Energy Programme JOULE III. References [1] B. Rummer, VDI Berichte 1314 (1997) 265. [2] B. Rummer, Simulation der Trocknung, Pyrolyse und Vergasung großer Brennstoffpartikel, PhD Thesis, Technische Universita¨ t Graz, 1998. [3] J. Petek, B. Rummer, V. Seebauer, G. Steiner, G. Staudinger, Fluidized Bed Technology VI, in: J. Werther (Ed.), Proc. 6th Int. Conf. on Circulating Fluidised Beds, Wu¨ rzburg, Germany, 22 – 27 August, 1999, p. 469.
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[4] H.-J. Mu¨ hlen, A. Sulimma, Thermochim. Acta 103 (1986) 163. [5] H.-J. Mu¨ hlen, A. Sulimma, Fuel Process. Technol. 15 (1987) 145. [6] V. Seebauer, Experimentelle Untersuchungen zur Pyrolyse von Kohle und Holz, PhD Thesis, Technische Universita¨ t Graz, 1999. [7] V. Seebauer, J. Petek, G. Staudinger, Fuel 76 (1997) 1277. [8] J. Petek, Experimentelle Untersuchung der Pyrolyse in inerter und reaktiver Atmospha¨ re unter den Bedingungen der Wurfbeschickung, PhD Thesis, Technische Universita¨ t Graz, 1998. [9] M.L. Boroson, J.B. Howard, J.P. Longwell, W. Peters, AIChE Journal 35 (1989) 120. [10] M.L. Boroson, Secondary Reactions of Tars from Pyrolysis of Sweet Gum Hardwood, PhD Thesis, MIT, University of Michigan, 1987. [11] W. Hochegger, Pyrolyse feuchter Biomasse, Master Thesis, Technische Universita¨ t Graz, 1999.
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Tar cracking from fast pyrolysis of large beech wood particles J. Rath *, G. Steiner, M.G. Wolfinger, G. Staudinger Institut fu¨r Apparatebau, Mechanische Verfahrenstechnik und Feuerungstechnik, Technische Uni6ersita¨t Graz, Inffeldgasse 25, A-8010 Graz, Austria Received 5 July 2000; accepted 24 November 2000
Abstract Pyrolysis experiments in a thermogravimetric analyser and in a muffle furnace were carried out with spruce and beech wood. The particle size of the samples of spruce wood was varied in the range 0.5–20 mm and the heating rate was varied in the range 5 – 60 K/min. Additionally, drop-in experiments in the muffle furnace were carried out with both spruce and beech wood. These experimental data were compared with the results from calculation of pyrolysis of ‘large’ particles with the simulation program PARSIM. Agreement could only be obtained when the tar decomposition outside of the particle was taken into account. The pre-exponential factor and the ultimate yield for cracking of tar from beech wood are given. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Tar cracking; Pyrolysis; Beech wood; Secondary reaction
Nomenclature A pre-exponential factor (s−1) activation energy (kJ·mol−1) E gas constant (kJ·mol−1·K−1) R t time (s) T temperature (K) V amount of tar unconverted (wt.%) V* amount of non-reactive tar (wt.%) * Corresponding author. Fax: + 43-3168737492. E-mail address: [email protected] (J. Rath). 0165-2370/02/$ - see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 5 - 2 3 7 0 ( 0 0 ) 0 0 2 1 5 - 1
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J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
1. Introduction For the simulation of the processes inside large solid fuel particles undergoing pyrolysis a computer program (PARSIM [1–3]) was developed. Results from validation experiments at low heating rates in a muffle furnace were in excellent agreement with simulation results for the mass loss, the temperatures in the centre and on the surface of the particle and also for the evolution of volatiles. In practice (e.g. fluidised bed technology) the fuel particles are dropped into the hot furnace atmosphere. Results from drop-in experiments with large solid fuel particles into a muffle oven are also in good agreement with simulation results for the mass loss and the temperatures in the centre and on the surface of the particle. However for the evolution of the volatiles, only poor agreement between simulation and experiment was found. The differences were assumed to be due to thermal degradation of the volatiles at the drop-in experiments. Detailed experiments were carried out to test this assumption.
2. Experimental setup Fig. 1 shows a diagram of the thermogravimetric analyser (TGA, manufactured by DMT) used. Detailed information can be found in Refs. [4,5]. This thermogravimetric analyser was used for experiments with small particles (0.5– 1.0 mm) in a packed bed and for experiments with single particles of a size of 6 mm [6]. The sample basket was made of Incoloy 800 wire mesh and is shown in Fig. 2. The samples were heated from 105 to 850°C at heating rates of 5, 20 and 60 K/min. The results from experiments with small particles at heating rates of 5 K/min were taken to derive kinetic data for the pyrolysis reactions. These kinetic data were then used in the simulation program PARSIM. During the experiments the reactor of the TGA was flushed with 2.4 l·min − 1 (s.t.p.) of nitrogen. In order to protect the microbalance of the TGA from the product gases, it was purged separately with 1.2 l·min − 1 (s.t.p.) of nitrogen during the experiments. The pyrolysis gases leaving the TGA were first cleaned in a tar trap. The tar trap was cooled by water to a temperature of about 15°C. The composition of the non-condensable product gases was analysed by a BOMEM MB 100 FTIR-spectrometer (Hartmann & Braun) and a CALDOS-hydrogen analyser (Hartmann & Braun). From the FTIR, quantitative information for carbon monoxide, carbon dioxide, methane and water was obtained. The data from the CALDOS-hydrogen analyser were corrected for their sensitivity with respect to other gas components (see [7]) using the data from the FTIR. Validation experiments with large particles were carried out in a muffle furnace (this being in fact a ‘TGA’ for large particles, see Fig. 3). This furnace is provided with a special sluice device which enables a quick drop-in of particles on a sample basket inside the preheated furnace. The sample basket is suspended by a balance above the furnace and enables the registration of the sample weight during the pyrolysis. With the muffle furnace, drop-in experiments [8] and exper-
J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
Fig. 1. Thermogravimetric analyser (TGA).
Fig. 2. Sample baskets for experiments with small particles in the TGA and in the muffle furnace.
85
86
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Fig. 3. Muffle furnace for experiments with large particles.
iments with constant heating rates were carried out. For the drop-in experiments large particles with a particle size of 10, 15 and 20 mm were used. For the experiments with constant heating rates small particles (0.5–1.0 mm) and large particles (20 mm) were used. Because of the wider measurement range and the lower accuracy of the muffle furnace balance compared to the TGA balance, higher sample masses in the muffle furnace were necessary. In order to get comparable conditions inside the basket of the TGA and the muffle furnace, a six basket arrangement for small particles in the muffle furnace was used. Fig. 2 shows the sample baskets used during the experiments in the TGA and in the muffle furnace. The small particles inside each basket of the six basket arrangement face conditions similar to the small particles inside the sample basket of the TGA. For experiments with large particles in the muffle furnace a flat basket, also made of Incoloy 800 wire mesh, was used. The experiments with constant heating rates in the muffle furnace were carried out in the temperature range from 105 to 850°C at heating rates of 5 and 20 K/min. For the drop-in experiments the muffle furnace was pre-heated to a constant temperature of 850°C. During all experiments the furnace was purged with 800 l·h − 1 (s.t.p.) of nitrogen. The pyrolysis gases leaving the muffle furnace were also cleaned in a tar trap at a temperature of –15°C because of the expected higher amounts of tar. In the experiments with constant heating rates the BOMEM MB 100 FTIR-spectrometer and the CALDOS-hydrogen analyser were used. With this setup, carbon monoxide, carbon dioxide, methane and hydrogen were detected. In the drop-in experiments, the analysers URAS (Fisher & Rosemount, carbon monoxide and carbon dioxide) and HYDROS (Hartmann & Braun, hydrogen)
J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
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Table 1 Experimental conditions: TGA and muffle furnace
Maximum temperature (°C) Pressure (atm) Heating rate (K·min−1) Maximum sample mass (g) Reactor volume (cm3)
TGA
Muffle furnace
1050 1 5, 20, 60 0.5 40
850 1 5, 20 4 900
were used because of the high concentrations expected. In the drop-in experiments a flame ionisation detector (FID, Testa) was used in addition, in order to get the propane-equivalent of the pyrolysis gases. The gas was sampled upstream of the tar trap for FID analysis. Table 1 gives an overview of the experimental conditions during the experiments in the thermogravimetric analyser and the muffle furnace.
3. Sample preparation Experiments were carried out with small particles of spruce wood and beech wood of the size fraction 0.5– 1.0 mm, with large particles (cubes) of spruce wood with a particle size of 6, 10, 15 and 20 mm and with large particles (cubes) of beech wood with a particle size of 20 mm. The preparation of small particles was done by means of a hammer-mill and sieving. The large particles were sawn by hand. All samples were dried for 2 h at a temperature of 105°C in a drying oven before the experiment. Table 2 shows the proximate and ultimate analysis of the spruce wood and the beech wood used.
Table 2 Proximate and ultimate analysis of spruce and beech wood
Volatiles Fixed C Ash C H O N S
Spruce wood (wt.% dry)
Beech wood (wt.% dry)
86.97 12.78 0.25 50.19 6.10 43.54 0.16 0.01
88.15 11.46 0.39 49.59 6.06 44.08 0.26 0.01
88
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Fig. 4. Influence of particle size and heating rate on the CO yield, the CO2 yield, the H2 yield, the yield of other volatiles (tar, methane, water,…) and the char yield, spruce wood, pyrolysis 105 – 850°C, nitrogen atmosphere.
4. Results and discussion
4.1. Experiments with spruce wood Fig. 4 shows the respective results, for experiments with constant heating rates (validation of experimental setup) and for drop-in experiments with spruce wood. It gives an overview of the experimental results for the yield of carbon monoxide, carbon dioxide, hydrogen, other volatiles and char from experiments with different heating rates and different particle sizes. The main component of ‘other volatiles’ is tar. The yield of total volatiles is the sum of the yields of the individual gases.
4.1.1. Validation of experimental setup In order to exclude the possibility of different conditions in the TGA and the muffle furnace, a number of tests under identical conditions were made with spruce wood in both installations. During these experiments the heating rates were low and constant. The results of experiments with small particles (0.5–1.0 mm) in the TGA on the one hand and in the muffle furnace on the other, are in good agreement. At heating rates of 5 and of 20 K/min the same respective yields for the total volatiles, for the different gas components and for char were achieved. The average residence time of the volatiles in the muffle furnace is about four times longer than in the TGA. In experiments with low heating rates the volatiles leaving the particle inside the reactor are only exposed to temperatures almost equal to the temperature of their formation. Most of the volatiles were formed at temperatures in the range 250–450°C. As can be seen in Fig. 4, the difference in the average residence time of the volatiles in the reaction zones of the TGA and of the
J. Rath et al. / J. Anal. Appl. Pyrolysis 62 (2002) 83–92
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muffle furnace does not produce different gases at low heating rates. The conclusion from this test series is, that the conditions for heat-up and reaction are the same in both devices. The different results from experiments with different but low heating rates and different particle sizes which can be seen in Fig. 4, are therefore not due to decomposition reactions of the volatiles inside the reactor, but are due to changes of the pyrolysis reaction process inside the particles. A detailed investigation of the influence of the particle size and the heating rate on the yield of different pyrolysis products can be found in [6]. The simulation program PARSIM takes account of these effects.
4.1.2. Drop-in experiments In the drop-in experiments the yield of total volatiles achieved was always slightly higher than in the experiments with constant heating rates. But the yields of carbon monoxide, carbon dioxide, hydrogen and other volatiles were completely different. The only meaningful explanation for these differences is the occurrence of decomposition reactions of the large molecules during their residence time inside the muffle furnace. In the drop-in experiments the volatiles are exposed to a temperature of 850°C for 1 s. Boroson [9,10] carried out experiments with sweet gum hardwood and found that at temperatures of 650°C and residence times of 1 s of the volatiles considerable decomposition of tar occurs in the reactor. He also found that this cracking of tar leads to an increase of the yield of carbon monoxide, carbon dioxide, hydrogen and small fragments of hydrocarbons. 4.2. Comparison between simulation and experimental results for beech wood A simple first order kinetic model (Boroson [9,10], see Eq. (1)) for the cracking of tar was used. This kinetic model was fed with a volatile composition as calculated by the simulation program PARSIM for beech wood. Table 3 shows a comparison of the kinetic data proposed by Boroson for sweet gum hardwood and our own data evaluated for the cracking of tar from beech wood. The value of the activation energy is the same as for sweet gum hardwood. From results of drop-in experiments using different residence times of the volatiles in the hot muffle furnace, the ultimate yield and the pre-exponential factor from our Table 3 Kinetic data for tar cracking
Biomass
Boroson [6,7] Sweet gum hardwood
This work Beech wood
A [s-1] E [kJ/mol] V* [wt.%, of primary tar]
104.98 93.37 11
105.14 93.37 22
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Fig. 5. Calculated and experimental temperatures and relative sample mass, pyrolysis of beech wood, cube 20 mm, muffle furnace, 850°C, 800 l·h − 1 (s.t.p.) nitrogen, drop-in experiment.
own measurements were determined. Boroson found a primary tar yield of 52.8 wt.% (of wood) and an ultimate tar yield at long residence times at high temperatures of 5.79 wt.% (of wood). So from the primary tar 11 wt.% was left. This ultimate yield was considered to be due to a non-reactive tar fraction. In this paper the fraction of non-reactive tar from primary tar from beech wood was determined to be 22 wt.% of primary tar. The rate of tar cracking is modeled as a first order reaction on the difference between the ultimate yield of tar V* and the amount of tar left at that time V(t). −
dV E =A·exp − ·(V(t) −V*). dt RT
(1)
Figs. 5 and 6 show a comparison between measurements from a drop-in experiment of a 20 mm particle (cube) of beech wood into the muffle furnace (preheated to 850°C) [11] and results from the simulation with PARSIM of the same drop-in experiment. Fig. 5 shows a reasonable agreement between measured and calculated data for the mass loss, the temperatures in the centre and on the surface of the particle during pyrolysis. Fig. 6 shows the measured FID-propane equivalent, the propane equivalent as calculated with PARSIM alone and the propane equivalent calculated with PARSIM combined with the above tar cracking model. Fig. 6 also demonstrates the ability of the tar cracking model to link the PARSIM results to the measured FID propane equivalent. For the calculation of the tar cracking reaction inside the muffle furnace, the residence time distribution obtained from step-response experiments was taken into account.
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Fig. 6. Calculated and experimental formation of tar (indicated as propane equivalent), pyrolysis of beech wood, cube 20 mm, muffle furnace, 850°C, 800 l·h − 1 (s.t.p.) nitrogen, drop-in experiment.
5. Conclusion From the results of experiments in a thermogravimetric analyser and a muffle furnace, an explanation was found for the differences between the simulation program PARSIM and validation experiments in our muffle furnace. The differences were caused by decomposition reactions of the volatiles in the hot oven atmosphere during drop-in experiments. In order to describe these decomposition reactions, a first order kinetic model was taken from the respective literature. The activation energy value proposed in the literature for sweet gum hardwood was found to be appropriate for describing the cracking of tar from pyrolysis of beech wood. The pre-exponential factor and the ultimate yield are slightly different for beech wood. Acknowledgements This research was supported by the Austrian Science Foundation FWF (Project No. S06803) and the European Commission in the Framework of the Non Nuclear Energy Programme JOULE III. References [1] B. Rummer, VDI Berichte 1314 (1997) 265. [2] B. Rummer, Simulation der Trocknung, Pyrolyse und Vergasung großer Brennstoffpartikel, PhD Thesis, Technische Universita¨ t Graz, 1998. [3] J. Petek, B. Rummer, V. Seebauer, G. Steiner, G. Staudinger, Fluidized Bed Technology VI, in: J. Werther (Ed.), Proc. 6th Int. Conf. on Circulating Fluidised Beds, Wu¨ rzburg, Germany, 22 – 27 August, 1999, p. 469.
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