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Influence of particle size on pyrolysis and gasification performance of municipal solid waste in a fixed bed reactor
Bioresource Technology 101 (2010) 6517–6520
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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Influence of particle size on pyrolysis and gasification performance of municipal solid waste in a fixed bed reactor Siyi Luo, Bo Xiao *, Zhiquan Hu, Shiming Liu, Yanwen Guan, Lei Cai School of Environmental Science & Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2009 Received in revised form 11 March 2010 Accepted 15 March 2010 Available online 3 April 2010 Keywords: MSW Pyrolysis Steam gasification Particle size
a b s t r a c t Pyrolysis and gasification of municipal solid waste (MSW) were carried out in a lab-scale fixed bed reactor in order to evaluate the effects of particle size at different bed temperatures on product yield and composition. The bed temperature was varied from 600 to 900 °C and the MSW was separated into three different size fractions (below 5 mm, 50–10 mm and above 10 mm). Particle size and temperature had integrated effects on product yield and composition: higher temperature resulted in higher gas yield with less tar and char, and, at the same temperature, dry gas yield increased with a decrease in particle size, and char and tar yield decreased. The differences due to particle sizes in pyrolysis and gasification performance practically disappeared at the highest temperatures tested. Smaller particle sizes resulted in higher H2 and CO contents for both pyrolysis and gasification of MSW. Minimizing the size of raw materials is an alternative method to improve the gas quality of MSW pyrolysis and gasification. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste (MSW) is composed mainly of paper/ cardboard, plastics, glass, metals, textile and garden waste. As such, the waste contains a high proportion of renewable materials which can be used for energy recovery or the production of solid, liquid and gaseous fuels (Buah et al., 2007). MSW conversion technologies especially composting, landfill, combustion and pyrolysis or gasification have been substantially studied to improve MSW energy utilization and solve partially the environmental issues (Alzate-Gaviria et al., 2007). Direct landfill and composting, due to their disadvantages, are gradually replaced by other technologies. In spite of the advantages derived from incineration of municipal solid waste, such as heat recovery, there are numerous disadvantages of incineration including production of large flue gas volumes, hazardous waste streams associated with the fly ash and a poor public image. MSW pyrolysis and gasification technology is an attractive way to treat MSW with less pollution emissions than other methods of treatment. Especially, it offers a potential of higher efficiency in energy production. Pyrolysis and gasification of MSW has been studied extensively (Björklund et al., 2001; Ridge et al., 2001; Rapagnà et al., 1998; Lv et al., 2003) in order to evaluate the influences of operating parameters (i.e. temperature, residence time, catalyst and steam
* Corresponding author. Tel.: +86 027 87557464. E-mail address: [email protected] (B. Xiao).
to MSW ratio, etc.) and type of MSW. However, the effects of particle size on pyrolysis or gasification of MSW has not been subjected to comprehensive study and thus this parameter is often neglected. The work we report here focused on the significance of the effect of MSW particle size as a function of temperature on the pyrolysis and gasification performance (with reference to the product yields and composition) in a fixed bed reactor. The aim was to gather experimental evidence for this effect in MSW pyrolysis and gasification for which there is still a shortage of comprehensive data.
2. Experimental setup 2.1. MSW samples MSW samples were collected from a transfer station, Wuhan, China. The samples were air-dried for a period of 7 days and the moisture content was reduced to about 10.2%. The samples were crushed in a self-designed crushing system composed of two parts: compaction chamber and shredding chamber (Luo et al., 2009). The products collected from the shredder were mixed to enhance homogeneity, and quartering was carried out to obtain approximately 20 kg MSW. The products were separated into three different size fractions by sieving. The particle size distribution was as follows: 33.4 wt.% below 5 mm, 40.1 wt.% 5–10 mm and 26.5 wt.% above 10 mm. The proximate and ultimate analyses of MSW are shown in Table 1.
0960-8524/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.03.060
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S. Luo et al. / Bioresource Technology 101 (2010) 6517–6520
Table 1 Proximate analysis and elemental analysis. Proximate analysis/wt.% (dry basis) Volatile matter Fixed carbon Ash Higher heating value (kJ/kg)
82.28 12.79 4.93 21,306
Ultimate analysis/wt.% C H O N S
51.81 5.76 30.22 0.26 0.36
2.2. Test facilities and procedures Pyrolysis and gasification tests were carried out in the same fixed bed reactor, which was surrounded by an electric heater. The schematic configuration is illustrated in Fig. 1. The oven of reactor was made of 1Cr18Ni9Ti stainless steel and was externally heated by an electrical ring furnace, which was covered with an insulation layer on the outside. The effective height of the reactor was 600 mm with an outside diameter of 219 mm. The furnace was heated at a rate 10 °C min1 to the desired final temperature and kept constant. A type K thermocouple was used to measure the temperature profile in the middle of oven. A filter in series was used as de-dusting unit for fuel gas cleaning. A condenser in series was used as cooling unit for fuel gas cooling and tar capture. The fuel gas from the reactor entered the following de-dusting and cooling units. A screw feeding system continuously fed MSW into the bed through a pipe which was connected at the top of the gasification reactor. MSW feeding rate was determined over a range of screw speeds prior to testing. To insure the reliability of test data, mass balance calculation was performed. At the start-up of each test, MSW was added to the screw feeder, and the controllers were set at the selected operating parameters. In each test the MSW material flow rate was 5 g/min. The gasification tests were performed at atmospheric pressure with steam as gasification agent. In gasification test, the steam to MSW ratio was set at 1.2. 7 13 9
6 5
For fast pyrolysis, the electrical heater was turned on first, when the desired temperatures were achieved in the reactor, the screw feeder was turned onto the desired rotation speed to feed material into the reactor. For steam gasification, the screw feeder was turned on at the set temperature, and the steam was injected from the bottom of the gasification reactor. Once the system was stable, the temperatures and the flow rate of steam were kept stable for about 30 min for gas sampling and analysis. The dry, clean, and cool fuel gas was sampled by a gas sampling bag every 10 min. And the gas compositions were analyzed by a gas chromatograph (Micro-GC 3000A, Agilent) with thermal conductivity detector and flame ionic detector. The oil produced as a result of the pyrolysis and gasification was collected in the condenser. The char was collected at the end of each experiment and weighed. Each experiment was repeated three times, and the data reported are the average values. The varied factors are pyrolysis/gasification temperature and particle size, the temperature range chosen for this study was 600–900 °C, whereas three particle diameter ranges were used, the fractions smaller than 5 mm, between 5 and 10 mm and between 10 and 20 mm. 3. Results and discussion 3.1. Effect of particle size on pyrolysis performance of MSW Fig. 2 shows the effect of particle size on dry gas yield at different temperatures. Fig. 3 shows an estimate of the percentage by weight of the MSW that remained in the reactor in the form of char, together with tar that adhered to the walls of the condenser, as a function of temperature for the different samples. It can be seen from Figs. 2 and 3, the dry gas yield and the production of char and tar were primarily influenced by the operating temperature. By changing the temperature from 600 to 900 °C, the gas yield increased significantly, while the char and tar decreased sharply for all particle sizes. The increase of gas yield can be attributed mainly to the decomposition of char and the tar vapor as temperature increased (He et al., 2009), since more char and tar can be converted into gas through Boudouard reactions and thermal cracking reaction, respectively. Pyrolysis product yields and composition are dependent on the heating rate of sample particles. Higher heating rate produced more light gases as well as less char and condensate. Smaller particles contributed to a larger surface area and faster heating rates (Di Blasi, 1996). As can be seen in Figs. 2 and 3, the effect of MSW size was substantial and systematic. At the same bed temperature, smaller particle size caused higher dry gas yield, presumably for two reasons. One is the influence of particle size on the
1
10
2
4
11
8
3
Fig. 1. The schematic of the MSW pyrolysis and gasification facility. 1, temperature controlled cabinet; 2, steam generator; 3, steam flow meter; 4, thermocouple; 5, electric heater; 6, pyrolysis and gasification reactor; 7, screw feeder; 8, condenser; 9, filter; 10, flue gas meter; 11, water-sealed bottle; 12, gas sampling bag; 13, fuel gas.
Gas yield Nm3/ Kg
12
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
10-20mm
600
5-10mm
700 800 Temperature
Below5mm
900
Fig. 2. Dry gas yield of MSW pyrolysis as a function of particle sizes at different bed temperatures.
6519
30
3.2. Effect of particle size on steam gasification performance of MSW
25
Figs. 4 and 5 show the dry gas yield and the weight ratio of char and tar to the MSW obtained at different MSW particle size by steam gasification, respectively. Particle size and temperature have integrated effects since, with the smallest particles, as the temperature was raised from 600 to 900 °C, the char and tar fell from 11.4% to 3.5%, and dry gas yield increased from 0.96 to 1.39 Nm3/kg. The differences in product yield and composition with temperature are due first to the greater production of gas in the initial pyrolysis (faster at higher temperatures), second to the endothermic reactions of gasification of the char and third to the steam cracking and reforming of the tars (Herguido et al., 1992). For the smallest particle size (below 5 mm), there is a negligible production of char and tars (6.6%) at lower temperature (700 °C) (Fig. 5). While for the intermediate particle size (5–10 mm) there is a large amount of residual solid at the same temperature, which falls with increasing temperature until, at 900 °C, it is down to 7% or 8%. However, for the largest particle size (above 10 mm), residual solids remain above 10% even at the maximum temperature (900 °C). This effect could be explained by the bigger surface area of the small particles that interacts with the gasification medium (steam) and formation of volatile products that leave the sample without undergoing secondary cracking reactions. In the case of larger particles, this phenomenon could be dominant, leading to additional char and tar formation. Mass and heat limitations are also more significant for larger particles. Table 3 shows the gas composition of MSW gasification by steam with different particle size at 900 °C. Smaller particle size significantly resulted in higher H2 and CO contents. It is known that water–gas shift reaction (Eq. (3)), carbon gasification reaction (Eq. (4)), the secondary cracking reactions of
20 15 10 5
10-20mm
5-10mm
Below 5mm
0 600
700 800 Temperature( )
900
Fig. 3. Percentage by weight of char and tar as a function of particle size and bed temperature (pyrolysis).
heat transfer since the pyrolysis process mainly occurs in the surface of MSW. Larger particles contain greater heat transfer resistance, and hence the actual temperature inside the particle is lower, which leads to the occurrence of a devolatilization process (Lv et al., 2004). Subsequently, incomplete pyrolysis results in a large amount of residual char. The other possible reason could be that the pyrolysis process of smaller particle was mainly controlled by reaction kinetics; as the particle size increased, the process was mainly controlled by gas diffusion, since the resultant product gas inside the particle had more difficulty to diffusing out (Lv et al., 2004). A reduction in the effect of particle size with increasing temperature was observed (Figs. 2 and 3). Once the temperature reached 800–900 °C, the difference in reaction products was minimal. Within this temperature range, the particle size only influenced reaction intensity and did not impact the reaction mechanism. This reduction could have been the result of the increase in the effective thermal conductivity (Di Blasi, 1996) due to the increase of the radiation contribution to heat transfer. In regard to the gas fraction, the gas component distribution profile from pyrolysis at 900 °C is plotted in Table 2 as a function of particle size. CO and CO2 were about one third each of the total volume in the gas. H2 contributed 18–22% of the total gas, and the contribution of hydrocarbons (C2H4 and C2H6) was less than 7% compared to approximately 10% of CH4. As particle size decreased, the content of H2 and CO increased from 18.3% to 22.4% and from 22.0% to 26.5%, respectively, which might be attributed to enhanced pyrolysis, described by the following equation:
Cx Hy Oz ! aCO2 þ bH2 O þ cCH4 þ dCO þ eH2 þ f C2þ
ð1Þ
1.6 Dry gas yield Nm3/ Kg
Percentage by weight of char and tar(%)
S. Luo et al. / Bioresource Technology 101 (2010) 6517–6520
1.4 1.2 1 0.8 0.6 0.4
10-20mm
0.2 0
600
The others gas components (CH4, C2H4 and C2H6) are not affected by the particle size in the range studied. Table 2 Gas composition of MSW pyrolysis with different particle size at 900 °C. Particle size/d (mm)
d<5
5 < d < 10
10 < d < 20
H2 CO CO2 CH4 C2H4 C2H6
22.4 26.5 34.2 10.1 5.3 1.5
20.6 24.7 37.1 12.6 3.3 1.7
18.3 22 43.2 11.5 4.3 0.7
800
900
Fig. 4. Dry gas yield of MSW gasification by steam as a function of particle sizes at different bed temperatures.
Percentage by weight of char and tar(%)
ð2Þ
700
Below5mm
Temperature
The CO2 content decreased gradully, which can be explained by improved Boudouard reactions (2) due to decreased particle size:
C þ CO2 ! 2CO
5-10mm
10-20mm
25
5-10mm
Below 5mm
20 15 10 5 0
600
700 800 Temperature( )
900
Fig. 5. Percentage by weight of char and tar as a function of particle size and bed temperature (steam gasification).
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Table 3 Gas composition of MSW gasification by steam with different particle size at 900 °C. Particle size/d (mm)
d<5
5 < d < 10
10 < d < 20
H2 CO CO2 CH4 C2H4 C2H6
32.8 31.1 22.5 8.7 3.7 1.2
30.6 25.7 34.5 5.6 2.6 1.0
26.5 23.3 42.8 4.6 2.2 0.6
with increasing temperature was observed. Smaller MSW particles produce more H2 and CO and are more favorable for gas quality and yield. Acknowledgements
tar (Eq. (5)), together with Boudouard reactions (Eq. (2)) are the main factors responsible for the increase in H2 and CO contents (Reed, 1981). It can be concluded that smaller particle size improved the above reactions
H2 O þ CO ! H2 þ CO2
ð3Þ
C þ H2 O ! CO þ H2
ð4Þ
Tar þ n1 H2 O ! n2 CO2 þ n3 H2
ð5Þ
CO2 content showed a decreasing trend with the decrease of particle size. As reported by Lv et al. (2004), this result is related to the fact that the fractions of the CO, CO2 and H2, are linked together by the equilibrium of the water–gas shift reaction under test conditions. A small increase in CH4 with the decreasing particle size was observed. CH4 is produced by the reactions (6)–(8), whose equilibrium constant increases with a decrease in particle size
C þ 2H2 ! CH4
ð6Þ
CO þ 3H2 ! CH4 þ H2 O
ð7Þ
2C þ 2H2 O ! CO2 þ CH4
ð8Þ
C2H4 and C2H6 content were relatively small, and slightly increased with decreasing particle size. 4. Conclusions Pyrolysis and steam gasification of MSW were primarily influenced by operating temperature. With increasing temperature, the gas yield showed increasing trend and char and tar decreased gradually. Smaller particles produce more gaseous products, less tar and char, but a reduction in the effect of MSW particles size
The authors wish to acknowledge the financial support received from the National Natural Science Foundation of China (No. 20876066). The authors would also like to thank the Analysis and Test Center of Huazhong University of Science and Technology for carrying out the ultimate analysis of MSW samples. References Alzate-Gaviria, L.M., Sebastian, P.J., Pérez-Hernández, A., Eapen, D., 2007. Comparison of two anaerobic systems for hydrogen production from the organic fraction of municipal solid waste and synthetic wastewater. International Journal of Hydrogen Energy 32 (15), 3141–3146. Björklund, A., Melaina, M., Keoleian, G., 2001. Hydrogen as a transportation fuel produced from thermal gasification of municipal solid waste: an examination of two integrated technologies. International Journal of Hydrogen Energy 26 (11), 1209–1221. Buah, W.K., Cunliffe, A.M., Williams, P.T., 2007. Characterization of production from the pyrolysis of municipal solid waste. Process Safety and Environmental Protection 85 (5), 450–457. Di Blasi, C., 1996. Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Industrial & Engineering Chemistry Research 35, 37–46. He, M.Y., Xiao, B., Liu, S.M., Guo, X.J., Luo, S.Y., Xu, Z.L., Feng, Y., Hu, Z.Q., 2009. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of steam to MSW ratios and weight hourly space velocity on gas production and composition. International Journal of Hydrogen Energy 34 (5), 2174–2183. Herguido, J., Corella, J., Gonzalez-Saiz, J., 1992. Steam gasification of lignocellulosic residues in a fluidized bed at a small pilot plant: effect of the type of feedstock. Industrial & Engineering Chemistry Research 31, 1247–1282. Luo, S.Y., Xiao, B., Hu, Z.Q., Liu, S.M., Guo, X.J., 2009. An experimental study on a novel shredder for municipal solid waste (MSW). International Journal of Hydrogen Energy 34 (3), 1270–1274. Lv, P.M., Chang, J., Xiong, Z.H., Huang, H.T., Wu, C.Z., Chen, Y., Zhu, J.X., 2003. Biomass air-steam gasification in a fluidized bed to produce hydrogen-rich gas. Energy & fuels 17, 677–682. Lv, P.M., Xiong, Z.H., Chang, J., Wu, C.Z., Chen, Y., Zhu, J.X., 2004. An experimental study on biomass air-steam gasification in a fluidized bed. Bioresource Technology 95 (1), 95–101. Rapagnà, S., Jand, N., Foscolo, P.U., 1998. Catalytic gasification of MSW to produce hydrogen rich gas. International Journal of Hydrogen Energy 23 (7), 551–557. Reed, T.B., 1981. MSW Gasification Principle and Technology. Noyes Data Corporation, Park. Ridge, N.J., Sorum, L., Grønli, M.G., Hustad, J.E., 2001. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 80 (9), 1217–1227.
Contents lists available at ScienceDirect
Bioresource Technology journal homepage: www.elsevier.com/locate/biortech
Influence of particle size on pyrolysis and gasification performance of municipal solid waste in a fixed bed reactor Siyi Luo, Bo Xiao *, Zhiquan Hu, Shiming Liu, Yanwen Guan, Lei Cai School of Environmental Science & Engineering, Huazhong University of Science and Technology, Wuhan 430074, PR China
a r t i c l e
i n f o
Article history: Received 14 July 2009 Received in revised form 11 March 2010 Accepted 15 March 2010 Available online 3 April 2010 Keywords: MSW Pyrolysis Steam gasification Particle size
a b s t r a c t Pyrolysis and gasification of municipal solid waste (MSW) were carried out in a lab-scale fixed bed reactor in order to evaluate the effects of particle size at different bed temperatures on product yield and composition. The bed temperature was varied from 600 to 900 °C and the MSW was separated into three different size fractions (below 5 mm, 50–10 mm and above 10 mm). Particle size and temperature had integrated effects on product yield and composition: higher temperature resulted in higher gas yield with less tar and char, and, at the same temperature, dry gas yield increased with a decrease in particle size, and char and tar yield decreased. The differences due to particle sizes in pyrolysis and gasification performance practically disappeared at the highest temperatures tested. Smaller particle sizes resulted in higher H2 and CO contents for both pyrolysis and gasification of MSW. Minimizing the size of raw materials is an alternative method to improve the gas quality of MSW pyrolysis and gasification. Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved.
1. Introduction Municipal solid waste (MSW) is composed mainly of paper/ cardboard, plastics, glass, metals, textile and garden waste. As such, the waste contains a high proportion of renewable materials which can be used for energy recovery or the production of solid, liquid and gaseous fuels (Buah et al., 2007). MSW conversion technologies especially composting, landfill, combustion and pyrolysis or gasification have been substantially studied to improve MSW energy utilization and solve partially the environmental issues (Alzate-Gaviria et al., 2007). Direct landfill and composting, due to their disadvantages, are gradually replaced by other technologies. In spite of the advantages derived from incineration of municipal solid waste, such as heat recovery, there are numerous disadvantages of incineration including production of large flue gas volumes, hazardous waste streams associated with the fly ash and a poor public image. MSW pyrolysis and gasification technology is an attractive way to treat MSW with less pollution emissions than other methods of treatment. Especially, it offers a potential of higher efficiency in energy production. Pyrolysis and gasification of MSW has been studied extensively (Björklund et al., 2001; Ridge et al., 2001; Rapagnà et al., 1998; Lv et al., 2003) in order to evaluate the influences of operating parameters (i.e. temperature, residence time, catalyst and steam
* Corresponding author. Tel.: +86 027 87557464. E-mail address: [email protected] (B. Xiao).
to MSW ratio, etc.) and type of MSW. However, the effects of particle size on pyrolysis or gasification of MSW has not been subjected to comprehensive study and thus this parameter is often neglected. The work we report here focused on the significance of the effect of MSW particle size as a function of temperature on the pyrolysis and gasification performance (with reference to the product yields and composition) in a fixed bed reactor. The aim was to gather experimental evidence for this effect in MSW pyrolysis and gasification for which there is still a shortage of comprehensive data.
2. Experimental setup 2.1. MSW samples MSW samples were collected from a transfer station, Wuhan, China. The samples were air-dried for a period of 7 days and the moisture content was reduced to about 10.2%. The samples were crushed in a self-designed crushing system composed of two parts: compaction chamber and shredding chamber (Luo et al., 2009). The products collected from the shredder were mixed to enhance homogeneity, and quartering was carried out to obtain approximately 20 kg MSW. The products were separated into three different size fractions by sieving. The particle size distribution was as follows: 33.4 wt.% below 5 mm, 40.1 wt.% 5–10 mm and 26.5 wt.% above 10 mm. The proximate and ultimate analyses of MSW are shown in Table 1.
0960-8524/$ - see front matter Crown Copyright Ó 2010 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2010.03.060
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S. Luo et al. / Bioresource Technology 101 (2010) 6517–6520
Table 1 Proximate analysis and elemental analysis. Proximate analysis/wt.% (dry basis) Volatile matter Fixed carbon Ash Higher heating value (kJ/kg)
82.28 12.79 4.93 21,306
Ultimate analysis/wt.% C H O N S
51.81 5.76 30.22 0.26 0.36
2.2. Test facilities and procedures Pyrolysis and gasification tests were carried out in the same fixed bed reactor, which was surrounded by an electric heater. The schematic configuration is illustrated in Fig. 1. The oven of reactor was made of 1Cr18Ni9Ti stainless steel and was externally heated by an electrical ring furnace, which was covered with an insulation layer on the outside. The effective height of the reactor was 600 mm with an outside diameter of 219 mm. The furnace was heated at a rate 10 °C min1 to the desired final temperature and kept constant. A type K thermocouple was used to measure the temperature profile in the middle of oven. A filter in series was used as de-dusting unit for fuel gas cleaning. A condenser in series was used as cooling unit for fuel gas cooling and tar capture. The fuel gas from the reactor entered the following de-dusting and cooling units. A screw feeding system continuously fed MSW into the bed through a pipe which was connected at the top of the gasification reactor. MSW feeding rate was determined over a range of screw speeds prior to testing. To insure the reliability of test data, mass balance calculation was performed. At the start-up of each test, MSW was added to the screw feeder, and the controllers were set at the selected operating parameters. In each test the MSW material flow rate was 5 g/min. The gasification tests were performed at atmospheric pressure with steam as gasification agent. In gasification test, the steam to MSW ratio was set at 1.2. 7 13 9
6 5
For fast pyrolysis, the electrical heater was turned on first, when the desired temperatures were achieved in the reactor, the screw feeder was turned onto the desired rotation speed to feed material into the reactor. For steam gasification, the screw feeder was turned on at the set temperature, and the steam was injected from the bottom of the gasification reactor. Once the system was stable, the temperatures and the flow rate of steam were kept stable for about 30 min for gas sampling and analysis. The dry, clean, and cool fuel gas was sampled by a gas sampling bag every 10 min. And the gas compositions were analyzed by a gas chromatograph (Micro-GC 3000A, Agilent) with thermal conductivity detector and flame ionic detector. The oil produced as a result of the pyrolysis and gasification was collected in the condenser. The char was collected at the end of each experiment and weighed. Each experiment was repeated three times, and the data reported are the average values. The varied factors are pyrolysis/gasification temperature and particle size, the temperature range chosen for this study was 600–900 °C, whereas three particle diameter ranges were used, the fractions smaller than 5 mm, between 5 and 10 mm and between 10 and 20 mm. 3. Results and discussion 3.1. Effect of particle size on pyrolysis performance of MSW Fig. 2 shows the effect of particle size on dry gas yield at different temperatures. Fig. 3 shows an estimate of the percentage by weight of the MSW that remained in the reactor in the form of char, together with tar that adhered to the walls of the condenser, as a function of temperature for the different samples. It can be seen from Figs. 2 and 3, the dry gas yield and the production of char and tar were primarily influenced by the operating temperature. By changing the temperature from 600 to 900 °C, the gas yield increased significantly, while the char and tar decreased sharply for all particle sizes. The increase of gas yield can be attributed mainly to the decomposition of char and the tar vapor as temperature increased (He et al., 2009), since more char and tar can be converted into gas through Boudouard reactions and thermal cracking reaction, respectively. Pyrolysis product yields and composition are dependent on the heating rate of sample particles. Higher heating rate produced more light gases as well as less char and condensate. Smaller particles contributed to a larger surface area and faster heating rates (Di Blasi, 1996). As can be seen in Figs. 2 and 3, the effect of MSW size was substantial and systematic. At the same bed temperature, smaller particle size caused higher dry gas yield, presumably for two reasons. One is the influence of particle size on the
1
10
2
4
11
8
3
Fig. 1. The schematic of the MSW pyrolysis and gasification facility. 1, temperature controlled cabinet; 2, steam generator; 3, steam flow meter; 4, thermocouple; 5, electric heater; 6, pyrolysis and gasification reactor; 7, screw feeder; 8, condenser; 9, filter; 10, flue gas meter; 11, water-sealed bottle; 12, gas sampling bag; 13, fuel gas.
Gas yield Nm3/ Kg
12
1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0
10-20mm
600
5-10mm
700 800 Temperature
Below5mm
900
Fig. 2. Dry gas yield of MSW pyrolysis as a function of particle sizes at different bed temperatures.
6519
30
3.2. Effect of particle size on steam gasification performance of MSW
25
Figs. 4 and 5 show the dry gas yield and the weight ratio of char and tar to the MSW obtained at different MSW particle size by steam gasification, respectively. Particle size and temperature have integrated effects since, with the smallest particles, as the temperature was raised from 600 to 900 °C, the char and tar fell from 11.4% to 3.5%, and dry gas yield increased from 0.96 to 1.39 Nm3/kg. The differences in product yield and composition with temperature are due first to the greater production of gas in the initial pyrolysis (faster at higher temperatures), second to the endothermic reactions of gasification of the char and third to the steam cracking and reforming of the tars (Herguido et al., 1992). For the smallest particle size (below 5 mm), there is a negligible production of char and tars (6.6%) at lower temperature (700 °C) (Fig. 5). While for the intermediate particle size (5–10 mm) there is a large amount of residual solid at the same temperature, which falls with increasing temperature until, at 900 °C, it is down to 7% or 8%. However, for the largest particle size (above 10 mm), residual solids remain above 10% even at the maximum temperature (900 °C). This effect could be explained by the bigger surface area of the small particles that interacts with the gasification medium (steam) and formation of volatile products that leave the sample without undergoing secondary cracking reactions. In the case of larger particles, this phenomenon could be dominant, leading to additional char and tar formation. Mass and heat limitations are also more significant for larger particles. Table 3 shows the gas composition of MSW gasification by steam with different particle size at 900 °C. Smaller particle size significantly resulted in higher H2 and CO contents. It is known that water–gas shift reaction (Eq. (3)), carbon gasification reaction (Eq. (4)), the secondary cracking reactions of
20 15 10 5
10-20mm
5-10mm
Below 5mm
0 600
700 800 Temperature( )
900
Fig. 3. Percentage by weight of char and tar as a function of particle size and bed temperature (pyrolysis).
heat transfer since the pyrolysis process mainly occurs in the surface of MSW. Larger particles contain greater heat transfer resistance, and hence the actual temperature inside the particle is lower, which leads to the occurrence of a devolatilization process (Lv et al., 2004). Subsequently, incomplete pyrolysis results in a large amount of residual char. The other possible reason could be that the pyrolysis process of smaller particle was mainly controlled by reaction kinetics; as the particle size increased, the process was mainly controlled by gas diffusion, since the resultant product gas inside the particle had more difficulty to diffusing out (Lv et al., 2004). A reduction in the effect of particle size with increasing temperature was observed (Figs. 2 and 3). Once the temperature reached 800–900 °C, the difference in reaction products was minimal. Within this temperature range, the particle size only influenced reaction intensity and did not impact the reaction mechanism. This reduction could have been the result of the increase in the effective thermal conductivity (Di Blasi, 1996) due to the increase of the radiation contribution to heat transfer. In regard to the gas fraction, the gas component distribution profile from pyrolysis at 900 °C is plotted in Table 2 as a function of particle size. CO and CO2 were about one third each of the total volume in the gas. H2 contributed 18–22% of the total gas, and the contribution of hydrocarbons (C2H4 and C2H6) was less than 7% compared to approximately 10% of CH4. As particle size decreased, the content of H2 and CO increased from 18.3% to 22.4% and from 22.0% to 26.5%, respectively, which might be attributed to enhanced pyrolysis, described by the following equation:
Cx Hy Oz ! aCO2 þ bH2 O þ cCH4 þ dCO þ eH2 þ f C2þ
ð1Þ
1.6 Dry gas yield Nm3/ Kg
Percentage by weight of char and tar(%)
S. Luo et al. / Bioresource Technology 101 (2010) 6517–6520
1.4 1.2 1 0.8 0.6 0.4
10-20mm
0.2 0
600
The others gas components (CH4, C2H4 and C2H6) are not affected by the particle size in the range studied. Table 2 Gas composition of MSW pyrolysis with different particle size at 900 °C. Particle size/d (mm)
d<5
5 < d < 10
10 < d < 20
H2 CO CO2 CH4 C2H4 C2H6
22.4 26.5 34.2 10.1 5.3 1.5
20.6 24.7 37.1 12.6 3.3 1.7
18.3 22 43.2 11.5 4.3 0.7
800
900
Fig. 4. Dry gas yield of MSW gasification by steam as a function of particle sizes at different bed temperatures.
Percentage by weight of char and tar(%)
ð2Þ
700
Below5mm
Temperature
The CO2 content decreased gradully, which can be explained by improved Boudouard reactions (2) due to decreased particle size:
C þ CO2 ! 2CO
5-10mm
10-20mm
25
5-10mm
Below 5mm
20 15 10 5 0
600
700 800 Temperature( )
900
Fig. 5. Percentage by weight of char and tar as a function of particle size and bed temperature (steam gasification).
6520
S. Luo et al. / Bioresource Technology 101 (2010) 6517–6520
Table 3 Gas composition of MSW gasification by steam with different particle size at 900 °C. Particle size/d (mm)
d<5
5 < d < 10
10 < d < 20
H2 CO CO2 CH4 C2H4 C2H6
32.8 31.1 22.5 8.7 3.7 1.2
30.6 25.7 34.5 5.6 2.6 1.0
26.5 23.3 42.8 4.6 2.2 0.6
with increasing temperature was observed. Smaller MSW particles produce more H2 and CO and are more favorable for gas quality and yield. Acknowledgements
tar (Eq. (5)), together with Boudouard reactions (Eq. (2)) are the main factors responsible for the increase in H2 and CO contents (Reed, 1981). It can be concluded that smaller particle size improved the above reactions
H2 O þ CO ! H2 þ CO2
ð3Þ
C þ H2 O ! CO þ H2
ð4Þ
Tar þ n1 H2 O ! n2 CO2 þ n3 H2
ð5Þ
CO2 content showed a decreasing trend with the decrease of particle size. As reported by Lv et al. (2004), this result is related to the fact that the fractions of the CO, CO2 and H2, are linked together by the equilibrium of the water–gas shift reaction under test conditions. A small increase in CH4 with the decreasing particle size was observed. CH4 is produced by the reactions (6)–(8), whose equilibrium constant increases with a decrease in particle size
C þ 2H2 ! CH4
ð6Þ
CO þ 3H2 ! CH4 þ H2 O
ð7Þ
2C þ 2H2 O ! CO2 þ CH4
ð8Þ
C2H4 and C2H6 content were relatively small, and slightly increased with decreasing particle size. 4. Conclusions Pyrolysis and steam gasification of MSW were primarily influenced by operating temperature. With increasing temperature, the gas yield showed increasing trend and char and tar decreased gradually. Smaller particles produce more gaseous products, less tar and char, but a reduction in the effect of MSW particles size
The authors wish to acknowledge the financial support received from the National Natural Science Foundation of China (No. 20876066). The authors would also like to thank the Analysis and Test Center of Huazhong University of Science and Technology for carrying out the ultimate analysis of MSW samples. References Alzate-Gaviria, L.M., Sebastian, P.J., Pérez-Hernández, A., Eapen, D., 2007. Comparison of two anaerobic systems for hydrogen production from the organic fraction of municipal solid waste and synthetic wastewater. International Journal of Hydrogen Energy 32 (15), 3141–3146. Björklund, A., Melaina, M., Keoleian, G., 2001. Hydrogen as a transportation fuel produced from thermal gasification of municipal solid waste: an examination of two integrated technologies. International Journal of Hydrogen Energy 26 (11), 1209–1221. Buah, W.K., Cunliffe, A.M., Williams, P.T., 2007. Characterization of production from the pyrolysis of municipal solid waste. Process Safety and Environmental Protection 85 (5), 450–457. Di Blasi, C., 1996. Kinetic and heat transfer control in the slow and flash pyrolysis of solids. Industrial & Engineering Chemistry Research 35, 37–46. He, M.Y., Xiao, B., Liu, S.M., Guo, X.J., Luo, S.Y., Xu, Z.L., Feng, Y., Hu, Z.Q., 2009. Hydrogen-rich gas from catalytic steam gasification of municipal solid waste (MSW): Influence of steam to MSW ratios and weight hourly space velocity on gas production and composition. International Journal of Hydrogen Energy 34 (5), 2174–2183. Herguido, J., Corella, J., Gonzalez-Saiz, J., 1992. Steam gasification of lignocellulosic residues in a fluidized bed at a small pilot plant: effect of the type of feedstock. Industrial & Engineering Chemistry Research 31, 1247–1282. Luo, S.Y., Xiao, B., Hu, Z.Q., Liu, S.M., Guo, X.J., 2009. An experimental study on a novel shredder for municipal solid waste (MSW). International Journal of Hydrogen Energy 34 (3), 1270–1274. Lv, P.M., Chang, J., Xiong, Z.H., Huang, H.T., Wu, C.Z., Chen, Y., Zhu, J.X., 2003. Biomass air-steam gasification in a fluidized bed to produce hydrogen-rich gas. Energy & fuels 17, 677–682. Lv, P.M., Xiong, Z.H., Chang, J., Wu, C.Z., Chen, Y., Zhu, J.X., 2004. An experimental study on biomass air-steam gasification in a fluidized bed. Bioresource Technology 95 (1), 95–101. Rapagnà, S., Jand, N., Foscolo, P.U., 1998. Catalytic gasification of MSW to produce hydrogen rich gas. International Journal of Hydrogen Energy 23 (7), 551–557. Reed, T.B., 1981. MSW Gasification Principle and Technology. Noyes Data Corporation, Park. Ridge, N.J., Sorum, L., Grønli, M.G., Hustad, J.E., 2001. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 80 (9), 1217–1227.