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Microwave co-pyrolysis of sewage sludge and rice straw
Energy 87 (2015) 638e644
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Microwave co-pyrolysis of sewage sludge and rice straw Yu-Fong Huang, Chun-Hao Shih, Pei-Te Chiueh*, Shang-Lien Lo Graduate Institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan Rd., Taipei 106, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 5 March 2015 Accepted 8 May 2015 Available online 6 June 2015
This study focused on the co-pyrolysis of sewage sludge and rice straw using microwave heating. The input microwave power level was a critical parameter. Sewage sludge was pyrolyzed without the addition of rice straw at the microwave power levels of 200e300 W, while lower or higher than this range led to only drying or over-heating. The addition of rice straw increased the performance of microwave heating, which allowed a higher maximum temperature. The calorific value of the pyrolyzed biomass increased with the addition of 30e40 wt.% rice straw. A maximum temperature of up to 500 C was measured for blends containing 20 wt.% rice straw, which could be attributed to the synergetic effect of the addition of rice straw and microwave heating. This high temperature may provide another way of thinking for the thermal treatment of waste sewage sludge. A fixed carbon content of up to 33 wt.% was obtained for blends containing 30e40 wt.% rice straw. The atomic H/C and O/C ratios were very close to those of anthracite coal. Therefore, the pyrolyzed blends should have a high potential to be co-fired with coal or even to replace it. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microwave Co-pyrolysis Sewage sludge Rice straw Biochar
1. Introduction The enormous worldwide demand for fossil fuels will soon deplete world energy reserves and cause more severe global climate change than can be imagined. Therefore, it is extremely important to immediately develop reliable and renewable energy to replace fossil fuels. Biomass is an abundant carbon-neutral renewable resource that is used for the production of biofuels and biomaterials [1]. Sewage sludge can be regarded as a biomass resource because of its considerable organic content, and thus energy and resource recovery from sewage sludge is promising [2e4]. Waste sewage sludge could be converted into a valuable biofuel with appropriate processes, which can solve the problem of sewage sludge disposal as well. Raw biomass has several disadvantages, such as low efficiency, high cost, and high rate of decomposition due to its moisture content, the presence of microorganisms, and the inconvenience of storage in practical applications [5]. Therefore, as-received biomass generally needs to be pretreated before it is stored, transported, and utilized. One pretreatment method of biomass is torrefaction, which is a mild pyrolysis performed at a relatively low heating temperature (200e300 C) and heating rate (<50 C/min) [6].
* Corresponding author. Tel.: þ886 2 3366 2798; fax: þ886 2 2392 8830. E-mail address: [email protected] (P.-T. Chiueh). http://dx.doi.org/10.1016/j.energy.2015.05.039 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Pyrolysis is a thermal degradation process for organic compounds in the absence of oxygen or air to produce various gaseous components as well as tar and char residues [7]. Significantly more solid residues (named biochar or torrefied biomass) are produced from biomass torrefaction than traditional pyrolysis. The biochar produced by torrefaction has a higher energy density and improved grinding characteristics, hydrophobicity, and homogeneous properties [8]. Additionally, significantly less energy is required to process torrefied biomass, and separate handling facilities are no longer necessary when it is co-fired with coal in existing power stations [9]. It is difficult to recover and utilize energy from several types of biomass (e.g., sewage sludge) because of their compositions and characteristics. However, these problems may be solved by blending several biomass feedstocks for co-pyrolysis. Blends of sewage sludge and another biomass feedstock could provide various advantages, such as enhanced reaction performance and increased calorific values of biochar with less inorganic and toxic content [2]. Several synergistic or coupling effects could occur during the co-pyrolysis of sewage sludge with rice straw to accelerate the release of volatile matter and shorten the processing time [10]. Rice straw is a valuable biomass feedstock because of its abundance [11] and high volatile content [12], and the addition of rice straw to sewage sludge could be workable to improve copyrolysis performance due to the synergistic or coupling effects as mentioned above.
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Compared with conventional heating, microwave heating is environmentally friendly and well-established and reduces energy consumption and reaction time [13]. Besides, microwave heating does not directly contact the heated materials [14]. Microwave heating has been widely used in many applications, including synthesis [15], digestion [16], extraction [17], sample pretreatment [18], and stabilization [19e21]. For large-sized biomass materials (e.g., wood and cornstalk), thermochemical reactions can occur rapidly due to the nature of fast, volumetric, and selective heating using microwave energy [22]. Materials that can absorb microwaves are called dielectrics, so microwave heating is also referred to as dielectric heating [14]. Potential applications of microwave heating are dependent on the dielectric properties of target materials [23]. Pyrolysis induced by microwave heating is one of the promising attempts because of the efficient heating of feedstocks that has been demonstrated using microwave dielectric heating [24]. Therefore, microwave heating has been utilized in various biomass pyrolysis [23,25e35] and torrefaction [36,37] systems. However, there has been no research on the co-torrefaction of biomass blends induced by microwave heating. In a previous work, it was reported that only 150 W microwave power and 10 min processing time were required to produce torrefied lignocellulosic biomass with a 70% mass yield and 80% energy yield, and the energy density of the torrefied biomass was higher than that of raw biomass by 14% [12]. Therefore, microwave torrefaction should be a feasible method to recover energy from biomass waste. In this study, the co-pyrolysis of sewage sludge and rice straw using a single-mode microwave device was investigated to evaluate the performance of microwave heating and energy recovery. The compositions and characteristics of sewage sludge, rice straw, and their blends were also studied. 2. Materials and methods 2.1. Materials The dry sewage sludge cake used in this study was obtained from the Dihua sewage treatment plant in Taipei, Taiwan. The moisture content of the as-received sewage sludge was approximately 85 wt.%. The sewage sludge was air dried for several months and then dried in an oven for three days. The rice straw was gathered on-site in Chiayi, Taiwan. After drying, the sewage sludge was grinded in a mortar. The rice straw was smashed and sieved using a 50-mesh screen. The characteristics of the raw sewage sludge and rice straw samples are listed in Table 1. Both of the biomass feedstocks contained a high content of volatiles. The volatile content of the rice straw was higher than that of the sewage Table 1 Characteristics of the raw sewage sludge and rice straw samples. Biomass
Sewage sludge
Rice straw
Moisture (wt.%) Proximate analysis (wt.%)a Volatiles Fixed carbon Ash Ultimate analysis (wt.%)b C H N O S Calorific value (MJ/kg)a
11.79 ± 3.82
10.14 ± 0.34
62.11 ± 1.86 10.00 ± 0.80 27.89 ± 1.69
79.71 ± 2.86 12.32 ± 2.55 7.97 ± 0.63
45.16 ± 0.37 7.20 ± 0.37 7.69 ± 0.22 27.50 ± 0.41 N.D. 16.18 ± 0.13
43.64 ± 0.48 5.32 ± 0.11 0.41 ± 0.04 18.86 ± 1.01 N.D. 18.40 ± 0.46
a b
Dry basis. Dry and ash-free basis.
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sludge by approximately 20 wt.%, whereas the ash content in the rice straw was lower than that of the sewage sludge by approximately the same magnitude. Consequently, it is not surprising that the calorific value of rice straw was significantly higher than that of the sewage sludge. In this study, the calorific value of the sewage sludge was compared with values presented in the literature. The obtained value was similar to 16.45 MJ/kg [38] and significantly higher than 9.69 MJ/kg [5]. Both of the biomass feedstocks were predominately composed of carbon. The content of oxygen and nitrogen in the sewage sludge was higher than that of the rice straw by 8.64 and 7.28 wt.%, respectively. Sulfur was not detected in either the two biomass feedstocks. 2.2. Experimental procedure This study used a single-mode (focused) microwave device with a 2.45 GHz frequency. The schematic diagram of the overall microwave pyrolysis set-up can be found elsewhere [36]. The shredded and sieved biomass feedstock was added to a quartz crucible and then placed inside a quartz tube that was located in the pathway of the microwaves. An infrared thermometer was placed at the top of the quartz tube to measure the temperature of the biomass sample. To maintain anoxic conditions, nitrogen gas was purged into the system at a flow rate of 25 mL/min. After sufficient purging was performed to maintain an inert atmosphere, the power supply was turned on and switched to the prescribed microwave power level for 20 min. The reflection microwave power levels were controlled to be as low as possible during the entire experimental period. When the prescribed processing time was reached, the power supply was turned off, the carrier gas purging was stopped, and the tar and gas collectors were removed and sealed. After self-cooling to approximately 100 C, the solid residues were removed and placed in a desiccator for several hours. All of the experiments were performed at least twice to obtain average values for the results. Because the maximum temperature would exceed 300 C at higher microwave power levels or under certain conditions, the terms of co-pyrolysis and pyrolyzed biomass were used in the following text. Ultimate, proximate, and thermogravimetric analyses (TGA) were performed, and the weight loss and calorific values of the pyrolyzed biomass were calculated to evaluate reaction performance and determine the optimum operating conditions for the microwave co-pyrolysis of sewage sludge and rice straw. 2.3. Product analysis Proximate analyses of the raw and pyrolyzed sewage sludge, rice straw, and blends were performed according to the standard test method D7582-12 of the American Society for Testing and Materials (ASTM). The ultimate analyses were performed with a PerkineElmer 2400 Elemental Analyzer. The calorific values were determined using a CAL2K ECO calorimeter. All of the samples were tested at least twice to obtain representative analytical results. The surfaces of the biochar were observed by using a JEOL JSM-7600F field emission scanning electron microscope (SEM) with an INCA X-Max Energy Dispersive Spectrometer (EDS). The SEM was operated at an acceleration voltage of 15.0 kV. The SEM images are shown in Fig. 1. The measurements of specific surface areas and micropore and mesopore size distributions were carried out by using a Micromeritics ASAP 2020 Analyzer with pure N2 at 77 K. Before the measurements, the samples were degassed in a vacuum at 373 K. The specific surface areas were calculated by the BrunauereEmmetteTeller (BET) model, and the pore size distributions were determined by the BarretteJoynereHalenda (BJH) model. The microporous properties were calculated by the t-plot method. The
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Fig. 1. SEM images of (a) raw sewage sludge, (b) raw rice straw, (c) pyrolyzed sewage sludge, and (d) pyrolyzed rice straw produced by microwave pyrolysis at a microwave power level of 200 W.
porous properties of pyrolyzed sewage sludge and rice straw are listed in Table 2. 3. Results and discussion 3.1. Thermogravimetric analysis Thermogravimetric analysis was utilized to understand the thermal characteristics of the sewage sludge and rice straw feedstocks. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves for the sewage sludge, rice straw, and a 1:1 blend are illustrated in Fig. 2. The heating rate was set to 20 C/min. The theoretical curves for the blends were derived from the average TG and DTG curves of the original sewage sludge and rice straw, and the experimental curves were obtained from the TGA experimental results. The rice straw thermally decomposed significantly faster than the sewage sludge. In the temperature range of 150e500 C, a weight loss of up to 62 wt.% was observed for the rice straw, whereas the weight loss of the sewage sludge was only approximately 45 wt.%. Both of them decomposed extremely slowly at temperatures higher than 500 C, which is when the thermally resistant and high molecular weight compositions began to gradually decompose. The highest rate of weight loss for the rice straw was approximately 0.8 wt.%/ C (16 wt.%/min) and occurred at approximately 330 C. The highest rate of weight loss for the
Table 2 Porous properties of pyrolyzed sewage sludge and rice straw.
Pyrolyzed sewage sludge Pyrolyzed rice straw a b c d
BET surface area. Micropore area. Total pore volume. Micropore volume.
Microwave SBET Smic Vt power level (m2/g)a (m2/g)b (cm3/g)c
Vmic (cm3/g)d
200 250 200 250
0.006 0.010 0.029 0.015
24.8 51.5 122.2 93.0
15.2 32.9 112.1 70.3
0.027 0.035 0.083 0.063
Fig. 2. TG (a) and DTG (b) curves for sewage sludge, rice straw, and a 1:1 blend.
sewage sludge was approximately 0.2 wt.%/ C (4 wt.%/min) and occurred in the temperature range of 260e360 C. Generally, both the sewage sludge and rice straw significantly decomposed in the temperature range of 200e500 C, which is consistent with the literature [39]. Extremely slow rates of decomposition were observed at lower or higher temperatures. The thermal decomposition of the blends was slightly worse than the average decomposition of the sewage sludge and rice straw. This result is not coincident with the co-pyrolysis of sewage sludge and rice straw, which can accelerate the release of volatile matter and shorten the processing time [2]. 3.2. Microwave heating The temperature profiles for the pyrolysis of sewage sludge at varying microwave power levels are illustrated in Fig. 3. The
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Fig. 3. The temperature profiles of sewage sludge pyrolysis at varying microwave power levels.
Fig. 4. The maximum temperatures of co-pyrolysis for blends containing varying weight ratios of rice straw.
Table 3 The maximum temperatures for the microwave pyrolysis of sewage sludge and rice straw at varying microwave power levels.
may provide another way of thinking for the thermal treatment of waste sewage sludge.
Feedstock
Microwave power level (W)
Maximum temperature ( C)
Sewage sludge
100 150 200 250 300 350 400 100 150 200 250
135 140 207 246 269 333 454 206 268 327 435
Rice straw
3.3. Mass yield and energy yield To clearly determine the weight changes and energy output of biomass feedstocks after microwave pyrolysis, the calorific values (CV), mass yields, and energy yields of the pyrolyzed sewage sludge were calculated, as listed in Table 4. The definitions of mass yield and energy yield are as follows [40]:
Mass yield ¼
Mass of pyrolyzed biomass 100% Mass of raw biomass
Energy yield ¼ Mass yield maximum temperatures for the microwave pyrolysis of sewage and rice straw at varying microwave power levels are listed in Table 3. A greater maximum temperature was observed at higher microwave power levels for both the sewage sludge and rice straw. For the sewage sludge, the maximum temperatures were 207, 246, and 269 C at microwave power levels of 200, 250, and 300 W, respectively. This demonstrates that there was only drying occurred for microwave power levels <200 W. The maximum temperature at the microwave power levels of >300 W was higher than that for biomass torrefaction (200e300 C). Therefore, microwave power levels of 200e300 W should be suitable for the microwave pyrolysis of pure sewage sludge. Lower or higher than this power level range would cause drying or over-heating. The heating profile at a microwave power level of 250 W can be a representative heating history of microwave torrefaction of sewage sludge. Since the maximum temperature was reached within approximately 10 min, the processing time of 20 min should be sufficient for sewage sludge torrefaction to produce biochar. The addition of rice straw to sewage sludge increased the maximum temperature of microwave pyrolysis, as shown in Fig. 4. At a microwave power level of 150 W, the maximum temperature increased by approximately 40 C for blends containing 0 to 40 wt.% rice straw. The effect of the addition of rice straw was more evident at a microwave power level of 250 W than 150 W. This may be because of the insufficient microwave heating of sewage sludge at low microwave power levels. Notably, the highest maximum temperature (approximately 467 C) at a microwave power level of 250 W was measured when the blend contained 20 wt.% rice straw. This implies that, under this condition, the synergetic effect of the addition of rice straw and microwave heating to accelerate the release of volatile matter could be more effective [10], and thus more microwave energy could be efficiently absorbed and utilized. Although this temperature was too high for biomass torrefaction, it
(1)
CV of pyrolyzed biomass 100% CV of raw biomass (2)
At low microwave power levels (100e200 W), mass and energy yields of up to approximately 80% were obtained. However, because of the low working temperature (<200 C), these pyrolyzed products could still be unstable and contain a high content of hemicellulose. The calorific values, mass yields, and energy yields of pyrolyzed sewage sludge were extremely low after microwave pyrolysis at high microwave power levels (300e400 W). This is attributed to the severe reactions caused by higher microwave power levels. Therefore, a microwave power level of 250 W is satisfactory for the microwave pyrolysis of sewage sludge. Without the addition of rice straw, the mass yield and energy yield of sewage sludge pyrolysis at 250 W were approximately 72% and 63%, respectively, and the calorific value was 14.2 MJ/kg. Adding rice straw to the sewage sludge sample decreased both the mass yield and energy yield. There were no obvious differences in the yields of blends containing 10e40 wt.% rice straw. The mass yield and energy yield were approximately 40% and 35%, respectively. The calorific value slightly increased with the addition of greater weight ratios of rice straw. However, both the calorific value and energy yield were significantly lower for blends with 20 wt.% rice straw. Fig. 5 illustrates the theoretical and experimental calorific values of the pyrolyzed blends containing varying weight ratios of rice straw. The theoretical calorific values were determined by the fractions of sewage sludge and rice straw and the calorific values of pyrolyzed pure sewage sludge and rice straw at a microwave power level of 250 W. The experimental and theoretical calorific values for blends containing 30 and 40 wt.% rice straw were similar. The experimental calorific value was slightly lower than the theoretical value for the blend containing 10 wt.% rice straw, whereas the experimental calorific value was significantly lower than the
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Table 4 Calorific values, mass yield, and energy yield of pyrolyzed sewage sludge at different microwave power levels and rice straw adding ratios. Microwave power level (W)
Rice straw ratio (wt.%)
Calorific value (MJ/kg)
100 150 200 250 250 250 250 250 300 350 400
0 0 0 0 10 20 30 40 0 0 0
17.05 17.25 17.46 14.20 13.50 12.22 14.72 15.11 12.87 9.25 8.90
± ± ± ± ± ± ± ± ± ± ±
0.70 0.42 0.15 0.12 1.27 1.34 2.48 0.45 1.22 0.10 0.23
Mass yield (%)
Energy yield (%)
76.46 75.29 77.75 71.80 42.41 38.83 39.64 38.71 56.31 44.59 40.89
80.58 80.27 83.90 63.03 34.90 28.55 34.65 34.26 44.81 25.49 22.49
Fig. 6. Proximate analysis results of pyrolyzed sewage sludge at varying (a) microwave power levels and (b) weight ratios of rice straw at 250 W.
rice straw. When the rice straw addition was 30e40 wt.%, the fixed carbon content of the pyrolyzed sewage sludge was approximately 33 wt.%. Therefore, the calorific value of the pyrolyzed sewage sludge was also higher, as listed in Table 4. Fig. 5. Theoretical and experimental calorific values of pyrolyzed blends containing varying weight ratios of rice straw (microwave power level: 250 W).
theoretical value for blends containing 20 wt.% rice straw. This could be correlated or attributed to the high maximum temperature occurred for blends containing 20 wt.% rice straw (Fig. 4). This phenomenon implies that the addition of a controlled amount of rice straw is advantageous for the blends to absorb microwave radiation. 3.4. Proximate analysis One of the primary purposes of torrefaction is to remove moisture and hemicellulose from biomass and thus to enhance its calorific value [41]. Therefore, the volatile fraction of biomass should decrease and the fixed carbon fraction should increase after torrefaction. The results of the proximate analysis for raw and pyrolyzed sewage sludge at varying microwave power levels and weight ratios of rice straw are illustrated in Fig. 6. Generally, the volatile fraction of sewage sludge decreased and the fixed carbon fraction increased with higher microwave power levels. Compared with the volatile fraction of raw sewage sludge (approximately 62 wt.%), the volatile fraction was only approximately 11 wt.% after microwave pyrolysis at 400 W. For microwave power levels between 300 and 400 W, the fraction of fixed carbon content reached a maximum value of 23 wt.% and the ash fraction was approximately 66 wt.%. Because microwave power levels that were either too low or too high were not favorable for microwave pyrolysis of sewage sludge, as mentioned above, the effect of the addition of rice straw was studied at a prescribed microwave power level (250 W). The results are shown in Fig. 6b. After the addition of rice straw, the volatile fraction decreased and the fixed carbon fraction increased. Moreover, the addition of rice straw allowed higher fractions of fixed carbon to be obtained compared with the samples containing no
3.5. Elemental composition A van Krevelen diagram, which is a plot of atomic H/C ratios versus atomic O/C ratios, is shown in Fig. 7 for coal, raw sewage sludge, and pyrolyzed sewage sludge with varying weight ratios of rice straw at a microwave power level of 250 W. The atomic H/C and O/C ratios of the raw sewage sludge were significantly higher than those of coal. During combustion or gasification processes, the use of the raw sewage sludge would lead to more formation of smoke [42]. Therefore, raw sewage sludge requires pretreatment before it can be regarded as an alternative fuel. After microwave pyrolysis at a medium microwave power level (250 W), both the atomic H/C and O/C ratios significantly decreased. These values further decreased with the addition of rice straw. When the rice straw addition ratio was 40 wt.%, the atomic H/C ratio was 0.28e0.41 and the atomic O/C ratio was 0.09e0.14. The atomic H/C ratio was lower than that of anthracite (0.43), whereas the atomic O/C ratio was slightly higher than that of anthracite (0.03) [43]. According to the
Fig. 7. The van Krevelen diagram for coal, raw sewage sludge, and pyrolyzed sewage sludge with varying weight ratios of rice straw (RS) at a microwave power level of 250 W. The data for the coal are taken from Ref. [43].
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elemental compositions, the thermal properties of the sewage sludge could be more similar to coal after microwave co-pyrolysis with agricultural residues. Therefore, the pyrolyzed blends should have a high potential to be regarded as an alternative fuel and thus to be co-fired with coal or even to replace it.
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Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council, Taiwan, ROC (contract no. NSC 1012221-E-002-109-MY3).
3.6. Prospects of microwave co-pyrolysis References The treatment of sewage sludge is an important issue for highly civilized cities. Sewage sludge can be regarded as a resource of biofuels. However, the use of sewage sludge would be inhibited because of its high moisture and mineral content and low calorific value. One feasible pretreatment method is the microwave copyrolysis of sewage sludge and another biomass feedstock. Rice straw is an abundant agricultural waste and can be appropriately pyrolyzed using microwave heating [44]. After the addition of rice straw, the pyrolysis of sewage sludge can be more severe, especially when 20 wt.% rice straw added. This means that, compared with single pyrolysis of sewage sludge, co-pyrolysis can provide various advantages, such as lower input energy, shorter processing time, and higher decomposition rate. By using a medium microwave power level and an optimized weight ratio of rice straw, sewage sludge can be quickly dried and volatilized to produce pyrolyzed biomass products with a high calorific value and low atomic H/C and O/C ratios. Compared with conventional heating, the input energy and processing time of microwave heating can be considerably lower [29], and the calorific value of the biochar produced by microwave pyrolysis can be significantly higher [45]. Therefore, for the purpose of energy recovery, microwave pyrolysis should be better than conventional pyrolysis. At a microwave power level of 250 W, the energy consumption per kg sewage sludge of microwave pyrolysis for 20 min processing time would be approximately 30 MJ/kg which is much higher than the calorific values of sewage sludge and rice straw, although the data were taken from lab-scale experiments. The energy consumption could be decreased in pilotor real-plant applications, because the quantity of feedstock can be much larger. Further studies investigating reaction conditions, blend composition, and characteristics of the pyrolyzed biomass are still necessary to develop and improve the applicability and feasibility of microwave co-pyrolysis, which may provide various advantages in waste treatment, bioenergy utilization, CO2 emission reduction, etc. 4. Conclusions Waste sewage sludge can be converted into a valuable biofuel using microwave pyrolysis, which can solve the problem of sewage sludge disposal as well. Sewage sludge was successfully copyrolyzed with rice straw at a moderate microwave power level by adding a controlled ratio of rice straw. For blends containing 20 wt.% rice straw, more microwave energy was effectively absorbed, and thus the maximum temperature was up to 500 C. This very high temperature may provide another way of thinking for the thermal treatment of waste sewage sludge. The fixed carbon content of the pyrolyzed biomass was approximately 33 wt.% for blends containing 30e40 wt.% rice straw. The atomic H/C and O/C ratios of the pyrolyzed sewage sludge were similar to those of anthracite coal when a 40 wt.% ratio of rice straw was added. Therefore, the pyrolyzed blends should have a high potential to be regarded as an alternative fuel. However, it is still necessary to study the reaction conditions, blend composition, characteristics of the pyrolyzed biomass, etc., to improve the applicability and feasibility of microwave co-pyrolysis.
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Microwave co-pyrolysis of sewage sludge and rice straw Yu-Fong Huang, Chun-Hao Shih, Pei-Te Chiueh*, Shang-Lien Lo Graduate Institute of Environmental Engineering, National Taiwan University, 71, Chou-Shan Rd., Taipei 106, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history: Received 16 July 2014 Received in revised form 5 March 2015 Accepted 8 May 2015 Available online 6 June 2015
This study focused on the co-pyrolysis of sewage sludge and rice straw using microwave heating. The input microwave power level was a critical parameter. Sewage sludge was pyrolyzed without the addition of rice straw at the microwave power levels of 200e300 W, while lower or higher than this range led to only drying or over-heating. The addition of rice straw increased the performance of microwave heating, which allowed a higher maximum temperature. The calorific value of the pyrolyzed biomass increased with the addition of 30e40 wt.% rice straw. A maximum temperature of up to 500 C was measured for blends containing 20 wt.% rice straw, which could be attributed to the synergetic effect of the addition of rice straw and microwave heating. This high temperature may provide another way of thinking for the thermal treatment of waste sewage sludge. A fixed carbon content of up to 33 wt.% was obtained for blends containing 30e40 wt.% rice straw. The atomic H/C and O/C ratios were very close to those of anthracite coal. Therefore, the pyrolyzed blends should have a high potential to be co-fired with coal or even to replace it. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Microwave Co-pyrolysis Sewage sludge Rice straw Biochar
1. Introduction The enormous worldwide demand for fossil fuels will soon deplete world energy reserves and cause more severe global climate change than can be imagined. Therefore, it is extremely important to immediately develop reliable and renewable energy to replace fossil fuels. Biomass is an abundant carbon-neutral renewable resource that is used for the production of biofuels and biomaterials [1]. Sewage sludge can be regarded as a biomass resource because of its considerable organic content, and thus energy and resource recovery from sewage sludge is promising [2e4]. Waste sewage sludge could be converted into a valuable biofuel with appropriate processes, which can solve the problem of sewage sludge disposal as well. Raw biomass has several disadvantages, such as low efficiency, high cost, and high rate of decomposition due to its moisture content, the presence of microorganisms, and the inconvenience of storage in practical applications [5]. Therefore, as-received biomass generally needs to be pretreated before it is stored, transported, and utilized. One pretreatment method of biomass is torrefaction, which is a mild pyrolysis performed at a relatively low heating temperature (200e300 C) and heating rate (<50 C/min) [6].
* Corresponding author. Tel.: þ886 2 3366 2798; fax: þ886 2 2392 8830. E-mail address: [email protected] (P.-T. Chiueh). http://dx.doi.org/10.1016/j.energy.2015.05.039 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
Pyrolysis is a thermal degradation process for organic compounds in the absence of oxygen or air to produce various gaseous components as well as tar and char residues [7]. Significantly more solid residues (named biochar or torrefied biomass) are produced from biomass torrefaction than traditional pyrolysis. The biochar produced by torrefaction has a higher energy density and improved grinding characteristics, hydrophobicity, and homogeneous properties [8]. Additionally, significantly less energy is required to process torrefied biomass, and separate handling facilities are no longer necessary when it is co-fired with coal in existing power stations [9]. It is difficult to recover and utilize energy from several types of biomass (e.g., sewage sludge) because of their compositions and characteristics. However, these problems may be solved by blending several biomass feedstocks for co-pyrolysis. Blends of sewage sludge and another biomass feedstock could provide various advantages, such as enhanced reaction performance and increased calorific values of biochar with less inorganic and toxic content [2]. Several synergistic or coupling effects could occur during the co-pyrolysis of sewage sludge with rice straw to accelerate the release of volatile matter and shorten the processing time [10]. Rice straw is a valuable biomass feedstock because of its abundance [11] and high volatile content [12], and the addition of rice straw to sewage sludge could be workable to improve copyrolysis performance due to the synergistic or coupling effects as mentioned above.
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Compared with conventional heating, microwave heating is environmentally friendly and well-established and reduces energy consumption and reaction time [13]. Besides, microwave heating does not directly contact the heated materials [14]. Microwave heating has been widely used in many applications, including synthesis [15], digestion [16], extraction [17], sample pretreatment [18], and stabilization [19e21]. For large-sized biomass materials (e.g., wood and cornstalk), thermochemical reactions can occur rapidly due to the nature of fast, volumetric, and selective heating using microwave energy [22]. Materials that can absorb microwaves are called dielectrics, so microwave heating is also referred to as dielectric heating [14]. Potential applications of microwave heating are dependent on the dielectric properties of target materials [23]. Pyrolysis induced by microwave heating is one of the promising attempts because of the efficient heating of feedstocks that has been demonstrated using microwave dielectric heating [24]. Therefore, microwave heating has been utilized in various biomass pyrolysis [23,25e35] and torrefaction [36,37] systems. However, there has been no research on the co-torrefaction of biomass blends induced by microwave heating. In a previous work, it was reported that only 150 W microwave power and 10 min processing time were required to produce torrefied lignocellulosic biomass with a 70% mass yield and 80% energy yield, and the energy density of the torrefied biomass was higher than that of raw biomass by 14% [12]. Therefore, microwave torrefaction should be a feasible method to recover energy from biomass waste. In this study, the co-pyrolysis of sewage sludge and rice straw using a single-mode microwave device was investigated to evaluate the performance of microwave heating and energy recovery. The compositions and characteristics of sewage sludge, rice straw, and their blends were also studied. 2. Materials and methods 2.1. Materials The dry sewage sludge cake used in this study was obtained from the Dihua sewage treatment plant in Taipei, Taiwan. The moisture content of the as-received sewage sludge was approximately 85 wt.%. The sewage sludge was air dried for several months and then dried in an oven for three days. The rice straw was gathered on-site in Chiayi, Taiwan. After drying, the sewage sludge was grinded in a mortar. The rice straw was smashed and sieved using a 50-mesh screen. The characteristics of the raw sewage sludge and rice straw samples are listed in Table 1. Both of the biomass feedstocks contained a high content of volatiles. The volatile content of the rice straw was higher than that of the sewage Table 1 Characteristics of the raw sewage sludge and rice straw samples. Biomass
Sewage sludge
Rice straw
Moisture (wt.%) Proximate analysis (wt.%)a Volatiles Fixed carbon Ash Ultimate analysis (wt.%)b C H N O S Calorific value (MJ/kg)a
11.79 ± 3.82
10.14 ± 0.34
62.11 ± 1.86 10.00 ± 0.80 27.89 ± 1.69
79.71 ± 2.86 12.32 ± 2.55 7.97 ± 0.63
45.16 ± 0.37 7.20 ± 0.37 7.69 ± 0.22 27.50 ± 0.41 N.D. 16.18 ± 0.13
43.64 ± 0.48 5.32 ± 0.11 0.41 ± 0.04 18.86 ± 1.01 N.D. 18.40 ± 0.46
a b
Dry basis. Dry and ash-free basis.
639
sludge by approximately 20 wt.%, whereas the ash content in the rice straw was lower than that of the sewage sludge by approximately the same magnitude. Consequently, it is not surprising that the calorific value of rice straw was significantly higher than that of the sewage sludge. In this study, the calorific value of the sewage sludge was compared with values presented in the literature. The obtained value was similar to 16.45 MJ/kg [38] and significantly higher than 9.69 MJ/kg [5]. Both of the biomass feedstocks were predominately composed of carbon. The content of oxygen and nitrogen in the sewage sludge was higher than that of the rice straw by 8.64 and 7.28 wt.%, respectively. Sulfur was not detected in either the two biomass feedstocks. 2.2. Experimental procedure This study used a single-mode (focused) microwave device with a 2.45 GHz frequency. The schematic diagram of the overall microwave pyrolysis set-up can be found elsewhere [36]. The shredded and sieved biomass feedstock was added to a quartz crucible and then placed inside a quartz tube that was located in the pathway of the microwaves. An infrared thermometer was placed at the top of the quartz tube to measure the temperature of the biomass sample. To maintain anoxic conditions, nitrogen gas was purged into the system at a flow rate of 25 mL/min. After sufficient purging was performed to maintain an inert atmosphere, the power supply was turned on and switched to the prescribed microwave power level for 20 min. The reflection microwave power levels were controlled to be as low as possible during the entire experimental period. When the prescribed processing time was reached, the power supply was turned off, the carrier gas purging was stopped, and the tar and gas collectors were removed and sealed. After self-cooling to approximately 100 C, the solid residues were removed and placed in a desiccator for several hours. All of the experiments were performed at least twice to obtain average values for the results. Because the maximum temperature would exceed 300 C at higher microwave power levels or under certain conditions, the terms of co-pyrolysis and pyrolyzed biomass were used in the following text. Ultimate, proximate, and thermogravimetric analyses (TGA) were performed, and the weight loss and calorific values of the pyrolyzed biomass were calculated to evaluate reaction performance and determine the optimum operating conditions for the microwave co-pyrolysis of sewage sludge and rice straw. 2.3. Product analysis Proximate analyses of the raw and pyrolyzed sewage sludge, rice straw, and blends were performed according to the standard test method D7582-12 of the American Society for Testing and Materials (ASTM). The ultimate analyses were performed with a PerkineElmer 2400 Elemental Analyzer. The calorific values were determined using a CAL2K ECO calorimeter. All of the samples were tested at least twice to obtain representative analytical results. The surfaces of the biochar were observed by using a JEOL JSM-7600F field emission scanning electron microscope (SEM) with an INCA X-Max Energy Dispersive Spectrometer (EDS). The SEM was operated at an acceleration voltage of 15.0 kV. The SEM images are shown in Fig. 1. The measurements of specific surface areas and micropore and mesopore size distributions were carried out by using a Micromeritics ASAP 2020 Analyzer with pure N2 at 77 K. Before the measurements, the samples were degassed in a vacuum at 373 K. The specific surface areas were calculated by the BrunauereEmmetteTeller (BET) model, and the pore size distributions were determined by the BarretteJoynereHalenda (BJH) model. The microporous properties were calculated by the t-plot method. The
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Fig. 1. SEM images of (a) raw sewage sludge, (b) raw rice straw, (c) pyrolyzed sewage sludge, and (d) pyrolyzed rice straw produced by microwave pyrolysis at a microwave power level of 200 W.
porous properties of pyrolyzed sewage sludge and rice straw are listed in Table 2. 3. Results and discussion 3.1. Thermogravimetric analysis Thermogravimetric analysis was utilized to understand the thermal characteristics of the sewage sludge and rice straw feedstocks. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves for the sewage sludge, rice straw, and a 1:1 blend are illustrated in Fig. 2. The heating rate was set to 20 C/min. The theoretical curves for the blends were derived from the average TG and DTG curves of the original sewage sludge and rice straw, and the experimental curves were obtained from the TGA experimental results. The rice straw thermally decomposed significantly faster than the sewage sludge. In the temperature range of 150e500 C, a weight loss of up to 62 wt.% was observed for the rice straw, whereas the weight loss of the sewage sludge was only approximately 45 wt.%. Both of them decomposed extremely slowly at temperatures higher than 500 C, which is when the thermally resistant and high molecular weight compositions began to gradually decompose. The highest rate of weight loss for the rice straw was approximately 0.8 wt.%/ C (16 wt.%/min) and occurred at approximately 330 C. The highest rate of weight loss for the
Table 2 Porous properties of pyrolyzed sewage sludge and rice straw.
Pyrolyzed sewage sludge Pyrolyzed rice straw a b c d
BET surface area. Micropore area. Total pore volume. Micropore volume.
Microwave SBET Smic Vt power level (m2/g)a (m2/g)b (cm3/g)c
Vmic (cm3/g)d
200 250 200 250
0.006 0.010 0.029 0.015
24.8 51.5 122.2 93.0
15.2 32.9 112.1 70.3
0.027 0.035 0.083 0.063
Fig. 2. TG (a) and DTG (b) curves for sewage sludge, rice straw, and a 1:1 blend.
sewage sludge was approximately 0.2 wt.%/ C (4 wt.%/min) and occurred in the temperature range of 260e360 C. Generally, both the sewage sludge and rice straw significantly decomposed in the temperature range of 200e500 C, which is consistent with the literature [39]. Extremely slow rates of decomposition were observed at lower or higher temperatures. The thermal decomposition of the blends was slightly worse than the average decomposition of the sewage sludge and rice straw. This result is not coincident with the co-pyrolysis of sewage sludge and rice straw, which can accelerate the release of volatile matter and shorten the processing time [2]. 3.2. Microwave heating The temperature profiles for the pyrolysis of sewage sludge at varying microwave power levels are illustrated in Fig. 3. The
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Fig. 3. The temperature profiles of sewage sludge pyrolysis at varying microwave power levels.
Fig. 4. The maximum temperatures of co-pyrolysis for blends containing varying weight ratios of rice straw.
Table 3 The maximum temperatures for the microwave pyrolysis of sewage sludge and rice straw at varying microwave power levels.
may provide another way of thinking for the thermal treatment of waste sewage sludge.
Feedstock
Microwave power level (W)
Maximum temperature ( C)
Sewage sludge
100 150 200 250 300 350 400 100 150 200 250
135 140 207 246 269 333 454 206 268 327 435
Rice straw
3.3. Mass yield and energy yield To clearly determine the weight changes and energy output of biomass feedstocks after microwave pyrolysis, the calorific values (CV), mass yields, and energy yields of the pyrolyzed sewage sludge were calculated, as listed in Table 4. The definitions of mass yield and energy yield are as follows [40]:
Mass yield ¼
Mass of pyrolyzed biomass 100% Mass of raw biomass
Energy yield ¼ Mass yield maximum temperatures for the microwave pyrolysis of sewage and rice straw at varying microwave power levels are listed in Table 3. A greater maximum temperature was observed at higher microwave power levels for both the sewage sludge and rice straw. For the sewage sludge, the maximum temperatures were 207, 246, and 269 C at microwave power levels of 200, 250, and 300 W, respectively. This demonstrates that there was only drying occurred for microwave power levels <200 W. The maximum temperature at the microwave power levels of >300 W was higher than that for biomass torrefaction (200e300 C). Therefore, microwave power levels of 200e300 W should be suitable for the microwave pyrolysis of pure sewage sludge. Lower or higher than this power level range would cause drying or over-heating. The heating profile at a microwave power level of 250 W can be a representative heating history of microwave torrefaction of sewage sludge. Since the maximum temperature was reached within approximately 10 min, the processing time of 20 min should be sufficient for sewage sludge torrefaction to produce biochar. The addition of rice straw to sewage sludge increased the maximum temperature of microwave pyrolysis, as shown in Fig. 4. At a microwave power level of 150 W, the maximum temperature increased by approximately 40 C for blends containing 0 to 40 wt.% rice straw. The effect of the addition of rice straw was more evident at a microwave power level of 250 W than 150 W. This may be because of the insufficient microwave heating of sewage sludge at low microwave power levels. Notably, the highest maximum temperature (approximately 467 C) at a microwave power level of 250 W was measured when the blend contained 20 wt.% rice straw. This implies that, under this condition, the synergetic effect of the addition of rice straw and microwave heating to accelerate the release of volatile matter could be more effective [10], and thus more microwave energy could be efficiently absorbed and utilized. Although this temperature was too high for biomass torrefaction, it
(1)
CV of pyrolyzed biomass 100% CV of raw biomass (2)
At low microwave power levels (100e200 W), mass and energy yields of up to approximately 80% were obtained. However, because of the low working temperature (<200 C), these pyrolyzed products could still be unstable and contain a high content of hemicellulose. The calorific values, mass yields, and energy yields of pyrolyzed sewage sludge were extremely low after microwave pyrolysis at high microwave power levels (300e400 W). This is attributed to the severe reactions caused by higher microwave power levels. Therefore, a microwave power level of 250 W is satisfactory for the microwave pyrolysis of sewage sludge. Without the addition of rice straw, the mass yield and energy yield of sewage sludge pyrolysis at 250 W were approximately 72% and 63%, respectively, and the calorific value was 14.2 MJ/kg. Adding rice straw to the sewage sludge sample decreased both the mass yield and energy yield. There were no obvious differences in the yields of blends containing 10e40 wt.% rice straw. The mass yield and energy yield were approximately 40% and 35%, respectively. The calorific value slightly increased with the addition of greater weight ratios of rice straw. However, both the calorific value and energy yield were significantly lower for blends with 20 wt.% rice straw. Fig. 5 illustrates the theoretical and experimental calorific values of the pyrolyzed blends containing varying weight ratios of rice straw. The theoretical calorific values were determined by the fractions of sewage sludge and rice straw and the calorific values of pyrolyzed pure sewage sludge and rice straw at a microwave power level of 250 W. The experimental and theoretical calorific values for blends containing 30 and 40 wt.% rice straw were similar. The experimental calorific value was slightly lower than the theoretical value for the blend containing 10 wt.% rice straw, whereas the experimental calorific value was significantly lower than the
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Table 4 Calorific values, mass yield, and energy yield of pyrolyzed sewage sludge at different microwave power levels and rice straw adding ratios. Microwave power level (W)
Rice straw ratio (wt.%)
Calorific value (MJ/kg)
100 150 200 250 250 250 250 250 300 350 400
0 0 0 0 10 20 30 40 0 0 0
17.05 17.25 17.46 14.20 13.50 12.22 14.72 15.11 12.87 9.25 8.90
± ± ± ± ± ± ± ± ± ± ±
0.70 0.42 0.15 0.12 1.27 1.34 2.48 0.45 1.22 0.10 0.23
Mass yield (%)
Energy yield (%)
76.46 75.29 77.75 71.80 42.41 38.83 39.64 38.71 56.31 44.59 40.89
80.58 80.27 83.90 63.03 34.90 28.55 34.65 34.26 44.81 25.49 22.49
Fig. 6. Proximate analysis results of pyrolyzed sewage sludge at varying (a) microwave power levels and (b) weight ratios of rice straw at 250 W.
rice straw. When the rice straw addition was 30e40 wt.%, the fixed carbon content of the pyrolyzed sewage sludge was approximately 33 wt.%. Therefore, the calorific value of the pyrolyzed sewage sludge was also higher, as listed in Table 4. Fig. 5. Theoretical and experimental calorific values of pyrolyzed blends containing varying weight ratios of rice straw (microwave power level: 250 W).
theoretical value for blends containing 20 wt.% rice straw. This could be correlated or attributed to the high maximum temperature occurred for blends containing 20 wt.% rice straw (Fig. 4). This phenomenon implies that the addition of a controlled amount of rice straw is advantageous for the blends to absorb microwave radiation. 3.4. Proximate analysis One of the primary purposes of torrefaction is to remove moisture and hemicellulose from biomass and thus to enhance its calorific value [41]. Therefore, the volatile fraction of biomass should decrease and the fixed carbon fraction should increase after torrefaction. The results of the proximate analysis for raw and pyrolyzed sewage sludge at varying microwave power levels and weight ratios of rice straw are illustrated in Fig. 6. Generally, the volatile fraction of sewage sludge decreased and the fixed carbon fraction increased with higher microwave power levels. Compared with the volatile fraction of raw sewage sludge (approximately 62 wt.%), the volatile fraction was only approximately 11 wt.% after microwave pyrolysis at 400 W. For microwave power levels between 300 and 400 W, the fraction of fixed carbon content reached a maximum value of 23 wt.% and the ash fraction was approximately 66 wt.%. Because microwave power levels that were either too low or too high were not favorable for microwave pyrolysis of sewage sludge, as mentioned above, the effect of the addition of rice straw was studied at a prescribed microwave power level (250 W). The results are shown in Fig. 6b. After the addition of rice straw, the volatile fraction decreased and the fixed carbon fraction increased. Moreover, the addition of rice straw allowed higher fractions of fixed carbon to be obtained compared with the samples containing no
3.5. Elemental composition A van Krevelen diagram, which is a plot of atomic H/C ratios versus atomic O/C ratios, is shown in Fig. 7 for coal, raw sewage sludge, and pyrolyzed sewage sludge with varying weight ratios of rice straw at a microwave power level of 250 W. The atomic H/C and O/C ratios of the raw sewage sludge were significantly higher than those of coal. During combustion or gasification processes, the use of the raw sewage sludge would lead to more formation of smoke [42]. Therefore, raw sewage sludge requires pretreatment before it can be regarded as an alternative fuel. After microwave pyrolysis at a medium microwave power level (250 W), both the atomic H/C and O/C ratios significantly decreased. These values further decreased with the addition of rice straw. When the rice straw addition ratio was 40 wt.%, the atomic H/C ratio was 0.28e0.41 and the atomic O/C ratio was 0.09e0.14. The atomic H/C ratio was lower than that of anthracite (0.43), whereas the atomic O/C ratio was slightly higher than that of anthracite (0.03) [43]. According to the
Fig. 7. The van Krevelen diagram for coal, raw sewage sludge, and pyrolyzed sewage sludge with varying weight ratios of rice straw (RS) at a microwave power level of 250 W. The data for the coal are taken from Ref. [43].
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elemental compositions, the thermal properties of the sewage sludge could be more similar to coal after microwave co-pyrolysis with agricultural residues. Therefore, the pyrolyzed blends should have a high potential to be regarded as an alternative fuel and thus to be co-fired with coal or even to replace it.
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Acknowledgments The authors gratefully acknowledge the financial support from the National Science Council, Taiwan, ROC (contract no. NSC 1012221-E-002-109-MY3).
3.6. Prospects of microwave co-pyrolysis References The treatment of sewage sludge is an important issue for highly civilized cities. Sewage sludge can be regarded as a resource of biofuels. However, the use of sewage sludge would be inhibited because of its high moisture and mineral content and low calorific value. One feasible pretreatment method is the microwave copyrolysis of sewage sludge and another biomass feedstock. Rice straw is an abundant agricultural waste and can be appropriately pyrolyzed using microwave heating [44]. After the addition of rice straw, the pyrolysis of sewage sludge can be more severe, especially when 20 wt.% rice straw added. This means that, compared with single pyrolysis of sewage sludge, co-pyrolysis can provide various advantages, such as lower input energy, shorter processing time, and higher decomposition rate. By using a medium microwave power level and an optimized weight ratio of rice straw, sewage sludge can be quickly dried and volatilized to produce pyrolyzed biomass products with a high calorific value and low atomic H/C and O/C ratios. Compared with conventional heating, the input energy and processing time of microwave heating can be considerably lower [29], and the calorific value of the biochar produced by microwave pyrolysis can be significantly higher [45]. Therefore, for the purpose of energy recovery, microwave pyrolysis should be better than conventional pyrolysis. At a microwave power level of 250 W, the energy consumption per kg sewage sludge of microwave pyrolysis for 20 min processing time would be approximately 30 MJ/kg which is much higher than the calorific values of sewage sludge and rice straw, although the data were taken from lab-scale experiments. The energy consumption could be decreased in pilotor real-plant applications, because the quantity of feedstock can be much larger. Further studies investigating reaction conditions, blend composition, and characteristics of the pyrolyzed biomass are still necessary to develop and improve the applicability and feasibility of microwave co-pyrolysis, which may provide various advantages in waste treatment, bioenergy utilization, CO2 emission reduction, etc. 4. Conclusions Waste sewage sludge can be converted into a valuable biofuel using microwave pyrolysis, which can solve the problem of sewage sludge disposal as well. Sewage sludge was successfully copyrolyzed with rice straw at a moderate microwave power level by adding a controlled ratio of rice straw. For blends containing 20 wt.% rice straw, more microwave energy was effectively absorbed, and thus the maximum temperature was up to 500 C. This very high temperature may provide another way of thinking for the thermal treatment of waste sewage sludge. The fixed carbon content of the pyrolyzed biomass was approximately 33 wt.% for blends containing 30e40 wt.% rice straw. The atomic H/C and O/C ratios of the pyrolyzed sewage sludge were similar to those of anthracite coal when a 40 wt.% ratio of rice straw was added. Therefore, the pyrolyzed blends should have a high potential to be regarded as an alternative fuel. However, it is still necessary to study the reaction conditions, blend composition, characteristics of the pyrolyzed biomass, etc., to improve the applicability and feasibility of microwave co-pyrolysis.
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