Study of Fuel Properties of Torrefied Municipal Solid Waste - Energy

Article pubs.acs.org/EF

Study of Fuel Properties of Torrefied Municipal Solid Waste Haoran Yuan,†,‡ Yazhuo Wang,*,†,‡ N. Kobayashi,§ Dandan Zhao,†,‡ and Shiyou Xing†,‡ Guangzhou Institute of Energy Conversion, and ‡Key Laboratory of Renewable Energy, Chinese Academy of Science, Guangzhou, Guangdong 510640, People’s Republic of China § Department of Chemical Engineering, Graduate School of Engineering, Nagoya University, Furocho, Chikusa-ku, Nagoya 464-8603, Japan

Energy Fuels 2015.29:4976-4980. Downloaded from pubs.acs.org by UNIV OF SUSSEX on 09/06/15. For personal use only.

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ABSTRACT: Incineration is one of the major methods for processing municipal solid waste (MSW) harmlessly; however, it could lead to low combustion efficiency and secondary pollution problems because of the high moisture content and low energy density of MSW. Torrefaction has been reported to be effective in removing moisture and improving combustion properties in various materials, such as feedstock and wood. The aim of this study was to investigate whether torrefaction could be used as a practical pretreatment method by improving the combustion properties and reducing the chloride content of MSW. Typical combustible components in MSW were chosen and torrefied at various temperatures (250, 300, 350, 400, and 450 °C), under a N2 atmosphere for 30 min. The calorific values of MSW became higher after torrefaction between 250 and 350 °C. At a torrefaction temperature of 300 °C, the torrefied MSW sample produced similar calorific values to those of standard coal. Under this condition, the chlorine content of the torrefied MSW sample was 57% lower than non-torrefied samples. In addition, the grindability of the torrefied sample was better than that of synthetic MSW. The improved grindability, lower chlorine content, higher heat value, and superior combustion characteristics of torrefied MSW suggested that torrefaction may be used as an effective MSW pretreatment, and the torrefied MSW is more suitable to be used as fuel than non-torrefied MSW.

1. INTRODUCTION Municipal solid waste (MSW) disposal is one of the main problems affecting sustainable development of cities in developing countries, such as China. According to the China Statistical Yearbook, 171 million tons of MSW were collected in China in 2013,1 and this value will increase at an annual rate of 8−10%. Among the three major MSW management technologies, i.e., landfilling, composting, and incineration, incineration is the most popular technology because it can reduce the weight (up to 75%) and volume (up to 90%) of the waste2 while producing electric energy. However, MSW is difficult to combust completely because of its complicated composition and high moisture content. This would result in uneven distribution of the temperature in the furnace as well as low thermal efficiency. Moreover, because of the high content of chlorine in polyvinyl chloride (PVC), which is the main composition of plastics in MSW, a large amount of dioxin precursors would be produced during combustion, leading to secondary pollution. Therefore, it is worthwhile to explore how to improve the combustion properties as well as reduce the chloride content of MSW. Torrefaction is a low-temperature pyrolysis process without oxygen that can serve as a thermal pretreatment in energy conversion.3 Thus far, many studies have been conducted on the mechanical properties, composition, structures, and reactivity changes of torrefied biomass. As early as 1988, Bourgois et al. reported that the energy density and ash content of pine had significantly increased after torrefaction, with the added benefit of torrefied pine being hydrophobic.4 Park et al. in 2005 further found that the fuel properties and economical efficiency of torrefied pitch pine were the best when the torrefaction conditions were 275 °C with 30 min.5 Bridgeman et al. in 2010 indicated that the combustion properties of reed © 2015 American Chemical Society

canary grass, wheat straw, and willow were greatly improved after being torrefied.6 Torrefaction also removes low-weight volatiles and the majority of moisture in the biomass and improves the energy density, bulk density, grindability, and material handling properties of the feedstock.7−9 However, studies on torrefied MSW have been sparse. In this study, the product composition and fuel properties of MSW torrefaction under various temperatures were investigated with the hope of finding an effective way of MSW pretreatment.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. We collected typical combustible parts of MSW, dried to a constant weight, simulated the MSW composition in Guangzhou, China, and produced the synthetic MSW. The composition and proximate and ultimate analysis results of the synthetic MSW are shown in Tables 1 and 2, respectively. The highheating value of the synthetic MSW is 23 537.4 kJ/kg.

Table 1. Components and Their Distribution in Mass Ratio (%) in the Synthetic MSW component

mass ratio (%)

polyethylene (PE) paper cloth vegetation kitchen waste PVC total

26 10 8 14 34 8 100

Received: October 9, 2014 Revised: July 11, 2015 Published: July 13, 2015 4976

DOI: 10.1021/ef502277u Energy Fuels 2015, 29, 4976−4980

Article

Energy & Fuels


Table 2. Proximate and Ultimate Analysis Results of the Synthetic MSW proximate analysis results of synthetic MSW

mass ratio (%)

ash volatile matter fixed carbon total ultimate analysis results of synthetic MSW

7.60 82.0 10.4 100 mass ratio (%)

C H O N S Cl total

56.95 8.61 19.72 1.00 0.20 5.92 92.40

Figure 2. (a) Synthetic MSW and (b) torrefied MSW (300 °C for 30 min) samples crushed in a pulverizer for 5 min.

content of volatile matter (from 76.7 to 27.7%) and an increase in ash (from 7.8 to 25.4%) as well as fixed carbon (from 15.5 to 46.9%) contents with the increase of the torrefaction temperature. The sulfur content of the samples remained almost stable under different torrefaction conditions. The hydrogen content of the samples was decreasing with the increase of the torrefaction temperature, with a dramatic reduction under the temperature of 450 °C. The chlorine content, as the main pollutant, was reduced to less than half of the content in non-torrefied samples (the content of chlorine in non-torrefied samples is 5.92%) and then rose to 3.39% under the temperature of 450 °C. 3.3. Energy Balance. The mass and energy yields of torrefied MSW were calculated on the basis of equations suggested by Bergman et al.10 mproduct Ymass = mfeed

2.2. Experimental Setup. The experimental setup is shown in Figure 1. The samples were torrefied in a quartz tube to keep good thermal stability.

Figure 1. Experimental setup: (1) nitrogen gas, (2) reducing valve, (3) flow meter, (4) rubber stopper, (5) tube furnace, (6) temperature controller, (7) material, (8) pipeline, (9) quartz tube, (10) tar collection system, (11) ice, and (12) gas analyzer.

⎛ HHVproduct ⎞ Yenergy = Ymass⎜ ⎟ ⎝ HHVfeed ⎠

The C, H, N, O, and S contents were measured using a vario EL III elemental analyzer. The heat value of MSW was measured using IKA C2000 oxygen bomb calorimeters. The thermogravimetric analysis (TGA)/differential thermogravimetric analysis (DTGA) was conducted using Netzsch STA 409C/PC under an air atmosphere; the heating rate was 20 °C/min; and the airflow rate was 20 mL/min. According to the sample mass requirement of the TGA analyzer, it is more accurate to conduct TGA when the sample mass is 10 ± 1 mg. The Cl content was measured in accordance with the national standard GB/T 3558-2014. 2.3. Torrefaction. The torrefaction process was finished under a N2 atmosphere. A total of 5 g of synthetic MSW were put into a quartz tube, which was then sealed with a rubber stopper. The nitrogen gas was purged for more than 30 min to make sure no air remained in the tube. When the temperature of the tube furnace reached the expected value, i.e., 250, 300, 350, 400, or 450 °C, the tube would be placed in the furnace. After 30 min, the tube was removed and the nitrogen gas was increased to an appropriate level to cool the tube faster. Finally, the torrefied samples were extracted.

where mproduct is the weight of the product, which was torrefied feedstock, mfeed is the weight of the feedstock, HHVproduct is the high-heating value of the product, which was torrefied feedstock, and HHVfeed is the high-heating value of the feedstock. As shown in Table 4, the mass yield and volume of the MSW sample were decreasing with the increase of the torrefaction temperature. The heat value of torrefied MSW was consistently increasing when the temperature increased from 250 to 300, 350, and 400 °C. The greatest heat value reached 31 162.7 kJ/ kg when the torrefaction temperature was 400 °C, which is even higher than the heat value of standard coal. Nevertheless, when the temperature had risen to 450 °C, the HHV dropped dramatically to 13 082.0 kJ/kg. For all samples of torrefaction under the temperatures between 250 and 400 °C, the energy yield was greater than the mass yield and both were decreasing with the increase of the temperature (Figure 3). However, when the temperature reached 450 °C, both yields rapidly reduced and the mass yield became greater than the energy yield. 3.4. Combustion Characteristics of Torrefied MSW. To evaluate the combustion characteristics of torrefied MSW, the torrefied samples were having thermogravimetric tests under an air atmosphere. The TGA and DTGA curves are presented in Figure 4. The combustible index C and burnout index Sn were calculated on the basis of the following equations:11

3. RESULTS 3.1. Influences of Torrefaction on MSW Grindability. As shown in Figure 2, the paper and cloth in synthetic MSW are very difficult to crush because the fibers formed links between particles (Figure 2a). To the contrary, torrefied synthetic MSW samples were isolated particles, where no fibers have been detected (Figure 2b). 3.2. Influences of Torrefaction Temperatures on Proximate and Ultimate Analysis Results of Samples. The proximate and ultimate analyses of torrefied MSW samples are presented in Table 3. This table shows a reduction in the

C = Wmax /Ti 2 4977


Article

Energy & Fuels Table 3. Proximate and Ultimate Analyses of MSW ultimate analysis (%) non-torrefied MSW temperature (°C) 250 300 350 400 450

C

H

O

N

56.95

8.61

19.72

1.00

61.83 65.47 65.59 66.57 51.50

8.43 8.38 8.30 7.97 3.18

17.77 13.54 11.79 8.59 14.14

1.30 1.35 1.36 1.35 2.13

proximate analysis (%) S

Cl

0.20 5.92 torrefied samples 0.23 2.64 0.25 2.51 0.30 2.46 0.29 2.53 0.26 3.39

ash

volatile matter

fixed carbon

7.60

82.0

10.40

7.80 8.5 10.2 12.7 25.4

76.7 74.0 69.3 63.4 27.7

15.50 17.50 20.50 23.90 46.90


Table 4. Mass, Volume, Mass Yield, and High-Heating Value (HHV) of MSW temperature (°C)

mass (g)

volume (mL)

mass yield (%)

HHV (kJ/kg)

synthetic MSW 250 300 350 400 450

5.00 4.34 3.68 3.51 2.92 1.49

25 22 20 18 12 11

100 86.8 73.6 70.2 58.4 29.8

23537.4 27720.3 29571.1 30749.3 31162.7 13082.0

Figure 3. Mass yield (%) and energy yield (%) under different torrefaction temperatures.

Sn =

WmaxWmean Ti 2Th

where Ti is the ignition point, Th is the burnout temperature, Wmax is the maximum weight loss rate, and Wmean is the average weight loss rate. The combustible index was used to determine the early stage of the combustion reaction ability, and the burnout index was used to determine the comprehensive combustion reaction ability. The higher these indexes, the better the combustion characteristics of the fuels. There are various methods to confirm the ignition point by TGA. Yu et al. used several methods, such as the DTGA method, TGA method, and curve cutoff point method to test the ignition point of different types of coal and concluded that the DTGA method was more accurate and convenient.12 Briefly, the ignition point was the point on the DTA curve at which the weight loss ratio was 0.1 mg/min. The indexes determining combustion characteristics are presented in Table 5. By comparison of the combustion

Figure 4. TGA and DTGA curves of torrefied MSW combustion (20 K/min).

indexes of torrefied MSW samples and those of coal samples presented in Table 6, we found that torrefied MSW samples have better combustion properties than coal. The best combustion characteristics of torrefied MSW samples were under 400 °C.

4. DISCUSSION This study shows that torrefaction had a great effect on the product as well as fuel properties of MSW. The torrefied MSW (300 °C for 30 min) was crisper and had better grindability than non-torrefied samples. The reason for this may be that the particles had become more spherical during the torrefaction process.9 Because of better grindability, the torrefied MSW 4978


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Energy & Fuels Table 5. Indexes of Combustion Characteristics MSW torrefaction temperature (°C)

Ti (K)

Th (K)

Wmax (mg min−1)

Wmean (mg min−1)

C (×10−6)

Sn (×10−10)

250 300 350 400 450 raw MSW

499 525 499 512 516 489

813 829 834 813 813 798

0.688 0.696 0.712 0.776 0.691 0.665

0.275 0.286 0.279 0.287 0.256 0.267

2.76 2.53 2.86 2.96 2.59 2.78

9.35 8.71 9.57 10.45 8.17 9.31

when the temperature reached 450 °C, which was due to the polyethylene content that began to pyrolyze at 450 °C. Last but not least, the combustible index C and burnout index Sn showed that the torrefied MSW had better combustion characteristics than coal, with the best combustion characteristics under 400 °C, indicating that it is very suitable to be used as fuel.


Table 6. Indexes of Coal13 coal sample

C (×10−6)

Sn (×10−10)

Wufeng coal Shixiajiang coal Xindong coal

0.52 0.98 0.97

1.68 3.26 4.40

could be crushed into a uniform size and be used in more types of furnaces, such as entrained-flow bed or tangentially fired furnaces. The proximate and ultimate analyses of torrefied MSW samples show that the chlorine content, as the main pollutant, first decreased with the increase of the torrefaction temperature and then rose slightly under the temperature of 450 °C. This may be due to the fact that PVC was easy to pyrolyze under low temperatures (below 400 °C) and a large amount of hydrogen chloride escaped at first. When the temperature increased to a high degree of 450 °C, the residual chlorine in MSW stabilized and other compositions began to pyrolyze; therefore, the ratio of chlorine rose. The low chlorine content of torrefied MSW suggests that torrefaction may be used as an effective pretreatment to prevent secondary pollution caused by MSW incineration. Previous research by Vikelsoe et al. showed that, when the PVC content in the MSW was doubled, the polychlorinated dibenzodioxin (PCDD) emissions increased by 32% during the MSW combustion process.14 Takeshita et al.15 indicated that dioxin emissions increase with the increase of the hydrogen chloride concentration when the combustion temperature remained constant. Moreover, Ruuskanen et al.16 observed that, when the chlorine content of fuel was 3.9%, the production of PCDD/Fs increased 10-fold; however, when the chlorine content of fuel was less than or equal to 2.7%, there was no correlation between the chlorine content and the production of PCDD/Fs. There are two parts of Cl in MSW: organic chlorine and inorganic chlorine. The organic chlorine mainly exists in the PVC content, and the inorganic chlorine mainly exists in the water-soluble salts, such as NaCl, KaCl, etc. The organic chlorine vaporization is dependent upon the PVC content in MSW, and the inorganic chlorine is stabilized in the torrefaction process. Table 3 shows that the chlorine content of torrefied MSW was 2.46% after torrefaction under 350 °C, almost 58% lower than that of the chlorine content of synthetic MSW, which was 5.92%. Therefore, when torrefied MSW is used as fuel, the PCDD/F emissions would be effectively reduced during the process of combustion or gasification. The heat value of torrefied MSW was consistently increasing when the temperature increased from 250 to 300, 350, and 400 °C. The greatest heat value reached 31 162.7 kJ/kg when the torrefaction temperature was 400 °C, which is even higher than the heat value of standard coal. The primary reason for the increase of the heat value was reduction in the oxygen content and increase in the carbon content during the torrefaction process. Both the mass yield and energy yield rapidly reduced

5. CONCLUSION In conclusion, the torrefied MSW had better grindability, lower chlorine content, higher heat value, and better combustion characteristics. All of these results suggest that the torrefied MSW was better adapted to be used as fuel and torrefaction was an effective method of MSW pretreatment.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-0-2087013240. E-mail: [email protected]. ac.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is financially supported by the National 973 Project of China (2011CB201501), the National Science and Technology Program of the Twelfth Five-Year Plan for Rural Development in China (2012BAD14B16-04), the National Natural Science Foundation of China (51161140330), the Pearl River Nova Program of Guangzhou (2013J2200008), and the Guangdong Science and Technoloy Program (2012B091400011).

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Study of Fuel Properties of Torrefied Municipal Solid Waste - Energy

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