A study on torrefaction of sewage sludge to enhance solid fuel qualities

Waste Management 40 (2015) 112–118

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A study on torrefaction of sewage sludge to enhance solid fuel qualities Jeeban Poudel a, Tae-In Ohm b, Sang-Hoon Lee c, Sea Cheon Oh d,⇑ a

Department of Mechanical Engineering, Kongju National University, 1223-24 Cheonan-Daero, Seobuk, Chungnam 330-717, Republic of Korea Department of Civil and Environmental Engineering, Hanbat National University, 125 Dongseo-Daero, Yuseong, Daejeon, 330-717, Republic of Korea c Korea Institute of Energy Technology Evaluation and Planning, 135-502 Teheran-ro 114gil 14, Gangnam-gu, Seoul, Republic of Korea d Department of Environmental Engineering, Kongju National University, 1223-24 Cheonan-Daero, Seobuk, Chungnam 330-717, Republic of Korea b

a r t i c l e

i n f o

Article history: Received 5 December 2014 Accepted 8 March 2015 Available online 23 March 2015 Keywords: Sewage sludge Torrefaction Energy yield Mass yield Elemental analysis HHV

a b s t r a c t Torrefaction is a treatment which serves to improve the properties of biomass in relation to thermochemical processing techniques for energy generation. In this study, the torrefaction of sewage sludge, which is a non-lignocellulosic waste was investigated in a horizontal tubular reactor under nitrogen flow at temperature ranging from 150 to 400 °C, for torrefaction residence time varying from 0 to 50 min. The torrefaction kinetics of sewage sludge was studied to obtain the kinetic parameters. The torrefied sewage sludge products were characterized in terms of their elemental composition, energy yield, ash content and volatile fraction. The energy and mass yields decreased with an increase in the torrefaction temperature. From an elemental analysis, the weight percentage of carbon in the sewage sludge increased with an increase in the torrefaction temperature. On the other hand, the weight percentages of hydrogen and oxygen tended to decrease. The gaseous products from torrefaction of sewage sludge were also analyzed. From this work, it was found that the compounds with oxygen were emitted at a temperature lower than that for hydrocarbon gases and the temperatures of 300–350 °C were the optimum torrefaction temperatures for sewage sludge. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The global problems associated with the intensive use of fossil fuels have increased interest in the use of renewable fuels worldwide where biomass is widely available at low cost. However, a pre-treatment process is required to convert biomass into a hydrophobic solid product with an increased energy density (Uslu et al., 2008). Raw biomass is generally characterized by its high moisture content and volatility, and by its lower higher heating value (HHV) and energy density levels compared to fossil fuels (Werther et al., 2000). Biomass has some disadvantages when used as fuel, such as its low HHV, high moisture content, hygroscopic nature, smoke emission during combustion, its heterogeneous and uneven composition, and transport difficulties (Uemura et al., 2011). Existing technologies to convert biomass to energy include thermochemical and biochemical processes (Goyal et al., 2008). The thermochemical conversion process is the most common technique, which include combustion, pyrolysis, gasification, and liquefaction (Uemura et al., 2011; Zhang et al., 2010). The treatment of biomass at lower temperatures ranging from 200 to 300 °C under an inert atmosphere was known to be effective for ⇑ Corresponding author. E-mail address: [email protected] (S.C. Oh). http://dx.doi.org/10.1016/j.wasman.2015.03.012 0956-053X/Ó 2015 Elsevier Ltd. All rights reserved.

improving the energy density and shelf life of the biomass (Uemura et al., 2011; Zhang et al., 2010; Arias et al., 2008). This treatment is referred to as ‘torrefaction’, and it has been widely applied to wood and grass biomass over the past few years. The main improvements of torrefied biomass include reduced moisture and an increased energy density; a reduced oxygen-to-carbon (O/ C) ratio, which increases the HHV; the strong fibers of the biomass becoming brittle, which improves grindability by reducing the cost and energy required for grinding; and the ignitability and reactivity is improved, which enhances the efficiency during gasification or pyrolysis (Chen and Kuo, 2010; Bridgeman et al., 2008). During this process the OH radicals of the polymer breaks away forming H2O and some carbon compounds enters into reactions forming CO2 and CO. Though this process involves significant loss in mass, the energy released in the above reaction is relatively small. Furthermore, H2O and CO2 do not carry away any chemical energy from the biomass. Thus, the biomass experiences a greater loss in mass and smaller loss in energy resulting in a net increase in energy density of the biomass even on dry basis (Dhungana et al., 2012). Torrefaction can be incorporated into a combined drying, torrefaction, and pelletization process, with both economic and energy efficiency benefits (Bergman et al., 2005a). However, this process requires a separate plant, the input of the process energy, and the production of gaseous and volatile streams, entailing

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capital costs, operating costs, and emission control efforts. A balance between these associated costs for fuels which are more grindable and have higher HHV is therefore critical for the future of torrefaction and requires thorough analysis and extensive, reliable data (Bridgeman et al., 2008). One of the materials which can be converted into fuel is sewage sludge (SS) (Atienza-Martinez et al., 2013). SS is a waste generated from wastewater treatment plants. During the last 2 decades, developments in municipal wastewater treatment strategies are characterized by a continuous effort to improve the quality of the effluent by upgrading existing treatment plants and designing and implementation of new more effective plants (Magdziarz and Werle, 2014). Municipal wastewater treatment can be considered as a continuous activity also in the future. It is organizationally, technically, and economically hardly possible to prevent or strongly reduce the amount of municipal wastewater. This means that the produced amount of sewage sludge will not change significantly in the future (Rulkens, 2007). In its dry form, SS could be considered a special type of renewable fuel, due to the high quantity of organics of sufficiently high calorific value, similar to that of brown coal (Werther and Ogada, 1999). There is therefore increased interest in utilization of SS, resulting also from limited reserves of fossil fuels, limited global security of energy supplies and environmental and climatic regulations on CO2 emissions. There are several works about torrefaction of lignocellulosic biomass (Uemura et al., 2011; Arias et al., 2008; Deng et al., 2009; Li et al., 2012; Chen and Kuo, 2010; Bridgeman et al., 2008) but torrefaction of non-lignocellulosic biomass, such as SS, has been studied to a lesser extent (Dhungana et al., 2012; Atienza-Martinez et al., 2013; A´brego et al., 2013). So, this study focuses on the effect of the operational conditions of SS torrefaction on the properties of the products obtained. Different torrefaction temperatures and torrefaction residence times were tested in a lab-scale horizontal tubular reactor.

2.2. Experimental device and procedure The dried sample was ground for a homogeneous experimental condition. Each experiment was carried out with 20 g of sample at atmospheric pressure. In this study, a horizontal tubular reactor with an internal diameter of 150 mm and a length of 600 mm was used for torrefaction, as schematically shown in Fig. 1. A prescribed amount of each sample was weighed and put in a crucible. Nitrogen flushing was done until the concentration of oxygen in reactor was less than 1%. After flushing the reactor with nitrogen (2 l/min), the temperature of the reactor was raised to different desired temperatures ranging from 150 to 600 °C at a constant heating rate of 10 °C/min using an electric heater. When the torrefaction temperature and torrefaction residence time reached the required experimental condition, the heating reactor was immediately stopped and the carrier gas was shut down. The torrefied sample was then instantly removed and was weighed. In order to investigate the influence of the residence time, the experiments for the torrefaction residence time have also been performed at various residence times varying from 0 to 50 min for 250, 300 and 350 °C. In this experiment, the torrefaction residence time means the residence time after reaching the required temperature. All the other experiments are performed at 0 torrefaction residence time. For each experiment, the moisture content, HHV, volatile fraction, and ash content were measured. The HHV was measured using a bomb calorimeter (Parr Instrument Co., Model 1672, Moline, IL, USA). The ash content was determined by the standard method developed by National Renewable Energy Laboratory (NREL) (Sluiter et al., 2008). Elemental analyses of the feedstock and torrefied sample were done by an elemental analyzer (Thermo Fisher Scientific Inc., Thermo FLASH 2000, Hudson, NH, USA). The emitted gas during the torrefaction of SS was measured using a gas analyzer (Greenline MK2, Eurotron Instruments, Chelmsford, UK). The mass and energy yields are defined by Eqs. (1) and (2), as used by Bridgeman et al. (2008).

2. Experimental

Mass Yield ðY mass Þ ¼ 2.1. Materials SS obtained from wastewater treatment plant in Cheonan, Korea was used as the raw material in this study. After collection, each sample was homogeneously mixed and dried at 105 °C for 24 h. Table 1 shows the properties of the SS sample used in this study. The moisture content of the raw SS was 82.2%, with 2.6% ash content. The HHV of the dried sample was determined to be 19.86 MJ/kg. Most of the compositions of ‘‘others’’ in Table 1 are inorganic components.

mass after torrefaction  100% mass of raw sample

Energy Yield ðY energy Þ ¼ Y mass 

ð1Þ

HHV ðtorrefied sampleÞ  100% HHV ðraw sampleÞ ð2Þ

To obtain the kinetic parameters of torrefaction reaction, a thermogravimetric analysis (TGA) of the SS was also conducted using a thermogravimetric analyzer (TA Instruments, Q50, New

Table 1 The properties of the SS sample. Elements (wt. %)a

Moisture (%)b Volatile fraction (%)b Ash (%)b HHV (MJ/kg, dry)a a b

Dry basis. Wet basis.

C H N O S Cl Others

46.93 6.83 7.40 23.23 0.54 0.07 15.0 82.2 15.2 2.6 19.86

Fig. 1. Schematic diagram of the horizontal tubular reactor used in this work.

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Castle, DE, USA). The sample was studied at various heating rates ranging from 10 to 30 °C/min and at temperatures ranging from ambient to 600 °C. The initial mass of each sample was 2 mg. All experiments were carried out in a nitrogen atmosphere under a flow rate of 60 ml/min.

The basic rate equation of conversion, a, for thermal decomposition can be expressed in the Arrhenius form as

  da E ð1  aÞn ; ¼ A exp  RT dt

ð3Þ

where A, E, T, R, n, and a are the frequency factor (1/s), the activation energy (J/mol), the reaction temperature (K), the gas constant (8.314 J/mol K), the overall reaction order, and the degree of conversion, respectively. If Eq. (3) is used and a heating rate b = dT/dt is employed, it can be shown that

    da A E exp  dTð1  aÞn : ¼ b RT dT

   ð1  aÞð1nÞ  1 A RT 2s E h ; ¼ exp  1 1n b E RT s Ts

ð5Þ

where h = T  Ts, and where Ts is defined as the temperature at which 1/(1  a) = 1/exp = 0.368. With an unknown reaction order n, a generally applicable method for determining Ts is to find where d(1  a)/dT is maximum or where d2(1  a)/dT2 = 0. When d2(1  a)/dT2 = 0, Eq. (5) becomes

! ð1  aÞð1nÞ  1 1 Eh ; ¼  ð1  as Þð1nÞ exp 1n n RT 2s

ð6Þ

when h = 0, and when (1  a) = (1  as), Eq. (6) yields 1

ð1  as Þ ¼ n1n :

ð7Þ

In Eq. (7), when as is known, we can obtain the corresponding value of n. Integral approximation and logarithms in Eqs. (4) and (6) yield:

" # 1  ð1  aÞðn1Þ Eh ¼ 2 ln 1n RT s ln½ lnð1  aÞ ¼

Eh RT 2s

For n – 1

For n ¼ 1

By plotting Y ¼ ln

h

1ð1aÞðn1Þ 1n

ð8Þ

ð9Þ i

versus h in Eq. (8) and

Y = ln[ln(1  a)] versus h in Eq. (9), we can obtain the activation energy, E, from the slope of the line. 3. Results and discussion 3.1. Thermogravimetric analysis Fig. 2 shows the TG curves of a dried SS sample in a nitrogen atmosphere. The inception of torrefaction can be observed at temperatures around 200 °C, upon the pyrolysis of the organic ingredients contained within the SS. The thermal degradation of the SS was completed at a temperature of approximately 500 °C. Generally, it is difficult to determine the exact torrefaction temperature which shows the best technical and economic feasibility levels. Parameters such as the energy yield, the composition of

0.8

0.6

0.4

0.2 0

100

200

300

400

500

600

Torrefaction temperature ( o C) Fig. 2. Typical TG curves of SS sample in an N2 atmosphere.

ð4Þ

In an integration using a series of applications and simplifications (Horowitz and Metzger, 1963), Eq. (4) becomes

10 oC/min 20 oC/min 30 oC/min

1.0

Weight fraction, 1- α

2.3. Kinetic analysis method

1.2

the torrefied products, and the HHV must be considered to obtain the optimum torrefaction temperature. A lower mass yield results in a lower energy yield. Therefore, in this study, a kinetic analysis of the torrefaction of SS was performed in the temperature range which led to a conversion of approximately 30%, after the complete removal of the moisture. The kinetic parameters obtained in this study is arranged and shown in Table 2. The activation energies obtained from Eqs. (8) or (9) was 65.2–70.1 kJ/mol for reaction order 2.1–2.7 according to heating rates. The average activation energy and reaction order for the torrefaction of SS was obtained as 68.1 kJ/mol and 2.4, respectively. It is well known that the kinetic investigation is very important to get information for rationally designing the reactor. There is no intention in this study to describe the fundamental chemical mechanisms for torrefaction of SS. This study focuses on the measurement of apparent kinetic parameters useful for the engineering design of chemical processes. 3.2. Energy yield, mass yield, HHV, volatile fraction, and ash content The torrefaction of biomass is usually performed at temperature of around 300 °C, and the thermal treatment at higher temperature is referred to as pyrolysis. In this work, we want to find the optimum torrefaction temperature for SS, and tried to find a temperature range which changes from torrefaction to pyrolysis. Therefore, the experiments with respect to torrefaction temperature were performed at temperature of 150–600 °C. The relationship between the HHV, the energy yield and the mass yield with respect to the torrefaction temperature is shown in Fig. 3(a). The increase in the HHV up to 350 °C is due to the removal of oxygen. A further increase in the torrefaction temperature resulted in a decrease of the HHV. This is due to the pyrolysis reaction that occurred at a higher temperature. Furthermore, since SS is non-lignocellulosic which contains thermally degradable organic components, it easily

Table 2 Summary of the kinetic parameters for torrefaction reaction of SS. Heating rate, °C/min

Reaction order, n

Activation energy, kJ/mol

10 20 30 Average

2.7 2.5 2.1 2.4

70.1 68.9 65.2 68.1

115

HHV Energy Yield Mass Yield

HHV (MJ/kg)

21

100 90 80

19

70 60

17

50 40

15

30 13 100

200

300

400

500

600

20 700

Torrefaction temperature (oC)

(b) 50

90

45

85

40

80

Ash content (%)

23

Energy Yield or Mass Yield (%)

(a)

35

75

30

Ash content Volatile fraction 70

25

65

20

60

15

55

10 100

200

300

400

500

600

Volatile fraction (%)

J. Poudel et al. / Waste Management 40 (2015) 112–118

50 700

Torrefaction temperature (oC)

Fig. 3. (a) The HHV, energy yield, mass yield and (b) ash content and volatile fraction contents of torrefied SS as a function of the torrefaction temperature.

breaks down with increase in torrefaction temperature which results into the decrease of the HHV. The torrefaction of SS at temperature higher than 350 °C has negative impact on its HHV. So, to minimize this loss, the pre-treatment of SS using torrefaction technology should be done at temperature below it. The energy yield is a useful measure during this process. It can be calculated from the mass yields, as described by Bridgeman et al (2008). Here, the energy yield decreased steadily from 98% to 25% with an increase in the torrefaction temperature. The energy yield in the torrefaction range corresponds to the mass yield. The energy yield calculated in this study is based on the HHV. The mass yield of the torrefied products at a temperature up to 300 °C showed that the mass after thermal treatment is nearly identical to that of the raw material. This indicates that the extent of torrefaction for the SS up to 300 °C was negligible compared to these values at higher temperatures. There are two main causes of the decrease in mass of the dried or torrefied products. One is moisture loss, with the other being thermal decomposition to form volatile gaseous products such as H2O, CO, CO2, acetic acid and other organics (Medic et al., 2012b). The reduction in the mass yield is attributed to the thermal effects (Sadaka et al., 2014). In other words, substantial torrefaction did not occur in the SS sample at a torrefaction temperature below 300 °C. For torrefaction at higher temperatures, the decrease is attributed to the thermal decomposition (Prins et al., 2006a,b). The variations in the volatile fractions and ash contents are shown in Fig. 3(b). The volatile fraction content gradually decreased while the ash content increased with the torrefaction temperature. 3.3. Elemental analysis The results of the elemental analysis of the torrefied products are shown in Fig. 4. Inferring to the lower HHV, energy yield and mass yield at higher torrefaction temperature, the elemental analysis was done for torrefaction temperature up to 400 °C. In this figure, the torrefaction residence times of all experiments were 0 min. From Fig. 4(a), it can be inferred that the nitrogen content increased slightly while the sulfur and chlorine contents did not vary much with an increase in the torrefaction temperature. This increase is a relative increase due to the decreased level of oxygen. From Fig. 4(b) it can be inferred that the weight percentage of C in the products increased with an increase in the torrefaction temperature. In contrast, the weight percentages of H and O had a decreasing trend. These decreases in hydrogen and oxygen are due to dehydration and de-carbon dioxide from the biomass during torrefaction (Prins et al., 2006a,b). It is clear that the emission of either CO2, CO or H2O will result in a decrease in the H and O contents of SS. The increase in the carbon content is only an apparent

increase due to the decrease in the oxygen content. These results indicate that the torrefaction method increases the energy density of the SS by removing oxygen. Fig. 5 shows the change in the O/C and H/C ratios with an increase in the torrefaction temperature. From this figure, it was found that the O/C ratio decreased with increasing torrefaction temperature after 200 °C. This decrease in the O/C ratio during torrefaction is attributed to the generation of volatiles rich in oxygen, such as CO2 and H2O (Prins et al., 2006b; Medic et al., 2012a). Also, the H/C ratio decreased with increasing temperature after 200 °C. This is due to the fact that the carbon was relatively increased in comparison with the other elements by decreasing of oxygen at temperature of 250–400 °C, and the hydrocarbon gases containing hydrogen generated after 350 °C. 3.4. Effect of the torrefaction residence time Figs. 6 and 7 shows the influence of the torrefaction residence time on the characteristics of SS torrefaction. Since Fig. 3(a) shows that torrefaction temperatures above 400 °C result in low energy and mass yields, the effect of the torrefaction residence time was studied for torrefaction temperatures below 400 °C. Fig. 6(a) and (b) indicate that the volatile fraction content decreases with an increase in the torrefaction residence time, while the ash content increases with an increase in the torrefaction resistance time. In Fig. 7(a) and (b), the energy and mass yields continually decrease with an increase in torrefaction residence time. This could be explained by the decrease in the moisture content and volatile matter content of the biomass. However, there was a significant mass loss at the beginning of the torrefaction process while the change of mass yield was not significant with a longer torrefaction residence time. This is due to the decomposition of more reactive components at the beginning of the torrefaction, while with a longer torrefaction time, the mass loss can be attributed to the decomposition of the less reactive component (Pimchuai et al., 2010). It can be inferred from Fig. 7(c) that the HHV decreased with an increase in the torrefaction time for higher torrefaction temperature. However, for lower torrefaction temperature, the HHV did not vary much with increase in torrefaction residence time. For 250 °C, as can be inferred from Fig. 3(a), the conversion at residence time of 0 min does not vary much from the raw material. So, the effect of torrefaction residence time was insignificant for torrefaction temperature below 250 °C. Several studies have investigated the influence of the torrefaction residence time on the torrefaction of biomass. Nevertheless, the torrefaction residence time has been shown to be less significant than the torrefaction temperature in all experiments

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(b)

10 8

Elemental analysis (wt %)

Elemental analysis (wt%)

(a)

6 4 N S Cl

2 0

60 50 C H O

40 30 20 10 0

50

50

100 150 200 250 300 350 400 450

100 150 200 250 300 350 400 450

Torrefaction temperature ( o C)

o

Torrefaction temperature ( C)

Fig. 4. Changes in elements as a function of the torrefaction temperature (a) N, S and Cl; and (b) C, H and O.

0.6

1.6

0.5 1.4 0.4

H/C Molar Ratio

O/C Molar Ratio

grindability at 240 °C if the torrefaction residence time was longer than 30 min. In a different study, Bergman et al. (2005b) concluded that torrefaction should be conducted for 17 min at 280 °C for cofiring applications.

1.8

0.7

3.5. Composition of the gaseous products

1.2 0.3 O/C Molar Ratio H/C Molar Ratio 1.0

0.2 50

100

150

200

250

300

350

400

450

Torrefaction temperature ( oC) Fig. 5. Changes of O/C and H/C as a function of torrefaction temperature.

conducted thus far (Sadaka et al., 2014). The minimum torrefaction residence time can vary depending on the torrefaction temperature, biomass types, the physical and chemical properties of the biomass, and its intended end use. However, there is a maximum torrefaction residence time after which any further increase does not affect the biomass properties significantly. Arias et al. (2008) concluded that there was little improvement in biomass

(a)

(b)

90 250 o C 300 o C 350 o C

40 35

Ash content (%)

Volatile fraction (%)

85 80 75 70 65 60 -10

Fig. 8 shows the composition of the gaseous products obtained from the torrefaction of SS. NOx and SO2 are first detected at a torrefaction temperature of approximately 200 °C while CO is detected at temperature around 300 °C as seen in Fig. 8(a)–(c). The detection of the hydrocarbon gases (CxHy) starts at a torrefaction temperature of approximately 350 °C as shown in the Fig. 8(d). This finding was attributed to the fact that compounds with oxygen are emitted at temperatures lower than those for hydrocarbon gases. These results show that the torrefaction of SS at a temperature lower than 350 °C is preferred; above 350 °C, the emission of hydrocarbon gases begins, which has a negative impact on the energy yield. Also, above this temperature the decrease of HHV starts which again has negative impact on the energy yield. From Fig. 9, torrefaction temperatures between 300 and 350 °C were considered as the optimum torrefaction temperature for SS though the torrefaction region varies with the initial condition of the raw material, the heating rate and the chemical composition. The HHV remained above 19 MJ/kg in this region and the energy yield remained above 80%. The tradeoff of the energy yield against the HHV limits the torrefaction region.

30 25 20 250 o C 300 o C 350 o C

15

0

10

20

30

40

50

Torrefaction residence time (min)

60

10 -10

0

10

20

30

40

50

Torrefaction residence time (min)

Fig. 6. Influence of the torrefaction residence time on the (a) volatile fraction, and (b) ash content.

60

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(a)

250 oC 300 oC 350 oC

250 oC 300 oC 350 oC

100

Mass yield (%)

100

Energy yield (%)

(b) 120

120

80

60

80

60

40

40

20 -10

0

10

20

30

40

50

20 -10

60

(c)

0

10

20

30

40

50

60

Torrefaction residence time (min)

Torrefaction residence time (min) 24

HHV (MJ/kg)

22

20

18 250 oC 300 oC 350 oC

16 -10

0

10

20

30

40

50

60

Torrefaction residence time (min) Fig. 7. Influence of the torrefaction residence time on the (a) energy yield, (b) mass yield, and (c) HHV.

(a) 3000

(b) 200

2500

NOx (PPM)

CO (PPM)

150 2000 1500 1000

100

50 500 0

0 0

100

200

300

400

500

600

0

Torrefaction temperature ( o C)

100

200

300

400

500

600

Torrefaction temperature ( o C)

(c) 600

(d) 1.6 1.4

500

1.2

Cx Hy (%)

SO2 (PPM)

400 300 200

1.0 0.8 0.6 0.4

100

0.2

0

0.0 0

100

200

300

400

500

Torrefaction temperature ( o C)

600

0

100

200

300

400

500

Torrefaction temperature ( o C)

Fig. 8. Gas analysis from torrefaction of SS as a function of the temperature.

600

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J. Poudel et al. / Waste Management 40 (2015) 112–118

3000

1.6

23 100

1.4

2500 21

90

1.2

1500

19

17

0.6

1000

HHV (MJ/kg)

CO (PPM)

CxH y (%)

0.8

Energy Yield CO Cx H y HHV

70 60 50

Energy yield (%)

80

2000 1.0

0.4

0.2

15

500

40 30

0.0

0 0

100

200

300

400

500

600

13 700

20

Torrefaction temperature (oC) Fig. 9. Torrefaction region of SS as obtained from this work.

4. Conclusions The torrefaction of SS was studied to investigate the effects of the torrefaction temperature and torrefaction residence time on the thermal and physical properties of the torrefied products. The average activation energy and reaction order for the torrefaction of SS was obtained as 68.1 kJ/mol and 2.4, respectively. The energy and mass yields decreased with an increase in the torrefaction temperature. The torrefaction of SS at temperature higher than 350 °C has negative impact on its HHV. So, to minimize this loss, the pre-treatment of SS using torrefaction technology should be done at temperature below it. It was also found that the torrefaction temperature had more of an effect on torrefaction of SS compared to the torrefaction residence time. From an elemental analysis, the weight percentage of C in the SS increased with an increase in the torrefaction temperature. On the other hand, the weight percentages of H and O tended to decrease. Therefore, the O/C and H/C ratios decreased with an increase in the torrefaction temperature. From an analysis of the gaseous products resulting from torrefaction, it was found that the compounds with oxygen were emitted at temperature lower than that for hydrocarbon gases. From this study, torrefaction temperatures between 300 and 350 °C are the optimum torrefaction temperatures for SS, although the actual temperature can vary depending on the initial condition of the raw material, the heating rate and the chemical composition of the raw material. Also it can be seen that the torrefaction at this temperature range has enhanced the solid fuel qualities of SS by the decrease of O/C ratio and the increase of HHV. Acknowledgements This subject is supported by Korea Ministry of Environment (MOE) as ‘‘ Waste-to-Energy Technology Development Project’’. References A´brego, J., Sa´nchez, J.L., Arauzo, J., Fonts, I., Gil-Lalaguna, N., Atienza-Martínez, M., 2013. Technical and energetic assessment of a three-stage thermochemical treatment for sewage sludge. Energy Fuels 27, 1026–1034. Arias, B., Pevida, C., Fermoso, J., Plaza, M., Rubiera, F., Pis, J., 2008. Influence of torrefaction on the grindability and reactivity of woody biomass. Fuel Process Technol. 89, 169–175. Atienza-Martinez, M., Fonts, I., Abrego, J., Ceamanos, J., Gea, G., 2013. Sewage sludge torrefaction in a fluidized bed reactor. Chem. Eng. J. 222, 534–545.

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