Effect of Calcium Hydroxide on the Pyrolysis Behavior of Sewage Sludge

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Effect of calcium hydroxide on the pyrolysis behavior of sewage sludge: Reaction characteristics and kinetics Siqi Tang, Sicong Tian, Chunmiao Zheng, and Zuotai Zhang Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 24 Apr 2017 Downloaded from http://pubs.acs.org on April 28, 2017

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Energy & Fuels

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Effect of calcium hydroxide on the pyrolysis behavior of sewage

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sludge: Reaction characteristics and kinetics Siqi Tanga, Sicong Tianb,c, Chunmiao Zhenga,b and Zuotai Zhang*,b,d

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a

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100871, P. R. China

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b

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China, Shenzhen 518055, P. R. China

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c

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University, Beijing 100084, P. R. China

Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing

School of Environmental Science and Engineering, Southern University of Science and Technology of

Key Laboratory of Solid Waste Management and Environment Safety (Ministry of Education), Tsinghua

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d

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Shenzhen 518055, P. R. China

Key Laboratory of Municipal Solid Waste Recycling Technology and Management of Shenzhen City,

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*Corresponding author: Tel: +86-755-88018019 E-mail: [email protected] (Prof. Zuotai Zhang)

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1 ACS Paragon Plus Environment

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Abstract

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The effect of Calcium hydroxide (Ca(OH)2), a promising additive to control the pollutants

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released during sludge pyrolysis, on the pyrolysis behavior and kinetics of sewage sludge was

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investigated in detail in this study. The obtained thermograms of Ca(OH)2-blended sludge showed that

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the addition of Ca(OH)2 influenced the thermogravimetric characteristics of sludge, especially in the

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temperature range of 340–700 oC where the decomposition of Ca(OH)2 happens. An increasing

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addition of Ca(OH)2 improved the pyrolysis conversion of sludge at temperatures of more than 600 oC,

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which was verified by the increase of the process heat flow. Importantly, the transformation of

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elements in sludge was promoted, resulting in a less content of impurities, which existed mostly in the

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thermally stable forms, in the remaining char. Kinetic analysis revealed that the pyrolysis behavior of

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sludge was influenced by the addition of Ca(OH)2 and reaction temperature. At low temperatures,

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Ca(OH)2 acted as the source of nuclei required for the establishment of reaction interface, and then

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induced the secondary cracking of the pyrolytic compounds in the sludge matrix when the reaction

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came to high temperatures. A retrofitted kinetic model, overcoming the drawback faced by most

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Arrhenius-derived models that the integral of temperature-induced item was resolved by

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approximation, is developed and exhibits superiority in describing the reaction characteristics of

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sludge pyrolysis.

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Keywords: Pyrolysis; Sewage sludge; Calcium hydroxide; Reaction kinetics; Model

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Energy & Fuels

1. Introduction Pyrolysis has been generally acknowledged as a promising option for sewage sludge treatment 1-3

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and energy recovery

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substantially reduced volume, but also produce value-added byproducts (e.g. char, tar, and syngas).

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Such substances derived have a big potential to be used as industrial feedstock and renewable fuel as

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long as suitable post-treatment measures are adopted

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for sewage sludge, such as landfilling, incineration, and land application, pyrolysis technique can

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avoid the production of highly toxic organic compounds (e.g. dioxins) and the release of particle

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matters, and make heavy metals in sludge largely immobilized 6. Therefore, it sounds convinced that

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pyrolysis tends to provide an alternative, environmental-friendly, and sustainable approach to dispose

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sewage sludge efficiently.

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. Upon pyrolysis at high temperatures, sewage sludge can not only show a

4, 5

. Compared with traditional treatment options

During the pyrolysis process of sewage sludge, N- or S-containing pollutants, mainly HCN and 5, 7-10

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NH3 and H2S, will be formed as the pyrolysis temperature rises

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introduced into the derived products (char, gas, and bio-oil), and subsequently, result in the

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degradation of product quality

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has been widely concerned by researchers 4, 7, 8. Calcium-based compounds, such as calcium hydroxide

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(Ca(OH)2) and calcium oxide (CaO), have been demonstrated to be satisfactory candidates to capture

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the aforementioned pollutants during sludge pyrolysis. When sludge was pyrolyzed after a blend with

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calcium-based additives, nitrogen (N) within the sludge was preferably transformed from tar-N to

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gas-N. Gas-N existed mainly in the form of N2, whereas tar-N was in the form of amine, nitrile, and

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heterocyclic hydrocarbons 4. After sludge pyrolysis, almost all the sludge sulfur was transformed into

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the form of sulfides and sulfates in the char matrix 4, 7. Therefore, a detailed investigation regarding the

2, 11

. These pollutants will be

. Thus, the control of pollutants in the process of sludge pyrolysis


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influence of calcium-based additives on the proceeding of sludge pyrolysis is of great significance for

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a deep knowledge on the control of N- and S-containing pollutants.

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Although it was found that the fate of Ca-based species was strongly associated with the 4, 7, 12

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pyrolysis behavior of sewage sludge blended with Ca-based additives

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calcium-based additives play in the pyrolysis process of sludge is still unclear, especially with regard

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to the reaction kinetics and the relation between reaction kinetics and microscopic changes occurring

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inside the sludge matrix due to the introduction of Ca-based additives. Essential work should be done

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to elucidate the mechanism of sludge pyrolysis before and after the introduction of calcium-based

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additives, at least in terms of industrial practice. Thermogravimetric methods, including TG, TG-DSC,

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and even TG coupled with Fourier transform infrared spectroscopy (FTIR) and/or mass spectrograph

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(MS) are capable of providing effective, accurate, and online-tracking information about the evolution

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of pyrolytic products during sludge pyrolysis 9, 13, 14. In particular, the thermogravimetric data acquired

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in real time is useful to investigate the process kinetics of sludge pyrolysis, based on the

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Arrhenius-type model

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approximations were made to calculate the integral of temperature-containing item, resulting in an

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inevitable discrepancy between the value of kinetic parameters and the genuine case

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necessary modifications to the model should be carried out to overcome this drawback.

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, the role that

. In the application of Arrhenius-derived kinetic models, different

17

. Therefore,

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The purpose of the present study is thus to probe the role of calcium hydroxide during the

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pyrolysis of sewage sludge, which not only determined the pyrolysis characteristics of sludge in the

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presence of calcium hydroxide with different mass ratios, but also elucidated the effect of calcium

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hydroxide on the mechanism of sludge pyrolysis. In addition, a retrofitted kinetic model based on the

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conventional Arrhenius-type equation, was developed and applied to the determination of kinetic


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parameters concerned during sludge pyrolysis. The outcome of this study will give a knowledge about

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the contribution of calcium hydroxide on sludge pyrolysis.

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2. Experimental

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2.1 Sewage sludge

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The sludge sample used in the present study, with a water content of 78 wt. % after dewatering,

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was obtained from a municipal wastewater treatment plant in Shenzhen, China. Prior to pyrolysis, the

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sludge sample was dried overnight at 100 oC in an oven and then ground. After that, the sludge sample

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with a range of particle size between 0.043 mm and 0.15 mm was collected by sieving and then stored

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in a plastic bag for further use. Proximate analysis, ultimate analysis, and ash composition analysis of

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the pretreated sludge revealed that the sample contained ~55% volatile matters and ~32% ash, while

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the main ash components were Al-, Si-, Fe-, and Ca-containing minerals together with other trace

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metals (Table 1).

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2.2 Sludge pyrolysis experiment

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Calcium hydroxide (Ca(OH)2, analytical grade, Xilong Chemical Industry, China) was blended

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with the pretreated sludge mechanically according to different contents of 15%, 30%, 50%, 70%, and

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85%, respectively. The other two samples, i.e., the pretreated raw sludge and Ca(OH)2 alone, were

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included for comparison. For simplicity, all samples were named according to the content of Ca(OH)2,

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namely S0, S15, S30, S50, S70, S85, and S100, respectively. The pyrolysis of sludge samples was

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conducted in a thermogravimetry–differential scanning calorimetry (TG-DSC, Q600 SDT, TA

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Instruments, America). During each run, 10±0.5 mg of the sludge sample was placed into the alumina

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crucible, and then heated from the ambient temperature to 1200 oC with a heating rate of 10 oC/min.

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Nitrogen (99.99%, v/v) with a flow rate of 100 mL/min was purged into the TGA furnace during the


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whole pyrolysis process. Another appropriate amount (2 ± 0.5 g) of the pre-treated sludge (identical

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to that in TGA furnace) was pyrolyzed in a lab-scale horizontal furnace. The pyrolysis was performed

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at 340 oC, 550 oC, and 700 oC, respectively, with a heating rate of 10 oC/min under a 250 mL/min flow

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of N2 (99.99%, v/v). The sludge char generated after pyrolysis was gathered for further analysis.

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2.3 Sludge-char characterization

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The content of C, H, and N in the sludge chars was analyzed with an elemental analyzer (Vario

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EL, Elementar CO., Germany). Mineral composition and chemical state of the chars was characterized

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by using X-ray diffraction (XRD, D8 advance, Brook, Germany) and X-ray photoelectron

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spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, America), respectively. Jade 5.0 software

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(Materials Data Inc., America) was used to process the XRD data, identify major crystalline phases in

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sludge-char, and quantify lattice parameters of the crystalline phases concerned. XPS data was

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processed using Thermal Avantage (Version 5.965) software to separate the spectra of concerned

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elements, including C 18, N 5, and S 19. The content of each species was semi-quantitatively determined

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based on the normalized fraction of peak area of each species in sludge-char.

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2.4 Kinetics analysis

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Arrhenius-type equation has been extensively used to express the reaction rate for sludge 16, 20, 21

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pyrolysis

, a function depending on reaction time (t), pyrolysis temperature (T), and extent of

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conversion ( ), as expressed below:

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⁄ = ) ) = − ⁄) )

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Where Ea, R, A, and ) represents the activation energy, gas constant, pre-exponential factor, and

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mechanism function, respectively. The extent of conversion is often defined as

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= − )⁄ − )

(1)

(2)


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Where , , and represents the mass of sludge sample before pyrolysis, pyrolysis at reaction

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time t, and after pyrolysis, respectively. In general, the can be described using the following

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equation:

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= , ) = ), )

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Therefore, the full derivative of against t can be expressed in eq. (4).

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⁄ = ⁄) + ⁄) ⁄

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In the linear heating process, the temperature of sludge matrix increased uniformly from an initial

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value To at a heating rate ". Thus at the reaction time t, the temperature T can be expressed as T0+"t,

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i.e.,

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(3)

(4)

" = ⁄

(5)

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It can be derived that the full derivative of against T is

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⁄ = ⁄" [1 + ⁄ 1 − ⁄) ]− ⁄) )

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By solving eq. (6) in the domain [T0, T], an analytical solution can be obtained as follow:

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Gα) = ⁄" − )exp − ⁄)

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The detailed deduction of eq. (6) and eq. (7) can be found in Text S1 of the Supporting Information.

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The possible mechanism considered in this study was listed in Table S1 of the Supporting Information.

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The non-linear least square algorithm was used to calculate kinetic parameters, by optimizing an

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object function (OF) as below to arrive at its minimum.

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+, = ∑[. ) − ⁄" − ) − ⁄)] /

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The optimization was evaluated using variant coefficient (VC) 22, defined as

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01 = 2345637+,)/9 − 2)?:;5. ) )

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Where N is the number of data point involved in the calculation of kinetic parameters, . ) is the

(6)

(7)

(8)


(9)

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experimental data of . ) for certain possible mechanism function. The smaller VC value of an OF

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indicated that it is more accurate to describe the pyrolysis kinetics. Therefore, the kinetic parameters

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corresponding to this OF was selected as a representative value in each region.

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3. Results and discussion

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3.1 Thermogravimetric behavior of the Ca(OH)2-blended sludge

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Temperature-programmed pyrolysis profiles of the sewage sludge with different additions of

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Ca(OH)2 are shown in Fig. 1, indicative of the weight change (TG curve), weight loss rate (DTG

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curve), and heat flow change (DSC curve) of the sludge sample, respectively. More than 80% of the

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-overall weight loss, as depicted in Fig. 1(a), occurred when the temperature was heated up to 700 oC,

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which is generally the highest temperature applied for sludge pyrolysis in practice 2. At this

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temperature, the total weight loss was influenced by the introduction of Ca(OH)2, and decreased with

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the increasing addition of Ca(OH)2; however, pyrolysis conversion of the Ca(OH)2-blended sludges

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was improved with the increasing addition of Ca(OH)2, as shown in Fig. 2. However, when the

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temperature was increased further, the weight loss of raw sludge (S0) was appreciably larger when

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compared to that of the Ca(OH)2-blended sludge, which could be attributed to the secondary cracking

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of polycyclic aromatic hydrocarbons (PAHs) deposited in the char

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carbonates 24.

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and the decomposition of

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As illustrated in Fig. 1 (b), three peaks were identified in the DTG curves of all Ca(OH)2-blended

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sludges, leading to the partition of these thermogravimetric curves. Accordingly, five temperature

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regions could be divided during sludge pyrolysis, namely, Region I (≤150 oC ), Region II (150-340

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o

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residual water in the sample contributed to the weight loss of sludge in Region I, while in Region V,

C ), Region III (340-550 oC ), Region IV (550-700 oC ), and Region V (≥700 oC ). The evaporation of


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the DTG-peak was almost disappeared in all Ca(OH)2-blended sludges, except for the case of raw

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sludge (S0), which indicated that the char derived from the blended sludge had a better thermal

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stability than that from the raw sludge. It is clear in Fig. 1(b) that Regions II, III, and IV were

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responsible for the main weight loss of either the raw sludge or the Ca(OH)2-blended sludge during the

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pyrolysis process, indicating that most sludge components were thermally stimulated to decompose

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within this temperature range.

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It is worth mentioning that, the weight loss in Region II was attributed to the sludge sample itself,

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this is because pure Ca(OH)2 (S100) did not decompose below 340 oC (Fig. 1(a)). A sharply increased

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peak of weight loss rate was observed in Region III during the pyrolysis of Ca(OH)2-blended sludge,

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whereas the peak of raw sludge was negligible, although the decomposition of raw sludge was still

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going on. It was clear that S100 exhibited the largest peak of weight loss rate, indicative of a drastic

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decomposition of calcium hydroxide in this temperature region. Additionally, the peak of weight loss

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rate for Ca(OH)2-blended sludges shifted to a higher temperature, when compared to that of the raw

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sludge, revealing that the pyrolysis behavior of Ca(OH)2-blended sludges in this region was a result of

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the interactions between pyrolytic products derived and Ca(OH)2 in the sludge matrix. Coming to

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Region IV, the peak of weight loss rate of all Ca(OH)2-blended sludges shrank significantly when

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compared to the case in Region III, indicating that the proceeding of sludge pyrolysis was approaching

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to the end. However, when compared to the case of S0 and S100, all the blended sludges had a higher

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weight loss rate. It was convincing that the presence of Ca(OH)2 enhanced the pyrolysis conversion of

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sludge matrix.

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Variations of the heat flow, which was the consequence of the total thermal effect of sludge

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pyrolysis, were recorded in the DSC curves, as depicted in Fig. 1(c). Compared with the case of S0


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and S100, the introduction of Ca(OH)2 influenced significantly the change of heat flow in the whole

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process of sludge pyrolysis. Since more than 80% of the total sludge weight loss took place when the

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pyrolysis was performed below 700 oC, changes of heat flow in Region II, Region III, and Region IV

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were concerned in particular. The heat consumed in a certain temperature region during sludge

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pyrolysis can be calculated by integrating the corresponding DSC signal with time. Fig. 3depicts the

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heat requirement of sludge in the three aforementioned temperature regions during the process of

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sludge pyrolysis. It can be seen that the heat requirement corresponding to per gram of weight loss in

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Region II and III increased proportionally with the increase of Ca(OH)2 addition. Particularly in

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Region IV, compared to the raw sludge (S0), the participation of Ca(OH)2 could significantly reduce

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the amount of heat required for sludge pyrolysis. In other words, the addition of Ca(OH)2 into sludge

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could promote deeply the pyrolysis conversion of sludge within the conventional sludge-pyrolysis

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temperatures.

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3.2 Characteristics of char derived from the Ca(OH)2-blended sludge

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To investigate the effect of Ca(OH)2 on the thermochemical reactions occurring inside the sludge

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matrix, the resulting chars were collected after the pyrolysis experiment, and then characterized using

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XRD and XPS, respectively. In addition to the pyrolysis of sewage sludge in thermogravimetric

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analyzer (TGA), the lab-scale furnace was also introduced to pyrolyze the sludge sample and harvest

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the sludge-chars in a required amount for characterization. Fig. 4 shows the comparison of the weight

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loss of sludge samples during pyrolysis in TGA and furnace, respectively. The temperatures used in

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the lab-scale furnace were determined in accordance with the peak of DTG curve as shown in Fig. 1(b).

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It was confirmed in Fig. 3 that the pyrolysis of sludge taking place in both reactors were generally

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consistent, suggesting that characteristics of the char obtained from the lab-scale furnace could


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represent that from TGA.

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3.2.1 XRD characterization

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XRD patterns of the chars derived from different Ca(OH)2-blended sludges were obtained as

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shown in Fig.5. The major crystalline phases identified were calcium-based inorganics (i.e., Ca(OH)2,

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CaCO3, and CaO) and SiO2. In the chars derived from the raw sludge (S0) at all given temperatures,

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SiO2 was the only major crystalline phase, whose diffraction intensity was increased with the increase

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of pyrolysis temperature. This was attributed to the fact that SiO2 is a common inert component in

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sewage sludge

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components was going on. After the addition of Ca(OH)2 into sludge, the chars produced had different

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crystalline compositions, which varied with the increase of pyrolysis temperature.

25, 26

, and thus, SiO2 content in chars can be increased as the pyrolysis of other sludge

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With regard to the chars derived from different Ca(OH)2-blended sludges at 340 oC, Ca(OH)2 was

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the only calcium-based crystallite identified (Fig. 5(a)), and the diffraction intensity of SiO2 decreased

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with the increasing addition of Ca(OH)2. The average lattice size of Ca(OH)2, calculated by using the

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Debye-Scherrer method 27 in Table 2, almost kept constant (~3.6 Å). The presence of CaCO3, as well as

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small amounts of (Mg0.03Ca0.97)CO3, in the chars derived from the Ca(OH)2-blended sludges at 550 oC

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(Fig. 5(b)), verified the occurrence of reaction between the blended Ca(OH)2 and pyrolysis-generated

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CO2 inside the sludge matrix. The average lattice size of CaCO3 also kept stable, regardless of the

238

dosage of Ca(OH)2. The Ca(OH)2 phase was also identified in the chars, especially when the addition

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of Ca(OH)2 was more than 50%. As for the chars derived from blended sludges at 700 oC (Fig. 5(c)),

240

CaCO3 and Ca(OH)2 were still identified, which was analogous to the case at 550 oC. However, the

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average lattice size of CaCO3 was clearly larger in the chars derived at 700 oC than those at 550 oC,

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indicating that the pyrolysis of sludge was still going on after 550 oC and the CO2 released was


Energy & Fuels

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probably due to the secondary cracking of the char derived

, which was likely associated with the

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catalytic effect of CaO at high temperatures around 700 oC 27. In addition, CaO was identified in the

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char from blended sludges, especially when the sludge with a higher addition of Ca(OH)2.Therefore,

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the Ca-based species in the char derived from blended sludges were governed by the interactions

247

between sludge and Ca(OH)2, which was subject to both the pyrolysis temperature and the addition of

248

Ca(OH)2.

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3.2.2 XPS characterization

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Chars derived from the pyrolysis of sludge samples S0, S30, and S70 were chosen to investigate

251

further the change of chemical state of elements during sludge pyrolysis. Table 3 shows the elemental

252

composition of different sludge-chars prepared. It was clear that the addition of Ca(OH)2 into sewage

253

sludge significantly influenced the content of elements in the sludge-chars. An increased addition of

254

Ca(OH)2 would obviously reduce the contents of C, H, and N in sludge-char, especially when the

255

chars were produced at high pyrolysis temperatures. However, in such chars, the C/H molar ratio was

256

decreased while the C/N molar ratio was increased, indicating a decreased aromaticity and

257

hydrophilicity, respectively, of the sludge-chars derived 28. It can be inferred that the introduction of

258

Ca(OH)2 promoted the thermal cracking of organic components in sludge.

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The content of different C-, N-, and S-species was determined using their C 1s, N 1s, and S 2p

260

state (Fig. S1 in the Supporting Information), respectively, based on the XPS spectra obtained as

261

shown in Fig. 6. It was seen that the effect of Ca(OH)2 on the evolution of such species depended

262

greatly on the pyrolysis temperature and the amount of Ca(OH)2 added.

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With regard to the carbon functionality, the chars derived via pyrolysis at 340 oC showed little

264

difference between the blended sludges and raw sludge; only except for the case of carboxyl (O-C=O),


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whose increase with Ca(OH)2 addition was attributed to the promotion of reactions between pyrolytic

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carboxylic products and Ca(OH)2 29. Chars derived at elevated temperatures had a lower fraction of

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C=O and C-OH for both the blended sludges and raw sludge, whereas the fraction of O-C=O showed

268

an increase. This contrast could be ascribed to the difference in the thermal stability of related

269

intermediates, i.e., compounds containing C=O or C-OH had a poorer thermal stability than

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O-C=O-containing compounds. A decrease in the fraction of C-(C, H) in the sludge-chars, with an

271

increasing addition of Ca(OH)2, indicated the breaking of different C-H bonds through cracking

272

reactions, especially at a higher temperature. This observation was in line with the aromaticity (C/H

273

molar ratio) change of chars in Table 3.

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As for the case of nitrogen functionality, it could be found that the fraction of pyridine-N in the

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chars derived from Ca(OH)2-blended sludges, when compared to raw sludge, was increased, especially

276

when the pyrolysis temperature was increased. However, an adverse trend was observed for the case of

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pyrrole-N, suggesting that the presence of Ca(OH)2 promoted the transformation of pyrrolic

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compounds into other N-species. The release of gaseous ammonia was resulted from the dissociation

279

of quaternary-N and protein-N under high temperatures and strong alkaline environment with the

280

addition of Ca(OH)2 5. In addition, the trend of quaternary-N in chars derived from calcium

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hydroxide-blended sludge indicated that inorganic N such as ammonium was unlikely to exist in the

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chars obtained due to the low thermal stability, especially when the addition of calcium hydroxide was

283

high (up to 70%). The increasing fraction of inorganic oxides-N in the char derived from blended

284

sludges, when compared to that from the raw sludge, indicated that oxides-N in sludge was thermally

285

stable. It is noticed that, the release of N as gaseous species is in the form of N2, which was verified

286

via the bench-scaled reactor pyrolysis in the previous study.4


Energy & Fuels

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Page 14 of 27

287

Now turning to sulfur-containing species, the fraction of sulfate-S and sulfoxide-S in chars

288

derived from Ca(OH)2-blended sludges increased slightly, when compared to the case of raw sludge, at

289

a lower temperature, whereas showed an adverse trend at higher temperatures. The dissociation of

290

both S-species at high temperatures would probably induce the release of SO2

291

fraction of sulfonic S at high pyrolysis temperatures was observed in chars derived from blended

292

sludges, resulting from its better thermal stability than other S-containing species. The reduced

293

fraction of aromatic S and aliphatic S in chars derived from blended sludges with an increasing

294

Ca(OH)2 addition, particularly at high pyrolysis temperatures, indicated that the added Ca(OH)2

295

promoted the cracking of organic S during sludge pyrolysis 7. Compared with the case of raw sludge,

296

the char derived from blended sludges exhibited an increasing fraction of inorganic sulfide-S, which

297

could be resulted from the capture of H2S, likely formed due to the cracking of organic S 19, by the

298

active CaO. In summary, the above mentioned speciation of C-, N-, and S-containing groups indicated

299

that the addition of Ca(OH)2 is closely related with the thermal stability of relevant species inside the

300

sludge matrix and the transformation of sludge elements during the pyrolysis process.

301

3.3 Kinetics analysis for the pyrolysis of Ca(OH)2-blended sludge

29

. An increasing

302

The pyrolysis conversion and the corresponding differential curve (as a function of the reaction

303

temperature) of the Ca(OH)2-blended sewage sludges, with that of the raw sludge (S0) and pure

304

Ca(OH)2 (S100) given for comparison, are presented in Fig. 2. The conversion profile of all

305

Ca(OH)2-blended sludges was very close, but differed from that of S0 and S100. This result indicated

306

that the addition of Ca(OH)2 led to the change of pyrolysis mechanism of sludge. The possible

307

mechanisms considered in the present study were categorized to five types, according to the study of

308

Galwey

16

. Key kinetic parameters required to investigate the pyrolysis mechanism of sludge,


Page 15 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

309

including the activation energy (Ea), pre-exponential factor (A), and the most possible mechanism

310

function ( )), were presented in Table 4, with the detailed calculation of related parameters shown

311

in Tables S2-S8 in the Supporting Information. It was found that the reaction mechanism for pyrolysis

312

of the raw sludge can be described by reaction order-based models, which agreed well with the

313

observation in previous studies 20, 24. Thipkhunthod, et al. 20 separated the sewage sludge into different

314

fractions and found that the overall decomposition contributed from the sum of the individual

315

compound decomposition, which could be simulated using reaction ordered-based model. However,

316

the pyrolysis mechanism of sludge would change with the addition of Ca(OH)2.

317

In the low temperature region (Region II), the DTG curve of S100 showed that calcium

318

hydroxide never started decomposition. The pyrolysis of S15 followed the mechanism function of

319

RO4, identical to that of S0 but showed lower apparent activation energy Ea. This reduction of Ea

320

indicated that Ca(OH)2 could catalyze the decomposition of sludge components, which coincided with

321

the previous study 25. However, as the mass ratio of Ca(OH)2 to sludge was up to 30% or even more,

322

such as S30, S50, S70, and S85, the conversion profile of sludge turned to obey the mechanism

323

function of A0.33, a model based on the Avrami-Erofeev equation (n=1/3) and describing the situation of

324

nucleation and then nuclei growth in Region II. This observation suggested that the role that Ca(OH)2

325

played in this pyrolysis regime, was a source of the nuclei needed for the initialization of nucleation. It

326

can be conceived that the volatiles generated in this temperature region were preferentially adsorbed

327

on the surface of Ca(OH)2 particles, which would have an adverse effect on the conversion rate of

328

pyrolysis

329

frequency factor A was decreased.

330

30

. Consequently, the Ea value for the pyrolysis of S30 and S50 was increased, while the

At a higher temperature, the pyrolysis reactions occurring inside the matrix of Ca(OH)2-blend


Energy & Fuels

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Page 16 of 27

331

sludges were complex, since both the sludge pyrolysis and Ca(OH)2 decomposition proceeded

332

concurrently as shown in DTG curve of S100. The decomposition of Ca(OH)2 would generate the

333

highly reactive CaO, which could effectively catalyze a secondary cracking of primary volatiles31. In

334

Region III, the pyrolysis of sample S15 was controlled by the diffusion step which could be described

335

using D3ZLT model; however, the reaction regime obeyed RO3 model for the pyrolysis of samples

336

S30 and S50. As for samples S70 and S85, the reaction mechanism complied with A0.75 model and

337

MP1 model, respectively. The difference between the two models was that the process of nucleation

338

obeyed random law for S70 but power law for S85, revealing that the quantity of calcium hydroxide

339

added influenced the proceeding of solid-state reactions during sewage sludge pyrolysis

340

Consequently, the variation of kinetic parameters of the pyrolysis reactions in Region III did not

341

correlate definitely with the dosage of Ca(OH)2 in sludge. This result supported that the presence of

342

Ca(OH)2 made inroads into the occurrence of pyrolytic reactions within the sludge matrix. In Region

343

IV, however, the mechanism of pyrolysis reactions taking place in both the blended sludges and raw

344

sludge complied with reaction order-based model. This result suggested that the reactions occurring

345

inside sludge matrix within the temperature range of 550-700 oC could be categorized as the same type.

346

It was clear that both the apparent activation energy Ea and frequency factor A were increased with the

347

increase of Ca(OH)2 addition, except for the case of sample S15. This result revealed that the excess

348

Ca(OH)2 (CaO) reacted further with sludge-char derived, leading to the weight loss of sludge in

349

Region IV.

350

4. Conclusions

15, 16

.

351

The introduction of Ca(OH)2 into sewage sludge showed a significant effect on the reaction

352

characteristics and kinetics of pyrolysis inside the sludge matrix, within the temperature range of


Page 17 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

353

340-700 oC investigated in this study. The lower addition of Ca(OH)2 (less than 15%) showed a

354

catalytic effect on the conversion of sludge components at low temperatures of ~340 oC, while a

355

higher addition (more than 50%) induced the secondary cracking of sludge at high temperatures of

356

550-700 oC; however, both cases influence the speciation of Ca-based species (i.e., Ca(OH)2, CaCO3,

357

and CaO). In particular, the addition of Ca(OH)2 is closely related with the thermal stability of relevant

358

species inside the sludge matrix and the transformation of sludge elements during the pyrolysis

359

process; the increasing addition of Ca(OH)2 made the sludge-char produced own a lower aromaticity

360

and hydrophilicity, in which the dominant speciation of C, N, and S was C-H group, pyridine N, and

361

sulfonic and sulfide S, respectively. Compared with the raw sludge, the blended sludge with a low

362

Ca(OH)2 addition (less than 15%) had lower activation energy and pre-exponential factor. Importantly,

363

as the temperature increased, the reaction mechanism of pyrolysis shift from one uniformed reaction

364

profile (reaction-order based model) for raw sludge to the compound profile (Avrami-Erofeev model

365

followed by reaction-order based model) for blended sludges.

366

Notes

367 368 369

The authors declare no competing financial interest. Acknowledgements This study was supported by National Science Fund for Distinguished Young Scholars (51522401)

370

and National Natural Science Foundation of China (51472007). This work was also supported

371

financially by Shenzhen Science and Technology Innovation Committee (ZDSYS201602261932201).

372

Supporting Information

373

The Supporting Information contains eight tables, one figure and one mathematical deduction.

374

References


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418

1. Manara, P.; Zabaniotou, A., Towards sewage sludge based biofuels via thermochemical conversion – A review. Renewable and Sustainable Energy Reviews 2012, 16, (5), 2566-2582. 2. Fonts, I.; Gea, G.; Azuara, M.; Ábrego, J.; Arauzo, J., Sewage sludge pyrolysis for liquid production: A review. Renewable and Sustainable Energy Reviews 2012, 16, (5), 2781-2805. 3. Bridle, T. R.; Pritchard, D., Energy and nutrient recovery from sewage sludge via pyrolysis. Water Science and Technology 2004, 50, (9), 169-175. 4. Liu, H.; Zhang, Q.; Hu, H.; Liu, P.; Hu, X.; Li, A.; Yao, H., Catalytic role of conditioner CaO in nitrogen transformation during sewage sludge pyrolysis. Proceedings of the Combustion Institute 2015, 35, (3), 2759-2766. 5. Wei, L.; Wen, L.; Yang, T.; Zhang, N., Nitrogen Transformation during Sewage Sludge Pyrolysis. Energy & Fuels 2015, 29, (8), 5088-5094. 6. Van Wesenbeeck, S.; Prins, W.; Ronsse, F.; Antal, M. J., Sewage Sludge Carbonization for Biochar Applications. Fate of Heavy Metals. Energy & Fuels 2014, 28, (8), 5318-5326. 7. Liu, H.; Zhang, Q.; Hu, H.; Xiao, R.; Li, A.; Qiao, Y.; Yao, H.; Naruse, I., Dual role of conditioner CaO in product distributions and sulfur transformation during sewage sludge pyrolysis. Fuel 2014, 134, 514-520. 8. Liu, H.; Zhang, Q.; Xing, H.; Hu, H.; Li, A.; Yao, H., Product distribution and sulfur behavior in sewage sludge pyrolysis: Synergistic effect of Fenton peroxidation and CaO conditioning. Fuel 2015, 159, 68-75. 9. Tian, K.; Liu, W.-J.; Qian, T.-T.; Jiang, H.; Yu, H.-Q., Investigation on the Evolution of N-Containing Organic Compounds during Pyrolysis of Sewage Sludge. Environmental Science & Technology 2014, 48, (18), 10888-10896. 10. Cao, J.-P.; Li, L.-Y.; Morishita, K.; Xiao, X.-B.; Zhao, X.-Y.; Wei, X.-Y.; Takarada, T., Nitrogen transformations during fast pyrolysis of sewage sludge. Fuel 2013, 104, 1-6. 11. Brown, R. C., Thermochemical Processing of Biomass: Conversion into Fuels, Chemicals and Power. 2011. 12. Hlavsová, A.; Corsaro, A.; Raclavská, H.; Juchelková, D., The effects of varying CaO content and rehydration treatment on the composition, yield, and evolution of gaseous products from the pyrolysis of sewage sludge. Journal of Analytical and Applied Pyrolysis 2014, 108, 160-169. 13. Shao, J.; Yan, R.; Chen, H.; Wang, B.; Lee, D. H.; Liang, D. T., Pyrolysis Characteristics and Kinetics of Sewage Sludge by Thermogravimetry Fourier Transform Infrared Analysis. Energy & Fuels 2008, 22, (1), 38-45. 14. Font, R.; Fullana, A.; Conesa, J. A.; Llavador, F., Analysis of the pyrolysis and combustion of different sewage sludges by TG. Journal of Analytical and Applied Pyrolysis 2001, 58–59, 927-941. 15. Galwey, A. K., What can we learn about the mechanisms of thermal decompositions of solids from kinetic measurements? Journal of Thermal Analysis and Calorimetry 2008, 92, (3), 967-983. 16. Galwey, A. K., Solid state reaction kinetics, mechanisms and catalysis: a retrospective rational review. Reaction Kinetics, Mechanisms and Catalysis 2015, 114, (1), 1-29. 17. Vyazovkin, S.; Burnham, A. K.; Criado, J. M.; Pérez-Maqueda, L. A.; Popescu, C.; Sbirrazzuoli, N., ICTAC Kinetics Committee recommendations for performing kinetic computations on thermal analysis data. Thermochimica Acta 2011, 520, (1–2), 1-19. 18. Naumkin, A. V.; Kraut-Vass, A.; Gaarenstroom, S. W.; Powell, C. J., NIST X-ray Photoelectron Spectroscopy Database, Version 4.1. In National Institute of Standards and Technology: Gaithersburg, 2012.


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19. Liu, S.; Wei, M.; Qiao, Y.; Yang, Z.; Gui, B.; Yu, Y.; Xu, M., Release of organic sulfur as sulfur-containing gases during low temperature pyrolysis of sewage sludge. Proceedings of the Combustion Institute 2015, 35, (3), 2767-2775. 20. Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Rirksomboon, T., Describing sewage sludge pyrolysis kinetics by a combination of biomass fractions decomposition. Journal of Analytical and Applied Pyrolysis 2007, 79, (1–2), 78-85. 21. Hayhurst, A. N., The kinetics of the pyrolysis or devolatilisation of sewage sludge and other solid fuels. Combustion and Flame 2013, 160, (1), 138-144. 22. Font, R.; Fullana, A.; Conesa, J., Kinetic models for the pyrolysis and combustion of two types of sewage sludge. Journal of Analytical and Applied Pyrolysis 2005, 74, (1–2), 429-438. 23. Dai, Q.; Jiang, X.; Lv, G.; Ma, X.; Jin, Y.; Wang, F.; Chi, Y.; Yan, J., Investigation into particle size influence on PAH formation during dry sewage sludge pyrolysis: TG-FTIR analysis and batch scale research. Journal of Analytical and Applied Pyrolysis 2015, 112, 388-393. 24. Thipkhunthod, P.; Meeyoo, V.; Rangsunvigit, P.; Kitiyanan, B.; Siemanond, K.; Rirksomboon, T., Pyrolytic characteristics of sewage sludge. Chemosphere 2006, 64, (6), 955-962. 25. Shao, J.; Yan, R.; Chen, H.; Yang, H.; Lee, D. H., Catalytic effect of metal oxides on pyrolysis of sewage sludge. Fuel Processing Technology 2010, 91, (9), 1113-1118. 26. Gascó, G.; Blanco, C. G.; Guerrero, F.; Méndez Lázaro, A. M., The influence of organic matter on sewage sludge pyrolysis. Journal of Analytical and Applied Pyrolysis 2005, 74, (1–2), 413-420. 27. Tsubouchi, N.; Ohtsuka, Y., Formation of N2 during pyrolysis of Ca-loaded coals. Fuel 2002, 81, (11-12), 1423-1431. 28. Zielińska, A.; Oleszczuk, P.; Charmas, B.; Skubiszewska-Zięba, J.; Pasieczna-Patkowska, S., Effect of sewage sludge properties on the biochar characteristic. Journal of Analytical and Applied Pyrolysis 2015, 112, 201-213. 29. Karayildirim, T.; Yanik, J.; Yuksel, M.; Bockhorn, H., Characterisation of products from pyrolysis of waste sludges. Fuel 2006, 85, (10–11), 1498-1508. 30. Zhang, Q.; Liu, H.; Liu, P.; Hu, H.; Yao, H., Pyrolysis characteristics and kinetic analysis of different dewatered sludge. Bioresource Technology 2014, 170, 325-330. 31. Tingyu, Z.; Shouyu, Z.; Jiejie, H.; Yang, W., Effect of calcium oxide on pyrolysis of coal in a fluidized bed. Fuel Processing Technology 2000, 64, (1–3), 271-284.

449 450

Table Captions

451

Table 1. The physicochemical properties of the pretreated sludge sample

452

Table 2. Calculated lattice size of Ca-based crystallites identified in the sludge-chars

453

Table 3. Elemental composition of different sludge-chars prepared in this study

454

Table 4. Kinetic parameters of the solid-state reactions occurring inside sludge matrix during pyrolysis

455


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

456

Figure Captions

457

Fig. 1. Thermogravimetric curves of the sewage sludge with different additions of Ca(OH)2 during the

458

temperature-programmed pyrolysis at a heating rate of 10 oC/min under a N2 atmosphere: (a) TG curve,

459

(b) DTG curve, (c) DSC curve

460

Fig. 2. Heat requirement per gram of weight loss during different temperature regions of sludge

461

pyrolysis

462

Fig. 3. Comparison of sludge weight loss in TGA and lab-scale furnace at several given temperatures

463

for sludge pyrolysis

464

Fig. 4. XRD patterns of the chars derived from the pyrolysis of different Ca(OH)2-blended sludges at

465

the temperature of (a) 340 oC, (b) 550 oC, and (c) 700 oC

466

Fig. 5. The relative content of different C-, N-, and S-species in the sludge-chars prepared at (a) 340

467

o

468

Fig. 6. The pyrolysis conversion of sewage sludge as a function of the reaction temperature

C, (b) 550 oC, and (c) 700 oC

469 470

Table 1. The physicochemical properties of the pretreated sludge sample Measurement Proximate analysisa Moisture Volatile matter Ash Fixed carbon

Content (wt.%, dry basis) 4.31 55.30 32.40 7.99

Ultimate analysisb C H N S

32.52 5.52 5.00 0.76

Ash compositionc


Page 20 of 27

Page 21 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

471 472 473 474

Energy & Fuels

Al2O3 11.01 10.14 SiO2 P2O5 5.38 Fe2O3 2.38 CaO 1.99 K2O 0.79 TiO2 0.25 SO3 0.22 ZnO 0.17 MnO 0.03 CuO 0.01 Cr2O3 0.02 PbO 0.003 a measured according to the Chinese Standard GB/T 17664-1999. Fixed carbon (FC) was calculated according to the formula: (FC(%) = 100%- Moisture(%)-Volatile matter (%)-Ash(%)). b measured using an element analyzer (Flash 2000, ThermoFisher, America). c measured using an X-Ray Fluorescence spectrometer (EDX, Shimadzu, Japan).

475 476 477 478 479

Table 2. Calculated lattice size of Ca-based crystallites identified in the sludge-chars Pyrolysis temperature (oC) 340

550

Sludge sample

S0 S15 S30 S50 S70 S85 S100 S0 S15 S30 S50 S70 S85 S100

Lattice size (Å)a Ca(OH)2

CaO

CaCO3

-b 3.2524 3.5495 3.5617 3.5528 3.5713 3.5695 3.5650 3.5530 3.4401 4.3516

8.0843

4.7471 4.4592 4.2053 4.0876 4.6556 -


Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 27

700

480 481 482

S0 S15 S30 S50 3.6213 S70 3.6212 6.9864 S85 4.3427 5.6223 S100 4.3405 4.0472 a Average lattice size measured using the Debye-Scherrer method 27. b Not determined.

5.7848 5.7814 5.7912 5.9055 5.5207 -

483 484 485 486 487 488 489 490 491

492 493 494

Table 3. Elemental composition of different sludge-chars prepared in this study Temperature (oC) Sample C (% wt)a 340 S0 29.83 S30 22.30 S70 10.35 550 S0 19.97 S30 13.36 S70 5.14 700 S0 20.08 S30 12.19 S70 4.24 a calculated on a dry basis. b the molar ratio of C to H in the char prepared. c the molar ratio of C to N in the char prepared.

H(%wt)a 3.80 3.44 2.91 2.03 1.59 2.25 1.34 1.41 2.27

N(%wt)a 4.75 3.07 1.32 3.11 1.38 0.40 2.30 1.02 0.26

495 496


C/Hb 0.66 0.54 0.30 0.82 0.70 0.19 1.25 0.72 0.16

C/Nc 7.33 8.47 9.14 7.50 11.26 15.17 10.21 14.01 19.00

Page 23 of 27

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

497 498

Table 4. Kinetic parameters of the solid-state reactions occurring inside sludge matrix during pyrolysis Temperature Parameter Region Region II Ea (kJ/mol) logA ) Region III Ea (kJ/mol) logA ) Region IV Ea (kJ/mol) logA )

S0

S15

S30

S50

S70

S85

S100

37.6

17.8

42.2

38.8

36.5

32.1

-

1.38 RO4 2.22 × 10DEF -3.00 RO4 4.44 × 10DEH -2.20 RO4

-3.39 RO4 0.248

-1.27 A0.33 2.22 × 10DEF -5.12 RO3 53.1

-2.59 A0.33 3.87

-6.59 A0.33 2.22 × 10DEF -7.02 MP1 172

15.3

-4.45 RO3 133

-4.06 A0.33 2.22 × 10DEF -6.35 A0.75 142

4.34 RO4

14.94 RO4

16.81 RO4

21.57 RO4

16.24 RO3

-7.94 D3ZLT 2.22 × 10DEF -2.39 RO4

499 500 501 502


-4.29 MP1/2 133

Energy & Fuels

Weight (mg)

Region I

Region II

Region III Region IV

Region V

10 8 6

Weight loss rate (%/oC)

4 2 0.6

S0 S15 S30 S50 S70 S85 S100

0.4 0.2 0.0

Heat flow (mw)

50 25 0

-25 -50 0

200

400

600 T (oC)

503

800

1000

1200

504

Fig. 1. Thermogravimetric curves of the sewage sludge with different additions of Ca(OH)2 during the

505

temperature-programmed pyrolysis at a heating rate of 10 oC/min under a N2 atmosphere: (a) TG curve,

506

(b) DTG curve, (c) DSC curve 150 1.0

300

450

600

750

900

1050

1200 S0 S15 S30 S50 S70 S85 S100

0.8

α

0.6 0.4 0.2 0.0 0.025

S0 S15 S30 S50 S70 S85 S100

0.020

dα/dT(1/oC)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 27

0.015 0.010 0.005 0.000 150

507 508

300

450

600

750

900

1050

1200

T (oC) Fig. 2 he pyrolysis conversion of sewage sludge as a function of the reaction temperature


Page 25 of 27

509

8.0x104

Region II Region III Region IV

Heat (J/mg)

6.0x104

4.0x104

2.0x104

0.0 S0

S15

S30

S50

S70

S85

S100

510 511

Fig. 3. Heat requirement per gram of weight loss during different temperature regions of sludge

512

pyrolysis

60

Weight loss in lab-scale furnace(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

340 oC 550 oC 700 oC

50

40

30

20

10

0 0

10

20

30

40

50

60

Weigth loss in TGA(%)

513 514

Fig. 4. Comparison of sludge weight loss in TGA and lab-scale furnace at several given temperatures

515

for sludge pyrolysis


Energy & Fuels

(a)

S-SiO2 O-Ca(OH)2

O

O

L-CaO C-CaCO3 M-(Mg0.03Ca0.97)CO3 O

O

Intensity (arbitrary units)

O

O

OO

O

S100

O

OO

O

S85

O

OO

O

S70

OO

O

O

O

O

O

O

O

O

O

O

S

O

O

O S O O

O

O

O

O

O

O

S30

O

O

O

S15

S50

O S O S

O

O

S

S0

10

20

30

40

50

60

70

80

90

2theta (o) (b)

O

LO

O


O

C O

O

C O

O

L

O

O

O

C

O

O

O

S85

C

O O

O

S70

S100

O

O

O,C O S C S

S

O O

OO

S50

O

C M C

M

CC

S30

C MM

CC

S15

S C S

S

S0 10

20

30

40

50

60

70

80

90

2theta (o) (c)

O L O C L O S


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 27

O

O

C

O

C

O

S S

O

L O

O

O

C

O

O O

S85

L C

O

O

S70

L

C

S100

O

O O

S50

C C

S30

C

S15

S C S

S0 10

20

30

40

50

60

70

80

90

o

516

2theta ( )

517

Fig. 5. XRD patterns of the chars derived from the pyrolysis of different Ca(OH)2-blended sludges at

518

the temperature of (a) 340 oC, (b) 550 oC, and (c) 700 oC


Page 27 of 27

(a) S0 S30 S70

60 50 40 30

50 40 30

40 30 20

10

10

0

0 C-OH

C-(O, N)

C=O

C-OH

C-(O, N)

O-C=O

C-(C, H)

80 70

60

60

Fraction (%)

70

50 40 30

(b)

40 30 20

10

10

Pyridine N

Pyrrole N

Protein N

Quaternary N

Quaternary N

Fraction (%)

30

40 30

Pyridine N

Oxides N

30

20 10

0

0 Aliphatic Inorganic sulfide

S0 S30 S70

30

10

Aromatic

Oxides N

40

10

Sulfoxide

Quaternary N

50

40

20

Sulfonic

Protein N

60

20

Sulfate

Pyrrole N

(c)

S0 S30 S70

50

40

50

70

60

50

S0 S30 S70

0 Protein N

(b)

S0 S30 S70

(a) 60

(c)

10

Pyrrole N

70

70

C-(C, H)

20

Pyridine N

Oxides N

C-(O, N)

70

0

0

C-OH

60

50

20

80

S0 S30 S70

Fraction (%)

S0 S30 S70

(a)

C=O

90

90

Fraction (%)

80

0 O-C=O

C-(C, H)

S0 S30 S70

50

10

C=O

(c)

60

20

90

Fraction (%)

70

S0 S30 S70

20

O-C=O

519

(b)

60

Fraction (%)

Fraction (%)

70

Fraction (%)

70

80

80

80

Fraction (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 Sulfate

Sulfonic

Sulfoxide

Aromatic

Aliphatic Inorganic sulfide

Sulfate

Sulfonic

Sulfoxide

Aromatic

Aliphatic Inorganic sulfide

520

Fig. 6. The relative content of different C-, N-, and S-species in the sludge-chars prepared at (a) 340

521

o

C, (b) 550 oC, and (c) 700 oC


Effect of Calcium Hydroxide on the Pyrolysis Behavior of Sewage Sludge

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