Subscriber access provided by University of Colorado Boulder
Article
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
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Energy & Fuels is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
Page 1 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
1
Effect of calcium hydroxide on the pyrolysis behavior of sewage
2
sludge: Reaction characteristics and kinetics Siqi Tanga, Sicong Tianb,c, Chunmiao Zhenga,b and Zuotai Zhang*,b,d
3 4
a
5
100871, P. R. China
6
b
7
China, Shenzhen 518055, P. R. China
8
c
9
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
10
d
11
Shenzhen 518055, P. R. China
Key Laboratory of Municipal Solid Waste Recycling Technology and Management of Shenzhen City,
12 13 14 15 16
*Corresponding author: Tel: +86-755-88018019 E-mail:
[email protected] (Prof. Zuotai Zhang)
17 18 19 20 21 22
1 ACS Paragon Plus Environment
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
23
Abstract
24
The effect of Calcium hydroxide (Ca(OH)2), a promising additive to control the pollutants
25
released during sludge pyrolysis, on the pyrolysis behavior and kinetics of sewage sludge was
26
investigated in detail in this study. The obtained thermograms of Ca(OH)2-blended sludge showed that
27
the addition of Ca(OH)2 influenced the thermogravimetric characteristics of sludge, especially in the
28
temperature range of 340–700 oC where the decomposition of Ca(OH)2 happens. An increasing
29
addition of Ca(OH)2 improved the pyrolysis conversion of sludge at temperatures of more than 600 oC,
30
which was verified by the increase of the process heat flow. Importantly, the transformation of
31
elements in sludge was promoted, resulting in a less content of impurities, which existed mostly in the
32
thermally stable forms, in the remaining char. Kinetic analysis revealed that the pyrolysis behavior of
33
sludge was influenced by the addition of Ca(OH)2 and reaction temperature. At low temperatures,
34
Ca(OH)2 acted as the source of nuclei required for the establishment of reaction interface, and then
35
induced the secondary cracking of the pyrolytic compounds in the sludge matrix when the reaction
36
came to high temperatures. A retrofitted kinetic model, overcoming the drawback faced by most
37
Arrhenius-derived models that the integral of temperature-induced item was resolved by
38
approximation, is developed and exhibits superiority in describing the reaction characteristics of
39
sludge pyrolysis.
40
Keywords: Pyrolysis; Sewage sludge; Calcium hydroxide; Reaction kinetics; Model
41 42 43 44
2 ACS Paragon Plus Environment
Page 2 of 27
Page 3 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
45 46
Energy & Fuels
1. Introduction Pyrolysis has been generally acknowledged as a promising option for sewage sludge treatment 1-3
47
and energy recovery
48
substantially reduced volume, but also produce value-added byproducts (e.g. char, tar, and syngas).
49
Such substances derived have a big potential to be used as industrial feedstock and renewable fuel as
50
long as suitable post-treatment measures are adopted
51
for sewage sludge, such as landfilling, incineration, and land application, pyrolysis technique can
52
avoid the production of highly toxic organic compounds (e.g. dioxins) and the release of particle
53
matters, and make heavy metals in sludge largely immobilized 6. Therefore, it sounds convinced that
54
pyrolysis tends to provide an alternative, environmental-friendly, and sustainable approach to dispose
55
sewage sludge efficiently.
56
. 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
57
NH3 and H2S, will be formed as the pyrolysis temperature rises
58
introduced into the derived products (char, gas, and bio-oil), and subsequently, result in the
59
degradation of product quality
60
has been widely concerned by researchers 4, 7, 8. Calcium-based compounds, such as calcium hydroxide
61
(Ca(OH)2) and calcium oxide (CaO), have been demonstrated to be satisfactory candidates to capture
62
the aforementioned pollutants during sludge pyrolysis. When sludge was pyrolyzed after a blend with
63
calcium-based additives, nitrogen (N) within the sludge was preferably transformed from tar-N to
64
gas-N. Gas-N existed mainly in the form of N2, whereas tar-N was in the form of amine, nitrile, and
65
heterocyclic hydrocarbons 4. After sludge pyrolysis, almost all the sludge sulfur was transformed into
66
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
3 ACS Paragon Plus Environment
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 4 of 27
67
influence of calcium-based additives on the proceeding of sludge pyrolysis is of great significance for
68
a deep knowledge on the control of N- and S-containing pollutants.
69
Although it was found that the fate of Ca-based species was strongly associated with the 4, 7, 12
70
pyrolysis behavior of sewage sludge blended with Ca-based additives
71
calcium-based additives play in the pyrolysis process of sludge is still unclear, especially with regard
72
to the reaction kinetics and the relation between reaction kinetics and microscopic changes occurring
73
inside the sludge matrix due to the introduction of Ca-based additives. Essential work should be done
74
to elucidate the mechanism of sludge pyrolysis before and after the introduction of calcium-based
75
additives, at least in terms of industrial practice. Thermogravimetric methods, including TG, TG-DSC,
76
and even TG coupled with Fourier transform infrared spectroscopy (FTIR) and/or mass spectrograph
77
(MS) are capable of providing effective, accurate, and online-tracking information about the evolution
78
of pyrolytic products during sludge pyrolysis 9, 13, 14. In particular, the thermogravimetric data acquired
79
in real time is useful to investigate the process kinetics of sludge pyrolysis, based on the
80
Arrhenius-type model
81
approximations were made to calculate the integral of temperature-containing item, resulting in an
82
inevitable discrepancy between the value of kinetic parameters and the genuine case
83
necessary modifications to the model should be carried out to overcome this drawback.
15, 16
, the role that
. In the application of Arrhenius-derived kinetic models, different
17
. Therefore,
84
The purpose of the present study is thus to probe the role of calcium hydroxide during the
85
pyrolysis of sewage sludge, which not only determined the pyrolysis characteristics of sludge in the
86
presence of calcium hydroxide with different mass ratios, but also elucidated the effect of calcium
87
hydroxide on the mechanism of sludge pyrolysis. In addition, a retrofitted kinetic model based on the
88
conventional Arrhenius-type equation, was developed and applied to the determination of kinetic
4 ACS Paragon Plus Environment
Page 5 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
89
parameters concerned during sludge pyrolysis. The outcome of this study will give a knowledge about
90
the contribution of calcium hydroxide on sludge pyrolysis.
91
2. Experimental
92
2.1 Sewage sludge
93
The sludge sample used in the present study, with a water content of 78 wt. % after dewatering,
94
was obtained from a municipal wastewater treatment plant in Shenzhen, China. Prior to pyrolysis, the
95
sludge sample was dried overnight at 100 oC in an oven and then ground. After that, the sludge sample
96
with a range of particle size between 0.043 mm and 0.15 mm was collected by sieving and then stored
97
in a plastic bag for further use. Proximate analysis, ultimate analysis, and ash composition analysis of
98
the pretreated sludge revealed that the sample contained ~55% volatile matters and ~32% ash, while
99
the main ash components were Al-, Si-, Fe-, and Ca-containing minerals together with other trace
100
metals (Table 1).
101
2.2 Sludge pyrolysis experiment
102
Calcium hydroxide (Ca(OH)2, analytical grade, Xilong Chemical Industry, China) was blended
103
with the pretreated sludge mechanically according to different contents of 15%, 30%, 50%, 70%, and
104
85%, respectively. The other two samples, i.e., the pretreated raw sludge and Ca(OH)2 alone, were
105
included for comparison. For simplicity, all samples were named according to the content of Ca(OH)2,
106
namely S0, S15, S30, S50, S70, S85, and S100, respectively. The pyrolysis of sludge samples was
107
conducted in a thermogravimetry–differential scanning calorimetry (TG-DSC, Q600 SDT, TA
108
Instruments, America). During each run, 10±0.5 mg of the sludge sample was placed into the alumina
109
crucible, and then heated from the ambient temperature to 1200 oC with a heating rate of 10 oC/min.
110
Nitrogen (99.99%, v/v) with a flow rate of 100 mL/min was purged into the TGA furnace during the
5 ACS Paragon Plus Environment
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 6 of 27
111
whole pyrolysis process. Another appropriate amount (2 ± 0.5 g) of the pre-treated sludge (identical
112
to that in TGA furnace) was pyrolyzed in a lab-scale horizontal furnace. The pyrolysis was performed
113
at 340 oC, 550 oC, and 700 oC, respectively, with a heating rate of 10 oC/min under a 250 mL/min flow
114
of N2 (99.99%, v/v). The sludge char generated after pyrolysis was gathered for further analysis.
115
2.3 Sludge-char characterization
116
The content of C, H, and N in the sludge chars was analyzed with an elemental analyzer (Vario
117
EL, Elementar CO., Germany). Mineral composition and chemical state of the chars was characterized
118
by using X-ray diffraction (XRD, D8 advance, Brook, Germany) and X-ray photoelectron
119
spectroscopy (XPS, Escalab 250Xi, Thermo Scientific, America), respectively. Jade 5.0 software
120
(Materials Data Inc., America) was used to process the XRD data, identify major crystalline phases in
121
sludge-char, and quantify lattice parameters of the crystalline phases concerned. XPS data was
122
processed using Thermal Avantage (Version 5.965) software to separate the spectra of concerned
123
elements, including C 18, N 5, and S 19. The content of each species was semi-quantitatively determined
124
based on the normalized fraction of peak area of each species in sludge-char.
125
2.4 Kinetics analysis
126
Arrhenius-type equation has been extensively used to express the reaction rate for sludge 16, 20, 21
127
pyrolysis
, a function depending on reaction time (t), pyrolysis temperature (T), and extent of
128
conversion ( ), as expressed below:
129
⁄ = ) ) = − ⁄) )
130
Where Ea, R, A, and ) represents the activation energy, gas constant, pre-exponential factor, and
131
mechanism function, respectively. The extent of conversion is often defined as
132
= − )⁄ − )
(1)
(2)
6 ACS Paragon Plus Environment
Page 7 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
133
Where , , and represents the mass of sludge sample before pyrolysis, pyrolysis at reaction
134
time t, and after pyrolysis, respectively. In general, the can be described using the following
135
equation:
136
= , ) = ), )
137
Therefore, the full derivative of against t can be expressed in eq. (4).
138
⁄ = ⁄) + ⁄) ⁄
139
In the linear heating process, the temperature of sludge matrix increased uniformly from an initial
140
value To at a heating rate ". Thus at the reaction time t, the temperature T can be expressed as T0+"t,
141
i.e.,
142
(3)
(4)
" = ⁄
(5)
143
It can be derived that the full derivative of against T is
144
⁄ = ⁄" [1 + ⁄ 1 − ⁄) ]− ⁄) )
145
By solving eq. (6) in the domain [T0, T], an analytical solution can be obtained as follow:
146
Gα) = ⁄" − )exp − ⁄)
147
The detailed deduction of eq. (6) and eq. (7) can be found in Text S1 of the Supporting Information.
148
The possible mechanism considered in this study was listed in Table S1 of the Supporting Information.
149
The non-linear least square algorithm was used to calculate kinetic parameters, by optimizing an
150
object function (OF) as below to arrive at its minimum.
151
+, = ∑[. ) − ⁄" − ) − ⁄)] /
152
The optimization was evaluated using variant coefficient (VC) 22, defined as
153
01 = 2345637+,)/9 − 2)?:;5. ) )
154
Where N is the number of data point involved in the calculation of kinetic parameters, . ) is the
(6)
(7)
(8)
7 ACS Paragon Plus Environment
(9)
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 8 of 27
155
experimental data of . ) for certain possible mechanism function. The smaller VC value of an OF
156
indicated that it is more accurate to describe the pyrolysis kinetics. Therefore, the kinetic parameters
157
corresponding to this OF was selected as a representative value in each region.
158
3. Results and discussion
159
3.1 Thermogravimetric behavior of the Ca(OH)2-blended sludge
160
Temperature-programmed pyrolysis profiles of the sewage sludge with different additions of
161
Ca(OH)2 are shown in Fig. 1, indicative of the weight change (TG curve), weight loss rate (DTG
162
curve), and heat flow change (DSC curve) of the sludge sample, respectively. More than 80% of the
163
-overall weight loss, as depicted in Fig. 1(a), occurred when the temperature was heated up to 700 oC,
164
which is generally the highest temperature applied for sludge pyrolysis in practice 2. At this
165
temperature, the total weight loss was influenced by the introduction of Ca(OH)2, and decreased with
166
the increasing addition of Ca(OH)2; however, pyrolysis conversion of the Ca(OH)2-blended sludges
167
was improved with the increasing addition of Ca(OH)2, as shown in Fig. 2. However, when the
168
temperature was increased further, the weight loss of raw sludge (S0) was appreciably larger when
169
compared to that of the Ca(OH)2-blended sludge, which could be attributed to the secondary cracking
170
of polycyclic aromatic hydrocarbons (PAHs) deposited in the char
171
carbonates 24.
23
and the decomposition of
172
As illustrated in Fig. 1 (b), three peaks were identified in the DTG curves of all Ca(OH)2-blended
173
sludges, leading to the partition of these thermogravimetric curves. Accordingly, five temperature
174
regions could be divided during sludge pyrolysis, namely, Region I (≤150 oC ), Region II (150-340
175
o
176
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
8 ACS Paragon Plus Environment
Page 9 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
177
the DTG-peak was almost disappeared in all Ca(OH)2-blended sludges, except for the case of raw
178
sludge (S0), which indicated that the char derived from the blended sludge had a better thermal
179
stability than that from the raw sludge. It is clear in Fig. 1(b) that Regions II, III, and IV were
180
responsible for the main weight loss of either the raw sludge or the Ca(OH)2-blended sludge during the
181
pyrolysis process, indicating that most sludge components were thermally stimulated to decompose
182
within this temperature range.
183
It is worth mentioning that, the weight loss in Region II was attributed to the sludge sample itself,
184
this is because pure Ca(OH)2 (S100) did not decompose below 340 oC (Fig. 1(a)). A sharply increased
185
peak of weight loss rate was observed in Region III during the pyrolysis of Ca(OH)2-blended sludge,
186
whereas the peak of raw sludge was negligible, although the decomposition of raw sludge was still
187
going on. It was clear that S100 exhibited the largest peak of weight loss rate, indicative of a drastic
188
decomposition of calcium hydroxide in this temperature region. Additionally, the peak of weight loss
189
rate for Ca(OH)2-blended sludges shifted to a higher temperature, when compared to that of the raw
190
sludge, revealing that the pyrolysis behavior of Ca(OH)2-blended sludges in this region was a result of
191
the interactions between pyrolytic products derived and Ca(OH)2 in the sludge matrix. Coming to
192
Region IV, the peak of weight loss rate of all Ca(OH)2-blended sludges shrank significantly when
193
compared to the case in Region III, indicating that the proceeding of sludge pyrolysis was approaching
194
to the end. However, when compared to the case of S0 and S100, all the blended sludges had a higher
195
weight loss rate. It was convincing that the presence of Ca(OH)2 enhanced the pyrolysis conversion of
196
sludge matrix.
197
Variations of the heat flow, which was the consequence of the total thermal effect of sludge
198
pyrolysis, were recorded in the DSC curves, as depicted in Fig. 1(c). Compared with the case of S0
9 ACS Paragon Plus Environment
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
199
and S100, the introduction of Ca(OH)2 influenced significantly the change of heat flow in the whole
200
process of sludge pyrolysis. Since more than 80% of the total sludge weight loss took place when the
201
pyrolysis was performed below 700 oC, changes of heat flow in Region II, Region III, and Region IV
202
were concerned in particular. The heat consumed in a certain temperature region during sludge
203
pyrolysis can be calculated by integrating the corresponding DSC signal with time. Fig. 3depicts the
204
heat requirement of sludge in the three aforementioned temperature regions during the process of
205
sludge pyrolysis. It can be seen that the heat requirement corresponding to per gram of weight loss in
206
Region II and III increased proportionally with the increase of Ca(OH)2 addition. Particularly in
207
Region IV, compared to the raw sludge (S0), the participation of Ca(OH)2 could significantly reduce
208
the amount of heat required for sludge pyrolysis. In other words, the addition of Ca(OH)2 into sludge
209
could promote deeply the pyrolysis conversion of sludge within the conventional sludge-pyrolysis
210
temperatures.
211
3.2 Characteristics of char derived from the Ca(OH)2-blended sludge
212
To investigate the effect of Ca(OH)2 on the thermochemical reactions occurring inside the sludge
213
matrix, the resulting chars were collected after the pyrolysis experiment, and then characterized using
214
XRD and XPS, respectively. In addition to the pyrolysis of sewage sludge in thermogravimetric
215
analyzer (TGA), the lab-scale furnace was also introduced to pyrolyze the sludge sample and harvest
216
the sludge-chars in a required amount for characterization. Fig. 4 shows the comparison of the weight
217
loss of sludge samples during pyrolysis in TGA and furnace, respectively. The temperatures used in
218
the lab-scale furnace were determined in accordance with the peak of DTG curve as shown in Fig. 1(b).
219
It was confirmed in Fig. 3 that the pyrolysis of sludge taking place in both reactors were generally
220
consistent, suggesting that characteristics of the char obtained from the lab-scale furnace could
10 ACS Paragon Plus Environment
Page 10 of 27
Page 11 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
221
represent that from TGA.
222
3.2.1 XRD characterization
223
XRD patterns of the chars derived from different Ca(OH)2-blended sludges were obtained as
224
shown in Fig.5. The major crystalline phases identified were calcium-based inorganics (i.e., Ca(OH)2,
225
CaCO3, and CaO) and SiO2. In the chars derived from the raw sludge (S0) at all given temperatures,
226
SiO2 was the only major crystalline phase, whose diffraction intensity was increased with the increase
227
of pyrolysis temperature. This was attributed to the fact that SiO2 is a common inert component in
228
sewage sludge
229
components was going on. After the addition of Ca(OH)2 into sludge, the chars produced had different
230
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
231
With regard to the chars derived from different Ca(OH)2-blended sludges at 340 oC, Ca(OH)2 was
232
the only calcium-based crystallite identified (Fig. 5(a)), and the diffraction intensity of SiO2 decreased
233
with the increasing addition of Ca(OH)2. The average lattice size of Ca(OH)2, calculated by using the
234
Debye-Scherrer method 27 in Table 2, almost kept constant (~3.6 Å). The presence of CaCO3, as well as
235
small amounts of (Mg0.03Ca0.97)CO3, in the chars derived from the Ca(OH)2-blended sludges at 550 oC
236
(Fig. 5(b)), verified the occurrence of reaction between the blended Ca(OH)2 and pyrolysis-generated
237
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
239
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
241
average lattice size of CaCO3 was clearly larger in the chars derived at 700 oC than those at 550 oC,
242
indicating that the pyrolysis of sludge was still going on after 550 oC and the CO2 released was
11 ACS Paragon Plus Environment
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 12 of 27
20
243
probably due to the secondary cracking of the char derived
, which was likely associated with the
244
catalytic effect of CaO at high temperatures around 700 oC 27. In addition, CaO was identified in the
245
char from blended sludges, especially when the sludge with a higher addition of Ca(OH)2.Therefore,
246
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.
249
3.2.2 XPS characterization
250
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.
259
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.
263
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),
12 ACS Paragon Plus Environment
Page 13 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
265
whose increase with Ca(OH)2 addition was attributed to the promotion of reactions between pyrolytic
266
carboxylic products and Ca(OH)2 29. Chars derived at elevated temperatures had a lower fraction of
267
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
270
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.
274
As for the case of nitrogen functionality, it could be found that the fraction of pyridine-N in the
275
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
277
pyrrole-N, suggesting that the presence of Ca(OH)2 promoted the transformation of pyrrolic
278
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
281
hydroxide-blended sludge indicated that inorganic N such as ammonium was unlikely to exist in the
282
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
13 ACS Paragon Plus Environment
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 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,
14 ACS Paragon Plus Environment
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
15 ACS Paragon Plus Environment
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 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
16 ACS Paragon Plus Environment
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
17 ACS Paragon Plus Environment
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.
18 ACS Paragon Plus Environment
Page 18 of 27
Page 19 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
419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448
Energy & Fuels
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
19 ACS Paragon Plus Environment
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
20 ACS Paragon Plus Environment
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 -
21 ACS Paragon Plus Environment
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
22 ACS Paragon Plus Environment
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
23 ACS Paragon Plus Environment
-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
24 ACS Paragon Plus Environment
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
25 ACS Paragon Plus Environment
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
Intensity (arbitrary units)
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
Intensity (arbitrary units)
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
26 ACS Paragon Plus Environment
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
27 ACS Paragon Plus Environment