Pyrolysis and Combustion of Typical Wastes in a Newly Designed

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Pyrolysis and combustion of typical wastes in a newly designed macro-TGA: characteristics and simulation by model components Yanqiu Long, Aihong Meng, Shen Chen, Hui Zhou, Yanguo Zhang, and Qinghai Li Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 25 May 2017 Downloaded from http://pubs.acs.org on May 25, 2017

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Energy & Fuels

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Pyrolysis and combustion of typical wastes in a newly designed

2

macro-TGA: characteristics and simulation by model components

3

Yanqiu Longa, Aihong Menga, Shen Chena, Hui Zhoub, Yanguo Zhanga,1, Qinghai Lia

4

a

5

Department of Thermal Engineering, Tsinghua University, Beijing 100084, P.R. China b

6 7

Key Laboratory for Thermal Science and Power Engineering of Ministry of Education,

Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA

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Abstract

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The pyrolysis and combustion characteristics of three biomass and five model components in a newly

10

designed macro thermogravimetric analyzer (macro-TGA) were compared, respectively. The

11

comparison of model components combustion in TGA and macro-TGA was conducted, too. The

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results showed that the behavior of model components combustion was significantly different between

13

TGA and macro-TGA. In macro-TGA, the maximum peak of DTG curves derived from model

14

components combustion occurred at similar temperature, compared to their pyrolysis; while for

15

biomass, the main peaks appeared earlier in combustion. The biomass could be simulated by main

16

components well both in pyrolysis and combustion in macro-TGA, while five model components

17

(hemicellulose, cellulose, lignin, starch and pectin) simulation fitted the experimental result better than

18

three ones (hemicellulose, cellulose and lignin). However, the fractions differed from each other

19

significantly in different atmosphere simulation.

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Keywords: Pyrolysis; combustion; simulation; macro-TGA; model components.

1. Introduction

21 22

Solid waste, including biomass and municipal solid waste (MSW), has been used as renewable

23

fuels in power or heat generation. Combustion, pyrolysis and gasification were the main thermal

24

processes in utilities. Pyrolysis was thought of the fundamental process of thermochemical utilities. 1

Corresponding author: TEL:+86-10-62783373, FAX:+86-10-6279-8047, E-mail: [email protected] 1

ACS Paragon Plus Environment

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Char, tar and combustible gases are the main products of pyrolysis, which might lead to quick soiling

26

in industrial processes [1]. Oxidative pyrolysis especially combustion would reduce char and tar

27

leading to much more energy intensive products. Generally speaking, hemicellulose accounts for

28

16-23%, cellulose accounts for 42-49%, and lignin accounts for 21-39% of biomass [2]. The other

29

components, such as pectins, starches, nucleic acids and minerals [3] are normally omitted or

30

ignorable for less fraction or complication. Many researchers have studied biomass pyrolysis and

31

combustion based on the three main components of biomass [4-15], in which thermogravimetric

32

analysis (TGA) is the most commonly applied technique, and through it, the continuous mass loss

33

characteristics of samples could be obtained by TGA to understand their pyrolysis or combustion

34

mechanisms. However, the mass of each sample in TGA is normally less than 20 mg. While MSW is

35

normally very complicated and uneven distribution in their components and shapes [16, 17], it’s

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difficult to keep consistent samples in different experiments in such a small quantity of samples. Due

37

to the limited sample mass, studies on pyrolysis and combustion behavior or their impact factors were

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also conducted on fixed bed reactor with a greater mass of samples [6, 18-20].

39

As well known, municipal solid waste (MSW) is a complicated mixture of food residue, paper,

40

plastics and some other components. Even though there was much research about MSW and biomass

41

waste, it is difficult to get universal principles in waste utilities research and design. Due to the

42

complicated properties of waste, it is huge challenging to investigate all real waste, so we aimed to

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establish a pseudo-components simulation system of MSW, in which the behavior of a single MSW

44

sample or MSW mixtures could be simulated by main components based on TG curves, because DTG

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curves are much more sensitive than TG curves to amplify the changes in the TG curves to highlight

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the differences between different experiments. Through the pseudo-components simulation system,

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the thermal behavior of complicated MSW or mixtures could be predicted. To obtain well repeated

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results for samples or their mixture, we developed a fixed-bed reactor system with real-time weight

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function as macro-TGA [21, 22]. The mass of a sample in macro-TGA could be 0.5-4 g, which allows

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us to investigate thermal degradation behavior of real MSW samples or its mixture by a similar way as

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TGA. To get more fundamental data for the simulation system and further research in thermal

2


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Energy & Fuels

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treatment of MSW, studies including pyrolysis, combustion and CO2 gasification of individual

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components or compounds and their mixtures were conducted.

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In our earlier studies, we had conducted some research on pyrolysis or gasification of typical

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waste in TGA [21-24], in which the comparison of pyrolysis in TGA and macro-TGA was conducted,

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as well as pyrolysis and CO2 gasification of components in macro-TGA [22, 23, 25]. The

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pseudo-components simulation of three biomass samples had been investigated to check its feasibility

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of the simulation system in different atmosphere, such as N2 and CO2. In this study, we focused on the

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comparison of thermal degradation in inert gas and oxidative atmosphere in macro-TGA. Comparison

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in biomass simulation by five model components (hemicellulose, cellulose, lignin, starch and pectin)

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and three ones (hemicellulose, cellulose and lignin) was conducted. To deeply understand the

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combustion behavior of components in macro-TGA, their combustion characteristics in TGA was

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analyzed and compared.

64 65

2. Materials and methods 2.1 Materials and their characterization

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Xylan, microcrystalline cellulose and dealkaline lignin were the representation of hemicellulose,

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cellulose and lignin, respectively. Cellulose, hemicellulose, pectin and starch samples were purchased

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from Sigma-Aldrich, and lignin samples from Tokyo Chemical Industry (TCI). The real biomass

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including poplar stem, orange peel and Chinese cabbage were used as feedstock to investigate the

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simulation of their pyrolysis or combustion by the main components. The samples were prepared in a

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granular form with a size of less than 150 µm. Before the experiments, all the samples were dried at

72

105 °C for at least 2h. The proximate and ultimate analyses of the samples were shown in Table 1.

73

Table 1 Proximate and ultimate analyses of the samples. Proximate analyses, % Ad Vd FCd 2.11 78.56 19.33

Cellulose

0.00

95.21

Lignin

16.15

Starch

0.14

Samples Hemicellulose

St,daf 0.01

Ultimate analyses, % Cdaf Hdaf Ndaf 39.18 6.32 0.00

Odaf 54.49

4.79

0.01

44.51

6.25

1.29

47.94

54.60

29.25

5.67

63.86

4.45

0.18

25.84

95.20

4.66

0.19

43.83

5.21

0.20

50.57

3


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Pectin Orange peel Chinese cabbage 74

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3.38 2.91

74.53 76.49

22.09 20.6

0.16 0.19

44.25 48.73

4.97 5.92

0.48 1.43

50.14 43.73

9.91 7.54

67.60 73.85

22.49 18.61

0.61 0.22

47.48 51.36

5.88 5.89

4.11 1.53

41.92 41.00

Poplar stem d: dry basis; daf: dry and ash free basis; St,daf: total sulfide at dry and ash free basis.

75

As seen in Table 1, the volatile in cellulose and starch is the most and their proximate analyses

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are similar. It is corresponding to that cellulose and starch has similar components, in which the

77

glucose molecules are linked via β-1-4 linkages and α-1-4 linkages, respectively. The ash and fixed

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carbon of lignin are the most and the volatile is the least, while those of hemicellulose are between

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those of cellulose and lignin. The proximate analysis of pectin is close to that of hemicellulose, while

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its ultimate analysis looks much more like starch and cellulose. Because of deakaline process, the

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sulfur in lignin is much higher than others. Chinese cabbage shows lowest volatile matter and highest

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ash in the three real biomasses.

83

2.2 Apparatus of pyrolysis and combustion

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As shown in Fig. 1 [22], the macro-TGA system includes carrier gas unit, reaction unit and

85

real-time weight unit. The carrier gas comes from high-pressure gas cylinder with two-stage

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pressure-release valves. In the pyrolysis and combustion of samples, nitrogen and air, respectively, is

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used as the carrier gas with the flow of 1 NL/min, which is fed from the bottom of the quartz tube to

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bring out the gas products immediately. The carrier gas is fed to system to be preheated 1 h before the

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samples feeding.

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The quartz tube is 1500 mm height with i.d. 60 mm, whose outside is surrounded by the tube

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heater with 5 kW. The maximum temperature is 1200 oC with the maximum heating rate of 40 °C/min.

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There are extra zones with 250 mm height of the quartz tube in the top and the bottom out of the

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heating zone, respectively. The tube heater is monitored by the temperature control & data acquisition

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unit.

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Energy & Fuels

1–crucible; 2–quartz tube; 3–metal wire; 4–electronic balance; 5–heater; 6–rotameter; 7– high-pressure gas; 8–outlet. Fig. 1. The schematic system of macro-TGA.

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During the experiments, each of the samples (approximate 1.5 g, except for starch approximate

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0.5 g) was placed manually in the quartz crucible, which was put in the constant temperature zone of

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the quartz tube. The quartz crucible is i.d. 30 mm with 30 mm height, and hanged by the metal wire of

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0.2 mm diameter to the electronic balance. The electronic balance of Metter Toledo ME204E with the

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maximum weight of 220 g and precision of 0.1 mg, is set on the platform top of the reaction unit and

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the weight data is real-time acquireded by the data acquisition system.

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The heating rate was 10 oC/min in all pyrolysis and combustion experiments. Before the

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experiments, the signal background of empty crucible was carried out. The background signal was

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subtracted in every experiment, and the real mass was calculated based on the relationship between

108

temperature and weight. The repeated experiments showed that the macro-TGA had good

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reproducibility.

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The TGA measurements were performed by a Netzsch STA409C in a gas flow of 100 ml/min of

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air or N2 and the heating rate was 10 oC/min.

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2.3 Biomass simulation

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Biomass simulation based on the assumption that thermal degradation of a compound or mixture

114

could be simulated by superposition of TG curves of its components. The detailed method was

115

described in our previous studies [14, 22, 26]. Actually, different compounds are consist of different

116

components, not only the five ones, or not all the five ones. Here, the combustion of biomass was

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simulated by five or three model components to verify this method, and simulation by five

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components and three components was compared.

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The weight loss of biomass could be denoted as m(t), representing that the relative mass m was a

120

function of temperature t. Eq. (1) could be derived according to mass conservation as follows:

121

m = ∑ m

i=1,2,…,n

(1)

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Here, m1, m2, m3 m4 and m5 denoted the hemicellulose mass, cellulose mass, lignin mass, starch

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mass and pectin mass, respectively. Supposing the interaction among m1, m2, m3 , m4 and m5 could be

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neglected，the optimal xi was obtained by the least square method using MATLAB calculation based

125

on m and mi. The constraint condition was that the mass fraction sum was 1, i.e.∑ = 1. When

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simulation by three components was conducted, n=3.

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To evaluate the overlap ratio of two TG curves, N sets of data were exported from the curves and

128

the overlap ratio was determined as follows [14]:

129

∑N ∆m(t ) area sandwiched by two curves R = 1− = 1− total area N

130 131

132

133

Here,

∆m (t )

(2)

denoted the absolute value of difference of two curves corresponding

temperature.

3. Results and discussion 3.1 Comparison of pyrolysis and combustion characteristics

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The pyrolysis and combustion characteristics, TG (in wt%) and DTG (in wt%/°C) curves of

135

the components in TGA and macro-TGA were compared in Fig. 2 and Fig. 3, respectively. Almost all

136

volatile matters were separated out and fixed carbons were burned in combustion, corresponding to

6


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Energy & Fuels

the proximate analyses shown in Table 1.

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In TGA, compared to pyrolysis, combustion of cellulose, starch and pectin looked like

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undergoing pyrolysis firstly, then char residue igniting [1, 15, 27], while it was more complicated for

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hemicellulose and lignin. Combustion of pure cellulose showed that it lost most mass before 500 °C,

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which was similar to the earlier studies [15, 27]. Pyrolysis and combustion of hemicellulose differed

142

from the prior data reported [15]. It suggested that the xylan samples were much more complicated

143

with random and amorphous structure. D. Shen et al.[15] reported that at higher temperature oxygen

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enhanced the mass loss of xylan and increased oxygen shifted the DTG peak to lower temperature. It

145

differed from what we observed. The degradation behavior of lignin in the present study also differed

146

distinctly from that showed in [15], in which Organosolv lignin degraded within 600 °C. It was due to

147

different kinds of lignin exhibit different thermal behavior as shown in our earlier pyrolysis

148

investigation [14]. Even though in combustion there were two obvious DTG peaks for two kinds of

149

lignin, it looked like that two significant DTG peaks in lignin pyrolysis compressed at a much narrow

150

temperature zone but were both amplified in combustion in the present study, as Figure 2 showed.

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Compared to their pyrolysis, there were an additional DTG peak in combustion of starch and pectin,

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while that of pectin combustion occurred lately at 765 °C. It indicated that the char residue of pectin

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was more difficult to degrade. Obviously, the presence of oxygen accelerated and enhanced the

154

decomposition of all component samples. However, behavior of samples differed from each other.

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Except for lignin, the first DTG peak of biomass components combustion occurred earlier or close to

156

that of pyrolysis. All the maximum mass loss rate of combustion increased and the residue decreased

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as shown in Fig. 2.

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158 159

Fig. 2 The TG/DTG curves of pyrolysis and combustion of biomass components in TGA

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In comparison of components pyrolysis and combustion in macro-TGA, their first stage of

161

degradation, normally within 400 °C (except of hemicellulose within 300 °C), matched each other

162

well. Two stages were identified in TG curves for all samples combustion in macro-TGA. The first

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stage of combustion of each component underwent almost the same mass loss as their pyrolysis in TG

164

curves to exhibit similar peak in DTG curves, as shown in Fig. 3. The second stage of combustion

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appeared to follow pyrolysis, which made the whole combustion process looked like ‘pyrolysis +

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combustion of the residue formed’ [29]. However, differed from that in TGA, existence of oxygen

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stimulated the decomposition slowly in macro-TGA so that there was no obvious DTG peak at higher

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temperature except for starch as shown in Fig. 3. In macro-TGA, except for starch whose combustion

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almost finished within 600 °C, combustion of all other samples lasted close to 1000 °C. It might be

170

due to less mass of starch and its drastic volumetric expansion during thermal process, which

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accelerated its reaction to oxygen. An obvious peak exhibited at 485 °C with the mass loss rate of

172

0.35 %/°C when starch combusted in macro-TGA. For other samples, oxygen enhanced the

173

decomposition gradually. For lignin, the peak at around 810 °C was significantly enhanced by oxygen

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and the mass loss rate was 0.28 %/°C, which was 0.13 %/°C in pyrolysis. As observed from Fig. 3,

175

decomposition behavior in macro-TGA of cellulose looked like that of starch, except of shorter

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combustion time for the later, and the maximum DTG peak occurred 35 °C earlier for starch. Similar

177

to this, the exhibition of hemicellulose and pectin looked alike.

40 2 20

60

3

40

2

20

0 0

200

178

400 600 o Temperature ( C)

800

0 1000

1

0 0

200


0 1000

800

5

0.8 100

0.6

0.4

40 0.2 20 0 0

179 180

200


800

Mass (%)

60

4

3

o

Air

Air

80 Mass loss rate (%/ C)

N2

Starch N2

100

Lignin

80

o

4 Mass (%)

Mass (%)

4

60

Mass Loss Rate (%/ C)

5

Air

80

o

Pectin-air

Hemicellulose N2

60 2

40

1

20 0

0.0 1000

0

200

400 600 Temperature (°C)

800

Mass Loss Rate (%/oC)

Cellulose-air Pectin-N2

80

6 100

6


Cellulose-N2

100

Mass (%)

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 1000

Fig. 3 The TG/DTG curves of pyrolysis and combustion of biomass components in macro-TGA.

181

Differed from what exhibited in TGA, the first DTG peak of all components in combustion

182

occurred at the same temperature or very close to that in pyrolysis in macro-TGA. The residue of all

183

samples was almost ash with the existence of oxygen, corresponding to Table 1.

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As shown in Fig. 4, all biomass samples in macro-TGA volatilize in a wider temperature than

185

biomass pyrolysis and combustion in TGA [1, 20]. In comparison to their pyrolysis, the first peak of

186

DTG curves appeared 6-11 °C earlier in combustion, as well as the second peak (17-21 °C earlier),

187

and there was an additional significant peak around 800-1000 °C. Chinese cabbage volatilized in a

188

wider range of temperature than other two biomasses in pyrolysis. Differed from components

189

pyrolysis and combustion, real biomass pyrolysis and combustion was affected by its intrinsic

9


Energy & Fuels

190

structure, components (e.g. main components, minerals), interactions between components, and many

191

other factors. Chinese cabbage began to decompose at around 200 °C might due to inert moisture

192

evaporation. The first peak of pyrolysis and combustion of orange peel and Chinese cabbage at

193

296-303 °C might attribute to hemicellulose or pectin, while that of poplar stem occurred at

194

330-338 °C might relate to starch. The second peak of pyrolysis or combustion DTG curves of orange

195

peel and poplar stem might relate to cellulose or lignin, while that of Chinese cabbage looked like that

196

of starch. As mentioned above, oxygen shifted the DTG peaks of real biomass to lower temperature, as

197

what happened in TGA [28]. 1.4

Orange peel

Chinese cabbage

1.0

0.6 40 0.4 20

80 Mass (%)

60

Air N2

o

0.8


Air N2

80 Mass (%)

100

200

198


800

1.0 0.8

60

0.6 40

0.4

0.2

20

0.0 1000

0

0.2

0 0

1.2 Mass Loss Rate (%/oC)

1.2 100

0.0 0

200


800

1000

1.4 Poplar stem Air N2

1.0

o

80

1.2 Mass Loss Rate (%/ C)

100

Mass (%)

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

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0.8

60

0.6 40 0.4 20

0.2

0 0

200


800

0.0 1000

199 200

Fig. 4 The TG/DTG curves of pyrolysis and combustion of real biomass in macro-TGA.

201

3.2 Comparison of model components combustion in TGA and macro-TGA

202

Pyrolysis of model components in TGA and macro-TGA has been compared and reported in early

203

study [22]. The comparison of components combustion in TGA and macro-TGA was performed, as

204

shown in Fig. 5. All samples began to degrade in TGA earlier than macro-TGA to indicate that there

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205

was heat transfer delay in macro-TGA with or without oxygen [22]. 6 6

Cellulose 5

3

40

Mass (%)

o

80 60

20

2

0

1

20

0 1000

0

-20 0

200

206


800

3

40

2 1

0

200


800

1.8

5

1.4

60

0.8 40

0.6 0.4

20

Mass (%)

Mass (%)

o

1.2 1.0

60

0

200

207


800

4

3

40 2 20 1

0

0.2 0

macro-TGA TGA

80

o

macro-TGA TGA

80

0 1000

Starch

100

1.6

Lignin

Mass loss rate (%/ C)

100

4

o

macro-TGA TGA


4

60 Mass (%)

Hemicellulose 5

Mass loss rate (%/ C)

macro-TGA TGA

80

100


100

0.0 1000

-20 0

200


800

0 1000

7 Pectin

6

macro-TGA TGA

5

o

80


100

Mass (%)

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

4

60

3 40 2 20

1

0 0

200


800

0 1000

208 209

Fig. 5 The TG/DTG curves of combustion of biomass components in TGA and macro-TGA.

210

The residue of combustion in macro-TGA was less than that in TGA to indicate their combustion

211

completed deeply, as Table 2 showed. As shown in Fig. 5, similar to their pyrolysis [22], with the

212

existence of oxygen, the DTG peaks in macro-TGA were also shifted to higher temperature in

213

comparison to those in TGA, while the shape of DTG curves of hemicellulose and lignin differed

214

significantly between macro-TGA and TGA. For cellulose, combustion in TGA finished within 600 °C

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215

and the maximum mass loss rate of 3.791 %/°C appeared at 328 °C; while that in macro-TGA took

216

place at the whole experiment and the maximum delayed approximate 60 °C. The delay was due to

217

heat transfer, alike to pyrolysis. Even though there was thermal clustering to inhibit the degradation of

218

the first stage for cellulose, oxygen exhibited stronger promotion on its ignition and combustion. For

219

hemicellulose, it took similar time to burn out in TGA and macro-TGA, but TG curve from TGA could

220

be regarded as several segments according to its slope. Besides a shoulder peak around 250 °C, there

221

were both two obvious peaks for lignin combustion in TGA and macro-TGA. But it took short time to

222

burn out for lignin in TGA to indicate oxygen promoted lignin degradation dramatically. The starch

223

degradation was shifted to higher temperature from TGA to macro-TGA; however, the char

224

combustion stage was promoted and accelerated in macro-TGA. Oxygen also promoted the

225

degradation of pectin in macro-TGA, but it looked like that char was ignited and burnt suddenly at

226

720-780 °C in TGA.

227

Table 2 The DTG characteristics and residue of model components combustion. Sample

DTG1 o

(wt%/ C)

TGA macro-TGA TGA Hemicellulose macro-TGA TGA Lignin macro-TGA TGA Starch macro-TGA TGA Pectin macro-TGA As mentioned in early study Cellulose

228

App.

T1(oC)

DTG2 o

(wt%/ C)

T2(oC)

3.791 328 0.102 1.852 387 0.069 0.673 246 0.663 3.592 300 0.246 0.552 406 0.534 0.285 391 0.281 2.591 316 0.368 2.299 351 0.35 1.373 236 0.775 3.758 289 0.08 [22, 25], in macro-TGA heat

DTG3 (wt%/oC)

T3(oC)

495 609 282 0.281 321 0.085 459 816 481 485 765 382 0.105 transfer shifted the

475 937

937 DTG

Residue (wt%) 2.25 0.05 5.69 3.91 18.97 18.52 6.56 0 7.21 3.16 peaks to

229

higher temperature and main peak temperature of components between TGA and macro-TGA

230

exhibited linear relationship on pyrolysis and CO2 gasification. But as mentioned above, with the

231

existence of oxygen, the main peaks temperature of components combustion varied complicated

232

between TGA and macro-TGA. As Table 2 showed, except of lignin, the maximum DTG peak

233

temperature of other components combustion in macro-TGA delayed 35-59 °C, compared to those in

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234

TGA. For lignin, even though oxygen showed dramatic promotion in its whole decomposition, the

235

first DTG peak delayed more than 60 °C in combustion in TGA, as Fig. 2 showed. Furthermore, the

236

temperature delay for its pyrolysis in macro-TGA was 40 °C . While compared to its pyrolysis in

237

macro-TGA, the delay of lignin combustion in macro-TGA was about 10 °C. Thus, it looked

238

unusually that the first DTG peak occurred earlier for combustion in macro-TGA.

239

3.3 Biomass simulation and its comparison

240

Because the components combustion behavior, TG and DTG characteristics in TGA significantly

241

and irregularly differed from their pyrolysis, which made it difficult to simulate and compare, here we

242

focused on the simulation of reactions in macro-TGA. Biomass pyrolysis or combustion was regarded

243

to relate to their main components. Biomass simulation was based on linear combination of TG curves

244

of the five model components and three ones (cellulose, hemicellulose and lignin), respectively. The

245

comparison of simulated curves and experimental curves was shown in Fig. 6. The simulation results

246

were shown in Table 3. As shown in Table 3, the overlap ratios of all samples were higher than 0.98,

247

indicating that they could be expressed and simulated by main components well. 100

100

Orange peel, N2 80

Orange peel, Air

60

40

20

60 40 20

0 0

248 249

Exp. Cal.5 Cal.3

80

Exp. Cal.5 Cal.3

Mass (%)

Mass (%)

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

200


800

1000

0 0

200

(a)

13



800

1000

Energy & Fuels

100 100

Chinese cabbage, N2 80

40 20

60

40

20

0

0 0

250 251

Exp. Cal.5 Cal.3

80

Mass (%)

Mass (%)

Chinese cabbage, Air

Exp. Cal.5 Cal.3

60

200


800

0

1000

200

400

600

800

1000

o

Temperature ( C)

(b) 100

100

Poplar stem, N2

Poplar stem, Air

Exp. Cal.5 Cal.3

80

Exp. Cal.5 Cal.3

80

60

Mass (%)

Mass (%)

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 18

40

20

60

40

20

0 0

200


800

1000

0 0

200

400

600

800

1000

o

Temperature ( C)

252 253 254

(c) Fig. 6 The simulation of biomass combustion and pyrolysis by main components: (a) Orange peel; (b)

255

Chinese cabbage; (c) poplar stem.

256

Table 3 The simulation of biomass combustion and pyrolysis in macro-TGA by main components. Sample

Atm. N2

Orange peel Air

Chinese cabbage

N2 Air N2

Poplar stem Air 257

mi

Overlap ratio

Cellulose 0.0881

Hemicellulose 0.2151

Lignin 0.1554

Starch 0.2194

Pectin 0.3220

0.1894

0.6695

0.1411

/

/

0.9863

0.0000 0.1873 0.0000 0.0829 0.0000 0.0107 0.2565

0.2752 0.7026 0.1888 0.5596 0.3172 0.5659 0.1860

0.1888 0.1101 0.3694 0.3575 0.4376 0.4234 0.2994

0.2156 / 0.1974 / 0.0452 / 0.2581

0.3204 / 0.2444 / 0.2000 / 0.0000

0.9907 0.9871 0.9850 0.9810 0.9861 0.9844 0.9878

0.4311

0.2922

0.2767

/

/

0.9850

0.0504 0.3434

0.1614 0.2940

0.4721 0.3626

0.3160 /

0.0000 /

0.9904 0.9857

/: not include.

14


0.9904

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Energy & Fuels

258

As observed from Fig.6, there were obvious differences between experimental TG curves and

259

fitted curves around 250-290 °C in orange peel simulation for both simulation methods, especially for

260

its pyrolysis. For Chinese cabbage, the mass difference around 170-220 °C was stable but it increased

261

sharply around 220-290 °C between experiment and simulation in its pyrolysis; there was similar

262

appearance around 230-290 °C in its combustion. In macro-TGA, moisture evaporated at around

263

200 °C because of heat transfer delay suggesting that there was inert water in Chinese cabbage more

264

or less. It suggested that there might be something decomposed earlier than hemicellulose and pectin

265

in fruit or vegetable waste like orange peel and Chinese cabbage. Furthermore, the mass loss of

266

Chinese cabbage above 800 °C in fitted curve was faster than that in TG curve might due to more

267

mass fraction of lignin in simulation. More cellulose in simulation resulted in faster mass loss around

268

390-500 °C, while less lignin made it slowly around 700-850 °C in poplar stem pyrolysis, compared to

269

experimental result. The obvious difference between two simulation methods appeared around

270

300-400 °C. In comparison of simulation results shown in Table 3, when the simulation was

271

conducted by three components, mass fractions of cellulose and hemicellulose increased significantly,

272

while lignin reduced. For orange peel, when simulation conducted by three components, mass lost

273

faster than experiment result and five components simulation around 300-350 °C, while it began

274

slowly around 350-400 °C. It might be due to much more hemicellulose fraction, which was more

275

than the sum of hemicellulose and pectin in five components simulation. Similar characteristics

276

exhibited on Chinese cabbage pyrolysis; while it was better for Chinese cabbage combustion

277

simulation. For poplar stem, mass loss of three components simulation deviated that of experiment

278

and five ones simulation around 300-390 °C due to more hemicellulose faction, too. Compared the

279

simulation results by five components and three ones, the difference between simulation and

280

experiment were less for five components than that of three ones, especially around 300-400 °C, as

281

shown in Fig. 6. It was supported by the overlap ratios. The overlap ratios from three components

282

simulation were less than those from five components simulation, as Table 3 showed. In much

283

pyrolysis or combustion, biomass decomposition mechanisms were analyzed based on cellulose,

284

hemicellulose and lignin [4, 5, 7, 30, 31]. As mentioned above, the decomposition behavior of

15


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

285

cellulose and starch looked alike, as well as hemicellulose and pectin. It indicated that three main

286

components (cellulose, hemicellulose and lignin) simulation could be applied to predict pyrolysis and

287

combustion behavior of biomass in macro-TGA. However, the various detailed characteristics brought

288

to more deviation. Undoubtedly, introduction of more main components, i.e. starch and pectin, could

289

help to refine the TG characteristics and increase simulation accuracy.

290

Actually, there were significant differences in mass fraction of main components between

291

pyrolysis and combustion of each sample. For different analysis or data basis, they also varied from

292

each other during decomposing in same atmosphere [22, 25], then the unified data were important. It

293

was because the simulation was only based on TG curves to seek the optimum overlap ratio. As

294

mentioned above, at N2 and air atmosphere, the behavior of different samples changed in various ways.

295

Because mass loss characteristics of different samples, especially component materials and real

296

biomass, varied differently in different atmosphere, TG or DTG characteristics of real biomass would

297

appear to match different components. For instance, the maximum DTG peak of orange peel pyrolysis

298

occurred at 296 °C, which was between that of hemicellulose (302 °C) and pectin (291 °C). While that

299

of its combustion appeared at 290 °C, which was much close to that of pectin (289 °C). The case of

300

Chinese cabbage pyrolysis and combustion was similar. The various mass loss changes between

301

pyrolysis and combustion of various samples would lead to difference in simulation results as Table 3

302

showed. Thus, the simulation results, mass fraction of pyrolysis and combustion was obvious different.

303

Furthermore, when the mass fraction of components derived from pyrolysis was applied to the

304

simulation of combustion of a biomass sample, the overlap ratio would decrease obviously. It was

305

because that the model components were not real ones. The detracted process of model components

306

would induce more or less deviation. Thus, the model components were also called

307

pseudo-components. However, in previous studies, even though the mass fraction of

308

pseudo-components varied more or less in different cases through optimum overlap ratio seeking, the

309

simulation results of biomass pyrolysis in TGA could be applied to macro-TGA based on the

310

pseudo-components model [22, 25], as well as pyrolysis and CO2 gasification in macro-TGA. It

311

indicated that the pseudo-components simulation results could be applied across in inert atmosphere

16


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Energy & Fuels

312

without bringing much more difference in simulating curves, while it was difficult to get coincident

313

results between inert and oxidative atmosphere. However, the simulation results could still help to

314

analyze and understand pyrolysis or combustion of a compound sample.

315

4. Conclusions

316

The behavior of mass loss of components and biomass combustion was significantly different

317

between TGA and macro-TGA. In TGA combustion, hemicellulose and lignin exhibited different

318

characteristics compared to pyrolysis. The maximum DTG peak from components combustion

319

occurred at similar temperature to their pyrolysis in macro-TGA. While for biomass, they appeared

320

6-21°C earlier. Biomass could be simulated by main components well, not matter pyrolysis or

321

combustion, and the simulation conducted by five model components (hemicellulose, cellulose, lignin,

322

starch and pectin) matched the experimental result better than by three ones (hemicellulose, cellulose

323

and lignin).

324 325

Acknowledgements The financial support from National Natural Science Foundation of China (No. 21376134) is

326

gratefully acknowledged.

327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347

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Pyrolysis and Combustion of Typical Wastes in a Newly Designed

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