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Influence of carbon dioxide on the thermal degradation process of representative components of combustible solid wastes using TG-MS Jing Gu, Yazhuo Wang, Haoran Yuan, Taoli Huhe, and Yong Chen Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b01226 • Publication Date (Web): 12 Jul 2017 Downloaded from http://pubs.acs.org on July 13, 2017
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Energy & Fuels
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Influence of carbon dioxide on the thermal degradation process of representative
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components of combustible solid wastes using TG-MS
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J. Gu,a,b, Y. Z. Wang,a,b H. R. Yuan,a,b * T. L. Huhe,a,b and Y. Chena,b,c
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a
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Academy of Sciences, Guangzhou 510640, China
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b
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Guangzhou 510640, China
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c
Guangzhou Institute of Energy Conversion, Key Laboratory of Renewable Energy, Chinese
Guangdong Key Laboratory of New and Renewable Energy Research and Development,
Guangzhou division Academy, Chinese Academy of Sciences, Guangzhou 510070, China
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* Corresponding author. Tel.: +86-20-87013240; fax: +86-20-87013240. E-mail address:
[email protected] 1
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Abstract
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The thermal mass loss, gaseous-phase reactions, and the reaction kinetics of five representative
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components of combustible solid wastes (CSWs), i.e., rice straw (RS), eucalyptus wood (EW),
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blank printing paper (BPP), high-density polyethylene (HDPE) and polyvinyl chloride (PVC),
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under argon (Ar) and carbon dioxide (CO2) atmospheres were studied by thermogravimetric–mass
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spectrometry (TG-MS) to observe the effect of CO2 on the thermal degradation process. The CO2
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atmosphere had bigger influence than the Ar atmosphere on the thermal mass loss of CSW; CO2
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affected the main gaseous product distribution, which was attributed to the complex gas reforming
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reaction that occurred in the gaseous phase, and the abundant CO2 promoted the formation of CO
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and CH4. When the temperature was over 650 °C, the CO2 reduction reaction occurred in the solid
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phase and the residue of RS and BPP was mainly silicon, calcium and potassium oxidizers. The
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best-fitting mechanism of the first main mass loss stage of CSW in Ar atmosphere was mainly the
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nucleation and growth model. The best-fitting mechanism of the second main mass loss of
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biomass-based CSWs (BCSWs) under CO2 atmosphere was the diffusion model, and that of
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plastic CSWs (PCSWs) was the reaction order model. Furthermore, dehydrochlorination of PVC
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occurred at 200-384 °C, wherein chlorine atoms were released in different compound forms under
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Ar and CO2 atmospheres. Therefore, the emissions from the heat treatment of CSW under a CO2
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atmosphere presented an improved environmental profile compared to those released from
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pyrolysis under an Ar atmosphere.
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Keywords: carbon dioxide; thermal degradation; combustible solid wastes; TG-MS; kinetics
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analysis
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1 Introduction
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With the development of the economy and the consequent improvement in living standards in
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the cities of China, the production of MSW is increasing precipitously. In 2015, the amount of
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MSW was as high as 191 million tons in China [1], and by 2025, this amount is predicted to reach
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510 million tons
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accounting for approximately 30 % of the total
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components of CSW are biomass and plastic, including vegetation (21 %), paper (15 %),
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polyethylene (PE) (39 %) and polyvinyl chloride (12 %) [4]. Due to the composition of CSW, it can
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be utilized in various waste-to-energy processes, thus, the development of processes that generate
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energy from CSW offers great potential.
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[2]
. Combustible solid waste (CSW) is an important component of MSW, [3]
. In Guangzhou, China, the representative
Several studies have examined the waste-to-energy processes of CSW using thermochemical [5]
, inert gases, i.e.,
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processes in various gaseous reaction media, such as air, i.e., combustion
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pyrolysis
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been suggested as feasible alternatives to combustion to inhibit emissions of dioxin precursors due
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to the reducing conditions, strong reduction of volume and mass, and the improvement of resource
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recovery
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greenhouse gases, has gained significant attention for waste management and energy recovery
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[10-12]
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oxygen, steam and CO2 in different compositions using a bubbling fluidized bed gasifier. Their
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results showed that using CO2 as a gasification agent may be a good option, either by recirculating
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part of the gasification gas or using gas captured from combustion processes. However, research
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on the effects of CO2 on the thermal mass loss, reaction kinetics and real-time analysis of the
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evolved gases of CSW have rarely been reported.
[6]
, and oxygen-enriched air, i.e., gasification
[7]
. Both pyrolysis and gasification have
[8-9]
. The utilization of carbon dioxide (CO2), which is the major component of
. Pinto et al. [13] studied the co-gasification of rice biomass wastes and PE with mixtures of air,
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Thermogravimetric analysis (TG) has been widely used to study the thermal mass loss
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characteristics and reaction kinetics of various solids (coal, biomass, plastic, etc.), which are
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influenced by the heating rate [14], atmosphere [15], pretreatment [16] etc. Lee et al. [17] analyzed the
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thermal degradation of biomass (i.e., red pepper stalk) in N2 and CO2 by TG and found that CO2
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promoted tar destruction due to CO2 reforming reactions and led to different extents of
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carbonization, which modified the physico-chemical properties of the biochar. Moreover, the
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combination of mass spectrometry (MS) and TG (i.e., TG-MS) is one of the best methods to 3
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analyze the evolved gases in real time to study the mechanisms of thermochemical processes [18-19].
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Zhang et al.
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components of MSW under N2 and steam atmospheres and found that the presence of steam
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caused the steam partial oxidation of solid residue, leading to an increasing amount of H2, CO and
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CO2 production in the high temperature range.
[20]
used TG-MS to investigate the thermal decomposition of six representative
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In this work, the thermal mass-loss characteristics and gas evolution data of five
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representative components of CSW under argon (Ar) and CO2 atmospheres were obtained using
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TG-MS, and the surface elemental analysis of the residue were characterized by the scanning
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electron microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS). This study attempted
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to compare their different behaviors with the thermal characteristics to determine the influence of
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CO2 on the thermal mass loss of CSW, the main gaseous product distribution and the residue
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composition, in order to demonstrate that the use of CO2 in the thermal degradation process is
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conducive to decreasing emissions.
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2 Materials and Methods
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2.1 Raw materials
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Rice straw (RS), eucalyptus wood (EW) and blank printing paper (BPP) were selected as
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representative biomass-based CSWs (BCSWs), and high-density polyethylene (HDPE) and
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polyvinyl chloride (PVC) were chosen as the representative plastic CSWs (PCSWs). The results of
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the proximate and ultimate analyses are shown in Table 1.
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Table 1 shows that RS has the highest ash content (~15 %) and the lowest volatile content,
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while EW has the highest fixed carbon, approximately 10 %. EW and BPP contain small amounts
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of sulfur. The oxygen content of the BCSWs is approximately 41-47 % higher than that of the
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PCSWs. HDPE is a thermoplastic resin with the highest volatile content, approximately 99 %.
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PVC is a type of polyethylene-based plastic polymer with a chlorine content of approximately
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56 %, which results in a large difference between its thermal degradation process and that of
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HDPE. These PCSWs contain no ash and a small amount of fixed carbon.
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2.2 Experimental methods All experiments were performed using a STA449 F3 TG analyzer (Erich Netzsch GmbH & 4
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Co., KG., Selb, Germany) coupled with an OMNISTAR GSD301 mass spectrometer (Pfeiffer
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Vacuum Technology AG, Asslar, Germany). A 15 mg sample with 150-200 µm average particle
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size was heated from 40 °C to 1000 °C at a constant heating rate of 30 °C/min with an Ar flow or a
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CO2 flow (100 mL/min). Before heating, the temperature was held at 40 °C for 1 h to remove the
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air and stabilize the baseline of MS. TG and MS data were recorded at the same time during the
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heating process, and a multiple ion detection (MID) model was adopted in the MS. This work
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examined the important ions [21] at 2, 16, 18, 28 and 44 (m/z), designated H2, CH4, H2O, CO and
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CO2, respectively. Moreover, the selected ion at 36.5 (m/z), which mainly accounted for HCl, was
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studied in the thermal process of PVC.
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The residue of BCSWs after the thermal treatment were collected and the scanning electron
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microscopy and energy-dispersive X-ray spectroscopy (SEM/EDS) analyses of residue were
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recorded on a Hitachi S-4800 FESEM/EDAX HORIBA EX-250 instrument.
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2.3 Kinetics model To compare the thermal degradation processes of the representative components of CSW
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[22-24]
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during the thermal processing, the Coats-Redfern method
was used to obtain the apparent
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activation energy, pre-exponential factor, and the best-fitting kinetics model. TG curves recorded automatically by the TG analyzer and Eq. (1) were used to calculate the
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conversion rate of sample ‘x’：
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x (t ) =
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where ‘t’ is the reaction time (in min); ‘x’ is the conversion of samples based on time; ‘m0’ is the
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mass in the thermobalance at time 0 (in mg); ‘m(t)’ is the mass in the thermobalance at a particular
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time in the reaction (in mg); and ‘mf’ is the mass in the thermobalance at the end of the reaction (in
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mg).
(1)
Under non-isothermal conditions, the heat treatment reaction rate of CSW can be written as:
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m0 − m (t) m0 − m f
dx = kf ( x ) dt
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(2)
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where ‘k’ is the Arrhenius speed constant, which can be represented as k =k 0 exp -
Ea . Here, RT
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‘k0’ is the pre-exponential factor (in min-1); ‘Ea’ is the apparent activation energy (in kJ/mol); ‘R’
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is the universal gas constant; and ‘T’ is the temperature (in K). The mathematical model f(x)
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depends on the reaction type or the reaction mechanism and is related only to the extent of the
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reaction (x). Therefore, Eq. (2) can be expressed as:
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dx Ea = k 0 exp( − ) f( x ) dt RT
(3)
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dT = dt Φ
(4)
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dx k0 Ea = exp( − ) f( x ) dT Φ RT
(5)
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where ‘Φ’ is the heating rate (in °C/min). The integrals on both sides of Eq. (5) can be converted
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as follows：
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2 RT Ea k R F(x) ）− ln 2 = ln 0 (1 − Ea RT T Φ Ea
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(6)
Because 2RT/Ea1000
18.78
Ar
216.21-548.25
363.77
73.89
548.25-757.44
733.47
7.27
CO2
218.10-550.95
363.60
73.60
550.95-910.60
883.12
17.78
Ar
406.41-616.33
491.14
97.94
-
CO2
406.80-559.81
496.81
99.63
-
Ar
203.33-388.28
299.24
64.40
388.28-572.18
478.09
27.45
CO2
200.74-383.73
295.54
64.44
383.73-588.65
483.72
27.86
sphere
-
RS 650.0-992.25
922.83
21.68
-
EW
BPP
HDPE
PVC
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Table 3 Different reaction mechanism Mechanism N1 N3/2
Mechanism description
f(x) （1-x）
Nucleation and growth model (n=1) Nucleation and growth model (n=3/2)
F(x) -ln(1-x)
3/2(1-x) [-ln(1-x)]
1/3
[-ln(1-x)]2/3
N2
Nucleation and growth model (n=2)
2(1-x) [-ln(1-x)]1/2
[-ln(1-x)]1/2
N3
Nucleation and growth model (n=3)
3(1-x) [-ln(1-x)]2/3
[-ln(1-x)]1/3
N4
Nucleation and growth model (n=4)
4(1-x) [-ln(1-x)]3/4
[-ln(1-x)]1/4
-1/4
[-ln(1-x)]5/4
N4/5
Nucleation and growth model (n=4/5)
4/5(1-x) [-ln(1-x)]
N6/5
Nucleation and growth model (n=6/5)
5/6(1-x) [-ln(1-x)]-1/5
[-ln(1-x)]6/5
R3/2
Reaction order model (n=3/2)
(1-x)3/2
2[(1-x)-1/2-1]
R2
Reaction order model (n=2)
(1-x)2
(1-x)-1-1
D1
One-dimension diffusion
1/(2x)
x2
D2
Two-dimension diffusion
[-ln(1-x)]-1 3/2[(1-x)
-1/3
-1]
x+(1-x)ln(1-x) -1
(1-2/3x)-(1-x)2/3
D3
Three-dimension diffusion (G-B)
D4
Three-dimension diffusion (Jander)
3/2(1-x)2/3[1-(1-x)1/3]-1
[1-(1-x)1/3]2
D5
Three-dimension diffusion (Z-L-T)
3/2(1-x)4/3[(1-x)-1/3-1]-1
[(1-x)-1/3-1]2
Table 4 Kinetic parameters of CSW in Ar and CO2 atmospheres
Sample
Atmosphere
First main mass loss stage Ea
k0 -1
R2
Second main mass loss stage Mecha
Ea
k0
R2
-1
(KJ/mol)
(min )
Ar
105.14
1.74E+09
0.9925
N4/5
CO2
104.46
1.55E+09
0.9928
N4/5
Ar
130.73
1.31E+11
0.9778
N4/5
CO2
127.86
7.10E+10
0.9856
N4/5
313.27
Ar
164.34
6.57E+13
0.9946
N4/5
CO2
170.16
2.23E+14
0.9942
N4/5
Ar
262.66
1.26E+18
0.9936
N3/2
-
CO2
273.58
4.79E+18
0.9933
N3/2
-
Ar
173.13
3.64E+15
0.9891
R3/2
184.06
CO2
173.23
4.58E+15
0.9867
R3/2
186.25
-nism
(KJ/mol)
(min )
Mechanism
-
RS 322.94
1.01E+13
0.9777
D4
1.64E+13
0.9603
D5
317.57
8.30E+16
0.9700
N4/5
317.49
7.14E+13
0.9809
D2
1.02E+13
0.9810
R3/2
1.51E+13
0.9763
R3/2
-
EW
BPP
HDPE
PVC
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