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O2 atmosphere from the perspective of TGA
Energy 36 (2011) 819e824
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A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA ZhiYi Lai*, XiaoQian Ma, YuTing Tang, Hai Lin School of Electric Power, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 15 December 2010 Accepted 16 December 2010 Available online 20 January 2011
In this paper, the combustion behavior of municipal solid waste (MSW) is carried out in a thermogravimetric analyzer under different N2/O2 and CO2/O2 atmospheres with temperature ranging from 100 C to 1000 C. TG (thermogravimetric) and DTG (derivative thermogravimetric) curves are analyzed. The nth order reaction fitting model is used to yield the activation energy of reduction process according to the degree of conversion. The results indicate that all samples lose most their weight between 200 C and 540 C. As the oxygen concentration increased, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are smaller than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere. With the increase of heating rate, the maximum weight loss rates of samples increase obviously. The three-step reaction of nth order reaction model fits the weight loss very well. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: MSW Oxygen-enriched combustion Thermogravimetry
1. Introduction With China’s accelerating urbanization, MSW (municipal solid waste) disposal has become a big trouble for all cities during urban development. The amount of MSW treated in China was 152 million tons in 2007. Three disposal methods, namely landfill, composting and incineration, account for 81.7%, 2.7% and 15.6%, respectively. Among these methods, waste incineration has become a major development trend of China’s MSW treatment owing to its characteristics of hygienic control, volume reduction, and energy recovery [1e4]. As of 2007, 67 MSW incineration plants had been established in China, with a processing capacity of 45800 t/d. Most of the waste incineration plants are located in coastal regions. CO2/O2 combustion technology is considered to be one of the most promising combustion technologies because of its easy CO2 recovery, low NOx emission and high sulfation efficiency. Liu et al. [5] found that sintering was much mitigated during direct sulfation of limestone in O2/CO2 coal combustion, and the diffusivity in the product layer demonstrated high temperature dependence and hardly changed with sulfation degree. Croiset et al. [6] found that the atmosphere had some impact on the NOx emission rate and the conversion of SO2. Singh et al. [7] made a techno-economic
* Corresponding author. Tel.: þ86 20 87110232; fax: þ86 20 87110613. E-mail address: [email protected] (Z. Lai). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.12.033
comparison of the performance of CO2 capture between amine scrubbing and O2/CO2 combustion, and found O2/CO2 combustion was a more attractive choice than MEA scrubbing. Liu et al. [8] found that the conversion ratio from fuel-N to exhausted NO in O2/CO2 pulverized coal combustion was only about one-fourth of conventional pulverized coal combustion. Liu et al. [9] found that up to 88 and 92% reductions of the recycled NO could be achieved with coal combustion in air and in 30% O2/70% CO2, respectively. Chen et al. [10] argued that CO2 concentration in both O2/CO2 and O2/recycled fuel gas combustion were much higher than that in O2/N2 combustion, while NOx concentration in O2/N2 combustion was higher than that in O2/CO2 or O2/recycled fuel gas combustion. Bejarano et al. [11] found that coal particles burned at higher mean temperatures and cost shorter combustion times in O2/N2 than in O2/CO2 environments at analogous oxygen mole fractions. Sheng et al. [12] found that O2/CO2 combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. Duan et al. [13] argued that replacing N2 with CO2 increased SO2 releasing rate. SO2 emission increased first and then decreased as O2 fraction increases in the O2/CO2 combustion. Li et al. [14] found that a higher O2 concentration in O2/CO2 atmosphere was needed to achieve the similar combustion characteristic to that in O2/N2 atmosphere. However, most literature focus on the coal combustion not MSW combustion. Oxygen-enriched combustion technology can effectively improve the combustion efficiency and reduce pollutant emissions [15e19],
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and it has also been successfully applied to some practical projects [20e22]. Several researchers have already studied combustion and pyrolysis characteristics of MSW by thermogravimetric analysis (TGA) [23e28]. Shen et al. [29] found that different catalysts exerted obvious influence on the MSW combustion. Several authors have studied the behavior of the co-combustion of coal and waste [30e34]. Murphy et al. [35] and Zdena et al. [36] proposed an integrated system to treat the MSW. Liu et al. [37] studied the MSW oxygen-enriched combustion characteristics in N2/O2 atmosphere. However, there are few literature expounding on the differences between combustion characteristics of MSW in CO2/O2 atmosphere and those in N2/O2 atmosphere. It is, therefore, significant to research the combustion of MSW in the two atmospheres. In this work, TGA is employed to investigate the combustion characteristics of MSW in N2/O2 and CO2/O2 atmosphere at oxygen concentration of 20%, 30%, 50%, 60% and 80%, respectively. The characteristics of MSW oxygen-enriched combustion in different atmospheres are discussed. The nth order reaction fitting model is used to yield kinetic parameters.
Table 2 Composition of original MSW on as received basis (wt%). Combustible
Noncombustible
Paper
Plastic
Leather
Cloth
Wood
Food waste
Metal
Glass
Sand
11.55
10.71
23.95
11.09
0.75
38.00
0.10
0.00
3.85
produced during combustion is ignored based on the following considerations. Firstly, with the evolution of combustion the CO2 production increases. The CO2 concentration of production varies as the temperature increases. Secondly, even if the sample is 6.5 mg (maximum mass), the amount of CO2 production is small the CO2 concentration of production is just 7.092 ml, while the mixed gas flow rate is 100 ml/min. Thirdly, what we can control is the gas we add into the STA 409 PC Luxx simultaneous thermal analyzer, and the gas should be kept constant in order to reduce error. The error percentage will be less than 1%. 2.2. Kinetic theory
2. Experimental
The degree of conversion of the reduction process is expressed as
2.1. Materials and measurements
a ¼
The experimental material of MSW was collected from Huizhou, Guangdong Province, China. The proximate analysis of original MSW is listed in Table 1. Metal, sand, paper, plastic, leather, cloth, wood, food waste were selected from the MSW as the test samples. The components of the MSW on received basis are listed in Table 2. The lower heating value of the MSW is 6726.6 kJ/kg. The components are dried, broken, grinded and screened. The sizes of all particles of the components were less than 1 mm. After treatment, all components were mixed together according to their original percentages. The samples were dried at 105 C for 3e4 h and stored in desiccators. The proximate analysis and ultimate analysis of the samples are shown in Table 3. The initial weight of the sample for all runs was 6 0.5 mg. The TGA was carried out on a NETZSCH STA 409 PC Luxx simultaneous thermal analyzer. The weight precision for the STA 409 is 0.01 mg. In the STA system, the sample was heated in a micro-furnace enclosed by a cooling jacket. Water is used as a cooling agent. The sample temperature is measured with a type S (PteRh10/Pt) thermocouple directly under the Al2O3 crucible. The control thermocouple, also type S, is integrated in the heating coil of the furnace. The sample weight was measured accurately using the sensor connected to the sample carrier by the connection pole underneath the sample carrier. The flow rate of the mixed gas was 100 ml/min. Several experiments without samples were carried out and used as a background in order to subtract the buoyancy effect. At heating rate of 20 C/min, samples were performed in different atmospheres: N2/O2 ¼ 8:2, N2/O2 ¼ 7:3, N2/O2 ¼ 5:5, N2/O2 ¼ 4:6, N2/O2 ¼ 2:8, CO2/O2 ¼ 8:2, CO2/O2 ¼ 7:3, CO2/O2 ¼ 5:5, CO2/O2 ¼ 4:6 and CO2/O2 ¼ 2:8. At heating rate of 10 and 40 C/min, samples were performed in the following atmospheres: N2/O2 ¼ 8:2, N2/O2 ¼ 7:3, CO2/O2 ¼ 8:2, CO2/O2 ¼ 7:3. MSW combustion would produce CO2 in N2/O2 atmosphere or CO2/O2 atmosphere. But the concentration of additional CO2 Table 1 Proximate analysis of original MSW on as received basis (wt%).
m0 mt m0 mN
(1)
where m0 is the initial mass of the sample, mt the mass of the sample at time t, and mN the final mass of the sample in that reaction. The kinetic equation of common type can be generally written as follows:
da ¼ kðTÞf ðaÞP n ðO2 Þ dt
(2)
where a is the conversion degree of sample, t is time, T is the absolute temperature, f(a) is a function, whose type depends on the reaction mechanism, P(O2) is the oxygen partial pressure, n is the power dependency of the oxygen partial pressure, and k(T) is the temperature dependent rate constant, which is usually described by the Arrhenius equation.
E k ¼ k0 exp RT
(3)
where k0 is pre-exponential or frequency factor, E is the activation energy, and R is the universal gas constant. There are different methods to carry out the analysis of kinetic data. According to the mathematical model, there are two possible approaches: model-fitting and isoconversional (free model) methods. In non-isothermal kinetics, the Friedman (FR), Kissingere AkahiraeSunose (KAS), FlynneWalleOzawa (FWO) and Vyazovkin (V) methods are the most popular representatives of the isoconversional methods. In model-fitting method, nth order reaction bases on the following equation:
da E ¼ A$ð1 aÞn $exp j dt a¼ai RT
(4)
Table 3 Ultimate and proximate analysis of MSW samples (wt%).
Proximate analysis
Ultimate analysis
Proximate analysis
Moisture
Volatile matter
Fixed carbon
Ash
C
H
O
N
S
Cl
Volatile matter
Fixed carbon
Ash
51.87
27.44
5.87
14.82
58.45
6.86
30.54
1.36
2.49
0.30
57.00
12.19
30.81
Z. Lai et al. / Energy 36 (2011) 819e824
a
Conversion (%)
Table 4 Results from themogravimetric analysis for MSW under different N2/O2 atmosphere.
1.0 0.8 0.6
N2/O2=2/8 N2/O2=8/2
N2/O2=4/6
N2/O2=7/3 0.4
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C)
0 -2 char
DTG (%/min)
-4 -6
plastic
-8
N2/O2=8/2 N2/O2=7/3
-10 -12
N2/O2=5/5 N2/O2=4/6
-14 -16
Sample
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
O2 ¼ 20% O2 ¼ 30% O2 ¼ 50% O2 ¼ 60% O2 ¼ 80%
272.2 271.9 270.9 269.0 268.8
732.7 729.0 727.3 726.1 714.2
12.24 12.96 14.51 15.08 17.54
310.8 305.9 304.4 302.4 298.6
N2/O2=5/5
0.2
b
821
volatile
N2/O2=2:8
-18 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 1. Combustion of MSW under different N2/O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
3. Results and discussion 3.1. Effects of oxygen concentration in N2/O2 atmosphere on combustion The TG signal is first transformed into the conversion variable. Fig. 1(a) and (b) shows the conversion rate curves and DTG (derivative thermogravimetric) curves of MSW with different oxygen concentration in the N2/O2 atmosphere at 20 C/min heating rate. Table 4 shows the results of TGA for MSW under different N2/O2 atmosphere. Tv is the onset temperature for volatile release. Final combustion temperature (Tf) is defined as the temperature of 99% conversion. DTGmax is maximum weight loss rate. TDTGmax is the temperature associated to DTGmax. As shown in Fig. 1(a), as the oxygen concentration increases, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape, indicating that MSW has a similar regular pattern in oxygen-enriched combustion. After 200 C, the sample has a significant weight loss. All samples lost most their weight in the temperature range from 200 C to 540 C, accounting for 94% of the total weight loss. About 6% weight loss occupies the temperature range of 540e1000 C. Fig. 1(b) shows the DTG curves of the TG, and the different steps of the decomposition process are more easily distinguished. As the temperature rises, there are four weight loss peaks in the DTG curves. The weight loss peaks centered on 305 C, 380 C and 465 C are obvious, while the weight loss peaks centered on 710 are relatively small. The first peak is attributed to the volatile release.
The second peak is mainly for plastic and third peaks is mainly for char combustion [27,38], while the last peak may corresponds to ash pyrolysis. Li et al. [31] find that for MSW combustion in air, a small hump appears within the ranges of 650e800 C, and explain it by the fixed carbon combustion or the gasification and combustion of the low reactive combustible materials. Further studies on the exact origin of this peak are needed. As shown in Fig. 1(b), the maximum weight loss rate appears between 200 C and 350 C, and at this temperature range, the oxygen concentration has an obvious effect on the reaction rate. As the oxygen concentration increases from 20% to 80% in N2/O2 atmosphere, the maximum weight loss rate increases from 12.24%/min to 17.54%/min. 3.2. Effects of oxygen concentration in CO2/O2 atmosphere on combustion Fig. 2(a) and (b) shows the conversion rate curves and DTG curves of MSW combustion with different oxygen concentration in the CO2/O2 atmosphere at 20 C/min heating rate. Table 5 shows the results of TGA for MSW under different atmospheres. As shown in Fig. 2(a) and (b), MSW conversion rate curves and DTG curves have an analogous trend in the CO2/O2 atmosphere and in the N2/O2 atmosphere. As Fig. 2(a) shows, with the oxygen content increases, conversion rate curve shifts to the lower temperature gradually. The final weight loss temperature appears between 850 C and 920 C. The main conversion focuses on 200e540 C, taking up about 94% of the total conversion. There is a significant offset between 20 and 30% oxygen concentration conversion rate curves, while the differences among conversion rate curves of 30, 50, 60 and 80% oxygen concentration are unconspicuous. Four obvious weight loss peaks are showed in Fig. 2(b). The first three peaks appear at 305 C, 390 C, 465 C, respectively. The weight loss rate reaches the maximum at the first weight loss peak, while the fourth weight loss peaks are unconspicuous. The oxygen concentration has a strong influence on the volatile release and combustion stage. Higher oxygen concentration enhances the volatile release and benefits ignition. Fig. 2(c) shows the DTG curves in the temperature from 800 C to 950 C. As the Fig. 2(c) shows, with increase of oxygen concentration, the fourthly weight loss peak shifts to a lower temperature obviously. The weight loss peak is 35 C lower in CO2/O2 ¼ 2/8 than that in CO2/O2 ¼ 4/6, 15 C lower in CO2/O2 ¼ 4/6 than that in CO2/ O2 ¼ 5/5, and 10 C lower in CO2/O2 ¼ 7/3 than that in CO2/O2 ¼ 8/2. The offset range prone to a lower temperature is progressively larger as the oxygen concentration increases. 3.3. The comparison of weight loss between N2/O2 and CO2/O2 atmosphere As shown in Fig. 3, the conversion rate curves and weight loss curves of 20% oxygen concentration between N2/O2 and CO2/O2 atmosphere are compared. Weight loss of the samples in N2/O2 and
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Z. Lai et al. / Energy 36 (2011) 819e824
a
1.0
1.0
0.8
0.8
-16
Conversion
0.6 0.4
CO2/O2=8/2 CO2/O2=7/3
0.2
CO2/O2=5/5 CO2/O2=4/6
-10
0.6
N2/O2=8/2 CO2/O2=8/2
0.4
-8 -6 -4
0.2
CO2/O2=2:8
-2
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature
b
-12 DTG (%/min)
Conversion (%)
-14
0 0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 3. Combustion of MSW under 20% oxygen concentration between N2/O2 and CO2/ O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
0 -2
DTG (%/min)
-4
loss peak. The difference of DTG curves in two different atmospheres before 600 C is not obvious, while after 600 C, the weight loss is obviously suppressed in CO2/O2 atmosphere. Perhaps because the high concentration of CO2 causes reduction of the reaction rate at this stage. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are smaller than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere.
char
-6
CO2/O2=8/2 CO2/O2=7/3
plastic
-8
CO2/O2=5/5 CO2/O2=4/6
-10 -12
CO2/O2=2:8
-14 volatile
-16
100 200 300 400 500 600 700 800 900 1000 Temperature
c
0.0
a
0.8
Conversion
-0.2 -0.4 -0.6 CO2/O2=8:2 CO2/O2=2:8 -0.8 CO2/O2=4:6 -1.0 800
850 Temperature
900
CO2/O2 atmosphere both focuses on a temperature range of 200e540 C. After 600 C, the beginning of weight loss in N2/O2 atmosphere is much earlier than that in CO2/O2 atmosphere. This variation can also be got by the appearance orders of the weight
0.4
10°C / min 20°C / min 40°C / min
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature
950
Fig. 2. Combustion of MSW under different CO2/O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
0.6
0.2
CO2/O2=7:3 CO2/O2=5:5
b
0
-5 DTG (%/min)
DTG (%/min)
1.0
-10
-15
10°C /min 20°C /min 40°C /min
Table 5 Results from themogravimetric analysis for MSW under different CO2/O2 atmosphere. Sample
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
O2 ¼ 20% O2 ¼ 30% O2 ¼ 50% O2 ¼ 60% O2 ¼ 80%
273.4 271.6 271.1 269.9 269.0
920.3 914.1 896.9 887.6 850.1
11.67 13.42 14.13 15.36 16.34
310.4 307.1 304.4 301.7 300.1
-20 100 200 300 400 500 600 700 800 900 1000 Temperature Fig. 4. Combustion of MSW under CO2/O2 ¼ 80/20 at b ¼ 10, 20 and 40 C/min: (a) conversion rate curves; and (b) DTG curves.
Z. Lai et al. / Energy 36 (2011) 819e824 Table 6 Results from themogravimetric analysis for MSW at different b under CO2/O2 ¼ 80/20 atmosphere.
10 C/min 20 C/min 30 C/min
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
263.0 272.4 280.9
919.2 920.3 925.2
5.88 11.67 20.84
297.5 310.4 322.8
3.4. Effect of heating rate on weight loss
4. Kinetic analysis NETZSCH-TA4-Kinetic2 software is used to study kinetic mechanism of MSW weight loss by employing nth order reaction fitting model. When using a single heating rate curve to study the loss mechanism model, there may have several groups of different models which all have a high degree of matching to the same experimental data. Burnham [39] argues that kinetic analysis using single heating rate method should no longer be considered acceptable in the thermal analysis community. In this paper, the data of N2/O2 ¼ 80/20, N2/O2 ¼ 70/30, CO2/O2 ¼ 80/20, CO2/O2 ¼ 70/30 at different b (10 C/min, 20 C/min and 40 C/min) are selected to be studied. The correlation coefficient is used to characterize the degree of matching between fitting results and experimental date. The fitquality is demonstrated by correlation coefficient. The sum of least squares is defined as: N X
wi ðYOi YRi Þ2
(5)
i¼1
Table 7 The kinetic parameters and correlation coefficients under nth order reaction model. Experimental N2/O2 ¼ 80/20 CO2/O2 ¼ 80/20 N2/O2 ¼ 70/30 CO2/O2 ¼ 70/30 condition 10.692 log A1/s1 E1 (kJ/mol) 137.871 Reaction 2.584 order 1 3.794 log A2/s1 E2 (kJ/mol) 80.180 Reaction 1.361 order 2 3 1 5.956 log A /s E3 (kJ/mol) 155.170 Reaction 0.319 order 3 Correlation 0.999864 coefficient
19.084 209.443 3.137
14.694 169.771 2.992
7.291 102.264 1.297
11.456 145.744 1.465
12.918 159.632 2.320
19.743 115.963 1.206
2.277 58.302 2.119
5.125 93.028 2.996
6.565 112.136 2.867
0.999647
0.8 0.6 0.4 0.2
The conversion rate curves and DTG curves at different heating rate (b) (10 C/min, 20 C/min and 40 C/min) in CO2/O2 ¼ 80/20 atmosphere are shown in Fig. 4(a) and (b). Table 6 shows the results of TGA for MSW under different b. The samples lose most their weight between 200 C and 540 C. In this temperature range, with the increase of b, the curves of conversion rate shift to a higher temperature. Fig. 4(b) shows that there are four weight loss peaks in the DTG profile in 10 C/min and 20 C/min, while in 40 C/min, there are only three weight loss peaks. This may result from the reaction overlap in 400e600 C temperature range under 40 C/min condition. The maximum weight loss rate increases from 5.88%/min to 20.84%/min when the heating rate varies from 10 C/min to 40 C/min. Therefore, the heating rate could affect the MSW combustion obviously.
SAQ ¼
1.0
Conversion (%/min)
Sample
0.999443
0.999432
823
the real value of 10 /min the real value of 20 /min the real value of 40 /min the calculated value
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 5. The comparison of the calculated conversion with those real in experiments.
where wi is the weight at point i. YOi stand for the calculated value at point i. YRi is the value of the real at point i (i is from 1 to N). The correlation coefficient:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u SAQ u CorrCoeff ¼ u1 u P 2 P t N N 2 N YO YO i i i¼1 i¼1
(6)
Table 7 shows the kinetic parameters and correlation coefficient under nth order reaction model. Fig. 5 shows the comparison between the calculated conversion and the real conversion in experiments. It serves as evidence that nth order reaction model could fit the weight loss very well. Several researchers study kinetic of the MSW combustion by different methods. Liu et al. [37] used three independent parallel reactions to model. The values of the activation energy obtained by Liu et al. were around 65.6, 55.0, and 7.4 kJ/mol for three reactions. Muthuraman et al. [40] divided the combustion into two regions and obtained the activation energy around 93.3 and 278.7 kJ/mol, respectively. Sanchez et al. [41] used the isoconversional methods to yield the organic fraction of MSW. The activation energy was around 173.96 kJ/mol. The variations among the various studies may result from the differences in material, operating condition and applications of kinetic method. 5. Conclusion The oxygen-enriched combustion characteristics and kinetic behavior of the MSW are studied at different N2/O2 and CO2/O2 atmospheres. The following conclusions can be drawn as a result of TGA and kinetic study. 1. All the samples lose most their weight between 200 C and 540 C. With the increase of oxygen concentration, the maximum weight loss rates of samples increase. The conversion rate curves and DTG curves shift to lower temperature without significant change in its shape as the oxygen concentration increases. 2. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are lower than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere. 3. Higher heating rate results in higher weight loss rate. In the b of 10 and 20 C/min, there are four peaks during the combustion;
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Z. Lai et al. / Energy 36 (2011) 819e824
in the b of 40 C/min, there are three. This may result from the reaction overlap in 400e600 C temperature range. 4. The three-step reaction of nth order reaction model could fit the weight loss very well. Acknowledgements The authors are grateful to the support given by the National Basic Research Program of China (973 Program) (2011CB201501) and Guangdong Key Laboratory of Clean Energy Technology (2008A060301002). References [1] Cherubini F, Bargigli S, Ulgiati S. Life cycle assessment (LCA) of waste management strategies: landfilling, sorting plant and incineration. Energy 2009;34(12):2116e23. [2] Baggio P, Baratieri M, Gasparella A, longo GA. Energy and environmental analysis of an innovative system based on municipal solid waste (MSW) pyrolysis and combined cycle. Appl Therm Eng 2008;28(2e3):136e44. [3] Shen BX, Lei Q. Study on MSW catalytic combustion by TGA. Energy Convers Manage 2006;47(11e12):1429e37. [4] Goh YR, Lim CN, Zakaria R, Chan KH, Reynolds G, Yang YB, et al. Mixing, modelling and measurements of incinerator bed combustion. Process Safety Environ Prot 2000;78(B1):21e32. [5] Liu H, Katagiri S, Kaneko U, Okazak K. Sulfation behavior of limestone under high CO2 concentration in O2/CO2 coal combustion. Fuel 2000;79(8):945e53. [6] Croiset E, Thambimuthu KV. NOx and SO2 emissions from O2/CO2 recycle coal combustion. Fuel 2001;80(14):2117e21. [7] Singh D, Croiset E, Douglas PL, Douglas MA. Techno-economic study of CO2 capture from an existing coal-fired power plant: MEA scrubbing vs. O2/CO2 recycle combustion. Energy Convers Manage 2003;44(19):3073e91. [8] Liu H, Okazaki K. Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation. Fuel 2003;82(11):1427e36. [9] Liu H, Zailani R, Gibbs BA. Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle. Fuel 2005;84(16):2109e15. [10] Chen JC, Liu ZS, Huang JS. Emission characteristics of coal combustion in different O2/N2, O2/CO2 and O2/RFG atmosphere. J Hazard Mater 2007;142 (1e2):266e71. [11] Bejarano PA, Levendis YA. Single-coal-particle combustion in O2/N2 and O2/CO2 environments. Combustion Flame 2008;153(1e2):270e87. [12] Sheng CD, Li Y. Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures. Fuel 2008;87(7):1297e305. [13] Duan LB, Zhao CS, Zhou W, Liang C, Chen XP. Sulfur evolution from coal combustion in O2/CO2 mixture. J Anal Appl Pyrol 2009;86(2):269e73. [14] Li QZ, Zhao CS, Chen XP, Wu WF, Lin BQ. Properties of char particles obtained under O2/N2 and O2/CO2 combustion environments. Chem Eng Process 2010;49(5):449e59. [15] Luo SY, Xiao B, Hu ZQ, liu SM, Guan YW. Experimental study on oxygenenriched combustion of biomass micro fuel. Energy 2009;34(11):1880e4. [16] Lacava PT, Carvalho JA, Pimenta AP, Ferreira MA. Thermal analysis of an enriched flame incinerator for aqueous residues. Energy 2006;31(4):528e45. [17] Muller-Roosen M, Ringel H. Studies on the high-temperature incineration of polymer-containing special waste with oxygen enrichment. Chem Eng Technol 2000;23(12):1069e72.
[18] Gohlke O, Busch M. Reduction of combustion by-products in WTE plants: O2 enrichment of underfire air in the MARTIN SYNCOM process. Chemosphere 2001;42(5e7):545e50. [19] Burdyny T, Struchtrup H. Hybrid membrane/cryogenic separation of oxygen from air for use in the oxy-fuel process. Energy 2010;35(5):1884e97. [20] Takuma M, Kaihara Y, Yamada A. Contribution of waste to energy technology to global warming. Tech Rev 2004;41(4):1e5. [21] Liszka M, Ziebik A. Coal-fired oxy-fuel power unit e process and system analysis. Energy 2010;35(2):943e51. [22] Hong JS, Chaudhry G, Brisson JG, Field R, Gazzino M, Ghoniem AF. Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor. Energy 2009;34(9):1332e40. [23] Buah WK, Cunliffe AM, Williams PT. Characterization of products from the pyrolysis of municipal solid waste. Process Safety Environ Prot 2007;85 (B5):450e7. [24] Wu CH, Chang CY, Tseng CH. Pyrolysis products of uncoated printing and writing paper of MSW. Fuel 2002;81(6):719e25. [25] Wu CH, Chang CY, Tseng CH, Lin JP. Pyrolysis product distribution of waste newspaper in MSW. J Anal Appl Pyrol 2003;67(1):41e53. [26] García A, Marcilla A, Font R. Thermogravimetric kinetic study of the pyrolysis of municipal solid waste. Thermochim Acta 1995;254:277e304. [27] Grammelis P, Basinas P, Malliopoulou A, Sakellaropoulos G. Pyrolysis kinetics and combustion characteristics of waste recovered fuels. Fuel 2009;88 (1):195e205. [28] Sorum L, Gronli MG, Hustad JE. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 2001;80(9):1217e27. [29] Shen BX, Wu CF, Qin L. MSW catalytic combustion by alkali and alkali-earth salts. Energy 2006;31(14):2900e14. [30] Xiao HM, Ma XQ, Lai ZY. Isoconversional kinetic analysis of co-combustion of sewage sludge with straw and coal. Appl Energy 2009;86(9):1741e5. [31] Li XG, Ma BG, Li X, Hu ZW, Wang XG. Thermogravimetric analysis of the co-combustion of the blends with high ash coal and waste tyres. Thermochim Acta 2006;441(1):79e83. [32] Lu R, Purushothama S, Yang XD, Hyatt J, Pan WP, Riley JT, et al. TG/FTIR/MS study of organic compounds evolved during the co-firing of coal and refusederived fuels. Fuel Process Technol 1999;59(1):35e50. [33] Otero M, Diez C, Calvo LF, Garcia AI, Moran A. Analysis of the co-combustion of sewage sludge and coal by TGeMS. Biomass Bioenergy 2002;22(4):319e29. [34] Folgueras AB, Diaz RM, Xiberta J, Prieto I. Thermogravimetric analysis of the co-combustion of coal and sewage sludge. Fuel 2003;82(15e17):2051e5. [35] Murphy JD, McKeogh E. The benefits of integrated treatment of wastes for the production of energy. Energy 2006;31(2e3):294e310. [36] Zsigraiova Z, Tavares G, Semiao V, Carvalho MD. Integrated waste-to-energy conversion and waste transportation within island communities. Energy 2009;34(5):623e35. [37] Liu GH, Ma XQ, Yu ZS. Experimental and kinetic modeling of oxygen-enriched air combustion of municipal solid waste. Waste Manage 2009;29(2):792e6. [38] Muthuraman M, Namioka T, Yoshikawa K. A comparison of co-combustion characteristics of coal with wood and hydrothermally treated municipal solid waste. Bioresour Technol 2010;101(7):2477e82. [39] Burnham AK. Computational aspects of kinetic analysis. Part D: the ICTAC kinetics project e multi-thermal-history model-fitting methods and their relation to isoconversional methods. Thermochim Acta 2000;355(1e2): 165e70. [40] Muthuraman M, Namioka T, Yoshikawa K. A comparative study on cocombustion performance of municipal solid waste and Indonesian coal with high ash Indian coal: a thermogravimetric analysis. Fuel Process Technol 2010;91(5):550e8. [41] Sanchez ME, Otero M, Gomez X, Moran A. Thermogravimetric kinetic analysis of the combustion of biowastes. Renew Energy 2009;34(6):1622e7.
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A study on municipal solid waste (MSW) combustion in N2/O2 and CO2/O2 atmosphere from the perspective of TGA ZhiYi Lai*, XiaoQian Ma, YuTing Tang, Hai Lin School of Electric Power, South China University of Technology, Guangzhou 510640, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 February 2010 Received in revised form 15 December 2010 Accepted 16 December 2010 Available online 20 January 2011
In this paper, the combustion behavior of municipal solid waste (MSW) is carried out in a thermogravimetric analyzer under different N2/O2 and CO2/O2 atmospheres with temperature ranging from 100 C to 1000 C. TG (thermogravimetric) and DTG (derivative thermogravimetric) curves are analyzed. The nth order reaction fitting model is used to yield the activation energy of reduction process according to the degree of conversion. The results indicate that all samples lose most their weight between 200 C and 540 C. As the oxygen concentration increased, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are smaller than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere. With the increase of heating rate, the maximum weight loss rates of samples increase obviously. The three-step reaction of nth order reaction model fits the weight loss very well. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: MSW Oxygen-enriched combustion Thermogravimetry
1. Introduction With China’s accelerating urbanization, MSW (municipal solid waste) disposal has become a big trouble for all cities during urban development. The amount of MSW treated in China was 152 million tons in 2007. Three disposal methods, namely landfill, composting and incineration, account for 81.7%, 2.7% and 15.6%, respectively. Among these methods, waste incineration has become a major development trend of China’s MSW treatment owing to its characteristics of hygienic control, volume reduction, and energy recovery [1e4]. As of 2007, 67 MSW incineration plants had been established in China, with a processing capacity of 45800 t/d. Most of the waste incineration plants are located in coastal regions. CO2/O2 combustion technology is considered to be one of the most promising combustion technologies because of its easy CO2 recovery, low NOx emission and high sulfation efficiency. Liu et al. [5] found that sintering was much mitigated during direct sulfation of limestone in O2/CO2 coal combustion, and the diffusivity in the product layer demonstrated high temperature dependence and hardly changed with sulfation degree. Croiset et al. [6] found that the atmosphere had some impact on the NOx emission rate and the conversion of SO2. Singh et al. [7] made a techno-economic
* Corresponding author. Tel.: þ86 20 87110232; fax: þ86 20 87110613. E-mail address: [email protected] (Z. Lai). 0360-5442/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.energy.2010.12.033
comparison of the performance of CO2 capture between amine scrubbing and O2/CO2 combustion, and found O2/CO2 combustion was a more attractive choice than MEA scrubbing. Liu et al. [8] found that the conversion ratio from fuel-N to exhausted NO in O2/CO2 pulverized coal combustion was only about one-fourth of conventional pulverized coal combustion. Liu et al. [9] found that up to 88 and 92% reductions of the recycled NO could be achieved with coal combustion in air and in 30% O2/70% CO2, respectively. Chen et al. [10] argued that CO2 concentration in both O2/CO2 and O2/recycled fuel gas combustion were much higher than that in O2/N2 combustion, while NOx concentration in O2/N2 combustion was higher than that in O2/CO2 or O2/recycled fuel gas combustion. Bejarano et al. [11] found that coal particles burned at higher mean temperatures and cost shorter combustion times in O2/N2 than in O2/CO2 environments at analogous oxygen mole fractions. Sheng et al. [12] found that O2/CO2 combustion did not significantly change the mineral phases formed in the residue ashes, but did affect the relative amounts of the mineral phases. Duan et al. [13] argued that replacing N2 with CO2 increased SO2 releasing rate. SO2 emission increased first and then decreased as O2 fraction increases in the O2/CO2 combustion. Li et al. [14] found that a higher O2 concentration in O2/CO2 atmosphere was needed to achieve the similar combustion characteristic to that in O2/N2 atmosphere. However, most literature focus on the coal combustion not MSW combustion. Oxygen-enriched combustion technology can effectively improve the combustion efficiency and reduce pollutant emissions [15e19],
820
Z. Lai et al. / Energy 36 (2011) 819e824
and it has also been successfully applied to some practical projects [20e22]. Several researchers have already studied combustion and pyrolysis characteristics of MSW by thermogravimetric analysis (TGA) [23e28]. Shen et al. [29] found that different catalysts exerted obvious influence on the MSW combustion. Several authors have studied the behavior of the co-combustion of coal and waste [30e34]. Murphy et al. [35] and Zdena et al. [36] proposed an integrated system to treat the MSW. Liu et al. [37] studied the MSW oxygen-enriched combustion characteristics in N2/O2 atmosphere. However, there are few literature expounding on the differences between combustion characteristics of MSW in CO2/O2 atmosphere and those in N2/O2 atmosphere. It is, therefore, significant to research the combustion of MSW in the two atmospheres. In this work, TGA is employed to investigate the combustion characteristics of MSW in N2/O2 and CO2/O2 atmosphere at oxygen concentration of 20%, 30%, 50%, 60% and 80%, respectively. The characteristics of MSW oxygen-enriched combustion in different atmospheres are discussed. The nth order reaction fitting model is used to yield kinetic parameters.
Table 2 Composition of original MSW on as received basis (wt%). Combustible
Noncombustible
Paper
Plastic
Leather
Cloth
Wood
Food waste
Metal
Glass
Sand
11.55
10.71
23.95
11.09
0.75
38.00
0.10
0.00
3.85
produced during combustion is ignored based on the following considerations. Firstly, with the evolution of combustion the CO2 production increases. The CO2 concentration of production varies as the temperature increases. Secondly, even if the sample is 6.5 mg (maximum mass), the amount of CO2 production is small the CO2 concentration of production is just 7.092 ml, while the mixed gas flow rate is 100 ml/min. Thirdly, what we can control is the gas we add into the STA 409 PC Luxx simultaneous thermal analyzer, and the gas should be kept constant in order to reduce error. The error percentage will be less than 1%. 2.2. Kinetic theory
2. Experimental
The degree of conversion of the reduction process is expressed as
2.1. Materials and measurements
a ¼
The experimental material of MSW was collected from Huizhou, Guangdong Province, China. The proximate analysis of original MSW is listed in Table 1. Metal, sand, paper, plastic, leather, cloth, wood, food waste were selected from the MSW as the test samples. The components of the MSW on received basis are listed in Table 2. The lower heating value of the MSW is 6726.6 kJ/kg. The components are dried, broken, grinded and screened. The sizes of all particles of the components were less than 1 mm. After treatment, all components were mixed together according to their original percentages. The samples were dried at 105 C for 3e4 h and stored in desiccators. The proximate analysis and ultimate analysis of the samples are shown in Table 3. The initial weight of the sample for all runs was 6 0.5 mg. The TGA was carried out on a NETZSCH STA 409 PC Luxx simultaneous thermal analyzer. The weight precision for the STA 409 is 0.01 mg. In the STA system, the sample was heated in a micro-furnace enclosed by a cooling jacket. Water is used as a cooling agent. The sample temperature is measured with a type S (PteRh10/Pt) thermocouple directly under the Al2O3 crucible. The control thermocouple, also type S, is integrated in the heating coil of the furnace. The sample weight was measured accurately using the sensor connected to the sample carrier by the connection pole underneath the sample carrier. The flow rate of the mixed gas was 100 ml/min. Several experiments without samples were carried out and used as a background in order to subtract the buoyancy effect. At heating rate of 20 C/min, samples were performed in different atmospheres: N2/O2 ¼ 8:2, N2/O2 ¼ 7:3, N2/O2 ¼ 5:5, N2/O2 ¼ 4:6, N2/O2 ¼ 2:8, CO2/O2 ¼ 8:2, CO2/O2 ¼ 7:3, CO2/O2 ¼ 5:5, CO2/O2 ¼ 4:6 and CO2/O2 ¼ 2:8. At heating rate of 10 and 40 C/min, samples were performed in the following atmospheres: N2/O2 ¼ 8:2, N2/O2 ¼ 7:3, CO2/O2 ¼ 8:2, CO2/O2 ¼ 7:3. MSW combustion would produce CO2 in N2/O2 atmosphere or CO2/O2 atmosphere. But the concentration of additional CO2 Table 1 Proximate analysis of original MSW on as received basis (wt%).
m0 mt m0 mN
(1)
where m0 is the initial mass of the sample, mt the mass of the sample at time t, and mN the final mass of the sample in that reaction. The kinetic equation of common type can be generally written as follows:
da ¼ kðTÞf ðaÞP n ðO2 Þ dt
(2)
where a is the conversion degree of sample, t is time, T is the absolute temperature, f(a) is a function, whose type depends on the reaction mechanism, P(O2) is the oxygen partial pressure, n is the power dependency of the oxygen partial pressure, and k(T) is the temperature dependent rate constant, which is usually described by the Arrhenius equation.
E k ¼ k0 exp RT
(3)
where k0 is pre-exponential or frequency factor, E is the activation energy, and R is the universal gas constant. There are different methods to carry out the analysis of kinetic data. According to the mathematical model, there are two possible approaches: model-fitting and isoconversional (free model) methods. In non-isothermal kinetics, the Friedman (FR), Kissingere AkahiraeSunose (KAS), FlynneWalleOzawa (FWO) and Vyazovkin (V) methods are the most popular representatives of the isoconversional methods. In model-fitting method, nth order reaction bases on the following equation:
da E ¼ A$ð1 aÞn $exp j dt a¼ai RT
(4)
Table 3 Ultimate and proximate analysis of MSW samples (wt%).
Proximate analysis
Ultimate analysis
Proximate analysis
Moisture
Volatile matter
Fixed carbon
Ash
C
H
O
N
S
Cl
Volatile matter
Fixed carbon
Ash
51.87
27.44
5.87
14.82
58.45
6.86
30.54
1.36
2.49
0.30
57.00
12.19
30.81
Z. Lai et al. / Energy 36 (2011) 819e824
a
Conversion (%)
Table 4 Results from themogravimetric analysis for MSW under different N2/O2 atmosphere.
1.0 0.8 0.6
N2/O2=2/8 N2/O2=8/2
N2/O2=4/6
N2/O2=7/3 0.4
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C)
0 -2 char
DTG (%/min)
-4 -6
plastic
-8
N2/O2=8/2 N2/O2=7/3
-10 -12
N2/O2=5/5 N2/O2=4/6
-14 -16
Sample
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
O2 ¼ 20% O2 ¼ 30% O2 ¼ 50% O2 ¼ 60% O2 ¼ 80%
272.2 271.9 270.9 269.0 268.8
732.7 729.0 727.3 726.1 714.2
12.24 12.96 14.51 15.08 17.54
310.8 305.9 304.4 302.4 298.6
N2/O2=5/5
0.2
b
821
volatile
N2/O2=2:8
-18 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 1. Combustion of MSW under different N2/O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
3. Results and discussion 3.1. Effects of oxygen concentration in N2/O2 atmosphere on combustion The TG signal is first transformed into the conversion variable. Fig. 1(a) and (b) shows the conversion rate curves and DTG (derivative thermogravimetric) curves of MSW with different oxygen concentration in the N2/O2 atmosphere at 20 C/min heating rate. Table 4 shows the results of TGA for MSW under different N2/O2 atmosphere. Tv is the onset temperature for volatile release. Final combustion temperature (Tf) is defined as the temperature of 99% conversion. DTGmax is maximum weight loss rate. TDTGmax is the temperature associated to DTGmax. As shown in Fig. 1(a), as the oxygen concentration increases, conversion rate curves and DTG curves shift to lower temperature without significant change in its shape, indicating that MSW has a similar regular pattern in oxygen-enriched combustion. After 200 C, the sample has a significant weight loss. All samples lost most their weight in the temperature range from 200 C to 540 C, accounting for 94% of the total weight loss. About 6% weight loss occupies the temperature range of 540e1000 C. Fig. 1(b) shows the DTG curves of the TG, and the different steps of the decomposition process are more easily distinguished. As the temperature rises, there are four weight loss peaks in the DTG curves. The weight loss peaks centered on 305 C, 380 C and 465 C are obvious, while the weight loss peaks centered on 710 are relatively small. The first peak is attributed to the volatile release.
The second peak is mainly for plastic and third peaks is mainly for char combustion [27,38], while the last peak may corresponds to ash pyrolysis. Li et al. [31] find that for MSW combustion in air, a small hump appears within the ranges of 650e800 C, and explain it by the fixed carbon combustion or the gasification and combustion of the low reactive combustible materials. Further studies on the exact origin of this peak are needed. As shown in Fig. 1(b), the maximum weight loss rate appears between 200 C and 350 C, and at this temperature range, the oxygen concentration has an obvious effect on the reaction rate. As the oxygen concentration increases from 20% to 80% in N2/O2 atmosphere, the maximum weight loss rate increases from 12.24%/min to 17.54%/min. 3.2. Effects of oxygen concentration in CO2/O2 atmosphere on combustion Fig. 2(a) and (b) shows the conversion rate curves and DTG curves of MSW combustion with different oxygen concentration in the CO2/O2 atmosphere at 20 C/min heating rate. Table 5 shows the results of TGA for MSW under different atmospheres. As shown in Fig. 2(a) and (b), MSW conversion rate curves and DTG curves have an analogous trend in the CO2/O2 atmosphere and in the N2/O2 atmosphere. As Fig. 2(a) shows, with the oxygen content increases, conversion rate curve shifts to the lower temperature gradually. The final weight loss temperature appears between 850 C and 920 C. The main conversion focuses on 200e540 C, taking up about 94% of the total conversion. There is a significant offset between 20 and 30% oxygen concentration conversion rate curves, while the differences among conversion rate curves of 30, 50, 60 and 80% oxygen concentration are unconspicuous. Four obvious weight loss peaks are showed in Fig. 2(b). The first three peaks appear at 305 C, 390 C, 465 C, respectively. The weight loss rate reaches the maximum at the first weight loss peak, while the fourth weight loss peaks are unconspicuous. The oxygen concentration has a strong influence on the volatile release and combustion stage. Higher oxygen concentration enhances the volatile release and benefits ignition. Fig. 2(c) shows the DTG curves in the temperature from 800 C to 950 C. As the Fig. 2(c) shows, with increase of oxygen concentration, the fourthly weight loss peak shifts to a lower temperature obviously. The weight loss peak is 35 C lower in CO2/O2 ¼ 2/8 than that in CO2/O2 ¼ 4/6, 15 C lower in CO2/O2 ¼ 4/6 than that in CO2/ O2 ¼ 5/5, and 10 C lower in CO2/O2 ¼ 7/3 than that in CO2/O2 ¼ 8/2. The offset range prone to a lower temperature is progressively larger as the oxygen concentration increases. 3.3. The comparison of weight loss between N2/O2 and CO2/O2 atmosphere As shown in Fig. 3, the conversion rate curves and weight loss curves of 20% oxygen concentration between N2/O2 and CO2/O2 atmosphere are compared. Weight loss of the samples in N2/O2 and
822
Z. Lai et al. / Energy 36 (2011) 819e824
a
1.0
1.0
0.8
0.8
-16
Conversion
0.6 0.4
CO2/O2=8/2 CO2/O2=7/3
0.2
CO2/O2=5/5 CO2/O2=4/6
-10
0.6
N2/O2=8/2 CO2/O2=8/2
0.4
-8 -6 -4
0.2
CO2/O2=2:8
-2
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature
b
-12 DTG (%/min)
Conversion (%)
-14
0 0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 3. Combustion of MSW under 20% oxygen concentration between N2/O2 and CO2/ O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
0 -2
DTG (%/min)
-4
loss peak. The difference of DTG curves in two different atmospheres before 600 C is not obvious, while after 600 C, the weight loss is obviously suppressed in CO2/O2 atmosphere. Perhaps because the high concentration of CO2 causes reduction of the reaction rate at this stage. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are smaller than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere.
char
-6
CO2/O2=8/2 CO2/O2=7/3
plastic
-8
CO2/O2=5/5 CO2/O2=4/6
-10 -12
CO2/O2=2:8
-14 volatile
-16
100 200 300 400 500 600 700 800 900 1000 Temperature
c
0.0
a
0.8
Conversion
-0.2 -0.4 -0.6 CO2/O2=8:2 CO2/O2=2:8 -0.8 CO2/O2=4:6 -1.0 800
850 Temperature
900
CO2/O2 atmosphere both focuses on a temperature range of 200e540 C. After 600 C, the beginning of weight loss in N2/O2 atmosphere is much earlier than that in CO2/O2 atmosphere. This variation can also be got by the appearance orders of the weight
0.4
10°C / min 20°C / min 40°C / min
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature
950
Fig. 2. Combustion of MSW under different CO2/O2 atmosphere at 20 C/min: (a) conversion rate curves; and (b) DTG curves.
0.6
0.2
CO2/O2=7:3 CO2/O2=5:5
b
0
-5 DTG (%/min)
DTG (%/min)
1.0
-10
-15
10°C /min 20°C /min 40°C /min
Table 5 Results from themogravimetric analysis for MSW under different CO2/O2 atmosphere. Sample
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
O2 ¼ 20% O2 ¼ 30% O2 ¼ 50% O2 ¼ 60% O2 ¼ 80%
273.4 271.6 271.1 269.9 269.0
920.3 914.1 896.9 887.6 850.1
11.67 13.42 14.13 15.36 16.34
310.4 307.1 304.4 301.7 300.1
-20 100 200 300 400 500 600 700 800 900 1000 Temperature Fig. 4. Combustion of MSW under CO2/O2 ¼ 80/20 at b ¼ 10, 20 and 40 C/min: (a) conversion rate curves; and (b) DTG curves.
Z. Lai et al. / Energy 36 (2011) 819e824 Table 6 Results from themogravimetric analysis for MSW at different b under CO2/O2 ¼ 80/20 atmosphere.
10 C/min 20 C/min 30 C/min
Tv ( C)
Tf ( C)
DTGmax (%/min)
TDTGmax ( C)
263.0 272.4 280.9
919.2 920.3 925.2
5.88 11.67 20.84
297.5 310.4 322.8
3.4. Effect of heating rate on weight loss
4. Kinetic analysis NETZSCH-TA4-Kinetic2 software is used to study kinetic mechanism of MSW weight loss by employing nth order reaction fitting model. When using a single heating rate curve to study the loss mechanism model, there may have several groups of different models which all have a high degree of matching to the same experimental data. Burnham [39] argues that kinetic analysis using single heating rate method should no longer be considered acceptable in the thermal analysis community. In this paper, the data of N2/O2 ¼ 80/20, N2/O2 ¼ 70/30, CO2/O2 ¼ 80/20, CO2/O2 ¼ 70/30 at different b (10 C/min, 20 C/min and 40 C/min) are selected to be studied. The correlation coefficient is used to characterize the degree of matching between fitting results and experimental date. The fitquality is demonstrated by correlation coefficient. The sum of least squares is defined as: N X
wi ðYOi YRi Þ2
(5)
i¼1
Table 7 The kinetic parameters and correlation coefficients under nth order reaction model. Experimental N2/O2 ¼ 80/20 CO2/O2 ¼ 80/20 N2/O2 ¼ 70/30 CO2/O2 ¼ 70/30 condition 10.692 log A1/s1 E1 (kJ/mol) 137.871 Reaction 2.584 order 1 3.794 log A2/s1 E2 (kJ/mol) 80.180 Reaction 1.361 order 2 3 1 5.956 log A /s E3 (kJ/mol) 155.170 Reaction 0.319 order 3 Correlation 0.999864 coefficient
19.084 209.443 3.137
14.694 169.771 2.992
7.291 102.264 1.297
11.456 145.744 1.465
12.918 159.632 2.320
19.743 115.963 1.206
2.277 58.302 2.119
5.125 93.028 2.996
6.565 112.136 2.867
0.999647
0.8 0.6 0.4 0.2
The conversion rate curves and DTG curves at different heating rate (b) (10 C/min, 20 C/min and 40 C/min) in CO2/O2 ¼ 80/20 atmosphere are shown in Fig. 4(a) and (b). Table 6 shows the results of TGA for MSW under different b. The samples lose most their weight between 200 C and 540 C. In this temperature range, with the increase of b, the curves of conversion rate shift to a higher temperature. Fig. 4(b) shows that there are four weight loss peaks in the DTG profile in 10 C/min and 20 C/min, while in 40 C/min, there are only three weight loss peaks. This may result from the reaction overlap in 400e600 C temperature range under 40 C/min condition. The maximum weight loss rate increases from 5.88%/min to 20.84%/min when the heating rate varies from 10 C/min to 40 C/min. Therefore, the heating rate could affect the MSW combustion obviously.
SAQ ¼
1.0
Conversion (%/min)
Sample
0.999443
0.999432
823
the real value of 10 /min the real value of 20 /min the real value of 40 /min the calculated value
0.0 100 200 300 400 500 600 700 800 900 1000 Temperature (°C) Fig. 5. The comparison of the calculated conversion with those real in experiments.
where wi is the weight at point i. YOi stand for the calculated value at point i. YRi is the value of the real at point i (i is from 1 to N). The correlation coefficient:
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u SAQ u CorrCoeff ¼ u1 u P 2 P t N N 2 N YO YO i i i¼1 i¼1
(6)
Table 7 shows the kinetic parameters and correlation coefficient under nth order reaction model. Fig. 5 shows the comparison between the calculated conversion and the real conversion in experiments. It serves as evidence that nth order reaction model could fit the weight loss very well. Several researchers study kinetic of the MSW combustion by different methods. Liu et al. [37] used three independent parallel reactions to model. The values of the activation energy obtained by Liu et al. were around 65.6, 55.0, and 7.4 kJ/mol for three reactions. Muthuraman et al. [40] divided the combustion into two regions and obtained the activation energy around 93.3 and 278.7 kJ/mol, respectively. Sanchez et al. [41] used the isoconversional methods to yield the organic fraction of MSW. The activation energy was around 173.96 kJ/mol. The variations among the various studies may result from the differences in material, operating condition and applications of kinetic method. 5. Conclusion The oxygen-enriched combustion characteristics and kinetic behavior of the MSW are studied at different N2/O2 and CO2/O2 atmospheres. The following conclusions can be drawn as a result of TGA and kinetic study. 1. All the samples lose most their weight between 200 C and 540 C. With the increase of oxygen concentration, the maximum weight loss rates of samples increase. The conversion rate curves and DTG curves shift to lower temperature without significant change in its shape as the oxygen concentration increases. 2. At the same oxygen concentration, the peak values in CO2/O2 atmosphere are lower than those in N2/O2 atmosphere, indicating that CO2 has a higher inhibitory effect than N2 on MSW combustion. After 600 C, the weight loss peak appears much later in CO2/O2 atmosphere than it does in N2/O2 atmosphere. 3. Higher heating rate results in higher weight loss rate. In the b of 10 and 20 C/min, there are four peaks during the combustion;
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in the b of 40 C/min, there are three. This may result from the reaction overlap in 400e600 C temperature range. 4. The three-step reaction of nth order reaction model could fit the weight loss very well. Acknowledgements The authors are grateful to the support given by the National Basic Research Program of China (973 Program) (2011CB201501) and Guangdong Key Laboratory of Clean Energy Technology (2008A060301002). References [1] Cherubini F, Bargigli S, Ulgiati S. Life cycle assessment (LCA) of waste management strategies: landfilling, sorting plant and incineration. Energy 2009;34(12):2116e23. [2] Baggio P, Baratieri M, Gasparella A, longo GA. Energy and environmental analysis of an innovative system based on municipal solid waste (MSW) pyrolysis and combined cycle. Appl Therm Eng 2008;28(2e3):136e44. [3] Shen BX, Lei Q. Study on MSW catalytic combustion by TGA. Energy Convers Manage 2006;47(11e12):1429e37. [4] Goh YR, Lim CN, Zakaria R, Chan KH, Reynolds G, Yang YB, et al. Mixing, modelling and measurements of incinerator bed combustion. Process Safety Environ Prot 2000;78(B1):21e32. [5] Liu H, Katagiri S, Kaneko U, Okazak K. Sulfation behavior of limestone under high CO2 concentration in O2/CO2 coal combustion. Fuel 2000;79(8):945e53. [6] Croiset E, Thambimuthu KV. NOx and SO2 emissions from O2/CO2 recycle coal combustion. Fuel 2001;80(14):2117e21. [7] Singh D, Croiset E, Douglas PL, Douglas MA. Techno-economic study of CO2 capture from an existing coal-fired power plant: MEA scrubbing vs. O2/CO2 recycle combustion. Energy Convers Manage 2003;44(19):3073e91. [8] Liu H, Okazaki K. Simultaneous easy CO2 recovery and drastic reduction of SOx and NOx in O2/CO2 coal combustion with heat recirculation. Fuel 2003;82(11):1427e36. [9] Liu H, Zailani R, Gibbs BA. Pulverized coal combustion in air and in O2/CO2 mixtures with NOx recycle. Fuel 2005;84(16):2109e15. [10] Chen JC, Liu ZS, Huang JS. Emission characteristics of coal combustion in different O2/N2, O2/CO2 and O2/RFG atmosphere. J Hazard Mater 2007;142 (1e2):266e71. [11] Bejarano PA, Levendis YA. Single-coal-particle combustion in O2/N2 and O2/CO2 environments. Combustion Flame 2008;153(1e2):270e87. [12] Sheng CD, Li Y. Experimental study of ash formation during pulverized coal combustion in O2/CO2 mixtures. Fuel 2008;87(7):1297e305. [13] Duan LB, Zhao CS, Zhou W, Liang C, Chen XP. Sulfur evolution from coal combustion in O2/CO2 mixture. J Anal Appl Pyrol 2009;86(2):269e73. [14] Li QZ, Zhao CS, Chen XP, Wu WF, Lin BQ. Properties of char particles obtained under O2/N2 and O2/CO2 combustion environments. Chem Eng Process 2010;49(5):449e59. [15] Luo SY, Xiao B, Hu ZQ, liu SM, Guan YW. Experimental study on oxygenenriched combustion of biomass micro fuel. Energy 2009;34(11):1880e4. [16] Lacava PT, Carvalho JA, Pimenta AP, Ferreira MA. Thermal analysis of an enriched flame incinerator for aqueous residues. Energy 2006;31(4):528e45. [17] Muller-Roosen M, Ringel H. Studies on the high-temperature incineration of polymer-containing special waste with oxygen enrichment. Chem Eng Technol 2000;23(12):1069e72.
[18] Gohlke O, Busch M. Reduction of combustion by-products in WTE plants: O2 enrichment of underfire air in the MARTIN SYNCOM process. Chemosphere 2001;42(5e7):545e50. [19] Burdyny T, Struchtrup H. Hybrid membrane/cryogenic separation of oxygen from air for use in the oxy-fuel process. Energy 2010;35(5):1884e97. [20] Takuma M, Kaihara Y, Yamada A. Contribution of waste to energy technology to global warming. Tech Rev 2004;41(4):1e5. [21] Liszka M, Ziebik A. Coal-fired oxy-fuel power unit e process and system analysis. Energy 2010;35(2):943e51. [22] Hong JS, Chaudhry G, Brisson JG, Field R, Gazzino M, Ghoniem AF. Analysis of oxy-fuel combustion power cycle utilizing a pressurized coal combustor. Energy 2009;34(9):1332e40. [23] Buah WK, Cunliffe AM, Williams PT. Characterization of products from the pyrolysis of municipal solid waste. Process Safety Environ Prot 2007;85 (B5):450e7. [24] Wu CH, Chang CY, Tseng CH. Pyrolysis products of uncoated printing and writing paper of MSW. Fuel 2002;81(6):719e25. [25] Wu CH, Chang CY, Tseng CH, Lin JP. Pyrolysis product distribution of waste newspaper in MSW. J Anal Appl Pyrol 2003;67(1):41e53. [26] García A, Marcilla A, Font R. Thermogravimetric kinetic study of the pyrolysis of municipal solid waste. Thermochim Acta 1995;254:277e304. [27] Grammelis P, Basinas P, Malliopoulou A, Sakellaropoulos G. Pyrolysis kinetics and combustion characteristics of waste recovered fuels. Fuel 2009;88 (1):195e205. [28] Sorum L, Gronli MG, Hustad JE. Pyrolysis characteristics and kinetics of municipal solid wastes. Fuel 2001;80(9):1217e27. [29] Shen BX, Wu CF, Qin L. MSW catalytic combustion by alkali and alkali-earth salts. Energy 2006;31(14):2900e14. [30] Xiao HM, Ma XQ, Lai ZY. Isoconversional kinetic analysis of co-combustion of sewage sludge with straw and coal. Appl Energy 2009;86(9):1741e5. [31] Li XG, Ma BG, Li X, Hu ZW, Wang XG. Thermogravimetric analysis of the co-combustion of the blends with high ash coal and waste tyres. Thermochim Acta 2006;441(1):79e83. [32] Lu R, Purushothama S, Yang XD, Hyatt J, Pan WP, Riley JT, et al. TG/FTIR/MS study of organic compounds evolved during the co-firing of coal and refusederived fuels. Fuel Process Technol 1999;59(1):35e50. [33] Otero M, Diez C, Calvo LF, Garcia AI, Moran A. Analysis of the co-combustion of sewage sludge and coal by TGeMS. Biomass Bioenergy 2002;22(4):319e29. [34] Folgueras AB, Diaz RM, Xiberta J, Prieto I. Thermogravimetric analysis of the co-combustion of coal and sewage sludge. Fuel 2003;82(15e17):2051e5. [35] Murphy JD, McKeogh E. The benefits of integrated treatment of wastes for the production of energy. Energy 2006;31(2e3):294e310. [36] Zsigraiova Z, Tavares G, Semiao V, Carvalho MD. Integrated waste-to-energy conversion and waste transportation within island communities. Energy 2009;34(5):623e35. [37] Liu GH, Ma XQ, Yu ZS. Experimental and kinetic modeling of oxygen-enriched air combustion of municipal solid waste. Waste Manage 2009;29(2):792e6. [38] Muthuraman M, Namioka T, Yoshikawa K. A comparison of co-combustion characteristics of coal with wood and hydrothermally treated municipal solid waste. Bioresour Technol 2010;101(7):2477e82. [39] Burnham AK. Computational aspects of kinetic analysis. Part D: the ICTAC kinetics project e multi-thermal-history model-fitting methods and their relation to isoconversional methods. Thermochim Acta 2000;355(1e2): 165e70. [40] Muthuraman M, Namioka T, Yoshikawa K. A comparative study on cocombustion performance of municipal solid waste and Indonesian coal with high ash Indian coal: a thermogravimetric analysis. Fuel Process Technol 2010;91(5):550e8. [41] Sanchez ME, Otero M, Gomez X, Moran A. Thermogravimetric kinetic analysis of the combustion of biowastes. Renew Energy 2009;34(6):1622e7.