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An experimental and kinetic study of thermal decomposition of phenanthrene
Journal of Hazardous Materials 365 (2019) 565–571
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An experimental and kinetic study of thermal decomposition of phenanthrene
T
⁎
Nana Peng, Cui Huang , Jun Su School of Public Policy and Management, Tsinghua University, Beijing 100084, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Phenanthrene Municipal solid waste to energy Activation energy Reaction mechanism Pollution control policies
Polycyclic aromatic hydrocarbons (PAHs) have enormous potential hazards. It is necessary for China to propose more internationally stricter standards for PAHs, in order to improve the country’s pollutant prevention and control policy system, and ultimately, provide institutional guarantees for implementing PAH emissions prevention and control. In this study, phenanthrene, a typical PAHs generated during municipal solid waste (MSW) to energy system, was applied as a model compound to study the thermal degradation mechanism during the combustion process. Combustion kinetics for the three major gaseous products, including hydrogen, methane, and carbon dioxide, were determined. Experimental results indicated that hydrogen was promoted compared to methane and carbon dioxide during the combustion of phenanthrene, especially in high oxygen concentrations. The apparent activation energy (Ea) of 8.299–11.51, 13.10–23.07, and 9.368–15.29 kJ/mol, pre-exponential factor (A) of 0.219–1.579, 5.034–10.12, and 6.553–15.51 s−1, and the reaction order (n) of 1.160–1.234, 1.059–1.305, and 1.636–1.774 were obtained for hydrogen, methane, and carbon dioxide, respectively. Research on combustion behavior of phenanthrene and reaction kinetics provides the theoretical basis for the high-temperature removal of PAHs as byproducts during the combustion of MSW in oxygen-rich atmosphere.
1. Introduction Tapping sustainable energy/fuels from renewable resources like MSW have been paid great attention due to the quick depletion of fossil
⁎
fuels and world-wide environmental pollution. Currently, the global generation of MSW is about 1.3 billion/year, in which around 13% is produced from China due to the rapid urbanization [1–3]. It is estimated that China will produce almost 0.5 billion/year at the year of
Corresponding author. E-mail address: [email protected] (C. Huang).
https://doi.org/10.1016/j.jhazmat.2018.11.026 Received 30 July 2018; Received in revised form 22 October 2018; Accepted 6 November 2018 Available online 10 November 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of the experimental facility.
by selecting typical model component of PAHs during the combustion of MSW. Our previous studies [15–17] found that the PAHs generated from the combustion of hydrochar obtained from MSW were mainly composed of 3- and 4-ring PAHs. Especially, phenanthrene accounted for 13–19% of the total PAHs, which is the dominant PAHs generated during the combustion of hydrothermally treated MSW. It is clear that the degradation mechanism of PAHs and the factors affecting degradation are beneficial for reducing the PAH emissions. Therefore, in the present work we selected phenanthrene, as the model compound of PAHs due to its high content. The effect of oxygen concentration on the distribution and evolution of incondensable gaseous products during phenanthrene combustion were studied. Combustion kinetics for the phenanthrene decomposition behavior under different conditions were further determined to better understand the degradation conditions of PAHs.
2025, which accounts for nearly one-quarter of the world's MSW [4]. Landfill, incineration and composting are commonly applied for the treatment of MSW [5,6]. Among all potential alternative techniques, incineration has attracted more attention because it can significantly reduce the waste volume (up to 90%)/weight (up to 70%) and provide extra energy, which is a economically feasible way for the effective energy recovery from MSW resources. However, the generation of toxic chemicals, such as soot, heavy metals, COx, NOx, SOx, furans and dioxins is a bottleneck problem during this thermo-chemical treatment, which may render air/soil/water pollution [7,8]. PAHs can be generated from MSW incineration, on top of the pollutants mentioned above [9,10]. Moreover, combustion of MSW incineration may produce more PAHs than that of coal [11]. The generation of toxic PAHs has become a global environmental issue because of the mutagenicity, teratogenicity and carcinogenicity [12]. People who have been exposed to PAHs for a sustained period of time are prone to diseases of the respiratory tract and skin. At the same time, PAHs have a significant inhibitory effect on the growth of animals and plants [13,14]. Therefore, it is necessary to develop a facile, green and cost-effective technology that can reduce the emission of PAHs during the combustion of MSW. Our previous study [15–17] observed that the total yields of PAHs and corresponding toxicity during MSW incineration can be effectively reduced by the pretreatment of MSW via hydrothermal carbonization. Nevertheless, a certain amount of PAHs were still detected during the combustion of hydrothermal treated MSW. Previous literature reported that the technology of oxygen-enriched combustion can effectively reduce the emissions of pollutants and increase the combustion efficiency [18,19]. Therefore, we envision that the oxygen-enriched combustion technology combined with hydrothermal treatments is a viable way which could effectively improve the combustion efficiency and reduce the pollutant emissions during MSW incineration. Nevertheless, the combustion behavior of the PAHs during the oxygen-enriched air-incineration atmosphere is still unknown, which may affect the PAH emissions during the oxygen-enriched combustion of hydrothermally treated MSW. Kinetic modeling is a promising way to gain fundamental insights into the plausible reaction mechanism for the combustion behavior of the PAHs. However, the decomposition kinetics and detailed reaction pathway of the PAHs during combustion of MSW in oxygen-rich atmosphere is still faced with lots of difficulties because PAHs are a complex mixture of organic components [20,21]. To investigate the reaction mechanism of combustion behavior of PAHs in oxygen-rich atmosphere, it is necessary to simplify the reaction process
2. Methods 2.1. Experimental procedure Phenanthrene, a typical 3-ring PAHs, was selected as surrogate of PAHs during the combustion of hydrothermal treated MSW and purchased from Sigma-Aldrich in this work for test. As shown in Fig.1, the experimental facility for the investigation of phenanthrene combustion is mainly composed of a gas mixture device, a laboratory-scale quartz tubular reactor, a gas purification kit, and a process mass spectrometer (Dycor ProLine, Ametek, USA). The inner diameter and length of the quartz tube were 30 mm and 500 mm, respectively. The flow rates of gases were modulated by mass flow meters. The produced gases can be on-line analyzed by the mass spectrometer. For each test, 300 mg of the phenanthrene was weighed and placed into a quartz boat. The whole reaction system was initially purged by N2 (100 ml/min). When the reactor was heated to desired temperature, the quartz boat was injected into the heating zone of the quartz tube by the rod and the N2 was switched to mixture of N2/O2 with a total flow rate of 100 ml/min. The produced gas was purified by the gas purification kit and then the relative content of the main gaseous products (H2, CO2, CH4) during the combustion of phenanthrene was on-line analyzed by the mass spectrometer. Our preliminary tests observed that the three main gaseous products (H2, CO2, CH4) accounted for more than 95% of the total volume of gases generated during the phenanthrene combustion process. It should be noted that on 566
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top of the three non-condensable gaseous products, a tiny amount of cyclic gaseous hydrocarbons or aerosol soot may be formed during the thermal decomposition of phenanthrene because the aromatic moieties of phenanthrene will undergo complex reactions like ring-opening and rearrangement during the thermal cracking process [22]. The reactions between the intermediates are extremely complex, and the secondary cracking reactions are still unknown due to the low content or short reaction time [23,24]. Consequently, the evolution of intermediates during the combustion of PAHs in oxygen-enriched atmosphere is recommended for future study. Combustion characteristics and gas composition are greatly affected by oxygen concentration and reaction temperature. According to the literature [25–27], the optimum temperatures for the thermal decomposition of aromatic hydrocarbons ranged from 500 to 1200 °C. In our previous work [15–17], it was observed that the PAHs generated from the combustion of hydrochar obtained from MSW were mainly composed of 3- and 4-ring PAHs. Especially, 3-ring PAHs such as phenanthrene were the most dominant PAHs at the combustion temperatures of 500–900 °C. Therefore, in this work the phenanthrene combustion temperatures were selected ranging from 650 to 850 °C with 50 °C intervals based on literature and previous experimental observations. In terms of oxygen concentration, Luo [28] investigated the oxygen-enriched combustion of biomass using oxygen concentrations ranging from 20% to 100% with 20% intervals. Liu [29] studied the oxygen enriched co-combustion characteristics of herbaceous biomass and bituminous coal. The oxygen concentration was selected as 20%, 40%, 60%, and 80%. Therefore, in this work we applied similar oxygen concentrations (30% to 90%) to study the effect of oxygen concentration on the thermal decomposition behavior of typical 3-ring PAHs. The flow rate of O2 was ranged from 30 to 90 ml/min with 20 ml intervals to make sure the oxygen concentrations range between 30% and 90%. All combustion tests were conducted in triplicates and the average results were reported.
x=
t ∫ts Vi × Vdt t ∫ts e Vi × Vdt
× 100% (3)
where t represents reaction time, ts denotes the start time of combustion reaction, te is the end time of reaction, V denotes the volume rate of all gas components, Vt is the volume rate of the gas product i. Substituting of Eq. (3) into Eq. (1) and then taking the logarithm of both sides can get the expression:
ln(dx / dt ) = −Ea/ RT + ln A + ln f (x )
(4)
For a series of measurements with varied combustion temperatures, the plot of ln (dx/dt) versus 1/T is a straight line with the slope corresponding to –Ea/R. Therefore, the apparent activation energy can be determined from the slope of the fitting line. Based on the probable reaction mechanisms commonly applied in pyrolysis/combustion process (Table S1), the reaction model of reaction order was applied in this work to describe the combustion process of phenanthrene. Therefore, Eq. (1) can be described as:
dx dt = k (T ) × (1 − x )n
(5)
where n represents reaction order. Taking the logarithm of both sides of Eq. (5) can lead to the expression:
ln(dx dt ) = ln(k (T )) + n (1 − x )
(6)
For a certain gas component, the value of reaction order (n) can be determined by the fitting line of the points of ln(dx/dt) versus (1-x) at different reaction temperatures. Taking the logarithm of both sides of Eq. (2) can lead to the expression:
ln(k (T )) = ln(A) − E RT
(7)
The plots between ln(k(T)) versus -E/RT generate a straight line with a intercept corresponds to ln(A) to determine the pre-exponential factor.
2.2. Kinetic methods 3. Results and discussion The global kinetic equation of phenanthrene combustion under the isothermal condition can be described by:
dx / dt = k (T ) × f (x )
3.1. Combustion behavior of phenanthrene
(1)
The relationship of the conversion fraction versus combustion time for the oxidation products (H2, CO2, and CH4) during the phenanthrene combustion at the temperature of 800 °C and in different oxygen concentrations ranging from 30 to 90% were presented in Fig. 2. The conversion fraction of 100% denotes the highest gas yield during the combustion reaction. It could be observed that the conversion fraction for all three gaseous products increased rapidly at the beginning of combustion reaction and it will reach almost 100% within the first 15 s for all the major products. During the initial thermal cracking of phenanthrene process, smaller organic intermediates including the oxygenated aromatic species such as ketones, phenols and aromatic acids were formed [32]. After further hydroxylation, decarbonylation and decarboxylation reactions, the intermediates were converted to carbon
where t is reaction time, T is absolute temperature (K), k(T) represents the rate constant of combustion reaction. Based on Arrhenius law, the rate constant k(T) in Eq. (1) is described as:
k (T ) = A exp(−Ea/ RT )
(2) −1
where Ea and A denotes the apparent activation energy (J·mol ) and pre-exponential factor (S−1), respectively. R is ideal gas constant (8.314 J·mol−1` K−1). f(x) in Eq. (1) denotes the differential reaction model, where x represents the conversion fraction of each gas component. It was defined by the generated gas volume against the total gas volume at the end of the reaction [30,31], which is expressed as:
Fig. 2. Evolution profiles of cracking products (a H2; b CH4; c CO2) during the combustion of phenanthrene at the temperature of 800 °C. 567
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illustrated in Fig. 3. The variation trends of Ea for different gas products are not identical. To be specific, the Ea of H2 and CH4 exhibited similar trends as a function of the conversion fraction. They both increased first and then decreased gradually. However, the variation trend of CO2 is totally different from that of H2 and CH4. It underwent a process of a rise after a fall. It can be explained that the generation of H2 and CH4 during the phenanthrene combustion may proceed with similar reaction mechanism, while the formation pathway of CO2 is totally different. The chemical reactions in terms of depolymerization, recondensation, repolymerization, ring-opening may require more energy to conquer the potential barrier, thus the value of Ea may significantly increase. Therefore, it accounts for a slow increase of generation of gaseous products in longer reaction time. Furthermore, it can be clearly seen in Fig.3 that increasing oxygen concentration from 30 to 90% may greatly decrease the value of Ea of H2, especially in higher ranges of conversion fraction (70–80%), which is totally different from CH4 and CO2. Therefore, we envision that to promote H2 production instead of CH4 and CO2 during the combustion of phenanthrene, a higher oxygen concentration is recommended. On the other hand, with increasing oxygen concentration from 30 to 90%, the average value of Ea for the three products were all gradually decreased, suggesting the reaction activity of phenanthrene combustion is increased and the formation of gaseous products is greatly promoted by the increased oxygen concentration, especially for CH4. Therefore, it accounts for the fact that for the same gas product, the time spent for reaching identical conversion fraction is greatly shortened by increasing the oxygen concentration, as shown in Fig.2. The curves of ln(dx/dt) versus ln(1-x) for the major gaseous species during the combustion of phenanthrene under the oxygen concentration of 30% in different combustion temperatures were illustrated in Fig.4. It can be seen that the combustion of phenanthrene was divided in three stages, including the first stage of rapid heating, the second stage of chemical reaction and the third stage of diffusion controlled phase. The calculated reaction order (n), pre-exponential factor (A) and R2 were summarized in Table 2. The pre-exponential factor of H2 formation ranged from 0.219–1.579 s−1, which was much shorter than that of CH4
monoxide and other gaseous products like methane. At the high-temperature reactions, the instability of carbon atoms with higher oxidation states is promoted, and thus a large amount of oxidizing radicals will be produced and a large amount of carbon dioxide due to the loss of carbonyl groups and the oxidation of carbon monoxide and other intermediates such as alkanes (e.g., methane), and alkenes that generated in the thermal cracking of phenanthrene that earlier generated during the combustion process [33]. As shown in Fig. 2, at each oxygen concentration, the time spent for reaching maximum gas yield is H2 < CO2 < CH4, indicating that the H2 is promoted compared to CO2 and CH4 during the combustion of phenanthrene. The dominate reaction pathways of thermal cracking of phenathrene in oxygen-enrich atmosphere include O-atom attack, Hatom abstraction, unimolecular decomposition, ipso-substitution reactions and so on [34,35]. The short reaction time for H2 than other gaseous products indicated that the desorption of H2 molecule via a unimolecular reaction [36] from the thermal decomposition of phenanthrene requires overcoming a relatively lower energy barrier than that of CH4 or CO2. Besides, the yield of H2 may be promoted by abstracting H atom from CH4 [37]. Moreover, the time spent for conversion fraction reaching 100% for the three products was further reduced with the increase of oxygen concentration, especially for the products of CH4. This is because the increase of oxygen concentration from 20 to 40% promoted the oxidative cracking of the phenanthrene molecule to smaller intermediates and a further increase of oxygen concentration from 40 to 50% favors the decomposition of smaller intermediates species to gaseous products, especially for carbon dioxide. Similar trends were reported in the literature [38]. 3.2. Kinetic analysis of combustion of phenanthrene The calculated apparent activation energy (Ea), correlation coefficient (R2) and stand derivation (SD) were summarized in Table 1. It can be seen that for each gaseous product, the value of R2 was higher than 0.9 and the value of SD was lower than 0.05, indicating a high accuracy of the determination of apparent activation energy. The variation of Ea for the three major gas components was
Table 1 Apparent activation energy for individual gaseous species from the combustion of phenanthrene. Conversion
O2 concentration 30%
H2
CO2
CH4
0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol)
50%
70%
90%
Ea
SD
R2
Ea
SD
R2
Ea
SD
R2
Ea
SD
R2
5.649 7.579 10.11 11.88 14.76 16.27 14.29 11.51 17.57 12.01 10.06 11.99 15.81 19.41 20.16 15.29 30.72 29.93 29.29 25.90 20.67 15.50 9.438 23.07
0.043 0.039 0.035 0.036 0.044 0.039 0.048
0.913 0.932 0.945 0.957 0.933 0.921 0.902
0.048 0.042 0.038 0.037 0.040 0.041 0.049
0.922 0.945 0.965 0.965 0.946 0.933 0.913
0.932 0.956 0.967 0.968 0.945 0.937 0.921
0.923 0.932 0.955 0.957 0.943 0.921 0.911
0.039 0.023 0.022 0.031 0.040 0.037 0.047
0.933 0.952 0.967 0.968 0.947 0.939 0.922
0.043 0.039 0.035 0.036 0.044 0.039 0.048
0.913 0.932 0.945 0.957 0.933 0.921 0.902
0.048 0.042 0.038 0.037 0.040 0.041 0.049
0.922 0.945 0.965 0.965 0.946 0.933 0.913
0.037 0.031 0.021 0.030 0.034 0.038 0.043
0.932 0.956 0.968 0.968 0.945 0.937 0.921
0.038 0.034 0.029 0.028 0.034 0.038 0.041
0.923 0.932 0.955 0.999 0.943 0.921 0.911
0.044 0.042 0.039 0.034 0.041 0.038 0.049
0.923 0.943 0.955 0.967 0.943 0.931 0.912
8.266 13.30 10.95 10.48 8.966 6.025 0.094 8.299 7.124 5.768 5.505 7.002 10.29 12.11 17.78 9.368 3.545 4.759 8.721 13.81 11.93 22.68 26.269 13.10
0.038 0.034 0.029 0.028 0.034 0.038 0.041
0.923 0.942 0.955 0.967 0.943 0.931 0.912
13.72 13.63 12.16 10.68 7.957 4.946 0.079 9.027 11.30 14.12 13.15 12.07 13.02 12.34 16.11 13.16 29.47 24.82 21.19 17.67 14.06 9.262 1.953 16.92
0.037 0.032 0.021 0.030 0.034 0.038 0.043
0.044 0.042 0.039 0.034 0.041 0.039 0.049
12.49 12.76 13.61 11.51 10.54 6.686 3.012 10.09 18.92 15.11 13.55 12.51 10.48 11.55 18.70 14.40 3.167 7.969 13.71 20.03 26.67 32.28 35.39 19.89
0.039 0.023 0.021 0.031 0.040 0.037 0.047
0.933 0.952 0.967 0.968 0.947 0.939 0.922
568
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Fig. 3. Variation trends of activation energy for major gaseous products (a H2; b CH4; c CO2) during the combustion of phenanthrene under different oxygen concentrations.
(5.034–10.12 s−1) and CO2 (6.553–15.51 s−1). It suggests that the production of H2 is more favored than CH4 and CO2 in identical oxygen concentration, which is in accordance with the results in that of apparent activation energy. Furthermore, for the same gaseous product, the pre-exponential factor was gradually decreased with the increase of oxygen concentration from 30 to 90%, verifying that the reaction activity of combustion of phenanthrene is greatly promoted by increasing oxygen concentration and therefore the formation of gaseous products is favored, especially for H2. As shown in Table 2, with the increase of reaction temperature, the reaction order for each gas product varied slightly instead of keeping constant, indicating that the reaction mechanism of production of gaseous product is greatly affected by the combustion temperature. And the variation trends for different gas species are different. For example, with the increase of temperature, a fall after a rise trend was observed for H2 and CH4 while the production of CO2 exhibited a gradual decrease after a rise, even in different oxygen concentrations. It indicates that the reaction mechanism of production of H2 and CH4 is totally different from that of CO2. According to previous literature, the thermal cracking of PAHs are dominated by the type of PAH radial sites such as armchair, free-edge and zigzag [39,40]. With the range of 750–800 °C, the reaction order for each gas species varied greatly than other temperatures, indicating that the critical temperatures affecting the evolution of gaseous products during combustion of phenanthrene were ranged from 750 to 800 °C A detailed density functional theory [41] may provide a theoretical basis for the decomposition pathways of phenanthrene, which is recommended for future study. This paper explores the thermal degradation mechanism of phenanthrene as surrogate of PAHs during MSW incineration in depth and aims to provide technical support for the reduction of PAHs. However, the control of PAHs requires both scientific and technological support as well as support from national policies. In general, the introduction of relevant policies has pointed out the future direction of pollutant control to a certain extent, and can effectively encourage society to work towards improving the quality of our natural environment. However,
compared to developed countries, China’s pollutant standards threshold is high. Key treatment technologies still need to be tackled and pollutant coverage is small, while systematic policy systems and sound policy implementation supervision mechanisms have not yet formed. In addition, the Chinese government should introduce more stringent standards, establish a systematic policy system, and enforce related policies, in order to provide institutional guarantees for pollution prevention and control of PAHs in the atmosphere. 4. Conclusions Combustion behaviors of phenanthrene, a typical PAHs from the combustion of MSW, were probed. The effect of combustion temperature and oxygen concentration on the evolution of gaseous products during the combustion of phenanthrene was studied and the combustion kinetics was determined. The results revealed that the production of gas product of H2 was promoted than other two major gaseous products, including CH4 and CO2, especially in high oxygen concentrations. The critical temperatures affecting the evolution of gaseous products were ranged from 750 to 800 °C. The apparent activation energy (Ea) of 8.299–11.51, 13.10–23.07, and 9.368–15.29 kJ/mol, pre-exponential factor (A) of 0.219–1.579, 5.034–10.12, and 6.553–15.51 s−1, and the reaction order (n) of 1.160–1.234, 1.059–1.305, and 1.636–1.774 were obtained for the H2, CH4, and CO2, respectively. The varied kinetic parameters indicated that the reaction mechanism of thermal decomposition of phenanthrene changed as a function of conversion fraction. As a result, the oxygen-enriched combustion technology combined with hydrothermal treatments is a viable way which could effectively improve the combustion efficiency and reduce the pollutant emissions during MSW incineration. In addition, the Chinese government should continue to encourage international integration, learn from advanced foreign degradation technologies, introduce more stringent standards, establish a systematic policy system, and enforce related policies, in order to provide institutional guarantees for pollution prevention and control of PAHs in the
Fig. 4. Curves of ln(dx/dt) versus ln(1-x) for the major gaseous species (a H2; b CH4; c CO2) during combustion of phenanthrene under the oxygen concentration of 30%. 569
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Table 2 Reaction order and pre-exponential factor for individual gaseous products from the combustion of phenanthrene. Temperature
O2 concentration (%)
°C
H2
CO2
CH4
650 700 750 800 850 A/s−1 650 700 750 800 850 A/s−1 650 700 750 800 850 A/s−1
30%
50%
70%
90%
n
R2
ln k(T)
n
R2
ln k(T)
n
R2
ln k(T)
n
R2
ln k(T)
1.260 1.124 1.243 1.192 1.204 1.204
0.999 0.998 0.998 0.997 0.997
−0.222 −0.207 0.006 −0.191 −0.090
1.220 1.199 1.231 1.290 1.230 1.234
0.998 0.995 0.994 0.999 0.997
−0.026 −0.239 −0.249 −0.067 −0.071
1.236 1.148 1.110 1.181 1.149 1.165
0.999 0.999 0.999 0.998 0.999
0.013 −0.147 −0.248 −0.068 −0.314
1.219 1.193 1.139 1.128 1.123 1.160
0.999 0.999 0.999 0.999 0.998
0.171 −0.114 −0.169 0.093 −0.286
1.579 0.994 0.981 0.978 0.988 0.993
−0.320 −0.101 −0.162 0.189 0.196
1.759 1.561 1.522 1.695 1.915 1.690
0.974 0.997 0.975 0.983 0.983 0.982
0.0202 −0.516 −0.613 −0.097 0.379
1.630 1.695 1.777 1.909 1.857 1.774
0.263 0.990 0.990 0.985 0.992 0.989
−0.340 −0.066 0.194 0.542 0.237
1.767 1.467 1.482 1.587 1.876 1.636
0.219 0.996 0.967 0.975 0.963 0.969
−0.019 −0.275 −0.175 −0.046 0.583
14.20 0.998 0.997 0.999 0.994 0.999
−1.349 −0.958 −0.862 −0.778 −0.657
1.006 1.225 1.489 1.005 0.996 1.144
6.553 0.994 0.990 0.998 0.996 0.987
−1.629 −1.062 −1.094 −1.075 −0.960
1.125 1.122 1.201 1.641 1.436 1.305
10.03 1.000 0.999 0.997 0.993 0.999
−1.150 −1.1549 −0.872 −0.482 −0.712
1.041 1.421 1.270 1.366 1.164 1.252
15.51 0.981 0.995 0.999 0.983 0.995
−1.346 −0.658 −0.745 −0.804 −0.581
1.823 1.691 1.566 1.852 1.783 1.743 0.807 0.992 1.062 1.205 1.231 1.059
10.12
5.034
10.03
atmosphere.
7.992
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Acknowledgements We are grateful for the financial support from China Postdoctoral Science Foundation [grant numbers 043202023], National Natural Science Foundation of China [grant numbers 71520107005], National Natural Science Foundation for Creative Research Groups [grant numbers 71721002], National Natural Science Foundation for Excellent Youth Scientists [grant numbers 71722002], and Tsinghua University Initiative Scientific Research Program [grant numbers 20165080054]. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jhazmat.2018.11.026. References [1] NBS, China Statistical Yearbook, China Statistical Publishing House, Beijing, 2015. [2] H. Ma, N. Du, X. Lin, C. Liu, J. Zhang, Z. Miao, Inhibition of element sulfur and calcium oxide on the formation of PCDD/Fs during co-combustion experiment of municipal solid waste, Sci. Total Environ. 633 (2018) 1263–1271. [3] J. Hong, Y. Chen, M. Wang, L. Ye, C. Qi, H. Yuan, T. Zheng, X. Li, Intensification of municipal solid waste disposal in China, Renew. Sustain. Energy Rev. 69 (2017) 168–176. [4] D. Hoornweg, P. Bhada-Tata, C. Kennedy, Environment: waste production must peak this century, Nature 502 (2013) 615–617. [5] J. Malinauskaite, H. Jouhara, D. Czajczynska, P. Stanchev, E. Katsou, P. Rostkowski, et al., Municipal solid waste management and waste-to-energy in the context of a circular economy and energy recycling in Europe, Energy 141 (2018) 2013–2044. [6] M.F. Mustafa, Y.J. Liu, Z.H. Duan, H.W. Guo, S. Xu, H.T. Wang, W.J. Lu, Volatile compounds emission and health risk assessment during composting of organic fraction of municipal solid waste, J. Hazard. Mater. 327 (2017) 35–43. [7] D.X. Xuan, C.S. Poon, Removal of metallic Al and Al/Zn alloys in MSWI bottom ash by alkaline treatment, J. Hazard. Mater. 344 (2018) 73–180. [8] D.Z. Shi, W.X. Wu, S.Y. Lu, T. Chen, H.L. Huang, Y.X. Chen, J.H. Yan, Effect of MSW source-classified collection on the emission of PCDDs/Fs and heavy metals from incineration in China, J. Hazard. Mater. 153 (2008) 685–694. [9] X. You, Polycyclic aromatic hydrocarbon (PAH) emission from co-firing municipal solid waste (MSW) and coal in a fluidized bed incinerator, Waste Manage. 28 (2008) 1543–1551. [10] G. Liu, Y. Liao, S. Guo, X. Ma, C. Zeng, J. Wu, Thermal behavior and kinetics of municipal solid waste during pyrolysis and combustion process, Appl. Therm. Eng. 98 (2016) 400–408.
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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat
An experimental and kinetic study of thermal decomposition of phenanthrene
T
⁎
Nana Peng, Cui Huang , Jun Su School of Public Policy and Management, Tsinghua University, Beijing 100084, China
G R A P H I C A L A B S T R A C T
A R T I C LE I N FO
A B S T R A C T
Keywords: Phenanthrene Municipal solid waste to energy Activation energy Reaction mechanism Pollution control policies
Polycyclic aromatic hydrocarbons (PAHs) have enormous potential hazards. It is necessary for China to propose more internationally stricter standards for PAHs, in order to improve the country’s pollutant prevention and control policy system, and ultimately, provide institutional guarantees for implementing PAH emissions prevention and control. In this study, phenanthrene, a typical PAHs generated during municipal solid waste (MSW) to energy system, was applied as a model compound to study the thermal degradation mechanism during the combustion process. Combustion kinetics for the three major gaseous products, including hydrogen, methane, and carbon dioxide, were determined. Experimental results indicated that hydrogen was promoted compared to methane and carbon dioxide during the combustion of phenanthrene, especially in high oxygen concentrations. The apparent activation energy (Ea) of 8.299–11.51, 13.10–23.07, and 9.368–15.29 kJ/mol, pre-exponential factor (A) of 0.219–1.579, 5.034–10.12, and 6.553–15.51 s−1, and the reaction order (n) of 1.160–1.234, 1.059–1.305, and 1.636–1.774 were obtained for hydrogen, methane, and carbon dioxide, respectively. Research on combustion behavior of phenanthrene and reaction kinetics provides the theoretical basis for the high-temperature removal of PAHs as byproducts during the combustion of MSW in oxygen-rich atmosphere.
1. Introduction Tapping sustainable energy/fuels from renewable resources like MSW have been paid great attention due to the quick depletion of fossil
⁎
fuels and world-wide environmental pollution. Currently, the global generation of MSW is about 1.3 billion/year, in which around 13% is produced from China due to the rapid urbanization [1–3]. It is estimated that China will produce almost 0.5 billion/year at the year of
Corresponding author. E-mail address: [email protected] (C. Huang).
https://doi.org/10.1016/j.jhazmat.2018.11.026 Received 30 July 2018; Received in revised form 22 October 2018; Accepted 6 November 2018 Available online 10 November 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic diagram of the experimental facility.
by selecting typical model component of PAHs during the combustion of MSW. Our previous studies [15–17] found that the PAHs generated from the combustion of hydrochar obtained from MSW were mainly composed of 3- and 4-ring PAHs. Especially, phenanthrene accounted for 13–19% of the total PAHs, which is the dominant PAHs generated during the combustion of hydrothermally treated MSW. It is clear that the degradation mechanism of PAHs and the factors affecting degradation are beneficial for reducing the PAH emissions. Therefore, in the present work we selected phenanthrene, as the model compound of PAHs due to its high content. The effect of oxygen concentration on the distribution and evolution of incondensable gaseous products during phenanthrene combustion were studied. Combustion kinetics for the phenanthrene decomposition behavior under different conditions were further determined to better understand the degradation conditions of PAHs.
2025, which accounts for nearly one-quarter of the world's MSW [4]. Landfill, incineration and composting are commonly applied for the treatment of MSW [5,6]. Among all potential alternative techniques, incineration has attracted more attention because it can significantly reduce the waste volume (up to 90%)/weight (up to 70%) and provide extra energy, which is a economically feasible way for the effective energy recovery from MSW resources. However, the generation of toxic chemicals, such as soot, heavy metals, COx, NOx, SOx, furans and dioxins is a bottleneck problem during this thermo-chemical treatment, which may render air/soil/water pollution [7,8]. PAHs can be generated from MSW incineration, on top of the pollutants mentioned above [9,10]. Moreover, combustion of MSW incineration may produce more PAHs than that of coal [11]. The generation of toxic PAHs has become a global environmental issue because of the mutagenicity, teratogenicity and carcinogenicity [12]. People who have been exposed to PAHs for a sustained period of time are prone to diseases of the respiratory tract and skin. At the same time, PAHs have a significant inhibitory effect on the growth of animals and plants [13,14]. Therefore, it is necessary to develop a facile, green and cost-effective technology that can reduce the emission of PAHs during the combustion of MSW. Our previous study [15–17] observed that the total yields of PAHs and corresponding toxicity during MSW incineration can be effectively reduced by the pretreatment of MSW via hydrothermal carbonization. Nevertheless, a certain amount of PAHs were still detected during the combustion of hydrothermal treated MSW. Previous literature reported that the technology of oxygen-enriched combustion can effectively reduce the emissions of pollutants and increase the combustion efficiency [18,19]. Therefore, we envision that the oxygen-enriched combustion technology combined with hydrothermal treatments is a viable way which could effectively improve the combustion efficiency and reduce the pollutant emissions during MSW incineration. Nevertheless, the combustion behavior of the PAHs during the oxygen-enriched air-incineration atmosphere is still unknown, which may affect the PAH emissions during the oxygen-enriched combustion of hydrothermally treated MSW. Kinetic modeling is a promising way to gain fundamental insights into the plausible reaction mechanism for the combustion behavior of the PAHs. However, the decomposition kinetics and detailed reaction pathway of the PAHs during combustion of MSW in oxygen-rich atmosphere is still faced with lots of difficulties because PAHs are a complex mixture of organic components [20,21]. To investigate the reaction mechanism of combustion behavior of PAHs in oxygen-rich atmosphere, it is necessary to simplify the reaction process
2. Methods 2.1. Experimental procedure Phenanthrene, a typical 3-ring PAHs, was selected as surrogate of PAHs during the combustion of hydrothermal treated MSW and purchased from Sigma-Aldrich in this work for test. As shown in Fig.1, the experimental facility for the investigation of phenanthrene combustion is mainly composed of a gas mixture device, a laboratory-scale quartz tubular reactor, a gas purification kit, and a process mass spectrometer (Dycor ProLine, Ametek, USA). The inner diameter and length of the quartz tube were 30 mm and 500 mm, respectively. The flow rates of gases were modulated by mass flow meters. The produced gases can be on-line analyzed by the mass spectrometer. For each test, 300 mg of the phenanthrene was weighed and placed into a quartz boat. The whole reaction system was initially purged by N2 (100 ml/min). When the reactor was heated to desired temperature, the quartz boat was injected into the heating zone of the quartz tube by the rod and the N2 was switched to mixture of N2/O2 with a total flow rate of 100 ml/min. The produced gas was purified by the gas purification kit and then the relative content of the main gaseous products (H2, CO2, CH4) during the combustion of phenanthrene was on-line analyzed by the mass spectrometer. Our preliminary tests observed that the three main gaseous products (H2, CO2, CH4) accounted for more than 95% of the total volume of gases generated during the phenanthrene combustion process. It should be noted that on 566
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top of the three non-condensable gaseous products, a tiny amount of cyclic gaseous hydrocarbons or aerosol soot may be formed during the thermal decomposition of phenanthrene because the aromatic moieties of phenanthrene will undergo complex reactions like ring-opening and rearrangement during the thermal cracking process [22]. The reactions between the intermediates are extremely complex, and the secondary cracking reactions are still unknown due to the low content or short reaction time [23,24]. Consequently, the evolution of intermediates during the combustion of PAHs in oxygen-enriched atmosphere is recommended for future study. Combustion characteristics and gas composition are greatly affected by oxygen concentration and reaction temperature. According to the literature [25–27], the optimum temperatures for the thermal decomposition of aromatic hydrocarbons ranged from 500 to 1200 °C. In our previous work [15–17], it was observed that the PAHs generated from the combustion of hydrochar obtained from MSW were mainly composed of 3- and 4-ring PAHs. Especially, 3-ring PAHs such as phenanthrene were the most dominant PAHs at the combustion temperatures of 500–900 °C. Therefore, in this work the phenanthrene combustion temperatures were selected ranging from 650 to 850 °C with 50 °C intervals based on literature and previous experimental observations. In terms of oxygen concentration, Luo [28] investigated the oxygen-enriched combustion of biomass using oxygen concentrations ranging from 20% to 100% with 20% intervals. Liu [29] studied the oxygen enriched co-combustion characteristics of herbaceous biomass and bituminous coal. The oxygen concentration was selected as 20%, 40%, 60%, and 80%. Therefore, in this work we applied similar oxygen concentrations (30% to 90%) to study the effect of oxygen concentration on the thermal decomposition behavior of typical 3-ring PAHs. The flow rate of O2 was ranged from 30 to 90 ml/min with 20 ml intervals to make sure the oxygen concentrations range between 30% and 90%. All combustion tests were conducted in triplicates and the average results were reported.
x=
t ∫ts Vi × Vdt t ∫ts e Vi × Vdt
× 100% (3)
where t represents reaction time, ts denotes the start time of combustion reaction, te is the end time of reaction, V denotes the volume rate of all gas components, Vt is the volume rate of the gas product i. Substituting of Eq. (3) into Eq. (1) and then taking the logarithm of both sides can get the expression:
ln(dx / dt ) = −Ea/ RT + ln A + ln f (x )
(4)
For a series of measurements with varied combustion temperatures, the plot of ln (dx/dt) versus 1/T is a straight line with the slope corresponding to –Ea/R. Therefore, the apparent activation energy can be determined from the slope of the fitting line. Based on the probable reaction mechanisms commonly applied in pyrolysis/combustion process (Table S1), the reaction model of reaction order was applied in this work to describe the combustion process of phenanthrene. Therefore, Eq. (1) can be described as:
dx dt = k (T ) × (1 − x )n
(5)
where n represents reaction order. Taking the logarithm of both sides of Eq. (5) can lead to the expression:
ln(dx dt ) = ln(k (T )) + n (1 − x )
(6)
For a certain gas component, the value of reaction order (n) can be determined by the fitting line of the points of ln(dx/dt) versus (1-x) at different reaction temperatures. Taking the logarithm of both sides of Eq. (2) can lead to the expression:
ln(k (T )) = ln(A) − E RT
(7)
The plots between ln(k(T)) versus -E/RT generate a straight line with a intercept corresponds to ln(A) to determine the pre-exponential factor.
2.2. Kinetic methods 3. Results and discussion The global kinetic equation of phenanthrene combustion under the isothermal condition can be described by:
dx / dt = k (T ) × f (x )
3.1. Combustion behavior of phenanthrene
(1)
The relationship of the conversion fraction versus combustion time for the oxidation products (H2, CO2, and CH4) during the phenanthrene combustion at the temperature of 800 °C and in different oxygen concentrations ranging from 30 to 90% were presented in Fig. 2. The conversion fraction of 100% denotes the highest gas yield during the combustion reaction. It could be observed that the conversion fraction for all three gaseous products increased rapidly at the beginning of combustion reaction and it will reach almost 100% within the first 15 s for all the major products. During the initial thermal cracking of phenanthrene process, smaller organic intermediates including the oxygenated aromatic species such as ketones, phenols and aromatic acids were formed [32]. After further hydroxylation, decarbonylation and decarboxylation reactions, the intermediates were converted to carbon
where t is reaction time, T is absolute temperature (K), k(T) represents the rate constant of combustion reaction. Based on Arrhenius law, the rate constant k(T) in Eq. (1) is described as:
k (T ) = A exp(−Ea/ RT )
(2) −1
where Ea and A denotes the apparent activation energy (J·mol ) and pre-exponential factor (S−1), respectively. R is ideal gas constant (8.314 J·mol−1` K−1). f(x) in Eq. (1) denotes the differential reaction model, where x represents the conversion fraction of each gas component. It was defined by the generated gas volume against the total gas volume at the end of the reaction [30,31], which is expressed as:
Fig. 2. Evolution profiles of cracking products (a H2; b CH4; c CO2) during the combustion of phenanthrene at the temperature of 800 °C. 567
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illustrated in Fig. 3. The variation trends of Ea for different gas products are not identical. To be specific, the Ea of H2 and CH4 exhibited similar trends as a function of the conversion fraction. They both increased first and then decreased gradually. However, the variation trend of CO2 is totally different from that of H2 and CH4. It underwent a process of a rise after a fall. It can be explained that the generation of H2 and CH4 during the phenanthrene combustion may proceed with similar reaction mechanism, while the formation pathway of CO2 is totally different. The chemical reactions in terms of depolymerization, recondensation, repolymerization, ring-opening may require more energy to conquer the potential barrier, thus the value of Ea may significantly increase. Therefore, it accounts for a slow increase of generation of gaseous products in longer reaction time. Furthermore, it can be clearly seen in Fig.3 that increasing oxygen concentration from 30 to 90% may greatly decrease the value of Ea of H2, especially in higher ranges of conversion fraction (70–80%), which is totally different from CH4 and CO2. Therefore, we envision that to promote H2 production instead of CH4 and CO2 during the combustion of phenanthrene, a higher oxygen concentration is recommended. On the other hand, with increasing oxygen concentration from 30 to 90%, the average value of Ea for the three products were all gradually decreased, suggesting the reaction activity of phenanthrene combustion is increased and the formation of gaseous products is greatly promoted by the increased oxygen concentration, especially for CH4. Therefore, it accounts for the fact that for the same gas product, the time spent for reaching identical conversion fraction is greatly shortened by increasing the oxygen concentration, as shown in Fig.2. The curves of ln(dx/dt) versus ln(1-x) for the major gaseous species during the combustion of phenanthrene under the oxygen concentration of 30% in different combustion temperatures were illustrated in Fig.4. It can be seen that the combustion of phenanthrene was divided in three stages, including the first stage of rapid heating, the second stage of chemical reaction and the third stage of diffusion controlled phase. The calculated reaction order (n), pre-exponential factor (A) and R2 were summarized in Table 2. The pre-exponential factor of H2 formation ranged from 0.219–1.579 s−1, which was much shorter than that of CH4
monoxide and other gaseous products like methane. At the high-temperature reactions, the instability of carbon atoms with higher oxidation states is promoted, and thus a large amount of oxidizing radicals will be produced and a large amount of carbon dioxide due to the loss of carbonyl groups and the oxidation of carbon monoxide and other intermediates such as alkanes (e.g., methane), and alkenes that generated in the thermal cracking of phenanthrene that earlier generated during the combustion process [33]. As shown in Fig. 2, at each oxygen concentration, the time spent for reaching maximum gas yield is H2 < CO2 < CH4, indicating that the H2 is promoted compared to CO2 and CH4 during the combustion of phenanthrene. The dominate reaction pathways of thermal cracking of phenathrene in oxygen-enrich atmosphere include O-atom attack, Hatom abstraction, unimolecular decomposition, ipso-substitution reactions and so on [34,35]. The short reaction time for H2 than other gaseous products indicated that the desorption of H2 molecule via a unimolecular reaction [36] from the thermal decomposition of phenanthrene requires overcoming a relatively lower energy barrier than that of CH4 or CO2. Besides, the yield of H2 may be promoted by abstracting H atom from CH4 [37]. Moreover, the time spent for conversion fraction reaching 100% for the three products was further reduced with the increase of oxygen concentration, especially for the products of CH4. This is because the increase of oxygen concentration from 20 to 40% promoted the oxidative cracking of the phenanthrene molecule to smaller intermediates and a further increase of oxygen concentration from 40 to 50% favors the decomposition of smaller intermediates species to gaseous products, especially for carbon dioxide. Similar trends were reported in the literature [38]. 3.2. Kinetic analysis of combustion of phenanthrene The calculated apparent activation energy (Ea), correlation coefficient (R2) and stand derivation (SD) were summarized in Table 1. It can be seen that for each gaseous product, the value of R2 was higher than 0.9 and the value of SD was lower than 0.05, indicating a high accuracy of the determination of apparent activation energy. The variation of Ea for the three major gas components was
Table 1 Apparent activation energy for individual gaseous species from the combustion of phenanthrene. Conversion
O2 concentration 30%
H2
CO2
CH4
0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol) 0.2 0.3 0.4 0.5 0.6 0.7 0.8 Average/(kJ/mol)
50%
70%
90%
Ea
SD
R2
Ea
SD
R2
Ea
SD
R2
Ea
SD
R2
5.649 7.579 10.11 11.88 14.76 16.27 14.29 11.51 17.57 12.01 10.06 11.99 15.81 19.41 20.16 15.29 30.72 29.93 29.29 25.90 20.67 15.50 9.438 23.07
0.043 0.039 0.035 0.036 0.044 0.039 0.048
0.913 0.932 0.945 0.957 0.933 0.921 0.902
0.048 0.042 0.038 0.037 0.040 0.041 0.049
0.922 0.945 0.965 0.965 0.946 0.933 0.913
0.932 0.956 0.967 0.968 0.945 0.937 0.921
0.923 0.932 0.955 0.957 0.943 0.921 0.911
0.039 0.023 0.022 0.031 0.040 0.037 0.047
0.933 0.952 0.967 0.968 0.947 0.939 0.922
0.043 0.039 0.035 0.036 0.044 0.039 0.048
0.913 0.932 0.945 0.957 0.933 0.921 0.902
0.048 0.042 0.038 0.037 0.040 0.041 0.049
0.922 0.945 0.965 0.965 0.946 0.933 0.913
0.037 0.031 0.021 0.030 0.034 0.038 0.043
0.932 0.956 0.968 0.968 0.945 0.937 0.921
0.038 0.034 0.029 0.028 0.034 0.038 0.041
0.923 0.932 0.955 0.999 0.943 0.921 0.911
0.044 0.042 0.039 0.034 0.041 0.038 0.049
0.923 0.943 0.955 0.967 0.943 0.931 0.912
8.266 13.30 10.95 10.48 8.966 6.025 0.094 8.299 7.124 5.768 5.505 7.002 10.29 12.11 17.78 9.368 3.545 4.759 8.721 13.81 11.93 22.68 26.269 13.10
0.038 0.034 0.029 0.028 0.034 0.038 0.041
0.923 0.942 0.955 0.967 0.943 0.931 0.912
13.72 13.63 12.16 10.68 7.957 4.946 0.079 9.027 11.30 14.12 13.15 12.07 13.02 12.34 16.11 13.16 29.47 24.82 21.19 17.67 14.06 9.262 1.953 16.92
0.037 0.032 0.021 0.030 0.034 0.038 0.043
0.044 0.042 0.039 0.034 0.041 0.039 0.049
12.49 12.76 13.61 11.51 10.54 6.686 3.012 10.09 18.92 15.11 13.55 12.51 10.48 11.55 18.70 14.40 3.167 7.969 13.71 20.03 26.67 32.28 35.39 19.89
0.039 0.023 0.021 0.031 0.040 0.037 0.047
0.933 0.952 0.967 0.968 0.947 0.939 0.922
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Fig. 3. Variation trends of activation energy for major gaseous products (a H2; b CH4; c CO2) during the combustion of phenanthrene under different oxygen concentrations.
(5.034–10.12 s−1) and CO2 (6.553–15.51 s−1). It suggests that the production of H2 is more favored than CH4 and CO2 in identical oxygen concentration, which is in accordance with the results in that of apparent activation energy. Furthermore, for the same gaseous product, the pre-exponential factor was gradually decreased with the increase of oxygen concentration from 30 to 90%, verifying that the reaction activity of combustion of phenanthrene is greatly promoted by increasing oxygen concentration and therefore the formation of gaseous products is favored, especially for H2. As shown in Table 2, with the increase of reaction temperature, the reaction order for each gas product varied slightly instead of keeping constant, indicating that the reaction mechanism of production of gaseous product is greatly affected by the combustion temperature. And the variation trends for different gas species are different. For example, with the increase of temperature, a fall after a rise trend was observed for H2 and CH4 while the production of CO2 exhibited a gradual decrease after a rise, even in different oxygen concentrations. It indicates that the reaction mechanism of production of H2 and CH4 is totally different from that of CO2. According to previous literature, the thermal cracking of PAHs are dominated by the type of PAH radial sites such as armchair, free-edge and zigzag [39,40]. With the range of 750–800 °C, the reaction order for each gas species varied greatly than other temperatures, indicating that the critical temperatures affecting the evolution of gaseous products during combustion of phenanthrene were ranged from 750 to 800 °C A detailed density functional theory [41] may provide a theoretical basis for the decomposition pathways of phenanthrene, which is recommended for future study. This paper explores the thermal degradation mechanism of phenanthrene as surrogate of PAHs during MSW incineration in depth and aims to provide technical support for the reduction of PAHs. However, the control of PAHs requires both scientific and technological support as well as support from national policies. In general, the introduction of relevant policies has pointed out the future direction of pollutant control to a certain extent, and can effectively encourage society to work towards improving the quality of our natural environment. However,
compared to developed countries, China’s pollutant standards threshold is high. Key treatment technologies still need to be tackled and pollutant coverage is small, while systematic policy systems and sound policy implementation supervision mechanisms have not yet formed. In addition, the Chinese government should introduce more stringent standards, establish a systematic policy system, and enforce related policies, in order to provide institutional guarantees for pollution prevention and control of PAHs in the atmosphere. 4. Conclusions Combustion behaviors of phenanthrene, a typical PAHs from the combustion of MSW, were probed. The effect of combustion temperature and oxygen concentration on the evolution of gaseous products during the combustion of phenanthrene was studied and the combustion kinetics was determined. The results revealed that the production of gas product of H2 was promoted than other two major gaseous products, including CH4 and CO2, especially in high oxygen concentrations. The critical temperatures affecting the evolution of gaseous products were ranged from 750 to 800 °C. The apparent activation energy (Ea) of 8.299–11.51, 13.10–23.07, and 9.368–15.29 kJ/mol, pre-exponential factor (A) of 0.219–1.579, 5.034–10.12, and 6.553–15.51 s−1, and the reaction order (n) of 1.160–1.234, 1.059–1.305, and 1.636–1.774 were obtained for the H2, CH4, and CO2, respectively. The varied kinetic parameters indicated that the reaction mechanism of thermal decomposition of phenanthrene changed as a function of conversion fraction. As a result, the oxygen-enriched combustion technology combined with hydrothermal treatments is a viable way which could effectively improve the combustion efficiency and reduce the pollutant emissions during MSW incineration. In addition, the Chinese government should continue to encourage international integration, learn from advanced foreign degradation technologies, introduce more stringent standards, establish a systematic policy system, and enforce related policies, in order to provide institutional guarantees for pollution prevention and control of PAHs in the
Fig. 4. Curves of ln(dx/dt) versus ln(1-x) for the major gaseous species (a H2; b CH4; c CO2) during combustion of phenanthrene under the oxygen concentration of 30%. 569
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Table 2 Reaction order and pre-exponential factor for individual gaseous products from the combustion of phenanthrene. Temperature
O2 concentration (%)
°C
H2
CO2
CH4
650 700 750 800 850 A/s−1 650 700 750 800 850 A/s−1 650 700 750 800 850 A/s−1
30%
50%
70%
90%
n
R2
ln k(T)
n
R2
ln k(T)
n
R2
ln k(T)
n
R2
ln k(T)
1.260 1.124 1.243 1.192 1.204 1.204
0.999 0.998 0.998 0.997 0.997
−0.222 −0.207 0.006 −0.191 −0.090
1.220 1.199 1.231 1.290 1.230 1.234
0.998 0.995 0.994 0.999 0.997
−0.026 −0.239 −0.249 −0.067 −0.071
1.236 1.148 1.110 1.181 1.149 1.165
0.999 0.999 0.999 0.998 0.999
0.013 −0.147 −0.248 −0.068 −0.314
1.219 1.193 1.139 1.128 1.123 1.160
0.999 0.999 0.999 0.999 0.998
0.171 −0.114 −0.169 0.093 −0.286
1.579 0.994 0.981 0.978 0.988 0.993
−0.320 −0.101 −0.162 0.189 0.196
1.759 1.561 1.522 1.695 1.915 1.690
0.974 0.997 0.975 0.983 0.983 0.982
0.0202 −0.516 −0.613 −0.097 0.379
1.630 1.695 1.777 1.909 1.857 1.774
0.263 0.990 0.990 0.985 0.992 0.989
−0.340 −0.066 0.194 0.542 0.237
1.767 1.467 1.482 1.587 1.876 1.636
0.219 0.996 0.967 0.975 0.963 0.969
−0.019 −0.275 −0.175 −0.046 0.583
14.20 0.998 0.997 0.999 0.994 0.999
−1.349 −0.958 −0.862 −0.778 −0.657
1.006 1.225 1.489 1.005 0.996 1.144
6.553 0.994 0.990 0.998 0.996 0.987
−1.629 −1.062 −1.094 −1.075 −0.960
1.125 1.122 1.201 1.641 1.436 1.305
10.03 1.000 0.999 0.997 0.993 0.999
−1.150 −1.1549 −0.872 −0.482 −0.712
1.041 1.421 1.270 1.366 1.164 1.252
15.51 0.981 0.995 0.999 0.983 0.995
−1.346 −0.658 −0.745 −0.804 −0.581
1.823 1.691 1.566 1.852 1.783 1.743 0.807 0.992 1.062 1.205 1.231 1.059
10.12
5.034
10.03
atmosphere.
7.992
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