thermogravimetricanalysisandkineticsofmixed...

Research ArticleThermogravimetric Analysis and Kinetics of MixedCombustion of Waste Plastics and Semicoke

Xiangdong Xing,1,2 Sha Wang ,1,2 and Qiuli Zhang 1,2

1School of Metallurgical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China2Research Center of Metallurgical Engineering & Technology of Shaanxi Province, Xi’an 710055, China

Correspondence should be addressed to Qiuli Zhang; [email protected]

Received 8 October 2018; Revised 4 April 2019; Accepted 15 April 2019; Published 12 June 2019

Academic Editor: Fateme Razeai

Copyright © 2019 Xiangdong Xing et al. ,is is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work isproperly cited.

,e thermogravimetric method was applied to study the combustion characteristics of waste plastics and semicoke mixture atdifferent heating rates with temperature ranging from room temperature to 1173K. Also, the kinetic parameters of combustionprocess were also estimated by fitting the experimental data to the calculated data. ,e results showed that the mixed combustionprocess of waste plastics and semicoke could be divided into volatile combustion stage and fixed carbon combustion stage. ,eaddition of waste plastics could increase the comprehensive combustion characteristic index (S) and flammability index (C). Itshowed synergistic effect in the mixed combustion process.When the additive amount of waste plastics was 60%, the S value andCvalue reached peak point at the heating rate of 20K/min. ,e heating rate had a promotion effect on combustion rate. ,e mixedcombustion process of waste plastics and semicoke could be well simulated by the n-order rate model of double parallel reactions.,e activation energies E in the first stage of combustion of the mixture were higher than that in the second stage, and thepreexponential factor k was opposite. Meanwhile, a marked kinetic compensation effect was presented between the activationenergy and the preexponential factor.

1. Introduction

,e enormous use of plastic products in daily life has causedmore and more serious “white pollution” problems, envi-ronmental pollution, and ecological damage. It has practicalsignificance to recycle waste plastics to solve environmentalproblems and ease energy supply [1]. Common wasteplastics treatment ways include incineration and landfill.However, both of the methods can produce secondarypollution. So, it is urgent to develop the new practicable andsustainable treatment technologies. Semicoke is a kind ofcoal conversion product with low fixed carbon content andsulfur content. Because of the good combustion perfor-mance, some iron and steel enterprises have replaced pul-verized coal for blast furnace injection [2, 3]. Waste plasticsare mainly composed of carbon and hydrogen elements,which can also be used as heating agent and reducing agentfor blast furnace ironmaking to make full use of its thermalenergy and chemical energy. Waste plastics injected to blast

furnace with semicoke is a new method for reusing wasteplastic [4].

In general, the research on combustion of combustiblematerials mainly focused on the combustion characteristicsof waste plastics, biomass, pulverized coal, and so on. Yi et al.[5] studied the cocombustion of biomass and biochar; it wascleared that the addition of biomass could promote thecombustion of biochar. When the mass fraction of mixedbiomass was 10%–30%, the combustion characteristic wasthe optimal. Zhang et al. [6], based on thermogravimetricanalysis and mass spectrometry analysis, analyzed thecocombustion characteristics and mixing optimization oftobacco stems and high-sulfur bituminous coal. It provedthat the combustion of tobacco stems was more complicatedthan that of high-sulfur bituminous coal, and when themixing ratio of tobacco stems was 0%–20%, the cocom-bustion characteristics was best. Fei et al. [7] studied thekinetics of cocombustion of biomass and plastic under thecondition of total oxygen by thermogravimetric analysis and

HindawiJournal of ChemistryVolume 2019, Article ID 8675986, 10 pageshttps://doi.org/10.1155/2019/8675986

mailto:[email protected]://orcid.org/0000-0003-4963-9123http://orcid.org/0000-0001-8961-8867https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://creativecommons.org/licenses/by/4.0/https://doi.org/10.1155/2019/8675986

found that plastic could reduce the combustion temperatureof wood chips. Fan-Fei et al. [8] analyzed the mixed com-bustion characteristics of biomass and coal with differentdegrees of deterioration. It concluded that biomass hadlower combustion characteristic temperatures and bettercombustion characteristics than coal. However, there are fewreports on the mixed combustion of waste plastics andsemicoke. ,e influence of waste plastics on the combustionof semicoke is not yet clear. It is of great significance to studythe combustion characteristics of waste plastics and semi-coke mixture.

,ermogravimetric analysis technology is widely usedbecause of its simple operation process and large amount ofdata [9–13]. Some combustion characteristic values such asignition temperature, burnout temperature, peak reactiontemperature, maximum combustion rate, average com-bustion rate, and combustion time can be obtained throughthermogravimetric analysis. In addition, according to theweight loss curves at different temperatures, activation en-ergy, preexponential factor, and other combustion kineticparameters can be determined. Kinetic analysis can deeplystudy the combustion mechanism of substances and predictthe combustion rate and reaction process. ,erefore, it ispromising to study the combustion of the mixture throughkinetic analysis.

In the present study, the similarities and differences ofwaste plastics and semicoke were analyzed by proximateanalysis and ultimate analysis. At the same time, the com-bustion characteristics of the mixture of waste plastics andsemicoke were discussed by thermogravimetric analysis atdifferent heating rates. In order to further investigate thecombustion mechanism, the ignition temperature, burnouttemperature, peak reaction temperature, ignition index, andcomprehensive combustion index of the mixture were ob-tained by TG andDTG curves.,e synergistic effect betweenthe combustion of the mixture was also researched.Moreover, the kinetic parameters during the combustionprocess were calculated by the n-order rate model of doubleparallel reactions. ,e studies provided theoretical basis forthe application of waste plastics with semicoke injected intoblast furnace and had a good benefit to relieve the pressure ofenvironmental protection caused by waste plastics at thesame time.

2. Materials and Methods

2.1. RawMaterials. Semicoke and waste plastic were used asraw materials in the experiment. ,e proximate analysis andultimate analysis according to Perkin Elmer EA-2400 ele-mental analyzer and 5E-MAC III infrared rapid coal qualityanalyzer are shown in Table 1. It can be seen from Table 1that the volatile content of waste plastics is far greater thanthat of semicoke, and the ash content is opposite. ,e sulfurcontent is 0, which means its pollution is small, and has goodadvantages for blast furnace injection.

2.2. (ermogravimetric Analysis. ,e tests were carried onTGA209F3 thermogravimetric analyzer produced by

German NETZSCH company under a flowing air atmo-sphere. ,e temperature changed from the ambient tem-perature to 1173K with the heating rates of 2.5 K/min, 5 K/min, 10K/min, and 20K/min, respectively. ,e sampleweight was approximately 10mg and was placed in an Al2O3crucible. ,en, the Al2O3 crucible was placed in a TGbracket.

,e samples used in the study were dried under the sunfor 7 days to separate out water. ,e semicoke was shreddedinto particles in sizes of about 5mm and subsequentlycrushed to −0.200mm in a self-designed crushing system.Afterward, the particle sizes of samples were selected to thatlower than 0.074mm by using standard test sieve. Wasteplastics were made of mineral water bottles commonly usedin life, after washing, drying, low-temperature soft melting,and cooling treatment, the −200mesh waste plastics powderwas obtained. ,e particle shape and surface structure ofsemicoke and waste plastics samples obtained with SEM areshown in Figure 1. ,e mixtures were prepared according tothe proportion of 100%, 80%, 60%, 40%, 20% and 0% ofwaste plastic powder. ,en they were ground for 20minutesto mix evenly.

From Figure 1, significant differences among twosamples can be seen. ,e semicoke sample exists in particleform with nonuniform surface, some of them with thesmooth surface and sharp edges and another in shape withsmaller randomly oriented pores. ,e waste plastic sample ismainly composed of irregular round particles with a smoothsurface. Small round particles gathered around largeparticles.

,e combustion rate of the mixture was calculated by

x �m0 −mt( m0 −m∞(

, (1)

where m0 represents the initial mass of the sample, mtrepresents the mass of the sample at time t during com-bustion, and m∞ represents the final mass of samplecombustion.

2.3. Kinetic Model. ,e kinetic model of the mixed com-bustion in this paper is based on two important assump-tions [14]. First, the sample consists of differentcomponents, and the property of every component is alsodifferent. ,e reactivity of the unit surface varies with thesample burning out. Assuming that the mixture of wasteplastics and semicoke has different reaction properties, themass of the mixture during combustion can be described asfollows:

Table 1: Proximate analysis and ultimate analysis of waste plasticand semicoke.

SamplesProximate analysis

(wt.%)Ultimate analysis

(wt.%)Vad Aad FCad C H O N S

WP 98.81 0.91 0.28 80.70 13.74 2.18 0.81 0NKC 7.48 16.87 75.65 77.66 1.20 0.79 4.75 0.24Note. WP is waste plastic, NKC is semicoke.

2 Journal of Chemistry

mt � n

i�1ci × xi + m∞, (2)

where ci is the fraction of combustible material in compo-nent i, xi is the reaction fraction of component i at time t, andn is the number of components.

Secondly, it is assumed that the mixed combustionprocess of waste plastics and semicoke is a gas-solid non-catalytic heterogeneous reaction, and the reaction rate of thesample can be expressed as follows:

dx

dt� k pg, T f(x), (3)

where k represents the apparent reaction rate constant and isrelated to the reaction temperature T and the gas phasepartial pressure Pg.

In this experiment, f(x) is assumed to be the n-th orderreaction rate model, which can be described by

f(x) � (1−x)n. (4)

During this process, if the gas phase partial pressure Pgremains invariant, the apparent reaction rate constant de-pends on the reaction temperature T and can be expressedby the Arrhenius equation, which can be described by

k � k0e−E/RT

, (5)

where k0 refers to the preexponential factor, E is activationenergy, and R is the gas constant. In the case of a single

independent parallel reaction, the total reaction rate iscalculated as follows:

dx

dt�

i

1ci

dxi

dt , i � 1, 2, 3, . . . . (6)

Substitute formulas (3)–(5) in (6) to obtain

dx

dt�

i

1ci · kie

−Ei/RT · 1− xi( ni , i � 1, 2, 3, . . . . (7)

,e combustion rate of the mixture can be obtainedfrom the average combustion rate values of the variouscomponents according to formula (7). ,e more thenumber of combustion components, the higher the accu-racy of the model. However, ci, ki, Ei, and ni are all requiredto calculate for every component, which requires a lot ofcalculation work. Due to the lack of experimental data andconvergence conditions, different results may be obtained,such as premature convergence, slow convergence, ornonconvergence.

By analyzing the combustion characteristic curves of themixture, the combustion process of waste plastics andsemicoke mixture can be divided into volatile combustionstage and fixed carbon combustion stage.,erefore, in orderto reduce the amount of calculation and maintain the ac-curacy of the model in combination with the conclusions ofBiney et al. [15], this paper uses the n-order rate model ofdouble parallel reactions to study the kinetics of the com-bustion process. So equation (7) can be converted into

(a)

(b)

Figure 1: SEM of the samples (a) semicoke (b) waste plastic.

Journal of Chemistry 3

equation (8) to simulate the two stages of the combustionprocess:

dx

dt�

i

1c1 · k1 · e

−E1/RT · 1−x1( n1

+ i

1c2 · k2e

−E2/RT · 1− x2( n2 , i � 1, 2, 3, . . . ,

(8)

where c1 and c2, respectively, represent the percentage ofvolatile content and fixed carbon content, and the re-lationship between them is shown in the following equation:

c1 + c2 � 1. (9)

Under nonisothermal conditions, the relationship be-tween sample temperature and combustion time is shown inthe following equation:

T � T0 + βt, (10)

where T0 is the initial temperature and ß is the heating rate,K/min.

Substituting formula (10) in formula (8) and integratinggives

x

0

dx

(1−x)n1�

c1k1

β

T

T0

e−E1/RTdT +

c2k2

β

T

T0

e−E2/RTdT.

(11)

When n� 1, equation (11) can be converted into:

x � 2

i�1ci · 1− exp

ki

βexp −

Ei

RT · T−T0( . (12)

When n≠ 1, equation (11) can be converted into

x � 2

i�1ci 1− 1− 1− ni(

ki

βexp −

Ei

RT · T−T0(

1/ 1−ni( ) .

(13)

Nonlinear least square fitting is performed on equations(12) and (13) to minimize the objective function of equation(14) to determine eight dynamic parameters (c1, c2, k1, k2, E1,E2, n1, and n2). ,e whole calculation process is executed bya program compiled by C++.

OF � M

m�1

xexp,m −xcalc,mxexp,m

2

, (14)

where xexp,m is the experimental value corresponding to thetemperature Tm, xcalc,m is the calculated value correspondingto the temperature Tm, and M is the number of data points.

,e mixed combustion process of waste plastics andsemicoke is very complex, and there is relative error betweenthe calculated value and the experimental value. By com-paring them, the relative error is calculated by the followingequation:

DEV(%) � 100 ×

Mm�1 xexp,m −xcalc,m

2/M

1/2

max(x)exp,

(15)

where DEV(%) is relative error, xexp,m is experimental value,xcalc,m is calculated value, max(x)exp is the maximum con-version rate of experiment, and M is the number of datapoints.

3. Results and Discussion

3.1. (ermogravimetric Analysis of Waste Plastic andSemicoke. ,e TG and DTG curves of waste plastics andsemicoke mixture at 10K/min heating rate are shown inFigures 2 and 3, respectively.

,e TG curves of the mixture were between individualwaste plastic and semicoke sample. With the increase of theamount of waste plastics, the TG curves of the mixturemoved to the low-temperature zone and varied with thecontent of waste plastic. However, the trend of the TGcurves of the mixture took into account the characteristicsof the individual sample. ,e combustion weightlessnessprocess of waste plastics and semicoke mixture could bedivided into volatile combustion stage and fixed carboncombustion stage. Because the semicoke was fully dried, thewater content of waste plastics was negligible which meantthere was no dehydration process. An initial stage ofcombustion existed between ambient temperature and654.86–678.26 K due to the softening of waste plastics andsemicoke as well as the precipitation of volatile matter.,en the largest peak was observed at 743.27–752.35 K. Inthis stage, the mixture of waste plastic and semicokethermally decomposed a large amount of volatile matterand burned rapidly. Finally, the temperature rose to910.24–918.27 K. ,e loss of weight mainly was due to thecombustion of fixed carbon in the mixture, which wascalled the second stage.

It was very complicated to analyze themixed combustionprocess of waste plastics and semicoke. In order to de-termine whether there was synergistic effect in the mixedcombustion process, the average conversion value of wasteplastics conversion and semicoke conversion in the singlecombustion process was compared with the experimentalvalue. ,e synergistic effect is positive when the experi-mental value is higher than the calculated value. When theexperimental value is lower than the calculated value, thesynergistic effect is negative [16]. When the experimentalvalue is the same as the calculated value, it means that thereis no synergistic effect. ,e existence of synergistic effect isrelated to the difference between the calculated value and theexperimental value.

Assuming that there was no interaction between themixture, the overall mass loss of the mixture was the averagemass loss of the individual (calculated by formula (16)):

Xcaculated � αxwp + βynkc. (16)

In formula (16), a and ß were the mass percentage ofwaste plastic and semicoke in the mixture, respectively.

,e comparison between the calculated data and theexperimental data is shown in Figure 3.

It could be found from Figure 4 that the calculateddata and the experimental data had good similarity, but


deviation between the calculated data and experimental datademonstrates that waste plastics interacted with semicoke.

ere was synergistic e�ect in the combustion process ofwaste plastics and semicoke mixture.e experimental valuein the �rst stage was lower than the calculated value, and thee�ect was negative. e experimental value in the secondstage was higher than the calculated value, and the e�ect waspositive.

In order to further investigate the e�ect of waste plasticsaddition on the combustion process of semicoke, thecombustion characteristic value was ascertained by TG andDTG curves. e comprehensive combustion characteristicindex (S) covers some main characteristic index duringcombustion and comprehensively reects the combustioncharacteristics of pulverized coal. e higher the value of S,the better the combustion characteristics, as de�ned in thefollowing equation:

S �Rmax × RmeanT2i × Th

, (17)

where Rmax is the maximum combustion rate, s−1, Rmean is

the average combustion rate, s−1, Ti is the ignition tem-perature, K, and Th is the burnout temperature, K.

C indicates the diculty of igniting the combustibles,and the larger the value, the better the ammability of thecoal. Flammability index C is mainly related to Rmax and Ti.It is de�ned as

C �RmaxT2i

. (18)

e combustion characteristic values of the mixture at aheating rate of 10K/min are shown in Table 2.

It could be seen from Table 2 that the ignition tem-perature (Ti) and burnout temperature (Tf ) of waste plasticswere lower than those of semicoke. As waste plastics fractionwas increased from 0 to 100%, Ti and Tf slightly decreasedfrom 568.25K to 563.554K and 978.257K to 798.554K. elower the initial temperature, the more ammable of thecombustible materials, which indicated that the addition ofwaste plastics could facilitate the combustion of semicoke.Meanwhile, Rmax−1 and Rmax−3 decreased, but Rmax−2 in-creased, while the maximum burning rate (Rmax) decreased.When 0% of waste plastic content is added, the maximumburning rate (Rmax) is 0.0068 s−1, which is the lowest. Withincrease of waste plastic content, ammability index C of themixture increased, while it showed the same change law ofcomprehensive combustion characteristic index S. Fromthese results, it could be concluded that comprehensivecombustion characteristics of semicoke could be improvedby addition of waste plastics, while it was the same as ig-nition property, which provided a theoretical basis for blastfurnace injection of waste plastics and semicoke.

WPNKC 20%/WP 80%

NKC 40%/WP 60%NKC 60%/WP 40%NKC 80%/WP 20%NKC

ExperimentalCalculated

0.0

0.2

0.4

0.6

0.8

1.0

TG

900 1200600Temperature (K)

Figure 4: Comparison between experimental data and calculatedvalues (10K/min).

NKC20% WP40% WP

60% WP80% WPWP

600 800 1000 1200400Temperature (K)

1.0

0.8

0.6

0.4

0.2

0.0

X

Figure 2: TG curves at 10K/min heating rate.

NKC20% WP40% WP

60% WP80% WPWP

0.000

0.005

0.010

0.015

0.020

0.025

dx/dt

(s–1

)

600 800 1000 120400Temperature (K)

Figure 3: DTG curves at 10K/min heating rate.


3.2. E�ect of Heating Rate on Combustion Process of Mixture.

e TG and DTG curves of the mixture adding 60% wasteplastics content at di�erent heating rates are shown inFigures 5 and 6, respectively.

As the heating rate increased from 2.5 to 20K/min, it wasfound that TG and DTG curves both moved to the high-temperature zone, resulting in thermal hysteresis phe-nomenon [17]. ere were two main reasons for this phe-nomenon. On the one hand, with the increase of heatingrate, the temperature raised faster, and the single reactiondid not have enough time to react and overlap with theadjacent high temperature reaction. On the other hand, theoverall temperature gradient of the sample cross sectionincreased at a higher heating rate, which resulted in a largetemperature di�erence between the inside and outside of thesample, and the surface combustion products were too lateto di�use, which hindered the combustion of internalcombustibles to a�ect the heat transfer between the ther-mogravimetric instrument and the sample.

e combustion characteristics parameters of the mix-ture adding 60% waste plastics content at di�erent heatingrates are shown in Table 3.

When heating rate increased from 2.5 K/min to 20 K/min, Ti and Tf, respectively, increased from 527.982 K to568.496 K and from 832.982 K to 933.496 K. But the rise ofboth Ti and Tf often means deterioration of combustionproperty. erefore, the e�ect of the heating rate on thecombustion process could not be judged only by the law ofTi and Tf. is paper also analyzed the inuence of heatingrate on the combustion performance of the mixture byammability index C and comprehensive combustioncharacteristic index S. Meantime, it could be concludedfrom Table 3 that C value and S value both increased withincreasing heating rate. When heating rate increased from2.5 K/min to 20 K/min, C increased from 4.61× 10−8 to5.28×10−8 and S increased from 3.72 ×10−13 to5.09×10−13. In a word, it could be con�rmed that withincreasing heating rate, Ti, Tf, C, and S increased. Althoughthe Ti and Tf increased which deteriorated the combustionproperty of the mixture, the obvious promotion e�ect on

Table 2: Combustion characteristic values of mixtures at a heating rate of 10K/min.

Samples WP WP 80% WP 60% WP 40% WP 20% NKCTi (K) 563.554 553.319 558.228 557.96474 563.79207 568.257T1 (K) 663.55 668.319 683.228 693.10 — —Rmax−1 (s−1) 0.00315 0.00316 0.00313 0.0041 — —T2 (K) 741.97 740.56 740.95 744.07 747.24 —Rmax−2 (s−1) 0.02062 0.0181 0.00998 0.00888 0.0077 —T3 (K) 793.55 838.32 853.23 858.46 863.49 885.17Rmax−3 (s−1) 0.00105 0.00149 0.00279 0.0047 0.00565 0.0068Tm (K) 741.97 740.56 740.95 744.07 747.24 885.17Th (K) 798.554 910.81731 918.10897 930.69712 933.492 978.257Rmax (s−1) 0.02062 0.0181 0.00998 0.00888 0.0077 0.0068Rmean (s−1) 0.00827 0.007583 0.0053 0.00589 0.005845 0.0068C× 10−8 6.49258 5.9119 3.20263 2.85233 2.42246 2.10581S× 10−13 6.72386 4.92195 1.8488 1.80512 1.73219 1.46378Note. Ti is the ignition temperature; T1, T2, and T3 are the reaction peak temperatures of the �rst, second, and third peaks; Th is the burnout temperature;Rmax−1, Rmax−2, and Rmax−3 are reaction rates corresponding to T1, T2, and T3; Rmax is the maximum reaction rate; Rmean is the average combustion rate; C isammability index; and S is the comprehensive combustion characteristic index.

2.5K/min5K/min

10K/min20K/min

1.0

0.8

0.6

0.4

0.2

0.0

TG

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

Figure 5: TG curves of adding 60% waste plastics content.

2.5K/min5K/min

10K/min20K/min

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

0.000

0.005

0.010

0.015

0.020

dx/dt

(s–1

)

Figure 6: DTG curves of adding 60% waste plastics content.


combustion rate was more significant. ,erefore, highheating rate could improve the combustion properties ofthe mixture.

4. Kinetic Parameter Analysis

In order to evaluate the effect of waste plastics proportion onsemicoke, the n-order rate model of double parallel reactionwas used to further understand the combustion kinetics. ,ecombustion rate x data of the mixture was fitted nonlinearlyaccording to equation (13), and the combustion rate curvesof the fitted data and the experimental data were obtained;the results are shown in Figure 7.

It could be seen from Figure 7 that the calculated data ofdifferent samples were in good agreement with the exper-imental data, which showed that the model had goodadaptability of the mixed combustion process of wasteplastics and semicoke.

,e kinetics parameters were calculated, and the resultsare shown in Table 4.

From Table 4, it could be seen that the correlation co-efficient R2 of the mixture was greater than 0.9986, whichproved that the kinetic model could better simulate themixed combustion of waste plastics and semicoke. ,eactivation energies of the first stage were higher than thoseof the second stage, which were 64.69–78.85 kJ·mol−1 and36.60–48.04 kJ·mol−1, respectively. It proved that thepyrolysis combustion of volatile occurred at the low-temperature zone in the first stage because the molecularmotion was slower and the energy required for molecularactivation was higher. While the second stage occurred inthe high-temperature range, the limiting steps changed fromthe interface reaction control of the first stage to the dif-fusion control [18], and the diffusion activation energy waslower than the interface reaction activation energy. ,epyrolysis and combustion process of volatile matter in thefirst stage produced a porous carbon structure, whichprovided good conditions for oxygen diffusion into thesurface in the second stage. So, the activation energy in thesecond stage was lower. Temperature had different effects on

interfacial reaction and diffusion.,e fact that the activationenergy in the low-temperature range is higher than that inthe high-temperature range is consistent with the researchresults of Okasha et al. [19].

Meanwhile, as waste plastics fraction decreased from100 to 0%, the activation energy of the two stages all in-creased. When the waste plastics mass ratio was 100%, thelowest activation energy was obtained with the number of64.69 kJ·mol−1 and 36.60 kJ·mol−1, respectively. As theaddition of waste plastics continued to increase, themagnitude of activation energy decrease was not very large.,ere existed a suitable addition of waste plastics in theprocess of promoting the burning of semicoke. And theactivation energy of the first stage had similar law with thesecond stage. ,e lower the activation energy, the higherthe reaction rate. However, it could conclude that thecombustion rate decreased with the decrease of wasteplastic content from Table 2, which was contrary to thechanging trend of activation energy. Generally speaking,the reaction rate was influenced by activation energy andpreexponential factor. In the same temperature range, thelower the activation energy, the more active the moleculesand the higher the reaction rate. ,e preexponential factorindicates the number of collisions of activated molecules.,e larger the preexponential factor, the higher the reactionrate is. From Table 4, it could be concluded that the pre-exponential factor was large corresponding to a low acti-vation energy, which was called compensation effect inkinetic studies. Compensation effect is a common phe-nomenon in gasification studies of coal, biomass, and othercarbonaceous materials [20, 21].

In order to quantify the error caused by the kineticmodel, the deviations DEV was calculated between theexperimental curves and the calculated curves by equation(14). ,e results are shown in Table 5. It could be seen thatthe deviations between the experimental value and thecalculated value were all lower than 1.09%. It once againproved that the n-order rate model of double parallel re-actions had good accuracy in representing the mixedcombustion process of waste plastics and semicoke.

Table 3: Combustion characteristics parameters of mixture adding 60% waste plastic content.

Heating rates(K/min) 2.5 5 10 20

Ti (K) 527.982 543.473 558.228 568.496T1 (K) 669.55128 672.86859 688.228 683.496Rmax−1 (s−1) 0.00329 0.00316 0.00318 0.00235T2 (K) 688.62179 688.73397 — 748.496Rmax−2 (s−1) 0.00754 0.00876 — 0.01353T3 (K) 717.98077 733.473 749.63141 763.01282Rmax−3 (s−1) 0.01286 0.01474 0.01631 0.01705T4 (K) 792.70833 823.91827 857.13141 897.4359Rmax−4 (s−1) 0.00317 0.00329 0.003 0.00312Tm (K) 719.21 729.7485 764.9969 773.1102Tf (K) 832.982 863.473 893.228 933.496Rmax (s−1) 0.01286 0.01474 0.01631 0.01705Rmean (s−1) 0.006715 0.007488 0.007497 0.009013C× 10−8 4.61 4.99 5.23 5.28S× 10−13 3.72 4.33 4.39 5.09


X

WP

1.0

0.8

0.6

0.4

0.2

0.0

2.5K/min5K/min10K/min

20K/minCalculated

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

(a)

80% WP

X

1.0

0.8

0.6

0.4

0.2

0.0


20K/minCalculated

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

(b)

60% WP

X

1.0

0.8

0.6

0.4

0.2

0.0


20K/minCalculated

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

(c)

40% WP

X

1.0

0.8

0.6

0.4

0.2

0.0


20K/minCalculated

400 500 600 700 800 900 1000 1100 1200Temperature (K)

(d)

20% WP

X

1.0

0.8

0.6

0.4

0.2

0.0


20K/minCalculated

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

(e)

NKC

X

1.0

0.8

0.6

0.4

0.2

0.0


20K/minCalculated

400 500 600 700 800 900 1000 1100 1200300Temperature (K)

(f )

Figure 7: Experimental conversion curves and n-order rate calculation curves of double parallel reactions for mixture combustion atdi�erent heating rates.


5. Conclusion

,e mixed combustion of the waste plastics and semicokewere investigated by thermogravimetric method. Resultsshowed that mixed combustion process of waste plasticsand semicoke included volatile combustion stage and fixedcarbon combustion stage. With addition of waste plastics,comprehensive combustion characteristics and ignitionproperty of semicoke could be improved. ,ere was syn-ergistic effect in the combustion process of waste plasticsand semicoke mixture. With the increase of heating rate,the mass loss peak corresponding to the mixture com-bustion moved to high temperature zone, resulting inthermal hysteresis. Moreover, the largest C and S values ofthe mixture were obtained when the heating rate was 20 K/min, which showed that the higher the heating rate was, thebetter the combustion performance of the sample would be.,e mixed combustion process of waste plastics andsemicoke could be well simulated by the n-order rate modelof double parallel reactions. ,e activation energy E in thefirst stage of combustion were higher than that in thesecond stage, while the preexponential factor k was op-posite, and there was obvious kinetic compensation effectbetween them.

Data Availability

,e data used to support the findings of this study weresupplied by Xiangdong Xing under license and so cannot bemade freely available. Requests for access to these datashould be made to Xiangdong Xing, [email protected].

Conflicts of Interest

,e authors declare that there are no conflicts of interestregarding the publication of this paper.

Acknowledgments

,e present work was financed by the National NatureScience Foundation of China (grant no. 51604209) andScientific Research Plan Projects of Shaanxi EducationDepartment (grant no. 16JK1442).

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Table 4: Combustion kinetic parameters of mixtures.

Sample k1 E1 (kJ·mol−1) n1 k2 E2 (kJ·mol−1) n2 R2

WP 1.06 64.69 1.24 0.80 36.60 0.95 0.999480% WP 1.02 65.54 1.23 0.71 37.26 0.94 0.998660% WP 0.92 66.16 1.30 0.64 38.86 0.93 0.998940% WP 0.75 70.13 1.25 0.43 42.77 0.93 0.999120% WP 0.52 74.92 1.26 0.24 46.12 0.90 0.9987NKC 0.38 78.85 1.24 0.02 48.04 0.49 0.9996

Table 5: Deviation between experimented and calculated values.

Sample WP 80% WP 60% WP 40% WP 20% WP NKCDEV(x) % 1.01 0.76 0.91 0.67 1.09 0.51


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