Kinetics of thermal conversion of gaseous products of polypropylene pyrolysis using several detailed kinetic mechanisms

Thermal conversion (TC) of polypropylene pyrolysis gaseous products was studied numerically using three detailed kinetic mechanisms (DKMs) for the initial temperatures T0 ranging from 700 K to 1300 K and the initial pressure of 1 atm. Numerical simulations showed that all DKMs predicted similar qualitative behavior of the reacting mixture, however, they gave significantly different rates of the increase of the mole fractions of CH4 and H2 along with simultaneous decrease in C2+ hydrocarbons. The temperature increase in course of TC process was also found to be predicted differently by the three DKMs. Its characteristics depended on the initial temperature, some calculations showed smooth increase while in others explosionlike regimes were observed. The conversion products, along with methane and hydrogen, contained aromatic compounds C6+. Various DKMs gave significantly different TC characteristic times.


Introduction
Polypropylene (PP) is present in substantial amounts in industrial and domestic waste and its efficient utilization is an open and important problem [1][2][3]. The utilization process should be safe for the environment and energy efficient both, but it is also potentially economically beneficial if chemicals produced by the conversion are suitable for further use in industry [4][5][6]. One of the promising methods of PP utilization is the pyrolysis of plastic waste that produces a variety of gas phase products [7][8][9][10]. The exact composition of gas phase depends on the specific method of pyrolysis, and by choosing the optimal conversion conditions energy rich chemicals can be obtained [11]. However the development of the optimal approaches is hindered by a number of factors, including the presence of other elements besides Н and С, the large variety of methods and conditions of pyrolysis, and difficulties of monitoring and control of the overall process of the decomposition of initial species and their subsequent transformation [8,12,13]. The kinetics of the chemical reactions of the pyrolysis products of different waste and their further conversion after the pyrolysis is also poorly understood [14][15][16]. In our previous work [16] we studied numerically the TC of gas phase products of PP pyrolysis (hereafter referred to as 'mixture') with several DKMs limited to the lower hydrocarbons up to C3. This approach has been widely used for modeling simplified chemical kinetics and determining the principal pathways of reactions [17,18]. This simplification was based on the assessment of the mass fraction of C4+ hydrocarbons in the gas phase products of the pyrolysis of various fuels and mixtures, considered to be less than 10 %. Moreover, it was difficult to identify precisely all C4+ isomers [19,20]. However, in our recent work [21] we found that it was important to take into account also C4+ components, that affected both the overall mode of the TC process and the specific outcome of H2. Therefore we decided to analyze the efficiency of different DKMs with more species for the numerical simulation of the mixture TC in terms of the composition of the products and the thermal behavior of the process.
The objective of this work is to study numerically the TC process of a mixture containing major gas phase components recorded at the exit of a pyrolytic reactor with special attention to the components not taken into account previously. The results of this study are useful both for deeper understanding of all the details of chemical reactions in the pyrolytic mixtures and for the development of optimal technologies of the utilization of various wastes in terms of their efficiency and compliance with environmental standards.

Numerical simulation
The source data for our study were taken from [22], where pure granulated PP was thermally decomposed and the composition of the gas phase was measured experimentally. The initial composition of the mixture supplied by a gas generator was analyzed chromatographically. The temperature of the mixture was constant and equal to 900 K, specific components are presented in Table. The temperature in the sampling zone was about 650 K. C4+ species were identified in [22] mainly as isobutene, while a small fraction of C5+ speciesas 2-Me-Buten-1. All numerical simulations used same initial mixture composition as per Table. We studied the TC process at different initial temperatures using CHEMKIN III software [23]. The initial temperature was T0 and the initial pressure was 1 atm, volume V assumed to be constant. Time integration was limited to no more than one hour. Only gas phase reactions were considered, thus condensation and soot formation were neglected. The process was assumed to be adiabatic (no heat losses) and heterogeneous reactions were absent.
All three DKMs predict initial slow increase of the C2H4 mole fraction. After reaching its maximum value, it drops sharply to less than 1 %. DKMs [25,26] predict that tс for [C2H4] is very close to tс for [CH4], however it decreases a little bit earlier (Fig. 1b, and 1c). DKM [24] predicts that tс for [С2Н4] is two times lower than tс for [СН4] (Fig. 1а). Mole fractions of other С2+ hydrocarbons present in the initial mixture either tend to zero monotonically or remain almost constant and then drop sharply to almost zero at the moment of tс for [CH4].
For Т0 = 650 K the shapes of the curves for the temperature, [CH4] and [H2] according to DKM [24] become similar to their respective shapes for DKMs [25], [26] calculated for higher Т0 = 900 K (Fig. 1b, 1c). On the other hand, the shapes of the curves for DKMs [25], [26] calculated for even higher Т0 ≥ 1000 K are similar to the shapes for DKM [24] but for lower values of T0 (Fig. 1а). Therefore all three DKMs describe the kinetics of the mixture TC in a similar way at least qualitatively. Even though the shapes look similar, numerical values are different.
The increase of T0 also affects the behavior of heat release. Fig. 2 presents the dependency of temperature vs time in course of TC for T0 = 1000 K and 1100 K. At the initial stage of the TC the  [25] predicts the smallest temperature decrease, while DKM [26] the largest one, occurring already for Т0 = 800 K before the temperature starts to rise and becomes higher than T0 (Fig.  1c). After the temperature reaches its maximum, the mixture cooling rate is different for the three DKMs. DKMs [25] and [26] predict insignificant cooling of the mixture after the maximum. However, DKM [24] shows more pronounced decrease of temperature at the last stage of the TC, eventually to the values slightly higher than Т0. The initial decrease of temperature can be attributed to the endothermic reactions of dissociation of propylene, the main component of the initial mixture [27]. Taking into account this initial decrease, the net heat release according to DKM [24] becomes low.
At the final stage of the TC process [CH4] and [H2] tend to their stationary levels, while C2+ species virtually disappear (Fig. 1). The temperature reaches its maximum. Therefore, tT can be considered as the characteristic time or the overall duration of the whole TC process. Fig. 3 shows the value of tT versus initial temperature for three DKMs. For high initial temperatures the duration of TC is almost same for DKMs [24] and [26], but for T0 < 900 K the predictions of the two mechanisms differ greatly, by the order of magnitude. The duration tT calculated by DKM [25] is about three times lower than that by DKM [26]. Fig. 1 demonstrates that the amount of C atoms in methane accumulated in course of TC is lower than its original amount in С2+ hydrocarbons that are consumed. According to all three DKMs [24][25][26], the excessive C atoms form a number of aromatic species С6+. However, each DKM predicts different its own variety of species. Only benzene C6H6 is present in all three DKMs. Other species are as follows. For DKM [24], they are: corannulene C20H10, dimethylbenzanthracene C20H16, and triphenylethylene C20H16. For DKM [25] − 1,1-bicyclopropyl C6H10 and naphthalene C10H8. For DKM [26] toluene С7Н8, naphthalene C10H8, and phenanthrene C14H10. Such discrepancy of the results of different DKMs limits us to making but qualitative conclusions with regards to the formation of aromatic species C6+. Because both "light", methane and hydrogen, and "heavy" aromatic compounds C6+ are formed in the TC process, they can be separated by cooling, and C6+ hydrocarbons can be transformed into acetylene and hydrogen as the products of further TC. Further TC of C6 + hydrocarbons can allow to obtain acetylene and hydrogen as products as it occurs in the case of benzene [28]. For this, it will be necessary to significantly increase the temperature in order to reach  [29,30] Comparison of our calculations using three DKMs [24][25][26] with other mechanisms limited to C3+ and lower hydrocarbons proves that the TC process in the absence of oxygen is strongly affected by the presence of C4+ hydrocarbons. Both the features of the conversion of species and the thermal behavior are different, therefore, more comprehensive DKMs are to be used for better understanding and modeling of the TC.

Conclusions
Our numerical simulations of the thermal conversion of the studied mixtures show that all three studied DKMs give qualitatively similar results. However DKMs differ significantly with regards to the time scale of the stages of TC. All calculations show the increase of CH4 and H2 and the decrease of higher hydrocarbons (C2+) initially present in the mixture. The CH4 concentration reaches its maximum and then drops, while the H2 concentration rises monotonically to its final value. The temperature is highest at the end of the active stage of TC, when higher hydrocarbons (C2+) are totally depleted.
Calculations taking into account C4+ hydrocarbons show more precise results and differ significantly from the calculations using simplified schemes limited to only C3 and lower hydrocarbons. With the increase of the initial temperature of the mixture the heat release of TC becomes lower due to the endothermic reaction of PP dissociation at the earlier stages of the conversion. In the TC process C2+ hydrocarbons are depleted, while some aromatic C6+ hydrocarbons are formed.