Study on the steam gasification mechanism of waste PP plastics based on ReaxFF-MD and DFT methods

doi:10.11949/0438-1157.20221000

Abstract

Abstract:

Gasification technology is an important method for treating the increasing amount of waste plastics and producing H₂-rich syngas. ReaxFF-MD and DFT methods were used to study the reaction mechanism of water vapor gasification of polypropylene (PP) plastics and the generation path of each product gas, and the effects of temperature and water content on the product distribution of syngas were further explored. The results show that the main reaction process in the early stage of steam gasification of PP plastics is the breaking of monomer bond with small dissociation energy at the places where the propylene (CH₃—CH $̿$ CH₂) monomer are formed. Subsequently, the monomer continue to dissociate into smaller C₁—C₂ hydrocarbon molecules, ·H, CH₃· and other free radical fragments. In the steam reforming reaction stage, there are many ways to generate H₂, but the collision of free ·H radicals generated from the C-containing structure in the early stage with H₂O molecular is the main way, accounting for 70% of the total H₂. At the same time, the accompanying generated ·OH radical is the main way of generating CO through the process of combining with small molecules and removing H. Increasing the temperature and water content can promote the steam reforming reaction of hydrocarbons, so as to improve the yield of H₂ and CO, but the improvement effect is gradually weakened. The above results can be a guide to master the plastic gasification process and select proper experimental parameters.

Key words: molecular simulation, gasification, waste plastics, syngas, chemical reaction

摘要：

气化技术是处理日益增多的废旧塑料并生产富H₂合成气的重要方法。利用ReaxFF-MD与DFT方法相结合研究了聚丙烯(PP)塑料水蒸气气化的反应机理及各产物气的生成路径，进一步探究了温度、水含量对合成气产物分布的影响。研究结果表明：PP塑料的水蒸气气化前期以解离能较小处的单体连接键断裂形成丙烯(CH₃—CH $̿$ CH₂)单体为主要反应过程。随后，单体继续解离生成更小的C₁~C₂烃类分子及·H、CH₃·等自由基片段。在水蒸气重整反应阶段，H₂的生成有多种路径，但由前期含C结构上脱掉的游离·H自由基与H₂O的结合是H₂的最主要生成路径，占据了H₂生成量的70%。同时伴随产生的·OH自由基通过与小分子结合，脱H等过程，是CO的主要生成方式。提高温度和含水量可促进烃类的水蒸气重整反应，从而提高H₂和CO合成气产率，但改善效果逐渐减弱。以上结果对于掌握塑料水蒸气反应进程，以及实验参数调整有重要指导意义。

关键词: 分子模拟, 气化, 废旧塑料, 合成气, 化学反应

CLC Number:

TQ 325

Weiwei XUAN, Yanwu DONG, Hailun WANG. Study on the steam gasification mechanism of waste PP plastics based on ReaxFF-MD and DFT methods[J]. CIESC Journal, 2022, 73(11): 5251-5262.

玄伟伟, 董彦吾, 王海轮. 基于ReaxFF-MD和DFT的废旧PP塑料水蒸气气化机理研究[J]. 化工学报, 2022, 73(11): 5251-5262.

Figures/Tables 15

Fig.1 Model construction process: (a) propylene molecule; (b) water molecule; (c) PP chain after optimization; (d) steam gasification system; (e) pyrolysis system

Table 1 Parameters of ReaxFF-MD simulation for PP gasification

体系	序号	PP链的数量(n=17)	H₂O的数量	水塑比/（g/g）	温度/K
1	1	1	102	2.56	3000
1	2	1	0	0	3000
2	3 4 5 6 7	20 20 20 20 20	2040 2040 2040 2040 2040	2.56 2.56 2.56 2.56 2.56	2500 2750 3000 3250 3500
3	8 9 10 11 12	20 20 20 20 20	0 398 795 1195 1590	0 0.5 1.0 1.5 2.0	3000 3000 3000 3000 3000

Fig.2 PP model under different steam plastic ratio: (a)S/P=2.56; (b)S/P=0; (c)S/P=0.5; (d)S/P=1.0; (e)S/P=1.5; (f)S/P=2.0

Table 2 Comparison of product gas components between this simulation and reference

气体组分	体积占比/%
气体组分	文献[40]	文献[41]	ReaxFF-MD
H₂	61.8	67.25	65.26
CO	28.7	25.24	31.45
CO₂	6.5	7.33	0.84
CH₄	2.5	0.18	2.27
C₂~C₄	0.8	0	0.18

Fig.3 Evolution of the products H2 and CH2̿ CH—CH3 during pyrolysis and gasification

Table 3 The initial breaking position of PP chain

C(n)	次数	C(n)	次数	C(n)	次数	C(n)	次数	C(n)	次数
C1	4	C6	3	C11	2	C16	0	C21	4
C2	1	C7	0	C12	6	C17	2	C22	2
C3	3	C8	1	C13	1	C18	4	C23	1
C4	0	C9	3	C14	0	C19	3	C24	1
C5	3	C10	0	C15	4	C20	2	C25	1

Fig.4 Main reactions of PP in thermal depolymerization stage

Fig.5 The main H2 production paths and proportions of different paths

Fig.6 The main CO paths and proportions of different paths

Fig. 7 Optimized C12H25· radical fragment

Table 4 C—C dissociation bond energy （BDE） of C12H25·example structure

键类型m	解离能/(kJ/mol)
键类型m	键位置n=1	键位置n=2	键位置n=3	键位置n=4
1	41.0568	83.4732	125.1554
2	67.0016	317.8537	324.8633	329.6969
3	410.2765	344.4268	348.8610	342.7835

Fig.8 Variation of the number of PP chains (a) and the number of H2O molecules (b) with temperature

Fig.9 Distribution of main products in PP steam gasification process

Fig.10 PP molecule (a) and water molecule (b) consumption under different water plastic ratio (S/P)

Fig.11 Influence of water content on distributions of the main gas products

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