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3.1 Chemical reactions in the thermal pyrolysis process I. Laws of pyrolysis reactions According to the sequence in which the reactions occur, they can be classified as primary reactions and secondary reactions. A one-step reaction is a reaction in which raw hydrocarbons undergo thermal cracking to produce lower alkenes such as ethylene and propylene. A secondary reaction mainly refers to the reaction in which lower hydrocarbons generated by a primary reaction undergo further reactions to produce various products, until coke or carbon is ultimately formed. (1) Thermal cracking of alkanes: For alkanes with the same carbon count, chain scission is easier than dehydrogenation ； b. The relative stability of alkanes decreases as the carbon chain length increases; c. The ease of dehydrogenation is related to the molecular structure of the alkane, with tertiary hydrogens being the easiest to remove, followed by secondary hydrogens, and primary hydrogens being the hardest to remove ； (2) Thermal cracking of naphthenes The cracking reaction of naphthenes follows the following pattern. The side-chain alkyl group is more easily cleaved than the hydrocarbon ring, resulting in a high yield of ethylene. The dehydrogenation of b-cycloalkanes is easier than ring-opening reactions, resulting in a higher likelihood of aromatic formation. When cleaving the side chains of cycloalkanes with long C-side chains, the five-membered rings break first at the center of the side cone; it is more difficult to open six-membered rings. Cutting straight through to the hydrocarbon ring without any side cones – overall, the difficulty level of cycloalkane cracking reactions is as follows: alkanes with side chains > hydrocarbon rings; dehydrogenation > ring opening. As the cycloalkane content in the feed increases, the ethylene yield decreases, while the yields of propylene, butadiene, and aromatics increase. (3) Thermal cracking of aromatic hydrocarbons: Aromatic rings in aromatic hydrocarbons have high thermal stability; therefore, ring-opening reactions do not occur easily during cracking. Instead, two types of reactions take place: one is the dehydrogenation and condensation of aromatic hydrocarbons, and the other is the cleavage of the side chains of alkylaromatic hydrocarbons to produce benzene, toluene, and xylene. The dehydrogenation reaction equation is as follows: In the cracking of aromatic hydrocarbons, only dehydrogenation and condensation reactions occur due to the stability of the aromatic ring, resulting in the formation of polycyclic aromatics or even coking. Aromatic hydrocarbons with side chains are prone to side-chain cleavage or dehydrogenation reactions. Aromatic hydrocarbons are not suitable as cracking feedstocks, as they cannot increase the ethylene yield; instead, they tend to cause coking and shorten the operating cycle. The ease of cracking varies among different types of hydrocarbons as follows: n-alkanes > isoparaffins > cycloalkanes (hexacyclic > pentacyclic) > aromatics. The secondary reactions in the hydrocarbon cracking process: after the first reaction, the products formed are mainly alkenes, which can undergo further reactions. (1) Olefin cracking, such as the breakdown of large-molecule olefins into smaller-molecule olefins; (2) Olefin polymerization, cyclization, and condensation reactions; (3) Olefin hydrogenation and dehydrogenation reactions; (4) Olefin decomposition to produce carbon. II. Mechanisms and kinetics of hydrocarbon cracking reactions: (1) Chain initiation – this is the beginning of the cracking reaction; in alkanes, initiation occurs mainly through the breaking of C-C bonds, with less initiation occurring at C-H bonds. (2) The chain growth reaction – can be divided into two types of reactions, namely the decomposition reaction of radicals and the hydrogen abstraction reaction of radicals. (3) Chain termination reaction—a reaction in which a free radical combines with another free radical to form a molecule.