| The polyethylene plastic has been widely used in various fields since its discovery, due to such good properties as low density, high tensile, anti-corrosion, easily-processed and so on. Meanwhile, a series of environmental problems have been aroused by the wide use of polyethylene plastic. However, plastic pyrolysis can not only reduce the volume of persistent plastics, but also effectively recycle the oil resources. Considering the deterioration of the “white pollution” in our country, and deeper national dependence on foreign oil, it is necessary to conduct the thorough research on the polyethylene pyrolysis.The pyrolysis experiments were carried out on a fixed bed tube furnace for 20 mins under three different temperatures of 400?, 450? and 500?. The pyrolysis tar collected from the reactor was different from its original state because of the secondary reactions. In order to compare the terminal tar and nascent tar, similar experiments were also conducted in a wiremesh reactor, setting heating rate as 10K/s and 100K/s, temperature as 400?, 450? and 500?, and holding time as 30 s. The components of nascent tar and terminal tar were specifically studied. The results showed that the distributions of carbon number were quite different. The average carbon atom number of the tar from the wire-mesh reactor was larger than that from the horizontal tube furnace. Moreover, under the temperature of 500?, 15.68 weight percent of naphthene was consisted in pyrolysis tar in the wire-mesh reactor while none of it was detected in the tar from the tube furnace. Therefore, it can be concluded that, due to the secondary reaction, naphthenes will convert to alkanes and alkenes and macromolecular hydrocarbons will decompose micromolecular ones.Comparative study found that there is a big difference between nascent tar and terminal tar no matter in the yields of the tar and char or in the composition and content of the tar. Therefore, the characteristics of nascent tar was further studied. The comprehensive pyrolysis experiments of polyethylene were carried out in the wire-mesh reactor. The heating rates were 1K/s, 10K/s and 100K/s and the final peak temperatures were 400?, 450? and 500? with different holding time of 0.1s, 10 s and 30 s. The results analysis was mainly about such factors as pyrolysis temperature, holding time and heating rate. The results showed that, under the same heating rate of 100 K/s and holding time of 30 s, the content of macromolecular hydrocarbon(C?31) in the nascent tar increased with the increment of temperature, like 0% at 400?, 3.64% at 450? and 14.01% at 500?. What's more, under the same heating rate of 100 K/s and peak temperature of 500?, the production of the macromolecular hydrocarbon decreased with the decline of holding time, like 14.01% during 30 s, 10.06 % during 10 s and 0% during 0.1s. Therefore, it is indicated that micromolecular hydrocarbon was will be first released at the preliminary stage of polyethylene pyrolysis and some macromolecular hydrocarbons will be gradually released with the increase of holding time. Moreover, the macromolecular hydrocarbon can be released more easily under a higher temperature.Above all, the polyethylene pyrolysis processes proposed in this paper are as follows: ? Random fracture of internal carbon chain; ? Release of the micromolecular hydrocarbons in primeval pyrolysis; ? The generation of macromolecular alkanes and alkenes; ? The generation of naphthenes under a higher temperature. |
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