Applications Of Photoionization Mass Spectrometry On Pyrolysis Study Of Typical Waste Polymers

Recently, in consideration of the depletion of fossil fuels and the environmental pollution, the potential use of the municipal solid waste (MSW) has become attractive to many countries, as its effective use can greatly ease the shortage of fossil fuels. As the largest developing country, China produces the largest amount of MSW worldwide per year. However, landfill and incineration are still the main processing methods of MSW in China, which can undoubtedly cause serious environmental problems. Pyrolysis, one of the thermal conversion technologies, could be a promising method for the recycling of MSW in the future. Fuels and chemical feedstock can be made from the polymers contained in MSW through pyrolysis. Previous studies on polymer pyrolysis mainly focused on thermal degradation behavior of polymers and distribution of pyrolysis products. Very little effort has been made on the on-line analysis of the pyrolysis products. In this dissertation, vacuum ultraviolet photoionization mass spectrometry (VUV-PIMS) which is an on-line detection method, was employed to study the pyrolysis of several typical polymers together with some other common methods. This dissertation consists of five chapters.In Chapter1, a brief introduction about the current situation of fossil fuels and MSW’s utility value was given. Previous studies on pyrolysis of polymers were also reviewed, on the basis of which the significance and the purpose of this dissertation were given.In Chapter2, the experimental methods were introduced in detail, which include the low-pressure synchrotron VUV-PIMS, atmospheric single-photon ionization mass spectrometry (SPI-MS), gas chromatography-mass spectrometry (GC-MS), thermogravimetry-fourier transform infrared spectroscopy-mass spectrometry (TG-FTIR-MS) and direct pyrolysis mass spectrometry (DP-MS).In Chapter3, the thermal degradation behaviors of polyethylene (PE) and polypropylene (PP) were studied with TG, which showed that temperature of the maximum mass loss rate of PP was lower than that of PE. This indicates that the presence of extra methyl makes PP more reactive. Furthermore, the pyrolysis products of both PE and PP detected with GC-MS could be classified into three groups:alkane, alkenes, dienes, and aromatics. In addition, SPI-MS was used to study the pyrolysis and the catalytic pyrolysis of PE and PP. The production of light hydrocarbons and aromatics become more intensive under a higher temperature. Aromatic products are greatly increased with the addition of HZSM-5zeolite. Besides, a two-stage catalytic degradation process taking place on the external surface and the micropores of HZSM-5was verified. A degradation mechanism was also proposed for the pyrolysis of these two kinds of polyolefin with a low HZSM-5content (20wt.%) based on the time-evolved profiles performed at a low temperature.In Chapter4, polyurethane (PU) and acrylonitrile butadiene styrene copolymer (ABS) were chosen for the pyrolysis study of N-containing polymers. Ammonium polyphosphate (APP) and expandable graphite (EG) were added to the PU as flame retardants. TG analysis shows that all the samples decompose in two major steps and more high-temperature residues are formed with the addition of APP and EG. Spectra recorded by FTIR reveals that the two thermal degradation stages could be assigned to the primary decomposition of PU and secondary reactions of primary products. On the basis of GC-MS analysis, the secondary pyrolysis products of PU are remarkably changed by APP. Large quantities of nitrogen-containing PAHs were found in the pyrolyzates of PU/APP, which could subsequently form the char layer to increase the flame retardancy of PU. However, the flame retardancy of EG mainly depends on its physical expansion character. These results were supported by the SPI-MS experiments. The pyrolysis and catalytic pyrolysis processes of acrylonitrile-butadiene-styrene copolymer (ABS) were also studied in this chapter. TG-FTIR results shows that ABS decomposes in only one step and a lot of aromatics and olefins are formed. Besides, the addition of HZSM-5and HUSY increase the formation of NH3, while decreasing the production of nitriles. All the products detected with GC-MS could be divided into three groups:aromatics, aliphatic hydrocarbons and nitriles. Comparison of the mass spectra recorded by SPI-MS indicates that the presence of HZSM-5and HUSY turns the N-containing products into NH3and HCN to a large extent. Based on the time-evolved profiles of pyrolysis products, the formations of benzene, indene, naphthalene and gaseous products were observed in two stages, which could be attributed to the thermal decomposition of polybutadiene and polystyrene within ABS.In Chapter5, the pyrolysis processes of waste papers, cotton and flax fabrics, which are typical kinds of biomass MSW, were studied under low pressure with TG, SVUV-PIMS and GC-MS. Two major peaks could be found through the TG curves of the paper samples, which could be assigned to decompositions of the paper itself (220～400℃) and the inorganic additives (600～700℃). Product identification performed with GC-MS indicates that the major fraction of these three paper samples is cellulose, while both the newspaper and the cardboard have a small fraction of lignin and could be made from soft wood. The comparison of the photoionization mass spectra of the pyrolysis products suggests that the formation of large compounds favors lower temperatures and these compounds are further decomposed as the temperature increases. As to the fabrics, the TG results reveals that the fast decomposition region of them is nearly the same (200～400℃). Main pyrolysis products were identified with mass spectra obtained at different photoionization energies together with GC-MS. Temperature-dependent mass spectra of the products shows that the most intensive pyrolysis products of cotton fabric were observed at500℃and the quantities of these products with large molecular weight would decrease under higher temperatures. Under the effect of hemicellulose and lignin components, flax fabric has a relatively wider temperature range.