Pyrolysis product distribution

Above 800 K, the Cl spectrum again became temperature independent, indicating that the final pyrolysis product distribution had been reached. The upper spectrum in Fig. 4.3 shows the result for 973 K. Note that virtually no (M — 1)+ was left, and the spectrum was dominated by (M + 1)+ and (C5H6 + H)+. These results show that the p Tolysis product distribution is dominated by C2H2 - - C5H6 and by isomerization to a more stable CrHg isomer. 

Mass conservation dictates that the pyrolysis products, distributed over the three phases, gas, liquid, and solid, consist of the same elements as the raw materials and that their relative amounts are conserved. There is a redistribution of relevant elements during pyrolysis, with hydrogen and chlorine emiching the gas phase, carbon in the coke. 

The observed pyrolysis product distribution of PP is developed by a free radical mechanism [28, 29] which begins with the homolytic breakage of the polymer chain drawn in Scheme 12.3 where X represent a methyl group in this case. Primary macroradical 1 is formed only in the initial step, thus its decomposition plays a minor role in PP. Secondary macroradicals may decompose to propene by depolymerization reproducing 2, or... 

In HIPS the butadiene content is in general low, the chemical structure of the polymer kept of vinyl type, thus the pyrolysis product distribution is very much like that of PS. However, some negligible components of PS pyrolysate such as toluene, a-methylstyrene, and 1,3-diphenylpropane are markedly produced from HIPS. These compounds originate directly from those volatile radicals which have been produced by the p-scission of the PS chain end. Presumably the intramolecular radical transfer necessary for the stabilization of the volatile radicals is facilitated due to the presence of butadiene segments in the copolymer. 

Recently the pyrolysis of polymer mixtures has become a focus of interest due to the increasing role of plastics recycling. Many researchers have investigated the thermal decomposition of various polymers in the presence of PVC. Kniimann and Bockhom [25] have studied the decomposition of common polymers and concluded that a separation of plastic mixtures by temperature-controlled pyrolysis in recycling processes is possible. Czegfny et al. [31] observed that the dehydrochlorination of PVC is promoted by the presence of polyamides and polyacrylonitrile however, other vinyl polymers or polyolefins have no effect on the dehydrochlorination. PVC generally affects the decomposition of other polymers due to the catalytic effect of HCI released. Even a few per cent PVC has an effect on the decomposition of polyethylene (PE) [32], HCI appears to promote the initial chain scission of PE. Day et al. [33] reported that PVC can influence the extent of degradation and the pyrolysis product distribution of plastics used in the... 

Figure 2. Pyrolysis product distributions from bituminous coal heated to different peak temperatures. (%) H20 and H2S (O) H20, H2S, CO, and C02 (X) H20, H2S, CO, C02, and all hydrocarbon gases (T) total weight loss, i.e., H20, H2S, CO, C02, all HC gases, tar, and liquids. Pressure = 1 atm (helium). Heating rate = 1000°C/sec. (14)...

Assuming different pyrolysis product distributions for almond shells according to the measurements of El Asri ct al. [5], Font et al. [12, 13], or Parodi et al. [14], different composition of the flue gases, in particular CO contents, have been predicted. It was found that the amount of char formed in the pyrolysis stage is of great importance. The fraction of fme particles that escape to the conical part of the reactor is of relevance for the CO production, but it doesn t affect the NO formation. The variation of the air splitting ratios between different blocks is of minor importance. 

Figure 3 Experimental. Predicted biomass reacted Table 4 Ranking of the traditional compositions on pyrolysis product distributions... 

Although under certain experimental condition, step 2 or step 3 may be ignored due to its non-conspicuous influence to the flnal pyrolysis product distribution, however, a good mathematical model for biomass pyrolysis should be versatile applicable to other pyrolysis conditions, and thus it should be involved the above-mentioned three steps of process, of course heat and mass transfer equations should be included also. This paper presents this kind of mathematical model. Although the model is constructed based on sawdust pyrolysis, it is quite straightforward to apply the same approach to other cases such as straw and municipal solid waste pyrolysis even if to biomass or coal gasification or metal ore reduction. 

Amorphous and semi-crystalline polypropylene samples were pyrolyzed in He from 388°-438°C and in air from 240°-289°C. A novel interfaced pyrolysis gas chromatographic peak identification system was used to analyze the products on-the-fly the chemical structures of the products were determined also by mass spectrometry. Pyrolysis of polypropylene in He has activation energies of 5-1-56 kcal mol 1 and a first-order rate constant of JO 3 sec 1 at 414°C. The olefinic products observed can be rationalized by a mechanism involving intramolecular chain transfer processes of primary and secondary alkyl radicals, the latter being of greater importance. Oxidative pyrolysis of polypropylene has an activation energy of about 16 kcal mol 1 the first-order rate constant is about 5 X JO 3 sec 1 at 264°C. The main products aside from C02, H20, acetaldehyde, and hydrocarbons are ketones. A simple mechanistic scheme has been proposed involving C-C scissions of tertiary alkoxy radical accompanied by H transfer, which can account for most of the observed products. Similar processes for secondary alkoxy radicals seem to lead mainly to formaldehyde. Differences in pyrolysis product distributions reported here and by other workers may be attributed to the rapid removal of the products by the carrier gas in our experiments. 

Finally, in many cases the pyrolysis product distribution is so complex that it is not possible to separate and identify all of the components. In such cases a chemometric method, simply treating the pyrogram as a pattern, can be used to compare one sample with another. This approach has foimd many applications in forensic analysis, for example, of paints and in analysis of authenticity of art works (54). It has even been used to type bacteria (55-57). These applications often take advantage of the fact that the high sensitivity of the analytical method allows minute samples to be analyzed they depend upon a database of pyrograms obtained under well-controlled conditions and analysis involves a statistical pattern-matching approach. 

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