Pyrolysis products factors affecting

The following discussion shows how the chemical composition, rate of formation, and heat of combustion of the pyrolysis products are affected by the variations in the composition of the substrate, the time and temperature profile, and the presence of inorganic additives or catalysts. The latter aspect, however, is discussed in more detail in Chapter 14. Combustion may be defined as complex interactions among fuel, energy, and the environment. Consequently, the combustion process is controlled not only by the above chemical factors, but also by the physical properties of the substrate and other prevailing conditions affecting the phenomena of heat and mass transport. Discussion of this phenomenon is beyond the scope of this chapter. 

Many excellent reviews (Howard,32 Gavalas,3 and Solomon and Serio33) have been published that discuss the factors affecting coal pyrolysis and product composition. In the following sections, major conclusions presented in the literature are discussed (as summarized in Table 19.14). 

A lthough coke formation is always of importance during pyrolysis processes that are used for production of ethylene and other valuable olefins, diolefins, aromatics, etc., relatively little is known about the factors affecting such coke formation. It has been found that operating conditions, feedstock, pyrolysis equipment, and materials of construction and pretreatments of the inner walls of the pyrolysis tubes all affect the production of coke. General rules that have been devised empirically at one plant for minimizing coke formation are sometimes different than those for another plant. It can be concluded that there is relatively little understanding of, or at least little application of, fundamentals to commercial units. 

The former variables affect the deposition of heat in the solid fuel and its transient temperature-profile, as well as the diffusion of the volatile pyrolysis products and their distribution and mixing with the surrounding atmosphere. The latter factors influence the nature and sequence of the primary and secondary reactions involved, the composition of the flammable volatiles, and, ultimately, the kinetics of the combustion. Consequently, basic study of the combustion of cellulosic materials or fire research has been channeled in these two directions. 

Although it has been noted that different types of metal surfaces of heating elements affect the pyrolysis process in a different manner (which is one of the reasons why the repeatability of the results in different laboratories is poor), it should be pointed out that the effect of the metal support on analytical pyrolysis must not always be regarded as a negative factor. Consider now some possible positive aspects of the influence exerted by the metal surface on pyrolysis (1) metal additives may improve the specificity of the pyrolysis products and enhance the selectivity of pyrolysis and (2) metal additives may in some instances improve the separation and simplify identification (as a result of the catalytic conversion of the pyrolysis products on metals). To enhance the effect of metal additives, one should not only use metals as the heating surface but also introduce them into the pyrolysed sample, e.g., by mixing the polymer with powdered metal. 

In coal pyrolysis the factor that most significantly affects the nature and proportions of the pyrolysis products is coal rank—i.e. (in general terms), the volatile matter content of a coal as measured under standard conditions. Beyond that, however, the products from even a single coal are very strongly affected by the pyrolysis temperature, rate of heating, particle diameter, and ambient atmosphere. 

The structures of some of the most common tobacco alkaloids found in smoke are shown in Figure XVII.B-3. The transfer rate for nicotine is generally about 10% to 12% but a number of factors, including the cut width of the tobacco, the shape of the cigarette, the moisture content of the tobacco, and various tobacco additives can significantly affect nicotine delivery to mainstream smoke [Kuhn and Klus (2231), Hecht et al. (1580)]. Many of the alkaloids of tobacco smoke consist of components originally in the tobacco, which transfer unchanged, and various pyrolysis products of the nicotine alkaloids. 

The above analysis of the factors affecting pyrolysis selectivity now permits the selection of a locus or conditions (i.e., a selectivity line) which will achieve a given pyrolysis selectivity and thus, a certain yield structure. Since this constant selectivity line covers a wide range of combinations of average residence times and average hydrocarbon partial pressures, the question remains what other factors should be considered by the designer to select a point on this selectivity line which is optimum for large-scale commercial production of olefins ... 

Wampler and Levy [1] have discussed factors affecting reproducibility in pyrolysis - gas chromatography (Py-GC) such as sample size, sample inhomogeneity, and pyrolyser design. There are two broad areas of application of Py-GC. The first is its use as a means of qualitatively identifying unknown polymers, for example, competitors products or in forensic investigations. This fingerprinting approach, useful as it is, is not pursued further in this book. 

The temperature profile is the most important aspect of operational control for pyrolysis processes. Material flow rates, both solid and gas phase, together with the reactor temperature control the key parameters of heating rate, highest process temperatures, residence time of solids and contact time between solid and gas phases. These factors affect the product distribution and the product properties. Solid residence time is another important factor in the bio-oil yields. A short residence time enhances biooil yields, while a longer residence time increases char production (Antal and Gronli, 2003). 

ZnO displays similar redox and alloying chemistry to the tin oxides on Li insertion [353]. Therefore, it may be an interesting network modifier for tin oxides. Also, ZnSnOs was proposed as a new anode material for lithium-ion batteries [354]. It was prepared as the amorphous product by pyrolysis of ZnSn(OH)6. The reversible capacity of the ZnSn03 electrode was found to be more than 0.8 Ah/g. Zhao and Cao [356] studied antimony-zinc alloy as a potential material for such batteries. Also, zinc-graphite composite was investigated [357] as a candidate for an electrode in lithium-ion batteries. Zinc parhcles were deposited mainly onto graphite surfaces. Also, zinc-polyaniline batteries were developed [358]. The authors examined the parameters that affect the life cycle of such batteries. They found that Zn passivahon is the main factor of the life cycle of zinc-polyaniline batteries. In recent times [359], zinc-poly(anihne-co-o-aminophenol) rechargeable battery was also studied. Other types of batteries based on zinc were of some interest [360]. 

Temperature is one of the most important factors that affect the process of plastics pyrolysis. The required temperature varies with different types of plastics and the desired composition of products. At a temperature above 600°C, the products are mainly composed of mixed fuel gases such as H2, CH4 and light hydrocarbons At 400-600°C, wax and liquid fuel are produced. The liquid fuel products consist mainly of naphtha, heavy... 

One feature of the pyrolysis reaction which is not fully understood is the role of reactor surface. Although the thermal decomposition of propylene has been at least implicitly assumed in some cases to be a completely homogeneous gas-phase reaction, the influence of reactor surfaces has been demonstrated frequently. Factors such as the material of construction, surface-volume ratio of the reactor, and chemical treatment of reactor surface often affect the rate of reaction and/or the product distribution. Many previous investigations are of limited value in determining the role of surface in the pyrolysis reaction. The literature on surface effects contains predominantly qualitative information and often contradictions and anomalies. Early studies... 

Acetate pyrolysis is normally carried out using a flow apparatus at temperatures in die region of 3(X)-600 °C. The syn elimination mechanism (equation 48) was established by pyrolysis of the stereospecifi-cally deuterium-labeled acetates (116) and (117), which gave labeled and unlabeled products as shown in equations (49) and (50). 1,2-Disubstituted acyclic alkenes are normally obtained with the ( )-stereo-chemistry, as for the other eliminations discussed in this chapter. However, the regioselectivity of acetate pyrolysis can be rather modest. This is because the regioselectivity is affected by several factors, in particular the stability of the product alkene and the number of available 3-hydrogens. If these two factors oppose each other only low regioselectivity is observed, e.g. a 50 50 mixture of alkenes (119) and (120) was obtained from the pyrolysis of acetate (118 equation 51). 

Jackson and Walker studied the applicability of pyrolysis combined with capillary column gas chromatography mass spectrometry to the examination of phenyl polymers (eg. styrene-isoprene copolymer) and polymer like phenyl ethers (eg. bis(m-(m-phenoxy phenoxy)phenyl)ether). They examined the effect of varying parameters affecting the nature of products formed and relative product distribution in routine pyrolysis. These parameters include the effects of pyrolysis temperature rise times, pyrolysis temperatures up to 985 C and pyrolysis duration. Temperature rise time (0.1 to 1.5 s) is not a critical factor in the Curie point pyrolysis of a styrene-isoprene copolymer, either with regard to the nature of the products formed or their relative distributions. Additionally, the variation of pyrolysis duration or hold time (2.0 to 12.5 s) at a fixed Curie temperature reflected no change in the nature of components formed however changes in product distributions were observed. Variations in Curie temperature at a fixed pyrolysis duration produced drastic changes in product distributions such as a three-... 

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