Temperature effects pyrolysis

It was established that carbons grafted with pyrolyzed Co(III)-Me complexes increase its electrocatalytic activity, and the temperature of pyrolysis is very important. This can be seen from the plots in Figure 3. The maximum catalytic effect is achieved for the samples annealed near 600°C, decreasing at higher temperatures and almost vanishing at 800°C (Figure 4). 

Thermogravimetric analysis (TGA) has often been used to determine pyrolysis rates and activation energies (Ea). The technique is relatively fast, simple and convenient, and many experimental variables can be quickly examined. However for cellulose, as with most polymers, the kinetics of mass loss can be extremely complex (8 ) and isothermal experiments are often needed to separate and identify temperature effects (9. Also, the rate of mass loss should not be assumed to be related to the pyrolysis kinetic rate ( 6 ) since multiple competing reactions which result in different mass losses occur. Finally, kinetic rate values obtained from TGA can be dependent on the technique used to analyze the data. 

The evidence suggests that temperature is the major parameter governing the extent to which a given fuel will soot under a particular flame condition or combustion process. As emphasized earlier, increasing the temperature under premixed fuel-air conditions decreases soot production, whereas the opposite is true for diffusion flames. The main effect of varying pressure must be manifested through its effect on the system temperature. Fuel pyrolysis rates are a... 

P.R. Westmoreland, J.B. Howard, J.P. Long well, and A.M. Dean. Predictions of Rate Constants for Combustion and Pyrolysis Pressure and Temperature Effects Reactions by Bimolecular QRRK. AIChEJ., 32(12) 1971-1979,1986. 

Previous studies of the decomposition of cellulose reported Ea for absorbent cotton as 54.3 kcal/mol at a high-temperature range of 270-310 °C (23). For temperatures below pyrolysis, Ea = 20 kcal/mol reflects the low-temperature degradation effects of loss of H and OH from adjacent carbon atoms in cellulose (dehydration) and the concomitant creation of C=C bonds (24). In another work Ea = 21 kcal/mol was estimated from Arrhenius plots of the degree of polymerization versus time for cellulose heated in air at 150-190 °C (25). 

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 23.7 The temperature effects on product phase yields of PS. (Reproduced from Journal of Analytical and Applied Pyrolysis, 60 (2), A. Karaduman, Flash pyrolysis of polystyrene wastes in a free-fall reactor under vacuum, 179-186(2001), with permission from Elsevier)...

A simple ranking of the relative activity of these functional groups showed that the relative activity of free carboxylic groups is strongest at 300 C at both pyrolysis and gasification conditions. This effect is more or less wiped out" at higher temperatures at pyrolysis. 

The concept of chemical modification (CM) is extremely popular in modern elec-trothermal-assisted atomic techniques. As per lUPAC s recommendations [47], in order to influence processes taking place in the atomizer in the desired way, reagents called chemical modifiers may be added. These can help to retain the analyte to higher temperatures during pyrolysis, remove unwanted concomitants or improve atomization in other ways . However, there is a tendency towards broadening the scope of this term, starting from the classical and still used term matrix modifier , matrix/analyte modifier or instrumental matrix modification to indicate the useful effects of the type, pressure and flow-rate of protective gas or gas mixtures internal matrix modifier for matrix constituent s) with favourable effects on processes in the atomizer, either by themselves (e.g. refractory components) or upon addition of suitable promoters permanent modifiers for... 

The ceiling temperature T can be considered the upper temperature at which a pyrolytic process will reach equilibrium. It may be seen, therefore, as a recommended temperature for pyrolysis. However, in practice, the application for macromolecules of the above relations is not straightforward. The theory was developed for ideal systems (sometimes in gas phase), and although in principle this theory should hold true for any system, its application to condensed phases or polymeric materials may be accompanied by effects difficult to account for (phase change, melting, cage effect [2], etc.). The reaction rate could also be low at calculated Tq values. For this reason, temperatures 50° C or 100° C higher than Tq must frequently be used as practical values of the temperature used in pyrolysis. 

Using the integrated form of this equation (see rel. (7)), the values for WA/Vq for cellulose can be obtained for different temperatures and pyrolysis times. Assuming a pyrolysis time of 10 s in isotherm conditions at different temperatures [in rel. (15). T should be expressed in ° KJ, several values for W/Wq expressed in % are given in Table 3.2.1. These calculated values for WA/Vq used E and A extrapolated for a wider range of temperatures than those reported in literature [8] for cellulose. The only purpose of these calculations is to illustrate the effect of temperature on the rate of pyrolysis. From Table 3.2.1 it can be seen that around 200° C cellulose is not significantly decomposed. Around 400° C the decomposition starts, and around 600° C the decomposition is practically complete. In analytical pyrolysis, 600° C and 10 s pyrolysis time (total heating time THT) could, therefore, be recommended for the experimental conditions of cellulose pyrolysis. 

A typical determination is that of Lossing and Tickner for the methyl radical. Methyl radicals were produced by the pyrolysis of mercury dimethyl diluted by helium, and the mass spectrum showed that only CH3, mercury, ethane and a trace of methane were formed. Sensitivity calibrations were obtained in the usual way for the stable substances, and then the net peak at mass 15, after subtraction of the contributions from mercury dimethyl, ethane and methane, was determined. At high temperatures of pyrolysis, where the methyl radicals were most abundant, the sensitivity for the mass 15 peak of the methyl radical could then be calculated on the basis of 100 % carbon balance. As discussed earlier, wall reactions may lead to appreciable disappearance of the radical under observation, and such effects must be taken into account when calculating the sensitivity of the apparatus. Corrections of these kinds were applied to the experiments described above when Ingold and Lossing discovered that part of the methane observed was produced by reaction in the ionization chamber. The smallest relative concentration of radicals which can be determined accurately i.e. where several species give rise to the... 

Figure 4.12 (A) ORR polarization curves obtained at an electrode rotation of 400 rpm in 0.5 M H2SO4 for a pyrolyzed CoTPTZ/C catalyst and (B) pyrolysis temperature effect on the ORR potential obtained at a current density of 0.5 mA cm (For color version of this figure, the reader is referred to the online version of this book.) Reprinted from Ref. 57, with permission from Eisevier.

The char is separated by cyclones, liquid is separated with a quench fluid and the gas is recycled or purified. The residence time in the reactor again is very short. The temperature of pyrolysis found effective is 850 F to 1050 F. Quench fluids are chosen for their immiscibility, low vapor pressure and rapid phase disengagement, A desirable fluidity in the product oil may be achieved by varying the moisture content. The moisture content of the pyrolytic oil is controlled by the quench oil temperature, A moisture content of 14-18% gives a suitable viscosity for handling municipal solid waste oil. 

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