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Tpicos Especiais: Degradao e flamabilidade de materiaisAula 02 aspectos ligados termodinmica do fogoIntroduoFalarIntroduoComo processo, o fogo pode ter varias formas: Reaes qumicasSe manipulado corretamente uma grande fonte de energiaPorm pode causar muitos prejuzosFogo: manifestao de uma reao qumicaModo de queima ou combusto depende do estado de disperso do e natureza qumica do combustvel, condies ambientais,

Exemplos simples: pedao de madeira e lascas de madeira3Natureza dos combustveisTabelas temperatura

5

Combustveis e o Processo de Combusto

Combustveis e o Processo de CombustoAquecimentoPiroliseIgnioCombusto e PropagaoExtino

Combustveis e o Processo de CombustoCombustveis e o processo de combusto

Natureza dos combustveis

Mecanismos de

Thermal stability can be quantified by determining how the rate of decomposition varies with temperature. These results may be expressed in a number of ways, the most common arising from the assumption that the pyrolysis proceeds according to a simple kinetic scheme such that:

where m represents the mass (or more correctly, the concentration) of the polymer. While this is a gross simplification, it does permit k , the rate coefficient, to be determined although this is of little direct value per se. However, it allows the temperature dependence of the process to be expressed in a standard form, using the Arrhenius expression for the rate coefficient, i.e.

where E A is the activation energy (J/mol), R is the universal gas constant (8.314 J/K.mol) and T is the temperature (K). The constant A is known as the pre-exponential factor and in this case will have units of s1 . Much research has been carried out on the thermal decomposition of polymers (Madorsky, 1964; National Bureau of Standards, 1972; Cullis and Hirschler, 1981; Hirschler and Morgan, 2008), but in view of the chemical complexity involved, combined with problems of interpreting data from a variety of sources and experimental techniques, it is not possible to use such information directly in the present context. Some activation energies which were derived from early studies (Madorsky, 1964) are frequently quoted (e.g., Williams, 1974b, 1982) and are included here only for completeness (Table 1.5). However, without a knowledge of A (the pre-exponential factor), these do not permit relative rates of decomposition to be assessed.

11Energia de ativao de polmeros

Polmeros

Aspectos Qumicos da combustowhere Q m is the heat flux supplied by the flame (kW/m2 ) Q represents the losses expressed as a heat flux through the fuel surface (kW/m2 ). Lv is the heat required to produce the volatiles (kJ/g) which, for a liquid, is simply the latent heat of evaporation.

There are two distinct regimes in which gaseous fuels may burn, namely: (i) in which the fuel is intimately mixed with oxygen (or air) before burning, and (ii) in which the fuel and oxygen (or air) are initially separate but burn in the region where they mix. These give rise to premixed and diffusion flames, respectively: it is the latter that are encountered in the burning of gas jets and of combustible liquids and solids (Chapter 5). Nevertheless, an understanding of premixed burning is necessary for subsequent discussion of flammability limits and explosions (Chapter 3) and ignition phenomena (Chapter 6), and for providing a clearer insight into the elementary processes within the flame (Section 3.2).In a diffusion flame, the rate of burning is equated with the rate of supply of gaseous fuel which, for gas jet flames (Section 4.1), is independent of the combustion processes. A different situation holds for combustible liquids and solids, for which the rate of supply of volatiles from the fuel surface is directly linked to the rate of heat transfer from the flame to the fuel (Figure 1.4). The rate of burning (m) can be expressed quite generally as:

where Af is the fuel surface area (m2 ), DHc (kJ/g) is the heat of combustion of the volatiles and is a factor (