Energy Relationships

No single value exists for the total energy required to py-rolyze any material. It depends upon the products formed which depend on the temperature, rate of heating, and sample size. Therefore, the reported values for the heat of pyrolysis conflict among various experimenters.

Figure 10.12.1 expresses the general energy requirements to pyrolyze a material as the amount of oxygen varies. The lower solid line represents the amount of heat added or removed from the system. The upper solid line represents the chemical energy of the pyrolysis products. For pure pyrolysis, no oxygen is available, and all energy for the pyrolysis reaction is supplied from indirect heating. The heat required is given by q, which represents the heat necessary to pyrolyze the solid feed and heat the products to the pyrolysis temperature. The value AH1 represents the chemical energy of the gas. As oxygen is made available, energy is released within the system, and less indirect energy is supplied.

Pure pyrolysis involves only the reaction in Equation 10.12(1), the destructive distillation in an oxygen-free atmosphere. This definition can be expanded to include systems in which a limited amount of oxygen is made available to the process to release enough chemical energy for the pyrolysis reaction.

Comparing the results of various experimental investigations on pyrolysis is difficult because of the many variables influencing the results. No reliable design methods have been developed that allow for the scale-up of the experimental results. However, certain guiding principles underlying all pyrolysis systems can help in the selection of a process that most likely satisfy a particular need.

The process and operating conditions vary depending upon the relative demand for the char, liquid, and gas from the process.

FIG. 10.12.1 General energy requirements for solid-gas reactions as a function of oxygen availability.

At point 2, an adiabatic condition is reached where the heat released from the oxidation of a portion of the py-rolysis products can furnish the energy required for the py-rolysis reaction as well as the energy necessary to heat the pyrolysis products, oxidation products, and nitrogen to the pyrolysis temperature. The value AH2 represents the total chemical energy of the gas under these conditions. The larger fraction of the energy goes to sensible heat if nitrogen is present and AH2 is smaller.

As the available oxygen increases, heat must be removed to maintain a constant reaction temperature. At point 3, the stoichiometric oxygen for complete combustion is reached, and the reaction products contain no chemical energy. Additional oxygen acts only as a coolant; therefore, less energy must be removed until point 4 is reached. This point is where the feed is being incinerated adiabati-cally, and no heat recovery is possible. This figure shows that the combined energy of the pyrolysis products is higher when the available oxygen is reduced. An advantage of the oxygen dependency is that it eliminates the limitation of pyrolysis systems on the rate of external heat demand. When enriched oxygen is used rather than air, the fraction of energy tied up in sensible heat is less, leaving more chemical energy in the pyrolysis products (the greater the fraction of chemical energy).

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