3. Calculations
3.1. Chemical equilibrium analysis of PHB ignition temperature
The problem is to determine the gas phase thermochemical
decomposition products as temperature of the polymer increases
due to an external heat source. The concentration of small organic
compounds generated by heating is determined by performing
chemical equilibrium calculations, based on the minimization of
Gibbs free energy (G ¼ Go þ nRT$lnP for ideal gases). The calculations
were done using the computer code STANJAN, written by the
late Professor Bill Reynolds of Stanford University. STANJAN calculates
the equilibrium species using the element-potential method
described by Reynolds [6], which solves for the element-potentials
(which are also Lagrange multipliers) subject to atomic constraints
while minimizing the Gibbs function of the system.
A local equilibrium is established near the surface of the polymer
with remote ambient air. An external heating source heats the
polymer to its ignition temperature, and beyond. Smaller chemical
species are produced by decomposition from heating of the polymer,
and are reported as the local equilibrium concentrations of
those species. Due to wall boundary conditions at the polymer
surface, the local gas phase equilibrium is assumed to have only one
fourth of the stoichiometric O atoms required for complete
combustion of the polymer, along with its corresponding ambient
nitrogen concentration. Chemical equilibrium is sensitive to the
initial molar atomic ratios of the polymer, as well as which product
species are selected. The molar atomic ratios of the polymers are
assumed to be only those for the polymer repeating unit for these
calculations. In other words, other ingredients that could be added
are neglected.
All polymers selected for analysis (listed in Table 1) except PVC
were run with 36 allowable organic chemical equilibrium species,
shown in Table 2. PVC was run with additional chlorinated species,
for a total of 67 different chemical species, listed in Table 3. All of
these species can be found in the past as typical combustion byproducts
from industrial sources, so the required thermodynamic
data as a function of temperature have been established for these
species. Results for the ten polymers are presented in Figs. 1e10
below, in units of mole fraction for equilibrium temperatures
between 0 and 1000 C except two of the polymers. Polyethylene