Myth of Pyrophoric Carbon.
Myth of Pyrophoric Carbon.
(1115 K)
Cuzzillo, B. R.; Pagni, P. J.
Fire Safety Science. Proceedings. Sixth (6th)
International Symposium. International Association for
Fire Safety Science (IAFSS). July 5-9, 1999, Poitiers,
France, Intl. Assoc. for Fire Safety Science, Boston,
MA, Curtat, M., Editor(s), 301-312 pp, 2000.
Sponsor:
National Institute of Standards and Technology,
Gaithersburg, MD
Keywords:
fire research; fire safety; fire science; self heating;
wood; ignition; thermal explosions; autoignition;
thermal inertia; permeability
Abstract:
The hypothesis of "pyrophoric carbon," that cooking
causes chemical enhancement of self-heating properties,
is inconsistent with measurements of the self-heating
parameters of cooked and uncooked wood chips. These
tests employed a recently-introduced experimental method
that offers advantages over traditional iterative
techniques. Predicted critical sizes and temperatures
are higher for the cooked material than the uncooked.
The additional effects of cooking-induced reductions in
the thermal inertia, due mainly to lowered density, are
quantified experimentally. The cause of delayed ignition
of whole wood is shown to be diffusion-controlled
self-heating facilitated by cooking. The delay is caused
by long-term effects of cooking on the physical
structure of wood cells, which consist of long slender
tubes. This microstructure, when undamaged, accounts for
high axial permeability but low transverse permeability,
with ratios on the order of 10(4): 1 for the softwoods
typically used as structural lumber. Cooking eventually
allows oxygen access deep into whole wood via greatly
enhanced transverse permeability caused by disruption to
the microstructure in the form of transverse cracks.
Lack of oxygen diffusion prior to cooking prevents
self-heating in whole wood except near end-grain
surfaces. These processes are demonstrated
experimentally.
Building and Fire Research Laboratory
National Institute of Standards and Technology
Gaithersburg, MD 20899