Rice Hulls for Composites - Introduction to Polymeric Composites with Rice Hulls - page 58

In their raw and unprocessed state, rice hulls constitute class-A insulation material.
Therefore, they can be used very economically to insulate walls, floors and roof
cavities in building construction. If used in panels or as reinforcement in composites,
they function as cost-effective insulating materials for most applications. With the
new emerging technology of polymeric composites with rice hulls, products made
by injection moulding and compression moulding have wide ranges of applications,
but the most versatile range is for products such as lumber made by extrusion. Until
recently, this humble gift of nature has not been exploited by scientists and researchers,
and only now are they beginning to understand the full potential of rice hulls.
Rice hulls are very tough and abrasive, consisting of two interlocking halves. These
halves encapsulate the tiny space vacated by the milled grain and, in proximity to
a myriad of other hulls, they form a thermal barrier that compares well with that
of other excellent insulating materials. Using rice hulls in their natural, loose form
is not practical, so ideally they should be made into products suitable for practical
applications. Do rice hulls burn? Yes, they do but with difficulty. In fact, if a mass
of loose rice hulls is set on fire (when used as a fuel for steam boilers), the ideal
would be to sprinkle a little highly inflammable liquid such as kerosene oil and, once
the mass is lit, it burns well with high heat content. Air cannot flow freely through
a pile of rice hulls to provide oxygen to sustain rapid combustion so they do not
combust easily. The bulk density of loose rice hulls is similar to that of baled straw
and anyone who has tried to burn a bale of straw understands the problem associated
with the availability of oxygen. Lack of oxygen may be one of the problems but is
not the main problem.
The high percentage of opaline silica within rice hulls is most unusual in comparison
with other plant materials. Some scientific evaluations have concluded that, during
the combustion of rice hulls, the silica ash may form a 'cocoon' that prevents oxygen
from reaching the carbon inside. Another school of thought among scientists is that,
because silica and carbon may be partially bonded at the molecular level, silicone
carbide is formed during high-temperature combustion and that the presence of this
heat-resisting compound impedes the easy combustion of rice hulls. Other scholars
say that, at certain temperatures, the molecular bond between the silica and carbon
in the hull is strengthened, thereby preventing the thorough and uniform burning
of the hull. The general conception is that, even if a pile of loose rice hulls can be
ignited, it will tend to smoulder rather than flame. However, practical experience
by the author (operation of steam boiler with rice hulls as fuel) can certify to the
fact that, once a pile of loose rice hulls is truly lit and a good flame stage has been
reached, it maintains this status with the constant feed of loose hulls and burns
with a good flame and high heat content. The value of the resulting ash, which
hitherto has not been exploited, is also emerging as material with many possibilities
(see later).