1.1 Benefits of Boron as a Fuel
Boron has great potential for use as an additive for an energetic material. Apart from being
a relatively common element, it has the greatest heating value of any other fuel, except for 3 beryllium, used with oxygen. Table 1.1 lists the
heating values for some of the more common
fuels. Beryllium, when it reacts with oxygen, forms BeO which is extremely toxic, so beryllium is
never considered a viable fuel. As can be clearly seen, boron has considerably greater heating
values per unit mass and per unit volume. If it is to be used as a rocket fuel, the reduction of
weight and volume of the fuel will result in higher payloads or longer durations of flight, not to
mention a more cost effective flight.
Also being considered is the use of boron for controlled, non-ideal detonations. The high
energy output with delayed reaction will generate an expanded pressure-volume process, which would result in more work output. However, having a high
heating value does not alone make a U material a desirable fuel.
1 1.2 Disadvantages of Boron as a Fuel
For a material to be a likely candidate for a fuel or fuel additive, it must be able to ignite,
burn, and release its energy within the combustor region of a rocket. Unfortunately, boron does
not meet this criteria for most applications [1]. The main reason that boron is difficult to ignite is 5 that the particle is coated with an oxide
layer, B20 3. The oxide layer, which is present whenever
the particle is in an oxygen containing atmosphere, inhibits further oxidation of the particle and
therefore restricts the ignition process.
Burning of the particle is also restricted by the oxide layer. After the particle reaches a
certain temperature, the oxide layer will liquefy. This allows some oxygen to slowly diffuse
through the liquid layer and to react with the boron. However, the reaction will then produce more
oxide and will increase the thickness of the oxide layer, which will retard the diffusion process as 3 well as the combustion of the particle.
Finally, full utilization of energy from the combustion reaction is difficult to achieve 3 because most of the energy is never released. The chemical
reaction of the boron and oxygen is
exothermic but most of that energy initially is used to continue the heat up of the particle. Also,
because boron has high melting and boiling temperatures, the heating of the particle can continue
for durations longer than most residence times in combustors. If the particle happens to react
completely, most of the products formed will be in the gas phase. The high energy output shown
in Table 1.1 is not achieved until the products condense to liquid phase. The trapping of the
products in the gas phase can potentially reduce the heating value of boron by up to 25 percent.
The condensing of the boron products is relatively slow until the temperature drops significantly,
and therefore the benefits of the high energy release are realized too late [1, 3]. |
