Technology of the DRAX burner
Simple pre-heating of the air has less effect than expected
Preheating of air to a gas burner increases flame temperature (see Kirk-Othmer
Encyclopedia of Chemical Technology, vol 4, page 285). However,
if for example the air temperature is increased by 500C the increase in flame
temperature is rather less, typically 300 C or 400 C. This occurs because
the gases in the flame are partially dissociated into molecular fragments at
high temperatures
Simple pre-heating of the gas supply has more effect than expected
In a typical burner there must be 25x more air than there is propane.
Thus, naively there may seem to be little point in preheating the
propane. However,
propane pre-heating markedly
improves the usefulness of the flame, not by simple transfer of heat but by
allowing an increase in the volumetric heat release
rate:
- Preheating of either the propane or air or both causes an increase in the
effective flame speed simply because the molecules are moving faster.
- Preheating can cause propane molecules to ‘crack’ into molecules which
are faster reacting, which further increases the effective flame speed.
- The products of cracking generally have higher heats of combustion than
the propane, thus increasing the heat given out by combustion in the flame.
This extra energy clearly originates from the pre-heater, but is carried to
the primary flame as chemical rather than thermal energy. At temperatures
above 500oC propane cracks into a range of products including
hydrogen and acetylene, both of which have higher flame speeds and
combustion heats than propane.
- The effect of the higher flame speed is to dramatically shorten the flame
brush, thus concentrating combustion into a shorter length, increasing the
volumetric heat release rate.
Electric pre-heating is preferred for
some smaller burners, while gas pre-heating
may be preferred for others. Both methods can achieve the 500-700 C
pre-heat levels used in DRAX burners. An additional heated air stream can
be used in a DRAX burner to provide the capability to carry out thermal cutting
of steel, a capability which can also be achieved without
oxygen gas.
Preventing sooting
In the past, in the rare cases where fuel was pre-heated, either the level of
pre-heat had to severely limited, or a degree of sooting had to be
endured. If sooting was to be controlled, large diameter tubing was
required, and burners had to be stopped periodically and cleaned. The DRAX
burners can achieve high levels of fuel pre-heat, but can avoid sooting, usually
by a controlled bleeding of air into the fuel supply. Catalytic techniques
have also been tested.
Turbulence and better heat transfer from the burner
With the intense DRAX flame, more heat can be transferred to the workpiece
without troublesome cooling-induced ‘quenching’ of the flame.
This is achieved by use of a turbulent ‘brush’ flame, which in combination
with the high flame temperature transfers sufficient heat to a steel workpiece
to melt it.
The broad range of applications for the DRAX torch has
rendered impractical an exhaustive development programme in all application
areas for the technology. Further development opportunities are
described in the links below, including specialised burners for
very high power, for alternative gases, and for thermovoltaic
power generation.
