Page 163 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

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Encyclopedia of Physical Science and Technology EN006K-933 July 12, 2001 15:6

Fuel Chemistry 273

with the increase in the air to fuel ratio, CO and hydrocar- the atmosphere by the fuel flow with which it mixes in the
bon emissions are reduced but NO x emissions increase. burner passage leading to the burner ports where the mix-
To reduce all the emissions, three-way catalysts are being ture is ignited and the flame stabilized. The induced air
used. They not only oxidize CO and HCs but also reduce flow is determined by the fuel flow-through momentum
NO x to N 2 . exchange and by the position of a shutter or throttle at the
air inlet. Hence, the air flow is a function of fuel velocity
as it issues from its orifice or nozzle, or of the fuel supply
XIII. GASEOUS FUELS
pressure at the orifice. With a fixed fuel flow, the equiv-
alence ratio is adjusted by the shutter, and the resulting
A. Combustion of Gaseous Fuels
induced air flow also determines the total mixture flow,
In any gas burner some mechanism or device (flame holder since desired air–fuel volume ratio is usually 7 or more,
or pilot) must be provided to stabilize the flame against depending on the stoichiometry. Burners of this general
the flow of unburned mixture. This device should fix the type with many multiple ports are common for domestic
position of the flame at the burner port. Although gas burn- furnaces, heaters, stoves, and for industrial use. The flame
ers vary greatly in form and complexity, the distribution stabilizing ports in such burners are not always round and
mechanism is fundamentally the same in most. By keep- maybe slots of various shapes conform to the heating task.
ing the linear velocity of a small fraction of the mixture Atmospheric industrial burners are made for a heat re-
flow equal or less than the burning velocity, a steady flame lease capacity of up to 50 kJ/sec −1 and even despite their
is formed. From this pilot flame, the main flame spreads varied designs their principle of stabilization is basically
to consume the main flow at a much higher velocity. The the same as that for the Bunsen burner. In some the mix-
area of the steady flame is related to the volume flow rate ture is fed through a fairly thick-walled pipe or casting of
of the mixture: appropriate shape for the application and the desired dis-
˙ tribution of flame. The mixture issues from many small
V mix = A f × S u
and closely spaced drilled holes, typically 1–2 mm in di-
where, ameter, and burns, as rows of small pilot flame, spark or
˙
V mix = volumetric flow rate heated wire, usually located near the first holes, to avoid
accumulationoftheunburnedmixturebeforeignition.The
A f = area of the steady flame
rate of total heat release for a given fuel–air mixture can
S u = burning velocity
be scaled with the size and number of holes—e.g., for
The volume flow rate of the mixture is, in turn, propor- 2-mm-diameter holes it would be 10–100 J/(hole) or in
tional to the rate of heat input: general 0.3–3 kJ cm sec −1 of port area, depending on
2
the fuel. The ports may also be narrow slots, sometimes
˙
˙
Q = V mix × HHV
packed with corrugated metal strips, to improve the flow
where, distribution and lessen the tendency to flashback.
˙
V mix = volumetric flow rate Gas burners that operate at high pressures are usually
intended for much higher mixture velocity or heating in-
HHV = higher heating value of the fuel
˙
Q = rate of heat input tensity and the stabilization against blowoff must therefore
be enhanced. This can be achieved by a number of meth-
In the simple Bunsen flame on a tube of circular cross ods such as (1) surrounding the main port with a number
section, the stabilization depends on the velocity varia- of pilot ports and (2) using a porous diaphragm screen.
tion in the flow emerging from the tube. For laminar flow In order to achieve high local heat flow the port velocity
(parabolic velocity profile) in a tube, the velocity at a ra- ofthemixtureshouldbeincreasedconsiderably.Inburners
dius, r, is given by: that achieve stabilization by causing pilot ports, most of
2
2
v = const(R − r ) the mixture can be burned at a port velocity as high as
100 Su to produce a long pencil-like flame, suitable for
where, operations requiring a high heat flux.
v = laminar flow velocity
R = tube radius SEE ALSO THE FOLLOWING ARTICLES
r = flame radius
const = experimental constant
CARBON CYCLE • COMBUSTION • COAL PREPARATION •
Most of the commercial gas–air premixed burners are COAL STRUCTURE AND REACTIVITY • ENERGY FLOWS IN
basically laminar-flow Bunsen burners and operate at at- ECOLOGY AND IN THE ECONOMY • ENERGY RESOURCES
mospheric pressure—i.e., the primary air is induced from AND RESERVES • FOSSIL FUEL POWER STATIONS—COAL