Note: Descriptions are shown in the official language in which they were submitted.
- 1092897
Background of the Invention
Pulverized coal has been introduced in the furnaces for
combustion in many different ways including tangential introduc-
tion into a furnace with flow upwardly or downwardly therefrom,
and vertical firing from the top of the furnace with a reversal
of the combustion gases in the furnace and exit through the top
of the furnace. Air also has been added to the furnace to
support combustion in a plurality of ways. In most cases the
air has been supplied in an effort to burn the fuel to the
fullest extent possible immediately. In other cases, staged
combustion has been used wherein a deficiency of air is supplied
with the coal in order to restrict the initial combustion rate.
By restricting -the initial combustion rate and inherently cool-
ing the flame by the furnace walls the time of combustion is
delayed and lower maximum combustion temperatures are achieved.
This reduces the formation of oxides of nitrogen which are due
to the nitrogen contained in the air.
It does not, however, appear to have a significant
effect on the oxides of nitroyen which are formed because of
the nitrogen in the fuel.
In the prior art combustion processes the procedure
was such that the volatiles which were driven off from the coal
by the initial heating continued to flow with the coal.
Therefore, as the newly introduced coal mixed with oxygen the
volatiles would tend to be the first ones burned. The remaining
carbon particles which are more difficult to burn must be
oxidized in air which is already partially depleted of oxygen.
5ummary of the Invention
It is an object of the invention to increase the
effectiveness of the burning of the carbon particles of the coal.
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It is a further object of the invention to decrease the oxides
of nitrogen formed in the combustion of fuel. Accordingly,
the invention broadly resides in a method of firing fuel in a
furnace having an upstream zone, an intermediate zone, and a
downstream zone, comprising: introducing fuel into said
furnace with a minimal amount of oxygen through said intermediate
zone in a direction and at a velocity to carry the fuel
particles into said ups.tream zone; introducing secondary air
tangentially into said upstream zone, and burning most of the
fuel therein, thereby forming combustion products; passing the
combustion products through said intermediate zone in the same
areas as the passage of fuel therethru to said downstream zone;
and introducing tertiary air into said downstream zone to
complete the burning of fuel.
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Coal is introduced with a minimum of oxygen, limited to that
which is required to convey the coal, as a high momentum stream through
an intermediate zone of the furnace. This intermediate zone contains
a relatively reducing atmosphere. The coal in pass;ng through this
at~osphere is exposed to the high radiant heat from other areas
of the furnace. Yolatile components of the coal are, therefore,
driven off. While the relatively heavy char particles continue
to the upstream zone of the furnace, the volatiles which are released
are stripped from the stream by friction with the surrounding
atmosphere and tend to remain in the intermediate zone. Conversion
to N0x of fuel nitrogen released with the volatiles would be
minimal, due to the reducing conditions.
From this zone the volatiles and products formed
th~refrom pass to a downstream zone. The char particles which have
continued to the upstream zone are therein mixed with fresh secondary
air which is tangentially introduced into this zone. In this manner,
the char particles are contacted by air which contains a maximum
oxygen content since the oxygen has not been depleted by reaction
with the volatiles from the coal. Some of the nitrogen contained
in the char will be converted to Nx
Thereafter the combustion products from the upstream
zone pass through the intermediate zone and continue to a downstream
zone wherein tertiary air is added to complete combustion of the
char particles and the volatiles which have been stripped in the
intermediate zone. The N0x formed in the upstream zone will be
partially reduced in passing through the intermediate zone.
While described with respect to coal firing, the invention
is also efficacious when firing other fuels, such as shredded
refuse, oil, etc.
Brief Description of the Drawing
Figure 1 is a schematic illustration of a furnace wherein
the coal is introduced with primary air,
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Figure 2 is a plan view taken through section 2-2 of
Figure 1 illustrating the direction of introduction of secondary
air and fuel, and
Figure 3 is a schematic arrangement of a ~urnace wherein
combustion products are used to convey the coal to the furnace.
Description of the Preferred Embodiments
A furnace indicated generally as 10 includes an inter~ediate
zone 12,an upstream zone 14 and a downstream zone 16. A duct 18
carries the combustion products from the furnace.
Pulverized coal which may be supplied either from a bin
system or directly fired from a pulverizer is supplied through coal
nozzles 20. The coal is introduced with high momentum in a direction
such that the coal particles pass through the intermediate zone 12
- into the upstream zone 14. As best seen in Figure 2, these nozzles
are preferably directed toward the vertical axis of the furnace. Since
a vortex action is set up in the furnace by introduction of air to
be described hereinafter there is less resistance to the downward
flow of coal in the center portion of the furnace and better penetration
may be achieved by directing the coal nozzles to this location.
In this intermediate zone 12 the coal stream is exposed
to radiation from the remainder of the furnace and the volatile
components of the coal are driven off. These components being gaseous
will be stripped from the stream by friction with the surrounding
environment and will, therefore, not proceed to the lower portion 14
of the furnace. Since the amount of oxygen in the intermediate zone
12 is restricted, these volatiles will not burn to completion at this
time and accordingly will create a reducing atmosphere in this zone.
In the upstream zone 14 the major portion of air is supplied
as secondary air through nozzles 22. The orientation of these
nozzles is such as to direct the flow to an imaginary circle 24
within the furnace thereby imparting a tangential action to the gases
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andeffecting a scrubbing action between the char particles and the
incoming air. The devolatilized char particles which are existing
: in the upstream zone 14 are the most difficult to burn. In this
i m ention these particles contact the incoming air directly which
has the maximum oxygen content thereby increasing the effectiveness
of burning these char particles.
This air is supplied by forced draft fan 26 and conveyed
through ducts 28. Regulating damper 30 may be used to control the
amount of air introduced through nozzles 22.
Air from fan 26 may also be introduced through lines 34 with
the flow regulated from damper 36 if primary air is to be used for
transportation of the coal into the furnace. Air from fan 26
is also used to supply tertiary air through nozzles 38 in an amount
controlled by dampers 40 as tertiary air.
The turbulent ~ixing between the char particles in the
upstream zone 14 and the incoming air should be effective to burn most
of the carbon present. The combustion products from this zone continue
; with a cyclonic action passing upwardly through the intermediate
zone 12. These gases should contain little or no oxygen since
extensive combustion of the coal particles has occurred in the
lower portion of the furnace.
The gases then continue upwardly into the downstream -
zone 16 along with volatile components and products thereof which
have been driven off from the fuel in the intermediate zone 12.
Tertiary air is added through nozzles 38 in a manner to mix the
air with the combustion products and to thereby permit final
burning of the remaining char particles and the volatiles.
Referring now to Figure 3, the operation is identical
with that of Figure 1 with the exception of the coal conveying
medium. Gas fan 42 withdraws a portion of the products of combustion
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from duct 18 and passes then through line 44. As schematically
illustrated the withdrawn portion of gas passes through the coal
nozzle 20 as the conveying medium for the coal to effect high
velocity injection of the coal into the furnace. While illustrated
as joining the nozzle near the furnace, it is to be understood
that the gas could pass through a pulverizer and carry the coal
particles from that location or could alternately be used to convey
coal from a bin storage system.
Since the flue gas inherently has a very low oxygen content,
the use of this gas for transporting the coal provides a stream
with very little oxygen therein. This avoids the burning of the
volatiles due to oxygen in the primary air. For a given air fuel
ratio on a boiler this increases the availability of oxygen to the
- upstream furnace zone 14 with the volatiles depending on the tertiary
air for combustion as previously described.
Fuels other than coal may be fired, according to the
invention, with the same principle being followed, and the same
advantages accruing.
What is claimed is.
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