Note: Descriptions are shown in the official language in which they were submitted.
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CASE 7105
OXY-FUEL REBURN: A METHOD FOR NOX REDUCTION
BY FUEL REBURNING WITH OXYGEN
Field and Background of the Invention
[002] The present invention relates generally to the field of NOx emissions
control in fossil fuel-burning boilers, and, in particular, to a new and
useful method of
reducing NOx emiissions by fuel reburning with oxygen.
[003] The combustion of fossil fuels generates oxides of nitrogen, such as NO
and NO2, cumulatively referred to as NO, Emissions of NOx in the atmosphere
are
increasingly becoming a health and environmental concern. The U.S.
Environmental
Protection Agency (EPA) has determined that regulation of NOx emissions is
necessary
and appropriate, thereby creating an urgent need to develop more efficient NOx
emissions control technologies.
[004] In a conventional fossil fuel combustor, combustion air and a fossil
fuel are
mixed and provided to a main flame zone within a furnace. NO, a byproduct of
the
combustion, is formed when naturally occurring nitrogen in the fuel and/or
molecular
nitrogen in the combustion air oxidize.
[005] Fuel reburning is a technology capable of reducing NOx emissions. The
technology includes providing an oxygen-deficient secondary combustion reburn
zone
above an oxygen-rich main combustion zone. Supplementary fuel provided to the
reburn zone generates hydrocarbon radicals, amines, and cyanic species that
react with
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incoming main combustion products to convert NO, to N2. Additional air may
then be
provided by overfire air (OFA) ports, placed above the reburn zone, to burn
combust the
remaining fuel and combustible gases.
[006] Fuel reburn applications generally utilize flue gas recirculation (FGR)
technology to reduce NO, emission. Flue gas from downstream of boiler is
recirculated
via conduits back to the secondary combustion zone as an oxygen-lean carrier
gas,
thereby maintaining a fuel-rich environment and enhancing the fuel penetration
and
mixing with the main combustion zone gases and products. Quenching, resulting
from
utilizing flue gas from downstream of the boiler outlet as a carrier gas,
further inhibits
NOx formation in the reburn zone.
Summary of the Invention
[007] It is an objective of the present invention to provide a method of NOx
reduction by fuel reburning with oxygen that exceeds NOx reduction levels
achieved
with conventional reburning.
[008] It is further objective of the present invention to provide a method for
NOx
reduction including combusting fuel with oxygen in a reburn zone located
downstream
of a main combustion zone wherein air is combusted with a fossil fuel.
[009] It is another objective of the present invention to provide a method for
NOx
reduction suitable for use with both wall-fired and cyclone fired boiler
configurations.
[0010] A method of NOx reduction according to the present invention comprises
the steps of creating a first combustion zone by combusting a first fuel with
a first air
stream, wherein the first combustion zone is provided with an excess of the
first air
stream to produce a fuel-lean combustion environment, producing a plurality of
NOx
species in the first combustion zone, creating a reburn zone downstream of the
first
combustion zone by combusting a second fuel with an oxygen stream, wherein the
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reburn zone is provided with an excess of the second fuel to produce a fuel-
rich
combustion environment, producing a plurality of NOX reducing precursors in
the reburn
zone during combustion, and reacting the plurality of NOX reducing precursors
with NOX
produced in first combustion zone, wherein a substantial portion of the NOX is
converted
to molecular nitrogen.
[0011] A method of NOX reduction according to the present invention provides
additional benefits over that of conventional reburning including but not
limited to higher
flame temperature via reducing the dilutent effect of nitrogen and other inert
gases
present in air, enhanced fuel pyrolysis, enhanced production of NOX
precursors, and
improved char burnout.
[0012] The various features of novelty which characterize the present
invention
are pointed out with particularity in the claims annexed to and forming a part
of this
disclosure. For a better understanding of the invention, its operating
advantages and
specific benefits attained by its uses, reference is made to the accompanying
drawings
and descriptive matter in which the preferred embodiments of the invention are
illustrated.
Brief Description of the Drawings
[0013] Fig. 1 is a schematic diagram of a conventional fuel reburning method.
[0014] Fig. 2 is a schematic diagram of an oxygen fuel reburning NOX emission
reducing method in accordance with an embodiment of the present invention.
[0015] Fig. 3 is a schematic view of a oxygen fuel delivery means capable of
being used in an embodiment of the present invention.
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Description of the Preferred Embodiments
[0016] Referring to the drawings, generally where like numerals designate the
same or functionally similar features, throughout the several views and first
to Fig. 1,
there is shown a schematic diagram view of a conventional fuel reburning
method. Fuel
and air are supplied to the boiler via burner apparatus 1 and combusted,
forming a
combustion zone 2. Excess air is generally supplied to the combustion zone in
order to
create a fuel-lean environment having a stoichiometric ratio (oxidant-to-fuel
basis)
above about 1Ø
[0017] A second fuel and air stream are supplied to the boiler via a second
burner apparatus 3 and combusted, forming a fuel-rich reburn zone 4 downstream
of
the combustion zone 2 having a combined stoichiometric ratio below about 1Ø
[0018] A third air stream is supplied to the boiler via an overtire air port
5, forming
a burnout zone 6, wherein the third air stream combusts remaining combustible
matter
for complete combustion.
[0019] Referring now to Fig. 2, a schematic diagram of an oxygen fuel
reburning
NOX emission reducing method in accordance with the present invention is
shown. A
first fuel and first air stream are supplied to the boiler via a burner
apparatus 7 and
combusted, forming a combustion zone 8. Excess first air is generally supplied
to the
combustion zone in order to create a fuel-lean environment having a
stoichiometric ratio
of about 1.0, and preferably between about 0.9 and about 1.2.
[0020] A second fuel and an oxygen stream are supplied to the boiler via a
second burner apparatus 9 and combusted, forming a fuel-rich reburn zone 10
downstream of the combustion zone 9 having a combined stoichiometric ratio
below
about 1.0, and preferably between about 0.6 and 1Ø
[0021] Injection of an oxygen stream into the reburn zone 10 increases the
local
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flame temperature via reducing the dilutent effect of nitrogen and other inert
gas of
which air comprises. Higher flame temperature enhances fuel pyrolysis and
generation
of NOX reducing precursors, including but not limited to hydrocarbon radicals,
amines,
and cyanic compounds.
[0022] Increased NOX reducing precursors concentrations facilitate improved
removal of NOX emission generated in the combustion zone 8. Additional NO,
emissions generated in the reburn zone 10 are minimized as nitrogen introduced
though
the prior art air stream is no longer present, thus not capable of producing
NO,
emissions which would otherwise compete for the limited supply of NOX reducing
precursors generated.
[0023] In an alternative embodiment of the present invention a burnout zone 12
may be positioned downstream of the reburn zone 10 for combusting remaining
combustible matter not combusted in either the combustion zone 8 or reburn
zone 10.
The burnout zone 12 may be formed by providing a second air stream to an
overtire air
port 13, and allowing combustion of remaining combustibles to ensue.
[0024] In another alternative embodiment, pulverized coal may be utilized as
the
second fuel and provided to the reburn burner 9 via a carrier gas. The second
fuel
carrier gas can be air, oxygen, recirculated flue gas, or any other gas known
to one of
skill in the boiler arts. Conduits may be installed anywhere along the boiler
system, in
recirculated flue gas embodiments, to allow desired quantities of flue gas to
be
recirculated into the boiler and obtain further NOX reduction via quenching of
the reburn
zone 10. The oxygen stream can then be premixed with the recirculated flue
gas, or
injected via an oxygen delivery means into the reburn zone 10.
[0025] In another alternative embodiment of the present invention oxygen is
delivered to the reburn zone 10 by an oxygen delivery means inside a reburn
burner 9.
The oxygen deliver means may be any spud or lance commonly know in the art.
Alternatively, referring to Fig. 3, the oxygen deliver means may by a multi-
bladed
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injection device wherein oxygen is provided to the reburn zone 10 through a
plurality of
hollow blade conduits 14 in fluid connection with a hollow guide pipe conduit
15.
[0026] In another embodiment wherein fuel oil is used as the second fuel, fuel
oil
can be sprayed into the reburn zone 10 using oxygen as the atomizing medium.
[0027] In yet another embodiment natural gas or other gaseous fuels can be
injected with oxygen by separate delivery means within a reburn burner 9.
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