Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02245294 1999-12-08
FIELD OF THE INVENTION
This invention is directed to a method and apparatus for improving combustion
and the
operation of new or retrofitted boilers in various ways. The adoption of the
proposed
method and apparatus can beg expected to reduce capital and operating costs.
The types
of boilers to which the invention applies are boilers burning biomass, wood
waste or
other solid fuel, and recovery boilers which burn liquor from various pulping
processes
which are employed in the manufacture of pulp and paper. These processes
include: the
kraft process, the soda process, the sodium-based sulphite process, the closed-
cycle
1o CTMP (chemical, thermal, mechanical pulp) process, the magnesium-based
sulphite
process and the ammonium-based sulphite process. Such recovery boilers, and
boilers
which burn biomass, wood waste or other solid fuel, generate steam for various
process
requirements. The rE~covery boilers, in burning the liquor, dispose of the
liquor and, in
most cases, the inorganic materials resulting from the combustion are
recovered to
regenerate the pulping chemicals. A prime function of a recovery boiler is to
convert
oxidised sulphur compounds such as, Na2S04, Na2S03, and Na2S203, to the
reduced
form Na2S, which is an activE~ component of the so-called white liquor which
is used in
the actual pulping process. The reduction efficiency of a recovery boiler is a
measure
of its ability to convert these .oxidised sulphur compounds to Na2S.
All these boilers require combustion air and generally have furnaces which are
rectangular in horizontal cross-section.
Ineffective combustion air syatems result in poor mixing of the combustion air
with the
combustibles in the furnace. (Poor mixing causes inefficient combustion, which
can lead
to excessive fouling of the heating surfaces, excessive erosion of the boiler
tubes and
overloading of the electrostatic precipitator. In certain recovery boilers,
inefficient
combustion also causes excessive emissions of TRS (total reduced sulphur),
carbon
monoxide and fume, unnecessarily low smelt-reduction efficiencies and may
cause
problems with char-bE:d control and smelt run-off. The manner in which the
combustion
air is introduced at thf~ various elevations in the furnace may generate
excessive fume.
Ineffective combustion air systems may also create a central column of rapidly-
upward-
flowing flue gases which entraiins particulate and, in recovery boilers,
liquor droplets and
particulate, and carries this material out of the furnace. This carryover
material can
cause fouling of the heating surfaces and overloading of ash hoppers.
Ineffective combustion air systems may have air jets which fail to penetrate
sufficiently
far into the furnace, thus starving the centre of the furnace of oxygen.
Alternatively, the
air jets may be too strong and impinge on the opposite furnace wall, causing
circulation
problems and/or tub.' damage. Some combustion air systems suffer lack of jet
penetration and/or e>;cessive jet penetration when operated at loads other
than the
design load.
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In boilers burning biomass, ~nrood waste or other solid fuel, the fuel is
generally burned
on a grate, or in a fluiclized bed and the combustion air is introduced both
undergrate and
through multiple air ports in the furnace walls. In this type of boiler, the
air introduced
through the wall ports is often termed overfire air, but the various air zones
may be given
the same terminology as the air zones in recovery boilers, as described below.
In recovery boilers firing liquor from the kraft process, the soda process,
the sodium-
based sulphite proceas, and the closed-cycle CTMP process, all the combustion
air is
introduced through multiple aiir ports in the furnace walls.
In recovery boilers firing liquor from the magnesium-based sulphite process,
and the
ammonium-based sulphite process, most of the combustion air is introduced as
so-called
primary air through liquor burners located in the walls or roof of the furnace
while the
remainder of the combustion air is introduced through multiple air ports in
the furnace
walls. These multiplE~ air ports may be arranged in several zones, or sub-
systems of
ports, and may be named, successively, from the burner region towards the
outlet of the
furnace, secondary aiir and tertiary air, etc.. The multiple ports of each air
zone may be
on one or more walls of the furnace.
The air ports in recovery boilers firing liquor from the kraft process, the
soda process, the
sodium-based sulphite process, and the closed-cycle CTMP process, and in
boilers
burning biomass, wood waste or other solid fuel, are arranged in several
zones, or sub
systems of ports, naimed, successively, from the furnace floor elevation,
upwards:
primary air, secondan~ air and tertiary air, etc.. The ports of each air zone
may be on one
or more walls of the furnace.
In recovery boilers which fire liiquor from the kraft process, the soda
process, the sodium-
based sulphite process, and the closed-cycle CTMP process, conventionally, the
primary
air is introduced through multiple ports in four walls, such that the quantity
of air
originating from each wall is approximately the same and the flow through all
the
individual ports is more or less equal. The primary air jets from these ports
create a
central column of rapiidly-upward-flowing flue gases which entrains liquor
droplets and
other particulate and carries tlhem out of the furnace. This carryover
material can cause
fouling of the heating ;surfaces and overloading of ash hoppers. In a similar
manner, the
other air zones of the boiler cam create or reinforce the central column of
rapidly-upward-
flowing flue gas which carries. liquor droplets and other particulate out of
the furnace.
This invention is directed to a method and apparatus for improving combustion
and the
operation of the boiler in various ways.
The method can be applied to new or retrofitted boilers.
The first embodiment: of the method in boilers firing all the above-mentioned
fuels
comprises introducingi a portion of the combustion air, or a portion of
recycled flue gas
together with a portion of the combustion air, at any elevation in the
furnace, from two
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opposing walls only, as jet:. arranged in a partially-interlaced manner, with
the jets
oriented in a more or less common plane which is not horizontal.
In recovery boilers, the first: embodiment of the method improves combustion
and
increases thermal efficiencies and, in specific cases, decreases TRS (total
reduced
sulphur) emissions and reduces fume generation, and also, minimizes the
extremes of
upward gas velocity, which, in turn, minimizes the carryover of particulate
such as liquor
droplets and fuel particles, miinimizes the build-up of deposits of unburned
liquor and/or
some of the products of combustion on the heating surfaces of the boilers, and
reduces
1o erosion of the tubular heating surfaces.
In boilers burning biomass, wood waste or other solid fuel, the first
embodiment of the
method improves combustion and increases thermal efficiencies, and also
minimizes the
carryover of particulate such as fuel particles and ash, which, in turn,
reduces erosion
of the tubular heating surface's.
A second embodiment of the method in recovery boilers which fire liquor from
the kraft
process, the soda process, the sodium-based sulphite process, and the closed-
cycle
CTMP process, comprises introducing at least 80 percent of the primary air,
that is, the
2o combustion air which is being introduced into the furnace at the lowest air
zone in the
furnace, in jets from first and second opposing walls and, where applicable,
that is, in all
but the 100 percent case, they remainder of the primary air in smaller jets
from the third
and fourth opposing vvalls of the furnace, with the ports from which all the
jets originate
located in a common plane which is not horizontal, and such that the larger
jets from the
first and second walls are directed more or less in the plane, while the
smaller jets from
the third and fourth walls may be directed steeply downwards, more or less in
the plane,
or slightly downwards., or slightly upwards relative to the plane.
In recovery boilers which fire liquor from the kraft process, the soda
process, the sodium
3o based sulphite process, and the closed-cycle CTMP process, the second
embodiment
of the method improves combustion as noted, increases reduction efficiencies,
decreases TRS (total reduced sulphur) emissions, and increases thermal
efficiencies
and, in some cases, reduces 'fume generation. The second embodiment of the
method
also improves char-bed control, reduces tube-wall metal temperatures and
attendant
metal wastage in the lower furnace, minimizes the carryover of particulate
such as liquor
droplets and fuel particles, which, in turn, minimizes the build-up of heavy
deposits of
unburned liquor and/or some of the products of combustion on the heating
surfaces of
the boilers, and reduces erosion of the tubular heating surfaces.
4o In the second embodiment, the higher velocity of air passing through the
ports of two of
the opposing furnace walls hE~lps to keep the ports clean, thus minimizing
port-rodding
requirements.
Thus, by employing the proposed method and apparatus, the operating and
capital costs
of the installation can be reduced.
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BACKGROUND OF THE INVENTION
Boilers are widely used to generate steam for numerous applications. All
boilers which
burn fuel (other than nucle<~r fuel) require combustion air. The combustion
air is
introduced into the furnace and, because the mixing of the combustion air and
the fuel
is imperfect, an air quantity in excess of the theoretical amount is required.
The
combustion air quantity which is employed in excess of the theoretical amount
of air is
termed "excess air". The theoretical combustion air and the excess air are
admitted to
1o the boiler system at ambient temperature and the excess air is exhausted to
atmosphere
with the other flue gases, at the temperature of the flue gases leaving the
stack. Excess
air thus reduces the thermal efficiency of boilers. One of the advantages of
the proposed
method is that the mixing of combustion air and combustibles in the furnace is
improved
and thus, the excess. air quantity may be reduced, thus the thermal efficiency
of the
boiler is increased.
Generally, the walls and, often, the floor of the furnaces in modern boilers
consist of
water-cooled tubes. Adjacenl: furnace tubes are fully-welded together along
their lengths
to form a gas-tight envelope which contains the furnace gases.
In the pulp and paper industry, recovery boilers are used to burn the waste
liquor
produced in a pulp-making process. The waste liquor is called black liquor in
the kraft
process, in the soda process, in the sodium-based sulphite process and in the
CTMP
process. In the socla procEas the liquor may also be called soda liquor. In
the
magnesium-based and ammonium-based sulphite processes, the liquor is called
red
liquor.
The liquor from theses pulpinc,~ processes consists of a mixture of the spent
chemicals
from the pulping procEases, and water; some of the spent chemicals are
dissolved, but
3o some are present in colloidal and particulate form.
Red Liquor Recovery Boilers
In recovery boilers firing red liquor from the magnesium-based sulphite
process, and the
ammonium-based sulphite process, the liquor is atomized using steam or
compressed
air and fired in liquor burners located in the walls or roof of the furnace.
4o Black Liquor RecovE:ry Boilers
In recovery boilers which fire' black liquor from the kraft process, the
sodium-based
sulphite process, and the closed-cycle CTMP process, the liquor is introduced,
without
atomization, through one or more liquor nozzles, or liquor guns, which are
inserted
through openings in the walls of the furnace, generally at a common elevation
some
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4 to 5 m above the furnace floor. The furnace height, from furnace floor to
furnace roof,
may be 10 to 40 or 50 m, depending on the capacity of the boiler. In the soda
process,
steam or air atomization of the liquor is employed.
When the black liquor is sprayed into the hot furnace, some in-flight drying
of the
droplets occurs, and some of the volatile combustible components vaporize.
Most or all
of these volatiles burin in the furnace. Small, light droplets may be carried
upwards by
the rising gaseous products of combustion. In the kraft process, the sodium-
based
sulphite process, anal the closed-cycle CTMP process, large, heavy droplets
fall to the
1o bottom of the furnace and form a bed of char where the combustion reactions
continue.
Molten smelt, together with imperfectly combusted solid materials including
carbon char
particles and unburned liquor, percolates through this char on the bed. In
cases where
the black liquor droplEas are sprayed on to the walls, the smelt also runs
down the walls
of the furnace. The molten srnelt is extremely corrosive; therefore the walls
of the lower
furnace, from the floor upwards, sometimes as high as the tertiary air ports
which are
generally somewhat above the elevation of the liquor-spraying nozzles, must be
protected from corrosion in various, expensive, ways. The smelt leaves the
furnace
through smelt spouts, located in one or more furnace walls just above the
floor tubes.
In the soda process, most of the combustion occurs in suspension and the ash
falls to
the furnace bottom and leaves the furnace as molten smelt in the same fashion
as the
other recovery units which fire black liquor.
The floor of the furnace can bE: flat, in which case the smelt-spout openings
are generally
located some 200-300 mm above the floor. Thus a pool of smelt collects in the
bottom
of this type of furnace, which is called a "decanting" or "flat-floor" hearth,
or "decanting"
or "flat-floor" furnace.
The floor of the furnace can be sloped, generally at an angle of 5 to 10
degrees to the
horizontal, towards one wall, in which case the smelt-spout openings are
located at the
lower end of the sloped floor.. Much less smelt is present in the bottom of
this type of
furnace, which is callE:d a "sloping-floor" hearth, or "sloping-floor"
furnace.
In some sloping-floor furnace;>, the smelt-spout openings are located some 100-
300 mm
above the floor. Thus a small pool of smelt also collects in this type of
furnace. For the
purposes of this discussion, this type of furnace is also designated a
"sloping-floor"
hearth, or "sloping-flo-or" furnace.
4o One or more liquor spray guns, or nozzles, may be employed in recovery
boilers firing
liquor from the kraft process, the soda process, the sodium-based sulphite
process, and
the closed-cycle CTMP process. The liquor guns are approximately 4 to 7 m
above the
furnace floor and are, generally, all at the same elevation. Generally,
multiple liquor
guns are distributed around the periphery of the furnace. Excessive local
deposition of
liquor on the char bed causes local combustion upsets which, although not
necessarily
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enough to disrupt the overall operation of the boiler, can cause local
temperature
variations and adversely affect the TRS emissions from the furnace, as
discussed below.
Two-wall primary air, as proposed in the method, minimizes such upsets.
When the liquor droplets in, or on, the char bed, or droplets in flight, are
sufficiently dry,
they pyrolize and burn, thereby forming combustion gases and releasing and/or
forming
other chemicals, some of vvhich are carried upwards, as chemical fumes, by the
combustion gases. The in-Might droplets that are too large to be carried out
of the
furnace by the up-flowing gases, fall to the bottom of the furnace on to the
char bed or
l0 are deposited on the lower furnace walls where most of the droplets
pyrolize and burn
and, at some point, fall on to the char bed. Some of the lighter droplets,
however, may
be entrained by the flue gases and carried upwards into the upper regions of
the boiler
where the pendent heating surfaces, such as the superheater, generating bank
and
economizer, are located.
If an excessive quantity of vvet liquor is deposited on any area of the char
bed, the
combustion in that area is suppressed, the bed builds up there and local
combustion
falters or ceases and causes a "black-out" in that area of the bed on which
the wet liquor
has been deposited. As a re:;ult of these black-outs, the bed temperature is
lowered in
these regions and this cause's an increase in the emission of sulphurous gases
from
these regions. If these sulphurous gases are imperfectly combusted, they
contribute to
the strength of rotten-egg-likes odour typically emitted from such boilers. If
the black-out
is severe, expensive ~>upport lfuel such as fuel oil or natural gas is
required to restore the
combustion and it may also be necessary to cease firing the liquor
temporarily. If the
char pile in the affected area becomes too high, it can topple over and block
the primary
air ports and/or cause' char and molten smelt to enter the primary air
registers. In such
instances, the boiler must generally be shut down to clean out the registers
and repair
any damage which may have resulted.
If the combustion in ;>ome area of the char bed is too intense, as a result of
the local
introduction of exce~;sive quantities of combustion air, the surface of the
char bed
becomes too hot and excessive chemical fume is generated. This fume can
condense
on the pendent heating surfaces of the boiler. The fume particles increase the
dust
loading in the flue gases, and add to the capacity requirements of the
electrostatic
precipitator, a device used to remove particulate from the flue gases before
they are
discharged to atmosphere.
It is important to efficiient boiler operation that the combustion of the
injected liquor is
completed as low down in the furnace as possible in order to minimize the gas
temperatures in the pendent heating surfaces of the boiler. Excessive gas
temperatures
are adverse because they cauise the gas-borne deposits to become sticky, semi-
molten,
or molten, in which state they can adhere strongly to the heating surfaces.
For the same
reason, the liquor droplets should be retained in the lower furnace and burned
there
rather than have then- carried' into the upper furnace, either to burn,
causing local high
temperatures, or to adhere to the heating surfaces as unburned liquor.
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Deposits which adhere to the heating surfaces reduce the heat-transfer to the
surfaces.
As much of these dE~posits as possible is therefore removed, using devices
such as
sootblowers which generally utilize steam or high-pressure air, at
considerable cost. It
is therefore important to minimize the entrainment, or carryover, of liquor
droplets and
chemical fume in the flue gases rising from the furnace and to complete the
combustion
at as low an elevation in they furnace as possible. Both embodiments of the
method
improve combustion ,and reduce the carryover of particulate.
The primary air ports in recovery boilers are particularly subject to fouling
and eventual
1o blockage from frozen smelt and dried liquor. The second embodiment of the
invention
reduces this port-fouling and minimizes the need for port rodding, either
manually or by
the use of automatic port-rodding equipment.
Combustion Air System in Boilers Burning Biomass, Wood Waste or Other Solid
Fuel
As noted above, the fuel in such boilers may burned on a grate, where the
grate may be
fixed, moving, horizontal or sloping, or in a fluidized bed.
The combustion air is admitted to the furnace of such boilers as undergrate
air and
overfire air.
There may be several overfire air zones at various elevations above the fuel
bed.
These zones are named aiccording to their elevations relative to the fuel bed.
Successively higher :ones are namely primary, secondary, tertiary, quaternary
air, etc.
Thus, the primary air is the air zone closest to the char bed. In some
instances, the
overfire air zones may be numbered in some fashion, depending on the
preference of
the boiler owner.
35
The gas-flow pattern in a funnace burning biomass, wood waste or other solid
fuel, is
created by the fuel distribution in the furnace, by the load-carrying and/or
auxiliary
burners and by the combustion air system.
Combustion Air System in (Recovery Furnaces Firing Black Liquor
As noted above, the combustion air is admitted to this type of recovery
furnace in several
zones which are named according to their elevations relative to the char bed.
Successively higher zones are namely primary, secondary, tertiary and, in the
latest
furnaces, quaternary .air. Thus, the primary air is the air zone closest to
the char bed.
Some older boilers have only two air zones: one below and one above the liquor
guns.
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Other older boilers and moss: modern boilers have at least three air zones:
two below
and one above the liquor guns.
An increasing number of modern boilers have four or more air zones: generally
two
zones below and the remaining zones above the liquor guns.
The primary air zone is generally about a metre above the surface of the char
bed and
is always below the elevation of the liquor guns.
1o The secondary air zone is generally one or two metres above the primary air
zone and,
except in older boilers of a certain design, is always below the elevation of
the liquor
guns.
The tertiary and quaternary air zones are almost always above the elevation of
the liquor
15 guns.
The air ports of each zone are generally at a common elevation, but need not
be. For
example, in sloping-floor furnaces, the primary air ports on the sidewalls are
generally
located along the sides of a flat, sloping plane parallel to the floor. The
primary air ports
20 on the front and rear walls are generally located along the other two sides
of the said
sloping plane. However, in such sloping-floor furnaces, the ports of the other
air zones
above the primary air zone are generally at a common elevation.
The openings through which the air is admitted, the air ports or nozzles, are
located on
25 one or more walls of the furnace, which is, typically, rectangular in
horizontal cross-
section. The ports on each wall are usually distributed evenly across the
width of the
wall and spaced according to the manufacturer's preference. The combustion air
enters
the ports from air registers which extend across all or part of each furnace
wall.
30 Primary air is almost universally introduced through air ports on all four
walls, as shown
in Figure 1, such that the quantity of air from each wall is approximately the
same and the
flow through individual ports its more or less equal. The inventors believe
that there is
only one recovery boiler in the world today which has primary air ports on two
walls only
namely at Tasman Pulp and F'aper Limited in New Zealand, discussed later under
Prior
35 Art.
One embodiment of one of the proposed method is that at least 80 percent of
the primary
air is introduced through ports ~on two opposing walls. In a conventional flat-
floor furnace,
the primary air jets are generally directed into the furnace at an angle of 0-
5 degrees
4o downwards from the horizontal, as shown in Figure 2. In a conventional
sloping-floor
furnace, the primary air jets are generally directed into the furnace at an
angle of
approximately 30 degrees downwards from the horizontal, and originate from air
ports
located along the sides of a flat plane, inclined more or less parallel to the
furnace floor,
as shown in Figure 3. -fhe primary air registers are generally short and each
register may
45 have 4 to 10 small air ports, each port typically rectangular and 50 mm
wide and 100 to
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CA 02245294 1999-12-08
200 mm high. Each register has, typically, a single damper which controls the
flow of air
to the register, but thE~re is most-often no constant-velocity control of each
port.
Some boilers are equipped with a high-primar)r air system. This is a system of
air ports,
perhaps as much as 1 m above the primary air elevation, and supplied with air
from
ducting tapped off the primar)~ air system. A booster fan may be employed for
the high-
primary air.
Secondary air may be introduced through air ports on all four walls, but is
often
l0 introduced through ports on two opposing walls. The secondary air registers
in a four-
wall system are often short and each register may have 4 to 10 small air
ports, each port
typically rectangular and 50 rnm wide and 100 to 200 mm high; each register
generally
has a single damper, as in the primary air system.
In a two-wall secondary air system, the registers may be continuous and extend
across
the full width of the wall; the sports are generally larger, say, 100 mm wide
and 300 mm
high and may have inclividual dampers which may, or may not, provide a
constant velocity
at the port opening.
In some older recovery boileirs, the secondary air is introduced above the
liquor guns
from air ports located close to the corners of a furnace which is often
square, or nearly
square in horizontal cross-section. The air from these corner ports creates a
cyclonic
action in the flue gasea in the furnace, such that the axis of the cyclone is
vertical.
Tertiary and quaterna air are generally introduced through air ports on two
opposing
walls, but may be introduced through ports on four walls or through ports on
one wall
only. These ports are typically the same size as, or larger than, those of the
secondary
air system and generally have individual dampers which may, or may not,
provide a
constant velocity at the port opening.
Again, in some boiler:;, the tertiary air is introduced from air ports located
close to the
corners of a furnace which is .often square, or nearly square. The air from
these corner
ports creates a cyclonic action in the flue gases in the furnace, such that
the axis of the
cyclone is vertical.
The gas-flow pattern in a recovery furnace is created largely by the
combustion air
system.
4o Primary Air Zones in Recovery Furnaces Firing Black Liquor
Where air jets issue from air ~>orts on four walls at any elevation, the air
jets from each
wall interfere with the jets from the adjacent walls at right angles and force
the air and the
flue gases into a central column of relatively-rapidly-upward-flowing gases.
This is shown
in plan view in Figure 1, and in elevation in Figures 2 and 3. With a two-wall
primary air
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CA 02245294 1999-12-08
zone, or "2wp" zone, as shown in Figure 4, as embodied in the proposed method,
the
same total primary air quantii:y as in the four-wall arrangement is used. In
the method,
the quantity of air through the ports of two opposing "inactive" walls is
significantly
reduced, in the limit, to zero, while the quantity of air through the ports of
the two
opposing "active" wal Is is therefore essentially doubled; thus, as the
quantity of air from
the inactive walls decreases, there is less and less interference with the
increasingly
stronger jets from the active vvalls. Also, in the limit, the velocity of the
jets issuing from
the ports of the two "active" vualls is therefore essentially double the
velocity of the jets
from the same walls in the four-wall arrangement. The more powerful jets of
the two-wall
arrangement create a column of relatively-rapidly-upward-flowing gases in a
region with
a rectangular horizontal cross-section, but, as explained below, the upward
velocity in this
region is lower than the upward velocity in the central column created by the
four-wall
arrangement of jets. This rectangular region extends across the full extent of
the furnace
width (or depth) with the longs axis of the rectangle parallel to the walls
from which the
large air jets originate. This is shown in Figure 4. The more powerful jets
entrain more
of the surrounding furnace gases, including combustible gases, into the air
jets, thereby
improving gas mixing and combustion.
Droplets from the liquor sprayers and particulate from the char bed can be
preferentially
captured and entrainf~d by the gases in these high-velocity regions and, as
described
previously, carried out of the 'furnace.
It can be seen from Figures 1 and 4, that the area of the rectangle in Figure
4 is greater
than the area of the central column in Figure 1. Since the amount of up-
flowing gases
is similar in both cases, the upward velocities in the larger rectangular
region are thus
slower than in the central column region. With lower upward velocities, the
flow pattern
created by the two-wall primairy air-jet arrangement is less likely to entrain
particulate in
the upward-flowing gases than the pattern created by the four-wall
arrangement.
Thus, it can be deduced that, in a system in which a large portion of the
primary air is
introduced from the ports in fiwo opposing walls, while the remainder of the
primary air
is introduced from the two remaining walls, the liquor-droplet carryover will
be less than
in a furnace with the same total primary air flow distributed such that the
flow from each
of the four walls is more or less equal, but will be greater than in a furnace
in which the
same total primary air quantity is introduced from ports on two opposing walls
only.
In a flat-floor furnace with a conventional combustion air system, the primary
air ports on
all four walls are generally all .at the same elevation, as shown in Figure 2.
The primary
air jets, directed horizontally or very slightly downwards at an angle of 0-5
degrees, are
4o directed in essentially the same horizontal plane P-P. The air velocity in
the primary air
ports is of the order of 25 to ;30 m/s and, since the jets are small, they
penetrate only
some 2 m into the furnace. The profile of the char bed is relatively flat in
the flat-floor
furnace, particularly around the periphery of the furnace where these small
primary air
jets sculpt the char bE:d, often forming a low char rampart around the
periphery of the
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CA 02245294 1999-12-08
furnace. Inside the pE~ripheral band affected by the primary air jets, the
char bed can be
higher, since this are<~ is unaffected by the relatively weak primary air
jets.
In a sloping-floor furnace with a conventional air system, as shown in Figure
3, the
primary air ports on the spout wall are all at one elevation, designated P5 on
Figure 3.
The primary air ports on the wall opposite the spout wall are also at a
single, higher,
elevation, designated P1 on I=figure 3. The ports on the other two walls,
designated the
sidewalls for the purposes of this discussion, are typically arranged in
horizontal groups
of several ports, each group served by its register, and arranged such that
the ports
1o served by each register are at a common elevation, while the registers are
at descending
elevations, designated P1 through P5 on Figure 3. The sidewall registers are
thus more
or less on the sides of a planes P1-P5 which is inclined and parallel to the
sloping floor of
the furnace. Typically, all the primary air jets are directed downwards at an
angle of
approximately 30 degrees as noted above.
Also in a sloping-floor furnace, as shown in Figure 3, the profile of the char
bed is not flat
like the bed in the flat-floor furnace. In a sloping-floor furnace, the small
primary air jets,
directed downwards act appro;Kimately 30 degrees as noted, keep the char
burned back,
away from the furnace walls around the periphery of the furnace, forming a
steep char
rampart about 1 to 1.~s m from the walls as shown in Figure 3. This rampart
impedes jet
penetration and deflects the ;sir jets upwards into the furnace. In the region
inside the
char rampart created by the primary air jets, the char bed is higher and this
area is
completely unaffectecl by the primary air jets which are contained by the char
rampart.
Thus, in both the flat-floor and sloping-floor furnaces that have conventional
four-wall
primary air, the primacy air is confined to a relatively small area around the
perimeter of
the furnace. Since i:he oxygen in the air jets is restricted to a confined
area, the
temperatures near the walls .are unnecessarily high, causing excessive local
NOx and
fume generation and metal wastage can occur. On the other hand, the centre of
the
3o furnace is relatively cooler. In this cooler region in the centre of the
char bed surface,
TRS emissions may b~e exce~;sive.
Conventional thinking suggests that, to minimize fume generation and metal
wastage in
the lower furnace, the combustion should be delayed and displaced to higher
elevations
in the furnace. Typically, the primary air flow is reduced and the other,
higher-elevation
air flows are increasE~d correspondingly. This reduces the temperature
immediately
above the smelt bed around tlhe perimeter of the furnace and reduces fume
generation
and metal wastage. However, the lower temperature generally results in lower
reduction
efficiencies and sometimes hiigher TRS emissions from the furnace. The
extremely
expensive heating surface of the furnace is under-utilized and the boiler
thermal
efficiency suffers.
Further, Prouty, Stuart and Caron indicated in their paper "Nitrogen oxide
emissions from
a kraft recovery furnace" (Tappi, Vol. 76, No.1 ) that, although the NOx
emissions were
reduced when the oxygen concentration was reduced, the carbon monoxide
emissions
(Pataircan, November 1999) 12 of 26
CA 02245294 1999-12-08
increased five-fold. In the technical paper "Novel air systems for kraft
recovery boilers",
by Colin MacCallum, presentE~d at a meeting of the Black Liquor Recovery
Boiler Advisory
Committee (BLRBAC), in Atllanta, GA, USA, on 6 October 1993, it is explained
that
decreases in the primary and secondary air quantities reduce both gas mixing
and
combustion at these elevations. The reduced gas mixing allows the oxygen-rich
zone
around the perimeter of the char bed (where the primary jets are), and the CO-
rich zone
in the centre of the furnace, to~ persist, rather than be broken up by the
secondary air jets
which would be more aggressive at a higher flow.
1o A two-wall primary air arrangement has more powerful jets issuing from the
two active
walls as noted above. These powerful jets burn the char bed back farther into
the
furnace and, where the jets are directed as proposed in the method,
essentially eliminate
the char ramparts otherwise formed by the four-wall arrangements. The stronger
jets
penetrate farther into the furnace and provide better gas mixing, as
described. Thus,
15 better gas mixing, as provided by two-wall primary air, would reduce the CO
emissions,
because, with two-wall primary air, the bed height is controlled by the
primary air jets
which penetrate deep into the furnace and consume the CO, whereas, with the
four-wall
system, the relatively weak jests form an oxygen-rich zone around the
perimeter of the
furnace and never penetrate to the CO-rich zone in the centre of the furnace.
The proposed methodl of achieving two-wall primary air in a boiler burning
liquor from the
kraft process, the soda process, the sodium-based sulphite process, and the
closed-cycle
CTMP process, applies primarily to sloping-floor furnaces, but could be
applied to flat-
floor furnaces where new primary air ports are installed in a plane which is
not horizontal.
In the method, as applied to an existing sloping-floor furnace with four-wall
primary air,
some, or most, of the primary .air would be shut off from two opposing walls.
The primary
air thus shut off would be directed to the other two walls in roughly equal
proportions.
Thus, the primary air quantity from the remaining two "active" opposing walls
would be
correspondingly increased, such that the total primary air quantity remains
substantially
3o the same as before. 'T'hat is, the velocity in the primary air ports of the
active walls would
be increased, or, in the limit, doubled. The remaining small quantity of
primary air, as
applicable, is essentially equally distributed between the two "inactive"
walls.
A three-month trial with two-wall primary air was conducted on a sloping-floor
boiler at
Mackenzie, BC. As explained in MacCallum's technical paper "Novel air systems
for kraft
recovery boilers", referred to earlier in this discussion, after some three
months it was
suddenly found that the steeply-sloping higher-velocity primary air jets from
the two active
walls were digging into the char bed and creating a very distinct rampart of
char some
1.8 to 2.1 m from the two active walls. This char rampart had probably existed
all the
4o time following the adoption of 2wp, but it suddenly appeared to the
operators as though
char particles were being swept up the face of the char rampart and entrained
in the flue
gases, so, conventional four-vvall operation was restored.
At the time of this trial, and until early 1999, the inventors were concerned
that, if the
method were applied t:o a sloping-floor furnace using the existing steeply-
sloping ports,
(Pataircan, November 1999) 13 of 26
CA 02245294 1999-12-08
the more powerful air jets frorn the two active walls might cut into the char
bed and could
damage the floor tubes. It was thought that, to avoid such damage, the jets
from the two
active walls must be directed more or less horizontally from the sidewalls or
essentially
parallel to the floor from the front and rear walls. Therefore, new primary
air ports in the
active sidewalls, directed more or less horizontally, would be installed; or
new primary
air ports directed essentially parallel to the floor would be installed in the
front and rear
walls. As an alternative to new air ports, inserts would be installed in
existing ports to
direct the primary air at the desired angle from the active walls. The
conventional
arrangement of such ports angled downwards at approximately 30 degrees is
shown in
1o Figure 11 and a simple insert to direct the air at the desired angle is
illustrated.
The inventors now believe that such inserts are not required. The inventors
have since
reviewed this concern about the floor tubes being endangered by steeply-
sloping air jets
and have concluded that, given an air-jet velocity of about 150 ft/s with 2wp,
combined
with the low angle of approach to the floor tubes, it is very unlikely that
any damage to
the floor could occur, even if it were bare. The fact that Mackenzie ran the
boiler with
2wp for three months. problem-free, supports this view.
The air ports in the inactive walls need not be modified, since the jets are
created by a
2o smaller quantity of air, for exaimple, leakage air through the dampers, and
are relatively
weak.
In the method, the plane of the primary air jets, that is, the plane which
passes through
the primary air ports of all walls from which the primary air jets originate,
is inclined, as
shown in Figure 5. The larger jets from the active walls can be along either
the horizontal
planes P1 and P5 on the front and rear walls, or on the sloping plane P1-P5 on
the
sidewalls, or, in a specific case, parallel to the sloping floor of the
furnace. Figure 6
shows the proposed active two-wall primary air jets from the spout wall and
from the wall
opposite the spout wall. Figure 7 shows a section through Register P3 of the
furnace
where the active two-wall primary air jets are introduced from the sidewall
registers at
elevations P1 through P5.
In the method, when the prirnary air flow is maintained at its four-wall flow
rate, but
injected through two walls only, the air velocity essentially doubles and the
jets sculpt the
bed profile more easily than the slower jets of the four-wall arrangement.
When the jets
are directed essentially horizontally into the furnace from the sidewalls, or
essentially
parallel to the floor from the front and rear walls, the jets penetrate
farther into the
furnace and sweep a~:,ross the surface of the bed, to the centre of the
furnace. This
results in more effective combustion across the entire cross-section of the
furnace and
leads to the higher average temperatures. The combustion is no longer
concentrated
around the perimeter, so the temperatures at the walls, especially the walls
with the
closed ports, or no ports, should be lower and the metal wastage should be
less.
With two-wall primary air direcl:ed in the proposed manner, the bed profile is
relatively flat,
with a central ridge parallel to 'the walls from which the jets issue. In a
furnace originally
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CA 02245294 1999-12-08
designed for four-wall primary air, and operating with two-wall primary air,
the height of
the central ridge of the char bed is generally somewhat higher than the
elevation of the
air ports on the "inactive" walls. In order to prevent the char and associated
smelt from
entering the ports at the centre of the inactive walls, some air is introduced
through the
air ports at the centre of the inactive walls; this air also sculpts the bed,
more weakly than
the stronger jets from the active walls, and pushes the central ridge away
from these
ports. That is, close to the inactive walls, the central ridge is lower than
the rest of the
ridge.
1o Obviously, in a furnace originally designed for two-wall primary air, there
would be no
ports on the inactive walls, so the central ridge of the char bed can extend
right to the
inactive walls.
As discussed above, in a conventional sloping-floor boiler utilizing four-wall
primary air,
15 the char bed is piled a p by the steeply-sloping primary air jets from all
four walls. Further,
the top of the char bc~d is cut off by the secondary air jets which have
relatively high
velocity in a modern system. This means that a large proportion of the
combustion air
is injected close to thE~ surface of the bed. Combustion close to the bed
promotes high
temperatures and fume generation.
On the other hand, tvuo-wall primary air creates a flat char bed, subjected
only to the
action of the primary air jets. -The surface of the char bed is well below the
secondary air
jets. That is, the bed surface is directly affected by less of the total air
quantity. Thus,
fume generation from the char bed is likely to be lower when two-wall primary
air is
employed.
With 2wp, the temperatures at the walls can be further reduced by reducing the
primary
air quantity in the same way as for the four-wall set-up. With 2wp, in the
method, a
reduction in the primary air quantity has fewer adverse effects than it would
have with
3o four-wall primary air. With the two-wall arrangement, even with a reduced
primary-air
quantity, the air velociiry from the active walls is significantly higher than
with the four-wall
arrangement, so the char bed is shaped much more easily with less primary air.
The
combustion will still be more effective than the combustion with the four-wall
mode of
operation and the furnace wiill still be utilized more fully, because the
combustion is
occurring lower in the furnace. Experience has shown that the primary air flow
(and total
air flow) can be reduced by some 5 percentage points with 2wp, while
maintaining the
same degree of char-bed control.
The added, expected bonuses of two-wall primary air operation are that the
furnace is
utilized more fully and the overall thermal efficiency is higher.
In a sloping-floor recovery furnace, the method comprises introducing the
primary
combustion air which is beings introduced into the furnace at the lowest air
zone in the
furnace, in jets from two opposing walls and, where applicable, in relatively
small jets
from the third and fourth walls of the furnace, such that the jets from the
first two
(Pataircan, November 1999) 15 of 26
CA 02245294 1999-12-08
opposing walls are formed by at least 80 percent of the air being introduced
at this lowest
air zone. Further, the jets from the first two opposing walls are essentially
opposed jets
as shown in Figure 10, with the jets directed essentially in an inclined
plane.
The inventors suspect that the most effective form of two-wall primary air
would be a true
two-wall arrangement: (that is, an arrangement with jets from two walls only)
employing
partially-interlaced air jets, discussed below. In this case, port-rodding
equipment is
required on two walls only - at a significant capital cost saving.
Secondary Air Zones in Recovery Furnaces Firing Black Liquor
The central column of rapidly-upward-flowing gases which is created by a four-
wall
primary air arrangement, and the larger rectangular region of somewhat-less-
rapidly
upward-flowing gases created by a two-wall primary air arrangement, can either
be
accentuated or dissipated to some extent by the air jets from the secondary
air zone,
depending on the arrangement of the air jets.
A four-wall secondary' air arrangement reinforces the central column of upward-
flowing
2o gases which is created by a four-wall primary air arrangement. However,
some two-wall
secondary air systems can dissipate the central column of gases.
Two particularly-effective two-wall secondary air arrangements are the subject
of this
discussion:
- a partially-interlaced arrangement of air jets
- a fully-interlaced arrangement of air jets.
3o A partially-interlaced air-jet arrangement consists of a two-wall pattern
of large and small
air jets. Each large jet: is opposed by a small jet. The jets are arranged so
that their size
alternates small/large/small/large, etc. across the width, or depth of a
furnace as shown
in Figure 8. The pattern may Ibe symmetrical, but need not be symmetrical. In
the prior
art discussed below, BlackwE~ll and MacCallum have shown that a partially-
interlaced
secondary air-jet arrangement in a horizontal plane minimizes the velocity
extremes in
the upward-flowing gases in a furnace. Further, Jones, Chapman and Mahaney, in
their
paper "Improved air port arrangements for the secondary air level" (Pulp &
Paper Canada
94:9 [1993]) have shown that: a partially-interlaced secondary air-jet
arrangement in a
horizontal plane improves gas mixing. In Figure 8, the corners of the plane of
the jets are
4o designated A, B, C and D. In the first embodiment of the method, the jets
are introduced
essentially in the non-horizontal plane ABCD; this plane can be inclined or
skewed,
depending on the particular embodiment of the method. The air jets may be
directed in
the inclined or skewed plane, or directed slightly downwards, or slightly
upwards from the
inclined or skewed plane. Further, the plane can be inclined in the direction
of jet flow,
or at right angles or skewed to the direction of jet flow.
(Pataircan, November 1999) 16 of 26
CA 02245294 1999-12-08
A fully-interlaced air-jet arrangement consists of a two-wall pattern of
similarly-sized air
jets which are unopposed by jets from the opposite wall. The jets are arranged
as shown
in Figure 9. The pattern may be symmetrical, but need not be symmetrical. In
the prior
art discussed below, Blackuvell and MacCallum have shown that a fully-
interlaced
secondary air-jet arr;angeme~nt in a horizontal plane does not minimize the
velocity
extremes in the upward-flowing gases in a furnace to the same extent as a
partially
interlaced air-jet arrangement in a horizontal plane. Jones, Chapman and
Mahaney,
have shown that a fully-interllaced secondary air-jet arrangement in a
horizontal plane
improves gas mixing slightly rnore than a partially-interlaced arrangement in
a horizontal
1o plane.
Clearly, by the addition of small air jets opposite the existing jets, the
fully-interlaced air-
jet arrangement can be converted to a partially-interlaced arrangement. In
this way, the
capacity of the particular air zone is increased. Similarly, a partially-
interlaced
arrangement can operate as a fully-interlaced arrangement by the simple
expedient of
closing the dampers associated with the small jets. In this way, however, the
capacity
of the particular air zone to supply combustion air is decreased.
As noted above, because the gas mixing in the furnace is ineffective, many
boilers
employ excessive quantities of combustion air in an attempt to complete the
combustion.
If recycled flue gas is introduced into the furnace in the said partially-
interlaced jets, alone
or in place of all, or some, of the portion of the combustion air which would
otherwise be
introduced through thEae jets, gas mixing is also improved, because the said
jets whether
or not they contain combustion air, still contribute to better gas mixing, by
entraining the
furnace gases surrounding tlhe jets, into the jets. The principle advantage of
using
recycled flue gas in place of some of the combustion air is that the excess
air is reduced
and, thereby, the thermal efficiency of the boiler is increased.
Partially-interlaced nir jet ~~rrangements in Boilers Firing Red Liquor or
Boilers
Firing Wood Waste, Biomass or Other Solid Fuel
The velocity of the combustion gases in boilers firing red liquor or boilers
firing wood
waste, biomass or other solid fuel, is generally not constant over the
horizontal cross-
section of the furnace. That ia, velocity extremes occur in these furnaces and
promote
entrainment of particulate into the combustion gas stream leaving the furnace.
The partially-interlaced air-jet arrangement described above, may also be
employed in
this type of boiler to minimize such velocity extremes and thereby minimize
entrainment
4o of particulate.
Again, if recycled flue gas is introduced into the furnace in the said
partially-interlaced
jets, alone or in place ~of all, or some, of the said portion of combustion
air, gas mixing is
also improved, because the said jets whether or not they contain combustion
air, still
(Pataircan, November 1999) 17 of 26
CA 02245294 1999-12-08
contribute to better gals mixing, by entraining the furnace gases surrounding
the jets, into
the jets.
Combination of Jets.
In any arrangement of air jet:., when two or more air ports are relatively
close together,
at a certain distance from the wall the jets combine to form a single jet. For
the purposes
of this discussion, this combining of jets is referred to as the "register"
effect. This
1o register effect can be used to~ create the desired partially-interlaced
arrangement of air
jets in the method. For example, Figure 12 shows a large jet created by the
combination
of two smaller jets. Thus, for ease of manufacture, the air ports can be all
the same size,
while the large jets are created by combining two or more small jets.
In the method, the large jets can be created by rows, columns, groups or
clusters of
smaller jets, or by increasing the pressure at the air port or combination of
ports. For
example, the partially-interlaced method can be applied at the primary air
zone in a
sloping-floorfurnace.~ In this case, ideally, the airwould be introduced from
two opposing
walls only. In practice, however, a small quantity of air might leak through
the closed
2o dampers of ports, if any, on the other two walls. Further, each small jet
could be created
by reducing the air flow to one register which, in turn, feeds several primary
air ports.
The small jets thus created, vvould combine to form a single "small" jet. Each
large jet
would be created by 'the combination of the several more powerful jets fed
from each
register for which the damper remained fully open.
In an air system, each of the .air jets can be opposed, partly opposed, or non-
opposed,
as shown in Figure 10. The opposing jets may or may not issue from ports at
the same
elevation, but they are directed such that they oppose, or partly oppose, or
do not oppose
the air jets from the opposite wall, as shown.
Coordination of Liquor-spraying and Air Systems in Boilers firing Black Liquor
When a central region of relatively rapidly upward-flowing gases is created by
a four-wall
or two-wall primary air-jet arrangement in boilers firing liquor from the
kraft process, the
soda process, the sodium-ba~~ed sulphite process, and the closed-cycle CTMP
process,
the region tends to persist to am elevation above the liquor guns, for many
arrangements
of secondary, tertiary and quaternary air ports. In this instance, it is
advantageous to
spray the liquor into the area:; of the furnace outside this high-upward-
velocity region,
where the furnace ga:>es tends to be flowing downwards.
The down-flow of the furnace cases around the central column of upward-flowing
gases,
as illustrated in Figures 2 and 3, is created by the entrainment of furnace
gases into the
primary air jets. The more powerful the primary air jets, the more pronounced
is the
down-flow.
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CA 02245294 1999-12-08
The particles and droplets sprayed into these regions of downward-flowing
gases tend
to be carried downwards on to the char bed. Thus, coordination of the liquor
spraying
and the two-wall arrangement of primary air jets can result in fewer liquor
droplets being
carried out of the furnace by 'the flue gas stream.
Two-wall primary air operation provides well-defined areas of downward-flowing
furnace
gases, along the active walls, into which areas the liquor droplets can be
sprayed to
minimize carryover of droplets and particulate in the flue gas leaving the
furnace.
Droplets which are inadvertently sprayed into the central, upward-flowing
region formed
1o by a two-wall primary air arrangement are less liable to be entrained than
with the four-
wall primary air arrangement, because the upward velocity in the central
region is lower
than with a four-wall arrangement.
Thus, coordination ofthe liquor spraying with the air system is particularly
complementary
to two-wall primary air operation because, with 2wp there are two well-defined
down-flow
regions - each being the full ~nridth of the furnace, above the primary air
jets which are in
use. The liquor can bE~ sprayed into these down-flow regions and the droplets
fall to the
char bed at places where a large amount of oxygen is supplied via the high-
velocity
primary jets; both the large oxygen supply and the high velocity of the jets
enhance the
burning of the char. This perrnits operation with a larger droplet size which
also helps to
reduce particle entrainment, o~r carryover. The high-velocity primary air jets
also facilitate
shaping of the bed, as mentioned.
PRIOR ART
By way of exemplification and not limitation, several examples of prior art
forms of two
wall primary air-jet arrangements and of partially-interlaced air-jet
arrangements are
described in the follovuing paragraphs. None of these examples presents the
concepts
3o embodied in the proposed invention.
The following patents, all entitled "Method and apparatus for improving fluid
flow and gas
mixing in boilers", describe <~ method and apparatus also invented by
Blackwell and
MacCallum, wherein the primary air in a recovery boiler is introduced
substantially from
two walls only, and another method and apparatus in which combustion air is
introduced
in a partially-interlaced mannESr:
- Canadian Patent No. 1,308,964: Serial No. 564,320; Issue date 20 October
1992
- Canadian Patens: No. 1,324,537; Serial No. 616,260; Issue date 23 November
1993
- US Patent No. 5,121,700: Issue date 16 June 1992
- US Patent No. 5,305,698: Issue date 26 April 1994.
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CA 02245294 1999-12-08
Canadian Patent No. 1,308,964 describes partially-interlaced air jets.
Canadian Patent 1,:324,537 describes a secondary air-jet arrangement with the
secondary air introdluced from two walls only, in combination with a
particular
arrangement of two-wall primary air in which there are large primary air jets
on two
opposing walls and small jets. on the third and fourth walls.
US Patent 5,121,700 describes partial-interlacing of the primary air jets from
a first and
a second opposing wall, with substantially no air from a third and a fourth
opposing wall,
partial-interlacing of the secondary air jets with substantially no air from
the third and
fourth walls, and partially-interlaced secondary air in combination with two-
wall primary
air which is created by large jets from two opposing walls and essentially no
air from the
remaining two walls.
US Patent 5,305,698 describes particular arrangements of two-wall primary air
in which
a small quantity of air' is introduced from the third wall, alternatively from
the third and
fourth walls. This patent alao describes secondary air introduced mostly from
two
opposing walls with somewhat less air, or substantially no air, being
introduced from the
third and fourth walls at the :;ame secondary elevation, in combination with
the afore-
2o mentioned (in this paragraph', two-wall primary air arrangements.
In all of these patents, the t~nro-wall primary air jets are from air ports
which are all at
substantially the samE~, first, Elevation while the secondary air jets are
from air ports, all
essentially at a common, second, elevation higher than the primary elevation.
None of
these patents describes the method or apparatus of the proposed invention in
which the
combustion air at either the primary or secondary elevations, or at any
elevation, is
introduced from air ports locai:ed along the sides of an inclined plane or a
skewed plane.
The proposed method and apparatus are improvements on the above-mentioned
patents.
As mentioned above, the technical paper "Novel air systems for kraft recovery
boilers",
by Colin MacCallum, ~>resente~d at BLRBAC, Atlanta, 6 October 1993, describes
a three-
month trial with two-wall primairy air conducted on a sloping-floor boiler at
Mackenzie, BC,
Canada. In the trial, the boiler was operated with steeply-sloping primary air
jets from the
front and rear walls, tlhat is, the walls at the upper and lower ends of the
sloping floor,
with a small quantity of air leaking through the steeply-sloping primary air
ports on the two
inactive sidewalls. The paper proposed directing the air horizontally from the
two active
front and rear walls by installing either new air ports or by installing
inserts in the existing
primary air ports. They paper did not disclose directing the air essentially
parallel to the
4o sloping floor, as opposed jets, from the front and rear walls by installing
either new air
ports or by installing inserts in the existing primary air ports. The paper
also did not
disclose directing the air essentially parallel to the sloping floor, as
opposing jets, from
the sidewalls.
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CA 02245294 1999-12-08
There is a wood-waste-fired boiler in Washington state, USA, in which the
tertiary air
ports were essentially fully-opposed and equally-spaced across the width of
the front and
rear walls of the furnace. Further, the front wall ports were at an elevation
somewhat
higher than the ports of the rear wall. In June 1997, MacCallum recommended,
to the
owner and to the boiler manufacturer only, that a partially-interlaced
tertiary air-jet
arrangement be installled and, to take advantage of the existing air ports,
suggested that
selected air ports be dampered to provide the desired partially-interlaced
arrangement
of jets on the sloping plane formed by the front and rear wall ports. The
modified boiler
started up in October 1997. -(here has been no public disclosure of this
feature.
At Tasman Pulp and F'aper Limited, in New Zealand, the principle of the two-
wall primary-
air-jet arrangement was suggested by the inventors as an improvement to a very
old
boiler, which originally started up in 1955 and, after a period of operation
with the primary
air shut off from two opposing walls, was rebuilt with a two-wall arrangement
around
1994. The boiler continues to operate well. With the two-wall primary-air mode
of
operation, the TRS emissions are significantly lower, and the reduction
efficiencies are
significantly higher, than they were with the original four-wall mode of
operation. The
furnace has a flat floor and ;all the primary air ports are at the same
elevation. The
primary air ports are angled downwards at 25 degrees, so the jets are not
horizontal, not
opposed or partly op~~osed, nor parallel to the floor.
SUMMARY OF THE INVENTION
Minimal entrainment of particulate and liquor droplets in the flue gases of a
recovery
furnace firing liquor from the kraft process, the soda process, the sodium-
based sulphite
process, and the clo:>ed-cycle CTMP process, can be achieved with the method
and
apparatus of the invention. Similarly, minimal particulate entrainment in the
flue gases
of a furnace fired with red liquor from the magnesium-based sulphite process,
or a
3o furnace fired with red liquor from the ammonium-based sulphite process, or
a furnace
fired with biomass, wood waste or other solid fuel can be achieved with the
method and
apparatus of the invention.
The first embodiment of the invention pertains to a recovery boiler furnace
firing black
liquor from the kraft process, a recovery boiler furnace firing black liquor
from the soda
process, a recovery boiler furnace firing black liquor from the sodium-based
sulphite
process, a recovery boiler furnace firing black liquor from the closed-cycle
CTMP
process, a recovery (boiler furnace firing liquor from the magnesium-based
sulphite
process, a recovery boiler furnace firing liquor from the ammonium-based
sulphite
4o process, and boiler furnaces burning biomass, wood waste or other solid
fuel, which
utilize injected air or flue gas, and comprises a method of introducing a
portion of the
combustion air, or sorne portion of recycled flue gas in place of all or some
of the said
portion of the combustion air, at any elevation into the furnace, such that
most or all of
the air, and/or recycled flue gas being introduced at the particular elevation
is introduced
from air ports located essentially along two opposing, so-called "active"
sides of a non-
(Pataircan, November 1999) 21 of 26
CA 02245294 1999-12-08
horizontal plane. This plane can be flat, or curved. The plane can be inclined
in the
direction of the jet flov~r, inclinE:d at right angles to the direction of the
jet flow, skewed, or
essentially parallel to the floor in a sloping-floor furnace. The air jets
from these ports on
the "active" walls are arranged in a partially-interlaced pattern of large and
small air jets.
The total air flow from each o~f the two "active" sides of the plane is more
or less equal.
Each large jet is opposed by a~ small jet originating from the opposite wall.
The large and
small jets alternate, i.e. they are arranged small/large/small/large, etc.
across the width,
or depth of the furnace. The pattern may be symmetrical, but need not be
symmetrical.
The air jets may be directed in the plane, or directed slightly downwards, or
slightly
upwards from the incliined or ;>kewed plane, such that the jets in each
opposing pair may
be fully opposed or partly opposed, as shown in Figure 10. The remaining small
quantity
of air, or no air, is introduced from air ports on the remaining two opposing,
so-called
"inactive" sides of the said plane. Alternatively, the remaining small
quantity of air and
recycled flue gas, or no air and no recycled flue gas, is introduced from air
ports on the
remaining two opposing, so-called "inactive" sides of the said plane. Where
all the air,
or all the air and recycled flue gas, is introduced through ports on the
"active" sides of the
plane, there need be no port:. on the "inactive" sides of the plane.
The second embodiment of the invention pertains to a recovery boiler furnace
firing black
liquor from the kraft process, a recovery boiler furnace firing black liquor
from the soda
process, a recovery k>oiler furnace firing black liquor from the sodium-based
sulphite
process, and a recovery furnace firing black liquor from the closed-cycle CTMP
process,
and which utilize injecaed air, and comprises a method of introducing the
primary air at
the lowest air zone into the furnace, such that at least 80 percent of the
primary air is
introduced, more or less equ<~Ily, from the air ports on two opposing, so-
called "active"
walls, the remainder of the air being introduced, more or less equally, from
the remaining
two opposing, so-called "inactive" walls, where all the air ports are located
essentially
along the sides of a non-horizontal plane. Where all the primary air is
introduced through
ports on the "active" slides of the plane, there need be no ports on the
"inactive" sides of
3o the plane. This plane can be flat, or curved. The plane can be inclined in
the direction
of the jet flow, inclined at right angles to the direction of the jet flow,
skewed, or
essentially parallel to i:he floor in a sloping-floor furnace. Further, the
larger air jets from
the active walls are directed more or less in the plane, or slightly
downwards, or slightly
upwards, while the smaller air jets from the inactive walls may be steeply
sloping
downwards, or directed morE; or less in the plane, or slightly downwards, or
slightly
upwards. The jets in each pair of opposite, active walls may be fully opposed
or partly
opposed as shown in Figure 10, while the jets in each pair of opposite,
inactive walls
may be fully opposed, or partly opposed, or non-opposed as shown in Figure 10.
In the method, there acre many ways of creating the large and small jets
featured in the
partially-interlaced palaern:
- the small and large jets can originate from corresponding small and large
ports
(Pataircan, November 1999) 22 of 26
CA 02245294 1999-12-08
- each small jet can origiinate from a group or cluster of small ports and
each large
jet can originate: from a group or cluster of large ports
- each small jet can originate from a group or cluster of ports and each large
jet can
originate from a larger group or cluster of similarly sized ports. For
example, each
small jet can originate from a single port and each large jet can originate
from a pair
of similarly sized port:>. Some or all of the area of the single port can be
substantially opposite to at least some of the area defined by the pair of
ports.
Some or all of the area of the single port can be opposite the area defined by
the
1o pair of ports.
- the ports can be~ of similar size and number and the large jets can be
created by a
higher air pressure than the pressure creating the small jets
The methods and apparatus can be applied to new or retrofitted boilers as
follows:
- a recovery boileirfurnace firing black liquor from the kraft process, a
recovery boiler
furnace firing black liquor from the soda process, a recovery boiler furnace
firing
black liquor frorn the sodium-based sulphite process, a recovery boiler
furnace
2o firing black liquor from the closed-cycle CTMP process, a recovery boiler
furnace
firing liquor from the magnesium-based sulphite process, a recovery boiler
furnace
firing liquor from the ammonium-based sulphite process, and boiler furnaces
burning biomass, wood waste or other solid fuel, which utilize injected air or
flue
gas, and comprising an arrangement of air ports for introducing the combustion
air
or some portion of recycled flue gas in place of all, or some of the said
portion of
the combustion .air, at any elevation into the furnace, such that most or all
of the air
and/or flue gas introduced at the particular elevation is introduced from air
ports
located essentially along two so-called "active" opposite sides of a plane
which is
not horizontal. -this plane can be flat, or curved. The plane can be inclined
in the
3o direction of the jet flow, inclined at right angles to the direction of the
jet flow,
skewed, or essentially parallel to the floor in a sloping-floor furnace. The
air jets
from these port:; on the "active" sides of the plane are arranged in a
partially-
interlaced pattern of large and small air jets. The total air flow from each
of the two
"active" sides of the plane is more or less equal. Each large jet is opposed
by a
small jet originating from the opposite wall. The large and small jets
alternate, i.e.
they are arranged small/large/small/large, etc. across the width, or depth of
a
furnace. The pattern m;ay be symmetrical, but need not be symmetrical. The air
jets may be directed in the inclined or skewed plane, or directed slightly
downwards, or :;lightly upwards from the inclined or skewed plane, such that
the
4o jets in each opposing pair may be fully opposed or partly opposed. Ports
may be
located on the other two opposing, so-called "inactive" sides of the plane,
through
which the remaining small quantity of air, or no air, is introduced.
Alternatively,
ports may be located on the other two opposing, so-called "inactive" sides of
the
plane, through which thE~ remaining small quantity of air and recycled flue
gas, or
no air and no recycled flue gas, is introduced. Where all the air, or all the
air and
(Pataircan, November 1999) 23 of 26
CA 02245294 1999-12-08
recycled flue gas, is introduced through ports on the "active" sides of the
plane,
there need be no ports on the "inactive" sides of the plane.
- A recovery boiler furnace firing black liquor from the kraft process, a
recovery boiler
furnace firing black liquor from the soda process, a recovery boiler furnace
firing
black liquor from the sodium-based sulphite process, and a recovery furnace
firing
black liquor frorn the closed-cycle CTMP process, and which utilize injected
air,
comprising a set of primary air ports at the lowest air zone in the furnace,
where at
least 80 percent: of the primary air is introduced, more or less equally, from
the air
to ports on two opposing, so-called "active" walls, the remainder of the air
being
introduced, more or leas equally, from the remaining two opposing, so-called
"inactive" walls, where the air ports are located essentially along the sides
of a non-
horizontal planE~. Where all the primary air is introduced through ports on
the
"active" sides oi' the plane, there need be no ports on the "inactive" sides
of the
plane. This plane can be flat, or curved. The plane can be inclined in the
direction
of the jet flow, inclined at right angles to the direction of the jet flow,
skewed, or
essentially paralllel to the floor in a sloping-floor furnace. Further, the
larger air jets
from the active walls arE~ directed more or less in the plane, or slightly
downwards,
or slightly upwards, while the smaller air jets from the inactive walls may be
steeply
2o sloping downwards, or clirected more or less in the plane, or slightly
downwards, or
slightly upwards. The jets in each pair of opposite, active walls may be fully
opposed or partly oppoaed as shown in Figure 10, while the jets in each pair
of
opposite, inactive walls may be fully opposed, or partly opposed, or non-
opposed
as shown in Figure 10.
In the furnace, there are many ways of arranging the air ports in order to
create the large
and small jets featured in the partially-interlaced pattern:
small ports can be used to create the small jets and large ports can be used
to
3o create the large jets
- groups or clusters of small ports can be used to create each small jet,
while groups
or clusters of large ports can be used to create each large jet
- groups or clusters of small ports can be used to create each small jet,
while larger
groups or clusters of similarly sized ports can be used to create each large
jet. For
example, each small jet can originate from a single port and each large jet
can
originate from a pair of :>imilarly sized ports. Some or all of the area of
the single
port can be substantially opposite to at least some of the area defined by the
pair
of ports. Some or all of tlhe area of the single port can be opposite the area
defined
by the pair of ports.
- the ports can be of similar size and number and the large jets can be
created by a
higher air pressure than the pressure creating the small jets
(Pataircan, November 1999) 24 of 26
CA 02245294 1999-12-08
The furnace can be that of a new or retrofitted boiler.
DRAWINGS
The following drawings illustrate specific embodiments of the invention, but
should not
be construed as restricting the spirit or scope of the invention in any way:
- Figure 1 is a schematic cross-sectional plan view of a typical furnace
showing the
1o primary air jets being admitted from all four walls and also showing the
cross-
sectional area o~ccupiedl by the central column of upward-flowing gases.
- Figure 2 is a schematic sectional side elevation of a typical recovery
furnace with
a flat floor and indicate:; the location of the primary air jets which are
directed at
15 0-5 degrees downwards from the horizontal on all four walls. The typical
profile of
the char bed is iindicated. Further, the central chimney of rapidly-upward-
flowing
gases, and they down-flow regions associated with the primary air jets, are
illustrated.
20 - Figure 3 is a schematic sectional side elevation of a typical recovery
furnace with
a sloping floor and indicates the location of the primary air jets which are
directed
at approximately 30 degrees downwards from the horizontal on all four walls.
The
typical profile of the char bed with its steep char rampart is indicated. The
various
typical elevations of the various air registers on the sidewalls are shown.
Further,
25 the central chimney of irapidly-upward-flowing gases, and the down-flow
regions
associated with the primary air jets, are illustrated.
- Figure 4 is a schematic cross-sectional plan view of a typical recovery
furnace and
indicates the location of the rectangular region of upward-flowing gases
created by
3o a two-wall primary air arrangement.
- Figure 5 is a schematic sectional side elevation of a typical recovery
furnace with
a sloping floor and indicates the horizontal plane P1-P1. It also shows the
Plane
P1-P5, from the sides of which the large primary air jets would be directed in
the
35 proposed method, in onE~ manner, from the wall opposite the spout wall at
elevation
P1 and from thE~ spout wall at elevation P5; or alternatively, in another
manner,
from the sidewall registers along the sides of the plane P1-P5.
Figure 6 is a schematic sectional side elevation of a typical recovery furnace
with
4o a sloping floor and indicates the large primary air jets proposed in the
method,
directed from the wall opposite the spout wall (the rear wall) at elevation P1
and
from the spout wall (the ifront wall) at elevation P5. The sculpted profile of
the char
bed with its central ridge, typical of two-wall primary air operation, is
indicated.
(Pataircan, November 1999) 25 of 26
CA 02245294 1999-12-08
- Figure 7 is a schematic sectional elevation of a typical recovery furnace
with a
sloping floor, where the section is taken through both sidewall registers at
Elevation
P3, looking towards the smelt-spout wall. The diagram indicates the large
primary
air jets proposed in the nnethod, directed from the sidewall registers at
elevation P3.
The corresponding large jets trom the other sidewall registers are not shown.
The
location of the ;>melt spouts on the front (or rear) wall is indicated. The
sculpted
profile of the char bed with its central ridge, typical of two-wall primary
air operation,
is indicated.
- Figure 8 is a schematic cross-sectional plan view of a typical furnace
showing
partially-interlaced air jets being admitted from any two opposing walls.
- Figure 9 is a schematic cross-sectional plan view of a typical furnace
showing fully-
interlaced air jeta being admitted from any two opposing walls.
- Figure 10 shows the juxtaposition, for example in plan view or elevation, of
pairs
of air jets that are opposed, partly-opposed, and non-opposed.
Figure 11 is a schematic sectional side elevation of a typical port register
in a
2o recovery furnace with .a sloping floor, indicating, on the left of the
figure, the
conventional design with the air jet issuing at approximately 30 degrees
downwards
from the horizontal and, on the right of the figure, the same register with an
insert
at the port opening to deflect the jet towards the horizontal.
- Figure 12 is a schematic plan or elevation of the register effect,
indicating the
combining of two jets from a pair of ports to form a single larger jet.
As will be apparent to those skilled in the art in the light of the foregoing
disclosure, many
alterations and modifications are possible in the practice of this invention
without
3o departing from the spiirit or scope thereof. Accordingly, the scope of the
invention is to
be construed in accordance with the substance defined by the following claims.
(Pataircan, November 1999) 26 of 26
