Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02688220 2009-12-11
SC File No: 12320P0123 CA01
FLUE GAS RECIRCULATION METHOD AND SYSTEM FOR
COMBUSTION SYSTEMS
FIELD OF THE INVENTION
The invention relates to improvements in the operation and efficiency of
furnaces for drying and
curing operations and related purposes, in particular those using high levels
of excess air. In
particular, the invention relates to a method and system of recirculation of
flue gas for such
furnaces, and has particular applicability for induration furnaces.
BACKGROUND OF THE INVENTION
It is well known that for boilers and heaters, high excess air results in
large thermal efficiency
losses, and the excess air should be monitored and maintained at specific
levels as required to
combust the fuel. Traditional excess air levels for fossil fuels are:
Oil 3 % (-1 % 02 by volume)
Natural Gas 5 % (-2 % 02 by volume)
Coal 20 % (-3% 02 by volume)
These excess air levels could vary slightly depending upon the application.
However, there are
large numbers of industrial heaters and furnaces that require significantly
higher excess air levels
which are well beyond what is needed to combust the fuel properly. These
levels have
conventionally been considered as acceptable and normal within the context of
certain processes.
There are numerous reasons for high excess air, including the following.
Firstly, and most commonly, the high excess air may be required to maintain
the heat transfer
rate of the process. Many applications require high convective rates to
transfer heat from the
flame and flue gas into a product or heat load, and the proportion of heat
transfer which is
convective or radiant will vary depending upon the furnace layout and
operation.
Secondly, excess air can be used to moderate flame and furnace temperatures.
For many
applications, typical flame temperatures at close to stoichiometric ratios
tend to be around
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3600 F, which exceeds the maximum refractory brick operating temperatures -
typically
2800 F or significantly less - so that excess air may be used to maintain the
integrity of
refractories and other structural elements of the furnace.
Thirdly, in drying and curing applications, the humidity level in the furnace
must be controlled,
which is conventionally done by increasing the excess air, and thereby
lowering the partial
pressure of moisture within the process.
Fourthly, excess air can be used to control the levels of flammable vapours
which may be
released in flue streams, by diluting these vapors well below the lower
flammability limit.
The types of combustion applications which typically use high excess air
include the following:
Spray and solids drying
Curing
Induration of ore pellets
Annealing
Forging
Heating
Large amounts of excess air generally result in very large losses within the
process. For example,
it has been found that the flue gas in iron ore furnaces may contain oxygen at
approximately
19.0 % to 19.5 % by volume, which corresponds to approximately 1400 % to 2100
% excess air.
The dry gas loss increases exponentially as the 02 in the flue gas approaches
the value for the
oxygen in air, which is approximately 21 % by volume.
In applications such as induration furnaces, very high excess air is required
in order to meet the
need for convective heat transfer in the various sections of the furnace. In
such furnaces, the key
heat transfer zone in the furnace is the combustion zone, within which are
three primary modes
of heat transfer to the product, that is, convection, radiation and
conduction. Radiation consists of
both direct luminous radiation from the flame envelope as well as cavity
radiation within the
physical geometry of the zone. Radiation is a strong function of both flame
and mean cavity
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temperature, whereas convection is a strong function of both flue gas velocity
and the
temperature of the flue gas passing through the pellet bed. The flame
temperature will increase if
less combustion air is used; however, the convection drops off dramatically.
The convection rate
must be maintained throughout the bed. The third mode of heat transfer is
conduction within the
pellet bed.
Within the other zones of the furnace convective heat transfer predominates;
however, it is both a
function of flue gas velocity through the bed and the temperature of the flue
gas. By increasing
temperature in these zones slightly it may be possible to maintain a similar
heat transfer
characteristic within the specific zones while decreasing velocity.
Clearly, any reduction in the amount of excess air used will result in
increased efficiency and
decreased fuel consumption, with consequent economic benefits. Further, a
decrease in excess
air will also decrease the amount of flue gas requiring treatment.
From prior art it is known that there are various methods of recirculation of
flue gases in
combustion systems which do not use high excess air. However, such methods are
directed at
controlling the flame temperature, NOx, and steam temperatures and are not
intended to improve
efficiency of high excess air systems.
It has now been found that for high excess air furnaces, such as induration
furnaces, controlled
and selective recirculation of exhaust gases from the drying, pre-heating, and
combustion zones
can substantially reduce the amount of excess air required by the system,
while maintaining the
required convective heat transfer.
The invention therefore seeks to provide a flue gas recirculation system for
combustion systems
which use high levels of excess air, such as drying, curing, induration and
other systems,
including but not limited to those noted above. In its most general conception
the system uses
the heat exhausted in the flue gas and re-introduces it into the process to
replace heat input from
fuel. The various methods for re-introduction can vary from one process to
another. The
invention is particularly advantageous for use for induration furnaces.
However, although
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particular reference is made in the discussion below to the specific features
and requirements for
such furnaces, the features of the invention are equally applicable to such
other excess air
systems in general.
The invention therefore seeks to provide a flue gas recirculation system for a
combustion system,
the combustion system comprising in sequence at least one pre-combustion
drying zone, at least
one combustion zone, and at least a first cooling zone, the recirculation
system comprising
(i) at least one exhaust gas outlet provided respectively to each pre-
combustion drying zone and
each combustion zone, and constructed and arranged to remove a gaseous flow
from each said
zone;
(ii) at least one cooling zone intake means provided to each cooling zone;
(iii) at least one flue gas delivery means each having at least one
recirculation intake means and
at least one delivery outlet, each exhaust gas outlet being operatively
connectable to one of the
recirculation intake means to selectively deliver at least part of each
gaseous flow as a
recirculation flow to the flue gas delivery means, and each delivery outlet
being selectively
operatively connectable to a selected one of the cooling zone intake means;
and
(iv) control means operatively connected to the flue gas delivery means to
selectively control and
direct the recirculation flow.
The invention further seeks to provide a method of recirculation of flue gas
for a combustion
system, the combustion system comprising in sequence at least one pre-
combustion drying zone,
at least one combustion zone, and at least a first cooling zone, the method
comprising the steps
of
(i) providing at least one exhaust gas outlet respectively to each pre-
combustion drying zone and
each combustion zone to allow a gaseous flow selectively to and through each
exhaust gas outlet;
(ii) providing at least one cooling zone intake means to each cooling zone;
(iii) providing at least one flue gas delivery means each having at least one
recirculation intake
means and at least one delivery outlet;
(iv) connecting each exhaust gas outlet to one of the recirculation intake
means;
(v) selectively delivering at least part of the gaseous flow from each exhaust
gas outlet as a
recirculation flow to the flue gas delivery means;
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(vi) selectively delivering at least part of the recirculation flow through
one of the delivery
outlets to the selected cooling zone intake means;
(vii) monitoring at least periodically the temperatures and pressures in each
cooling zone
receiving the recirculation flow to determine temperature and pressure values;
and
(iv) adjusting the recirculation flow in response to the determined
temperature and pressure
values.
The recirculation systems of the invention are compatible with the
conventional configurations
of many or most high excess air systems, and are particularly advantageous for
systems wherein
the at least one pre-combustion drying zone comprises an updraft drying zone
and a downdraft
drying zone; and/or wherein the combustion system further comprises at least
one pre-heating
zone and/or multiple cooling zones, such that the flue gas delivery means is
preferably
constructed and arranged to deliver the recirculation flow to the cooling zone
intake means of the
first of the cooling zones.
As noted above, the recirculation systems of the invention can be used for a
wide range of
operational end uses, such as at least one of curing, drying, induration,
heating, annealing and
forging.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described in relation to the drawings, in which
Figure 1 shows a typical configuration of a conventional high excess air
combustion system of
the prior art, without recirculation of the flue gas;
Figure 2 shows a configuration of a recirculation system in an embodiment of
the invention; and
Figure 3 shows a configuration of a recirculation system in a second
embodiment of the
invention.
DETAILED DESCRIPTION OF THE DRAWINGS
Referring first to Figure 1, a typical configuration of a high excess air
combustion system 10 of
the prior art is shown, in this case exemplifying an induration furnace, for
treating a stream of
pellets 70, carried through the system in the direction of arrow Z. The
combustion system 10
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comprises in sequence (shown here from left to right) a pre-combustion drying
zone 20, which is
shown as having an updraft drying zone 22 and a downdraft drying zone 24; a
pre-heating
zone 26; a heating zone 30 comprising a first heating zone 32 and second
heating zone 34; and a
cooling zone 40 comprising a first cooling zone 42 and a second cooling zone
44. A separate air
intake line, respectively 43 and 45, is provided to each of the cooling zones,
providing air from a
source 46.
Exhaust gas from the second cooling zone 44 is carried through line 60 to the
updraft drying
zone 22; and exhaust gas from the first cooling zone 42 is carried through
line 62 selectively to
the heating zone 30 or the pre-heating zone 26. Exhaust gas from the second
heating zone 34 is
carried via wind box recuperator fan 50 and delivered selectively to the
downdraft drying
zone 24, the updraft drying zone 22 and the pre-heating zone 26. Exhaust gas
from the first
heating zone 32, the pre-heating zone 26 and the downdraft drying zone 24 is
carried through
multi-clone 52, wind box exhaust fan 54, through electrostatic precipitator 56
to the stack (not
shown). Other particulate removal means (not shown), depending on the
operating environment,
can be provided, such as further electrostatic precipitators, a cyclone
system, baghouse or settling
chamber. Exhaust gas from the updraft drying zone 22 is carried through hood
exhaust fan 58 to
the electrostatic precipitator 56 and thence to the stack (not shown).
Figure 2 shows a first embodiment of the invention, as applied to a combustion
system 210,
having components mostly corresponding to those shown in Figure 1, and
corresponding exhaust
gas flows leaving each of the components. Thus, combustion system 210
comprises pre-
combustion drying zone 220, heating zone 230, cooling zone 240, air intake
lines 243, 245, wind
box recuperator fan 250, multi-clone 252, wind box exhaust fan 254,
electrostatic
precipitator 256, and hood exhaust fan 258. Exhaust gas from the second
cooling zone 244 is
carried through line 260 to the updraft drying zone 222, and exhaust gas from
the first cooling
zone 242 is carried through line 262 selectively to the hearing zone 230 or
the pre-heating
zone 226.
However, each of the lines carrying the various exhaust gases from the
components of the system
is split, to produce recirculation lines. Thus, the exhaust gas from the
downdraft drying zone 224,
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the pre-heating zone 226 and the first heating zone 232 is divided after wind
box exhaust
fan 254, to produce a first recirculation flow R1 which is carried back to
intake 243 of the first
cooling zone 242, and the remaining flow exits the system through
electrostatic precipitator 256
as in the combustion system 10 of Figure 1. Similarly, the exhaust flow from
the updraft drying
zone 222 is split after hood exhaust fan 258 to produce a second recirculation
flow R2 which is
carried back to intake 243 of the first cooling zone 242; the exhaust flow
from the second heating
zone 234 is split after wind box recuperator fan 250 to produce a third
recirculation flow R3; and
the exhaust flow from the electrostatic precipitator 256 is split to produce a
fourth recirculation
flow R4. The paths of each of the four recirculation flows RI, R2, R3 and R4
can
advantageously be combined with each other as shown in Figure 2, before being
carried to the
intake 243. Ambient air is provided to the second cooling zone 244 from source
46 as in the
combustion system 10 of Figure 1, and to the first cooling zone 242 by
selective delivery to the
combined paths of the four recirculation flows Rl, R2, R3 and R4, prior to
recirculation fan 272 .
Figure 3 shows an alternative embodiment of the invention, in which the
combustion system 310
has the same general configuration as the system 210 of Figure 2. Thus,
combustion system 310
comprises pre-combustion drying zone 320, including updraft drying zone 322
and downdraft
drying zone 324, pre-heating zone 326, heating zone 330, including first
heating zone 332 and
second heating zone 334, cooling zone 340, air intake lines 343, 345, wind box
recuperator
fan 350, multi-clone 352, wind box exhaust fan 354, electrostatic precipitator
356, and hood
exhaust fan 358. Exhaust gas from the second cooling zone 344 is carried
through line 360 to the
updraft drying zone 322, and exhaust gas from the first cooling zone 342 is
carried through line
362 selectively to the hearing zone 330 or the pre-heating zone 326.
However, in the embodiment of Figure 3, the first cooling zone 342 is divided
into two sections,
342A and 342B, and each of the four recirculation flows R1, R2, R3 and R4 can
be divided into
two sub-flows, R1A and RIB, R2A and R2B, R3A and R3B, R4A and R4B, so that one
set of
sub-flows R1A, R2A, R3A and R4A is delivered to intake 343A for the first
section 342A of the
first cooling zone 342, and the second set of sub-flows RIB, R2B, R3B and R4B
is delivered to
intake 343B of the second section 342B of the first cooling zone 342.
Generally it will be
preferable to recirculate the hotter of the two combined recirculation flows
into the first
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section 342A, with selective addition of ambient air from source 46 prior to
recirculation fans
372A, 372B respectively.
In each embodiment, the recirculation fans are controlled to maintain the
desired pressure
balance and temperatures in the heating zones 232, 234, 332, 334 respectively,
by the provision
of conventional means such as inlet dampers, variable inlet vanes or variable
speed drives
separately or in combination. In addition, oxygen sensors (not shown) can be
installed at
appropriate locations such as the stack, to monitor overall oxygen
concentrations which are
indicative of the flue gas recirculation rate.
In operation, for start-up until the system is stabilized, make-up ambient air
is provided from
source 46. As soon as the system is stabilized, flue gas recirculation is
introduced gradually.
Thereafter, at each zone in the system, continuous or periodic monitoring of
operating
conditions, in particular the temperature, will be performed, and adjustments
made to the portion
of each exhaust flow which is recirculated, the relative flow as between each
recirculation flow
or sub-flow, and the amount and location of input of ambient air to temper the
flue gas or control
flow at any particular location.
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