Note: Descriptions are shown in the official language in which they were submitted.
CA 02354876 2001-07-27
REGENERATIVE THERMAL OXIDIZER
TECHNICAL FIELD OF THE INVENTION
The present invention relates generally to regenerative thermal oxidizers.
More
specifically, the present invention relates to regenerative thermal oxidizers
with high gas
flow rates (for cement plants, refineries, power plants, etc.).
BACKGROUND OF THE INVENTION AND PRIOR ART
Regenerative thermal oxidizers (RTOs) are used in a number of industries to
reduce the
quantity of combustible contaminants in process effluent gases. In a RTO, the
process
effluent gases are oxidized in a combustion chamber. A heat recovery section
of RTO is
filled with loose or structured ceramic packing. A mass of heat-resistant
ceramic packing
in heat exchanger stores heat from the hot gases exiting the combustion
chamber while
another preheated heat exchanger releases heat into the relatively cold gases
entering
the combustion chamber. In the heat exchanger, up to 95% of the heat is
transferred
from the gases to the ceramic packing. The flow of gases is then reversed to
preheat
another heat exchanger (regenerator). Typical cycle time range is from 30 to
120
seconds.
In a RTO having three or more heat exchangers, one heat exchanger sequentially
serves
as a standby heat exchanger. A RTO having two heat exchangers does not have
any
standby heat exchangers. A RTO having three heat exchangers, various types of
valves
(butterfly, wafer, etc) are employed. In the case of two heat exchangers RTO
the most
frequently used valves are poppet calves.
Typical RTOs are shown, for example, in the following patents granted in
Canada:
2,161,860 Wilhelm
2,211,924 Gribbons
2, 251, 768 Blazejewski
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CA 02354876 2001-07-27
The prior art does not address the issue of high power consumption. However,
the fan
for RTO having high gas flow rates (above 10,000 scfm) requires hundreds of
kilowatts of
electricity. Therefore it wauld be desirable to address the high electrical
energy demand
SUMMARY OF THE INVENTION
The present invention provides a RTO having a plurality of heat exchangers.
Each heat
exchanger comprises a process gas inlet, a process gas outlet, a clean gas
inlet, and a
clean gas outlet. Each heat exchanger is filled with ceramic packing. The
valve means
are provided to direct a process gas and clean gas through heat exchangers.
Depending
on process requirements, various types of valve means can be used (poppet
valves,
butterfly valves, dampers, etc). Heating means produce sufficient heat input
to cover the
thermal losses and maintain high temperature range of process gas sufficient
for
oxidizing of combustible components. As an example of said heating means, a
combustion chamber with a burner can be used. A combustion chamber is
connected by
valves or dampers to gas flow path between process gas outlets and clean gas
inlets of
heat exchangers. A combustion chamber is usually constructed from steel with
internal
refractory lining. A burner is connected to external air and fuel sources. A
centrifugal fan
compresses a process gas to overcome pressure losses in a RTO. During start
up, a
bypass valve of centrifugal gas motor (CGM) is open and CGM is not operating.
After
start up of a RTO, a process gas is compressed by a fan to a higher pressure.
It results
in additional power consumption by a fan. A bypass valve of centrifugal gas
motor
(CGM) is closed and excessive gas pressure is used by CGM to generate
mechanical
energy by expansion of the gas. The mechanical energy generated by a CGM
exceeds
said additional power consumption by a fan due to higher volume flow of
preheated gas.
It results in significant net savings in RTO power consumption, and, in some
cases, in
generation of additional power.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of a RTO according to one embodiment of the
present
invention, having three heat exchangers, a CGM being connected to outlet of
combustion
chamber, and a fan being connected to inlet of a RTO.
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CA 02354876 2001-07-27
FIG. 2 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to inlet of combustion chamber, and a fan being connected to inlet
of a RTO.
FIG. 3 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, and a fan being connected to outlet
of a
RTO.
FIG. 4 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to inlet of combustion chamber, and a fan being connected to outlet
of a RTO.
FIG. 5 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO, and
a purging line being connected to the fan inlet.
FIG. 6 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to inlet of combustion chamber, a fan being connected to inlet of a
RTO, and a
purging line being connected to the fan inlet.
FIG. 7 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to outlet of
a RTO,
and a purging line having additional fan and being connected to the process
gas inlets of
heat exchangers.
FIG. 8 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to inlet of combustion chamber, a fan being connected to outlet of a
RTO, and
a purging line having additional fan and being connected to the process gas
inlets of heat
exchangers.
FIG. 9 is a diagrammatic view of a RTO having two heat exchangers, a CGM being
connected to outlet of combustion chamber, and a fan being connected to inlet
of a RTO.
FIG. 10 is a diagrammatic view of a RTO having two heat exchangers, a CGM
being
connected to inlet of combustion chamber, and a fan being connected to inlet
of a RTO.
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CA 02354876 2001-07-27
FIG. 11 is a diagrammatic view of a RTO having two heat exchangers, a CGM
being
connected to outlet of combustion chamber, and a fan being connected to outlet
of a
RTO.
FIG. 12 is a diagrammatic view of a RTO having two heat exchangers, a CGM
being
connoted to inlet of combustion chamber, and a fan being connected to outlet
of a RTO.
FIG. 13 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO, and
additional air heater being connected to outlet of CGM.
FIG. 14 is a diagrammatic view of a RTO having two heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO, and
additional air heater being connected to outlet of CGM.
FIG. 15 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO, and
additional air heater being connected to inlet of CGM.
FIG. 16 is a diagrammatic view of a RTO having two heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO, and
additional air heater being connected to inlet of CGM.
FIG. 17 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to inlet of combustion chamber, and an electrical generator being
mechanically connected to a CGM.
FIG. 18 is a diagrammatic view of a RTO having three heat exchangers, a CGM
being
connected to outlet of combustion chamber, a fan being connected to inlet of a
RTO and
a gaseous fuel inlet being connected to process gas inlets of heat exchangers.
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CA 02354876 2001-07-27
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 of the drawings, a RTO is shown having heat exchangers 1,
2, and 3,
having process gas inlets 12 with associated valve means, process gas outlets
11, clean
gas inlets 10, and clean gas outlets 13. _ _The_.heat.sxchangers _are filled.
yvithcerami~_-
elements. As heating means, a combustion chamber 4 with internal refractory
lining and
burner 5 is provided. An outlet of combustion chamber is connected to the
inlet of
centrifugal gas motor (CGM) 8. Clean gas inlets 10 of heat exchangers 1, 2,
and 3 are
connected to CGM 8 outlet. A consumer of mechanical energy comprises a fan 6
being
in flow communication with process gas inlets 12 of heat exchangers 1, 2, and
3.
Electrical motor 7 rotates a fan 6 during start up or in the modes of
operation when
mechanical energy generated by CGM 8 is not sufficient to rotate a fan 6. A
fan fi can be
placed anywhere upstream of RTO, including upstream of a source of process
gas.
A process gas containing combustible components (products of incomplete
combustion
or other contaminants) enters preheated heat exchanger 1 through process gas
inlet 12
and associated valve. The preheated process gas passes directly from process
gas
outlet 11 of heat exchanger 1 to a combustion chamber 4 where it is subjected
to
temperatures and times delays sufficient for oxidizing of combustible
contaminants.
Additional heat is provided by a burner 5 to maintain sufficient temperature
range of
1500°F to 1900°F if required. The oxidizing temperature depends
on contaminant type,
required destruction efficiency, and available residence time. A hot clean gas
having
higher temperature and higher volume flow than initial volume flow of
relatively cold
process gas enters a CGM 8 and expands. After expansion, a clean gas enters
heat
exchanger 3 through clean gas inlet 10, preheating a heat exchanger for the
next heating
cycle. A clean gas leaves heat exchanger 3 through clean gas outlet 13 at a
temperature
higher than initial temperature of process gas. A power generated by CGM 8 is
used by
consumer of mechanical energy, a fan 6, to compensate additional power
consumption
by a fan 6 due to additional compression of a process gas in a fan, equal to
pressure
drop across a CGM 8, and to compensate at least a part of power consumption
required
to overcome a pressure drop across heat exchangers. If a power generated by
CGM 8
CA 02354876 2001-07-27
exceeds power consumption of a fan 6, excessive power can be converted to
electricity
in another consumer of mechanical energy, a motor-generator 7, or it can be
used to
develop additional head in a fan 6 in order to reduce the load of other
process fans
installed upstream andlor downstream of RTO.
Later the flow of gases is redirected to preheat a heat exchanger 2 and to use
a heat
stored in a heat exchanger 3, etc. Typical cycle range is from 30 to 120
seconds.
As can be seen from Fig. 2, a CGM 8 can be placed upstream of combustion
chamber 4
to reduce temperature of gas entering CGM 8. It results in insignificantly
reduced power
generation by CGM 8. A fan 6 can be provided downstream of a RTO seen in Fig.
3 and
Fig. 4. In this case a RTO is always under negative pressure. It guarantees
zero leaks
of a process gas to the environment.
A RTO seen in Fig. 5 and Fig. 6 differs from RTO seen in Fig. 1 and Fig. 2 by
having
additional valve means 14 provided to remove a process gas left in heat
exchangers
when a gas flow changes its direction. The valve means 14 connect process gas
inlet 12
of one of heat exchangers 1, 2, or 3, which is not being heated or cooled at
this moment,
to a fan 6 inlet and further to process gas inlet 12 of one of heat exchangers
1, 2, or 3,
which is being cooled by a process gas at this moment.
A RTO seen in Fig. 7 and Fig. 8 differs from RTO seen in Fig. 3 and Fig. 4 by
having
additional valve means 14 and additional fan 15 provided to remove a process
gas left in
heat exchangers when a gas flow changes its direction. The valve means 14
connect
process gas inlet 12 of one of heat exchangers 1, 2, or 3, which is not being
heated or
cooled at this moment, to the inlet of additional fan 15 and further to a
process gas inlet
12 of one of heat exchangers 1, 2, or 3, which is being cooled by a process
gas at this
moment (heat exchanger 1 ). It results in removal of process gas with
contaminants from
the idle heat exchanger (heat exchanger 2, Fig. 5, Fig. 6, Fig. 7, and Fig. 8)
and returning
the contaminated gas for further processing by heating it in heat exchanger 1
and
combustion chamber 4. A fan 15, Fig. 7 and Fig. 8, is provided to overcome a
pressure
difference between clean gas inlets 10 and process gas inlets 12 of heat
exchangers 1,
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CA 02354876 2001-07-27
2, and 3, and develop sufficient gas flow rate to purge a contaminated gas
from idle heat
exchanger.
The RTOs seen in Fig. 9, Fig. 10, Fig. 11 and Fig. 12, are similar to RTO seen
in Fig. 1,
Fig. 2, Fig. 3 and Fig. 4 respectively, with exception of total number of heat
exchangers
provided.
An air heater 16 and additional fan 17, Fig. 13, Fig. 14, Fig. 15, and Fig.
16, are provided
to preheat the air required for combustion of fuel in a burner 5. Comparing
with RTOs
seen in Fig. 1, Fig. 2, Fig. 3 and Fig. 3, preheating the air in air heater 16
reduces a fuel
consumption by the burner 5 having the given heat output, or increases a heat
output if
fuel consumption remains the same.
As seen in Fig. 17, a RTO is provided with electrical generator 18 as a
consumer of
mechanical energy. It is effective if a fan cannot be used as a consumer of
mechanical
energy. The electricity generated by an electrical generator 18 can be used by
other
consumers of electrical energy such as press equipment, which produces a
process
gas to be cleaned in RTO.
Referring to Fig. 18, a RTO is shown having gaseous fuel inlet 19 connected to
a process
gas flow path upstream of process gas inlets 12 of heat exchangers 1, 2, and
3. Being
mixed with a process gas, a gaseous fuel passes through heat exchangers, fully
using
recovery heat and eventually increasing the temperature above autoignition
temperature
and being oxidized by excessive oxygen containing in process gas. The heat
release
results in higher thermal efficiency of RTO due to greatly reduced burner heat
output
required to maintain sufficient temperature in combustion chamber.
The efficiency of RTO having process gas flow of 60,000 acfm, temperature in
combustion chamber of 1800°F, nominal pressure drop across RTO of 20'
w.c. (when
CGM is not in service), clean gas pressure at RTO outlet of 0' w.c., and
mechanicat
efficiency of a fan and a CGM of 80%, is illustrated by following data:
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CA 02354876 2001-07-27
No. RTO mode Gas Fan power CGM Total RTO
of pressure consumption,mechanicalelectrical
at power
operation process hp output, consumption,
gas hp hp
inlet, inch
W~c.
1 Valve 9 20 892 0 892
open,
CGM is
not
operating
2 Valve 9 36 1513 1070 441
closed,
CGM in
service
3 Valve 9 48 2018 1847 172
closed,
CGM in
service
4 Valve 9 56 2360 2360
closed,
CGM in
service
A RTO will have zero electrical power consumption and generate excessive
mechanical
energy if gas pressure at process gas inlet exceeds 56" w.c.
The efficiency of RTO having process gas flow of 30,000 acfm, temperature in
combustion chamber of 1800°F, nominal pressure drop across RTO of 10'
w.c. (when
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CA 02354876 2001-07-27
CGM is not in service), clean gas pressure at RTO outlet of 0' w.c., and
mechanical
efficiency of a fan and a CGM of 80%, is illustrated by following data:
No. RTO mode Gas pressureFan power CGM Total RTO
of operationat press- -consumption,---mechanical---electrical
- -power
gas inlet, hp output, consumption,
hp hp
inch w.c.
1 Valve 9 10 223 0 223
open, CGM
is not
operating
2 Valve 9 18 378 268 110
closed,
CGM in
service
3 Valve 9 24 505 462 43
closed,
CGM in
service
4 Vaive 9 28 590 590 ~0
closed,
CGM in
service
A RTO will have zero electrical power consumption and generate excessive
mechanical
energy if gas pressure at process gas inlet exceeds 28" w.c.
9
