U.S. patent application number 12/374646 was filed with the patent office on 2009-12-17 for oxygen enhanced combustion in industrial processes.
Invention is credited to Dante Patrick Bonaquist, Vijayaraghavan Srinivasan Chakravarthy, Raymond Francis Drnevich, Stefan Laux, Minish Mahendra Shah, Monica Zanfir.
Application Number | 20090308073 12/374646 |
Document ID | / |
Family ID | 38982044 |
Filed Date | 2009-12-17 |
United States Patent
Application |
20090308073 |
Kind Code |
A1 |
Bonaquist; Dante Patrick ;
et al. |
December 17, 2009 |
OXYGEN ENHANCED COMBUSTION IN INDUSTRIAL PROCESSES
Abstract
The present invention relates to a system for carrying out
oxygen-enhanced combustion in an industrial process wherein the
industrial process, an oxygen supply system or a source of oxygen,
a heat recovery network, and an alternative Rankine cycle system
based on a working fluid other than steam are integrated to achieve
improved throughput and efficiency, and a method for
oxygen-enhanced combustion in an industrial process using said
system. Examples of industrial processes include cement production,
steel reheat applications, glass production, aluminum and copper
melting, as well as any industrial process that uses process
heater, furnaces where combustion is carried out using an oxidant
stream with oxygen content higher` than that in ambient air and up
to 100%.
Inventors: |
Bonaquist; Dante Patrick;
(Grand Island, NY) ; Shah; Minish Mahendra; (East
Amherst, NY) ; Chakravarthy; Vijayaraghavan Srinivasan;
(Williamsville, NY) ; Zanfir; Monica; (Amherst,
NY) ; Drnevich; Raymond Francis; (Clarence Center,
NY) ; Laux; Stefan; (Williamsville, NY) |
Correspondence
Address: |
PRAXAIR, INC.;LAW DEPARTMENT - M1 557
39 OLD RIDGEBURY ROAD
DANBURY
CT
06810-5113
US
|
Family ID: |
38982044 |
Appl. No.: |
12/374646 |
Filed: |
July 25, 2007 |
PCT Filed: |
July 25, 2007 |
PCT NO: |
PCT/US07/16709 |
371 Date: |
July 6, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60833258 |
Jul 26, 2006 |
|
|
|
Current U.S.
Class: |
60/645 ; 110/297;
431/2; 60/651; 60/671; 62/640 |
Current CPC
Class: |
F23L 7/007 20130101;
F25J 3/04533 20130101; F25J 3/04593 20130101; F25J 2230/22
20130101; F25J 3/04551 20130101; F22B 1/16 20130101; F23C 9/00
20130101; F25J 3/04527 20130101; F01K 13/00 20130101; Y02E 20/34
20130101; F25J 3/04557 20130101; F01K 25/10 20130101; Y02E 20/344
20130101; F25J 3/04521 20130101; F25J 2240/70 20130101; F23L
2900/07005 20130101 |
Class at
Publication: |
60/645 ; 110/297;
431/2; 60/651; 60/671; 62/640 |
International
Class: |
F23L 7/00 20060101
F23L007/00; F25J 3/04 20060101 F25J003/04; F01K 25/08 20060101
F01K025/08 |
Claims
1. A system for carrying out oxygen-enhanced combustion in an
industrial process comprising the industrial process system, an
oxygen supply source, a heat recovery unit, and an alternative
Rankine cycle system based on a working fluid other than steam,
wherein a) the oxygen supply system supplies oxygen to the
industrial process, b) the industrial process generates waste heat
as at least one heat source which is sent to the heat recovery
unit, c) the waste heat is then sent from the heat recovery unit to
the alternative Rankine cycle system, d) the alternative Rankine
cycle system converts the waste heat to power, which is utilized by
the oxygen supply system or the industrial process or is exported
to a utility system.
2. The system of claim 1 wherein the industrial process system
comprises a process heater or furnaces where combustion is carried
out using an oxidant stream with oxygen content in the range from
higher than that in ambient air to up to 100%.
3. The system of claim 2, wherein the industrial process is
selected from the group consisting of cement production, steel
reheat applications, glass production, aluminum melting and copper
melting.
4. The system of claim 1, wherein the oxygen supply source is an
air separation unit.
5. The system of claim 4, wherein the air separation unit is a
cryogenic unit, a pressure swing adsorption system, a vacuum
pressure swing adsorption system or a membrane system.
6. The system of claim 1, wherein the oxygen supply source is a
liquid delivery system or a pipeline supply system.
7. The system of claim 1, wherein the heat recovery unit comprises
heat exchangers that enable heat transfer between the heat source
and the working fluid of the alternative Rankine cycle system and
may employ an intermediate heat transfer fluid.
8. The system of claim 7, wherein the intermediate heat transfer
fluid is thermal oil or air.
9. The system of claim 1, wherein the alternative Rankine cycle
system has at least two heat sources which are generated from the
industrial process.
10. The system of claim 1, wherein flue gas from the heat recovery
unit is partially recirculated to the industrial process
system.
11. The system of claim 1, wherein the working fluid of the
alternative Rankine cycle system is an organic substance.
12. The system of claim 11, wherein the working fluid of the
alternative Rankine cycle system is a refrigerant, a hydrocarbon,
an aromatic, or an aromatic perfluorocarbon.
13. The system of claim 12, wherein the working fluid of the
alternative Rankine cycle system is siloxane or a siloxane
mixture.
14. The system of claim 1, wherein the system for carrying out
oxygen-enhanced combustion further comprises an additional firing
heater.
15. The system of claim 1, wherein the industrial process is a
cement manufacturing process.
16. The system of claim 1, wherein the industrial process is a
steel reheat process.
17. A system for carrying out oxygen enhanced combustion in an
industrial process comprising the industrial process system and an
oxygen supply source, wherein there is 100% oxygen enhanced
combustion and partial recirculation of flue gases.
18. A method for carrying out oxygen-enhanced combustion in an
industrial process using the system of claim 1.
19. A method for carrying out oxygen-enhanced combustion in an
industrial process using the system of claim 17.
Description
RELATED APPLICATION
[0001] This application claims the priority to U.S. Provisional
Application No. 60/833,258, filed on Jul. 25, 2006, the entire
contents of which are incorporated by reference herein.
TECHNICAL FIELD
[0002] The present invention generally relates to the field of
cogeneration of power and heat, and particularly to recover heat
lost to ambient air using oxygen enhanced combustion.
BACKGROUND OF INVENTION
[0003] Oxygen enhanced combustion is utilized in industrialized
furnace applications to increase throughput and has additional
advantages such as lower emissions, improved flame stability and
heat transfer. Although increased thermal efficiency is claimed for
processes that use oxy-fueled combustion, making use of the
unavoidably generated waste heat remains a challenge. Unlike
air-fired processes, where the excess thermal energies from the
flue gases are used to preheat the air for combustion, in oxygen
enhanced combustion, the hazards of handling hot oxygen-enriched
streams limit this type of heat integration. Therefore, in some
processes, conversion to oxygen enriched combustion (e.g., cement
manufacturing processes) may lead to a higher waste heat rejection
resulting in a decrease of thermal efficiency.
[0004] Cogeneration of power and heat by production of electricity
and/or steam stands as an alternative. However, for heat sources
having low temperatures (e.g., lower than 500.degree. C. for gas
streams or 150.degree. C. for liquid streams) energy recovery and
power generation through steam becomes inefficient. Limitations
related to low-grade heat recovery to generate power can be
overcome by making use of alternative Rankine cycles, where the
working fluid is not steam, but rather an organic substance or a
mixture. Power generated from such low grade heat sources can be
utilized to meet oxygen supply system needs, strengthening the
incentive for conversion to oxygen enhanced combustion.
[0005] In the past, organic Rankine cycles (ORC) have been used to
recover heat. For example, U.S. Pat. No. 6,701,712 B2 discloses a
method and an apparatus based on an ORC to recover the heat from
the hot air used to cool the clinker in cement manufacturing in
order to produce power. The waste heat recovery unit contains
specific equipment to extract particulate matter from the hot air,
a combination of heat exchangers where waste heat is transferred
from the source to the ORC working fluid by means of an
intermediate fluid, such as thermal oil or air. Examples of
applications disclosed in U.S. Pat. No. 6,701,712 B1 are reported
by Baatz et al. (Baatz E., Heidt G., ZKG Intl., Vol. 8, pp.
425-436, 2000) and by Claus et al. (Claus W., Kolbe T., ZKG Intl.,
Vol. 55, pp. 78-86, 2002). These describe the implementation of an
ORC at Heidelberg Cement in Lengfurt, Germany. The heat source is
represented by the hot air exiting the clinker cooler, as shown in
FIG. 1 of U.S. Pat. No. 6,701,712 B2. For a cement plant with a
capacity of 3000 tpd clinker, the hot air flow rate exiting the
clinker cooler is about 193,100 kg/h, at an average temperature of
275.degree. C. The available thermal energy carried by this flow
rate is 14 MW (assuming that it is discharged at 25.degree. C.).
From this, 8.2 MW can be recovered by cooling the hot air from
275.degree. C. to 125.degree. C., generating about 1.15 MW power.
The amount of power generated reduces the power demand of cement
manufacture by 10%. Typical temperatures of air available after
clinker cooling are usually less than 350.degree. C. (see Baatz et
al.). In a cement plant, heat can be recovered not only from the
hot air used for cooling the clinker, but also from the flue gases,
and oxy-enhanced combustion is utilized for increasing the
throughput of cement plants. However, the benefit of capacity
increase is penalized by a decrease in thermal efficiency of the
plant. Substituting air with oxygen for combustion makes less use
of the hot air available after clinker cooling. Therefore, the heat
removed due to clinker cooling is not efficiently shifted up-stream
to preheat the raw materials. This makes available more low-grade
heat in the hot air exiting the clinker cooler and therefore a
higher amount of power is generated. Furthermore, the generated
power is integrated with the oxygen supply system.
[0006] U.S. Pat. No. 7,062,912 identifies the need for increased
efficiency of oxygen-enriched combustion in industrial furnaces
through integrated heat recovery strategies; main emphasis is on
power generation using steam Rankine cycles from flue gases
produced in oxy-enhanced combustion. Further, mechanical power is
generated, which is integrated with an air separation unit that
supplies the oxygen for oxy-enhanced combustion, in order to
partially cover the demand for power of air separation. However,
U.S. Pat. No. 7,062,912 uses steam as working fluid for the Rankine
cycle and therefore for power generation and it does not teach a
method or system of heat recovery at lower temperatures where steam
is not an appropriate working fluid. Furthermore, it does not teach
a method or system for generating electrical power.
[0007] U.S. Pat. No. 6,077,072 discloses a firing scheme that uses
at least one injector for oxidant and fuel in a cement rotary kiln,
which allows an increase in the amount of heat released toward the
load, resulting in significant increases in kiln efficiency and
production. However, unlike in the present invention where the
exhaust flue gases are further used for power generation, U.S. Pat.
No. 6,077,072 uses oxy-enhanced combustion only for throughput
increase.
[0008] Cement production is an energy intensive process. According
to the method of preparation of raw materials, cement manufacturing
can be classified in wet-processes and dry-processes. In
dry-process, the raw materials are fed to the kiln in a dry state,
whilst in a wet-process a slurry is formed by adding water (see
LEA's Chemistry of Cement and Concrete, ed. Hewlett PC, New York,
1998). Nearly 33% additional kiln energy is consumed in evaporating
the slurry water. Although the drying process makes a better use of
the available heat by preheating the raw materials and using flue
gases, still the thermal efficiency of a cement plant is low. The
average thermal efficiency reported for cement kilns used in the US
is 37% for dry-kilns and 27% for wet-kilns (see Choate, W. T.,
"Energy and Emission Reduction Opportunities for Cement Industry",
U.S. Dept. of Energy, Energy Efficiency and Renewable Energy,
prepared under contract for Industrial Technology Program,
2003).
SUMMARY OF INVENTION
[0009] The present invention relates to a system for carrying out
oxygen-enhanced combustion in an industrial process wherein the
industrial process, an oxygen supply system or a source of oxygen,
a heat recovery network, and an alternative Rankine cycle system
based on a working fluid other than steam are integrated to achieve
improved throughput and efficiency, and a method for
oxygen-enhanced combustion in an industrial process using said
system. Examples of industrial processes include cement production,
steel reheat applications, glass production, aluminum and copper
melting, as well as any industrial process that uses process
heater, furnaces where combustion is carried out using an oxidant
stream with oxygen content higher than that in ambient air and up
to 100%. The oxygen supply system can be any type of air separation
unit (e.g., cryogenic, pressure swing adsorption (PSA), vacuum
pressure swing adsorption (VPSA), membrane, etc.), or other type of
oxygen supply (e.g., liquid delivery, pipeline supply). The heat
recovery network comprises heat exchangers that enable heat
transfer between the heat source and the working fluid of the
alternative Rankine cycle, and may employ an intermediate heat
transfer fluid such as thermal oil or air.
[0010] Also, the invention seeks to integrate in an efficient
manner the utilization of the power generated with the oxygen
supply system, or any demand of electrical power within the
industrial process considered. The power output of the alternative
Rankine cycle could be in the form of electricity, and the
alternative turbine of the alternative Rankine cycle can be
directly coupled to one of the power consuming devices through a
shaft or through a motor/generator assembly for reducing equipment
cost associated with generating electricity and distributing it to
different devices. Having a motor/generator assembly will provide
flexibility in operation.
[0011] Use of oxygen enhanced combustion increases the availability
of low-grade heat sources, which could become valuable
opportunities for power generation. However, steam power generating
systems are less efficient when heat source is available at
temperatures lower than 400.degree. C., due to lack of economic
viability caused by poor achievable efficiency typical to steam
processes at such low temperature. Generation of electrical energy
by alternative Rankine cycles (including organic Rankine cycles),
which can make use of low temperature heat, represents a feasible
alternative. In addition, using a working fluid other than steam
has other advantages, such as low mechanical stressing of the
turbine as a result of the low speed of the turbine impeller,
direct drive of the electrical generator without any reduction gear
unit, no erosion of the turbine blades due to absence of moisture
in the vapor, and thus, simple maintenance and operation and longer
service life of the plant.
[0012] Oxygen enhanced combustion is utilized in many industrial
applications to increase throughput, which decreases the need for
building new plants and thus minimizing capital investment. In
addition, the power generated from heat sources produced as a
consequence of oxy-fuel combustion reduces the net demand of
electricity of the oxygen supply system. Therefore, the present
invention helps to lower the cost of power and/or lower the cost of
oxygen for oxy-fuel conversion. The oxygen demand for a typical
cement plant for a 25% throughput increase is 650-1450 tpd, whilst
the power generated can range between 4-7 MW.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] For a more complete understanding of the present invention
and the advantages thereof, reference should be made to the
following Detailed Description taken in conjunction with the
accompanying drawings in which:
[0014] FIG. 1 is a schematic of an industrial process employing
oxygen enhanced combustion integrated with an oxygen supply system,
heat recovery network, and alternative Rankine cycle system.
[0015] FIG. 2 is a schematic of an industrial process employing
oxygen enhanced combustion integrated with an oxygen supply system,
heat recovery network, and alternative Rankine cycle system having
at least two heat sources 106, 201 generated from the industrial
process 10.
[0016] FIG. 3 is a schematic of an industrial process employing
oxygen enhanced combustion integrated with an oxygen supply system,
heat recovery network, and alternative Rankine cycle system wherein
the flue gas is recirculated 203 for additional heat recovery.
[0017] FIG. 4 is a schematic of an industrial process employing
oxygen enhanced combustion integrated with an oxygen supply system,
heat recovery network, alternative Rankine cycle system, and an
additional firing heater 13 used to enhance the temperature level
of the recoverable heat.
[0018] FIG. 5 is a schematic showing an industrial process wherein
the industrial process 10 is a cement manufacturing process.
[0019] FIG. 6 is a schematic of an industrial process employing
oxygen enhanced combustion integrated with an oxygen supply system,
heat recovery network, and alternative Rankine cycle system wherein
at least one heat source stream 201 is represented by flue gases
generated by oxygen enhanced combustion.
[0020] FIG. 7 is a schematic showing an industrial process
employing oxygen enhanced combustion integrated with an oxygen
supply system, wherein there is no heat recovery and partial
recirculation of flue gases.
[0021] FIG. 8 is a schematic showing an industrial process
employing oxygen enhanced combustion integrated with an oxygen
supply system, heat recovery network, and alternative Rankine cycle
system, wherein the industrial process is a steel reheat
process.
DETAILED DESCRIPTION OF THE INVENTION
[0022] The present invention relates to a system for carrying out
oxygen-enhanced combustion in an industrial process wherein the
industrial process, an oxygen supply system or a source of oxygen,
a heat recovery network, and an alternative Rankine cycle system
based on a working fluid other than steam are integrated to achieve
improved throughput and efficiency, and a method for
oxygen-enhanced combustion in an industrial process using said
system, wherein
[0023] a) the oxygen supply system supplies oxygen to the
industrial process,
[0024] b) the industrial process generates waste heat as at least
one heat source which is sent to the heat recovery unit,
[0025] c) the waste heat is then sent from the heat recovery unit
to the alternative Rankine cycle system,
[0026] d) the alternative Rankine cycle system converts the waste
heat to power, which is utilized by the oxygen supply system or the
industrial process or is exported to a utility system. Examples of
industrial processes include cement production, steel reheat
applications, glass production, aluminum and copper melting, as
well as any industrial process that uses process heater, furnaces
where combustion is carried out using an oxidant stream with oxygen
content higher than that in ambient air and up to 100%. The oxygen
supply system can be any type of air separation unit (e.g.,
cryogenic, pressure swing adsorption (PSA), vacuum pressure swing
adsorption (VPSA), membrane, etc.), or other type of oxygen supply
(e.g., liquid delivery, pipe line supply). The heat recovery
network comprises heat exchangers that enable heat transfer between
the heat source and the working fluid of the alternative Rankine
cycle, and may employ an intermediate heat transfer fluid such as
thermal oil or air.
[0027] Also, the invention seeks to integrate in an efficient
manner the utilization of the power generated with the oxygen
supply system, or any demand of electrical power within the
industrial process considered. The power output of the alternative
Rankine cycle could be in the form of electricity, alternative
turbine of the alternative Rankine cycle can be directly coupled to
one of the power consuming device through a shaft or through a
motor/generator assembly for reducing equipment cost associated
with generating electricity and distributing it to different
devices. Having a motor/generator assembly will provide flexibility
in operation.
[0028] In the present invention, the working fluid of the
alternative Rankine cycle system can be refrigerants (e.g., R11,
R123, HCF 245fa), hydrocarbons (e.g., ethanol, iso-butane,
n-pentane, iso-pentane), aromatics (e.g., toluene, p-xylene),
aromatic perfluorocarbons (e.g., hexafluorobenzene), or siloxane
and siloxane mixtures.
[0029] FIG. 1 is a simplified schematic of an industrial process
employing oxygen enhanced combustion integrated with an oxygen
supply source, heat recovery network, and alternative Rankine cycle
system. Any industrial process 10, where oxygen enhanced combustion
is used to supply the required thermal energy can be a suitable
application. For example cement production, steel reheat
applications, glass production, aluminum and copper melting, as
well as any industrial process that uses a process heater or
furnaces where combustion is carried out using an oxidant stream
with oxygen content higher than that in ambient air and up to 100%.
The industrial process 10 uses at least one fuel stream 101,
combustion air 102 if required, and adequate raw materials 103. The
oxygen used for oxygen enhanced combustion is delivered by an
oxygen supply system 11, which can be any air separation process
(e.g., cryogenic, PSA, VPSA, membrane, etc.) or liquid oxygen or
oxygen pipe delivery system. An oxidant stream 104, with an oxygen
concentration higher than that in ambient air and up to 100%,
supplies the necessary amount of oxygen for oxygen enhanced
combustion. In addition to the main product 105, from the
industrial process 10, at least one heat source is generated 106.
The heat source 106 can be liquid or gas stream, produced either as
flue gases as a direct result of oxygen enhanced combustion, or as
any stream utilized within process 10 for cooling in order to meet
the process needs. The heat source temperature is lower than
600.degree. C. most preferably between 400-100.degree. C. Heat
recovery unit 12, comprising at least one heat exchanger or a
network of heat exchangers, extracts the recoverable heat from the
source 106, which is further used in the alternative Rankine cycle
30 in order to generate a certain amount of power 107. The power
generated 107 can be either exported to the utility system, 108, or
utilized to meet electrical energy demand of the oxygen supply
system, 110, or process demand, 109, or a combination of the above,
for example the turbine of the alternative Rankine cycle can be
directly coupled to one of the power consuming device through a
shaft or through a motor/generator assembly for reducing equipment
cost associated with generating electricity and distributing it to
different devices. Having a motor/generator assembly will provide
flexibility in operation.
[0030] FIG. 2 is another simplified schematic showing at least two
heat sources 106, 201 generated from the industrial process 10.
Each heat source is sent to a separate heat recovery unit 12 and 13
respectively. An intermediate fluid 112, (e.g., thermal oil, air,
or pressurized water) is utilized to transfer the recoverable heat
to the alternative Rankine cycle 30. The intermediate fluid circuit
112 can operate in various modes to recover the available heat in a
series, parallel or alternative manner.
[0031] FIG. 3 is yet another simplified schematic, wherein at least
one heat source stream, 201 is represented by flue gases generated
by oxygen enhanced combustion. These are sent to the heat recovery
unit 12. After exiting the heat recovery unit, they are partially
re-circulated, as stream 203, within process 10, for dilution of
the oxidant stream in order to meet combustion temperature
conditions. The remaining part of flue gases is vented as waste
stream 202.
[0032] FIG. 4 is another simplified schematic showing an additional
firing heater 13, which uses additional fuel 201 and combustion air
202 to increase the temperature level of the recoverable heat sent
to the alternative Rankine cycle 30. The recoverable heat from the
flue gas 203 generated in the firing heater 13 is further used in
the heat recovery unit 12.
[0033] FIG. 5 is a simplified schematic wherein the industrial
process 10 is a cement manufacturing process. Partial oxygen
enhanced combustion is used in precalciner 20 and cement kiln 21.
Fuel 101 is added to the precalciner as 101a and cement kiln as
101b. The raw materials are preheated in countercurrent by the flue
gases. The calcined dust 403 is further fed to the rotary kiln 21.
The hot clinker 404 exiting the kiln 21 is further cooled in the
clinker cooler 22, using an air stream 102. Some of the air heated
in the first part of the clinker cooler 22a is utilized as
combustion air for kiln, 402, and precalciner, 401, in addition to
the oxygen enriched streams 104a and 104b to achieve oxygen
dilution requirements. Therefore, oxygen concentration in kiln and
precalciner can range between 20-60%. The exhaust heat sources are
on one hand the flue gases after they are used for raw materials
preheating, 201, and the remaining hot air 106 obtained from the
clinker cooler 22b. The power generated 107 can be either exported
to the utility system, 108, or utilized to meet the electrical
energy demand of the oxygen supply system, 110 or cement
manufacturing demand 109, or a combination of the above.
[0034] FIG. 6 is yet another simplified schematic, wherein at least
one heat source stream 201 is represented by flue gases generated
by oxygen enhanced combustion. These are sent to the heat recovery
unit 12. Flue gases 202 after exiting the heat recovery unit 12 are
partially recirculated in kiln 21 as stream 502 and in precalciner
20 as 501 to adjust oxygen dilution requirements. The remaining
part of flue gases is vented as waste stream 203.
[0035] FIG. 7 is a schematic showing an industrial process
employing oxygen enhanced combustion integrated with an oxygen
supply system, wherein there is no heat recovery and flue gases 202
are partially recirculated in kiln 21 as stream 502 and in
precalciner 20 as 501.
[0036] Cement production is an energy intensive process. According
to the method of preparation of raw materials, cement manufacturing
can be classified in wet-processes and dry-processes. In
dry-process, the raw materials are fed to the kiln in a dry state,
whilst in a wet-process a slurry is formed by adding water (see
LEA's Chemistry of Cement and Concrete, ed. Hewlett PC, New York,
1998). Nearly 33% additional kiln energy is consumed in evaporating
the slurry water. Although, the dry process makes a better use of
the available heat by preheating the raw materials, using flue
gases, still the thermal efficiency of cement plant is low. The
average thermal efficiency reported for cement kilns used in the
U.S. is 37% for dry-kilns and 27% for wet-kilns (see Choate, W. T.,
"Energy and Emission Reduction Opportunities for Cement Industry",
U.S. Dept. of Energy, Energy Efficiency and Renewable Energy,
prepared under contract for Industrial Technology Program, 2003).
Therefore, the present invention can be applied regardless of the
wet or dry process type to improve the energy efficiency. Moreover,
cement manufacturing is not only energy intensive, but also capital
intensive, requiring large-scale equipment in order to be
economically competitive. Practicing oxygen-enhanced combustion in
order to increase the kiln-throughput avoids investment in new
plants. Power generation in oxygen-enhanced combustion cement
plants becomes even more attractive, since conversion to oxygen
enhanced combustion is associated with a drop in thermal efficiency
compared to air-fueled cement plants.
EXAMPLES
[0037] Several examples are given and explained below. The energy
and material balances have been obtained using an in-house model
for the cement process, whilst Aspen HYSYS.RTM. was used to
simulate the alternative Rankine cycle, having n-pentane as working
fluid.
Example 1
[0038] Benchmark: An optimized air fueled cement plant has the
following throughput and fuel consumption:
TABLE-US-00001 Plant Capacity: 4000 tpd Fuel Consumption: 131.7 MW
Oxygen consumption: 0.0 kg/h
[0039] The heat sources from kiln and clinker cooler are given in
Table 1, if no heat is recovered from these streams, the total
exhausted heat amounts to 23.5% of the fuel consumed (calorific
input).
TABLE-US-00002 TABLE 1 Air fuel combustion without heat recovery
Kiln Clinker Cooler Stream Exhaust Exhaust Flow rate, kg/h 274 000
172 400 Temperature .degree. C. 332 169 Composition, % vol. N.sub.2
58.0 79.0 O.sub.2 2.0 21.0 CO.sub.2 34.2 0.0 H.sub.2O 5.8 0.0 Heat
Flow*, MW 24.4 6.6 Exhausted heat as % 18.53 5.0 Fuel Calorific
Input *calculated for a temperature discharged of 35.degree. C.
[0040] Moreover, low temperature of the hot air exiting the clinker
cooler of 169.degree. C. makes heat recovery using alternative
Rankine cycle less feasible. Only the heat from flue gases is
recoverable. Table 2 shows the amount of recoverable heat from each
stream, the power generated considering an overall efficiency of
18%, and recalculates the exhausted heat as percent of fuel
calorific input.
TABLE-US-00003 TABLE 2 Air fuel combustion with heat recovery Kiln
Clinker Cooler Stream Exhaust Exhaust Recoverable Heat Flow*, MW
14.9 0.0 Power Generated, MW 2.7 0.0 Exhausted heat as % 7.2 5.0
Fuel Calorific Input *calculated for cooling at 150.degree. C.
Example 2
Partial Oxygen Enhanced Combustion
[0041] An increase by 25% in cement plant throughput can be
achieved by increasing the fuel input and at the same time using
about 47.5% of the oxygen required for combustion as pure oxygen,
stream 104 as shown in FIG. 4.
TABLE-US-00004 Plant Capacity: 5000 tpd Fuel Consumption, 101:
191.7 MW Input Oxygen, 104: 28,460 kg/h
[0042] The heat streams 106, 201 generated in this case are given
in Table 3.
TABLE-US-00005 TABLE 3 Oxygen enhanced combustion without heat
recovery Exhaust kiln Exhaust Clinker Stream (201) Cooler (106)
Flow rate, kg/h 287,000 317,300 Temperature .degree. C. 332 403
Composition, % vol. N.sub.2 45.3 79.0 O.sub.2 2.0 21.0 CO.sub.2
44.7 0.0 H.sub.2O 8.0 0.0 Heat Flow*, MW 24.7 41.8 Exhausted heat
as % 12.9 21.8 Fuel Calorific Input *calculated for a temperature
discharged of 35.degree. C.
[0043] Table 4 shows the amount of recoverable heat from each
stream, the power generated considering an overall efficiency of
18%, and recalculates the exhausted heat related to fuel
consumption.
TABLE-US-00006 TABLE 4 Oxygen enhanced combustion with heat
recovery Exhaust kiln Exhaust Clinker Stream (201) Cooler (106)
Recoverable Heat Flow*, MW 15.6 24.0 Power Generated, MW 2.8 6.0
Exhausted heat as % 4.8 9.3 Fuel Calorific Input *calculated for
cooling at 150.degree. C.
Example 3
100% Oxygen Enhanced Combustion with Flue Gas Recirculation
[0044] (a): No heat recovery through power generation.
[0045] A similar increase in throughput, by 25%, can be obtained
switching to 100% oxygen enhanced combustion, and partial
recirculation of the flue gases, to account for oxygen dilution as
shown in FIG. 6, where no heat recovery is utilized.
TABLE-US-00007 Plant Capacity: 5000 tpd Fuel Consumption, 101:
184.7 MW Input Oxygen, 104: 60,000 kg/h
[0046] The heat streams 106 and 201, 202 and 203, generated in this
case are given in Table 5.
TABLE-US-00008 TABLE 5 Complete oxyfuel conversion without heat
recovery Exhaust Exhaust Clinker kiln Recirculated Vented Cooler
Stream (201) (202) (203) (106) Flow rate, kg/h 331,600 126,100
205,500 275,900 Temperature .degree. C. 332 523 Composition, % vol.
N.sub.2 3.7 79.0 O.sub.2 2.0 21.0 CO.sub.2 81.6 0.0 H.sub.2O 12.8
0.0 Heat Flow*, MW 36.3 -- 22.5 39.8 Exhausted heat as % -- -- 12
21.5 Fuel Calorific Input *calculated for a temperature discharged
of 35.degree. C.
[0047] (b): Heat recovery through power generation.
[0048] Table 6 summarizes the flow rate, temperature and
composition of streams 106, 201, 202, and 203 when 100% oxygen
enhanced combustion is used at the same time with power generated
as shown in FIG. 5. It also summarizes the amount of recoverable
heat from each stream, and the power generated considering an
overall efficiency of 18%.
TABLE-US-00009 TABLE 6 Complete oxyfuel conversion with heat
recovery Exhaust Exhaust Clinker kiln Recirculated Vented Cooler
Stream (201) (202) (203) (106) Flow rate, kg/h 331 600 126 100 205
500 275900 Temperature .degree. C. 332 150 150 440 Composition, %
vol. N.sub.2 3.7 79.0 O.sub.2 2.0 21.0 CO.sub.2 81.6 0.0 H.sub.2O
12.8 0.0 Recoverable Heat 17.9 -- -- 23.8 Flow*, MW Power
Generated, MW 3.2 -- -- 4.3 Exhausted heat as % 6.2 4.9 Fuel
Calorific Input *calculated for cooling at 150.degree. C.
[0049] Table 7 gives a comparison of air-fueled and oxygen enhanced
combustion cement processes with and without power generation from
heat sources available in the process. Column (1) gives the plant
capacity. An increase in throughput can be obtained by either using
a partial conversion to oxygen enhanced combustion, for example
47.5% of the oxygen needed for combustion is provided as pure
oxygen (see FIG. 4), or by total conversion to oxy-combustion, when
100% of the required oxygen is delivered by the oxygen supply
system and part of the flue gases are recirculated (see FIG. 5). As
it can be seen in column (2) the increase in throughput comes with
a penalty in fuel consumption. About 10% increase of fuel consumed
is required for 25% throughput increase. Column (3) gives the
exhausted heat (amount of discharged heat with the flue gases and
hot air from the clinker, considering a discharged temperature of
35.degree. C.) as percent of fuel calorific input. Relative to air
fuel case, conversion to oxy-enhanced combustion leads to about 1.5
times increase in exhausted heat. Coupling of oxy-combustion with
power generation makes available significant amount of electricity
and at the same time reduces exhausted heat by 66%.
TABLE-US-00010 TABLE 7 Comparison of Air and Oxygen Enhanced
Combustion Cases (3) Exhausted (2) Heat Fuel as % (5) (1) Calorific
of Fuel (4) Gross Plant Input Calorific Oxygen Power Power Capacity
GJ/tc Input Consumption Output Case Generation tpd linker % tpd MW
Air- No 4000 2.9 23.5 0.0 0.0 Fuel Yes 4000 2.9 12.2 0.0 2.7
Partial No 5000 3.2 34.7 683 0.0 oxy- Yes 5000 3.2 14.1 683 4.3
combustion (47.5%) Total No 5000 3.3 33.5 1440 0.0 oxy- Yes 5000
3.3 11.1 1440 7.5 combustion (100%)
[0050] Another potential application of the present invention is
related to steel reheat furnaces. The primary concerns in the steel
industry are productivity, energy efficiency, and reduced
emissions. These demands can and indeed have been satisfied by the
use of oxy-fuel combustion in a wide range of both batch and
continuous type furnaces. Continuous furnaces such as pusher,
walking beam or roller hearth are designed so that the exhaust
gases flow counter-current to the in-coming product so that the
energy contained can be used in the pre-heat zone at the entrance
to the furnace, thus improving the thermal efficiency. The use of
oxy-fuel in such furnaces however offers a step change increase in
fuel efficiency and productivity not attainable by air-fuel
combustion techniques. In addition the exhaust temperature of the
flue gases, around 450.degree. C., makes the application of the
present invention suitable for this type of furnace.
Example 4
Steel Reheat Furnace
[0051] FIG. 8 is a schematic for a steel reheat process using the
present invention, integrating an oxygen supply system, a heat
recovery unit and an alternative Rankine cycle. Table 8 summarizes
the characteristics of the heat source and the power generated for
a steel reheat furnace with a capacity given below.
TABLE-US-00011 Steel Reheat Furnace Capacity: 8200 tpd Oxygen
consumption: 140 tpd
TABLE-US-00012 TABLE 8 Steel reheat furnace characteristics Stream
Flow rate, kg/h 168,380 Temperature .degree. C. 482 Heat Flow*, MW
32 Available Heat**, MW 18 Power Generated, MW 3 *calculated for a
temperature discharged of 35.degree. C. **calculated for a
temperature discharged of 150.degree. C.
[0052] Although the invention has been described in detail with
reference to certain preferred embodiments, those skilled in the
art will recognize that these are other embodiments within the
spirit and the scope of the claims.
* * * * *