OXYGEN ENHANCED COMBUSTION IN INDUSTRIAL PROCESSES

RELATED APPLICATION

This application claims the priority to U.S. Provisional Application No. 60/833,258, filed on July 25, 2006, the entire contents of which are incorporated by reference herein.

TECHNICAL FIELD 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

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.

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 ⁰ C for gas streams or 150 ⁰ 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 steeim, 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.

In the past, organic Rankine cycles (ORC) have been used to recover heat. For example, U.S. Patent 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. Patent No. 6,701,712 Bl 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 Figure 1 of U.S. 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°C. The available thermal energy carried by this flow rate is 14 MW (assuming that it is discharged at 25°C) . From this, 8.2

MW can be recovered by cooling the hot air from 275 ⁰ C to 125 ⁰ 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 ⁰ 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.

U.S. Patent 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. Patent 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.

U.S. Patent 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 ftirther used for power generation, U.S. Patent No. 6,077,072 uses oxy-enhanced combustion only for throughput increase.

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

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.

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.

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 ⁰ 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.

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 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:

Figure 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.

Figure 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.

Figure 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.

Figure 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 .

Figure 5 is a schematic showing an industrial process wherein the industrial process 10 is a cement manufacturing process .

Figure 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.

Figure 7 is a schematic showing an industrial process employing oxygen enhanced combustion integrated with an oxygen supply system, whesrein there is no heat recovery and partial recirculation of flue gases.

Figure 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

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 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. 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.

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.

In the present invention, the working fluid of the alternative Rankine cycle system can be refrigerants (e.g., RIl, 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 .

Figure 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 ⁰ C most preferably between 400-100 ⁰ 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.

Figure 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 .

Figure 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.

Figure 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.

Figure 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.

Figure 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.

Figure 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.

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

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 ^® was used to simulate the alternative Rankine cycle, having n-pentane as working fluid.

EXAMPLE 1

Benchmark: An optimized air fueled cement plant has the following throughput and fuel consumption: Plant Capacity: 4000 tpd

Fuel Consumption : 131.7 MW Oxygen consumption : 0.0 kg /h

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 1: Air fuel combustion without heat recovery

calculated for a temperature discharged of 35 ⁰ C

Moreover, low temperature of the hot air exiting the clinker cooler of 169°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 2 : Air fuel combustion with heat recovery

calculated for cooling at 150 ⁰ C EXAMPLE 2: Partial Oxygen Enhanced Combustion

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 Figure 4.

Plant Capacity: 5000 tpd

Fuel Consumption, 101: 191.7 MW

Input Oxygen, 104: 28,460 kg/h

The heat streams 106, 201 generated in this case are given in Table 3.

Table 3 :. Oxygen enhanced combustion without heat recovery

calculated for a temperature discharged of 35 ⁰ C

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 4 : Oxygen enhanced combustion with heat recovery

calculated for cooling at 150 ⁰ C

EXAMPLE 3: 100 % Oxygen Enhanced Combustion with Flue Gas Recirculation

(a) No heat recovery through power generation.

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 Figure 6, where no heat recovery is utilized.

Plant Capacity: 5000 tpd

Fuel Consumption, 101: 184.7 MW

Input Oxygen, 104 : 60,000 kg /h

The heat streams 106 and 201 _# 202 and 203, generated in this case are given in Table 5.

Table 5: Complete oxyfuel conversion without heat recovery

Stream Exhaust Recirculated Vented Exhaust kiln Clinker Cooler

(201) (202) (203) (106)

Flow rate, kg /h 331,600 126, 100 205, 500 275 , 900

Temperature ⁰ C 332 523

Composition, % vol .

N ₂ 3.7 79 .0

O ₂ 2.0 21 .0

CO ₂ 81.6 0 .0

H ₂ O 12.8 0 0

Heat Flow ^* , MW 36.3 - 22.5 39 8

Exhausted heat 12 21 .5 as % Fuel Calorific Input calculated for a temperature discharged of 35°C (b) : Heat recovery through power generation. 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 Figure 5. It also summarizes the amount of recoverable heat from each stream, and the power generated considering an overall efficiency of 18%.

calculated for cooling at 150 ⁰ C

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 Figure 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 Figure 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°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 7 Comparison of Air and Oxygen Enhanced Combustion

Cases

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 ⁰ C, makes the application of the present invention suitable for this type of furnace .

EXAMPLE 4 : Steel Reheat Furnace

Figure 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.

Steel Reheat Furnace Capacity: 8200 tpd Oxygen consumption: 140 tpd

Table 8: Steel reheat furnace characteristics

calculated for a temperature discharged of 35°C calculated for a temperature discharged of 150 ⁰ C

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.