Patent Application
20090308073
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.
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° C. for gas streams or 150° 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.
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° 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° C. to 125° 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° 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. 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.
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.
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).
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° 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.
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:
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., 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.
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.
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.
Benchmark: An optimized air fueled cement plant has the following throughput and fuel consumption:
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).
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.
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
The heat streams 106, 201 generated in this case are given in Table 3.
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.
(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
The heat streams 106 and 201, 202 and 203, generated in this case are given in Table 5.
(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
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
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° C., makes the application of the present invention suitable for this type of furnace.
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.
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.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/US07/16709 | 7/25/2007 | WO | 00 | 7/6/2009 |
| Number | Date | Country | |
|---|---|---|---|
| 60833258 | Jul 2006 | US |
