Patent Application
20070183966
The present invention relates to a method of recovering waste heat discharged from various industrial facilities, and in particular, to a waste heat recovery apparatus for recovering waste heat of an exhaust gas discharged from a combustion apparatus such as a furnace, an internal combustion engine, or electric power generation equipment, and a method therefor.
There have been continued efforts focused on recovering waste heat of an exhaust gas discharged from combustion apparatuses such as various furnaces examples of which are a combustion furnace that burns fuel, an internal combustion engine, and electric power generation equipment to achieve satisfactory energy saving.
In general, as a method of recovering waste heat of an exhaust gas, a method of recovering the sensible heat of the exhaust gas as thermal energy is employed. Specifically, a heat exchanger is disposed in an exhaust system of a combustion apparatus, a heat recovery medium passes through the heat exchanger, and the heat recovery medium is heated by the exhaust gas, thereby recovering the sensible heat of the exhaust gas as thermal energy. For example, in a waste heat boiler, water is used as a heat exchange medium, and the sensible heat of the exhaust gas is recovered as thermal energy of low-pressure steam or hot water. In a recuperator and a regenerative burner, air is used as a heat exchange medium, and the sensible heat of the exhaust gas is recovered as thermal energy of heated air that has been preheated.
A characteristic of waste heat that affects the waste heat recovery technique is its low energy density. In some cases, the temperature of an exhaust gas is 300° C. or lower. In such a case, when heat exchange is performed from the exhaust gas to a heat exchange medium, at most, thermal energy in the range of 200° C. to 250° C. is recovered. Thermal energy having a low energy density can be used only as a low-temperature heating source. In order to recover waste heat with high efficiency to overcome this problem, a heat exchanger with a large heat transfer area is required, and thus the size of the waste heat recovery apparatus tends to increase.
Another problem with the waste heat recovery technique is that the application of the waste heat recovery technique is limited to combustion air, a low-temperature heating source, or the like, and the application site may be limited to areas near the recovered heat source (when heated air or steam is transported over a long distance with a pipe or the like, heat dissipation is increased and the recovered heat may not be effectively used). For this reason, in particular, when the temperature of an exhaust gas is low, it is extremely difficult to economically establish the waste heat recovery technique.
On the other hand, recently, a fuel cell in which H2 fuel is electrochemically reacted with oxygen in air to directly generate electric power has been practically used. As a method of obtaining the H2 fuel of the fuel cell, a method of reforming CnHm components contained in coke oven gas in a steel works utilizing the sensible heat of the coke oven gas to recover H2 and CO has been developed (see Japanese Unexamined Patent Application Publication No. 2002-212575). In this method, however, the H2 fuel cannot be produced without a coke oven. Therefore, this method cannot be used for a general-purpose H2 fuel source for fuel cells or the like in the future.
It is an object of the present invention to provide a novel waste heat recovery apparatus, a waste heat recovery system, and a method of recovering waste heat that can recover the sensible heat of an exhaust gas discharged from a combustion apparatus such as a furnace, an internal combustion engine, or electric power generation equipment as fuel such as H2 fuel.
To achieve the above object, the present invention provides the following waste heat recovery apparatuses, waste heat recovery systems, and methods of recovering waste heat:
According to the inventions described in any one of items (1) to (7), waste heat of the combustion apparatus is recovered not as steam, hot water, heated air, or the like, but as H2 fuel or H2 and CO fuel. Therefore, the application of the recovered energy is not limited (that is, since the waste heat is recovered as combustion heat of the fuel, the recovered energy is not decreased even when the fuel is transported over a long distance). In addition, dimethyl ether is reformed to produce H2 fuel using the sensible heat of an exhaust gas, the energy conversion efficiency in the conversion of the dimethyl ether to H2 can be increased (for example, even when the temperature of the exhaust gas is low, i.e., in the range of 250° C. to 300° C., the energy conversion efficiency reaches 90% or more). Furthermore, since waste heat can be recovered as fuel with a high added value, even when the temperature of the exhaust gas is low (for example, even when the temperature of the exhaust gas is 300° C. or lower), economically efficient heat recovery can be performed. Furthermore, since only the presence of an exhaust gas, dimethyl ether, and H2O or CO2 can generate H2 fuel, a general-purpose H2 fuel generator can be provided and a social system in which a large amount of H2 fuel is consumed can be realized.
According to the invention described in item (8), the H2 fuel or H2 and CO fuel can be used as fuel for a gas turbine, a gas engine, or a diesel engine.
According to the invention described in item (9), since the composition and the calorific value of the resulting H2 fuel or H2 and CO fuel are similar to those of a by-product gas in steel works, the fuel can be used as an alternative to the by-product gas in the case where the amount of by-product gas generated is low because of stopping of the operation of a blast furnace or the like without remodeling existing combustion equipment. Thus, the operation efficiency of the by-product gas can be increased in the steel works.
According to the invention described in item (10), the resulting H2 fuel or the H2 and CO fuel can be used as fuel of a fuel cell.
According to the invention described in item (11) or (12), a waste heat recovery system that can recover waste heat of a combustion apparatus as H2 fuel or H2 and CO fuel can be provided.
According to the invention described in item (13), CO2 generated together with the H2 fuel recovered in the second heat exchanger is separated, and the separated CO2 is supplied to the first heat exchanger. Accordingly, CO2 is circulated, and thus the CO2 emission can be reduced.
According to the invention described in item (14), by using LNG, whose carbon emission is lower than that of dimethyl ether, as the fuel of the combustion apparatus, the CO2 emission can be reduced.
According to the invention described in item (15), the recovered H2 and CO fuel can be used as fuel of a combustion apparatus such as a heating furnace or a boiler.
According to the invention described in item (16), steam to be supplied to the second heat exchanger can be recovered with the waste heat boiler.
According to the invention described in item (17) or (18), waste heat is recovered not as steam, hot water, heated air, or the like, but as H2 fuel or H2 and CO fuel. Therefore, the application of the recovered energy is not limited (that is, since the waste heat is recovered as combustion heat of the fuel, the recovered energy is not decreased even when the fuel is transported over a long distance).
According to the invention described in item (19), CO2 generated together with the H2 fuel recovered in the second heat exchanger is separated, and the separated CO2 is supplied to the first heat exchanger. Accordingly, CO2 is circulated, and thus the CO2 emission can be reduced.
As described in item (20) or (21), dimethyl ether may be directly fed to an exhaust system.
Embodiments of the present invention will now be described.
Various types of heat exchanger such as tubular type, plate type, extended heat transfer surface type, regenerative type, fluidized bed type, and heating medium circulating type can be used as the heat exchanger 2.
In the recuperator shown in
The heat exchanger shown in
A catalyst used for reforming DME to produce hydrogen is filled in the side of the heat exchanger 2 to which the mixed gas of DME and H2O is supplied. A known catalyst such as alumina, silica; or titania can be used as this catalyst. The type of catalyst used is not particularly limited, and any type of catalyst may be used so long as DME can be reformed to produce hydrogen.
Instead of supplying the mixed gas of DME 6 and H2O to the heat exchanger 2, a mixed gas of DME and CO2 may be supplied to the heat exchanger 2, thereby pyrolyzing the mixed gas to H2 and CO. Thus, the sensible heat of the exhaust gas of the combustion furnace 1 may be recovered as H2 and CO fuel. This reaction in which DME and CO2 are reformed to produce the H2 and CO fuel will be described in detail in a second embodiment shown in
a shows a fluidized-bed heat exchanger, and FIG. 5b shows a heating-medium-circulating heat exchanger. In the fluidized-bed heat exchanger shown in
In the heating-medium-circulating heat exchanger shown in
In the above-described heat exchanger 2, a reaction represented by CH3OCH3+3H2O+29 kcal→6H2+2CO2 is progressed by the catalyst. DME is pyrolyzed to H2 by this reaction, and the sensible heat of the exhaust gas can be recovered as H2 fuel.
In the above case, when the temperature of the exhaust gas is low (e.g. 300° C. or lower), the reaction in which the mixed gas of DME and H2O is decomposed to H2 does not easily occur. Furthermore, since the reaction in which DME and H2O are decomposed to H2 and CO2 is an endothermic reaction, disadvantageously, the temperature of the exhaust gas may be further decreased. Consequently, as shown in
When the temperature of the exhaust gas is in the range of 350° C. to 400° C., the reaction in which the mixed gas of DME and H2O is decomposed to H2 proceeds. Therefore, in such a case, DME need not be superheated. In addition, the use of a heat exchanger having high efficiency enables DME to be pyrolyzed even at an exhaust gas temperature of 300° C. or lower.
In this example, in order to maintain the temperature of the exhaust gas 7 at 300° C. or higher, DME 5 serving as a heat source is superheated at a rate of 0.1 kg/h (688 kcal/h) in the exhaust system 4. In order to provide the exact quantity of heat required for the reaction, the amount of DME 5 fed for the superheat is equal to an amount that supplements the heat of reaction in the endothermic reaction in which the DME 5 is pyrolyzed to H2 fuel. The temperature of an exhaust gas 10 is increased to 394° C. by the superheat of the DME 5.
DME 6 and H2O 8 at a temperature of 30° C. are supplied to the heat exchanger 2. The sensible heat of the exhaust gas 7 is used to increase the temperature of the DME 6 and the H2O 8 to 300° C. Since the temperature of an exhaust gas 11 discharged from the heat exchanger is 248° C., a quantity of heat corresponding to about 50° C. (=300° C. (the temperature of the exhaust gas 7) −248° C. (the temperature of the exhaust gas 11)) of the exhaust gas 11 is used for heating the DME 6 and the H2O 8.
In the heat exchanger 2, the exhaust gas 10 and the mixed gas of the DME 6 and the H2O 8 are subjected to heat exchange to reform the DME 6 to produce H2 fuel 9. In the heat exchanger 2, the DME 6 is fed at a rate of 1.05 kg/h (7,224 kcal/h). The H2 fuel 9 is obtained at a rate of 2.91 Nm3/h (7,499 kcal/h).
In this example using the exhaust gas 7 at 300° C., the energy conversion efficiency when the DME 6 is converted into the H2 fuel 9 is calculated as 7,499/(7,224+688)×100=94.5%. Thus, the conversion is performed with a high thermal efficiency. The energy conversion efficiency reaches 90% or more even at an exhaust gas temperature in the range of 250° C. to 300° C. Herein, the energy conversion efficiency η is defined as η=(combustion energy of resulting H2 fuel)/(combustion energy of DME used).
In terms of this energy conversion efficiency, 95% of the combustion heat of DME is changed to the combustion heat of H2. For example, when the H2 fuel is used in a fuel cell, and the fuel cell itself has a power generation efficiency of about 60%, the combined efficiency including energy used for the generation of H2 is calculated as 60%×95%=about 57% (wherein the combined efficiency is represented by power generation energy/fuel (DME) energy). Thus, the power generation can be performed at the above combined efficiency. Consequently, the same power generation efficiency as that of large-scale gas turbine electric power generation equipment in existing electric power plant can be achieved.
As a comparative example, the thermal efficiency in the case where the exhaust gas at 300° C. is not used will be estimated. In this case, all the quantity of heat of 2,788 kcal of the exhaust gas 10 fed to the heat exchanger 2 is covered by the combustion heat of DME. That is, DME of a first type is combusted, and DME of a second type is pyrolyzed by the resulting heat. The energy conversion efficiency in this case is calculated as 7,499/(7,224+2,788)×100=74.9%, and thus decreased to 80% or less. This is because the heat source used for pyrolyzing the DMF of the second type is obtained by combusting the DME of the first type.
Instead of supplying the mixed gas of the DME 6 and the H2O 8 to the heat exchanger 2, a mixed gas of DME and CO2 may be supplied to the heat exchanger 2, thereby pyrolyzing the DME to H2 and CO. Thus, the sensible heat of the exhaust gas of the combustion furnace may be recovered as H2 and CO fuel. This reaction in which DME is reformed to produce the H2 and CO fuel will be described in detail in the second embodiment described below.
The temperature of the exhaust gas of the gas turbine 16 is high, and the flow rate of the exhaust gas thereof is also high. A heat exchanger 18 is disposed in the exhaust system of this gas turbine 16. A mixed gas of DME and CO2 is fed to the heat exchanger 18. Thereby, the DME is pyrolyzed, and the sensible heat of the exhaust gas is recovered as H2 and CO fuel. The recovered H2 and CO fuel is mixed with, for example, a by-product gas of a steel works, and is then used as the fuel of the gas turbine 16.
The heat exchanger 18 is filled with a catalyst used for reforming DME to produce the H2 and CO fuel. A known catalyst such as ruthenium or nickel can be used as this catalyst. The type of catalyst used is not particularly limited, and any type of catalyst may be used so long as DME can be reformed to produce H2 and CO.
In the heat exchanger 18, a reaction represented by CH3OCH3+CO2+58 kcal→3H2+3CO proceeds. Since the heat of reaction in this endothermic reaction is larger than that in the reaction in which a mixed gas of DME and H2O is fed to produce H2 fuel, the temperature of the exhaust gas must be high. Since the temperature of the exhaust gas of the gas turbine 16 is as high as, for example, 600° C., this exhaust gas can be suitably used for the pyrolysis of DME. Instead of the heat exchanger 18, for example, the heat exchanger 2 shown in
An exhaust gas discharged from the heat exchanger 18 is supplied to a waste heat boiler 19. The waste heat boiler 19 heats water (H2O) to recover the sensible heat of the exhaust gas as water vapor (H2O). The water vapor (H2O) thus obtained is used for rotating a steam turbine 20 combined with the gas turbine 16. When the gas turbine 16 and the team turbine 20 are rotated, the power generator 17 generates electric power.
The first heat exchanger 25 is filled with a catalyst used for reforming DME to produce the H2 and CO fuel. A gas prepared by mixing DME with CO2 is fed to the first heat exchanger 25. In the first heat exchanger 25, the sensible heat of the exhaust gas is recovered as the H2 and CO fuel. The recovered H2 and CO fuel is then supplied to, for example, a by-product gas system in a steel works or the like. In order to increase the temperature of the exhaust gas fed to the second heat exchanger 26, a part of the DME may be fed to the exhaust system between the first heat exchanger 25 and the second heat exchanger 26 and may be combusted (i.e., superheated).
The second heat exchanger 26 is filled with a catalyst used for reforming DME to produce the H2 fuel. A mixed gas of DME and H2O is fed to the second heat exchanger 26. In the second heat exchanger 26, the sensible heat of the exhaust gas is recovered as the H2 fuel. CO2 discharged from the second heat exchanger 26 together with the H2 fuel is separated in a separating unit 28. The separated CO2 is supplied to the first heat exchanger 25 as a raw material. Since the concentration of the separated CO2 is high, a gas other than the CO2 need not be heated in the first heat exchanger 25, and thus the decomposition reaction of DME can be efficiently performed. In contrast, when an exhaust gas is fed in the first heat exchanger 25 together with DME, N2 and O2 are also heated in addition to CO2, resulting in a decrease in the recovery efficiency of waste heat. This apparatus may be used instead of the heat exchanger 18 of the gas turbine combined system shown in
The composition and the calorific value of the H2 and CO fuel obtained in this embodiment are similar to those of a by-product gas in a steel works, the fuel can be used as an alternative to the by-product gas in the case where the amount of by-product gas generated is low because of stopping of the operation of a blast furnace or the like without remodeling existing combustion equipment. Consequently, the operation efficiency of the by-product gas can be increased in the steel works. The reason for this will be described below.
The by-product gas is a mixed gas containing 4,500 kcal/Nm3 of coke oven gas, 750 kcal/Nm3 of blast furnace gas, and 2,000 kcal/Nm3 of converter gas and is primarily composed of H2 and CO. The fuel gas obtained in this embodiment contains 2,800 kcal/Nm3 of H2 and CO fuel and 1,800 to 2,580 kcal/Nm3 of H2 fuel. The components and the calorific value of the fuel gas of this embodiment are similar to those of the by-product gas. Therefore, when this embodiment is applied to an exhaust gas system of a combustion furnace in a steel works and the resulting H2 fuel or H2 and CO fuel is mixed in a by-product gas system, the fuel can be used as an alternative to the by-product gas without remodeling existing combustion equipment.
In steel works, which are considered to be an example of high-energy-consumption plants, the amount of waste heat is also large, and about 40% of the energy generated in each production process is discharged as a by-product gas. However, since the amount of by-product gas significantly fluctuates, it is difficult to use a high-efficiency gas turbine for power generation that converts the by-product gas to fuel in steel works. Instead, the amount of steam generated is stabilized by combusting fuel oil or the like so as to compensate for the fluctuation of the amount of by-product gas, and electric power is generated with a steam turbine. When the H2 fuel or H2 and CO fuel obtained in this embodiment is mixed in the by-product gas system, the amount of by-product gas generated can be stabilized. Consequently, a machine with high efficiency, such as a gas turbine for power generation, can be easily introduced, and thus the operation efficiency of the by-product gas in steel works can be increased.
The first heat exchanger 25 is filled with a catalyst used for reforming DME to produce H2 and CO fuel. A gas prepared by mixing DME and CO2 is fed to the first heat exchanger 25. In the first heat exchanger 25, the sensible heat of the exhaust gas is recovered as the H2 and CO fuel. On the other hand, the second heat exchanger 26 is filled with a catalyst used for reforming DME to produce H2 fuel. A mixed gas of DME and H2O is fed to the second heat exchanger 26. In the second heat exchanger 26, the sensible heat of the exhaust gas is recovered as the H2 fuel. CO2 discharged from the second heat exchanger 26 together with the H2 fuel is separated in a separating unit 28. The separated CO2 is supplied to the first heat exchanger 25 as a raw material.
The H2 and CO fuel recovered in the first heat exchanger 25 is used as fuel of a heating furnace 32. A combustion apparatus such as a boiler can also be used instead of the heating furnace 32. A waste heat boiler 33 is provided at the downstream side of the second heat exchanger 26, and water vapor recovered in the waste heat boiler 33 is supplied to the second heat exchanger 26.
In the waste heat recovery system of this embodiment, the gas engine power generator 31 is used as a combustion apparatus, and not DME but liquefied natural gas (LNG) is used as the fuel of the gas engine power generator. In this embodiment, since DME is supplied to the first and second heat exchangers 25 and 26, it is assumed that DME may also be used as the fuel of the gas engine power generator 31. However, when DME is combusted as fuel, the CO2 emission is larger than that of LNG by about 18% (the carbon emission factor of fuel of LNG is 64.06 g-C/Mcal, whereas that of DME is 75.60 g-C/Mcal). Accordingly, when DME is used as the fuel of the power generator 31, carbon emission is increased from the standpoint of the above relationship of the carbon emission factor of the fuels. Therefore, in this embodiment, LNG is used as the fuel of the power generator 31 in order to reduce the carbon emission.
A description will be made of the comparison between the waste heat recovery system in which not DME but LNG is used as the fuel of the power generator 31, which consumes a large amount of fuel, (hereinafter, this waste heat recovery system is referred to as “combined waste heat recovery system of LNG and DME”) and a waste heat recovery system in which only LNG is used (see
As a comparative example,
Users who use waste heat recovery systems desire that both the fuel consumption and the CO2 emission can be reduced. A reduction of fuel consumption can achieve a reduction in cost. Furthermore, now that COP3 (the 3rd Session of the Conference of the Parties to the United Nations Framework Convention on Climate Change) has been ratified in the Kyoto Protocol, CO2 emission must also be reduced from a social standpoint. In view of this background, the combined waste heat recovery system of LNG and DME shown in
Referring to Table 2, the combination of LNG and DME can reduce both the CO2 emission and the fuel consumption compared with those in the process in which only LNG is used. It is believed that the CO2 emission can be reduced because waste heat of the gas engine power generator 31 is changed to fuel, and the CO2 emission corresponding to this waste heat is reduced. That is, CO2 generated from the second heat exchanger 26 is separated and is supplied to the first heat exchanger 25, and the CO2 is then mixed with DME in the first heat exchanger 25, thereby converting it to CO fuel again. Accordingly, CO2O circulates in the waste heat recovery system, and thus the CO2 emission is reduced.
Table 3 summarizes the comparison of the above two waste heat recovery systems. Case 1 represents the waste heat recovery system in which only LNG is used, and case 2 represents the waste heat recovery system in which LNG and DME is combined. In case 2, the fuel can be reduced by 9% and the CO2 emission can also be reduced by 4%.
A reduction in fuel consumption brings users cost benefits. If the money saved by this reduction in fuel consumption can be used as a financial resource for popularizing DME obtained from biomass, wastes, petroleum residues, coal bed methane, or coal (hereinafter referred to as “DME derived from biomass”), the DME derived from biomass can be commercially distributed on the basis of the economical efficiency thereof in the form of fuel in which a certain percentage of the DME derived from biomass is mixed with DME derived from natural gas. Popularization of the DME derived from biomass can further reduce CO2 generated by society as a whole. The DME derived from biomass is produced by gasifying biomass or the like in a gasifier to CO+H2 followed by a known DME synthetic reaction in the presence of a catalyst.
The application of the waste heat recovery apparatus of the present invention is not limited to the fields of power generation and mining industry. The waste heat recovery apparatus of the present invention can be provided in an exhaust system of combustion apparatuses such as furnaces used in various industries, e.g., petrochemical, paper and pulp, nonferrous metal, cement, ceramic, and transportation industries; internal combustion engines; and electric power generation equipment. The waste heat recovery apparatus of the present invention can also be provided in an exhaust system of burning devices for air-conditioning, kitchens, or hot-water supply in the consumer and commercial fields. Furthermore, the waste heat recovery apparatus of the present invention is preferably applied to an exhaust system in which the temperature of the exhaust gas is 300° C. or higher, but can also be effectively applied to an exhaust system in which the temperature of the exhaust gas is 300° C. or lower.
The application of the waste heat recovery system of the present invention is not limited to the case where a power generator, a heating furnace, and a hydrogen reformer are present. The waste heat recovery system of the present invention can be applied to various waste heat recovery systems as long as a waste heat recovery apparatus is installed in an exhaust system of an internal combustion engine or electric power generation equipment. For example, in an aspect of the present invention in which steam is not necessary, a waste heat boiler may not be present. Furthermore, the heating furnace in the waste heat recovery system of the present invention may be a burning device for air-conditioning, a kitchen, or hot-water supply instead of an industrial heating furnace, or may be replaced with a fuel cell.
Furthermore, in the method of recovering waste heat of the present invention, DME may be reformed to produce H2 fuel or H2 and CO fuel by directly feeding DME to an exhaust system filled with a catalyst without providing a heat exchanger.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-193249 | Jun 2004 | JP | national |
| 2005-116222 | Apr 2005 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP05/12440 | 6/29/2005 | WO | 2/7/2007 |
