| 1. | A method for operating a heat exchanger with a heat generator wherein a fuel, containing pollutants, is burned with an inflow of air to generate a hot combustion gas which, after transfer of heat therefrom, exhausts as a flue gas at an elevated temperature and contains par¬ ticulates and gaseous pollutants, comprising the steps of: passing the flue gas and the inflow of air through the heat exchanger for the transfer of heat from the flue gas to said inflow of air for a preheating thereof; during said transfer of heat applying a flood¬ ing amount of spray of liquid onto the heat exchanger at a discrete portion thereof to enable said liquid to remove particulates from the heat exchanger while protecting said heat exchanger from corrosive pollutants, said spray of liquid being effectively moved along a path over the heat exchanger with the path being selected so as to clean the entire heat exchanger during said transfer of heat. |
| 2. | The method for operating the heat exchanger as claimed in claim 1 wherein the liquid is a neutralizer liquid so as to protect the heat exchanger from corrosive pollutants. |
| 3. | The method for operating the heat exchanger as claimed in claim 2 wherein the heat exchanger is a rotary heat exchanger having a rotor and the liquid is moved along a generally radial zone of the rotary heat exchanger. |
| 4. | The method for operating the heat exchanger as claimed in claim 2 wherein the liquid is applied in a manner sufficient to remove particulates without a significant cooling effect on the heat transfer process. |
| 5. | The method for operating the heat exchanger as claimed in claim 4 wherein the liquid is applied between time intervals selected to prevent corrosive damage and to reduce the cooling effect of the liquid. |
| 6. | The method as claimed in claim 5 wherein the heat exchanger is a rotary heat exchanger having a rotor which is entirely cleaned by said liquid between intervals whose durations are equivalent to several or more rotations of the rotor. |
| 7. | The method for operating the heat exchanger as claimed in claim 4 wherein the liquid is 'applied in termittently at any same portion of the heat exchanger. |
| 8. | The method for operating a heat exchanger as claimed in claim 3 wherein the liquid is applied along a zone of the rotary heat exchanger where the rotor leaves the heat exchanger part through which combustion air passes. |
| 9. | The method for operating a heat exchanger as claimed in claim 1 wherein the heat exchange relationship with the inflow of air is sufficient to reduce the tem¬ perature of the flue gas from a transfer of heat therefrom to the inflow of air for an enhanced thermal efficiency of the heat generator, with the temperature of the flue gas being reduced by said transfer of heat by said heat ex¬ changer to a level where at least one of said pollutants condenses out from the flue gas within the heat exchanger, and wherein said liquid is applied between time intervals selected to prevent corrosive damage by timely removal of condensed corrosive pollutants and to reduce the cooling effect of the liquid on the heat transfer process. OMPI_ " ./IP . |
| 10. | The method as claimed in claim 9 for operating a rotary heat exchanger wherein the liquid spray is re¬ ciprocated between the inner and outer diameter of the rotor of the rotary heat exchanger. |
| 11. | A rotary heat exchanger for use in a heat generator wherein a fuel, containing pollutants, is burned with an inflow of air to generate a hot combustion gas which exhausts as a flue gas at an elevated temperature and contains particulates and gaseous pollutants, comprising: rotary heat exchange means having a rotor for transferring heat from the flue gas to the inflow of air; means for applying during said heat transfer a flooding amount of spray of liquid onto said heat exchange means at a discrete portion thereof to enable said liquid to remove particulates from the discrete portion of the heat exchanger, and means for moving the flooding amount of spray relative to the rotor along a zone selected so as to clean the entire rotor' and protect said heat exchange means against accumulating particulates and corrosive effects of pollutants. |
| 12. | The rotary heat exchanger as claimed in claim 11 wherein the rotary heat exchange means is selected so as to reduce the temperature of the flue gas from a transfer of heat therefrom to the inflow of air for enhanced thermal efficiency of the heat generator, with the temperature of the flue gas being reduced by said transfer of heat by the heat exchanger means to a level where at least one of said pollutants condenses out from the flue gas within the heat exchange means. |
OP A HEAT GENERATOR
Field of the Invention
This invention relates to heat generators in which combustible fuels such as fossil fuels, refuse or other materials are burned. More specifically, this invention relates to a method and system for improving the ef¬ ficiency of such heat generators and particularly for better utilization of heat produced in the thermal section for a large electric power plant using a combustible fuel.
Background of the Invention
Heat generators using combustible fuels such as oil, coal, gas or refuse materials and the like, generate a substantial quantity of waste materials in the form of pollutant gases and particulates. Federal and state environmental requirements have imposed maximum emission standards for these waste materials. Compliance with
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these emission standards involves substantial investments for appropriate pollution control equipment, the costs for which can be prohibitively high.
For example, large systems are available to remove particulates using a dry flue gas treatment. Typical devices used for this purpose may involve electrostatic precipitators, bag houses and the like. These devices are suitable for the removal of the particulates, but gaseous pollutants are not removed and as can be appreciated, the addition of these devices increases cost and reduces the efficiency of the heat generator.
The magnitude of gaseous pollutants generated from the combustion of fuel throughout the world is enormous. As a result, many techniques have been described for the removal of thesepollutants from flue gases exhausted from heat generators. A general statement of various wet scrubbing processes for pollutant removal from flue gases exhausted from large scale electric power plants can be found in a chapter entitled "Wet Scrubbing Process - S0 X and N0 X Removal Chemistry" by R. G. Nevill, at page 9-312 of "Energy Technology Handbook" edited by D. M. Considine and published by McGraw-Hill Book Company.
Flue gas wet scrubbing techniques also involve substantial investments with complex systems. For ex- ample " , in the U.S. patents 3,320,906 to Domahidy and 3,733,777 to Huntington, wet scrubbers are described in which flue gases are passed through a filter bed for intimate contact with a wash liquid. The wash liquid may be an aqueous bisulfite salt solution such as described in the Huntington patent or such alkaline scrubbing liquors indicated as useful with the wet scrubber described in U.S. patent 4,049,399 to Teller.
Since corrosive liquid droplets are likely to be entrained by the scrubbed flue gas, special techniques such as described by Teller or in the U.S. patent to Brandt 3,844,740 may be used to avoid corrosion on subsequent equipment such as an induced draft fan located at the stack where the flue gas is exhausted to atmosphere.
Another technique for the removal of pollutants may involve cooling of the flue gas to such low tem¬ peratures that gaseous pollutants such as SO2 and SO3 condense out. One such system is described in the U.S. patent to Maniya 3,839,948, in which the flue gas is cooled to about 10 * C to condense out the sulfurous pollutants after which the flue gas is reheated before discharge to atmosphere. These and other techniques for the removal of waste materials from flue gas involve a substantial amount of energy, much of which is irretrievably lost. As a result, the overall efficiency, i.e. the energy available for sale from a power plant is significantly reduced. Techniques for preheating of air have been known and used for many years in connection with boilers to improve combustion. One such preheating technique em¬ ploys a Ljungstrom air preheater. This uses as shown in Fig. 3 herein a rotor 2 through which on one side 3 flue gas is passed while an inflow of combustion air is passed through the other side 4, with the two gas flows being in opposite directions. Air preheaters, however, are op¬ erated at sufficiently high temperatures to avoid con¬ densation inside the heat exchanger of pollutants such as SO3 present in the flue gas.
Summary of the Invention
In a technique in accordance with the invention for the operation of a heat generator in which combustible fuels are burned, the thermal efficiency is improved by combining the preheating of the air for the heat generator with the removal of pollutants.
For example, as described herein with respect to one embodiment in accordance with the invention for the operation of a heat generator using combustible fuels, both an inflowof air and the flue gas from the combustion are passed through a heat exchanger, which is simul¬ taneously flooded with a scrubbing liquid for removal of particulates and gaseous pollutants in the flue gas. Heat from the flue gas is transferred through the heat ex- changer to the inflow of air for its preheating while the flue gas pollutants are removed by collecting the liquid after its passage through the heat exchanger.
It is well well known that below about 700 * F SO3 will combine with water vapor molecules to form H2SO4. At temperatures above approximately 300 * F in the flue gas the H2SO4 is a gas. The cooling of the flue gas can be carried out to a temperature at which apollutant may condense out. For example, the flue gas may be cooled in the heat exchanger to a temperature at which SO3 in the form of H2SO4 condenses out. By employing a suitableneutralizing scrubbing liquid, the corrosive effective of the con¬ densed SO3 is avoided, yet a substantial part and even virtually all of the SO3 in its H2SO4 form in the flue gas is removed. With a technique in accordance with the invention for operating a heat generator, its net thermal efficiency can be significantly increased. The technique can be applied to improve operating efficiencies of existing heat generators such as may be used in electric power
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plants, steel manufacturing furnaces, sulfur producing plants and the like.
It is, therefore, an object of the invention to improve the thermal efficiency of a heat generator using combustible fuels while removing pollutants from flue gas generated from the generator. It is a further object of the invention to enable the cleaning of an air preheater during its use as a heat exchanger in a heat generator using combustible fuels. These and other advantages and objects of the invention can be understood from the following descrip¬ tion of one illustrative embodiment in accordance with the invention and described in conjunction with the drawings.
Brief Description of Drawings
FIGURE 1 is a schematic representation of a con¬ ventional thermal section for a power plant; and
FIGURE 2 is a schematic representation of a ther¬ mal section improved in accordance with the invention. FIGURE 3 is a schematic representation of a rotary air preheater that has been modified in accordance with the invention.
Detailed Description of Embodiment
With reference to FIGURE 1, the thermal section 10 of a conventional power plant is shown with a boiler 12 in which a suitable fuel such as fossil fuel in the form of coal, oil, or gas or other fuel such as a waste material is burned. An inflow of combustion air is provided, as suggested by arrows 1.6, through suitable ducts 14 into the boiler 12. The boiler 12 includes suitable heat exchange elements (not shown) in which a working fluid (water or
steam) is circulated for heating by the combustion gases generated in the boiler 12. Flue gas, as suggested by arrows 18, emerges at discharge 20 from the boiler 12 at a high temperature, typically in the range of about 650 * F, 5 and is passed through a heat exchanger 22 to preheat the inflowof air 14. After passage through heat exchanger 22, the flue gas 18 is discharged to atmosphere at a stack 24. Air flow through the thermal section 10 is obtained with a forced draft fan 26 and an induced draft fan 28. 0 The flue gas 18 may include pollutant materials in the form of particulates such as fly ash and gases such as SO2, SO3 and others. Techniques for removal of the pollutants are usually a part of the thermal section 10, though for purposes of simplicity of FIGURE 1, these 5 pollution controls have been left out of the schematic representation. Suffice it to say that techniques and devices for collecting particulates and pollutant gases from flue gases have been extensively described in the art. Q It is generally recognized that, particularly in large electric power plants, the exhaust temperature of the flue gas should preferably be kept above the dewpoint of the acid H2SO4 to avoid corrosive effects from contact by precipitated SO3 with equipment such as the induced 5 draft fan 28. Hence, the amount of heat recaptured from the flue gas is usually limited to maintain the flue gas temperature above the acid (H2SO4) dew point, i.e. at about 300 * F. As a result, the temperature of the inflow of air 16 at the boiler 12 is usually about 450 β F and the 0 thermal efficiency of thermal section 10 is not as high as it could theoretically be made.
With a technique for operating a heat generator in accordance with the invention, a substantially greater amount of heat from flue gas is recaptured to achieve a 5 high thermal efficiency while simultaneously extracting
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pollutants. This can be achieved with a thermal section 30 as described for a power plant as illustrated in FIGURE 2.
In FIGURE 2, the flue gas 18, after passage through preheater 22, is passed through a heat exchanger 32 where a substantial portion of the heat in the flue gas 18 is extracted for transfer to the inflow of air 16.
The heat exchanger 32 operates with a working liquid which is applied through an inlet 34 from a supply (not shown) to sprayers 36 into the heat exchanger portion 38 through which the flue gas 18 is passed. The sprayers 36 flood portion 38 to enable intimate and direct contact between the flue gas and the liquid. The liquid is applied in such volume as to collect particulates in the flue gas while also acting as a protective sheath for the heat exchanger to prevent its damage from corrosive consti¬ tuents in the flue gas.
Although the liquid could be water, the liquid is preferably formed with ingredients suitable for absorbing and neutralizing various pollutants in the flue gas. These pollutants may be SO2, SO3 (in the form of H2SO4) and others, for which absorption and neutralizing techniques are well known, see for example, some of the aforementioned prior art publications. An alkaline wash liquid may be used to, for example, neutralize condensed SO3 (H2SO4) and absorb SO2.
In some Cases the heat exchanger 32 may be sprayed with a powder at the same time that the liquid is applied. The powder may be of a type which neutralizes corrosive components. Use of stich powders to protect heat exchangers against corrosion is known in the art.
While recuperative, regenerative, and heat pump types of heat exchangers 32 can be used, the heat exchanger 32 preferably is of the rotary regenerative type. The large surface area in such heat exchangers enhances mixing and
8
contact of flue gas pollutants with the absorbent alkaline wash liquid. The flue gas 18 in such case is passed through a hot zone portion of a rotor used such as 2 in Fig. 3 in heat exchanger 32 and the liquid spray is directed at that 5 hot zone with a flow rate selected to prevent a build-up of particulates and corrosive effects from condensed and absorbed pollutants. The spent liquid is collected at a drain 40 for processing in a suitable conventional scrub¬ bing cycle.
10 Preferably the liquid is applied to the rotary heat exchanger in amanner sufficient to timely remove condensed corrosive constituents, i.e. before corrosive damage to the heat exchanger occurs, while reducing the cooling effect of the liquid on the heat exchange process. One
*L5 preferred technique for applying the liquid involves the intermittent application of a flooding amount of liquid to discreet portions of the rotary heat exchanger at suf¬ ficiently short intervals to prevent corrosive damage. The intervals may be related to the rotation of the rotary heat
20 exchanger such as by effectively flooding the entire rotor 2 once every several or more rotations of the rotary heat exchanger 32. The liquid is preferably applied to that part of the rotor where it is about to leave the "cold" region where combustion air passes through to move into the "hot"
25 region where flue gas passes through.
Another technique for applying the liquid involves, as shown in Fig. 3, the application of a high pressure spray, through a nozzle 70 that reciprocates with the aid of a drive 71 at a desired speed, along a generally radial
30 part or zone 72 of the rotor 2 between its inner and outer diameters 74, 76. In this manner the entire rotor 2 is cleansed of condensed pollutants along a spiral path as the rotor 2 rotates in the direction of arrow 78 below the reciprocating nozzle 70.
35 The application of liquid in accordance with the invention to a preheater type rotary heat exchanger such as
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22 advantageously removes flue gas materials that clog the preheater to thus reduce the load on combustion air fan 26. As a result, the intervals between heat generator down times for the cleaning of preheater 22 can be considerably lengthened. Furthermore, in certain heat generators an additional combustion air preheater that is located ahead or upstream of the preheater 22 is used. Such additional preheater serves to assure a minimum temperature for the combustion air and thus avoid condensation of corrosive constituents, such as H2SO4, in the flue gas in preheater 22 during cold ambient air temperatures. The application of liquid to preheater 22 may thus allow a deletion of such additional air preheater for an improvement in the heat generator thermal efficiency. Sufficient heat is transferred from the flue gas by the rotor 2 in the rotary heat exchanger 32 to the inflow of air 16 to increase the latter's temperature signifi¬ cantly while the flue gas 18 is considerably cooled when it emerges at the outlet 42 of heat exchanger 32. The flue gas temperature may in fact be so low that if discharged to atmosphere, the water vapor in the flue gas would create a visible plume. Since this is undesirable, a visibility suppression technique is used whereby the flue gas 18 from heat exchanger 32 is reheated with a heat exchanger 44, which thus also promotes rise of the flue gas from the stack. Appropriate moisture separators 46, 48 are placed at the outlets 42, 42* of heat exchanger 32 to collect and enable removal of droplets entrained by the gas flow through heat exchanger 32. A significant improvement in the overall efficiency of the thermal section 30 is obtained with a heat exchanger such as 32 with which a substantial portion of heat in the flue gas 18 is recovered while pollutants are removed and the heat exchanger 32 is protected against corrosive ef- fects of the removed pollutants. The thermal efficiency of
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the heat generator may be increased by about three and a half percent (3.5%). The net thermal efficiency, i.e. after allowing for additional energy requirements to im¬ plement the improvement of the invention, being about two and sixteenth of a percent (2.6%) . In addition, less costly high sulfur containing fossil fuels may be used so that the operating costs of the thermal section 30 can be sig¬ nificantly reduced.
As a result of the thermal efficiency improvement, the temperature of the inflow of air 16 to boiler 12 at 50 is increased. It is estimated that the air flow can be raised to within 90 * F or even less from the temperature of the flue gas 18 at the outlet 52 of boiler 12.
The efficiency advantage of the invention can be illustrated with the following table of normal temper¬ atures encountered in the prior art system of FIGURE 1 in comparison with temperatures estimated to be generated in a system of FIGURE 2.
Table 1
Temperatures
Places FIGURE 1 FIGURE 2
At air inlet 54 70 # F 70" * F
At outlet 42' -na- 266" • F
At boiler inlet 50 450 * F 630' 'F
At boiler outlet 52 650 # F 650" • F
At reheater inlet 56 -na- 320' ■ F
At reheater outlet 58 -na- 300" • F
At outlet 42 -na- 120' • F
At stack 24 300'F 140' • F
The efficiency advantage of the invention can fur¬ ther be illustrated with the following tables 2 and 3 normalized for a heat generator using one pound of com¬ bustion air and assuming a mass of flue gas of 1.06 pounds for a number 6 type fuel oil.
Table 2 FIG. 1 FIG. 2
Places Temperature Temperature
At air inlet 54 70 # F 70 * F
At outlet 42' -na- 253 * F
At boiler inlet 50 450 * F 611 * F
At boiler outlet 52 650 β F 650 * F
At reheater inlet 56 -na- 320 * F
At reheater outlet 58 -na- 300 # F
At outlet 42 -na- •120 * F
At stack 24 300 * F 139'F
Table 3
Heat Transfer BTU Recovered BTU Transferred Efficiency From Flue Gas to Combustion Air Or Reheated Flue Gas
FIG 1 FIG 2 FIG 1 FIG 2 FIG 1 FIG 2
Air Pre¬ heater 22 95% 95% 95.8 90.3 91.2 85.8
Reheater 44 na 95% na 5.5 na 5.2
Condenser- Heat Ex¬ changer 32 na 89% na 49.3 na 43.9
The incremental efficiency improvement achieved with the invention using the data from Tables 2 and 3 is obtained using the relationship of:
Efficiency increment % -= ,24(611"450) x 100 = 3.4%
1130 where .24 is the specific heat of air, 611-450 reflects the higher temperature of the flue gas into the boiler with the invention over the embodiment of FIGURE 1, and 1130 is the
amount of BTU released by the complete combustion of 0.06 pounds of #6 fuel oil with one pound of air to produce flue gas with 15% excess air in the resulting flue gas.
Having thus described an illustrative embodiment in accordance with the invention for improving the efficiency of the thermal section for a power plant, the advantages of the invention can be appreciated. The invention can be advantageously used without a preheater 22 and reheater 44 and for different heat generators such as thoseused in blast furnaces, municipal waste burning plants, chemical process¬ es and the like. Variations from the described embodiment can be made, such as in the selection of the washing liquid and the heat exchangers without departing from the scope of the invention.
What is claimed is:
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