IMPROVED HEAT GENERATOR

Abstract: A method and apparatus for improving the efficiency of a heat generator in which combustible fuels such as fossil fuels, refuse, or other materials are burned is described. Flue gas (18) generated in the heat generator and combustion air (16) are passed in heat exchange relationship in a heat exchanger (22,32). The heat exchanger (22,32) is selected so that it removes a substantial amount of heat from the flue gas (18), whose temperature is consequently lowered to a level where a corrosive pollutant is likely to condense out. A pollutant removing liquid is applied (36) onto the portion of the heat exchanger (38) where the flue gas (18) passes through both to scrub the flue gas (18) and protect that portion of the heat exchanger (38) from corrosive effects from condensed pollutants. Particulates and gaseous pollu­ tants are removed from the flue gas (18) and the heat exchanger (32) by the liquid, which may be a neutralising liquid such as an alkaline solution to provide a protective sheath for the heat exchanger against the effects of corrosive pollutants. In the described illustrative embodiment, the invention is shown in use with a thermal section for a large scale power plant.

Description: IMPROVES HEAT GENERATOR 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 efficiency 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. 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 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 these pollutants 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 - SOx and NOx 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 example, 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 temperatures 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 100C 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 employs a Ljungstrom air preheater. This uses a rotor through which on one side flue gas is passed while an inflow of combustion air is passed through the other side, with the two gas flows being in opposite directions. Air preheaters, however, are operated at sufficiently high temperatures to avoid condensation inside the heat exchanger of pollutants such as SO3 present in the flue gas. 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 from the flue gas. 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 inflow of air and the flue gas from the combustion are passed through a heat exchanger, which is simultaneously 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 exchanger 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. The cooling of the flue gas can be carried out to a temperature at which a pollutant may condense out. For example, the flue gas may be cooled in the heat exchanger to a temperature at which SO3 is avoided, yet a substantial part of the SO3 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 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. These and other advantages and objects of the invention can be understood from the following description of one illustrative embodiment in accordance with the invention and described in conjunction with the drawings. FIGURE 1 is a schematic representation of a conventional thermal section for a power plant; and FIGURE 2 is a schematic representation of a thermal section improved in accordance with the invention. 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 16, 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, and is passed through a heat exchanger 22 to preheat the inflow of 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. The flue gas 18 may include pollutant materials in the form of particulates such as fly ash and gases such as S02, S03 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 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. It is generally recognized that, particularly in large electri'c power plants, the exhaust temperature of the flue gas should preferably be kept above the dew point of the fas03 to avoid corrosive effects from contact by precipitated SO3 with equipment such as the induced draft fan 28. Hence, the amount of heat recaptured from the flue gas is usually limited to maintain the flue gas temperature above the SO3 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 F and the 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 high thermal efficiency while simultaneously extracting 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 constituents in the flue gas. The liquid is preferably formed with ingredients suitable for absorbing the neutralizing various pollutants in the flue gas. These pollutants may be S02, S03 and others, for which absorption and neutralizing techniques are well known, see for example, some of the aforementioned prior art publica tions. An alkaline wash liquid may be used to, for example, neutralize condensed S03 and absorb S02. 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 such 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 contact of flue gas pollutants with the absorbent alkaline wash liquid. The flue gas J8 in such case is passed through a hot zone portion of a rotor used in heat exchanger 32 and the liquid spray is directed at that 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 scrubbing cycle. Sufficient heat is transferred from the flue gas by the rotor in the rotary heat exchanger 32 to the inflow of air 16 to increase the latter's temperature significantly 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 effects of the removed pollutants. The thermal efficiency of 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 implement the improvement of the invention, being about two and six-tenth 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 significantly 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 500F 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 temperatures encountered in the prior art system of Fig. 1 in comparison with temperatures estimated to be generated in a system of FIGURE 2. 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 0F At outlet 42 -na- 120'F At stack 24 300 F 140 F 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 for different heat generators such as those used in blast furnaces, municipal waste burning plants, chemical processes 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.

Claims: CLAIMS 1. A method for operating a heat generator wherein a fuel, containing pollutants, is burned with an inflow of air (16) to generate a hot combustion gas which, after transfer of heat therefrom, exhausts as a flue gas (18) at an elevated temperature and contains particulates and gaseous pollutants, characterised by passing the flue gas (18) and the inflow of air (16) through a heat exchanger (32) for the transfer of heat from the flue gas to said inflow of air for a preheating thereof; and applying pollutant removing liquid (36) onto the heat exchanger (32) at a location (38) enabling said liquid to remove, from the flue gas, pollutants and particulates while protecting said heat exchanger from corrosive removed pollutants and particulates for an enhanced thermal efficiency of the heat generator. 2. A method for operating the heat generator as set forth in claim 1 characterised in that the heat exchange relationship with the inflow of air (16) is sufficient to reduce the temperature of the flue gas (18) within said heat exchanger (32) to a level where at least one of said pollutants condenses out from the flue gas within the heat exchanger and the condensed pollutant is removed by said liquid. 3. A method for operating the heat generator as set forth in claims 1 or 2 characterised in that the step of applying said liquid includes applying a liquid which neutralises and absorbs pollutants from said flue gas (18) in said heat exchanger (32). 4. A method for operating the heat generator as set forth in any preceding claim and further characterised by passing flue gas (18) from which said pollutants and particulars have been removed in heat exchange relationship (44) with the flue gas (18) available prior to said removal step to reheat said pollutant and particulate depleted flue gas to a temperature sufficiently high for the suppression of a plume upon discharge of flue gas to atmosphere. 5. A method for operating the heat generator as set forth in any preceding claim characterised in that said flue gas (18) is passed in heat exchange relationship with the inflow of air with a rotary regenerative heat exchanger. 6. A method for operating the heat generator as set forth in any preceding claim characterised in that said liquid applying step consists of flood spraying (36) the heat exchanger (38) with said liquid to establish a running liquid condition in the heat exchanger (32). 7. A method for operating the heat generator as set forth in any preceding claim and further characterised by including the steps of separating moisture (46) entrained by the inflow of air (16) emerging from the heat exchanger (32); and separating moisture (48) entrained by flue gas (18) emerging from the heat exchanger (32). 8. A method for operating the heat generator as set forth in any preceding claim characterised in that said liquid is formed of an alkaline neutralising solution. 9. A heat generator wherein a fuel, containing pollutants, is burned with an inflow of air (16) to generate a hot combustion gas which exhausts as a flue gas (18) at an elevated temperature and contains particulates and gaseous pollutants, characterised by heat exchange means (32) for transferring heat from the flue gas (18) to the inflow of air (16); and means (36) for applying a pollutant removing liquid onto said heat exchange means (32) at a location (38) selected to enable direct contact by the flue gas with said liquid for removal of pollutants and particulates from the flue gas and protection of said heat exchange means against corrosive effects of removed pollutants. 10. A heat generator as set forth in claim 9 characterised in that said heat transferring means is a rotary heat exchanger having a hot zone through which said flue gas is passed, and wherein said liquid is applied to said hot zone. 11. A heat generator as set forth in claims 9 or 10 and further characterised by moisture separators (46,48) respectively located in the path of the air inflow (16) and the flue gas (18) as they emerge from said heat exchanger. 12. A method for operating a heat generator in a large electrical power plant in which a fuel, containing pollutants, is burned with an inflow of air (16) to generate a hot combustion gas in a boiler (12) for heat exchange with a working fluid, and wherein the combustion gas is exhausted as a flue gas (18) at an elevated temperature with particulates and gaseous pollutants, characterised by passing the flue gas (18) and the inflow of air (16) through a heat exchanger (32) for the transfer of heat from the flue gas to said inflow of air for preheating therebf to a temperature selected to enhance the thermal efficiency of the heat generator; ; simultaneously applying pollutant removing liquid to the heat exchanger at a location (38) enabling said liquid to remove from the flue gas (18), pollutants and particulates while protecting said heat exchanger from corrosive removed pollutants and particulates for an enhanced thermal efficiency of the heat generator in said large electrical power plant. 13. A method for operatina the heat generator for a power plant as set forth in claim12 characterised in that said step of transferring heat to the inflow of air (16) includes the raising of the temperature of the inflow of air (16) to a level generally within about fifty degrees fahrenheit of the temperature of said flue gas where it exhausts-from said boiler. 14. A method for operating the heat generator for a power plant as set forth in claims 12 or 13 characterised in that said step of passing the flue gas (18) through said heat exchanger further includes transferring sufficient heat from the flue gas to the inflow of air to lower the temperature of said flue gas to a level where a pollutant condenses out from said flue gas and is flushed off the heat exchanger by said pollutant removing liquid.

Inventors: MONRO, RICHARD J.
Applicants: MONRO, RICHARD J.
Application year: 1981
Main classification: F23L15
IPC codes: F23L15, F23J15