US2699758A - Method of preheating combustion supporting air for steam generating plants - Google Patents

Description

Jan. 18, 1955 2,699,758

, D. DALIN METHOD OF PREHEATING COMBUSTION SUPPORTING AIR FOR STEAM GENERATING PLANTS Filed July 11, 1950 2 Sheets-Sheet l Jan. 18, 1955 D. DALIN METHOD OF PREHEATING COMBUSTION SUPPORTING AIR FOR STEAM GENERATING PLANTS 2 Sheets-Sheet 2 Filed July 11, 1950 United States Patent METHOD OF PREHEATING COMBUSTION SUP- ;OEEING AIR FOR STEAM GENERATING L TS David Dalin, Stenlrullen, Ronninge, Sweden, assignor to A/B Svenska Maskinverken, Sodertalje, Sweden, a corporation of Sweden Application July 11, 1950, Serial No. 173,068

Claims priority, application Sweden February 2, 1946 13 Claims. (Cl. 122--1) This application is a continuation in part of my copending application, Serial No. 725,493, filed January 31, 1947, now abandoned; and the invention relates to steam generating plants and has more particular ref erence to a method and apparatus for improving combustion in the furnaces of steam boilers by preheating the combustion supporting air to relatively high temperatures using the flue gases as a source of heat for this purpose.

The conventional air preheater in which the combustion supporting air is heated by direct transfer of heat from the flue gases through the walls of tubular air passages which comprise the heat exchanger of the preheater structure has serious disadvantages, especially when fuel of poor quality such as pulverized coal, sulphite lye and other so-called pulver-rich fuels, are burned. With such fuels it is difficult to obtain complete combustion. Hence, the flue gases contain large quantities of solid and gaseous substances prone to adhere to any surface with which they come in contact and if the flue gases condense on these surfaces, destructive corrosion results due to the ensuing chemical reaction between the condensate and the foreign substances.

To preclude such destructive corrosion and to main' tain efficient operation, the heating surfaces must be frequently cleaned but since the tubular air passages of the conventional air preheater are closely spaced, fixed structures, they are very difficult to keep clean.

While these disadvantages of the conventional air preheater have been appreciated in the past and attempts have been made to overcome them, as in the patent to Roe, No. 1,833,130, issued November 24, 1931, wherein an intermediate heat vehicle or medium is circulated in heat exchange relation with the hot flue gases and the air to be heated, all prior efforts to solve the problem have fallen short of reaching their goal, especially where low quality fuel is being burned.

The present invention, like the patent to Roe, employs an intermediate heat vehicle and thus enables the use of vibratable heating surfaces in the flue gas duct so that the problem of keeping the heating surfaces clean is greatly minimized. In other words, the heating surfaces over which the flue gases flow may be serpentine coils supported as disclosed in the copending application to David Dalin, Serial No. 622,832, filed October 17, 1945, now Patent No. 2,550,676, so that the tubes may be readily vibrated to dislodge foreign matter accumulating thereon. v

However, merely utilizing an intermediate heat vehicle and employing vibratable heating surfaces in the flue gas duct is not sufficient. To achieve its primary purpose of improving the combustion of poor quality fuel, the present invention provides for heating the combustion air to a very high temperature and with a view toward achieving efficiency the invention further contemplates the abstraction of as much as possible of heat energy from the flue gases. These objectives are accomplished through a novel two-stage heating of the combustion air, the air being preheated by heat abstracted from the flue gases at the coolest portion of the flue gas duct and being further heated by heat abstracted from a much hotter zone of the flue gas duct spaced from the first heatabstraction zone by an intermediate heat absorbing unit or exchanger.

The invention contemplates still another important point and that is the control of the temperature of the heating surfaces in the flue gas duct and especially those which provide the heat for the first preheating stage to assure that the temperature of these surfaces will at all times be higher than the dew point of the flue gases flowing thereover.

In summation therefore, the purpose and objective of this invention is to provide a method and apparatus for utilizing the heat contained in the flue gases to preheat the combustion air to a temperature high enough to enable poor quality fuel and fuel having a high moisture content to be successfully burned, and to achieve this heat transfer from the flue gases to the combustion air by the boiler fluid contained in the steam generating plant in a manner which assures the abstraction of the maximum amount of heat energy from the flue gases all without danger of condensation of the flue gases upon the heat abstracting surfaces over which they flow, notwithstanding a possible high S02 and S03 content in the gases, the presence of which tends to lower the dew point of the gases and, of course, results in a very serious corrosion problem.

With the above and other objects in view, which will appear as the description proceeds, this invention resides in the novel method and sequence: of operations substantially as hereinafter described and more particularly defined by the appended claims, it being understood that such changes in the precise embodiment of the hereindisclosed invention may be made as come within the scope of the claims.

The accompanying drawings illustrate two complete examples of the method of this invention propounded in accordance with the best procedure so far devised for the practical application of the principles thereof, and in which:

Figure 1 is a diagrammatic view illustrating the preferred method for preheating the combustion air flowing into the furnace of a steam generating plant; and

Figure 2 is a diagrammatic view similar to Figure 1 but illustrating a slightly modified method.

Referring now more particularly to the accompanying drawings in which like numerals indicate like parts, the numeral 5 designates the furnace of a steam generating plant equipped with steam generating surfaces or boiler tubing (not shown) and having the customary steam dome 6 associated therewith.

The furnace is provided with a grate 7 in its lower regions to support a bed of burning fuel, and has an inlet 8 in the side of the furnace a distance above the grate providing for the charging of wood or other solid fuel into the furnace to be burned upon the grate 7. A pair of inlet ports 9 in the side of the furnace a suitable distance above the grate provide for the injection of pulverized coal or other pulver-rich fuel into the interior of the furnace, while additional inlet ports 10 beneath the pulverized coal ports provide for injecting sulphite lye or the like into the interior of the furnace.

Primary combustion air is supplied to the furnace through a duct 12 opening to the interior of the furnace through a port 13 in the sidewall of the furnace at a zone adjacent to the grate 7. A blower 14 adjacent to the inlet of the duct is provided to force-feed the air into the furnace.

The flue gases emanating from the furnace are conducted away from the upper part thereof through a flue gas passage or duct 15, and in their passage through this duct flow across a plurality of heat exchangers 16 through which boiler fluid is circulated. Thus a primary superheater 17 is located in the hottest portion of the flue gas passage adjacent to the inlet thereof and a secondary superheater 18 is located directly downstream therefrom. Next in line is an economizer 19 23d beyond the economizer is a primary heat exchanger It is important to note that the economizer 19 is positioned between the primary heat exchanger 20 and the superheaters 17-18. This assures a definite temperature differential between the two zones from which the superheaters and the primary heat exchanger derive their heat, and since the primary heat exchanger 20 is located farthest downstream the flue gases flowing thereover are appre ciably cooler than those flowing over the superheaters but still contain much heat energy, which should be abstracted if waste is to be avoided.

The primary heat exchanger comprises a group of coils having their oppositeends connected to inlet and outlet headers 21 and-'22, respectively, and suspended to bereadily vibrated. An-y suitable manner of suspending the coils maybe employed but that shown and described in the aforesaid pending application, now Patent No. 2,550,676, is preferable. The *boiler fluid circulated through and heated in the coils .of the primary heat .exchanger 20, in accordance with the method of this inventionQis in a liquid state (water) and constitutes an intermediate heat vehicle or medium by which heat is indirectly transmitted-from the flue gases to the combustion air flowing hrough h 1'2- .FQf hi pu pose secondary h a e hang d! is p s tioned inside theduct 12 to have heated fiuid from the primary heat exchanger 20 circulated therethrough. The secondary heat exchanger 24 maybe of any suitable const u ti but u t mar y comprise a er o pip of tubes connecting inlet and outlet headers 25 and 26, respectively. The inlet header 25 is connected with the outlet header 22 of the primary heat exchanger 26 by a ee 1111921. n th ou le h ader .26 of h s cond ry heat exchanger is connected with an accumulator 28 through a pipe l ne .29- T he acc mula or h r P p 39 'leadingfrorn a point below its normal water level to the inlet of a pump 31, the discharge end of which is connected with the inlet header 21of the primary heat exchangerlt), through a pipe 32.

The pump 31 thus circulatesthe liquid boiler fluid heatedin the prirnaryheat exchanger 20 through the secondary heat exchanger 24 to thereby effect an indirect transfer of hea from th was e fl gas s t the combustion air to preheat, the lat er.

' While it is desirable to abstract as much as possible of the heat ener y f om the flu es, i is impor nt h the temperature of the liquid circulating through the primary heat exchang r, and henc the heati g rfa s here f, be kept safely a v h d point of the the gases flowing thereover. To this end a bypass pipe 33 connects the outlet side of the primary heat exchanger, specifically the ipe Withthe inlet of the pump through a thermostatically controlled valve 34which controls flow through thepipe 3 and, has its thermally responsive control e1cmentj35 located in the pipe 32 between the discharge ,end .of the pump 31 and the inlet header 21 of the primary heat exchanger 2%). Whenever the temperature of the fluid returning to the primary heat exchanger is below a predetermined value, the thermally responsive element 35 functions to open the valve .34 allowing heated fluid in the pipe, 27 to bypass the secondary heat exchanger 2'4 and flow directly to the inlet of the pump to mix with the cooler fluid from the accumulator and thereby raise the temperature of the fluidreturningto the primary heat exchanger. In this manner the temperature of. the fluid entering the. primary heat exchanger 20, and consequently its heating surfaces, ismaintained above the dew ,point of the flue gases flowing thereover.

A pipeline- 36 connecting the accumulator 2,8 with the steam dome 6 serves, through condensation. of steam in the line 36, to maintain an adequate supply of boiler fluid in the air preheating system just described and also affords a .relieffor the circuit which is otherwise closed.

The'ternperature of the air which is preheated by passage over the secondary heat exchanger 24 may be high enough for use with furnaces burning relatively high grade fuels, but it is too low to overcome the objection of incomplete combustion with the lower grade fuels mentioned hereinbefore and fuels. of high moisture content. Thus, the method of this invention, contemplates further heating of the airflowing through the duct 12 to a .tem-

perature. of atleast between 200 to 300 C. For this purpose .a second stage ,air preheater 40 is located in the duct 12 near the furnace to provide-second stage heating oftheair. This airheater, like the first stage heater (the heat exchanger 24), derives its heat indirectly from the flue gases, but through the ,rnedium of the much hotter superheated steam coming from the superheater 1,7.

structurally the second stage air heater 40 may be of any suitable design, but like the heat exchanger 24 preferably comprises a group of tubes .or coils having one end connected. to a comrnoninlet header 41 and their opposite ends connected with a common outlet header 542. A pipe ine 4.3dilz etly connects th inle head r 4. with th ou let header 4 f he primary superhea er .17 sothat superheated steam from the primary superheater flows by its own pressure through the coils of the heater 40.

The inlet header 45 of the primary superheater 17 is connected by means of a pipe 46 with the steam dome 6 from which the superheater is supplied with steam; and while the superheated .steam naturally loses both pressure and temperature in its circulation through the air heater 49, this loss is restored by reheating the steam in the secondary superheater 18 and to this end the outlet header 42 of the air heater 40 is connected by means of a pipe line 48 with the inlet header 49 of the secondary superheater 13. The outlet header 50 of the secondary superheater may be connected to one or more locations at which the superheated steam is to be used.

The modified embodiment of the invention illustrated in Figure 2 differs from that of Figure 1 mainly in that the superheated steam used. in the second stage of combustion air preheating is allowed to condense as it circulates through the ,air heater .40. Hence, :the superheated steam givesoif its heat of evaporation to thceonibust'ion air and thereby raises the temperature ofwthe air :to a suitably highdegree. Also, only partofathe superheated steam issuing from the primary superheater 1'7 is utilized for the heating .of the combustionair. The outlet'header 44 of the superheater '17 in addition to being connected with the inlet header 41 of the heat exchanger .40 has a line 43 leading therefrom to one or more points at which the superheated steam is to be utilized. The condensate issuing from the outlet header. 42 of the heat exchanger 40 is led to .a reservoir 52 by means .of a-pipe 54. A purnp 55' and a pipe line 56 leading from the discharge side of the pump feeds the condensate from theaccumulator to the inlet header S7 of the economizer 19 located-inthe flue gas duct between the primary heat exchanger 20 and a steaming section 18 which corresponds in location to the secondary steam superheater 1.8 vof the Figure leni-- bodiment. .In .its passage through the economizer 19 :the condensate is, of course, reheated before ,it is returned to .the steam dome through a pipe line 518 leading from the outlet header 59 of the .economizer.

The steaming coils 18 have boilerfluid circulated therethrough directly from and back to the steam dome by means of a pump 60 and feed and return lines 61' and 62, respectively,.connectedwith the inlet and outlet headers 63 and 64', ofthe steaming coils 18'.

The circulatory system forythe liquid heatvehicle by which the initial or first stage air preheating is effected is similar to that shown in Figure -1 except that the accumulator 28 is not connected with the steam dome, but instead a safety valve 65 is mounted in the top of the accumulator to relieve excessive pressure in the system. Since the system is closed except for the possible opening of :the relief valve, replenishment of the liquid circulating therein is seldom necessary but when required, maybe fed thereto through a feed line 66 provided with a suitable valve .67 and adapted to be connected with any suitable source of heated water as, for instance, the feed water in the steamdome.

From the foregoing description taken in connection with the accompanyingdrawings, it--will be readily apparent;to those skilled in :the art that the method of =this invention enables the heating of the combustion air to the high emperature required to successfully burn poor quality fuel and fuel of high moisture content and that it also abstracts the maximum heat energy from the flue gases without dangerofdestructive corrosion of the heat-- ing surfaces over which the :flue gases flow which would result if condensation of .the flue gases .on these surfaces were permitted.

The explanation for the success .of this invention perhaps will he more readily appreciated from alconsideration of the following temperature values whichuobtain in a typical installation of this invention. The flue gas temperature in such a typical installation at entrance of. the flue gasduct is about 1000 C. and the temperature of the superheated steamissuing from the primary superheater 17 is about 400 :C. :In view of this high temperature of Ihesunerhetaed steam, t Combustion air already preheatedv in'its passage over the first stage air heater, namelytheheat exchanger 24, is readilyheated at thesecond stage air preheater A0 to a temperature sufliciently highas to :assure rapid andcomplete-combustion of practically any ;fuel :that might be charged into. the=furnace r I I For the fuels herein mentioned, the combustion air must be heated to between 200 C. and 300 C. and this, as stated, is readily accomplished. In raising the temperature of the combustion air to this value the temperature of the superheated steam is proportionately lowered and in the Figure 1 embodiment is reheated to about 425 C. or 450 C. in its passage through the secondary superheater 18. In the Figure 2 embodiment of the invention the superheated steam is allowed to condense as it heats the combustion air to thus further assure raising the temperature of the air to the desired value by virtue of the steam giving up its heat of evaporation.

In the superheating and reheating of the steam in the Figure l embodiment and in the superheating of the steam and the heating of the boiler fluid in the section 18 in the Figure 2 embodiment, the flue gas temperature is reduced from its initial 1000 C. to approximately 650 C., and in its passage over the economizer the flue gas temperature is further reduced to between 250 C. and 300 C. Hence, the temperature of the flue gases which provide the heat source for the first stage of the air preheating is considerably less than that which prevails in the zone which affords the heat for the second stage of preheating of the air; and finally in its passage across the primary heat exchanger 20 the temperature of the combustion air is further reduced to approximately 125 C. From this it is evident that maximum heat energy is abstracted from the flue gases and that at the same time the temperature of the combustion air is raised to its desired high value.

It will also be apparent that the spacing of the two zones which provide the heat sources for the first and second stage of air preheating, by means of the intermediate heat exchange surfaces, constitutes an important feature of the invention.

What I claim as my invention is:

l. The method of preheating the combustion air being fed to the furnace of a steam generating plant having a flue gas duct through which hot flue gases discharge from the furnace, which comprises: withdrawing steam from the plant; superheating the steam so withdrawn by circulating it in indirect heat exchange relation with the flue gases flowing in a relatively hot zone of the flue gas duct; circulating the superheated steam in indirect heat exchange relation with the combustion air being fed to the furnace to heat the same; abstracting heat energy from the flue gases leaving said relatively hot zone to thereby materially reduce the temperature of the flue gases below that prevailing in said relatively hot zone; and circulating a liquid heat exchange medium in indirect heat exchange relation both with the flue gases flowing through a zone of the flue gas duct downstream from that zone at which said heat abstraction occurs and with the combustion air before said combustion air is heated by the superheated steam in the manner stated to thereby pre-v heat the air and further reduce the temperature of the flue gases, whereby the combustion air is heated to a high degree while at the same time the temperature of the flue gases is reduced to a value below that which could be attained by the abstraction of heat therefrom by the steam or the liquid heat exchange medium alone.

2. The method of claim 1 further characterized by regulating the rate at which said liquid heat exchange medium is circulated in heat exchange relation with the combustion air in accordance with the temperature of said liquid heat transfer medium as it begins its circulation in indirect heat exchange relation with the flue gases to thereby preclude condensation of the flue gases as they flow in indirect heat exchange relation with said liquid heat transfer medium.

3. The method of preheating air for supporting combustion in the furnace of a steam generating plant having a flue gas duct through which hot flue gases discharge from the furnace, said method comprising: continuously circulating some of the liquid boiler fluid of the plant serially in indirect heat exchange relation both with the flue gases in a zone of the flue gas duct remote from the combustion zone of the furnace and with the air for combustion to thereby effect first stage preheating of said air; withdrawing steam from the plant and superheating it by heat exchange with the flue gases in a zone of the flue gas duct at which the minimum temperature is considerably higher than the maximum temperature in the zone where the said liquid boiler fluid is heated; and circulating the superheated steam in indirect heat exchange relation with the combustion air already heated by the liquid boiler fluid to thus eifect second stage preheating of the air.

4. The method set forth in claim 3 further characterized by circulating the steam cooled by such heat exchange, in heat exchange relation with the flue gases in a zone of the flue gas duct intermediate said other two zones.

5. The method of preheating the combustion air being fed to the furnace of a steam generating plant having a flue gas passage through which hot flue gases discharge from the furnace, said method comprising: continuously and serially circulating some of the liquid boiler fluid from the plant in indirect heat exchange relation both with the flue gases in a zone of the flue gas passage remote from the combustion zone of the furnace and with the combustion air being fed to the furnace to thereby eifect first stage preheating of said air; withdrawing steam from the plant and superheating it by circulating it in indirect heat exchange relation with the flue gases: in a zone of the flue gas passage at which the minimum temperature of the gases is considerably higher than the maximum temperature of the gases at said other zone; circulating a portion of the steam so superheated in. indirect heat exchange relation with the preheated combustion air to eflect second stage preheating of said air and to condense the steam; circulating the condensed steam in indirect heat exchange relation with the flue gases in a zone of the flue gas passage between said two other zones to regenerate it into steam; returning the steam so regenerated back to the steam plant; and conducting the remaining portion of the superheated steam to a point at which it may be utilized as desired.

- 6. The method of preheating air for supporting combustion in the furnace of a steam generating plant having a flue gas passage through which hot combustion gases discharge from the furnace, said method comprising: continuously and serially circulating some of the liquid boiler fluid from the plant in indirect heat exchange relation both with flue gases in a zone of the flue gas passage remote from the combustion zone of the furnace and with air for combustion to thereby'effect first stage preheating of said air; bypassing some of the boiler fluid which would otherwise circulate in said heat exchange relation with the combustion air and returning it into heat exchange relation with the flue gases in said zone to thereby maintain the temperature of said liquid boiler fluid above the dew point of the flue gases flowing through said zone; withdrawing steam from the plant; superheating the steam so withdrawn by circulating it in heat exchange relation with the flue gases in a zone of the flue gas passage at which the minimum temperature is considerably hotter than the maximum temperature at said first zone; and circulating the superheated steam in indirect heat exchange relation with the combustion air already heated by the liquid boiler fluid to thereby effect second stage preheating of the air.

7. The method of preheating air for supporting combustion in the furnace of a steam generating plant having a flue gas passage through which hot combustion gases discharge from the furnace, said method comprising: continuously circulating some of the boiler water from the plant in indirect heat exchange relation both with the flue gases in a zone of the flue gas passage remote from the combustion zone of the furnace and with air for combustion to thereby effect first stage preheating of said air; recirculating some of said heated boiler water directly back into heat exchange relation with the fiue gases in said zone without first circulating it in heat exchange relation with the combustion air; controlling the quantity of boiler water being so recirculated; mixing the recirculated boiler water with the continuously circulating boiler Water to maintain the temperature of the water passing in heat exchange relation with the flue gases above the dew point of the flue gases in said zone; withdrawing steam from the boiler plant; spperheating the steam so withdrawn by circulating it in heat exchange relation with the flue gases in a relatively hot zone of the flue gas passage; and circulating the superheated steam in indirect heat exchange relation with the combustion air already heated by the boiler water to thereby effect second stage preheating of the air.

8. The method of preheating air for supporting combustion in the furnace of a steam generating plant having a flue gas passage through which hot combustion 9 10 References Cited in the file of this patent 2,170,345 Bailey et a1 Aug. 22, 1939 UNITED STATES PATENTS 2,424,587 Smlth et a1. July 29, 1947 1,741,567 Heaton Dec. 31, 1929 FOREIGN PATENTS 1,753,363 Coghlan et a1 Apr. 8, 1930 5 299,436 Great Britain 1,819,174 Jacobus Aug. 18, 1931 (Not accepted but printed in 1930) 1,833,130 Roe Nov. 24, 1931 729,409 France July 23, 1932 1,837,713 Jacobus Dec. 22, 1931 575,509 Great Britain Feb. 21, 1946 1,840,836 Davis Jan. 12, 1932 629,298 Great Britain Sept. 16, 1949 1,975,519 Rudorfi Oct. 2, 1934 10 633,361 Great Britain Dec. 12, 1949

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