CA1092522A - Process and means for gas conditioning - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

The specification describes a method and an apparatus of preconditioning a flue gas mixture containing fly ash emanating from a coal burning boiler for the efficient removal of the fly ash by an electrostatic precipitator, the method comprising the steps of: directing a flow of air into a sulfur burner; directing a flow of sulfur into the sulfur burner; sensing the rate of coal combustion in the boiler; selectively varying the flow of sulfur in response to the sensing; combusting the sulfur within the sulfur burner to create a fluid mixture including the combustion products of the sulfur; passing the fluid mixture through a catalytic converter to produce a conditioning mixture; and combining the conditioning mixture with the flue gas mixture with the flue gas mixture prior to passing the flue gas mixture into an electrostatic precipitator.

Description

This is a di~ision of copending Canadian Patent Application Serial No. 238,448, filed ~ctober 28, 1975 and assigned to the present assignee.
In many cases, removal of fly ash from the flue gases of fossil fuel combustion by means of electrostatic precipitators has proven to be highly satisfactory. Hcwever, scme fossil fuels such as low sulfur coal may produce an ash that does not satisfactorily respond to certain electrostatic precipitating techniques.
For example and regarding coal in particular, virtually the entire sulfur content of coal, which may vary from less than one percent to approximately six percent, oxidizes to sulfur dioxide during combustion of the coal, and from one to five percent of such sulfur dioxide further oxidizes to sulfur trioxide. Typically, as the flue gases cool after co~bustion the sulfur trioxide component thereof combines with entrained moisture to form sulfuric acid which condenses on fly ash particles. It is well known that the surface electrical properties of ~ ?
the fly ash may be largely controlled by the amount of sulfuric acid -present. Thus, in the case of low sulfur coal often there is little -~
sulfuric acid present and ash surface resistivity is correspondingly high. As a consequence collecting efficiency may ke degraded considerably, particulæ ly in precipitators which are sensitive to surface resistivity of the fly ash as for example a precipitator receiving flue gases at temperatures corresponding to normal stack -exit temperatures, e.g, approximately 250F to 320F, It is well known that fly ash collecting efficiency may be ~
vastly improved when, prior to entry of the flue gas stream into the ~-electrostatic precipitator, the flue gas is conditioned by the augmentation of the sulfuric acid content thereo, The acid condenses upan fly ash particles as the flue gas cools to advantageously decrease ash surface resistivity as hereinabove noted. Recently, with the advent of strict statutory emission regulations fly ash conditioning ~ -has kecame increasingly commonplace and necesary as many users of fossil ~ ~ -.,~, , km:~ D ,`
~' :' ', fuels have turned fro~ high sulfur to low sulfur fuels.
In the prior art various processes have been proposed for sulfuric acid conditioning of fly ash to facilitate electrostatic precipitation thereof. For example, one proposed conditioning method entails the storage of relatively large quantities of liquid sulfur trioxide which is metered as required into the flue gas stream upstream of the precipitator to complement the naturally generated sulfur trioxide and thereby react with the moisture in the flue gas to generate sufficient sulfuric acid for effective conditioning. This method has proven unacceptahle inasmuch as liquid sulfur trioxide is an extremely corrosive chemical the proper storage, use-and handling of which requires extreme care and expensive, complex equipment.
For example, expensive dehumidifying apparatus is required to ensure that the air mixed with the sulfur trioxide is campletely dry.
Another conditioning technique used in the prior art comprises the heating of concentrated sulfuric acid in a closed system to generate sulfuric acid vapor which may be injected directly into the flue gas stream or alternatively may be mixed with dry air for subsequent injection into the flue gases. In view of the many serious hazards of storing and using sulfuric acid in quantity such as the threat of acid spills and leaks, the deficiencies of this method are self evident.
In U. S. Patent No. 3,689,213 issued Septem~er 5, 1972 to Guerrieri, a method of flue gas conditioning is disclosed whereby liquid sulfur dioxide is vaporized and admixed with an excess of co~pressed and dehumidified oxygen-containing gas. The mixture is introduced into a converter wherein the sulfur dioxide is converted to sulfur trioxide and the gaseous products from the converter are introduced into a flue gas prior to the passage thereof through an electrostatic precipitator. I~ is also suggested in the patent that the sulfur dioxide may be obtained by the combustion of organic sulfur :- ~ . , - ~. .

compounds; however, no practical means are disclosed for perfonmmg or controlling a continuous process of producing the requisite quantity of sulfur dioxide for proper gas conditioning, which quantity varies with boiler load.
The present invention relates to an i~proved method and apparatus for sulfuric acid conditioning of flue gas, Sulfur, preferably in liquid form, is directed into a sulfur burner where in the presence of air it is burned to form crlnbustion products containing sulfur dioxide. The ccmbustion products are then passed through a catalytic converter where a major portion of the sulfur dioxide is converted into sulfur trioxide. The sulfur trioxide containing gases exiting from the converter are introduced into the flue gases to augment the sulfur trioxide content of the flue gases to thereby advantageously lower the surface resistivity of the fly ash contained therein as described hereinabove.
In order for the system to operate efficiently it is required that the quantity of sulfur dioxide formed in the sulfur ~ -burner be accurately controlled and varied in response to the j , varying sulfur trioxide requirements of the system while maintaining the temperature in the sulfur burner akove a predetermined minimum, Also, it is necessary to maintain the temperature of the ~ases entering the catalytic converter within a predetermined range to provide efficient conversion of the sulfur dioxide to sulfur trioxide and to prevent deterioration of the catalytic converter. ~, -According to the present inventionr the rate of sulfur combustion is varied in response to tha boiler load, Since the ~
rate of sulfur combustion effects the temperature within the ~ -sulfur burner, means are provided to maintain the temperature --within the sulfur burner above the temperature necessary for ~ ~
3Q immediate comkustion at all times while the temperature of the ~ -., ~oszs22 gases exiting from the sulfur burner ~re kept within the range acceptable for efficient operation of the catalytic converter.
It is therefore an object of the invention to provide a method and means for continuously forming sulfur dioxide at a rate and temperature necessary for efficient conversion to sulfur trioxide and acid conditioning of flue gases.
It is another object to provide a method and means of forming sulfur dioxide in a burner wherein the temperature is maintained at all times the system is in operation above that required for combustion, One aspect of the present invention is defined as a method of preconditioning a flue gas mixture containing fly ash emanating from a coal burning boiler for the efficient removal of the fly ash by an electrostatic precipitator comprising the steps of: directing a flow of air into a sulfur burner; directing a flow of sulfur into the sulfur burner;
sensing the rate of coal combustion in the boiler; selectively varying the flow of sulfur in response to the sensing;
combusting the sulfur within the sulfur burner to create a fluid mixture including the combustion products of the sulfur;
passing the fluid mixture through a catalytic converter to produce a conditioning mixture; and combining the conditioning mixture with the flue gas mixture prior to passing the mixture into an electrostatic precipitator.

Another aspect of the present invention is defined as an air pollution control apparatus providing for the efficient removal of fly ash from a boiler flue gas by electrostatic precipitation comprising: a sulfur burner;
means for directing at least a portion of a source flow of bm:¦~

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air into the sulfur burner; means for directing sulfur into the sulfur burner; means for combusting such sulfur with the sulfur burner to ereate a fluid mixture exiting from the sulfur burner; first detecting means for detecting the temperature of such fluid mixture exiting from the sulfur burner; means for selectively varying the quantity of such portion of such source flow of air in response to the first of such portion of such source flow of air in response to the first deteeting means; catalytic converter means for changing such fluid mixture to a conditioning mixture; and means for combining such conditioning mixture with sueh boiler flue gas ~ -prior to passing such flue gas through an electrostatic precipitator. ~
These and other objects and advantages of the present -invention are more fully specified in the following description -with reference to the drawings wherein:
Fig. 1 is a schematic illustration of a flue gas ~
conditioning apparatus whieh operates in accordance with the , ~`
principles of the invention; and ,~ -Fig. 2 is a sehematie illustration of a preferred flue gas conditioning apparatus aecording to the invention. ~
In order to faeilitate a eomplete understanding of ~ ' the unobvious advantages inherent in the preferred embodiment of the invention depieted by Fig. 2, a description of an ~'~-';
earlier developed form of the invention will first be ', '~
described.
The appratus 10 schematieally shown in Fig, 1 represents a flue gas conditioning apparatus according to one form of the invention. The apparatus 10 comprises: an air fan ~' 12 preferably a eonstant speed fan, the inlet of which eommunicates,with the atmosphere via an inlet conduit 20, a fully modulatable heater portion 30 which communicates with the .
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109ZS~Z -outlet of fan 12 via a conduit 18 for the purpose of receiving a flow of intake air therefrom; a sulfur burner 32 which communicates via a conduit 34 and an adjustable proportioning valve 35 with the outlet of heater 30 to receive heated -4b-bm:~O

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air therefrom, and which further communicates via a suitable feed conduit 36 and a sulfur feed pump 38 with a liquid sulfur storage tank 40; a catalytic converter 42 which communicates with the outlet of burner 32 via a conduit 44; and a manifold injector 46 disposed within a boiler flue 48 and adapted to receive sulfur trioxide from converter 42 via a communicating conduit 52 for injection into the flue 48 to condition flue gases conveyed therethrough from a boiler 50 to a conventional ~;
electrostatic precipitator 80. -The apparatus 10 further includes a bypass conduit :
54 which communicates proportioning valve 35 and conduit 44 thereby producing a bypass loop for air flow around the burner 32, and suitable automatic control lines 56, 58, 60 and 72 which monitor and control the operation of valve 35, :-heater 30 and pump 38 in a manner and for such purposes as described hereinbelow. .
It is of course to be understood that the tank 40 is suitably heated and insulated to be capable of maintaining ~-the sulfur contained therewithin in a liquid state; that is, ~: .
above its melting point, whereby liquid sulfur is ~ :
continuously available to be pumped into burner 32.
Initially during operation of apparatus 10 intake .-~
air is drawn preferably at a constant rate by fan 12 via conduit 20 through a filter 14 to ensure intake air cleanliness, and additionally through a preheater 16 which ~ - .
provides consistent intake air temperature thereby further - -controlling intake air volume and mass flow rate. The fan 12 thence impels the intake air via conduit 18 into heater 30 wherein the air is heated as required for burner startup~
for example to a temperature of approximately 800F.
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~9Z522 The hot air is thence di~ected via conduit 34 into valve 35 wherein the air flow is divided between the inlet of burner 32 and the bypass conduit 54, for example in the ratio of approximately 75% to burner 32 and 25% to bypass 54. The hot air directed through burner 32 heats the interior thereof and thence exits burner 32 via conduit 44 to be rejoined with the bypass air flow in conduit 54 at junction 62. The combined air flow thence flows through converter 42, conduit 52 and the manifold 46, and into the flue 48.
The hereinabove described continuous flow of heated air may in practice constitute a standby mode of operation of the apparatus 10 wherein hot air flow continuously maintains burner 32 at a temperature substantially above the ignition temperature of sulfur to permit sulfur ignition upon initiation of sulfur flow. Ordinarily the apparatus 10 would be maintained in the standby mode when not actively operating, -that is, when sulfur is not being fed to burner 32, in order to prevent difficulties resulting from repeated cooling and heating of burner 32 and to keep the flow path through the system clear of debris, In practice, a particular boiler load generally indicates that rate of coal combustion, and therefore generally `
determines the sulfur trioxide requirement to condition the resulting fly ash. Accordingly, in response to a boiler load sensed by a sensor 74 in a well known manner and transmitted -via controller 75 and line 60, the pump 38 is regulated to deliver liquid sulfur at the required rate from tank 40 via conduit 36 into burner 32. Of course it is to be understood that controller 75 is suitably calibrated to control sulfur ~
flow into burner 32 according to the known percent of sulfur -content of the coal being burned. Within the burner 32, which bm:~o . ~ .. . . .. . .. . .. . .. .

lO9Z522 is shown as a well known cascade or checkerwork type burner, the liquid sulfur cascades downwardly over a brick checkerwork 64 countercurren~ly to the upward rush of heated air from the burner inlet. Due to the high temperature maintained within burner 32 by the hot air flowing therethrough the sulfur ignites immediately and rapidly oxidizes to form a sulfur dioxide and air mixture containing, for example, approximately 5% sulfur dioxide by volume. Baffles 66 are provided within ~ ~
burner 32 downstream of checkerwork 64 to create turbulence ~ -in the air-sulfur flow thereby ensuring complete combustion ~; -of the injected sulfur within burner 32. -Combustion of the sulfur as described generates substantial heat within burner 32; however, it is desirable to maintain the burner within a predetermined temperature range during sulfur burning for optimal operation, for example -~ ~
approximately 1200F to 1400F for many known burners. To ~ -help achieve this condition the control loop 56 includes a `-sensor 68 located adjacent conduit 44 intermediate burner 32 and junction 62, and is adapted to sense the outlet temperature of the burner 32. Inasmuch as burner outlet temperature is indicative of burner operating temperature, the ~
loop 56 ~ay be utilized to control burner operating temperature. ~ -Accordingly, sensor 68 communicates via the loop 56 with a suitable controller portion (not shown~ of valve 35 to increase or decrease the proportion of intake air flow directed through burner 32 in response to a burner outlet temperature that is-too high or too low~ respectively since increasing the amount of air in the burner will decrease burner operating temperature. Preferably the valve 35 is capable of dividing the total air flow in a wide range of proportions between burner 32 and bypass 54 in order to maximize burner temperature bm:~v lO9Z52Z

regulating capability.
The hot air a~ld sulfur dioxide mixture exits burner 32 and is quickly cooled by combining at junction 62 with the cooler bypass air flowing within bypass 54. Subsequently the combined air and sulfur dioxide flow is directed through the converter 42, which may be any suitable conventional catalytic converter such as a vanadium pentoxide catalytic converter, to further oxidize the sulfur dioxide to sulfur troixide in the well known manner. ~ -Inasmuch as the converter 42 may commonly have an optimal operating temperature well below 1200F for example approximately 780F to 850F and may even be damaged at temperatures in the order of 1200F, means are provided in control loop 58 to regulate converter operating temperature by regulating the temperature of the air-sulfur dioxide mixture flowing into the converter 42. ~ccordingly, loop 58 includes a sensor 70 located adjacent conduit 44 intermediate junction 62 and converter 42 and is adapted to monitor the `

air-sulfur dioxide mixture temperature adjacent the converter inlet. The converter inlet temperature thus sensed is - utilized by a suitable controller portion (not shown) of --heater 30 to regulate the output of heater 30 whereby the temperature of the air flow into burner 32 and bypass 54 may be increased or decreased in response to a converter inlet temperature which is too low or too high, respectively.
Typically, during sulfur burning at maximum capacity heater 30 will be regulated to heat the air flowing there-through only slightly, for example to approximately 100F

inasmuch as the heat of sulfur combustion under such 30 conditions will be quite sufficient to maintain the desired ~ - -burner operating temperature. Moreover, under such conditions a high proportion of a relatively cool bypass air flow is bm:~-o - . . -. ~ , ~ ,. , .. . . .

required, for example 40~, versus 60% through burner 3~, to cool the burner output from the burner operating temperature to the desired cohverter inlet temperature.
If during system operation the rate of sulfur burning is changed by action of the sensor 74 and controller 75 in response to a changing boiler load~ the sensor 68 of loop 56 will sense the resultant change in burner outlet temperature and will automatically adjust valve 35 accordingly to bypass a greater or lesser proportion of the total air flow through conduit 54 as required. Such adjustments of the sulfur burning rate will also alter the temperature sensed by sensor 70, and in response thereto loop 58 will atuomatically adjust heat output of heater 30 to provide the desired converter inlet temperature. However, in response to such automatic adjustment of heater 30, outlet temperature of the burner 32 may be altered whereupon valve 35 will again be adjusted by action ;
of sensor 68 to maintain proper burner operating temperature. -Furthermore, it is to be noted that the signal used to regulate heater 30 is also communicated to controller 75 from loop 58 via a connection 72 as a feed back signal to controller 75 so that the output of pump 38 may be varied in response to the signal in loop 58 which is an indication of the actual amount of sulfur being delivered to the sulfur burner, Thus it will be seen that the control loops including sensors 68~ 70 and 74 continuously interact to maintain desired thermal conditions ~
and sulfur trioxide production rate as required, Of course ; ~;
it is to be understood that the control loops include suitable conventional control circuitry (not shown) to account for ~6 ~-inherent time lags in the system, and to preclude undersirable temperature overshoots, oscillations and similar irregularities.

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From converter 42 the air-sulfur trioxide mixture formed therewlthin is directed via conduit 52 and the manifold ~6 into the flue gas stream within flue 48. As is well known, within flue 48 the injected sulfur troixide combines readily with the moisture in the flue gas steam to produce sulfuric acid which advantageously condenses upon fly ash particles thereby effectively conditioning the fly ash for subsequent arrest by conventional electrostatic precipitating means 80, In Fig. 2 there is shown a preferred flue gas conditioning apparatus which is simpler in operation and less expensive to manufacture than the system of Fig. 1. The apparatus 10' is the same as the apparatus 10 of Fig. 1 except that the proportioning valve 35, bypass 54, sensor 68, feedback ~-72 and control loop 56 have been eliminated. As stated herein- ; ; --~
above the main purposes of these elements were to control the sulfur burner operating temperature and to help cool the exhaust of the sulfur burner down to the optimal operating temperature of the known catalytic converter, However, it has been found by the applicant that if the sulfur burning rate is kept below a predetermined maximum, it is possible by only varying the temperature of the inlet air passing through heater 30' to maintain the exhaust of the sulfur burner 32' at the optimal converter temperature of approximately 850F, The preferred form of the invention depicted by Fig,

2 operates as follows, Initially, intake air is forced into heater 30' by constant speed fan 12' where it is heated to a `~ -temperature above the ignition temperature of sulfur, The hot air is directed into the sulfur burner 32' wherein the interior brickwork is maintained above the ignition temperature at ~`
approxiately 800F, and the air flows out of the sulfur burner ' '. " ' -1 0- .
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through pipe 44', converte~ 42', mainfold 46' and up through flue 48'.
When a boiler load is sensed by sensor 74' indicating that fuel is being burned and flue gases are being generated and expelled through flue 48', a signal is sent via controller 75' (which is calibrated according to the sulfur content of the fuel being burned) to commence a metered flow of liquid sulfur from pump 38' into the burner 32', Since the temperature within the sulfur burner 32' is above the ignition temperature of sulfur combustion occurs oxidizing the sulfur to sulfur dioxide. Since heat is generated by this ~:
chemical reaction the temperature of the combustion products -will be higher than that desired for efficient opearation of the catalytic converter 42'; however, the sensor 70' immediately upon sensing this undesirable temperature will :
reduce the temperature of (or shut down) the air heater 30' so that the temperature of the combustion products exiting from suflur burner 32' will soon be reduced to a temperature -within the range of efficient converter operation, It has been found, for example, that to produce a burner output temperature between the 780-850F desired range the constant -flow of air should be set to provide approximately 100 parts air at 100F to 5 parts sulfur by weight, If sensor 74' senses a reduced boiler load which indicates a reduced; ``
requirement of sulfur trioxide, to condition the flue gas, pump 38' would react to restrict the flow of sulfur to burner 32'. This change in the sulfur to air ratio in the burner will produce less heat of combustion which is sensed by sensor 70'.. In.response to the decrease in temperature at sensor 70' the heater 30' will increase the temperature of the incoming air so that the ignition temperature of sulfur will : -~

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: . ~ , , . , ., . - . , iO9Z5Z2 be maintained in the burner and the exiting combustion products will be within the desired converter temperature range. As previously stated the sulfur dioxide formed in the burner 32' will be converted to sulfur trioxide in converter 42' for introduction into flue 48' and acid conditioning of the flue gases will result for efficient removal of fly ash in the -electrostatic precipitator 80', Thus the apparatus according to Fig, 2 provides a simplified conditioning system which will operate efficiently in those cases where the sulfur dioxide 10 requirement does not exceed the amount which will produce a -~
temperature above the operating limit of tne converter.
According to the foregoing recitation there is ~ -provided means for sulfuric acid conditioning of the flue gases comprising a known sulfur burner adapted to receive thereinto a flow of air and liquid sulfur for the combustion of such sulfur to produce combustion products including sulfur dioxide, and further comprising a known catalytic converter adapted to convert the sulfur dioxide from the sulfur burner into a mixture containing sulfur trioxide for immediate injection into the flue gas stream to be conditioned, According to this invention such conditioning apparatus is operable by an improved method whereby the stream of intake air may be heated prior to injection thereof into the sulfur burner to provide ignition heat for injected sulfur~ and to maintain standby temperature conditions in the system, There are provided automatic control means adapted to regulate ~ ~
the heat output from the air heater located upstream of the air ~ ~ ;
inlet to the sulfur burner whereby catalytic converter operating temperature may be controlled, and automatic control means adapted to regulate the sulfur trioxide production rate by controlling the sulfur feed rate in response to the boiler load.

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The system hereinabove described includes capability for total turndown from the maximum sulfur trioxide requirement to zero by employing a pump 38 which is operable over a continuous range of sulfur delivery rates between shutdown and a maximum delivery rate.
Notwithstanding the reference hereinabove to particular embodiments of the present invention, it is to be understood that this invention may be practiced in various other embodiments with numerous modifications thereto without `~
departing from the broad spirit and scope thereof. For example: any of a number of sulfur containing compounds may be burned in place of the preferred liquid sulfur; a separate heater could be placed in line 54; line 54 could have a separate source; burner 32 may be other than a cascade type burner, for example a pool type burner; and may comprise a plurality of burners adapted to serve a like plurality of gas ;~
conditioning systems 10; converter 42 may be any of a variety of catalytic converters; a control loop is comtemplated ;
whereby total air flow is reduced to decrease heater power ~-requirements during standby as by a suitably adjustable valve in conduit 18 which is regulated by a control circuit capable of sensing a no-flow condition in pump 38j various additional controls, monitors and alarms in the apparatus are comtemplated; and the like.
These and other modificatlons and embodiments having `-been envisioned and anticipated it is requested that this invention be interpreted broadly and limited only by the scope of the claims appended hereto.
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Claims (8)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of preconditioning a flue gas mixture containing fly ash emanating from a coal burning boiler for the efficient removal of the fly ash by an electrostatic precipitator comprising the steps of: directing a flow of air into a sulfur burner; directing a flow of sulfur into said sulfur burner; sensing the rate of coal combustion in said boiler; selectively varying said flow of sulfur in response to said sensing; combusting said sulfur within said sulfur burner to create a fluid mixture including the combustion products of said sulfur; passing said fluid mixture through a catalytic converter to produce a conditioning mixture; and combining said conditioning mixture with said flue gas mixture prior to passing said flue gas mixture into an electrostatic precipitator.

2. An air pollution control apparatus providing for the efficient removal of fly ash from a boiler flue gas by electrostatic precipitation comprising: a sulfur burner;
means for directing at least a portion of a source flow of air into said sulfur burner; means for directing sulfur into said sulfur burner; means for combusting such sulfur within said sulfur burner to create a fluid mixture exiting from said sulfur burner; first detecting means for detecting the temperature of such fluid mixture exiting from said sulfur burner: means for selectively varying the quantity of such portion of such source flow of air in response to said first detecting means; catalytic converter means for changing such fluid mixture to a conditioning mixture; and means for combining such conditioning mixture with such boiler flue gas prior to passing such flue gas through an electrostatic precipitator.

3. The apparatus specified in claim 2, further comprising: means for combining a second flow of air with such fluid mixture subsequent to such fluid mixture exiting from said sulfur burner.

4. The apparatus specified in claim 3, further comprising: second detecting means for detecting the temperature of such fluid mixture subsequent to the combining thereof with such second flow of air; and means for selectively varying the temperature of the air in such second flow of air in response to said second detecting means.

5. The apparatus specified in claim 4, further comprising: means for selectively varying the quantity of such second flow of air in response to said first detecting means.

6. The apparatus specified in claim 5, wherein:
such second flow of air is another portion of such source flow of air.

7. The apparatus specified in claim 6, wherein:
such source flow of air is a substantially constant flow of air.

8. The apparatus specified in claim 7, further comprising: means for selectively varying the temperature of the air in such source flow of air in response to said second detecting means.