Combustion Control By Additives Introduced In Both Hot And Cold Zones

Abstract

By burning fuel in the presence of manganese and magnesium, and by then additionally adding to the products of combustion at a relatively low temperature zone an additional amount of magnesium, noxious and undesirable emissions are greatly reduced and internal boiler conditions are greatly improved.

Description

The present invention relates to a method for improving fuel combustion in furnaces, thereby to greatly improve stack emission problems and to minimize boiler fouling.

There are two general areas where fuel combustion presents problems. One general area involves the nature and amount of chemicals which are discharged into the environment. The substances emitted are often corrosive or otherwise damaging to any surfaces on which they fall. In many instances they are harmful to human or plant life, and in many instances they contribute to the formation of smog. These problems are today very generally recognized as quite serious, and strenuous efforts are being made to reduce the environmental pollution attendant upon combustion. The other general area, boiler fouling as a result of the formation of various substances in the boiler which coat the walls or the tubes of the boiler, constitutes a direct economic problem, since it reduces the efficiency of heat transfer and, when the build-up of materials becomes too great within the boiler, necessitates that the boiler be shut down from time to time for cleaning purposes, an obviously uneconomical procedure.

In general, different fuels present different problems. With sulphur-containing fuels, one of the major problems is the concentration of sulphur dioxide and sulphur trioxide in the stack gases. These compounds are extremely deleterious from a pollution point of view. When fuels contain vanadium in addition to sulphur, the production of undesired sulphur oxides is accentuated; the vanadium, probably in combination with the exposed iron on the tubes in the boiler, is able to catalyze the formation of undesirable sulphur oxides. Since both sulphur and vanadium are present in many of the commonly available industrial fuels, these problems are very pressing from a pollution control standpoint.

One standard approach to minimizing pollution problems is to add various substances to the fuel with a vew to having those substances enter into chemical combination with the undesired products of combustion in order to render them less undesirable or more readily removable from the stack emissions. Many different substances have been proposed to this end, including manganese and magnesium, usually introduced into the fuel in the form of compounds such as oxides and hydroxides. It is the manganese and magnesium which are the active ingredients, the oxides ane hydroxides being chosen as the addition media because they are more readily available and handleable than the active metals themselves.

With these additives, as with other additives, problems often arise. In some instances the additives, while entering into the expected reactions, also enter into side reactions the products of which present their own individual problems, which sometimes outweigh the problems which are intended to be cured. Also, in some instances particular additives, especially when used in large quantities, cause such fouling of the interior of the boilers as to make them undesirable from an economic point of view. Moreover, all additives are costly, and if especially large amounts of a particular additive are required in order to produce a given improvement the cost may be prohibitive from a commercial point of view.

It is the prime object of the present invention to improve the effects of fuel combustion, particularly with regard to emitting sulphur trioxide in the stack gases and improving the condition of the boilers where the combustion is carried out.

It is a further prime object of the present invention to achieve that improvement through the use of a minimal amount of additive, thereby reducing the expense of the fuel combustion improvement process.

It is another object of the present invention to provide a fuel combustion improvement process which is particularly adaptable for use in conjunction with commercially available fuels, and which can be carried out in existing combustion installations with a minimum of difficulty.

It is a further object of the present invention to provide a process for improving the effects of fuel combustion which inhibits the formation of slag in the boiler and which minimizes the emission of many acid substances in addition to sulphur trioxide.

The process of the present invention, involving as it does the presence of manganese or magnesium at the time of combustion, not only inhibits the formation of hard slag within the boiler, thereby to reduce boiler fouling, but also, by combining with vanadium in the fuel to form a relatively soft coating on the iron tubes within the boiler, reduces the production of sulphur trioxide by minimizing the availability of iron and vanadium to catalyze the formation of SO.sub.3. This is done by burning the fuel in the presence of magnesium or manganese additives in minimal amounts. This alone is known in the prior art; it produces a reduction of SO.sub.3 by about 25-40 percent. It is important to note, however, that this prior art approach only incompletely reduces the SO.sub.3 content of the stack emission, and still leaves that emission with a very substantial SO.sub.3 content. Adding additional quantities of manganese or magnesium to the fuel have been ineffective in SO.sub.3 reduction, have caused fouling problems within the boiler and have involved excessive cost.

I have discovered that if, in addition to the employment of manganese or magnesium in the hot zone of the furnace where the fuel is burned, one also adds magnesium to the combustion products at a zone in the furnace which has low temperature relative to the temperature of the combustion zone, several highly advantageous results are achieved: the ash is made less acidic (its pH is raised), the hygroscopic nature of the flue gas particulates is reduced, acid smut is effectively eliminated, boiler fouling is reduced because lesser amounts of additive need to be applied at the combustion station, and, most importantly, the SO.sub.3 content of the fuel gas is very radically reduced by as much as 80 percent.

The method in question can be used with many different types of fuel and many different types of furnaces. It may be used in oil-fired boilers such as those employed by utility companies, refineries and large industrial plants, with the additive feed to the relatively low temperature zone (hereinafter sometimes called cold-end feed) occurring at the economizer outlet, for example. The combustion of both residual fuel and crude oil is greatly improved in that manner. The process may also be used with coal-fired and waste gas-fired boilers with a cold end feed occurring at the up-takes, for example. The process is also applicable for use in steel mills burning waste gases, either alone or with Bunker C fuels, by refineries burning waste gas in boilers, and in refinery process heaters burning waste gas or waste gas in combination with Bunker C fuel. This list of examples is not intended to be all-inclusive.

When the magnesium-containing substance is added to the combustion products at a relatively low temperature station, it reacts directly and catalytically with the SO.sub.3 in the flue gas. It dramatically reduces acid particulates, acid condensation and dew point of the flue gas.

The cold zone supplemental treatment has been found to be more effective in controlling acid conditions than the standard oil treatment methods. One reason for this greater reactivity is that the cold zone additive does not have to first pass through the flame zone before combining with SO.sub.3 in the colder zones of the boiler. In the flame, oil dispersed additives, such as MgO or MgO:Al.sub.2 O.sub.3, do not react with SO.sub.3 at the high temperatures involved. At high boiler temperatures the sulfate complexes decompose, actually releasing SO.sub.3. The dryer and less hygroscopic ash resulting from the cold-end feed reduces cold end corrosion and at the same time will often eliminate acid smut emission problems. Moreover, an improvement in the stack plume appearance will often result from cold-end feed, with the consequent elimination of nuisance and legal complaints, particularly when the plant is in a residential area.

The most practical and least costly way of carrying out the method here disclosed is by initial treatment of the fuel oil with a manganese- or magnesium-containing substance in order to minimize the amount of SO.sub.3 in the stack gases and thus to minimize the amount of magnesium containing substance to be added at the cold zone. When this is done, the amount of additive can be reduced considerably, by 20 or 50 percent or more, over what was normally thought to be required with a particular fuel, yet the overall reduction in SO.sub.3 content in the exit gases is very greatly improved over what had previously been possible through the use of relatively large amounts of additive in the fuel oil.

In some boilers a portion of the combustion gases, after they leave the combustion station and their temperature drops, is recirculated back to the combustion station. In boilers of this type the method of the present invention can be practiced by adding a magnesium-containing substance to the combustion products in advance of the point where recirculation takes place. This not only constitutes the "cold-end feed," but also, by reason of the recirculation of a portion of the combustion products back to the combustion zone, serves in effect to add the magnesium-containing substance to the fuel oil in the combustion chamber.

By combining the operative use of a magnesium- or manganese-containing additive at the combustion station with a magnesium-containing substance at a station having a relatively low temperature, SO.sub.3 is very substantially removed from the flue gas, the SO.sub.3 formerly in the gas forming a dry and non-corrosive powder. Those materials entrained in the flue gas are generally rendered non-acidic, thus preventing damage to paint surfaces, equipment and other objects in areas surrounding the combustion plant. The visible plume from the stacks is often reduced, and the acridity of the odor from the stacks is minimized. The effectiveness of soot blowers and stack collectors, when employed, is enhanced, the overall fallout from stack gases is localized and the possibility of plume "hang-up" in the effect of an atmospheric inversion is minimized.

With many fuels it is preferred that the combustion chamber additive be a manganese-containing substance. Such a substance reduces the amount of carbon in the fly ash because manganese is known to be a carbon-destroying catalyst. This in itself is an advantage, since wet carbon leaving the boiler tends to absorb SO.sub.3. Moreover, the presence of manganese tends to cause the formation of SO.sub.2 rather than SO.sub.3. The addition of the magnesium-containing substance thereafter in the low temperature zone is effective to remove virtually all of the SO.sub.3 which does form in the boiler.

To the accomplishment of the above, and to such other objects as may hereinafter appear, the present invention relates to a method of improving the effects of fuel combustion, as defined in the appended claims and as described in this specification, taken together with the accompanying drawings, in which

FIG. 1 is a schematic representation of an exemplary fuel-burning boiler installation of the non-recirculating type; and

FIG. 2 is a schematic representation of an exemplary fuel-burning boiler installation in which a portion of the products of combustion are recirculated back to the combustion chamber.

The illustrations are provided simply for purposes of facilitating explanation of the process of the present invention. The illustrated boiler arrangements per se form no part of the present invention, are not novel in and of themselves, and are merely exemplary of many different types of constructions and arrangements with which the process of the present invention can be practiced. Since the process of the present invention involves introducing additives at different stations in the boiler, and particularly at a high temperature station such as the combustion chamber and at a low temperature station downstream of the combustion chamber, some appreciation of the general arrangement of boiler systems is of assistance in understanding the process of the present invention and the results achieved thereby, and it is for that reason that FIGS. 1 and 2 are here presented.

FIG. 1 represents a particular boiler installation where no recirculation of the products of combustion takes place. An appropriate fuel, such as fuel oil, coal or combustible gas, is introduced into the furnace 1 in any appropriate manner as through burner guns 24 in the case of fuel oil. Air, preferably heated, is supplied to the furnace in any appropriate manner to combine with the fuel. Combustion of the fuel takes place in the furnace 1, the portion of the heat energy produced by that combustion being transmitted to the tubes (not shown) covering the furnace walls, thus converting the water in those tubes to steam. Combustion of the hot gas may be completed by means of the addition thereto of secondary heated air from the heated air duct 21, air being supplied to that duct by air inlet 16, blower 17, air duct 18, air preheater 19 and air ducts 20 and 21. The products of combustion then pass through the platen superheater 2 and reheater 3, pendant superheater 4, and the horizontal superheater 5. When the combustion products leave the horizontal superheater 5 their temperature, which in the furnace 1 was about 2,400.degree.-2,800.degree. F, has been reduced to 800.degree.-900.degree. F. The products of combustion then flow through the economizer 6 which preheats the water entering the steam-producing tubes inside the furnace 1. The products of combustion then flow into the gas duct 7 at a temperature of 650.degree.-700.degree. F. They then flow through duct 8 and air heater 9, the air heater 9 tending to transfer the heat from the exiting gases to the air preheater 19. At this point the temperature of the products of combustion is approximately 300.degree. F. The products of combustion then flow through duct 10 and precipitators 11 where ash is removed from the stream of gas. The thus cleaned gas flows through duct 12 and induced draft fan 13 into breeching and then out through the stack 15.

One of the most important problems in connection with reducing air pollution caused by products of combustion is the presence of acidic SO.sub.3 in the combustion products. That compound is itself deleterious, and in addition it tends to be taken up in the particulate matter which escapes from the stack to produce an acid smut often referred to as "green rain."

The conventional approach to minimize this situation is to introduce into the furnace a suitable quantity of a magnesium compound, such as magnesium oxide. This is done by mixing it with the fuel or by applying it to the coil before the latter is burned or by adding the substance to the furnace while combustion takes place. Such an additive has several effects. It reacts with the vanadium in the fuel to prevent high temperature corrosion and the formation of hard slag inside the furnace. It acts itself to coat the superheater tubes and thus insulate the products of combustion from the iron surfaces of those tubes. Since iron catalyzes the formation of SO.sub.3 from SO.sub.2, this results in a reduction in the formation of SO.sub.3. In addition, the magnesium compound reacts with vanadium, thus reducing the amount of vanadium oxide which is formed, that vanadium oxide also tending to catalyze the formation of SO.sub.3. For these purposes, the magnesium oxide can be added to the fuel in any suitable form, such for example as a pre-mix with the fuel, as a liquid slurry added to the fuel, or as a powder injected into the furnace proper.

However, there is a limit to the amount of magnesium compound which can be provided at the high temperature combustion zone in the furnace. If too much such material is provided large amounts of ash will result; this ash will build up in and eventually block the furnace, requiring that it be shut down and cleaned. Moreover, the greater the amount of ash, the greater the amount of inorganic particulate matter emitted through the stack. In addition, the mechanism by which magnesium reduces the amount of SO.sub.3 in the products of combustion involves the formation of magnesium sulphate. Magnesium sulphate decomposes at temperatures above 1,500.degree. F, and since the temperatures in the furnace are well above that value the decomposition of magnesium sulphate undoes what the added magnesium initially accomplishes. Indeed, the nature of the reactions involving SO.sub.2 are such that the introduction of massive amounts of magnesium oxide into the hot end of the furnace may actually increase the production of SO.sub.3 rather than decrease it.

It has been proposed in the past that certain substances be added to the products of combustion at a relatively low temperature station. However, insofar as magnesium-containing oxides and any effect they may have in reducing SO.sub.3 are concerned, this approach is ineffective, because the magnesium compounds by themselves are too inert to produce the desired result. They are in solid form and must react with gaseous products. Reaction rates in such conditions are generally very low. To use a magnesium compound such as magnesium oxide only in conjunction with cold end feed would require so much magnesium oxide that particulate matter would escape from the stack in tremendous volume, and a pollution problem would be created rather than eliminated.

I have found that by combining a magnesium-containing substance added at the cold end with a magnesium-containing or manganese-containing substance added at the hot end, greatly improved results are obtained insofar as SO.sub.3 reduction is concerned. A normal amount of magnesium oxide when added to the combustion zone, in accordance with known practice, results in an SO.sub.3 reduction of 15-25 percent, and that reduction cannot be increased to much more than 40 percent no matter how much magnesium oxide is added to the furnace and no matter what the deleterious effects of adding that magnesium oxide to the furnace may be. The same amount of magnesium oxide added only to the cold end of the furnace (for example, at the outlet of the economizer 6 in FIG. 1, a preferred place for effecting the cold-end feed in accordance with the present invention) will reduce the SO.sub.3 content by 35-40 percent. However, if the same total amount of magnesium oxide is used, but with 25 percent thereof added to the combustion zone and 75 percent added to the cold end (e.g. the outlet of the economizer 6 of FIG. 1) 50-80 percent of the SO.sub.3 is removed.

If the substance added to the combustion zone is a manganese-containing substance such as manganese oxide, and if it is added solely to the furnace, the total SO.sub.3 removed is 40 percent or better, under exceptional circumstances sometimes going as high as 75 percent. If, in conjunction with the addition of such manganese oxide to the combustion zone, a magnesium-containing compound such as magnesium oxide is added at the cold end, the total SO.sub.3 emitted is easily reduced by 80-95 percent. Thus it is seen that the combination of hot-end feed and cold-end feed as here disclosed results in greatly improved SO.sub.3 reduction, to a degree not achievable through the use of hot-end addition alone or cold-end addition alone, and through the use of moderate and economically feasible amounts of the additive materials.

The active components of the additive materials here under discussion are manganese and/or magnesium. However, the handling of those metals is not particularly convenient, nor are they commercially available in quantity at reasonable prices. Accordingly, the preferred additives are compounds of magnesium and/or manganese, usually the oxides or hydroxides thereof because of their ready and economic availability and ease of handling.

I set forth below a number of specific examples illustrating the manner in which my new method of improving the effects of fuel combustion can be carried out and showing the advantages thereof over that which had formerly been thought to be achievable. It will be understood that these examples are in no way limiting, and that the method in its broader application can be practiced in specifically different ways and in different environments.

Bunker C fuel containing an average of 225 parts per million vanadium, a sulfur content of an average of 2.05 percent and an ash content of 0.07 to 0.10 percent was burned in a front fired boiler with a 375 megawatt output and a superheat steam temperature of 1,050.degree. F.

At 2 percent excess oxygen, the SO.sub.3 content without any treatment was 60 parts per million.

Example 1

A slurry of magnesium oxide was injected directly into the fuel oil being burned at a treatment rate of 3.1 lbs, MgO/8,000 lbs. of fuel oil. The temperature in the flame zone was in excess of 2,300.degree. F.

The reduction of the SO.sub.3 in the flue gas was from 60 to 50 parts per million.

Example 2

The MgO was aspirated into the economizer outlet of the boiler at a temperature of 700.degree. F. rather than being injected into the fuel oil. On a dry powder basis, 3.1 lbs. of magnesium oxide was injected for each 8,000 lbs. of fuel oil burned in the above boiler.

The SO.sub.3 was reduced from 60 to 45 parts per million.

Example 3

A slurry of the magnesium oxide was introduced into the boiler to provide 1.5 lbs. of magnesium oxide for each 8.000 lbs. of fuel burned. At the same time the dry, powdered, MgO was concurrently aspirated into the economizer outlet, to provide 1.6 lbs. MgO for each 8,000 lbs. of fuel in the same boiler. In other words, although the same amount of MgO was used in Example 3 as in either Example 1 or 2 above, the quantity was split up so that half of the MgO was added to the boiler proper and the other half was added to the outlet section of the boiler in the economizer region at temperatures of 500.degree. to 600.degree. F.

Under these conditions, the SO.sub.3 was reduced from 60 to 30 parts per million.

Example 4

In this case, the slurry of MgO was added exactly as in Example 1 so as to provide 3.1 lbs. MgO for 8,000 lbs. of fuel oil. In addition, 1.6 lbs. MgO as a dry powder was aspirated into the economizer outlet.

The SO.sub.3 was reduced from 60 to 27 parts per million. In other words, increasing the amount of MgO added to the fuel oil did not significantly reduce the SO.sub.3 content of the flue gas. Stated in another way, there was a limiting factor in how far one could reduce the SO.sub.3 when injecting the magnesium oxide through the furnace by direct addition to the fuel oil.

Example 5

The addition of 1.5 lbs. of MgO as a slurry to the fuel oil was supplemented by the direct injection of dry powdered magnesium oxide into the economizer outlet, to provide a total of 3 lbs. of MgO aspirated directly into the flue gas through the economizer outlet.

The SO.sub.3 was reduced from 60 to 21 parts per million. This further shows that once a minimum amount of MgO is added to the fuel oil to provide a coating within the boilers, then the further addition of MgO as a dry powder into the flue gas has a much greater effect in reducing SO.sub.3 than adding that equivalent amount or adding that same amount of MgO to the fuel oil through the burner. This could indicate that cold-end injection (at temperatures of 200.degree. to 1,000.degree. F.) is more effective than the addition of the MgO through the furnace where temperatures reach 2,000.degree. to 3,300.degree. F. or thereabouts.

Example 6

In this example, and in subsequent ones, the magnesium oxide was added to the fuel oil to provide 1.5 lbs./8,000 lbs. of fuel oil resulting in a reduction of the SO.sub.3 from 60 to 55 parts per million. In addition the fuel was further treated with a slurry of a manganese oxide to provide 16.5 parts per million of manganese to the fuel oil.

The SO.sub.3 in the flue gas was reduced from 60 to 47 parts per million. This shows that the manganese is even more effective than the magnesium for reducing SO.sub.3 when added to the fuel, but the reduction of the SO.sub.3 in the flue gas is still insufficient to prevent low temperature corrosion.

Example 7

The fuel was treated with both the magnesium and manganese as in Example 6 above and magnesium oxide, as a cold dry powder, was aspirated into the economizer outlet at 700.degree. F. to provide an additional 1.6 lbs. of MgO for each 8,000 pounds of fuel oil. The SO.sub.3 was reduced from 60 to 26 parts per million.

Example 8

The fuel was treated with a slurry of manganese oxide to provide 40 parts per million manganese to the fuel. The SO.sub.3 in the flue gas was reduced from 60 to 48 parts per million.

Example 9

The fuel was treated with the manganese oxide slurry to provide 40 parts per million manganese to the fuel oil. At the same time, magnesium oxide was aspirated as a dry powder into the economizer outlet so that the flue gas was being treated with 1.6 lbs. of MgO for each 8,000 lbs. of fuel burned. The SO.sub.3 was reduced from 60 to 23 parts per million. This shows that the combination of manganese addition to the fuel oil with magnesium oxide powder added to the flue gas is effective for substantially reducing the SO.sub.3 and even more effective than the combination of magnesium oxide addition to the fuel supplemented with magnesium oxide addition to the flue gas.

Example 10

In this example, a slurry of manganese oxide was added to the fuel oil as in Example 8. This then was supplemented by the injection of a magnesium oxide powder into the economizer outlet. However instead of adding the magnesium oxide on a continuous basis, the total daily amount of magnesium oxide was split into two portions and half of this quantity was injected at 12 hour intervals one hour each time. A coating of the magnesium oxide was thus formed on the air preheater section of the boiler.

The total amount of additive was equivalent to 0.32 lbs. of MnO added to the fuel oil and 1.6 lbs. of MgO aspirated into the economizer (per each 8,000 lbs. of fuel burned). The sulphur trioxide in the flue gas when the MgO powder was not being fed remained at 48 parts per million but during the period when the MgO powder was being fed the SO.sub.3 reduced to 30 ppm.

Although the intermittent use of MgO was not sufficient to prevent the SO.sub.3 completely, the use of the MgO in this fashion did almost completely protect preheaters against corrosion. This is quite significant because it shows how to prevent low temperature corrosion of the exit sections of the boiler, where there are no other problems such as acid smut emissions and for other reasons where it would not be desirable to continuously treat the flue gas with the neutralizing powder. It also shows the possibility of reducing the amount of neutralizing powder required by aspirating it intermittently into the zone in cases where it is only necessary to lay down a coating on the furnace surfaces.

Example 11

This is similar to Example 10 above except that the additive is added directly to the fuel oil was a magnesium oxide slurry. Again the intermittent use of the magnesium oxide powder protected the air heaters from corrosion to a greater extent than use of magnesium oxide slurry added only to the fuel oil.

TABLE 1 __________________________________________________________________________ COLD END FEED vs ADDITION TO FUEL OIL FURNACE CHAMBER __________________________________________________________________________ Fuel = Bunker C of 225 ppm V, 2.05% S and 0.085% Ash Addition of: Injection Sulfur Acidity Condition Appearance Total Lbs. of Mg:V Ex- Agent of: Point Trioxide of Ash of of Stack Lbs. of Additive Weight ample in Flue Deposits Air Additive Ton of Ratio Gas on Air Heater (on dry Sulfur (parts Heater basis) per Outlet per million) 8,000 lbs. of fuel Equal to Lbs. Addi- tive/1,000 Gals. Fuel __________________________________________________________________________ -- None None 60 1.9 Heavily Distinct -- -- -- Corroded Blue Plume 1 Magnesium Fuel Oil 50 2.4 Corroded Distinct 3.1(as 37.8 1:1 Oxide Blue Plume MgO) Slurry 2 MgO Economizer 45 3.0 Fair, Blue Plume, 3.1 37.8 1:1 Powder Outlet only slightly trace of reduced in Corrosion intensity 3Gi a) Magnesium Fuel Oil ) 1.5 18.3) 0.5:1 Oxide ) Slurry ) 30 4.0 Negligible Slight ) Plus ) Corrosion Blue Plume ) 37.8 (b) Mgo Economi- 19.5) 0.5:1 Powder zer Out- 1.6 let 4(a ) Mgo Fuel Oil ) 3.1 37.8 1:1 Slurry Plus ) 27 4.0 Negligible White-grey, ) 57.3 (b ) Mgo Economi- ) Dense Plume 1.6 19.5) 0.5:1 Powder zer Out- ) let ) 5(a ) Mgo Fuel Oil ) 1.5 18.3) 0.5:1 Slurry ) White-- Plus ) 21 4.5 Good --slightly ) 54.9 (b) MgO Economi- ) grey cast 3.0 36.6) Powder zer Out- ) let 6( a) Mgo Fuel Oil ) 1.5 18.3 0.5:1 Slurry ) ) Plus ) 47 2.9 Slight Slight ) 19.9 (b) Mangasese ) Corrosion Blue Plume 0.132 lbs 1.6) -- Oxide ) MnO Slurry 7( a) Slurry of Fuel Oil ) 1.5 18.3) 0.5:1 Mgo ) Plus ) ) (b) Slurry of Fuel Oil ) 26 4.2 Good,only Almost 0.132 lbs 1.6) 39.4 -- Manganese trace of clear Oxide Plus Corrosion ) (c) Mgo Economi- ) 1.6 19.5) 0.5:1 Powder zer Out- ) let ) 8 Slurry of Fuel Oil 48 2.7 Slightly Slight 40 ppm Mn Manganese Corroded Blue Plume (0.32 lbs. 3.9 -- Oxide MnO) 9( a) Slurry of Fuel Oil ) 40 ppm Mn Manganese ) (0.32 lbs 3.9) -- Oxide ) 23 4.6 Negligible Almost MnO) ) 23.4 Plus ) Clear ) (b) MgO Economi- ) 1.6 ) 0.5:1 Powder zer Out- ) 19.5) let ) 10( a) Slurry of Fuel Oil ) 40 ppm Mn 3.9) -- Manganese ) (0.32 lbs ) 23.4 Oxide ) MnO) ) Plus ) 48 3.8 Negligible Slight ) (b) Mgo Inter- ) (except Blue Plume 1.6 ) 0.5:1 Powder mittently ) during 19.5 into econo- ) MgO mizer outlet feed (at 12 hr cycle, intervals) when it dropped to 30 ppm) 11(a) Magnesium Fuel Oil ) 1.5 18.3 0.5:1 Oxide ) ) Slurry ) ) 15.1 Plus ) 55 3.8 Satis- Slight ) (b) Mgo Inter- ) (except factory Blue Plume 1.6 19.5) 0.5:1 Powder mittently ) during into econo- ) the Mgo mizer outlet ) feed (as in cycle, 13b) when it dropped to 30 ppm)

Example 12

A novel feature of the present invention is the adding of a magnesium oxide powder into a furnace in such a way that we get complete control and arresting of corrosion of the air heaters, complete elimination of acid smut emissions, the capabilities of reducing the exit gas temperature by 50.degree. to 100.degree. F. (which is a fuel savings of 2.5 percent for each 100.degree. drop in exit temperature), as well as a means of keeping the furnace clean. As previously shown, using an equal weight of magnesium oxide added to the fuel oil itself, or injected with the primary air going to the furnace, or direct addition of MgO powder or an MgO containing slurry to the hot furnace box, does not give this benefit of keeping the superheater tubes clean in the furnace, because (1) any MgSO.sub.4 that forms decomposes in the hot furnace zones, and (2) the MgO, as it goes through the flame, is exposed to temperatures of 3,000.degree. F. This causes the MgO to melt and accordingly it becomes fluxed and is not available as a dry powder on the surface tubes in the superheaters.

The method described in this Example is a unique one because the SO.sub.3 is contacted with fresh, dry and unfluxed (active) MgO; then residual MgO that is recirculated through the furnace is in direct contact with the superheater tubes so that it is never exposed to surface temperatures in excess of 2,000.degree. F.

Reference to FIG. 2, which shows an exemplary furnace with recirculation of a portion of the products of combustion, will be helpful in understanding this example. Fuel, fuel oil or coal is injected into the furnace 1' through burner guns 19'. Heated air from heated air duct 18' combines with the fuel and helps to atomize it, that heated air being sucked in from the atmosphere at air inlet 14', blown through duct 16' by fan 15', passing through air heater 17' and then flowing through duct 18' into the furnace 1'. Combustion of the fuel takes place in the furnace at a high temperature, and the heat produced thereby is transmitted to the tubes (not shown) covering the walls of the furnace, coverting the water in those tubes to steam. The products of combustion pass through a secondary superheater 2', a reheater 3', a primary superheater 4', a duct 5', an economizer 6' and a duct 7' into heater 8' which transmits some of the heat from the exhaust products to the heater 17'. The exhaust products then flow through duct 10' to a point where duct 11' and induced draft fan 12' are located. There some of the products of combustion are forced through the duct 11' back into the furnace 1' (this being the recirculated portion of the products of combustion) while the bulk of those products of combustion pass through duct 13' to a collection device and thence to the stack.

With a furnace of the type shown in FIG. 2, the magnesium-containing additive, such as magnesium oxide or magnesium hydroxide may be added completely at a relatively low temperature zone, such as the outlet of the economizer 6'. The bulk of the material thus added remains with low temperature combustion products, but that portion of the additive entrained with the recirculated combustion products passing through duct 11' are returned to the furnace or combustion zone 1', where they are subjected to the high temperatures of combustion. Thus, although the magnesium additive is introduced into the system only at a single location, in fact some of that additive finds its way to the combustion station, while another portion of the additive remains in contact with the low temperature combustion products, and hence in effect some of the additive is introduced at a high temperature zone and another portion of the additive is subjected only to low temperature conditions.

The following Table 2 discloses the results obtained in connection with a furnace of the type disclosed in FIG. 2 in which fuel oil was burned, the fuel oil containing 0.97 percent sulphur and having a vanadium content of 100 parts per million and an ash content of 0.035 percent.

TABLE 2 ______________________________________ MgO MgO lbs./8000 Condition of Addition lbs. of Fuel Superheater Tubes ______________________________________ 1) Added as a 50/50 slurry directly to the fuel oil. 1.75 Furnace walls heavily slagged and superheater tubes completely bridged over with a heavy coating but porous and removable with manual lancing. 2) Added as a 50/50 slurry directly to the fuel oil. 1 Some fouling of furnace box, but not a continuous sheet. Hard deposits on superheater tubes. Impossible to remove by manual cleaning. 3) MgO added as a powder into economizer outlet and recirculated back into furnace. 2 Furnace walls have only a minimum of deposit buildup. Superheater tubes show soft white deposits that are easily removed by manual or air lancing. Completely cleaned to bare metal by air lancing. 4) MgO added as a powder into economizer outlet and recirculated back into furnaces. 1 No deposits on furnace walls. Superheater tubes show a lightly crusted slag, removable by air lancing. ______________________________________

Example 13

This example, like Example 12, involves cold-end feed of magnesium oxide and recirculation of a portion of the additive back to the furnace through the gas recirculation duct 11'.

A coal to oil converted boiler with a mechanical cyclone collector, a 20 megawatt design unit, was experiencing difficulties following conversion when fired with Bunker C fuel. The stack emissions had a low pH of 1.5 and when the particles descended onto the cars in the employees' parking lot, as well as on adjoining homes near the utility generating plant, greenish-black, wet, corrosive, deposits penetrated the paint surfaces of the cars and homes. The surface of the cars were covered with black-greenish dots or splotches.

The superheat temperature within the furnace was 900.degree. F. and particularly when burning fuel oils having a sulfur content of 2 to 2.5 percent and vanadium contents of 100 to 400 ppm, there was a further problem of hard deposits formed on the superheater tubes, which were difficult to clean off.

As a first attempt to solve this problem, the plant tried the use of oil slurried additives consisting of 50:50 weight ratio of MgO in the fuel oil. Using quantities as high as 1 gallon of this 50:50 ratio for each 500 gallons of fuel oil, the persistent acid emissions were not materially reduced. Moreover, at these high rates, the plant experienced serious operating difficulties because of blockage by the magnesium-containing deposits that had built up between the tubes.

Reverting to a blend of oil additive containing various ratios of aluminum oxide with the magnesium oxide of 1:10, 1:6, 1:3 and 1:1 and continuing with rates as high as 1 gallon/500 gallons of fuel did not alleviate the acid emission problem but resulted in even harder and more tenacious deposits on the superheater tubes.

Using combinations of manganese oxide slurries added to the fuel oil, as well as mixtures of manganese oxide with magnesium oxide of various weight ratios from 1:5 to 1:10 of Mn:Mg likewise did not reduce the emissions problem or the blockage problem within the boiler.

Since the boilers were equipped with gas recirculation, a blend of the additive of this invention was added to the economizer outlet with approximately 50 percent of the powder being recirculated through the boiler into the superheat cavity. The additive blend consisted of 70 percent magnesium oxide, 20 percent calcium oxide, 5 percent manganous oxide and 5 percent urea. The addition rate was 14 lbs./hour at a steam level of 300,000 lbs./hour. Utilizing this cold-end feed, the pH of the deposits collected in the mechanical hoppers as well as on the surrounding plant was 4.2 and the deposits were light and fluffy with a whitish appearance. They did not adhere to the paint surface of the cars or homes.

Quite unexpectedly, the boiler tubes remained extremely clean when treated in this manner. The boilers remained on the line for a period of 6 months as compared to 3 months prior to the use of the additive either when the fuel was untreated or treated with any of the magnesium oxide, aluminum oxide or manganese containing oxide slurries described above.

Repeating this test, but using MgO, resulted in a considerably improved emitted deposit with a pH of 3.8, and the boilers showed considerably cleaner boiler tubes, without any hard deposits on the tubes.

Further, it has been found that even in furnaces that do not have a serious acid emissions problem, this combination of aspiration of MgO with gas (and powder) recirculation back to the furnace is a considerably improved way to prevent hard, tenacious and corrosive vanadium deposits from fouling the superheater tubes and passages. The same amount of MgO added to the fuel does not produce these dramatic results, and instead will often require a more frequent shutdown of the furnace for clean-down.

Example 14

This example illustrates the use of the method of the present invention in connection with a furnace which had been converted from coal firing to oil firing, utilizing Bunker C fuel oil with an 80 megawatt rating.

It was experiencing great difficulty in utilizing the electrostatic precipitators when burning the liquid fuel. Fouling of the air heater elements occurred and sticky adherent deposits were present, which resulted in frequent shutdowns. Further, the pH of the deposits collected on the precipitators, as well as the deposits collected 100 feet from the stack, showed a value of 1.5 to 1.9.

An intimate mixture consisting of 60 percent magnesium oxide, 15 percent sodium bicarbonate, 10 percent calcium oxide and 15 percent manganous oxide was injected into the economizer outlet of the furnace before the electrostatic precipitators.

The particle size of the powder mixture averaged 25 to 40 microns. The addition rate of the powder was 7.5 lbs./8,000 lbs. of fuel burned, the fuel having a sulphur content of 2.7 percent, a vanadium content of 375 parts per million and an ash of 0.14 percent.

With this dry powder addition, the pH of the deposits collected 100 feet from the stack was found to be 4.1. There was negligible sticky deposits on the electrostatic precipitators which functioned, virtually uninterruptedly, as long as the powder blend was fed continuously into the economizer outlet at temperatures of 400.degree. to 850 F., the normal temperature of the flue gas at the economizer outlet.

The addition of this powder, or the same amount of MgO of 7.5 lbs./8,000 lbs. of fuel, directly to the fuel or to the furnace did not raise the pH of the stack deposits above 2.5 to 3.0, an ineffective result.

Where, as is optimally the case, the cold-end additive is used to minimize the presence of SO.sub.3 in the flue gas, at least enough additive should be used to accomplish the maximum neutralization of SO.sub.3. This will, of course, depend upon the amount of sulphur in the fuel to begin with, the percent of conversion of that sulphur (as SO.sub.2) to SO.sub.3 in the course of combustion (generally ranging from 1 to 10 percent depending upon the particular combustion conditions and hot-end additives used) and also the amount of excess air present. This if a fuel oil contains 0.25 percent sulphur, for optimum minimum use of a 1:1 equivalent neutralizing mole ratio of additive to SO.sub.3, then 0.2 lbs. of cold-end additive in the form of MgO per 8,000 lbs. of fuel oil would be required if the conversion of SO.sub.2 to SO.sub.3 amounted to 1 percent, and 2.0 lbs. of that additive would be required if the conversion figure were 10 percent. Stated in terms of ratio of additive per tons of sulphur in the fuel oil, 1.7 lbs. of MgO additive per ton of sulphur would be required where the conversion figure is 1 percent and 17 lbs. of that additive would be required where the conversion figure was 17 percent. If the fuel oil contained a greater proportion of sulphur, then the amounts of additive would increase but the ratio of additive would remain the same.

If it is desired to remove SO.sub.2 from the flue gases, then additional amounts of cold-end additives will be required, with a lower limit of perhaps 2.5 lbs. of MgO additive per 100 lbs. of fuel for each 1 percent of sulphur content in the fuel.

Of course, there is nothing to prevent the use of lesser amounts of cold-end additive than those set forth above in order to obtain some benefit from the method of the present invention, even though that benefit is not maximally obtained. Comparably, additional amounts of cold-end additive may be used than those here set forth, although it is not believed that any additional benefit can be obtained thereby, except perhaps by way of a safety factor to compensate for variations in the sulphur content of the fuel, for inaccuracies in determining the magnitude of that sulphur content, or for changes which may occur in the combustion conditions which in turn may give rise to variations in the percentage conversion to SO.sub.3. In other words, there is nothing critical in the amounts of magnesium-containing material employed for cold-end feed, and in a given installation the determination of the optimal amount of additive to be used may well be arrived at empirically, by varying the amount of additive, analyzing the content of the stack gases, and selecting that amount of additive which gives the best results.

The cold-end feed additive may be introduced into the system continuously or intermittently, depending upon economic and environmental needs and operating conditions to which the plant in question is subject, but in general it is preferred that the cold-end addition occur continuously, since only in that way will the undesirable SO.sub.3 emissions from the plant be fully minimized.

The additives used in accordance with the present invention may contain substances other than the manganese- and magnesium-containing substances here specified, those other substances sometimes adding combustion control effects of their own and sometimes enhancing the effect of the manganese and/or magnesium here involved.

The additives may be introduced into the cold end in any convenient form e.g., as a dry powder, as a liquid slurry, either aqueous or non-aqueous, or as a solution.

While only a limited number of embodiments of the present invention have been here specifically described, it will be apparent that many variations may be made therein, all without departing from the spirit of the invention as defined in the following claims.

Claims

I claim:

1. The method of improving the effects of fuel combustion which comprises:

a. Burning the fuel in the presence of a first effective amount of an additive comprising a substance from the group consisting of manganese, magnesium, compounds thereof, and combinations thereof;

b. Conveying the combustion products to a relatively low temperature station;

c. Adding to said combustion product at said station a second amount of additive comprising a substance from the group consisting of magnesium, magnesium compounds and combinations thereof; and

d. Conveying the resultant products to an exhaust station.

2. The method of claim 1, in which said fuel is burned in a boiler and then conveyed through superheater means, said low temperature station being located after said superheater means.

3. The method of claim 1, in which the temperature at said relatively low temperature station is about 200.degree. to 1,000.degree. F.

4. The method of claim 1, in which said fuel is burned at the combustion station and means are provided for recirculating a substantial portion of said combustion products from said relatively low temperature station to said combustion station, and in which a member from a substance from the group consisting of magnesium, magnesium compounds and combinations thereof is physically added to said combustion products at said low temperature station and in an amount at least equal to the sum of said first and second amounts, said recirculation accomplishing the step (a) addition of said substance.

5. The method of claim 1, in which said additive step (a) comprises a manganese compound.

6. The method of claim 1, in which said additive of step (c) comprises a substance selected from the group consisting of magnesium oxide and magnesium hydroxide.

7. The method of claim 1, in which said second amount is at least about 1.7 pounds of magnesium oxide or the equivalent thereof in the case of other substances, per ton of sulphur in the fuel.

8. The method of claim 1, in which said second amount is from about 1.7 to 5.2 pounds of magnesium oxide, or the equivalent thereof in the case of other substances, per ton of sulphur in the fuel.

9. The method of claim 1, in which said second amount is from about 1.7 to 17 pounds of magnesium oxide, or the equivalent thereof in the case of other substances, per ton of sulphur in the fuel.

10. The method of claim 5, in which said additive of step (c) comprises a substance selected from the group consisting of magnesium oxide and magnesium hydroxide.

11. The method of claim 10, in which said second amount is at least about 1.7 pounds of magnesium or magnesium compound, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

12. The method of claim 10, in which said second amount is from about 1.7 to 5.2 pounds of magnesium substance, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

13. The method of claim 3, in which said additive step (a) comprises a manganese compound.

14. The method of claim 13, in which said additive of step (c) comprises a substance selected from the group consisting of magnesium oxide and magnesium hydroxide.

15. The method of claim 14, in which said second amount is at least about 1.7 pounds of magnesium or magnesium compound, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

16. The method of claim 14, in which said second amount is from about 1.7 to 5.2 pounds of magnesium substance, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

17. The method of claim 3, in which said additive of step (c) comprises a substance selected from the group consisting of magnesium oxide and magnesium hydroxide.

18. The method of claim 17, in which said second amount is at least about 1.7 pounds of magnesium or magnesium compound, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

19. The method of claim 17, in which said second amount is from about 1.7 to 5.2 pounds of magnesium substance, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

20. The method of claim 4, in which said additive of step (c) comprises a substance selected from the group consisting of magnesium oxide and magnesium hydroxide.

21. The method of claim 20, in which said second amount is at least about 1.7 pounds of magnesium or magnesium compound, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

22. The method of claim 20, in which said second amount is from about 1.7 to 5.2 pounds of magnesium substance, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

23. The method of claim 6, in which said second amount is at least about 1.7 pounds of magnesium or magnesium compound, based on magnesium oxide as said substance, per ton of sulphur in the fuel.

24. The method of claim 6, in which said second amount is from about 1.7 to 5.2 pounds of magnesium substance, based on magnesium oxide as said substance, per ton of sulphur in the fuel.