US3103486A

US3103486A - Method for preventing slag formation in

Info

Publication number: US3103486A
Authority: US
Publication date: 1963-09-10
Anticipated expiration: 1980-09-10

Description

Sept. 10, 1963 w. A. WALLS ETAL METHOD FOR PREVENTING SL AG FORMATION IN HIGH TEMPERATURE BOILERS Filed Dec. 29, 1960 mu; 2 wou m 96 8.6; 2233 7 mm 3 w vlmv E5: N. mu v23 5 Szfid w r o 2 m v.23 52:25 515 52% Om monnzmkzmu William AWGlIs William S. Procror ATTORNEY United States Patent 3 103 486 METHOD FOR PREVIlNTlNG SLAG FORMATKGN 1N HlGH TEMPERATURE BQHJERS 'William A. Walls, Drexel Hill, and William S. Proctor,

This invention relates to a method for preventing slag formation and improving the fuel combustion characteristics in high temperature boilers fired with heavy fuel oils and, more particularly, to the removal of sodium Compounds fromfuel oils such as residual oils containing sodium compounds and vanadium compounds as impurities to render such fuels suitable for use in high temperature boilers.

During recent years there has been a trend toward the use of high temperature, oil fired boilers, particularly high temperature marine boilers in which superheateroutlet temperatures of 850 F. up to as high as 1020 F. or higher are employed. The fuels normally used are of the residual type, such as bunker C, which contain various metallic compounds as impurities. In particular, these fuels contain sodium compounds predominantly the chloride and vanadium compounds probably in the form of complex organo-vanadium compounds. When such fuels are burned in thesehigh temperature boilers, serious slagging occurs on the tubes and adjacent surfaces. It is believed that slagging arises from a complex combination of sodium and vanadium compounds, some of which have relatively low fusion temperatures. Thus, in high temperature boilers these compounds deposit rapidly as a semi-molten mass which adheres to the tube surfaces until bridg'ng occurs between the tubes which eventually leads to plugged air passages through the boiler. The geometry of marine boilers resulting from the compactness dictated by shipboard space limitations requires that the tube spacing of the boilers be small which permits particularly quick slag bridging and also interferes with soot-blower effectiveness.

The slagging has to be removed by cooling the boilers and manually chipping off the slag which operation frequently results in serious tube and header damage. This slagging problem is so serious that frequently it has been found necessary to operate the boilers well below their normal high temperature design operating temperatures. In addition, it has been found necessary to avoid bunkering at certain ports because of previous bad slagging experience with the fuels available at such ports. Other efforts to solve the slagging problem have involved the use of additives to the fuel. None of these measures has proved to be successful, thus, for example, when the boilers were operated at temperatures below their design temperatures they obviously were operating at a less efficient level. Moreovenwhen it became necessary to avoid bunkering at certain ports, inefficient scheduling resulted with attendant increased costs and, finally, additives did not alleviate the slagging problem.

It now has been found that if the sodium content of a fuel oil containing both sodium compounds and vanadium compounds is reduced by at least about 65 percent to 70 percent by weight based on the original weight of the sodium in the oil, slaggin-g in high temperature boilers is substantially completely eliminated and the fuel combustion characteristics are greatly improved.

It is an object of this invention to provide a method for preventing slag formation and for improving the fuel combustion characteristics in high temperature boilers fired with fuel oils containing sodium compounds and vanadium compounds as impurities.

It is another object of this invention to provide a 3-,lh3,48 Patented Sept. 10, 1963 "ice method for reducing the sodium content of a fuel oil containing both sodium compounds and vanadium compounds by at least about 65 percent to 70 percentby weight based on the original weight of the sodium in the oil.

It is another object of this invention to provide a method for preventing slag formation and for improving the fuel combustion characteristics in high temperature boilersfired with fuel oils containing both'sodium cornpounds and vanadium compounds by reducing the sodium content of the fuel oil by at least from 65 percent to 70 percent by weight, based on the original Weight of the sodium in the oil, utilizing water extraction for reducing the sodium content.

Other objects of this invention will be apparent from the description and claims that follow.

In accordance with this invention, heavy fuel oil of the residual type, such as bunker C fuel oil, is admixed with water for a suflicient time to extract at least about 65 percent to 70 percent by weight of the sodium compounds based on the original Weight of the sodium compounds in the oil and thereafter separating the water from the oil by centrifugation.

In order for a more complete understanding of the invention, reference is hereby made to the accompanying drawing which is a diagrammatic view of an apparatus embodying the invention.

With reference to the drawing, numeral 1 indicates a pipe originating at a storage tank for feeding the raw oil, bunker C fuel oil for example, to the intake side of raw oil feed pump 2 from which it is pumped through pipe 3 into pipe '4 which is surrounded by a heating jacket 5 where the oil is heated to a temperature of from 190 F. to 210 F. A demulsifier or emulsion breaker, for example the demulsifier known commercially as Tretolite, contained in tank 6 is added through pipe 7 by demulsifier feed metering pump 8 through pipe 9 to the raw oil contained in pipe 1. The heated oil after leaving jacket 5 is passed through pipe 10 into mixer 11 where it is thoroughly mixed with water by agitator 12.

Fresh water from storage is furnished to the system through pipe :13 to the intake side of fresh water feed metering pump 14 from which'it is pumped into pipe 15 which is surrounded by heating jacket '16 Where the water is preferably heated to a temperature of from 180 F. to 210 F. The heated water is passed through pipe 17 into mixer 11 wherein it is thoroughly mixed with the oil as has been described. The mixture of oil and water is passed from mixer 11 through pipe 18 through a strainer or filter 19 where any large lumps or particles are removed The oil and water mixture is then passed requires that the through pipe 249 into centrifuge '21 \where it is separated 7 into an oil phase and an aqueous phase which latter phase contains sludge particles as well as the extracted sodium salts.

The aqueous sludge phase leaves the centrifuge through pipe 22 and is delivered into settling tank 23. This tank which is of conventional design fitted with conventional dams or weirs permits the sludge to settle and allows the sludge together with Waste water to be discharged from the tank through pipe 24 to the intake side of pump 25 which discharges the separated water and sludge through pipe 26 to waste. If centrifuge 21 is of the type which final oil-Water separation be carried out in a settling tank, oil may be recycled to the centrifuge by means of pipe 27 through recycle pump 28 and pipe 29. If the centrifuge 21 is of the type in which the oil- Water. interface is maintained within the centrifuge itself, water is recycled through pipe 27, pump 28, pipe 29 into centrifuge 21 Where it supplies the necessary quantity of water to maintain the seal in the centrifuge. The net waste water together with the sludge is then discarded through pipe 24, pump 25, and pipe 26 as has been described.

The cleaned fuel oil is ejected from the centrifuge through pipe 30 into cleaned fuel oil tank 31 from where it is pumped through pipe 32 by pump 33 into pipe 34 which is cooled by jacket 35 to lower the temperature of the oil to approximately 120 F. and thereafter the cooled oil is transferred by pipe 36 to the cleaned oil storage.

This invention is particularly applicable to fuels of the residual type containing a high level of sodium compounds and vanadium compounds. Such levels comprise from 30 parts per million to 100 parts per million or higher of sodium and from 50 parts per million to 500 parts per million or higher of vanadium. In general, the average for such fuels is of the order of 60 parts per million of sodium and 270 parts per million of vanadium, i.e. a sodium to vanadium ratio of approximately 0.22.

When such higher metal content fuels are burned in high temperature boilers, i.e. a boiler operating at 850 F. or higher, serious slag problems are encountered as has been described. If, however, in accordance with this invention the sodium content of such fuels is reduced to approximately 20 parts per million, i.e., reduced from 65 percent to 70 percent of the original sodium content, slagging is substantially completely eliminated. This reduction in sodium content reduces the average sodium to vanadium ratio to a value of 0.074.

The instant invention, however, is not applicable to fuels having a specific gravity greater than 0.99 at 60 F., i.e. not less than 11.5 API since, with fuels having a specific gravity greater than this limit, the gravity differential between the extraction water and the fuel is not sufficiently large to permit clean separation by the centrifuge or by gravity settling.

The type and quantity of demulsifier employed is not a critical feature of this invention. Numerous compounds are available commercially for breaking oil and Water emulsions with the preferred dosages being specified for each compound by the manufacturer. With the aforementioned Tretolite compound a dosage of 0.6 cubic centimeter per gallon of fuel was found to be completely suitable in aiding the separation of water and oil in the centrifuge.

The oil after being admixed with the demulsifier is heated to a temperature of from 190 F. to 210 F. and thereafter is admixed with water which has been preheated to approximately the same temperature of the oil.

The heating of the oil serves two purposes. It reduces the viscosity of the oil so that it may be more easily admixed with the water and it permits the use of water at equally high temperatures under which conditions the sodium salts are more soluble and therefore more easily extracted from the oil by the water.

The quantity of water utilized may range from 3 volume percent to 15 volume percent based on the volume of the oil. In general, it has been found that volume percent of water provides the optimum treat for removal of the desired quantities of sodium. The mixing of the oil and water is carried out in a conventional mixer equipped with a high speed propellor and, if desired, internal baffles. The mixer is sized so that the hold up time ranges from 2 to 3 minutes although the time of mixing obviously will be a function of the efiiciency of agitation, the viscosity of the oil and other factors. Such mixing should be suflicient to provide efficient contacting of the oil with the water so that the desired quantities of sodium compounds are extracted from the oil. These engineering principles are well understood and therefore need not be further elucidated here.

The mixture of oil and water is pumped from the mixer through a conventional strainer or filter which is provided to remove any large particles of solid material which may be found in the mixer and which would subsequently interfere with the operation of the centrifuge. The strained oil-water mixture is pumped into a conventional high speed centrifuge, for example, the self-cleaning type unit provided with internal reflux of water which permits passage of solid sludge and dirt particles from the peripheral nozzles while maintaining the oil-water interface within the machine. The position of the interface in such a centrifuge may be controlled by interchangeable dams which are changed when fuels of different gravities are used successively. The interface also may be adjusted with the machine in operation by raising or lowering the temperature of the recycle water. The source of such recycle water will be described hereinafter. The exact type of centrifuge which is employed for separating the oil and water mixture is not a critical feature of this in vention and one conventional type has been described but other commercially available machines are equally suitable. In addition to containing the extracted water soluble salts, the water phase contains some heavy sludge particles. The combined water and sludge leaving the centrifuge is introduced into a settling tank. In the settling tank the sludge settles to the bottom and may be removed along with the net waste water. The purified water from which the sludge has settled is recycled to the centrifuge in amounts sufficient to maintain the centrifuge seal. This quantity is normally of the order of the oil being separated from the oil-water mixture. The quantity of recycle is controlled by the dynamic balance of the machine such that only enough recycle water is accepted to satisfy the demand of the machine with excess recycle water being bypassed to the settling tank. The extracted fuel oil after being ejected from the centrifuge is pumped through a cooler and then to storage. It is generally necessary to cool the oil to approximately 120 F. since the pumps which operate to feed the boilers cannot handle but oil of the order of 180 F. which is the temperature of the oil discharged from the centrifuge.

In order to demonstrate that the instant invention substantially completely eliminated slag formation and improved the combustion characteristics in a high temperature boiler fired with conventional high sodium and high vanadium bunker C fuels, an apparatus similar to that described in the drawing was installed in a modern supertanker of approximately 30,000 tons deadweight capacity at summer load draft. This tanker was equipped with two Combustion Engineering SM sectional header type marine boilers which operated at a total steam temperature of 1020 F. This tanker operated on conventional bunker C type fuel.

Bunker C fuel or No. 6 fuel as it is also designated is the high viscosity bottoms fraction of crude oil. In general, it is specified that such fuel has a maximum Furol viscosity of 300 seconds at 122 F. and a minimum Furol viscosity of 45 seconds at 122 F. The specifications for the fuel burned in this tanker, however, was a Furol viscosity of 200 seconds at 122 F. maximum and seconds at 122 F. minimum.

The tanker had a history of serious slag deposition on the boiler tubes and consequently high tube renewal requirements. .Prior to beginning the test run on the low sodium fuel experimental program, the boilers were cleaned completely and tube renewals made where necessary. After installation of the apparatus described in the drawing, the tanker was operated in a normal manner for several months on bunker C fuels which varied in gravity from about API to 15.8 API, Furol viscosities ranging from 102 seconds at 122 F. to 204 seconds at 122 F., sodium contents ranging from 29 parts per million by weigh-t to 96 parts per million by weight (average, 60 parts per million) and vanadium contents ranging from 27 parts per million by Weight to 539 parts per million by weight (average, 270 parts per million).

The treating equipment was designed to process up to 20 gallons per minute of bunker fuel and consequently since the ship burned only about 580 barrels per day of fuel the equipment built up an inventory of treated fuel during each voyage so that the ship was always operated on treated fuel. Treating temperatures of approximately 200 F. were utilized together with a 5 volume percent treat of fresh water. It was found that the sodium content of the fuel was reduced from 65 percent .to 70 percent or higher with the 70 percent level being maintained more frequently toward the end of the experimental period as experience was gained in operating the equipment. The vanadium content of the fuel was reduced by only about 16 percent maximum indicating that, \as was known heretofore, the vanadium compounds are not present predominantly in water soluble form. After approximately 11 months of operation utilizing treated fuel the boilers were inspected and found to be free of slag deposits and with only a very thin layer of exceedingly brittle deposit which was so friable, thin, and loosely adhering that it could be removed by the fingers.

During the time treated fuel was utilized it was found that the combustion characteristics of the fuel had been improved since impingement against the rear wall of the furnace, drooling at the burner, oil on the furnace floor and collection of oil in the superheater tanks disappeared. Along with these changes refractory temperatures dropped which prolonged the life of the brickwork. In addition, flame photographs made at the time untreated fuel was being used compared with photographs made when the treated fuel was being used showed that a superior flame pattern was obtained with the treated fuel.

These full-scale experiments demonstate that the method of the instant invention prevents slag formation in high temperature oil-fired boilers and also improves combustion characteristics of the fuel in such boilers.

We claim:

1. A method for preventing slag formation and improving the fuel combustion characteristics in high temperature boilers fired with heavy fuel oils having a specific gravity not greater than 0.99 at 60 F. and containing sodium compounds and vanadium compounds as impurities, which comprises adding a demulsifier and from 3 volume percent to 15 volume percent of fresh Water to the impuritycont-aining oil, mixing the impurity-containing oil, demulsifier, and water at a temperature of from 190 to 210 F., whereby a two-phase mixture is obtained in which the water soluble sodium compounds are extracted by the water phase and the sodium content of the :oil phase is reduced by at least 65 percent to 70 percent, and centrifugally separating the aqueous phase from the oil phase.

2. A method for preventing slag formation and improving the fuel combustion chamacteristics in high temperature b oilers fired with heavy fuel oils having a specific gravity not greater than 0.99 at 60 F. and containing sodium compounds and vanadium compounds as impurities, which comprises adding a demulsifier and about 5 volume percent of fresh Water to the impurity-containing oil, mixing the impurity-containing oil, demulsifier, and Water at a temperature of from 190 F. to 210 F, whereby a two-phase mixture is obtained in which the water soluble sodium compounds are extracted by the water phase and the sodium content of the oil phase is reduced by at least 65 percent to 70 percent, and centrifugally separating the aqueous phase from the oil phase.

3. A method for preventing slag formation and improving the fuel combustion characteristics in high temperature boilers fired with heavy fuel oils having a specific gravity not greater than 0.99 at F. and containing sodium compounds and vanadium compounds as impurities, which comprises adding a demulsifier and about 5 volume percent of fresh water to the impuritycontaining oil, mixing the impurity-containing oil, demulsifier, and water at a temperature of about 200 F., whereby a two-phase mixture is obtained in which the water soluble sodium compounds are extracted by the water phase and the sodium content of the oil phase is reduced by at least percent to percent, and centrifugally separating the aqueous phase from the oil phase.

4. A method for preventing slag formation and improving the fuel combustion characteristics in high temperature boilers fired with heavy fuel oils having a specific gravity not greater than 0.99 at 60 F. and containing sodium compounds and vanadium compounds as impurities, which comprises adding a demulsifier and from 3 volume percent to 15 volume percent of fresh water to the impurity-containing oil, mixing the impurity-containing oil, demulsifier, and Water at a temperature of from 190 F. to 210 F., whereby a two-phase mixture is obtained in which the water soluble sodium compounds are extracted by the water phase and the sodium content of the oil phase is reduced by at least 65 percent to 70 percent, centrifugally separating the aqueous phase from the oil phase, and cooling the separated oil phase to a temperature of about F.

References Cited in the file of this patent UNITED STATES PATENTS 2,682,496 Richardson et a1 June 29, 1954 2,728,714 Winkler et a1. Dec. 27, 1955 2,774,722 Burkland et al. Dec. 18, 1956 2,785,120 Metcalf Mar. 12, 1957 2,789,083 Hardy Apr. 16, 1957 2,825,678 Iahn'ig et al. Mar. 4, 1958

Claims

1. A METHOD FOR PREVENTING SLAG FORMATION AND IMPROVING THE FUEL COMBUSTION CHARACTERISTICS IN HIGH TEMPERATURE BOILERS FIRED WITH HEAVY FUEL OILS HAVING A SPECIFIC GRAVITY NOT GREATER THAN 0.99 AT 60*F. AND CONTAINING SODIUM COMPOUNDS AND VANADIUM COMOOUNDS AS IMPURITIES, WHICH COMPRISES ADDING A DEMULSIFIER AND FROM 3 VOLUME PERCENT TO 15 VOLUME PERCENT OF FRESH WATER TO THE IMPURITYCONTAINING OIL, MIXING THE IMPURITY-CONTAINING OIL, DEMULSIFIER, AND WATER AT A TEMPERATURE OF FROM 190* TO 210* F., WHEREBY A TWO-PHASE MIXTURE IS OBTAINED IN WHICH THE WATER SOLUBLE SODIUM COMPOUNDS ARE EXTRACTED BY THE WATER PHASE AND THE SODIUM CONTENT OF THE OIL PHASE IS REDUCED BY AT LEAST 65 PERCENT TO 70 PERCENT, AND CENTRIFUGALLY SEPARATING THE AQUEOUS PHASE FROM THE OIL PHASE.