INDUSTRIAL AND ENGINEERING CHEMISTRY
620
For speeds up to 500 cc. per minute this catalyst gives higher results a t 450' than a t 500" C. At speeds between 200 and 400 cc. per minute conversions of 98 and 99 per cent are obtained. At 600 to 700 cc. per minute conversions of 97.3 per cent were obtained a t 500" C. These values are for the 12 by 2 cm. mass. Figuring the conversive ability of this catalyst from the industrial standpoint, the following result is obtained: 0.15 pound of ammonium vanadate will make 1 pound of catalyst which will occupy 75 to 80 cubic inches. By operating this mass a t 450" C. and using a flow of 17 to 35 cubic feet per hour of 8 per cent gas, a conversion of 98.0 to 98.8 per cent will be obtained, which will produce 0.38 to 0.75 pound of sulfuric acid per hour. If a flow of 50 cubic feet per hour is used with a temperature of 500" C., a conversion of 97.3
Vol. 23, No. 6
per cent will be obtained, producing 1.00 pound sulfuric acid per hour. Figuring the above on the basis of the vanadium used, 1 pound of vanadium will produce 5 to 10 pounds per hour a t 450" C. and 98 per cent conversion, or 13 pounds per hour a t 500" C. and 97 per cent conversion. These calculations are based, of course, on results obtained in the laboratory. Under commercial conditions a higher conversion is possible, owing to the better conditions of control obtainable in the industry. Literature Cited (1) Holmes and Elder, IND.ENG.CHBM.,PP, 471 (1930). (2) Scott, Standard Methods of Chemical Analysis, Vol. 11. pp. 1264, 1266, Van Nostrand, 1925.
Reactions of Sulfur Compounds in Boiler Furnaces'sa H. F. Johnstone UNIVERSITY OF ILLINOIS ENGINEERING EXPERIMENT STATION, URBANA, ILL.
I n a furnace the sulfur in coal is converted mainly into sulfur dioxide. Only about 2 per cent is oxidized to t h e trioxide, regardless of the temperature or oxygen content of t h e gases. The concentration of sulfur trioxide in t h e stack gases is no greater, therefore, t h a n t h a t in the furnace gases. Flue dust has only slight catalytic action in t h e oxidation of sulfur dioxide. When t h e sulfur in t h e fuel exists as sulfuric acid, as, for instance, in petroleum residues, about 85 per cent of t h e acid is reduced in t h e furnace t o sulfur dioxide. The gases contain only slightly more trioxide t h a n those from highsulfur coal. When coal is fired on a stoker, about 30 per cent of t h e sulfur remains in t h e ash, a t least a part of which exists a s iron sulfide. Particles of dust containing t h e sulfide adhere readily t o one another and t o metal surfaces, so t h a t hard deposits build u p readily both on boiler a n d economizer tubes. On boiler tubes most of t h e sulfur in
t h e slag is lost by oxidation of t h e sulfides a n d decomposition of t h e sulfates. A t lower temperatures t h e sulfates are stable a n d t h e slag contains a large proportion of sulfate sulfur, even above t h e condensation temperatures of t h e gases. Concentrations of sulfur trioxide in t h e gases a s low as 0.015 per cent raise t h e dew point to 80-100' C. The hygroscopic nature of deposits containing ferric sulfate also causes moisture to condense a t temperatures considerably above t h e dew point of t h e gases calculated from t h e partial pressure of water vapor in t h e gases. As solutions containing ferric sulfate act a s strong catalysts for t h e oxidation of sulfur dioxide t o sulfuric acid, t h e existence of these sulfates in t h e flue dust is responsible for a n increase in the temperature range of corrosion by flue gases. Increased moisture content of the gases caused by leaks or by t h e use of steam soot-blowers will produce the same effect.
HE small quantities of sulfur compounds present in
gators is somewhat higher than the values reported here. The discrepancy was found in each case to be due to errors inherent in the analytical methods previously used.
T
some fuels are the source of many of the undesirable properties of flue gases. In an investigation of the corrosion of power-plant equipment by flue gases a study was made of the reactions which these sulfur compounds undergo during combustion. Previous interest in this problem has been shown by a t least two investigators, who have attempted to find the extent of oxidation of the sulfur in flue gases. Sherman and his co-workers (IO) a t the Bureau of Mines, in a study of refractories service conditions, determined the sulfur dioxide and trioxide in the gases a t several power plants. More recently Ardern and Wheeler (1) made analyses for the same constituents in experiments on the purification of flue gases by a washing procedure. The extent of oxidation of the sulfur dioxide to sulfur trioxide found by both these investi1 Received February 19, 1931. Presented before the Division of Industrial and Engineering Chemistry a t the 81st Meeting of the American Chemical Society, Indianapolis, lnd., March 30 to April 3, 1931. 2 Published by permission of the Director of the Engineering Experiment Station, University of Illinois. This paper contains results of a cooperative investigation with the Utilities Research Commission on "The Corrosion of Power Plant Equipment by Flue Gases." The complete results of the investigation will be published as Bullefin 22s of the Experiment Station.
Reactions of Sulfur in Combustion of Coal Sulfur exists in three forms in coal-viz., in pyritic, organic, and sulfate combinations. The relative amounts of the first two forms vary considerably, although in Illinois coals they are usually present to about an equal extent. The percentage of sulfate sulfur is usually very small, amounting in general to less than 0.1 per cent. I n some coals, however, notably those of low grade, this proportion may be much larger. The first reaction which the pyrite and marcasite undergo is decomposition with liberation of one-half of the sulfur. Both the free sulfur and that which remains combined with the iron are then oxidized to sulfur dioxide (4, 16,7). The iron is, of course, a t the same time converted to the oxide, in which state, or in combination with silica, it remains in the ash. The initial reaction of the organic sulfur compounds is also probably one of decomposition to sulfur, carbon disulfide, hydrogen sulfide, etc., which are readily oxidized to sulfur dioxide. The sulfates that exist in coal are easily
June, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
decomposed in the presence of silica and iron oxide a t the high temperatures reached in the furnace. This decomposition yields the metallic oxide and sulfur trioxide, the latter being partially reduced to the dioxide. I n the furnace, therefore, sulfur dioxide will be the principal product of the combustion of sulfur in coal, with small percentages existing as the trioxide in the gas and as sulfates and unoxidized iron sulfide in the ash. It will be shown herein that the small amounts of the sulfide play an important part in the sulfur problem. Oxidation of Sulfur Dioxide in Flue Gases
It has generally been assumed that further oxidation of the sulfur dioxide occurs as the temperature of the gases decreases-i. e., in the last pass of the boiler, in the economizer, the pre-heater, and the stack. This question was thoroughly studied because of the great influence of small concentrations of sulfur trioxide on the dew point of the gases. The catalytic effect of flue dust on the oxidation was first determined in the laboratory and then a thorough survey was made of the sulfur distribution in flue gases a t five power plants. The apparatus used for the study of the oxidation of sulfur dioxide in the presence of flue dust was similar to that used by Holmes, Ramsay, and Elder in their study of platinized silica gels as catalysts for the same reaction (6). A mixture of air and sulfur dioxide containing 0.428 per cent SOz, a composition which corresponds to that of the flue gases from a highsulfur coal, was passed over the finely pulverized flue dust suspended on asbestos. The temperature of the catalyst was maintained within *3' C. of constancy. The ash selected was collected from the economizer of a power plant burning coal from Kincaid Mine, Christian County, Illinois. The composition of the ignited ash was: SiOz, 37.6; &03, 16.1; Fez03,39.5; CaO, 4.5 per cent; and small percentages of compounds of magnesium, the alkali metals, and other undetermined substances. The rate of gas flow was approximately 250 cc. per minute per gram of dust. The results of the experiments are shown in Figure 1. Although the values obtained deviate somewhat from a smooth curve, the oxidation is small in all cases and apparently reaches a maximum near 400" C. (750"F.) when 1.8 per cent of the sulfur dioxide is converted to the trioxide. The gases used in the catalysis experiments were dried before coming in contact with the dust. This expedient was necessary because of the absorption of sulfuric acid vapor formed when moisture was present. I n some of the runs made with moist gases a small amount of sulfur trioxide was found in the gases from the catalyst chamber. An attempt was made to drive off any adsorbed sulfuric acid vapor by heating the catalyst to red heat in a current of air, but no additional sulfur trioxide was obtained. At low temperatures it was evident that the oxidation of sulfur dioxide was high when moist gases were used. This indicated that the catalysis by solutions of iron salts, formed when sulfuric acid condensed on the flue dust, might be much larger than that by the dry ash. This fact was verified by passing a mixture of air and sulfur dioxide containing 0.325 per cent SO2 through a solution of ferric sulfate containing 4.2 grams of iron per liter. The absorption of the sulfur dioxide was approximately 95 per cent and the absorbed gas was completely converted to sulfuric acid. The absorption and oxidation of the sulfur dioxide continued with only slight decrease in efficiency over acid concentrations up to 10 per cent H2S04. This indication of the catalytic action of dissolved iron salts on the oxidation of sulfur dioxide has an important relation to the cause of corrosion by flue gases. When deposits of ferric sulfate exist a t temperatures below the dew point of the gases, they contain large quantities of sulfuric acid formed by oxidation of the sulfur dioxide. The combined action of
621
the acid and of the oxidizing ferric ions makes such a condition exceedingly active towards steel. From the standpoint of corrosion, therefore, the total sulfur in the coal may be considered as potential corrosive material, although the actual percentage of the sulfur present as sulfur trioxide in the gases is exceedingly small. Analytical Methods
The methods proposed for the determination of small percentages of sulfur dioxide and trioxide present in gaseous mixtures mere thoroughly studied in order that the results obtained in the gas analyses could be interpreted correctly. Because of the oxidation of dissolved sulfur dioxide, the absorption of the gases in aqueous solutions, with subsequent determination of sulfur trioxide and total sulfur, always gives high results for sulfur trioxide except when an efficient inhibitor is present. This procedure was used by Sherman and by Wheeler and Ardern, and accounts for the high values of sulfur trioxide obtained by them as compared with the values quoted below. The rate of oxidation of the sulfur dioxide depends upon the extent and character of the impurities in the water that may act as catalysts.
Figure 1-Oxidation of Sulfur Dioxide in Sulfur Dioxide-Air Mixtures i n Presence of Flue Dust f r o m Illinois No. 6 Coal
The test devised to determine the extent of this oxidation in solutions made up with distilled water consisted in drawing a mixture of air and a known volume of sulfur dioxide through 50 cc. of standard 0.2 N sodium hydroxide The solution was then titrated with standard acid to the methyl orange end point, a t which the excess hydroxide is completely neutralized and the sulfite converted to the bisulfite. The solution was then treated with neutral hydrogen peroxide to convert the sulfite to sulfate and titration was continued to the methyl orange end point with standard alkali. The values obtained were as follows:
cc. SO1 taken, a t 0' C., 760 mm. SO1 found, without HlOz SO2 found, with Hz02 Oxidation
87.5 146.7 86.1
67.6%
cc. 86.8 141.7 87.7 63.3%
Other tests using the benzyl alcohol-benzidine method described below gave similar values. I n a previous paper (11) a method was described which was based on the separation of the sulfur trioxide as sulfuric acid fog by filtration through a fine alundum thimble. This method gave good results except when the concentration of trioxide was less than 0.005 per cent, when there was evidence of a large percentage error due to a slight reaction of the sulfuric acid with the thimble and t o a possible passage of some of the acid vapor through the thimble. Trouble was also encountered in clogging of the thimbles with the very fine dust from pulverized fuel furnaces. This method was used, however, for analysis of the gases a t four power plants. I n order to eliminate the difficulties and errors of the filtration method, use was made of the absorption method
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
622
mentioned by several previous workers (5, 2 , 9). The inhibitor chosen was pure benzyl alcohol. This substance was found to inhibit the oxidation of sulfur dioxide t o such an extent that no more than a trace of sulfate ions could be detected when gases containing 0.3 per cent sulfur dioxide were drawn through the absorbing solution. I n order to use this method for flue gases, it is necessary to analyze for sulfate and total sulfur to determine the sulfur trioxide and dioxide, rather than for sulfite and total sulfur, on account of the relatively small quantity of sulfur present as trioxide. Since the solutions could not be warmed or allowed to stand for several hours without danger of oxidation, even in the presence of the inhibitor, the usual gravimetric method could not be used. Table I-Determination of Sulfur Dioxide a n d Trioxide i n Gas-Air Mixtures w i t h Benzyl Alcohol Inhibitor a n d t h e Benzidine Method SL*LFUR DIOXIDE SULFURTRIOXIDE So. Taken Found Error Taken Found Error CC. Cr. Jfg. Sfg. 9 1 88.3 85 3 -3.4 19.93 20.10 '0.8 2 88.5 85.5 0.0 19.93 20.15 71.1 3 87.8 88.6 4-2 1 0.00 0.19 85 8 85.4 f3.0 31.83 33 39 41'6 a 90 9 69.4 -1 6 32.3,5 30.8'2 -4.4 f, 85.1 65.2 fO.l 30 65 31.53 +2.9 , 86 1 64 8 -1 5 29 04 28.5s -1.6 Average a1.i *2 1
cr
With a slight modification in the concentration of the reagents, the benzidine volumetric method as described in several textbooks was found to be suitable. The accuracy of the method is indicated by the results shown in Table I. In these tests the volume of air containing the sulfur dioxide and trioxide in each run was approximately 85 liters and the concentration of the gases was about that in flue gases. The method was later used for the analyses of the gases in the catalysis experiments and of the flue gases a t the fifth power plant. I n applying the method to flue gases the usual precautions for sampling flue gases should be observed. In addition, the gases should not come into contact with any metal after they are cooled below the dew point. For sampling gases in economizers and preheaters a '/2-inch (1.3-cm.) pipe of any desired length may be used. A 12-mm. Pyrex tube inserted through a packing gland at the outer end of the pipe should extend several centimeters inside the setting. Rubber connections between glass tubes may be used if the
Vol. 23, No. 6
exposed surface of the connection is small. -1tuft of glass wool should be placed before the end of the glass tube in the iron pipe to filter out any dust carried by the gas sample. The gases are drawn from the sampling tube through a 250-cc. wide-mouthed bottle containing exactly 50 cc. of standard 0.2 N sodium hydroxide and 3 cc. of pure benzyl alcohol. A sintered-glass immersion filter of medium porosity serves to break up the bubbles and bring about rapid absorption of sulfur dioxide and trioxide. The gases may be drawn by means of an aspirator and measured with a flowmeter of the orifice type. If this combination is used, it is necessary to keep the rate of flow constant by adjusting a relief valve ahead of the aspirator. The volume of gases drawn is computed from the average rate of flow and the time of sampling. In many cases, however, i t will be found more convenient to draw the gases through the absorption bottle by means of a small rotary booster pump. The exhaust of the pump is connected to a wet-test gas meter and the volume of the flue gases read directly. The size of the sample varies between 1 cubic foot (0.02832 cu. m.) for high-sulfur gases and 3 cubic feet for low-sulfur gases. The gases may be drawn as fast as 0.2 cubic foot per minute. After the sample has been drawn, the Pyrex tube is removed from the iron pipe and rinsed with distilled water into the absorption bottle. The bubbler should be removed and rinsed. The solution in the absorption bottle should be kept below 25' C. by immersion in cold water while the sample is being drawn. This precaution was found necessary on account of a slight oxidation of the dissolved sulfur dioxide in warm solutions, even in the presence of the inhibitor. It is also best not to allow the solution to stand more than 3 hours after sampling before titration. The first titration is made with standard 0.2 N hydrochloric acid to the end point of bromocresol green, a t which the excess hydroxide is neutralized and the sulfite is converted to bisulfite. The volume of sodium hydroxide neutralized by the SO2 SO3. The solution is then gases is equivalent to treated with 10 to 15 cc. of benzidine hydrochloride solution made up as follows: 30 grams of benzidine are shaken with 30 cc. of water; the suspension is then diluted with water to 960 cc. and 40 cc. of concentrated hydrochloric acid are added, which should completely dissolve the benzidine. A silky precipitate of benzidine sulfate forms a few seconds after the
+
Table 11-Composition of F l u e Gases a t Plant A COALANALYSIS. "As RECEIVED" BASIS
%
Oxygen.. . . . . . . . . Nitrogen... . . . . . . Sulfur..
Moisture.. . . . . . . 1 4 . 5 Carbon.. . . . . . . . . 5 5 . 6 Hydrogen . . . . . . 3 . 8 6 BOILER 1
2
POINTOF SAMPLING First pass of boiler First pass of boiler First Dass of boiler First pass of boiler First pass of boiler First pass of boiler First pass of boiler First pass of boiler First pass of boiler Second pass of boiler Second pass of boiler Economizer outlet Economizer outlet Induced fan inlet
.........
DISTANCE FROM: Front Side Feet Feet 4 5 4 10 6 10 2 5 2 10 5 4 10 4 5 6
6
10 10 2 2
..
4 4 6 6
10 5 10 5 10 2
5 10 5 10 5
10 10 10 Second pass of boiler 20 5 Economizer outlet 2a 10 Economizer outlet 20 15 Economizer outlet Averaee From top of duct between economizer and preheater. b All percentages by volume. e In all tables ratios are by weight.
%
Thermal value. . . . . . . . . . . . . 10,100 B. t. u. per Ib. Fusion point of ash . . . . . . . 1085' C.
7.01 1.13 4.45
NO.
SAMPLES 2 2 2 2 3 3 3 4 2 3 3 3 2 6 2 3 2 3 3 3 3 3 3
SO2 %b
SO3
%
%
0.289 0,313 0.300 0.295 0.347 0.384 0.373 0.279 0.328 0.268 0.346 0.270 0.274 0.100 0.156 0.197 0.207 0.319 0.268 0.274 0.240 0.272 0.817 0.279
0.0060
8.6 7.8
0.0086 0.0122 0.0060 0.0018 0,0030 0.0143 0.0123 0.0148 0.0154 0.0126 0,0050 0.0050 0,0019 0.0016 0.0059 0.0035 0.0086 0.0035 0.0152 0.0071 0.0068 0.0079 0.0082
0 2
so .. ..
4.1 , .
.. .. 9:0 9.5 15.6 10:s 11.8 7.5 9.3 9.0 11.4 9.7 8.0
..
COI
% 10.8 11.6 11.5 12.6 14.2 13.1 15.0 12.1 12.8 9.6 13.0 10.9 10.0 4.4 5.4 8.0 7.8 11.5 10.1 10.6 8.6 10.2 11.7 10.7
0.073 0.074 0.072 0.064 0.067 0.079 0.069 0.064 0.071 0.078 0.074 0.068 0.075 0.063 0.078 0.075 0.073 0.074 0.072 0.073 0.073 0.073 0.074 0.072
2 0 2.7 3.9 2.0 3.3 0.8 3.8 3.9 4.3 5.5 3.6 1.8 1.8 1.9 1.2 2.7 1.6 2.6 1.4 5.3 2.9 2.4 2.4 2.8
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
June, 1931
solution is added. The solution is allowed to stand for about 10 minutes and is then filtered by suction. The precipitate is washed four times with 5 cc. of water each time. Care should be taken not to use too much water, because the sulfate is slightly soluble. The precipitate and filter paper are then placed in a flask containing 50 cc. of water and heated to boiling. The suspension is titrated with 0.02 N sodium hydroxide using phenolphthalein as the indicator. The benzidine sulfate precipitate dissolves rather slowly, so that care must be taken to see that it is all dissolved. A good procedure is to add sufficient sodium hydroxide that the red color of the basic solution remains after boiling for 3 minutes. The solution is then cooled and titrated to the end point with 0.02 N acid in the usual manner. The calculations are made as follows: Per cent SOSby volume =
determinations were used for establishing the minimum operating temperatures. They also illustrate the reactions of the sulfur compounds in a furnace burning fuel containing sulfur in a form distinctly different from that in coal. The data for the sulfur composition of the flue gases are shown in Tables I1 to VI. Each value represents an average of all the analyses performed for any one point in the gases. I n most cases three samples were drawn from each point. In order to eliminate errors due to any variation in excess air, and also, for the most part, those due to stratification in the gases, the sulfur-carbon ratio by weight is shown. These values for any two points in a boiler and for any two plants are directly comparable. From them also the approximate fraction of the sulfur in the fuel passing into the gases may be obtained as follows: Ratio S:C in gas = fraction of S in gas Ratio S:C in fuel
cc. 0.02 N NaOH X 0.000791 273 pb - P w
“760
Per cent SO1 by volume =
~ r ( ~ ~ . ~ ~ - cc. 0.2 N HC1) Pa - fim
v x 7 6 0
t1-273
cc. 0.02 N NaOH X-
to
In
1
The extent of the oxidation of the sulfur in the gases was also determined by calculating the percentage of the sulfur existing as sulfur trioxide.
o.oo,91
Table 111-Composition of Flue Gases at P l a n t B COALANALYSIS, “As RECEIVED” BASIS % Thermal value.. . . 11,280 Moisture. . . . . . . . 9.2 B. t. u. per Ib. Carbon, 67.7 Fusion point of a s h . . 1090’ C. 5.22 Hydrogen. Oxygen. .. , . . . . . . 11.82 1.40 Nitrogen. . . . . . . . . Sulfur . . . . . . . . . . 3.18
273 i- 273
where V = volume of gas drawn, cu. f t . Pb = atmospheric pressure, mm. Hg - vapor pressure of water a t temperature of meter temperature of meter, ’ C.
$
623
......... .......
:
The values may be converted to grains per cubic foot as follows:
POINTOF NO. SAMPLING SAMPLES SOY
Grains SO2 per cubic foot = per cent SO2 X 12.48 Grains SO8 per cubic foot = per cent SO3 X 15.60
Economizerinlet Economizer outlet Average
6
6
SO3
COT
Oz
%
%
%
%
0.108 0.0914 0.100
0.0032 0.0025 0.0028
11.4
7.8
11.4
7.8
. . . . .
RATIO 5 AS S:C SO3
0.026 0.022 0.024
% 3.0 2.7 2.9
Analyses of Flue Gases
The slight oxidation of sulfur dioxide in flue gases as shown by the gas analyses agrees with that found in the laboratory experiments on catalysis. The maximum per cent of coal sulfur converted to sulfur trioxide was 5.5, while the average for the four plants was 2.2 per cent. The data also show that there is no increase in the extent of the oxidation as the
A survey was made of the sulfur content and distribution in the flue gases a t five power plants. The plants selected were representative of modern central-station design and operation. The sulfur content of the coals a t four of the plants (A, B, C, and D) varied from 2.1 to 3.8 per cent.
Table IV-Composition of F l u e Gases at P l a n t C COALANALYSIS,“AS RECEIVED’’ BASIS
........
%
13.2 Moisture. Carbon. . . . . . . . . . 63.9 Hydrogen. . . . . . . . 4.33 POINTOF SAMPLING
Oxygen.. . . . . . . . . Nitrogen.. . . . . . . . Sulfur.. . . . . . . . . .
DISTANCE FROM: Front Side Feel Feel
No. SAMPLES
In superheater loop 3 5 In superheater loop 3 10 12 5 In second pass of boiler 12 10 I n second pass of boiler 2” 4 Economizer outlet Economizer outlet 2” 8 Average a From top of passage between economizer and preheater.
%
Thermal value.. . . . . . . . . . . . 11,450B. t.u. per Ib. Fusion point of a s h . . . . . . . . 1205’ C .
6.22 1.33 2.1
RATIO
so1
sot
COI
01
%
%
%
%
0.117 0.131 0.194 0.148 0.038 0.098
0.0049 0.0006 0.0005
13.3 14.5 14.2 13.6 5.8 13.4 12.5
6.0 4.9 50 6.0 14.8
0.121
The other plant, E, used as part of the fuel the residues from a near-by petroleum refinery. The gas analyses a t this plant are particularly interesting because the sulfur in part of the fuel was present almost entirely as sulfuric acid, which a t times amounted to 25 per cent of the residue. The petroleum residues consisted of pulverized coke, acid lubricating sludge, acid tar, fuel oil, soda bottoms, neutral sludge, and wax tailings. Practice has shown that certain combinations of these cannot be fired together. I n all cases natural gas constituted the main portion of the fuel. This plant was designed to operate with the highest boiler efficiency attainable without trouble from corrosion in the economizers and preheaters. The results of the gas analyses and dew point
0.0004
0,0009 0.0019 0.0015
n 7
7.1
s:c
s AS SOa
% 0.0233 0.0281 O.OBR.5 ......
0.0290 0.0180 0.0196 0.0268
4.1 0.5 0.3 0.3 2.2 1.9 1.6
temperature of the gases decreases and, therefore, furnace reactions are the only ones of importance as far as the formation of sulfur trioxide is concerned. It was noticed that the extent of the oxidation was practically independent of the oxygen content of the gases and of the location of the sampling tube in respect to slag-covered walls or tubes. A comparison of the data from plants A and D, where the same coal is fired, in the former case chain-grate stokers being used and in the latter pulverized fuel burners, shows that less sulfur trioxide is produced in a pulverized-fuel furnace than in a stoker-fired furnace. The reason for this is not entirely apparent, but it may be assumed that the smaller percentage of excess air and the larger zone in which carbon
624
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
POINTOF SAMPLING First pass of boiler First pass of boiler First pass of boiler First pass of boiler Economizer outlet Economizer outlet Economizer outlet Induced fan inlet Average
Table V-Composition of Flue Gases a t Plant D, Pulverized Coal analysis, same as at Plant A DISTANCE FROM: No. Front Side SAMPLES SOI SOa On Reef Feet % % % 4 5 0.414 0.0046 5.0 10 4 0.387 0.0027 4.0 8 5 0.421 0.0032 5.9 10 S 0.453 0.0034 3.6 5 0.479 0.0036 5.0 10 0.416 0.0032 6.3 5 0.347 0.0020 8.2 4 0.385 0.0028 7.6 0 413 0.0032 5.7
monoxide and hydrogen are present ( I O ) in the former tend to prevent the oxidation. The quantity of sulfur trioxide in flue gases is no more than that equivalent to the sulfate in the coal. It is doubtful, however, that the sulfates are the only source of the trioxide, first, because the decomposition of the sulfates must take place in the fuel bed where the carbon would bring about reduction to sulfur dioxide, or perhaps to the sulfide; and second, because the flue gases from the combustion of natural gas alone contain a very small amount of sulfur trioxide. From the standpoint of physical chemistry, it is not surprising that the oxidation of sulfur dioxide in flue gases is small. Application of the equilibrium constants for the reaction given by Bodenstein and Pohl (3) shows that no increase in the ratio of sulfur trioxide to dioxide above that found to exist can take place above 1000" C. (1832" F.). The interval of time during which the gases are cooled from 1000" C. to stack temperature is less than 2 seconds. Thus, the approach to equilibrium ratios for this slow reaction is not probable. The sulfur-carbon ratios indicate that about 70 per cent of the sulfur of the coal enters the gas when fired on a stoker and that this is increased to about 90 per cent when the coal is fired in the pulverized form. The data in Table VI show that the extent of the reverse reaction-i. e., the reduction of sulfuric acid vapor to sulfur dioxide-approaches the same ratio of sulfur trioxide to dioxide as was found in the burning of coal sulfur. Approximately 85 per cent of the sulfur in the gases exists as sulfur dioxide. The sulfur trioxide concentrations are only slightly higher than the corresponding values in flue gases from highsulfur coal. The reducing reaction evidently takes place in the furnace where the high temperatures, together with carbon monoxide, hydrogen, and free carbon, react readily with sulfuric acid vapor. Whether further reduction of the acid can be brought about by favorable changes in furnace design so that flue gases with lower sulfur trioxide content will be produced is a question worth consideration. Importance of Sulfur Trioxide in Flue Gases
The small percentages of sulfur trioxide in flue gases have a large effect on the properties of the gases. Serious corrosion
PLANT
DESCRIPTION
Vol. 23, No. 6
Fuel Boiler
s:c
so3:sO;
RATIO
s AS
0.078 0.068 0.085 0.078 0.090 0.086 0.085 0.087 0.082
0.011 0.007 0,008 0.007 0.008 0.006
1,-1 0.7 0.8 0.7 0.8 0.6 0.6 0.7 0.8
RATIO
cor
%
14.3 15.3
13.3 15.6 14.2
12.9
11.1 11.8 13.6
0.006
0.007 0,008
is found only when the temperature of the metal is below the dew point of the gases. Three conditions affect the condensation temperature-the partial pressure of water in the gases, the partial pressure of sulfur trioxide, and the vapor pressure of saturated solutions (hygroscopicity) of the material on the surface of metal. Direct measurements of the dew point of flue gases made by means of an electrical device (8) showed that condensation on a clean metal surface may take place as high as 80-100" C. when the gases contain as little as 0.015 per cent sulfur trioxide. High moisture introduced by leaks, by steam soot-blowers, or by fuels with high hydrogen or moisture contents, cause high dew points with consequent corrosion at higher temperatures. While the exact effect of deposits containing ferric salts on the dew point is not known, a n increase in the condensation temperature of 20" to 30" C. above that on clean metal surfaces may be expected. Table VI-Dew Point a n d Composition of Flue Gases a t Plant E All samples drawn from between economizer and preheater, 4 feet from side wall FUEL
NO. SAMPLES
Acid sludge, natural gas Sodabottoms,naturalgas Acid tar. natural cas Acid tar, soda Kottoms, natural gas Natural gas alone Fuel oil alone
DEW POINT
sot
so3
OC.
%
%
%
Con %
RATIO S
3
56 63 64
0 145 0 0240 10.5 0.044 0 034 0 0073 10 5 0 008 0 159 0 0101 11.2 0 040
1 3 3
66 61 46
0.108 0.0130
0,031 0.0018 0,011 0.0003
AS
s:c so3
8 4
% 14 3 17 9 6.0
9 . 6 0 009 5.4 9 . 3 0,003 2 . 4 1 1 . 5 0 , 0 2 8 10.6
Sulfur Content of Fly Ash and Deposits
The composition of the fly ash carried by the gases varies with the coal, the method of firing, and the source of the sample. Table VI1 shows the analysis of several representative samples collected a t various plants. When the dust was taken directly from the gases of a stoker-fired furnace and analyzed immediately, a considerable quantity of sulfide sulfur was found. On standing, this was rapidly oxidized t o sulfate. Sherman and Rice ( I O ) have noted the greater proportion of iron and sulfur in deposits of flue dust on dust-sampling tubes as compared with the composition of ash-pit refuse. They interpreted this as being due to a selective adherence of the particles containing large percentages of iron and sulfur. The low agglomerating temperature of iron sulfide particles
Table VII-Composition of Fly Ash a n d Deposits IGNITION LOSS" Si02 ~TOISTURE SOURCE
5%
SO8
%
% I
S~JLFID~;
s AS s
..
%
%
11.66 0.096 16.37 10.01 ... ... Dry Economizer outlet Fly ash 9.25 0 148 .. 5.28 17.77 , . . ... Economizer outlet Fly ash Dry 12.00 0 088 9 . OA 1 9 . 4 2 Economizer outlet Fly ashb Dry 37.58 i:is . . 7.67 14.36 13 00 19:30 Boiler outlet Hard deposit Dry 16.10 2.86 26.15 10.25 23.96 17.60 2.62 Economizer outlet Ash 11.04 3.64 11.42 37.54 29 83 ... 0.40 Boiler tubes Slag 1.65 4 1 . 1 2 16 88 4 . 4 1 31.46 0.25 Economizer inlet Ash B 1.86 2:ao 42.70 16.78 2.86 29.22 5.20 Economizer outlet Ash 38.50 . . . 0 . 6 0 2 09 33.91 1 92 . . . 13.37 Economizer Hard deposit ... 2.58 6.66 32.51 10.65 38.96 6.48 0.93 Economizer outlet Ash C 26.39 ... 0.66 11.94 16.24 18.10 45.92 4.08 Economizer Hard deposit 2.81 ... 6 02 44.19 4.84 32.87 5.50 1.71 Preheater outlet Fly ash D 7.92 1 . 6 1 21.61 40 PO 15.87 9 . 1 4 6 . 2 2 Economizer Hard deposit 7.71 5.28 27.33 13.11 40 89 3.51 0.19 Boiler Slag 8 .40 . . . 4 . 7 3 4 3 . 7 8 3 02 37 07 0. 5s 0.13 Superheater Slag 20.52 ... 2.94 38.86 6.03 20.13 3.10 2.10 Preheater Hard deposit iroii siilfate. sulfuric acid. Loss on ignition represents the carbonaceous material as we!l as other volatile constituents such as sulfur trioxide present as or aluminum sulfate. b Average of 19 samples collected direct from flue gases. A
I . .
INDUSTRIAL A N D ENGINEERING CHEMISTRY
June, 1931
compared with that of the silicates indicates that these unoxidized sulfides may be responsible for the hard deposits in economisers and preheaters. Because the temperatures here are low compared with those in the boiler the sulfur remains after oxidation as sulfate. A large part of the sulfur in these deposits, therefore, comes from the ash particles, and not from condensation of sulfuric acid vapor in the gases. Only traces of sulfide sulfur in pulverized-fuel ash were found and the deposits in the economizers and preheaters, for the most part, are not adherent as in stoker-fired boilers. This may be attributed to the more complete oxidation of the sulfur in pulverized fuel. Acknowledgments
This investigation was conducted under the supervision D. B*Keyes, head Of the Division Of Industria’ to whom the author expresses his appreciation for many kind Of
62 5
suggestions and criticisms concerning the work. Acknowledgment is also made to L. F. Dobry for assistance in the laboratory work on catalysis. Literature Cited (1) Ardern and Wheeler, Interim Report, Ministry of Transport, London, 1929. ( 2 ) Derl, Chcm.-Zfg.,46,693 (1921). (3) Bodenstein and Pohl, Z.Elcklrochcm., 11, 373 (1905). (4) Fichter and Shaffner, Helo. chim. acta, 3, 869 (1920). (5) Haller, J . SOC.Chem. I n d . , 38, 52 (1919). (6) Holmes, Ramsay, and Elder, IND. ENG.CHEM.,21, 850 (1929). (7) Huttia and Lurman, Z . anaew. Chem., 89, 759 (1926). (8) Johnstone, University of Illinois Eng. Expt. Sta., Circ. 20 (1929). (9) Ries and Clark, IND. ENC. CHBM.,18, 747 (1926). (10) Sherman et al., Bur. Mines, Bull. 834 (1931);M d . En&, 48, 1115, 1389 (1926); see also Barkley, Bur. Mines, Tech. P o p n 486 (1928). (11) Taylor and Johnstone, IND. ENG.CHRM.,Anal. Ed., 1, 197 (1929). (12) Thompson and Tilling, J . SOC. Chem. I n d . , 48, 37 (1924).
Heat Transfer to Liquids in Viscous Flow’ C. G. Kirkbride2and W. L. McCabe DEPARTYEKT O F CHEMICAL ENGINEERING, UNIVERSITY
OF
MICKIGAN, ANN ARBOR,Mxcn.
New data are reported on the transfer of heat into The mechanism of such flow NDER ordinary conliquids flowing at-velocities below the isothermal critiis the same as that of natuditions] when heat is cal velocity. These and other data are correlated by ral convection, and the two t r a n s m i t t e d from a the Nusselt-Grober theory, which expresses the film processes are controlled by solid surface to a non-boiling coefficient in the form: the same variables. Therfluid that is in forced flow, mal t u r b u l e n t flow apparthe v e l o c i t y of the fluid is greater than the correspondently forms a lower limit to where h = point film coefficient, D = tube diameter, t h e v e l o c i t y range correi n g i s o t h e r m a l critical veL = tube length, k = thermal conductivity, Y = mass locity as determined by Reysponding to viscous flow, just velocity, and c = heat capacity. The experimental as viscous flow sets a lower nolds’ criterion. As a result results are compared with the theoretical calculations many able i n v e s t i g a t i o n s v e l o c i t y limit to turbulent of Nusselt and Grober. have c e n t e r e d on this case, flow. AS shown by Colburn and Hougen, it is questionbut v e r y l i t t l e work h a s been reported on heat transfer in the analogous case able whether or not thermal turbulent flow is of much signifiwhere the fluid moves a t a velocity below the isothermal cance in horizontal flow, where the movement of fluid due to critical velocity. This paper reports new experimental data natural convection is a t right angles to the forced flow. on the latter subject, and correlates these and other existing Nusselt-Grober Theory data with the Nusselt-Grober theory of heat transfer to fluids in viscous flow. Nusselt (9) carried through a theoretical solution of the following problem: Assume a fluid flowing through a horiExperimental and Theoretical Results to Date zontal tube a t a constant mass velocity (below the isothermal critical) and a t a constant entrance temperature. Let the Colburn and Hougen (Z), in a paper on the flow of fluids a t fluid be cooled by maintaining the tube wall a t a constant low velocities, have described the work previous to their temperature. In order that entrance effects may be elimiown covering the problem of this article. I n addition to the nated] assume that the fluid has passed through a “calming references cited by them, it should be noted that some of section” before reaching the cooling surface. the low-velocity data on heat transfer t o oils determined by (1) How will the mean fluid temperature vary along the hlorris and Whitman (8) fall in the range of viscous flow. tube? These data will be mentioned later. How much heat will be transferred through a given length Colburn and Hougen ( 2 ) used a vertical 3-inch tube 7 feet of (2) tube? long. Their experiments were confined to water flowing a t (3) How will the film coefficient vary with the tube length very low velocities. These investigators were studying the and velocity and properties of the fluid? mechanism of heat flow from vertical walls to fluids exhibiting In his attack of the problem Nusselt makes these assumpthe type of flow described by Colburn and Hougen as “thermal tions: turbulent.” Under such conditions the mean velocity of (1) The tube wall is smooth. the fluid is of minor consequence, and the only effective motion (2) The flow is viscous, and the velocity distribution over of the fluid is that set up by natural convection due to the any cross section is a paraboloid of revolution, as given by the differences in density caused by the temperature gradient. eauation
U
1 Received
February 2, 1931. Present address, Standard Oil Co. (Indiana), Whiting, Ind.
