The Combustion of Gaseous Fuels. - Industrial & Engineering


The Combustion of Gaseous Fuels. George F. Moulton. Ind. Eng. Chem. , 1923, 15 (6), pp 583–587. DOI: 10.1021/ie50162a011. Publication Date: June 192...
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June, 1923

INDUSTRIAL AND ENGINEERING CHEMISTRY

are: (1) pressure distillate (crude gasoline), ( 2 ) residuum (fuel oil), (3) carbon (usually coke formation used as fuel), and (4)uncondensable gas (also used as fuel). The pressure distillate is sulfuric acid-treated, water-washed, caustic-soda-treated, and then steam-distilled into marketable gasoline, kerosene, and gas-oil bottoms. There are no difficulties involved in producing water-white, sweet-odor, cracked gasoline commercially. The residuum oil resulting from the cracking reactions is a highly desirable fuel, for it contains more B. t. u. to the gallon than the charging stock from which it was produced, whether the charging stock is gas oil, fuel oil, or heavy crudes.

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This residuum (cracked} has a low cold test and viscosity and has many advantages over an ordinary fuel oil. The carbon formed is usually of a black honeycomb structure, shot through with pitchy hydrocarbons and quite dry, having a high B. t. u. content per pound, and is suitable for use under the stills and boilers, preferably mixed with lowgrade coal as it gives a very hot fire. The uncondensable gas ranges from 1100 to 1200 B. t. u. per cu. ft., showing an unsaturated hydrocarbon content of 8 per cent, hydrogen 4 per cent, and paraffin hydrocarbons of 87 per cent, although these values vary widely as a function of the type of oil being cracked.

T h e Combustion of Gaseous Fuels’ By George F. Moulton BUREAU OF

STANDARDS, WASHINGTON,

Gas fuels commonly distributed for domestic and industrial purposes vary in heating value from 450 to 1100 B. t . u . per cu. f t . , in specific graoity from 0.35 to 0.70. and the distribution pressures range from 2 to 12 in, of water pressure at the consumer’s appliance. Manufacturers of gas appliances are compelled to make those which will operate over these wide omiations in heating value, specific gravity, and pressure. The present appliances are largely the product of practical experience, because little has been known of the relation of these various factors as applied to the design of burners. The knowledge of correct adjustment of appliances is very vague, and modification of existing standards has been made dificult because the effect of these changes on quality of service, eficiency of

D. C.

utilization, and safety of operation has been dificult to determine. The quality of the service that can be secured with different gases depends primarily on the characteristics of the flame. This paper shows, for one type of appliance, how the flame characteristics are modified by a change of injection of primary air. which may be caused by variations in gas rate, pressure, composition of gas, etc. The data and figures that are here presented have been gathered from extensive investigations made by the Cas Engineering Section of the Bureau of Standards on the design of atmospheric-gas burners.2 and an investigation made for the Public Service Commission of Maryland to determine the proper heating-value standard for the City of Baltimorc3

EFFECT OF PRIMARY AIR ON FLAME CHARACTERISTICS to completely burn the gas, the velocity of combustion increases less rapidly, and the cone will lengthen, as is shown HE amount of air required for complete combustion by the cone-height curves in Fig. 1. of any gas increases with increase in the heating value. EFFECTOF CHANGESIN COMPOSITION OF GAS ON THE For water gas of 300 B. t. u. per cu. ft. the volume of FLAME CHARACTERISTICS air theoretically required for complete combustion is about 2.5 cu. ft.; for 400 B. t. u. water gas, 3.4 cu. ft.; for 500 A great deal has been said by practical gas men concerning B. t. u. water gas, 4.5 cu. ft.; and for 600 B. t. u. water gas 5.5 cu. ft. are required. In the case of 525 B. t. u. coal the incompleteness of combustion of the illuminants in gas 4.6 cu. ft. are required, while natural gas, in the Pitts- manufactured gases of high heating value, which is claimed to burgh district, of about 1080 B. t. u. per cu. ft. requires 10.3 result in lower utilization efficiency. There appears to be cu. ft. of air. Incidentally, a convenient rule for rough cal- very little basis for this argument, as will be shown by a culations of theoretical air required for combustion of gas study of the flame characteristics of these gases and the of any heating value is to take the heating value of the gas, analyses of the products of combustion. The type of flame required for different processes will vary point off two places, and subtract 0.50. These tests with domestic range burners have shown that considerably, but for ordinary domestic range burners the the flame height continually decreases as the ra;tio of primary flames may be roughly classified into three types-soft air to gas is increased. The inner-cone height also decreases, flame, normal flame, and hard flame. From practical exbut above a certain air-gas ratio the cones begin to lengthen perience the appliance men have discovered that with a again. This lengthening of the cone is more pronounced very soft flame there is danger of blackening the utensils, with water gas of 400 B. t. u. than with water gas of higher and with a hard flame the burners are likely to flash back heating value. The surface of the cone niarks the beginning when operated a t the smaller gas rates, or when the gas of combustion, and the height of the cone depends on the pressure is reduced. The result is that most burners are ratio of the mixture, the velocity of the mixture, and the rate adjusted to what is called a normal flame. A careful study of combustion of the gas. When the air-gas ratio is increased of the adjustments made by several gas-appliance experts from a low value, the increase in the velocity of combustion showed that a normal adjustment for a typical, standard-size is a t first greater than the increase of the velocity of the gas-range burner resulted in a consumption of about 9000 mixture through the ports, and the height of the cone is de- B. t. u. per hr. and the air adjustment was such that the creased. As the primary air approaches the amount required mixture in the burner was practically constant a t about 175 B. t. u. per cu. ft., irrespective of whether the heating 1 Presented before the Section of Gas and Fuel Chemistry a t the 64th

T

Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922.

2

Bur Standavds, Tech. P a p e r 193.

3

Report now in press.

INDUSTRIAL A N D ENGINEERING CHEMISTRY

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J

.5

/

2

a

3

R~+io:

3 Primary Air

Vol. 15, No. 6

\I

4

Gus FIG.1 - c O N E

AND

FLAME HEIGHTS WITH

AIR-GASRATIOSWHEN REGULAR STAR BURNERIS OPERATED C U . FT. AT RATESOF 7000, 9000, AND 11,000 B. T. U . PER HR.

VARYING

value-of the gas was 400 or 600 B. t. u. per cu. ft. By B. t. u. per cu. ft. of mixture is meant the British thermal units contained in a cubic foot of primary air and gas within the burner. With water gas, the yellow tips appear at about 250 B. t. u. per cu. ft. of mixture, and for the very hard flame, which occurs near the condition where the flames tend to leave the ports, it is about 150 B. t. u. per cu. ft. of mixture. These values will vary slightly with the design of burner being used. In Fig. 2 are shown graphically the cone heights for 400, 500, and 600 B. t. u. water gas, when the gas was burned in a typical gas-range burner. The values are shown for three conditions of adjustment representing 150, 200, pnd 250 B. t. u. per cu. ft. of mixture. These three conditions of adjustment, covering the range in consumption from 7000 to 11,000 B. t. u. per hr., represent the maximum variation that would be found in adjustment of burners of standard size. A rate of 7000 B. t. u. per hr. represents the average rate of consumption over the whole cooking period, and the curves show that a t this rate the difference in cone height is very small. The difference in flame height is a little greater than the difference in cone height, but it is not so great that it would be noticed with the present types of burners in use. An observation of the flame characteristics of the different gases does not, therefore, show any marked differences that might be expected to result in pronounced differences in utilization efficiency.

EFFECTOF CHANGEIN SPECIFIC GRAVITY,GASRATE, AND PRESSURE ON THE INJECTION OF PRIMARY AIR The original work2 contains a very complete discussion regarding the effect on air injection in burners when the

WITH W A T E R

GAS O F 500 B. T. U.

PER

specific gravity, gas rate, and pressure are changed. The results of that work are summarized here very briefly and show what effect this variation in primary air has on the flame characteristics and consequently on burner operation. The variations in air injection for changes in specific grayity and gas rate are shown clearly in Fig. 3. These curves show that the heavier gas has a greater injecting power, and the relative volume of air injected is shown by the ordinates. If the specific gravity of the gas is changed and no adjustment is made in the orifice, the rate of flow through the orifice will vary inversely as the square root of the density. The air injection will then correspond with the values given for this gas rate. The way in which the ratio of air injection changes with change in gas rate is apparent from these curves. Fig. 4 shows the effect of change in gas pressure on air entrainment. flumerous tests have shown that without any change in the orifice allowing the gas rate to change with a change in pressure, the air-gas ratio remains almost constant over the usual range of distribution pressure (from 2 to 6 in.). I n Fig. 5 is shown a graphical illustration of tests made with a range burner of regular size. Water gas of 550 B. t. u. per cu. ft. requires about 5 cu. ft. of air for complete combustion, and the curves show that within a very narrow range in gas rate it is possible to operate the burner above the theoretical mixture. At the smaller gas rates the flame has a tendency to flash back into the burner, which occurs a t a definite ratio of air and gas. At the larger gas rates if too much primary air is injected the flame will leave the burner ports. At small ratios of primary air injection the flames will burn with a yellow tip. I n this particular burner a t normal

INDUSTRIAL A N D ENGINEERING CHEMISTRY

June, 1923

rates of consumption more than one volume of air to one of gas is required to remove the yellow tip from the blue inner cone.

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The method of analysis is based on the reaction of carbon monoxide with iodine pentoxide. The gas sample is drawn through the apparatus by suction, and passes successively through absorption towers containing hot chromic acid, sodium hydroxide solution, sulfuric acid, and then through U-tubes containing, respectively, granular sodium hydroxide, phosphorous pentoxide, and alternate layers of iodine pentoxide and glass wool heated by means of an oil bath to 155' C. The liberated iodine is absorbed in a solution of potassium iodide and titrated with a standardized solution of sodium thiosulfate. From this titration the amount of carbon monoxide in the products of combustion as sampled is readily calculated. TABLE I-CARBON

MONOXIDE I N THE PRODUCTS OF COMBUSTION FROM 500 AND 600 B. T . U. W A T E R GAS (Regular-Front Star Burner) Dis--CARBON MONOXIDE PRODUCED-CU. FT./HR.-tance Soft Flame of Uten- Hard Flamea Medium Flameb Yellow Tipc B. t. u . si1 from 500600500600500600Delivered Burner B. t. u. B. t. u. B. t. u. B. t. u. B. t. u. B. t. u. Gas Gas Gas Gas Gas per Hr. In. Gas 0 0.05 0.05 0.17 0.20 0 7,000 0 0 0 Trace Trace Trace (1.25 0 0 0 0 0 0 0 0 0 1.50 0 0 0 0.07 0.15 0.11 0.08 0.17 0.20 0 0 0 Trace 0 Trace 9,000 'I 1 . 2 5 0 0 0 0 0 0 (1.50 0 0 0 0 0 0 (0.75 0.29 0.22 0.26 0.24 0.27 0.18 Trace Trace Trace Trace Trace 1.00 0 11,000 { 1.25 0 0 0 0 0 0 L1.50 0 0 0 0 0 0 a 150 B. t. u. per cu. f t . of mixture. b 200 B. t. u. per cu. ft. of mixture. c 150 B. t. u. per cu. ft. of mixture.

p;: E:;:

Consumption

- Thousands o f Bfu Perflour

HEIGHTSWITH 400, 500, AND GAS AT 250 B . T . U . PER C U . FT. OF MIXTURE WITHIN THE BURNER (SOFTFLAME, YELLOW TIP), 200 B. T. u. PER Cu.FT.O F MIXTURE(MEDIUM FLAME), AND 150 B T U. PER CU.FT. O F MIXTURE (HARDFLAME) F I G . 2-COMPARISON

600 B. T .

OB C O N E

U. W A T E R

COMPLETEXESS OF COMBUSTION WITH FLAMES OF DIFFERENT CHARACTERISTICS Good flame contact is essential for high efficiency in many operations, and the question of correct location of the burner in relation to the object being heated becomes an extremely important one. Tests on natural-gas burners have shown that by raising the gas burner from a distance of 2.5 in. to within 1in. of the vessel, the efficiency has been increased from an average of 28 per cent a t the lower position to about 45 per cent a t the higher position. However, where the products of combustion are allowed to escape into the room, as is the case with most domestic appliances, it is important that the burrier be placed so that carbon monoxide is not produced in dangerous quantities. The question naturally arises whether the formation of carbon monoxide is the same with each kind of gas, when the appliance is operated at the best adjustment for each gas. If the amount of carbon monoxide produced is found to be less with any one gas, then the distance between burner and utensil could be reduced, resulting in an increase in efficiency. I n the initial work an apparatus of the type used by the Bureau of Mines for analyzing mine-air samples was used, but this method was not entirely satisfactory. The iodine pentoxide method of analysis was finally adopted, with the apparatus arranged as shown in Fig. 6. This has proved exceedingly reliable and satisfactory. The general arrangement of the apparatus is practically the same as that used by Larson and White, who did considerable work on the quantitative determination of carbon monoxide while in the Chemical Warfare Service.

JO

m

Rdr

-

50

60

Cubic Feef per H e w

70

80

90

FIG.Z-CURVES SHOWING EFFECT OF C H A N G E O F SPECIFIC GRAVITY OB GAS ON RATIOOF PRIMARY AIR TO GAS INJECTED INTO THE BURNER

FIG. CURVES SHOWING EFFECT OF CHANGE OF GAS PRESSURE ON RATIO OF

PRIMARY AIR TO GAS INJECTED

INTO THE

BURNER

IA'D USTRIAL A N D ENGIhrEERING C H E X I S T R Y

586 ThOUJOn&

(550 Biu Fer Cubic Foof)

of i3tu Per Hour

Vol. 1.5, No. 6

of burners. The contact of the flame with the utensil 'was varied by decreasing the distance between the burner and the utensil from 1.6 to0.76 in. At the positions where carbon monoxide was found it was observed that the utensil was in contact with the blue inner cones of the flame. The tests show conclusively that when the burner was operated at the same B. t. u. rate, position of burner, and B. t. u. per cu. ft. of mixture, the combustion was equally good with 600 B. t. u. 0 as it was with 500 B. t . u. gas.

CONCLUSION

Rote FIG. 5-CURVES

Cubic f e e t Per Hour

SHOWING PRIMARY

AIR-GAS RATIO- O B T A I N E D WITH AND REGULAR-SIZE RANGE

W A T E R GAS O F

550 B .

B U R N E R AT THE

DIFFERENT CONDITIONS OF

T. U .

PER

C U . FT.

OPERATION

I n Table I are given the results of tests of the products of combustion for carbon monoxide when water gases of 500 and 600 B. t. u. per cu. f t . were consumed in a star burner of standard size. Star burners are most widely used. Tests were made at rates of consumption of 7000, 9000, and 11,000 B. t. u. per hr. The character of the flame was also changed. Tests were made at 160, 200, and 250 B. t. u. per cu. ft. of mixture, representing the maximum variation in adjustment

F I G . 6-APPARATUS

The figures presented in this paper show how the injection of primary air in gas burners is modified by variation in the gas rate, pressure, and specific gravity, resulting in different types of flame. A comparison of the gases of different heating value shows that the flame and cone heights of 600 B. t. u. gas are only slightly greater than those of 400 B. t. u. gas. Therefore, they should give almost identical results with domestic appliances. An analysis of the products of combustion from 500 and 600 B. t . u. gases, when each was burned under the best conditions of adjustment for each gas, showed equal completeness of combustion. These observations of the flame characteristics and the completeness of combustion with gases of different heating value show, independently of any calorimetric determination, that when an appliance is supplied with the same number of British thermal units per hour practically the same service should be secured.

USED FOR ANALYZING PRODUCTS O F COMBUSTION OF C A R B O N MONOXIDE

June, 1923

IKDUSTRIAL A N D EKGIXEERISG CHEMISTRY

In this paper the principal consideration has been the relative results that may be secured with flames produced from different gases, without consideration of how these data may influence the design of appliances. A number of manufacturers are already making use of this information and are making changes in their burners to secure higher effi-

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ciency, greater flexibility, and increased safety of operation. G~IENT ACKXOWLED

The author wishes to express his appreciation for the advice and suggestions of Walter AI. Berry, gas engineer of the Bureau of Standards.

T h e Examination of Low -Temperature Coal Tars-I’ By Jerome J. Morgan and Roland P. Soule COLUMBIA UNIVERSITY, N E W

YORK,K. Y.

ISTILLATION a t tion to isolate the phenolic The composition of iars produced in the low-temperature carbonicompounds from the alka500” to BOO” C. of zation of coal presents a problem of analysis with which no standard line solution, but the tar a bituminous coal methods2 are designed to deal, and in this paper and a subsequent one acids recovered by simple containing about 35 per a critical review is made of available methods. neutralization with sulfuric cent volatile matter gives The phenols present a special problem, in view of the presence acid have been found in rise to a tar3 of specific of comparatioely large quantities of higher homologs which must be most cases to consist engravity 1.068 a t 15.5”/15.5” removed before phenol and the cresols may be determined. Methods tirely of phenols. ExtracC. Dry distillation of this based on freezing points and specific gravities of mixtures, together tion by benzene or ether tar yields 68 per cent by with Raschig’s nitration method, are satisfactory for determining of the residual acid liquor weight of pitch melting a t the latter, once they are separated. remaining after the separa53” C. in air. The distilThe hydrocarbons may be separated into two groups, one contion of the bulk of the late contains about 43 per taining parafins and naphthenes, and the other aromatic and unphenols thus liberated, percent phenols, 2 per cent saturated compounds, by the use of 98 per cent sulfuric acid. The mits a more nearly quantinitrogen bases, and 55 per selective solvents, dimethyl sulfate, sulfur dioxide, and selenium tative recovery. Distillacent hydrocarbons, threeoxychloride, as well as oleum and nitrating mixtures, gaoe low results. tion of the combined phequarters of which are cyclic, The specific-graoity and aniline-point modifications of the sulfuric nols and extract then reunsaturated compounds. acid method are inapplicable in the presence of unsaturated hydromoves the solvent together Almost two-thirds of the carbons. Physical methods depending on refractive index and spesaturated hydrocarbons are with any water present. cijic gravity were preferred for determining the relative proportions GENERALEXAMINATIOX naphthenes, while the rest of naphthenes and paraflns to solubility in sulfur dioxide, misci-The composition of the are paraffins. Aromatic bility with aniline, and ultimate analysis. A subsequent publicapurified phenol mixture hydrocarbons are only prestion will discuss the unsaiurated hydrocarbons. ent in traces in most lowt h u s obtained depends temperature tars. principally upon the teniSuccessive extractions with 10 per cent sodium hydroxide perature of carbonization of the coal. Higher temperaand 20 per cent sulfuric acid solutions divide the tar distillate tures result in a decrease in the total quantity of phenols and into three parts: (1) the phenols or “ta.r-acids,” ( 2 ) the in the percentage of higher homologs. Thus, while the tar nitrogen bases, and (3) the neutral compounds, chiefly acids from high-temperature tar consist almost entirely of hydrocarbons. The absence in low-temperature tars of solid phenol and the cresols in the ratio of about 1 to 2 by weight, compounds, such as naphthalene, makes these extractions low-temperature tar acids have been found containing only easier than in the case of ordinary tars. Since the phenols 38 per cent3 phenol and cresols, or in some cases7 even less form compounds4 with the nitrogen bases, however, the large than 2 per cent cresols, with little or no phenol. quantity of phenols present in low-temperature t’arsmakes it Fractional distillation of these mixtures of low-temperature necessary to perform the alkali treatment first, becau,qe ex- phenols gives results capable of only a very general interpretraction of the bases of sulfuric acid is very incomplete in the tation. While binary mixtures of phenol and the cresols presence of an excess of phenols.5 have been showns to possess normal boiling-point curves, the resolution by simple distillation, with a Vigreux or other ANALYSIS OF PHENOLS ordinary column, of this complex mixture of isomers and P u ~ r ~ ~ c a ~ ~ o r v -tar T h eacids are ordinarily purified by closely related homologs into its components is exceedingly steam distillation of the alkaline extract’, or by washing it difficult and unsatisfactory. Fractionation is helpful, however, in obtaining relatively with bcnzene or ether to remove dissolved or entrained hydrocarbons or bases. The presence in some lowtemperature simple mixtures for subsequent examination-i. e., in sepatars6 of carboxylic acids requires a carbon dioxide precipita- rating groups of compounds, such as the cresols, from the xylenols. As far as the determination of the relative quan1 Presented before the Section of Gas and Fuel Chemistry a t the 64th tities of these groups is concerned, two or three distillations Meeting of the American Chemical Society, Pittsburgh, Pa., September 4 to 8, 1922. are nearly as accurate as ten or more, since experiment shows * Weiss, TIZIS JOURNAL, 10 (1918), 732, 817, 911, 1006, for an outline that if the cut is made midway between the boiling points of of standard practice in the examination of high-temperature coal tars. two components, the quantity of the higher-boiling compound a Morgan and Soule, Chem. Met. Elag., 26 (1922), 923, 977, 1025; the primary tar of the “Carbocoal” process, cf. Curtis, I b i d . , 23 (1920), 499; appearing in the lower fraction is very nearly balanced by THISJOURNAL, 13 (1921), 23. the amount of lower-boiling compound in the higher fraction. 4 Bramley, J . Chem. SOC. (London), 109 (19161, 10, 434,469, 496. Quantitative methods of analysis have been developed KHatcher and Skirrow, J. A m . Chem. SOC.,39 (1917), 1939; Skirrow

D

and Binmore, I b i d . , 40 (1918), 1431. 8 Marcusson and Picard, Z.angew. Chem., 34 (1921), 201; cf. Tropsch, Byelanstof Chem., 2 (1921), 251, 312.

7 F i s h e r , Brennstoff Chem., 1 (1920). 31, 47; temperature of carbonization 400’ to 500° C. 8 Fox and Barker, J . SOC.Chem. I n d . , 36 (1917). 842; 37 (1918), 268T.