March, 1923
INDUSTRIAL AiVD ENGINEERING CHEMISTRY
The second curve shows that with the particular furnace the highest economical rate of heat evolution is about 23,000 B. t. u. per cu. ft. of effective combustion space. If the rate of heat evolution were carried beyond this point, more particles of only partly burned coal would be carried out of the furnace with the gases. The third curve from the bottom shows that with a fur-
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nace of the proper size powdered coal can be burned nearly completely with an excess of air of 10 to 25 per cent. The two highest curves give the thermal efficiency of the furnace and boiler. The efficiency of boiler and superheater is well above 80 per cent, and that including the economizer reaches 90 per cent, which is about the highest efficiency so far attained on a large power-plant, steam-generating unit.
Thermal Operation of Modern Regenerator Coke Ovens' By D.W. Wilson,2 H.0.Forrest,B and C. H.Herty, Jr.8 BUFFALO STATION, SCHOOL OF CHEMICAL ENGINEERINGPRACTICE, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, LACKAWANNA, N. Y .
Several careful thermal e.@ciency tests have been made on coke owns by students in the School of Chemical Engineering Practice of the Massachusetts Institute of Technology, and the figures thus obtained form the basis of this paper. The equipment studied was one sixtyoven battery of regenerator ovens fired by a part of the gas produced f r o m the coal. The coal used consisted of coal of 33 per cent volatile matter and coal of 18 per cent volatile matter so mixed as to produce about 27 to 28 per cent volatile in the mixture. The coke produced was for use i n blast furnaces. The coke plant was completely equipped with apparatus for recovery of the usual by-products. The excess colpoven gas was used for raising sfeam or i n steel-reheating furnaces. The immediate objects of these tests were-first, to obtain a comprehensive heat balance on the ovens: second, to determine f r o m this the thermal eficiency of the battery; and third, to obtain a n y information possible regarding the heat eflecl of coking this particular coal mixture.
ITH conditions of the past few gears the question of fuel economy has been rapidly becoming vital to all producers and users of fuels. Probably, more efficient use of coal would be the greatest factor tending to reduce necessary fuel consumption, and consequently effect a considerable fuel economy. With this in mind, in its field work, the School of Chemical Engineering Practice of the Massachusetts Institute of Technology has taken advantage of all opportunities to study the use of coal in its various forms, particularly as coal, as coke, as producer gas, as coke-oven gas, and as blast-furnace gas. A f i s t consideration in investigating the thermal operation of any machine is a knowledge of the distribution of the heat energy supplied to the unit. This best takes the form of a comprehensive heat balance. This method has the following distinct advantages: First, the reliability of the experimental methods and results is known by the agreement obtained between total heat input and total heat output; and, second, from a thermal standpoint the operation of the particular unit is covered completely, and, consequently, unforeseen future uses for the results are usually provided for. Of most importance, however, a heat balance makes possible a comparison between heat distribution on similar types of furnaces which will often indicate the directions in which efforts can profitably be expended in striving for improvement. 1 Presented before the Section of Gas and Fuel Chemistry at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa , September 4 t o 8, 1922. 2 Director. 3 Assistant Director.
DATANEEDED Obviously, in lnaking the tests it was first necessary to define the system under consideration. This was done as follows: The system studied was the entire battery of sixty ovens, starting with the coal as charged to the ovens, the fuel gas and air as delivered to the battery, and ending with the hot coke as pushed out of the ovens, the crude coke-oven gas as it left the ovens, and the waste gases as they entered the stack. This, then, involved a knowledge of the following information : HEATINPUT:
HEATOUTPUT.
Coal 1. Temperature 2 . Heating value 3. Analysis 4 . Quantity Air Humidity 1. Temperature 2. 3. Quantity Fuel Gas (purified or debenzolized coke-oven gas), 1. Temperature 2 . Analysis Quantityvalue 3. Heating 4. (A) Coke 1. Temperature 2 . Specific heat 3 . Analysis 4. Heating value 5 . Quantity Crude coke-oven gas (foul gas) 1. T a r Temperature b) Heating value c) Quanti'ty 2 . Ammonia ( a ) Heating value ( b ) Quantity 3. Light oils Heatinz value ( b ) QuantiFy 4 . Water condensed (both free and combined water in coal) ( a ) Quantity 5 . Purified gas ( a ) Temperature ( b ) Analysis ( c ) Heating value Id) Ouantitv %Taste (chimney) gages 1. Temperature 2 . Analysis 3. Humidity 4. Quantity (D) Radiation and convection 1. Temperature of unit areas 2 . Wind velocities 3. Surface exposed
I")
The quantity unit used throughout was one net ton d r y coal charged. The length of test was necessarily some multhis case 54 hrs., o r tiple of the coking time of 18 hrs.-in three consecutive cycles.
EXPERIMENTAL METHODS Representative samples of coal were obtained as t h e charging car was filled from the bins a t the ovens. Each
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INDUSTRIAL AND ENGINEERING CHEMISTRY
car was carefully sampled and the entire coal sample was then thoroughly mixed and quartered down to laboratory size entirely by hand. Coke is separated a t the coke ovens into three grades, according to size, known as “furnace,” “domestic” and “breeze.” Each of these grades was sampled as it was loaded into cars. The samples were taken from the coke produced from the coal which previously had been sampled. The three samples of the different grades of coke were mixed in proportion to the weights of each grade produced. The mixing and quartering were the same as for the coal, except that for the coke preliminary crushing and further final grinding were necessary. On both coal and coke, moisture determinations were made on a 10- to 20-lb. sample, special care being taken that the results should represent the moisture in the material as weighed in the plant. Coal and coke weights were obtained from track scales calibrated previous to the test and found to be reliable. The heating values of coal, coke, and tar were determined by an Emerson bomb calorimeter, and experimental values for moisture, sulfur, and nitrogen were corrected where necessary. The usual proximate analysis of coal and coke was carried out. Coke temperatures were measured in two ways-fist, by inserting a thermocouple into the coke and noting its temperature just before the oven was pushed; and second, by using an optical pyrometer just before pushing out the coke. I n the latter case, care was taken to break the outer skin of comparatively cool coke before taking a reading. Good agreement was observed between the two methods. Since no reliable values were available from the literature for the specific heat of by-product coke, this was experimentally determined over the temperature range involved. Foul gas temperatures were measured with thermocouples inserted in the gas line just before the gas left the oven proper. Readings were taken over the entire coking period and the arithmetical average used in calculating the results, no attempt being made to allow for the varying quantity of gas during the coking period. Admittedly, this is not correct, but the difference between such an average and the correct figure is so small that it can safely be neglected for a heat balance. The quantity of gas produced was measured by a recording Venturi meter on the debenzolized gas line. Amounts of light oil and ammonia produced were small in proportion to the total gas, and these were obtained from the pIant operation figures. The tar and weak ammonia liquor were measured by carefully noting the change in level in all necessary tanks, decanters, and scrubbing equipment during test, and subtracting from this total volume the fresh water added to the system in the same period. The coke-oven gas used to heat the ovens was measured by a recording Venturi meter. The air for combustion of this gas was measured by a recording instrument connected to an orifice installed on the inlet air line. The volume as obtained in this way was checked against that calculated from the coke-oven gas analysis, the waste-gas analysis, and the volume of coke-oven gas. The temperature of the flue gas was measured by thermocouples inserted a t the same point that gas samples were withdrawn for the waste-gas analysis. All gas samples were taken over water previously saturated with the gas so as to avoid error by solution in the water. So far as possible, all gas samples were continuous over several hours. However, owing to 15-min. reversals, this could not be done on flue gas, so that figures were obtained by averaging a Iarge number of analyses each covering about a 10-min. period. Stack-gas samples were analyzed in the usual portable Orsat, using the customary absorption methods. The analysis of coke-oven gas was accomplished by the use of absorption
Vol. 15, No. 3
methods for carbon dioxide, illuminants, and for oxygen, while carbon monoxide and hydrogen were determined together by slow combustion with copper oxide, subsequently measuring the contraction and absorbing the carbon dioxide formed. I n this analysis the methane was determined by combustion with oxygen by means of a hot platinum wire. Heating values of this gas were calculated from its analysis and also were determined directly by a Junkers gas calorimeter. For all gases gross heating values at 60” F., 30 in. of mercury, and saturated with water vapor are the figures used. Miscellaneous methods of measurements, such as temperature of coal, of air, of air humidity, are not described in detail here, since in every case the method employed was the obvious one. I n all cases it was attempted to obtain enough data so that the final average value would be representative. DATAOBTAINED I n Table I are included experimental data obtained during the test and on the basis of which are calculated the heat balances given later. DISCUSSION OF EXPERIMENTAL DATA I n Table I the observed higher heating value of the coal has been corrected by 110 €3. t. u. per lb. for its sulfur and nitrogen content. This correction, largely due to sulfur, does not include all the sulfur and nitrogen of the coal bub only that part which gives up heat in the bomb calorimeter, but is not present in the coke. I n the results in Table I1 the corrected higher heating value of 14,470 €3. t. u. per Ib. was used. The weight of drags per oven-that is, coal removed from the oven during leveling-was practically negligible. I n quenching the hot coke, the water used carries with it some $ne cokewhich settles out in a sump and is removed a t intervals and burned under the boilers. It is tkis item which has been estimated a t 11 Ibs. per ton of dry coal, or 0.5 per cent. It is largely through chance that the close agreement shown in Table I was obtained between the calculated fuel-gas heating value and the calorimeter value, since the experimental error was greater than the difference between 549 and 551 B. t. u. per cu. ft. It is thought that the fuel-gas consumption of 4070 cu. ft. per ton of coal coked, or of 36.5 per cent of the total gas produced, represents excellent coke-plant practice. The difference between the value of 588 B. t. u. per cu. ft. for the foul gas and that of 549 for the fuel gas is accounted for by the light oil and the ammonia. The tar yield of 9.5 gal. per ton seems low, probably caused by lack of precision in tank measurements due to the short length of the test with respect t o volume of tar produced. The measurement of condensate was much more accurate than that of the tar. The figure of 18.45 gal. of condensate does not include all the water produced from the coal, since after cooling the gas is still saturated at its temperature with water vapor originally derived from the coal. By deducting from the total condensate produced that made from the known free moisture, it was found that 2.G per cent combined oxygen appeared as condensed water. Reference to the data on ais shows that on the one draft actual measurement gave 40,500 cu. f t . at 32” F., while calculations from gas analysis give 39,600 cu. ft. of air under the same conditions. On the other draft the measured volume was 38,000 cu. ft., while the calculated volume was 37,200 cu. ft . When gas-sampling and gas-analysis errors are considered, these checks of about 3 per cent would indicate both results to be trustworthy.
March, 1923
INDUSTRIAL A N D ENGINEERING CHEMISTRY
TABLE I-GENERAL DATA (Dates of Test. December 8 t o 10. 1921. Coking time, 18 hrs.) COALCHARGED: Average weight per oven as charged 30,130 lbs. Weight of drags per oven 25 lbs. 49 2 Tons per hour (dry basis) Free moisture (wet basis) 4 . 6 per cent Volatile (dry basis) 2 6 . 0 per cent Ash (dry basis) 6 . 0 9 per cent Temperature of coal 3 9 . 0 " F. Higher heating value (uncorrected dry basis) 14,580 B. t. u./lb. Higher heating value (corrected for sulfur a n d nitrogen 14,470 B. t. u./lb. Specific heat 0.314 COKE PRODUCED: Average weight per oven, furnace coke as produced 20,070 Ib. Average weight per oven, total coke as produced 21,625 lbs. Average moisture (wet basis) 3 . 1 2 per cent Volatile (dry,basis) 1 . 1 4 per cent Ash (dry basis) 9 . 1 2 per cent Average yield furnace coke (dry basis coal andloke) 6 7 . 5 per cent Average ield total coke (dry basis coal and coce) 7 2 . 8 per cent Coke breeze in pond (lbs. per ton dry coal estimated) 11 lbs. 0 . 5 per cent Per cent coke breeze in pond Temperature as pushed 2140' F. Heating. value 13,020 B. t. u./lb. Specific heat 0.42 FUELLGASUSED: Volume at 60' F., 30 in. H g (satd.) per net ton (dry coal) Gross heating value (calorimeter) Gross heating value (calculated) Per cent total gas used as fuel gas B. t. u. a e r lb. drv coal coked FUEL GASANALYSES: COI 1 . 4 Der cent Illurninants 2 . 9 per cent 0 . 5 per cent Oxygen cn 5 . 1 per cent i%$drogen 5 7 . 4 per cent Methane 2 8 . 5 per cent Witroaen 4 . 2 per cent FOUL:GAS PRODUCED: Volume at 60° F.. 3#O in. Hg (satd.) per net ton (dry coal) 11 150 cu f t . Average temperature i430' E.'. Gross heating value including light oil 588 B. t. u./cu. f t . TAR:PRODUCED : Amount per ton 9 . 5 gal. Specific gravity 1.2 Specific heat 0.4 Heat of vaporization (estimated) 162 B. t. u./lb. Gross heating value 15,865 B. t. u./lb. CONDENSATE PRODUCED: Total net amount condensed (not including tar) 1 5 . 4 5 gal./ton From free moisture in coal 11.50 gal./ton Fiom combined moisture in coal 6 . 9 5 gal./ton Oxygen per cent in coal appearing in condensate as combined moisture 2.6 3 . 3 gal./ton LIGHTOIL PRODUCED: AIR~USED : Volume per ton of dry coal, updraft, a t 32' F. (from orifice reading) 40,500 cu. f t . Volume per ton of dry coal updraft a t 32' F. (calculated at Last bulkhead) 39,600 cu. ft. Stack draft. uDdraft 42 mm. kerosene Volume pei
