August, 1924
I N D U S T R I A L A N D ENGINEERING CHEMISTRY
779
Low-Temperature Distillation of Farmville, N. C., Coal‘ By W. E. Giles and F. C. Vilbrandt UNIVERSITY OF NORTH CAROLINA, CHAPEL HILL, N. C.
T
HE coal from the
This low-temperature distillation study of the Farrnville coals of nature of the volatile Products distilled from different Farmville mines of North Carolina indicates that this excessively smoky and high volasamples of coal at low ternt h e Deep River tile coal can be converted into commercial gas and a low volatile Peratures in the early stages Valley field of east-central residue suitable for domestic consumption. The presence in the coal of heating varies according North Carolina is rated a of complex paraffin hydrocarbons easily liberated at low temperatures to the smoke-producing good gas coal with respect to is girren as the cause for the smolpproducing tendencies andfor the t e n d e n c i e s of the coal. its volatile matter, accordcharacteristic atmospheric disintegration of this coal i n storage. A c c o r d i n g 1y , the coals ing to the classification of from the Farmville mines, coals by Bone,2 who rates coals having a volatile matter content between 32 and 38 being excessively smoky, should liberate large quantities of per cent a t 900” C., with a n oxygen to hydrogen ratio of heavy hydrocarbons very rapidly when subjected to low1:2. The Farmville coal falls in this class if only volatile temperature distillation. The gases liberated should be rich matter is considered, but its oxygen to hydrogen ratio is 1:1. in the heavy hydrocarbons, and at very low temperatures No studies are recorded that show any data on laboratory or would be unsuited for gas for ordinary domestic heating plant tests on the applicability of this coal for such purposes as purposes, because of their high sooting tendencies. gas coals. RAWMATERIAL The outstanding characteristic of this coal is that of giving off excessive amounts of smoke and frequent “flare backs.” The coal used in this investigation came from the Farmville On this account it is in disfavor as a household fuel. It is said mines of the Deep River coal field, comprising parts of that the use of this particular coal on the fleet blockade run- Chatham, Moore, and Lee Counties, North Carolina. This ners of the Confederacy enabled the Union forces to spot and coal field has been estimated to contain 100,000,000 tons capture them readily because of the large amount of smoke available a t depths not exceeding 1600 feet. The sample used pouring from the stacks. The engineers on locomotives using in this investigation was a face sample taken from different this coal claim that the flues become quite clogged with the parts of the mine. The coal is brilliant black, retaining its soot and also with some fine coal dust accompanying the soot. luster on pulverizing; the fracture is conchoidal. Sulfur apThe fines thus produced are the result of disintegration of the pears mainly as “ball” sulfur. When mined the coal is hard coal on being heated, for the coal shows this property of dis- and lumpy, but on exposure to the air it disintegrates after a integrating from lump into fines on exposure to the air. short time t o “fines,” leaving but a small portion as lumps. The efficiency of boilers depends largely upon the quantity APPARATUS and nature of those constituents of the coal that can be gasified a t a low temperature, but which do not condense I n order to maintain the different temperatures accurately upon the boiler surfaces, or which are not reduced below the and for long periods of time, an electric furnace was built with temperature for their complete combustion. An evaluation electrical control. The furnace was of sufficient size to hold of the products of low-temperature distillation of these coals a retort of 1 kg. capacity. assists in understanding why this Farmville coal is notoriously The retort (Fig. 1) was made of 3.2-mm. (‘/V-inch) steel, smoky. electrowelded 162 X 102 X 264 mm. (6 X 4 X 10 inches) inside Inasmuch as the coal as mined cannot be used efficiently dimensions, calculated to hold 1kg. of the coal. On using the either in the boiler room or in the homes, some heat treatment coal that was screened to pass 13-mm. (‘/2-inch) mesh and be is necessary to convert the 100,000,000 tons that are in sight retained on the 3.2-mm. (l/&xh) mesh, there remained an ininto illuminating gas and the residue into coke, which is sufficient air space above the charge, so the charge was made to finding much favor in the households a t the present time. equal 907 grams (2 pounds). The top of the retort was I n addition to the above, further studies were outlined to made of 13-mm. (l/%-inch)steel provided with holes to bolt the investigate the smoke-producing tendencies of this coal, top to the container and also with three holes, one for charging, which is notorious in this characteristic. The coal was one for the pyrometer well, and the third for the stillhead, subjected to a series of low-temperature distillations in a for carrying off the gases and the volatile matter. The conlarge retort a t temperatures maintained constant by electrical denser system consisted of a tar trap with a tapping petcock control. The products obtained a t these low temperatures, a t the bottom, and three glass condensers sealed into a unit which ranged from 200” to 660” C., were carefully analyzed. with glass-blown joints. Receivers were supplied a t the From studies on some British coals by Burgess and Wheeler,3 bottom of each condenser. The gases were collected over in which the smoke-producing tendencies of the several water containing sulfuric acid. The entire set-up consisted of types by low-temperature heat treatment were investigated, the electric furnace, A , the steel retort, stillhead, tar trap, it was concluded that this property was due entirely to the three glass condensers, four glass receivers, and a gasometer presence or formation of the higher hydrocarbons of the assembled as shown in Fig. 2. paraffin series, while the extent of the smokiness was due DISTILLATION to the rapidity with which these hydrocarbons were disengaged by the heat applied. Porter and Ovitz4 claim that the A 907-gram mmple of the freshly mined, graded coal was 1 Received December 4, 1923. charged into the retort and the retort sealed with steam 2 “Coal and Its Scientific Uses,” 1918, p. 68. Longmans, Green & ‘20. gasket and sodium silicate to prevent leakages. This sys8 J . Chem. SOL. (London), 97, 1917 (1910); 99, 649 (1911); 106, 131, tem was then tested for leaks. The furnace was previously (1914). heated to approximately the desired temperature before the 4 J . Gas Lighting, 107, 343 (1908).
INDUXTRIAL A N D ENGINEERING CHEMISTRY
780
retort was inserted therein. The heating was regulated after the retort was placed and the temperature maintained by means of electrical control until all evidences of decomposition or reactions were a t an end. The stopping of gas bubbling through the tar trap, the absence of further evolution of gas into the gasometer, and the cooling of the gooseneck were all used as evidences of completion of the distillation at~each heat treatment.
8"-
chiefly air and some paraffin hydrocarbons that are liberated from this coal a t this low temperature. At 300" C. the heat begins to have a marked effect on the coal. The water fractions vary but slightly throughout the series from 6.66 per cent a t 300" C. to 7.68 per cent at 540" C. TABLE I-DISTILLATIONDATAON FARMVILLE, N. Temp. of Runs C 200 300 360 420 480 540 600
660
-.
FIG. DETAILS
OF
RETORT
The distillations were carried out at atmospheric pressures a t 200", 300", and at intervals thereafter to 660" C. These temperatures were lower than those used by Burgess and Wheeler,3and are comparable with the results of Taylor and Porter,5 who made studies on the high volatile coals from Wyoming and Illinois a t reduced pressures to eliminate as much as possible any secondary reactions. Any change taking place in the course of the distillation a t low temperatures of the coal from the Farmville mine should make itself evident by exerting an influence upon the nature, amount, and properties of the products obtained. The, distillation data obtained in this study are recorded in Table I. The data are calculated on the yields from the original coal on the moisture-free basis. The course of the reactions that occurred during the heat treatments a t increasingly higher temperatures is clearly shown from the quantitative data in Table I. Subsequent analysis of the gas obtained a t this point indicates that it is 6
Bur. Mines, BUZZ. 140 (1914).
Vol. 16, No. 8
Total Volatile Matter Per cent 10.33 25.67 27.32 30.30 31.50 34.00 34.00 34.50
Tar Water Fraction Fraction Per cent Per cent 2:i3 4.69 5.98 7.04 10.62 10.72 11.19
6:66 6.82 7.26 7.12 7.68 7.48 7.24
c., COAI,
Coke Ammonium Volume Residue Sulfate of Gas Per cent Per cent Liters ,~ 89.67 0.17 2.5 1.23 13.5 74.33 1.86 72.68 33.0 2.20 69.70 42.0 2.00 68.50 55.5 2.25 66.00 72.8 2.10 93.6 66.00 3.00 65.50 136.4 ~~~
The ammonium sulfate obtained is the sum of the ammonia retained in the tar and that absorbed from the gases by passing through sulfuric acid in the fourth receiver. After the small quantity of ammonium sulfate that made itself evident a t 200" C. the variations in this constituent are very small. The fact that this coal gave off ammonia a t 200" C. must be taken as further evidence of its ease of decomposition. The loss in the treated sample is termed "total volatile matter." Casual study of the data in Table I reveals a decided difference in this total volatile matter or loss on heat treatment and the sum of the products obtained from the coal, assuming extrapolated gravities for the gases where such data are lacking in Table 111. This difference is large a t the low temperatures and becomes increasingly smaller as the temperature to which the coal is subjected is increased until it almost disappears between 540" and 660" C. This large difference at the low temperatures of distillation with the decreasing difference a t the increasingly higher temperatures considered due to the fact that the gases liberated a t the lower temperatures are mainly the heavy, wet, complex paraffin hydrocarbons, which are increasingly diluted a t the higher temperatures by the formation and liberation of larger quantities of the light hydrocarbon gases. This assertion is borne out by the analyses of the several gases obtained during the distillations. Closely balanced low-temperature distillation analyses are difficult to obtain. At 540" C. there is a decided increase in the volume of gas liberated, as well as in the tar fraction and in the total volatile matter, indicating a change in the course of the distillation. This reaction is one of prime importance, since it indicates the first critical point a t which this co'al undergoes a decided change on heating. Below this temperature the change in the coal is uniform, but slight. Further heat treatment after 540' C. up to 660" C. does not change the residue, the decomposition that occurs being due to the decomposition of the bodies evolved from the coal at 540" C. The coke residue does not materially change between these temperatures. The slight increase in the tar fraction after 540" C. indicates that decomposition of the tar occurred on increasing the temperature and, together with a decomposition of the constituents of the gases evolved a t 540" C., materially increased the volumes of gases a t the higher heats.
TARFRACTIONS The tar fractions were analyzed by redistillation according to the standard tar distillation tests6 The results of these redistillations are tabulated in Table 11. No tar distillate was obtained a t 200" C. The cuts made in redistillation were weighed and calculated over to percentage of the individual tar fraction. The quantities and order of increase of the different cuts follow the uniformity indicated in the coal 6
U.S. Dept. Agr., Bull.
314, 21 (1915).
I.VDUSTRIAL B.VD E N lPINEERING CHEMIGTRY
August, 1924
distillation data. Again at 540" C. the tar fraction analysis indicates a decided change in composition due to a change in the course of the decomposition of the coal on increased heat treatment. TABLBII-COMPOSTTION os VAS T e . DLSTILLII~SS Temperaiure of %un
c.
:300
380 420 480
540 ROO 660
Cut 1
Cut2
50'to
170* to 230'C.
1 7 0 O C. Per cent 31.01
Per cent
10.01 9.83
25.83 19.68 10.56 9.98
25.62 30.64
19.15 11.90 8.86 8.83
8.08
Cut 3
230'to
270'C. Per cent 18.79 17.40 13.32 12.05 7.09 10.61 7.09
Cut4 270Oto
315" C. Per cent
Cut 5 315Oto
355-C. Residue Per cent Per cent
13.26
9:27 13.79 18.61
7.86 7.20 6.91
23.53
12139
12.30
32.41
14.00
7.06 18.32 20.54 $7.05 38.81 54.88 10.85
Te*perat"re
The composition of the gases obtained at each heat treatment indicated the course of the reaction of the coal during the heating. The analyses were conducted according to the procedure described by Mahin'-heavy hydrocarbons by ahsorption in absolute alcohol; carbon dioxide and hydrogen sulfide in potassium hydroxide solution; ethylene and its homologs in fuming sulfuric acid; oxygen in alkaline pymgallol; carbon monoxide in acid cuprous chloride; hydrogen by fractionalcombustion over palladinired asbestos; methane and ethane by slow combustion; and nitrogen by difference. The samples were well mixed before analysis. The air in the apparatus was evacuated until a head of 18 inches was maintained in the gasometer. No hydrogen was found in the gases evolved. h w temperature distillation of this coal revealed a well-defined primary decomposition point at 540" C. in addition to the decomposition point between 700" and 800" C., as found by Wheeler and associates and by Taylor and Porter.s The latter identified the presence of a decomposition point below 500" C., but found no definite point. Although tho distillations were not carried above 660"C., the trend of the compositions of the different gases evolved at increasingly higher temperatures indicates that the decomposition following the one at 540" C. would occur slightly ahovethelastheatmadei. e., 660' C. The initial decomposition point in this distillation occurs at 540' C. Although the percentage of heavy hydrocarbons at 540" C. totals 22.40, the methane 51.62, and the next 60degree rise in temperature brings about a reduction to 1.49 per cent heavy hydrocarbons and 74.50 per cent methune, the retort distillation data indicate but slight variation in the constituents above this temperature, except in the volume of gas liberated and the residue from the tar analyses. The ethane content does not materially change during the distillation, the decrease above 540" C. probably being due to a greater formation of methane from t.he decomposition of the liquid tars a t this primary decomposition point. At 200" C. the small quantity of gas consisted mainly of air that WRS contained in or adsorbed upon the coal. The libcration of this &sorbed or retained air and smaIli percentage of heavy hydrocarbons at low temperatures indicates a loose combination of distillable bodies in the coal and accounts for * -yunntitativeA ~ ~ IZllded., ~ ~ ISIS, ~ ~ p.a4~. , - ~ ~ ~ ~~ ~~c,,.~~ TABLEIII--PER,RCGNTAFE Temperature of Run
*
c.
200 300 360
420
480 540
Specii;;Gfa;itga at 15.8' C.
..
1.767
..
1.099
CoMPOsITrON OB Ethyleae and Itr Homoloss Heavy Hy- or UnPaturated drocarbonn Hydrocarbons 14.90 0.74 48.09 0.30 36.42 0.20
26.60 23.00 22.40 1.49
0.30 0.14 0.15 0.17
of
K""
0
AEh Prc cent
c.
Original
01.COALA W D
ma/
a5 mined
200 300 360 420 480 540
GOO 600
37.80
9.03
26.82
0.25
8.95 3.40 2.63 1.61 0.98 0.37 0.12
9.31 9.80
Cogs RESIDVBS nixed Heating Carbon Vaiue Per emt B. t. U.
sulfur Per eeot
0.22 0.22 0.04 1).03
10.05
0 . 03
10.02 10.49 111.77
0.03
10.29
0.03
C O K E REslnulrs The undesirability of using this coal as mined for domestic con.umption on account of its excessive sinokinrss neeessitated a treatment of the same to make available the 100,000,000 tons of coal in sight. An examination of the rcsidues obtained from the low-temperatare di&llat,ions showed that the coal could be comverted into a suitable residue or coke. The residues obtained up to the 300' C. heat treatment retained the appearance of the original coal.
Pic 2 - . 4 5 F I ~ ~ u r , ~os DISTILLATIONAPPARATUS
At 360" C. sufficienttars had been liberated to cement the residueintolumps. As the t.emperaturewas increased the residue lost more of the binder and began to take on the appearance and characteristics of eomrnercial coke. At alld abo7.e 540" . l ~ i C. i the l residues i were hard, flinty, and honeycombed.
LOW-TSMPERATUG DISTILLATION GASSS FROM FARMVILLE COALS Ethane 14.41
17.11
17.14 17.93 17.31 17.55 16.46 15.73
600 0.952 660 1.08 0.12 O1 Data only given on sares where agreeing data could be obtained.
..
the ready disintegration of the coal on exposure t u the atmosphere. Heating causes an easy liberation of large volumes of the heavy hydrocarbons and other gases, and permits the too ready disintegration of the coal and a subsequent smokiness. This characteristic of the Famville coal calls for precaution in mining because of the dangerous explosibility of the fines and gases liberated a t low temperatures. Such explosions are frequent in heating stoves being charged with this coal. According to the analyses of the gases a heat treatment ahovej40"C.mnstbegiven tothecoaltouse thegas obtained therefrom for cominercial purposes. T ~ s r aIV-ABALVSBS
COMPOSITION OF G A ~ EEvoLvEn S
781
Methane 19.09 24.15 36.40
44.89 48.20 31.62 74.50 76.64
Carbon Monoxide 0.00
0.M
0.00 0.40 0.40 0.40 0.55
0.40
Carbon Dioxde and Hydropen Sulfide
0.00
0.65 0.%& 1.24
Nitrogen
40.01 7.62
2.08
7.VA
1.98
6.16
2.09 1.47
1.24
6.81 7.62
1.46
4.2s
1.25
Oxygen 10.86
3.82 1.55 Schilliog method for determination of specific gravity used.
1.83
1.14 0.86
INDUSTRIAL A N D ENGINEERING C;%IEMISTRY
782
These residues were analyzed according to the methods for proximate coal analysis as given by Mahin,* and their heating values on the Parr calorimeter according to instructions accompanying the instrument. The results are tabulated in Table IV. The volatile matter, sulfur, and B. t. u. decrease with increasing still temperatures, while the fixed carbon and ash increase. The heating values of these residues are fairly high, the coke obtained above 540” C. having a heating value slightly above 14,000B. t. u. Heating just above 360” C. produces a residue from this,coal with a volatile matter value of but 8.95 per cent. Above 540” C., a t which quantities of gases are produced, a coal residue with less than 1 per cent volatile matter is obtained. I n this manner a product is formed that can be used in the household without fear of crumbling to a dust or producing dangerous back fires and excessive smoking and sooting, thereby increasing the safety and efficiency in the use of the fuel. 8
“Quantitative Analysis,” 2nd ed., 1919, p. 308.
Vol. 16, No. 8
CONCLUSIONS 1-The excessive smokiness of Farmville, N. C., coal is due to the liberation of complex paraffin hydrocarbons a t relatively low temperatures. 2-The characteristic disintegration of this coal on exposure to the air is due to the absorbed gases and the loose combination of the heavy hydrocarbons in the coal. 3-A positive decomposition point occurs a t 540” C. in addition to the secondary change occurring above 700’ C., as found by other investigators. 4-No marked decomposition point occurs below 540” C., the coal yielding complex hydrocarbons and other by-products without a well-defined decomposition point. 5-This high volatile and smoky coal can be converted into a safer, more efficient, and cleaner fuel for domestic consumption by low-temperature distillation. 6-Low-temperature distillation of this coal produces a commercial gas.
Producer Gas’ Apparent Equilibrium between Its Constituents and Influence of Depth of Fuel Bed By R. T.Haslam MASSACHUSETTS INSTITUTEOP TECHNOLOGY, CAMBRIDGE, MASS.
The constituents of producer gas come to an apparent equilibrium value dependent on the thickness of the fuel bed alone and independent of gas velocity (rate of firing). ratio of pounds of coal to pounds of steam. or temperature of the exit gases. A hypothesis explaining this phenomenon is gioen, based on the reaction HzO CO = COZ Hz being catalyzed by hot surfaces which the gases can reach only by digusion. The relationship between the apparent equilibrium oa of fuel bed may be expressed as
+
+
The amount of water in producer gas may be calculated from the
+
HE importance of the reversible reaction HzO GO = COZ Hz to indicate the direction in which producer gas composition is changing has been recognized by all investigators in this field, but one of the unsettled questions is the time necessary to complete the change. Pu’euman2 considers that the time of contact between the gas and fuel is far shorter than that required for equilibrium, which according to his findings depends not only on time of contact and temperature, but also on the nature of the boundary surface between the solid and gaseous phases. He further noted in all his investigations (seventeen in number) that the producer gas came to an apparent equilibrium corresponding ~ ,the ~ other hand, to a temperature around 600” C. H U S S Oon basing his deduction on the work of LeChatelier, assumes in his mathematical treatment of this subject that the gas is in equilibrium with the coke a t the temperature of the gas. Haber4 states that in a free flame above 1500” C. the equi-
T
+
1 Presented in part under the title, “Calculation of the Amount of Water in Producer Gas from the Orsat Analysis,” before the Section of Gas and Fuel Chemistry at the 66th Meeting of the American Chemical Society, Milwaukee, Wis., September 10 t o 14, 1923. 2 Stahl u. Eisen, 83, 394 (1913). a Rev. ind. mindvale, 1922, 373. 4 “Thermodynamics of Technical Gas Reactions,” 1908.
usual Orsat analysis to within 10 per cent by the equation
where IHzO] equals the oolumes of moisture in the producer gas per 100 volumes of dry gas, (COz), (Hz), and (CO) equal the percentages of these gases as determined by the Orsat analysis, and L equals the depth of fuel bed in feet. The use of this equation aooids the necessity for the direct determination of water in producer gas, a relatioely dificult analysis to carry out in the plant. Its use also brings out the important point that greater dilution of producer gas is brought about by the presence of the undecomposed water than by the carbon dioxide.
librium value for this reaction shifts rapidly with temperature. Below this temperature numerous writers have pointed out the great effect of catalytic surfaces. While going over the recent investigations of Clements5 it was noticed that the producer gas from each series of experiments, in which the entering air was humidified to varying degrees, came to an apparent equilibrium constant independent of the degree of humidification, but that the constant differed for each series. The only variable differentiating the two series of experiments was the depth of fuel bed, 3.5 feet in the first series and 5 feet in the second. Therefore, a search of the literature was made to bring out the effect of depth of fuel bed on the apparent equilibrium between the constituents of producer gas, and two other satisfactory investigations were found-those of Bone and Wheeler6 and recent work of Hunt, Johnson, and Willi~.~ Data from all these sources are tabulated in Table I. Engineering, 116, 8 J . Iron Steel Inst. (London), 1928 (advanced proof); 597 (1923). 4 J . Iron Steel Inst. (London),76, 126 (1907). 7 “The Determination of Optimum Operating Conditions for a Commercial Morgan Gas Producer;’ M. I. T. Thesis, 1923.
