May 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
LITERATURE CITED Broughton and Wentworth, J . Am. Chem. SOC.,6 9 , 741 ( 1 9 4 7 ) . Calvert, Ann. Physik, (4) 1, 4 8 3 ( 1 9 0 0 ) . Cuthbertson and Maass, J . Am. Chem. Soc., 52, 4 8 9 (1930). Fiske and Subbarow, J . Biol. Chem., 66, 3 7 5 (1925). International Critical Tables, Vol. VI, p. 2 3 0 , New York, McGraw-Hill Book Co., 1929. Joyner, Z . anorg. C h e m . , 7 7 , 103 (1912). Maass and Hatcher, J . Am. Chem. SOC.,42, 2548 ( 1 9 2 0 ) . Maohu. "Das Wasserstoff Peroxid und die Perverbinduneen." -Vienna, Julius Springer, 1937. Regnault, H., and Le Noir de Carlan, R., Congr. chim. ind., Cornpt. rend. 18me Congr., Nancy, 1 9 3 8 , 7 6 6 . Reichert, J. S., Chem. Eng. News, 2 1 , 4 8 0 (1943). Reichert, J. S.,et al. (to E. I. du Pont de Nemours & Go.), U. S. Patent 1 , 9 5 8 , 2 0 4 (1932). Ibid., 2 , 0 0 4 , 8 0 9 . Ibid., 2 , 0 0 8 , 7 2 6 . Ibid., 2 , 0 2 1 , 8 3 4 .
1003
(15) Ibid., 2 , 0 2 7 , 8 3 8 . (16) Ibid., 2 , 0 2 7 , 8 3 9 . (17) Ibis., 2 , 0 9 1 , 1 7 8 , (18) Ibid., 2 , 2 2 4 , 8 3 5 . (19) Ibid.. 2.316.487. > - - , (20) I b i d . : 2:347;434 (1941). CHEM.,3 9 , 1536 ( 1 9 4 7 ) . (21) Shanley and Greenspan, IND.ENG. (22) Wolffenstein, Ber., 2 7 , 3307 (1894). RECEIVED June 28, 1948. A portion of the work was carried out under N a v y Bureau of Ordnance Contract NOrd 9107, Task C ; the rest was under Contract Njori-78; T.O. XIX, Office of Naval Research, from which permission to publish was received. General supervision was by H. C. Hottel, G. C. Williams, T. K. Sherwood. and C. N. Satterfield, Department, of Chemical Engineering. Experimental work was carried out by the following group in the Department of Chemistry under Projects D I C 6351 and 6552 of Division of Industrial Cooperation: R. C. Young, C. D. Turner. R. Knodel, Mrs. P. H. Blackall, Misses E. M. Bickford, H. 34. Stickley, and C. L. Tucker.
Low Temperature Carbonization
of Alberta Subbituminous Coal RESULTS OF PRELIMINARY TESTS OF THE NEW STANSFIELD RETORT J. GREGORY AND A. MCCULLOCH Research Council of Alberta, Alberta, Canada has been the object of many OTENTIALLY the An account is given of the construction and operation investigat,ions; some of these largest home market for of the new Stansfield retort for the low temperature carhave been developed on t h e Canadian coal is in the Provbonization of Alberta subbituminous coal. The principles industrial scale. T h e subinces of Ontario and Quebec of operation of the retort have been established on a basis bituminous coal may be carwith their relatively high of the yields and compositions of the low-temperature bonized at a low temperadensity of population and carbonization products of an Alberta subbituminous coal. ture and the char so protheir comparatively heavy The necessary data have been obtained from stepwise carduced briqueted with a suitaindustrial c o n c e n t r a t i o n . bonization of the coal, in stages of approximately 100" C. ble binder, or under certain This market, however, is more from 320" to 610" C., in a Gray-King low temperature circumstances, b r i q u e t i n g carbonization assay apparatus. Details are given of the easily and economically supmay precede carbonization. plied with coal imported from results of preliminary tests of a retort built on a pilot As early as 1921, Bone (8) the coal fields of Pennsylplant scale. The yields and compositions of the chars pointed out t h a t if immature vania and Ohio. The greater obtained under varying conditions of oneration of the coals are carbonized up t o cost of transporting coal from retort are described. temperatures of the order of Alberta is the principal rea400' C., a large proportion of son why coal producers there the oxygen present is evolved as steam and as oxides of carbon find it difficult t o compete successfully with coal produced in (principally carbon dioxide) and the calorific value of the solid the United States. Nonetheless, with the aid of a Dominion material thus substantially increased. The carbonization of coals Government subsidy on transport, some coal mined in Alberta is at temperatures as low as 400" C., however, is not economically marketed in Eastern Canada. Apart from the cost of transport, practicable, and in most investigations temperatures not exceedcoal quality is a n important factor in the situation. Generally ing about 700' C. have been employed. I n Canada, considerable speaking, the bituminous coals of Alberta are of high quality experimental work of both a small and large scale nature was and, after preparation for the market, admirably suited t o most carried out by Stansfield and others ('7) for the Canadian Lignite industrial purposes. On the other hand, although possessing Utilization Board. Various types of retorts were designed and properties such as cleanliness in handling, low ash content, and eventually one was adopted for the construction of a battery of the production of only small quantities of smoke on combustion, six retorts, each having a daily production capacity of 16 tons of which make them attractive solid domestic fuels for use locally, char. The plant, however, was found t u be unsatisfactory in the subbituminous coals have other properties which are discontinuous operation. Meanwhile, American investigators had advantageous when these coals are considered for use in distant developed the Hood-Ode11 system for the carbonization of the markets. Their calorific values are low due t o their high moisture lignites of North Dakota (6). The Hood-Ode11 plant (Figure 1) and oxygen contents, and since certain of them slack readily, operated somewhat o n the principle of a simple gas producer. considerable quantities of finely-divided material may be proThe lignite was dried in the upper section of the plant before its duced in transit and during storage before use. carbonization and partial combustion in the lower section. T h e T o devise a n economical method of carbonizing subbituminous air for combustion was admitted through tuyhres in the lower coals or lignites to produce a solid residue of enhanced heating section and the heat developed sufficed for the drying and carvalue and possessing good storage and weathering properties,
P
INDUSTRIAL AND ENGINEERING CHEMISTRY
1004
Vol. 41, No. 5
bonization of the lignite. mutely 40% of the total output of cos1 i n Alberta in 1946 was Baffles were provided classified as subbituminous (9). Fourteen per cent of the output centrally in order to diof subbituminous coal was slack coal which, generally speaking, FIRECLAY A N D rect the lignite towards was below 0.i5-inch size. A large proportion of the slack coal, CEMENT TILE the walls of the diyer amounting in some instances t o as much as 75%, was less than FIRECLAY AND and carbonizer and it 0.375-inch size. The market value of this material was low, but CEMENT TILE 9AFFLE was thus brought more from it there might conceivably be manufactured a char, which intimately into contact after briqueting, would find a sale either locally or elsewhere. 17B n-ith the upward-flowSecondly, the use of slack coal containing so large a proportion ing stream of gas. The of fine material, would probably give rise t o considerable working topmost baffle consisted difficulties in any of the then existing carbonization plants emi of a pipe, which was ploying internal heating. Presumably channeling would occur Figure 1 (6) slotted underneath, and the resulting irregular flow of gas produce uneven carbonizat,hrough which gas and tion and partial combustion of the coal. Moreover, the inStaltar vapors could be lation of such a plant as the Lurgi-Spulgas, had t h a t even been possible, n-ould involve substantial capital outlay, and the emwithdrawn and recovered. A Hood-Ode11 plant was continuously ployment of more or less experienced operators. What appeared operated successfully for some time carbonizing Saskatchewan t o be required was a carbonization unit of relatively simple and lignite (8). Later this plant was replaced by one of the older inexpensive construction, automatically controlled, which could designs of the Lurgi plant, which by this time had been developed readily be duplicated. For these reasons, especially in view of in Germany. the low cost of the original material and since tar recovery would T h e modern Lurgi system, known as the Lurgi-Spulgas ( 3 ) introduce a more complicated plant, it was decided t h a t the tar employs a unit built in two sections (Figure 2). Partially-dried vapors might well be burned as fuel along with the gas necessary !ignite, after briqueting, is completely dried in the upper section for drying and carbonization. As the coal might contain as much before passing into the lower section for carbonization. I n one as 30% water, there was also some doubt as t o whether the heat iarge German installation, the lignite is first dried t o about 12 to available in the clean gas would suffice for this purpose. I n any 13% moisture content, and then briqueted in extrusion presses case, it was desirable t o determine whether a somewhat higher without the addition of any binder, before charging to the Lurgiyield of char could be obtained than that normally produced Spulgas carbonizer. by the carbonization processes available Such a char would Both drying and carhave a higher content of volatile matter than the chars usually bonization are carried produced from subbituminous coals in existing carbonization out b y means of the processes. sensible heat of the As they occur naturally, the subbituminous coals of Alberta products of combuscontain from about 12.5% to 30 moisture, the amount varying tion of the gas obaccording t o the locality in which the coal is mined. When the tained from carbonicoals are exposed t o the atmosphere, laminar cracks develop and, zation of t h e lignite; in time, as the coals dry out, considerable quantities of h e coal light oils and t a r are are produced. The principal mining districts center in Drumrecovered from this heller, Edmonton, and Tofield. Representative analyses of the gas before burning it coals produced in these districts are given in Table I ( l a ) ,reported in t h e combustion on the as-mined basis. chambers. Separate According t o the Canadian classification ( I ) the coals are subcombustion chambituminous B or subbituminous C coals. When carbonized at bers are provided for low temperatures, the individual pieces of coal shrink in size but the drying and carbonization sections. Figure 2 (3) retain, more or less, their original shape. If the coal is in movement during the carbonization process, some disintegration of Figure 2 shows t h a t the pieces of coal occurs. The char is friable and breaks rapidly. baffles, arranged horizontally in series across both the drying and carbonization sections, are provided to ensure the even disLaboratory Tests. The carbonization of small quantities of a n tribution of the combustion gases. The carbonization gas, Edmonton subbituminous coal in the laboratory, gave the readmitted at the base of the retort in order t o cool the char before sults shown in Tables I1 t o v, inclusive. Table I1 gives the it is discharged, is distributed in a like manner. The installation referred t o above is reoorted as being- practically selfTABLE1. CoIMPOSITION O F ALBERTAsUBBITUXINOUS COALS _ supporting from the point of. District Edmonton Tofield Drumheller .I___ _____ view of the heat required for I I1 Ill IV v I I1 I11 I I1 Proximate composition, wt. 7% 25.3 25.0 23.3 28.1 28.4 19.6 20.2 drying and carbonization, but Moisture 18.8 18.0 18.8 7.5 6.5 5.4 7.1 6.2 6.0 7.6 6.6 6.1 6.9 Ash under normal working condiVolatile matter 31.2 30.3 28.0 27.9 28.5 28.6 28.4 30.2 30.1 31.3 44.2 4 4 . 8 4 3 . 0 4 4 . 2 4 2 . 1 3 9 . 0 40.4 41.7 38.5 36.2 tioiis there is little surplus gas. Fixed carbon ---____----I _ _
100.0
LLBERTA SUBBITUMINOUS COALS
Two major considerations determined the commencement of the experimental work on t h e low temperature carbonization of the subbituminous coals of Alberta described hereafter, Approxi-
100.0
Calorific value gross, B.t.u. 10,020 9,870 per lb. Ultimate composition, wt. % 18.8 Moisture 18.0 6.6 6.1 .4sh Carbon 57.6 57.1 3.7 3.7 Hydrogen Nitrogen 1.2 1.2 Sulfur 0.4 0.4 Difference (oxygen, undetermined and errors) 12.5 12.7 100.0 100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
100.0
9,780
9,730
9,280
8,640
8,860
9,160
9,520
8,070
18.8 6.9 56.7 3.8 1.3 0.5
19.6 6.0 56.7 3.7 1.2 0.4
20.2 7.6 54.5 3.6 1.2
25.0 6.2 51.6 3.4
23.3
0.5
25.3 7.1 50.2 3.4 1.0 0.3
0.3
1.0
53.3 3.5 1.1 0.3
28.1 5.4 49.6 3.3 0.9 0.4
28.4 7.6 47.3 3.2 0.9 0.6
12.0 12.4 100.0 100.0
12.4 100.0
12.7 100.0
12.5 100.0
11.8 100.0
6.5
12.3 12.1 100.0 100.0
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
1005
stituent gases, and the composition of the chars with the final temperatures of carbonization are shown in Figures 3,4,and 5 , respecWeight % tively. The yields of char fall a t a rapid and consistent rate up to As received Dry, ash-free 500" C., but between 500" and 600' C. this rate diminishes conProximate composition ... 19.3" siderably. The oxygen present originally in the coal, is evolved Moisture 13.3 Ash principally as water and carbon dioxide and presumably also as 26.8 39:s Volatile matter 4 0.6 60.2 Fixed carbon __ oxygenated bodies in the tar. The fall in the yield of tar be:oo.o 100.0 tween 500' and 600" C. suggests that some cracking of tar takes Wltimate composition place between these temperatures; this results in a marked inMoisture 19.3a ... Ash 13.3 ... crease in the quantity of saturated hydrocarbons in the gas. CarCarbon 50.2 74.5 bon monoxide continues to be evolved in substantial amounts Hydrogen 3.6 5.3 Nitrogen 1.3 1.9 between 500" and 600' C. Up t o 600' C. the gross calorific Sulfur 0.3 0.4 Difference (oxygen, undetervalue of the char increases at a fairly uniform rate as the volatile mined and errors) 12.0 17.9 matter steadily decreases with the increasing temperature of 1oo.o 100.0 carbonization. Calorific value, gross, B.t.u. per lb. 8,480 12,580 Stansfield and others (IO)carried out the stepwise carbonization Calorific value, net, B.t.u. of a sample of Black Diamond coal from the same mine in a per lb. 7,960 11,810 quartz crucible placed in a cylindrical iron retort which was then a l h e tolerance in duplicate capacity moisture dctcrniinntione on the >ani? coal with coals containing over 10% capavity moisture. may be considered heated in a lead bath. Approximately 10 grams of the coal to b6 0.2%; duplicate tests on the same coal in tho Gray-King, low temperature carbonization assay apparatus should give agreement in both the sample were carbonized in steps of 50°, from 350' to 800' C., weights of the solid residue and the combined weights of tar and liquor and the char obtained a t each separate stage was analyzed. respectively, within 0.2 gram. Under these conditions of carbonization, the char obtained at TABLE111. GRAY-KING,Low TEMPERATURE CARBONIZATION 600" C. was found to be of higher calorific value than the chars ASSAYOF BLACKDIAMOND SLACKCOAL obtained a t either lower or higher temperatures of carbonization. Yields of Carbonization products at The calorific value of the char, however, did not decrease markTemp., C. edly until the carbonization temperatures fell below 550" C., but 510 610 320 410 with still lower carbonization temperatures, the decrease in the 61.6 57.6 79.7 70.9 Char weight yo 8.2 4.5 nil Tar height % 4.2 calorific value of the char was rapid. Above 600" C., the calorific 21.5 26.5 19.05 19.7 Liqlor weight % Gas (dther than oxygen and value of the char decreased relatively slowly up to a temperature nitrogen), ml. a t N.T.P.b of about 650' C. Thereafter, as the carbonization temperature 1955 5510 8740 per 100 g. of coal as received 298 increased up to 800" C., the calorific value of the char decreased a Same as footnote Table 11. b Normal temperature and pressure. a t a fairly rapid rate. I n the carbonization process t o be described, 600' C. may thus be considered as approximately the TABLEIV. VOLUMESOF CONSTITUENT GASES(OTHER THAN optimum temperature of carbonization for the production of a OXYGBNAND NITROGEN) char of maximum calorific value. It might be found, however, MI. (N.T.P.") per 100 g. of Coal at more economic to carbonize the coal a t lower temperatures, Carbonization Temp., C. dependent on the yields and properties of the chars produced under 320 510 410 610 272 2220 Carbon dioxide 1420 2950 these conditions, provided, of course, that the heat available in the 165 Unsaturated hydrocarbons 80 210 gas and tar liberated during carbonization sufficed to meet the 920 Carbon monoxide '26 295 1355 Hydrogen 495 ... 940 heat requirements of the carbonization process. Saturated hydrocarbons ... 'iio 1710 3285
TABLE 11. COMPOSITION OF BLACKDIAMONDSLACKCOAL
~
(I
-
a
298 At normal temperature and pressure.
- - 1855
5510
8740
CONSTRUCTION O F PILOT RETORT
The pilot retort, shown diagrammatically in Figure 6 was designed by Stansfield. It was rectangular in cross section, the outside dimensions being 16.75 X 24.25 inches; the over-all height of the complete structure was approximately 23 feet.
proximate and ultimate compositions and the calorific value of the Black Diamond slack coal used. The yields of the carbonization products from the Gray-King, low tem erature carbonization assays are shown in Table 111. Table IF gives the volumes of the constituent gases (other than oxygen and nitrogen) in ml. a t normal temperature and pressure per 100 grams PERCENT of coal, as calculated from the yields and compositions of the gases produced on carbonization of the coal over the respective ranges of temperature. The proximate and ultimate compositions and the calorific values of the chars obtained are shown in Table V. Separate 20-gram lots of a sample of Black Diamond subbituminous coal slack (obtained from the Clover Bar mine of the Great West Coal Company, Edmonton), which had passed a 65-mesh standard Tyler screen, were carbonized under standard conditions to temperatures ranging from 320' to 610" C. in a Gray-King, low p temperature carbonization assay apparatus (4). The final carbonization temperature was attained in the hour and f carbonization was continued a t this temperature for another : hour. The yields of char, tar, liquor, and gas were measured i in the usual way and the volume of gas was calculated to the basis of normal temperature and pressure. Samples of gas ' were analyzed in a modified Bone and Wheeler, gas-analysis apparatus and the volumes of the constituent gases, other than oxygen and nitrogen, were then calculated. The ultimate compositions of the chars were determined,
Yields and Composition of Carbonization Products. The variations of the yields of the products, the volumes of the con-
liT N I P Ob'
OAA8ONIZATION
Figure 3
TEMPERATURE
.t.
1006
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 41, No. 5
retort operation. The total capacity of the baffle smtem from the top covpr plate of Temp. of carbonization, C. Basis the retort, and including the char cooler, was approximately Proximate composition, a t . % ... ... ... ... ... ... ... , . . Moisture 200 pounds of coal packed inta 16.8 , . , 18.7 ., , 20.3 .., 22.1 .,. Ash the column by simple flow from 31.4 37.8 24.0 29.6 14.7 18.4 9.1 11.7 Volatile matter 51.8 62.2 67.3 70.4 65.0 81.6 68.5 85.3 Fixed carbon the coal hopper 100.0 100.0 100.0 100.0 100.0 100.0 100.0 100.0 Air Supply and Combustion Cltimate composirion, wt. 7 0 Controls. On the two sides of ... ... ,.. ... ... Moisture ii:? . . . 15.7 . . 20.3 ... 22.1 ... Ash the retort supporting the baffle 69.0 86.6 71.5 91.9 65.2 80.1 60.9 73.8 Carbon 2.2 2 8 2.7 3.4 4.3 4.7 3.5 3.9 Hydrogen plates, the refractory lining 1.7 1.4 1.8 1.5 1.8 1.4 1.4 1.2 Nitrogen 13-as 3.75 inches in thickness. 0.2 0.2 0.3 0.4 0.2 0.3 0.3 0.4 Sulfur On the other two sides, t h e Difference (oxygen, undetermined and errors) 16.1 19.7 10.9 13.6 6.4 8.0 2.5 3.2 thickness of the lining was re100.0 100.0 1 0 0 . 0 100.0 100.0 1oo. o 1oo.o 1oo.o duced a t G and F to make Calorific value, gross, B.t.u. per lb. 10,160 12,210 10,890 13,400 11,300 14,190 11,730 15,060 some allowance for the water Calorific value, net, B.t.u. per v a p o r a n d carbon dioxide lb. 9,800 11,790 10,570 13,000 . 11,060 13,880 11,520 14,780 evolved in large quantities in upper portion of the carbonization section. Below G, air passages, i%f, constructed in the refractory lining, served for the The retort was enclosed in the sheet-iron casing, C, lagged on introduction of the air for combustion of the volatile matter the exterior and interior with asbestos sheets, between the casing from the coal. The air was supplied t o M through the 2-inch. and the refractory lining. Essentially, the retort consisted of feed pipes, P and V , respectively, at a pressure of 10 inches water. three parts-namely, a hopper, a carbonizing section, and a char gage by a blower driven by a 0.6-hp. motor. These air passages, cooler. The sheet metal hopper, A , 20 inches square and 8 inches . each divided at the top into three separate distribution flues, in height, was tapered to a 8-inch square neck which was welded through which the air was introduced across the combustion to the top cover plate, B; this in turn was welded t o the metal flues through a number of openings 0.5-inch in diameter spaced a casing, C, of the retort. Slack coal from the hopper flowed a brick apart over a vertical distance of approximately 4 feet. directly between two series of baffle plates which were built Turbulence and good mixing of the combustion air and the volacentrally and ran lengthwise through the carbonizing sect'ion. tile matter were thus obtained. The flow of air supplied for The baffle plates, each 9 X G X 0.75 inch, were made of Carcombustion was measured by means of a robmeter, R, placed borundum. They were supported in slotted Carborundum end between the air blower, D,and the feed pipes, P and V . T h e plates a t a n angle of sixty degrees; the end plates were built in two streams of flue gas passed into a common waste gas main the refractory lining of the shorter sides of the carbonizing fitted with a condenser trap and then through a n exhauster into a section. The two vertical series of baffle plates m-ere st,aggered stack. The speed of this exhauster could not be varied. Acso that slack coal would flow freely between them without cordingly, pressures in the combustion flues were controlled by spilling over into the flues. The spacing apart of the baffle varying the size of a n opening cut in t'he common mastc gas main plates was 2.5 inches for a distance of 34 inches from the hopper outlet and 3G inches upwards from the base of the carbonizing section; between, the spacing there was 1.5 inches. When the retort was designed it. was thought that spacing the baffle plates 12,OOP in this way would give a rather longer period for the drying of the coal and the cooling of the char. Operating conditions largely Il,BOD determine the lengths of the drying, carbonization, and cooling zones in the column of coal in the retort, and no clearly marked effect due t o this differential spacing could be observed in actual , ,&io T.4BLE
1'.
COMPOSITION
F R O M BLACKD I A M O N D COAL 320 410 510 610 As proDry, As proDry, As proDry, As proDry, duced ash-free duced ash-free duced ash-free duced ash-free OF
CHARS
,
~~
. 1 .
.
I
.
3
',4OO
B 3.000
w ll,030$
e.000
lO.800q:
C,4002
ic,zca
400 CARBOHITbTIOH
Figure 4
500 TEUPERITURE
600 oc.
CARBOIIIIATICN
I E M P E R A I U R E 'C.
Figure 5
May 1949
INDUSTRIAL AND ENGINEERING CHEMISTRY
arid adjusting the main valve, S, in the air supply line from the blower. The pressures in the combustion flues were measured b y means of calibrated water gages which recorded pressures 16.5 inches below the center of one flue outlet, Pf, and 27.25 inches above the base of the other, Pd. T o make possible t h e o b s e r v a t i o n of combustion conditions when the retort was in operation, o p e n i n g s were m a d e a t t h e bottom of each of the combustion flues and fitted with short lengths of %inch iron p i p e as extensions; each pipe was provided with a glass coverslip, L. For heating up the retort from cold with natural gas, two burners, K, were k e d horizontally, one in each flue, 3 feet above the base of the flue and 6 inches below the lowest air inlet. C o o l i n g and Disc h a r g i n g t h e Char. The char was cooled in a sheet-iron, w a t e r H j a c k e t e d cooler, W, which had two 3-inch * passages with a superficial cooling area of 1770 square inches; Figure 6 the rate of movement of the char through these passages was controlled by the speed of the discharge mechanism. At the outset, the discharge mechanism consisted of a reciprocating rake, constructed of 0.5-inch iron strips, spaced 4 inches apart and held in a metal framework. The rake had a Pinch stroke. Owing t o the rake permitting air to pass into the cooler, the char was discharged in a n incandescent condition. Spraying the char with water simply resulted in the rake ploughing ridges through the wet material and failing t o remove it. Later the cooler was lengthened t o give a superficial cooling surface of 3400 square inches and the rake mechanism was replaced by a rotary compartmented extractor, E , and a small char hopper, H (fitted with a hand-operated sliding door), which could be emptied from time t o time. With low retort throughputs, the temperature of the char on discharge from the hopper was about 60" C. but at high retort throughputs the temperature of the char was as high as 120" C. No difficulty was found in handling the char on discharge, but occasionally the char fked in storage. Temperature Measurement. In the expectation t h a t it might be possible t o follow the course of the drying and carbonization of the coal in its passage through the retort, sheathed chromelalumel thermocouples were spaced centrally at six points along the length of the coal stream. The lowest thermocouple, T6, was 9 inches from the base of the retort, the next, Ta,24 inches
1007
above it, and so on up to all but the last thermocouple. The last thermocouple, TI, was 40 inches below the top cover plate of the retort. Temperature measurements showed fluctuations of as much as 50' C. over a period of 1minute, and, except when low rates of coal throughput and air supply were used, failed to indicate either where drying of the coal neared completion or the progress of carbonization. With low rates of coal throughput and air supply, however, averaged readings of temperatures taken by the top thermocouple, TI, indicated the onset of carbonization, whereas temperatures a t lower levels in the coal stream showed regular and systematic increases and decreases as though carbonization was proceeding in a stepwise manner. Flue gas temperatures, referred to below as T , and T s , respectively, were measured by means of two sheathed chromelalumel thermocouples, one placed centrally in each flue, 9.5 inches below the actual flue gas outlet. These thermocouples were connected through compensating leads t o a Leeds-Northrup temperature recorder; temperatures in the coal stream were measured b y means of a potentiometer, using EI multipoint switch. PRINCIPLES OF OPERATION O F T H E RETORT
After the heating up of the retort with natural gas and completely charging it with coal, the whole of the charge above the lowest air inlet commenced t o carbonize. The volatile matter passed into the combustion flues and burned with the air supplied by the air blower. As partially carbonized and later, carbonized material was discharged from the retort, the heat generated by combustion of the volatile matter carbonized fresh coal flowing into the retort from the coal hopper. Thereafter, the carbonization process could be maintained only if the quantity of heat developed, less that incidental t o the process, met t h e
(00
BO
eo
70
60
50
40
30
20
l6'
.
o
Figure 7 PERCENT
l
o
Cos
requirements for drying and carbonizing the coal. The conditions for the continuous operation of the retort will be understood from a consideration of the relations derived from t h e laboratory data given in the previous tables. Figure 7 shows the percentages of carbon dioxide in the flue gases which would be produced on combustion of the volatile matter obtained by
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1008
I
---
TIEORLIICAI
AIR
100 P E Q C E N T E X C E S S A I R
carbonization of the coal a t 410 ', 510 O, and 610 C., respectively, plotted against the corresponding percentage of excess air in the flue gases. Even under theoretical combustion conditions, the quantities of carbon dioxide in the combustion gas from the volatile matter at, 410" C. and the volat,ile matter a t 610" C. differ.only by just over 1%. The carbon dioxide content of t,he theoretical combustion gas from the volatile matter a t 510' C. is approximately 0.4% lower than t h a t for the theoretical combustion gas at 610" C. As the quantity of air in excess of t,hat theoretically required increases, these differences, of course, will decrease. It will be obvious, therefore, that although the carbon dioxide content of the flue gas will serve to measure the quantities of excess air used, it is unlikely, under plant conditions, to give a satisfactory indication of the kmperature of carbonization of the coal. The greatest quantity of heat lost to the process is tha,t in the due gas; this loss depends not simply on the flue gas volume, which in turn is determined by the quantity of excess air used, but also on the temperature of the flue gas. I n the actual process, other heat losses occur as sensible heat in the char entering the cooler and as heat radiated and convected from the outer surfaces of the retort. These losses, however, are relatively low as conipared with the heat lost in the flue gas. The total heat developed in the process is the difference between the heat available in the coal and t h a t available in the char produced from it, apart from a small quantity of heat which may be derived from exothermic reactions known t o occur when high oxygen-cont'aining coals are carbonized at low temperatures ( 6 ) . In Figure 8 the heat, per pound of coal carbonized, lost t o the ~ ~ O G C SasS heat, in the flue gas is related t o the flue gas temperature. Curves have been constructcd for theoretical combustion conditions and for the use of 100% excess air. For t,hese conditions, three sets of curves are given, based on the data given in the previous tables, for combustion of the volatile matter produced when the coal is carbonized at 410°, 510°, and 610' C., respect,ively. Keglecting the minor heat losses enumerated above and any heat derived from exothermic reactions in the carbonization of the coal, the heat available for drying and carbonizing the coal limits broadly the temperatures of the flue gas a t the outlet above which, under particular condit,ions of air supply, the process cannot conceivably continue in operation. These theoretical limits of flue gas temperatures are indicated in Figure 8. The actual limits of flue gas temperatures, of course, will be considerably lower. O
Vol. 41, No. 5
Thus, if the coal is carbonized a t 510" to 610" C., the outlet flu? gas temperaturrs must be substantially beloa 860" and 880" C., respectively, if i n combustion 100% excess air is used. The lower the quantity of excese air, the higher the permissible outlet flue gae temperature With the aid of the. Rosin and Fehling intensity-temperature diagram (11) an attempt has been made t o approximatc t h e t e m p e r a t u r e s to be anticipated in the comFigure 9 b u s t i o n flues, given that combustion of the volatile matter is complete. Figure 9 shows the result,s obtained in relation t o the quantity of excess air used. As t,hc quantity of excess air increases, the temperat.ures decrease, and any advantage dcrived from carbonization of the coal a t 610" C., as compared with carbonization at 510" C., becomes less nia,rked. Summarizing, the optimum operating conditions of thc new Stansfield retort will be obtained when the flue gas contains the maximum percentage of carbon dioxidc consistent with as coinplete combustion of t,he volatile matter of the coal as is attainable. Under these conditions, heat lost in the flue gas will be minimizer! and the maximum temperatures attained in the combustion flues. The latter, presumably, will determine largely the rate of drying and carbonization of the coal and the carbonizing capacity of the retort. PILOT RETORT T E S T S
Initially it Fas possible to operate the retort only ovci a period of a working day. D a y tests sufficed to show t h a t the retort could be operated continuously, and later, tests were carried out over longer periods. It was found convenient t,o time the comrneneenient of a test from the moment when the retort was first charged completely with coal, but several hours elapsed before steady conditions of carbonization were in any way approached. The average temperatures in the retort before charging it with coal were: Thermocouple Temperature,
C
Tr
Tz
Tg
TI
Ta
Ta
R90
930
1160
1050
560
60
?'I 770
TR 770
As soon as volatile matter distilling from the coal was observed to ignite in the vicinity of the natural gas burners, the supply of natural gas was cut o b and the char extractor and air blower were started a t predetermined rates. The exhauster having been switched on, pressures in the retort flues were adjusted to 0.03inch water gage a t the top of one flue and 0.10-inch water gage a t the base of the other; these pressure conditions then were maintained over the remainder of the test period. Slack coal bclow 1-inch square mesh size, in its usual dry condition as normally delivered from the mine, flowed freely through the retort. T h r presence of as little as 2% additional superficial water, however. caused the free flow of coal to be interrupted and blockage of the baffle system. Throughout the tests, Black Diamond slack coal was used of a composition similar to t h a t given in Table I1 but with a somewhat higher moisture content and lower ash content.
INDUSTRIAL AND ENGINEERING CHEMISTRY
May 1949
TABLE VI. COMPOSITION OF COAL As Charged to Retort
Proximate composition, wt. Moisture Ash Volatile matter Fixed carbon
Yo
Wltimate composition, wt. 3 ‘, Moisture Ash Carbon Hydrogen Nitrogen Sulfur Difference (oxygen, undetermined and errors) Calorific value, gross, B.t.u. per Ib. Calorific value, net, B.t.u. per lb.
24.9 10.8 26.1
1q.2 100.0
1009
TABLE VII. SCREEN ANALYSES O F COAL AND CHAR Dry, A s k Free Basis
... 40.6
59.4 100.0
.__
24.9 10.8 47.9 3.2 1.3 0.4
74.5 5.0 2.0 0.6
11.5 100.0
17.9 100.0
8,210 7,660
12,880 11,910
Coal,
Tyler screen mesh size Below 0.742, over 0.525 inch Below 0.525, over 3 mesh Below 3 mesh over 4 mesh Below 4 mesh’ over 8 mesh Below 8 mesh: over 14 mesh Less than 14 mesh
% by wt.
Char,
% by wt.
...
2.3 23.2 12.1 20.8 le.!
25,O 10.00
12.7 12.3 26.2 20.4 28.4 100.0
...
TABLE VIII. TESTCONDITIONS OVER PERIOD OF ONE WORKING DAY Duration of test from oomwletion of charging retort, hr. . Average coal throughput, lb./hr. Rate of air supply Average COz content of flue gas, 3 ’% Average flue gas temperature,
O
C.
13.5 65 Falling from 2 5 cu. ft. per minute a t s t a r t of test t o 12.5 cu. f t . per minute after 1 hour operation 12.3 for 12 hours: 14.1 for 1.5 hours Two hours after s t a r t of test, teinperature fell consistently from approximately 924 to 730
Average temperature of char enterTo guard against the chance inclusion of exceptionally large ing cooler after 6 hours operaC. 430 tion pieces, the coal was passed through a 1-inch square mesh screen 52 Averake yield of char, wt. % before charging t o t h e retort. Otherwise the coal was exactly as received from the mine. Deliveries of coal were sampled: Composition of Char analysis of the samples showed t h a t there was little substantial D r y , Asbis Produced Free Basis change in the various deliveries, except t h a t the moisture content Proximate composition, wt. 9% ,Moisture nil varied somewhat. 21 6 Ash As soon as a test period commenced, the temperatures measVolatile matter 10.0 12 8 Fixed carbon 68.4 87.2 __ ured in the coal stream were recorded a t 30-minute intervals, 100.0 100.0 three separate readings being taken 1 minute apart and averaged Calorific valuc, gross, B.t.u. per Ib 11,240 14,320 to give the recorded reading. Grab samples of the flue gas were analyzed every 30 minutes TABLE IX. VARIATIONOF AIR SUPPLY for oxygen and oxides of carAverage Average bon, using the Hay’s modificaAir Rate, Average COz Discharge of CO Content Average Flue Char, Lb. of Flue Gas, Cu. Ft. Time, Content of Flue tion of the Orsat apparatus. Period per Min. Hours Gas, % ’ Gas Temp., C. per Hour % Remark? Immediately after charging 1 20 2 Falling t o 9 880 ... 0,s Starting-up period the retort with fresh coal, the 2 15 9 5 hr. above 12, Falling from 880 .. 0.5 Period over whole of the charge above the 4 hr. from 8 t o 600 which COX content indjt o 10 lowest air inlet commenced to 3 11 2 Rising to 12 and 600 ... 0.4 cated air then falling t o rate regulacarbonize. With sufficient air tion required 10 supplied for more or less com4 9 29 . Fairly steady at Slowly fell from 29.6 0.8’ Fairly steady 12 for 7 hr,L 600 t o 500 operation plete combustion of the volalater u p to 1 0 6 11 12 14 Again reached 600 30.4 Excess air intile matter, as partially carboncreased in and remained ized coal was removed by the steady steps over 6 13 6 13 600 34.8 0.6 periods 5, 6, extractor and fresh coal ad7 I5 6 13 Slowly fell from 34.4 0.51 and7 600 to 540 mitted from the hopper, the 0.2 High excess air 8 20 0 . 5 Falling to 8 550 30.0 9 15 2 11 540 30.0 amount of air admitted t o the 0.2) tO 17 7 11 t o 13 Fell slowly from 33.6 0 5 Period before retort was regulated as indi640 t o 350; shutdown of cated necessary by the carflame in one flue plant expired bon dioxide content of the flue gas, until reasonably steady carbonization and combustion TABLE X conditions were approached. Once these conditions were obtained, Air R a t e , Composition of Char, Wt. % ’ Gross CaloriSample Cu. Ft./ Volatile Fixed fio Value the rate of discharge of the char was measured by weighing the NO. Min. Ash matter carbon B.t.u. per Lb. char over a suitable discharge period. The discharge rate 609-48 9 21.5 10.0 68.5 11,370 varied considerably and accordingly rates of coal throughput 610-48 11 21.6 9.8 68.6 11 350 are given as average rates. The char was sampled periodically 611-48 15 21.0 12.6 66.4 11:420 and the samples were analyzed. The average composition of the coal used throughout the retort tests is given in Table VI. Representative screen analyses of the coal’used and of char 1YN.s varied in regular a manner as the conditions of plant produced from i t throughout the tests are given in Table VII. operation permitted. The average coal throughput was 70 The results of three separate tests are selected as representative pounds per hour and the duration of the test was 78 hours, The of the performance of the present plant. t,est conditions are summarized in Table IX. Test No. 1. Test No. 1 indicates the nature of the operating Figure 11 represents the operating data obtained over periods oonditions with the retort over a working day (Table VIII). 4 and 5 in this test. Analyses of the samples of char taken during The operating data obtained during the test are represented the test are shown in Table X. OnsERvATIoNs. The variation in the rate of discharge of the in Figure 10. char was 4 pounds per hour, equivalent t o approximately 10% Test No.2. In this test, the rat,e at which the air was supplied O
1
o’81
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1010 16
E8
Vol. 41, No. 5
7
000
300
800
9 w cc
c 3
700
"a a c w
600
500
0
I
2
3
4
6
5
T I M E 7
-
H O U 8R S
10
9
le
II
400
13
Figure 10
of the average coal throughput during the test. This variation over periods of constant air supply would appear to be the principal reason for the carbon dioxide content of the flue gas varying by about 2%. Generally speaking, the carbon nionoxide content of the flue gas varied between 0.2 and 0.8%; the percentage diminished as the air rate was increased. No precise correlation is evident between the quantity of air supplied and the flue gas temperature, but over a considerable period of the test, the flue gas temperature tended t o fall by about 100' C.; it increased rapidly by about the same amount when the air rate was increased: and then once again decreased slowly. Flue gas temperatures in general, were about 600" C. The analyses of the samples of char given in Table X may be considered t o be fairly representative of the quality of the char produced under the steady conditions of operation described. Test No. 3. I n Test No. 3, the rate of coal throughput was increased in steps from 75 t o 144 pounds of coal per hour. As the coal throughput increased, the rate of air supply was correspondingly increased t o meet the air requirements of the larger quantities of volatile matter produced. The complete test period was 40.5 hours. The results are summarized in Table XI. What may be termed the effective carbonization temperatures at the several rates of coal throughput may be approximated by comparison of the volatile matter contents and calorific values of
the chara, 011 the dry, ash-free basis, with the corresponding values for the chars obtained in the laboratory carbonizations. Figure 12 indicates t h a t the effective carbonization temperatures a t the different rates of coal throughput employed in test KO.3 are approximately: Throughput,
Sample No.
Lb. Coal/Hour 75 92 121 144
Effective Carbonieation Temp., C. 565 600
510 440
The char was cooled effectively for handling immediately but, as in previous tests, on storage, firing of portions of the material
occurred. The bulk density of the char was determined by filling a vessel of known volume, tapping the vessel repeatedly to attain maximum packing, and weighing. One cubic foot of dry char weighed 48.7 pounds. With a coal throughput of 92 pounds per hour, after the air rate was adjusted to 13 cubic feet per minute, the temperat>ure of the char immediately entering the cooler, as indicated by thermocouple T4, increased slowly from 600" to 700" C. When the air supply was again adjusted, this temperature increased further to approvimately 760" C. Increasing the rate of coal throughput to 121 pounds per hour resulted in a fairly rapid fall in the temperature (to about 650" C.) of the char entering the cooler; this temperature was more or less maintained TABLE XI until the maximum rate of VARIATION OF COALTHROUGHPUT 813-48 614-48 615-48 616-48 coal throughput of 144 pounds Sample S o . Throughput, lb. of coal per hour 45 92 121 144 per hour was reached. ThereAir rate cu. f t . per minute 10 15 20 25 after, following a further necesAverage'flue gas temperature, C. 460 375 495 500 Average flue pas composition, Yo by volume sary adjustment of the rate of Carbon dioxide 14.6 15.6 16.5 16.3 Oxpen 2.7 0.9 1.3 1.0 air supply, the temperature CarbDon monoxide 1.0 1.0 0.6 0.1 once more incrcased t o over Yield of char, % by wt. of coal 56 50 54 23 700" C. Apart from variaCOMPOSITION o r CHAR 613-48 614-48 615-48 616-48 tions in the rate of discharge Sample S o . Basis As proDry, As proDry, As proDry, -4s PSODry, of the char due to the variable duced ash-free duced ash-free duced ash-free duced ash-free of the char extractor, operation Proximate composition, wt. yo 23.3 21.2 . . , 19.5 .. . 16.9 .. these changes in the temperaAsh Volatile matter 10.3 i3:4 8.2 10.5 14.5 18.0 21.1 26.2 ture of the char appear to be 6 6 ._ 4 -8 6.6 - 7 0 .0 - 89.5 66.0 82.0 62.0 74.8 Fixed carbon _ 100.0 100.0 100.0 100.0 100.0 100 0 100.0 100.0 brought about mainly- by - the adjustments of the air supply Gross calorific value, B.t.u. per lb. 11,170 14,580 11,530 14,630 11,420 14,180 11,340 13,650 required for the larger quan'
.
I
__
May --... lY4Y
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 11
tities of volatile matter evolved a t higher rates of coal throughput. If at a low rate of coal throughput, the active carbonization zone in the retort is some distance above the position of'thermocouple T4, as the rate of coal throughput is increased, the active c a r b o n i z a t i o n zone must descend slowly in the baffle system and the length of the zone, in turn, must increase until its lower limit reaches the position of thermocouple T4.This stage represents the maximum rate of coal throughput a t a particular effective carbonization temperature. Apparently, as steady conditions of carbonization are approached, there is a tendency for the temperature * near t h e bottom of the carbonization zone to increase, possibly due to some oxidation of the char by the air admitted t o the retort. A decrease in the temperature of the char entering the cooler was indicated by thermocouple Tq when the rate of coal throughput was increased t o 121 pounds per hour. This occurrence suggests that a t high rates of coal throughput the coal is in effect carbonized at lower temperatures. The rate of heat transfer from hot gas t o the coal decreases with the lower effective tempera. t u r e of c a r b o n i z a t i o n . , When the rate of coal throughput is increased still further, eventually a position is reached at which insufficient heat is developed to dry and carbonize the coal and t o meet the heat losses in the retort. After the rate of coal throughput was increased to 144 pounds per hour and the rate of air supply had been adjusted accordingly, C a r b o n i z a t i o n appeared to continue normally for a period of 4 hours. At the end of this period, however, and over a period of 2 hours, the carbon dioxide content of the flue gas
1011
Figure 12
fell. The approaching cessation of the process was indicated earlier by the slow, continuous fall of the temperature indicated by thermocouple TI. The flue gas temperature, on the other hand, was well maintained until within 0.5 hour of the cessation of active carbonization. Owing t o the rapid decrease in the heat available on the combustion of the volatile matter, when carbonization temperatures fell below 500' C., maintenance of steady carbonization at lower effective carbonization temperatures in t h e retort appeared difficult, if not impossible, since relatively slight variations in operating conditions on the plant exercised a marked influence on the continuance of the process. Under these conditions the extent of the heat lost a6 sensible heat in the char and as heat radiated and convected from the outer surfaces of the retort became critical. ACKNOWLEDGMENT
The authors are indebted to J. Fryer for much of the analytical work carried out in connection with this investigation and t o S. J. Groot for the preparation of all the figures. Thanks must be expressed t o V. F. Parry for his helpful criticism in the preparsr tion of this paper. LITERATURE CITED
(1) American Society for Testing Materials, Part 111-A, A.S.T.M. Designation: D 388-38 (1946). (2) Bone, W. A., Proc. Roy. SOC.(London) A99,236 (1921).
(3) British Intelligence Objectives Sub-committee, London, Final Report No. 626, Item No. 30 (1946). (4) Gray, T., and Xing, J. G . , Dept. Scientific and Industrial Research, London, Fuel Research Technical Paper No. 1, (1921); King, J. G., Tasker, C., and Edgcombe, L. J., I b i d . , No. 21 (1928). (5) Hollings, H., and Cobb, J. W., J. Chem. SOC.,107,1106 (1915); Davis. J. D.. Place. P. B.. and Edeburn, P., Fuel, 4, 286 (1927); Burke, S. P., and Parry, V. F., IND.ENG.CHEW, 19, 15 (1927). (6) Hood, 0. P., and Odell, W. W., U . S. Bur. Mines, Bull. 255 (1926). (7) Lignite Utilization Board of Canada, Montreal, First General Report, p. 154, (1924). (8) I b i d . , p. 219.
(9) Mines Branch, Dept. Lands and Mines, Alberta, Canada, Annual Report (1946). (10) Mines Branch, Dept. Mines, Ottawa, Canada, Summary Reports (1918-19). (11) Rosin, P. O., J. I n s t . Fuel, 19,63 (1945). (12) Stansfield, E., and Lang, W. A., Research Council of Alberta, Canada, Rept. No. 35 (1944) I
RECEIVED September 17, 1948. Presented before the Division of Gas and Fuel Chemistry at the 114th Meeting of the AMERICAN CHEMICAL SOCIETY.
Bt. Louis, Ma.
