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The Technology of Low Temperature Carbonization
by
Frank M. Gentry
[ Chapter 6: Processes of Low Temperature
Carbonization ]
Table of Contents
Preface & Table of
Contents
Chapter I ~ Fundamentals
Chapter II ~ Low Temperature
Coal Gas
Chapter III ~ Low Temperature
Coal Tar
Chapter IV ~ Low Temperature
Coke
Chapter V ~ Nitrogenous &
Other By-Products
Chapter VI ~ Processes of Low Temperature
Carbonization
Chapter VII ~ Operation, Design,
& Materials of Construction
Chapter VIII ~ Economics &
Conclusion
Bibliography
Name Index
Subject Index
Processes of Low Temperature Carbonization
Adaptability of Processes
Classification of Processes
Carbocoal-McIntire Processes
Coalite Processes
Freeman Processes
Fuel Research Board Processes
Fusion Process
Greene-Laucks Process
K.S.G. Process
Maclaurin Process
McEwen-Runge Process
Nielsen Process
Piron-Caracristi Process
Sutcliffe-Evans Process
Tozer Process
As may be expected in any new industry, and particularly in one where the public imagination has been fired by the potentialities of by-product recovery in a field where tremendous quantities of raw material are consumed annually, a number of the processes that have been proposed in the past for low temperature carbonization have been prima facie promotion enterprises. Most of the processes, however, which have been developed to effect primary distillation are creditable attempts to solve the problem, both technically and commercially. A number of the retorts, so designed, have failed for lack of adequate technical knowledge of the conditions underlying the art of low temperature carbonization. The absence of strong financial backing, an item of paramount importance in the inauguration of new industrial departures, where markets are unreceptive and where profits accrue largely from full-scale operation, has also been a contributory factor in the failure of a number of other processes.
Aside from laboratory experimental apparatus, with daily throughputs of several hundred pounds of coal, many retorts have been built which are capable of handling from 5 tons to 25 tons of coal per day. Several plants have been erected with a daily capacity of over 100 tons of raw material. In one case, a low temperature carbonization plant with a throughput of 500 tons of coal per 24 hours was erected and several of about this same capacity are now (1928) under construction.
In all, the author has investigated over 175 different processes for the low temperature carbonization of carbonaceous materials and Brownlie (233) states that there are at least 25 more, of which he has selected 50 as typical of the application of low temperature methods to the processing of bituminous coal. The literature, past and present, abounds with methods for the treatment of brown coal, lignite, peat, shale, oil sands, wood, and other materials, rich in distillable hydrocarbons, for the recovery of oils, gas, or carbonaceous residuum, as well as for the coking of coal.
No process of low temperature carbonization is applicable to all raw fuels, both with regard to type and to physical condition. Some retorts are operative with non-coking materials, but not with coking materials; some will function with low grade materials such as shales and lignites, whiles others are restricted to particular coals or to blended charges. As regards physical condition, some processes require a pulverized raw material, some use briguets, and some merely crush the coal to suitable size. There is a corresponding variation in the physical condition of the carbonization residuum, as delivered from the low temperature retort, but this, of course, is a function of three variables, the initial physical condition of the charge, the fuel type, and the manner of distillation, Some processes deliver a semi-coke which is suitable only for pulverization as a power char or for briguetting; others give a material fitted for a variety of purposes after sizing, including a domestic size which is adapted to household fuel.
Finally, no method of low temperature carbonization is adapted to give maximum yields of all the products of distillation. This same statement applies also to the quality of the products. Some retorts are particularly adapted to the production of a high calorific coal gas; others are suitable for the recovery of oils; some strive for domestic fuels and artificial anthracites; and finally, there are those which propose to extract the valuable by-products in the preparation of boiler fuel. In every case, therefore, it is seen that the selection of a process of low temperature carbonization, or the design of a retort to attain that end, is not a matter of simple consideration, but one which, at best, is a compromise and requires considerable knowledge.
The simple classification of low temperature carbonization processes is a difficult matter, because of the numerous processes and their complex nature. Simple comparisons could be made with regard to motion, in which the retorts are found to be static or dynamic; in relation to position, in which they may be horizontal, vertical or inclined; in regard to method of heating, in which the charge may receive its heat internally or externally; upon the subdivision of the charge, in which the processes may involve pulverization, crushing, or briquetting; upon operational procedure, in which the retorts may be single or multi-stage processes; upon the bases of heat transfer, involving the principles of carbonization in thin layers by heat conduction, or by convection; and finally, upon any generalized point of difference of sufficient import to become the basis of classification. Hardly any low temperature process can fall in only one of these simple categories, so that more than one point of difference must be considered in any attempt to reach a satisfactory classification.
Possibly the best classification of low temperature carbonization processes has been given by Fieldner (233) and is reproduced herewith:
Classification of Low Temperature Carbonization Systems
A. Externally heated retorts. Coal in thin layers, not stirred.
1.Vertical layers of coal in narrow retorts.
2. Horizontal thin layers of coal.
B. Externally heated retorts. Coal stirred in contact with
heated surfaces.
1. Vertical retorts.
2. Horizontal retorts
a. Stationary retorts
with interior stirrers
b. Rotating cylinders
C. Internally heated retorts. Coal in direct contact with hot
fluids.
1. Hot gases generated by air or steam blown
into the retort.
a. Coal charged in
lumps or briquets.
b. Coal charged in
pulverized form.
c. Complete
gasification.
2. Hot gases or vapors generated outside the
retort.
a. Combustion
products.
b. Producer gas.
c. Water gas.
d. Coal gas.
e. Superheated steam.
f. Combination of the
foregoing.
3. Melted lead in contact with coal.
D. Two-stage carbonization to control the sticking properties of
coal.
This classification, however, is not the last word in the matter, for obviously no provision is made for processes involving the carbonization of pulverized coal by radiant heat; multi-stage processes may include either or both internal and external heating; and no distinction is made between intermittent and continuous operation, as well as in regard to other distinctive points of difference.
In the final analysis, every low temperature carbonization process has its points of novelty, is gifted with certain advantages, and is confronted with its own peculiar operating difficulties. It is beyond the scope of this book to enter into a comprehensive exposition on the numerous low temperature retorts, but certain processes which are considered representative of an outstanding type and which have been fully tested in operation will be discussed.
Carbocoal-McIntire Processes ~
The Carbocoal process was based upon the patents granted to Smith and was developed by the International Coal products Company. Under the financial support of the US government in 1918, a large plant with a daily capacity of 575 tons of raw coal was erected at Clinchfield, VA by the Clinchfield Carbocoal Company. The data necessary for the design of this plant were secured from the operation of a semi-commercial plant at Irvington, NJ from as early as 1915. Operating difficulties, however, together with changing economic conditions, culminated in discontinuance of the Clinchfield plant in the latter part of 1922. The patent rights were acquired by the Consolidated Coal Products Company, who erected a large-scale experimental plant at Fairmont, WV where the Carbocoal process has been further developed and greatly improved by McIntire.
The Carbocoal (189) process is a multi-stage system involving primary carbonization in externally heated horizontal retorts in which the coal is stirred by paddles; briquetting of the primary char; and secondary carbonization of the briquets in an externally heated inclined retort. The Clinchfield plant consisted of a total of 24 primary retorts grouped into four batteries, as shown in the frontispiece, and 30 secondary ovens arranged in 10 benches. One point of novelty to be noted in this system is that it combines the methods of low and high temperature carbonization to obtain a high yield of oil from the former and to produce an anthracized briquet from the latter. A very good description of the Carbocoal process, its development and operating difficulties, has been given by Curtis (213) and his associates.
The primary Carbocoal retort, used at Clinchfield, is shown in Figure 57. A screw conveyer fed the crushed raw coal to the carbonization chamber, which was of cardiodal cross-section and built of carborundum shapes which were supported on fireclay saddles. A combustion flue surrounded the muffle and two rows of 10 burners heated the flue from below. The entire retort rested upon two heat recuperators in which the combustion air was preheated before being introduced to the burners. Two cast steel shafts, equipped with revolving paddles set at an angle, served to transport the charge to the delivery end and to stir the coal during coking, thus bringing fresh material in contact with the hot sole of the muffle and allowing volatile matter, entrapped within the mass, to escape. When primary carbonization was complete, the semi-coke was pushed into a discharge chute, at the bottom of which it was crushed by breaker arms, and finally delivered by means of a spiral extractor through a water seal to a continuous conveyer. The volatile products driven off during distillation collected in the crown of the muffle and were removed from the carbonization chamber through a gas offtake and scrubber standpipe at the discharge end. The muffle end plates were suspended by springs from the top to provide for heat expansion. The muffle of this retort was about 7.3 feet in maximum width and 16 feet long, while the overall dimensions were roughly 27 feet high, 12 feet wide, and 37 feet long, including machinery, recuperators, etc. The primary retorts operated at a temperature of about 760° C and about 3.5 hours were required from carbonization, giving a normal capacity of about 20 tons per 24 hours per retort.
The semi-coke was received from the primary retort in the form of black, friable, irregular lumps which were prepared for briquetting by crushing to 1/8 inch mesh while hot. The briquets were formed in a roll press, using hot pitch obtained from fractionation of the low temperature tar. This stage yielded a hard dense briquet, suitable for a fuel, but it was not smokeless because of the tar which was introduced for binding purposes. Accordingly, the raw briquets were delivered to the secondary retort for further treatment.
Figure 57 ~ Carbocoal Retort
The secondary Carbocoal retort is illustrated in Figure 58. The selection of an inclined oven for this purpose depended upon its ease of charge and discharge, together with reduction of excessive breakage by elimination of the weight of thick layers of the briquets themselves. The secondary retort was charged with raw briquets at the upper end and, after carbonization, the finished briquets were discharged into a quenching car at the lower end. An inclined horizontal partition divided the retort into two carbonization chambers in order to reduce the crushing pressure until the briquets had been thoroughly hardened by heat. The distillation gases were removed from the crown of the oven at the lower end by a standpipe connected with the hydraulic main. The oven was heated externally on each side by vertical flues, fired from burners located in the crown. The combustion gases passed downward through a heat recuperator, which supported the carbonization chamber and in which the combustion air was preheated. Each of the two carbonization chambers was approximately 14 inches wide, 4 feet high, and 25 feet long. The secondary retort was built of silica brick and measured about 45 feet high, 4 feet wide, and 40 feet long in overall dimensions. The secondary retort operated at about 980° C and approximately 8 hours were required for distillation. The finished briquets were delivered as a hard silvery product, being reduced in size about 25% through shrinkage.
The additional equipment a Clinchfield consisted of a very extensive by-product recover plant for extraction of light oil and ammonia from the gas and for the condensation of primary tar. There was also provided a complete tar distillery for fractionating the low temperature tar.
A very great number of different coals from all over the world were tested in the Carbocoal process. Table 103 gives the ultimate analyses of two coals which were carbonized by this method, as well as the analyses of the corresponding Carbocoal briquets. The high volatile sample of bituminous coal was from Ohio, while the medium volatile specimen, with high ash, was from England. Analysis of another sample of Carbocoal has already been given in Table 78. These analyses should be compared with those of Table 1 to see how early the raw bituminous coal was converted to a fuel resembling anthracite. Reference has been made in Chapter IV, under the subject of power char, to tests made on the use of these briquets as a fuel for locomotives and for marine boilers.
The primary and secondary yields from carbonization of the coals given in Table 103 are tabulated in Table 104, where it is to be observed, as expected, that the high volatile coal was the richest in by-products. Space does not permit a detailed discussion of the character of the products, but certain particulars in this connection have been presented heretofore. In Table 62, the fractionation of a tar from the Carbocoal process has been given; in Table 72, the yield of crude light oil has been reported; and in Table 50 the distribution of various tar acids, as determined by Morgan and Soule (48) has been reproduced. A distillation analysis of Carbocoal tar was given in Figure 24, while the phenolic content of the various tar fractions was shown in Figure 22, to which reference should now be made.
It has been mentioned that the Carbocoal retort was subsequently improved by McIntire (234) at Fairmont, WV. The McIntire primary low temperature retort is shown in Figure 59. It will be noted that there are two distinct departures in this retort from the older design, apart from the many minor improvements relating to accessibility of parts, method of heating, etc. In the first place, the costly carborundum shapes were replaced by a corrugated metal semi-cylindrical muffle floor, which is more durable and gives better heat transmission. And, in the second place, the rotating paddles were replaced by an oscillating shaft whose paddles sweep back and forward through the grooves in the muffle door. The corrugations are cast in sections and are held together by tension produced with springs, so that there is the utmost freedom for expansion, no bolts being used to hold the sections together. The floor of the retort is operated at 650° C, but the maximum temperature of the charge is about 455° C, while the temperature of the gas in the roof of the retort never exceeds 345° C. This retort is said to operate satisfactorily and, up to 1926, some 22,000 tons of coal had been successfully carbonized in it.
Test results in this retort indicate that one net ton of Pennsylvania coal, from the Pittsburgh seam, containing 36% volatile matter, yields about 1,480 pounds of semi-coke, 31 gallons of tar, 2 gallons of light oil, about 10 pounds of ammonium sulfate and approximately 3000 cu ft of gas with a calorific value of 950 BTU/cu ft. The semi-coke contains 10% to 14% volatiles. An analysis of the gas which is evolved from the McIntire retort, as determined by Fieldner (36) has been given in Table 6.
McIntire has conducted a number of experiments with a view of developing a satisfactory oven for the secondary carbonization of briquets, manufactured from the semi-coke discharged by the primary retort. A tunnel kiln and several internally heated retorts were built and abandoned. The carbonizer finally adopted consists of 5 superimposed horizontal retorts into which the briquets are charged on trays by special machinery. The briquets are heated for 30 minutes at a temperature of about 650° C by radiation from the crown and sole of the retort. By this means, breakage is reduced to a minimum and carbonization is said to be complete. The carbonized briquets contain more volatiles than the old Carbocoal briquets, 8% to 10% volatile matte being allowed to remain.
We have already seen in Chapter I that the Coalite process is based upon patents issued to Parker (28, 29), one of the pioneers in low temperature carbonization. Some of these patents were granted as early as 1890, but real progress was not made until 1906, when he introduced the principal of carbonization with superheated steam. The processes are controlled by Low Temperature Carbonization, Ltd., the successor to several other companies engaged in its development. The Coalite retorts are all static, externally heated processes, involving carbonization in thin layers.
After parker's initial experiments, the Eticoal Syndicate erected a plant at Barugh, near Barnsley, England, with a daily capacity of about 50 gross tons of coal. The installation consists of 32 retorts, each composed of a single iron casting, containing 12 vertical tubes in a nest of two rows. The tubes are 9 feet long and taper from 4.5 inches in diameter at the top to 5.5 inches at the bottom. The casting is mounted in a firebrick flue and supported over a discharge chamber, provided beneath for each two retorts. During distillation, the charge is retained within the tubes by top and bottom covers. The temperature of the retorts is maintained at about 650° C for approximately 4.5 hours to carbonize the coal completely. The semi-coke is finally rammed from the carbonizing tubes and allowed to cool in the closed chamber beneath the retort until the next charge is ready for removal.
A test of the Coalite retorts at Barugh was conducted by the Fuel Research Board (235) and the yields obtained from the carbonization of Dalton main, a medium caking coal, are given in Table 105, along with the yields indicated from an assay of the same fuel in the Gray and King assay apparatus, described under the subject of coal assay in Chapter I. The factors given in the table are the proportionality constants that must be applied to yields obtained in the standard laboratory assay apparatus in order to convert them to those expected with the same fuel from the Coalite retorts.
The low temperature gas from the test had a calorific value of 765 BTU/cu ft before scrubbing, and 705 BTU/cu ft after washing to remove 2.13 gallons of light oil per gross ton of coal. The yield of ammonium sulfate agrees with that reported by Armstrong (212) in Table 88. Tests by Brittain, Rowe, and Sinnatt (125) on the distribution of phenols in Coalite tar have been given in Table 50, while the percentage of bases in the various tar fractions was reported in Table 53 and the sulfur distribution in Table 54.
In 1911, the British Coalite Company erected a plant of different design at Barking, near London. This plant consisted of 20 retorts with a daily throughput of 32 gross tons of raw coal. This is a wide narrow oven built of fireclay. It is 11.5 feet high, about 6 inches wide, and is tapered in length from 2.5 feet at the top to 3 feet at the bottom. A sliding door at the bottom and a hinged lid at the top keep the retort closed during coking. After about 7 hours of distillation, the coke is dumped into a triangular shaped cooling chamber beneath the setting, where it remains until the temperature is reduced below the ignition point, so that the semi-coke can be removed without combustion.
The most recent design of the Coalite retort is a modification by Davidson (187) of the retorts at barking, described above. Brownlie (236) has described a setting of 20 retorts of this description at Barugh, where the other setting of tubular retorts, previously described, is erected. With an operating temperature of about 650° C and a carbonizing period of about 8 hours, the maximum throughput of this setting is approximately 36 tons of coal per 24 hours. Figure 60 illustrates the Coalite retort, a modified by Davidson. It is built of firebrick with dimensions of 9.5 feet high, 7.5 feet long at the base, and 11 inches wide. The crushed raw coal is charged from twin hopers at the top through a rotary valve. Just beneath the retort is a cooling chamber, shaped as a right angular sector and built of steel plate, provided with a water-jacket for cooling the coke. A cast-iron rocking cylindrical valve closes the bottom of the carbonizing chamber and supports the charge during carbonization. Firebrick combustion flues and heat recuperators are placed in the bench between adjacent retorts.
The most unique feature of this modification of the Coalite process is the mechanical device which facilitates escape of the gases during coking and final discharge of the semi-coke. Two perforated cast-iron plates are connected by linkage and suspended within the carbonization chamber in parallel position so that the charge is separated into two thin layers about 3.5 inches thick against each side wall of the retort. The center of the retort is thus segregated into a gas-collecting chamber about 4 inches wide by the two perforated plates. As the plastic layer of the charge moves to the center during coking, the low temperature gas finds an easy path through the cooler layers of the charge into the space between the plates, from which it is withdrawn at the top. When coking is complete, the linkage mechanism permits the plates to be collapsed so that, even though the charge may have swelled considerably during coking, it can be removed easily and dropped into the cooling chamber.
It is said that an average coal containing 25% to 35% volatile matter, when distilled in the Davidson modified Coalite retort at 550° C, yields per gross ton approximately 6,000 to 6,500 cu ft of 700 to 750 BTU/cu ft gas, without deduction for heating of the retorts; about 24 gallons of oil, fractionating to 3.6 gallons of light oil, 9.6 gallons of diesel oil, and 9.6 gallons of lubricating oil; 15 pounds of ammonium sulfate, and 1,568 pounds of semi-coke smokeless fuel, containing 8% to 10% volatile matter. A proximate analysis of the semi-coke has already been given in Table 78.
This process bears the name of its inventor, Freeman, and is controlled by the British Oil and Fuel Conservation, Ltd. The Freeman retort belongs to the general class known a multiple retorts. It is an externally heated vertical carbonizing machine in which the raw material is progressively distilled in thin layers upon rotating horizontal plates. This type of retort is suitable for non-coking and weakly coking coals, shales etc., but soon plugs in operation with untreated sticky materials.
According to Tupholme (237), the Freeman retort which was installed at Willesden, London, but now dismantled, was 27 feet high, about 5 feet in diameter, and consisted of 6 stages, as shown in Figure 61. A central shaft carried a series of rotating disks, one to each distillation stage. The cast-iron shell of the retort formed alternate carbonization and combustion chambers, in such a way that the floor of each distillation zone was heated by gas burners from below. A small, annular, gas-collecting chamber was formed above each carbonization chamber and connected with it through perforations, so that the volatile products from each stage could be separately removed and condensed to effect partial fractionation of the oils.
It will be seen from Figure 61 that the crushed raw fuel is fed by a sprocket valve into the top of the retort, where it falls upon the first rotating disk. A set of stationary scrapers, not shown in the illustration, is fitted to the top of each carbonization chamber and a second set of moving plows, also omitted in the illustration, is fastened to the bottom of each rotating platform. The charge is this raked from the center of each disk to the periphery, where it falls to the floor of the carbonization chamber. It is then carried back to the center of the retort by the rotating plows to be dropped, finally, through an annular passage onto the second rotating disk of the next distillation chamber. The material thus zig-zags its passage during coking from the top to the bottom of the retort. The semi-coke is finally ejected in pulvurent form from the last chamber to a discharge chamber.
The Freeman multiple retort at Willesden was tested by the Fuel Research Board (23) before being dismantled to make room for further experiments. The average temperatures maintained in the various chambers during the tests were approximately as follows: 105° C in the first chamber; 215° C in the second chamber, 460° C in the third chamber, 420° C in the fourth chamber, 460° C in the fifth chamber, and 500° C in the sixth chamber. The first distillation chamber serves merely to preheat and dry the coal, while the last chamber may be used either as a heating or cooling stage, as desired. Proximate analyses of two coals and the semi-coke produced therefrom are given in Table 106.
The Kirby coal given in Table 106 was typical of non-caking slack, while the Brynna gas coal was indifferently caking. The maximum temperature of carbonization was 500° C in each case. The products yielded in the carbonization of these two fuels are given in Table 107. The coke was not incandescent when discharged, but it readily ignited due to its extreme combustibility. It was too small for sue as a domestic fuel, but was an excellent material for briquetting or pulverizing. It is interesting to note the low yield of ammonium sulfate, although Table 106 shows the coals to have contained sufficient quantities of elementary nitrogen to give a fair yield, provided the temperature of carbonization was sufficiently high. This was apparently not the case, or else such ammonia as was evolved was rapidly decomposed.
The figures given in Table 107 are based upon a gross ton of raw fuel as charged, which included 13.7% moisture in the Kirby coal and 5.6% in the Brynna coal. The gas from Kirby slack contained 10.9% hydrogen and had a gross calorific value of 895 BTU/cu ft, while that from the Brynna gas coal contained 12.6% hydrogen and had a gross thermal value of 978 BTU/cu ft. Comparison of the yields with those obtained from the Gray and King assay apparatus at the somewhat higher temperature of 600° C showed the following conversion factors for Kirby slack: 0.89 for semi-coke, 0.51 for gas, 0.81 for tar, and 0.24 for ammonium sulfate. The conversion factors for the Brynna gas coal were: 0.95 for semi-coke, 0.53 for gas, 0.74 for tar, and 0.20 for ammonium sulfate.
Fuel Research Board Processes ~
In 1917, the British Government provided for the organization of the Fuel Research Board as a branch of the Department of Scientific and industrial Research, and the establishment of a Fuel Research Station at Greenwich, London. This station, which was initially under the directorate of Beilby and later of Lander, was charged with the task, among other duties, of conducting experiments on the low temperature carbonization of coal and of testing such private plants as requested this service. A number of tests conducted under this later function have been referred to under the discussion of individual processes, so that now only the experiments carried on for the purpose of developing operative retorts will be dealt with.
The Fuel Research Board (114) inaugurated its work with the erection of a battery of 9 horizontal retorts of the type shown in Figure 62. After 6 years of satisfactory operation, all but two of these have now been dismantled to provide room for further experiments. The retorts were built of mild steel convex plates, riveted to two steel channels, which formed the sides. They were 9 feet long. 2.5 feet wide, and 5 to 7 inches deep. The gas offtake connected with the rear wall, while the front was closed by a self-sealing door. The crushed coal was placed in thin layers upon shallow metal trays, two of which were charged at a time to each retort. The trays were formed into a grid of 96 cells by vertical partitions, so that the coke was formed into briquets of convenient domestic size. All the retorts were set in a single heating flue and they were charged and discharged in an order which minimized the cooling effect produced by these operations.
In these retorts, the maximum temperature was not allowed to exceed 600° C, under which condition from 3 to 4 hours were required for carbonization. The yield of products, obtained from carbonizing two coals, which yielded semi-coke of satisfactory coherency when distilled in the Fuel Research Board horizontal retorts, is given in Table 108. The factors for conversion of yields from the Gray and King assay apparatus to the yields from full-scale horizontal retorts were found to be 1.00 for coke, 0.57 for tar, and 0.97 for gas. Figure 17 showed the rate of gas evolution and the calorific value of the gas obtained from these retorts, as a function of the time of carbonization, while Table 26 showed the composition of the low temperature gas. A proximate analysis of the semi-coke was given in Table 78, while fractionations of the low temperature tar were reported in Tables 62, 63, 64, and 72.
The Fuel Research board (239) attempted to effect low temperature carbonization in a battery of standard Glover-West continuous vertical retorts by the simple procedure of lowering the flue temperature. It was not expected, however, that any economical operation could be obtained from such a retort built to function at high temperatures. The retorts were built of oval silica brick shapes. They were 21 feet high and 33 inches by 10 inches in cross-section. Their normal capacity was 2.5 gross tons per day, but when the carbonizing temperature was reduced to an average of 780° C, the daily throughput fell to 1.8 gross tons per 24 hours. A short spiral extractor continuously removed the carbonized charge from the retort and delivered to a cast-iron cooling chamber, from which it was removed intermittently. Four such retorts were placed in a common setting, separated into pairs by a vertical partition, and heated by 7 separately fired horizontal flues. Provision was made for the introduction of steam during carbonization, and the beneficial results of this on the distribution of heat through the retort has been shown in Figure 9.
Table 100 gives analyses of the coal and semi-coke which was obtained in four tests of the Glover-West retorts by the Fuel Research Board, the mean temperature in the combustion chambers being approximately 770° C in each case. The charge consisted of a mixture of 60% Mitchell Main gas nuts and 40% Ellistown Main breeze. Test # 1 was conducted in the absence of steam; 7.24% steam was admitted in Test # 2; 13.47% in test # 3; and 20% in Test # 4. The products yielded from each of these tests are tabulated in Table 110. The composition of the low temperature gas from these tests has already been given in Table 43. It is interesting to note, that increasing percentages of steam give increasing yields of tar.
The effect of various percentages of steam on the composition
of the gas from high temperature carbonization in
the Glover-West retorts has been noted from tests by the Fuel
Research Board (115) given in Table 41. A number of experiments
on the carbonization of peat in Glover-West retorts were also
carried out by the Fuel Research Board (106) and these have
previously been referred to in Table 27, where the yields and
gas composition were given; in Table 60, where a fractionation
of the peat tar was recorded; and in Table 77, where ultimate
and proximate analyses of the carbonization residuum were
reported.
A second battery of 4 externally heated vertical retorts were tested by the Fuel Research Board (240). These retorts consisted of a standard Glove-West spiral extractor connected by a special iron casting with three upper cast-iron sections. The retorts were of rectangular cross-section 15 feet high, tapering from 28 inches by 9 inches at the top to 33 inches by 15 inches at the bottom. All 4 retorts were mounted in a common firebrick setting. With flue temperatures of 650° C, a throughput was attained of 6 tops of briquets and about 5 tons of crushed coal per day. After a good deal of difficulty with the formation of breeze by the extraction mechanism and with bridging of the charge, due both to design and to heat distortion, these retorts were finally dismantled and the information gained from their operation used to improve the retort construction.
The Fuel Research Board finally erected a battery of 4 narrow cast-iron continuous vertical retorts of the design shown in Figure 63. These retorts are 21 feet high and taper in length from 6.5 feet at the top to 6.92 feet at the base. Two of these, designated as "D" retorts, taper in width from 4 inches at the top to 8 inches at the bottom, wile two, designated as "E" retorts, tapered in width from 7 inches at the top to 11 inches at the bottom. In all other respects, the two modifications of the narrow vertical retorts are identical. The retorts are built in three sections of good gray cast-iron one inch thick. A modified Woodall-Duckham extractor, consisting of a curved comb, which sustains the charge, and a toothed wheel, which slowly rotates between the prongs, is used. An outrushing of coke from the side of the extractor is prevented by the pressure of weighted hinged rods. A number of vertical ribs were cast onto the outside onto the outside of the retorts in an attempt to prevent warping. The combustion chamber consists of a plain firebrick setting surrounding each pair of retorts and heated by 48 gas burners arranged in three stages of 16 each. Care is taken to place the burners close to the flue wall, in order to prevent the flame from impinging directly upon the retort castings. Two standpipes at the top of each retort remove the volatile products and deliver them to an hydraulic main.
These retorts have been operated by the Fuel Research Board (241) for over a year at flue temperatures up to 625° C. The thinner "D" retorts were badly distorted by growth of the cast-iron after prolonged operation at 650° C. After carbonization of over 1,350 tons of raw coal of various types in the model "E" retorts, they have been declared successful and arrangements have been completed to give them a commercial trial by the erection, at London, of a battery of these retorts, with a daily capacity of 100 tons, by the Fuel Production Company, Ltd., organized under the joint auspices of the British government and the Gas Light and Coke Company.
Tests demonstrated that, with carbonization temperatures ranging from 605° C to 625° C, the throughput per day of each "E" retort varied from 4.1 to 2.7 gross tons of coal, depending on the fuel used. A number of different coals, both coking and non-coking, as well as blended and briquetted fuels, have been successfully distilled. The yields obtained form three different materials are given in Table 111. The briquets consisted of a mixture of 74% Durham coal, 20% semi-coke breeze, and 6% pitch. Instead of working the retorts continuously, it was found more satisfactory to work them intermittently, fresh charges being introduced every hour, in the case of Dalton Main, and at 2-hour intervals in the case of the other two fuels.
The Fusion low temperature retort is the invention of Hutchins and is controlled by the Fusion Corporation, Ltd. Much of the development was carried out in a 5 gross ton per day unit, installed at the works of the Electro-Bleach By-Products, Ltd., Clifford, England. This system belongs to the type of externally heated rotary kilns which contain an internal deice for agitating the charge. According to Tupholme (242), two types of retorts have been designed, the single and the double-tube type.
The single-tube Fusion retort is shown in Figure 64. It consists of a mild steel cylinder which is mounted horizontally upon roller at the ends. The crushed fuel is introduced at one end of the rotating tube by an automatic feeding device and the coal is transported through the kiln by the combined action of rotation and the head of raw material which is built up by fresh coal at the charging end. At the discharge end of the retort, there is a stationary chamber, into the bottom of which the semi-coke falls to await delivery, while the gas is removed by an offtake and conducted successively to a dust-catcher, condenser, scrubber, and finally top the gas-holder. A star-shaped breaker is placed within the cylinder, but in no way connected with it, so that the rotation of the retort causes the breaker to tumble over and over, stirring the charge and chipping off the hard crust which tends to form on the inner wall when carbonizing sticky fuels. The retort is surrounded by a firebrick flue, containing on one side a combustion chamber from which the hot products of combusted producer gas are extracted by heating flues and circulated around the rotating cylinder.
The double-tube Fusion retort operates on essentially the same principle as the single-tube design. In this modification, two cylinders are mounted concentrically. The inner and smaller cylinder contains the breaker. This construction has two advantages. In the first place, it permits charging and discharging at one end, thus eliminating one gas-tight gland between the rotating and stationary parts. In the second place, the coal is preheated in the inner tube before actual carbonization in the outer cylinder. It has already been demonstrated in Chapter IV, under the subject of preheating and oxidation of the charge, that this thermal pretreatment destroys the coking power of the coal to prevent adhesion of the plastic material to the retort walls, as often occurs in kilns of this design. Ordinarily, if some means is not provided to prevent the formation of this crust, when dealing with sticky materials, it becomes so thick that sufficient heat cannot be conducted through the refractory material to carbonize the charge without raising the flue temperature to regions which endanger the retort shell.
A test of the semi-commercial single-tube Fusion retort at Cledford was made by the Fuel Research Board (243). The tube of the retort is 25 feet long, 2.5 feet in diameter, and contains 5 breakers, 20 inches in diameter, placed end to end. The temperature of carbonization ranged from a minimum of 325° C, in the flue, to a maximum of nearly 625° C, just past the middle of the retort. Under these conditions, the daily throughput was nominally 5 tons, depending on the character of the fuel treated. The Welback Cannel, used in the test by the Fuel Research board, contained 1.8% moisture, 46.6% volatile matter, 36.8% fixed carbon, and 14.8% ash. The carbonaceous residuum from distillation contained 9.7% volatile matter, 64.8% fixed carbon and 25.5% ash. The products yielded by carbonization, per gross ton of raw fuel, consisted of 1196 pounds of semi-coke, 60.2 gallons of oil, 2,740 cu ft of gas, 4.5 gallons of light oil scrubbed from the gas, and only about 3 pounds of ammonium sulfate. It is said that about 65 gallons of tar per gross ton of coal could be obtained by rearranging the discharge chamber. The above yields gave the following conversion factors for yields, as determined in the gray and King assay apparatus: 0.98 for semi-coke; 0.73 for tar; ad 0.86 for gas. The gross calorific value of the gas was estimated as 1,110 BTU/cu ft after scrubbing. The solid residuum was discharged in small pieces of about the size it was introduced into the retort and was, therefore, suitable for briquetting and pulverization.
This retort bears the name of its two inventors, Greene and
Laucks, but it is controlled by the Old Ben Coal Corporation,
Chicago. The experimental work was carried out by the Denver
Coal By-Products Company at Denver, CO but subsequently a
semi-commercial plant was built at Waukegan, IL, according to
Greene (244). The Greene-Laucks process belongs to the general
class of vertical externally heated retorts with an internal
spiral conveyer. It is operated primarily to produce a domestic
fuel from low-grade bituminous coal screenings.
This retort has undergone a number of modification with respect
to minor details, but it remains essentially the same in
principle of design. Figure 65 shows two of the battery of 4
retorts erected at Denver. These retorts consist of a cast-iron
shell about 12 inches in diameter and 18 feet long. Mounted
centrally within this casing is a hollow shaft, 8.5 inches in
diameter, which has a 1.5-inch spiral web cast on its exterior.
Each pair of retorts is fed from a common hopper by a screw
conveyer which introduces the raw coal at the bottom of the
retort. The central core is mounted upon a thrust bearing at the
bottom and is geared to rotate slowly, so that the charge is
forced upward by the motion and discharged at the top after
carbonization. The speed of rotation varies from 1 rpm to 3 rpm,
depending upon the material being processed and upon the
percentage volatile matter which is permitted to remain in the
coke. Each retort is mounted in a firebrick setting, leaving a 4
inch heating flue surrounding the metal shell. Sticking of the
charge to the rotor is prevented by heating the interior walls
of the shaft by a gas burner, consisting of a perforated gas
pipe. The height of this retort was subsequently reduced to 12
feet, while the Waukegan modification was again increased to 18
feet high and enlarged to 3 feet in diameter.
With a shell temperature of about 450° C, extensive tests on bituminous coals showed a yield of low temperature tar amounting to 0.876 gallons per 1% of volatile matter in the raw fuel. The average yield of products per net ton from bituminous coal was 1,400 pounds of semi-coke, 35 gallons of tar, 4000 cu ft of gas, and 12 pounds of ammonium sulfate. About half of the gas, which had a calorific value of approximately 700 BTU/cu ft, was required for heating the retort. A low grade Colorado shale, containing 28% volatile matter and 30% ash, yielded upon carbonization 1,500 pounds of solid residuum, 30 gallons of tar, 4000 cu ft of 793 BTU/cu ft gas per gross ton, and 15 pounds of ammonium sulfate.
The Kohlensheidungs Gesellschaft, Essen, Germany, is responsible for this process, which was invented by Cantieny (245). The development work was carried out at the Mathias Stinnes Colliery at Karnap. A plant of this type, with a large daily capacity, is now (1928) under construction at New Brunswick, NJ by the International Coal Carbonization Company, a subsidiary f the International Combustion Engineering Company, who have acquired the process. The K.S.G. process belongs to the general type of externally heated rotary kiln retorts in which the coal is preheated before actual distillation.
As shown in Figure 66, the K.S.G. retort consists of two coaxial cylinders securely fastened together and mounted at an inclination upon rollers, so that they rotate as a unit. The entire weight of the retort is carried by the cooler inner cylinder, which, according to Mueller (246), is an important advantage, in that the supporting structure never exceeds 300° C, and sagging of the retort, caused by reduction of tensile strength through heating of the steel shell, is avoided.
The K.S.G. kiln, which is installed at Karnap, is 76 feet long. Its inner cylinder is 5.75 feet in diameter, while it outer shell is 10 feet in diameter. The charge requires about 2.5 hours to pass through the retort when the speed of revolution is approximately once in 1.5 minutes. A retort of this size has a daily throughput ranging from 60 to 80 tons. It will be seen from Figure 66 that the crushed raw fuel is introduced by a special screw conveyer, which projects well into the lower end of the inner or preheating cylinder. Spiral flights, fastened to the inner wall of the smaller cylinder, convey the coal to the upper end of the retort, where it falls through discharge ports into the outer or carbonizing cylinder. It is claimed that this pretreatment sufficiently removes the sticking quality of the fuel, so that it does not stick to the walls of the outer chamber. The retort s mounted in an ordinary firebrick combustion chamber in such a way that the hottest zone of the retort is at the point where the pretreated coal falls into the carbonizing cylinder. By this procedure, the pretreated coal is rapidly brought through the plastic stage long before the semi-coke is discharged, thus giving the material sufficient time to cement the coal particles together and harden, with the result that the breeze is reduced to a minimum. The pretreated charge, having been deposited at the upper end of the outer chamber, descends to the charging end under the combined action of gravity and rotation. At the bottom end of the retort, the finished semi-coke is elevated by a plow into a short extremity of the inner cylinder, where reverse spiral flanges transport the material to the discharge exit. The volatile products are withdrawn through a hollow shaft at the upper end of the retort. Provision is made for introducing steam to the carbonizing cylinder through 12 longitudinal steam chests which are arranged around the periphery of the outer cylinder and connected with a rotating valve so that steam is introduced only while coal is above the steam chests. The temperature of preheating ranges from 200° C to 300° C, while that of carbonization ranges from 600° C to 700° C. The superheated steam is introduced at a pressure of about 7.5 psi and at a total temperature of approximately 450° C.
Using a weakly coking German coal, containing 25% volatile matter, 57% fixed carbon, about 15% ash, and 3% moisture, the average yield was 1837 pounds of semi-coke, 11.3 gallons of dry tar, 2420 cu ft of low temperature gas, and slightly over one gallon of light oil scrubbed from the gas. The thermal value of the low temperature gas ranged from 750 to 830 BTU/cu ft. About 85% of the semi-coke was in lumps, ranging in size from 0.5 inch to 4 inches, and proximate analysis showed it to contain 9.3% volatile matter and 18.6% ash. When coals containing a higher percentage of volatile matte were used, a greater yield of tar was obtained, but the throughput of the retort was correspondingly reduced.
The patents on this process are controlled by Maclaurin Carbonization, Ltd. The Maclaurin (188) retort is an internally heated process of the partial gasification type. It combines both the principles of low temperature carbonization and of the manufacture of producer gas.
After his initial small-scale experiments, Maclaurin (247) erected a semi-commercial plant with a daily capacity of 20 gross tons at the Port Dundas plant of the Glasgow Corporation, Scotland. The Port Dundas retort consisted of a gas producer and a carbonizing shaft built into a single setting with a connecting conduit, so that the hot producer gas could be led directly from the top of the gasification chamber into the bottom of the carbonizing shaft. The hot producer gas, rising up through the layers of fresh coal, distilled it with its sensible heat. The semi-coke was extracted at intervals from the bottom of the carbonizing shaft and partly charged to the combustion chamber for complete gasification. The top of the retort was built of steel, as an inverted cone, and the volatile products were led over the top of the funnel to a gas-collecting chamber, from which it was withdrawn to the condensers. The gasification and carbonization chambers were segregated in the Port Dundas retort to reduce the ash that would appear in the semi-coke, and both operations were carried out in a single chamber. It was subsequently discovered, however, that the ash from partial gasification can be very easily separated from the semi-coke, so that segregation of the gasification chamber from the distillation shaft was entirely unnecessary. An analysis of the semi-coke obtained from this retort has been given in Table 78.
Following the experience gained at Port Dundas, a retort of the design shown in Figure 67 was erected and tested at Grangemouth, Scotland, and the results are said to have been sufficiently satisfactory to warrant the installation of a battery of 5 Maclaurin low temperature retorts at the Dalmarnock Gasworks of the Glasgow Corporation for the production of smokeless fuel.
The Grangemouth retort, which somewhat resembles a blast furnace in shape, has a daily capacity of 20 tons. It is square in cross-section and stands about 45 feet tall overall. Externally, the retort tapers from about 8 feet at the top to about 13 feet at the bottom, but internally the maximum width of 8 feet occurs about 7 feet from the base. Air is blown into the retort at the point of its maximum internal width, so that this section of the retort constitutes the partial gasification zone, while the superincumbent layers of fuel constitute the carbonizing shaft. The top of the firebrick shaft is surmounted by a double-walled metal cylinder, into which the raw coal is introduced from a charging hopper immediately above. Any oils, which are distilled from the coal and condense on the cold metal walls, are collected by a trough at the top of the refractory shaft, thus preventing the low temperature oil from trickling down the walls to the hot gasification zone where they would be cracked. A V-shape dividing wall extends across the retort just below the combustion zone and seems to divide the charge so that it can be withdrawn from two discharge doors at the bottom of the retort. The mixture of producer and low temperature gas is withdrawn at the top of the retort, the metal cylinder being built with hollow walls to form an annular gas-collecting chamber. The temperature in the retort ranges from about 900° C in the combustion zone to about 70° C at the top of the retort, the bulk of the volatile products, however, being removed at temperatures from 300° C to 500° C.
A test on bituminous coal at Grangemouth showed a yield, per gross ton of coal, amounting to 1,096 pounds of semi-coke, 18.7 gallons of oil, 27,731 cu ft of gas, and 14.5 pounds of ammonium sulfate. About 74% of the coke was larger than one-inch lumps. The composition of the gas, which had a gross heating value of 247 BTU/cu ft, has already been given in Table 26. The original coal contained 30.5% volatiles, 53.7% fixed carbon, 8.1% ash, and 1.3% moisture, whereas proximate analysis of the large lumps of the residuum showed 3% volatile matter, 81.2% fixed carbon, 13.5% ash, and 2.3% moisture. Most of the ash from combustion was concentrated in the breeze, which contained from 20 to 35% ash.
Tupholme (248) reports some yields when the Grangemouth retort was used to completely gasify the coal for the purpose of recovering the low temperature oil, ammonia, and gas without the production of coke. The fact that the solid fuel is completely gasified, of course, has very little bearing on the characteristics of the tar produced, as long as the tar is distilled out by the sensible heat of the gas before the coke reaches the combustion zone. Table 112 gives the proximate analyses of three low-grade fuels completely gasified in the Maclaurin retort, without the production of smokeless fuel. The thermal efficiency of complete gasification was 81% for the cannel coal, 88% for the bituminous refuse, and 72% for the anthracite refuse.
This process, which is based on the invention of McEwen, is controlled by the International Combustion Engineering Corporation and was largely developed by Runge (249) at the Lakeside Station of the Milwaukee Electric Railway and Light Company. The McEwen-Runge process is an internally heated static vertical retort of the treatment of pulverized coal.
McEwens's initial attempt at the low temperature carbonization of powdered coal was made in a closed circuit of cast-iron pipe 6 inches in diameter and approximately 70 feet in length. In this apparatus the pulverize coal was held in suspension by the rapidly moving superheated gas, which acted both as the carbonizing medium and as the means of transport for the fuel. The completely carbonized particles were finally separated from the heating medium by a cyclone dust-catcher. The original scheme was eventually abandoned when it was found impossible to maintain the dust in suspension at the temperature required for carbonization without maintaining excessive gas velocities, which would necessitate an undue length for the carbonizing circuit.
The Milwaukee development unit, shown schematically in Figure 68, consists of two superimposed steel cylinders, each about 30 feet in length and 6 feet internal diameter. The cylinders are lined with refractory to protect them from the heat. The upper cylinder constitutes the primary retort, where the coal is treated by preheating or partial oxidation to destroy its coking property, while the lower cylinder forms the secondary retort, where the coal is carbonized. The pulverized coal is introduced at the top of the primary retort through 4 feed pipes which project about 8 feet into the cylinder and which are fed by screw conveyers. The raw pulverized coal then falls freely to the bottom of the primary retort against a current of hot air, or products of combustion, introduced by means of a bustle pipe at the bottom of the chamber. By the time the pretreated coal collects in the bottom of the primary retort its agglomerating power has been entirely destroyed and the hot material is fed by screw conveyers through four additional deed pipes into the top of the secondary retort, where it again falls counter-current to a current of ascending hot inert heating gas introduced in the same manner at the bottom of the cylinder. Two combustion chambers are provided, one for the primary and one for the secondary retort. A certain proportion of the coal gas is combusted in the primary heating chamber and the hot glue gas is used to raise the temperature of the primary retort to approximately 320° C. It is found that, if a certain amount of excess air is present, the treatment is far more effective than if preheating alone is used. Very little volatile matter is removed in the primary retort, so that the gas is vented to the atmosphere. A certain proportion of the scrubbed low temperature gas is preheated in the secondary combustion chamber to bring the temperature of the secondary retort to approximately 570° C. The hot semi-coke, which is pulvurent in form, collects in the cooling chamber at the bottom and may be fed hot to a pulverized coal boiler or reduced in temperature below its ignition point by a heat exchanger before being discharged by a spiral extractor. The volatile products are removed at the top of the retort by two gas offtakes which connect with a hydraulic main. Both the primary and secondary retorts are expanded to a diameter of about 13 feet at the top, to reduce the velocity of the gas to such an extent that none of the pulverized material is carried through the gas offtake. The capacity of the Milwaukee installation exceeds 200 tons of raw coal per day.
Using a high volatile Youghiogheny coal, containing 34.6% volatiles, 57.6% fixed carbon, and 7.8% ash on a dry basis, the yield per net ton in extensive tests was approximately 1,400 pounds of semi-coke, 25 gallons of tar, 3 gallons of light oil scrubbed from the gas, and about 4,000 cu ft of surplus gas with a calorific value of approximately 540 BTU/cu ft. When the raw coal was pulverized, so that 60% passed 200 mesh, 85% passed 100 mesh, and 100% passed 40 mesh screens, the resultant semi-coke showed that 7% passed 200 mesh, 22% passed 100 mesh, and 100% passed 10 mesh screens.
The low temperature retort designed by Nielsen (91, 250) is a horizontal rotary kiln, internally heated by the sensible heat of hot gas. The process, originally controlled by Messrs. Laing, Marshall, and Nielsen, is now owned by the Sensible Heat Distillation, Ltd, London. A plant of this type, with a daily capacity of 100 gross tons per day, has been erected in India by the Carbon Products Company. While the general scheme of internal heating is always adhered to in this system, there are a number of variations in the method of applying it. Thus, distillation may be effected by the sensible heat of unignited producer or water gas, by combusted gas, by recirculated and superheated coal gas, and by combusted powdered coal.
The Nielsen process, outlined diagrammatically in Figure 69, is arranged for the use of superheated producer gas, mixed with superheated steam, as the heating medium. A portion of the producer gas is combusted in a gas-fired superheater to raise the temperature of the rest of the producer gas to the required degree. A waste-heat boiler, surmounting the superheater, serves to generate steam for mixing with the heating gas. Once the process is started, a part of the low temperature gas can be drawn from the system and recirculated to replace the lower calorific producer gas, giving a final product of superior heating quality.
Tupholme (252) gives the dimensions of the unit, which was erected in India with 100 gross tons daily capacity, a 90 feet long, 7 feet in diameter, for the first half of its length near the charging end, and nearly 9 feet in diameter for the remainder of its length. Near the discharge end there is an annular cooling chamber 8 feet long and 14 feet in diameter. A hand-operated sliding valve connects the discharge chamber with the retort, the end of the kiln stopping abruptly in a bulkhead. A second sliding door permits the cooled semi-coke to be extracted from the apparatus. The kiln is mounted upon rollers at each end and in the center, being tilted at a slight angle so that the combined action of rotation and the inclination causes the charge to travel through the retort. The kiln is built of plate steel sheets and is lined with refractory to protect the metal from the influence of hot heating gas, introduced at the lower end of the cylinder to secure the benefit of counter-current heating. The installation of the Carbon Products Company is of the simple hot producer gas type.
The average temperature of the heating gas as it enters the Nielsen retort is 750° C and it emerges at a temperature ranging from 120° C to 245° C. The refractory lining, which is of double thickness over the length of the kiln that is especially large in diameter, prevents the metal shell from exceeding 45° C. About 2.5 hours are required for the charge to pass through the kiln. A part of the semi-coke is used to fire the gas producer.
Tests on the Nielsen retort, with a bituminous slack as the raw fuel and producer gas as the heating medium, showed a gross tonnage yield of 21.6 gallons of oil, 44,000 cu ft of gas, and 15.2 pounds of ammonium sulfate. In addition there was a gross production of 1,475 pounds of semi-coke, of which 620 pounds were consumed in the gas producer, giving a net yield of 855 pounds. The gas from the process consisted of a mixture of about 5,000 cu ft of true low temperature coal gas, with a calorific value of about 735 BTU/cu ft, and approximately 39,000 cu ft of producer gas, with a thermal value of about 140 BTU/cu ft. Thus the mixture amounted to 44,000 cu ft, wit a heating value of 230 BTU/cu ft. Analysis of the mixed gas showed it to contain: 5.6% carbon dioxide, 22.7% carbon monoxide, 10.5% methane, 14.1% hydrogen, 45.6% nitrogen, and about 1.6% hydrocarbons and free oxygen. A fractionation of the low temperature tar from the Nielsen process has been given in Table 62 and a proximate analysis of the semi-coke is reported in Table 78. The proximate analysis of several other fuels tested in the Nielsen retort, together with analyses of the semi-coke, are tabulated in Table 113. The Barnsley coal, given in Table 113, was a coking slack which yielded a fairly soft coke; the cannel coal was a coking fuel, and the brown coal was a sample from Germany, which yielded a semi-powdery solid residuum.
This process bears the name of its two inventors, Piron (252) and Caracristi (253). The Piron-Caracristi process belongs to the class of internally heated retorts in which carbonization is effected by means of molten lead. It consists essentially of a long horizontal tunnel kiln, through which the fuel is transported in thin layers by means of an endless conveyer. Two units of this design, each with a daily throughput of approximately 50 tons of raw coal, were erected by the Ford Motor Company, one at their River Rouge, Detroit, plant and one in Canada at their Walkerville, Ontario, plant. Both of these installations, however, were dismantled because of operating difficulties, which are now said to have been overcome.
A Piron-Caracristi retort, designed for a daily capacity of about 50 tons of fuel, is illustrated by longitudinal and cross-sectional views in Figure 70. Its overall dimensions are approximately 47 feet long, 33 feet high, and 22 feet wide. The distillation chamber consists of an arched brick tunnel about 14 feet wide and 6.5 feet high. The floor of the distillation chamber is formed by molten lead, which is maintained in a liquid state by a number of transverse U-shaped cast-iron flues of rectangular cross-section which are placed side by side beneath the lead. The cast iron flues are fastened down to prevent them from floating on the molten lead. The heating flues, which are gas-fired, are built into the firebrick setting alongside the distillation chamber, so that the hot combusted gases can be drawn through the transverse cast-iron flues to maintain the lead at the required temperature. An endless conveyer, built of hinged plates, passes over sprocket wheels at each end of the tunnel, the upper part of the conveyer floating on the surface of the molten lead and the lower part passing through a conveyer tunnel built immediately below the distillation chamber. This serves to transport the charge through the retort. A second apron conveyor is placed in the lower tunnel to receive the finished semi-coke at the opposite end of the retort from which is is charged and return it to the charging end for delivery after it has cooled sufficiently to prevent its ignition. A regenerator or recuperator is placed on top of the setting to preheat the combustion gas by extracting heat from the spent flue gas.
The crushed raw coal is fed in thin layers to the apron conveyer and carbonized in about 5 minutes, during which time it passes through the distillation chamber. The entire heat for distillation is transmitted by metallic conduction from the molten lead, maintained at about 650° C, through the plates of the apron conveyer and into the thin layer of raw fuel. At the far end, the semi-coke falls in slabs upon the return conveyer on which it is cooled, during the return passage through the lower tunnel, before discharge.
Among all molten metals available as a heating agent, lead has a number of advantages which render it particularly suitable. In the first place, it is not attacked by sulfur in the coal gas, except at temperatures considerably higher than those prevailing in the distillation chamber. Furthermore, it has a low melting point and a high boiling point, which prevents freezing and loss of vapor respectively, thereby eliminating the necessity of close temperature regulation. Experience has shown that practically no lead is lost by oxidation and no difficulty is experienced in retaining the molten metal within a refractory structure. The lead losses consist almost entirely of small quantities lifted out of the bath by the conveyer at the discharge end of the retort and most of this can be recovered later.
A test in the Piron-Caracristi process installed at Walkerville, Ontario, using a bituminous coal containing approximately 38% volatile matter, 5.4% moisture, and 4% ash, yielded a semi-coke containing an average of 9.7% volatile matter. The products obtained per net ton of coal amounted to 26 gallons of tar, 3,973 cu ft of gas, and 1,425 pounds of semi-coke. The low temperature gas had a calorific value of about 793 BTU/cu ft. The semi-coke was pulverized and used as a fuel for firing steam boilers.
This process for the low temperature carbonization of coal is frequently referred to as the Pure Coal Briquet process, as well as by the name of its inventors, Sutcliffe and Evans (254). It was developed by Pure Coal Briquets, Ltd, at Leigh, England. The Sutcliffe-Evans process is an internally heated continuous vertical retort, whose chief novelty resides in preliminary preparation of the coal, particularly in super-pressure briquetting before carbonization.
This process depends largely for success upon the washing, grinding and blending of the raw fuel. The crushed coal is usually washed, so that its ash content does not exceed 6%, dried to less than 3% moisture, and then ground to a size determined by the quality of fuel desired. Ordinarily the coal is prepared to pass a 30 mesh screen, which gives satisfactory results without excessive grinding cost. Non-coking coal or coke breeze is next mixed with eh charge in suitable proportions to prevent swelling of the briquets during carbonization. Most British coals give best results with about 22% coke breeze. Finally the mixture is formed into briquets, without the use of a binder, under the extreme pressure of approximately 20,000 psi. A special rotary briquetting press, designed by Sutcliffe and Speakman, Ltd., is used in this stage of the process.
The Sutcliffe-Evans retort, shown in Figure 71, consists of a vertical steel cylinder lined with refractory and provided with a charging hopper and a discharge chamber at top and bottom, respectively. The raw briquets descend the carbonization shaft under their own weight. Two vertical checkerwork regenerators are used in connection with the retort. Part of the low temperature gas is burned in the combustion chamber of one regenerator, the hot spent gas being drawn through the brickwork and finally through a steam superheater. Meanwhile the superheated steam, mixed with another portion of the low temperature gas, is circulated through the other regenerator until the mixture of steam and gas is intensely superheated. The hot heating gas is then led to the base of the carbonization shaft and introduced to the retort, where it carbonizes the briquets with its sensible heat. As rapidly as one generator is cooled down, circulation of the heating gas is diverted to the other, which has meanwhile been reheated. The mixture of volatile products and steam is removed by a gas offtake at the top of the retort.
In a test with Lancashire coal in the Sutcliffe-Evans retort at a temperature of approximately 400° C, there was obtained per gross ton of briquets, 1,400 pounds of semi-coke briquets, 53 pounds of ammonium sulfate, 18 gallons of tar, 4.8 gallons of light oil, and about 17,000 cu ft of gas. The gas had a calorific value of 437 BTU/cu ft and approximately 9,000 cu ft of the gross yield was required for heating the regenerators. The high yield of ammonium sulfate is remarkable. The carbonized briquets contained approximately 5% volatile matter, whereas the raw coal contained 35%. The percentage of volatile matter remaining in the carbonized briquets can be reduced to as little as 1.5% by raising the temperature of the heating gas to 1,000° C.
The Tarless Fuel Syndicate, London, developed and control the
process invented by Tozer (190). In principle, the Tozer system
is an intermittent static vertical cast-iron retort. It is
externally heated and depends upon the method of thin layers and
upon heat conduction by metal walls to effect carbonization at
low temperatures. Beginning as early as 1909, a number of
different retorts were designed, culminating with that shown in
Figure 72.
A battery of Tozer retorts, of the design shown in Figure 72,
with a daily capacity of 25 tons, was erected at Battersea,
London, and tested on many different carbonaceous materials.
This retort is built of silicon cast-iron, which was found to
stand up well after 8 years of service at temperatures as high
as 650° C. The casting is about 10 feet high and consists of 3
coaxial cylinders held together by 4 radial fins. The inner
chamber is kept empty during charging of the retort by means of
a removable stopper, thus providing a gas-collecting passage.
Two quartered annular cells constitute the distillation
chambers, into which the coal is delivered. The retort tapers
slightly from top to bottom to facilitate discharge. The
cast-iron cylinder is provided with a hinged bottom to support
the charge and to provide a communicating passage between the
carbonization chamber and the gas-collecting zone. A removable
lid permits charging at the top and maintains the system
gas-tight. The casting is surrounded by a refractory setting,
which provides an external heating flue adjacent to the retort.
Even though the coal may be as far as 10 inches from a heating
surface at certain points in the retort, sufficient heat is
conducted by the metal walls and fins to completely carbonize
the charge in layers approximately 5 inches thick. When the
temperature of carbonization is maintained at 540° C, about 4.5
hours are required to complete the distillation.
Marshall (255) has reported the yields obtained from a number of different fuels in the Tozer retort, as well as the composition of the residual coke. These data are reproduced in Table 114 and Table 115. The bituminous coal referred to in these tables was a Silkstone slack, the cannel was from Wigan, and the lignite and shale were from Spain. The temperature of carbonization was approximately 650° C in each case. An analysis of another sample of semi-coke, manufactured in the Tozer retort from a strongly caking coal, was given in Table 78.
In addition to the yields per gross ton of raw fuel tabulated in Table 115, the Tozer process gives approximately 4,500 cu ft of 450 BTU stripped gas, from 2 to 4 gallons of light oil being removed in the scrubbing process. A distillation analysis of a low temperature tar, obtained in the carbonization of an ordinary bituminous coal, has been given in Table 62. One point of particular interest in Table 115 is the notably high yield of tar obtained from Wigan cannal. The semi-coke from the Tozer process is said to be suitable for domestic uses and tests have demonstrated its value as a fuel for complete gasification.
