The conversion of the beehive coke plants, in this country, to byproduct plants has been slow, because the coal supplies were near the centers of the steel industry. With the growth of this industry, especially with its development around Chicago, it became necessary to transport large tonnages of coal from the eastern districts and then convert it into coke. The losses due to transportation costs were partly offset by the value of the byproducts recovered.
To save the transportation costs, it was desirable that the coal deposits of Indiana and Illinois be utilized. This coal had always been classified as non-coking; it was also considered unsuitable for the metallurgical field because of the high ash and sulfur contents. The conditions, however, were promising enough to start experimental and development work, which crystallized in the design of the Roberts coke oven.
The fundamental features of most coke ovens, with respect to the application of heating gas and the recovery of byproducts, are the same and, in the last few years, the tendency has been toward the better application of heating to the walls, higher thermal efficiency, and, by the control of heating conditions, the increase of byproduct yields. Structural features also have been improved and the use of high-grade refractory material has allowed the use of higher temperatures with the resulting higher rated capacity per oven per day. Great improvements have been made in both recuperators and regenerators, particularly with reference to the individualizing of each oven with respect to its adjacent oven, so that each can be operated as a unit if desired. The highest development of the flue type of oven has been applying individual regenerators for the recovery of waste heat.
After some research work, it was found that the high-volatile Illinois coals could be utilized for coke by obtaining the better application and control of the heating conditions in the oven. Most of this high-volatile coal that had been used gave a good coke structure under certain conditions. The fact that coke was occasionally made from this coal, demonstrated the possibility of making coke from it at all times provided the conditions under which the coke cell or structure formed could be isolated.
One problem in coke-oven design is to heat uniformally a surface as large as a coke-oven wall so that the results of the heat distribution will be uniform throughout the entire coking mass. Yet, the necessity of this is demonstrated by the fact that when a coke-oven wall is uniformly heated and a sufficient quantity of heat made available for the coking mass a good coke structure will be made from a large number of coals that otherwise do not give good results.
A coke-oven wall of standard size is approximately 43 ft. long and 14 ft. high, or about 600 sq. ft. With about 1200 sq. ft. of surface exposed to a cake of coal approximately 14 in. thick and weighing 30,000 lb., it is necessary that each square foot of the surface exposed to the coking mass be as nearly the same temperature as possible. One of the conditions in the coke-oven chamber that influences the quantity of heat required is the taper, which will vary from 1¼ to 2½ so that there is approximately 15 per cent, more thickness in the discharge side of an oven than on the pusher side, which means that at least 15 per cent, more heat must be made available at that zone. As this variation in thickness is gradual, the heat supply along the wall must be graduated. The variable heating conditions in the height of the wall must also be compensated for both, because of structural features in design and because of the different heat requirements in each vertical zone.
The Roberts oven is designed in three types: regenerative, recuperative, and the combination-regenerator oven, which can be heated by coke-oven gas or by blast-furnace and producer gas; or where it is necessary to preheat both the air and fuel gas. The fundamental principles of these designs are the same. The regenerative type necessitates the reversing of the flow of the air and gas; the recuperative type permits the flow of air and gas in one direction continuously. The ovens are mounted on concrete foundations, which are simply large flat pads of reinforced concrete of sufficient strength and area to support a number of ovens in each battery.
Roberts Recuperative Oven
In the recuperative oven, the foundation contains the ducts through which all the air required for operation passes. By this means the tendency to overheat the concrete foundation is overcome in a simple and effective manner; the incoming air absorbs the heat as rapidly as it is transferred to the foundation, so that the foundation is maintained at approximately atmospheric temperature.
A longitudinal section of the ovens and a cross-section of the concrete pad are shown in Fig. 1. Seven ducts passing through the concrete, the full length of the battery, carry air at atmospheric temperature in the bottom of either the recuperator or regenerator oven, depending on which type is used; Figs. 1 and 2 show how these ducts are connected to the recuperator.
The foundation of a battery of coke ovens must be stable for, if because of expansion and contraction the foundation is ruptured, there is great possibility of rupturing the brick structure above, which would allow short circuiting of air and gas, which might result in bad operating conditions.
Directly on top of the concrete pad are two courses of fireclay brick. These act as heat insulation for the concrete and form a surface on which the silica brick slides when expanding or contracting. From the top of this fireclay course to the bottom of the battery nothing but high-grade silica brick is used. The supporting walls between recuperator or regenerator are made up of silica straights and shapes. They are 18 in. thick and are placed directly under the oven heating walls, so that the maximum support is provided for the entire structure above. Expansion joints are provided for the lower part of the oven chamber, the location of which is shown by the heavy black lines in (a), Fig. 3. This construction effectively prevents leakage from the oven chamber in the recuperator or regenerator chambers; the expansion joint also allows for the vertical expansion of the recuperator. The linear expansion is controlled by buckstays on the ends of the oven and by auxiliary buckstays at the end of the recuperators. The small buckstays are fastened to the main buckstays but may move independently. This arrangement permits the free movement, either vertically or horizontally, of the recuperator brick
regardless of the movement of the oven brick or the supporting walls. The recuperator is made of silica brick, which always expands when going from atmospheric temperature to working temperatures thus permitting the control of the joints, so that leaks are reduced to a minimum. The heavy silica blocks are so laid that all vertical joints are broken. In this part of the structure are located the sole flues and the waste-gas equalizing flues for each heating wall.
From the sole of the oven to the top of the combustion-chamber wall, two adjacent ovens are made up of two heating walls and an intermediate wall. This triple wall is about 30 in. thick and forms a structure of exceptional strength between adjacent oven chambers.
The center of the separating wall is composed of large flat blocks and shapes through which pass the ducts for air and the supply of secondary-fuel gas. On either side of this separator wall are the heating walls of one side of two adjacent ovens. In these walls is incorporated the space for the combustion of fuel gas.
The shape of the brick of which this wall is constructed and the way they are assembled form one of the features of the Roberts oven. The brick is shown in Fig. 4; the manner of laying is shown at (b) and (c), Fig. 3. Each wall is backed by the center wall; the three walls together are sufficiently strong to withstand any stress placed upon them by operating conditions. From (b), Fig. 3, it is evident that the only stresses the bricks have to resist are in the direction of the arrows D, and they will resist these the same as if solid. They are so laid in the wall that the ends in registering form an arch and the pressure along the line EE is on this arch. The entire wall, therefore, is practically as strong as if there were no openings in it; at the same time ample open space is allowed for the passage of the burning gases through the wall.
In laying, each course is offset half the width of a brick, breaking the vertical joints, while at the same time the centers of the brick in one course come directly over the openings of the course below, as shown at (c), Fig. 3. From the standpoint of strength, the effect of this is the same as if each brick were reinforced over 50 per cent, of the area exposed to the coal, and the next brick above or below has the same amount of reinforcement offset on a new center, thereby increasing the strength
enormously. The strength of this interlocked wall structure and, the comparatively weaker construction of flues is at once apparent and the effect upon the extraction and disposal of the heat from burning gases in this wall is influenced in a marked degree, as compared to the flow of gases through a flue.
Experience has shown that it is difficult to hold a flue-like structure tight, there always being a tendency to develop leaks between the combustion and coke-oven chambers, causing overheating by the introduction of gases from the oven chamber in the combustion flues. This uncontrolled supply of gas easily causes hot spots and uneven heat effect upon the coking mass.
A flue structure is unsupported throughout its length, amounting to more than 700 sq. in. in the smaller type of ovens, while the greatest unsupported surface in the Roberts wall brick is only a little more than 14 sq. in.
A common comparison is to measure the total length of a joint exposed to the coke in a coke-oven wall, which is all right when comparing flue-type ovens, but this comparison does not hold true with a structure such as the Roberts oven, because the strength of the wall structure is dependent on the fact that the combustion chamber and the two faces of heating wall are incorporated in one brick, the rupture of which depends on the cracking of this brick in the center of the combustion chamber.
The inherent weakness in the flue-type structure is overcome by the use of thicker tiles between the combustion and coke-oven chambers, this tile in some cases being 6 in. thick and the average is well over 4 in. The Roberts wall is only 3 in. thick between the burning gases and the coal charge; this thickness is uniform from the top to the bottom, and from end to end of the wall. All the parts of this wall are uniform in strength, heat absorption, and conductivity. With the thin wall through which the heat is conducted to the coal, there is still the full strength of a solid wall 30 in. thick. This particular construction permits the apparent contradiction of the belief that thin coke-oven walls are fragile.
Near the top of the Roberts wall, and immediately below the nozzle for the introduction of the primary gas, is a short space in which the baffle brick are omitted; this forms the mixing chamber. While this chamber is short and comparatively narrow and the necessity for great strength is reduced at this point in the wall, the same keyed construction is used, only the baffles being omitted. Above the combustion chamber, the wall is solid except for the openings through which pass the gas ducts and the openings for the top air adjustment.
The crown of the oven is silica but, as the temperature above this point is low, clay shapes form the top of the battery. Clay is a better heat insulator than silica and is also more resistant to the weather. It is also possible to introduce high-grade heat insulating material in portions of the battery top, thereby lessening radiation losses.
Above the clay top brick are the gas headers that supply gas to the primary and secondary burners. These headers are rectangular in shape and the spaces between them and the brick are filled with a cement mixture so that the top of the battery is perfectly smooth.
The ends of the ovens are faced with clay shapes, which act as heat insulation and are also a protection from the weather. These clay shapes lie directly behind the buckstays and cover the entire space between the oven doors. A layer of silica, 33, Fig. 1, between the clay and the wall brick, protects the clay from direct contact with the heating gases. The clay shapes at the ends are made as large as possible, in order to reduce the number of joints; also they are so designed that all joints may be readily pointed up in case the cement falls out from weathering.
As the expansion of clay and silica are quite different, the clay used for these ends will not move vertically to as great an extent as the silica in the heating walls. The silica will, therefore, slide on the clay and, because of the great friction caused by the pressure of the buckstays, the clay will have a tendency to follow the silica in a vertical direction, thereby opening the joints between the clay shapes. This will cause leakage of the burning gases from the wall to the atmosphere, which leakage has caused considerable annoyance and damage, particularly when starting up new batteries.
This difficulty is overcome, in the Roberts oven, by placing on the buckstays a steel angle 34, Fig. 1, that extends over the top of the clay insulating brick. The bottom of the buckstay is secured to the concrete pad, so that the clay shapes are securely held in place and their vertical movement is prevented. The silica slides on the clay without breaking open the joints in either the heating or the insulating walls. The entire surface of the insulating brick remains perfectly bonded and unbroken.
At no time during the heating up or when operating can leaks be detected in this part of the oven. The buckstays remain cool and are unwarped; the alignment of the exterior of the oven is most noticeable. The system of rigidly holding the clay insulating brick in place lessens the labor necessary in keeping the jambs pointed, thus making the luting of the doors much easier.
The door consists of a cast-iron frame tapered to fit the oven chamber. The brick forming the lining of the door are placed in the frame from the back and, being wedge shaped, are firmly keyed into place; the space in the frame behind the brick is filled with powdered insulating material and a steel plate is bolted to the frame. This gives a door that fits in its frame in the oven and has the lining brick firmly keyed so that they will not shift. The insulation prevents radiation and the backs of the doors are so cool that the hand may be placed on them.
The jamb into which this door fits is also of cast iron; the cast-iron jamb is preferred as it can be accurately set and so cemented in place that it will not move or leak. It does not spall, as does the brick jamb, and always presents a smooth surface for luting the door. This care in fashioning the door and jamb prevents leaks around the doors and lessens the labor in luting.
The buckstays used on the Roberts oven are heavy, for the strain placed on them is enormous. In most of the early installations, where the buckstays were of insufficient strength, they bent to such an extent that it was difficult to place the doors in the oven.
The usual practice in the United States is to carry the charging car, or larry, on rails laid directly on top of the brickwork of the ovens. As this car carries from 13 to 16 tons of coal, it sets up considerable vibration in the brickwork of the ovens when moving over the top.
In the Roberts system, the larry is carried on rails laid on top of the buckstays, so that the weight is carried direct to the foundation of the battery and the top is relieved of the shock from a heavy moving load. In addition, the battery top is cleared of all obstructing rails, there is less danger to the men on top from the moving larry, also the rails carrying the current for operating the car are placed on the side, well out of the way.
Operation of the Roberts Oven
The operation of the Roberts oven is extremely simple and may be readily followed in Figs. 1 and 2. The air enters through the tunnels 30 and passes up the smaller ducts 1 into the air equalizing duct 2. The openings 1 are regulated by dampers 3, which are controlled from the outside and are easily seen through openings left in the ends of the recuperator walls for this purpose. From the equalizing ducts 2, the air passes up around the outside of the recuperator tile. This passage is alternately between the tile and the supporting pier and then between the tile so that each tile is surrounded on three sides by the ascending air. In this manner, the air is thoroughly heated and the tile maintained at an even temperature differential, and at the same time the supporting piers are kept at a temperature no higher than the recuperators.
As the air flows countercurrent to the waste gas, it meets progressively hotter tile as it ascends and, on reaching the outlet ports 4, has attained the temperature of the waste gases at this point. The air then flows through the ducts 6 to the top of the heating wall, horizontally through the passage 7, then downward through the air ports 9 that surround the primary-gas nozzles 10. The quantity of air admitted to the air ports 9 is controlled by the slide brick 8. This brick is reached from the top of the battery through the openings 35 so that, by means of a short iron rod, the operator may accurately regulate the size of the opening 9.
As the recuperator tile is of silica with a high heat conductivity, the air is raised to 2000° F. at the point where it leaves the tile and enters the duct 6. Passing upward through this duct, the air arrives at the tip of the burner at a constant temperature throughout the entire oven. The advantage of having the air reach the burner at a uniform temperature at all times and throughout the entire battery cannot be over-rated as uniformity of heating can only be obtained, when conditions governing combustion are uniform. The falling off in air temperature because of the cooling of a regenerator is well known; its effect has been measured and is admitted to be a factor in the heat effect of a reversing oven. The uniformity of preheating the air in the Roberts oven is but one of the points attained by the designers in the effort to eliminate “average conditions.”
After the byproducts have been extracted, the fuel gas returns to the battery and then enters a large gas header, which is supported on steelwork attached to the buckstays on the coke side of the battery, occupying on this side approximately the same position as the collecting mains on the pusher side. From this header, the gas is distributed to the individual gas headers 11 which are embedded in the top brickwork so that the top of the battery is smooth and unbroken
These headers B, Fig. 5, are of square cross-section and are divided into two pipes of equal capacity. One side carries the primary gas and the other side carries the secondary gas.
The delivery of the gas to the individual headers is through a manifold, Fig. 6, equipped with shut-off cocks; from the rectangular headers the gas passes to the burners through the burner cock 12. Thus the gas may be shut off from the entire oven or from any individual burner cock. All the cocks mentioned are only for shutting off the supply and not for the regulation, as this regulation is by means of orifices in the manifolds supplying the burner headers and in the body of the burner cocks.
The regulation system of the Roberts oven is based on the fact that the flow of gas through an orifice is proportional to the pressure of the gas. A disk is inserted in each manifold supplying primary and secondary gas just below the shut-off cock. This disk has an accurately drilled hole of such size as to pass the required quantity of the primary and secondary gases. If the amount of secondary gas is to be 20 per cent, greater than that of primary gas, the disk for the secondary gas will have an orifice 20 per cent, larger than the orifice through which the primary gas passes. The proportions of the two gases are accurately known and all the disks are drilled before the ovens are heated and when once set in place need never be changed. To change the coking time, it is only necessary to raise or lower the pressure on the main battery header, and the primary and secondary gas orifices will pass the exact proportions of gas required. The supply of gas to the individual headers is thus correctly proportioned at all times and is independent of setting a valve or cock to what must be only an approximate position.
The gas supply through the individual burner cocks is cared for in a similar manner; a cross-section of the gas cock is shown in Fig. 5. The core of this cock is drilled with two holes set at 180° apart; as the core may be turned through 360°, either hole may be made to register with the gas inlet A. If the core is turned to a position midway between the two openings, the gas supply will be entirely closed. In practice, one of the holes is made the full size of the gas inlet but the other is drilled, with extreme accuracy, to the size necessary to compensate for the taper of the oven.
The oven chamber is wider at the coke end than at the pusher end, so that the coke will readily push from the oven. In the Roberts oven, this taper is generally 2 in.; that is, the oven will be 2 in. wider at the coke end than at the pusher end but this taper is uniform throughout the oven as the walls are built without offsets. It is obvious, therefore, that the thickness of coal at the coke end will be greater than at the pusher end; and this thickness will vary uniformly between the ends of the oven, so that it is necessary to burn more gas at the wider end if the entire mass of coal is to be coked in the same number of hours. Also the quantity of gas burned in any part of the oven should be graduated to this increasing taper of the oven from end to end.
The small holes drilled in the core of the cocks gradually increase in size from the pusher side to the coke side of the oven. This increase in size is accurately proportioned to the increase in the amount of the coal charged. As the cores are iron, they may be drilled to the thousandth of an inch and the areas of the holes graduated with great exactness. In the case of a 2-in. taper and a charge of 15 tons per oven, the Roberts oven will have approximately 15 per cent, more coal at the coke end than at the pusher end. The gas burned at the coke end will, therefore, be 15 per cent, greater than at the pusher end of the oven and each intermediate burner will supply the exact amount necessary to coke the coal in the portion of the oven heated by that particular burner.
As the increase due to taper is known, these cocks are drilled before the oven is put in operation and are not changed; for, as is the case with the supply to the headers, all that is necessary to change the coking time is to change the pressure. As the pressure is changed, each burner cock will carry the correct proportion of gas necessary to do the work at that point in the oven. The result is uniformity of heating and the coking of the entire charge in exactly the same time. Such uniformity is impossible without accurate regulation, such as is attained in the Roberts oven.
The vertical-flue reversing type of oven cannot accurately allow for this taper in the oven for it can only be regulated for average conditions. There is an attempt at regulation by increasing the pressure on the coke-side gas header, but as the individual gas nozzles cannot be regulated, the supply to each nozzle is not under close control.
In the Roberts oven, the fuel gas is introduced at numerous points in small, accurately measured quantities, so that the relative temperature of one part of the wall compared to the other is under control, and no point is subject to overheating from the rapid combustion of a large quantity of gas introduced at one point as in other ovens.
Combustion and Flow of Combustion Products
After passing the accurately graduated gas cock 12, the gas is conducted to the burner nozzle 10 where it meets the air flowing through the air ports 9 and combustion starts in the short mixing chamber 13. The gas introduced at this point is called the primary gas and is approximately one half of the total supplied to the heating wall. The rest of the gas, called secondary gas, is introduced into the secondary-gas ducts 31 through the same type of graduated cock as the primary gas. The secondary-gas ducts pass downward through the center wall to a point 32 about midway to the bottom of the oven and there enter the heating wall.
The initial combustion of primary gas is in the mixing chamber 13, as it is introduced with the total amount of air necessary for combustion of the total gas supplied to that heating wall; that is, this gas will meet twice the amount of air required for its combustion. This large excess of air acts as a depressent to the flame temperature. As the air is not preheated to flame temperature, there will be a reduction in the temperature of the flame corresponding to the amount of heat required to raise the excess air to the average temperature of the mixture of burning gas and air.
The gas and air are also introduced at a neutral pressure so that they mix very slowly, with the result that combustion takes place quietly and evenly. The mixing chamber aids in extracting the small amount of radiant heat in the burning gas. The gas used for heating a coke oven usually contains but a small percentage of the illuminants, therefore, the radiant heat generated is proportionately small.
At the bottom of the mixing chamber 13, the burning gas meets the standard checkered-wall typical of the Roberts construction. The stream of gas in each mixing chamber will here be broken into three parts by the wall brick. As there are 24 primary burners in each heating wall, there will be 72 streams of burning gases in the wall from the bottom of the mixing chambers downward. Such an extremely uniform distribution of the heating gases is not found in any other type of construction.
By impinging on the brick at the bottom of the mixing chamber, the gas and air are more intimately mixed and the extraction of heat generated by this combustion is increased. Sweeping around the brick in the top row of checkers, the burning gas flows downward directly on top of the brick in the next tier below, moving in this way to the bottom of the wall. The result is complete combustion; and as the brick are entirely surrounded by these gases and also present the maximum surface for the absorption of the heat, a high degree of heat extraction is attained.
As a matter of fact, the extraction of heat is so complete that at a point 32 about midway down the wall the primary gases cannot heat the wall to coking temperature. At this point the secondary gas is admitted in the proper proportion to continue the generation of heat in the lower portion of the wall. Sufficient air is introduced with the primary gas to support also the combustion of the secondary gas; the oxygen in this air will, therefore, be available for combustion with the secondary gas at its point of introduction. At this point the temperature of the secondary gas is high, also the temperature of the waste gases from the primary combustion. The combustion of the secondary gas, however, will be subdued by the high proportion of inert gases present. These inert gases are the carbon dioxide and water vapor from the primary combustion and the nitrogen present in the air introduced with the primary gas. The combustion of the secondary gas will, therefore, be quiet and even. As this combustion takes place directly in the same checker brick of the wall, it is complete, and the extraction of the heat is as perfect as in the case of the primary gas.
As nearly all the available heat is extracted from the waste gases by the time they reach the bottom of the walls, the two streams are combined by the ducts 14 and this combination produces sufficient heat to maintain the sole of the oven at coking temperature. The coal in the sole of a Roberts oven is coked in a vertical direction by this method. The heat is thus extracted from the burning gases to such an extent that on reaching the lower sole flue 15 the gases contain only sufficient heat to be used for preheating air.
The upper sole flues 16 are connected to the lower sole flue 15 by six openings, each of which is controlled by dampers 17. By these dampers, the differential in the heating wall is maintained uniformly from end to end. These dampers are readily reached through openings left for this purpose.
From the lower sole flue 15, the waste gases pass down through 18 and through the top pass of the recuperators to the downcomer 19, then through the lower pass into 20 and offtakes 21 to the waste-gas tunnel 22. The waste-gas offtakes 21 are equipped with butterfly dampers 23 by which the draft for the oven is regulated; these dampers are readily set from the exterior of the offtake.
The ideal sought in any coke oven is the completion of the coking of the entire charge at one time; this can only be attained when there is uniform distribution of heat from the top to the bottom of the oven and a gradual distribution of heat from end to end to compensate for the taper of the oven and the increased thickness of the charge toward the coke side because of this taper.
The progress of heat, and consequent heating in an oven, in the vertical planes should be parallel to the walls of the coking chamber and the rate of this progress should be proportional to the thickness of the coal charge between the wall and the center of the chamber. The walls of the chamber are perpendicular, therefore, the thickness of the charge will be practically the same throughout its height, but the thickness will vary from the pusher end to the coke end in proportion to the taper of the oven.
Temperature in the Oven Top
The temperature of the space above the coal charge should be as low as possible, for it is through this space that the gases distilled from the coal pass on the way to the collecting mains. These gases are composed of hydrocarbons, some of which are readily broken down by heat; this breaking down results in free carbon and the destruction of valuable byproducts. The initial application of heat in the Roberts oven is at a point well below the top of the coal charge. The combustion is then downward, so that the tendency to overheat the top is eliminated. The temperature of the upper zone of the chamber may be changed by changing the amount of primary gas burned; this is done by changing the size of the orifice or the gas pressure.
Generation of the Heat
The heat necessary for coking the coal is generated by burning gas in the walls of the ovens. Usually this is a lean coke-oven gas of about 525 B.t.u. per cu. ft. Sometimes producer gas is used, but then it is necessary to preheat the gas as well as the air in order to maintain the necessary flame temperature.
The principal factors governing the burning of a gas are the ignition temperature and the rate at which the particles of combustible matter in the gas combine with the oxygen present. This rate is dependent on the quantity of oxygen present and its dilution by inert gases, and the velocity of the gas and air streams during combustion.
All coke ovens operate at a temperature sufficiently high to ignite the gas. Combustion will, therefore, be sustained as long as air and gas are admitted to the walls. The temperature at the point of admission will, however, have considerable effect on the rapidity of the combustion. If the temperature is high and there is just sufficient oxygen to combine with all the combustibles, a high flame temperature will result and the evolution of heat will be rapid.
Rate of Combination of Gas and Oxygen
The lean coke-oven gas generally used to heat up an oven ordinarily contains a high percentage of hydrogen. There is also a high percentage of methane, which, on burning, decomposes, producing more hydrogen. The calorific power of hydrogen is not great but it is rapidly combustible , and produces a high flame temperature.
The introduction of this gas into a highly heated chamber, such as the heating wall of a coke oven, with the theoretical amount of highly heated air necessary to burn it, produces a high local temperature. If, in addition, the gas and air have a comparatively low velocity, the mixing will proceed at a high rate with the evolution of large volumes of heat.
The rate at which the gas is introduced (the quantity per unit of time), sufficient air for combustion being present, will determine the rate at which heat is generated. In those ovens, both vertical- and horizontal-flue types, having but a few points for the introduction of the gas, the quantity of gas burned at each point will be far greater than in an oven using many points of introduction. In the Roberts oven, there are 96 points of gas introduction while most ovens of the flue types introduce gas at not more than 16 points. It is obvious that, in the same coking time and using the same quantity of gas per ton of coal carbonized, the oven using but 16 points for the introduction of gas will burn six times as much gas at each point as the Roberts oven will burn at each of the 96 points. The danger from local overheating in the latter oven is further lessened by the method of transmitting the heat to the coal.
The temperature produced by the combustion of the gases sets up a heat flow that tends to balance the temperature difference between the heating gases and the oven wall with which they are in contact. This balance is influenced by the area of the wall in contact with the heating gases and the rapidity with which the heat is transmitted through the wall to the coking mass. In the Roberts wall, the area for the absorption of the heat is two and one-half times as great as the area that distributes this heat to the coking mass. It is believed that the Roberts oven will transmit the heat generated by the combustion of the heating gases almost twice as rapidly as is done by other types.
The air supply in the Roberts oven is controlled by the damper 3 at the entrance to the recuperators and by the slide brick 8 over the air port 9. By means of the damper 3, the flow of air through the recuperator is equalized so that all parts will receive the same amount of air to preheat; this assures uniform temperatures in the recuperator and so reduces movement of the brick as the result of expansion and contraction. These dampers also regulate the quantity of air supplied to each oven. In other types of ovens, the air port is used first for the passage of air and, on the reverse, for the passage of waste gases. The quantities of air and waste gas are not the same, therefore, the conditions under which they should be governed are different.
In the Roberts oven, all the air is introduced at the top of the heating wall with the primary gas. As the latter is approximately only one half the gas required in the oven, there is no question of there being sufficient air for perfect combustion of the primary gas. The control of the heat generated by the primary gas is, therefore, dependent on the adjustment of the primary burners alone.
After leaving the mixing chambers, the burning gases enter the zone of checked brick typical of the Roberts oven. As this portion of the wall is practically one single chamber with inter-communicating passages, the air and gas are free to move from one part to the other so that not only is combustion of the primary gas complete but the secondary gas will come, into contact with sufficient air to make combustion in the secondary zone complete. The control of the heat generated by the secondary gas is, therefore, dependent only on the amount of secondary gas and this control is consummated with the same accuracy as in the case of the primary gas.
Control of the Heating Gases
The control of the heating gases in the heating walls plays an important part in the distribution of the air, and this movement is controlled by the distribution of the draft. The regulation of the draft or differential (that is, difference in pressure between the point of admission of the gas and air and the pressure of the outgoing waste gases) through the various parts of the heating walls of the ovens is of great importance, because, in conjunction with the air slide regulation, it determines the volume of air introduced also the velocity of flow and distribution of the heating gases through various parts of the combustion chambers.
The Roberts oven is clear of controls and obstructions from the point of admission of the gas and air to the point where the waste gases leave the recuperators. The controls are located at the points of admission of the gas and air and can be accurately adjusted. The gases are unobstructed in their flow after their admission, except for the accurately designed motion through the wall. The products of combustion then pass through the damper-controlled openings that regulate the distribution of draft. These dampers 17 are easily accessible and may be readily set to obtain the proper distribution of draft. The essential feature of this regulation is the maintenance of the proper differential from end to end of the oven necessary to move the graded quantities of heating gases; the maximum differential is maintained at the coke end and the minimum at the pusher end.
The method of controlling the gas supply to the headers and individual burners has been described. The effect of this control and the distribution of the gas through many burners may be summed up as follows:
- The number and arrangement of the points of introduction gives accurate control of the heat at all points in the oven walls.
- The quantity of fuel gas introduced at any one point is reduced to a minimum, and the amount of heat generated in a restricted space is proportionately small.
- The arrangement of these burners is such that a relatively large area of brickwork is exposed to the gases for the absorption of heat as it is generated.
- There is 100 per cent, excess air on the basis of the air required for the fuel gas introduced at the top or primary burners, which serves to temper the flame temperature, the entire body of gases in the wall absorbing heat and tending to attain the same temperature.
- The combustion of the secondary gas takes place in an atmosphere containing a high percentage of inerts (produced by the combustion of the primary gas) with the production of a low flame temperature at the point of introduction of the secondary gas and a progressive combustion with a sustained heating effect as the gases pass down through the wall.
- The introduction of the gas from the top, by means of the evenly distributed ducts, serves to reduce the temperature of the brickwork in the top of the ovens protecting the products of the distillation from decomposition during their travel through this portion of the oven.
Extraction of Heat from the Heating Gases
One result of introducing the gas in small quantities at many points is the production of a very small amount of radiant heat. The gas used is a lean coke-oven gas with a low percentage of illuminants. This radiant heat is extracted in the short duct at the top of the wall in which the initial combustion of the primary gas takes place.
The transmission of the sensible heat of combustion is by conduction, which will take place more rapidly if the particles of the burning gases come directly in contact with the brick. In the flue structure, only part of the gases come directly in contact with the brick, the rest sweep past the flue walls some distance from the surface of the brick so that their heat can reach the brick only by conduction through the outer layers of gas. Practically, all gases are poor conductors of sensible heat so that there is poor extraction of the heat generated by the combustion.
In the Roberts oven, the heating gases are brought into actual contact with the brick in the heating chambers. They impinge directly on top of the wall brick, slide off at a slight angle, and drop to the tier below where they again impinge directly on top of the brick in this tier. By this method, the brick is entirely surrounded by the burning gases and each part of the brick will receive a uniform amount from them. The effect is the same as the baffling of the tubes in a boiler or the checker brick in a regenerative chamber. The passages for these streams of gases are but 3 in. wide, whereas in the flue-type oven the flues are often 15 in. or more in width.
Transmission of Heat to the Coal
There are two principal factors in heat transmission through a solid medium: The temperature differential and the conductivity of the transmitting medium.
The generation and extraction of heat in the Roberts oven has been so well worked out that the heat is supplied to the heating wall progressively and it is transmitted to the coal at the same rate, thereby maintaining a constant temperature differential between the combustion chamber and the coking mass without any tendency toward excessive temperature in any zone.
The production of heat is uniform from top to bottom of the wall and from end to end of the oven. As the wall is of uniform thickness, the transmission of heat will be uniform in all parts. The absorption of heat and its transmission to the coking mass will also be proportional to the surface exposed to the heating gases and the ratio of this surface to the surface in contact with the coal. From the shape of the brick and the method of laying them, a far greater surface is exposed to the heating gases than is possible with a flue construction.
Recovery of Heat from the Waste Gases
The recovery of the heat from the waste gases, after leaving the heating walls of the oven proper, is accomplished in three ways: (1) By waste-heat boilers. (2) By regeneration, (a) common regenerators; (b) individual regenerators. (3) By recuperation, (a) common recuperators; (b) individual recuperators.
The amount of heat reclaimed from the products of combustion is dependent entirely on the temperature to which the gases going to the stack can be reduced, and at the same time have them sufficiently hot to eliminate them from the oven system without the aid of auxiliary power. The choice between regenerative and recuperative settings is a matter of individual opinion.
The object of heating a coke oven is the effect on the coking mass and the more uniform the application of heat the more uniformly will the coking be carried on. To produce coke with uniform cell structure, the heat must be applied evenly and continuously throughout the entire oven. Heat-effect curves of the Roberts oven are shown in Fig. 7.
A section of the oven showing the points of admission for air and primary and secondary gas is illustrated at (a). Curve AA is the theoretically perfect curve for the application of heat to the coal charge; curve BB is the curve produced in the Roberts oven. Curve GG represents what would occur if all the gas and air were admitted at the top of the combustion space.
Curve DDEE is what would be expected if the gas and air were introduced at two points with just sufficient air at each point for complete combustion and this combustion took place in a flue. The dotted portion of curve BB represents the rise in temperature as the gas passes through the top of the oven to the mixing chamber. At this point, the gas meets the highly heated air, the volume of which is sufficient for the combustion of both the primary and the secondary gas. There is, then, a great excess of air over that required for the primary combustion, as a result the flame temperature is depressed and the curve flattens out as shown from L to M. At the point M, the burning gases meet the checkered construction, and the curve continues in a practically straight line to the point where the secondary gas is introduced.
At this point, it might be supposed that the introduction of the secondary gas would make the curve take the form EE but, as this gas is introduced into an atmosphere containing a high percentage of inert gases resulting from the primary combustion, the reaction between the combustibles and the oxygen present is subdued and the generation of heat is continued along the line of curve BB. A tendency to follow EE is prevented by the baffled construction.
There is, then, in the case of the primary gas, a reduction of the heat effect of the initial combustion because of the large excess of air present. Beyond the point M, the heat effect is strengthened by the baffled-wall structure. The initial combustion of the secondary gas overlaps the final combustion of the primary gas but is subdued by the inert gases present, and the heat effect of the latter part of the secondary combustion is strengthened by the baffled-wall structure. The curve BB, which has been accurately checked by thermocouples, therefore, follows closely the ideal curve for heat effect. It must be remembered that this heat effect is applied continuously, for there are no reversals with the attendant fluctuations in temperature as shown at RR and SS (b) and (c), which are curves taken on reversing ovens. The effect of the reversals is evident.
Roberts Individual Regenerator
The adaption of individual regenerators to the Roberts oven is possible in most forms of common practice. According to Fig. 8, which is the longitudinal section of the oven illustrated in Fig. 9, a new principle in design is incorporated in this oven regardless of whether the oven is recuperative or regenerative. This plan makes it possible to cool the
ovens to atmospheric temperature without rupturing the brickwork after it has been at working temperature.
The expansion of silica brick, such as is used in American coke-oven practice, is about 1/8 in. per lin. ft., when heated to 2600° or 2700° F.; therefore, a coke-oven wall 40 ft. long, when heated from atmospheric to working temperature expands 5 inches.
If it is necessary, because of the desirability of closing down the plant, to have the oven go back to atmospheric temperature, this wall must contract approximately 5 in. As the brick is not strong enough to withstand this contraction, shrinkage cracks result; and as there is no way of controlling the ruptures thus caused, they are irregular and form passages between the air and gas flues and between the coke-oven chamber and gas flues.
To overcome this fault, one type of Roberts oven is so designed that it can be built in sections; that is, it would be equivalent to a number of short ovens placed end to end for the required length of a complete unit. Each section can contain three or four burners and be complete within itself. As these sections are about 7 ft. long the total expansion of each section will be 7/8 in.; and as the brick will expand equally in all directions, the expansion from the center to either end of a section will be 7/16-in. Experience has shown that a coke-oven wall can expand and contract to this extent without damage, so that with this sectional construction a wall may be repeatedly expanded and contracted without developing cracks that result in operating difficulties.
This method of construction is illustrated in Fig. 8. The expansion joints are indicated at XX, YY and ZZ. These joints are intercommunicating between adjacent ovens, but are bulkheaded off from the sections in which are incorporated the combustion space and also from the center wall.
While the utilization of expansion joints in this manner permits circulation back and forth between adjacent ovens, when the ovens are first charged, this condition exists only a short time because the expansion of the brickwork will tightly close the joints and after the oven has been charged a few times any small leaks will be hermetically sealed by carbon deposits. The coking chambers will then be completely isolated from one another.
The other features of this oven are not different from those of the recuperative type. The primary difference, of course, is the admittance of gas into the heating wall at three points, the top, middle, and bottom. The fuel gas is admitted from the top and carried to intermediate and bottom burners through ducts, the same as the secondary gas is put into the wall in the recuperative oven.
In the reversal of the gas flow, this oven utilizes the dividing wall for eliminating the waste gas from the heating wall when the oven is burning in an upward direction; and, vice versa, the waste-gas ducts become air ducts when the gases are burning downward in the heating wall.
The arrangement of the checker brick in these regenerators is optional. They can be divided into zones or made two pass or single pass in a horizontal direction.
The possibility of keeping the gas ducts, particularly the secondary-gas ducts, free of carbon from the decomposition of the fuel gas passing through them is often questioned. Generally, these ducts are of such diameter that it would take many hours continuous flow of rich hydrocarbon gas to decompose sufficient hydrocarbon to cause a stoppage, and it has been found that by shutting off the gas supply in these secondary ducts at regular intervals and running air through in them for 15 to 20 min. all carbon accumulations on the walls are burned out. The common practice is pass air through these ducts once every 8 hr. When using lean, debenzolized coke-oven gas, once every 24 hr. is sufficient to keep the ducts free of carbon. The ovens are provided with an auxiliary air header (the upper pipe, Fig. 6), which contains air at atmospheric temperature at 1 lb. pressure, supplied by a rotary blower.
Results Obtained with Roberts Recuperative Oven
A plant of eighty ovens of the recuperative type has been in operation since January, 1921. During this period, these ovens have run continuously with more than satisfactory results. The plant was designed to operate on 15-hr. coking time, but when the market would absorb the products it has been operated continuously on 12-hr. gross coking time, using either Illinois or Indiana coal exclusively, or mixtures of the two coals. The resultant coke has been used for the usual purposes, such as the operation of a 500-ton blast furnace, lead smelting furnaces, water-gas practice, and foundry cupola work.
It is believed that this character of coal is not used in any other byproduct plant for the production of metallurgical coke. These coals have been used in other plants, when mixed with coking coals in which the coking coal in the mixture predominated. In the Roberts ovens, the Illinois coal has always formed the larger part of the mixture and when the Illinois coals low enough in ash and sulfur could be secured, they have been used successfully without the mixture of any coking coals.
The average operating results in this plant are as follows:
Average quantity of coal per charge per oven, tons…………….14.5
Average coking time, hours………………………………………………………….12
Average coke yield, per cent………………………………………………………..67.25
Average breeze yield, per cent…………………………………………………….3.7
Average tar yield, gallons……………………………………………………………….8.5
Average ammonium sulfate yield, pounds……………………………….27.6
Average total gas yield, cubic feet………………………………………………10,284
Average surplus gas yield, cubic feet…………………………………………4200
Average B.t.u. rich gas……………………………………………………………………..593
Average B.t.u. lean gas…………………………………………………………………….508
Average quantity of coal per oven per day, tons……………………..29
The benzol plant was put in operation on Oct. 1, 1922, and has produced an average of 2.9 gal. of motor fuel per ton of coal.
The blast furnace operated in conjunction with this plant has produced approximately 260,000 tons of pig iron on an average coke consumption of 1806 lb. The ovens have been operated with the normal crew that would be used on any type of coke oven and have shown no operating difficulties and, after practically two years of continual operation, show no inherent weaknesses from a structural or operating standpoint. There has been no charge for repairs or maintenance on the ovens and, as far as it is possible to judge, the brickwork is in as good condition as when the oven was started.
It is thought by the owners of this plant that the only replacement necessary on the ovens will be the replacement of the false bottom in the sole of the oven; this work is easily done. These false bottoms are 3 in. thick, made of silica slabs, and are held in place in the oven by the curbs on each end.
There has been no difficulty in keeping the gas ducts clean and clear of obstructions, including carbon.
The relative value of recuperators and regenerators is a matter of individual opinion. In the operation of the plant mentioned, it is still a question whether the recuperators are as economical of gas in the operation of the ovens as regenerators would be. It is impossible at the present time to get a comparison of the amount of heat required to carbonize this character of coal as compared to the use of 100 per cent, coking coals. However, if the question of costs is eliminated, recuperators made of silica brick can be constructed that will be as efficient in heat recovery as regenerators. The men operating this plant favor the recuperator because of the elimination of the reversing feature and continuous flow of heat in one direction. As most of these men have had considerable experience in the operation of the reversing type of oven, considerable weight should be given their opinion.
While it is true that more brick is used in the construction of this type of oven than in the single-wall type, the added cost is offset by the greater capacity of the oven. Several demonstrations have been made with these ovens in which it has been proved that a 14-in. oven of this construction can be operated continuously on 10.5-hr. gross coking time. As the tendency in the United States for the last few years has been to build ovens with greater cubical content and operate them at faster coking times, based on the results obtained by the use of the Roberts oven, it is believed that it will be only a short time when ovens that will carbonize 20-ton charges of coal in from 10 to 12 hr. will be in use.