Carbonization of Coal in Heat Recovery Coke Oven Battery
Carbonization of Coal in Heat Recovery Coke Oven Battery
One of the present trends in the production of the metallurgical coke is the comeback of non-recovery ovens. The ovens are called non-recovery since the by-products are not recovered and are burnt during the process of coal carbonization. This is driven due to the less interest in by-products, smaller investment per annual ton, and better environmental performance. The development of non-recovery coke ovens took place in 1980s and 1990s. This technology arises from the classic beehive ovens which supplied since the eighteenth century the coke for the industrial revolution. The beehive ovens were manually operated, with small heat recovery, just for heating the oven. Now, non-recovery ovens are modern construction, with highly mechanized operation, and automated to a certain degree. The non-recovery ovens are called heat recovery ovens when the energy of the exit gases is recovered in the form of steam for generation of power. The schematics of the HR coke oven process are shown in Fig 1.
Fig 1 Schematics of the process of a heat recovery coke oven
The basis for the heat recovery (HR) coke ovens is the so called ‘Jewell-Thomson oven’. These ovens were developed in 1960 when three test ovens were successfully built at Vansant, VA. Several of these ovens are grouped together to form one battery. Gases generated by the combustion of the volatile matter are sent through the down-comers and further burnt to heat the oven bottom and sides. The hot flue gas is used for steam production and power generation.
Jewell-Thomson oven is shaped with a rectangular ground area. The oven brick lining is composed of silica refractory material. Coal is charged onto the oven floor at the beginning of the cycle. The carbonization process is started by the heat which exists from the previous carbonization cycle. The released raw coke oven gas is partially combusted in the crown by the addition of ambient air through the oven doors and the gases pass through the down-comers into the heating flues situated in the oven sole. This flue system is beneath the oven floor and here by way of a further supply of the ambient air, the complete combustion of the raw gas takes place at temperature ranging from 1200 deg C to 1400 deg C. The gases then pass into an afterburner tunnel where any remaining not combusted gases are oxidized. The afterburner tunnel system routes the hot gases to the heat recovery steam generators. Fig 2 shows a cut-away drawing of a Jewell-Thompson heat recovery oven.
Fig 2 Cut-away drawing of a Jewell-Thompson heat recovery oven
The coking of the input coal takes place by direct heating from the oven crown and by indirect heating from the refractory floor. The whole system is being operated under negative (sub atmospheric) pressure. The time of coking in Jewell-Thomson oven is around 48 hours. After completion of coking, the coke is pushed out and wet quenched.
Process of carbonization
In the heat recovery coke ovens, the coal/coke mass is located in the large oven chamber in the upper part of the oven. The coal/coke fixed mass is of a parallelepiped shape. The carbonization process of the coal begins at the top and at the bottom of the fixed mass. Then the carbonization front moves continuously through the coal mass. The heat required for thermal decomposition and carbonization is generated by combustion of volatile matter released from the coal mass. The volatile matter is partially combusted in the free space above the fixed coal mass using air entrained through the various ports/openings (channels) located in both side doors and the oven roof.
The gas formed in the upper oven is transported through the down comer passages to the sole flue (bottom part of the oven). Additional channels in the sole flue provide the required amount of air to complete the combustion process. In the course of the process of coking, which proceeds under negative pressure, the amount of air provided to the spaces above and below the coal mass is controlled by under-pressure (suction) generated using either a fan or through natural draft caused by a stack. The entire coke making process can be controlled just by varying the suction at the outlet of the oven. This is usually not enough to obtain the same high quality product (coke) in the entire coke mass.
For improving the homogeneity of the coke product, a uniform heating rate of the fixed coal mass has to be ensured. In the upper part of the oven (above coal mass), heat is transferred to the coal mass mainly through radiation from the oven walls and from hot gases, which are products of partial combustion of volatile matter. The heat produced in the sole flue is transferred to the coal charge through the sole flue brick ceiling. The uniform heating rate of coke charge through the sole flue ceiling can be obtained if the temperature profile in the sole flues is uniform. This can be achieved by changing the size and position of the gas and air channels.
The coking proceeds from the top through direct heating by the partial combustion of the volatile matter over the coal mass, and from the bottom by heat coming from full combustion of gases escaping from the oven. The coke oven is provided with sufficient free space between the oven top and the coal mass where the volatile matter coming out from the coal charged during carbonization gets combusted. The adequate free space as well as controlled supply of air ensures efficient combustion of the hydrocarbons present in the volatile matter. The unburnt volatile matter along with the hot flue goes to the sole via the chamber provided at the side walls of the oven where secondary air is injected to facilitate the complete combustion of the remaining hydrocarbons. The burning of the volatile matter in the sole increases the temperature of the coke oven mass which increases the efficiency of the coke oven. Also an additional combustion chamber is normally provided for complete combustion of unburnt volatile matter and for settling down of particulate matters. All the above mentioned pollution control devices are inbuilt in the coke oven system. The clean flue gas is utilized for the heat recovery.
The coal carbonization is the physico-chemical process and depends on the coking rate, operating parameters, coal blend properties and the transport of thermal energy. The heating rate of coal influences the strength and the fissuring properties of coke. In order to arrive at a homogeneous quality, the heating of the coal mass in a coke oven is to be uniform over the total length and height of the oven. In addition to this, the plastic layer migration rate influences the level of thermal stress in the re-solidified mass and hence, the level of fissuring. The strength of coke depends to a large extent on the thermal condition prevailing during carbonization. The thermal conditions are influenced by oven crown and sole flue temperature, negative pressure (suction) and carbonization time.
The oven temperature and the time for coking adopted in a normal operation are not independent factors and they vary inversely. At constant temperature, extension of the time for coking beyond what might appear strictly necessary is known as the soaking time and allowing the coke to remain in the hot oven during this time is called soaking of coal.
During the process of coking, the maximum coke temperature is partially decided by the total heat supplied between charging of the coal mass in the oven and discharging the coke. Heating rate is strongly associated with the pattern of heat supply during coke making process. A suitable heat requirement and the pattern of heat supply need to be selected from the point of view of coke quality as well as minimizing the total heat consumption. During the process of coking a plastic layer is formed. The plastic layer of coal is a highly heterogeneous where intricate physical and chemical equilibria between solid, liquid and gaseous components take place.
Softening, devolatilization, swelling and re-solidification are closely related. These all mentioned phenomena are highly dependent on the degree of heating rate. It is normally seen that all coals irrespective of their rank, can be de-volatilized without showing any swelling provided the heating rate is sufficiently slow. The carbonization process can be schematically characterized by the following typical equations
Coal –> Metaplast
Metaplast –> Semi-coke + primary volatile matter
Semi-coke –> Coke + Secondary volatile matter
Heat transmission rate to a coal charge in a coke oven is affected by several factors, such as, coal blend, moisture content, bulk density, oven crown and sole temperature, etc. These factors influence the thermal phenomena. The most important ones are shown in Fig 3.
Fig 3 Factors influencing the thermal phenomena in HR coke oven
Design and construction of heat recovery coke oven batteries
There are several suppliers for the heat recovery coke oven batteries. The main features of some of the designs are described below.
The Sun Coke battery – Present Sun Coke battery design is based on in-house experience since 1960. In 1989, the basic design was renewed and the Jewell-Thompson name was adopted for the oven type. Then, in 1998, new changes were made to the design. During this time, a power plant to recover heat in the off gas is included. Present oven configuration is shown in Fig 2.
Typical dimensions of the chambers of the coke ovens are 14 m length, 3.5 m to 3.7 m width, and 2.4 m to 2.8 m height. 40 tons to 50 tons of coal is charged per oven. Typical charging height is 1000 mm. The ovens are built with 23 refractory brick shapes. Coal blend is charged through one side, by means of a so called -pusher charger machine’ (PCM) moving over rails close to the ovens. Immediately after charging, the coal blends absorbs the heat from the refractories and the combustion of volatile matter starts.
Below the oven roof, partial combustion of volatile matter takes place, on top of the coal mass. Soon afterwards, gases are suctioned to the oven hearth, where more air is introduced to complete the combustion. The coking front advances from the bottom and from the top, joining somewhere in the middle. There is no pressure buildup occurs as in by-product coke ovens, so expansible low volatile coals can be blended.
Temperature, pressure and inner combustion are controlled in the ovens. The time of coking is of the order of 48 hours. Coke withdrawal is carried out with the same PCM used for charging. Coke is quenched with water. All water used for coke quenching is recovered, with the exception of evaporation loss.
Process hot gas, after going through the bottom of the oven, goes up to a duct built on top of the ovens. They may be driven to the boilers, for steam production, or sent to the stacks. In both cases, desulphurization is carried out by aspersion of lime slurry on the gas. At least 80 % of the SO2 generated during coal carbonization is eliminated. This equipment generates solid calcium sulphate and sulphide as a waste.
The Chinese designed battery – The Chinese have built also, besides standard horizontal heat-recovery coke ovens, a vertical-type non-recovery oven. As regards the horizontal ovens, the oven roof is a 120 deg arch structure. Adjustable primary air inlets are evenly installed in the arch, forming a waste-gas-protecting layer between the coal and burning zone in the oven roof. Four linked arches are used at the oven bottom. On the base of the arches, adjustable secondary air inlets are installed to distribute the air in the flue, for further combustion of the exit gas to heat the oven bottom. Flues inside wall and bottom can be coordinated. A ventilation layer between the foundation of the oven and the sole prevents the base plate from overheating. Main wall is equipped with suction-adjusting facilities.
Oven door is divided into two sections, the upper one is fixed and the lower movable, in order to prevent soot leakage. They are made in cast iron and lined with ceramic fiber.
The dimensions of carbonization chamber of the horizontal heat-recovery coke oven battery of one of the design consists of 13,340 mm length, 3596 mm width, 2758 mm height with 4292 mm of centre to centre distance of the chambers. The effective dimension of the coal cake is 1300 mm length, 3400 mm width, and 1100 mm height. The bulk density of the charge coal is 1.0 tons/cum to 1.5 tons/cum. The oven has a coal charge capacity of around 50 tons and has 72 hours coking time.
There are two more designs available. The dimensions of carbonization chamber in these two designs consist of 13,334 mm and 15,440 mm length, 3,598 mm and 3,700 mm width, 2,888 mm and 2,693 mm height with 4,530 mm and 4300 mm of centre to centre distance of the chambers respectively. The effective dimension of the coal cake is 12,750 mm and 14,850 mm length, 3,500 mm and 3,600 mm width, and 1,050 mm and 1000 mm height respectively. The ovens have a coal charge capacities (on dry basis) of 42.393 tons and 51.856 tons and has coking time of 72 hours and 70 hours respectively.
The first vertical coke oven heat recovery battery has been built in 2002 in China. In comparison with horizontal-type, these coke ovens require less space and 20 % to 30 % less construction cost. More important, it is said that the separation between coking chamber and combustion chamber avoids the burning of the coke which may occur in horizontal-type oven. Heat comes only through refractories, as in conventional batteries. There are two layers of air cooling channels at the bottom of the batteries. The temperature of the foundation is between 100 deg C to 150 deg C, preventing failure. Main dimensions and features of the vertical ovens are listed in Tab 1.
Tab 1 Technical parameters of vertical heat recovery coke ovens
|2||Charge weight||tons||Around 24|
|3||Cake bulk density||t/cum||1.0-1.1|
|4||Time of coking||hours||38|
|5||Number of ovens||numbers||4×35|
|6||Partition wall thickness||mm||100|
|7||Thickness of the oven sole||mm||1182|
|9||Centre temperature||deg C||1000 +/- 50|
|10||Pushing coke weight||tons||Around 18|
|11||Exit gas||N cum/h||350,000|
|12||Exit gas temperature||deg C||950 +/- 50|
The coke ovens design of Sesa Goa – Sesa Goa initially entered into a joint venture with Kembla Coal and Coke (now Illawarra Coal and Coke), under the name of Sesa Kembla Coke Company, in 1993. The Australian coke maker contributed their in-house developed technology. The 84 ovens battery failed. A systematic analysis of the reasons for the failure was carried out and the battery was rebuilt. Then, agreements were ink to license the technology. Ovens are narrower than the coke ovens of Sun Coke. This makes possible to use roman arch for the roof (Fig 4). They are built with aluminous refractories, as a difference with Sun Coke ovens and this implies a smaller width. The oven is having 10760 mm length and 2745 mm width. 21 numbers of ovens are connected to a stack. The ovens are with top charging.
Fig 4 Transverse cut of non-recovery oven of Sesa Goa
The use of aluminous refractories instead of silica refractories is due to their better behaviour under oxidizing atmosphere, better resistance to thermal shock and less volume changes upon cooling, when there is some delay in recharging the oven.
Sesa Goa is using vibro compaction, for higher charge density. The operation is carried out in a separate station. Vibration and compaction are applied simultaneously to the coal blend with a determined moisture and grain size, within a box, in three successive layers. To this end, plates covering the full surface of the ‘cake’ are actuated during two minutes for each layer, to achieve the strength required for the transportation to the oven (Fig 5). The density achieved is of the order of 1.14 tons/cum.
Fig 5 Essential parts of a compacted charging machine
The compacted charging is done in a stationary compacting station and the compacted cake is then transferred to a charge car on a steel plate. The charging car transfers the coal cake into the oven with the help of a winch.
The Uhde design – Thyssen Still Otto Anlagentechnik GmbH (TSOA), then ThyssenKrupp Encoke, now part of Uhde, signed in the late 1990s an exclusive license agreement with Pennsylvania Coke Technology Inc. (PACTI). This company has developed a concept of non-recovery oven and built a pilot plant with two full-size ovens in Nueva Rosita, Mexico. On this base, TSOA redesigned the oven and through an agreement with the Illawarra Coke Company, Australia, built two ovens there and carried out several coking experiments with different coals.
The present Uhde design is based on its strength of accumulated knowledge of companies building conventional batteries since the beginning. These companies through mergers and acquisitions are all under the same roof.
The coke ovens are stamped charged. But the charging machine does not enter the oven. The dimensions of the ovens are having 3.8 m width and 15 m length. The lining is of silica bricks.
The tunnel for exit gas runs laterally below the oven floor level, instead of over the ovens, as in Sun Coke design. Another difference is that charge and discharge are carried out with two different machines. The charge being previously stamped, there is no need for the machine to enter into the oven, avoiding water cooling and water to humidify coal. For discharge there is no fall of the coke, keeping the cake without breaking, thus favouring lower emissions.
SJ 96 coke oven -This oven has been developed in-house by Shanxi Sanjia and is characterized by the exceptional weight of the coal charge which 120 tons. Hence, there is a need for long coking time of ten days in comparison with the 48 hours to 72 hours of typical coking time of the other processes. Coal mass height is 1.8 m and leveling is done at 90 cm and 180 cm levels. Both charge and discharge are manual and with the oven cooled. Temperatures are of the order of 1200 deg C in the upper coal layer and 1150 deg C in the lower coal layer. Gas is burnt completely in the under flues below the oven, and its temperature is used to produce steam for power generation.
Recently, a thermal modeling exercise has been carried out. This study provided some interesting conclusions. These coke ovens are to be kept at as high a temperature as possible if the optimum gross coking times are to be achieved. Hence, the volatile matter content in the coal blend is limited, to achieve an adequate heat balance with a bed height of 1 m, densities higher than 1.05 tons/cum and gross coking times less than 60 hours. Hence, the heat recovery coke oven is to be airtight. Adjustment of the primary and secondary air flow quantities at the correct points in time is of great significance with respect to the temperature regime in the oven. Supplying the primary air through the oven top promotes the surface heating of the charge in ovens longer than 10 m and is an advantage over air supplied through the oven doors at the sides.
Some of the features of different processes are compared in Tab 2.
|Tab 2 Comparison of different types of HR coke ovens|
|Process||Charge preparation||Refractories||Charging||Discharging||Width X length in meters|
|Sun Coke||Standard||Silica||Horizontal (PCM)||Fall to wagons||3.5 to 3.7 x 14.0|
|Chinese horizontal||Stamped charging||Horizontal||Fall to wagons||3.596, 3.598, 3.7 x13.34,13.334,15.44|
|Chinese vertical||Stamped charging||Top||Fall to wagons||3.2 to 4.4 height x 12.57|
|Sesa Goa||Vibro compaction||Alumina||Horizontal||Fall to wagons||2.745 x 10.76|
|Uhde||Stamped charging||Silica||Horizontal||Push to wagon||3.8 x 14|
|SJ -96||With cold oven||Alumina||Manual||Manual, inside cooling||3 x 22.6|
Blend design and coke quality
Non recovery/heat recovery coke oven battery produces a quality coke for blast furnaces, cupolas, and ferroalloy furnaces etc. These ovens are useful to obtain high quality coke for blast furnace operation with high PCI (pulverized coal injection), where better properties of coke are needed, or to obtain standard quality based on blends with some proportion of non-coking coals.
In some batteries, the aim was to increase the percent of non-coking coals in the blend, obtaining a coke with reasonable quality for their blast furnaces. Optimizing coal moisture content and using vibro-compaction, a charge density of 1.1 g/cc was achieved. Upto 35 % of non-coking and weakly-coking coals were introduced, obtaining a coke with CSR greater than 64 %, a coke reactivity of less than 25 % and an abrasion strength index M10 of less than 6 %.
The content of volatile matter in the blend is an important parameter in non-recovery coking, as the energy required by the process is contributed by their combustion. So, a certain minimum content is required. But if volatile content is too high, coke may have high porosity, being too reactive to CO2 and with low post reaction strength. As in by-product coke making, coking power, expressed by the free swelling index (FSI) plays an important role. The range of coal, in terms of the reflectance of vitrinite, is important, as well as the rheological properties, in terms of maximum fluidity. Tab 3 gives typical specification of coal being used in the HR coke ovens.
Tab 3 Typical specification of coal for HR battery
|1||Ash||%||less than 10|
|3||Sulphur||%||less than 0.6|
|4||Free swelling index||greater than 6.5|
|5||Vitrinite||%||greater than 60|
|6||V9-V13||%||greater than 55|
|8||Mean maximum reflectance||%||1.1 – 1.2|
A relevant experience is that of the Sun Coke battery of Indiana Harbor Coke Company, from 1998 to 2000. They started charging a blend of 30 % volatile matter (dry basis), 3097 ddpm maximum fluidity and 1.11 % maximum vitrinite reflectance. Then the blend evolved to less fluidity (200 ddpm), less volatile matter (22 %) and higher reflectance (1.42 %), with an important content of low volatile coal, which normally results in wall damage in a conventional battery, due to expansion at the end of coking typical of these coals. Coke quality continued to be high, despite of the changes.
Chinese experience shows that upto 40 % anthracite or 50 % to 60 % lean coal can be included in the blend. The coke quality obtained in Chinese vertical-type non-recovery ovens is M 25 value greater than 90 %, CSR (coke strength after reaction) values in the range of 65 to 68, M 10 value less than 7 %, CRI (coke reactivity index) value in the range of 23 to 25 and ash content less than 11.5 %.
Comparisons have been carried out by coking the same blend in conventional and non-recovery ovens, and it has been noted that in this case an increase in post-reaction strength and in cold mechanical properties.
The substantial differences from the environmental point of view between non-recovery and conventional process come from two aspects namely (i) the operation of the ovens under negative pressure, and (ii) the non-existence of byproducts plant. In the coke oven batteries, emissions to air are related to coal handling, oven charging, oven doors, process stacks, discharging, coke quenching and coke handling. By-products plants are a source of air emissions, too. Differences between processes are reflected first in coal charge and oven operation.
Standard emissions of SO2, NOx, CO and VOC are smaller for non-recovery ovens, while TSP (total suspended particles) and PM10 (particulate matter) are higher. The residence time of gas in the oven, the high temperature, turbulence and oxygen, enough to destroy HAP, imply very low emissions in HR coke oven batteries.
The main advantage of a non-recovery coke oven battery is that it does not generate liquid effluents. Cooling water for equipments and washing is collected and used for coke quenching.