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Technologies for Improvement in Coking process in Byproduct Coke Ovens

• satyendra
• July 31, 2013
• 13 Comments
• bulk density, carbonization, coal blend, Coal moisture control, coke, Coke dry quenching, coking, Pre-screening, selective crushing, stamp charging,

Technologies for Improvement in Coking process in Byproduct Coke Ovens

Coking coals are converted to coke in byproduct coke oven batteries. The coking process consists of heating the blend of the crushed coking coals in the absence of air to drive off the volatile compounds. The resulting coke is a hard, but porous carbon material which is used for the reduction of iron bearing materials in a blast furnace. The byproduct coke oven also recovers volatile chemicals in the form of coke oven gas, ammonium sulphate, tars, and oils. In last three to four decades several technologies have been developed which have not only resulted into (i) the use of inferior coals in the coal blend, (ii) vast improvements in the process of coking, (iii) improved the quality of produced metallurgical coke, (iv) improved yields, (v) recovery of waste energy, and (vi) improved control of emissions at the battery. Some of the major technologies in this regards are given below.

Selective crushing of coals

Coal is a heterogeneous material. Its various components have a different hardness, so that during breakage by mechanical means whether the inevitable breakage in mining or crushing during coal preparation for coking, the weaker components tend to concentrate in the fine fractions and the others in the coarse fractions. These various fractions are expected to have different coking properties. This triggers the concept of selective crushing which aims at controlling the degree of crushing of different constituents of the coal blend. This technology is helpful when coals are petrographically heterogeneous.

This technology is a theoretically sound technology and aims at controlling the degree of crushing of the different constituents of coal.  It aims to improve homogeneity of reactive and inert components in coal. The reactive components of coals are primarily vitrinites and are the softest constituents while the mineral matters of coals are the hardest components. In conventional coal crushing units, where the entire coal is crushed together, the vitrinites get crushed to a relatively finer size compared to mineral matter constituents. For producing coke of higher quality, it is desirable to crush the mineral matter finer than the vitrinite component of the coal so that during the process of coking, when the coal charge softens, the mineral matter is assimilated better, leading to the improved strength. This is normally carried out by crushing of each coal type separately.

Selective coal crushing comprises of the surge bin, crushing station, blending station, coal mixer and routes the coal upto the coal tower at the coke oven batteries. The coal is transported by belt conveyors from the coal storage yard to the surge bin, which buffers the high mass flow from the coal yard against the reduced mass flow to the crushing station. At the crushing station, the coals are crushed type by type with individual crusher settings. After the crushing process, the crushed coal is transported to the blending station. Each coal type is stored in a separate blending bin. Adapted to the number of coal types and their percentage relative to the coal blend composition, different amounts and sizes of bins are designed.

Downstream of the blending bins, the different types of coal are fed to the outgoing conveyor by belt weighing feeders. From the blending station, the coal is transported to the mixer. Additives, like oil, water, and coke dust etc., are added to the coal in the upstream transfer chute before the mixer. In the mixer, the coal blend is homogenized. After the mixing process, the coal is fed to the coal tower. The moisture measuring of the coal takes place upstream at the end of the belt conveyor to the coal tower.  The basic concept for selective crushing of the coal is shown in Fig 1.

Fig 1 Basic concept of selective crushing

In selective coal crushing concept, the adjustment of the coal crusher is changed automatically following pre-settings for every coal type. This provides the possibility, to achieve the optimum grain size of every single coal type in consideration of the coal blend composition and to react on changing coal properties at any time. For example, coal types with weak coking abilities are crushed to a larger average grain size to ensure a lower specific surface. Soft coking coals with a large amount of inerts can be ground to a lower average grain size in relation to the coal blending composition. The coal blend is more homogenized than in case of blend crushing. The inerts as well as the reactive contents of the coals are uniformly distributed in the coal mixture and not concentrated in certain grain fractions. This results in less fragile spots in the coke and avoids differences in terms of shrinkage by inhomogeneously distributed inerts.

Pre-screening before crushers

A recommendable additional process step is the pre-screening of coal before it is routed to the coal crushing station. Most of the raw coal types have 30 % to 50 % of a grain size less than 3 mm, which does not need to be crushed further. Removing this grain size from the coal before the crusher has the positive effect and it helps in reducing the capacity of the crushers considerably. Further, the portion of fine particles can be controlled more effectively.

The separation of the fine coal can be effected by pneumatic classifiers or by flip-flop screens. The additional investment for this equipment is partly compensated by the reduction of the capacity of the coal crushers.

By the reduction of fine particles, the average grain size of the coal blend is reduced effectively, increasing the bulk density with a positive effect on the coke oven productivity. Further, the coking properties of the coal blend are improved by reducing the specific surface. This allows the use of a higher percentage of low-grade coking coals in the blend.

Oil addition

It is possible to adjust the bulk density of the coal by the addition of oil in relatively small amounts of 0.1% to 0.5%. This shows positive effects on coke properties. In cases of adding oil to compensate the reduction of the bulk density due to increasing percentage of particles smaller than 0.5mm, it has been found that the gas pressure of the coal does not increase simultaneously. In cases of constant particle size distribution, the gas pressure of the coal is even slightly decreasing because of the addition of oil. In addition, a better flow characteristic inside of the chamber during charging has been confirmed in many test runs by coke oven battery operators. Hence, a more even coal distribution inside the chamber with a constant coal line can be achieved and a reduction of carryover is there. This leads to less spillage coal and uniform carbonization of the coal over the entire coke oven chamber. Furthermore, also the heat consumption is optimized and overheating of the gas collecting space above the coal can be avoided. This reduces the building of carbon from cracking reactions which takes place at temperatures above 850 deg C.

Coal moisture control

Coal moisture control is carried out by the drying of the coal to a constant moisture level. It is now a common feature. The constant moisture level provides for a stable operation of the coke oven batteries. The drying is done by means of rotating tube dryers or fluidized bed dryers which are arranged behind the blending station. Coal moisture control uses the waste heat from the coke oven gas to dry the coal used for coke making.

The moisture content of coal blend for charging in coke ovens normally varies in the range of 8 % to 10 %. Drying of coal blend reduces the moisture content in the coal blend to a level of around 6 %. This in turn reduces fuel consumption in the coke ovens. The coke is dried using the heat content of coke oven gas, low pressure steam, or any other waste heat source. Schematic flow chart of coal moisture control is given in Fig 2.

Fig 2 Schematics of coal moisture control

The benefits of coal moisture control are (i) saving of fuel by around 71,700 kcal/ton, (ii) improvement in the quality of coke by 1.7 %, (iii) increase in production of coke by around 10 %, (iv) reduction in coking period, and (v) decrease in water pollution.

Another favourable, but not yet realized alternative, is the combination of pneumatic classifiers for removal of the coal fines and drying of the coal by the air stream in front of the crusher station. Such a system combines the advantages of pre-screening and predrying in one process station. The basic equipment needed required for such process is available in the market, but its combined application has not yet been practically adopted in coal treatment plants.

Stamp charging of coal

The widespread use of pulverized coal injections in blast furnace has the requirement of higher quality of coke. Further, as the coking coals have become more costly, with volatile price and relative availability, the introduction of cheaper coals in the coal blend has become a necessity. This has led to the use of stamp charging of coal which was originally developed for the use of high volatile poorly coking coals.

The technique of charge preparation consists in preparing a cake with the coal blend in a metallic box, then charging it in the coke oven. The higher charge density implies better coke quality when compared with conventional charging. So, depending on the situation, either better coke quality can be achieved, or weakly coking coals can be included in the blend.

Stamping of coals increases the bulk density of charge by 30 % to 35 % to around 1,150 kg/cum. Crushing of coals to more than 90 % below -3 mm size and 40 % to 50 % below – 0.5 mm size is needed for a stable cake. With stamp charging, low rank, weakly coking, and high volatile coals can be used to the extent of around 20 %, but since the coal charge is compacted to high bulk density there can be problem of increased wall pressures. In order to ensure that the refractory oven walls are not damaged, the coal blend used is to be carefully chosen by optimum balancing between high and low volatiles coals.

The stamp charging process had been used since the early 20th century. This technology was born in Silesia and Poland more than 100 years ago. In the earlier plants, the stamping station was located in the space between the two batteries. Straw was used as an aid to cake strength. Straw was used as a binder and a specially designed charger car / ram built to load the charge into the oven from the back. Coke made by the stamp charging process was of a denser and larger variety than that made by other methods, hence making it better suited for ironmaking in foundries where strength is an important factor. Another advantage noted was that a much larger range of coals could be used with the limits of (high) volatility and coking properties much increased.

Modern process development took place in Fuerstenhausen coke plant, Volklingen, Germany, focused in the use of high volatile coal. In 1978, after intensive research and development the first 6 meters high cake was produced, overcoming a bottleneck for the economical implantation of this technology. The first plant of this dimension was started-up in 1984 at Germany. Both conventional slot ovens and non-recovery / heat recovery ovens make use of this technology.

The technology basically involves formation of a stable coal cake with the finely crushed coal by mechanically stamping outside the oven for carbonization. In stamp charging, the bulk density of the coal charged in the oven is increased by physically stamping the charge into a cake. The cake, which is almost similar in size to the oven, is then inserted in the oven. Stamping is carried out in a stamping cum charging cum pushing machine which uses drop hammers for stamping.

Stamping equipment can be located in a building build for this purpose or in the charging / pushing machine. The stamping process normally consists of introducing the coal blend previously ground at a specific size, within a steel box, as successive layers which are rammed mechanically. It can be applied vertically or horizontally. In addition, vibration can be applied to facilitate the accommodation of the particles. A horizontal box is filled with the coal blend, with defined grain size distribution and moisture content, in three equal layers. Compaction and vibration is applied, through a number of drop hammer plates covering entire of the surface of the cake, for two minutes for each layer, to support the transfer from the box to the oven. It is said that in this case fine grain size is not needed as for conventional stamping.

Two aspects have to be taken into account. These are (i) densification, and (ii) mechanical properties. Densification is needed for the process of coking. The denser is the cake, the better is the coke quality, taking into account both cold mechanical strength and behaviour at high temperature. Mechanical properties are to be sufficient to support transport the cake for its charge into the coke oven.

When the densification starts, coal particles yield under the stress applied by the stamping machine, filling the interstitial voids with smaller particles. The rearrangement of the particles is supported by the surface moisture, which reduces the internal friction. With further strain an elastic-plastic deformation of the particles takes place partly resulting in particle breakage and filling of small pores with the fragments. While the pore volume decreases the pore saturation with water rises causing a damping effect.

Besides the influence of the capillary water on the densification process itself, the mechanical properties of the compacted mass are also determined by the surface water as it serves as a binding agent in the formation of adhesive forces. Within the systematic of process engineering the stamp cake can be called as so-called wet agglomerate which is characterized by the adhesive forces resulting from liquid bridges within the capillary pore system.

Cake density is aimed at 1,100 kg/cum 1,150 kg/cum and depends around on moisture and grain size of the coal blend and compacting energy applied. Mechanical properties of coals have also an influence. Wet density increases continuously when moisture increases from 6 % to 13 %. For higher moisture, the energy applied is used to expel water out of the cake. Normally two important variables of the coal blend are moisture and grain size for the achievement of the strength needed for cake transport and charging. The operating variable is the compacting energy applied and the relevant mechanical properties are compressive strength and shear strength.

For the charging of the cake to the coke oven, different techniques are used depending on the design of the coke oven (vertical slot oven or horizontal non-recovery / heat recovery coke oven. For vertical coke ovens, cake charging takes place through the pusher side doors. This procedure causes high emissions. In order to decrease such emissions, several systems have been experimented during coal cake charging, especially through the use of sealing frames. However, the emission control system only partially reduces the emission without completely eliminating it. The solution adopted recently in the new batteries to decrease emissions during the oven charging process is to create a strong depression (minus 400 Pa) in the collecting main during the charging phase. During the coking process when no charging process is ongoing, the collecting main is set to a nominal negative pressure. The switch to the higher depression set-point is done before starting the pushing process of the cake.

Normally the specification of the coal blends includes 25 % to 32 % volatile matter (ash free dry base) and a free swelling index of 3 minimum. However, the coal blend specifications changes from plant to plant, depending on coal availability and costs. In some plant, coal blend includes petroleum coke and coke fines.

The advantages of stamp charging are (i) increased throughput of 8 % to 10 % due to higher bulk density, and (ii)improved strength of coke (micum and CSR value) due to closer packing of the individual coal particles during carbonization, The produced coke is denser, smaller and more uniform in size.

Large chamber top charged coke ovens

Large chamber coke ovens are 7.6 m tall. Large chamber coke oven battery complex represents the state of the art and consolidated technology for coke making plants. Tab 1 shows a comparison table between large chamber top charged coke oven batteries with 6.25 m tall coke oven batteries based on a coke production of 1.9 million tons annual production.

Reduced number of pushing per day of the large chamber coke oven batteries allows having only 1 set of operating machine in operation instead of 2 sets for the 6.25 m tall coke oven batteries, with advantages in terms of (i) investment cost of the coke oven machines, and (ii) operating cost. Tab 1 also shows that considering an average number of pushing that each oven can perform during its life, the result is that the expected lifetime is considerably increased.

Each heating wall is composed by 38 heating flues having the configuration consisting of (i) twin flue, with partial re-circulation of waste gases for low NOx production, (ii) three levels of staggered air inlet in order to minimize NOx formation and keeping a proper vertical temperature distribution, and (iii) mixed gas and air flow rate easily adjustable by means of regulation plate placed in the bottom part of the regenerator level.

High pressure ammonia liquor aspiration system

The high pressure ammonia liquor aspiration system is effective for controlling charging emissions in coke oven batteries. In this system, the ammoniacal liquor, which is a byproduct, is pressurized to around 35 kg/ sq cm to 40 kg/sq cm and injected through special nozzles provided in the goose neck at the time of charging. This creates sufficient suction inside the oven thereby retaining the pollutants from being released to the atmosphere. The system consists of high pressure multistage booster pumps, sturdy pipe work, specially designed spray nozzles, suitable valves, and control instruments. This system emissions control results in saving in quantity of process steam and increase in the yield of raw gas.

Coke dry quenching

Coke dry quenching is an alternative to the traditional wet quenching. It is an energy saving process used during the production of coke in the coke oven battery. A coke dry quenching plant is also called coke dry cooling plant. In the traditional coke wet quenching process, the red-hot coke which is pushed from the coke oven is cooled by spraying water on the hot coke. The water used for cooling is vapourized and released into the atmosphere. An issue with this conventional system is the energy loss when the thermal energy of the red-hot coke is converted into the steam which is vapourized and released unused. Another drawback is that the coke wet quenching process also produces airborne coke dust, and hence, the process is associated with high carbon dioxide emissions and thermal energy loss.

During the coke wet quenching process for cooling of the run of oven coke, sensible heat of the hot coke is dissipated into the atmosphere and is lost. In addition, there are air borne emissions (0.5 ton of steam per ton of coke, which is laden with phenol, cyanide, sulphide and dust) and a large quantity of water (around 0.6 cum per ton of coke) is needed for wet quenching. The contaminants in water are also discharged in the environment.

In a coke dry quenching plant, red hot coke is cooled in specially designed refractory lined steel cooling chambers by counter currently circulating inert gas media in a closed circuit consisting of a cooling chamber, a dust collecting chamber, a waste heat boiler, dust cyclones, a mill fan, a blowing device and circulating ducts. The heat energy from the red hot coke is recovered in the waste heat boiler for use as steam, resulting in energy conservation as well as a reduction in coke particle emissions. Tab 2 shows the comparison of typical properties of coke produced by the two processes.

Hot coke after its pushing is brought from the coke oven battery to the coke dry quenching plant in bottom opening bucket kept on the quenching car. This bucket is lifted at the coke dry quenching plant by a hoisting / charging device to the top of the coke dry quenching chamber and red hot coke is discharged into the pre-chamber by the charging device.  Hot coke (temperature around 1,000 deg C to 1,100 deg C) is cooled in the chamber by the circulating gas. In the chamber the circulating gas moves counter-current to the coke movement, i.e. the circulating gas moves upwards while the coke moves downward by the gravity.

The circulating gas in a continuous running coke dry quenching plant is a mixed gas which consists of mainly nitrogen along with small amounts of carbon dioxide, carbon mono-oxide, and hydrogen. The hot coke, while descending in the chamber, is cooled to a temperature which is less than 200 deg C by the circulating gas blown from the lower zone of the cooling chamber and is discharged from the discharging facility at the bottom of the chamber. The passage time of the coke through the chamber is around 5 hours to 6 hours.

The high-temperature circulation gas (at around 800 deg C to 850 deg C) after a heat exchange process in the cooling chamber passes through the primary dust catcher and is supplied to the boiler. The circulation gas after a heat exchange process in the boiler is cooled down to around 180 deg C. The steam generated in the boiler is used either as a general-purpose process steam, or converted into super heated high pressure steam for the generation of electric power through a turbine generator.

The circulating gas passes through the secondary dust catcher into the gas circulation blower, by which its pressure is boosted, and its composition is corrected by addition of nitrogen gas and then the circulating gas is injected at the bottom of the coke dry quenching chamber. If necessary, a sub-economizer is installed to decrease the temperature of circulation gas to around 130 deg C, improving the cooling efficiency of the cooling chamber.

There are some auxiliary facilities which include cut off device for cutting out the coke from the chamber, dust removing system for removing the dust in the circulating inert gas, and the secondary dust catcher installed before the circulation blower. The flowsheet of the coke dry quenching process is shown in Fig 3.

Fig 3 Flowsheet of the dry quenching process

In the coke dry quenching process, the red-hot coke is cooled by gas circulating in an enclosed system, thereby preventing the release of airborne coke dust. The thermal energy of the red-hot coke, which is lost in the conventional coke wet quenching process, is collected and reused as steam in the coke dry quenching process. This technology uses less fossil fuel and results in lower carbon dioxide emissions, thereby contributing to the prevention of global warming. Nowadays, coke dry quenching plants have gathered a lot of attention from the world due to its efficient energy recovery and the characteristic of reducing the environment pollution. They are being regarded as an essential facility for the counter-measure against environmental problems like global warming by carbon dioxide and air pollution. In a study, in which the energy saving calculation has been carried out based on the operation of an existing coke dry quenching process, it has been shown that 85 % of the waste heat generated during coal carbonization is being recovered by the coke dry quenching process.

Dry quenching also improves the coke strength. Other advantages of coke dry quenching are reduction in green house gas (GHG) emissions and improved water efficiency.

Modern leak proof doors

Leaking doors of a coke oven battery are always a major source of pollution. The design of oven doors has gone through a process of evolution, starting from luted doors to the present generation self regulating zero leak doors. The imported features of the leak proof doors are (i) a thin stainless steel diaphragm with a knife edge as a sealing frame built in between the door body and the brick retainer, (ii) spring loaded regulation on the knife edge for self sealing, (iii) provision for air cooling of the door body, and (iv) large size gas canals for easier circulation of gas inside the oven.

The advantage of leak proof doors are minimization of door leakages, regulation free operation, longer life due to less warping of the air cooled door body and reduced maintenance.

Land based pushing emission control system

The emissions generated during the pushing of red hot coke contain a large amount of coke dust (around 11 % of total pollutants in coke ovens. Land based pushing control systems mitigate this pollution. It consists of (i) a large suction hood fixed on the coke guide car and moving with the coke guide, directing the fumes to the coke side dust collecting duct (ii) dust collection duct and (iii) and the equipment cleaning of the fumes. The large amount of paroxysmal high temperature fumes are collected under the hot float fan into the large gas suction hood installed in the coke guide car, and enters the dust collection duct through the other equipment. The air is dissipated into the atmosphere after purification by the pulse duct collector and after being cooled by the accumulator cooling. The system is controlled by programmable logic controllers (PLC).

Automation and process control system

The automation of coke oven is structured in the classic levels, from Level 0 (field Level) up to Level 3 (management Level). The automation design of a coke oven plant is normally divided into six basic equipment layers. Fig 4 shows hierarchy of coke oven plant automation system.

Fig 4 Hierarchy of coke oven plant automation

The electrical equipment, the control elements and the instrumentation are normally connected to redundant remote I/O (input/output) units done by standard 4-20 mA and 24 DC interfaces. Intelligent subsystems are normally coupled with Profibus or Modbus. All automation equipment is connected via a fibre optic plant network which runs through all plant locations in which relevant equipment is placed. All data are collected and distributed through this network, whereby data source and data target can be flexible connected with each other using physical connections by patch panels and switches as well as logical connections using a network management system. Through this network all systems are able to communicate with each other.

In the coke oven plant area, an integrated ‘distributed control system’ (DCS) is used on the process control level. Many applications in the coke oven plant are sequence control functions, which are best executed by PLCs.

Automation and process control for the coke oven battery heating and machines is achieved using a level 2 control system which conducts various process model calculations based on the processed data collected from a level 1 automation system. The level-2 control system provides coke oven operators with an advanced, accurate and easy-to-use support tool, which can be successfully used to improve both the operational and environmental performances of the plant.

The benefits of the automation and process control system include lower energy consumption through reduction in fuel gas consumption, stabilize condition and operation of coke oven battery, consistent quality of coke, reduced emissions, increase in battery life and ease in reporting and analysis of operational and maintenance data.