Automation, Control, and Measurement System of Coke Oven Plant
Automation, Control, and Measurement System of Coke Oven Plant
Metallurgical coke is used in iron and steel industry processes (primarily in blast furnaces) for the reduction of e iron ore to iron and in foundries. Over 90 % of the total coke production is dedicated to blast furnace operations. Foundry coke comprises most of the balance and is used by foundries in furnaces for melting metal for casting. Foundry coke production uses a different blend of coking coals, longer coking times, and lower coking temperatures relative to those used for metallurgical coke.
Coke causes upto 50 % of the costs during the hot metal production. The cost effective production of high quality coke is thus of prime importance for the competitive ability of the iron production. Majority of coke is produced in the world using the by-product coke oven batteries and most of the coke oven plants are integrated with iron and steel production facilities. Under the present conditions of a sharp competition and fluctuating coal bases, the reduction in the of production costs of coke is one of the most important challenges which is faced by the iron and steel industry.
The basic process of coke production in the coke oven plant (COP) is quite complex. The thermal distillation takes place in groups of ovens called batteries. A battery consists of a number of adjacent ovens with common side walls which are made of high quality silica and other types of refractory bricks. The wall separating adjacent ovens, as well as each end wall, is made up of a series of heating flues. At any time, half of the flues in a given wall are burning gas while the other half are conveying waste heat from the combustion flues to a ‘checker brick’ heat exchanger and then to the combustion stack. Every 20 minutes to 30 minutes the battery ‘reverses’, and the waste heat flues become combustion flues while the combustion flues become the waste heat flues. This process provides more uniform heating of the coal mass.Automation, control
The operation of each oven is cyclic, but the battery contains sufficiently large number of ovens to produce an essentially continuous flow of raw coke oven gas. The individual ovens are charged and emptied at approximately equal time intervals during the coking cycle. Coking proceeds normally for 15 hours to 18 hours to produce blast furnace coke. During this period, volatile matter of coal distils out as coke oven gas. The coking time is determined by the coal blend, moisture content of the coal, rate of under firing, and the desired properties of the coke. When demand for coke is low, coking times can be increased to 24 hours. Coking temperatures normally range from 900 deg to 1,100 deg C and are kept on the higher side of the range to produce blast furnace coke. Air is prevented from leaking into the ovens by maintaining a positive back pressure in the gas collecting main. The ovens are maintained under positive pressure of around 10 mm water column in batteries by maintaining high hydraulic main pressure. The gases and hydrocarbons which evolve during the thermal distillation are removed through the off take system and sent to the by-product plant for recovery.
The coking is complete when the central temperature in the oven is around 950 deg C to 1,000 deg C. At this point the oven is isolated from hydraulic mains and after proper venting of residual gases, the doors are opened for coke pushing. At the end of coking period the coke mass has a high volume shrinkage which leads to detachment of mass from the walls ensuring easy pushing.
The important characteristics of the coke production are (i) coking process is a batch process and operation of each coke oven is to be based on integrated operational planning, (ii) coking process needs a minimum time which is quite large and hence coke oven plant has a high inertia, (iii) coking process has a directional flow and it is temperature and time dependent, (iv) there are large number of variables available which interferes with the coking process, (v) coking process is non linear with a number of parameters affecting the process, and (vi) coking process takes place in a closed chamber with indirect heating through heating walls making the measurement of temperatures very complex.
COP is an important unit since it produces coke for the reduction of iron ore in a blast furnace. It also produces coke oven gas which is used as fuel gas in various units of integrated steel plant. The quality and output of the products of COP are important since it provide stability to the operation of the iron and steel plant. Further COP is highly energy intensive and there are several environmental issues which are associated with the plant. COP is required to be equipped with automation, process control, and measurement system for achieving optimum efficiency. The modular design of the automation, control, and measurement system provides it flexibility for meeting the specific requirements needed by a particular COP. Automating of the process sequences also facilitates the long adjustments needed for meeting environmental protection requirements.
The automation, control, and measurement system is a modern user friendly tool which helps in improving the productivity and stability of the COP. It helps in improving the performance of the COP by addressing to the needs of the plant. It stabilizes production of coke oven, helps in reaching of the anticipated result and has an immense practical value. Its benefits include lower energy consumption through reduction in fuel gas consumption, stabilize condition and operation of coke oven battery, produce consistent quality of coke, reduce environmental emissions, increase battery life, and result into ease in reporting and analysis of operational and maintenance data.
Automation, control, and measurement system is structured in the classic levels, from Level 0 (field Level) up to Level 3 (management Level). The automation design of the COP is normally divided into six basic equipment layers. Fig 1 shows hierarchy of coke oven plant automation system.
Fig 1 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.
Modern instrumentation and control equipment connected to ‘programmable logic controllers’ (PLC) or ‘distributed control system’ (DCS) (Level 1) with operation stations are standard facilities in the present day COP. Some of the COPs have additional automation for advanced control and optimization functions. The automation of the by-product plant (BPP) is normally carried out by another DCS which is specialized on continuous control functions. Both the systems are normally coupled with each other through Modbus and are designed as integrated control systems for achieving the control for both, electrical and instrumentation equipment.
The automation, control, and measurement system enables an operation of the COP through operator control stations, located in separate control rooms. Besides having operator control stations for all major plant units, there are normally a number of control rooms which are equipped with large size video screens, including split screen capacity, audio paging systems, and intranet-access etc. The complete ‘network and system configuration’ for the COP and the BPP include the COP remote I/O (input / output) level, the COP-PLC-level, the COP-operation level and the system administration level with server and network equipment. Also, there is the interface to the Level 3 systems. .
In the area of COP, an integrated DCS is used at the process control level. Many applications in the COP 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. Fig 2 shows Level 2 automation and process control system for the COP.
Fig 2 Level 2 automation and process control system for coke oven plant
The process control technologies which are commonly used for the automation, control, and measurement system of the COP are described below.
Oven pressure regulation system
At any given point of time, pressure inside individual ovens is different since they are at different stages of coking periods. It is a known fact that ovens which are freshly charged witness highest pressure while ovens which are nearing their coking time witness lowest pressures. This has two effects namely (i) ovens at a higher pressure in comparison to gas collecting main are more prone to have fugitive emissions, and (ii) adjacent ovens operating at different pressures have different levels of stress on the oven chamber walls thus, reducing the refractory life considerably. In order to overcome these serious issues, it is important to regulate pressures in dividual ovens so as to maintain a slightly negative pressure throughout the coking period. This can be easily achieved by installing oven pressure regulation system.
Within the 1990s, a first version of a single oven pressure regulation system was developed by the DMT (Deutsche Montan Technologie GmbH) company in Germany. Since then the system has been improved continuously based on the practical experience. The improved oven pressure regulation system helps in the reduction of fugitive emissions at the COP. Oven pressure regulation system has been accepted as a ‘best available technique’ (BAT).
The most important technological improvements because of the oven pressure regulation system are (i) the oven pressure is decoupled from the collecting main pressure, (ii) the collecting main operates with negative pressure, (iii) the pressure inside each oven is controlled individually, (iv) charging gases are sucked off by negative collecting main pressure, and (v) the conventional valve is replaced by a so called ‘fix cup’ valve.
In the oven pressure regulation system, the ‘fix cup’ is installed between the standpipe and the crude gas collecting main. By means of the closure plug is equipped with a regulating device and connected to a control rod. In its extension, the standpipe gooseneck terminates in a so called crown tube, protruding with the crown slots existing therein into the ‘fix cup’. Also installed in the standpipe gooseneck are two spraying nozzles which on the one hand provide for cooling the hot crude gas and on the other hand for wetting the gas collecting main to prevent encrustation of tar and other deposits. In addition, by way of the quick filling valve, the ‘fix cup’ can be quickly flooded while a coke oven has been disconnected from the gas collecting main. Fig 3 shows oven pressure regulation system.
Fig 3 Oven regulation system
The regulation of the oven pressure is carried out by a variable pressure resistance for the generated crude gas, created by slots in the crown tube. The slots are opened more or less by means of a variable water level in the ‘fix cup’. The water level is influenced by the overflow regulation device, which maintains a certain water level within the ‘fix cup’ depending on the set-point of the oven chamber pressure. The water level in the ‘fix cup’ is directly related to the position of the passage piston of the overflow regulation device. The drive of the overflow-regulation device is a pneumatic cylinder which is connected with the overflow regulation device by a rod. The pneumatic cylinder is controlled by a both side working positioner, receiving its information from a control system, which processes the oven pressure measurement.
The oven pressure is measured within the gooseneck, from where it is transmitted to the control system. During the carbonizing time the oven pressure is increased stepwise from around +3 mm H2O (0.3 mbar) at the beginning of the carbonizing time, when the amount of generated crude gas and the danger of emissions are on its highest level, to around +16 mm H2O (1.6 mbar) at the end of the carbonizing time, when the amount of generated crude gas is dropping against zero and hence the danger of emissions is very low. The final adjustment of the set-points is normally carried out after pressure measurements behind the oven doors at oven sole level at the time of commissioning of the COP. The objective is to adjust the oven pressure in such a way that the lowest possible pressure in the oven can be achieved at all times without creating suction behind the doors at oven sole level.
The gas collecting main is normally located on the pusher side of the coke oven battery. It consists of three sections. Each collecting main section is normally equipped with two gas bleeders to be able to discharge crude gas directly at the battery in case of an emergency. Water sealed valves form the closure between gas collecting main and the atmosphere. The bleeder valves are pneumatically driven and open automatically at a pre-defined maximum pressure in the gas collecting main. Ignition of the crude gases is effected by an electrical arc system which starts ignition immediately before opening of the bleeder valves. The collecting main pressure is controlled by a control valve in each of the off-take mains. The negative pressure provided from the exhauster, is throttled upstream of the control flap so that merely as much crude coke oven gas is released as is needed to maintain the defined pressure in the gas collecting main.
The HMI (human machine interface) of the oven pressure regulation system consists of multiple operator displays which enables the battery operator to monitor and adjust the system (in automatic mode) and if necessary to operate the system in manual mode (i.e. in case of emergency). All process values like oven pressure, water level inside the fix-cup, status of all control elements, last coking time, status messages, etc. are shown. If switched to manual operation, all operation functions like ‘connect to charge’, ‘back to regulate’, ‘close the standpipe lid’ etc. can be manually initiated within the proper operation sequence. Some interlocking sequences are still active to avoid harmful operation mistakes. Manual operation without PLC-control and interlocking sequences can only be done from the pneumatic control panel which is located directly in front of the respective standpipe. A trend display for each oven can be selected at the HMI, which shows the main process values in terms of time.
Coal moisture analyzer
A number of moisture measurement systems are available. However, the reliable method to measure the coal moisture on-line is to use ‘microwave with area weight compensation’. Microwaves are a highly accurate way to measure moisture due to the fact that microwaves are highly selective to water. They penetrate the material to be measured. Water molecules are naturally polar which causes the microwaves to weaken and slow down significantly. The dielectric constant of the material indicates the influence on the microwaves. The dielectric constant of water is 20 times larger in comparison to other materials. This results in a strong interaction of the microwaves with water which are then measure as attenuation and phase shift.
For ensuring that reflection and resonance do not affect the measurement, multiple frequencies are used and evaluated. Hence irregular influences of geometry changes, as the layer thickness of the material in spite of a compensation for area weight are nearby eliminated. The phase shift measurement is additionally needed since it is less influenced by several disturbances and results thus in a better accuracy. Hence a combination of attenuation and phase-shift further results in a reduction of disturbances, which additionally improves the accuracy. By combining to measure phase shift and attenuation, a precision better than 0.2 % can be achieved which from the measurement ‘point of view’ is sufficient to use the moisture value for heating control.
If the bulk density varies, which is the case operating with different coal blends and different grain size distributions, an additional radiometric measuring unit is needed. The layer thickness and bulk density has an impact on the measurement results. It can be largely eliminated by normalizing attenuation and phase shift to the mass per unit area, which is determined by gamma-ray transmission measurements. In this transmission measurement, the weakening of the gamma-ray intensity, which depends on the area weight, is measured. As a result, a density-independent moisture signal can be obtained, ensuring the highest possible precision for optimal process control. If at the same time the coal layer thickness is measured close to the gamma-ray source e.g. with an ultrasonic level sensor, the bulk density of the coal can be determined (area weight multiplied with the layer thickness is the bulk density). The set-up of coal moisture measurement system is shown in Fig 4. Besides, the equipment shown in the set-up, some more items are needed to make the system work.
Fig 4 Set up of coal moisture measurement system
The microwaves are transmitted using a pair of so called horn antennas. One is installed above the belt and the other is installed below the belt. Due to this transmission geometry, a large percentage of the whole volume is measured. This provides a very accurate representation of the moisture content throughout the coal layer. Hence, the moisture inside the full coal layer is measured and not only the surface moisture.
The gamma ray source (Nuclide Cs 137) is installed below the belt. It is to be as close as possible to the microwave emitting horn antenna so that the same coal portion at the same place and time are referenced with each other (attenuation and phase shift are correlated with area weight in real time). Vertically centered to the gamma ray source is the gamma ray detector (Scintillation detector) installed above the belt. The two horn antennas, gamma-ray source and the gamma ray detector are connected with special high frequency signal cables to an evaluation unit which correlates and calculates moisture and bulk density in real time. Reference curves obtained from multiple calibration tests (on-line moisture over laboratory moisture) are stored in the evaluation unit for multiple coal consistence or blends. These integrated reference lines ensures reliable compensation of environmental influences. In this way the water content and bulk density of the coal can be very accurately determined.
The measurement works best, if the surface of the coal is straight and flat. Hence, it is necessary to put some flattening equipment in front of the measurement set-up. The flattening of the coal surface is done in two steps. First, a heavy steel plate is used as a scraper. The maximum excursion of the scraper is limited by chains to avoid that the scraper comes into contact with the rubber belt. Weights can be added to the scraper to set the scraping force and adjust the paving path. Second, a sledge, also limited in its movement by chains, levels the remaining bumps. If the coal level on the belt is very high or piles of coal are approaching, the scraper or the sledge can spill coal from the belt. Hence, containments made from rubber-belt material are placed on each side of the belt. A flat coal surface is necessary for getting reliable signals.
Automatic chamber wall temperature measurement system
The automatic chamber wall temperature measurement system consists of a coke chamber wall temperature measurement system through air cooled fibre optic cables and attached pyrometers mounted on the ‘cold’ rear end ram beam of the pusher car. The temperatures of the walls are measured when the ram passes through the oven. They are converted and evaluated to enable the supervision of the temperature and heat distribution of the battery in longitudinal, transversal and vertical direction. This can be performed by checking cross wall temperatures, longitudinal battery temperatures, vertical heat distribution, temperature development in terms of time, wall-heating checks etc. Fig 5 shows the principle of the automatic chamber wall temperature measurement system.
Fig 5 Principle of automatic chamber wall temperature measurement
The light intensity radiated from the oven wall is detected by a fibre optic cable at each measuring point. This measuring point consists of a housing thermally insulated against radiation and heat conducted by the ram head. The housing accommodates the fibre optic cable holder, air routing system for an optimum cooling effect, fibre optic cable, and compressed air feed connection. The fibre optic cable is permanently attached in relation to the ram. Compressed air is allowed to pass along the fibre optic cable protecting it against overheating and dirt, and clearing the passage between light the guide housing as it blows out into the oven chamber.
Several hundred data points (raw data) are measured by each pyrometer during one push process transmitted to a PLC in the electrical room of the pusher machine and correlated with the related distance information from the ram drive system. The raw values are compressed to build one average temperature value per heating flue for each pyrometer. These values (in total 6 x number of heating flues) together with the oven number of the respective push, the time of pushing, and the levelling are temporarily saved in the storage medium of the designated automatic chamber wall temperature measurement system PLC station on the pusher machine. The values so determined are transmitted through the fibre optics from the pusher machine to the COP PLC for oven machines which receives the data and stores those data in a database.
Whenever needed, the operator can select and evaluate temperature data from the archive by using a comprehensive menu system which is integrated in the HMI of the COP PLC for oven machines available on all the server client PCs. Automatic alarms are generated if threshold values are exceeded. The operator is able to check at regular intervals or in case of an alert the temperature distribution within the battery block to detect maladjustments of the under firing system which can lead to under coking of the coal in specific areas of the coke mass resulting in bad coke quality and pollution during pushing.
Besides alerting to problems in the crosswall, the vertical temperature distribution is especially important in high oven chambers. This is achieved by a long flame over the full height of the flue. The flame is influenced by the gas and air distribution to the heating flue which has to be properly adjusted. Changes in the air distribution (i.e. changes in the stack draft) without proper counter-measures can have disturbing influences to the length of the flame (vertical heat distribution) and ultimately can lead to uneven coking, to roof carbon, and worst of all ultimately to ‘sticker ovens’. Automatic chamber wall temperature measurement system is able to quickly detect vertical heating problems while taking temperatures in three levels of the oven chamber during each push. Detection of these problems helps to improve the heating system which leads to better environmental protection, higher coke quality, higher production efficiency (gas / energy savings) and less stress to the brickwork (longer service life of the battery).
The heating evaluation of single walls or ovens, the oven wall temperatures from automatic chamber wall temperature measurement system can be condensed to provide a mean battery temperature, which can be used as an input for battery heating control.
Automatic monitoring system of the pushing force
Together with the measurement of the chamber wall temperatures, while the pusher ram pushes the coke out of an oven, concurrently the torque needed for this action is measured on the ram drive motor. These values are measured while the pusher ram pushes the hot coke out of the oven. The torque is provided from the frequency converter unit which controls the motor speed and motor torque and is converted in the PLC for oven machines into a pushing force. The system is called ‘automatic monitoring system of the pushing force’ and provides outstanding information about the mechanical maintenance situation of the ram drive system and the coking condition of the coke cake. If the ram force increases over a period of time, a mechanical or a heating problem can be expected which calls for attention and further evaluation for trouble shooting.
Graphics of the automatic monitoring of the pushing force can be called up on the HMI of the PLC for oven machines by the operators for process control and as a trouble-shooting tool. The plant managers select the data of the automatic monitoring of the pushing force from a long term archive for process monitoring, optimization and historical surveys. The pushing force curves show the same profile which means a pushing force peak in the beginning to break the coke loose from the wall and get the coke cake moving. After this initial peak, the pushing force is much lower, just enough to keep the coke cake moving along the length of the oven. As soon as the pusher ram shoe enters the oven, a new but smaller peak develops. This support shoe slides over the oven sole and put additional friction onto the bricks which have to be counteracted by the ram drive, leading to an increase of the pushing force needed. This is the normal situation during each pushing.
If the graphic stands out of the regular profile with multiple pushing peaks along the pushing path then the first peak repeats itself multiple times during one push. The reason can be the pushing stopped several times and resumed again as the ram travelled through the oven. Mechanical problems on the coke guide needs these stops. With each restart, the pusher drive system has to regain the force to get the coke cake moving again. Four peaks indicate that the pusher ram stopped and restarted four times after the initial ‘break off peak’. This example shows that the pushing force measurement is a useful tool to detect and document operational problems during pushing.
Automatic scheduling and control system of oven machines
Process control and monitoring of operation of COP also includes the preparation of a pushing schedule and screen display of the oven machines operation performance. For this purpose a very advanced pushing and charging schedule program called ‘automatic scheduling and control system of oven machines’ as part of the COP automation system is used. Pushing and charging times for each oven are calculated and optimized, transferred to the oven machines and signalled to the operators. The actual data of the pushing and charging operation is feed back to the scheduling system to update the calculation.
The ‘automatic scheduled and control system of oven machines’ can handle normal production planning as well as all types of special operation (i.e. compensation of breakdown or decreased production). A re-calculation can be triggered and remade anytime when there is a change in production data or there is any operating trouble. Several strategies are available to handle a loss of production. The loss can be accepted or made up by increasing production with shortening the coking time in a careful and secure manner for keeping best heating performance and production. Hence, changes in the schedule automatically influence the calculated nominal heat within the heating control model.
The pushing and charging schedule can be calculated for several days in advance in a special simulation mode for advanced production planning. The system is interlocked with the oven pressure regulation system to handle the disconnection from the collection main for pushing and reconnection to the collection main for charging. Fig 6 shows the main function and operation system philosophy as well as the operation displays. The computer screen on the right side has normally three displays. The first display is called ‘oven status’, which displays for each oven the next push / charge-times, last push / charge times, time in cycle as bar graphs with multiple colours, and charging weight etc. The second display is for the calculated schedule which shows the pushing and charging cycles in chronological order for the next few days in advance. The third display shows the pushing and charging history as a report.
Fig 6 Automatic scheduling and control of oven machines
Automatic control system of battery heating
Automatic control system of battery heating is a theoretical calculation model which determines the needed energy for heating of the battery. The model is dynamically updated by the actual production performance (adapting to delays, ‘speed up’, lost production, etc.) and the actual heating performance (adapting is based on actual heating flue, coke or wall temperatures which are outside of the target range). The energy requirements determined by the ‘automatic control system of battery heating’ model are the set point for the heating system. The energy needed for the battery heating in this case is controlled by changing the heating time (varying a pause time between reversals).
Automatic control system of battery heating is shown in Fig 7. The figure also shows a trend graph of the heating control results. Whenever the coking time changes (green arrow), the energy quantity control reacts by creating a new set point for the energy input (yellow arrow), mainly by changing the pause time (blue arrow). In the example given, the pause time is increased from around 200 seconds to around 275 seconds by the model to match a declining energy demand (red curve)) due to a general increase in the coking time (green curve) from 28.9 hours to 30.1 hours. The rise of the ‘mean battery temperature’, measured by the automatic chamber wall temperature measurement system (pink dotted arrow), also needs a reduction of heat, which lowers the energy set point even further. The total reduction of energy (yellow curve) leads to a reduction of the ‘mean battery temperature’ (pink arrow). However, this happens with a time delay due to the reaction time needed to bring the energy from the heating flue to the coke.
Fig 7 Automatic control system of battery heating
For making both temperatures measured at different places in the brickwork comparable to each other, the heating flue temperatures are extrapolated by the heat transfer rate to oven wall temperatures. The ‘automatic control system of battery heating’ model is able to keep the quantity of heating energy under control, ensures less energy consumption and a quick and automatic response to operation troubles which holds the battery temperatures in balance, reducing heating problems and pushing emissions.