Blast Furnace Process Automation, Measurement, and Control System
Blast Furnace Process Automation, Measurement, and Control System
The efficient operation of the modern blast furnace needs a high degree of automation in conjunction with a measuring system and a monitoring and control system. The blast furnace process control system in combination with the blast furnace optimization system creates a high level in intelligent blast furnace automation. The optimal interaction between sophisticated models and the expert system provides comprehensive assistance to the plant operators and minimizes the risk of human errors.
The problems related to the blast furnace process automation, measurement and control system which are to be dealt range from problems of classical control theory of linear and nonlinear, single, and multivariable systems in process control, to questions of operational and production control. For this, complex operating systems are to be applied. These systems have to start and stop special program modules (‘tasks’) automatically, without the operator so as to fulfill the so-called ‘real-time condition’ with the computer must have reacted completely in a clearly defined maximum time (deadline) to an event in the process. The deadlines range from some minutes to some seconds.
Blast furnace process automation, measurement, and control system is one of the main contributors to the successful blast furnace operation and belongs to the key factors of its economic effectiveness. Blast furnace process complexity in combination with growing demand for the effectiveness and reducing environmental impact has necessitated a change in the process control strategy. In the past, static calculations based on black-box principles were used to predetermine some fundamental set points with limited feed-back from the process. This type of control can give acceptable results only when process has small deviation from stationary operating point. Because of process instability small disturbances can cause considerable deviation from the operating point that needs set point correction for which AI (artificial intelligence) methods are normally used.
With variation in the charge composition and various operating practices the dynamics of the process is to be taken into account providing recalculation of set points and immediate feed-back in real-time. One precondition for the dynamic process control was the development of the sensors and measuring techniques which gives real time information about the process state. The evaluation of dynamic models makes it possible to go from process supervision to proactive real time control. Different approaches reflect specific situation and control philosophy. Presently blast furnace process control system is based on dynamic models and allows for on-line control. The basic approach is to consider blast furnace as a system including water cooling, where material and energy are supplied, exothermic and endothermic chemical reactions take place, and energy is dissipated in the form of heat losses to water cooled surfaces and as sensible heat in off gas.
Cost-optimized operation, process improvements which do not compromise burden-material selection, the highest product quality, and achievement and maintenance of the desired productivity are the core challenges for advanced blast furnace process automation, measurement, and control system. The optimized burden calculation with precise chemical targets and subsequent closed-loop controls form the basis for producing hot metal and slag of high quality, while simultaneously reducing energy consumption. The optimization system provides easy access to all process parameters, material properties, and productivity data which include charging information, chemical and physical burden material, and process measurements so that the optimal conditions for producing hot metal and slag at the best quality level can be determined.
The blast furnace process automation, measurement, and control system is a high accuracy process control with prompt online graphical information. It provides a stable, reproducible operation of the blast furnace with constant hot metal quality. It ensures shift independent plant operation where manual interactions are reduced to a minimum. The result is smooth blast furnace operation all the time, increased equipment lifetime, and reduced production costs. Fig 1 shows overview of blast furnace process automation, measurement, and control system.
Fig 1 Overview of blast furnace process automation, measurement, and control system
The advantages of the blast furnace process automation, measurement, and control system include (i) high productivity which means maintaining the running of the blast furnace at peak performance while minimizing consumption of electrical energy and fuel, (ii) high and uniform product quality which means maintenance of the chemical properties of hot metal and slag at the desired levels, (iii) reduced fuel consumption while keeping the temperature of the hot metal constant through small modifications of the fuel rate, based on the thermal conditions of the blast furnace, (iv) stable, shift independent, and the best practice blast furnace operation during the entire day for ensuring efficient production, (v) easy integration of a comprehensive range of metallurgical models and packages with the automation, measurement, and control system, (vi) quick and flexible reactions to the change in the requirements, (vii) possibilities for future system extensions, and (viii) very quick return on investment.
The blast furnace process automation, measurement, and control normally consists of PLC (programmable logic controller) and HMI (human machine interface) systems at Level-1 automation which are the control for all critical processes. These control means are completed by Level-2 solutions (process models, expert systems, and program tools), covering either the overall BF operation or specific parts of the ironmaking process (hot blast stoves, and blast furnace charging etc.). An expert system is used for the integrated Level-2 solutions for the blast furnace. This advanced process assistance system includes blast furnace control and real time data analysis and process optimization as well as deferred blast furnace data analysis. It allows operators to optimize hot metal production, to be assisted by a knowledge based system and to report performance indicators and production figures.
In overall process automation, measurement, and control system, computers, programmable controllers, and micro controllers are used which are connected in the form of a local area control network to perform all communications from the enterprise level down into the plant and vice versa in an optimal way. The intelligent motor control systems, integrated with the control system, provide distributed control and additional maintenance data for the increased diagnostics and field equipment performance.
Blast furnace process automation, measurement, and control system has (i) high speed, open, and redundant capable networks, (ii) complete field I/O (input/output) cabling and design considerations to reduce the installed cost, (iii) as a minimum, HART (Highway Addressable Remote Transducer protocol) instrumentation with HART interfacing capability in the control system, (iv) an asset management system capable of maintaining all plant assets including the control system, (v) seamless interface to Level 2 applications for process optimization (if not already performed in Level 1) and historical data with Level 2 easily growing with the Level 1 control platform, (vi) a Level 0/1 data is stored on the history file, for use at the HMI, or at Level 2/3, (vi) process and operational reports, and (vii) operator and maintenance data / diagnostic systems. The HART communication protocol is a hybrid analog + digital industrial automation open protocol. Its most notable advantage is that it can communicate over legacy 4–20 mA analog instrumentation current loops, sharing the pair of wires used by the analog-only host systems.
A typical blast furnace automation system uses process controllers, operating stations integrated across high speed ‘Modbus plus’ peer-to-peer network. This results into high accuracy process control with prompt online graphical information. Mainly hot blast stove and burden optimization with the fool proof interlocks for reliable and safe blast furnace charging and burden distribution result into stable, reproducible operation of the blast furnace with constant quality of the hot metal
Since blast furnace is a very simple reaction vessel, it is difficult to control the conditions inside it directly and delicately. In addition, because the processes in the blast furnace change very slowly, once stable operation of the furnace is disturbed, it is very difficult to recover a normal state. It is hence necessary for maintaining stable operation to monitor very small changes in the condition inside the furnace and take appropriate timely action. Towards this end, a process computer collects and calculates data from sensors provided at various positions of the blast furnace, and thus monitors the condition of the furnace in real time. When it detects any change which can adversely affect the stable operation of the blast furnace, it outputs action guidance for the furnace operators.
More specifically, the monitoring and controlling functions of a process control system of a blast furnace include (i) charging control of ore and coke by doing calculation of the charging ratio of iron ore and coke and the sequential order of their charging, (ii) charging operation control by setting of the operation mode of the rotating charging chute such that an adequate circumferential distribution of ore and coke is realized, (iii) hot blast stove control by the control of the combustion in hot stoves such that hot blast is supplied stably, (iv) furnace control by the estimation of the condition inside the furnace based on information from various sensors, and (v) control of the tapping of hot metal and liquid slag with control of the amount and quality of tapped hot metal.As seen above, the blast furnace process automation, measurement, and control system has very important roles in the operation of a blast furnace.
The present day blast furnace has nearly a thousand sensors installed in it. This huge number of sensors in conventional analog instrumentation is difficult to install in the wide ironmaking area. The progress of micro-electronics and data communication systems such as the data highway makes it possible to introduce distributed digital instrumentation. There are several advantages of digital instrumentation over the conventional analog instrumentation such as (i) able to build noise free systems, especially electromagnetic noise, (ii) able to use advanced signal processing and advanced control functions, (iii) reliability of the system can be improved by the use of dual functions, (iv) by using a CRT (cathode ray tube) display it is possible to receive more information from a compact control desk than the previous huge analog instrument panel, (v) it is much easier to change or improve the systems or functions, (vi) it is easier to exchange information with host computer systems, and (vii) installation cost is much cheaper than the conventional analog instrumentation from the standpoint of increased functions. For these reasons, digital instrumentation is normally used for the blast furnace process automation, measurement, and control system.
Efficient blast furnace control needs reliable measurements of the conditions inside the furnace. The temperatures in the lower half of the furnace can increase to more than 2,000 deg C, where most intrusive measurement technologies are unreliable, so most of the in-furnace measurements are carried out above or near the burden surface. Reliable probe operation is important to support, on a regular basis, data processing, furnace process models, and furnace operation supervisory systems. Reliable probes help the blast furnace operator to establish (i) top gas temperature profile and top gas chemistry, (ii) material falling trajectories, stock-line level and profile, (iii) burden layer build-up, mixing and descent behaviour, (iv) gas and temperature profiles in the burden column, (v) raceway and coke bed conditions, and (vi) hot metal quality and tapping operations. The most important techniques for direct or indirect quantification of the burden distribution include (i) above-burden probe, (ii) in-burden probe, (iii) stock-line detector, (iv) profile meter, (v) vertical probe, (vi) thermocouples, (vii) pressure gauges at the furnace wall, and (viii) miscellaneous measurements. Fig 2 shows typical layout of some of the probes used in the blast furnace.
Fig 2 Typical layout of some blast furnace probes
Above-burden probe – The above-burden probe has a number of thermocouples attached to the device to measure the gas temperatures at different radial positions above the burden surface. This provides the information about the gas flow conditions in the furnace. The regions with lower permeability allow less gas to flow which reduces the gas temperature compared to regions with higher permeability. Hence, the temperature readings give the information about the permeability conditions in the furnace.
An issue with the above burden probe is that the gas coming out of the burden surface mixes before it reaches the probe. Hence, some temperatures can be either underestimated or overestimated. The probe is hence to be mounted closer to the burden surface, which is difficult to realize as the burden surface can change during the process and with the production rate. Furthermore, a sudden increase in the stock-line caused by fluidization can damage the probe.
In-burden probe – In-burden probes are normally installed at any height below the burden surface and above the cohesive zone. Hence, these probes have to survive higher temperature and abrasion compared to the above-burden probes. This is the reason why they are normally retractable and only inserted when sampling is done. The in-burden probe measures the gas temperature and the composition at different radial points. The measurements are, in general, more accurate than the signals from the above-burden probe as mixing does not occur to the same extent. However, strictly speaking, the result depends on the layer in which the sampling point is at the moment of measuring.
Stock-line detector – Stock-line detectors are used to obtain information about the height of the burden surface, known as ‘stock-line’, after charging each dump into the furnace. Blast furnace is programmed so that a dump is charged into the furnace only when the burden surface has descended beyond a certain vertical level. Stock-line detectors can be mechanical devices (stock-rods) where a weight at the end of a chain or wire is lowered until resistance in the form of burden surface is reached. Present day furnaces use non-contact techniques, such as radar systems which eliminate the time loss while lowering the weight into the furnace. A sudden drop in the stock-line is an indication of a slip, which can be a concern for the furnace operator.
Profile meter – Profile meters have been mechanical devices originally but now they have been replaced by non-contact methods, e.g., movable radars (moving probe) along a horizontal channel which measure the burden surface height at various radial points. The profile meter can also estimate the burden descent velocity. Present day profile meters have radars fixed on rotary joints and 3D burden surfaces can be estimated, which gives a much better understanding than by measurements along a single direction. Non-contact level measurement with radar in blast furnace has several advantages which include (i) reliable measurement, independent of dust, material composition and high temperatures, (ii) high measurement certainty even during filling, and (iii) wear and maintenance-free operation.
Vertical probe – Vertical probes are used to provide the temperature and the gas composition along the height of the blast furnace. These probes can consist of cables at different radial positions which are lowered to the burden surface and are dragged down by moving solids until the tip is damaged, as the cables reach high temperatures in the lower part of the furnace. The probes normally measure temperature and pressure and can sample gas for composition. These probes can be equipped with a camera for particle size distribution. The lengths of the eroded probes also indicate the location of the cohesive zone in the furnace. Although vertical probes provide maximum information about the furnace, they are seldom used as they are expensive and need complex feeding equipment.
Thermocouples – Blast furnace walls are lined with thermocouples which also provide crucial information about the furnace operation. For example, sudden changes in thermocouple readings can indicate dropping of skull, which is a stagnant solidified mass formed at the furnace walls.
The harsh environment caused by high temperatures and high pressures encountered in the stove considerably reduces the lifetime of the thermocouples, due to the contamination and migration of the tip materials. The metal sheaths used to protect conventional thermocouples are not viable above 1,200 deg C. Alternative sheaths are easily broken or damaged by the expansion and contraction of the ceramic brickwork during the heating cycle. In addition, sudden pressure relief (or ‘snorting’) can cause a drop in the temperature reading of between 20 deg C to 30 deg C for about 30 seconds. This makes the thermocouple signal unsuitable for use in automatic stove reversal control systems. Correctly installed, an infrared pyrometer system provides accurate temperature measurements for stove application while overcoming several of the problems encountered with the use of thermocouples.
Pressure gauges at the furnace wall – Gas pressure is measured at different points on the walls. As the gas flows through the coke slits the direction is horizontal, so it affects the pressure at the walls. Hence, the pressure information can be used to estimate the cohesive zone shape.
Other measurements – Some of the other measurements at the blast furnace include (i) pressure, temperature, and composition of the top gas, (ii) flow rate and rise in temperature of the cooling water, (iii) blast conditions, (iv) hot metal and slag variables, (v) use of belly probe etc. (vi) infrared cameras to measure burden surface temperature, and (vii) skin flow thermocouples (or mini-probes). These measurements are indirectly affected by the burden distribution.
Some of the additional measurement probes which are used in the blast furnace are (i) material scanning probe (ii) tuyere probe, (iii) core sampler, (iv) impact probe (v) torpedo ladle level radar probe, and (vi) resistance measuring in the furnace shell.
Core functionality for blast furnace optimization
Blast furnace optimization is an innovative process optimization system which lifts blast furnace process automation, measurement, and control system to a completely new level. The typical solution based on a well tested and proven basis system ensures the highest availability and efficiently combines data acquisition, processing, and visualization. The system demonstrates reliability and cost savings.
A broad spectrum of raw data sources (including front end signals, amount of material charged, laboratory data, events, model results, and cost data) is stored throughout the entire plant lifetime. Specialized tools are provided which allow process information to be linked to analysis data and burden matrices. Flexible interfaces, modularization, and reliable software architecture provide the means to easily adapt and maintain the system in an ever changing environment of raw materials, operation philosophy, and connectivity to third party systems.
In addition to the robust basis system, a number of interacting process models support operators and line managers in their daily decisions. The metallurgical process models make the blast furnace process transparent. Plant specific requirements are normally incorporated into these metallurgical process models.
The purpose of dynamic models is to continuously display and predict process status. It also allows controlling various process parameters with the applicable process strategy. The process status determination is based on information retrieved from field instruments. Some of the examples of the models which have been developed are (i) material distribution and position of particular material zones, (ii) gas distribution, (iii) temperature distribution, (iv) thermal state of the furnace and its parts, (v) hot metal and slag chemical composition in the dropping zone, (vi) hearth liquid level, (vii) geometry of material zones (cohesive zone, dropping zone, dead man, and long term thermal resistance), and (viii) shaft geometry (scaffolds). Developed models are of analytical, empirical, and heuristic nature. The calculation runs cyclically and the output from these models are geometrical, thermal, and material state of the furnace and its characteristics.
The predictive model makes real-time simulation based on actual data about the furnace inputs. Model is of zonal type. The modelled processes are gas flow, material flow, thermal process, chemical process, physical process, and geometrical process. The furnace state is determined for each element with heat and material balance. The calculation is based on information retrieved from the laboratory and field instrumentation allowing the closing of the balances. The basic chemical reactions are used in the model.
End-to-end transparency in real time through upto date data visualization and metallurgical process models lead to better collaboration, improved work flows, and reduced errors while also supporting the decision making. Even important process parameters such as the flame temperature and indirect reduction percentage are implemented as soft sensors, which makes them indistinguishable from conventional measurements.
The information technology of the iron and steel industry is shifting, and mobile access is expected to outpace desktop based access. The HMI is to be designed to be responsive and flexible, whether it is a desktop or a new multi-touch interface. It is to be simple for allowing the operators to work more efficiently and effectively.
With the use of different measurements together and with past experience, operators can get a holistic view of the conditions in the blast furnace and identify the cause of improper furnace conditions. The process automation, measurement, and control of different areas of blast furnace is briefly described below.
Automation of stock-house and furnace charging – Automation of stock-house and furnace charging can be implemented from scale car systems to fully automatic conveyor / screen/ feeder systems, providing ore coke and miscellaneous materials to skip or belt fed furnaces. This includes the ability to create the batch recipes remotely or through the HMI with a fully automated ‘charge master’ programme. The programme tracks the flow of material from the stock house storage bins to delivery to the furnace top, complete with dry weight-error correction and weigh-error compensation, to maintain correct burden composition and level. An essential component for a fully automated system is the addition of a dust collection system to remove and extract emissions produced by the material handling process.
The material-based charging system allows for the dynamic assignment of different materials to the stock house bunkers. The charging matrix is related to available materials only and not to the bunkers. The effect is a more flexible plant operation with no need for program adjustments. Alternatively, a more elaborate version of the conventional bunker-based charging matrix can also be used. Independent of the type of charging matrix, an intelligent compensation of dosing deviations is considered a matter of course. The stock house control calculates all possibilities of material overlapping on the charging conveyor belt in the case of belt fed furnace. Along with comprehensive material tracking functions from the stock house to the furnace top, a smooth and efficient operation is achieved.
Automation of furnace top charging – Automation of for furnace top charging includes (i) control system programming of small bell / large bell material delivery as well as control for the bell-less systems, (ii) stock-line level monitoring and control, (iii) furnace top pressurizing and relief systems, (iv) lubrication systems, (v) as a minimum, HART instrumentation with HART interfacing capability in the control system, and (vi) secure and safe design for the control of furnace top pressure relief bleeder valves, including the associated hydraulic systems.
Different charging equipments provide different degrees of control over the charging process, which ultimately determines the burden distribution. Even with a few options, though, the charging process can become very complicated and can be counter-intuitive at times.
Smart distribution solutions for all standard types of material distribution systems are normally provided in case of bell less top charging. A smart version of the most common ring distribution logic is improved by the options to enable spiral charging, full rings, and weight or time distribution. As a result, flexible and marginal distribution modifications between individual batches are possible. Spot and sector charging offer a flexible and easy opportunity to react to the actual furnace status under demanding blast furnace conditions. Smooth free shape distribution is a combination of ring and spot distribution applying speed variations to the distribution device. This mode combines the stable ring-mode distribution with the flexibility of free-shape distribution.
Closed-loop burden distribution control is a unique feature of the automation system for blast furnace process stabilization and reduction of the fuel consumption. Based on radial temperature measurements in the blast furnace shaft, the model calculates modifications of the distribution pattern in order to achieve a target temperature profile. The system supports either an in-burden probe or above-burden temperature measurements based on conventional and acoustic techniques.
Automation of furnace proper and hearth – Automation of furnace proper and hearth includes controls for process temperature monitoring, trending and alarming, off-gas analysis, and above-burden and in-burden probe systems. As a part of the monitoring system, process and production calculations and third-party models are used to provide operational guidelines. Some of this data includes (i) hearth and sidewall isotherms, (ii) burden distribution, (iii) coal grinding and pulverized coal injection, (iv) tuyere leak detection and heat loss, (v) process calculations such as adiabatic flame temp, permeability, and tuyere velocity, and (vi) data to / from Level 2 systems (history data files and process models).
Optimized blast furnace operation needs accurate charging of the raw materials, including sinter, pellets, ores, coke, fluxes, and additives. For modifying the charging set-points, the coke rate, slag basicity, and actual raw material analyses and their influence on the blast furnace parameters are to be taken into consideration. This procedure is complex and needs assistance of a computer model. Burden control model calculates the charging matrix based on the optimized burden composition while the burden distribution model improves the gas utilization through precise material distribution.
With the blast furnace optimization system, operators have complete data transparency across the entire process. This enormous variety of process and meta-data (for example, shifts, alarms, and materials) is made transparent, accessible, and understandable through the concise reports generated by the system. Smart tiles serve as glazed doors which automatically display live information from the favourite applications of the operator, even if the application is not running. In the event of a significant deviation from normal process parameters, related production data come alive on the home screen, enabling the operator to make important decisions and take corrective action for the situation.
The purpose of the burden control model is to establish a precise burden composition which fulfils the assigned target values for coke and fuel injectant rates, slag basicity, hot metal quality, and burden feed rates. The final result of the burden control model is a charging matrix which can be transferred to the basic automation system for execution with a single mouse click. In combination with the expert system, the burden control model is the central part of the fully automatic burden composition optimization in the operation of the blast furnace.
The benefits of the burden control model include (i) constant product quality which means maintaining the chemical compositions of hot metal and slag at the desired levels, (ii) shift-independent burden modifications since the calculation of the new burden composition is performed automatically using the latest raw material analyses and standardized calculation procedures, and (iii) no manual operator interaction needed for calculating and activating a new charging matrix.
Bell-less charging chutes as well as bell-type charging devices with moveable armour enable a precise distribution of ore and coke layers into the blast furnace. The burden distribution model assists the operators and / or line managers to modify the actual distribution in order to improve the gas-flow pattern and burden permeability according to the actual process requirements. The model simulates the burden descent through the blast furnace shaft and calculates the actual shape of the material layers in the upper part of the shaft. It also computes the radial volume, chemical properties, and particle size distribution, taking into account material segregation.
The on-line burden distribution model performs the calculation based on actual charging data and actual measurements of the stock-line and calculates the current burden distribution in the upper shaft of the furnace. This gives the operator the opportunity to detect irregularities in the burden distribution in a timely manner. In the off-line mode, the model calculation is based on a charging matrix and a pre-defined stock-line. The off-line burden distribution model is a valuable tool for the design of new distribution matrices for optimized gas-flow patterns and burden permeability.
3D hearth lining monitoring – 3D hearth lining monitoring is for safe, durable, reliable production at the blast furnace. The campaign duration of a blast furnace is mainly determined by the lifetime of its hearth. Hence, it is clear that monitoring the refractory thickness in the hearth wall and bottom areas is important for estimating the lifetime of the hearth lining.
The hearth wear model includes mathematical algorithms which solve the inverse heat transfer problem in 3 spatial dimensions based on the statistical evaluations of the thermocouple measurements and the heat conductivities of the refractories. The model calculates the erosion profile and the formation of the solidified skull layer. The computed wear velocity together with the remaining wall thickness allows the blast furnace operators and line managers to predict the lifetime of the hearth refractory.
The 3D hearth lining monitoring model includes user interfaces and reports for visualizing the results of the model over the entire life of the blast furnace. For every calculation, the HMI screens show 3D graphs of the actual and maximum wear lines. This means that the contour can be efficiently compared with the original lining, a single isothermal area with configurable temperature (e.g. 1,150 deg C) can be displayed, and horizontal and vertical angle ranges can be selected.
Thermal index calculation model – The energy consumption and productivity of the blast furnace are reflected by the thermal index, which can subsequently be used to predict the development of the hot metal temperature and silicon content. The model result is used as an input to the blast furnace expert system for controlling the thermal state of the blast furnace process.
Shaft calculation model – The model performs a mass-balance calculation based on actual charging data using the materials of one charge, consisting of one coke and one burden layer. These individual charges are tracked from the furnace top down to the tuyere area. The results are displayed graphically and allow the operator to track burden composition and burden distribution changes. The model also computes the time when burden changes become effective on hot metal and slag.
Mass and energy balance plausibility model – This model automatically generates reports based on actual charging, process, and production data over a pre-defined time period, taking into account the material retention time in the blast furnace. The mass and energy balance calculation is used to detect the build up of alkaline and zinc circuits or to identify systematic measurement inaccuracies. The subsequent balance plausibility algorithm indicates the most probable sources of measurement faults.
Automation of cast house – Stable and reproducible tapping operations are necessary for both the hot metal quality and the establishment of a smooth, efficient blast furnace process. Further, a clean and safe working environment in the cast house is achievable with good layout, accessibility and ergonomics. The cast house machines are to work together to provide this environment. Cast house automation provides for consistent, safe, operation of the cast house. It includes (i) control of mud gun and tap hole drill equipment, (ii) trough and runner temperature monitoring, (iii) tilting runner operation and monitoring of the level of hot metal in the ladle, (iv) process parameters of slag granulation, (v) monitoring of the cast house fume collection systems, and (vi) control schemes include radio operated belly box designs to fully automated gun-up and automated drilling logic.
The tapping management model calculates the actual hot metal and slag production rates as well as the drainage rates through the open tap holes. This allows it to continuously compute the actual amount and level of hot metal and slag in the hearth. The model result is used as an input to the blast furnace expert system, which makes a recommendation on opening a tap hole.
Automation of furnace cooling system – From shell plate spray cooling to closed loop stave cooling, the various options for furnace cooling and temperature monitoring and control are managed through a well instrumented and integrated control system. Heat flux monitoring and water treatment systems are integral to proper furnace cooling operations and longevity.
Leakage detection system for critical cooling circuits – Leakage detection system is essential from the point of view of safety. There is risk involved if there is no quick detection of the leakages. Expanding cracks or small water leaks in cooling system pipes can affect the quality of hot metal, lead to stoppage of production, damage to furnace, or in the worst case, loss of life.
A system which effectively detects leakage is hence not just one of the most important parts of the blast furnace process automation and control system, but more importantly, an integral part of the at the blast furnace. For this reason, the leakage detection is designed so as to meet the requirements of the safety standard for the instrumented systems for process industries sector.
Automation of stoves and hot blast delivery – Stove automation is for providing automatic cycling of stove valves to supply uninterrupted and consistent hot blast to the blast furnace. Stove cycling systems is designed for 2, 3, and 4 stove operation. Stove automation includes control of the process gasses and firing strategies, including Level 1 control optimization, and burner management capabilities. In addition, preheated air and gas supplies are used to reduce the amount of enrichment gas used in the stove heating cycle. Cold blast monitoring and control and hot blast temperature control, fuel injection control is included within the hot blast delivery system.
Present day blast furnaces are typically operated very close to the maximum hot blast temperature which the stoves can sustain. If the dome temperature does not increase rapidly enough, sophisticated controls are provided to enrich the blast furnace gas with a fuel of higher calorific value to achieve a faster heating rate. The use of the optimized combustion control consists of a number of features such as (i) control of excess air, (ii) consideration of flue gas oxygen or chemical combustibles analysis, or both, (iii) dome temperature influences on the gas enrichment ratio, and (iv) sequencing for either three or four stoves.
Optimization of hot stove is needed for achieving high efficiency, flexibility, and energy saving. The blast heating process offers considerable energy saving potentials. The challenge for the plant operators and the line managers is to optimize the energy input to the hot blast stoves while keeping the blast temperature at given targets of the blast flow rate and the blast time. The stove model ensures stoves optimization, and energy savings by increasing stove efficiency.
The hot stoves control model combines short term direct control and longer self-tuning algorithms. Rapid control is used to correct the firing rate for maintaining the proper stoves operation parameters. The fast controls reduce carbon di-oxide (CO2) emissions and maximize stove efficiency. Artificial intelligence algorithms are used to optimize the efficiency performance of the hot blast stoves. These self-learning algorithms enable operators to identify and correct measurement errors.
The hot blast stoves control model supports all operation modes in combination with various rich gas types. All the types of stoves (such as Cowper or Kalugin) as well as pre-heating and heat-recovery systems are normally covered in the model.
Automation of gas cleaning plant – Blast furnace gas is cleaned using a variety of methods including a cyclone or dust catcher to remove large particles and either annular gap water sprays or electrostatic precipitators. Semi-cleaned gas has also been used to drive a TRT (top gas recovery turbine) system for energy recovery. In either case, gas cleaning systems provide furnace top pressure control, as well as cleaned gas to be re-used for various processes including stove heating, and for use in the boiler house of the power plant.
Closed loop blast furnace expert system
The closed-loop blast furnace expert system is normally designed according to the principle ‘as few actions as possible, as many as essential’. The objective is to optimize blast furnace operation and reduce operator interactions to a minimum. The expert system, which is normally designed as a rule-based decision system, counteracts process fluctuations caused by changes in burden material composition and quality, human factors, and process conditions. The sooner the system responds to an abnormal or changing process situation, the smoother is the overall blast furnace operation. Timing control activities accurately and anticipating disturbances are both of the utmost importance in order to avoid critical process conditions and to maintain a high production rate at low costs. The closed-loop blast furnace expert system ensures considerable improvements to product quality and reduced fuel consumption with the available burden materials.
The expert system recommends operational changes in a two-step process. The first step is an analysis of the current situation, called process diagnosis. The expert system studies the occurrence of phenomena in the blast furnace using a variety of technical calculations based on a huge amount of process measurements and analysis data which are collected continuously. In the second step, corrective actions are proposed if needed. An extensive rule set forged by experienced blast furnace process experts and operators on the basis of cause-and-cure relationships results in recommendations for the best-practice and shift independent operation. Corrective actions to achieve and maintain the smooth operation of the blast furnace are reported to the operators. The actions can be executed either in closed-loop mode or after operator confirmation.
There are a set of major corrective actions which results in a continuous, shift-independent blast furnace operation. The guidance of the expert system, especially during start-up and shut-down periods, leads to energy savings and minimized production losses. This uniform operation contributes to prolonging the lifetime of the blast furnace. The set of major corrective actions are described below.
Fuel rate and injection control – The expert system observes the thermal state of the blast furnace hearth and suggests a change of the fuel rate as soon as significant deviations from optimal conditions are recognized. According to the rules defined in the knowledge base, either a change of injected fuel or coke rate is suggested and can be executed fully automatically.
Slag basicity control – On the basis of recent slag analyses and hot metal temperature data, the expert system recommends changes in the burden composition as soon as a deviation from the target slag basicity is detected. Working with the burden control model, a new charging matrix is calculated automatically which can be transferred to the process control system for execution.
Control of oxygen enrichment and steam addition – The oxygen enrichment control calculates an optimized oxygen addition rate to achieve the target hot metal production. Critical situations caused by rapidly increasing production rates can be avoided. The expert system uses steam addition to maintain the burden permeability at the targeted level. Because of the fast control cycles, it is possible to precisely adjust the addition of steam to the amount needed by the process. In this way the steam input is reduced whenever possible directly leading to energy-savings.