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Pusher reheat furnace at Metinvest Trametal

Metinvest Trametal’s new slab reheating furnace with Tenova’s FlexyTech technology provides low NOx emissions, reduced fuel consumption and uniform heating of slabs thanks to its flameless burners and a level 2 control model which calculates the heat distribution in the furnace zones and charged slab.

As emission controls become increasingly stringent following the signing of the Kyoto protocol in 2002,

and as the cost of fuel continues to rise, the furnace industry has devoted most of its research into the development of new burner firing techniques to obtain better thermal performance and reduced emissions.

FlexyTech furnaces are engineered to achieve top performances in all conditions while providing a special focus of inflicting only a low impact on the environment. The principal fields of innovation for these furnaces are new flameless burners and a level 2 computer model for offline simulation of different design solutions and for best online furnace regulation.

FLEXYTECH® FURNACE CONCEPTFlexyTech burners Thermal NOx formation is controlled by flame temperature, oxygen content in the reaction zone and the residence time of the combustion products in the high-temperature zone of the flame. The techniques previously used to reduce NOx emissions such as staged combustion (air and fuel), flue gas recirculation (FGR) and partially premixed combustion, are not capable, in many cases, of limiting the emission levels to the increasingly restricted limits set by various international regulations.

Only an understanding of the basics of ‘flameless’ firing open up the possibility of reaching these new goals and significantly reduce emissions while still ensuring even distribution of temperature inside the furnace chamber. For this reason the flameless approach has been selected by Tenova as the main path for the improvement of FlexyTech furnaces.

Tenova’s flameless TSX burners (see Figure 1) have been engineered for low NOx emissions, high fuel efficiency and very good temperature uniformity of the reheated steel.

Their main characteristics are: ` NOx emissions below 40ppm @ 3% O2 in dry flue

gases

Authors: M Carbonaro, M Fantuzzi, S Gnemmi and C MoriTenova LOI-Italimpianti

Forming Processes

` Ultra low CO emissions (below 5ppm)` No valves on hot air for air staging` Lowest excess air operations for maximum fuel

efficiency` NOx emissions not affected by air temperature or

turn-down` Air preheating up to 550°C` Combustion air butterfly valve (to allow on–off firing)` Refractory baffle` Gas lance` Gas manual shut-off valve

Thousands of such burners are now installed on many types of reheating and heat treatment furnaces all over the world.

FlexyTech level 2 control The slab discharge temperature must not only be in the correct range but the temperature differentials within the body of the reheated steel slab must also be minimised, but it is difficult to manage a

r Fig 1 TSX burners at Metinvest Trametal

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precisely the reheating process since there is no way of obtaining this information through direct measurement.

To solve this problem Tenova provides a reheat furnace level 2 process control system which includes a precise mathematical model of the heat transfer within the reheat furnace. This model calculates the temperature of the steel at each node of a two-dimensional matrix so that bulk temperature as well as temperature distribution can be evaluated. The accuracy of this model has been tested and confirmed in many applications. Since its first use in a production environment in 1980 the model has been substantially enhanced and deployed to several computing environments. The modern version of this online thermal model is the heart of the level 2 reheat furnace process control system and has been designed to achieve the following goals:` Reheat the charge according to the ‘optimum cycle’

predetermined for each type of material; this is accomplished for the complete range of furnace production rates including transitions and delays

` Improve the accuracy of control of the heat supply to the regulation zones using the knowledge of the actual temperatures of the steel

As a consequence the level 2 control, compared with traditional set-point temperature control set by the furnace operator, has reduced fuel consumption, reduced scale production giving a direct effect on yield and overall mill productivity improvement and precise control of the target discharge temperature.

It is a computer-based system that can be built on several hardware and software platforms such as Windows Server, OpenVMS and HP-UX. As shown in Figure 2 all the modules of the system store and retrieve data to and from an internal data area, where data is exchanged with remote computers and terminals through an Oracle database and OPC server.

The modules are:` Communication layer, from and to the other devices in

the plant

Burners Zone Location Type Number Heat input [MW] Zone heat input [MW] 1 Top preheating Side 8 1.40 11.162 Bottom preheating Side 8 1.86 14.883 Top heating Side 8 1.40 11.164 Bottom heating Side 8 1.86 14.885 Top soaking Side 8 0.96 7.676 Bottom soaking Side 8 0.96 7.67 Top zones total heat capacity 30.00 Bottom zones total heat capacity 37.44 Total furnace installed power 67.44

r Fig 2 Level 2 architecture

r Table 1 Distribution of heat input

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a

` Mathematical model to compute the thermal profile of the slabs

` Predictive control logic to compute the set points of the furnace

` User interface with real-time and historical data stored by the system

` Technological editor` Remote diagnostic and assistance

METINVEST TRAMETAL FURNACEThe pusher furnace supplied to Metinvest Trametal is shown in Figures 3 and 4. It is fuelled with natural gas supplying top and bottom burners and has a nominal throughput of 100t/h of slab. Its dimensions are 29.5m long, 8.1m wide and 1.3–2.0m high (tunnel to centre and soak zone), and is fitted with six fixed beams.

It is charged from the front and discharged by a discharging machine, the method being designed to maintain the right tolerances to avoid bending of the slabs.

The combustion system is a recuperative type to provide preheated combustion. The furnace is divided into four physical zones (starting from the charging side):` Recuperative (zone without burners)` Pre-heating` Heating` Soaking

It also has six temperature zones. Table 1 shows how the thermal power is distributed in the six zones. Table 2 reports some data relevant to the processed slabs.

MixSlab thickness 150–400mmSlab length 1,350–5,150mm in double or triple rowsSlab width 850–2,550mmSteel grade Carbon steel Ship building High strength low alloy API-5L-X-80Charging temperature 20°CDischarging temperature 1,340°C (max)

r Fig 3 FlexyTech® Furnace at Metinvest Trametal

r Table 2 Product data

r Fig 4 Furnace cross-section

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` Protection against high recuperator temperatures: inlet flue gas temperature is limited by dilution by air injection

` Protection against high combustion air temperatures: outlet combustion air temperature is limited by a bleeder valve installed on the hot air manifold

` Automatic reduction in sequence of preheating zone should the temperature of the furnace outlet waste gases be too high

AUTOMATION CONTROL SYSTEMThe automation system for furnace control includes basic automation (level 1) and the furnace computer (level 2). Two separate programmable logic controller (PLC) systems are provided for process control at level 1. One system controls all the mechanical functions (handling system), and a second is dedicated to combustion control. The PLC type used is a Siemens type S7 model 400 series, with remote I/O type ET200. The PLC I/O is connected to field sensors (via remote I/O linked by Profibus DP) and the processor is connected through a dedicated local bus (Ethernet connection) to the operators’ stations based on two PCs and can run independently and simultaneously. This will guarantee that, in the unfortunate case of failure of one computer, the total control of the furnace will be available from the second unit. Both PCs constantly monitor the status of the other PC and of the network. The level 1 units have a UPS power supply, thus assuring constant monitoring of the furnace plant even during a power failure.

Combustion control system The control of furnace combustion is by means of a supervisor program running in the PCs, which constitute the Human Machine Interface (HMI) in the control room and can run in automatic or manual operation modes:

` Automatic mode The set point of the respective controller is determined and fixed by the operator via the HMI. The controller derives its output from the PID algorithm.

` Manual mode The operator, through the HMI, sets the output of the selected controller. The remaining controllers maintain their status unless control or safety logic is violated. At all times control is allowed to pass from automatic to manual mode.

In each mode interlocks and internal conditions are respected at all times. Furthermore, the HMI display of each zone’s video page shows the operator the mode currently functioning. Seamless switching from manual to automatic mode is also foreseen. The manual set output of the PID algorithm must be always be tracked to match the automatic output when in automatic mode.

Emissions The waste gases leave the furnace through an uptake duct and are then conveyed to a recuperator and finally to a forced draught stack. The O2 content is measured with sampling points at the waste gas duct by a zirconium-oxide cell. The average measured concentration of NOx emissions is lower than 150mg/Nm3 at 5% O2 content. Figure 5 plots NOx emissions measured during the performance tests.

Combustion system This is designed to provide sufficient heat input to maintain the design production levels with a skid insulation loss of 10% (bare pipe). Each of the temperature control zone’s fuel and combustion air headers is equipped with a metering device coupled to a transmitter and with a flow control valve sized for design flow conditions. Combustion airflow and gas metering are by orifice plate. Flow control valves are butterfly type.

The primary variables in the combustion air-fuel flow control systems are the zone temperatures. The secondary variable is the fuel:air ratio. The flow control system is double cross limited such that the combustion airflow and fuel flow are within a prefixed gap controlled in any operating condition. Also, the control system automatically limits fuel supply to the available air at the selected ratio.

Recuperator A convectional metallic recuperator preheats the combustion air to 550°C. It consists of an encased metallic pipe-bundle heat exchanger, arranged in such a way so as to allow thermal expansion during operation. The main elements in contact with the waste gases are made of heat and corrosion-resistant steel. Waste gases coming from the ducts connecting the furnace to the recuperator flow directly through the recuperator into ducts connected to the stack. The recuperator is protected against overheat as follows:

r Fig 5 NOx emissions at Metinvest Trametal

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also supervises the hydraulic station of the furnace. Video pages are provided on the HMI units showing the status of the pumps, valves and sensors. The discharging machine (extractor) is controlled in the same way as the charging machine. Discharge of the slabs from the furnace takes place in accordance with the planned rolling programme. The discharging machine removes the slab from the furnace and places it on the roller table where it travels to the mill. Level 1 automation performs the tracking of the charge from the charging roller table until discharging from the furnace to the discharging roller way.

LEVEL 2The model described earlier is oriented so it can determine the cooling at skid marks caused by contact of the slab with the fixed and moveable beams. The furnace charge heat exchange, representing the boundary conditions of the Fourier equation, is evaluated for each surface of the slab as a function of furnace zone temperatures, zone fuel and air flow rates and calculated charge temperature. Figure 6 shows the operator’s HMI displaying the thermal map of the slabs inside the furnace.

CONCLUSIONSThe 100t/h Metinvest Trametal pusher furnace has operated for over a year with satisfactory results, very good temperature uniformity and NOx emissions much better than the guaranteed values and in accordance with environmental standards. MS

M Carbonaro, M Fantuzzi, S Gnemmi and C Mori are with Tenova LOI-Italimpianti, Genova, Italy.

CONTACT: info@loi-italimpianti.it

The temperature controller of each zone operates in cascade on the set points of the air fuel flow controllers. The output provides set values in parallel for the fuel and air flow controllers which are controlled by the Double Cross Limit (DCL) method. The high–low selection obtained from the DCL values and the feedback values from the air and fuel flow provides an upper and lower limit to air and fuel variation. According to the safety sequence followed by the master controller, on demand (when there is a load request from the temperature master controller) the air supply leads the rise, on release (when there is a request of decrement by the temperature master controller) the fuel supply will lead the decrease. This safety sequence will thus produce during any transient situation in the furnace excess of air to avoid the presence of unburnt gas.

Material handling control system The various interlocking and sequencing functions required for the furnace equipment will be carried out automatically by means of one PLC which performs the following functions:` Slab centring in front of the furnace` Charging machine movement` Charging door movement` Discharging door movement` Discharging machine control (extractor)` Hydraulic system control` Tracking of the charge inside the furnace

The PLC performs the automatic, semi-automatic and manual running of all movements from the furnace to the roller table. These are provided with a safety system of interlocks to avoid damage during handling operations. The furnace is completed by a pre-furnace area, which consists of the approaching roller table (in front of the furnace). For each slab delivered to the charging roller way (in front to the furnace), the following data will be sent to the furnace PLC: length, width, identification code.

The slab that has to be charged is positioned in front to the furnace, according to the relevant charging scheme.

The furnace is characterised by a high level of flexibility, normally uncommon for pusher furnaces. In fact it is possible to charge slabs in three alternative modes: three independent rows, two adjacent coupled rows and one independent row, or three coupled rows for long slabs.

To prevent critical lateral shifting of the charge due to shape irregularities (eg, tapering and carving) during travelling onto the beams, a laser system scans each slab’s profile and, by means of a mathematical model, warns the furnace operator, suggesting relevant corrective actions.

The charging machine is driven by an electrical motor to the furnace and lifting to sill height is by hydraulic cylinders. At the discharging side, the position of a slab is detected by a photocell with a backup for emergency. The PLC

r Fig 6 Level 2 HMI slab thermal map