Blast Furnace Top Charging Systems
Blast Furnace Top Charging Systems
In earlier days, blast furnace (BF) top used to be open with the gas from the furnace escaping into the atmosphere and burning, causing environmental issues while wasting considerable energy. By 1850 as the furnace size increased, the furnace top could be closed. A single bell and hopper arrangement could be used for charging the furnace that kept the top of the furnace closed and sealed. The single bell and hopper system permitted large quantity of gas to escape every time the bell was opened. Soon a second bell and hopper was added above the first so that a gas tight space could be provided between the two bells to prevent the blast furnace gas escaping when the small bell was opened. The upper bell and hopper did not have to be as large as the lower one because several charges could be deposited through it on the lower bell and the upper bell could be closed before the lower bell was opened for dumping the charges in the furnace.
The two bell system continued to be the only charging system for the blast furnaces around the world till S.A. Paul Wurth in Luxembourg, developed bell less top (BLT) charging system and the first successful industrial application of BLT charging system was in 1972. Soon BLT charging system took over from two bell charging system since it provided a number of advantages to BF operators. During 2003, Siemens VAI introduced Gimbal concept of charging. This charging system has been successfully used for Corex and Finex processes for charging. The first application of Gimbal for charging a blast furnace was in 2009, when it has been used for C blast furnace of Tata steel.
Two bell charging system
The two bell charging system consists of a revolving material distribution, a small bell and a large bell as shown in Fig 1. The diameter of large bell is usually smaller than the stockline diameter. The lower edge of the upper face of the bell forms a seal against the bottom edge of the large bell hopper. The bells are connected by a rod and move in the vertical direction by means of air cylinders.
Fig 1 Two bell charging system
The furnace charging is done in four steps (Fig 2).
- Step 1 – The charge material is taken to the furnace top either by a skip car and hoist or by a conveyor belt and is delivered to a receiving hopper. Small bell and large bells both are in closed condition. The charge materials from skip or conveyor are dumped in hopper above the small bell. Gas flowing from top of furnace through uptakes located in the dome (top cone).
- Step 2 – With the large bell closed, the small bell is lowered and the charge material is dropped on the large bell. This is repeated several times.
- Step 3 – The small bell is closed to prevent escape of gas to atmosphere. The large bell is lowered and the charge material is discharged into the blast furnace.
- Step 4 – Both the bells are closed and the system is ready for repeat charging.
Fig 2 Four steps of furnace charging
With each charge of the material from skip or conveyor, the small bell and hopper rotates to a selected position before the material is discharged. This provides an improved distribution of materials on the large bell. The bells, seating surface of the bells and hopper are hard surfaced. The rod supporting the large bell passes through the hollow rod supporting the small bell, thus permitting independent operation of the bells. In this system of charging, the small bell, large bell and hopper are subjected to heavy impact and require replacement 2-3 times during a campaign of the BF lining. In this charging system, it is extremely difficult to maintain a gas tight seal for a top pressure higher than 1 Kg/Sq cm. Further two bell charging system has limitations towards burden distribution in the blast furnace. Burden distribution plays a big part in achievement of high productivity in the blast furnace.
To overcome the limitations of two bell system, numbers of modified systems were tried. These are two bells top with seal valves and revolving chute and bell type with adjustable throat armor etc.
Bell less top charging system
The development of Bell less top charging system by S.A. Paul Wurth was a big quantum jump in technology and hence this system rapidly gained in popularity. The BLT charging system has the following advantages.
- It allows nearly continuous charging of the BF. While the rotating chute is distributing the contents of one lock hopper bin, the other can be filled.
- It solves the problem of gas sealing under a high pressure operation
- It provides flexibility in the distribution of BF burden. It can carry out one ring charging, multi ring charging, spiral charging, sector charging and point charging both in manual and automatic mode. Charge regulating valve provides accurate and constant distribution of burden materials.
- It provides improved BF operational stability and efficiency leading to better hot metal chemistry control.
- It contributes to increase in the BF productivity.
- It reduces BF coke rate and helps in achieving higher injection rates of pulverized coal.
- It contributes to higher campaign life due to reduced BF wall heat loads.
- It greatly reduces the maintenance time and frequency of maintenance of top equipment. The chute can be replaced within a short period of time.
- The top equipment is of light and compact construction compared to other high pressure top charging system.
However the height of the BLT top equipment is more than the two bell type charging system. BLT charging system can be integrated with skip hoist or conveyor belt charging system. BLT charging system has the following main component parts.
- A movable receiving hopper.
- One or two material lock hoppers equipped with upper and lower seal valves and a material flow control gate.
- A central vertical feeding spout
- A rotating adjustable angle distribution chute
- A rotational and tilting drive mechanism
- Hydraulic, lubrication and cooling systems
- Monitoring and control systems
During the operation of the blast furnaces equipped with BLT charging equipment, the skip or conveyor brings the charge material to the receiving hopper. The material is then filled in the lock hopper which is then sealed and pressurized to the furnace top operating pressure. The lock hoppers are used alternately, that is one is being filled while other is being emptied. By design, the seal valves are always out of the path of material flow to prevent material abrasion. This reduces the probability of sealing problem. The flow control gate open to predetermined positions for the various types of charge materials to control the rate of discharge. Lock hoppers are lined with replaceable wear plates. The lower seal valves and material flow gates are in a common gas tight housing with the material flow chute, which directs the material through a central discharge spout located in the main gear housing. The scheme of charging through a BLT charging system is at Fig. 3.
Fig 3 Scheme of charging through a BLT charging system
Gimbal system of Charging
The Gimbal system, developed by Siemens VAI in 2003, is an elegant, simple and rugged charging system designed for high temperature and pressure operation. The system facilitates controlled distribution of charge material into the blast furnace through a Gimbal type oscillating chute (Fig 4) through a holding hopper and variable material gate opening such that the pressurized charging system above can operate independently of the distribution system. Gimbal utilizes a conical distribution chute, supported by rings in a Gimbal arrangement producing independent and combined tilting of the chute axis.
The tilting chute is driven by two hydraulic cylinders, mounted 90 deg apart. This type of suspension and drive arrangement results not in a rotation of the tilting chute, but in a circular path by superposition of both tilting motions. Independent or combined operation of the cylinders allows the chute axis to be directed to any angle, or even along any path. Motion is supplied by two hydraulic cylinders, each operating through a shaft, connecting rod and universal joint in order to drive the Gimbal rings.
Through the movement of the hydraulic cylinders, the distribution chute allows precise material distribution with potential for an infinite number of charging patterns at varying speeds. These include rings, spiral, spot, segment or sector charging, providing complete control of material charging into the BF. The whole distributor assembly is enclosed in a gas tight housing, which is mounted directly on the top flange of the BF top cone. The housing contains a fixed inlet chute and a tilting distribution chute supported by rings in a Gimbal arrangement allowing independent and combined tilting of the chute axis. Gimbal material distributor is shown in Fig.4.
Fig 4 Gimbal material distributor and charging system
The key features of Gimbal design are as follows:
- It has simple rugged design using levers driven by hydraulic cylinders.
- Drive cylinders are mounted outside pressure envelope, hence not subject to hot and dusty service conditions.
- Gimbal ring arrangement gives simple tilting motion in two planes, which when superimposed gives 360 deg distribution.
- Wear on the tilting chute is equalized around its circumference giving a long extended operational life.