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Energy optimised conveying solutions for DRI

2010-06-22   作者：佚名   网友评论 0 条

Although the global steel industry has not escaped the economic downturn the industry remains fundamentally strong and leading international institutes forecast a steady growth of worldwide steel production at a higher pace than in recent decades.
 
The demand for direct reduced iron (DRI) has increased steadily, year on year, with some blast furnaces now using pellets to replace sinter, more DR alternatives to the BF route for iron production being built, as well as increased use in the EAF.
 
There has been a steady increase in the percentage of steel produced in electric arc furnaces, however, with the decreasing quality of scrap it becomes more difficult to keep impurity contents low. High quality scrap is only available at prices that have increased to record heights in recent years so the industry now looks for alternatives like DRI, hot-briquetted iron (HBI) or hot compacted iron (HCI) to upgrade the charging mix in order to obtain steel qualities with a predefined composition.
 
DRI demand and production have grown almost continuously since 1970. The vast majority is used in EAFs and the rise in demand has been due to the strong growth in EAF production, the move by mini-mills into low residual products and the limited supply of high quality scrap. Most new DR installations and all new Midrex plants feature hot charging options or are planned for future installation of hot charging into the subsequent melting facilities. The target is to allow the melt shops to operate with hot material charging into the EAF. Hot charging provides major benefits in energy consumption and productivity and today most melt shops specify hot charging systems for their plants. The latest projects provide for even more flexibility by installing feeding systems which are able to choose between cold or hot material. This allows to manage market changes and to meet the future needs of the steel industry.
 
With the success of the direct reduction technologies, AUMUND began to develop conveyors for hot DRI / HBI some 12 years ago. This includes systems for transport of hot DRI from the direct reduction stage to the further processing stage, the transport of HBI before and after the briquetting process, and the continuous feeding of material into the EAF with hot DRI. 
 
THE CHALLENGES OF HOT DRI TRANSPORT
 
The major benefits of charging hot DRI into the EAF are reduced power consumption and shorter melting cycle times. The greatest temperature loss of hot materials transported occurs at the transfer points, i.e. during charging onto the conveying equipment and discharging into the melting vessel, but not during the transport itself. With the use of conventional large buckets or pots there are various sources of temperature loss: the buckets or pots are handled without being covered and heat radiation via the surface is high. Much higher losses occur during opening and swinging of the furnace roof during conventional charging. In addition, the time required for opening and closing is lost for metallurgical operation or energy entry.
 
The challenge for hot DRI transport is not just that the material is hot, but also that it must be kept in a non-oxidising atmosphere.
 
Depending on the plant layout, the conveying distance and the conveying capacity mechanical conveying of hot material represents the most adequate solution for modern applications.
 
Pneumatic transport systems are well suited for small capacities, but require much more power and maintenance, feature rather complicated start and stop procedures and a higher investment cost. Mechanical conveying combines the advantages of a closed system with the benefits of hot charging into the furnace vessel. The inert gas system integrated into the conveyor prevents re-oxidation of the material.
 
During operation the conveyor adapts to the operation sequences. A push-button is all that is needed to start and stop conveying and no preparation, warm-up time or slowdown of fluidised materials in special bins is required.
 
HOT TRANSPORT CONVEYOR DESIGN FEATURES
 
The conveyor capacity is linked to the geometry of the conveying system: the greater the lift, the smaller the conveying capacity. With limited space the conveyor is usually as steep as possible, but steep slopes do have an adverse impact on the ratio of dead load to live load. Along horizontal sections, a capacity of 1,200 tph represents no problem, but when vertical lift is required, the capacity drops proportionally. Currently systems are designed with an inclination up to 60 °.
 
Conveying capacities of 210 tph up to an elevation of nearly 100 m and 400 tph up to 80 m have already been realised. The design always targets maximum filling of the conveyor buckets to optimise the ratio of volume to surface. The lift limit is dictated by the chain strength since the whole installation is essentially attached to the chains. It is not feasible to simply enlarge the chain links, because such a measure would increase the dead weight. For physical reasons only one (single) or two (double) chains can be used and the capacity of the installation cannot be raised by using three or more chains. Hence there is a physical limit to the system capacity and lift, but in comparison with other material transport the potential is several times higher.
 
Over the past few years Aumund has developed chains which offer a specific breaking strength of each single chain of 3,000 kN minimum, the strongest in the world. Special know-how, particularly with material temperatures of up to 1,100°C, is required to design such a chain and many years of research and development have been put into this feature. A multitude of issues have to be considered, including materials employed, lubricants, drives, mechanical components and safety devices.
 
A further important feature, the air seal provided by the inert gas shrouding system, has been constantly improved such that dust can be contained inside the system and that oxygen is kept outside. A completely closed conveying system was developed, using special covering and sealing combined with the inert gas system.
 
No dust is emitted from the conveyor and no spillage is generated underneath. Dust can be collected at defined areas with a common exhaust air system. The precise amount of inert gas, such as nitrogen or off-gases, non containing oxygen or carbon monoxide, can only be determined during operation and for safety reasons it is recommended to use an excess of gas on start-up. Special sensors monitor the safe operation and monitor the oxygen content inside the system. The off-gas temperature, within certain limits, reflects the material temperature. For operation no special tools, no specialised staff and no special equipment is required.
 
With increased charging of DRI to electric arc furnaces to substitute for use of expensive low residual scrap, so the benefits of charging this material hot rather than cold have been highlighted. Most DR installations now feature hot charging options or are planned for the future so that EAF melt shops can operate with hot material charging. Efficient transport of hot material with minimum temperature and dust losses and no oxidation is key to this technology.
 
The major benefits of hot charging of DRI to EAF on a pan conveyor are
 
u Lower energy consumption for melting
u Increased productivity due to shorter melting cycle times
u Minimum degradation of DRI
u No reoxidation of the DRI because of inertisation
u Low maintenance equipment
 
AUMUND has installed about 1,500 conveying systems for various applications worldwide.
 
In addition to DRI and HBI, these systems are also used to transport hot materials such as sinter, coking coal, aluminium clinkerat temperatures to 1,100°C.
关键词：DRI

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