An innovative solution to maximize scrap and HBI rates in converter steelmaking
Fluctuating prices of scrap and hot-briquetted direct-reduced iron (HBI) in recent years — along with growing pressure from public authorities to further reduce the CO footprint — has heightened the interest of integrated steel producers to increase the share of scrap and HBI that can be used in converter steel making. The new Jet Process, a highly efficient technological development from Siemens VAI, now allows the portion of solid materials charged into the converter to be dramatically increased.
Hot metal, scrap and HBI are the three primary iron carriers that are used in steelmaking processes. The inherent energy of the hot metal and the heat that is released during the oxygen-blowing step make it possible to charge up to approximately 20% solid materials in steelmaking converters. If post-combustion techniques are applied, this rate can be increased to around 30%. For a higher solid- material charging rate, additional energy for melting and heating is required. This extra energy can be provided by electrical power or by taking advantage of the chemical energy of coal. The generation and use of electricity, however, normally incurs considerable energy conversion losses and relatively high costs. The Jet Process was therefore developed to utilize the chemical energy of coal in a highly efficient manner to enable higher portions of solid materials to be directly used in converter steelmaking.
Process equipment and description
The centerpiece of the Jet Process is a converter that is equipped with bottom blowing tuyeres for the injection of oxygen, coal and lime into the converter. It is complemented with a hot-blast lance system that is positioned above the converter mouth for post-combustion purposes. Figure 1 depicts the main equipment items of the Jet Process and Figure 2 shows the arrangement of tuyeres and piping of a typical bottom blowing converter.
The coal injected into the hot metal in addition to the coal at ready in solution is combusted in two steps: combustion of C to CO in the bath, and post-combustion of CO to COC above the bath by a hot blast. Two thirds of the chemical energy stored in the coal is released in the second step of this combustion process. Hence, it is essential to ensure good post-combustion and an efficient transfer of the generated heat to the hot-metal bath. This is made possible by the hot blast, which is heated to approximately 1,300°C in a pebble heater, enriched with oxygen up to 30°+, and blown at near-sonic speed onto the bath surface in the converter. As a result of the velocity, volume and momentum of the jet blast, a significant portion of the surrounding gases with their huge heat content are conveyed to the bath surface. This leads to a high input of heat into the liquid metal, intensive bath mixing and extremely efficient process reactions. An excellent utilization rate of the chemical energy contained in the coal is thus achieved, typically in excess of 50°/. This figure is in contrast to the in much lower efficiency rate of electric steelmaking due to the large energy losses that occur during electrical power generation.
Oxygen for decarburization of the hot metal is blown via tuyeres through the converter bottom into the bath. These tuyeres act like flame cutters, and large pieces of scrap can be melted quickly and efficiently. Similar to the hot blast of the Jet Process, bottom blowing intensively mixes the bath as welI; hence, aII reactions are accelerated and rapidly reach the equilibrium state. Furthermore, bottom blowing reduces the percentage of iron and iron oxide that is lost with the slag, and the tendency for slopping is decreased compared to standard top-blowing converters. Lime powder blown through tuyeres into the converter accelerates desulfurization and slag formation, and a smaller slag volume is generated. These factors result in increased productivity and process yield.
Maximum process flexibility
The Jet Process is easily adapted to different scrap or HBI rates by adjusting the amount of coal injected into the hot metal. Theoretically, solid-charge rates from 0 to 100% are possible with this flexible process. With up to 30% scrap charges, no coal injection is required because the heat contained in the hot blast combined with CO post-combustion heat provides enough energy. For scrap or HBI rates close to 100%, step-wise or continuous charging in combination with hot heel operation is necessary. To further increase operational flexibility, a modular converter design was developed that a allows a conventional LD (BOF) converter bottom and an oxygen blowing lance to be quickly installed so that the converter can be operated as a typical LD converter, should this be more feasible.
Areas of application
There are a number of economically attractive market opportunities for the Jet Process. For example, if prices for scrap or HBI are lower than the costs for hot metal, application of this process with a converter charge of 50% scrap/HBI and 50% hot metal could be a highly feasible option.
Other scenarios where the Jet Process offers unique advantages are during a hot metal bottleneck resulting from a blast furnace blow down, a furnace standstill for revamping purposes, or to increase the total steelmaking output. In such cases, the Jet Process can be used to maintain or increase the overall converter output with higher scrap/HBI charging rates. The only investment necessary is to adapt existing converters to the Jet Process, which is far less expensive than installing added hot-metaI capacity and the associated coking and Wintering plant expansions.
Finally, the Jet Process can contribute to a notable reduction in CO emissions. In a typical integrated plant, huge amounts of CO are generated during hot-metal production. Replacing a portion of the hot metal with scrap considerably reduces CO emissions per ton of steel tapped from the converter, even though carbon is used in the Jet Process as the energy source. Plant operators can thus reduce CO emissions or increase production while keeping COC emissions constant.
Efficient and economic
The Jet Process is ideally suited for medium to high scrap/HBI rates in converters of any size. It represents an attractive upgrading option for producers to flexibly respond to varying prices for hot metal, scrap and HBI. Converters equipped with this solution thus close the gap between conventional LD (BOF) plants and electric arc furnaces. As demonstrated at the steelworks of one of the world‘s largest producers, the Jet Process is very reliable and economically promising.
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