Iron smelting below the smelting point
Steel without coke - is it possible? Current developments in iron smelting and steel production are heading in exactly this direction. Instead of reducing the iron oxide in the ore with coke to elementary iron, steelworkers are more and more often going another way. It was only in autumn 2016 that the world’s biggest direct reduction plant was commissioned in Texas. Instead of coke, reduction gas is used, mostly generated from natural gas. In many plants all over the world, positive displacement blowers made by AERZEN are taking over the gas supply of the reduction towers.
Iron sponge is the substance generated from direct reduction - also called the Midrex-procedure. While the generation of raw iron is usually connected with fiery images, direct reduction plants go quite another way as far as process technology is concerned. First of all, this technology has been designed for smaller lots, which results in smaller dimensioned plants. These can be started up and shutdown more flexibly compared with traditional blast furnaces. There is another difference between direct reduction and “steel making”: the independence from coke - and thus from coal. The background: as reducing agent no fixed carbon as bulk material is used - instead, there is a gas mixture consisting of carbon monoxide and hydrogen. Accordingly, the material flow for the combustible material differs: Conveyor belts and screw conveyors in traditional steel mills; blowers and compressors in direct reduction plants. In this field of application, AERZEN has a market presence all over the world, mainly with its positive displacement blowers.
Before natural gas can be used to reduce the positively charged iron ions in the iron oxide to molecular iron, methane has to be treated in a so-called reformer. For the chemical steam reformation, natural gas with water and oxygen injection is modified into carbon monoxide and hydrogen in an endothermic reaction and using heat. Both gases are perfectly suitable for dissolving out the oxygen retained in the iron oxide. The reaction products are iron, water and carbon dioxide.
Iron sponge briquettes with high purity
Large multi-stage blowers made by AERZEN convey the initial gases with a volume flow of up to 300,000 cubic metres per hour into the reformers. From there, the separated gas is channelled in the counterflow into the shaft furnace also by means of positive displacement blowers. Reduction of iron ore to iron sponge begins. This is then cooled down immediately, to prevent the hot material, which has a temperature of approaching 1,000 degrees C, from oxidising immediately in the air. Furthermore, it is processed mechanically, in a hot state, into briquettes or pellets. The intermediate product, on the way to becoming high-grade steel, now has the quality of raw iron and can be processed accordingly in the steel works.
Different process, different requirements, depending on the location: As already mentioned, due to their smaller production volume compared with conventional steel works, Midrex plants are suitable for smaller locations. Moreover, their requirements for raw materials are different. In its heyday, the Ruhr area benefited from access to coal as an energy source and redox agent for its ironworks, but for the Midrex process the key element is access to cheap natural gas. Therefore, it is hardly surprising that the currently biggest direct reduction furnace was commissioned in 2016 in the U.S.A. The capacity of the plant in Texas is two million tons of iron sponge, processed into briquettes while hot (Hot Briquetted Iron).
Durable and reliable
In view of the importance of gas availability in such plants, demands for durability and operational reliability of the blowers are accordingly high. For conveying and compression of process gases, AERZEN developed positive displacement blowers series GR and GQ. In sizes 12 to 21, type GR covers intake volume flows from 100 to 50,000 cubic metres per hour. Type GQ, with sizes between 17 and 22, covers the volume range between 1,500 and 100,000 cubic metres per hour. Both performance classes convey the gas based on the positive displacement principle.
“Our machines are among the biggest available in the market,” explains Pierre Noack, Head of Process Gas Division at AERZEN. Their size, in connection with a high performance density, offers the advantage that fewer machines need to be installed to handle the required gas volume. Thus, Midrex-plants can be specified more easily and commissioned faster. Another advantage is availability. In engineering, and the evaluation of the most suitable technology, calculations of the mean time between failures (MTBF) play an increasingly important role - particularly for plants which work continuously. If bigger compressors are used, fewer of them are required, which means that less piping needs to be built, fewer connections need to be installed, and less sensor technology and actuator technology needs to be integrated into the overall control system. Against this background, higher acquisition costs may play a subordinate role - in the evaluation of the technical components, it is MTBF figures, average maintenance intervals (MTTR - mean time to repair) and lifecycle costs which play the key role. “Our packaged units still have a longer service life,” says Noack, and he mentions “very satisfied customers.”
Oil-free conveying process gas blowers have been designed as robust high performance machines
Water injection for maximum washing effect
Their long working life and operational reliability result primarily from the fact that the oil-free conveying process gas blowers have been designed as robust high performance machines. Types GR and GQ are able to withstand contaminations in gas very well, and are also highly resistant to iron dust. In addition, water can be injected into the packages. This detail has a significant impact, particularly in the conveying of aggressive process gases. With the specific injection of water, both gas and blower package can be cooled very efficiently. Another advantage is the washing effect of the water. During operation, this prevents positive displacement blowers from sustaining damage from the deposits of highly viscous residues of process gases. The two-lobe design of the machine allows an additional self-cleaning effect.
This may not be important when conveying pure natural gas into a reformer, but it is a significant aspect when primary energy sources are blended with other gases. Here, coke oven gas is first on the list. The by-product of the pyrolysis of coal and coke contains, depending on the quality of the coal, about 55% hydrogen, 25% methane, 10% nitrogen and 5% carbon monoxide. Except for nitrogen, the mixture is excellent for reducing iron ore. However, the coke oven gas also contains minor components, including tar, hydrogen sulphide, ammonia, benzene, as well as aromatic compounds, such as naphthalene. In general, in Midrex processes, it is worthwhile to use the volatile components of coal for direct reduction of iron ore, as its share of coal is 25 % of the initial weight. One tonne of coal produces 280 cubic metres of pure gas from coking plant and approximately 55 kg of sticky coal tar.
Gas treatment is not necessary
Positive displacement blowers made by AERZEN are always capable of conveying fail-safe the indicated contaminations. Thus, users of Midrex plants do not need separate gas treatment. This would be necessary if turbo compressors were used instead of positive displacement blowers. However, due to the high rotation speeds, this technology does not tolerate any foreign particles, no matter how small, in the gas flow. Thanks to the different operating principles of forced displacement, positive displacement blowers operate at considerable lower surface speeds. Just to recap: The working principle of positive displacement blowers allows maximum tip speeds which are seven to ten times lower than the tip speeds of turbo compressors. Thus, the injection of water leads to low erosion of the positive displacement blowers, whereas turbo compressors are not equipped with continuous water injection.
Easy modernisation with Positive Displacement Blowers
In order to apply major industrial blower solutions in other application fields of raw iron production, Pierre Noack explains that “our machines have a broadband design.” During operation, when the capabilities of the displacement machines are not being fully utilised, either calculated theoretically or facturing in optimum characteristic curves, they nevertheless work quite efficiently due to the speed control of frequency converters. In comparison with centrifugal compressors, which have a relatively small control range at constant back pressure, positive displacement blowers adapt perfectly to changing working conditions. Their speed variance creates flexibility. It makes a difference that AERZEN positive displacement blowers can operate in highly variable modes. Existing plants can be retrofitted easily, and modernised, thanks to these machines.