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H2-Change: Refractories under Attack of Challenging Atmospheres during Transformation Process

Common and advanced lining materials for gas driven DRI shaft furnaces are discussed. Based on reformed natural gas as a standard a closer look is taken to the transformation route via hydrogen boosted coke oven gas in an integrated steel plant up to the pure hydrogen use in a pilot plant, especially by using different gases such as hydrogen, methane, carbon monoxide and water vapour and mixtures of these gases as attacking agent.

1 Introduction Our modern industrialised society is focussing on a new challenge regarding our own living environment. Therefore, a change in mind and a change in economy is unavoidable. One can call this a global green deal as a basis for a sustainable economy. To reach this aim, the emissions of greenhouse gases have to be reduced – only one part but of very high importance. Consequently, a lot of transformation of industrial processes is needed. Caused by a long existing alternative process and therefore a high possible decrease of emissions, the steel industry gets into this focus of transformation. Namely, the direct reduction process will be one of the major production routes for iron and steel in the future. In this paper, common and advanced lining materials for gas-driven DRI shaft furnaces are discussed. Based on more than 15 years of experience, STEULER Linings shows some examples of lining solutions for the different major processes. Using reformed natural gas as a standard, a closer look is taken to the transformation route via hydrogen boosted coke oven gas in an integrated steel plant up to the pure hydrogen use in a pilot plant, especially by using different gases such as hydrogen, methane, carbon monoxide and water vapour and mixtures of these gases as attacking agent. Therefore, the first result of tests made in a STA equipment where the authors are able to use these different gases under different pressures up to approx 5 bar and temperatures up to 1400 ÆC are shown. Whereas the transformation process with different gas compositions will be the real challenge, the authors focus in the first step on the hydrogen attack of standard and advanced brick material. In another part of the paper, the usual wear of lining materials for DRI shaft furnaces will be discussed. This will be done for both gas-driven DRI processes. 2 Lining concepts 2.1 Improvement of gas inlet area This paper focuses on the two major gasdriven processes: one in a non-pressurized and the second one in a pressurized vessel. Both types of DRI shaft furnaces were lined with more or less a comparable refractory material. Most critical lining areas are the lower and upper shaft lining and especially the gas inlet area. The lining of this area in a non-pressurized reactor was always done with some special-shaped so-called port blocks. This area in a pressurized vessel was originally done by using small-format special shapes with small-size interlocking. Due to high mechanical loads, damage often occurs in this area because the “noses” break off and the brickwork loses stability. On the following pages, the steps of improvement during a time frame of 5 years starting with the insufficient original lining design are shown. During a certain time of design review and inspections of old and new lining design, STEULER finally installs nowadays only the below mentioned lining design shown in Figs. 4–6. Very impressive to see was the behaviour of the old design after approx 18 month of operation. The brick arrangement in the gas inlet area was shifted by around 5 cm inside the reactor. Usually not possible, but indeed it happened. On the other side, it can be seen that the brick material itself shows fortunately more or less no wear. Anyway, the authors came to the conclusion to provide a new design for the repair work based on special shaped bricks in a double layer design corresponding to the usual lining concept of a pressurized vessel. Installation was done in 2016 and the inspection after 2 years in 2018 showed no issues regarding the design and the material used for this modification. The original brick material was a kind of mullite fireclay brick whereas the special shaped gas inlet bricks of the new design were made as high fired mullite/corundum brick. 2.2 Expansion joint and support rings Due to the thermal conductivity of the support ring made of steel, one can observe a higher temperature on the steel shell outside. The areas with an increased risk for hotspots are clearly visible. Detailed adjustments to the lining concept of these areas can be implemented and evaluated on the 3D model in order to minimise these hotspot areas. 2.3 Reducing gas transfer line inlet Due to the inlet hot gas stream, the temperature on the upper half of the transfer line inlet increases. Therefore, the installation of the inside refractory material has to be done very carefully to get a lining with sufficient tightness. Experience shown in the last paper led to the best solution by implementing a ring of special shaped bricks for the connection of transfer line and ring channel of the reactor. 3 Typical wear of lining 3.1 Wear of shaft lining On the following Figs. 9–10, one can see the typical wear of a shaft lining based on mullite-fireclay bricks. Sometimes, this happens when a chemical reaction of the pellets and or the coating of the pellets lead to a sticking of the feed to the lining material. Usually, only in areas where there is a minor movement of the pellet bed accompanied with a possible temperature excursion. In the area with the highest gas velocity and therefore most turbulence, this usually does not occur. So at least the area of the gas inlet nozzle bricks and approx half a meter below one doesn’t see an impact. Fig. 11 shows a possible situation after a shutdown for maintenance issues. Before restart it is necessary to clean up these joints and remove the pellets. If this happens some measures have to be obtained to avoid a refilling with pellets during the restart operation. 3.2 Mechanical impact Another impact may occur if thermal expansion of the lining is not calculated in the right way. Due to uncompensated expansion of the lining the thermal stress can induce a spalling effect on the top surface of the brick lining. Fig. 12 shows this effect above the gas inlet area. Above this spalled area one can see also an advanced wear of the joints. To avoid this, the alignment of mortar and brick material has to be done in a proper way. 4 Post mortem analysis 4.1 Analysis of upper shaft material The samples for the analysis were taken out of the upper shaft lining above the gas inlet area. In this case the lining was done with a usual blast furnace brick based on fireclay (super duty). Figs. 13–15 show the pieces for the analysis get out of partially spalled material which was already attached to the brick surface. Besides the brick material we also see a lot of metallic material attached to the brick surface. Based on the chemical analysis Tab. 2, one is able to state that there is no real chemical impact on the upper shaft lining due to the process and due to the possible occurring reduction gases in this area. Analysis of Lab No. 6705-18 compared to the mean value of this brand out of production are more or less the same. Definitely the difference in the major components based on alumina and silica shows more or less no deviation. Therefore one can be very sure that there is no real attack by the reducing gases like carbon monoxide or hydrogen. Remaining reducing gases should also occur in this area, nevertheless they should have reacted in the reducing part of the shaft which is in a lower part of the vessel. Seen from the point of chemical analysis the iron content of the spalled piece is slightly higher which is not really surprising. On the other side, one can see a very interesting analysis of the metallic flakes. It was not possible to separate the metal completely from the remaining brick material and therefore the authors also got alumina and silica in the analysis. The proportion of alumina and silica is contrary to the proportion in the brick. This may be related to the forming of iron silicates which will be attached in a very small amount to the metallic flakes. This may explain the small difference in alumina and silica analysis between the mean value of production and the spalled brick part out of the reactor. Furthermore, one has to accept a temperature excursion in this area with the effect of creating molten metal flakes. There is still no idea how this could happen in this area but it is a fact. Seen from the point of molten metal, the temperature should have reached a level close to 1290 ÆC which is far away from a supposed temperature of approx 1040–1070 ÆC in the gas inlet area which is several meters below the investigated area. Additionally the chemical analysis of the metallic flakes shows especially also a higher content of lime (CaO) and magnesia (MgO) which can be led back to the use of cement as coating material for the pellets. 5 Starting new investigations 5.1 STA analysis in different atmospheres Testing of raw materials and refractory material in atmospheres with hydrogen, carbon monoxide, methane and water vapour needs a fully new equipment which STEULER installed this year. Additionally to the possibility for testing in different atmospheres, it is also possible to apply different pressures up to 5 bar. The system is able to run at temperatures up to 1400 ÆC and flexible heating-up rates and holding ramps. During the first testing period, tests were done only in 100 % hydrogen atmosphere at atmospheric pressure to get a first impression on the effectiveness of the new equipment. An example of a measurement given as TG analysis is shown in Fig. 19. The abovementioned diagram shows the graphs of the measurement of a mullite fireclay brick. The brick material was milled and then fractionised. 2000 mg of this powder was given into the TG-crucible. The graphs show from up to down the pressure, the temperature, the weight difference and the hydrogen throughput in [cmÑ/min]. Additionally, it is also possible to run a DSC analysis of such a sample. Between 950–1000 ÆC one can see a weight increase of approx 0,01 mass-%, whereas afterwards up to 1400 ÆC a slight weight loss of approx 0,2 mass-% is determined. Currently, the authors have no idea regarding the first effect, but maybe we see more after DSC running. The second effect shows a minor affected material. This slight effect is not really surprising cause this material is currently used in a lot of DRI plants and is obviously a suitable material for this kind of application. 6 Conclusion This paper shows the importance of a careful consideration of the refractory engineering and design. Calculations of thermal stresses etc. are very helpful for getting a suitable lining concept. Combined with the experience in the use of especially developed refractory material for this kind of application, the authors see currently no difficulties in the built up of DRI plants for the hydrogen case. Common systems based on reformed natural gas are running with more or less the same lining material and design. For the transformation process with a lot of different possible gas atmospheres one is now able to run specialised tests. And one of the most important components in this field could be the addition of water vapour. Due to current knowledge, this could change the measured attack in the lab. With pure hydrogen, one can see a minor effect but pure hydrogen will never go to the DRI-shaft. Also some water will be in the gas due to the heating up of the reducing gas. But one has also to take this challenge.