05 Low Temperature Corrosion in Cement Plants Christian Suchak Volker Hoenig Manuscript

July 15, 2017 | Author: maria_joao_botelho | Category: Corrosion, Gases, Sulfuric Acid, Chemical Reactions, Electrochemistry
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Low temperature corrosion in cement plants Christian Suchak, Volker Hoenig; VDZ, Duesseldorf, Germany Introduction Corrosion describes a wide topic and summarizes all the miscellaneous corrosion types within one term. The speed and severity of corrosion depend on a number of factors, which in the end define the actual corrosion mechanism. Looking at the corrosion problem in a cement plant, two basic types of corrosion can be distinguished. Corrosion at temperatures above 400 °C is described as high temperature corrosion and beneath 400 °C as low temperature corrosion. In case of low temperature corrosion it is always implied that an aqueous electrolyte takes part in the electrochemical reaction, so that a cathodic and anodic reaction takes place. This rough classification allows allocating the different components of a cement plant to these corrosion types. Basically all aggregates downstream of the preheater, the bypass filter as well as the clinker cooler filter are subject to low temperature corrosion. Major equipment parts such as dedusting units or chimneys have often high maintenance requirements related to corrosion damages which cause additional costs for the operating companies. Figure 1 & 2: Corroded sidewalls of a bag house

Possible corrosion mechanisms The gas way of cement plants are subjected to corrosion damage. In the process gas are gaseous and solid chloridic and sulphatic compounds present which have corrosive properties. The oxygen and water vapour in the gas atmosphere provide for oxidizing conditions. Carbon dioxide, originating from the decarbonisation reaction of calcium carbonate and fuel combustion, has both oxidizing and carburizing characteristics. As soon as water and in particular different kinds of acids are condensing, the metal parts are subject to corrosion. In table 1 hypothetical acids present in the waste gases of cement plants are summarized. Table 1: Acids with corrosive potential occurring in the waste gas path of cement plants during running operation Acid

Gas Agent

Relevance

Nitrous acid HNO2

NO NO2

Low

Nitric acid HNO3

NO2

Low

Hydrochloric acid HCl

Cl2

Medium

Sulphurous acid H2SO3

SO2

Low

Sulphuric acid H2SO4

SO3

High

&

For the formation of nitrous, nitric and sulphurous acid condensed water is required. The gas agents are physically dissolved in water and form the various acids and could oppose a corrosive threat to the metal.

Figure 3: SO3-Measurement in a low dust gas environment

Corrosion during kiln operation Inspections of different precipitators during major maintenance stops showed in general signs of both extensive and uniform damages as well as sign of pitting. Both these types of corrosion damages can be caused by acids (cf. Figure 1&2). A possible explanation for this corrosion occurrence could be the condensation of sulphuric acid during the running clinker production. If a considerable concentration of SO3 is present in these flue gases, it reacts with the water vapour beneath 500 °C to H2SO4. Then the acid can condensate on all open metal surfaces by reaching the sulphuric acid dew point. Until today, the widely observed phenomenon of corrosion and the hypothesis of the existence of sulphuric acid corrosion in cement plants have not been investigated systematically yet. Therefore, hardly any scientific studies have been conducted in this matter so far, also no systematically verified measurements of SO3 in high dust atmospheres at a cement plant have been reported. Additionally, the existing measuring techniques for determining SO3 and H2SO4 respectively in flue gases are

exclusively designed for power plant environments (cf. Figure 3). Such measurements are regularly carried out in the power industry at dust loads of max 1 g/Nm³. In comparison, the flue gases of cement plants can have - depending on the point of process - a dust load of 30 – 500 g/Nm³. The regular measuring equipment is not usable for such conditions. A new measuring technique was therefore necessary to be able firstly to measure and secondly assess the possibility of the existence of SO3/H2SO4 and furthermore to determine whether the prevailing concentrations are harmful or can be neglected. The determination of acidic components in high dust flue gases All conventional measuring techniques used to determine gas components in flue gases consist of probes which are equipped with ceramic and sintered metal filters respectively. Over time a considerable amount of dust builds up on the filter surface and in filter pores and a closed dust layer is formed. The measured gas has to pass this dust layer and is subject to the risk of chemical reactions. This problem was confirmed by several measurements done with a conventional probe and a continuously working SO2-analyzer. After five minutes the dust layer starts to absorb the major amount of SO2 (cf. Figure 5). It is therefore not possible to rule out a false measurement of highly reactive gas components such as SO3/H2SO4. Therefore, an electrostatic precipitator was developed to avoid this problem. With this equipment several measurements in different cement plants were successfully executed (cf. Figure 4). Figure 4: SO3-Measurement in a high dust gas environment

The measurements done with the newly developed measuring probe showed considerable amounts of SO3 (measured as H2SO4). In some cases

Figure 5: Continuous SO2-measurement with a conventional gas probe

concentrations up to 90 mg/Nm³ were found. Therefore qualitative and quantitative proof of H2SO4 existence in flue gases has been verified. This underlines the possibility that H2SO4 can occur even in environments with nearly pure carbonate particles, particularly considering the common dust loads of 30 - 500 g/m³. With regard to the confirmation of the H2SO4 existence in the clinker burning process, it is suggested that this compound is generated in the top cyclone stages of the preheater. Then, at temperatures of 400 – 600 °C the geogenic sulphides are oxidizing to SO2. Furthermore high amounts of catalytic effective oxides like Al2O3, SiO2 and Fe2O3 [1], as part of the kiln feed, are suspended in the atmosphere of the cyclones. Figure 6: H2SO4 isothermal dew point curves for a cement plant important partial pressure ranges and plant measurements

Through the heterogenic catalytic chemical reaction SO2 is been oxidized to SO3. During the cooling in the conditioning tower or in the raw mill, the SO3 reacts at temperatures beneath 500 °C with water in the gas atmosphere to H2SO4 [2]. The calculations of the sulphuric acid dew points of the performed SO3 measurements have been executed with the following empirical determined equation [3]: 122.4

27.6 lg p H 2O

18.7 lg p H 2 SO 4

With ϑ as the acid dew point in °C, pH2O as the partial pressure of water and pH2SO4 as the partial pressure of sulphuric acid, both in mmHg. The results of the plant measurements are shown in figure 6. The graph displays a selection of measurements in comparison with the theoretical acid dew point at different temperatures. Several measurements are well below the acid isothermal curve of 130 °C. Since in many cement plants the

exhaust temperature is below 130 °C, especially in mill on operation, precipitation of acid takes place. Correspondingly under these conditions the sulphuric acid is present as a condensate and can cause active corrosion. This supports the earlier mentioned hypothesis that the observed corrosion could be partially caused by sulphuric acid. Measures against low temperature corrosion in cement plants Considering the measured amount of sulphuric acid and compared to the observed extend of corrosion it is to consider that also other factors could cause these partially heavy damages. Cement plants without considerable amounts of H2SO4 in the flue gases show in some cases also severe damages. This leads to the assumption that the observed corrosion is not solely caused by the presence of sulphuric acid. During periods of time of short or long down-times the temperatures of the flue gases are dropping considerably and water condensates freely. In all likelihood other corrosive compounds and corrosive agents in the flue gases are also taking part in the active metal corrosion. But these corrosion mechanisms are not discussed any further, because it goes beyond the scope of this paper. In general three possibilities to avoid low temperature corrosion caused by H2SO4 condensation can be defined as following measures:  electrochemical protection  influencing the properties of the reactants and/or changing the reaction conditions respectively  separation of the metal from the corrosive environment However not all mentioned possible actions are economically feasible. Especially the first measure is much too costly and practically not achievable. In regard of the second point not much can be done effectively too. It is not feasible to lower the SO3generation by using different sources of raw materials, merely if alternative materials contribute significantly to low volatile sulphur input.

Figure 7: Protective coating in a bag house [5]

Also the conditions in the waste gas path are somewhat fixed and cannot be varied much. It´s only in cases when SO2 abatement is applied, that this could be adapted to SO3 minimization by means of Ca(OH)2 injections directly after the preheater. The neutralisation reaction of SO3 works chemically as of SO2. So by applying effective SO2-reduction systems immediately after the preheater, the absolute amount of acids in the flue gas can be reduced and therefore the corrosive potential.

amount of preparation before implementation. The different coefficients of thermal expansion of the coating and metal have to be considered too. So the surfaces of all areas which are to be coated have to be especially prepared to avoid localized mechanical stress and spalling of the coating (cf. figure 8). The application of high alloyed acid resistant steels is a last possible measure to avoid high material losses. But it also implicates high material and erection costs which are in most cases not expedient.

Special attention could be given to the temperature level of the exhaust gas. As long the temperature is well above the correspondent acid dew point, no acid will condensate and cause corrosion. In some cases the increase of the waste gas temperature of 10 °C would be sufficient to avoid any condensations. It´s also important to make sure that a proper insulation of all affected aggregates is maintained. Therefore it is crucial to avoid any false air leakages so that no low gas temperatures can occur locally, which could lead to major water and acid condensations.

References

With the application of a protective coating of the affected areas by organic coatings an effective corrosion protection can be achieved (cf. figure 7) [4]. But this measure is also cost-intensive and requires a high

[4] Mazeika L., Sherin B., Cardwell B.: Repairing a corroded baghouse filter at the Durkee cement plant by coating the surface; Cement International 5 (2004) 2; pp. 100-103

Figure 8: Spalling of an organic coating because of too high mechanical stress [5]

[1] Wickert K.: Chemische Umsetzungen im Feuerraum der Schmelzkammerkessel; BWK 9 (1957); pp. 105-118 [2] Rolker J.: Zur elektrischen Messung des Säuretaupunktes von SO3-haltigen Rauchgasen; Dissertation; Universität Stuttgart; 1973 [3] Haase R., Borgmann H.-W.: Präzisionsmessungen zur Ermittlung von Säuretaupunkten; Mitteilungen der VGB 76 (1962) 2; S. 16-19

[5] Personal information: Solnhofer Portland-Zementwerke GmbH & Co. KG, Germany

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