Circulation Phenomena

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1 Circulation Phenomena 2 Circulation Phenomena in the Clinkerization Process Ren Hasler, Daniel Brassel PT 99/14503/E 1. INTRODUCTION........................................................................................................ 510 2. MECHANISM OF THE CIRCULATION PHENOMENA ............................................ 513 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM ............................................... 520 3.1 Input of Circulating Elements ............................................................................ 520 3.2 Enrichment of Circulating Elements / Endangered Zones for Encrustation Formation ................................................................................................................... 520 3.3 Output of Circulating Elements ......................................................................... 521 3.4 Volatility of Circulating Elements....................................................................... 525 3.5 Condensation of Circulating Elements .............................................................. 536 4. TYPICAL APPEARANCE OF BUILD-UPS.................................................................. 538 5. KILN OPERATION PROBLEMS DUE TO CIRCULATING ELEMENTS..................... 547 6. IDENTIFICATION OF PROBLEMS WITH ENCRUSTATIONS AND BUILD-UPS ..... 547 6.1 Material Balance ............................................................................................... 548 6.2 Criteria and Indicators to Assess the Build-up Problem.................................... 549 6.3 Example of a Circulation Phenomena Problem ................................................ 553 7. MEASURES AGAINST BUILD-UP FORMATION ....................................................... 556 7.1 General Measures............................................................................................. 556 7.2 Intelligent Cleaning ......................................................................................... 556 7.3 Measures against Chloride Problems ............................................................... 561 7.4 Measures against Sulfur Problems ................................................................... 561 7.5 Measures against Alkali Problems ................................................................... 564 8. MATHEMATICAL MODEL TO SIMULATE THE CYCLES OF THE CIRCULATING 3 ELEMENTS................................................................................................................... 565 4 SUMMARY This chapter describes the circulation of volatile elements in the kiln system. It indicates the tolerable inputs of circulating elements so that no excessive build-up and clogging problems arise. In particular it shall serve as guideline how an encrustation problem, caused by volatile elements, is systematically solved. 1. INTRODUCTION Alkali, sulfur and chlorine compounds (hereafter called circulating or volatile elements, see below) in raw materials and fuels utilized for the cement manufacture, when present in high concentrations often given rise to difficulties in kiln operation with build-up formation, mainly in the preheater and the kiln inlet section. Volatile Elements (VE): Sulfur SO3 Potassium K2O Sodium Na2O Chlorine Cl The build-up formations cause cyclone blockages or restrictions of the kiln inlet area so that the kiln has to be stopped for cleaning. In extreme cases more than 200 annual kiln stops due to blockages may occur, so that the impact on kiln availability and productivity can become a severe issue. Suspension preheaters armed with dozens of shock blowers (Fig. 1a, 1b) to prevent build-up formation illustrate the importance of this problem. 5 Fig. 1a 6 Fig. 1b 7 2. MECHANISM OF THE CIRCULATION PHENOMENA Depending on the degree of volatility, the circulating elements evaporate in the sintering zone of the cement kiln and are carried by the gases to colder zones, where they mainly condense on the raw meal and partly also on the surrounding walls. Afterwards they return with the raw meal into the sintering zone where they partly reevaporate depending on the degree of volatility. This repeated process through the kiln leads to the establishment of internal cycles (see Fig. 5). Finally the cycles reach equilibrium so that the output of circulating elements is equal to their input by the raw aterials and the fuels (cp. Fig. 2). Fig. 2 Circulation of Volatile Elements within the Kiln System 8 Almost all the circulating elements finally leave the system with the clinker. However, this is only the case when beforehand sufficiently high internal cycles of the volatile elements have been formed. The extents of these cycles depend on the degree of volatility of the circulating elements. As the latter recondense on the colder raw meal and the surrounding walls, the formed sticky molten salts are able to reduce the fluidability of the raw meal and, if present in sufficient quantities, to glue it finally on the walls. From time to time, especially during a change of the temperature profile, pieces of build-ups fall down and mainly block the cyclone outlets (Fig. 3). 9 Fig. 3 Build-ups due to Circulating Elements in the Various Kiln Systems 10 If the amount of molten salts becomes too high, either because of an excessive input of volatile elements or due to a high degree of volatility, the installation of a kiln gas bypass becomes necessary in order to extract part of the circulating elements from the kiln system (Fig. 4a, 4b, 4c). Fig. 4a Conventional Bypass with Water Injection 11 Fig. 4b Hot Gas Bypass without Water Injection Fig. 4c Bypass with Gas Feedback 12 A small part of the circulating elements leave the kiln system with the main exhaust gas dust. The latter is efficiently precipitated in the dedusting unit and is normally reintroduced into the kiln system. This is called the external cycle of the circulating elements (Fig. 5). Fig. 5 Circulation Phenomena: Internal and External Cycle In normal cases a negligible amount of circulating elements is emitted by the stack into the atmosphere. However, the emission of SO2 is not negligible anymore, when the sulfur in the raw material is present in form of sulfide (FeS2, PbS, ZnS) or organic compounds. In this latter case the sulfides are volatilized in the temperature range of 400 to 600C and leave the kiln system partly as gaseous SO2 emission (Fig. 6 and section 3.3.4). 13 Fig. 6 SO2 Emission in case of Sulfides 14 3. CIRCULATING ELEMENTS IN THE KILN SYSTEM 3.1 Input of Circulating Elements In the following the typical inputs of circulating elements by the raw materials and the fuel(s) are indicated. It is differentiated between input ranges where usually no encrustation and build-up problems arise and input ranges that usually lead to severe clogging problems. Important: All figures may serve as rough guidelines only. The real limits for the build-up and clogging formation depend on a lot of individual parameters like the degree of volatilization, the temperature profile, the completeness of combustion and the excess air factor as well as the kiln system itself. The indicated values are guidelines for suspension preheater kiln only. 3.1.1 Input by the Raw Materials (loss free basis) Alkalis (K, Na): Generally appear as interlayer cations in the clay minerals and the feldspars. Sulfur: The sulfur is introduced in several mineralogical forms: - as sulfate: gypsum CaSO4 2H2O anhydrate CaSO4 - as sulfide: pyrite FeS2, organic compounds In the following only the sulfates are considered. Chlorine: The chlorides are mainly introduced as NaCl (from seawater) or KCl. Note: Thekiln feed normally includes already external cycle (see Fig. 2) so that its concentration on circulating elements is higher than the one of the pure raw mix. Typical limits for 4 to 5 stage SP kilns are listed in section 6.2.2. 3.1.2 Input by the Fuel(s) Mainly sulfur is introduced by the fuels such as Coal, Coke, fuel oil and so on. In Addition alternative fuels often contain a reasonable amount of sulfur. Typical limits for 4 to 5 stage SP kilns are shown in section 6.2.2. 3.2 Enrichment of Circulating Elements / Endangered Zones for Encrustation Formation Depending on the individual circulating element and its compounds, the condensing point lies in the temperature range of 650 to 1000C. Condensation: 650 800C : Chlorides and its compounds 800 1000C : Sulfates Therefore the endangered zones for the formation of build-ups by the condensed circulating elements depend on one hand on the circulating element and its compounds and on the 15 other hand on the kiln system itself (see also Fig. 3). In the following the build-up zones for the various kiln systems are indicated. At the colder end usually the encrustations by the chlorides are found whereas at the hotter end the ones of the sulfates are met. 3.3 Output of Circulating Elements 3.3.1 Clinker The chlorides are so volatile that they hardly leave the kiln via clinker. In exceptional cases, when the sintering zone has largely cooled down or when embedded in big material lumps, the chlorides may leave the kiln in major quantities. However, normally the chlorides form a large cycle within the kiln system and they need to be extracted by a kiln gas bypass. The sulfur and the alkalis leave the kiln system normally via clinker either as definite compounds: K2SO4, K3Na(SO4), Na2SO4 Ca2K2(SO4)3 CaSO4 (rare!) or in solid solution in clinker minerals: K with the Belite, Aluminate Na with the Aluminate SO3 with the Belite The calciumanhydrate CaSO4 is rather volatile (decomposes at temperatures > 1000C) and forms therefore a large sulfur cycle in the kiln system. Therefore it is very important that there is sufficient alkalis to combine with the sulfur