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BY PASS SYSTEM

BY PASS SYSTEM. CIRCULATION PHENOMENON. Normal atmosphere 96-99% 60-90% 30-70% 20-40%. Reducing atmosphere 97-99% 90-100% 30-70% 20-40%. Kiln Chemistry. Kiln Volatile Cycles. Cl SO 3 K 2 O

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BY PASS SYSTEM

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  1. BY PASS SYSTEM CIRCULATION PHENOMENON

  2. Normal atmosphere 96-99% 60-90% 30-70% 20-40% Reducing atmosphere 97-99% 90-100% 30-70% 20-40% Kiln Chemistry Kiln Volatile Cycles Cl SO3 K2O Na2O

  3. Preheater exit gas 360 C Preheater exit gas 440 C Normal operation 760 net kcal/kg clinker Abnormal operation 830 net kcal/kg clinker Calciner exit gas 1000 C Calciner exit gas 880 C Back-end Fuel Kiln fuel High BET and risk of blockages AIR SEPARATE CALCINER

  4. BY-PASS GAS REMOVAL PREHEATER P.H. PH VOLATILES KILN HORNO PRECALCINER P.C, VOLATILES PH HIGHER CONCENTRATION OF VOLATILES BY PASS P/C KILN HORNO

  5. Air Separate Pre-Calciner Calciner vessel Back-end Fuel Tertiary Air duct

  6. PROBLEMS OF BLOCKAGES IN KILN RISER AND KILN BACK-END UPPER STAGE 3 MEAL FEED UPPER STAGE 3 MEAL FEED EFFECT OF LOWER ENTRY FOR STAGE 3 MATERIAL BUILD UP AREAS

  7. Five stage 310 490 630 750 820 1000-1100 330 - 360 °C 530 - 560 °C 680 - 720 °C 800 - 840 °C 1010 - 1100 °C

  8. 350-400 ° C Build-ups 500-550 °C 650-700 °C Chloride based build-up 800-850 °C Sulphur based Build-up 1500-1600 °C 1150-1200 °C Ash rings

  9. process adverse materials • These harmful materials are sulfur,chlorine and alkali elements i.e. sodium and potassium. Their behaviors in the kiln and preheater atmosphere leads to build-up of layers of these components and trapping huge quantity of kiln dust. This build-up forces the kiln operator to shutdown the kiln system to clear this build-up. The kiln operation suffers because the build-up in the riser pipes and cyclones increases pressure drop in the system

  10. Circulation phenomenon This term is used to represent the phenomena caused by the presence of the volatiles in system i.e. alkali chlorides, Sulphates and other related components in the dry kiln system with preheater and precalciner

  11. Internal and ExternalCirculation Phenomena • 1-INTERNAL CIRCULATION PHENOMENON is between preheater tower lower stages and the kiln-burning zone. • 2-EXTERNAL CIRCULATION PHENOMENON Their cycle from preheater to electrostatic precipitator to kiln feed and back again to the kiln system

  12. % By-pass = % chlorine as Cl in the raw materials x 100 • This procedure is applied to chlorine because it is nearly impossible to control the evaporation of chlorine in the kiln burning zone or control its concentration in the process by any mean.

  13. Example • The raw materials contain chlorine [raw meal basis] =0.25 %. What is the bypass %? • The required bypass% = 0.25x100=25%

  14. The main compounds made are: • Alkali sulfate [K, Na]2 SO4 • Alkali chloride KCl, NaCl • Alkali carbonate [K, Na]2 CO3 • Calcium sulfate Anhydrite CaSO4 • Sulfate spurrite 2C2 S CaSO4 • Sulfo-spurrite [K, Na] 2 SO4

  15. Circulation Mechanism • The circulating elements enter the kiln with the kiln feed that travels through the preheater to the kiln inlet. • Starting from lower most cyclones the temperature starts reaching 800ºC in the kiln system. From this temperature, part of these elements is volatilized and becomes part of the kiln atmosphere. • KCl NaCl CaCl2 K2SO4 Na2SO4 CaSO4 775 772 801 1069 884 1450ºC [melting temperature]

  16. Circulation Mechanism • When the material reaches burning zone, all the chlorine will be evaporated with part from sulfate, sodium, and potassium [the harder the kiln feed to burn the higher will be the evaporation rate of the volatiles and this takes place also in case of a very strong flame in the main burner]

  17. Chloride reacts primarily with the alkalis, forming NaCl and KCl. Any excess of chlorides will react with calcium oxide available in the system to form CaCl2. A part of the alkalis in excess of chloride combine with sulfur to form Na2SO4, K2SO4 and double salts as Ca2K2(SO4)2. Alkalis not combined with chloride or sulfur is present as Na2O and K2O embedded in the clinker mineral

  18. An Example • If we introduced 1 kg of potassium each hour with the feed and when the reactions achieve a state of equilibrium of volatiles in the system, then we will have the following condition: • 0.811 kg of potassium will leave with the clinker • In the system the concentration of potassium will be 2.573 kg • In the by-pass dust the concentration of potassium will be 0.221 kg. • In the exhaust gas dust the concentration will be 0.042 kg of potassium.

  19. Evaporation Rates of Different Elements • The evaporation factorE = • 1 – (% within the clinker / % at kiln inlet loss on ignition free basis )

  20. Example: 1 • The concentration of the sulfate in the clinker in one sample is 1.05 %, SO3 concentration in cyclone 4 materials which is collected from the material pipe of cyclone 4 to the kiln inlet is 2 % and the loss on ignition of this same sample is 3.5% • What is the evaporation factor of sulfate [SO3] in this system?

  21. Evaporation Rates of Different Elements • % SO3 at the kiln inlet loss on ignition free basis = (2/100-3.5) * % = ( 2/96.5 ) * % = 2.0725 • Evaporation factor E = 1 – (1.05/2.0725 ) = 1 - 0.507 = 0.493

  22. Example 2 • The loss on ignition of the sample for chlorine is 3.8 %. Its concentration in the clinker is 0.03 % and its concentration in the hot meal [kiln feed] from cyclone 4 to the kiln inlet is 0.65 %. What is the evaporation factor of chlorine in this system?

  23. Solution • % Chlorine at the kiln inlet loss on ignition free= ( 0.65 / 100-3.8) * % = ( 0.65/96.2) * % = 0.676 Evaporation factor E of chlorine = 1 - ( 0.03/0.676) = 1 - 0.04 = 0.96

  24. Example 3 • The loss on ignition of the sample for potassium in kiln feed is 3.5% and the concentration of potassium in the clinker 0.29% and its concentration in the hot raw meal from cyclone 4 to the kiln inlet 0.39%. What is the evaporation factor of potassium in this system?

  25. Solution: • The % of potassium at the kiln inlet loss on ignition free = (0.39/ 100-3.5) * 100 = ( 0.39/96.5) * 100 = 0.4 Evaporation factor E of potassium = 1 – (0.29/0.4) = 1 - 0.725 = 0.275

  26. When E= 1 indicate that all volatile elements evaporate and none leave with the clinker • This is clearly indicated in the case of Example 2 of chlorine where the solution proved in a very unmistakable way this fact. [E in the example is nearly one].

  27. When E = 0 indicate that none of the volatile elements evaporate and all leave with the Clinker. • This is clearly indicated in the case of Example 3 of potassium where the solution proved in a very unmistakable way this fact. [E in the example is very small )

  28. Average evaporation factors • Alkali SO3 0.2 - 0.9 [have a relatively high melting point of 1074ºC, boiling at 1689ºC] • Excess SO3 0.75 • KCl 0.990 - 0.996 [have low melting point of 768ºC, boil at 1411ºC]

  29. KCl, CaCl2 and NaCl • Chloride compounds KCl, CaCl2 and NaCl are seen to have an evaporation factor of 0.990-0.996 in the kiln at 800ºC. These compounds melt and boil at 1400ºC

  30. Excess sulfur • Alkali sulfates have evaporation factors from 0.2 to 0.90 but they are mostly in the lower part of the range, while excess sulfur that cannot find alkali to react with has an evaporation factor of 0.75, therefore it is best that all sulfur react with alkalis to the highest extent

  31. Molecular Ratio of Sulfur and Alkalis • If the alkalis are in the right proportion with the sulfur in the system, both will combine together and become built in salts in the clinker minerals. But in the absence of alkalis i.e. if there is excess sulfur in the system, the more volatile calcium sulfate will be formed in the kiln system, and it has a higher evaporation factor

  32. SO3 /Alk = ( SO3/ 80) K2O /94 + 0.5 * ( Na2O / 62 ) = 1.1 Estimation of optimum molecular ratio between sulfur and alkalis in the system:

  33. Estimation of optimum molecular ratio between sulfur and alkalis in the system: • If the sulfur and alkalis ratio exceeds 1.1 it means that the amount of sulfur present in the kiln feed material that react with the alkalis is in excess and the remaining excess sulfur will react to form CaSO4

  34. Example 1 • A kiln feed sample contains the following concentration • SO3=0.45 % K2O=0.37 % Na2O=0.38 % • What is the sulfur and alkalis molecular ratio in this system?

  35. Solution The result is indicating that there is no excess sulfur in the system.

  36. Example2 • A kiln feed sample contain the following concentration • SO3=0.57% K2O =0.21 % Na2O=0.15 % • What is the sulfur and alkalis molecular ratio in this system?

  37. Solution The result is indicating that there is excess sulfur in the system that will react to form CaSO4

  38. The amount of excess sulfur is expressed in gram SO3 per 100kg clinker • E.S = 1000x SO3 –850x K2O – 650x Na2O [gram SO3/100kg clinker] The limit on the excess sulfur is given to be in the range of 250-600g/100clinker

  39. Example • A kiln feed sample contain the following concentration • SO3=0.57% K2O =0.21 % Na2O=0.15 %

  40. Solution • E.S = 1000 x SO3 –850x K2O – 650 x Na2O [gram SO3/100kg clinker] E.S = 1000x 0.57 – 850 x 0.21 –650 x 0.15 = 570 – 178.5 –97.5 = 294 gramSO3/100kg clinker

  41. comment • This kiln feed contains a relatively small amount of excess sulfur. But if the material of the kiln feed is hard to burn or the flame is very strong a coating problem may cause some trouble due to build-up in the preheater and the pressure loss may increase in the preheater

  42. Optimum range of molecular of sulfur and alkalis in the presence of chlorine Therefore the optimum range is nearly 0.8 to 1.1.

  43. comment • Since the chlorine affinity for reaction with alkalis is higher than the sulfate therefore the following equation is applied to determine the optimum sulfate alkali ratio where the chlorine is subtracted from the alkalis

  44. Coating and Ring Formation in Kiln and Preheater In certainstage of build-up a new material starts to exist and causes more trouble in the system. The formation of spurrite [2 C2S . CaCO3] and sulfo spurrite [2 C2S . CaSO4] in case of the excess sulfur will exist in abundance

  45. Where does the build-up occur in the kiln and preheater system? Cyclone preheater Preheater with Precalciner

  46. Why? • What is the reason that makes suspension-preheater-kilns with precalciner more sensitive to the volatiles problem than the suspension –preheater kilns?

  47. How to Decrease the Effect of Volatile Matters on the Kiln System? • Frequent kiln stops due to cyclones blocking which need additional time for cooling and cleaning. • Higher heat consumption due to this frequent stops, additional fuel used for reheating the system and higher kiln’s brick consumption. • Reduced kiln production since the operator will try to continue work with less draft in the kiln and in most cases in reducing atmosphere with much CO in the system.

  48. Reducing the burning zone temperature • This means the reduction of the volatility of the alkalis, chloride and sulfate components. This can be done by reducing the burning zone temperature. The volatility of the sulfur compounds especially calcium sulfate is a function of the burning zone temperature. Calcium sulfate starts to decompose at 1220ºC and this thermal decomposition can be avoided by lowering burning zone temperature

  49. This can be done also by other means as Decreasing the silica ratio of the kiln feed and thus making the kiln feed easier to burn. Finer grounding of coarser particles especially the free silicates if present in the kiln feed therefore easier to burn kiln feed. The result will be lower sintering temperature in the burning zone decreasing the volatility . Accepting higher free-lime in the clinker. This requires less fuel in the burning zone, and there will be no overheating of the burning zone.

  50. Controlling volatile content • Controlling volatile content inthe raw material used for grinding and used as kiln feed. • That means observing the optimum molecular of sulfur to alkali and ensuring that the excess sulfur is minimized

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