Causes for bag house corrosion Frequent stop and start of kiln HAG operation for False air ingress though raw mill without feed The rotary air locks and doors of Bag housePresence of sulfurous gases BH operated close to the dew point Poor gas distribution inside the bag house
COST OF CORROSIONThe cost of corrosion goes beyond the obvious such as replacing corrodedtube sheets, walls in hoppers and the actual replacement of the entire dustcollector with the required kiln shutdowns.They include other costs that are not so obviously caused by corrosion. Theseother costs manifest themselves in short bag life. This short bag life leads toother items listed in the introduction of this paper.• High bag replacement costs• Environmental fines• Curtailed production• Personnel problems due to the difficult task of replacing bags• Safety hazards• Management distraction from making cement
Types of corrosionThese reactions do not take place in the gaseous phase. The presence ofliquid H2O is necessary to cause the reactions to take place. Therefore, threethings are needed for corrosion to occur in a bag house:1. Acid producing compounds2. Condensation3. Plain carbon steelEliminating any one of the three will solve the problem. Since the first twoitems can be at least partially eliminated in most cases, will focus heavily onthose. The replacement of plain carbon steel by more expensive alloys orexpensive coatings will only be touched upon only if there is no solution
Acid Producing Compounds The major sources of acid producing compounds are as follows: 1. Pyritic Sulfur in raw materials 2. Sulfur in fuels 3. Chloride in raw materials 4. Chloride from the addition of CaCl2 5. Chloride in fuelsPyritic Sulfur burns to form SO2 in the temperature range of 400 – 600°C (750 – 1110°F). Theamount of SO2 from pyritic sulfur that actually makes it to the kiln baghouse is dependent uponthe kiln system, kiln efficiency, and the handling of the kiln exit gases. During the combustion offuels containing sulfur, it is expected to generate a certain amount of SO2 gases. Again, theamount that actually makes it to the kiln baghouse is highly dependent upon the system design,efficiency, and operational parameters.Chloride containing compounds require high temperatures to decompose. These hightemperatures can be found in the lower stages of a preheater/precalciner system and in theburning zone of the kiln. The gaseous chloride will then circulate throughout the entire system.The amount of chloride that actually makes it to the kiln baghouse is dependent upon the systemdesign and the efficiency of the system
CondensationThe acid producing compounds need to be in aqueous form to function as an acid, andthereby promote corrosion. Water in the cement process can come from a number ofsources. All kiln gases have H2O from the combustion of fuel and from the inherent moisturein the kiln feed. In some cases, additional H2O is added in water spray form. In systemsdesigned to utilize the kiln exit gases for drying in an in-line raw mill, the amount of H2Ovapor to the kiln baghouse can be greatly increased. The total amount of water vapor in thegas stream as well as the temperature of the gas dictates the dew point temperature. If theprocess gas temperature is within 20°C (36°F) of the dew pointtemperature, localized condensation of water and acid formation is a potential problemThe third item present to have corrosion is plain carbon steel. As everyone knows, plaincarbon steel corrodes slowly in ambient conditions (lying in the maintenance yard) dueprimarily to weak carbonic acid formed by the reaction of water (condensation) and theCO2 in the air reacting with the iron. The corroded parts will then lead to shorter lifetimesand require sooner than expected maintenance. This same carbon steel located inside abuilding will last much longer if it is kept dry. In a baghouse that same carbon steel even inthe presence of acid producing components will last a long time if it is kept hot and dry.In some processes where the opportunity to control temperature in the presence of acidproducing compounds does not exist, plain carbon steel is eliminated from the equation by:• Replacing with other material such as stainless steel or various plastics• Extensive coatingsIn most cement kiln applications other strategies can reduce the problem significantlyenough to eliminate the need for these extreme measures.
Materials of ConstructionThe third item present to have corrosion is plain carbon steel. As everyone knows, plaincarbon steel corrodes slowly in ambient conditions (lying in the maintenance yard) dueprimarily to weak carbonic acid formed by the reaction of water (condensation) and theCO2 in the air reacting with the iron. The corroded parts will then lead to shorter lifetimesand require sooner than expected maintenance. This same carbon steel located inside abuilding will last much longer if it is kept dry.In a baghouse that same carbon steel even in the presence of acid producing componentswill last a long time if it is kept hot and dry.In some processes where the opportunity to control temperature in the presence of acidproducing compounds does not exist, plain carbon steel is eliminated from the equation by:• Replacing with other material such as stainless steel or various plastics• Extensive coatingsIn most cement kiln applications other strategies can reduce the problem significantlyenough to eliminate the need for these extreme measures.
Scrubbing action in millsThe exhaust gases from a wet and long dry system are usually vented directly to abaghouse. Suspension preheater kilns and precalciner kilns typically send their exhaustgases to the raw mill area for drying of the raw materials. In some instances where theraw materials are very dry, a simultaneous bypass of kiln gases around the mill andrecycle of mill exit gases is utilized. In most cases when all the kiln gases are passedthrough the roller mill, the majority of the sulfur and chloride containing compounds arescrubbed out of the gases due to the intimate contact of raw materialand gases found inside a raw mill. Even though the baghouse runs at low temperatureswhen the mill operates (80 – 100°C / 176 – 212°F), corrosion does not occur as rapidlydue to the lack of acid compounds.
STRATEGIES TO REDUCE CORROSIONIf baghouse corrosion is suspected, the inside of the baghouse should be inspected. During theinternal inspection, careful notation must be made about the location of all corrosion sites. Adetailed corrosion inspection along with other plant data will often answer what is causing thecorrosion. In general the following advice can be considered as an effective guide to minimizekiln baghouse corrosion.Keep it hot or get it out! That is, keep the cement kiln baghouse well above acid dew point orget rid of the acid producing compounds.Keep it hot!Dew point is the temperature where water vapor condenses out of the gas stream. The dewpoint is highly dependent upon the percentage of water vapor and temperature of the gasstream. Very small amounts of water in terms of percentage of the gas stream build up to largequantities of water in the baghouse over a period of time. Acid dew point is the temperaturewhere water that contains acid producing compounds condenses. The Sulfuric acid dew point isusually much higher than the water dew point, whereas the hydrochloric acid dew point istypically lower than that of water
Proper InsulationEven when the gas passing through thebaghouse is hot enough, if the baghouse isnot insulated properly the steel surfaceswill usually be cold enough to causecondensation. It is absolutely necessaryto insulate all process baghouses ventingproducts of combustion. During start-upconditions this condensation will be at suchlarge quantities that the hoppersand material handling equipment mayfill up with water. The details of insulationare very important in preventingcondensation. The areas around doors andon doors may be the most difficult. It is anadvantage to have a double doorarrangement – one door to seal the dustcollector and another door to sealthe insulation around the dust collector
Conclusion1. Run the raw mill only when the kiln reaches the full production so that the gases vented should me more than 90 deg C. And for the safe side the temperature at the stack is more than 100 deg C ie ., 40deg C above the dew point.2. False air ingress through the inspection doors must be avoided where localised condensation can occur. This is visually seen in big dust collection vessels like kiln ESP near the doors and tail end drum of drag chain etc and inspection doors. Double doors can reduce the false air ingress. Reduction in no of doors and double rotary air locks too reduce false air ingress.3. Top stage cyclone can have top door to vent out the sulfurous gases when kiln is heated after the refractory lining job. Our coal has sulfur as well as moisture4. And hence this low temperature gases must not be sent to bag house. Reducing the coal moisture can reduce the corrosion.5. Tripping of bag house fan must be avoided as moisturous gases inside the bag house can condense. Stoppage of fan stops the venting. Mind that we send 30 tons of water vapor from raw mill and 3 tons of moisture from coal firing and kiln feed6. Proporting gate so that kiln feed feed enters 4 th stage as well as fifth stage So that preheater out let temperature can be maintained depending on the moisture in the lime stone.7. HAG operation must be avoided absolutely for raw mill.
Hence only way to reduce the corrosion is torun the plant continuously with, not less than the rated Production , reduce the false air ingress though the inspection doors and rotary air locks ofthe bag house . 4 th stage operation or boosterfan operation will further reduce the corrosion .CO can induce the corrosion as more SO2 can esapefrom preheater and hence avoid CO.Baghouse outletTemperature must be more than 100 Deg C
ThanksK.P.PRADEEP KUMARSr G M OperationChettinad cementsAriyalore