Kiln Cycling Produces that Dam Ring

Anyone that’s grown up in the Snow Belt is familiar with scraping ice off the windshield of their car and using the defroster to defog the inside of the windshield. (Even if you don’t live in the Snow Belt, you’re still probably familiar with a fogged up car window.) Both of these phenomena are actually very analogous to the alkali and sulfate cycles in a rotary cement kiln.

The ice that forms on the windshield of a car is caused by the freezing of moisture vapor against the glass surface. The longer the car sits and the more moisture there is the more ice that forms. (Of course that’s true only as long as the outside temperature is below freezing.)

The fog on the inside of the windshield is a slightly different story. The vehicle occupants exhale carbon dioxide and water vapor. The water vapor rapidly condenses from body temperature because of the cold interior onto the cool interior surface of the windshield. The cars defroster removes the fog by blowing hot dry air onto the surface. That increases the temperature gradient and removes the moisture from the air. That’s also why the quickest way to defrost the inside of a car is to turn on the air conditioning. The air conditioning removes the moisture from the air. (In most cars, turning the defroster on also automatically turns on the air conditioning.)

Hmmm… rapid temperature changes, evaporation, freezing and melting, air movement. What makes sense for your car also makes sense for your kiln…

Sulfates in the kiln system are brought in either through fuel or the raw materials or both. Alkalis enter through raw meal. As these materials heat up to about 800ºC they volatilize (roughly similar to evaporates) in the kiln atmosphere in the form of alkali chlorides and sulfates. These alkali sulfates are then pushed to the back of the kiln by the counter current gas stream and into the lower temperature areas of the kiln system where they then condense… just like the situation during freezing weather where the car’s windshield fogs up on the inside when everyone first gets in and before the defroster is turned on. After these materials condense towards the front end of the system, they then travel back towards the burning zone with the raw meal. Here they’re once again heated up and re-volatilized into the kiln atmosphere. If we don’t remove these alkali sulfates or alkali chlorides they’ll continue to build up more and more material… just like when a bunch of people are sitting in a car without the engine running in freezing weather. They keep producing moisture vapor. Local accumulations of condensate in the form of melt wetting partially clinkered raw meal causes the meal to get sticky. This material then accumulates along the kiln walls. This is how clinker rings form.

Once a ring forms it will impede the progress of raw meal passing down the kiln. That’s why you’ll also hear the term “dam ring.” Freezing of the molten sulfates becomes an even greater problem as the dam rings grow in size. The growth in size also means growth in material that is more difficult to heat. (Think of the windshield that continues to gather ice because the supply of moisture and freezing temperatures continues. The more ice you have… the more difficult the removal problem.)

The same principles behind getting ice off your windshield or defrosting your car’s interior apply to the kiln as well. You can scrape ice off the windshield… you can blast the dam rings out. Both alternatives take time, risk damage, and only treat the symptom. You can also remove the source of the buildup; either the moisture vapor or the sulfates. Not very realistic since there’s water vapor in the atmosphere and sulfates in most materials. You can make sure that the material condenses where you want. That’s the whole point of an alkali bypass.

As noted in PCA’s Innovations in Portland Cement Manufacturing:

The bypass takeoff is usually located in front of the riser duct because kiln gases coming from below have a tendency to hit the back wall and create an area with low dust concentration at the front of the riser pipe. The kiln gases sucked out of the riser pipe are then quenched to about 350°C (660°F) to provide the alkali condensation.

We could also control the temperature to make sure that the material just doesn’t condense. In the car scenario, we’d just continue to blast hot air from the defroster onto the inside of the windshield. (In practical terms we’re keeping the rate of evaporation greater than the rate of condensation.) A precalciner system has an inherent advantage over other kilns systems when it comes to dealing with the cycle of alkali accumulation. In the precalciner we just divert the gas stream with the alkalis before it can condense.

Going back to PCA’s Innovations in Portland Cement Manufacturing:

When fuel is burned in two locations, the only process heat requirements in the kiln are the completion of calcination and the maintenance of the exothermic clinkering operation. When the calcination level reaches 85% to 90%, the kiln fuel rate is nearly independent of the disposition of the kiln exit gas. If all or part of the kiln gas is wasted, the additional fuel needed to compensate for the heat losses is provided through the calciner burners, and the stability of the kiln operation is not compromised at any bypass level (Warshawsky and Porter, 1979).

For more information on alkali bypasses and kiln cycles take a look at PCA’s Innovations in Cement Manufacturing (CD400).