Build ups in pyro-processing

December 2019
Build ups in pyro-processing

It is a common problem to face build ups in the cement making process. The difficult ones are in pyro process, because you have to deal with high temperatures and unsafe conditions. It is always a challenge for plant management to handle such situations.

Undoubtedly pyro processing is the most important part of cement manufacturing. It is high energy intensive next only to grinding. However the pyro process faces some problems like jamming due to build ups. Let us look into some of the causes of build ups.

Generally these build ups are hurdles in the smooth running of plant. Sometimes these even turn out to hazards for safe working. It is necessary to go to the root cause of the build up formations. Though each and every build up can be attributed with a specific cause yet there are some common causes which can be viewed as generic problems.

The most common problems of build up formation are... "Snowman", "Rhino horn", and "Boulder", Red River in kiln system. We shall discuss about few of those. The first two build ups are formed from small clinker nodules and clinker dust particles. Their chemical and mineralogical composition is identical to clinker, but in some cases enrichment up to 3.5 percent K20 and 3.0 percent S03 has been noticed. Snowman is a formation of large build-up on cooler first grate (static grate) or kiln discharge wall where the clinker falls from the kiln. Snowman may eventually grow to reach the mouth of the kiln, thereby blocking the discharge of clinker from the kiln.

Snowman causes poor clinker distribution thereby poor heat exchange between clinker and secondary air. Snowman formation mechanism may divide to two:

1)Freezes of the clinker liquid phase as the clinker passes through the first cooling zone in the rotary kiln or on falling down the chute into the grate cooler. The clinker dust particles carried back by the secondary air stream from the clinker bed grate into the interior of the rotary kiln also play an important role on formation snowmen. The clinker dust particles, having a superficial liquid phase layer, strike against the chute wall and the refractory lining at what is the formation mechanism of "Snowman", "Rhino horn", and "Boulder" in kiln system?

2).Occasionally large lumps of coating discharge from the kiln, these lumps of kiln coating act as "seeds" for the formation of snowman. Snowman form when fines fall from the kiln onto the top surfaces of these lumps, on top of the clinker bed within the cooler. As layer after layer of the fines fuse onto the lump, snowmen "grow" upwardly into stalagmite-like structures.

Rhino horn: is a build-up on the top of kiln burner pipe. Rhino horn formation mechanism: Clinker dust carried back to the kiln with the secondary combustion air loses velocity around the burner and that causes clinker dust to settle and build up on the top of the burner.

Boulder: Generally, clinker ball/ boulder formation is related to low clinker SM with high AM or related to very thick coating. Figure is showing a boulder with 1.80 m in diameter.

Boulder formation mechanism: Large clinker balls initiate and grow behind a thick coating or ring. Build up often fall and slide into the kiln as large slabs and coming to rest behind a ring (if it is large enough). The slab can roll around behind the ring forming a ball which can then grow larger due to accretion.

Snowman formation
Dusty clinker melts due to alkali or sulphur recycle distorted flame disintegration of clinker due to reduced conditions. Blow back of dust from cooler to kiln and recycling high air flow rate in first portion of clinker When the kiln is dusty the snow man problem is common. The dust falls on the cooler and blows back into the kiln which is carried away by secondary air stream. If secondary velocity is more than 5 m/s the finer dust is carried into the kiln and the velocity of combustion air and combustion products further shoots up near the flame which carries the dust further inside the burning Zone. This dust steals some liquid from the burning zone and falls on the cooler and the cycle continues between cooler to kiln. It reaches a stage when the clinker dust becomes so sticky that it sticks to the side and vertical wall of the cooler and dust build - up called snow man forms. If sulphur and alkali recycle is there inside the kiln the melt increases, which do not reflect in the calculated liquid, the minor low melting compounds concentrate more on dusty fraction of the clinker. Some critical parameters can be controlled by operating the kiln with good quality liquid and good flame with oxidised conditions. A short radiant flame forms relatively better clinker. A long flame increases the burning zone length which allows the low melting constituents to volatilize and recycle.

It is often misunderstood that long flame gives good refractory life and hence to form a lazy flame . This makes the kiln more dusty. The momentum for conventional burner is totally different from the multi channel burner. The momentum is maintained to improve the rate of mixing of coal and combustion air, there by improve the combustion efficiency. The momentum with low primary air percentage and high velocity is much better than with high Primary air and low velocity. The shorter black jet (plume) and good radiant convergent flame make the kiln much efficient. The flame is the victim of many hands and so the kiln. Flame has to be monitored often and so the shell temperature which is good reflection of the good shaped flame.

If flame is wrong everything goes wrong . We immediately blame the chemistry and demand more liquid to form good nodules which aggravates the situation. If coal falls on the charge, the liquid quality changes One thing we have to remember is that with a good flame, short radiant flame we can reduce the liquid percent. Short radiant flame gives dense and better shaped nodules than long lazy flame.

Why the dust blow back? The dust is the main culprit of the snowman formation and then the uneven distribution of air further aggravates the situation. The red river area is aerated more, the dust Is blown back into the kiln. In modern beam aerated cooler the sandblasting effect must be low. If it is so then the cooler plates profile needs check. Air flow in the front portion of the cooler has to be redistributed to avoid blasting. More airflow does not mean more recuperation. Burner can be pushed inside the kiln by 500 to 1000 mm if there is margin in percent calcination. If calciner does not have margin it leads to loss of production. Avoid CO formation at any cost. 

Source: Nael Shabana

Compiled by VIKAS DAMLE