If a kiln operates 330 days a year at 4,000 tonnes of clinker a day, what is its annual clinker production?
Answer: A
Four thousand multiplied by 330 gives 1.32 million tonnes. Availability, the proportion of the year a kiln actually runs, is one of the most closely watched figures in the industry. A percentage point of availability is worth a great deal of money.
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Kiln availability is important chiefly because
Answer: B
Correct answer: a stopped kiln produces nothing while still costing money
A cement plant is capital-intensive and its costs are largely fixed, so the hours the kiln runs determine whether they are covered. A stopped kiln also cannot be restarted quickly, which multiplies the cost of every unplanned outage - and is why maintenance is planned around the kiln rather than the reverse.
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State whether the following is true or false.
A cement kiln can be started and stopped quickly like a boiler.
Answer: C
Correct answer: False
A kiln contains hundreds of tonnes of refractory that must be heated and cooled slowly, or it will spall and crack from thermal shock. Warming up takes a day or more, and cooling down as long. That is why an unplanned stop is so expensive and why plants run continuously for months between outages.
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Consider the following statements about kiln operation.
1. Gas and material flow counter-current to one another.
2. A precalciner allows a shorter kiln for the same output.
3. Rapid cooling of clinker reduces the strength of the cement.
Answer: D
The first two are correct. The third reverses the effect, since rapid cooling preserves the alite that gives the cement its strength and slow cooling is what harms it. Cooler performance therefore bears directly on cement strength.
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The kiln hood is the enclosure
Answer: A
It contains the discharge, houses the main burner and directs secondary air from the cooler into the kiln. Conditions inside it are among the most severe anywhere in the plant. Temperature, dust and radiant heat all combine in that one enclosure.
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Tertiary air ducting carries hot air from the cooler to the
Answer: B
Combustion in the calciner needs air, and taking it hot from the cooler rather than cold from outside saves a great deal of fuel. Secondary air, by contrast, enters the kiln itself through the hood. The two streams serve two burners in two different places.
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State whether the following is true or false.
Secondary and tertiary air are both drawn from the clinker cooler.
Answer: C
Correct answer: True
Both are cooler air recovered from the clinker. Secondary air passes into the kiln hood to feed the main flame; tertiary air is ducted separately up to the calciner to feed the fuel burnt there. Taking both from the cooler is what recovers the clinker’s heat back into the process.
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Assertion (A): A precalciner kiln requires a tertiary air duct that a simple preheater kiln does not.
Reason (R): Fuel burnt in the calciner needs its own supply of combustion air.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. In a simple preheater kiln all the fuel burns at the main burner, which draws its air through the kiln. A calciner burns fuel of its own, well upstream of the kiln, so it needs its own hot air - and the tertiary duct carries that air from the cooler directly to it.
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The oxygen content of the kiln exhaust gas is monitored in order to
Answer: A
Too little oxygen leaves fuel unburnt and produces carbon monoxide, while too much wastes heat in warming surplus air. The operator works within a narrow band between the two faults. Continuous oxygen measurement is what makes holding that band possible.
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State whether the following is true or false.
Running a kiln with a large excess of combustion air improves efficiency.
Answer: B
Every extra volume of air has to be heated and then carries that heat out of the stack. Excess air is kept to the minimum consistent with complete combustion for exactly this reason. Excess air is a direct and avoidable loss of fuel.
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