Pyroprocessing and Kilns Questions and Answers

Answer: A

Correct answer: high temperature, chemical attack and mechanical abrasion

All three act at once, which is what makes burning zone refractory so demanding. It sits at 1,450 degrees against material that includes a liquid phase attacking it chemically, and the charge grinds across it continuously as the kiln turns. Magnesia-spinel bricks are used because they resist the combination rather than any one of the three.

Answer: B

Correct answer: generally desirable, as it protects the brick

A layer of clinker freezing onto the lining insulates the brick and takes the abrasion in its place, so a stable coating greatly extends refractory life. The operator works to keep it: too cold and it will not form, too hot and it melts away, and the loss of coating shows at once as a hot spot on the shell.

Answer: C

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. A scanner traverses the shell continuously and plots its temperature along the length. Where coating or brick has gone, the steel is no longer insulated and its temperature rises sharply - so the scan sees the loss immediately, long before the shell itself is damaged.

Answer: D

Correct answer: False

A hot spot means the shell is being heated far beyond its design temperature, and steel loses strength rapidly as it heats. Left alone the shell distorts permanently or fails, which is a far larger repair than replacing brick. The response is to cool the spot externally and to plan an early stop, not to wait.

Answer: A

Solid fuels dominate because they are cheap and give a luminous flame that radiates well to the charge. Electricity and steam are quite unsuited to raising a large mass of solids to 1,450 degrees. A luminous flame also transfers heat by radiation far better than a clear one.

Answer: B

Correct answer: waste tyres, solvents and processed municipal waste

A cement kiln burns these well because the gas residence time is long, the temperature is very high, and the alkaline charge neutralises acid gases. The ash is incorporated into the clinker rather than left over, which is why the kiln is used for wastes that would otherwise need dedicated incineration.

Answer: C

Both are true and the reason gives the technical basis: the conditions exceed those of a dedicated incinerator, and the alkaline charge neutralises acid gases, though the input still has to be controlled for chlorine and metals.

Answer: D

Correct answer: False

There is no separate ash. Whatever mineral matter the waste fuel contains ends up combined into the clinker, which is one of the principal reasons the kiln is used for waste destruction - nothing is left to dispose of. It does mean the ash composition must be allowed for in the raw mix.

Answer: A

Thirty per cent of 12 million is 3.6 million megajoules. Substitution rates of this order are routine in Europe and considerably higher at some individual plants. A few kilns run almost entirely on wastes and secondary fuels.

Answer: B

Correct answer: a stable flame of controlled length and shape

The flame determines where the peak temperature falls along the kiln, and that in turn determines where the coating forms and how the clinker is burnt. The burner is designed so that length and shape can be adjusted through the primary air and the momentum, and stability matters as much as either.