Pyroprocessing and Kilns Questions and Answers

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. Insufficient oxygen leaves fuel unburnt and produces carbon monoxide, wasting fuel and creating reducing conditions that harm the clinker. Excess oxygen means excess air, all of which is heated and sent up the stack. The optimum is a small definite margin, typically two or three per cent, and it is controlled continuously.

Answer: D

Three divided by twenty-one is about one seventh, so most of the oxygen has been consumed. The calculation is approximate because the gas volume changes during combustion and calcination. The figure is used as an operating guide rather than as an exact balance.

Answer: A

It represents both wasted fuel and a hazard, particularly where an electrostatic precipitator is fitted. A rising reading normally prompts an immediate increase in the draught or reduction in fuel. Carbon monoxide near a precipitator is an explosion risk as well as a loss.

Answer: B

The first two describe standard practice. The third is wrong, since recovering heat from the hot clinker is precisely what the cooler exists to do. Recovered cooler air returns to the kiln and calciner as combustion air.

Answer: C

Gas leaving the preheater and cooler still carries useful heat, and waste heat recovery boilers driving steam turbines are now common. The economics depend on the local price of electricity and on how much heat the raw mill already takes.

Answer: D

Both need the preheater exhaust, so a works with wet raw materials has less heat available for power generation. The trade-off is settled case by case rather than by any general rule. Raw material moisture is usually the deciding factor.

Answer: A

Both are true and the reason gives the conflict: the same stream of hot gas cannot do both duties, so the drying requirement takes precedence and leaves too little heat for a worthwhile turbine. Drying the feed always takes precedence over generating power.

Answer: B

The shell is short and the material moves through it fairly quickly once calcination has been done in the tower. Gas passes through the whole system in a matter of seconds. The contrast between the two residence times is very large indeed.

Answer: C

The gas takes seconds while the meal takes around half an hour, which is what makes counter-current heat exchange so effective. The two residence times are of completely different orders. This disparity is what makes counter-current heat exchange work so well.