Pyroprocessing and Kilns Questions and Answers

Answer: A

Correct answer: the charge travels slowly down the length as it turns

The slope gives the material a component of gravity along the axis and the rotation keeps lifting and dropping it, so the charge advances a little with every revolution while being continuously turned over. The residence time is set by the slope and the speed together, and both are adjusted to suit the burning.

Answer: B

Correct answer: in opposite directions, counter-current to one another

The meal enters cold at the upper end and the flame burns at the lower, so the gas travels up the kiln against the descending charge. That arrangement puts the hottest gas against the hottest material and the coolest against the coolest, which is what makes the heat transfer efficient over the whole length.

Answer: C

Both are true and the reason gives the design principle: the gas gives up heat progressively as it travels towards the cold end, so it leaves the system at a far lower temperature than it would if it travelled with the charge.

Answer: D

Correct answer: a tower of cyclones through which meal and gas pass

Meal is injected into the gas duct between stages, heats almost instantly because the particles are fine and fully suspended, and is then separated by the cyclone and dropped into the next duct down. Four to six stages take the meal to around 800 degrees using the kiln’s own exhaust.

Answer: A

Burning part of the fuel in a separate vessel drives off most of the carbon dioxide before the material reaches the rotating shell. This allows a much shorter kiln for a given output and improves thermal efficiency considerably.

Answer: B

Correct answer: 10 per cent

\[ 100 - 90 = 10\ \mathrm{per\ cent}. \] Calcination is the most heat-hungry part of the process, so moving nine tenths of it out of the kiln and into the calciner is what allows a much shorter kiln to carry the same output - the kiln is left with the burning zone and little else.

Answer: C

Correct answer: False

A precalciner kiln is considerably shorter. Drying, heating and almost all the calcining have been moved into the tower, so the kiln has only to complete the last of the calcination and carry out the clinkering. Length to diameter ratios fall from around 15 to nearer 11.

Answer: D

Correct answer: calcium carbonate decomposes to lime and carbon dioxide

\[ \mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2}. \] The reaction absorbs a great deal of heat and proceeds from about 800 degrees, which is why it dominates the plant’s energy consumption. The combination of lime with silica to form the silicates is a separate, later stage at a much higher temperature.

Answer: A

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. Calcining a tonne of limestone releases carbon dioxide directly from the rock, and that process emission is roughly 60 per cent of the total - more than everything the fuel produces. It is why efficiency and alternative fuels alone cannot decarbonise cement, and why the effort has turned to capture and to alternative binders.

Answer: B

Correct answer: 1,450 degrees Celsius

The material reaches about 1,450 degrees in the burning zone, at which a portion of the charge melts and the liquid phase lets lime and silica combine into alite. The gas and flame are hotter still, around 2,000 degrees, which is what makes the refractory and cooling arrangements so demanding.