Energy Conservation in Industry Questions and Answers

Answer: A

Correct answer: hot metal produced

The coke rate is kilograms of coke per tonne of hot metal, so it measures reductant consumed against useful output. It is the blast furnace’s principal efficiency indicator, and its reduction - from around a tonne per tonne of iron historically to the region of 300 kg with coal injection - is the main story of the furnace’s development.

Answer: B

Correct answer: replaces part of the coke charged at the top

Pulverised coal injected at the tuyeres supplies carbon for reduction at a fraction of the cost of coke, so less coke need be charged. Coke cannot be replaced entirely, because it also has to bear the burden and keep the bed permeable, which coal cannot do - hence a replacement ratio rather than a substitution.

Answer: C

A high top pressure furnace lets its gas down through a turbine rather than a valve, generating electricity from energy otherwise thrown away. The gas's calorific value is used separately in the stoves and the boilers.

Answer: D

Correct answer: avoiding the reheating of cooled ingots

In the older route the steel was cast into ingots, allowed to cool, and then reheated to rolling temperature in a soaking pit, so the heat put into the melt was thrown away and bought again. Continuous casting produces semi-finished shapes directly and passes them to the mill still hot, saving that whole cycle.

Answer: A

Every degree the stock retains from casting is a degree the furnace need not supply, so scheduling the two processes together is worth a large fraction of the reheating fuel. It demands the mill and the caster be coordinated.

Answer: B

Correct answer: melts scrap rather than reducing ore

The blast furnace route must reduce iron oxide to iron, which is chemically expensive, and it must make the coke to do it. The arc furnace starts from metal that has already been reduced and needs only to melt it, so it consumes a small fraction of the energy per tonne. The saving is chemical rather than thermal.

Answer: C

Correct answer: electrolysis of its oxide

Aluminium is too reactive to be reduced by carbon, so it is won by passing a very large current through alumina dissolved in molten cryolite. Electrolysis is inherently costly in energy, and at around 13 to 15 MWh per tonne the smelter is generally sited where electricity is cheap rather than where the ore is.

Answer: D

Correct answer: raises the cell voltage sharply

An anode effect occurs when the alumina in the bath runs low and a gas film forms under the anode, and the cell voltage jumps from about four volts to thirty or more. The current continues to flow, so the power drawn multiplies while no metal is produced - and perfluorocarbons are emitted besides.

Answer: A

Correct answer: melting the metal

Recycling skips the electrolysis entirely: the metal has already been reduced and needs only to be remelted, which takes about five per cent of the energy of primary production. This is why aluminium scrap has so high a value and why collection rates for it are among the highest of any material.

Answer: B

Fibre breaks and static build up if the air is too dry, so the mill must be kept humid whatever the weather, and the plant may take a fifth of the mill's electricity. Controlling it to the actual requirement rather than a fixed setting is the saving.