Energy Conservation in Industry Questions and Answers

Answer: C

Correct answer: 453 kWh/tonne

Split the consumption into a fixed part and a part proportional to output. At full load the fixed part is 20 per cent, so at 60 per cent output \[ E = 0.2 + 0.8(0.6) = 0.68 \text{ of the full-load energy,} \] against 0.6 of the output. The specific figure is therefore \[ \frac{0.68}{0.6} \times 400 = 453\ \mathrm{kWh/t}. \] This is why specific consumption must be normalised for capacity utilisation before it is benchmarked.

Answer: D

Correct answer: 7.5 per cent

The saving is proportional to the cullet added, and the increase is thirty percentage points, or three lots of ten: \[ 3 \times 2.5 = 7.5\ \text{per cent.} \] Note that the question asks for the saving from the change, not the saving relative to no cullet at all, which would be 12.5 per cent at fifty per cent cullet.

Answer: A

Correct answer: 1-Q, 2-P, 3-R

Each sector has its own principal lever. In cement it is the clinker factor, since clinker is what took the kiln fuel. In iron and steel it is the coke rate, coke being both the reductant and the largest energy input. And in pulp and paper it is recovery of the black liquor, which is burned to raise steam and to regenerate the pulping chemicals at the same time.

Answer: B

Correct answer: 1-Q, 2-P, 3-R

Three different reasons for a large energy bill. Aluminium is intensive because it must be won by electrolysis, being too reactive for carbon reduction. A cement kiln is intensive because limestone must be calcined, which is a large endothermic reaction. And paper drying is intensive because water must be evaporated, at 2260 MJ per tonne.

Answer: C

Correct answer: 2, 1, 4, 3

Measure first: establish the specific consumption and benchmark it, since nothing can be judged without a baseline. Then take the housekeeping measures, which cost nothing and can be done at once. Then identify and cost the remaining options, and only then sanction the capital projects that still look justified once the free savings have been taken.

Answer: D

Correct answer: 1 and 2 only are correct

Statements 1 and 2 are correct: specific consumption rises at part load because the fixed loads are spread over less output, and one certificate is one tonne of oil equivalent. Statement 3 is out by a factor of ten - recycled aluminium takes about five per cent of the primary energy, not half, because the electrolysis is skipped entirely.

Answer: A

Correct answer: raising the clinker factor of the product

The clinker factor is the proportion of the cement that is clinker, and clinker is what consumed the kiln fuel, so raising it raises the energy per tonne of cement. The other three all reduce it: extra preheater stages recover more heat from the exit gases, a roller mill grinds more efficiently than a ball mill, and a precalciner moves the calcination out of the kiln into a more efficient vessel.

Answer: B

More water entering the dryers means more evaporation at 628 kilowatt hours a tonne of water, which is exactly what the press section exists to avoid. The other three each reduce the energy per tonne of saleable paper.

Answer: C

Correct answer: what happened to production over the same period

A ten per cent fall in absolute consumption means nothing until it is set against output: if production fell fifteen per cent the plant has become less efficient, not more. Efficiency is the ratio, and a claim based on the numerator alone is not a claim about efficiency at all.

Answer: D

Stock arriving cold rather than hot from the caster must be heated the whole way, which can add a third to the furnace's fuel. It is a scheduling failure rather than a furnace fault, and no burner adjustment will correct it.