Fuels and Combustion Questions and Answers

Answer: C

Carbon, hydrogen, nitrogen, sulphur and oxygen are found by chemical means, oxygen usually by difference. It is what a combustion calculation and Dulong's formula require, whereas the proximate analysis guides the choice of firing equipment.

Answer: D

Correct answer: carbon content

The coalification series - peat, lignite, sub-bituminous, bituminous, anthracite - is a progressive loss of moisture and volatile matter and a corresponding enrichment in fixed carbon. Carbon content therefore rises monotonically along the series, and it is the property rank is defined by. Moisture and volatile matter fall along the same series, and ash content depends on the deposit rather than on rank.

Answer: A

The particles agglomerate as the volatiles are driven off, which blocks the air passages of a grate but is exactly what coke making requires. A coking coal is a caking coal whose residue is hard and strong enough for the blast furnace.

Answer: B

It burns to give heat, provides carbon monoxide to reduce the oxide, and must hold the column of ore open so the blast can pass. No other material combines all three functions, which is why coke has not been displaced.

Answer: C

Correct answer: metallurgical coke

High temperature carbonisation at about 1000 °C drives off almost all the volatile matter and leaves a hard, strong, porous coke able to bear the burden in a blast furnace - that strength is what the high temperature buys. Low temperature carbonisation at around 700 °C is the process that gives a soft domestic coke and a tar richer in the lighter oils.

Answer: D

Correct answer: boiling ranges

A fractionating column separates crude oil by boiling point: the vapour rises, each fraction condenses on the tray where the temperature matches its boiling range, and the cuts are drawn off from gas at the top through naphtha, kerosene and gas oil to residue at the bottom. Density and octane number correlate loosely with the cut, but they are consequences of the separation and not the property it works on.

Answer: A

Correct answer: breaking large molecules into smaller ones

Cracking breaks the carbon-carbon bonds of heavy, high boiling molecules to give smaller ones in the petrol and diesel range. It exists because the demand for light products far exceeds the proportion the crude naturally contains, so a refinery must convert the residue rather than merely separate it. Joining small molecules is polymerisation or alkylation; rearranging chains into rings is reforming.

Answer: B

Correct answer: higher octane number at milder conditions

A zeolite catalyst promotes the ionic route to cracking, which favours branched and aromatic products - exactly the high octane species petrol needs - and it does so at lower temperature and pressure than thermal cracking requires. Thermal cracking proceeds by free radicals and gives more straight chain product of lower octane, and it needs the severer conditions to go at all.

Answer: C

Branched paraffins and aromatics resist pre-ignition far better than the straight chains they replace, and hydrogen is a valuable by-product. It changes the structure rather than the molecular size, which distinguishes it from cracking.

Answer: D

Correct answer: premature explosion of part of the charge

As the flame front advances the unburnt charge ahead of it is compressed and heated, and if it autoignites before the flame arrives, that portion explodes spontaneously. The resulting pressure wave strikes the piston and cylinder wall and produces the characteristic metallic knock, together with loss of power and, if sustained, mechanical damage. It is a matter of the fuel igniting too readily, which is what octane number measures.