Combustion Calculations 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 necessity. Fuel and air do not mix perfectly in the time available, so with exactly stoichiometric air some fuel would find no oxygen and leave unburnt while some oxygen left unused - wasting fuel and producing carbon monoxide and smoke. Excess air buys the margin, and the optimum is the least that keeps the carbon monoxide low, since every extra kilogram is heated and thrown up the stack.

Answer: D

The first two are correct and between them define the optimum. The third is inverted, since the extra nitrogen and oxygen absorb heat and the flame temperature falls. Around fifteen per cent excess is typical for gas firing.

Answer: A

Correct answer: counts the latent heat of the water formed

The gross value assumes the water vapour produced is condensed and its latent heat recovered; the net value assumes it leaves as vapour. The difference is substantial for a hydrogen-rich fuel - several per cent for coal and around ten for natural gas - so an efficiency is meaningless unless the basis is stated. Conventional boilers exhaust above the dew point and are quoted on the net value; condensing boilers recover that heat and can exceed 100 per cent on it.

Answer: B

The difference is the latent heat of the water formed from the hydrogen in the fuel, which the net value does not count as recovered. A fuel with no hydrogen would have equal values, which is very nearly the case for carbon monoxide.

Answer: C

Correct answer: the volatile matter content

Volatile matter belongs to the *proximate* analysis, which reports moisture, volatiles, fixed carbon and ash - a practical description of how the coal behaves. The ultimate analysis reports the elements: carbon, hydrogen, nitrogen, sulphur, oxygen and ash. The two are complementary and answer different questions.

Answer: D

Correct answer: the ultimate analysis giving the elements

Combustion calculations balance atoms, so they need the elemental composition: carbon and hydrogen determine the oxygen required and the products formed, sulphur adds its own demand, and the fuel-bound oxygen reduces what must be supplied. The proximate analysis cannot serve, since volatile matter and fixed carbon are not elements and their oxygen requirements are unknown.

Answer: A

Correct answer: 44 kilograms

Carbon has a relative mass of 12 and carbon dioxide 44, so \[ 12 \times \frac{44}{12} = 44\ \mathrm{kg}. \] Every kilogram of carbon becomes 3.67 kilograms of carbon dioxide, and the added mass is oxygen from the air - which is why fuel combustion produces so much more carbon dioxide by weight than the fuel itself weighs.

Answer: B

Everything follows from the fuel analysis, and the air cannot be found before the theoretical requirement is known. The flue gas is the last step because it needs both the fuel and the air figures. A basis of a hundred units keeps the arithmetic tidy.

Answer: C

The fuel brings some of its own oxygen, so less need come from the air. The correction is subtracted before the air requirement is computed and is significant for oxygenated fuels such as wood. Its effect on the air requirement can be several per cent.

Answer: D

Correct answer: carbon monoxide in the flue gas

Carbon monoxide is the signature of *incomplete* combustion - carbon that found too little oxygen, or too little time or mixing to reach it. Complete combustion of a hydrocarbon in air gives carbon dioxide and water, together with the nitrogen that passed through unchanged and any excess oxygen. Carbon monoxide in the flue gas is therefore both a loss of unreleased heat and the signal that the air setting is wrong.