Combustion Calculations Questions and Answers

Answer: A

Correct answer: exactly satisfies the combustion stoichiometry

Theoretical air - also called stoichiometric air - is the quantity the balanced equations require, with no more and no less oxygen than the fuel can consume. It is a calculated reference, not what is actually supplied: real furnaces run on more than it, and the difference is the excess air on which every efficiency calculation turns.

Answer: B

Correct answer: 9.52

From \( \mathrm{CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O} \), two moles of oxygen are needed, and air is 21 per cent oxygen: \[ \frac{2}{0.21} = 9.52\ \mathrm{mol\ air}. \] The 4.76 offered is the air per mole of *oxygen*, which is the conversion factor rather than the answer, and 2.00 is the oxygen requirement with the nitrogen forgotten.

Answer: C

Both are true and the reason gives both uses: it lowers the flame temperature substantially and it relates the air supplied to the flue gas produced without any further information. Roughly four moles accompany every mole of oxygen.

Answer: D

Twenty per cent excess air means one point two times the theoretical amount. Using the supplied air as the denominator gives a smaller number and is the most frequent error in these problems. The denominator is always the theoretical requirement.

Answer: A

Two moles above ten is twenty per cent excess. Dividing the two by the twelve supplied gives sixteen point seven per cent, which is the answer to a different question entirely. Reading the definition carefully settles which is wanted.

Answer: B

The theoretical figure is fixed by the fuel composition and does not depend on how completely the fuel actually burns. Carbon monoxide in the flue gas signals incomplete combustion but leaves the excess air definition untouched.

Answer: C

The four appear together in almost every combustion problem, and confusing any pair leads to an answer that is internally consistent but wrong. Each has a precise definition worth memorising. Each appears in almost every furnace problem.

Answer: D

Nitrogen fixes the total air that entered, and the free oxygen remaining is what was not needed, so the ratio of the two gives the excess directly. This is why a single dry gas analysis is enough to check how a furnace is being run.

Answer: A

The three must sum to a hundred on the dry basis, so nitrogen is eighty-five per cent. The nitrogen figure is then used as the tie component to find the air actually supplied. It is the only species that passes through unchanged.

Answer: B

Carbon that reaches only the monoxide releases about a third of the heat it would have given as dioxide, so it is both a loss and a hazard. More air or better mixing is the usual remedy. Its presence also makes the flue gas toxic.