Fuels and Combustion Questions and Answers

Answer: A

Condensing the steam recovers its latent heat and so gives the larger figure, which is what a bomb calorimeter measures. The net value leaves the water as vapour and is the figure relevant to a boiler whose flue gas leaves hot.

Answer: B

Hydrogen in the fuel becomes water, and unless the flue gas is cooled below its dew point that latent heat leaves up the stack. A hydrogen-rich fuel therefore shows a larger gap between the two values than a carbon-rich one.

Answer: C

One part of oxygen is taken to have already satisfied an eighth of its mass in hydrogen, so only the remaining hydrogen is available to release heat. The correction is why the term is written as hydrogen minus oxygen over eight.

Answer: D

Complete combustion is assured by the excess of compressed oxygen, and the heat is measured from the rise in temperature of a known mass of water. The water formed condenses in the bomb, so the result is the gross value.

Answer: A

The gas burns continuously and its heat is transferred to a measured flow of cooling water, so the value follows from the two flow rates and the temperature rise. The condensate is collected, which allows the net value to be worked out as well.

Answer: B

A fuel too easily ignited is a hazard in storage and one too difficult to ignite is troublesome to start, so a middling figure is wanted. Sulphur is undesirable because its oxides corrode the plant and pollute the atmosphere.

Answer: C

Fixed carbon is obtained by subtracting the other three from a hundred rather than being measured directly. Total carbon, hydrogen, nitrogen, sulphur and oxygen belong to the ultimate analysis, which is a chemical determination.

Answer: D

Correct answer: burn with a long smoky flame

Volatile matter is the gas and tar driven off when coal is heated, and it burns in the space above the fuel bed rather than on it - so a high volatile coal gives a long luminous flame that smokes if the combustion space or the air supply is inadequate. It is easy to ignite, which is the compensating advantage, but the furnace must be tall enough for the flame to complete its burning.

Answer: A

Correct answer: lowers the calorific value and must be handled and disposed of

Ash is incombustible mineral matter, so every kilogram of it in the coal is a kilogram that contributes no heat - the calorific value falls in proportion. It then has to be removed from the grate, carried away and disposed of, and it fouls heat transfer surfaces on the way. Both the loss of heating value and the handling cost count against a high ash coal.

Answer: B

Correct answer: absorbs heat in evaporating and leaves as vapour

The water in the coal must be evaporated before the coal can burn, and the latent heat that takes is supplied by the fuel itself and then carried out of the stack in the flue gas as vapour. It is a double loss - heat consumed and heat exported - and it is why the net calorific value falls as moisture rises, and why a wet coal both reduces boiler output and slows ignition.