Fuels and Combustion: Types, Calorific Value and Flame

The correct answer is B, spontaneous. In spontaneous combustion a substance catches fire by itself, without any external flame being brought near it. This happens when the slow oxidation of the substance in air releases more heat than the substance needs to reach its own ignition temperature, so the heat accumulates until the material bursts into flame. White phosphorus is the standard laboratory example, and heaps of coal dust, oily rags and damp haystacks catch fire in the same way. A, rapid combustion, is wrong because there the fuel burns quickly only after it is ignited by an outside source, as cooking gas does when a match is brought to it. C, complete combustion, describes the quality of burning, not how it starts: it means the carbon of the fuel is fully converted to carbon dioxide because oxygen is plentiful. D, explosive combustion, is wrong because it describes a reaction so fast that gas, heat, light and sound are produced in an instant, as in a firecracker.

The correct answer is A, Ghee. Calorific value is the heat released on completely burning one gram of a substance, and among the four foods listed ghee has the highest, because it is almost entirely fat. Fats yield roughly nine kilocalories per gram, more than twice what carbohydrates and proteins give, since the carbon and hydrogen in fat are far less oxidised to begin with and so have more energy to release on oxidation. B, maize, and C, rice, are wrong because both are mainly starch, a carbohydrate, which gives about four kilocalories per gram. D, meat, is wrong because it is largely protein and water; protein also gives about four kilocalories per gram, and the water in meat contributes nothing at all. The same reasoning explains why oils and butter are called energy-dense foods and why hydrogen, the lightest and least oxidised fuel, tops the calorific value table for fuels.

The correct answer is C, the outermost blue non-luminous zone. In this zone the supply of air is unrestricted, so combustion is complete and all the carbon of the fuel is converted to carbon dioxide, releasing the maximum heat. It gives little light and is faintly blue, which is why it is called the non-luminous zone, and a goldsmith directs exactly this part of the flame on gold or silver through a blowpipe to melt it. A is wrong because the innermost dark zone around the wick holds unburnt vaporised wax and receives the least oxygen, making it the coldest part of the flame. B is wrong because the middle yellow zone shows only partial combustion, where glowing unburnt carbon particles give the flame its light but not its greatest heat. D is wrong because the three zones differ sharply in both temperature and composition, and the whole point of studying flame structure is that difference.

The correct answer is A, Hydrogen. Its calorific value is about 150 kilojoules per gram, the highest of any known fuel, roughly three times that of petrol, and its only combustion product is water vapour, which makes it a clean fuel as well. For these reasons hydrogen is called the fuel of the future, although it is costly to produce and hard to store and transport safely because it is a very light gas that must be kept at high pressure or very low temperature. B is wrong because liquefied petroleum gas, a mixture of butane and propane, gives about 55 kilojoules per gram. C is wrong because petrol gives about 45 kilojoules per gram, the same order as kerosene and diesel. D is wrong because coal gives only about 25 to 33 kilojoules per gram, depending on its grade, and also leaves ash and smoke behind.

The correct answer is B, butane and propane. Liquefied petroleum gas is obtained during the refining of petroleum and from natural gas processing, and it is chiefly a mixture of butane and propane, which liquefy under moderate pressure at ordinary temperature and so can be stored in a cylinder. Because the gas has no smell of its own, a strong-smelling substance called ethyl mercaptan is added so that a leak can be detected. A is wrong because methane with some ethane is the composition of natural gas, which is supplied as compressed natural gas or through pipelines, not of cylinder gas. C is wrong because a mixture of carbon monoxide and hydrogen is water gas, an industrial fuel, and coal gas has a similar origin. D is wrong because ethylene and acetylene are industrial gases, acetylene being used with oxygen for welding because of its very hot flame.

The correct answer is D, Anthracite. Coal forms by the slow burial and compression of plant matter, and the longer and deeper the process goes the more carbon is left behind as moisture and volatile matter are driven off. Anthracite is the final and hardest stage, with the highest carbon content and the highest calorific value, and it burns with a short blue flame and almost no smoke, which makes it the best quality coal. A is wrong because peat is the first stage, barely coal at all, with a very high moisture content and the lowest carbon. B is wrong because lignite or brown coal is the next stage, soft and still moist. C is wrong because bituminous coal, the commonest variety and the one used to make coke for metallurgy, lies between lignite and anthracite. Remember the rising order: peat, lignite, bituminous, anthracite.

The correct answer is B, oil floats on water and the fire spreads further. Petrol and kerosene are lighter than water and do not mix with it, so water poured on an oil fire sinks below the burning layer and carries it outwards, enlarging the fire instead of putting it out. Such fires are smothered with carbon dioxide, foam or dry chemical powder, which cut off the supply of oxygen. A is wrong because ignition temperature is a fixed property of a substance and no extinguisher changes it; water works on ordinary fires by lowering the temperature of the fuel below that fixed value. C is wrong because water does not react chemically with petroleum fuels at all; that behaviour belongs to metals such as sodium and to burning magnesium. D is wrong because water does absorb a great deal of heat, having a high specific heat and a high latent heat of vaporisation, which is precisely why it works so well on wood and paper fires.

The correct answer is C, it is heavier than air, covers the fire and cuts off the oxygen supply. Carbon dioxide is about one and a half times as dense as air, so when released it forms a blanket over the burning material and separates it from atmospheric oxygen, removing one side of the fire triangle. It is a poor conductor of electricity and leaves no residue, so it does not damage electrical equipment or electrocute the user, unlike water. Its sudden expansion from the cylinder also cools the fuel, which helps. A is wrong because carbon dioxide is denser than air, not lighter; a lighter gas would drift away and be useless. B is wrong because carbon dioxide does not react with ordinary fuels; it works physically, by exclusion of oxygen. D is wrong because ignition temperature is a fixed property of a substance and cannot be raised by any extinguisher.

The correct answer is C, ignition temperature. It is the lowest temperature to which a combustible substance must be heated before it begins to burn, and it is a fixed property of that substance; substances with a very low ignition temperature, such as petrol, alcohol and liquefied petroleum gas, are called inflammable substances and are stored with care. A is wrong because the boiling point is the temperature at which a liquid turns to vapour at a given pressure and has nothing to do with catching fire. B, flash point, is the closest distractor and is a related but different idea: it is the lowest temperature at which the vapour above a liquid fuel gives a momentary flash when a flame is brought near, without the liquid itself continuing to burn. D is wrong because the calorific value is the heat released on burning a unit mass of fuel, a measure of energy and not of temperature.

The correct answer is B, carbon monoxide. When the oxygen supply is short, the carbon of the fuel is only partly oxidised and carbon monoxide is formed instead of carbon dioxide. Carbon monoxide is colourless and odourless, so it gives no warning, and it binds to the haemoglobin of blood far more strongly than oxygen does, forming carboxyhaemoglobin and starving the tissues of oxygen; this is what kills people who sleep in a closed room with a coal fire or a burning angithi. A is wrong because carbon dioxide is the product of complete combustion; it is a greenhouse gas and can suffocate in very large amounts, but it is not the poison meant here. C is wrong because sulphur dioxide comes from the sulphur in coal and petroleum and is a cause of acid rain. D is wrong because the oxides of nitrogen form at the high temperatures of engines and also cause acid rain and smog.

The correct answer is A, coal tar and coal gas. Destructive distillation means heating coal strongly in the absence of air, so that it decomposes instead of burning. The solid left behind is coke, which is almost pure carbon and is used as a reducing agent in the extraction of iron and other metals and as a smokeless fuel. The liquid that condenses is coal tar, a black mixture of about two hundred substances used to make dyes, drugs, explosives, plastics and naphthalene. The gas that escapes is coal gas, once piped for street lighting. B is wrong because petrol and diesel come from the fractional distillation of petroleum, not of coal. C is wrong because producer gas is made by passing air over hot coke and biogas by the anaerobic digestion of dung and waste. D is wrong because bitumen and paraffin wax are the heavy fractions of petroleum refining.

The correct answer is C, it has a very low ignition temperature. A good fuel should have a moderate ignition temperature, not a very low one. A fuel that catches fire at a very low temperature is an inflammable substance and is dangerous to store, carry and handle, because a small spark or even a hot day can set it alight; petrol is useful but precisely for this reason it needs sealed containers and strict precautions. A is wrong as the answer because a high calorific value is certainly desirable: more heat is obtained from the same mass of fuel. B is wrong as the answer because a low ash content is desirable too, since ash is useless residue that has to be removed and that lowers the effective heat output. D is wrong as the answer because cheapness and ready availability are standard requirements of a good fuel, along with ease of storage and transport and the absence of harmful combustion products.