By-product Utilisation Questions and Answers

Answer: C

Correct answer: Molasses, its sugars needing no pretreatment

The table shows why. Molasses has the lowest sugar content of the first three, yet it needs no pretreatment at all and carries the lowest cost. Rice straw has more fermentable material but demands severe pretreatment and enzymes before any of it can be reached.

Sugar content is not what governs the choice; the cost of getting at the sugar is. That is why molasses dominates wherever cane is grown, despite the far greater tonnage of straw available, and why lignocellulosic ethanol has been slow to become competitive.

Answer: D

Correct answer: Burn below about 700 degrees

Reading down the table, pozzolanic activity falls from high at 500 degrees to negligible at 1100, exactly as the silica passes from amorphous to crystalline. The recommendation follows directly: keep the combustion below about 700 degrees.

Crystalline silica cannot react with the calcium hydroxide released during cement hydration, so the ash becomes an inert filler. Burning hotter does not purify the silica or increase its yield; it destroys the one property that gives the ash its value.

Answer: A

Correct answer: Briquetting, at a margin of 400 per tonne

The margin column settles it: briquetting returns 400 per tonne against 240 for digestion, 200 for ethanol and 180 for selling loose.

The ethanol route is the instructive case. Its product is worth 2100 per tonne, by far the highest figure in the table, yet its processing cost of 1900 absorbs almost all of that. Product value alone is a poor guide, and the margin is what the decision must rest on - which is why simple routes so often beat sophisticated ones on a modest stream.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason is precisely why the practice exists. Rice husk ash is pozzolanic only while its silica is amorphous, because amorphous silica reacts with the calcium hydroxide released as cement hydrates.

Above about 700 degrees the silica crystallises to cristobalite and tridymite, which are unreactive, and the ash becomes an inert filler. Controlled combustion - limiting both temperature and residence time - is therefore what separates a saleable pozzolan from a waste to be disposed of.

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the second explains the first. At about 100 kg/m\(^3\) loose husk fills a lorry's volume long before it approaches the weight limit, so the vehicle carries a fraction of the tonnage it could and the cost per tonne is correspondingly high.

Briquetting raises the density to around 650 kg/m\(^3\), so the same lorry carries several times the mass and the cost per tonne falls in proportion. The calorific value per kilogram is barely altered; the whole gain is in volume.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason gives the arithmetic. Evaporating water takes about 2.3 MJ per kilogram, so a residue at 85 per cent moisture requires roughly 2 MJ of drying energy for every kilogram of wet material - against perhaps 2.5 MJ released by burning the dry matter it contains.

The margin is negative or negligible once losses are counted, so combustion cannot pay. Anaerobic digestion takes place in water and needs no drying at all, which is why wet residues go that way.

Answer: A

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason is the mechanism. Milling ruptures the bran cells and brings lipase into contact with the oil it was previously separated from.

The enzyme hydrolyses the oil into free fatty acids at several per cent a day, and once that content passes a few per cent the oil is no longer fit for edible use and its value collapses. Stabilisation by extrusion or steam heating denatures the lipase, and it must follow milling within hours rather than days.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the preference. Residues are bulky and low in density, so gathering them from a scattered source and moving them to a processing point often costs more than the material is worth.

A residue arising continuously at a mill avoids that cost almost entirely: it can be handled by conveyor at the point where it appears. This is why rice husk and bagasse have well established outlets while straw, far more abundant, is still largely burnt in the field.

Answer: C

Correct answer: A is false but R is true

The assertion is false. Briquetting compacts the material without altering its chemistry, so the energy available from a given mass is essentially unchanged - and slightly reduced overall, once the energy consumed by the press is counted.

The reason is true and identifies what actually improves: the bulk density rises several fold, so the energy per cubic metre rises with it. The gain is entirely in transport, storage and handling, which is precisely the obstacle that made distant use uneconomic in the first place.

Answer: D

Correct answer: Both A and R are true, and R is the correct explanation of A

Both statements are true and the reason explains the restriction. Free gossypol is toxic to monogastric animals, causing depressed growth and, at higher intakes, damage to the liver and reproductive organs.

Ruminants tolerate it because rumen microorganisms bind it before absorption, which is why untreated cake has traditionally gone to cattle. For poultry the cake must first be heat treated, binding the gossypol to protein, or solvent extracted to remove it - and that processing is what the price difference reflects.