By-product Utilisation Questions and Answers

Answer: C

Correct answer: It raises the calorific value per kilogram

This is the false statement. Compaction changes no chemistry, so the energy in a kilogram of material is what it always was - and marginally less overall, once the press's own energy is counted.

The other three are true: briquetting raises the bulk density several fold, which is what makes distant transport economic, and the dense uniform blocks are far easier to store, handle and stoke than loose husk. The gain is in volume and handling, never in energy content.

Answer: D

Correct answer: Burning the husk at 1000 degrees

Burning at 1000 degrees crystallises the silica, and crystalline silica cannot react with the calcium hydroxide released during cement hydration. The ash becomes an inert filler worth very little.

The other three all raise the value: controlled combustion below about 700 degrees keeps the silica amorphous, grinding to a high fineness increases the surface area available for that reaction, and keeping the silica amorphous is the property on which everything else depends.

Answer: A

Correct answer: Its high calorific value as a solid fuel

Molasses is a syrup, not a solid fuel, and burning it would be an absurd use of a fermentable sugar stream worth far more.

The other three are the genuine reasons for its dominance: its sugars are already fermentable so no saccharification or pretreatment is needed, that absence of pretreatment removes most of the cost that burdens other feedstocks, and it arises concentrated at sugar factories so collection costs nothing.

Answer: B

Correct answer: Unsound: drying would cost more than it yields

At 85 per cent moisture, every kilogram of wet peel holds 850 grams of water, and evaporating it takes about 2.3 MJ per kilogram - roughly 2 MJ - against perhaps 2.5 MJ released by burning the 150 grams of dry matter left. Once losses are counted the balance is negative.

Scale does not change the arithmetic, and a solar dryer only moves the energy cost to land and time rather than removing it. Anaerobic digestion works in water and needs no drying at all, which is the route such a residue should take.

Answer: C

Correct answer: Free fatty acids rise several per cent a day

Milling ruptures the bran cells and brings lipase into contact with the oil, and the enzyme hydrolyses it into free fatty acids at several per cent a day. Once past a few per cent the oil is unfit for edible use.

That is why bran must be stabilised within hours of milling. The other options are all wrong: bran carries 15 to 20 per cent oil, silica belongs to the husk rather than the bran, and bran is anything but stable - its instability is its defining commercial problem.

Answer: D

Correct answer: Availability scattered thinly across many farms

Scattered availability is what defeats most by-product schemes. Collection and transport dominate the cost of using a residue, and gathering low-density material from many separate sources often costs more than the material is worth.

The other three all help: a residue arising concentrated at a single mill costs almost nothing to collect, a high content of one valuable constituent gives the process something worth recovering, and arriving already dry avoids the drying energy that rules out combustion for wet residues.

Answer: A

Correct answer: It is protein extracted from oilseed cake

This is the false statement. Single cell protein is not extracted from anything - it is microbial biomass, grown deliberately, and the cells themselves are the product.

The other three are true: it is grown on a waste substrate such as molasses or whey, which is what makes it a by-product route; and its nucleic acid content limits the proportion that can safely be included in a diet, since the uric acid produced from it burdens the animal. Protein from oilseed cake is a quite different thing.

Answer: B

Correct answer: Rice husk

Rice husk is roughly 18 per cent silica and almost entirely indigestible. It supplies essentially no nutrition, and the abrasive silica damages the animal's digestive tract, so it has no place in a ration at all.

The other three are established feeds: groundnut cake is a protein concentrate at 40 per cent or more, wheat bran supplies fibre and protein, and defatted rice bran retains useful protein and fibre once the oil has been extracted. Husk is the one whose principal constituent has no feed value whatever.

Answer: C

Correct answer: Partial oxidation gives a producer gas

Gasification supplies less oxygen than complete combustion requires, so the carbon is only partly oxidised and the products are combustible - carbon monoxide, hydrogen and some methane, diluted by nitrogen from the air.

It is a thermochemical process, not a biological one, and it requires a dry feed, since water in the material must be evaporated at the expense of the reaction's heat. The gas is a low-calorific mixture rather than pure methane; producing methane is the business of anaerobic digestion.

Answer: D

Correct answer: Processing cost may absorb the extra value

The margin is what matters, not the product price. A route yielding a product worth 2100 per tonne but costing 1900 to run returns 200, while a simple route yielding 780 at a cost of 380 returns 400 - twice as much from a far less demanding process.

Sophisticated conversions carry heavy capital and operating costs, and on a modest residue stream those costs frequently swallow the additional value. High-value products are not inherently hard to sell, and simple routes do not yield more product; it is the cost that decides.