By-product Utilisation Questions and Answers

Answer: C

Correct answer: 180 kg

Two stages again:

\[ \text{ash} = 1000 \times 0.20 = 200\ \mathrm{kg} \]

\[ \text{silica} = 200 \times 0.90 = 180\ \mathrm{kg} \]

The ash fraction of about 20 per cent is remarkably high - most biomass leaves a few per cent - and it is entirely due to the silica the plant deposits in the husk. Whether that 180 kg is worth anything depends on the combustion temperature, since only amorphous silica has pozzolanic value.

Answer: D

The volatile solids are \( 8000 \times 0.12 = 960 \) kg, and \( 0.96 \times 55 = 52.8 \) cubic metres of gas. Yield per tonne of volatile solids rather than per tonne of wet waste is the meaningful basis for comparison.

Answer: A

Correct answer: 270 kg

Multiply the substrate by the yield coefficient:

\[ 600 \times 0.45 = 270\ \mathrm{kg\ dry\ biomass} \]

A yield of 0.45 means rather less than half the sugar's mass ends up as cells; the rest leaves as carbon dioxide and as the energy the organism uses to grow. Single cell protein turns a low-value sugar stream - molasses, whey, or a hydrolysate - into a protein feed, and the yield coefficient is what determines whether that conversion pays.

Answer: B

Correct answer: 2.8 tonnes of briquettes

Match the energy rather than the mass:

\[ \frac{42}{15} = 2.8 \]

So 2.8 tonnes of briquettes deliver the energy of one tonne of oil. The ratio is what any fuel substitution must be judged on, and it has consequences beyond the purchase price: nearly three times the mass must be transported, stored and handled, and the ash from it must be disposed of, none of which arises with oil.

Answer: C

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second is the reason for the first. Rice husk ash is pozzolanic only while its silica is amorphous, since amorphous silica reacts with the calcium hydroxide released during cement hydration.

Above about 700 degrees the silica crystallises into cristobalite and tridymite, which are unreactive. The ash then becomes an inert filler worth very little. Controlled combustion - holding the temperature down and the residence time short - is therefore what separates a valuable product from a waste.

Answer: D

The second is correct and it shows why the first is wrong: the energy in a kilogram is unchanged and only the energy in a cubic metre rises. That distinction is what makes transport rather than combustion the real benefit.

Answer: A

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second explains the first. Anaerobic digestion takes place in water and is indifferent to a wet feedstock; combustion requires the water to be evaporated first, and the latent heat of vaporisation is about 2.3 MJ per kilogram.

For a residue at 85 per cent moisture the drying energy can exceed the energy released by burning what remains, so the process consumes more than it delivers. That crossover is the single most useful rule for choosing a route: wet residues to digestion, dry residues to combustion.

Answer: B

Correct answer: Both Statement I and Statement II are correct

Both statements are correct and the second gives the mechanism. Milling ruptures the bran cells and brings the oil into contact with lipase that was previously kept apart from it.

The enzyme hydrolyses the oil into free fatty acids at several per cent a day, and once the free fatty acid content passes a few per cent the oil is no longer fit for edible use. Stabilisation - usually extrusion or steam heating to denature the lipase - must therefore follow milling within hours, not days.

Answer: C

The second is correct and it displaces the first: many technically sound routes fail on the cost of gathering scattered material. Residues arising concentrated at a mill are far more attractive for exactly this reason.

Answer: D

Correct answer: Both Statement I and Statement II are correct

Both statements are correct. Gossypol is a natural phenolic compound in cottonseed that is toxic to monogastric animals such as pigs and poultry, causing reduced growth and, at higher intakes, organ damage.

Ruminants tolerate it because rumen microorganisms bind it, which is why untreated cake has traditionally gone to cattle. Appropriate heat treatment during processing binds the free gossypol to protein, and solvent extraction removes it, which is what allows the cake to be fed more widely.