By-product Utilisation Questions and Answers

Answer: C

Correct answer: Only three

Three pairs are right. Loose rice husk suffers from very low bulk density; rice bran from a rapid rise in free fatty acids once lipase is released at milling; and cottonseed and rapeseed cake from antinutritional factors, gossypol and glucosinolates respectively.

The fourth is wrong. Rice husk leaves the mill dry, at around 8 to 10 per cent moisture, which is one of the few things in its favour as a fuel. High moisture is the difficulty of fruit and vegetable residues, not of husk.

Answer: D

Correct answer: Only three

Three pairs are right. Molasses ferments to ethanol, whey serves as a substrate for single cell protein, and a lignocellulosic hydrolysate ferments to ethanol.

The fourth is wrong on the crucial qualification. Rice husk cannot be fermented without pretreatment: its cellulose is locked within lignin and, in the husk's case, behind a silica layer as well, so neither yeast nor enzymes can reach it. Pretreatment is not an optional refinement here but a precondition.

Answer: A

Correct answer: Bagasse

Bagasse is the fibrous residue from crushed cane, almost entirely lignocellulose with very little protein or digestible energy. It is burnt in the mill's boilers, where it typically supplies all the steam and power the mill needs.

The other three are feeds. Oilseed cake is a protein concentrate, wheat bran a fibre and protein source, and defatted rice bran still carries useful protein and fibre once the oil has been taken. Burning any of them would waste a far higher value.

Answer: B

Correct answer: Anaerobic digestion

Anaerobic digestion is a biological process: a mixed microbial population breaks the organic matter down in the absence of oxygen, producing methane and carbon dioxide over days to weeks at modest temperature.

The other three are thermochemical, using heat to transform the material. Combustion oxidises it fully to carbon dioxide and water; gasification uses a restricted oxygen supply to produce a combustible gas; pyrolysis heats it without oxygen to give char, oil and gas. The biological route is the one that tolerates a wet feedstock.

Answer: C

Correct answer: Arising concentrated at a single mill

A residue arising at one point is far easier to use, because collection is the cost that defeats most by-product schemes. Material that appears continuously at a mill can be handled by conveyor with no gathering cost at all.

The other three all make use harder: a very low bulk density inflates transport cost per tonne, high moisture rules out combustion and forces a wet route, and scattered availability across many fields imposes exactly the collection burden that concentration avoids.

Answer: D

Correct answer: Molasses

Molasses comes from sugar manufacture, not from rice milling. It is the syrup remaining once no further sucrose can economically be crystallised from cane or beet juice.

The other three are all rice milling by-products: husk from hulling, at about 20 per cent of the paddy; bran from whitening, at 8 to 10 per cent and carrying the oil; and broken rice from the grading stage, sold at a discount to whole grains or used for starch, flour and brewing.

Answer: A

Correct answer: Burning at 900 degrees or above

Burning at 900 degrees or above crystallises the silica into cristobalite and tridymite, which are chemically unreactive. The ash then has no pozzolanic activity and is worth little more than an inert filler.

The other three all preserve or add value: controlled combustion below about 700 degrees keeps the silica amorphous, grinding to a high fineness increases the surface area available to react, and it is the amorphous state itself on which the whole value depends.

Answer: B

At about a hundred kilograms a cubic metre a lorry fills up long before it is loaded to weight, so the freight is being paid on air. Compaction raises the density severalfold and brings the delivered cost down accordingly.

Answer: C

Pozzolanic activity depends on the silica remaining amorphous, and above about 700 degrees it converts to crystalline forms that do not react with lime. Combustion control is what separates a valuable ash from a filler.

Answer: D

Correct answer: Anaerobic digestion, the peel being far too wet

At 85 per cent moisture the peel is mostly water, and evaporating it would consume about 2.3 MJ per kilogram - more energy than burning the dry matter left behind would release. Every thermal route fails on the same arithmetic.

Anaerobic digestion works in water and needs no drying at all, so a wet feedstock is an advantage rather than an obstacle. It yields biogas for the plant's own heat and a digestate usable as fertiliser. The rule of thumb is simple: wet residues to digestion, dry residues to combustion.