By-product Utilisation Questions and Answers

Answer: A

Correct answer: A high proportion of silica

Rice husk is roughly 15 to 20 per cent silica, drawn up from the soil by the plant and deposited in the husk. No other common agricultural residue carries anything approaching that.

The silica governs everything the husk is used for. It makes the husk abrasive and hard to handle, it leaves an ash of about 20 per cent by mass where most residues leave a few per cent, and it makes that ash a valuable pozzolan for cement. The husk contains almost no oil, no starch and very little protein, so it is a fuel and a silica source rather than a feed.

Answer: B

Correct answer: A pozzolan in cement and concrete

Rice husk ash is nearly all silica, and when that silica is amorphous it reacts with the calcium hydroxide released as cement hydrates, forming additional binding compounds. That is what a pozzolan does, and it improves strength and durability while replacing part of the cement.

The value depends entirely on the ash being amorphous, which means burning below about 700 degrees. Above that the silica crystallises and the pozzolanic activity is lost, leaving an inert filler. The ash is not a fermentation substrate, a feed or an oil source.

Answer: C

Correct answer: 13 to 16 MJ per kg

Rice husk has a calorific value of roughly 13 to 16 MJ/kg - about a third of fuel oil's 42 MJ/kg, but ample for raising steam. A mill burning its own husk is generally self-sufficient in process heat.

The figure is lower than wood's because of the large mineral fraction: the silica contributes mass but no heat, and it leaves as ash. The value per kilogram is therefore respectable, but the value per cubic metre is poor, which is the real obstacle to using husk away from the mill.

Answer: D

Correct answer: Fifteen to twenty per cent oil

Rice bran carries 15 to 20 per cent oil, which is what makes it worth processing rather than simply feeding. Rice bran oil is edible, stable at high temperature and rich in oryzanol, and the defatted bran that remains is still a useful protein and fibre source for feed.

The oil is also the reason bran must be stabilised within hours of milling: the same milling that releases it releases lipase, which splits the oil into free fatty acids at several per cent a day and quickly renders it unfit for food use.

Answer: A

Once the oil is removed the residue is concentrated in protein, and soybean meal in particular is the backbone of intensive livestock feeding. Some cakes need treatment to remove toxic or antinutritional factors first.

Answer: B

A sugar mill generally burns its own bagasse to raise the steam and power it needs, which makes it close to self-sufficient in energy. Surplus bagasse goes to paper, particleboard and increasingly to cogeneration for export.

Answer: C

Correct answer: Fermentation to ethanol

Molasses is the syrup left when no more sucrose can economically be crystallised, and it still holds around 50 per cent fermentable sugar. Those sugars are already in a form yeast can use, so fermentation needs only dilution and nutrient adjustment rather than any hydrolysis step.

That is why molasses became the classic ethanol feedstock: the substrate arrives ready to ferment, concentrated at the mill, as a by-product of a process running anyway. It is also used in cattle feed and as a fermentation medium for yeast and other products.

Answer: D

Both residues are rich in the pectic substances of the middle lamella, and acid extraction followed by alcohol precipitation recovers them. Pectin's gelling behaviour makes it valuable to the jam and confectionery trades.

Answer: A

Compaction does not change the energy in a kilogram at all, but it transforms the energy in a cubic metre, which is what makes transport and stoking practical. Binderless screw and piston presses are the machines usually employed.

Answer: B

Partial oxidation in a restricted air supply converts the solid to a low calorific value gas that can run an engine or a burner. Tar removal is the practical difficulty that has limited how widely the technology is used.