By-product Utilisation Questions and Answers

Answer: C

Roughly fifty-five to sixty-five per cent of the biogas is methane, and the digestate remains a useful fertiliser. Wet residues suit digestion far better than combustion, since drying them would consume more energy than they contain.

Answer: D

Yeasts, bacteria and fungi grown on molasses, whey or lignocellulosic hydrolysate multiply rapidly and yield a high protein biomass. Nucleic acid content and consumer acceptance are the standing obstacles to its wider use.

Answer: A

Correct answer: Release fermentable sugars

Lignocellulose is cellulose and hemicellulose locked inside a lignin matrix. Enzymatic hydrolysis uses cellulases to break the cellulose chains into glucose and other simple sugars that yeast can then ferment.

The step cannot work on untreated material, because the lignin physically bars the enzymes from reaching the cellulose - which is why a pretreatment must come first. This two-stage requirement is precisely what makes ethanol from lignocellulose so much more demanding than ethanol from molasses, where the sugars are already free.

Answer: B

The sugars are already fermentable, so molasses needs none of the pretreatment a lignocellulosic feedstock demands. That simplicity is precisely why molasses remains the dominant substrate for fuel alcohol in cane growing countries.

Answer: C

Correct answer: A dietary fibre and feed source

Wheat bran is the outer layers removed in milling, and it is valued for its fibre, its protein and its mineral content. It goes to animal feed in bulk and, increasingly, into human food where dietary fibre is wanted.

It carries only a small amount of oil, so there is no extraction industry around it as there is for rice bran. It has no significant silica, and its calorific value would make it a poor fuel - and a wasteful one, since its feed value is far higher than its energy value.

Answer: D

Correct answer: Maize oil

Maize germ is unusually large, around 10 to 12 per cent of the kernel, and roughly a third oil. Degermination in dry milling recovers it as an intact fraction, and it is pressed or solvent-extracted for maize oil, leaving a protein-rich cake for feed.

Removing the germ serves the main product as well: its oil would otherwise turn rancid and ruin the shelf life of the grits and meal. The starch stays with the endosperm, and neither pectin nor silica is present in any useful quantity.

Answer: A

Aerobic microbial action stabilises the organic matter and destroys pathogens as the pile heats. The product is bulky and low in nutrients compared with mineral fertiliser, so its value lies in soil structure rather than in nutrition.

Answer: B

Correct answer: Its very low bulk density

Loose rice husk has a bulk density around 100 kg/m\(^3\), so a lorry fills its volume long before it approaches its weight limit. The transport cost per tonne is consequently high, and it rises with distance until it exceeds the value of the fuel being carried.

The calorific value per kilogram is perfectly respectable at 13 to 16 MJ/kg, and the husk is dry as it leaves the mill. It is the value per cubic metre that fails, which is exactly the problem briquetting solves by raising the density several-fold.

Answer: C

Correct answer: Product value exceeds collection and processing cost

A by-product route is worth pursuing when it pays, and that means the value of the product must exceed the cost of gathering the residue and converting it. Technical feasibility is necessary but nowhere near sufficient - a great many routes work in the laboratory and lose money at scale.

Collection cost is what defeats most schemes, because residues are bulky, low in density and often scattered. This is why residues arising concentrated at a mill are far more attractive than the same material spread across many fields.

Answer: D

Gossypol in cottonseed and glucosinolates in rapeseed both respond to appropriate processing, and untreated cake can be toxic to monogastric animals. Over-treatment damages the protein, so the step is a balance rather than simply heating hard.