Incineration and Energy Recovery Questions and Answers

Answer: A

The match is a-iii, b-iv, c-i, d-ii.

List-II gives typical energy contents of solid waste components, in kJ per kg:

• Plastics: about 32,500, the highest, since they are made from petroleum.
• Paper: about 16,500.
• Food waste: about 4,500, low because of its high moisture content.
• Wood: about 18,500.

Plastics, with the highest value, must take 32500 (iii), and only option A does that. The other codes give plastics the low values of food or paper.

Answer: B

The component with the highest heating value is plastics.

Plastics are made from petroleum and are mostly carbon and hydrogen, so their energy content is high, about 30,000 to 40,000 kJ/kg on an as-discarded basis, close to that of fuel oil.

Paper and wood are cellulose-based and partly oxidised already, giving about 15,000 to 19,000 kJ/kg. Food waste is very wet and gives only about 4,000 to 5,000 kJ/kg. This is why plastics raise the calorific value of refuse-derived fuel.

Answer: D

The step that does not belong is cooling of MSW at −80°C.

Refuse-derived fuel is made by sorting and processing the combustible part of municipal solid waste. The usual steps are removal of inerts, magnetic separation of ferrous metals, shredding to a uniform size, drying, and densification by pelletising or briquetting.

Freezing waste to −80°C plays no part; it would cost a great deal of energy and add nothing to a fuel product.

Answer: C

The process is pyrolysis.

Pyrolysis is thermal decomposition in the absence of oxygen. Heating waste plastics to about 300-400°C over a catalyst in an inert atmosphere cracks the long polymer chains into smaller molecules that can be recovered as fuel oil, gas and char.

Gasification uses a limited supply of oxygen or steam at higher temperatures to make syngas. Biodegradation is breakdown by microorganisms, and co-processing is burning waste as a substitute fuel and raw material in cement kilns.

Answer: B

The green fuel is Refuse Derived Fuel [RDF].

RDF is made from the combustible fraction of municipal solid waste, such as plastics, paper, textiles and wood, after removing inerts, metals and wet organics. It is shredded and dried, sometimes pelletised, and burnt in cement kilns, power plants or waste-to-energy units. It reduces the waste going to landfills and provides energy, which is why it is presented as a solution to both problems.

'Toxic', 'hazardous' and 'pharma' waste-derived fuels are not recognised fuel categories, and burning toxic or pharmaceutical waste is handled under hazardous waste rules, not promoted as green fuel from municipal waste.

Answer: A

The keyed range for the heating value of municipal solid wastes is 9,300 to 12,800 kJ/kg. This is the typical range quoted in waste management texts for mixed municipal waste that is rich in paper, plastics and other combustibles, as in the United States. The ranges of 1,000-2,500, 1,780-2,830 and 530-780 kJ/kg are far lower than any typical MSW and would not sustain combustion. Indian waste with a high moisture and inert content has a lower value, but among the options only the first fits the heating value of MSW.

Answer: D

Pelletisation (refuse derived fuel), biomethanation (biogas) and pyrolysis all recover energy from solid waste. Composting converts organic waste into manure, which recovers nutrients and not energy, so it is not an energy recovery method. The answer is composting.

Answer: D

Pyrolysis involves heating in the absence of air.

Pyrolysis is the thermal decomposition of waste at high temperature without oxygen, which breaks it down into gas, oil and char instead of burning it.

Autoclaving is steam sterilisation. Heating in the presence of air is combustion or incineration, and heating with acetic acid has nothing to do with the process.

Answer: C

The minimum stack height of an incinerator should be 30 m, option (C). The pollution control rules for hazardous and biomedical waste incinerators require a stack of at least 30 m above the ground so that the flue gases disperse high in the air. Heights of 10 m (A) and 15 m (B) are too low for adequate dispersal, and a minimum of 60 m (D) is more than the rule requires. A tall stack lets the gases rise and mix with the air, which lowers the concentration of pollutants at ground level near the plant.