UPSC Prelims 2024 Q46: Science & Technology – Energy & Environment
Consider the following materials:
1. Agricultural residues
2. Corn grain
3. Wastewater treatment sludge
4. Wood mill waste
Which of the above can be used as feedstock for producing Sustainable Aviation Fuel?
- (a) 1 and 2 only
- (b) 3 and 4 only
- (c) 1, 2, 3 and 4
- (d) 1, 3 and 4 only
Answer: (c) 1, 2, 3 and 4
Explanation
Sustainable Aviation Fuel (SAF) — feedstocks
SAF is a drop-in jet fuel made from biological or waste sources. It can cut life-cycle CO₂ emissions by up to about 80%.
| Feedstock | Used for SAF? | Pathway |
|---|---|---|
| 1. Agricultural residues | Yes | Straw and stalks → cellulosic ethanol → alcohol-to-jet |
| 2. Corn grain | Yes | Corn ethanol → alcohol-to-jet (ATJ) |
| 3. Wastewater treatment sludge | Yes | Sludge → hydrothermal liquefaction or gasification → jet fuel |
| 4. Wood mill waste | Yes | Sawdust and bark → gasification + Fischer-Tropsch |
The US Department of Energy lists all four as SAF feedstocks.
India's push:
| Detail | Fact |
|---|---|
| Global Biofuels Alliance | Launched at the G20 summit, 2023 |
| Blending targets | 1% SAF by 2027, 2% by 2028 for international flights |
Answer: (c) 1, 2, 3 and 4.
Why each option is right or wrong
- (a) Incorrect. It leaves out sludge and wood mill waste, both approved SAF feedstocks.
- (b) Incorrect. It leaves out crop residues and corn grain, which are common feedstocks.
- (c) CORRECT. Crop residues, corn grain, sewage sludge and wood mill waste can all be turned into SAF.
- (d) Incorrect. It leaves out corn grain, used through the alcohol-to-jet pathway.
Key point
SAF feedstocks include crop residues, food crops (corn), sewage sludge, wood waste, used cooking oil and animal fats.
Remember
Crop residues, corn, sludge and wood waste can all be made into sustainable aviation fuel.
Common mistakes
- Thinking only waste can make SAF; food crops like corn are also used, though they raise food-security concerns.
- Not knowing that sewage sludge can be turned into fuel through hydrothermal liquefaction.
- Confusing SAF with ethanol blending in petrol; SAF is for jet engines.
Exam tip
If it contains carbon from living matter, it can usually become SAF. 'All of the above' is safe here.
Sources
- US Department of Energy — Sustainable Aviation Fuel: Review of technical pathways
- Ministry of Petroleum and Natural Gas — SAF blending targets (2024)
- ICAO — CORSIA eligible fuels