UPSC Prelims 2024 Q46: Sustainable Aviation Fuel Feedstocks Explained
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?
- 1 and 2 only
- 3 and 4 only
- 1, 2, 3 and 4
- 1, 3 and 4 only
Answer: (c) 1, 2, 3 and 4
Statement-wise explanation
- 1: correct — Agricultural residues such as crop straw, corn stover and sugarcane bagasse are lignocellulosic biomass. They can be gasified and turned into jet fuel by the Fischer-Tropsch route, or broken down into sugars, fermented to alcohol and upgraded by the alcohol-to-jet (ATJ) route. Indian government statements call surplus crop residue India's largest untapped feedstock for SAF.
- 2: correct — Corn grain is mostly starch. Its starch is fermented into ethanol, and the alcohol-to-jet pathway turns that ethanol (or isobutanol) into jet-range hydrocarbons. Corn grain is a food crop, but it is still a recognised SAF feedstock, mainly in the United States, which makes its SAF from corn ethanol.
- 3: correct — Sludge from wastewater treatment is a 'wet waste' rich in organic carbon. Hydrothermal liquefaction turns it into a bio-crude that can be refined into jet fuel. Anaerobic digestion can also turn it into biogas, which can then be upgraded. The US Department of Energy names wastewater treatment sludge among its SAF feedstocks.
- 4: correct — Wood mill waste is woody biomass: sawdust, bark, chips and offcuts. Like crop residue, it can be gasified for Fischer-Tropsch synthesis or converted to alcohols for the ATJ route. Forestry and mill residues are standard second-generation SAF feedstocks.
How to eliminate
The question rests on one principle: SAF can be made from almost any carbon-rich biological material, whether that is oils and fats, sugars and starches, lignocellulose or organic wet waste. The approved conversion routes (HEFA, Fischer-Tropsch, alcohol-to-jet) were built to accept different kinds of feedstock. Once you know this, look at the options. Options with 'only' leave out one or more materials, so each would need a reason why a carbon-rich biomass cannot be used. No such reason exists. Statement 2 is the likely trap: aspirants link SAF with 'wastes' and drop a food crop. But food crops like corn and sugarcane are first-generation biofuel feedstocks, and corn ethanol is the main SAF input in the US. Sludge (statement 3) also looks odd, but it is organic waste that biogas and waste-to-fuel schemes already use. The rule to remember for biofuel questions: if a material is biological and carbon-rich, assume it can be a feedstock unless you know a specific reason it cannot. Questions like this usually have 'all of the above' as the answer.
Know more
Sustainable Aviation Fuel (SAF) is jet fuel made from biological or waste carbon instead of fossil crude. It is a 'drop-in' fuel: it is chemically almost the same as conventional Aviation Turbine Fuel (ATF), so engines and airport pipelines need no changes. Current standards allow up to 50% SAF in a blend with ATF. Its climate benefit is measured over the full life cycle. The CO₂ released in flight was recently taken out of the air by the plants or wastes that made the fuel.
Main production pathways
| Pathway | Feedstock | Process |
|---|---|---|
| HEFA (hydroprocessed esters and fatty acids) | Used cooking oil, animal fat, non-edible oils | Hydrogen treatment of oils |
| Alcohol-to-Jet (ATJ) | Corn, sugarcane, molasses; ethanol from residues | Ferment to alcohol, then dehydrate and oligomerise |
| Fischer-Tropsch (FT) | Crop residue, wood waste, municipal solid waste | Gasify to syngas, then synthesise liquid hydrocarbons |
| Hydrothermal liquefaction | Wet wastes: sewage sludge, manure, food waste | High heat and pressure give a bio-crude, which is then refined |
Generations of feedstock: first-generation feedstocks are food crops (corn, sugarcane, edible oils). Second-generation feedstocks are non-food residues and wastes. Third-generation feedstocks are algae. Policy now favours wastes because they avoid the 'food vs fuel' conflict and land-use change.
India's policy: the government has set indicative SAF blending targets for international flights: 1% in 2027, 2% in 2028 and 5% in 2030. These are timed to match the start of the mandatory phase of ICAO's CORSIA offsetting scheme in 2027. India launched the Global Biofuels Alliance at the G20 New Delhi Summit (September 2023). Its scope covers SAF along with ethanol and compressed biogas. In 2026, SAF-blended ATF was brought under the ATF (Regulation of Marketing) Order, 2001.
In the news
SAF was in the news through 2023. India's first domestic commercial flight using SAF made from indigenous feedstock flew in May 2023. India launched the Global Biofuels Alliance, which covers SAF, at the G20 New Delhi Summit in September 2023. The question's wording closely follows the US Department of Energy's standard list of SAF feedstocks.
NCERT Concepts to Revise
- Maize: Statement 2 asks about corn grain. Knowing maize is a starchy cereal grown for food and fodder helps you judge whether it can yield fuel.
- Ethanol blending: Ethanol from fermented starch and sugar is the same intermediate the alcohol-to-jet pathway upgrades into aviation fuel.