Q.24 Write an informative note on biodiesel. (asked in MU June 2025 KT)
Best answer
Answer: Biodiesel is a mixture of fatty acid methyl (or ethyl) esters made by transesterifying vegetable oils or animal fats with methanol in presence of a base catalyst; it is a renewable, biodegradable, low-sulphur substitute for diesel.
Raw materials: edible and non-edible vegetable oils (jatropha, karanja, soybean, palm, sunflower), used cooking oil and animal fats. These are triglycerides (esters of glycerol with long-chain fatty acids).
Preparation (transesterification): the oil is heated at about 60–65 °C with excess methanol in presence of NaOH or KOH as catalyst.
\[\text{Triglyceride}+3\,\text{CH}_3\text{OH}\xrightarrow{\text{NaOH/KOH}}3\,\text{RCOOCH}_3\ (\text{biodiesel})+\text{C}_3\text{H}_5(\text{OH})_3\ (\text{glycerol})\]
The heavier glycerol layer is separated, and the ester layer is washed and dried. Transesterification lowers the high viscosity of the oil so that it can be used in diesel engines.
Advantages: renewable and biodegradable; almost free of sulphur and aromatics; lower emissions of CO, unburnt hydrocarbons and particulates; nearly carbon-neutral; higher cetane number and better lubricity than diesel; can be blended with diesel (e.g. B20) without engine changes; glycerol is a useful by-product.
Limitations: slightly lower calorific value; may increase NOₓ emissions; thickens or gels in cold weather; can attack some rubber seals; using edible oils competes with food supply.
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