To increase fuel efficiency and reduce emissions, the Indian government mandates an ethanol-petrol blend rate of 20 percent by 2030. Producing alcohol from food grains is easy, but it will threaten food security.
Ethanol can be produced using cellulose, a sugar-polymer, highly abundant in crop waste. But cellulose lies within a matrix of hemicellulose and lignin. The matrix needs to be loosened, using physical or chemical pretreatment, to allow access to cellulase enzymes, which break it down to release sugars for yeast-based fermentation.
Sathyanarayana N. Gummadi of IIT-Madras, in collaboration with researchers from Iran, recently came up with an inexpensive method for pretreating wheat straw, a major agro residue. They chose ionic liquids — special salts with low melting points. These are good solvents and remain stable at high temperatures. Unfortunately, the most efficient ones are not economically viable.
So, the scientists synthesised an alternative, triethylammonium hydrogen sulphate, an ionic liquid gaining popularity for its low cost. They synthesised and characterized the chemical using nuclear magnetic resonance to ensure that the chemical was indeed triethylammonium hydrogen sulphate.
After pretreating wheat straw with the chemical, they tested the activity of standard cellulase enzymes. Cellulose breakdown was high, allowing higher loads of biomass than usual.
They experimented with durations of pretreatment and found that shorter durations also work well. The ionic liquid was not only capable of separating lignin and cellulose in wheat straw, but could also be reused many times, improving the economic viability of the process. What’s more, it was able to dissolve the sugars released.
The scientists are hopeful that these findings can be upscaled to suit large-scale biorefineries.
India generates nearly 500 million tonnes of agro waste. Using such waste to make ethanol will not only fulfill fuel needs and improve income for farmers, but can also help reduce pollution caused by stubble burning, say the researchers.
Freelance writer, Mumbai
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