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Converting wet organic municipal waste to biofuel & biogas without pre-drying

India’s humongous waste problem has a very specific issue: a greater portion of urban waste generated is organic. This part is very high in moisture content, which, during the monsoons, can rise up to 60-65%. Indian cuisine is dominated by gravies and daals, which keeps the water content of organic waste high even during the dry season.

If such wet organic waste is traditionally incinerated, the water component brings down the calorific value of the waste. Municipal corporations have to devote time, money and space to dry waste before incineration, or set up a separate bunker for the same.

A team of researchers from IIT Kharagpur has successfully developed a mechanism to process wet Municipal Solid Waste (MSW) without the need for drying it first. The products of the process are all usable, or re-usable.

The team has adapted a process called Hydro Thermal Carbonization (HTC) for Indian conditions, which converts organic MSW into hydrochar using a batch reactor. After waste is collected and segregated, the organic component is subjected to high temperatures (200-250 degrees Celsius) in a pressurised chamber for about two hours. The water in the waste is actually essential to the process.

Organic MSW is high in both carbon and water. HTC is a way to accelerate what is already happening in nature over millions of years – the conversion of organic matter to coal.

According to Professor Brajesh Kumar Dubey of the Department of Civil Engineering, existing processes convert only 20-30% of the organic fraction of municipal solid waste to biofuel. But in the reactor used by the researchers, for example, 1gm waste and 4gm water was converted to 1gm biofuel, and the water remained available for reuse.

The solid component of the output of the process is carbon-rich, and can be used as biofuel. Its calorific value has been determined to be 24.59 MJ/ kg, which is comparable to lignite. The liquid component can be used as a bio-oil, or anaerobically digested to produce biogas.

Hydrochar — the solid component — has a high surface area and can also be used as an adsorbent for contaminant removal at industrial sites or landfills. With some modification, it can be used like activated charcoal.

The 4-5% residue that remains at the end of HTC is due to impurities like plastic. The process cannot tackle plastic, concrete or bricks. Wet and dry waste also need to be segregated. This technique is already being used successfully in Western European countries and Canada, in agro-processing.

Professor Dubey says that two to three Indian companies have evinced interest in the technology. A large airport in South India, as well as a Smart City mission have also made enquiries. He adds, “We need to test this on a bigger scale and are looking for investors from the industry. We are currently using batch reactors; ideally, we will use a continuous feed process.”

As India pivots to Waste-to-Energy plants to tackle its waste burden, the IIT-Kharagpur team’s innovation may be the silver bullet that bypasses the need for drying waste before processing it, and which generates completely useful products.

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