Thermo-chemical upcycling of crop residues

Extracting heat and making biochar (charcoal) from agri residues has been known to mankind for millennia. Recent innovations are helping do this in a clean and efficient way. Over the last eight years, our laboratory in IIT-Madras has been developing ‘thermo-chemical’ technologies to convert crop residues into value-added products.

Activated carbon is a highly porous form of charcoal. Porosity is the ‘intrinsic surface area’. It is 1,500 sq m per gram for activated carbon, compared with about 20 sq m per gram for charcoal. Such high porosity is achieved by reacting high-temperature steam with charcoal (known as activation) — the steam strips away carbon from charcoal in the form of carbon monoxide, leading to microscopic pores and an increase in intrinsic surface area. Although it appears straightforward, the activation step requires careful control of parameters.

Products of significantly greater value compared with heat energy and charcoal include liquid hydrocarbons, alcohols and ethers. Extracting these involves complex processes with multiple steps. The complexity arises because the carbon and hydrogen in agro-residues co-exist with oxygen as cross-linked polymeric chains made of cellulose, hemi-cellulose and lignin.

One way of making value-added products is by breaking down these polymeric chains into simpler units by exposing them to high temperatures and then assembling the simpler units in a specific order to form the required products. Catalysts are needed for the latter step. These two steps together are known as a thermo-chemical process; the ‘thermo’ refers to the use of high temperatures to break down the polymeric chains, and ‘chemical’ is a reference to the catalyst-driven assembly of the simpler units.

Yet another useful product is syngas, which is the end-product of the thermal breakdown of agro-residues in the presence of oxygen. That is, syngas is the product of the ‘thermo-chemical’ process minus the catalytic assembly. This process is popularly known as gasification. The useful part of syngas is carbon monoxide, hydrogen and traces of methane and other higher hydrocarbons; the rest is a mixture of carbon dioxide, water vapour and nitrogen, whose proportion depends on the source of oxygen used for gasification, namely, atmospheric air, enriched air (atmospheric air combined with pure oxygen), or a mixture of air, pure oxygen, carbon dioxide and steam. Steam is used to enhance hydrogen in syngas, while carbon dioxide is used to enhance carbon monoxide. Liquid hydrocarbons like gasoline and diesel, and alcohols like methanol and ethanol can be obtained through catalytic assembling with carbon monoxide and hydrogen as starting material.

We now have three processes for clean and efficient extraction of biochar, activated carbon, and syngas from agro-residues. The biochar process has been scaled up to one tonne per hour and is in commercial operation. The activated carbon and syngas processes are being scaled up.

(The writer is Associate Professor, Mechanical Engineering, IIT-Madras)

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