Value addition to crop-residues through the thermo-chemical route

Crop-residues like coconut shells, sugarcane bagasse, paddy straw etc, are good alternate sources of heat-energy, carbon and hydrogen; alternative to fossil fuels like coal, crude and shale gas & oil. Over the last 8 years, our laboratory has been involved in developing what are known as ‘thermo-chemical’ process technologies for converting crop-residues to value added products. In this essay, the basics of our research and development activities are outlined in a way accessible to the general audience. For readers interested in further details, a section titled ‘Further Reading’ containing a list of research articles and patents from our laboratory is included at the end of this essay.

Heat-energy can be extracted from agro-residues by simply burning it – something that has been known to mankind for a long time. This heat-energy can be used for cooking, electricity generation through the steam route etc,. Extracting carbon in the form of charcoal (a.k.a biochar) has also been in practice for centuries. Charcoal can be obtained by quenching the embers left over after the flaming of agro-residues. Both heat-energy and charcoal are considered easy to extract, though there are recent innovations from our laboratory in doing it in a clean and efficient way [see ‘Further Reading’ refs. 1 and 2].

Activated carbon is the next in the increasing level of difficulty of extraction. Activated carbon is nothing but a highly porous form of charcoal. Porosity is quantified using intrinsic surface area – it can be as high as 1500 m2/g for activated carbon as compared to about 20 m2/g for charcoal. Such high surface area 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 the creation of microscopic pores and a corresponding increase in intrinsic surface area. Although it appears straightforward, the activation step requires careful control of process parameters and must be efficient. An example of a recent innovation in this process from our lab can be found in ref. 3 in ‘Further Reading’.

Products of significantly greater value compared to heat-energy and charcoal include liquid hydrocarbons, alcohols and ethers. Extracting these requires complex processes involving several steps. The reason for the complexity of these processes is that 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 to make value-added products is to break down these polymeric chains into simpler units by exposing them to high temperatures and build the required products by assembling the simpler units in a certain specific order. Catalysts are needed for the latter step to ensure that the simpler units are assembled in the correct order. These two steps together are referred to as a thermo-chemical process; ‘thermo’ referring to the use of high temperatures for breaking down the polymeric chains and ‘chemical’ referring to the catalyst driven assembly of simpler units into required products.

Another useful product not mentioned so far issyngas. It is the end product of thermal breakdown of agro-residues in the presence of oxygen. That is, syngas is the product of the ‘thermo-chemical’ process without the catalytic assembly step. 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 remaining part is a mixture of CO2/water-vapor/N2. The relative fractions of CO2/water-vapor/N2 in syngas depends on the source of oxygen used for gasification, which could be atmospheric air, enriched air (i.e atmospheric air + pure oxygen) or mixtures of air/pure-oxygen/CO2/steam. Steam is used to enhance hydrogen in syngas while CO2 is used to enhance CO. Liquid hydrocarbons like gasoline and diesel and alcohols like methanol and ethanol can be obtained by the catalytic assembling process mentioned above with CO and H2 as starting material.

Gasification is also a well known technology; one of its first uses was in Germany during World War II. A more recent example is its use in South Africa to make liquid fuels from coal. Gasification was also widely deployed between 1990-2000 in India as a technology for rural electrification. For this, the syngas generated by gasification was burnt in a reciprocating IC engine to generate electricity. Although this technology has now been replaced by solar and wind power, gasification continues to be of interest in the context of producing value added products like liquid fuels for which syngas is the starting material. There have been several recent innovations in gasification from our laboratory [see in ‘Further Reading’ refs. 4-8]. These innovations are mostly focussed on increasing the yield of CO and H2, while minimizing or eliminating condensable higher hydrocarbons and oxygenated aromatics collectively known as tar (which are detrimental to catalysts and reciprocating internal combustion engines).

In summary, work in our lab at IIT Madras led to the development of three processes for clean and efficient extraction of biochar, activated carbon and syngas from agro-residues. The biochar process is already scaled up to 1 ton/hr field system and is currently in commercial operation. The activated carbon and syngas processes are under scale-up currently. With these ‘thermo’ based technologies out of the lab and enroute to commercial scale production, we are venturing into the ‘chemical’ part of the process. We hope to have proven scalable technologies for extracting liquid hydrocarbons and other high value products from agro-residues in the coming years.

Further reading

  1. Varunkumar, S. (2012). Packed bed gasification-combustion in biomass based domestic stoves and combustion systems (Doctoral dissertation, Indian Institute of Science Bangalore).
  2. Varunkumar S and Muthu Kumar K. Self-sustained controlled oxidative flash de-volatilization system for biochar synthesis. Indian patent # 408924. Granted on 12, October, 2022.
  3. Varunkumar S, Syed Mughees Ali and Muthu Kumar K. Self-sustained single-step activation in situ process for activated carbon synthesis from agro-residues. Indian patent # 389137. Granted on 14, February, 2022.
  4. Jaganathan V M, Mani Kalyani Ambatipudi & Varunkumar S (2020) The Phenomenon of Flame Jump in Counter–current Flame Propagation in Biomass Packed Beds – Experiments and Theory, Combustion Science and Technology, DOI: 10.1080/00102202.2020.1804886
  5. Jaganathan, V. M., and S. Varunkumar. “A novel self-sustained single step process for synthesizing activated char from ligno-cellulosic biomass.” Fuel Processing Technology 208 (2020): 106516.
  6. Ambatipudi Mani Kalyani, Jaganathan V. M. & Varunkumar S. “Mechanisms governing the gasification to char oxidation transition in counter-current flame propagation in packed beds: insights from single-particle experiments”. Particulate Science and Technology (2020): DOI:10.1080/02726351.2020.1767245
  7. V.M. Jaganathan, Omex Mohan, S. Varunkumar, Intrinsic hydrogen yield from gasification of biomass with oxy-steam mixtures. International Journal of Hydrogen Energy 44, Issue 33, (2019):17781-17791.
  8. V M Jaganathan and S Varunkumar. Net carbon-di-oxide conversion and other novel features of packed bed biomass gasification with O2/CO2 mixtures. Fuel 244 (2019): 545-558.

Author – Varunkumar S, Associate Professor, Mechanical Engineering, Indian Institute of Technology, Madras

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