The concerning issue of air pollution in Delhi, due to stubble burning in winter, has been making news headlines. The escalating situation has triggered an inclination towards the application of foreign technologies. One such technique from Sweden is ‘torrefaction’, which can potentially solve this crisis by transforming rice stubble into bio-coal. The Indian government is evaluating the effectiveness of this technology through a pilot project at the National Agri-Food Biotechnology Institute in Mohali, Punjab, in collaboration with a Swedish company.
Understanding Torrefaction: Converting Stubble into Bio-coal
Torrefaction is essentially a thermal technique that transforms biomass into a substance similar to coal. This process improves the fuel traits of the initial biomass. It involves heating materials such as straw, grass, sawmill residue and wood biomass to a temperature range of 250-350 degrees celsius. This heat treatment alters the constituents of biomass, forming ‘coal-like’ pellets. These pellets, combined with coal, are suitable for combustion applicable in industrial activities like steel and cement manufacturing.
Benefits of Torrefaction
The project holds the potential to convert 150-200 kilograms of paddy straw into bio-coal every hour, significantly reducing CO2 emissions by 95%. Other advantages of torrefied biomass include its brittleness, which simplifies grinding and reduces energy consumption. The storage of torrefied material becomes considerably straightforward because the technique minimizes biological degradation and water intake. Additionally, torrefied pellets are ideal for replacing coal due to lower shipping and transport costs and reduced sulfur and ash content as compared with standard coal.
| Aspect | Amount/Effect |
|---|---|
| Paddy straw conversion capacity per hour | 150-200 kg |
| Reduction in CO2 emissions | 95% |
| Reduction in shipping and transport costs | Significant |
| Lower sulfur and ash content (compared with coal) | Yes |
Shortcomings of the Torrefaction Process
Despite its numerous advantages, the torrefaction process also has a few drawbacks. Firstly, the volume of torrefied biomass is only slightly reduced, about 10-20% lower than the dried feedstock during the procedure. Additionally, even though it offers higher calorific values, there is no significant improvement in energy density. Lastly, torrefaction does not reduce the corrosion of machinery, particularly boiler tubes.
The Concept of Bio-coal
Bio-coal, also known as synthetic coal, is produced through the torrefaction of biomass. It possesses characteristics akin to traditional, fossil-based coal. Therefore, bio-coal is considered a viable alternative to mitigate greenhouse gas emissions.