How can biochar help in cutting CO2 emissions from sponge iron production? BigMint explores

• Coal-based DRI has high emissions intensity of 1.85 tCO2/t-DRI
  
  
• Biochar is a sustainable partial substitute for coal in domestic DRI industry
  
  
• Tata Steel's trial achieves 36% reduction in emissions with biochar in rotary kiln
  
  

Morning Brief: With surging domestic demand, India's crude steel production is spiralling. But amid dearth of ferrous scrap the domestic industry's dependence on sponge iron, a metallic widely used as a substitute for scrap in induction furnace steel production, is steadily increasing.

BigMint data reveal that India's sponge iron or direct reduced iron (DRI) production reached almost 55 million tonnes (mnt) in CY'24 which is projected to exceed 60 mnt by the turn of the current decade. From a sustainability perspective, however, it is vital to note that around 84% of total sponge iron production is based on coal, which is highly emissions intensive.

Domestic DRI scenario

India is the world's largest producer of sponge iron and also the top producer of coal-based sponge iron. Due to high volatile matter and moisture content in domestic coal, the value in use of domestic coal is significantly lower compared to imported coal, mainly from South Africa. While 0.8-0.9 t of imported non-coking coal is required for the production of 1 tonne of DRI, around 1.3-1.4 t of domestic coal is required for the same purpose.

The CO2 emissions intensity of domestic coal-based sponge iron production is around 1.85 tCO2/t-DRI. Considering the fact that the IF steel production route is highly reliant on electric power (the grid emission factor in India is over 0.7 tCO2/MW) and due to high embedded CO2 in the DRI, it is to be expected that total carbon emissions from this route is quite high compared with scrap-based production, and even higher than BF-BOF steel production in India.

During an informative presentation at the All India Steel Conclave 2.0 (AISC 2.0) held recently in Raipur, Chhattisgarh, Varun Mishra, Head of Tata Steel's DRI plant, observed that the share of coal in total emissions in domestic DRI production is 95% compared to just 3% for electricity and 1% for dolomite.

Biochar - a sustainable material

Therefore, the need of the hour is to find an alternative to coal in domestic DRI production. Tata Steel has taken a lead in this by trialling biochar, a biomass derivative, for partial substitution of coal in sponge iron production.

"There are many challenges in pivoting to biochar from coal. From the operational point of view, we are uncertain about the chemical kinetics of biochar at high temperatures. Another challenge relates to the method of charging biochar into the rotary kiln as it has much lower density than coal. Additionally, there is no proper supply chain of biochar in the country," said Mishra.

Tata Steel's laboratory results have shown that biochar and coal have similar kinetic behaviour till a temperature of 750 degrees. After that biochar shows faster decomposition and at a temperature of 1,050 degrees it gets wholly burnt out. This is due to the higher porosity and reactivity of biochar vis-a-vis coal, informed Mishra.

Tata Steel prepared the detailed trial and key performance indicator (KPI) reports in September last year. The company has initiated the patenting process of this unique trial of biochar at a such a huge industrial scale.

Trial & emissions reduction

Within a seven-day period, Tata Steel used around 450 t of biochar to replace coal in sponge iron production, gradually increasing the share of biochar replacing coal from 3% on day 1 to 30% on day 7. Due to the higher reactivity of biochar carbon utilisation in the kiln was better compared to coal and there was no accretion formation inside the kiln. The product quality remained unchanged although the percentage of sulphur was reduced.

"The trial threw up remarkable results," said Mishra. "With 30% replacement of coal by biochar, CO2 emissions fell by 36%. This was a huge result. If half of the total coal used in DRI production is substituted with biochar, total CO2 emissions at Tata Steel can be reduced by 2.75%."

For widespread adoption of biochar in the domestic DRI industry, proper supply chains and logistics are essential. Different state governments are making efforts to promote babool-based biomass which can create stable supply chains in the future.

Tata Steel is planning a longer trial in the near term for one whole month with approximate consumption of 6,000 t of biochar. The previous trial has shown that at a landed cost of INR 22,000/t, the value in use of biochar becomes positive in comparison with coal and can be used to substitute 30-60% of coal used in sponge iron production.