Torrefaction is the process of making biomass a better quality fuel for utilisation in combustion and gasification processes. The final product is dry and hydrophobic, so it can be stored both inside and outside without causing any harm, such as rotting, to the fuel. This torrefied biomass can be used for co-firing (partial substitution of coal by torrefied biomass) in thermal power plants, decentralised heating, steel manufacturing and the production of bio-based fuels like syngas.
Biomass torrefaction entails pyrolysis of biomass at a temperature of about 200 °C, which produces a dense black product called biocoal. The untreated biomass is dried before being torrefied as the amount of moisture content decides the amount of energy required in the torrefaction plant. Torrefaction is an exothermic process wherein volatile gases are burnt in a combustor to generate heat for drying and subsequent torrefaction. The process makes biomass more compact and dense for the production of torrefied biomass pellets or briquettes. Though some of the volatile matter is lost in the process, this helps in reducing the chances of explosion when fired.
Torrefied biomass can prove to be one of the most cost-effective ways to reduce carbon emissions from thermal power plants (TPPs) and increase the use of biomass as a source of renewable energy. It can be used in the existing coal-based power plants in the country, and prove to be a huge leap towards decarbonisation of energy in the country, at very little additional costs. Coal is a fossil fuel and its cost is expected to increase further in the future. However, biomass has unlimited reserves and the availability of torrefied biomass pellets on a large scale will make it cost competitive.
Torrefaction makes biomass more brittle, leading to improved grindability. Further, the gross calorific value of the fuel is increased, and the torrefied material can be handled by the same infrastructure that is used for coal. Torrefied biomass does not contain any hazardous materials like mercury and no heavy metals. Even the content of sulphur in torrefied biomass is very little compared to traditional coal. The efficiency of boilers increases with torrefied biomass, as compared to normal biomass, as the moisture content decreases.
The main drawback in the production of torrefied biomass is that most of the agricultural residue in the country is wasted. This value is as high as 140 million tonnes, which can instead be used to generate about 28,000 MW of power, if a systematic approach is taken in biomass collection and pelletisation. At present, pellet manufacturing is being done on a small scale by a few developers, while torrefied biomass production is even less in India, and is restricted to wood torrefaction.
Application and outlook
With increasing air pollution in the country, the Ministry of Power has issued a policy directing TPPs in all states and union territories to use at least 5-10 per cent biomass pellets along with coal. NTPC is working towards starting commercial scale co-firing of biomass and coal to promote the biomass pelletisation and torrefaction industry. NTPC has already issued a tender for the procurement of 500 tonnes per day (tpd) of pellets and 500 tpd of briquettes, all to be produced from agricultural waste. Farmers who burn their crops may actually take up this option, to generate additional revenue from their agricultural waste. This would also help curb pollution. Torrefied biomass will earn farmers better revenue as compared to non-torrefied pellets and briquettes as it is a fuel with higher calorific value. In future, the 10 per cent co-firing limit can be removed, allowing TPPs to use greater volumes of torrefied biomass.