Biomass torrefaction for production of bio-coal used as a substitute for pulverised coal in blast furnaces (TORERO)

This innovative technique enables the conversion of biomass (i.e. waste B-wood) into bio-coal using a torrefaction process. The bio-coal produced is subsequently used as a substitute for pulverised coal injection in blast furnaces. This technique has been developed and tested in a large-scale demonstration plant at ArcelorMittal in Ghent. This development was financially supported under the EU Horizon 2020 project Torero, over the period 2017 - 2024.

TECHNICAL DESCRIPTION:
The demonstration plant uses specifically waste wood, in particular B-wood. Typical wood-based products which are found in B-wood include:
• Particle boards: manufactured from particles of wood glued together to form a board that are often used for furniture, shelving, and other applications.
• Veneer: thin slices of wood glued together onto core panels used to produce e.g. doors, parquet floors or furniture parts
• Plywood and laminated wood: composite material manufactured from thin layers of wood stacked and glued together.
• Fibre board: wood product made from wood fibres that are bonded together using a synthetic adhesive. It is typically used in various applications, including furniture, flooring, and construction.

The B-wood is processed through a mild pyrolysis process, known as torrefaction, to produce torrefied wood. The torrefied wood is then re-grinded and used as a replacement for pulverised coal in the blast furnaces.

At the demonstration plant, about 88,000 tonnes of waste wood is converted into 37,500 tonnes of bio-coal each year.

The demonstration plant operates as follows:
• Receiving / sorting: B-wood (wet) is received and sorted (e.g. oversized/undersized wood is removed). The plant requires 17 trucks of B-wood per day, which is sourced from domestic residues. The B-wood is processed to remove metals and other contaminants and is then fed into the torrefaction reactor. The plant uses approximately 13% of the entire Belgium's B-wood supply and is currently looking for alternative sources to meet its feedstock requirements.

• Storage / transportation: B-wood is stored in a wet silo and transported using a buck-et elevator and chain conveyor towards screening / torrefaction.

• Screening and drying: B-wood is screened for impurities (e.g. metals) and dried in a continuous belt dryer.

• Torrefaction process: The torrefaction process is the key step for conversion of B- wood into bio-coal. During torrefaction, the dry wood is heated in the absence of oxygen to a temperature of around 250-320°C inside the torrefaction reactor in an inert environment (i.e. atmospheric pressure in the absence of oxygen). In this process, the wood undergoes a series of physical and chemical changes, resulting in the production of bio-coal which has higher energy density, is pulverisable, hydrophobic, and has a lower oxygen to carbon (O/C) ratio. Torrefied wood exhibits a similar thermal degradation process as char-coal. The torrefaction process also produces torrgas, a combustible gas that can be used to generate energy.

• Energy generation: The torrgas produced during the torrefaction process is very flammable (at 300°C) and rich in tars. It is removed from ash inside a dust-cyclone and then oxidised in a thermal oxidiser at about 900°C for ca. 2 seconds in order to reduce the amount of VOCs and other pollutants. This process generates heat which is in turn is re-covered to dry the wet wood and produce steam.

• Flue gas treatment: After the thermal oxidiser, the flue gas (about 200ºC) requires further treatment using fabric filters (e.g. at Arcelor Mittal Ghent demonstration plant, the flue gases are directed to the sinter plant fabric filters).

• Bio-coal processing and transportation: The bio-coal is cooled, pulverised, stored in a silo, then transported to the blast furnaces for substituting pulverised coal.

CROSS-MEDIA EFFECTS (trade-offs)
Since waste wood is today used for renewable energy generation the use as alternative reductant can result in less renewable energy generation using biomass. However, the use of biomass for energy generation is less and less incentivised in Europe; therefore the impact on energy generation might be less relevant in the future.

BARRIERS TO IMPLEMENTATION:
• B-wood availability with adequate quality and acceptable price is difficult to source at economical pricing.
• Pollution of the wood with other waste: remainders of plastic foil are not pulverised and can block the pneumatic transport of the pulverised bio-coal.

Basic information about the technique

Participant Companies