Torrefaction

Torrefaction is a thermochemical treatment applied to upgrade lignocellulosic biomass into a solid bioenergy carrier (torrefied biomass) with superior properties in terms of logistics (handling, transport, and storage) and end use (combustion, gasification, and chemical processing). The word "torrefaction" is derived from the French verb torrefier, which means roasting (as in the roasting of coffee beans). As in most thermochemical treatments, torrefaction results in a combination of products, namely, solid torrefied biomass, condensable liquids, and permanent gases.

Feedstock

Origin and composition

In general, woody biomass and other lignocellulosic biomass are used for torrefaction. Heat transfer to the biomass lies at the heart of the torrefaction process as the temperature of the biomass must be increased sufficiently to allow the thermochemical degradation reactions to take place. In case biomass is not predried, moisture evaporation creates an additional heat demand. Since the heat requirement for drying is much larger than for torrefaction, and the heat used in drying the biomass cannot be readily recovered, applying dry torrefaction for upgrading biomass feedstock with high inherent moisture levels is typically less attractive. For this reason, moisture content of incoming biomass should normally not exceed 15%.

Pre-treatment

• Drying

Process and technologies

Torrefaction process

In torrefaction, the feedstock is subjected to thermal treatment at a relatively low temperature of 200°C-300°C in the absence or reduced oxygen level. Torrefaction could be wet or dry. During wet torrefaction, a hot compressed water is used in heating the biomass. During dry torrefaction, direct heating or hot inert gas (e.g. argon, helium and neon) is normally applied. The latter has been the most common practice for commercial purposes.

During torrefaction, as the temperature of the biomass is increased, the evaporation of physically bound water starts as the temperature approaches 100°C. At more elevated temperatures above 160°C, the structural biopolymer constituents (i.e., cellulose, hemicellulose, and lignin) within the biomass begin to degrade, forming gases and vapours. Hemicellulose typically decomposes at temperatures above 220°C, whereas cellulose starts to decompose at a higher temperature, typically above 300°C.

Torrefaction technologies

• Rotary drum reactor
• Screw reactor
• Multiple Hearth Furnaces (MHF)
• Torbed reactor
• Moving bed reactor
• Belt reacor
• Microwaves reactor

Product

Torrefaction has been employed to improve on biomass properties, such as lower moisture content, higher energy density, improved hydrophobicity, and better grindability.

Properties of biomass before and after torrefaction

Raw biomass	Torrefied biomass
Higher moisture	Lower moisture
Higher O/C and H/C ratios	Lower O/C and H/C ratios
Lower heating value	higher heating value
Hygroscopic	Hydrophobic
Poor grindability	Better grindability
Non uniform properties	More uniform properties
Energy density: 1 MJ/kg	Energy density: 1.28 MJ/kg

Torrefaction process essentially aimed at making biomass suitable for subsequent processes such as pyrolysis, hydrolysis and densification. Densification is one of the most prominent paths for processing torrefied biomass. Densification involves applying mechanical force on biomass to compact it into solid particles of uniform size such as pellets, briquettes and logs. Another area of application of torrefied biomass is in iron making. Biomass has the potential to cause reduction in fossil fuel usage in blast furnaces either as metallurgical coke production, iron ore agglomeration or pulverised coal in tuyere injection.