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
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.
| 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. It can also be employed in gasification and iron-making processes.