During the process, the biomass properties are changed to obtain a much better fuel quality for combustion and gasification applications. During this process, oxygen-rich compounds are volatilized from the biomass. Torrefied wood is mainly composed of cellulose and lignin. The torrefaction reaction begins at 200⁰C, but the practical range is 250⁰C to 280⁰C. Care must be taken not to go much higher in temperature as carbonization begins in the range of 280⁰C to 300⁰C. The volatilized materials are combusted to provide the heat for the process. An optimum control point is autothermal operations where the energy of the volatiles is enough to supply the energy for the process.
The advantages of torrefied biomass pellets are:
- Product is water-resistant; can be stored outdoors on a coal pile; and generally does not reabsorb moisture after drying.
- The fibrous nature is reduced and the grindability is improved.
- Energy density is higher than a wood pellet, especially on a volume basis.
- No binder is necessary to form the pellet.
- Biological degradation is greatly reduced.
- Various biomass feedstock can be utilized.
- Uniform quality improves combustibility.