A good fuel for gasifiers is biomass that has been formed into pellets. The bulk density of pellets can be 6 to 15 times higher than the bulk density of loose biomass. Biomass pellets are easy to transport and store. If stored properly under dry conditions, pellets do not emit gases. They do not catch on fire or explode during storage. A much higher gasification temperature is achieved with pellets than with loose biomass.
As the bulk density of biomass increases through the use of pellets, reactor height can be much more efficiently utilized. This results in shorter reactors or longer burn times. Gasifiers become much cheaper to fabricate. A reactor of a height of only 20 cm or 8 inches has a burn time on pellets of over 75 minutes. A reactor of a height of one meter has a burn time of over 6 hours.
Pellets of a diameter of 4 mm and a length of about 6 mm length are ideal. Uniform pellets allow for the uniform flow of air up through the reactor. No channeling occurs, no hot spots are formed within the reactor, and very little carbon dioxide is produced. When pellets are gasified, the biochar produced is also quite uniform, and biochar retains a pelleted shape
With pellets there is much less escape of fine particles during gasification than with loose biomass. There are small-scale combined grinding and pelletizing machines, such as this all-in-one unit from Ecoworxx.
Bamboo internodes can be split using a hand-held splitter into uniform sticks resembling chopsticks and placed vertically in the reactor of the gasifier
Making bamboo chopsticks is cheaper than making bamboo pellets. Bamboo chopsticks can easily be cut to a short pieces, resembling in every respect a pellet.
The best pellets are those produced from torrefied biomass . When biomass is torrefied, it undergoes Maillard reactions. Biomass gets roasted, very much as in the roasting of coffee. Water is removed from biomass, and biomass can no longer absorb water. Also, superfluous oxygenated volatile compounds are released, and this provides the energy needed for the torrefaction process, provided the moisture content of the incoming biomass is below 15%.
In the torrefaction process, biomass might lose 20% of its mass and 10% of its heating value. But the energy density of torrefied pellets can increase to over 5,000 Kcal/kg (Status overview of torrefaction technologies) and A review on biomass torrefaction process and product properties for energy applications.
The energy needed to grind can be reduced by as much as 90%, and the energy need to pelletize can be reduced by a factor of two. Some recommend pelletization before torrefaction (Torrefaction: Pre- or Post-Pelletization)
The torrefaction of pellets can be accomplished using superheated steam.
Torrefaction of biomass, e.g., wood or grain, is a mild form of pyrolysis at temperatures typically between 200 and 320 °C. Torrefaction changes biomass properties to provide a better fuel quality for combustion and gasification applications. Torrefaction produces a relatively dry product, which reduces or eliminates its potential for organic decomposition. Torrefaction combined with densification creates an energy-dense fuel carrier of 20 to 21 GJ/ton lower heating value (LHV). Torrefaction makes the material undergo Maillard reactions.
Biomass can be an important energy source. However, there exists a large diversity of potential biomass sources, each with its own unique characteristics. To create efficient biomass-to-energy chains, torrefaction of biomass, combined with densification (pelletisation or briquetting), is a promising step towards overcoming the logistical challenges in developing large-scale sustainable energy solutions, by making it easier to transport and store. Pellets or briquettes have higher density, contain less moisture, and are more stable in storage than the biomass they are derived from.