Biomass energy is considered a renewable form of energy because the organic materials used to produce it are never-ending. The only carbon released into to the environment from biomass fuels is the quantity that was absorbed by plants throughout their life cycle. This aspect renders biomass cleaner than all other forms of energy production. Arundo donax (or giant reed) is way ahead of its competitors, with around 50-80 dry tonnes of biomass yield per hectare every year. Let’s look at how Arundo fits into the process of torrefaction.
What is torrefaction?
Through torrefaction, biomass is converted into a coal-like material. It involves heating the biomass (in the absence of oxygen) to a temperature of typically 200 to 400°C. The main product is the solid, torrefied biomass. During the torrefaction process a combustible gas is released, which is utilised to provide heat to the process.
The final product has a much higher energy density than the raw biomass. As an extra benefit of this higher density, the distance over which the biomass can be economically transported for use or further processing is higher than that non‐torrefied biomass.
Earlier research into torrefied Arundo donax indicates it has a higher gravimetric BTU value (by approximately 15%) over typical coal. Also, the same tools to process coal can be used to process torrefied biomass.
The torrefaction of Arundo donax
In a 2012 study, a conceptual 100 ton per day biomass torrefaction plant for processing Arundo donax was used to calculate the potentials of the plant. Such a facility would yield 52.7 tons per day of torrefied Arundo chips, yielding 1,160 GJ of energy per day. The energy density of torrefied giant reed is then 22.0 GJ/ton or 10,400 BTU/lb. It is worth mentioning that Arundo donax offers significantly higher energy densities than other proposed bio‐crops such as switchgrass (7,741 BTU/lb), wheat straw (7,978 BTU/lb) and hybrid poplar (8,178 BTU/lb).
In the study, the following theories for mass and energy balances were brought up: in a unit with a capacity of 100 tons/day for torrefaction, the moisture content of biomass received from Arundo donax would be 6 mass %. As for biomass elemental composition (on dry basis, mass %), carbon would be 48.8, hydrogen 5.9, oxygen 43.7, and ash 1.6.
The outstanding yields, fast growth rate and low input need of Arundo donax make this energy crop a perfect candidate for biomass and thus biocoal production, and investing in the giant reed means investing in the fight against climate change.
