CO2 Sequestration Potential of Arundo Donax

As CO₂ emission is becoming one of the major threats to the planet, alternative and green energy sources are greatly sought after and the number of companies planning to go green in the following decade is increasing. An extensive study was carried out by four universities (University of Washington, Washington State University, Oregon State University and Portland State University) to reveal the great potential of Arundo donax (or giant reed), an energy plant, which could supply power plants with biocoal, thus mitigating CO₂ emission with high level CO₂ sequestration  

Arundo donax, the candidate for replacing coal

Arundo donax is a tall, perennial plant, belonging to reed species. It has an outstanding growth rate and yield (upwards of 35 dry metric tons/acre/year) and the giant reed produces a hollow cane, which can be easily processed and handled. It is called bamboo reed as well, since it is often mistaken for bamboo.

In 1998, in a 28-day experiment with Arundo donax plants, measurements of CO₂ exchange and transpiration were carried out with a gas exchange device, in a chamber where the conditions were set so that they tracked the outside climate, that is 27 Celsius degrees.

Maximum photosynthetic CO₂ sequestration ranged between 19.8 and 36.7 micromoles depending on leaf age, and the amount of sun-ray the plant got (otherwise, the weather). Based on this data, it was concluded that CO₂ sequestration rates in Arundo donax were unusually high compared to other locally co-occurring species. The results are summarized in the following table:

The study for Portland General Electric

Portland General Electric is planning to stop burning coal at its 600 MW power plant in Boardman Oregon at the end of 2020, which is 20 years ahead of earlier predictions. Past this date, the operation of the plant would rest on the use of biomass, however it requires many critical items to be in place. First is the need for biomass sources with high yields that could meet the needs of the existing power plant. A 600 MW power plant would call for large amounts of biomass, and thus potentially large acreages of dedicated biomass crops.

That is why high-producing biomass crops, such as Arundo donax would be attractive for power plants. The Boardman facility uses over 2.5 million tons of coal to generate 4.3 million MWh of electricity in a year, resulting in a net production of 4.6 million tons of CO₂e (CO₂ equivalent) annually. Coal to Boardman is transported via railway, which is responsible for adding another 77.000 tons of CO2e.

Although PGE would imagine to use a mix of biomass sources, for research purposes, the study suggested that, beginning in 2021, the Boardman facility could completely replace coal with torrefied Arundo.

CO2 sequestration: The potentials of Arundo donax

For the sake of the calculations of the above mentioned study, an estimated yield of 33 tons per acre per year was used. The below‐ground production in the root mass was estimated to be 22 percent of the above ground production. Therefore, a yield of 33 tons per acre per year would produce circa 7.3 tons belowground in the root mass.

Creating an Arundo plantation would require the plantlets and other necessities being shipped to the location. CO₂ from shipping of Arundo for plantation establishment is minimal in comparison to the total amount of CO₂ involved in the whole procedure.

The study also assumed that the power plant would operate at an annualize 300MW, producing power under optimal economic conditions. Based on the BTU content of torrefied Arundo (10,400 BTU/lb), 1.25 million tons of torrefied Arundo would be used by the power plant to provide 2.6 million MWh annually (300MW annualized) under this assumed scenario. This is the BTU equivalent of 1.5 million tons of Powder River Basin coal, which is currently being used.

To produce 1.25 million tons of torrefied chips and support torrefaction, a total of 67.6 thousand acres of Arundo would be needed, assuming 33 dry tons per acre per year.

All things considered (farming, transport, torrefaction and combustion) would result in the emission of around 4.05 million tons of CO2e annually. Thus, the study suggested that the farming of Arundo would result in the CO₂ sequestration of around 4.34 million tons/year, resulting in an annual net CO₂ sequestration of around 0.29 million tons.

CO2 Sequestration: summary and numbers

The Portland General Electric study for an Arundo fuelled power station involved an Arundo plantation of a size of 27,356 hectares (67.600 acres). This plantation would have a capacity of CO₂ sequestration up to 4,340,000 tons per year.

Based on these figures one hectare of Arundo would sequester 160.1 tons of CO₂ per year

however, this depends on the Arundo yields. The study assumed that the annual yield of green Arundo would be 81.5 tons/hectare (33 tons/acre).

Following these assumptions, the sequestration rate of Arundo per ton is 1.963 tons of CO₂ per year.

It should also be noted that Arundo’s total sequestration can be divided into aboveground and belowground amounts. The study calculated that the aboveground part of the plant equates to 82% of the total CO₂ captured. Therefore, from the total 1.963 tons of CO₂ sequestered per ton of Arundo per year, 1.61 ton is captured aboveground, whilst 0.353 ton is seized below the soil.

Find out more about Arundo’s remediation capabilities on our website at www.arundobioenergy.com/bioremediation/