Biogas is an increasingly important biofuel and several feedstocks, such as energy crops, agricultural residues and organic wastes are suitable for producing it. Finding the appropriate feedstock for a biogas powerplant is hard. Nevertheless, the most used biomasses are originated from annual food crops, which require relevant agronomic inputs and are not well suited to marginal soils, thus resulting in competition for land with food production.
However, perennial rhizomatous grasses like giant reed (Arundo donax) show some relevant advantages compared with annual crops. Giant reed is a no-food crop, capable of high aboveground biomass yields, which shows good adaptability to marginal lands, as well as low input requirements. In a study carried out in Italy, Arundo donax was compared with conventional annual crops for fueling a biogas powerplant. See the results in this article below!
Arundo donax, the needed feedstock for a biogas powerplant
Apart from offering a solution to the food vs. fuel dilemma, utilising giant reed as a feedstock for biogas is backed up by many factors. To start with, Arundo donax was proven to be able to persist in environments in which seasonal drought occurs and not only does it recover from water scarcity, but even brings high yields.
Furthermore, among the considered alternatives (like corn or sorghum), double harvests and single late harvests of giant reed gave the highest methane outputs, as a consequence of maximised biomass yields. In fact, comparing corn (yielding about 20 tonnes of DM/hectare) with giant reed harvested once in September the results were:
6750 Nm3 of methane/hectare for corn
11,280 Nm3 of methane/hectare for Arundo donax
Further comparing Arundo donax to other feedstocks, it can be seen that giant reed enables more biogas production with about 50% lower input considering price. The explanation is simple: corn is a plant one has to take care of every year with high input: buying and planting seeds every year and treating it with pesticides as it is highly sensitive. On the contrary, Arundo donax – once established – do not require chemicals nor weed controlling, it has no pests typical for this species, and as it is a perennial, soil preparation is only needed once.
Fuelling a biogas powerplant: the numbers
The theoretical land use of each crop assessed in this particular study, expressed as the required area to fuel 1 kWe, derives directly from the methane yields per hectare. Therefore, the less productive crop (like forage sorghum) would require more land to fuel a biogas powerplant, while the most productive one, giant reed (preferably under double harvest systems) is expected to be the most preservative. As it was calculated, a biogas powerplant with an installed capacity of 500 kWe would require about 100 hectares of Arundo donax managed under a double harvest system, or about 120 hectares of the same crop cut once in September. As for corn, fibre sorghum and forage sorghum it would be about 135, 160, 185 hetcares, respectively. It must be also pointed out, that the latter require much more input and arable lands than giant reed.
Overall, it could be stated that harvesting giant reed may lead to marginal gains in land saving, unless the crop is cut early enough to allow its resprouting. Alternatively, harvesting close to the full biomass potential should also lead to reduce land requirements. At opposite, a steep decrease in methane potentials can be expected at autumn and winter harvests , when giant reed slows its growth and undergoes senescence.
Dragoni, Federico & Ragaglini, G. & Corneli, Elisa & Nassi o Di Nasso, Nicoletta & Tozzini, Cristiano & Cattani, Sergio & Bonari, Enrico. (2015). Giant reed (Arundo donax L.) for biogas production: Land use saving and nitrogen utilisation efficiency compared with arable crops. Italian Journal of Agronomy. 10. 192. 10.4081/ija.2015.664.