The technology utilised for biogas production has been around for years. Other than the fact that it is a well-known and established technology, electricity generated of biogas is an eco friendly solution. Unlike other green energy such as wind and solar,  biogas can be quickly accessed on demand. The global warming potential can also be greatly reduced when using biogas as the fuel instead of fossil fuel. But what can biogas be made of? Is it worth investing into a feedstock used for this kind of bioenergy?

Turning biogas to electricity

Biogas refers to gases that are derived from the composition of organic materials like plant remains, resulting from a so called anaerobic digestion process. The main composition of biogas is methane. Biogas possesses chemical energy, and therefore electricity from biogas comes as a result of converting this chemical energy to mechanical energy and finally into electricity.

As we know of its evident benefit of reducing pollution, having a cleaner impact on the environment, and releasing biogenic carbon dioxide, biogas is an overall carbon neutral fuel.

In most cases, biogas is used as fuel for combustion engines, which convert it to mechanical energy, powering an electric generator to produce electricity. Appropriate electric generators are available in virtually all countries and in all sizes. The technology is well known and maintenance is simple. In most cases, even universally available 3-phase electric motors can be converted into generators. Technologically far more challenging is the first stage of the generator set: the combustion engine using the biogas as fuel.

In theory, biogas can be used as fuel in nearly all types of combustion engines, such as gas engines (Otto motor), diesel engines, gas turbines and Stirling motors.

What plants are used for biogas production?

Having established that technology is available, let’s see what kind of plants can be used as a feedstock for biogas. Several studies focused on testing alternative crops for biogas purposes, like Arundo donax (or giant reed) in comparison to conventional annual crops (corn, fibre sorghum and triticale), aiming to evaluate their suitability.

It is very common to use edible crops for biogas production especially where intensive farming systems are established. However, this will not solve the food vs. fuel dilemma, not to mention that using these kind of crops is not sustainable on the long run.

In addition to environmental and economic reasons, numbers also speak for themselves. High yielding, perennial energy crops, like Arundo donax, have the potential to outnumber the conventional annual crops, considering the amount of biogas that can be gained from:

Arundo donax: total production on 1 ha = 16,000 m3 / ha,100 metric tons of silage / ha x 160 m3 of biogas / metric ton)

Corn: total production on 1 ha = 11,000 m3/ ha, 50 metric tons of silage / ha x 220 m3 of biogas / metric ton)

Triticale: total production on 1 ha = 4,800 m3/ ha, 30 metric tons of silage / ha x 160 m3 of biogas / metric ton

Further comparing Arundo donax to other feedstocks, it can be seen that giant reed enables 150% more biogas production than corn, and with 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.

Furthermore, good productivity under low input conditions has been observed in the case of giant reed. In contrast, corn has to be cultivated under a high input regime, while giant reed takes advantage of much lower fertilisation levels (250 vs 100 kg of N/hectare) and they can be rainfed. Arundo donax can be even irrigated with wastewater as well, without seriously damaging the plant or lessen its yield.

Regarding the growth of giant reed as tested by cutting at different times, a modest yield increase was recorded from June to early August. Giant reed is able to persist in environments in which seasonal drought occurs and to recover from water scarcity thanks to its large carbohydrate reserves in the rhizomes.

To sum it up, there are several reasons to invest into Arundo donax as a feedstock for biogas:

  • has an outstanding performance compared to food crops
  • it is not edible, therefore it is not in competition with crops for human consumption
  • it does not compete for lands used for food production, since it adapts to marginal lands
  • its cultivation is sustainable
  • it has low input requirements and low cost production

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