Perennial grass crops - a carbon neutral biofuel?


Source: European Commission, Environment DG

Perennial crops, such as grasses, are attracting increasing interest as potential biofuel crops. Perennial crops have significant advantages over many annual crops because they require less energy input during growth than annual crops which not only need to be planted each year, but typically require more fertiliser, herbicide and pesticide input. Research on farm-scale cultivation of the perennial grass, switchgrass (Panicum virgatum L), has shown that it can produce five times more energy per hectare than is needed to grow, harvest and process it into ethanol.

Previous research on 'cellulosic' crops, such as grasses, has only been conducted on small scales, leaving open the question of how feasible and efficient farm scale production would be. In this new study, farmers planted switchgrass on ten farms, in fields sized between 3-9 hectares. Inputs which release CO2 directly or during manufacture, such as nitrogen based fertilisers or diesel used in farm equipment and for transporting the harvested grasses, were tracked over a five year period, as was the amount of grass harvested. The researchers found that the ethanol produced from switchgrass yielded on average 540 per cent more energy than was required to grow the crops.

Perennial crops, such as switchgrass, are attracting increasing interest as biofuel crops because they require less intensive farming practices. Factors such as reduced ploughing and lower fertiliser input mean that less CO2 is produced to grow these crops than first generation annual crops such as rape and maize.

For many sources of biofuels, the carbon absorbed from the atmosphere is the same as the carbon emitted during burning, making them 'carbon neutral'. However, for some crops, especially annual crops grown in the Northern Hemisphere, studies suggest that they are 'carbon positive'. Biofuel crops can be said to be carbon positive when they rely on farming practices that result in significant greenhouse gas emissions, for example crops requiring a heavy input of fertilisers, such as maize. In this case, more greenhouse gases are emitted as a result of the farming practices and later burning of the biofuel than are absorbed by the crops during growth. Carbon positive biofuels may release fewer greenhouse gases per unit of power generated than fossil fuels, but they still contribute to global greenhouse gas emissions, when considered over their entire lifecycle.

The researchers in this study also carried out a lifecycle analysis for the production and use of ethanol from switchgrass. This indicated that, on average, using bioethanol from switchgrass is nearly carbon neutral if carbon sequestration in the soil is included in the model and if the waste biomass is also used to produce energy. The researchers are hopeful that further improvements will be possible, through the use of genetic engineering techniques as well as improvements in processing technology.

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