Sep 12, 2007
Energy crops: crunching the numbers
Research carried out in Belgium has shown that using maize and grass as energy crops to feed anaerobic digesters saves a significant amount of CO2 emissions from fossil fuels despite the fact that they need to be transported to the plants by road (Journal of Bioresource Technology; doi:10.1016/j.biortech.2007.04.049).
Researchers assessed the net balance of CO2 emissions and renewable energy production for maize and grass crops produced in several agricultural systems relevant for southern Belgium and surrounding areas. They found that after conversion to electricity, the specific CO2 emissions range from 31 to 104 kg of CO2 per MWh of electricity. This corresponds to a significant reduction in CO2 emissions compared with the current reference gas-steam technology, which produces 456 kg of CO2 for every MWh of electricity.
Patrick Gerin, from the Unit of Bioengineering at the Catholic University of Louvain, told environmentalresearchweb: "We took into consideration several agricultural options for these energy crops and wanted to clarify the energy and CO2 gains that can be expected under the regional conditions. The values of these gains are needed by the farmers and the public authorities to calculate the value of green certificates for electricity production from these energy crops. These stakeholders were using values that were in some way arbitrarily established. The work aimed to clarify the calculation of these values, and also to demonstrate the wide range of values that can be reached, depending on the details of the crop production scheme."
The researchers found that energy and CO2 balances can vary over a relatively wide range as a function of the way the crops are grown and handled. "Practitioners should take care of the way they manage their energy crops in order to get the best of it," said Gerin.
In spite of the fossil energy consumed for their production and transformation to biogas, maize and grass energy crops allow a net production of renewable energy together with a significant reduction in fossil-energy-related CO2 emission. Owing to their high-energy contents, maize and tedded grass can be transported over a certain distance (e.g. to a central anaerobic digestion plant) while keeping the energy and CO2 balance positive. However, increasing the transportation distance decreases the advantage of the energetic and environmental balance. Maize presents a better renewable-energy productivity and yield. Grass, while being less productive, offers good energy balances and has several other agricultural and environmental advantages.
"The results of our work will help the stakeholders, such as public authorities and farmers, to adequately assess the green certificates that can be allocated to green electricity production, and to better consider plant-to-plant differences in the way energy crops are managed. The results will also help to improve the practice towards the best management of energy crops," says Gerin.
About the author
Nadya Anscombe is a contributing editor on environmentalresearchweb.