The Concept of Agroenergy

by Marie DHUICQ on April 19, 2012

The term agroenergy was coined at the end of 1970’s and is used to refer to the science that studies the cultivation of plants intended to be used for producing energy as well as the necessary technology to transform organic matter (i.e. biomass) into fuel.

Crops that are cultivated to be used as biomass fuel are favored by the existence of large areas of uncultivated farmland, the need to fight greenhouse effect and the fact that it could be an alternative for countries to ensure their energy independence.

Every year the earth absorbs 3850 Zettajoules (=3850*1021 J) of energy from solar radiation. Plants convert this electromagnetic energy into biochemical energy (redox potentials) through the process of photosynthesis. The yield of conversion is about 20%. Considering the fact that not all the land is covered by plants, only 3 ZJ are captured by plants. The annual production of biomass is estimated at 170.2 * 109 t (dry matter).

The advantages of using plants to capture solar energy compared to other   renewable sources are numerous. Plants produce their own captors of the solar photons(the molecules of chlorophyll which can be considered as semiconductors). Furthermore, plants  have developed the ability of orientating themselves towards solar radiation. During the entire period of growth, they can accumulate energy in the form of sugar which they will transform into carbohydrates, acids and oil. When harvest, they can restitute this energy to different forms of energy by using them as fuel .

The concept of agroenergy could be seen as a circle of energy production if the production of biomass is followed by its use as a fuel to produce energy that ends up to an end user such as an industrial plant, a house or a city. Biomass during its production captures solar energy and CO2 thus accumulating chemical energy that can be used if processed properly. Processed Biomass will then be used to produce electricity, heat or biofuels. Part of this energy can be exported to other regions, but it is usually more profitable if it is used locally, because there are no losses of energy due to transportation. After processing biomass, the only remaining is ashes which can be used to fertilize part or all of the fields that it came from. In addition, CO2 and water that are produced can be used to enhance the crops.

The ways that organic matter can be transformed (i.e. biomass) into usable fuels, are numerous, because of the diversity of biomass forms. We can separate biomass fuels into two main categories: biofuels, (solid, liquid or gaseous) and solid ones (burning wood, sawdust and crushed, briquettes or charcoal). Biomass often needs some mechanical treatment and sometimes a chemical treatment before being used to produce energy. Among the liquid biofuels, there are alcohols, biogasoline (biodiesels, diester), vegetable oils, oils from pyrolysis and biohydrocarbons. To obtain liquid biofuels, most of the time we need to set up an extraction process coupled with a chemical process so as to change the chemical function of molecules. Among gaseous biofuels, we can find hydrogen (obtained after a chemical extraction) and biogas (obtained through anaerobic fermentation or digestion). Biofuels serve also to produce heat. The electricity production from biomass is always coupled with heat emission. In some cases, the heat simply dissipate in the air, as waste, but nowadays cogeneration plants exist giving us two main products: electricity and heat. As a result the process can reach better yields. Both electricity and heat can be exported to the public distribution grid or stay at the local grid to run autonomously.

Nowadays, agroenergy extends to other sources of biomass, like urban, industrial or cattle feces, but the key principle remains the same: to utilize all the sources we have at our disposal, and if possible without affecting the production of food and arable land. Thus if agroenergy is properly used, it leads towards sustainable development and energy independence from fossil fuels.

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