Renewable Energy

The use of the biomass as a renewable energy has the advantage of being neutral from the point of view of CO2, contrary to the use of fossil matters (coal, oil, natural gas, uranium). Here are some explanations.

It is important to understand that these reflexions about the greenhouse gas emissions refer to the apparent disordered state of the total cycle of carbon. To summarize, in the cycle of carbon, the carbon dioxide transports carbon. This biogeochemical cycle of carbon takes place simultaneously with other cycles, such as that of oxygen, nitrogen or silicon. These cycles are closed and are imbricated one in another.

The cycle of carbon breaks up into two cycles which, certainly, interact but on scales of different times and space. Thus, we speaks about:

• short-term cycle for the cycle of the organic carbon (alive species, biomass) which takes place over a few decades,
• long-term cycle for the inorganic carbon (not-alive elements, fossil carbon) which achieves on thousands even million years.

At the time of the short-term cycle, the ecosystems, whose forests and plants, synthesize vegetable matter which stores CO2 taken in the atmosphere. Thanks to photosynthesis, the biomass is considered as a genuine pump of CO2, thus constituting a well, a stock, of carbon. Also, CO2 rejected by the energetical use of the biomass is reabsorbed by the plants for their growth. Emitted CO2 thus does not constitute a net contribution since it is integrated in the short-term cycle of carbon. The carbon balance is thus null. If the biomass is not exploited, the organic matter breaks up at the end of the lifetime and releases CO2 sequestered again available for the plants. In this manner, the carbon assessment is null too.

The bioenergies, the green coal, the vegetable carbon are thus renewable and without apparent environmental impact.

Several remarks can emerge from these explanations.

The first relates to the assumption of a normal replacement of the biomass exploited for energy: consumed biomass = regenerated biomass. While simplifying, with each cut tree, a new tree must be planted. In the worls, the land area dedicated to the forests has been in constant increase for two centuries. The sustainable development of the biomass implies to create sufficient carbon stocks to compensate the CO2 emission induced by the bioenergies.

Moreover, one second remark can be made about the apparent balance between CO2 storage and emissions by the biomass in the bioenergy network. During the valorization of the biomass in bioenergy, the totality of CO2 is released in only one time, while the plants which replace the consumed biomass will reabsorb it gradually: one year for cereals, ten years for a tree. So, while this “brutal” increase in CO2 is attenuated by photosynthesis, CO2 present in the atmosphere can contribute to the greenhouse effect. Also, it is important to adjust the cycles of cultures and reafforestations according to the biomass collect to reduce significantly the CO2 rate. The introduction of agricultural waste into the cycle of valorization of the biomass will make possible to increase considerably the proportion used of vegetable matter from carbon short-term cycle.

To these observations we can added the competition begining between crop products dedicated to agroalimentary and those dedicated to the bioenergies, competition we can already observe in several countries. It is thus of primary importance to conceive an adapted production not to have to choose between food and fuel. Many actions to carry out jointly to satisfy the needs are to found cultures allowing an adequate control of vegetable carbon and to develop processes of transformation of agricultural waste in biofuels of second generation.

For a last point, we can take into account the emissions generated for the production of bioenergies. For this purpose, it is necessary to consider the cycle of the overall production, namely: production of the biomass (culture, extraction, refining, conditioning), its transport, its preparation, its valorization in energy and possible treatment of the waste produced by these operations. Although these stages are today relatively close to those employed in the energy network of fossil carbon, powerful processes have to be developed and technological innovations have to be created to minimize the potentially harmful effects (dust, varied gases) and to optimize the output.