The BioEnergy Cycle

Bioenergy is produced in a cycle. Bioenergy is produced in a cycle. Sustainable use of natural energy mimics the Earth's seasonal variations and minimizes the emission of pollutants into the air, rivers and oceans. Most of the carbon to create it is taken from the atmosphere and later returned to the atmosphere. The nutrients to create it are taken from the soil and later returned to the soil. The residues from one part of the cycle form the inputs to the next stage of the cycle.

Carbon dioxide (CO2) is withdrawn from the atmosphere by the process of plant growth (photosynthesis) and converted into vegetation biomass (trees, grasses, and other crops). Harvested biomass, together with forestry and crop residues, can be converted into building materials, paper, fuels, food, animal feed and other products such as plant-derived chemicals (waxes, cleaners, etc.). Some crops may be grown for ecological purposes such as filtering agricultural run-off, soil stabilization, and providing habitat for animals as well as bioenergy. The solid biomass processing facility (represented by the factory building at the bottom left) may also generate process heat and electric power. As more efficient bioenergy technologies are developed, fossil fuel inputs will be reduced. Organic by-products and minerals from the processing facility may be returned to the land where the biomass grew, thereby recycling some of the nutrients such as potassium and phosphorus that were used for plant growth.

Selected residues from the town may be combined with forestry and crop residues, animal wastes, and biomass crops to provide the feedstocks for a different type of biomass processing (represented by the factory at the top right). This new biomass processing facility (or biorefinery) could make a range of products—fuels, chemicals, new bio-based materials, and electric power. Animal feed could be an important co-product of some processes. Such biomass processing facilities would use efficient methods to minimize waste streams and would recycle nutrients and organic materials to the land, thereby helping to close the cycle.

Biomass products (food, materials, and energy) used by the human population are represented by the town at the bottom of the diagram. The residues from the town (scrap paper and lumber, municipal refuse, sewage, etc.) are subject to materials and energy recovery, and some may be directly recycled into new products.

Throughout the cycle, carbon dioxide from biomass is released back into the atmosphere—from the processing plants and from the urban and rural communities—with little or no net addition of carbon to the atmosphere. If the growing of bioenergy crops is optimized to add humus to the soil, there may even be some net sequestration or long-term fixation of carbon dioxide into soil organic matter. The energy to drive the cycle and provide for the human population comes from the sun, and will continue for many generations at a stable cost, and without depletion of resources.

For additional information, contact the Bioenergy Feedstock Development Program, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6422, (865) 574-576-5132.