Biomass and the Natural Carbon Cycle
In the natural carbon cycle, carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the earth. During photosynthesis, atmospheric carbon dioxide (CO2) is converted into plant biomass. When carbon in the biomass is combusted or used for energy and materials for industry and agriculture, the biomass carbon is released back in different ways, but mostly as CO2 to the atmosphere. When biomass species are reproduced again by photosynthesis, the cycle repeats (Figure 12.1). For a steady operation, the concentration of CO2 in the atmosphere remains constant. However, the heavy use of fossil fuels, which is not part of the regular carbon cycle, has caused increase in the atmospheric CO2 concentration.
The conversion of biomass into energy is receiving increasing interest due to the need for maintaining the carbon cycle, and the depletion of petroleum fuels. Biomass is likely to play an important role in limiting CO2 emissions while supplying energy needs related to electric power, transportation fuels, and heating. Compared with the redesign of fossil fuel processes for lower CO2 emissions, biomass has a higher effectiveness in reducing CO2 emissions due to potential net zero carbon emission considering the production/use cycle (Hall, Mynick, and Williams, 1991; Overend, 1996; Williams and Larson, 1996). Given the growing shortage of petroleum fuels and environmental pollution (especially the greenhouse gas effect), the need for developing a carbon-neutral renewable source of energy is greater now than ever before (Lal, 2005).