The terminology “renewable carbon resource” for virgin and waste biomass is actually a misnomer because the earth’s carbon is in a perpetual state of flux. Carbon is not consumed in the sense that it is no longer available in any form. Many reversible and irreversible chemical reactions occur in such a manner that the carbon cycle makes all forms of carbon, including fossil carbon resources, renewable. It is simply a matter of time that makes one form of carbon more renewable than another. If society could wait several million years so that natural processes could replenish depleted petroleum or natural gas deposits, presuming that replacement occurs, there would never be a shortage of organic fuels as they are distributed and accepted in the world’s energy markets. Unfortunately, this cannot be done, so fixed carbon-containing materials that renew themselves over a time span short enough to make them continuously available in large quantities are needed to maintain and supplement energy supplies. Biomass is a major source of carbon that meets these requirements.

The capture of solar energy as fixed carbon in biomass via photosynthesis, during which carbon dioxide (C02) is converted to organic compounds, is the key initial step in the growth of biomass and is depicted by the equation

C02 + H20 + light 4- chlorophyll —> (CH20) 4- 02.

Carbohydrate, represented by the building block (CH20), is the primary or­ganic product. For each gram mole of carbon fixed, about 470 kj (112 kcal) is absorbed. Oxygen liberated in the process comes exclusively from the water, according to radioactive tracer experiments. Although there are still many unanswered questions regarding the detailed molecular mechanisms of photo­synthesis, the prerequisites for virgin biomass growth are well established; C02, light in the visible region of the electromagnetic spectrum, the sensitizing catalyst chlorophyll, and a living plant are essential. The upper limit of the capture efficiency of the incident solar radiation in biomass has been variously estimated to range from about 8% to as high as 15%, but in most actual situations, it is generally in the 1% range or less (Klass, 1974).