ABSTRACT
Biomass conversion refers to the process of converting biomass into energy that will in turn be used to generate electricity or heat. The principal categories of biomass conversion technologies for power and heat production are direct-fired and gasification systems. Within the direct-fired category, specific technologies include stoker boilers, fluidized-bed boilers, and co-firing. Within the gasification category, specific technologies include fixedbed gasifiers and fluidized-bed gasifiers. Anaerobic digesters are also considered a biomass conversion technology; however, in this text only a brief, introductory discussion of anaerobic digestion is provided. (Note: Readers desiring an in-depth discussion of aerobic and anaerobic digestion should refer to F.R. Spellman, Handbook of Water/Wastewater Plant Operations, 2nd ed., CRC Press, 2008.)
Anaerobic digestion is an efficient way to refine biomass. Biomass fermentation transforms sugars into an ethanol/water mixture from which the alcohol has to be recovered by means of an energy-intensive distillation process. In contrast, anaerobic digestion transforms many animal and vegetable substances into methane gas with very little expenditure of energy. Methane gas has low solubility in water, evolves naturally, and, as mentioned, can be collected with a minimal expenditure of energy. Anaerobic digestion is the traditional
method for managing waste, sludge stabilization, or releasing energy. The process uses bacteria that thrive in the absence of oxygen. It is slower than aerobic digestion but has the advantage that only a small percentage of the wastes are converted into new bacterial cells; instead, most of the organics are converted into carbon dioxide and methane gas. Anaerobic digestion is a process that is still being actively researched on many levels; its use and operation are well understood in wastewater (sewage) treatment but less is known about potentially important industrial applications.
