Fixed Bed types produce low Btu gases and can use updraft or downdraft approaches. The updraft pulls hot air up from the bottom through the fuel where pyrolysis (decomposition of organics by heat in the absence of oxygen) occurs first, followed by reduction and oxidation. The syngas is not considered “clean” because it contains tars, un-combusted solids, and moisture. An advantage of updraft systems is that they are very scalable; they can be made for small or large systems. Downdraft gasifiers differ in that the oxidation step occurs after pyrolysis. This sequence creates a system where the char filters the gas. This is also a very scalable system that produces clean gas that can go right into a pipeline. A disadvantage in manure management is that manure needs to be pelletized which greatly increases the cost of using a downdraft system.
Fluidized bed gasifiers produce a more energy-dense syngas than fixed bed systems. A fluidized bed consists of heated inert materials through which combustion air rises upward and creates a mass of suspended solids through which the fuel can intermingle. When the temperature is high enough, the gasification reaction occurs. These systems are generally more complex than fixed bed systems and require more careful attention to operation and maintenance.
One of the areas of great interest and research in research years is in developing gasification system that can handle wet biomass, such as raw pig manure, algal biomass, or municipal sewage. These systems require a metallic catalyst and are referred to as catalytic wet gasification or hydrothermal gasification. In addition to a high-energy syngas, this system also produces ammonia, which can be recovered and utilized as a crop fertilizer. These systems require efficient heat-recovery components and monitoring for potential catalyst-poisoning materials (especially sulfur-containing compounds) for optimum performance.
To see an example of current research in this area, visit: auger reactor gasification
Author: Jill Heemstra, University of Nebraska firstname.lastname@example.org
This information is part of the program “Integrated Resource Management Tool to Mitigate the Carbon Footprint of Swine Produced In the U.S.,” and is supported by Agriculture and Food Research Initiative Competitive Grant no. 2011-68002-30208 from the USDA National Institute of Food and Agriculture. Project website: http://www.extension.org/71201.