Submitted to: Industrial and Engineering Chemistry Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 3, 2007
Publication Date: December 31, 2007
Citation: Boateng, A.A. 2007. Characterization and Thermal Conversion of Charcoal Derived from Fluidized-Bed Fast Pyrolysis Oil Production of Switchgrass. Industrial and Engineering Chemistry Research. 46:8857-8862. Interpretive Summary: One approach to convert biomass into renewable fuels is to heat it in the absence of oxygen to produce liquids, called bio-oil, that can be subsequently converted into transportation fuels and chemicals. A byproduct of this process is carbon-enriched charcoal. Applying this charcoal into soils has been suggested as an easy means to "sequester" carbon, a process thought to improve soil and also possibly mitigate greenhouse gas emissions, and to bind toxic metal ions in contaminated soils. However, the economics of producing the bio-oil improves when the charcoal, about 15-30% of the original biomass, can be used as a valuable combustion fuel or gasification feedstock. We tested some key fuel properties of the charcoal produced when bio-oil was made from switchgrass and found that it had a lower surface area than common charcoals used for activated carbon applications. However, we also found the charcoal to have a high heating value similar to some coals. Additionally, the charcoal was easily gasified into gaseous products with high heating content. From these studies, it appears that the charcoal could be better used as combustion fuel or gasification feedstock rather than for metal sorption in soils as this may require higher surface and interfacial areas. Although more work is needed to ascertain all its usefulness as a soil amendment substrate, the study provides useful information on the use of switchgrass for renewable fuel production and may advise farmers on economic use of associated co-products.
Technical Abstract: The char coproduct of pyrolysis oil production can have many potential uses. Applications including soil amendment, use as combustion fuel, or gasifier feedstock have been proposed but not extensively studied. In this work the charcoal produced from making pyrolysis oil (bio-oil) from switchgrass in a fluidized bed reactor was characterized to establish its usefulness as combustion fuel or gasification feedstock. It was found that the surface areas were low, typically 7.7 m2/g and 7.9 m2/g, two orders of magnitude of that encountered in activated charcoal. The crystallinity of the structure measured by X-Ray diffraction was high, thereby precluding its usefulness as a sorption agent without further activation, although it may be used for other soil applications including carbon storage and as a nutrient delivery substrate. Further pyrolysis in helium yielded equal amounts of CO and CO2 with kinetics similar to that of coal pyrolysis. Reactivity in CO2 and in air atmosphere resulted in activation energies of 8411 and 11487 J/mol respectively. It appears that the charcoal could be better used as combustion fuel or gasification feedstock rather than for metal sorption in soils as this may require higher surface and interfacial areas.