Submitted to: Journal of Analytical & Applied Pyrolysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 2005
Publication Date: May 11, 2005
Citation: Boateng, A.A., Hicks, K.B.,Vogel, K.P. 2006. Pyrolysis of switchgrass (panicum virgatum) harvested at several stages of maturity. Journal of Analytical & Applied Pyrolysis. 75: p.55-64. Interpretive Summary: Interpretive Summary: Switchgrass, a fast growing perennial crop widely cultivated in the United States has been earmarked as a crop that can be used to produce fuels and chemicals on an economically competitive level with fossil fuel in the near future. This can be done by fermenting the sugars in it to make ethanol or by partial burning (pyrolysis) to yield a gaseous fuel called syngas that can burn cleaner than burning the grass as is. Either way the time at which the grass is harvested is important because that determines the sugar level and the condensable residues in the smoke (tar) containing combustible gases. We carried out pyrolysis experiments to investigate how the maturity of the grass and pyrolysis temperature affect the quality and the amount of the combustible gas produced. We determined that the quantity of the gas we measured was lesser for the grass at the senescent stage than for the young plant. For all stages of maturity the quality of the gas i.e., its heating value improved with higher pyrolysis temperatures. These results will be useful to those who will be growing and harvesting switchgrass or using it as a pyrolysis feedstock to generate gaseous fuels.
Technical Abstract: Technical Abstract: The pyrolysis of switchgrass (Panicum virgatum) of the same cultivar, the lowland ecotype 'Cave-in-Rock,' harvested at three stages of maturity was studied in a PY-GC/MS system at the 600 to 1050 oC temperature range. Under these conditions, the decomposition was complete within 20 s yielding char, and two sets of pyrolysis gas, condensable and non-condensable. The former consisted of acetaldehyde (CH3CHO), acetic acid (CH3COOH) and higher molecular weight compounds possibly from the hydroxyl group and from the methoxy groups of the cell wall components. The non-condensable gases were mainly CO, CO2 and C1-C3 hydrocarbons. For these, there was a 900 oC temperature boundary where dramatic change occurred in their evolution rates. Prior to this temperature CO2 decreased but CO and the C1-C3 hydrocarbons also increased almost linearly with temperature. Beyond this temperature boundary the hydrocarbons leveled off but there was a rapid rise in CO and CO2 evolution at a constant CO/CO2 ratio. These suggest the appearance of secondary or tertiary pyrolysis reactions involving rearrangement and release of CO and hydrocarbons prior to this temperature boundary and the release of CO and CO2 from the tightly bond oxygen functionalities including C-C bonds thereafter. At < 750 oC there were modest increases in condensable gas yield and decrease in non-condensable gas due to plant maturity. However the maturity effect on gas yield was statistically insignificant at high temperatures. The energy content of the non-condensable gas measured was about 68% of the gross energy content of the biomass for the early harvest crop and 80% for the mature crop. The activation energy, pre-exponential factor and the reaction order for the decomposition reactions kinetics showed no definitive correlation with plant maturity because of the rapid high-temperature degradation of any lignin bonds that might be associated with cell wall development. The results can assist in understanding the synergies of crop cultivation, harvesting and processing of this dedicated energy crop during its thermochemical conversion.