Pyrolysis of switchgrass (Panicum virgatum) harvested at several stages of maturity

The pyrolysis of switchgrass (Panicum virgatum) of the cultivar, "Cave-in-Rock" harvested at three stages of physiological maturity was studied in a PY-GC/MS system at the 600-1050 degrees C 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 C-1-C-3 hydrocarbons. For these, there was a 900 degrees C temperature boundary where dramatic change occurred in their evolution rates. Below this temperature, CO2 decreased but CO and the C-1-C-3 hydrocarbons increased almost linearly with temperature. Above 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 degrees C, there were modest increases in condensable gas yield and decrease in non-condensable gas due to differences in plant maturity at harvest. However, the effect of switchgrass physiological maturity 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 for the decomposition, estimated assuming first order reaction kinetics, showed a linear increase with plant physiological maturity. The results demonstrate that physiological maturity at harvest of switchgrass biomass can result in different concentrations of pyrolysis products at different temperatures. These results also demonstrate the need for additional research with a broader array of herbaceous biomass materials to develop a better understanding of the synergies of crop cultivation, harvesting and processing of dedicated herbaceous biomass energy crops during their thermochemical conversion. (C) 2005 Elsevier B.V. All rights reserved.