Devolatilization and fragmentation of solid lignin-rich residues from bioethanol production in lab-scale fluidized bed reactors

The waste stream of second generation bioethanol production plant mainly consists of lignin - a polymer of several units of not fermentable phenylpropane - coming from lignocellulosic biomasses. This stream can be used only partly to energetically support the process of bioethanol production (about 40%), whereas the fate of the remaining 60% has to be found. Fluidized bed combustion technology can be considered as a viable option to recover thermal power from this waste stream. However, the understanding of the mechanisms of thermo-conversion and attrition of these lignin-rich particles in fluidized beds is scarce and it deserves further investigations. To this end, the combustion of lignin-rich residues were studied in lab-scale fluidized beds with the aid of different diagnostic and experimental protocols to analyze devolatilization, char burn-out and fragmentation of single fuel particles. Comparing devolatilization times with transversal mixing time of typical industrial-scale fluidized bed combustors, wet fuel particles larger than 10mm can be fed directly to the combustor chamber, whereas pre-dried fuel was extremely reactive with the risk of localized emissions of heat and micro- and macro-pollutants once fed to the fluidized bed reactors. Particles do not undergo primary fragmentation, whereas probably secondary and not percolative fragmentation occurs during the late stage of char burn-out.