A kg/h pilot plant has been built and is currently in operation to convert switchgrass and other biomass into stable fractions suitable for transportation fuel use or further refining. This plant is composed of a solid feeder, a fluidized bed reactor, and several gas-solid separations and cooling-condensing stages that include different solvent sprays. The plant has been designed to control the thermal reactions of the lignin, cellulose and hemicellulose present in the feed biomass to maximize liquid yields (minimum gas and char) boiling in the gasoline-diesel ranges. The cascade system used for condensation permits the separation of product by boiling point and by solubility/compatibility with solvents and will allow the addition of selective catalytic stages. The plant produces several liquid and solid samples that were analyzed using different techniques to perform a mass balance and to characterize the reactions that took place in the fluidized bed reactor. In particular the plant produces some solids called char, soot, micro-char and pitch (soluble precipitate) that needs to be valorized for improving the operational cycle and, ultimately, the economics of the process. These solids and pitch were characterized by using elemental analyses, solvent extractions and subsequent liquid and solid characterization by 13CNMR, GC-MS, TGA-MS, SEM, particle size distribution and XRD analyses.
Operation of the plant showed the importance of the pitch and microcoke deposition in some of the cooling and separation zones of the pilot plant that produce an increase in pressure drop with increasing time on stream. The intensity of this effect, observed by other authors during the operation of large pyrolysis units, depends upon the type of feed and operating conditions used. The characterization of the solid indicates the presence of some partially converted grass into char (retaining structural features of the original biomass) and the formation of soot type particles. The formation of soot particles are a characteristic of the thermal conversion and condensation reactions of aromatics. Micro-char particles may be formed by attrition of char, by the pyrolysis of non vaporized aerosol drops of organics and by the condensation and thermal cracking of high molecular weight fragments produced in the fluidized bed reactor. The pitch may be formed by condensation reactions of organic fragments initially formed by thermal cracking of cellulose and hemicellulose. Further study of formation mechanisms should lead to reduction of the solid fractions and improved operability of such plants.