Production of Mesophase Pitch from Gasified Lignin-Derived Tars

Lignin tars are created as a by-product of lignin gasification. Significant effort has gone into minimizing tar production. However, no process has been shown to completely eliminate its formation. As a result, the tar is being used to manufacture low value products such as asphalt, or is being used as a fuel in fixed combustion systems. The research presented here is part of an effort to increase the viability of using lignocellulosic biomass as a feedstock to produce high value products. We tested the postulate that tars derived from lignin could be processed into a mesophase pitch allowable for conversion into high quality carbon fibers. Literature demonstrates that coal tar and petroleum pitch can be transformed into a high quality mesophase pitch. However, the processes involved are complicated and expensive. This is because the impurities - ash content for example, in these materials lead to defects in the pitch and must be removed. Since lignin is without these impurities, its transformation into mesophase pitch should be a less complicated and less expensive process than those for coal or petroleum analogs. Tars were generated using a lab-scale gasification vessel, at an overall conversion rate of 16% tar from kraft lignin. The tar was subsequently filtered and fed to a residue pitching reactor. The pitching reactor was used to generate mesophase pitch under inert conditions. During the pitching step, an unexpected temperature increase likely due to partial oxidation of lignin carbon units with inherent oxygen released during the process was observed. Characterization of both the tar and pitch products was performed by thermogravemetric analysis and differential scanning calorimetry. Further characterization of the pitch products was performed using SEM. A mixture of apparent coke and mesophase pitch products were produced in the pitching reactor. However, the TGA profiles of both products were very similar while their morphologies differed slightly. The condition of ~215°C with a vacuum pressure ranging from ~0.03-0.13 kPa was at the threshold of coke formation while allowing mesophase pitch formation for lignin tar. These results demonstrated that it is possible to generate a mesophase pitch from lignin-derived tars. However, further work is required to perfect the process such that a commercially viable pitch product is generated