(440c) An Experimental and Kinetic Modeling Study of Soot and Tar Formation from Fast Pyrolysis of Lignin

Gasification offers the utilization of biomass to a wide variety of applications such as heat, electricity, chemicals and transport fuels in an efficient and sustainable manner. High soot yields in an high-temperature entrained gasification lead to intensive gas cleaning and can cause a possible plant shut down. The reduction of soot formation increases the overall production system efficiency and improves the economic feasibility and reliability of the gasification plant. Soot (or tar at low temperature) is formed through gas phase reactions of volatile compounds, which evolved from pyrolysis at high temperatures and at high heating rates.

The aim of this work is to present a kinetic model for the estimation of tar, soot and gas yields in both gas phase and over char surfaces in a temperature range of 800-1250°C. The model was validated against data from fast pyrolysis in the drop tube reactor. The tar yields and composition from pyrolysis of lignin and extractives (resins) were investigated at four residence times to obtain the kinetic data. The conversion and product distribution of primary, secondary and tertiary tar compounds were related to the yield and morphology of soot particles. The influence of lignin derived compounds (syringol, guaiacol, p-hydroxyphenol) and alkali on the soot formation was studied to predict the soot yield more accurately. The results showed that the proposed kinetic model for the fast biomass pyrolysis and gas phase reactions is relatively simple and predicts the tar and soot yields from pyrolysis of lignin and lignin derived compounds reasonably accurately.

The present results indicated that both lignin samples from softwood and wheat straw obtained higher soot yields than extractives due to the higher amounts of PAHs formed during pyrolysis. The wheat straw lignin showed lower soot and tar yields and higher yields of char and gas when impregnated with potassium, mainly due to the catalytic effect of alkali metals that increase polymerization / cross-linking. Interestingly, the impregnation of wheat straw lignin with potassium decreased the size of soot particles to that from original wheat straw. Particle sizes of soot from pinewood and pinewood lignin were similar. Both wheat straw and pinewood resulted in similar nanostructures of soot from original sample and derived lignin. The pyrolysis of p-hydroxyphenols led to the lower soot yields due to the presence of many hydroxyl groups compared to pyrolysis of syringol and guaiacol.

The model was used to predict the product yields and composition from non-isothermal particles considering the intra-particle heat transport limitations as well as the change in reaction kinetics due to catalytic effect of alkali, heating rate and heat treatment temperature. The total yield of tar was simplified by lumping the different compounds into catechol, acetol, toluene, benzene and naphtalene to predict the volatile yields in both gas phase and over char surfaces. The catalytic effect of potassium on the product yields was related to the concentration of catechol which is higher in the wheat straw compared to wood samples. In this study, the kinetic model was validated against the own experimental results.