Stabilization and upgrading of pyrolysis oil, mixed model compounds, and catalyst deactivation

Doktorsavhandling, 2024

Pyrolysis oil derived from fast pyrolysis of lignocellulosic biomass, as well as hydrogenated vegetable oils (HVO) from waste sources, are promising alternative energy sources used to produce renewable biofuels. However, the high reactivity of unsaturated oxygenated compounds in pyrolysis oil results in aging during storage as well as charring and deactivation of the catalyst under the hydrotreatment upgrading conditions. Catalytic mild hydrotreatment is suggested as a means to stabilize the oil before proceeding to hydrodeoxygenation (HDO) for upgrading. This thesis focuses on reactions during stabilization and upgrading of such oils, specifically, the undesirable carbon loss to solid char as well as aspects of catalyst deactivation during biofuel production. The selection of reaction parameters, such as residence time, temperature, oil composition and type of catalyst are crucial for the process.

In this context, a set of catalysts; Pt/Al2O3, Rh/Al2O3, Pd/Al2O3, Re/Al2O3, and NiMo-S/Al2O3 was used for the mild catalytic hydrotreatment of a simulated pyrolysis oil, containing a comprehensive mixture of various oxygenated groups in a batch reactor. The solid products were extracted into soluble and insoluble solid fractions to determine the degree of polymerization. Firstly, it was found that at elevated temperatures (180-300°C) using NiMo-S/Al2O3 the soluble solids transformed into the bio-liquid product and solid insoluble yield was suppressed and its composition changed to fully developed char. It was also accompanied by a higher degree of HDO causing the stabilization of light oxygenates, and a significant decrease in the formation of heavy oligomers in the liquid phase. Further, furan and sugar compounds were identified to significantly influence the yield of liquid products containing stabilized compounds and contributed to solid formation through oligomerization and polymerization reactions. Then, the catalyst screening results showed that Pd/Al2O3 at 180°C was significantly better in terms of producing a high yield (66 wt%) of stabilized oil with the highest selectivity to low molecular weight products and a high char-suppressing potential (3.3 wt% of solids) in which soluble polymers were more pronounced. The results also indicated that NiMo-S/Al2O3 showed a good performance in catalyzing the conversion of reactive compounds, except furans, into stable light oxygenates with the formed solids rich in heavy insoluble solids (13.6 wt%). The Rh/Al2O3 was comparable to NiMo-S/Al2O3, however, Pt/Al2O3 and particularly Re/Al2O3 rendered poor performances with the lowest yields and qualities of the liquid products, consisting mainly of heavy soluble oligomers, which resulted in a high degree of polymerization. Also, the results indicate that accelerated aging converted 79% of the simulated pyrolysis oil into large oligomers, increasing solid formation to almost double after catalytic stabilization. This suggests that aging before stabilization is highly detrimental to the industrial process. Additionally, dewatered pyrolysis oil can be derived by azeotropic distillation, and the treated bio-oil was compared to pyrolysis oil. The dewatering resulted in the elimination of certain volatile compounds or those highly soluble in mesityl oxide/water, particularly acids. Our findings suggest that dewatering the pyrolysis oil before an upgrading process can open new reaction pathways which lead to improved hydrodeoxygenation efficiency.

Furthermore, sulfided metal catalysts, NiMo/Al2O3, NiMo/SiO2-Al2O3, and NiW/Al2O3 along with bare supports (Al2O3, SiO2-Al2O3, and zeolite Y) were investigated after being used in a refinery green hydrotreating unit for several months. Phosphorus, alkali metals like potassium and sodium, common in renewable feedstocks, were identified as major poisons affecting these catalysts. It revealed that metal catalysts specially the NiW catalyst exhibited an increased affinity for poisons compared to the bare supports, leading to a significant decrease in HDO activity for fatty acid HDO in the lab-scale experiments. Attempts to recover NiMo/Al2O3 catalyst activity through solvent washing with DMSO were not successful as this led to further reduced surface area, although the poisons declined.